Special Paper 422: Geology of México: Celebrating the Centenary of the Geological Society of México Volume 422 || The Mojave-Sonora megashear: The hypothesis, the controversy, and

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Roberto S. Molina-Garza and Alexander Iriondo state of knowledgeThe Mojave-Sonora megashear: The hypothesis, the controversy, and the current

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233

Molina-Garza, R.S., and Iriondo, A., 2007, The Mojave-Sonora megashear: The hypothesis, the controversy, and the current state of knowledge, in Alaniz-Álvarez, S.A., and Nieto-Samaniego, Á.F., eds., Geology of México: Celebrating the Centenary of the Geological Society of México: Geological Society of America Special Paper 422, p. 233–259, doi: 10.1130/2007.2422(07). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved.

The Geological Society of AmericaSpecial Paper 422

2007

The Mojave-Sonora megashear: The hypothesis, the controversy, and the current state of knowledge

Roberto S. Molina-Garza*Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, 76230, México

Alexander IriondoCentro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, 76230, México, and

Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA

ABSTRACT

The Mojave-Sonora megashear model, which implies left-lateral strike-slip motion of northern México in Jurassic time, remains one of the most infl uential ideas concerning the geology of México. A comprehensive review of the literature related to this topic does not yet allow resolution of the controversy over the validity of this hypothesis. A clear conclusion is that the original hypothesis was based on a relatively simplistic model of the geology of Sonora, as the basement of the Caborca terrane is not simply a fragment of the Mojave Precambrian basement province of eastern California. Attempts to use quantita-tive techniques in testing the model have yielded results contrary to the hypothesis, such as clockwise rotations indicated by paleomagnetic data, and the diversity and complexity of the basement of Caborca indicated by geochemical and geochronological data. Other quantitative methods such as zircon provenance studies in quartzites of the sedimentary cover yield inconclusive results. The main conclusion of the studies of detrital zircons is that Grenvillean zircons are relatively abundant, but that their presence cannot be attributed solely to sources in the Grenville province in a fi xist model. Stratigraphic cor-relations of upper Paleozoic and Mesozoic rocks in Caborca with similar sequences in California and Nevada do not provide convincing arguments of large displacement, but should be evaluated in more detail. Elements that have the potential to test the hypoth-esis with greater certainty include detailed studies of basement rocks, a refi ned stratigra-phy of the Jurassic volcanic and volcaniclastic arc rocks south of the inferred fault trace, and an increased understanding of depositional trends in the miogeoclinal sequence. Structural studies are sparse in this region. It is particularly important to gain a better understanding of the effects in time and space of Late Cretaceous–Tertiary contraction-al deformation. A tectonic evolution model that does not confl ict with the existing data is the proposal that displacement of a para-autochthonous Caborca terrane may have occurred in the late Paleozoic. Nonetheless, available data and geologic relations in the Caborca region do not require Late Jurassic slip of several hundred kilometers.

Keywords: Mojave-Sonora megashear, Caborca terrane, northwestern México, Proterozoic.

*[email protected]



234 Molina-Garza and Iriondo

INTRODUCTION

The Megashear Hypothesis

The Mojave-Sonora megashear hypothesis is one of the most infl uential models concerning the geology of México. The hypothesis proposed in the early 1970s (Silver and Anderson, 1974) suggests that there is a NW-SE–oriented system of left-lateral strike-slip faults extending from the Mojave Desert, in California, across northern México. As known today, this fault system connects with a hypothetical spreading center in the present-day Gulf of México (Anderson and Schmidt, 1983). The fault system was active in Middle to Late Jurassic time. The original hypothesis was based on the observation of the apparent juxtaposition of two Precambrian provinces of differ-ent age and geologic history in northwest Sonora, in the Caborca region (“This zone ... disrupts two northeasterly trending oro-genic and magmatic belts of Precambrian age” [Anderson et al., 1979. p. 59]). These two provinces were, however, defi ned on the basis of U-Pb zircon ages determined in a few localities, two

of them located to the north of the Mojave-Sonora megashear trace (Anderson and Silver, 1981, 2005; Fig. 1).

The original argument of the juxtaposition of Precambrian basement provinces is more complex than originally perceived. Basement rocks in northwest Sonora, south of the hypothetical trace of the megashear, belong to both the Mojave and Yavapai provinces (Iriondo and Premo, 2003), and they are indeed differ-ent from basement rocks exposed in northeast Sonora. Rocks in northeast Sonora belong to the Mazatzal province (Conway and Silver, 1989). But to this day, the age, tectonic limits, and affi nity of Sonoran basement rocks are not known in suffi cient detail. This is not only because the analytical data of the original publication (Silver and Anderson, 1974) were few and have become avail-able only recently (Anderson and Silver, 2005), but also because only a few other studies have been conducted since that time. The most recent publications on this topic (Iriondo and Premo, 2003; Iriondo et al., 2004; Nourse et al., 2005) indicate that the base-ment of a miogeoclinal Precambrian-Paleozoic sequence in the Caborca region is composed of various blocks of differing affi n-ity. This basement includes elements of the Mojave and Yavapai

RESUMEN

El modelo de la megacizalla Mojave-Sonora, el cual implica desplazamiento lat-eral izquierdo en el norte de México durante el Jurásico, permanece como una de las ideas más infl uyentes en la geología del país. Una revisión general de la literatura relacionada con el tema no permite aún resolver la controversia sobre la validez de la hipótesis, pero una conclusión clara es que la hipótesis original estaba basada en un modelo relativamente simplista de la geología de Sonora, ya que el basamento del terreno Caborca no es un simple fragmento de la corteza Mojave del este de Califor-nia. Intentos de utilizar métodos cuantitativos han dado resultados contrarios a la hipótesis, como el de las rotaciones horarias indicadas por el paleomagnetismo y la diversidad de basamentos en Caborca que sugieren la geocronología y geoquímica; otros métodos producen resultados indeterminados, como la proveniencia de circones en las cuarcitas de la cobertura del terreno Caborca. La conclusión más relevante de esos estudios es la abundancia de circones de edad Grenvilleana, pero su presencia no puede simplemente atribuirse a fuentes en la Provincia Grenville en un modelo fi jista. Las correlaciones estratigráfi cas entre secuencias Paleozoico tardío y Mesozoico en Caborca y secuencias similares en California y Nevada no producen argumentos convincentes a favor de grandes desplazamientos, pero deben considerarse con datos más detallados. Elementos que podrían evaluar la hipótesis con mayor contundencia son estudios más detallados del basamento, una estratigrafía fi na del arco volcánico Jurásico y de las rocas volcanoclásticas al sur de la traza inferida de la falla y un mejor conocimiento de la secuencia miogeosinclinal. Son pocos los estudios estructurales en la región y en particular un problema importante es resolver en tiempo y espacio los efectos de la deformación compresional Cretácico-Terciario. Un modelo que no entra en confl icto con la evidencia existente es la propuesta de que el desplazamiento del terreno parautóctono Caborca haya ocurrido en el Paleozoico tardío. Sin embargo, los datos existentes y las relaciones geológicas en la región de Caborca, no requieren de un desplazamiento de cientos de kilómetros en el Jurásico Tardío.

Palabras clave: megacizalla Mojave-Sonora, terreno Caborca, noroeste de México, Proterozoico.



The Mojave-Sonora megashear: Analysis of the hypothesis 235

provinces, as well as rocks with characteristics of the transition between these provinces (Iriondo et al., 2004). This conclusion is solid as it is based on crystallization ages, model ages, and the age of metamorphism of these basement rocks.

The timing of activity and details of the displacement along the Mojave-Sonora megashear were defi ned in the Caborca region in northwest Sonora. Various authors recognized the miogeo-clinal Neoproterozoic and lower Paleozoic sequence (Gamuza beds and overlying units), and they proposed that the sequence can be correlated, formation by formation, with the miogeoclinal sequence of eastern California and southern Nevada in the Inyo and San Bernardino Mountains (Eells, 1972; Stewart et al., 1984; Stewart, 2003). The correlation of the Caborca sequence with the

section exposed in the Mojave Desert has been used to estimate ~800 km of displacement along the Mojave-Sonora megashear. The approximate age of the fault is based on the apparent juxta-position of Middle Jurassic rocks north of the hypothetical trace of the megashear against rocks of the miogeoclinal sequence south of the trace (Anderson and Silver, 1979).

Geotectonic Signifi cance

Since originally proposed, the Mojave-Sonora megashear hypothesis has been controversial. Yet, the hypothesis was readily adopted by researchers interested in late Paleozoic paleo geographic reconstructions of western equatorial Pangea,

30°N 111°W

Hermosillo

28°N

Caborca

USA

Nogales

Bámuri (1.61-1.75 Ga)

7 Hills (1.64-1.74 Ga)

Fed. Hwy. 2 (1.62-1.66 Ga)

San Antonio (1.65-1.71 Ga)

Cerro Prieto (1.61 Ga)

Mababi (1.63-1.66 Ga)

CityPluton ca. 1.1 GaPluton ca. 1.4 GaLocality in the MazatzalprovinceLocality in the Caborcaterrane

Proposed extension ofCaborca basement

Proposed extension ofNorth America basement

50 km

Cananea

Santa Ana

B. Hill Cucurpe

Opodepe

Mazatán

Guaymas

B. Kino

Aconchi

ElCapitán

Cratonal facies Pz

S. Los Ajos

CaborcaTerrane

NorthAmerica Block

Eugeoclinal facies Pz

C. CobachiS. el Aliso

S. el Encinal

Aprox. limit Cortés Terrane

MSM

110˚ 100˚New Mexico

Texas

Arizona

Oua

chita

Suture

North AmericaBlock

PacificOcean La Paz

Block

Cortés T. CoahuilaBlock

MSM

Caborca T.

Alisitos-Guerrero Terrane

México

PápagoTerrane

CortésTerrane

Figure 1. Proterozoic basement provinces in Sonora and location of U-Pb zircon dates used to present the original hypothesis for the Mojave-Sonora megashear. Modifi ed after Anderson and Silver (1979). Also shown are the Paleozoic cratonal de-posits in northeastern Sonora and the eugeosinclinal rocks from south-central Sonora. The inset shows tectono-stratigraphic terranes and other tectonic features in NW México. MSM—Mojave-Sonora megashear; S.—Sierra; C.—Cerro.




because it offers an elegant solution to the problem posed by the overlap between South America and most of México when clos-ing the Gulf of Mexico (Pindell and Dewey, 1982; Pilger, 1978; and others). The overlap is inevitable and requires repositioning most of México, but as Figure 2 shows, reconstructions proposed more recently do not require large-scale displacement of north-ern México. The overlap is only evident between northern South America and southeast México. The hypothesis as viewed today has been presented in an elaborate model for the evolution of the circum-Gulf region (Anderson and Schmidt, 1983). In this model, cogenetic faults along the Trans-Mexican Volcanic Belt and along the Mesoamerican trench accommodated motion of cortical blocks in central and southern México in a similar direc-tion (southeastward) as the Mojave-Sonora megashear (Fig. 3).

Other lineaments in northern México with a general north-west-southeast orientation may or may not be linked to the Mojave-Sonora megashear hypothesis (Fig. 4). Authors like De Cserna (1971, 1976) have suggested the existence of such struc-tures; one example is the Torreón-Saltillo lineament, which fol-lows the trend of faults and folds along the front of the Sierra Madre Oriental. Of these lineaments, the San Marcos fault (McKee et al., 1984, 1990), and perhaps the La Babia fault (Charleston, 1981; Padilla y Sánchez, 1986), both in Coahuila and north of the trace of the megashear, are the best documented. La Babia fault, also known as the Boquillas lineament, marks the limit between the Sabinas basin and the Burro-Peyotes platform. A component of left-lateral slip along this fault is inferred by the apparent displacement between the Burro-Peyotes platform and the Tamaulipas arch (Charleston, 1981; Padilla y Sánchez, 1986). The fault strikes S55°E. The San Marcos fault, which is also known as the Las Delicias arc, is located along the southern margin of the Sabinas basin, and separates the Sabinas basin from

Coahuila Island. The fault was active in Late Jurassic and Early Cretaceous time as a normal fault (Chávez-Cabello et al., 2005). Yet another structure linked to the Mojave-Sonora megashear, at least conceptually, is the Texas lineament. This structure is mani-fested as a series of east-west fractures and Tertiary volcanism, exemplifi ed by the Balcones fault, across southwest Texas.

The eastward continuation of the megashear is inferred from scarce exposures of pre-Oxfordian rocks in central México (i.e., Jones et al., 1995). The trace is projected eastward below the vol-canic cover of the Sierra Madre Occidental, along the southern margin of Coahuila Island, to the south of exposures of the upper Paleozoic Las Delicias arc, and continues eastward over the Laramide-age Parras basin. In this region, the Torreón-Saltillo or the Torreón-Monterrey lineaments (De Cserna, 1976) may repre-sent the continuation of the Mojave-Sonora megashear. Nonethe-less, these structures are associated with Laramide thrusting of the frontal section of Sierra Parras and Sierra Jimulco, as well as the Monterrey curvature, over the Parras–La Popa basin. The Cenozoic cover of the coastal plain masks the possible continu-ation of the Mojave-Sonora megashear east of the Sierra Madre Oriental. The San Tiburcio lineament in northern Zacatecas, with a general northwest-southeast orientation, has been interpreted as a regional fault with left-lateral slip (Mitre-Salazar, 1989) per-haps linked to the megashear.

Similarly, the hypothetical trace of the Mojave-Sonora megashear is projected to the northwest, to the desert region of southeast California and southern Nevada. Various authors have noticed that the proposed trace of the megashear in the Mojave

Figure 2. Reconstruction of continental masses in North America and South America on the Pangea confi guration of Lottes and Rowley (1990) showing the overlap (gray) of South America over parts of southern México.

NorthAmerica

South America

Maya B.

Chortis B.

México north

Méxicosouth

Mojave-Sonora megashear

Gulfof

Mexico

0˚

10˚N

10˚S

Figure 3. Paleogeographic reconstruction of the Circum-Gulf of Mexico after Anderson and Schmidt (1983). In more recent reconstructions (Lottes and Rowley, 1990), today’s Gulf region is occupied by the Yuca-tan block and the continental masses are much closer. The continental platforms without attenuation effects are shown in light gray.




Desert cuts through apparently continuous late Paleozoic–Triassic stratigraphic trends and patterns (such as isopachs and facies changes; Cameron, 1981; Walker and Wardlaw, 1989). Stone and Stevens (1988) and Walker (1988) proposed instead that the Cor-dilleran margin of California was truncated in late Paleozoic time in the context provided by the Sonoman Orogeny. In the Mojave Desert, the evidence of continental margin truncation includes sedimentation patterns in Mississippian and Pennsylvanian rocks, and the apparent abrupt termination of stratigraphic trends in lower Paleozoic strata in the region of southwestern Nevada (Stevens et al., 1992). Even if the Cordilleran continental margin was truncated by lateral slip, the activity of the Mojave-Sonora megashear in Middle–Late Jurassic time is an element of the ini-tial hypothesis that has proved diffi cult to demonstrate.

The purpose of this article is to present a compilation of existing information on the topic of the Mojave-Sonora megashear since the idea was fi rst introduced. We attempt to weigh the evidence that favors and contradicts the hypothesis. A defi nitive conclusion on the validity of the megashear hypoth-esis, from the point of view of the authors, is not yet within reach. We also present alternatives to the megashear model that explain general stratigraphic, structural, geophysical, and geo-chemical observations.

GEOLOGY OF SONORA

Crystalline Precambrian basement in northwest México was fi rst reported by Cooper and Arellano (1946), who identifi ed a Middle Cambrian fauna in the sedimentary sequence of Caborca overlying a metamorphic basement. Damon et al. (1962) reported the fi rst radiometric dates (K-Ar) in Paleoproterozoic basement rocks, also identifying Grenvillean age magmatism (ca. 1.0 Ga). Several years later, Anderson and Silver (1977) described the base-ment rocks as a Paleoproterozoic metamorphic series assigned to the Bámuri Complex. These authors depicted the metamorphic complex as a volcano-sedimentary sequence in amphibolite to greenschist facies, metamorphed ca. 1.65 Ga, and intruded by 1.4 and 1.1 Ga granitoides.

The fi rst geological map and a general stratigraphic frame-work for northwest Sonora were published in studies conducted by Merriam (1972) and Merriam and Eells (1979); whereas a detailed stratigraphy and correlation of this stratigraphy with the known Neoproterozoic and Paleozoic miogeoclinal sequence of Nevada were established by Stewart et al. (1984) and Longoria and Pérez (1979). In general, the pre-Jurassic geology of Sonora allows distinction between three provinces: northern Sonora, the region of Caborca to Hermosillo, and the region of south-

Torreón-Monterrey lineamentDe Cserna (1976)

Torreón-Monterrey

Texas lineament Caltham lineament

La Babia fault

San Marcos

Mojave-Sonora

Mojave-Sonora megashearSilver and Anderson (1974)

unnamed lineamentPilger (1978)

La Babia or Boquillas faultPadilla y Sánchez (1986)San Marcos faultMcKee et al. (1990)San Tiburcio lineamentMitre-Salazar (1989)

San Tiburcio

Texas lineamentAlbritton (1957)

TMVB lineamentAnderson and Silver (1981)TMVB lineamentAnderson and Silver (1981)

TMVB

Figure 4. Regional structures and lineaments in northern México that may have a bearing on the Mojave-Sonora megashear hypothesis. TMVB—Trans-Mexican Volcanic Belt.




central Sonora. These provinces have been incorporated into the modern literature in the context of tectonostratigraphic terranes (Fig. 1, inset), corresponding to the North America, Caborca (or Seri), and Cortés terranes, respectively (Campa and Coney, 1983; Sedlock et al., 1993).

Caborca Terrane

The most characteristic lithologies of the Caborca-Hermosillo region belong to the Neoproterozoic and Paleozoic. Here the pre-Mesozoic geology consists of carbonate and siliciclastic rocks, considered part of the cordilleran miogeocline, exposed along the western margin of the craton. The sequence rests disconformably on Proterozoic basement. It has an estimated thickness of 3.3 km, and was deposited in shallow-marine environments on the pas-sive margin of Laurentia that developed after Rodinia’s breakup in the Neoproterozoic (Stewart et al., 2002).

Sequences of the mid-Paleozoic (Siluro-Devonian) are only locally preserved in the Caborca terrane. Also, a hiatus that includes most of the Silurian is evident (Poole et al., 2000), and an apparent disconformity in the Hermosillo region separates the Permian from the rest of the Paleozoic sequence (Stewart et al., 1997). The Paleozoic and Neoproterozoic sequences are folded—folds have an east to northeast vergence and locally overthrust Mesozoic sequences (De Jong et al., 1988). This has led some authors to suggest that the trace of the Mojave-Sonora megashear is regionally covered by a Cretaceous age fold-and-thrust belt.

Upper Paleozoic rocks in the Caborca terrane include plat-form carbonate rocks of Devonian, Mississippian, and Pennsyl-vanian age (Brunner, 1976; Stewart et al., 1997). At Sierra Santa Teresa, close to Hermosillo (Fig. 5), the upper Paleozoic is over 2000 m thick. A deep-water siliciclastic sequence ~600 m in thickness, assigned to the Lower Permian, may represent fore-deep to foreland deposits of an orogen developing to the south (in the present paleogeography), or development of a marginal basin concurrent with transtensional faulting (Stewart et al., 1997).

Overlying the Paleozoic sequence rests, disconformably, a Mesozoic succession of transitional to deep-marine environments (Santa Rosa Formation and stratigraphic equivalents such as the Antimonio Formation; González-León, 1980). The discon formity between Paleozoic and Mesozoic strata is not well exposed except at localities east of Cerro Pozos de Serna (Calmus et al., 1997; Lucas et al., 1999), to the south of Cerro El Rajón (Stewart et al., 1984), in Sierra Santa Rosa (Hardy, 1981), and perhaps in the area of Rancho Placeritos (Poole et al., 2000, Fig. 5).

As originally defi ned, the Antimonio Formation has a thick-ness of ~3–4 km (González-León, 1980). The unit was subdi-vided by this author into a Triassic lower member and a Jurassic upper member. From the lower member, Lucas and Estep (1999) have identifi ed an ammonoid fauna of the Spathian (Lower Trias-sic) to the Norian. In their work, Lucas and Estep (1999) revise the stratigraphy of the Antimonio Formation, and restrict the use of Antimonio Formation to the lower member of González-León (1980), with a stratigraphic range from the Spathian to the

upper Carnian. This interval is dominated by siliciclastics (red mudstone and fi ne-grained sandstone) with intercalations of bio clastic limestone. This unit also contains conglomerate with clasts of quartzite, and sandstone petrology suggests a crystalline continental source (Stanley and González-León, 1995). Rocks of the redefi ned Antimonio Formation were deposited in deep-marine environments that become shallower in the upper part of the sequence; conglomerates are interpreted as shelf-slope deposits. Lucas and Estep (1999) assigned shallow-marine facies of the uppermost part of the lower member sensu González-León (1980) to the Río Asunción Formation, which is marked by inter-vals of limestone and sandstone with faunas from the Norian to the Lower Jurassic. Marine to transitional environments are typical of the Río Asunción Formation. High-energy fl uvial and alluvial fan environments may apply to a distinct conglomeratic interval in the Río Asunción Formation containing clasts of quartzite, chert, metamorphic quartz, and granitic porphyry. A Jurassic sequence that was originally assigned to the upper member of the Antimonio Formation is reassigned by Lucas and Estep (1999) to the Santa Rosa Formation as defi ned by Hardy (1981) in Sierra Santa Rosa. This sequence contains a diverse, abundant Liassic fauna, pos-sibly reaching the Pliesbachian (Stanley and González-León, 1995). The Santa Rosa Formation, sensu Lucas and Estep (1999), was derived from the erosion of a volcano-plutonic terrane, and consists of basinal facies dominated by siliciclastics.

The Jurassic sequence is well exposed at Sierra del Álamo. Equivalent strata are exposed at Pozos de Serna (Calmus et al., 1997; Lucas et al., 1999) and Sierra Santa Rosa (Hardy, 1981). These facies are characterized by turbidites that indicate deep-marine environments. Possibly correlative lower Mesozoic strata crop out at Sierra López, Sierra Santa Teresa, and Sierra la Flojera (in the Hermosillo region; Lucas and González-León, 1994; Stewart et al., 1997), and at Cerro El Rajón southeast of Caborca (Fig. 5; Longoria and Pérez, 1979). At this later locality the volcanic component is relatively important.

The tectonic environment of the Santa Rosa Formation at Sierra del Álamo and Sierra Santa Rosa is probably a fore-arc basin developed between a continental volcanic arc and a trench complex (González-León, 1997). The apparent lack of a fore-arc basin and trench-related rocks south of the trace of the hypo-thetical Mojave-Sonora megashear was interpreted by Silver and Anderson (1974) as evidence supporting truncation of the Juras-sic continental arc of northern Sonora by the megashear. This interpretation must be reconsidered.

Cortés Terrane

In central and eastern Sonora, Paleozoic strata consist of eugeoclinal deep-water facies, which include siliciclastic and car-bonate rocks, chert, and less abundant volcanic rocks (Stewart et al., 1990). The eugeoclinal sequence is well exposed in the Barita de Sonora, Sierra el Aliso, Cerro Cobachi, and Sierra el Encinal areas (Fig. 1). Eugeoclinal facies have also been recognized at Tiburón Island and in Baja California. The basement of the sequence is not



SSR

SA

SST

CAL

CBACP

QUI

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USAMEXICO

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BenjaminHill

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Precambrian basement rocks 29˚N

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T. Caborca

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MojaveDesert

México

United Statesof America

Map area

SLG

Figure 5. Pre-Tertiary geology of NW México and SW Arizona (modifi ed after Molina Garza and Geissman, 1998). Abbreviations for localities: SLT—Sierra Los Tanques; QUI—Quitovac; SLG—Sierra La Gloria; CAL—Cerro Álamo; CP—Cerro Prieto; SA—Sierra del Álamo; CR—Cerro El Rajón; SSR—Sierra Santa Rosa; BIZ—Bizani; RB—Rancho Bámuri; SL—Sierra de López; SST—Sierra Santa Teresa; BA—Aconchi batholith; PS—Sierra Pozos Serna; LF—Sierra La Flojera; RP—Rancho Placeritos; SAz—Sierra Azul; SV—Sierra la Víbora; CBA—Cerro la Basura; BM—Baboquivari Mountains; MSM—Mojave-Sonora megashear.




known; the oldest strata are Ordovician in age. Deep-water facies crop out in a belt ~140 km long and ~50 km wide, roughly oriented east-west (Stewart et al., 1990). This belt consists of at least two facies associations, which are both similar and different from eugeo-clinal facies of the North America western cordillera. In the Barita de Sonora region, the sequence has a minimum thickness of 700 m. It consists of (1) Lower to Middle Ordovician graptolitic shale and deep-water limestone; (2) banded chert, shale, and dolomite from the Upper Ordovician; (3) Devonian chert, shale, sandstone and barite; (4) Lower Mississippian limestone; (5) Upper Mississip-pian conglomerate, shale, and chert; and (6) Pennsylvanian silt, sandstone, chert, and barite (Poole et al., 2000, 2005).

The abrupt change from shallow-water platform facies to deep-water facies in the Lower Permian, overlying the miogeo-clinal sequence at Sierra Santa Teresa near Hermosillo, has been interpreted as evidence of the tectonic emplacement of what has been called the Sonoran allochthon (Stewart et al., 1997; Poole et al., 2005). This event marks the accretion of eugeoclinal rocks of the Cortés terrane over the margin of the Caborca terrane. The work of Valencia-Moreno et al. (1999) on the isotopic signa-ture of Laramide granitoides suggests that the Cortés terrane is thrusted over the Caborca terrane.

While juxtaposition of the Cortés and Caborca terranes is well documented in central Sonora, exposures of deep-water Permian strata (Monos Formation) in northwest Sonora at Sierra del Álamo are anomalous in the sense that similar facies have not been identifi ed in other ranges of this area. The westward continu-ation of the Sonoran allochthon is obscured by younger tec tonics and plutonism in both western Sonora and Baja California.

The eugeoclinal sequence is covered in a regional angular unconformity by Upper Triassic strata of the Barranca Group. The Barranca Group thus postdates juxtaposition and thrusting of rocks from the Cortés terrane over miogeoclinal rocks. The Barranca Group comprises a continental red beds unit assigned to the Arrayanes Formation, a coal-bearing marine-transition marine cyclothemic unit assigned to the Upper Carnian Santa Clara Formation, and a thick unit of quartz-cobble conglomerate of the Coyotes Formation (Stewart and Roldán-Quintana, 1991). The Barranca Group was deposited in fl uvial (Arrayanes Forma-tion), fl uvio-deltaic and shallow marine environments (Santa Clara Formation), and alluvial fans (Coyotes Formation). An erosional surface separates the Santa Clara and Coyotes Formations. The Coyotes Formation may provide a stratigraphic link between post-Triassic strata in Sonora. Exposures of quartz-cobble conglomer-ate crop out from Sonoita in northwest Sonora to the oriental limit of exposures of the Barranca Group in east-central Sonora.

North America Block

In northeast Sonora, Paleoproterozoic basement crops out only locally. It consists primarily of schists and associated metavolcanic rocks, intruded by Mesoproterozoic granitoids (ca. 1.4 and 1.0 Ga). The sedimentary cover in northeastern Sonora is thinner than in the Caborca region; it consists mostly of

shallow cratonal facies, and outcrops are also restricted (Krieger, 1961; Anderson and Silver, 1977). The oldest strata include the Bolsa Quartzite and carbonate rocks of the Abrigo Formation. The overlying upper Paleozoic succession is analogous to the well-known Paleozoic rocks of northern Chihuahua and southern Arizona. Overlying Paleozoic and Precambrian rocks is a Middle to Upper Jurassic volcanic sequence, coeval Jurassic granitoids intrude this sequence. These rocks are considered an extension of the Jurassic arc of southern Arizona. The region of southwestern Arizona and northwestern Sonora is characterized by the lack of exposures of basement rocks, and this region has been assigned to the Papago terrane (Haxel et al., 1984; Tosdal et al. 1989), more recently defi ned as the Papago Domain by Anderson et al. (2005). The volcanic sequence includes pyroclastic and volcano-clastic rocks, tuffs, and lava fl ows with relatively evolved com-positions. Locally, volcanic clast conglomerates, sandstone, and carbonate rocks are present. Volcanic rocks range from andesitic to rhyolitic compositions; the volcanic arc they embody was built on continental crust. These rocks have been mapped in Sonora by Corona (1979), Nourse (1995), Rodríguez-Castañeda (1984, 1994) and Iriondo (2001), while Haxel et al. (1984) and Tosdal et al. (1989) mapped these rocks in Arizona. The arc sequence crops out extensively in the ranges north of Caborca and Santa Ana, although they are affected by intense deformation and meta-morphism in the Late Cretaceous and again in the mid-Tertiary. According to Anderson and Nourse (1998, 2005), parts of the Jurassic arc rocks were affected by NW-SE–trending faults in the Late Jurassic. These faults, parallel to the trace of the Mojave-Sonora megashear, formed elongated basins fi lled by syntectonic conglomerate and marine sedimentary rocks of the Lower Creta-ceous (Bisbee Group and correlative strata in Sonora).

Post-Jurassic Stratigraphy and Tectonics

In northern Sonora, overlying rocks of both the North America and Caborca blocks, there is a nonmarine, transitional, and shallow-marine sequence of Upper Jurassic and Lower Cre-taceous strata assigned to the Bisbee Group and partly equiva-lent stratigraphic units (such as Cerro del Oro, El Represo, and Arroyo Sasabe Formations; González-León and Lucas, 1995; Jacques-Ayala et al., 1990; and others). The Bisbee Group is widely distributed in Sonora, Arizona, and New Mexico, and it is interpreted as a sequence deposited in extensional backarc envi-ronments. Common lithologies include volcanic rock conglom-erate, red sandstone and mudstone, carbonate rocks, and locally volcanic rocks. Notable is the lack of both crystalline basement– and Neoproterozoic strata–derived clasts (Jacques-Ayala, 1995) in spite of studies that indicate north-directed paleocurrents or southerly sediment sources. Overlying Lower Cretaceous strata intermediate composition volcanic fl ows are common (Jacques-Ayala et al., 1990); similar rocks crop out extensively in central and eastern Sonora, where they are assigned to stratigraphic units such as the Palma and Tarahumara Formations (Amaya-Martínez et al., 1993; García-Barragán and Jacques-Ayala, 1993).




Bisbee Group strata and older units of the Caborca and North America blocks are intruded by plutonic rocks. Studies have rec-ognized stocks and subvolcanic bodies that range from small to intermediate sizes, as well as Late Cretaceous to early Tertiary batholiths. These rocks were included in the Laramide Sonora batholith of Damon et al. (1983), and were emplaced between ca. 90 and 40 Ma. The vast majority of these intrusions are gran-itoides of the calc-alkaline series, and include peraluminous two-mica granites. The batholiths, typifi ed by the Aconchi batholith (Fig. 5), crop out primarily along N-S–oriented ranges, which are a product of Tertiary extensional tectonics.

De Jong et al. (1988) recognize a Mid-Cretaceous compres-sional event, which does not affect the granodiorite intrusion dated ca. 80 Ma at Sierra la Víbora east of Caborca (Fig. 5). Coeval deformation in western North America is assigned to the Sevier Orogeny. Late Cretaceous to early Tertiary deformation has been identifi ed in southern Arizona, in northwestern Sonora, and in the Altar region (Hayama et al., 1984; Damon et al., 1962; Iriondo, 2001; Iriondo et al., 2005). Late Cretaceous–early Ter-tiary Laramide deformation produced greenschist facies metamor-phism and ductile deformation affecting the Jurassic volcanic arc sequence, the Bisbee Group strata, and Late Cretaceous granites. Metamorphic rocks resulting from this event have been assigned to the Altar Schist, and they could represent an eastward extension of the Orocopia Schist in California (Jacques-Ayala and De Jong, 1996). In fact, the area between Altar and Sonoita (Fig. 5) corre-sponds to a low-grade metamorphic belt, for which a unique asso-ciation with Jurassic activity along the Mojave-Sonora megashear was once theorized. Nonetheless, an Upper Cretaceous protolith for the Altar Schist invalidates this interpretation (García-Barragán et al., 1998). The Altar-Sonoita metamorphic belt is of economic importance as it is host to “orogenic” gold mineralization (Clark, 1998; Jacques-Ayala and Clark, 1998; Iriondo and Atkinson, 2000; Iriondo, 2001). In the eastern portion of Sonora State in the region of Sahuaripa, Pubellier et al. (1995) recognize a Mid-Cretaceous thrust system. This deformation is partly contemporaneous with deposition of the Tarahumara Formation and correlative strata. In northeast Sonora in the Cabullona basin, compressive deformation affected Upper Cretaceous strata. Deformation is similar in style to the thick-skin Laramide deformation of the Rocky Mountains (González-León and Lawton, 1995).

The main events affecting the region during Tertiary time are (1) emplacement of silicic volcanic rocks of the Sierra Madre Occidental; (2) development of metamorphic core complexes (Nourse, 1990, 1995; Nourse et al., 1994); and (3) Basin and Range–style extension—linked too to the opening of the Gulf of California extensional province (Henry and Aranda-Gómez, 2000). Although the Tertiary geology is outside the scope of this article, it is important to mention, for example, that the tectonic fabric imposed on Precambrian through Mesozoic rocks by Late Cretaceous contraction and Tertiary extension may complicate the interpretation of sequences exposed along the hypothetical trace of the Mojave-Sonora megashear, to the north, northwest and east of Caborca, as well as in the Opodepe region.

DISCUSSION OF THE GEOLOGICAL EVIDENCE RELATING TO THE MOJAVE-SONORA MEGASHEAR HYPOTHESIS

Juxtaposition of Precambrian Basement Provinces in Northwestern Sonora

As we mentioned earlier, metavolcanic and metasedimentary rocks from the Bámuri igneous-metamorphic complex (green-schist to amphibolite facies) are intruded by calc-alkaline plu-tonic rocks with crystallization ages between 1.71 and 1.75 Ga (Anderson and Silver, 1981, 2005). All of these rocks were sub-sequently metamorphosed at ca. 1.66 Ga and later intruded by Mesoproterozoic ca. 1.4 and ca. 1.1 Ga granitoids that present neither deformation nor metamorphism (Anderson and Silver, 1981, 2005). This basement is quite different from the present one north of the Mojave-Sonora megashear in Sonora, where crystallization ages for the oldest rocks range between 1.6 and 1.7 Ga and were later affected by a magmatic pulse at ca. 1.4 Ga (Anderson and Silver, 1981, 2005) (Fig. 1). A simplistic model for the distribution of Precambrian basement provinces with respect to the Mojave-Sonora megashear for northern México is presented in Figure 6.

The metamorphic basement rocks present in Sonora have not been studied in detail, but similar, better-characterized rocks from southern Arizona and New Mexico (Yavapai and Mazatzal provinces) have been interpreted to have originally represented volcanic island arc sequences that were accreted to the North America craton during Paleoproterozoic time. In the case of the Mojave province in southern California, the arc has a conti-

200 km

Mazatzal (1.6-1.7 Ga)

Grenville (1

.0-1.2 Ga)

Yava

pai

1.7-

1.8

Ga

Gulf of C

alifornia

114˚W

T28˚N

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106˚W

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ton

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gin (Pz)

CoahuilaBlock

N

1.1-1.3 Ga1.6-1.7 Ga1.7-1.8 Ga

Traza MMS

Moj

ave

S

C

H

SL

Figure 6. Proterozoic basement provinces in SW North America and localities of interest in northern México. A rectangular box depicts the area of geological transect by Cameron et al. (1989). Abbrevia-tions: H—Hermosillo; C—Caborca; S—Sonoita; SL—San Luis Río Colorado; T—Tomochic Caldera; MMS—Mojave-Sonora megashear. Modifi ed after McDowell et al. (1999).




nental nature with magmas incorporating the crustal signature from preexisting Archean crust. There is no doubt this is one of the key aspects for testing any potential correlation between the Paleoproterozoic Caborca terrane rocks with the ones present in the Mojave province. The fi rst study to test that possible cor-relation was attempted south of the megashear in the Quitovac region (Fig. 5; Iriondo, 2001; Iriondo et al., 2004). These authors report U-Pb zircon crystallization ages for Caborca igneous rocks between 1.78 and 1.69 Ga. The inheritance from older crust is not evident in dated zircons for these granitoids (Iriondo 2001; Iriondo et al., 2004), but whole-rock Sm-Nd isotopic determi-nations on the same lithologies provide evidence for the inter-action of these magmas with slightly older crust as inferred by the Nd model ages (2.07 and 1.88 Ga) and the εNd values (+0.6 to +2.6) determined for these rocks. However, Pb-Pb isotopic determinations for these rocks from Quitovac plot away from the fi eld of the Mojave province rocks (Iriondo 2001; Iriondo et al., 2004). These authors conclude that the Caborca basement in the Quitovac region might be correlated with the Yavapai province, as it has been defi ned in northwestern Arizona, or perhaps to a transitional crust between the Mojave and Yavapai provinces, but not directly correlative to the Mojave crust.

In sharp contrast to the Caborca terrane, the basement from northeastern Sonora, also known as the North America terrane or block, is correlated with the Mazatzal province represented in southern Arizona as the Pinal Schist (Silver, 1965). Outcrops of this Precambrian basement are scattered along northern Sonora; some of the localities include areas around Cananea, Sierra San Jose, Sierra Los Ajos, Las Mesteñas, Nacozari, and Anibacochi (Anderson and Silver, 2005). The predominant lithology is a mus-covite schist that locally is intruded by ca. 1.4 Ga gran itoids (i.e., Herrera-López et al., 2005). The basement in northern Sonora is even less understood than the Bámuri igneous-metamorphic com-plex south of the Mojave-Sonora megashear, but it is assumed to have rocks with crystallization ages between 1.6 and 1.7 Ga, sim-ilar to rocks from southern Arizona (Anderson and Silver, 1977, 1981, 2005; Karlstrom and Williams, 1995).

In Quitovac, the Caborca terrane (Yavapai? province) is jux-taposed against the North America terrane (Mazatzal province) along Laramide ductile thrust faults (Iriondo, 2001; Iriondo et al., 2004, 2005). The age range for rocks from this terrane, based on U-Pb zircon geochronology, is 1.71–1.66 Ga. The age overlap between the Caborca and North America terranes for Quitovac rocks presented by Iriondo et al. (2004) proves that the distinc-tion of one province from the other cannot be achieved based solely on crystallization ages, as originally suggested by Silver and Anderson (1974) when stating the Mojave-Sonora megashear hypothesis. Iriondo et al. (2004) suggest that geochemical and isotopic determinations (specifi cally Sm/Nd systematics) will be necessary to distinguish them. In the case of the North America terrane in Quitovac, the Nd model ages (1.8–1.74 Ga) and εNd values between +3.4 and +3.9 indicate that the basement rocks are more juvenile, that is, less enriched than their counterparts on the Caborca terrane. These characteristics were used by Iriondo

et al. (2004) to propose that the North America terrane in Quito-vac could be correlated to the Mazatzal province as defi ned in southern Arizona.

The presence of at least two distinct, juxtaposed Precam-brian basement provinces in northwestern Sonora is indisputable. However, the Caborca terrane rocks in the Quitovac area do not appear to correlate well with the Mojave province; instead these Precambrian rocks have geochemical and isotopic signatures that resemble the rocks from the Yavapai province of northern Arizona or rocks with a transitional signature between those in the Mojave and Yavapai provinces. In addition, it is important to emphasize that the juxtaposition between these basement blocks in northwestern Sonora appears to be a thrust-related to Laramide compressional tectonism, instead of Mid- to Late Jurassic strike-slip displacement. Future work is needed to demonstrate if the Caborca terrane is a composite terrane made up of several of the aforementioned provinces present in southwestern North Amer-ica (Fig. 7; i.e., Iriondo and Premo, 2003).

Displacement of the Cordilleran Miogeoclinal Sequence

The cordilleran miogeoclinal sequence is widely exposed in the Caborca region (Stewart et al., 1984). It is assigned to 14 formations (Fig. 8) consisting primarily of carbonate rocks, ultra-mature quartzite, and fi ne siliciclastic rocks. The Neoproterozoic sedimentary sequence lacks age diagnostic fossils that would permit precise age determinations; it rests disconformably on Precambrian crystalline basement near Aibó ranch southwest of Caborca (Aibó granite; 1.1 Ga; Damon et al., 1962; Anderson and Silver, 1977, 2005; Iriondo et al., 2003a; Farmer et al., 2005). At Aibó ranch the Neoproterozoic sequence overlies well-exposed diabase dikes with K-Ar radiometric dates ca. 900 Ma (Damon et al., 1962), thus indicating the maximum age of the sequence.

Lower Paleozoic rocks include the Proveedora Quartzite and the Puerto Blanco Formation with Lower to Middle Cambrian faunas (Cooper et al., 1952). The existence of the Proveedora Quartzite in northwestern and central Sonora permits the correla-tion of the Caborca sequence with the miogeoclinal section of California and Nevada, where it is equivalent to the Zabriskie Quartzite (Fig. 8). Similarly, an informal unit within the Johnnie Formation, the Johnnie oolite, of the Neoproterozoic has been identifi ed in the Clemente Formation of the Caborca terrane. Ketner (1986) also recognized an equivalent to the Middle Ordo-vician Eureka Quartzite of California and Nevada in the Peña Blanca quartzite (informal) in the region of Cerro Cobachi in the Cortés terrane of central Sonora (Fig. 1). The notable simi larity in grain size and accessory mineral composition between the Eureka and Peña Blanca quartzites supports the notion that they are indeed the same unit. It is important to note, however, that the age of the Peña Blanca quartzite has not been precisely deter-mined. The abrupt disruption of isopachs of the Eureka Quartzite in eastern California supports the hypothesis that, after deposi-tion, this unit was removed tectonically from its original posi-tion. Ketner (1986) suggests that the Eureka Quartzite–displaced




equivalent is located in the Cortés terrane. According to this model, the correlation of these units indicates a southeastward displacement of the Cortés terrane (and by inference the Caborca terrane) of ~1200 km.

In summary, not only are the miogeoclinal successions of Caborca and eastern California correlative at a gross scale, but also comparable individual stratigraphic units exist in both suc-cessions. Stewart (2003) points out other elements that link the Caborca sequence to the rest of the cordillera, such as the distri-bution of Neoproterozoic diamictites, the distribution of Lower Cambrian archaeocyathids, and stratifi ed barite deposits of the Ordovician and Devonian in both the Roberts Mountains and in

the Sonoran allochthon (Fig. 9). An essential test to demonstrate that the miogeoclinal sequence has been displaced, and does not represent a sequence that borders and surrounds a salient in the geography of the craton, is determining the trends in stratigraphic patterns in the Caborca terrane (facies changes, thickness varia-tions, paleocurrents, etc.). In the western Cordillera, stratigraphic patterns trend in a general north-south (actual) orientation, but stratigraphic data for the Caborca sequence is insuffi cient to determine those trends with reasonable precision. Stewart (2003) suggests that existing data for the Caborca terrane indicate a north-south trend of stratigraphic patterns, supporting the notion that the miogeoclinal sequence was displaced. Nevertheless, res-toration of the ~40° post–Middle Jurassic rotation of the Caborca block determined by paleomagnetic data (Molina-Garza and Geissman, 1999) changes the trends to a NW-SE orientation, sup-porting the hypothesis that the sequence is autochthonous, wrap-ping a salient of the Laurentia margin (i.e., Poole et al., 2005).

The observation of (1) thin shallow-water cratonal facies in the extreme northeast of Sonora, (2) thick miogeoclinal platformal facies in central and northwestern Sonora, and (3) deep-water eugeoclinal facies of variable thickness in south-central Sonora is common to the western cordillera (Stewart et al., 1984). This suggests a paleogeography that does not require large-scale modifi cation of the southwestern margin of North America. However, transitional facies between shallow-cratonal strata and platform strata have not yet been recognized in Sonora. In those regions of Sonora that have been mapped with suffi cient detail, eugeoclinal facies are found structurally below the miogeoclinal strata (Ketner and Noll, 1987), or the sequences are juxtaposed by normal faults. The juxtaposition of the migeoclional and eugeoclinal sequences in Sonora occurred before depositon of the Upper Triassic Barranca Group, pos-sibly in the Early Permian (Stewart et al., 1997).

The stratigraphy of the miogeoclinal sequence of Caborca allows direct comparisons with California and Nevada, but Stewart et al. (1990) points out that correlations are not perfect. The Upper Cambrian Dunderberg Shale, with ample distribu-tion in eastern California and southern Nevada, has no correla-tive equivalent in the Caborca sequence. The youngest Cambrian units in the Caborca terrane are the Abreojos and el Tren Forma-tions of the Middle Cambrian.

Silurian platform facies in west-central Sonora (i.e., Rancho Placeritos area, west of Hermosillo) are signifi cant to the discus-sion of the allochthoneity of the Caborca terrane because they could represent the westward continuation of a belt of Silurian platform facies that wraps around the southern North America craton from south Texas through Chihuahua to Sonora (Poole et al., 2000, 2005). Reconstruction of the Caborca terrane to the northwest of its present position, according to the Mojave-Sonora megashear model, would result in a wide gap of Silurian platform facies west of exposures of these rocks in Chihuahua.

Recent work in northern Sonora and southern Arizona suggests that the Cambrian succession of Caborca is autoch-thonous. According to Strickland and Middleton (2000), the

n = 21

n = 31

n = 20

n = 37

U-Pb zircon Age (Ga)

Nu

mb

er o

f S

amp

les

1 .71 1.65

1.74 1.66

1.61.71.8

Mojave

(USA)

Mojave-YavapaiTransition zone

(USA)

Yavapai

(USA)

ProvinceMazatzal

(USA)

"North America"

Caborca Terrane (Mexico)

n = 19

n = 7

Terrane (Mexico)

Province

Nu

mb

er o

f S

amp

les

Province

Figure 7. Histograms for U-Pb zircon ages of igneous rocks from the Proterozoic basement provinces in SW North America. Modifi ed after Iriondo et al. (2004).




Cambrian sequence represents deposition along a recess in an otherwise continuous continental margin wrapping the south-western portion of the craton. The general distribution of the Bolsa Quartzite and the Abrigo Formation, in northern Sonora, suggests that this recess opened to the southwest during Mid-Cambrian times. Cambrian strata in the Caborca region in this

paleogeography represent continuation of that trend to some-what deeper waters (Strickland and Middleton, 2000). It is important to note that detrital zircon populations in the Bolsa and Proveedora Quartzites are indistinguishable (Kurtz et al., 1998). In particular, these units share the dominance of zir-cons ca. 1.1 Ga (n = 14 out of 21 zircons analyzed in the Bolsa

Figure 8. Comparison of stratigraphic columns for Neoproterozoic-Paleozoic rocks from Caborca in Sonora and the Death Valley region in California. Modifi ed after Stewart et al. (1984).

Greenstone

Bonanza King Fm.

Fm. Carrara

Zabriskie QuartziteWood Canyon Fm.

Delamar Mountain Fm.

GreenSpot Fm.

Moonridge Quartzite

LightingGulch Fm.

Wildhorse Meadows Quartzite

Johnnie oolite

Tren Fm.

Arrojos Fm.

Cerro Prieto Fm.Buelna Fm.Proveedora Quartzite

Puerto Blanco Fm.

La Ciénega Fm.

Tecolote Quartzite

Papalote Fm.

Gamuza Fm.

Pitiquito Quartzite

Clemente Fm.

Caborca Fm.

El Arpa Fm.

Greenstone

CaborcaSan Bernardino

Mountains

Basement

Basement

Dunderberg Shale



500 m

Limestone conglomerate

Sandy and silty bioclasticlimestone

Cherty limestone

Limestone

Calcareous siliciclastic rocks

Siltstone and shale

Sandstone and siltstone

ConglomerateP

erm

ian

Wol

fcam

pian

Leon

ardi

an

Penn.

Mississ.

G.

Tr

Per

mia

n

Leon

ardi

an

Pen

nsyl

vani

anM

issi

ss.

Mor

row

an-D

esm

ones

ian

Che

ster

ian

SonoraSierra Santa Teresa(Stewart et al., 1997)

CaliforniaInyo Mountains

(Stevens and Stone, 1988)

Figure 9. Comparison of stratigraphic columns for upper Paleozoic rocks from the southern Inyo Moun-tains in California (Stone and Stevens, 1988) and Sierra Santa Teresa in Sonora (Stewart et al., 1997).




Quartzite, and three grains ca. 1.2 Ga); this observation makes the Bolsa and Proveedora Quartzites distinct from similar strata in California, and suggests that the Caborca terrane is autoch-thonous with respect to the North America terrane.

A sedimentary sequence that is diffi cult to incorporate into the models that displaced the Caborca miogeoclinal sequence to the northwest of its present position crops out at Ejido Aquiles Cerdán near San Luis Río Colorado in the most northwestern sec-tion of Sonora (Fig. 6). This sequence essentially represents cra-tonal facies of the Colorado plateau (Fig. 6), including the Supai Group, Coconino Sandstone, and Kaibab Limestone (Leveille and Frost, 1984). The sequence is metamorphosed to greenschist facies, but the age of metamorphism is not well understood. If this sequence is autochthonous, as it suggests its remarkable sim-ilarity to the sequence of the Colorado plateau, it implies that the trace of the Mojave-Sonora megashear should lie further to the south of this locality.

Zircon Provenance Studies from Quartzites

One important method for testing the correlation between miogeocline rocks from Caborca with the ones from the rest of the cordillera is the zircon provenance studies from Paleozoic quartzites (Gehrels and Stewart, 1998). Such studies are not defi nitive but can be used to obtain some preliminary conclu-sions that are noteworthy. Zircons from the Cambrian Proveedora Quartzite located in the Caborca block were primarily derived from crystalline rocks ca. 1.1 Ga, 1.40–1.45 Ga, and 1.6–1.8 Ga (Gehrels and Stewart, 1998). The most abundant population is ca. 1.1 Ga (20 out of 35 zircons studied) with a few grains at ca. 1.24 Ga. This is very important because there is a lack of abun-dant igneous rocks in the Mojave Desert in eastern California and southwestern Nevada that could have served as the source of these zircons. Gehrels and Stewart (1998) note that the presence of 1.1 Ga plutonic rocks in the Caborca region (i.e., Aibó Granite ) could indicate a local source for the detrital zircons present in the Proveedora Quartzite. However, we cannot discard the possibil-ity that some of these zircons could have been derived from the nearby Grenville province in south Texas and Chihuahua.

Detrital zircons from Mid-Ordovician miogeocline rocks present in Sierra López just west of Hermosillo (Fig. 5) appear to have distal sources with ages between 1.77 and 2.07 Ga (9 grains), as well as ages between 2.47 and 2.90 Ga (5 grains). An upper Devonian quartzite from the miogeocline section at Agua Verde, north of Mazatán in central Sonora (Fig. 1), has detrital zircons with ages at ca. 1.43 Ga (10 grains), between 1.62 and 1.78 Ga (14 grains), and two more grains at 2.07 and 2.47 Ga. These provenance results are very intriguing but are not defi nitive as far as assessing the paleogeographic position of the Caborca block when the miogeocline rocks were formed. Perhaps future zircon provenance studies of more miogeocline rocks will help clarify if these Caborca rocks were formed in the Mojave Desert or instead were formed close to where we fi nd them today in Sonora.

Eugeoclinal Sequence—The Sonoran Allochthon

A notable inconsistency in the Mojave-Sonora megashear model noted fi rst by Poole and Madrid (1988), and later devel-oped by Stewart et al. (1990), involves the stratigraphy and tim-ing of deformation of the eugeoclinal sequence of central Sonora (Cortés terrane, Fig. 10). This sequence is allochthonous, and shares several characteristics with other eugeoclinal sequences along the North America cordillera. This includes general facies associations and zircon provenance data, with a high abundance of zircons with ages > 2.4 Ga. Based on current knowledge of eugeoclinal sequences in Sonora and the western United States, fi nding arguments that clearly support or invalidate the Mojave-Sonora megashear hypothesis is diffi cult at best. The age of thrusting of the Roberts Mountains allochthon in Nevada is not well constrained and the Antler Orogeny itself may have occurred diachro nously along the cordilleran margin. Nonetheless, in con-trast to eugeoclinal sequences in Nevada, which were juxtaposed in the mid-Paleozoic, the eugeoclinal and miogeoclinal sequences of central Sonora were juxtaposed in Permian time (Stewart et al., 1990, 1997). Poole and Madrid (1988) point out that there are some similarities between the Sonoran eugeoclinal allochthon and the Ouachita orogen of western Texas. The most notable of these is the general southwestern tendency of stratigraphic trends in Texas. These authors also draw attention to the apparent west-ward continuation of the Ouachita front into Sonora.

In a recent article, Poole et al. (2005) further developed the hypothesis that the Ouachita-Marathon belt can be extended westward from exposures in the Big Bend region of western Texas toward central Sonora. These authors interpret sedimen-tary facies and structures along the southern margin of Laurentia as the result of diachronous oblique collision, so that the fore-deep facies and foreland basin depocenters become progressively younger toward the west. According to the collisional model of Poole et al. (2005), the deformation along the margin initiated in the Mississippian, and culminated in the late Pennsylvanian in the Ouachita segment of the orogen, in the Early Permian in the Marathon segment, and lastly, in the Late Permian in the Sonoran segment. The age difference is interpreted as the result of the progressive counterclockwise rotation of Gondwana dur-ing collision. In turn, this model requires that Sonora remain in a paleogeography similar to the paleogeography present during the late Paleozoic.

Comparison of the Upper Paleozoic Marine Sequences of East-Central California and the Caborca Terrane

During the late Paleozoic, carbonate platforms developed along the southwest margin of North America. In northeast Sonora, these facies are represented by the Escabrosa, Horquilla, El Tigre, Earp, and Epitah Formations from the Carboniferous to the Permian. These formations are well known in northern Chi-huahua, southern Arizona, and southwestern New Mexico. In the Caborca terrane, the sequence is represented by a thick succes-



25°N

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115°W 110°W 105°W

0 500 1000

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SB

SM

M

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SA GF

SierraNevada

Nevada MesozoicMarine Province

Tr-J Volcanic arc

ColoradoPlateau

PeninsularRangesbatholith

Las Deliciasvolcanic arc

Ouachi ta-Mara thon B

elt

Sevier B

elt

?

Neoproterozoic-Paleozoicmiogeoclinal sequence

Paleozoic eugeoclinalsequence

?

?

P-Tr volcanic arc sequences

Mesozoic batholithic series

Nevada Tr-J marine sequences

Volcanic arc sequences (includes localities in central Mexico)

PacificOcean

?

Nazasvolcanic arc

MSM

Z

CH

SMO(~150 Ma)

A (~205 Ma)SD

T M

H

CA (~158Ma)

IM

KM

CortésTerrane

CH

CaborcaTerrane

CJ

RM

SMh

Figure 10. Map with tectonic elements of the southwestern United States and northern México. Also shown are the localities for the Triassic-Jurassic magmatic arc of north-central México (open hexagons). Abbreviations: Localities in the Mojave Desert: SA—San Andreas fault; SB—San Bernardino Mountains; EP—El Paso Mountains; SM—Soda Mountains; GF—Garlock fault; IM—Inyo Mountains. Localities in México: C—Caborca; H—Hermosillo; CH—Chihuahua; SMO—Santa María del Oro; CJ—Ciudad Juárez; SD—Sierra del Diablo; A—Acatita, Sierra Las Delicias; T—Torreón; CA—Caopas-Pico de Tera; Z—Zacatecas; H—Valle de Huizachal; M—Pozo Mayrán I; MSM—Mojave-Sonora megashear. Localities in Nevada: RM—Roberts Mountains; SMh—Shoshone Mountains. TR-J—Triassic-Jurassic; KM—Klamath Mountains; P-TR—Paleozoic-Triassic.




sion of seven informal units exposed at Sierra Santa Teresa near Hermosillo (Stewart et al., 1997). The Sierra Santa Teresa upper Paleozoic section is ~1500 m thick; it includes shallow-marine car-bonate platform facies (mudstone, wackestone, and packstone) of the Upper Mississippian to the mid-Pennsylvanian, a hiatus from the late Pennsylvanian to Wolfcampian, 100 m of fusulinid pack-stone and briozoan fragments, and 600 m of calcareous siltstone, fi ne sandstone, and calcarenita with Leonardian to Guadalupian fauna. This sequence bears no resemblance to the upper Paleo-zoic sequence of the Inyo Mountains (Stevens and Stone, 1988). The Inyo Mountains sequence is characterized by the presence of Mississippian shales, Pennsylvanian calcareous sandstones and bioclastic cal careous siltstone, and an expanded Wolfcampian section of calcareous limolite (nearly 2000 m thick), and by cal-carenita, bioclastic limestone, and limestone conglomerate of the Leo nardian, covered by siliciclastic rocks (Fig. 9).

Correlation of the Marine Triassic Section of Northwest Sonora with the Mesozoic Marine Province of Nevada

Stanley and González-León (1995) have suggested that the Antimonio and Río Asunción Formations of Triassic to lowermost Jurassic age (González-León, 1980; Lucas and Estep, 1999), and exposed at Sierra del Álamo Muerto west of Caborca (Figs. 6 and 10), may be correlated with the Triassic-Jurassic sequence of central Nevada (Luning and Gabbs Formations; Oldow, 1984; Oldow et al., 1993).

At Sierra del Álamo, the Antimonio Formation overlies the Monos Formation, which consists of limestones bearing giant fusulinids characteristic of allochthonous terranes of the cordillera, such as the Klamath terrane of northern California. González-León and Stanley (1993) proposed including rocks of the Antimonio and Monos Formations in the suspect Antimonio terrane. The Antimo-nio terrane has been interpreted as allochthonous with respect to the miogeoclinal sequence of Caborca (González-León and Stan-ley, 1993; Molina-Garza and Geissman, 1996). The allochthoneity of the terrane cannot be clearly demonstrated as there is no clear evidence of a suture, but Lucas and Estep (1999) show the Río Asunción and Santa Rosa Formations in thrust contact over the Antimonio Formation. Based on the correlation of the marine Tri-assic sequences, González-León and Stanley (1993) infer that the Antimonio terrane is a fragment of the marine Mesozoic basin of Nevada, which has been tectonically transported ~1000 km south-ward until reaching its present position.

The basal coarse siliciclastic member of the Río Asunción Formation (quartzite and metamorphic quartz boulder conglom-erate) is inserted between two marine horizons with ammonoid faunas; Hettangian below and Sinemurian above. This con-glomeratic member has been recognized at Pozos de Serna and Sierra Santa Rosa (Fig. 5) and it is possibly equivalent to the Coyotes Formation of the Barranca Group of central Sonora. The Barranca Group overlies miogeoclinal and eugeoclinal strata in that region. Outcrops of the Liassic conglomerate are exposed in a band ~300 km possibly from Quitovac, in north-

western Sonora, to the Barranca Group basin in central Sonora. This correlation suggests that the inferred allochtho neity of the Antimonio terrane may be only valid for pre–Middle Jurassic time. This is because the Liassic boulder conglomerate over-lies sequences characteristic of the Caborca terrane at various localities (i.e., Calmus et al., 1997).

The correlation of the Lunning Formation with Triassic rocks of Sierra del Álamo in Sonora is based, however, on a diverse subtropical reef fauna of the Norian, which lacks diagnostic pro-vincialism. This fauna is present in a biostromal interval at the top of the Antimonio Formation, and contains coral, bivalve, sponge, and chephalopod taxa with affi nities that allow this fauna to be equally associated with Norian faunas of the Mesozoic Marine province of west-central Nevada, northern Peru, or the western Thethys region; although the Antimonio fauna share more taxa with rocks from Nevada. Rocks from Sierra del Álamo also share taxa with the east Klamath and Stikinia terranes. Stanley and González-León (1995) suggest that thickness and lithofacies at Sierra del Álamo compare favorably with the Nevada sequence. For instance, in Sonora as in Nevada limestones dominate dur-ing Norian time. The succession in Nevada contains, however, expanded Lower and Middle Norian sections with much thicker carbonate intervals than at Sierra del Álamo. Also, at Sierra del Álamo, Early Norian rocks are dominated by siliciclastics.

Recent reviews of the ammonoid sequence of Sierra del Álamo (Gómez-Luna and Martínez-Cortés, 1997; Lucas and Estep, 1999) do not support the correlation of Stanley and González-León (1995). Oldow et al. (1993) show that the oldest rocks in the Lun-ning interval are from the Middle Triassic (Ladinian), while Lucas and Estep (1999) recognize ammonoids from the Smithian and Spathian stages (Lower Triassic) in basal strata of the Antimonio Formation. The ammonoid fauna in the Sierra del Álamo section also includes components of the Dilleri biozone (Carnian), the biozones Suessi, Columbianus, Cordil leranus (Upper Norian), and the Crickmayi zone of the latest Triassic (Rhaetian). According to Stanley and González-León (1995), this fauna resembles that reported for the middle part of the Gabbs Formation in west-central Nevada. We note that the Mesozoic marine sequence from Nevada contains a unit that may be correlated with the coarse silici clastic interval (of quartz-boulder conglomerate) of the Río Asunción Formation. However, lithologically, the Dunlap Formation and the Sierra del Álamo sections do not resemble each other; there is a greater abundance of carbonate facies in the Liassic shallow-water facies from Nevada, whereas these facies are nearly absent in northwestern Sonora.

In their correlation of sequences from Nevada and the Anti-monio terrane, Stanley and González-León (1995) point out that the ammonoid fauna characteristic of the Crickmayi zone resembles one reported for the Gabbs Formation; when these authors refer to ammonoid fauna that conforms to the Dilleri zone, they fi nd affi nity with the fauna present in the Hosselkus Limestone of California. Ammonoides of the Dilleri zone of the Late Carnian occur at Sierra del Álamo in thin micrite facies, bioclastic limestone, and fi ne-grained sandstone, interpreted as




deep-water facies (González-León et al., 1996). Overlying this interval, the section includes sandstone, siltstone, and scarce limestone beds, capped by an interval of impure carbonates (biostromes) interstratifi ed with shale. Near the base of this sec-tion there is a fauna of the Middle and Upper Norian (Colum-bianus or Cordilleranus zone of the lower Upper Norian). In turn, overlying this package there are general representatives of the Amoenum zone. These strata are covered by bioclastic lime-stone, impure limestone, and siltstone with an Upper Norian fauna (Crickmayi zone), and above this interval ammonoids of the Upper Hettangian were reported.

Rocks of the Lunning association were originally defi ned in the central region of the Pilot Mountains in Nevada as a sequence of dark dolomite, limestone, argillite, and shale, interstratifi ed with coarse clastic rocks. In the Shoshone Mountains, three ammonoid associations have been recognized: the “Carnites” fauna of the Carnian is concordantly overlain by a fauna char-acteristic of the Tropites zones (Tropites welleri subzone) of the Late Carnian. The third association corresponds to the Guem-belites fauna of the Late Carnian–Early Norian (Muller et al., 1936). The Guembelites zones were referred by Silberling and Tozer (1968) to the Kerri zone. In turn, the Gabbs Formation is defi ned as successions of shale and siltstone interstratifi ed with black limestone overlain by bioclastic sandy and shaley lime-stone. The fauna in the lowest strata is from the Late Norian, assigned by Silberling and Tozer (1968) to the Suessi zone. Tozer (1980) indicates that the fauna from this zone occurs throughout the Late Norian, and he assigned instead basal strata of the Gabbs Formation to the Amoenum zone. Faunas forming the middle and upper members of the Gabbs Formation were correlated with the Crickmayi zone (Tozer, 1980).

In the standard zonation, the Dilleri zone underlies the Welleri zone. Thus, there is no correspondence between the Nevada ammonoid succession and that from Sierra del Álamo. Furthermore, the sequence in Nevada at the Shoshone Mountains is much more complete than in Sonora, including, in ascend-ing order, the Welleri, Macrolabatus, Kerri, and Magnus zones (Gómez-Luna and Martínez-Cortés, 1997). Gómez-Luna and Martínez-Cortés noted that biostromes in Nevada at the Pilot Mountains that share coral and sponge taxa with Sonora occur between the Kerri and Magnus zones, while in Sonora they are restricted to a younger interval between the Columbianus and Cordilleranus zones. The later zones have no equivalent in Nevada. Thus on the basis of the Cepahlopode zonation, Gómez-Luna and Martínez-Cortés (1997) fi nd no support for the correla-tion of Stanley and González-León (1995).

Ductile Deformation with NW-SE Orientation Representing the Juxtaposition of a Triassic Intrusive against Jurassic Volcanics Close to Sonoita, Northwest Sonora

Campbell and Anderson (1998, 2003) report the presence of a ductile deformation zone in Sierra Los Tanques, close to the town of Sonoita in NW Sonora. They interpret this zone as the origi-

nal trace of the Mojave-Sonora megashear (Fig. 5). Ductile fab-rics are represented by banded ultramylonite and coarse-grained mylonite with NW-SE orientations and a variable thickness from 1 to 3 km. Triassic age granitoids (225 Ma; U-Pb zircon, reported in Stewart et al., 1986) are juxtaposed against Jurassic volcanic arc rocks. Subhorizontal lineations present along the ductile zone suggest lateral displacements; however, our own assessment of kinematic indicators in the classic outcrops of the Mojave-Sonora megashear in Sierra Los Tanques was inconclusive in determin-ing the real sense of displacement along the ductile fault zone.

Caudillo-Sosa et al. (1996) propose that Proterozoic base-ment rocks and Jurassic volcanic arc rocks in the Quitovac area just SW of Sonoita were deformed during a transpressional event directly associated with the Jurassic Mojave-Sonora megashear. This age constraint is based on the argument that Late Cretaceous granitoids in the area were not affected by this ductile defor-mation event. However, more detailed work by Iriondo (2001) and Iriondo et al. (2005) suggests that these intrusive rocks in Quitovac, dated with U-Pb zircon geochronology at ca. 79 Ma, are affected by the thrust-related, ductile deformation event. Sup-porting this idea, Jurassic volcanic and volcanoclastic rocks from the Comobabi and Baboquivari Mountains in Arizona, ~100 km north of Quitovac, present a similar style of deformation and metamorphism that has been proposed to be of Late Cretaceous to early Tertiary age (Haxel et al., 1984). Similar fabrics are also found at Cerro Prieto, ~50 km south of Quitovac, where Calmus and Sosson (1995) found Proterozoic rocks thrusting Jurassic arc volcanic rocks.

Campbell and Anderson (1998, 2003) report mylonites with subvertical foliations from Sierra Los Tanques. In addition, some of the lineations are locally subvertical with kinematic indi cators suggesting both NE and SW thrusting. These compressional structures, forming a positive fl ower structure, are interpreted by these authors to represent transpression during Jurassic strike-slip movement associated with the Mojave-Sonora megashear.

There is general agreement about the geometry of the struc-tures, but the main problem is defi ning the age of the deforma-tional event forming them. While Campbell and Anderson (1998, 2003) propose a Jurassic age for the ductile fabrics in Sierra Los Tanques, Iriondo (2001), Iriondo et al. (2005), and Haxel et al. (1984) propose a Late Cretaceous age for the fabrics in NW Sonora and SW Arizona. In particular, a structural and geo-chrono logical study by Iriondo (2001) and Iriondo et al. (2005) in the Quitovac area suggests that the ductile deformation pres-ent in the region, including the mylonite zone from Sierra Los Tanques, is the result of Laramide compressional tectonism (Late Cretaceous–early Tertiary). The different thrust units proposed in Quitovac were later tilted as a result of movement along Basin-and-Range normal faults (Iriondo, 2001; Iriondo et al., 2005). It is possible that some of the ductile fabrics in the region are the result of reactivation of older fabrics, but studies by Iriondo and colleagues basically proved that the ductile fabrics are not all exclusively associated with Jurassic tectonism in connection with the Mojave-Sonora megashear as previously proposed.




Ductile Deformation in the Opodepe Region: Additional Structural Evidence

In the Opodepe region northeast of Hermosillo (Fig. 5), duc-tile and fragile deformation has been reported to be associated with the hypothetical Jurassic Mojave-Sonora megashear ( Rodríguez-Castañeda, 1996). Proterozoic rocks with NE-SW foliation, and variable dips (~60°), present kinematic indicators suggesting dextral slip along a ductile structure. This zone of deformation in Opodepe has been interpreted as an antithetic structure to the main Mojave-Sonora megashear fault. The deformation is considered pre–ca. 36 Ma because an alkaline granite of that age intrudes the ductile fabrics; however, the pluton itself presents ductile fabrics with the same NE-SW orientation. The proposed Jurassic age for the deformation in Opodepe is based solely on “regional tectonic considerations” (Rodríguez-Castañeda, 1996).

It is important to consider the possibility that this mylonite zone in Opodepe could be related to late Tertiary metamorphic core complex deformation. However, Calmus et al. (1997) ques-tion the presence of a metamorphic core complex in Opodepe, and instead propose that the ductile fabric may be of Proterozoic age. Obviously, the presence of ductile fabrics in Sonora does not imply that the resulting mylonites have any particular age.

Even more speculative is the inference made by Araiza-Martínez (1998) that a gneissic fabric present in Proterozoic rocks in Estacion Llano, ~150 km north of Hermosillo, is attrib-uted to the Jurassic Mojave-Sonora megashear based solely on its proximity to the proposed trace of the Mojave-Sonora megashear. Precambrian gneisses assigned to the San Francisco Group over-thrust Jurassic volcanoclastic rocks with fairly low metamorphic grade. Locally, in the San Francisco gold mine, deformation with the same characteristics is associated with metamorphism in amphibolite facies, perhaps indicating that the deformation could be Proterozoic. There is no evidence that metamorphic grade associated with Late Cretaceous–early Tertiary compressional deformation, or for that matter Jurassic deformation, exceeded greenschist facies in the Altar region (Hayama et al., 1984). The age interpretations presented by Araiza-Martínez (1998) are a good example of the infl uence of the Jurassic Mojave-Sonora megashear hypothesis on many geologists working in México.

Interpretation of the Barranca Group as the “Displaced” Deltaic Facies of the Chinle Group from Southwest North America

Marzolf and Anderson (1996, 2000) have suggested that the Triassic Barranca Group, in central Sonora, was displaced from its original position in southern Nevada via the Mojave-Sonora megashear. The Barranca Group was interpreted earlier as a con-tinental rift basin fi ll (i.e., Stewart and Roldán-Quintana, 1991). The rift would have developed over a late Paleozoic orogen that deformed the Sonora miogeocline and eugeocline sequences. Deposition of the Barranca Group in an elongated basin bordered by normal faults, or a series of basins with these characteristics, is

diffi cult to demonstrate. Furthermore, most of the sandstones in the Barranca sequence are relatively mature with abundant quartz and the section lacks volcanic rocks typical of continental rifts; only the Santa Clara Formation of the Barranca Group contains immature arkoses. Paleocurrent indicators suggest south-fl owing streams, and these are not consistent with the expected east-west fl ow that the orientation proposed for the hypothetical Barranca rift basin (Stewart and Roldán-Quintana, 1991).

Detrital zircons in sandstones of the Santa Clara Forma-tion (Gehrels and Stewart, 1998) form groups with ages ca. 1.42 Ga (17 grains), 250–280 Ma (14 grains), and 225–235 Ma (3 grains). The abundance of Permo-Triassic zircons indicates a relatively close magmatic source of that age, and the probable source is Permo-Triassic rocks of the continental arc of Chihua-hua, Coahuila, and northeast México (Torres-Vargas et al., 1999; McKee et al., 1990). Triassic zircons can also be derived form the Coahuila block to the east. Sources of magmatic zircons with those ages in the Mojave Desert are scarce but do exist (Miller et al., 1992; Barth et al., 1997). González-León (2005, personal communication) notes that igneous rocks in the age range from 240 to 280 Ma are common in southeast California and west-central Nevada. Triassic plutons have been also recognized near Sonoita (Stewart et al., 1986; Campbell and Anderson, 1998). A test for the proposed correlation of Triassic rocks in Sonora with the Chinle Group would be the presence of zircons with ages between 500 and 525 Ma, which are characteristic of the Osobb Formation (of the Auld Lang Syne Group in Nevada) in what has also been interpreted as the westward continuation of the Chinle Group drainages (Riggs et al., 1996).

Apparent Truncation of the Jurassic Continental Arc Sequence of Northern Sonora and Southern Arizona in the Caborca Region

Anderson and Schmidt (1983) and Anderson et al. (1979) have suggested that the Sonoran Jurassic continental arc was truncated by the Mojave-Sonora megashear. This segment of the Jurassic arc is typically included in the Papago terrane of Haxel et al. (1984) or Papago Domain of Anderson et al. (2005). The interpretation of truncation of the arc is based on the premise that outcrops of Jurassic volcanic rocks do not exist south of the inferred trace of the megashear. This argument is not entirely valid and needs revision. For instance, the arc sequence north of Caborca at Cerro la Basura and Cerro Álamo (Fig. 5) con-tains marine rocks (Corona, 1979) with Sinemurian ammonites ( García-Barragán and Jacques-Ayala, 1993); this indicates the presence of a Liassic marine basin south of the arc.

In the mountains of Sierra Santa Rosa, Cerro El Rajón, Sierra del Álamo, and Pozos de Serna, there are outcrops of marine sequences with similar Liassic fauna (Hardy, 1981; Lucas et al., 1999; Stanley and González-León, 1995; Longoria and Pérez, 1979), thus correlative with rocks in the southern part of the arc sequence. Besides sharing an ammonoid fauna, rocks in those ranges show a continuity that suggests the structural integ-




rity of the basin in which they were deposited. These sequences have been interpreted as fore-arc basin deposits (González-León, 1997). The sequence, as mentioned earlier, is characterized by turbidites and tuffaceous horizons; lateral facies changes are also important. Locally, there are conglomerates with abundant clasts of volcanic rocks and granite. Sandstones of the Santa Rosa Forma tion have a sedimentary petrology that indicates the rock components were derived from a volcanic arc (Stanley and González-León, 1995). For this reason, the present distribution of the arc in northern Sonora and a fore-arc basin in the Caborca region do not require displacement between these two elements. We must add that several outcrops of volcanic rocks of Jurassic age exist south of the trace of the megashear. Jurassic porphyry crops out overlying the Paleoproterozoic Bámuri Metamorphic Complex near Cerro Bámuri (P. Castiñeiras, 2004, personal communication), Jurassic andesites have also been reported for Cerro Chino (Longoria and Pérez, 1979), and Jurassic rhyolites are exposed near Cabo Tepoca (Anderson et al., 2005). These authors entertain the idea that these rocks were thrust southward, although thrusts with this vergence have not been reported.

Other authors have proposed that rocks of the volcanic arc extended, at least partially, south of the trace of the hypothetical megashear (Jacques-Ayala, 1995). This interpretation is based on observations of conglomerates of the Bisbee Group along the trace of the Mojave-Sonora megashear with abundant clasts of volcanic rocks. Paleocurrent indicators in the conglomerates suggest sources to the south. This, combined with the low fre-quency of clasts derived from the miogeoclinal sequence sug-gests, as Jacques-Ayala (1995) inferred, that rocks of the vol-canic arc covered the mioclinal sequence during deposition of the Bisbee conglomerates.

Yet another important contribution to the debate over trunca-tion of the Jurassic arc is the observation of a Middle Jurassic vol-canic sequence in tectonic contact with Precambrian basement at a locality south of the trace of the Mojave-Sonora megashear (Calmus and Sosson, 1995). This locality, east of Puerto Peñasco in northwest Sonora (Fig. 5), clearly demonstrates that the Mojave-Sonora megashear, if it exists, must be located south of these exposures.

Jones et al. (1995) interpret volcanic rocks in north-central México in northern Zacatecas and Durango as the “displaced” counterpart of the volcanic arc of Sonora. Volcanic and volcano-clastic rocks in north-central México include the Caopas schist and the Nazas Formation (Grajales-Nishimura et al., 1992). Although the age and correlation of the Nazas Formation present a complex stratigraphic problem (Barboza-Gudiño et al., 1999), the presence of a Triassic-Jurassic volcanic arc in north-central México in southeast Chihuahua, southern Coahuila, and southern Tamaulipas (Fig. 10) is reasonably well documented. Rhyolites at the Mayran-1 well in Coahuila produce a two point Rb-Sr iso-chron with an age of 220 ± 20 Ma (Grajales-Nishimura et al. [1992]). Also, a K-Ar date on intrusive rocks at Santa María del Oro in Durango is also Jurassic (ca. 150 Ma; Grajales-Nishimura et al., 1992). U-Pb and Ar-Ar data for intrusive rocks at Sierra

Las Delicias in Coahuila give dates ca. 215 Ma (Molina-Garza, 2005). Finally, a discordant U-Pb date on zircons from an intru-sive related to the Caopas schists in northern Zacatecas is of 158 ± 4 Ma (Jones et al., 1995).

The hypothesis that rocks in north-central México were displaced by the megashear is inconsistent with the age of the basement in Durango, Zacatecas, and San Luis Potosí (Rudnick and Cameron, 1991). This hypothesis is also inconsistent with the association of volcanic rocks of the Nazas Formation, in Durango, with a thick continental sequence of red beds, and not with Liassic marine strata as rocks of the Sonora segment of the volcanic arc at Cerro la Basura (Fig. 5). Furthermore, the vol-canic sequence in north-central México overlies Upper Triassic marine strata of the Zacatecas and La Ballena Formations, sub-marine fans with a restricted stratigraphic range. Similar strata are unknown in the arc of northern Sonora.

Rocks of the Nazas Formation and equivalent strata in the Mexican Altiplane are overlain by an Oxfordian transgressive sequence (Zuloaga and La Gloria Formations and other Juras-sic strata). In the region of Caopas, Anderson et al. (1991) reported a deformation event that they assign to the Late Juras-sic. A northwest-striking nappe is interpreted to be the result of transpression related to the Mojave-Sonora megashear. The nappe involves rocks of the Caopas schist and the Nazas Forma tion. Anderson et al. (1991) suggest that some deformation affects the Zuloaga Formation and interpret the deformation event as contem-poraneous with deposition of this unit. We note that the Caopas schist is diffi cult to correlate with the Jurassic arc of northern Sonora, because the age of the protolith (a quartz porphyry) is unknown. Also, phillites in the region record a Cretaceous age of metamorphism (ca. 79 Ma; Iriondo et al., 2003b). Mylonites in the same region also suggest Laramide ages, but their Ar-Ar spectra are disturbed. The Jurassic age (158 ± 4 Ma) of the gran-ite mentioned earlier, presumably related to the Caopas schist, is also problematic; the age is too close to the depositional age of the Oxfordian transgressive sequence.

Finally, in seeking to fi t the model of displacement of the volcanic arc, Jones et al. (1995) draw the trace of the Mojave-Sonora megashear to the south of the Coahuila paleo-island (Las Delicias arc). This excludes exposures of calc-alkaline igneous rocks of the Acatita series at Sierra Las Margarita (Molina-Garza, 2005) and rhyolites of pre-Cretaceous age at Sierra del Diablo (McKee, et al., 1990; Fig. 10).

Transtension and Pull-Apart Basins along the Trace of the Mojave-Sonora Megashear in Sonora and Arizona

Early Cretaceous basins in Sonora and southern Arizona record an interval of nonmarine to marine deposition that followed activity along the hypothetical Mojave-Sonora megashear and associated faults. According to Nourse (1995, 2001), deformation linked to left-lateral shear along a broad region resulted in areas of localized subsidence, defi ned by outcrops of the Bisbee Group. According to this author, the margins of these areas of localized




subsidence are NW- and E-trending faults that can be delineated by deposition of the Glance Conglomerate, the basal unit of the Bisbee Group. Thus, the basins have been interpreted as pull-aparts developed at releasing bends along the megashear. More recently, Anderson and Nourse (2005) elaborate on this hypothesis. Nourse et al. (1994) and Nourse (1995) have postulated the metamorphic core complexes in the same region developed along those areas that were stretched and fractured by transtensional faulting in the Late Jurassic and Early Cretaceous. These authors also suggest that the same discontinuities created by transtensional faulting accom-modated compressional deformation in the Late Cretaceous.

McKee and Anderson (1999) have interpreted a Lower Cre-taceous sequence near Sierra Azul north of Cucurpe (Fig. 5) as gravity-slide deposits emplaced in the Early Cretaceous in a basin with high-relief margins; they interpreted the basin as a pull-apart basin linked to Jurassic strike-slip. Although the basin in question is some 50 km north of the trace of the Mojave-Sonora megashear, these authors suggested that a transtensional regime existed in northern Sonora and southern Arizona. Other basins that have been incorporated into this model include the McCoy basin, in SW Arizona and deposits in the Altar–Santa Ana region of Sonora (Nourse, 2001). Similarly, Rodríguez-Castañeda (1994) and other authors have suggested that deposition of the Bisbee Group was controlled by the Mojave-Sonora megashear in northern Sonora, even if activity on the fault had presumably ceased by this time.

Interpreting the Bisbee Group as the fi ll of pull-apart basins is not straightforward. For instance, gravity-slide deposits in the Agua Prieta region studied by McKee and Anderson (1999) had been previously interpreted as a Paleozoic sequence thrusted on rocks of the Bisbee Group (Rangin, 1982). An important criticism of the model suggesting that NW-SE–trending faults affecting the region of the continental Jurassic arc of northern Sonora and southern Arizona are linked to the Mojave-Sonora megashear, as well as the models suggesting that these faults are concurrent with pull-apart basins and deposition of the Bisbee Group, is that it has been demonstrated that faults with this orientation (i.e., Sawmill Canyon fault in Arizona) are active in the Late Creta-ceous and early Tertiary. The faults affect rocks as young as Late Cretaceous, producing rotations about vertical axes (Hagstrum and Sawyer, 1989; Sosson, 1990). However, one could argue that the faults were established in the Late Jurassic, but were reacti-vated during the Late Cretaceous. Jurassic slip on these faults has been suggested by Drewes (1996).

The Basement in the Northern Mexican Altiplane

Cameron et al. (1989) undertook an isotopic study of mid-Tertiary andesites along a 700-km NE-SW transect through Chi-huahua. These andesitic lavas, with relatively low K/P values (<7) and variable Ba/Nb (50 versus 18), show similar isotopic values throughout the transect, suggesting that their isotopic sig-natures are not controlled by assimilation of signifi cant amounts of crustal material. Along the transect, basaltic rocks have εNd

and 87Sr/86Sr values close to bulk earth and isotopic relationships for

206Pb/

204Pb and

207Pb/

204Pb that plot on the 1.7 Ga pseudo-

isochron. This NE-SW transect crosses the trace of the inferred Mojave-Sonora megashear. At this latitude, the megashear is considered a lithospheric discontinuity that separates Proterozoic basement to the NE from Phanerozoic basement to the SW. The geochemical/isotopic changes across the inferred fault are very smooth and appear to be gradational rather than abrupt. This iso-topic uni formity for Sr and Nd determinations across the Mojave-Sonora megashear could be interpreted in at least three ways: (1) The subcontinental lithosphere for both the Proterozoic and the Phanero zoic basement blocks is indistinguishable for their Sr and Nd isotopic values. (2) The Mojave-Sonora megashear is not a lithospheric boundary separating two different basements at least not in the study area of the transect. (3) The isotopic signatures were acquired at the asthenosphere and not from the subconti-nental lithosphere. McDowell et al. (1999) presented a similar conclusion based on the study of rocks from the Tomóchic vol-canic fi eld in central Chihuahua (Fig. 6). These authors suggested that a simplistic hypothesis in which Laurentian basement was displaced along the megashear was not possible.

Paleomagnetic Data

Paleomagnetic data for the Caborca terrane were cited as evidence in support of the Mojave-Sonora megashear. The data cited include preliminary data for the Triassic-Jurassic Anti monio Forma tion (3 sites) published by Cohen et al. (1986). More recently, Molina-Garza and Geissman (1996, 1999) reported paleomagnetic data for the Antimonio Formation, for the rocks of the Neoproterozoic-Paleozoic mioclinal sequence, and for rocks of the Jurassic volcanic arc assigned to the Fresnal Can-yon sequence. Molina-Garza and Geissman (1999) also present paleomagnetic data for Lower Cretaceous strata and for intrusive rocks of the Laramide age Sonoran batholith.

The paleomagnetic data for the Antimonio Formation and for an unnamed yet correlative sequence at Barra Los Tanques (Fig. 5), ~50 km west of type locality of the Antimonio Forma-tion at Sierra del Álamo, clearly show that the study of Cohen et al. (1986) cannot be used to support the Mojave-Sonora megashear hypothesis. First, the characteristic magnetization of the Antimonio Formation is of secondary origin; the age of the magnetization is pre–Early Cretaceous, but is not Triassic. Sec-ondly, there is structural complexity at Sierra del Álamo, which Cohen et al. (1986) failed to recognize, such as the westward tilt of Cretaceous rocks overlying the previously folded Triassic-Jurassic sequence.

The mean paleomagnetic direction reported by Molina-Garza and Geissman (1996) for the Antimonio Formation was inter-preted in terms of the Jurassic accretion of the Antimonio ter-rane to the margin of North America or the para-autochthonous Caborca terranes. The mean direction (D = 186.7.0°, I = -28.4°; n = 14 sites; k = 28.2; α95 = 7.5°) is discordant; it does not resemble the expected direction from Early Jurassic to Early Cretaceous




cratonic reference poles. This magnetization is interpreted as the record of accretion of the Antimonio terrane to Caborca in the Late Jurassic. Alternatively, this magnetization may refl ect deformation in an essentially autochthonous fore-arc basin. The declination of the characteristic magnetization indicates post–Middle Jurassic clockwise rotation of the Antimonio terrane with respect to North America. Similar clockwise rotations have been observed in other regions of Sonora and are explained below.

The mean paleomagnetic direction for the mioclinal sequence is also a secondary magnetization restricted by fi eld and other evidence to date from the Middle to Early Jurassic (Molina-Garza and Geissman, 1999). The observed direction (D = 15.0°, I = 10.0°; n = 28 sites; k = 23.0; α95 = 5.8°) indicates that the Caborca terrane experienced a clockwise rotation with respect to North America. According to paleomagnetic data for Lower Cre-taceous strata, the rotation occurred in Middle to Late Jurassic. These paleomagnetic data also argue against latitudinal displace-ment of the Caborca terrane from north to south (Fig. 11).

The Fresnal Canyon volcanic sequence from localities north of Caborca has a characteristic magnetization for which the mean (D = 15.0°, I = 4.0°; n = 10 sites; k = 12.4; α95 = 14.3°) is essen-tially indistinguishable from the direction observed in mioclinal rocks of the Caborca terrane. The magnetization of the volcanic rocks indicates a similar clockwise rotation and suggests that no relative displacement exists between the Caborca terrane and the Jurassic continental arc. The paleomagnetic data for the volcanic rocks at Cerro la Basura and Cerro Álamo predate deformation

of the sequence, which suggest but do not prove that this is a primary magnetization. These rocks also pass a conglomerate test (Molina-Garza and Geissman, 1998), further supporting the interpretation of the magnetization as primary.

The mean declinations of both the secondary magnetiza-tion in mioclinal rocks and of the primary magnetization in rocks of the Jurassic arc indicate a clockwise rotation of Sonora with respect to the craton. It is diffi cult to give a tectonic interpreta-tion to secondary magnetization, unless the timing of remanence acquisition can be established with some certainty. In the region of Caborca, both Lower Cretaceous sedimentary rocks and Late Cretaceous intrusive rocks contain concordant magnetizations. That is, these rocks indicate no rotation or latitudinal displace-ment of the Caborca area with respect to North America. The youngest rocks of the Caborca terrane containing a discordant magnetization are latest Early Jurassic; thus the age of the regional remagnetization event is constrained between the Middle and Late Jurassic.

If the regional remagnetization event that affects rocks of the Caborca terrane had occurred in the Late Jurassic, because the observed inclination is more shallow than expected, it would indi-cate a latitudinal displacement from south to north of ~10° ± 7° (opposite of the Mojave-Sonora megashear proposed displace-ment). The observed declination would indicate a clockwise rotation of up to 50° for both the Jurassic volcanic arc and the Caborca terrane. On the other hand, if the remagnetization event affecting rocks of the Caborca terrane is Middle Jurassic, the interpretation is more uncertain. The low latitude track of the apparent polar wander path proposed by May and Butler (1986) predicts latitudes for a site in NW Sonora near the equator (1°S to 14°N), following the Jurassic northward drift of North America. The apparent polar wander path at high latitudes proposed by Van Fossen and Kent (1990) predicts latitudes of 1°S to 22°N. The observed paleolatitude of ~5°N (Fig. 11) may indicate displace-ment from north to south, but only if the remagnetization event is early Middle Jurassic. However, this displacement must apply to both the Caborca terrane and the Jurassic volcanic arc NE of the hypothetical Mojave-Sonora megashear. The evidence of a primary magnetization in rocks of the Jurassic volcanic arc north of Caborca (Molina-Garza and Geissman, 1998) and the reported isotopic ages for rocks within the arc (ranging from 153 to 170 Ma; Stewart et al., 1986; Anderson et al., 2005) suggest, however, that the age of the magnetization is late Middle Jurassic.

Together, the observation of pre-Cretaceous clockwise rota-tion indicated by NNE-directed declinations, the lack of evidence for north-south latitudinal displacement indicated by relatively shallow inclinations, and the observation of similar directions for localities north and south of the inferred trace for the megashear do not support reconstructions of the Caborca terrane north of its present position with respect to North America. The paleo-magnetic data for relatively distant localities are similar, sug-gesting that the Caborca terrane behaves as a rigid block. In our view, paleomagnetic data provide perhaps the strongest evidence against the Mojave-Sonora megashear hypothesis.

Cretaceous Jurassic100 120 140 160 180

10˚

20˚

30˚

40˚

50˚

Exp

ecte

d in

clin

atio

n

CaborcaI obs

0˚

Pal

eola

titud

e

5˚

10˚

20˚

30˚

Figure 11. Expected (dotted line) and observed (I obs) inclinations for the Jurassic and Cretaceous times in the Caborca region. Modifi ed after Molina-Garza and Geissman (1999).




Geology of the Mojave Desert Region

Upper Paleozoic and Lower Mesozoic rocks from the Mojave Desert region are very important to consider when assessing the hypothetical Mojave-Sonora megashear. The proposed trace is located west of the Soda Mountains in the central part of the Mojave Desert in California. Cameron (1981) was the fi rst to note the apparent continuity of the Neoproterozoic and Paleozoic miogeosyncline and cratonal facies in the region (Stewart and Poole, 1975) that would hamper the presence of the hypothetical fault in the Mojave Desert. The apparent relationship between miogeosynclinal facies east and west of the Mojave Desert could be still consistent with the Mojave-Sonora megashear model only if the rocks from the San Bernardino Mountains were tectonically translated along the megashear at a later time; however, there is no evidence for such a geological relationship.

Following the Mojave-Sonora megashear model, Upper Paleozoic and Triassic sequences from the Soda Mountains and surrounding areas (Fig. 10) should have been present in Sonora if they were displaced along the megashear. However, rocks of these ages in the Caborca terrane are quite distinct from the ones in the Mojave Desert (Walker and Wardlaw, 1989). Late Permian rocks are scarce in Sonora, and Triassic rocks from the Barranca Group correspond to shallow continental to transitional facies with coal layers. In contrast, the lower Triassic in the Mojave Desert is represented by deep-water facies rocks (Silver Lake Formation) and other deep-marine units in the southern Sierra Nevada. In the Soda Mountains region, the Triassic rocks are rep-resented by subaereal volcanic rocks, volcanoclastic sandstones, and eolian quartzites.

ALTERNATIVES AND CONCLUSIONS

In summary, evidence that clearly contradicts the Mojave-Sonora megashear model includes the paleomagnetic results (Molina-Garza and Geissman, 1999), the stratigraphic relation-ships between the Jurassic volcanic arc and the pre-arc basin present just south of the volcanic arc (poorly documented in the literature), and the impossibility of tracing the hypothetical fault through the Mojave Desert. However, we cannot ignore the apparent stratigraphic correlation between the miogeocline sequence in Caborca and the paleogeography of eastern Califor-nia. Although, these correlations have been recently questioned in Sonora because of the apparent continuity between the Bolsa Quartzite (NE Sonora-Mazatzal province) and the Cambrian Proveedora Quartzite from the Caborca terrane, the correlation of Caborca miogeocline rocks with rocks of the Mojave Desert is a solid observation.

The apparent impossibility to trace the Mojave-Sonora megashear through the Mojave Desert in the lower Paleozoic suggests that the possible displacement of the miogeocline sequence occurred in the late Paleozoic as proposed by Stone and Stevens (1988) and Walker and Wardlaw (1989). The evidence that correlates the Triassic stratigraphic sequence from Nevada

and the sequences from the Barranca Group and/or the Anti-monio Forma tion in Sonora appears to be more equivocal and probably requires additional work to be tested. Another line of evidence to support the Mojave-Sonora megashear that requires further study is the correlation between Paleoproterozoic base-ment from Sonora and the basement from eastern California and southern Arizona. In addition, the hypothesis that a transpres-sional tectonic environment controlled the deposition of the Bis-bee Group in southern Arizona and northern Sonora should be studied in greater detail. Even if these hypotheses were proven correct, this would not demonstrate the required, large latitudi-nal displacements of the Caborca terrane in the Mojave-Sonora megashear model. Stratigraphic, paleomagnetic, structural, and geochronological data that question the existence of the Mojave-Sonora megashear, however, do not preclude displacements of the miogeocline rocks during the late Paleozoic or even older dis-placements of the Proterozoic basement prior to deposition of the miogeocline sequence.

Some of the important observations that prop up the hypoth-esis for the Pennsylvanian to lower Permian truncation of the SW margin of Laurentia include the strike change for the con-tinental margin, and the abrupt interruption of sedimentation patterns for lower Paleozoic rocks (Stevens et al., 1992, 2005). In Nevada, both the upper Paleozoic continental margin and the calcareous platform have a southwestern strike whereas this direction changes to southeast in southeastern California in the Mojave Desert region. In the lower Permian, eastern California was exposed to an extensional event with important subsidence (associated turbidite deposits) that was contemporaneous to local thrusting. This combination of contrasting deformations could be explained by transtension associated with a system of strike-slip faulting similar to the younger Jurassic Mojave-Sonora megashear. Stone and Stevens (1988) proposed that displace-ment of the Neoproterozoic and lower Paleozoic miogeocline rocks from the Caborca terrane occurred sometime in the late Paleozoic instead of Jurassic time. A similar reasoning for the tectonic evolution of SW North America is presented by Dick-inson and Lawton (2001), where they propose Triassic displace-ment of the Caborca miogeocline rocks along what they call the California-Coahuila transform fault. These authors also proposed that the California-Coahuila fault system connected the conver-gence event occurring along the Sonora orogen in California and Nevada with the subduction taking place in east-central México at approximately the same time.

In conclusion, the unbiased evaluation presented here for the different sets of data, including stratigraphic relationships, struc-ture, geochemistry, age constraints, and paleomagnetism, does not favor the original model of the Caborca terrane displaced along the Jurassic Mojave-Sonora megashear; however, these results cannot totally discredit the hypothesis. The impossibility of demonstrating latitudinal displacements during Middle to Late Jurassic time is perhaps, as shown in the paleomagnetic studies, the main weakness of the Mojave-Sonora megashear model. It is true that paleogeographic reconstructions of the Circum-Gulf of




Mexico require a different position for the Mexican subcontinent with respect to North America, but the Jurassic Mojave-Sonora megashear model or a similar set of faults are not the only alter-natives for such reconstructions.

ACKNOWLEDGMENTS

This study has been supported by PAPIIT-UNAM project IN-1221002 and partly fi nanced through a CONACyT project T-3286. We are thankful to T. Calmus, C. González-León, and T.H. Anderson for their reviews, which greatly improved an older version of this manuscript.

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