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Tectonic framework of Tiburon Basin, Gulf ofCalifornia, from seismic reflection evidenceEduardo Mar-Hernández a , Mario González-Escobar b & Arturo Martin-Barajas ba Instituto Tecnológico de Ciudad Madero , C.P. 89440 , Ciudad Madero , Tamaulipas , Méxicob División de Ciencias de la Tierra , Centro de Investigación Científica y de EducaciónSuperior de Ensenada (CICESE) , Ensenada , Baja California , C.P. 22860 , MéxicoPublished online: 16 Dec 2011.
To cite this article: Eduardo Mar-Hernández , Mario González-Escobar & Arturo Martin-Barajas (2012) Tectonic frameworkof Tiburon Basin, Gulf of California, from seismic reflection evidence, International Geology Review, 54:11, 1271-1283, DOI:10.1080/00206814.2011.636988
To link to this article: http://dx.doi.org/10.1080/00206814.2011.636988
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International Geology ReviewVol. 54, No. 11, August 2012, 1271–1283
Tectonic framework of Tiburon Basin, Gulf of California, from seismic reflection evidence
Eduardo Mar-Hernándeza*, Mario González-Escobarb and Arturo Martin-Barajasb
aInstituto Tecnológico de Ciudad Madero, C.P. 89440, Ciudad Madero, Tamaulipas, México; bDivisión de Ciencias de la Tierra, Centrode Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, Baja California, C.P. 22860, México
(Accepted 3 October 2011)
Tiburon Basin is characterized by a thick sedimentary fill that records the evolution of one of the rift segments of the EastPacific Rise. Its structure corresponds to an echelon pull-apart basin bounded by two dextral-oblique faults. Unlike basinsin the southern Gulf of California that are underlain by oceanic crust, rift basins in the northern Gulf of California containsedimentary thickness (up to 6 km) that masks the structure of the crust. To study the architecture of the Tiburon Basin,two-dimensional, multichannel seismic reflection data collected by Petróleos Mexicanos (PEMEX) in the early 1980s wereused. The data base is a grid of lines, 5–20 km apart, with 6 s of record in 48 channels. Additional seismic data of the Ulloa99 project were also interpreted. Our results indicate that the general structural pattern of the Tiburon Basin is controlled bytwo dextral-oblique faults: De Mar and Tiburon. De Mar lies to the east and ends in elevated basement transferring the stressto the Desemboque fault. The latter borders the incoming basement from the Sonora and Tiburon faults to the west, endingto the north in an antiform. Four structural domains are recognized: (1) the northern Tiburon domain is a high basementthat divides the Delfin Basin to the northeast and exhibits extensional folds with their axes parallel to the basement and itsflanks; (2) the Libertad domain is a sheared basement high along the margin of Sonora and forms the right step of the TepocaBasin to the north; (3) the Tiburon central domain defines a broad sag cut by a dense NE-striking pattern of normal faultswith opposed dips in the depocentre and abruptly ends to the west against the Tiburon fault; and (4) the southern Tiburondomain forms a basement ramp offshore Isla Tiburon and is controlled by a pattern of NNE-striking normal faults on thesouth that likely connect at an oblique angle (∼60◦) to the De Mar fault. We propose a rhombochasm basin model withmore than 6 s of sedimentary record in the depocentre, in which the basement is not recorded. The NW-trending faults inthe Libertad domain possibly continue towards the Sonora coastal plain. The principal NW-trending dextral faults and thesecondary NNE-striking pattern of normal faults cut the shallow strata of this domain.
Keywords: Gulf of California; rift; pull-apart basin; seismic reflection; Tiburon Basin
Introduction
The Gulf of California is located in the divergent bor-der between Pacific and North American tectonic plates(Figure 1). This border plate has a dextral strike–slip thatdominates the oblique movement of the Baja CaliforniaPeninsula in relation to the continent. The deformationis primarily accommodated due to the transform faults,while the extension is accommodated in normal faultsforming oblique basins (Fenby and Gastil 1991; Nagy andStock 2000; Aragón-Arreola and Martín-Barajas 2007).The plate boundary south of the lower Delfin Basin isdefined by narrower zones of deformation. The faultzone Canal de Ballenas strain transferred to the GuaymasBasin along a strip narrow deformation is orientedN30◦–46◦W close to the orientation of Pacific–NorthAmerican relative motion to N33◦W (Atwater and Stock1998; Figure 1).
*Corresponding author. Email: [email protected]
Towards the northern Gulf, the Cerro Prieto fault defor-mation transfers the stress between the diffuse zone ofdeformation (Wagner–Delfín Basins) and the system ofthe San Andreas fault, which includes the Imperial faultin the valley of the same name (Figure 1). In the mouthof the Gulf, the Alarcon Basin has an oceanic crust withmagnetic anomalies, indicating that the formation of oceanfloor began at 3.5 Ma (Lonsdale 1989; DeMets 1995).In contrast, in the northern Gulf, it has not been possi-ble to document the presence and the age of the oceaniccrust because the basins are filled with thick sediments.However, in recent years, the results of the interpreta-tion of seismic data of reflection and refraction have beenpublished which have contributed to the understanding ofthe structure and the geometry of this region (Persaudet al. 2003; González-Escobar et al. 2006, 2009; Aragón-Arreola and Martín-Barajas 2007).
ISSN 0020-6814 print/ISSN 1938-2839 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2011.636988http://www.tandfonline.com
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Figure 1. (A) Map of the Gulf of California. The main structures are transform faults that connect a series of pull-apart basins. AlarconBasin and the basins as far north as Guaymas Basin present an ocean floor from 3.5–0 Ma (Lonsdale 1989). The northern basins (Delfin,Consag, and Wagner) are covered with sediments and their structures are still unknown in detail. The system of active faults and basins isshown in red lines. The Tiburon Basin is located between Tiburon Islands (IT) and Angel de la Guarda (IAG). FCP = Cerro Prieto Fault,E = Ensenada, Cabo San Lucas CSL = (modified from González-Escobar et al. 2009). (B) Study area and seismic lines of PEMEX(black) and Ulloa 99 (orange), with a record time of 6 and 2 s, respectively.
The Tiburon Basin has a sigmoidal shape bounded onthe west by the Tiburon fault and to the east by the De Marfault (Figure 1; Aragón-Arreola and Martín-Barajas 2007).The southwestern part consists of a basement with a sharpdrop that extends into the Gulf from the coast of the Isla
Angel de la Guarda, and the northern end of the basin isa structural high that extends from the northern tip of IslaAngel de la Guarda to the northeast towards the TepocaBasin (Figure 1). The Tiburon Basin is part of a systemcontrolled by transform faults connecting pull-apart basins
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in the axial zone of the Gulf. This system has created aseparation of the margins of the basin to the northwest,produced by transtensional efforts acting on the rift zoneof the Gulf of California.
Based on a seismic refraction profile, González-Fernández et al. (2005) estimated that the Tiburon Basinhas a crust thickness of 19 km off the coast of Sonora,decreasing to 17 km beneath the basin, with a sedimentthickness of approximately 6 km. This study suggested thatthe filling seismostratigraphy sediment consists of threemain stratigraphic sequences that Aragón-Arreola (2006)called ‘pre-rift, syn-rift, and post-rift’. Due to the largethickness of sediments in the basin, it is difficult to interpretthe structure of the basement and the type of crust underthe basin, thus the analysis of gravity maps published andmagnetic anomalies is essential to the understanding of thedeep structure.
The region between the Cerro Prieto and Canal deBallenas faults is structurally characterized by wide zonesof distributed deformation in multiple and oblique nor-mal faults in the lower Delfin, upper Delfin, Consag, andWagner Basins, and all are tectonically active (Persaudet al. 2003; González-Escobar et al. 2009). Due to the highrate of sedimentation (1–4 mm/year, Schmitt and Vazquez2006) produced by the contributions of the Colorado River,the basins have several kilometres of sedimentary fill, andthe kind of crust beneath them is unknown. The lack of evi-dence of seafloor can suggest that the crust of the northernGulf may be considered a transitional crust.
In the eastern margin of the northern Gulf fromnorth to south, the basins Adair, Tepoca, and Tiburon arelocated. These basins are tectonically inactive (Aragón-Arreola and Martín-Barajas 2007). Several studies (Phillips1964; Lonsdale 1989; Fenby and Gastil 1991; Stock2000; González-Fernández et al. 2005) lacked the data toresolve the structural pattern of the basins. Aragón-Arreolaand Martín-Barajas (2007) used a Petróleos Mexicanos(PEMEX) data base in the Tiburon Basin, reporting newstructures in the Tiburon Basin including the depth of thebasin in time. In this work, we present new seismic datathat provide significant knowledge of the structure in thisinactive basin (Figure 1A).
We evaluated the seismic reflection data (propertyof/provided by PEMEX) with the purpose of obtaining abetter definition of the geometry and structural character-istics of the Tiburon Basin. Our ultimate aim is to improvethe understanding of the geometry and the tectonic role ofthe faults in this basin.
Data analysis
From 1978 to 1980, PEMEX implemented an explorationprogram in the northern Gulf of California during whichseismic reflection data were collected over the TiburonBasin as part of the San Felipe–Tiburon campaign (Pérez-Cruz 1982). In this work, these seismic data have been
reprocessed and reinterpreted. The data base comprisestwo-dimensional, marine multichannel seismic reflectionrecords covering approximately 1500 km along lines ori-ented S60◦W and N30◦W (Figure 1B).
The data were acquired with a 1341 cubic inch air gunfired at 6.144 s intervals and recorded by a 48-channelhydrophone streamer over a 4 s scan, the individualchannels being spaced 50 m apart. The data-processingsequence was as follows: (1) editing seismic records,(2) filtering, (3) velocity analysis based on semblance coef-ficients, (4) normal move out and stack, (5) sphericaldivergence correction, (6) predictive deconvolution (widtha filter length of 30 ms and gap of 16 ms), (7) migration,and (8) time-variant filtering (Yilmaz 1987). Data process-ing and interpretation are based on Badley (1985) and usedProMax and Seisworks of the Landmark software package.
Results
Seismics
All the seismic lines show a good resolution from 0.5 to 6 stwo-way travel time (TWTT). Through the seismic lines atthe north of Tiburon Basin (Figures 2 and 3), the strata onthe basement high present folds of low amplitude super-imposed on the antiform structure (basement high). Theseare elongated mainly in the same direction of the crest ofthe antiform but also occur in the southern flank of theantiform. These folds are continuous over 15 km orientednorth–northeast (average N10◦E) and occur at a distanceof between 5 and 27 km of the axis of the main antiform.The secondary folds are better defined in the southeasternpart of the antiform in comparison to the west, where thesecondary folds are on the crest of the structural high at adepth of 1000 ms.
The majority of the northern Tiburon faults do notcut the newer stratigraphic sequence, but generally cut thestrata of those under, on average, 400 ms. However, somefaults in the northwestern flank cut the recent deposits onthe crests of the anticlines and/or control the location oftidal channels in the shallow zone between the Tiburon andlower Delfin Basins to the northwest (Figure 3). The dipof the strata in the northern domain is towards the north-west on the northern flank of the antiform. On the axis ofthis structure and on its southern flank, the strata have localvariations due to the secondary folds observed. The stratain general dip to the depocentre in a SSE direction as shownin Figure 4. Also shown is the fault density, where the high-est density faults are located to the northwest and southeastof the basin, which correspond to the northern and southerndomains, respectively.
Structural domains definition of Tiburon Basin
The rift basins are characterized by a geometry governedby master faults and vertical displacement is generallycontrolled by sedimentary fill. The Tiburon Basin geometry
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Figure 2. Seismic line 5097 through the northern Tiburon domain and part of the Libertad domain to the west. Tiburon fault (blue), DeMar fault (pink), and acoustic basement interpreted (yellow). Note great fault concentration of high angle (red), which presents a dominantnorth–northeast orientation, oblique to De Mar and Tiburon faults. An antiform is defined in the projection zone of the Tiburon fault tothe east of the structural high. The yellow line is the acoustic basement and red lines are faults.
is controlled primarily by the dextral-oblique faults Tiburonand De Mar to the east and west, respectively (Aragón-Arreola and Martín-Barajas 2007). The Tiburon fault hasan approximate length of 80 km, oriented N33◦W, whilethe De Mar fault has a length of approximately 62 km andis subparallel with a strike of N50◦W. Although these arethe master faults, the Tiburon Basin and its margins can bedescribed in terms of four structural domains: (1) TiburonNorth, (2) Libertad, (3) Tiburon Centre, and (4) TiburonSouth, as shown in Figure 6. These domains have failurepatterns of density and orientation (Figures 5 and 6) anddeformation characteristic styles that are used for descrip-tion. Figure 6A shows the apparent dip of the faults for eachof the domains.
Tiburon North domain
This domain represents the northeastern end of the TiburonBasin and is characterized by the presence of a structural
high of acoustic basement, that lifts up to 1500 ms(TWTT), with an elongated axis oriented N10◦E with30 km length (Figures 2 and 3). The structural high definesan antiform structure reaching 24 km wide, as shown inFigure 6. This structure separates the Tiburon Basin at thesoutheast from the lower Delfin Basin and the northwestof the upper Delfin Basin. The northern Tiburon domainends with the fall of the north flank of the structural highthat deepens in a series of normal faults with northeastorientation dipping towards the northwest Delfin Basin.At the east, the De Mar fault separates Tiburon Basin ofthe high basement of the Sonora margin. The west limit isthe prolongation of the trace of the Tiburon fault, whereexpression loses its vertical displacement.
The folding presents a pattern of normal faults in theNNE direction (Figure 7), with dip opposed with respect tothe folding axis, and is observed in the south flank of thestructural high. Using the seismic lines of Ulloa 99 witha NW–SE direction that cross the dominating structural
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Figure 3. Seismic line 5093 that crosses the ending north of Tiburon Basin. The axis of the antiform is not at the centre of the structuralhigh (between black lines), suggesting the presence of a low-angle fault which produces a drag fold ‘roll over’ on the southeast flank of thestructural high. The fault’s pattern in NE–SW (in red) is parallel to the orientation of the folds. The yellow line is the acoustic basementand red lines are faults.
pattern in a NNE direction in the northern Tiburon domain,the density of faults is relatively higher than the otherdomains, becoming more than 30 faults per 5 km length(Figures 2 and 3). The major fault density in this domaincould be due to better seismic line resolution of Ulloa 99,even though PEMEX lines also show a high fault densityin this area. In general terms, the apparent dip of the faultsshown in Figure 5A has a predomination of high-anglefaults (>60◦).
In the northwest flank of the structural high, the faultdips are at west–northwest, while the faults that dip towardssouth–southeast in the south flank of the structural highcontrol the basement fall unto the depocentre of theTiburon Basin. However, the secondary folds on the south-ern flank produced locally reverse failure on the depth ofthe fold axis. To the south, the fall of the acoustic base-ment fits into a pattern of faults that falls to the southeaston the southern flank of the structural high. This patternof faults is manifested in an area over 30 km long towards
the basin depocentre, where dominate the faults of TiburonCentre, which have the dip in the opposite direction (NW)even though they have the same orientation. As there is atransition zone between the northern Tiburon domain andthe southern Tiburon domain, it is defined by the change inthe direction of the fall of the faults, which may define theposition of the depocentre of the basin.
Libertad domain
This structural domain is located east–northeast of theTiburon Basin (Figure 5) and is characterized by an incom-ing acoustic basement that extends over 35 km to the west,with a depth of between 300 and 2500 ms (Figures 8and 12). This outcoming of the margin of Sonora’s base-ment separates Tepoca Basin to the north. The morphologyof the acoustic basement close to the coast is defined bynarrow and shallow grabens (1300 ms) oriented to thenorthwest. Some of these grabens are controlled by faults
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Figure 4. Regional domains of the strata lying between 0 and 2 s. In general, strata tend to dip towards the basin depocentre. In the northend, the strata change direction towards the depocentre of the lower Delfin Basin. Fault density is included. The highest density faults arelocated to the northwest and southeast of the basin, which correspond to the northern and southern domains, respectively.
with significant vertical displacement that control an asym-metric geometry of the sedimentary fill. Some of thesegrabens or valleys are of little relief but are visible due tothe vertical exaggeration of the seismic lines (Figure 5B).
The interpreted faults in the northern part of Libertaddomain, where the acoustic basement deepens to the north-west, form a pattern in the direction of approximatelyN15◦E, lying west–northwest towards Tepoca Basin, wherethe main fault is Libertad (first time documented in thiswork), being approximately 50 km in length. The south-southeast part of the basement and sedimentary cover ischaracterized by being cut by faults in a N50◦–60◦Wdirection, possibly dextral, which form the narrow grabens
near the coast of Sonora. These structures are well definedat a depth between 1300 and 1650 ms (Figures 8 and 9).
In the PEMEX seismic lines, the density of faults inthis domain is high, with seismic line segments having upto 20 faults in a 5 km length, while the estimated apparentdip in this region indicates that the faults have moderateto high angles (Figure 5B). The apparent graphic dip ofthe faults in the Libertad domain is dominated by high-angle faults between 60◦ and 90◦, although there were also40% of moderate angle faults. The high-angle faults in theLibertad domain (Figure 5B) usually do not intersect therecent layers (∼0–300 ms). In several of the seismic linescan be observed horizontal layers not affected by the faults
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Figure 5. Delimitation of the four structural domains of Tiburon Basin and apparent dip graphic of Tiburon Basin. (A) Northern Tiburondomain. The majority of the faults are high angle (60◦–90◦) even though there are a considerable number of faults with a moderate angle(30◦–60◦). (B) Libertad domain. Dominated by high-angle faults between 60◦ and 90◦, although there were 40% moderate angle faults.(C) Tiburon Centre domain, which has the highest density faults. Most angle faults are moderate (30◦–60◦), and high-angle faults (60◦–90◦)were also observed. (D) Southern Tiburon domain. This domain is characterized by a high density of moderate angle faults (30◦–60◦) andvery high density of high-angle faults (60◦–90◦).
(Figures 8 and 9). Below the horizontal strata (>300 ms),there are some strata dips generally to the west due to thepresence of faults that form grabens. The faults control thelocal dip and there are cases with encounter strata in thecentre of the graben (Figure 4). A pattern of failure to thenorth–northeast (Libertad fault) controls the basement fallinto the Tepoca Basin, but the upper strata dip towards theNNW.
Tiburon Centre domain
This domain includes the central part of the Tiburon Basin,where the depocentre is located/present (Figure 6). Thedomain is bounded by the Tiburon fault to the west, andthe De Mar fault and the Infiernillo fault to the east; thislatter is documented here for the first time with a lengthof approximately 35 km. The domain is characterized bytwo patterns of subparallel normal faults (Figures 5 and 10)
with NE–SW direction, but falling in opposite directions.The Tiburon Centre domain has an abrupt boundary to thewest controlled by the Tiburon fault, which has a significantvertical fall in its central part. This abrupt fall of the base-ment to the east indicates that the Tiburon fault is obliqueand generated space for the accumulation of sediments afew kilometres from the fault zone making it not possibleto determine the depth of the basement in the area of thedepocentre. The T-1 exploration well located on PEMEX5113 line of this domain is 4.8 km deep and does not cut thebasement. The density of faults in this domain is the high-est in this study, reaching >50 faults in 5 km. The apparentdepth of the faults shows a dominating pattern of moderateangle faults (30◦–60◦) (Figures 4 and 5C).
Tiburon fault does not cut recent sediments in the cen-tral and southern part of the basin, but in the north, in thelast 10 km, this fault zone cuts the recent sediments and
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Figure 6. Seismic line Ulloa-29 on the high structural NW–SE direction. This line shows an open fold with its axis pointed to the blueline. The other blue line represents the projection of one of the axes of the folds. Faults (red) show the opposite lying on both sides of theantiform.
Figure 7. Seismic panel (lines 5117-5032-5121). The abrupt basement (in yellow) on both sides of the figure is controlled byDesemboque fault (in blue), which was reported as a normal fault that deepens towards the depocentre of Tiburon Basin (González-Escobar et al. 2006). Desemboque fault is not a continuity of De Mar fault to the South; instead it is perpendicular to De Mar fault.A pattern of normal faults centring on Infiernillo fault (in yellow on the panel) accommodates most of the basin subsidence. Towardsthe centre, it is not possible to determine the presence of acoustic basement, although there are irregular reflectors discontinuous atapproximately 4.5 s (left side panel).
controls the formation of tidal channels. The depth of thestrata converges towards the depocentre area (Figure 12)with some local inversions in the dip direction. In theseismic lines, it was observed that the faults cut the stratathat lie beneath the seabed of 200 ms (Figure 10). The area
of the depocentre of the basin has less coverage of seismiclines, but the overall trend is consistent. To the east of theTiburon fault, the strata dip towards the east–northeast, andin the seismic line 5040 the strata are inclined towardseast–southeast.
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Figure 8. Seismic panel lines interpreted (5085, 5024, and 5079). It shows the fall of the basement (yellow line) towards Tepoca Basin,which marks the northwest boundary of Libertad domain. Further south, the incoming basement margin of Sonora (yellow) extends overapproximately 35 km to the west. Note that the adjacent faults to the left vertex converge at depth, indicating that they are the same faults.
Southern Tiburon domain
The southern Tiburon domain is controlled by a NNE-oriented fault pattern that apparently forms a continu-ous pattern of normal faults with the central domain(Figure 11). The faults in this domain are character-ized by an apparent depth of moderate and high angles(45◦–90◦), as shown in Figure 6D. The fault densitybecomes >12 faults in 5 km, mainly along the areas wherethe faults are of moderate angle (>30◦–60◦) and reach-ing >30 faults in 5 km length for high angles (Figures 4and 10). In the area near the coast of Sonora, the faults donot cut into the upper strata (000–300 ms) and produce thelocal plummet of the strata, and below this level it is notaffected by the faults.
Structural settings of Tiburon Basin
The structure of Tiburon Basin is controlled by two dextralfaults, De Mar and Tiburon, N50◦W and N33◦W, respec-tively, which produce the downfall of the basement on bothsides of the basin (Figure 12). However, the consistentinclination of the strata towards the depocentre in the first2 s of record suggests that a majority part of the extension
of the subsidence of the basin fits between a two subparallelfault patterns, with north–northeast orientation, that end upin the master faults. These fault patterns define a pull-apartbasin, over 6 s of sedimentary fill in the depocentre, wherethe basement is not observed (Figures 5, 11, and 12).
Including the southern part of the basin, there is a grad-ual descent of the basement of the northwestern marginof Tiburon Islands (Figure 12). This descent of the base-ment is controlled by a NNE-oriented fault pattern thatconnects at an angle (∼60◦) to the south end of the De Marfault (Figures 4 and 12). The southeast of the basementramp is interrupted to the coast by NNW faults forminga shallow and narrow graben (Figure 12). The structuralsouthern closure of the Tiburon Basin is not defined due tothe lack of seismic data. However, the acoustic basement isclearly seen in the 5131 line, which is the last complete lineavailable in this work.
The NW faults, mainly the Libertad fault, which isdocumented for the first time, suggest the likely continu-ation of these dextral faults to the coastal plain of Sonora(Figure 12). As the principal (dextral) faults NW–SEand the pattern of normal faults (NNE) cut the youngerstrata in some areas, this activity was possibly recent or,
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Figure 9. Detail of seismic line 5101 (CDP 3600–4700) showing/illustrating the graben’s presence controlled by fault depths of1300 ms formed on the basement of Sonora’s margin. The Libertad presents less relief apparently controlled by normal faults, andlateral displacement (in red) controls the geometry of the sedimentary fill. Note that only some faults cut the upper strata.
Figure 10. Seismic panel (lines 5111-5032-5113). Subparallel faults (red) with NE–SW direction, which have opposite dip, allowing thecorrelation of the same panel, are represented in green in both sides of the panel Infiernillo fault as well as De Mar fault (in purple) andthe correlation of secondary faults in the depocentre is represented in pink. Acoustic basement interpreted is represented in yellow.
alternatively, it could mean that the strata are older thanthe fault. The south projection of the Tiburon fault is deter-mined to be to the area between the San Esteban and SanLorenzo Islands and not between Tiburon and San EstebanIslands as inferred in published papers (Lonsdale 1989).This latest projection requires a change in the orienta-tion of the fault at approximately 15◦–30◦ to the project
direction towards the southern border of Tiburon Islands.The Tiburon fault could join the transform fault systemsomewhere between San Esteban and San Lorenzo Islands.The general dip of the strata above 2 s in the TiburonBasin can be summarized in three points: (1) the strataconverge towards the depocentre area located in TiburonCentre domain, with some local changes produced by
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Figure 11. Seismic panel (lines 5131-5032-5127). The stepped basement ramp of Tiburon Islands is controlled by north–northeastnormal faults that connect at an oblique angle (60◦) with De Mar fault. Desemboque fault is represented in blue.
growth faults; (2) the strata dip to the southeast throughthe structural high; and (3) the coastal area of Sonora con-tains sub-horizontal strata in the first 300 ms which werenot affected by the faults that produce the basement relief(Figures 4 and 12).
Discussion and conclusions
The processing and interpretation of multichannel seismicreflection lines that are the property of PEMEX and in thepublic domain (Ulloa 99) suggest a structural map of theTiburon Basin, where the acoustic basement (Figure 12)has two downfalls associated with the Tiburon fault onthe western edge and the De Mar fault on the easternedge. These two faults define a rhombochasmic basin withnormal NNE-oriented faults (average N10◦E) which con-trol the fall of the basement in the northern and southerndomains, respectively. This geometry defines a pull-apartbasin, although it is possible that part of the deformationalso occurs on the coast of Sonora, possibly through agradient of increasing deformation towards the De Marand Tiburon faults. The basement of Sonora’s marginhas step geometry with two segments, one controlled bythe La Cruz fault system in the southern end and anotherby the De Mar fault. These faults produce two incoming
basements over 35 km to the west: one is Tiburon Islandsand the other is the Libertad domain. The northern endof the basin is oriented north–northeast to the structuralhigh of approximately 20 km wide, which reaches 2.5 s(in contrast to the >6 s depth of the depocentre). The sedi-mentary sequence on the high structural and in its southernflank shows open folds with normal faults on the ridges.It is speculated that drag folds developed on a detachmentfault at depth with transportation of the top plate towardsthe southeast. This low-angle fault is only documented inthe PEMEX 5085 line. The depocentre of the basin hasa slightly elongated north–south shape and is deeper than6 km. The area ‘without basement’ is approximately 57 kmin the NW direction, and it is possible that in this region ofthe basin the sediments lie on new oceanic crust. However,there were no volcanic intrusions, similar to those found inthe active basins, Delfin, Wagner, Cerro Prieto, and Salton,which are considered incipient spreading centres (Fuis andKohler 1984; Lonsdale 1989; Persaud et al. 2003). TheTiburon fault has a N33◦W orientation and is projectedto the south in an oblique form to the Canal de Ballenasfault. Their orientation indicates an important componentof extension and it is possible that this difference inorientation has caused its abandonment and the initiationof the fracture zone of the Canal de Ballenas. The Tiburon
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Figure 12. Tiburon Basin and proposed basement structural model (in milliseconds). Master faults governing the basin clearly indicatea pull-apart system, in addition to the basement that is not interpretable in the middle. Fault density in the northern part is considerableand is probably caused by the antiform structure present in that area, forming the northern structural closure. The central part has a higherdensity of faults, which may be caused by the master fault De Mar and its possible extension Desemboque, reported by González-Escobaret al. (2006).
fault may not continue to the south, and lateral wastransferred to the dextral faults bordering Tiburon Islands(La Cruz fault) or other shear zones between the SanLorenzo Islands and San Esteban Islands. The Tiburonfault in the north domain cuts the most recent sediments,which could indicate that this fault zone has had a recentseismic activity. The geology studies in the coast of Sonoraand in the San Felipe region suggest/indicate shorteningthe age of the deformation in the Tiburon Basin, to aninitial sedimentary fill that was documented as possiblyduring the middle Miocene (Helenes et al. 2009). Futureseismic work in the southwestern part of the basin wouldestablish the connection of Tiburon fault and the faultsystem that transfers the deformation to the north fracturezone of the Guaymas Basin.
AcknowledgementsWe are indebted to CONACYT, México, for the financial sup-port through scholarships, to PEMEX Exploración y Producción
for allowing the use of seismic data, and to LandMarkTM forthe use of their software through the University Grant Programto CICESE, Baja California. This work was partially funded byNSF-Margins project EAR-0739017 to M. Oskin through UCDavis-CICESE sub-award No. 08-004375-01. We thank SergioArregui and Martin Pacheco for technical support.
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