Temporal constraints on the eastward migration of the …rmolina/documents/valenciamorenoetal2006.pdfMartı´n Valencia-Moreno a,*, Alexander Iriondo b,c, Carlos Gonza´lez-Leo´n

www.elsevier.com/locate/jsames

Journal of South American Earth Sciences 22 (2006) 22–38

Temporal constraints on the eastward migration of the LateCretaceous–early Tertiary magmatic arc of NW Mexico basedon new 40Ar/39Ar hornblende geochronology of granitic rocks

Martın Valencia-Moreno a,*, Alexander Iriondo b,c, Carlos Gonzalez-Leon a

a Estacion Regional del Noroeste del Instituto de Geologıa, Universidad Nacional Autonoma de Mexico, Apartado Postal 1039,

Hermosillo, Sonora 83000, Mexicob Centro de Geociencias, Universidad Nacional Autonoma de Mexico, Campus Juriquilla, Juriquilla, Queretaro 76230, Mexico

c Department of Geological Sciences, University of Colorado at Boulder, Boulder CO 80309, USA

Received 1 May 2004; accepted 1 February 2006

Abstract

Hornblende step-heating 40Ar/39Ar dating for granitic plutons along an E–W transect of central Sonora was carried out to constrainthe Late Cretaceous–early Tertiary migration of the cordilleran magmatic arc across northwestern Mexico. Geochronological data fromprevious studies offer a good estimate of the overall process, but because they come from different dating schemes performed on a varietyof rocks and/or minerals with a wide range of closure temperatures, the ages largely overlap when plotted on a map. Previous data sug-gest that the Cordilleran magmatic arc was nearly static in the western portion of the Peninsular Ranges batholith in Baja California(140–105 Ma), then the axis of magmatism migrated east at approximately 10 km/Ma and reached coastal Sonora approximately90 Ma ago. The locus of the plutonic emplacement continued to migrate inland during the Laramide magmatic pulse (80–40 Ma), pene-trating up to central Chihuahua. New argon data indicate that granitic plutons intruded the region northeast of Bahıa Kino, in coastalSonora, approximately 77 Ma ago. Magmatism subsequently moved east to the area surrounding the city of Hermosillo approximately69 Ma ago and continued its easterly migration, reaching the Sonora–Chihuahua state boundary 59 Ma ago. However, the granitic rocksof east-central Sonora yield ages in a relatively wide range of 62–56 Ma. Synchronic plutons reported farther east in central Chihuahuasuggest an unusually broad magmatic arc, which appears difficult to explain on the basis of the traditional subduction model assumed forsouthwestern North America during this time and may reflect particular – and little understood – tectonic conditions derived from therelatively flat subduction regime prior to the extinction of the Laramide magmatic arc. Moreover, volcanic rocks exposed in east-centralSonora yield fairly old U–Pb zircon dates of 90–70 Ma, which have no known contemporaneous plutons, and complicate the scenario forthe Laramide event in Sonora, perhaps requiring the existence of a second volcanic arc. Considering solely the evidence from graniticplutons, the data provide a systematic way to evaluate the shift of magmatic activity across Sonora. It needs a proper restitution forthe conspicuous Cenozoic extension affecting the region. After restoring the cumulative extension of 90% estimated for east-central Sono-ra, a rate of approximately 8.5 km/Ma of eastward migration can be roughly estimated for the Laramide arc across Sonora.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Arc migration; 40Ar/39Ar dating; Laramide; Granitic rocks; NW Mexico

1. Introduction

During the Early Cretaceous–early Tertiary, northwest-ern Mexico underwent intense magmatic activity associated

0895-9811/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jsames.2006.08.006

* Corresponding author. Fax: +52 662 217 5340.E-mail address: [email protected] (M. Valencia-Moreno).

with the relatively long-lived subduction of the Farallonplate beneath the North American continent (Coney andReynolds, 1977). This magmatic event occurred in twomain phases (Busby, 2004), but for simplicity, we refer toit as the ‘‘Cordilleran arc.’’ One phase is an extensional-fringing, nearly static, oceanic arc developed during theEarly Cretaceous in the western part of the Peninsular

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M. Valencia-Moreno et al. / Journal of South American Earth Sciences 22 (2006) 22–38 23

Ranges (western arc), and the other is a continental migra-tory arc developed in Late Cretaceous–early Tertiary in theeastern part of the Peninsular Ranges and mainland Mex-ico (eastern arc). The abundant granitic bodies producedduring this event are widely exposed along nearly continu-ous NW–SE–oriented belts, characterized by the Peninsu-lar Ranges batholith of Baja California to the west andthe Sonora and Sinaloa batholiths to the east and south-east, respectively (Fig. 1). The age and tectonic setting ofthis great production of igneous rocks have been the targetsof important studies published since the late 1970s (e.g.,Coney and Reynolds, 1977; Gastil and Krummenacher,1977; McDowell and Clabaugh, 1979; Damon et al.,1983a,b; Silver and Chappell, 1988; Kimbrough et al.,2001; Staude and Barton, 2001; Henry et al., 2003; Orte-ga-Rivera, 2003). The evidence provided by these studiesindicates arc magmatism occurred in northern and north-western Sonora since the Late Triassic–Late Jurassic(Anderson and Silver, 1979; Stewart et al., 1986). Subse-quently, the axis of magmatic shifted westward to theregion of Baja California and remained there during mostof the Early Cretaceous (Silver and Chappell, 1988). Dur-ing the Late Cretaceous–early Tertiary, the magmatic arcmigrated eastward from eastern Baja California to approx-imately 1000 km away from the paleotrench and finally

Fig. 1. Map of northwestern Mexico showing the distribution of Late Cretac1992; Fernandez-Aguirre et al., 1993). Main cities shown by open squares are

extinguished 40 Ma ago (Coney and Reynolds, 1977).Next, a magmatic gap was abruptly interrupted in the Oli-gocene by a great ignimbrite flare-up along the Sierra Mad-re Occidental (Fig. 1), which built one of the largest silicicvolcanic provinces recorded on Earth (McDowell andClabaugh, 1979; Ferrari et al., 2005). This massive volcanicevent has been interpreted as the initiation of a fast regres-sion of the volcanic arc to the Pacific coast (Damon et al.,1983a). However, the tectonic mechanism involved in pro-ducing such an enormous plateau of ignimbrites may haveincluded much more complex processes (e.g., Ferrari et al.,2002).

The age database available to estimate the progress ofCordilleran arc magmatism in Sonora includes differentisotopic dating methods performed on a variety of rocktypes and mineral separates, as compiled in Table 1. Subse-quently, we refer to particular dated localities according totheir corresponding code number in this table. In mostcases, the resulting dates are cooling ages that represent arelatively large window of closure temperatures (�700–300 �C), which often deviate from the time of crystalliza-tion. As a result, there is a notable age overlap when thedata are plotted on regional maps (Fig. 2). Thus, the indis-criminate use of these dates may induce incorrect assump-tions about the temporal constraints of the granitic

eous–early Tertiary granitic rocks (adapted from Ortega-Gutierrez et al.,Hermosillo (H), Culiacan (C), and La Paz (LP).

Table 1Compiled dates of Late Cretaceous and early Tertiary intrusive rocks in Sonora

Sample Locality Rock N–W Lat-Long(�)

Age (Ma) ± Method Min. Ref.

1 Not labeled Borderline Hill Gd 32.32 114.40 95.0 3.0 U–Pb Zr 12 UAKA 77-40 Tinajas Altas Gr 32.23 114.05 53.1 1.3 K–Ar Bi 13 Not labeled NW of Caborca Di 32.25 114.00 70.0 – Ar–Ar Hb 34 PED 11-58 Adobe Blanco Gr 31.78 113.01 53.2 1.6 K–Ar Mu 15 PED 10-58 Cerro Cipriano Gr 31.83 112.91 52.7 1.3 K–Ar Mu 16 UAKA 82-50 Cerro Cobabi Gr 31.72 112.69 53.8 1.4 K–Ar Mu 17 PED 10-66 Mina Leones Gr 31.20 112.12 70.9 2.1 K–Ar Bi 28 PED 11-66 Mina La Margarita Gr 31.15 112.08 67.6 2.0 K–Ar Bi 29 PED 11-59 Puerto Blanco Qz-m 30.68 112.27 71.2 1.8 K–Ar Bi 1

10 PED 7-59 Altar Peg 30.53 112.12 74.3 1.8 K–Ar Bi 111 Not labeled Caborca Gr 30.50 112.20 66-70 – Rb–Sr WR–Bi 412 Not labeled Rancho Los Alamos Gd 31.26 111.48 74.0 2.0 U–Pb Zr 513 UAKA 82-65 San Juan Gd 31.08 111.53 42.5 1.0 K–Ar Hb 114 Not labeled Sierra Guacomea Gr 30.98 111.05 78.0 3.0 U–Pb Zr 515 Not labeled Cuitaca Gd 30.98 110.28 64.0 3.0 U–Pb Zr 616 UAKA 73-157 El Alacran Qz-lat 30.85 110.18 56.7 1.2 K–Ar Bi 217 not labeled Tinaja Qz-m 30.98 110.28 63.0 0.4 K–Ar Hb 718 Not labeled Marıa Mine Rhy-P 31.11 110.43 54.2 2.0 K–Ar Bi 719 Not labeled Marıa Mine KF-P 31.11 110.43 57.4 1.6 Re–Os Mo 720 RM 4-64 La Colorada Bx 30.98 110.29 59.9 2.1 K–Ar Flo 221 PED 39-60 Torreon Qz-m 30.96 110.59 68.7 1.7 K–Ar Bi 122 B-59 El Manzanal Gd 30.68 110.27 64.8 1.0 Ar/Ar Hb 823 B-83 Vaquerıa Qz-md 30.42 110.23 55.0 0.7 Ar/Ar Hb 824 UAKA 73-148 Batamote Qz-m 30.45 109.45 56.8 1.2 K–Ar Bi 225 DEL 4-70 La Caridad Qz-di 30.32 109.57 54.5 0.9 K–Ar Ser 226 PED 57-66 La Caridad Peg 30.31 109.34 55.2 1.6 K–Ar Bi 227 UAKA 77-123 La Bella Esperanza Qz-m 30.26 109.70 55.9 1.2 K–Ar Bi 228 UAKA 77-124 Mina La Lily Gr 30.38 109.73 52.4 1.1 K–Ar Ser 129 UAKA 83-03 Sierra Oposura Gd 29.86 109.27 62.7 1.4 K–Ar Bi 130 UAKA 74-151 Sierra Oposura Gd 29.86 109.45 59.6 1.3 K–Ar Ser 131 UAKA 76-68 San Judas Gd 29.81 109.82 40.0 0.9 K–Ar Bi 232 UAKA 76-35 Mina Washington Gd 29.91 110.07 45.7 1.0 K–Ar Ser 233 UAKA 76-36 Mina Washington Gd 29.91 110.07 56.4 1.2 K–Ar Bi 234 Pc-M Padercitas Gd 29.92 110.08 56.8 1.1 K–Ar Mu 935 UAKA 74-162 Mina San Felipe Rhy-P 29.88 110.3 51.0 1.1 K–Ar KF 236 UAKA 77-125 Mina El Creston Gr 29.89 110.66 53.5 1.1 K–Ar Ser 237 Ant-1H El Jaralito Gd 29.68 110.28 46.6 1.3 Ar/Ar Hb 938 Not labeled Puerto del Sol Gr 29.47 110.25 57.0 3.0 U–Pb Zr 539 Not labeled Sierra Mazatan Gd 29.16 110.22 58.0 3.0 U–Pb Zr 540 UAKA 81-06 Cerro Mariachi Gr 29.09 112.94 64.1 1.4 K–Ar Hb 141 He-14 Hermosillo Gd 29.06 110.95 64.9 1.3 K–Ar Bi 942 UAKA 80-05 Granito Hermita Gr 28.87 110.75 62.9 1.5 K–Ar Hb 143 UAKA 80-20 Cobachi Gd 28.84 110.21 66.7 1.6 K–Ar Bi 144 VP-5H La Venada Gd 29.03 109.94 56.9 1.2 Ar/Ar Hb 945 Not labeled Rancho El Estribo Gr 28.90 109.90 61.0 1.0 U–Pb Zr 1046 UAKA 81-01 Rebeico Mz 28.88 109.82 61.2 1.4 K–Ar Gm 147 UAKA 81-02 San Antonio Di 28.60 109.60 57.4 1.4 K–Ar Gm 248 PED 3-70 Aurora Qz-m 28.56 109.61 55.8 1.8 K–Ar Bi 249 UAKA 80-07 East of Tecoripa Gd 28.62 109.89 62.0 1.7 K–Ar Hb 150 UAKA 77-127 Suaqui La Verde Gd 28.42 109.82 58.8 1.3 K–Ar Hb 251 UAKA 77-128 Suaqui La Verde Qz-di 28.41 109.80 56.7 1.1 K–Ar Ser 252 UAKA 77-126 Lucıa Gd 28.43 109.86 56.9 1.2 K–Ar Ser 253 UAKA 80-11 San Nicolas Gd 28.41 109.24 49.6 1.2 K–Ar Bi 154 UAKA 80-12 Santa Rosa Gd 28.41 109.18 49.5 1.0 K–Ar Hb 155 SR-83 Santa Rosa Gd 28.45 109.11 56.7 0.4 Ar/Ar Hb 1156 TP-2B Tres Piedras Gd 28.42 109.18 53.7 0.9 Ar/Ar Bi 957 TP-5M Tres Piedras Gd 28.42 109.18 56.1 1.0 Ar/Ar Mu 958 TP-2B Tres Piedras Gd 28.42 109.18 53.3 1.1 K–Ar Bi 959 TP-2H Tres Piedras Gd 28.42 109.18 63.3 3.3 K–Ar Hb 960 Ma-1 Maicoba Gd 28.40 108.63 63.6 1.0 K–Ar Bi 1261 UAKA 80-18 Santa Ana Gd 27.75 109.72 66.1 1.4 K–Ar Hb 162 SA-8H Mina San Alberto Gd 27.35 108.95 56.4 1.8 Ar/Ar Hb 963 SO59 El Bayo Gd 28.22 110.97 76.9 2.8 K–Ar Bi 13

24 M. Valencia-Moreno et al. / Journal of South American Earth Sciences 22 (2006) 22–38

Table 1 (continued)

Sample Locality Rock N–W Lat-Long(�)

Age (Ma) ± Method Min. Ref.

64 SO25 San Antonio Gd 28.00 110.16 82.7 1.7 K–Ar Hb 1365 SIG-5 NW Bahıa Kino And 28.87 112.03 85.1 1.7 K–Ar Hb 1466 S2G-23 Arroyo La Noriega Gr 28.13 111.90 64.7 1.3 K–Ar KF 1467 S2R-19 NW Isla Tiburon Tn 29.21 112.46 85.2 1.7 K–Ar Bi 1468 S2W-45 NE Isla Tiburon Tn 29.01 112.32 90.4 2.7 K–Ar Hb 1469 Not labeled NE Isla Tiburon Tn 29.17 112.33 82.0 – Rb–Sr WR-Bi 470 SOH-281 Punta Cuevas Gd 29.72 112.54 71.7 1.4 K–Ar Bi 1471 SIG-6 Cerro Bolo Gd 29.90 112.73 91.0 1.8 K–Ar Bi 1472 SOK-3714 Puerto Libertad Gd 29.96 112.75 70.1 1.9 K–Ar Bi 1473 SOK-276 Puerto Libertad Dac-P 29.91 112.70 63.9 2.0 K–Ar Bi 14

Age sources: 1, Damon et al., 1983b; 2, Damon et al., 1983a; 3, Nourse et al., 2000; 4, Schaaf et al., 1999; 5, Anderson et al., 1980; 6, Anderson and Silver,1979; 7, Wodzicki, 1995; 8, Gonzalez-Leon et al., 2000; 9, Mead et al., 1988; 10, Poole et al., 1991; 11, Gans, 1997; 12, Bockoven, 1980; 13, Mora-Alvarezand McDowell, 2000; 14, Gastil and Krummenacher, 1977. And, andesitic dike; Dac, dacite; Di, diorite; Gd, granodiorite; Gr, granite; Mz, monzonite;Peg, pegmatite; Qz-di, quartzdiorite; Qz-lat, quartzlatite; Qz-m, quartzmonzonite; Qz-md, quartzmonzodiorite; Rhy, rhyolite, Tn, tonalite; -P, porphyryphase. Bi, biotite; Bx, breccia; Gm, groundmass; Hb, hornblende; KF, K feldspar; Mu, muscovite; Mo, molybdenite; Ser, sericite; WR-Bi, whole rock-biotite isochron; Zr, zircon.

Fig. 2. Distribution of the granitic rock-dated localities in Sonora in various time slices. Numbers refer to codes in Table 1.


magmatism. To reduce the age uncertainty due to signifi-cant differences in the blocking temperatures of the datedmaterials, we present a geochronological analysis basedon new 40Ar/39Ar hornblende dates of granitic plutonsexposed along an E–W transect in central Sonora(Fig. 3), which, combined with available U–Pb zirconand 40Ar/39Ar plus K–Ar hornblende dates, may provide

a more homogeneous evaluation of Cordilleran arc migra-tion by northwestern Mexico. Because hornblende is rela-tively high in the experimentally measured closuretemperatures of minerals with respect to argon diffusion,as we explain subsequently, we predict that the 40Ar/39Arand K–Ar hornblende dates provide a good approximationof the crystallization age of granitic rocks.

Fig. 3. Map of northwestern Mexico showing the location of the study transect with the Baja California peninsula restored to its prerifting position(adapted from Stock and Hodges, 1989). Numbers refer to the studied sample localities in Table 2 but without the prefixes BC or MV. The black areasrepresent exposures of Late Cretaceous and Tertiary granitic rocks. The localities shown correspond to Bahıa Kino (BK), Hermosillo (H), Isla Tiburon(IT), La Colorada (LC), Suaqui Grande (SG), and San Nicolas (SN). SPM and SR-83 are samples studied by Ortega-Rivera (2003) and Gans (1997) in theSierra San Pedro Martir pluton and Santa Rosa granodiorite, respectively.


2. Characteristics of the study area and goals of the study

The studied area comprises an E–W transect approxi-mately 450 km long by 125 km wide that lies across themain trend of the eastern arc (Fig. 3) and includes animportant fraction of existing dates. If the Baja CaliforniaPeninsula is restored to its position prior to the opening ofthe Gulf of California (e.g., Stock and Hodges, 1989), theSierra San Pedro Martir (SPM) pluton, one of the beststudied sites of the Mexican Peninsular Ranges batholith,lies to the west of the study transect (Fig. 3; see alsoFig. 3 of Oskin et al., 2001), which provides a more region-al view of the Cordilleran arc in northwestern Mexico fromthe middle part of Baja California to the west to theSonora–Chihuahua state boundary to the east.

The age inventory of the Late Cretaceous–early Tertiarygranitic rocks in Sonora is relatively large (Table 1) butrather incomplete for a conclusive review of the temporalevolution of magmatism in this region. To reinforce thegeochronological support in the studied area, we dated five

selected plutons by means of 40Ar/39Ar geochronology atlocations indicated in Fig. 3. The advantage of this tech-nique over conventional K–Ar dating is that it can detectsubsystems related to postemplacement tectonic episodes,which can make K–Ar ages unrealistic. However, if the iso-topic composition in the sample is not disturbed by a tec-tonic or magmatic event, the K–Ar and 40Ar/39Arhornblende should yield concordant ages. On the basis ofthis assumption, we consider the K–Ar ages are very usefulfor this study but prefer argon ages in localities whereimportant age differences existed.

An analysis of the main geochemical and isotopic char-acteristics of granitic rocks in Sonora, including the newdated samples, appears in works by Valencia-Morenoet al. (2001, 2003). In general, the plutons are more tonalit-ic in composition near the coastal region and more grano-dioritic to the east. The silica content ranges from 56 to75 wt%, showing no regular variations in major and mosttrace elements but a notable decrease in K2O toward thecoast (Valencia-Moreno et al., 2003). Similar chemical


behavior is observed in the Peninsular Ranges batholith,where plutons emplaced in the eastern migratory arc dis-play greater K2O enrichments than plutons of the westernstatic arc (Gromet and Silver, 1987). In both cases, theinvolvement of K2O-rich crustal materials in the magmasource for the eastern region is considered a likely explana-tion for the variation. At a more regional scale, the potas-sium contents in Cretaceous and Tertiary magmatic rocksof southwestern North America are believed to reflect thedepth of the subducted oceanic slab beneath a correspond-ing center of volcanic activity (Keith, 1978, 1982). The K2Ovariation also was coupled with relatively larger strontiuminitial ratios to the east (e.g., Gromet and Silver, 1987; Sil-ver and Chappell, 1988; Valencia-Moreno et al., 2001),which may invoke the contribution of an older and moreisotopically evolved continental lithosphere. In general, ini-tial isotope values characterized by 87Sr/86Sr �0.7060 andeNd of �3.4 are considered to represent an important iso-topic breakup of regional implications (Valencia-Morenoet al., 2003): lower Sr ratios and higher eNd values charac-terize the western portion of the Cordilleran arc, whereashigher Sr ratios and lower eNd values characterize graniticrocks exposed in the eastern part. This trend has been inter-

Fig. 4. Map of northwestern Mexico showing major basement domains. The bValencia-Moreno et al. (2001, 2003). Labeled localities as in Fig. 3.

preted to reflect the influence of a relatively younger andmore mantle-related crust in the western region and an old-er, much thicker continental crust to the east (e.g., DePaoloand Farmer, 1984; Valencia-Moreno et al., 2001). SampleBC-25 located in the western part of the study transect(Fig. 3) lies right on this isotopic breakup. The studied gra-nitic plutons located east of sample BC-25 yield strontiumand epsilon neodymium initial values ranging from 0.7064to 0.7089 and from �3.4 to �5.4, respectively (Valencia-Moreno et al., 2001, 2003), which suggests a large influenceof old North American crust (Fig. 4). Mesozoic rockassemblages of island arc and oceanic affinity underlyingmost of Baja California, coastal and southern Sonora,and Sinaloa (Fig. 4) likely imprinted the relatively moreprimitive isotopic signatures observed in the plutons ofthe western arc (Gromet and Silver, 1987; Silver and Chap-pell, 1988; Valencia-Moreno et al., 2001, 2003; Henry et al.,2003).

3. Variations in closure temperatures of minerals

A critical assumption in this study is that the differentminerals crystallizing from a cooling granitic magma form

oundary between the two regions indicated in the figure was adapted from


in sequential order. The time that these minerals becomeclosed systems to crystal-liquid exchange of a determinatepair of parent–daughter isotopes thus is controlled by thecooling rate, which in turn depends on internal and exter-nal physical factors, such as the emplacement depth, size,and shape of the intrusion, and its capacity to transfer heatto wall rocks (e.g., Winter, 2001). To illustrate the problemof closure temperature, Fig. 5 shows the cooling history oftwo samples of dated plutons considered in this study; oneis the SPM pluton in Baja California, and the other is theSanta Rosa granodiorite (SR-83) exposed near San Nicolasin eastern Sonora (Fig. 3). In the first case (Fig. 5a), thecooling history of the SPM pluton was studied by Orte-ga-Rivera et al. (1997) on the basis of data from a samplecollected in the western part of the intrusion. A U–Pb zir-con date of approximately 97 Ma yielded the age of crystal-lization, inferred to be the time when the magma cooleddown to 700 �C and zircon became a closed system to Uor Pb isotope exchange. Hornblende and biotite acted asindependent 40Ar/39Ar clocks, operating at a lower temper-ature window of 500–300 �C. A simple inspection of Fig. 5asuggests that the magma took approximately 5 Ma to crossthis temperature window; however, it required 2 Ma tocool down from the U–Pb closure temperature of zirconto the closure temperature of hornblende with respect toAr diffusion. This interpretation implies a relatively smallage difference between the two isotope clocks and suggeststhat hornblende argon dates may represent an approxima-tion of the pluton crystallization age. At temperatures low-er than 300 �C, 40Ar/39Ar plagioclase and apatite fissiontrack data display a cooling curve that gets flatter withtime. Therefore, cooling is more rapid during the earlystages of crystallization, which suggests that the magmatransfers heat to the intruded crust more efficiently justafter its emplacement. Fig. 5 shows that hornblende liesin a middle position within the relatively fast cooling sideof the curve, whereas biotite lies right on the bending pointbetween fast and slow cooling.

Fig. 5. Diagrams for the cooling history of two plutons emplaced in the weMexico: (a) San Pedro Martir batholith in central Baja California (Ortega-R1997).

In the second example (Fig. 5b), the Santa Rosa grano-diorite was studied by Gans (1997) to examine an apparentconflict with isotopic dates by Damon et al. (1983b) andMead et al. (1988) for this pluton, who report K–Ar horn-blende ages of 49.5 Ma (54) and 63.3 Ma (59), respectively.Gans assumes the crystallization age for the pluton to beapproximately 60 Ma. Hornblende and biotite, represent-ing a closure temperature window for Ar isotope diffusionof 525–325 �C, yield ages of approximately 57 and 52 Ma,respectively. Also in this example, the resulting argon horn-blende date does not deviate greatly from the crystalliza-tion age (�60 Ma), and under proper restrictions, it maybe set as an approximation. Because 40Ar/39Ar dating candetect disturbed subsystems, we posit that the hornblendeargon age of 57 Ma obtained by Gans (55) may be moreaccurate. The rest of the cooling history of the Santa Rosagranodiorite shows a flat slope between 325 and 275 �C,revealing a slow cooling stage between approximately 52and 25 Ma. However, the pluton cooled down more rapidlybetween 26 and 20 Ma due to the crustal unroofing associ-ated with the late Cenozoic extension that affected most ofsouthwestern North America. The total extension estimat-ed by Gans for this region was approximately 90%, muchgreater than the surrounding regions to the south (i.e.,50%, Henry et al., 2003) and west (i.e., as much as 18%,Lee et al., 1996), and may explain the considerably widerexpression displayed by the plutonic belt in Sonora (Fig. 1).

4. Geochronologic background and age distribution

Studies based on U–Pb zircon dating suggest that anearly stationary arc developed in the western portion ofthe Peninsular Ranges batholith of Baja Californiabetween approximately 140 and 105 Ma (Silver and Chap-pell, 1988). The associated magmatism is characterized by acomplex of volcanic and volcaniclastic rocks with coeval,commonly mafic intrusions that comprise the Alisitos ter-rane (Campa and Coney, 1983), which is also considered

stern and eastern parts of the Cordilleran magmatic arc in northwesternivera et al., 1997); (b) Santa Rosa granodiorite in eastern Sonora (Gans,


part of the Guerrero superterrane (Dickinson and Lawton,2001). The Alisitos arc extended more to the south alongSinaloa, where combined K–Ar hornblende and U–Pb zir-con data indicate an age range of 139–101 Ma (Henryet al., 2003). The locus of magmatism in Baja Californiastarted to migrate inland across the eastern portion ofthe Peninsular Ranges batholith approximately 105 Maago (Silver and Chappell, 1983), reaching the region ofthe present-day coast of Sonora at approximately 90 Ma(Gastil and Krummenacher, 1977; Damon et al., 1983b).The plutonic activity remained in this region up to approx-imately 82 Ma ago (Schaaf et al., 1999; Mora-Alvarez andMcDowell, 2000), then continued to migrate eastwardacross Sonora and farther east into Chihuahua duringthe Laramide (80–40 Ma). In the southern continuationof the plutonic belt in Sinaloa, granitic intrusions associat-ed with the migrating arc yielded ages that suggest nearlycontinuous magma emplacement between 90 and 45 Ma(Henry et al., 2003).

a b

c d

Fig. 6. Distribution of Late Cretaceous–early Tertiary isotopic ages in Sonoraeastward advance of the batholithic front at various times: (a) 80 Ma, (b) 70 Mamagmatic arc migration. Numbers next to the dated localities (open circles) rTable 1.

Cordilleran arc magmatism in Sonora is difficult to eval-uate in terms of the available ages. Without reflecting onthe implications of mixing dates calculated by different iso-tope dating methods, the granitic rocks older than 80 Maare all distributed near the Sonoran coast, including aU–Pb date of approximately 95 Ma for a pluton exposedin the northwestern tip of the state (Fig. 6a). Ages in therange 79–70 Ma also occur in the coastal region, but theyextend up to north-central Sonora (Fig. 6b). Younger agesspread through the entire state and show no consistent timepatterns; however, there is an apparent limit for agesbetween 69 and 60 Ma near the eastern boundary of Sono-ra (Fig. 6c). Plutons younger than 60 Ma are abundant inthe eastern portion of Sonora (Fig. 6d) and should contin-ue to the east beneath the Sierra Madre Occidental volcaniccarpet (Fig. 1). The notable age overlap observed in Fig. 6may reflect discrepancies produced by the dating methodand mineral used. However, overlapped intrusions appearcommonly across the magmatic arc; thus, care must be

compiled in Table 1. Time contours indicate estimations of the maximum, (c) 60 Ma, and (d) younger than 60 Ma. Arrows illustrate the direction ofepresent the age; numbers in parenthesis indicate corresponding codes in


taken to interpret the meaning of the geochronologicaldata. Despite this problem, the highly idealized lines inFig. 6, though not properly constrained, appear to resem-ble the expected progressive inland younger ages of thebatholithic front.

5. Sample location and results

Regarding the isotopic age of the selected samples with-in the study area, the new step-heating 40Ar/39Ar geochro-nology on hornblende mineral concentrates yields agesin the range from 77.6 ± 1.6 to 56.3 ± 1.3 Ma. Analyticalresults appear in Table 2, and we discuss individual sampledetails and data interpretation next.

5.1. Sample BC-25

Sample BC-25 is a coarse-grained granodiorite collectedfrom a pluton exposed approximately 10 km north of Bah-ıa Kino (Fig. 3). The step-heating experiments do not pro-vide a plateau age (Fig. 7a), which requires at least threeconsecutive steps that account for more than 50% of thetotal 39Ar gas released and whose ages overlap (see theAppendix A). However, a very good isochron age at77.11 ± 0.40 Ma is defined by steps A–E, which accountfor 81.5% of the total 39Ar released (Fig. 7b). A likely pla-teau age is defined by steps B–D, but they carry onlyaround 30% of the total 39Ar released. The resulting iso-chron age constitutes a sort of breakpoint in this part ofSonora, because granitic rocks exposed just to the west inIsla Tiburon (Fig. 3) yield ages between approximately 90and 82 Ma (Gastil and Krummenacher, 1977; Schaafet al., 1999), whereas granitic rocks exposed to the eastyield dates younger than 70 Ma (e.g., Damon et al.,1983a,b).

5.2. Sample MV-15

The sample MV-15 is a coarse-grained granodiorite con-taining abnormally large euhedral titanite crystals. Thissample was collected from an outcrop on the central-eastside of the city of Hermosillo (Fig. 3). In this case, the dif-ferent heating steps do not display an acceptable plateau(Fig. 7c). The age fitting line in the isochron diagram(Fig. 7d) is very poor, with a mean square weighted devia-tion (MSWD) of approximately 18.8, well above the statis-tically accepted value of <2.5 (Baksi, 1999). Thus, our bestguess for this sample is an average step age of60.51 ± 0.33 Ma. This average age differs significantly fromthe previous K–Ar hornblende date of 64 Ma reported byDamon et al. (1983b) for a granodiorite pluton exposed2 km south of the sample locality (40).

5.3. Sample MV-11

Sample MV-11, also named the Hermita Granite byDamon et al. (1983b), is a medium- to coarse-grained gran-

ite collected 39 km E-SE of Hermosillo and 6 km west ofLa Colorada (Fig. 3). The argon data obtained from thestep-heating analyses of this sample do not yield a plateauage (Fig. 7e). The first two heating steps in the age spectraindicate relatively young apparent dates, coupled withabnormally high K/Ca ratios for hornblende (>1), whichwe interpret as biotite contamination (Fig. 7e). However,a rather good isochron age at 68.77 ± 1.14 Ma is obtainedfrom steps C–F (Fig. 7f). Compared with previous data,this age appears much older than the hornblende K–Ardate of 63 Ma reported by Damon et al. (1983b) (42).

5.4. Sample MV-5

Sample MV-5 is a coarse-grained granodiorite collected7 km southwest of Suaqui Grande (Fig. 3). The step-heat-ing apparent age spectra do not offer a consistent plateau(Fig. 7g), but a simple inspection of the step ages suggeststheir average may provide a good approximation of theage. However, the best estimate is an isochron age definedby a very good fitting line of the argon data, which indicate56.32 ± 0.45 Ma (Fig. 7h). This age is in good agreementwith a previous K–Ar hornblende age of 59 Ma reportedby Damon et al. (1983a,b) in the Suaqui Grande area(50) and almost identical to an 40Ar/39Ar hornblende agefor a granodiorite pluton exposed 60 km north of theMV-5 sample site near La Venada mine (44).

5.5. Sample 59-96

Sample 59-96 is a medium-grained granite with localvariations to quartzdiorite, collected east of San Nicolasin easternmost Sonora (Fig. 3). The hornblende argonspectra for this rock do not offer a good plateau age(Fig. 7i). The isochron diagram yields an age of60.10 ± 0.32 Ma with steps A–C, which accounted for66.2% of the total 39Ar gas released (Fig. 7j); however,the corresponding initial 40Ar/36Ar intercept is approxi-mately 279.8, which is significantly lower than the presentatmospheric argon ratio of 295.5, which thereby makes thisage unsuitable (Baksi, 1999). Therefore, our best guess forthis sample is an average of steps A–G, which indicates anage of 59.10 ± 0.33 Ma. A K–Ar biotite age of 64 Ma forthis pluton has been reported by Bockoven (1980) (60);however, we consider it relatively old for this locationand particularly old for a biotite age.

6. Analysis of the magmatic arc migration

The geochronological record of granitic rocks in thestudy area, on the basis of U–Pb zircon and K–Ar and40Ar/39Ar dates (Fig. 8), reveals an overall west-to-east pro-gression between approximately 97 and 59 Ma, which sug-gests that the Cordilleran arc took around 38 Ma tomigrate from central Baja California across Sonora. Afterrestoring the maximum of 18% of Cenozoic extension esti-mated by Lee et al. (1996) for Baja California, the magmatic

Table 240Ar/39Ar step-heating data for hornblendes from granitoids along an E–W transect by central Sonora, northwestern Mexico

Step Temp. (�C) %39Arof total

Radiog.yield (%)

39Ark

(moles · 10�12)

40Ar�39Ark

ApparentK/Ca

ApparentK/Cl

ApparentAge (Ma)

±1r (Ma)

BC-25 Bahia Kino Granodiorite Hornblende J = 0.003738 ± 0.50% wt = 173.5 mg #28KD28A 1100 46.2 86.8 0.220403 12.163 0.07 9 80.21 0.04B 1125 17.7 94.4 0.084421 11.877 0.07 10 78.37 0.06C 1150 7.1 94.4 0.033610 11.822 0.07 11 78.01 0.13D 1175 5.0 94.0 0.024019 11.842 0.06 8 78.14 0.15E 1200 5.4 94.0 0.025920 11.911 0.06 10 78.58 0.17F 1250 11.6 94.6 0.055213 12.048 0.06 9 79.47 0.12G 1300 6.9 95.1 0.032980 12.094 0.06 9 79.77 0.24

Total gas 100.0 90.9 0.476566 12.041 0.07 10 79.42

MV-15 Hermosillo Granodiorite Hornblende J = 0.003738 ± 0.50% wt = 172.9 mg #26KD28A 1100 14.1 91.9 0.080323 9.091 0.08 16 60.29 0.21B 1150 57.4 97.7 0.326381 9.128 0.08 14 60.53 0.05C 1175 6.0 95.8 0.034397 8.647 0.08 11 57.39 0.28D 1200 6.3 95.9 0.035941 8.748 0.08 15 58.05 0.25E 1250 16.2 96.8 0.091927 9.106 0.08 15 60.38 0.14

Total gas 100.0 96.5 0.568969 9.066 0.08 14 60.1371.5% of gas in

steps A through BAverage age: 60.51 0.33

MV-11 Hermita Granite Hornblende J = 0.003739 ± 0.50% wt = 158.4 mg #25KD28A 900 4.1 90.4 0.024709 7.918 2.74 136 52.63 0.56B 1000 4.4 90.6 0.026408 8.123 0.77 96 53.97 0.55C 1100 13.4 88.8 0.079970 10.258 0.09 23 67.90 0.19D 1150 35.4 95.8 0.211576 10.588 0.08 11 70.04 0.07E 1175 10.9 95.8 0.065014 10.270 0.09 10 67.97 0.21F 1200 7.5 95.9 0.044828 10.333 0.09 12 68.39 0.30G 1250 17.6 97.0 0.104956 10.880 0.07 10 71.93 0.16H 1300 6.7 96.7 0.039953 11.071 0.06 10 73.17 0.30

Total gas 100.0 94.7 0.597414 10.354 0.22 21 68.52

MV-5 Suaqui Grande Granodiorite Hornblende J = 0.003737 ± 0.50% wt = 163.5 mg #27KD28A 1100 27.0 67.6 0.062682 8.642 0.03 11 57.34 0.19B 1125 23.2 89.6 0.053959 8.563 0.04 10 56.82 0.26C 1150 7.1 86.3 0.016588 8.232 0.04 11 54.66 0.72D 1175 5.5 89.1 0.012753 8.570 0.03 7 56.87 0.87E 1200 7.0 87.6 0.016287 8.299 0.03 10 55.10 0.60F 1250 14.6 90.1 0.033860 8.640 0.03 10 57.33 0.31G 1300 6.0 90.3 0.013930 8.446 0.03 10 56.06 0.70H 1450 9.7 92.1 0.022454 8.788 0.03 10 58.29 0.44

Total gas 100.0 83.6 0.232513 8.568 0.03 10 56.86

59-96 Maycoba Granite Hornblende J = 0.003742 ± 0.50% wt = 159.0 mg #29KD28A 1100 18.6 49.2 0.064904 8.584 0.04 6 57.04 0.15B 1125 22.9 85.3 0.079915 8.963 0.05 6 59.51 0.08C 1150 24.6 92.8 0.085687 9.053 0.06 6 60.10 0.25D 1175 6.4 88.4 0.022151 8.540 0.05 5 56.75 0.29E 1200 6.3 89.0 0.021966 8.474 0.05 7 56.31 0.24F 1250 12.6 90.3 0.043839 8.901 0.05 6 59.11 0.17G 1300 8.6 89.0 0.029809 9.327 0.05 5 61.89 0.21

Total gas 100.0 81.8 0.348271 8.880 0.05 5.833 58.97100% of gas in steps A through G Average age: 59.10 0.33

Ages calculated assuming an initial 40Ar/36Ar = 295.5 ± 0. Ages of individual steps do not include error in the irradiation parameter J. No error iscalculated for the total gas age. Average age errors repeated at two sigma.


arc advanced inland from the western edge of the SPM plu-ton to the northeast tip of Isla Tiburon in coastal Sonora ata relatively fast rate of approximately 10.9 km/Ma between

97 and 90.4 Ma (Fig. 9). This rate of migration agrees withthe 10 km/Ma calculated by Silver and Chappell (1988) forthe eastern portion of the Peninsular Ranges batholith.

0.01

0.1

K/Ca

App

aren

t age

(Ma)

Isochron Age:77.11 ± 0.40 Ma

20

60

100

140

A B C DE F G

A

BCDE

FG

Inverse-isotope correlation diagram

0.14

0.28

0.42

0.01

0.1

K/Ca

Average Age:60.51 ± 0.33 Ma

A B CD EAverage

A

B

CDE


0.08

0.16

0.24

AB

C

DEFG

H

IA: 68.77 ± 1.14 MaAri: 283.24 ± 66.4MSWD: 1.451C-F: 67.2% of 39ArK

0.12

MV-11hornblende

Hermita Granite0.24

0.36

0.01

0.11

10

ABC D E F G H

K/Ca


*

A

BCDEFGH


39Ar/40Ar

1

2

3

0.02 0.06 0.10

0.01

0.1

K/Ca


A B CD E F G H

36A

r/40

Ar

(x10

-3)

0.01

0.1

Cumulative % 39ArK released

K/Ca

Average Age:59.10 ± 0.33 Ma

20 40 60 80 100

A

Average ageB C DE F G

*

A

BCDEFG


1

2

3

IA: 56.32 ± 0.45 MaAri: 306.68 ± 4.71MSWD: 1.615A-H: 100% of 39ArK

MV-5hornblende

Suaqui Grande

39Ar/40Ar0.02 0.06 0.10


39Ar/40Ar0.02 0.06 0.10

IA: 60.10 ± 0.32 MaAri: 279.85 ± 1.04MSWD: 1.153A-C: 66.2% of 39ArK

59-96hornblende

Maicoba

39Ar/40Ar0.02 0.06 0.10

IA: 58.37 ± 0.52 MaAri: 437.52 ± 37.45MSWD: 18.805A-E: 100% of 39ArK

MV-15hornblendeHermosillo

39Ar/40Ar0.02 0.06

IA: 77.11 ± 0.40 MaAri: 371.98 ± 3.43MSWD: 1.380A-E: 81.5% of 39ArK

BC-25hornblendeBahía Kino

36A

r/40

Ar

(x10

-3)

36A

r/40

Ar

(x10

-3)

36A

r/40

Ar

(x10

-3)

36A

r/40

Ar

(x10

-3)

BC-25hornblendeBahía Kino

MV-15hornblendeHermosillo

App

aren

t age

(Ma)

20

60

100

140

App

aren

t age

(Ma)

20

60

100

14059-96

hornblendeMaicoba

0

Cumulative % 39ArK released20 40 60 80 100

App

aren

t age

(Ma)

20

60

100

140

0

MV-5hornblende

Suaqui Grande

MV-11hornblende

Hermita Granite

App

aren

t age

(Ma)

20

60

100

140

IA: Isochron AgeAri: 40Ar/36Ar initialMSWD: mean squares of weighted deviatesA-H: heating steps considered for the isochron

a

b

c

d

e

f

g

h

i

j

25

5

15

59-9611

Fig. 7. 40Ar/39Ar age spectra, K/Ca, and inverse-isotope correlation diagrams for hornblende samples of Laramide granitic rocks from the studiedtransect along central Sonora. The ages in bold are considered the most suitable hornblende cooling ages determined for the different samples.


Igneous activity may have remained in coastal Sonora atleast for another 5 Ma, and then it resumed 77 Ma ago(sample BC-25). The arc continued to migrate and reachedthe region near Hermosillo at approximately 69 Ma (sampleMV-11) and the eastern limit of Sonora at 59 Ma (sample59-96). However, age data in the eastern section of the mag-matic arc suggest that plutonism was very active in thisregion between 64 and 56 Ma (Figs. 8, 9). The relativelyyoung date of approximately 49.5 Ma reported by Damonet al. (1983b) near the eastern edge of the area (54) probablyreflects biotite contamination (Gans, 1997). In this sense, alower age limit at 56 Ma may be constrained by the data forthe eastern portion of the studied transect.

After restoring 90% of cumulative Cenozoic extension,as calculated by Gans (1997) for eastern Sonora, andassuming this value as representative of the transect upto coastal Sonora, a rate of magmatic arc migration ofroughly 8.5 km/Ma can be estimated (Fig. 9). This rateof inland arc migration is relatively similar to that of10 km/Ma obtained for the eastern Peninsular Rangesof Baja California by Silver and Chappell (1988) and

the simple two-point migration rate of 10.9 km/Ma cal-culated here for the SPM pluton and Isla Tiburon(Fig. 9). However, this rate of migration is relativelygreater than the rate of 1.5 km/Ma considered for thesouthern extension of the Cordilleran arc in Sinaloa(Henry et al., 2003).

7. Discussion

The main reason to exclude K–Ar biotite ages from thisstudy is because they may represent cooling ages too youngto estimate the age of crystallization of the granitic plutons(Fig. 5). Hornblende dating yields cooling ages that, as wehave noted, may provide an approximation of the U–Pbzircon dates and thus of the crystallization age. In thesouthern extension of the Cordilleran arc in Sinaloa(Fig. 1), Henry et al. (2003) observe that biotite and horn-blende from single samples yield K–Ar ages that are con-cordant within the analytical uncertainty. Moreover, theynote that in four of five cases, the ages are coupled withslightly younger but nearly concordant U–Pb zircon dates.

Fig. 8. Distribution of Late Cretaceous–early Tertiary K–Ar and 40Ar/39Ar hornblende and U–Pb zircon dates in Sonora. Bold numbers are samplesdated in this study. Numbers followed by numbers in parenthesis represent ages and corresponding codes in Table 1, respectively. The age range for theSierra San Pedro Martir pluton is from Ortega-Rivera (2003).


Henry and colleagues conclude that this coherence in theU–Pb and K–Ar ages yielded by the plutonic rocks associ-ated with the eastward migrating arc (90–45 Ma) impliesrapid cooling after emplacement. In Sonora, just a few ofthe K–Ar or 40Ar/39Ar dated rocks include both biotiteand hornblende, and none of them addresses U–Pb zircondating. The available examples indicate discordant biotiteand hornblende ages ranging from approximately 1 to>10 Ma (e.g., Damon et al., 1983a,b; Mead et al., 1988),with relatively large age differences dominant (>3 Ma).Therefore, cooling of the Laramide granitic plutons inSonora may not have been very rapid. However, the fewU–Pb dates in central and northern Sonora appear to bein good concordance with contiguous K–Ar and 40Ar/39Arhornblende ages (1–4 Ma age differences, Fig. 8). The clo-sure temperature of hornblende with respect to the K andAr isotopes (530 ± 40 �C, Harrison, 1981) likely wasreached by the magma not long after its emplacement.The discordant, often much younger biotite ages may indi-cate that further cooling to less than 300 �C was relativelyslower.

In addition to the age overlap observed in the Laramidegranitic rocks of Sonora (Fig. 2), another major objectionexists to the time patterns expected for the Cordilleranmagmatism in this region. In east-central Sonora, wheremany of the data are concentrated (Fig. 4), McDowellet al. (2001) report ages of 90–70 Ma for volcanic rocksassociated with the Tarahumara Formation (Wilson andRocha, 1949), which is regionally considered the volcaniccomponent of the Laramide magmatic arc. These agesare much older than predicted in previous works andbecame an intriguing problem for constraining a conceptu-al tectonic model for the Laramide magmatism in Sonora.So far, equivalent ages for plutonic rocks are not knownfor this region, though biotite and hornblende K–Ar datesof approximately 83 Ma have been reported for a granodi-orite pluton exposed near Batopilas, in southwestern Chi-huahua close to the intersection with Sonora and Sinaloa(Bagby et al., 1981). Moreover, McDowell and Mauger(1994) report volcanic and plutonic rocks with ages of68–55 Ma in central Chihuahua, which also appear exceed-ingly old for the general eastward-decreasing age pattern

Fig. 9. Age variation across the study transect of central Sonoraaccording to ages in Fig. 8. The diagram represents an E–W line thatdivides the transect in the middle. The sample ages nearby were projectedto the section according to the regular NW–SE trend observed for themagmatic arc. Labels in bold are samples dated in this study. Othernumbers refer to codes in Table 1. The eastward migration rates werecalculated assuming the Cenozoic extension rates indicated for each case.The solid line represents the earliest arrival of the magmatic arc activity toeasternmost Sonora. The dashed line represents the best fitting line thatcrosses the newly dated samples BC-25, MV-11, and 59-96 and theprevious dates indicated by 45, 49, 55, and 59 (Table 1).


previously visualized for Laramide magmatic activity innorthwestern Mexico and, in general, for southwesternNorth America (e.g., Coney and Reynolds, 1977; Damonet al., 1983a). The unexpectedly old ages obtained ineast-central Sonora led McDowell and colleagues to con-sider the Laramide magmatic event a tectonic episode char-acterized by a series of eastward-stepping, long-lived,overlapping arcs rather than a continuous transgressivearc. The evidence from the Tarahumara volcanic rocks isvery strong and suggests that the presence of equivalentplutonic rocks could be demonstrated. For now, the avail-able plutonic rock ages indicate an apparently sustainedeastward migration of the magmatic arc across Sonora dur-ing Laramide times.

Regarding the rate of migration of the Cordilleran arcacross northwestern Mexico, it is clear that it occurred inat least two episodes: a first episode characterized by anearly static western arc that operated between approxi-mately 140 and 105 Ma in Baja California (Silver andChappell, 1988) and 139–101 Ma in Sinaloa (Henry et al.,2003), and a second episode characterized by a migratingeastern arc that migrated inland between 105 and 80 Main Baja California (Silver and Chappell, 1988) and 90 and45 Ma in Sinaloa (Henry et al., 2003). The eastern arcarrived at coastal Sonora approximately 90 Ma ago andapparently remained there for a period of at least 10 Ma.The arc restarted migration at approximately 77 Ma andreached eastern Sonora at 59 Ma.

In addition to geochronology, uncertainties in the anal-ysis of the arc migration are complicated by the Late Ceno-

zoic extensional tectonics. In particular, the tectonicdisplacements of the rifting of the Gulf of California andthe northwest translation of the Baja California peninsulalargely fragmented the magmatic belts associated with theCordilleran arc. After restoring these motions, the Peninsu-lar Ranges batholith of Baja California would lie in frontof central and southern Sonora and northwest of thebatholithic rocks of Sinaloa (Henry et al., 2003). Also,the Los Cabos block (�129–115 Ma), located in the south-ern tip of the Baja California peninsula south of La Paz(Fig. 1), would lie just northwest of the Jalisco block(Schaaf et al., 2000), which is slightly south of the map areashown in Fig. 1.

The sections of the long-lived, nearly static western arcin western Baja California and Sinaloa commonly wereemplaced in a relatively young, thin crust dominated bymantle-derived volcanic and volcaniclastic successions(Fig. 4). The differential rates of eastward migration ofthe eastern arc in Baja California and Sonora, comparedwith the much slower rate calculated for Sinaloa, may havebeen determined by important physical aspects of the crustabove the subducted plate. In Baja California, as plate con-vergence proceeded, the magmatic arc axis encountered acrustal front characterized by the Proterozoic North Amer-ican basement (Fig. 8). This possibility is more obvious forSonora, where the melting zone was overlain by this thickcrustal structure. The arc migration largely depended onthe rate of the Farallon/North American plate convergenceand subduction angle (e.g., Coney and Reynolds, 1977);however, the relatively large area of plate coupling expect-ed in such a flat subduction scenario (e.g., English et al.,2003) may have been critical for controlling the locus andamplitude of the magmatism. In Sinaloa, the arc did notcome across such a thick old structure, and the oceanic slabapparently was not forced to develop flat subduction geom-etry, which resulted in much more static magmaticmechanism.

8. Conclusions

Our new 40Ar/39Ar hornblende dates for Laramide gra-nitic rocks from central Sonora reveal important temporalconstraints for evaluating the eastward migration of theLate Cretaceous–early Tertiary magmatic activity in thesouthern part of the North American Cordillera. Becauseof the relatively high closure temperature of hornblendeto argon diffusion, we argue that these dates give a goodapproximation of the crystallization ages. The results indi-cate an age range between 77.6 ± 1.6 to 56.3 ± 1.3 Ma,which roughly constrains the timing of plutonic activityassociated with the Laramide event in central Sonora.The extension-restored calculation of the eastward arcmigration, using a line that connects samples BC-25,MV-11, and 59-96 (Fig. 9), indicates a rate of approximate-ly 8.5 km/Ma, which is slightly slower but comparable tothat developed by the arc in its course from central BajaCalifornia to coastal Sonora. Moreover, we include the

Fig. 10. Regional time contours for Cretaceous–early Tertiary granitic rocks of northwestern Mexico. The dated localities are arranged by time slices,indicating the employed dating method. Dates for the Sinaloa batholith are from Henry et al. (2003); time contours for the Baja California plutons arefrom Ortega-Rivera (2003).


nearby samples – referred to as numbers 45, 49, 55, and 59in Table 1 – and a best fitting line (dashed line in Fig. 9)yields a likely rate of eastward migration of approximately7.6 km/Ma.

The new ages and existing K–Ar and 40Ar/39Ar horn-blende plus U–Pb zircon dates constrain various regionaltimelines (Fig. 10), which are sustained on a relativelyhomogeneous basis. Time contours between 120 and80 Ma were previously determined for the PeninsularRanges batholith of Baja California by Ortega-Rivera(2003). These contours, defined for each 10 Ma, are rela-tively tight and evenly spaced across a distance of approx-imately 100 km. In Sonora, the westernmost time limit isfor 80 Ma and extends along the coastal region in centraland southern Sonora, continues south along Sinaloa, andvirtually dissects the state by the central region. Fig. 10shows that the connection with the 80 Ma and possiblythe 90 Ma timelines in eastern Baja California does notmatch properly with the available data for coastal Sono-ra. The actual age database may not be sufficient, andmore geochronological work is needed, particularly inSonora; however, the figure also suggests that a tectonicreconstruction placing the Baja California peninsula a lit-

tle more to the east or north could yield a better agefit.

The 70 Ma contour is constrained far more to the east inSonora (150 km from the coast in central Sonora), follow-ing a NW–SW direction along the central part of the state;however, in Sinaloa, it lies basically in the same position asthe 80 Ma contour, which suggests that eastward arcmigration between these timelines was almost indistin-guishable for this region. The last time contour that maybe reasonably constrained is 60 Ma, which lies east of the70 Ma line and describes a similar trajectory along Sonoraand Sinaloa. The distance between both contours is actual-ly of 75 km in northern Sonora and decreases progressivelyto around 25 km in Sinaloa. Dates younger than this ageare known farther east into Chihuahua, but constraintsto evaluate the rate of migration are complicated by theextensive cover of the mid-Tertiary Sierra Madre Occiden-tal volcanic province (Fig. 1).

Acknowledgements

This research was partially funded by Consejo Nacionalde Ciencia y Tecnologıa (CONACYT, Grant I29887T to


Valencia-Moreno). We thank Arturo Martın-Barajas andThierry Calmus for helpful reviews and comments, whichbenefited the article. A fresh sample chunk of the Maicobapluton (sample 59-96) was kindly provided Jaime Roldan-Quintana. We express our appreciation to Michael Kunkfor access to and close supervision of the step-heating40Ar/39Ar geochronology at the US Geological SurveyThermochronology Laboratory in Denver, Colorado. Wealso thank Roy George and Seth Fetters, former Universityof Colorado at Boulder students, for their careful horn-blende mineral separations for the geochronology studies.

Appendix A. 40Ar/39Ar geochronology

Five samples from fresh, Late Cretaceous–early Tertiarygranitoids from central Sonora were collected to perform40Ar/39Ar geochronology (Table 2 and Fig. 5). We pro-duced 150–125 lm hornblende separates with magnetic sep-aration, heavy liquids, and hand-picking techniques with apurity of >99%. The separates were washed in acetone,alcohol, and deionized water in an ultrasonic cleaner toremove dust and then resieved by hand using a 125-lmsieve. Hornblende aliquots of approximately 150 mg werepackaged in copper capsules and vacuum sealed in quartztubes. The samples were irradiated for 15 h in the centralthimble facility at the TRIGA reactor (GSTR) at the USGeological Survey, Denver, Colorado. The monitor mineralused in the package was Fish Canyon Tuff sanidine (FCT-3)with an age of 27.79 Ma (Kunk et al., 1985; Cebula et al.,1986) relative to MMhb-1 with an age of 519.4 ± 2.5 Ma(Alexander et al., 1978; Dalrymple et al., 1981). The typeof container and the geometry of samples and standardsare similar to those described by Snee et al. (1988).

Finally, the samples were analyzed at the US GeologicalSurvey Thermochronology Laboratory in Denver, Colora-do, on a VG Isotopes Ltd. model 1200 B mass spectrometerfitted with an electron multiplier using the 40Ar/39Ar step-heating method of dating. For additional information onthe analytical procedure, see Kunk et al. (2001). The argonisotopic data were reduced using an updated version of thecomputer program ArAr* (Haugerud and Kunk, 1988).We used the decay constants recommended by Steiger andJaeger (1977). Table 2 shows 40Ar/39Ar step-heating datafor the hornblende separates and identifies individual steps,the existence of plateaus, and total gas ages. Total gas agesrepresent the age calculated from the addition of all mea-sured argon peaks for all steps in a single sample. The totalgas ages are roughly equivalent to conventional K/Ar ages.No analytical precision is calculated for total gas ages. Pla-teau ages emerge when three or more contiguous steps inthe age spectrum agree in age, within the limits of analyticalprecision, and contain more than 50% of the 39ArK releasedfrom the sample. Average ages are calculated from contigu-ous steps forming no plateau but containing more than 50%of the gas, with the intention to obtain the best age approx-imation for the sample.

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Documents

Temporal constraints on the eastward migration of the …rmolina/documents/valenciamorenoetal2006.pdfMartı´n Valencia-Moreno a,*, Alexander Iriondo b,c, Carlos Gonza´lez-Leo´n