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TECTONICS,VOL. 8, NO. 1, PAGES99-115, FEBRUARY1989

PRE-PLIOCENE EXTENSION AROUND THE GULF OF CALIFORNIA AND THE TRANSFER OF BAJA CALIFORNIA TO THE PACIFIC PLATE

J. M. Stockand K. V. Hodges

Departmentof Earth, Atmospheric,and PlanetarySciences, MassachusettsInstitute of Technology,Cambridge

Abstract. Late Miocene (12-5 Ma) extensionaround the Originally, the protogulfconcept was usedto explain an area edgesof the Gulf of Californiahas been alternatively attributed of anomalouslyold oceanic crust adjacentto the Mexican to "Basin and Range" extension, back arc extension, or margin at the mouth of the Gulf of California [Moore and developmentof the Pacific-NorthAmerica plate boundary. Buffington, 1968]. More recently, this concept has been This extensionwas ENE directedand similar in structuralstyle expandedto include late Miocene extensionalfaulting and to extensionin the Basin and Range province. Timing marine sedimentsfrom areas surroundingthe northern and constraints permit nearly synchronous onset of this centralparts of the Gulf of California [e.g., Karig and Jensky, deformationin a belt extendingSSE from northernmostBaja 1972; Moore, 1973; Gastil et al., 1979]. These late Miocene Californiato the mouthof the gulf. Where this extensional extensionalstructures and sedimentsare exposedaround the faulting continuedthrough Pliocene time to the present, gulf in the "Gulf ExtensionalProvince" [Gastil et al., 1975] synchronouswith motion on the modern transformplate on the east side of Baja California and the west coast of boundaryin the Gulf of California,no changein directionof mainland Mexico (Figure 1). The amountand direction of extension can be resolved. Revised constraints on Pacific- extensionof the pre-5.5 Ma protogulf, and its relation to North Americaplate motionsupport the developmentof this Pacific-North America motion, is not well known. late Miocene extensionas a componentof Pacific-North Several causes have been proposed for late Miocene Americadisplacement that couldnot be accommodatedby circumgulfextension. Karig andJensky [1972] suggestedthat strike-slipdisplacement along the existing plate boundary west it was back arc extension,but more preciseconstraints on the of the Baja Californiapeninsula. This scenarioimplies that timing of cessationof subductionwest of Baja California transferof BajaCalifornia from the North America plate to the [Mammerickxand Klitgord, 1982; Lonsdale,1989b] showthat Pacific plate was a gradualprocess, beginning about 12-10 it was not contemporaneouswith active subduction. The Ma, when motion of the Pacific plate relative to North geographiccontinuity, and similarityin extensiondirection, of Americawas partitioned into separateregimes of strike-slip the Gulf Extensional Province and the southern Basin and anddip-slip displacement on opposite sides of BajaCalifornia. Range extensionalprovince (in Arizona and Sonora)invited suggestionsthat this entire Miocene extensionalbelt resulted INTRODUCTION from the sameprocess of "Basinand Range extension" [Gastil, 1968;Dokka andMerriam, 1982;Curray and Moore, 1984]. In Openingof the Gulf of Californiais often attributedto two both of thesescenarios, late Miocene circumgulfextension is sequential extensional events: middle to late Miocene assumedto have beenoblique to Plioceneextension associated "protogulf"extension [Moore and Buffington, 1968; Karig and with the developmentof the Pacific-North America plate Jensky,1972; Moore, 1973]and the Pliocene development of boundary,and it is thoughtto have weakenedthe crust and the Pacific-NorthAmerica plate boundary, from about5.5 Ma hencefacilitated the propagationof the Pacific-NorthAmerica to the present[Larson et al., 1968;Curray and Moore, 1984]. plate boundary into the gulf. By contrast, Gastil and Krummenacher[1977] viewedpost-10 Ma extensionin Sonora as a continuous interval of "rhombochasmic extension", implicitly related to the developmentof the Pacific-North Copyright 1989 America plate boundary. Spencerand Normark [1979] and by the AmericanGeophysical Union. Hausback [1984] proposedthat late Miocene circumgulf extension accommodatedthe componentof Pacific-North Papernumber 88TC03753. Americadisplacement perpendicular to the preexistingstrike- 0278-7407/89/88TC-03753510.00 slip faults west of Baja California, after counterclockwise 100 Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California

/ !

I O0 200 m, i ß 0 I00 200 km

RIVERA PLATE I

PACIFIC PLATE

Fig. 1. Map of the Baja Californiapeninsula and adjacentmainland North America,showing the coastline,the southernBasin and Range province, and the Gulf ExtensionalProvince. The Tosco-Abreojosfault zone is located accordingto Spencerand Normark [ 1979];other borderland faults are generalized from Krause [ 1965]. Gulf plate boundaryfaults and limits of thecircumgulf extensional province (dark ticked lines) are generalized from references in the text. Bathymetriccontours (500 m, 2000 m) andbase map are from the North AmericanMap Committee [1965]. ENS is Ensenada. Stockand Hedges: Pre-Pliocene Extension Around the Gulf of California 101 rotationof the direction ofPacific-North America plate motion. Possibleexplanations forthe widespread Tertiaryextension in westernNorth America have been summarizedby Stewart [1978] and Christiansenand McKee [1978]. Some of these,as \$ YUM well as more recentexplanations, invoke mechanismsrelated '\ to the geometryand kinematicsof the Pacific-NorthAmerica • ß PHX plate boundary:extension within a broadzone of right-lateral shear [Atwater, 1970], extension above a "slab window" •lJ [Dickinsonand Snyder,1979], extensiondue to the geometric "'-.ME instability of triple junctions [Ingersoll, 1982], or extension ENS/'• , due to flexure above the Mendocino fracture zone [Glazner and X ß TUC Bartley, 1984]. Othersinvoke lithosphericor asthenospheric '\ causes unrelated to the development of the Pacific-North America plate boundary'extension due to relaxationof crust ß MD •-. thickened under compression [Molnar and Chen, 1983; c $ Wernicke et al., 1987] or extensiondue to steepeningof a formerly shallowly dipping subductedslab [Davis, 1980]. 400 k rn ----.,,-y , '" ' ' .,. Probably, Tertiary extensionin western North America had ß ,, Y f multiple causes. Observation of extension in areas ß .,.•?')' ? ) surroundingthe Gulf of California, where both "Basin and G '• Range"extension and Pliocene extension related to thePacific- ,, y y NorthAmerica plate boundary have occurred, may provide y someconstraints onpossible causallinks between extension _ and the developmentof the Pacific-NorthAmerica plate boundaryelsewhere. Thispaper examines (1) the differences instyle and direction betweenlate Miocene circumgulfextensional structures and " later structuresof the Pacific-NorthAmerica boundary zone in regionswhere Miocene to Recentdeformation has occurred, (2) the timing of late Miocene circumgulfextension and cessation of subduction,and (3) the direction and amount of Pacific- North America plate motion in Miocene time. Extensionand tectonic disruption in this area were much simpler than in -4 areasto the north, so the earlier shapeof the plate margincan = 4 bereconstructed andincorporated inthe analysis. 4 We suggest that late Miocene extension in the Gulf ExtensionalProvince wassimilar instyle tothat in the Basin m--• component of Pacific-North America plate boundary MAZ 4 anddisplacement.Rangeprovince This impliesbutmay that, have in latebeen Miocene kinematically time, plate a boundarydisplacement occurred both on the borderlandfaults - • 0 100 200 KM west of Baja Californiaand on extensionalfaults east of Baja California,isolating Baja Californiaas a rigid block within the Pacific-NorthAmerica plate boundary zone. 4 (• Geologictie point: -! distinctiveconglomerate LATE MIOCENE EXTENSION IN TI-• GULF TEP I {Gastil et al., 1981) EXTENSIONAL PROVINCE

The Gulf Extensional Province [Gastil et al., 1975] encompassesthe easternedge of the Baja Californiapeninsula and the part of mainland Mexico bordering the Gulf of California, includingparts of the statesof Baja California, Fig. 2. Reconstructed width of the Gulf Extensional Baja California Sur, Sonora,Sinaloa, Nayarit, Jalisco,and Province,after closureof the gulf transformfaults by 300 km Durango(Figures 1 and 2). We draw the westernlimit of the (usingthe 5.5 Ma reconstructiondiscussed in the text). Base Gulf Extensional Province at the western limit of known map as in Figure 1. MD, MZ are Sonoran"metamorphic core continuousor pervasive Miocene or Pliocene normal faults complexes"[Anderson et al., 1980]. Other abbreviations:BC along the west side of the gulf, inferred from geologic is Bahia Concepci6n;ENS is Ensenada;LA is Los Angeles; summariesreferenced in the text. In the centralpart of the LOR is Loreto; LP is La Paz; MAZ is Mazatlfin; MEX is Baja California peninsula there is no evidence of Miocene Mexicali; P is Puertecitos;PHX is Phoenix;SB is San Borja; extension; Miocene extensional structureson the west side of SF is San Felipe; SJC is San Jos6del Cabo Trough; SR is thepeninsula and in thecontinental borderland therefore belong Santa Rosalfa; SRB is Santa Rosa Basin; TEP is Tepic, in to a separategeologic province. The westernlimit of the Gulf Nayarit; TIB is Isla Tibur6n; TIJ is Tijuana; TUC is Tucson; ExtensionalProvince (GEP) in the stateof Baja California VCH is Valle Chico; YUM is Yuma; 3V is Tres Vfrgenes; coincidesapproximately with the Main Gulf Escarpment,a 3M is Islas Tres Marias. Geologictie point acrossthe gulf topographicbreak of varyingsteepness that follows the length (circledx) is a distinctiveconglomerate [Gastil et al., 1981]. 102 Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California of the Baja California peninsula and is generally fault- developmentof an extensionalaccommodation zone, NE Baja controlled [Gastil et al., 1975; Lindgren, 1888, 1890]. We California, Mexico, submitted to Journal of Structural draw the eastern limit of the GEP as the eastern limit of Geology,1988]. Extensionin thisarea occurred by high-angle known extensionalstructures east of the gulf, as deducedfrom normal faulting east of listric faults along the Main Gulf referencesin the text; there is no pronouncedtopographic Escarpment,which had no topographicrelief until after 11 Ma. escarpmentalong this easternboundary. On mainlandMexico About 10% extension occurred in this area, with about half the northern and southern limits of the Gulf Extensional priorto 6 Ma andhalf since,on thesame set of NNW striking Province are gradational;the GEP mergeswith extensional normalfaults. A changein extensiondirection with time was structures of the Trans-Mexican volcanic belt on the south and not recognizedbut cannotbe ruled out. In this area,Holocene with the southernBasin and Range provinceon the north. On scarpsindicate continued normal faulting on the NNW striking the west side of the gulf the northernlimit of extensional SanPedro Mfirtir fault (Main Gulf Escarpment) [Gastilet al., structures is the San Andreas fault. 1975; Brown, 1978]. In much of the Gulf Extensional Province the structures and Puertecitosregion. Sommerand Garcia [ 1970] and Gastil et timing of Tertiary extensionhave not been studiedin detail. al. [1975, 1979] reportedages of 3.1+0.5 Ma and 5.9+0.2 Ma Regionalsyntheses of availabledata [Gastilet al., 1975, 1979] for flat-lyingrhyolites overlying a seriesof steeplydipping and more recent mapping and geochronologyprovide the acidic volcanic rocks yielding ages from 7.3+1.5 Ma to constraints summarized below. For most of these areas, 8.3+0.8 Ma [Gastilet al., 1975, 1979]. They thusinferred detailedfault slip dataand regional transport directions are not that the major phaseof tilting in this area is late Miocene available; thus, althoughfault orientationsare known, the (post-9Ma) in age. About 6 km northof their studyarea, exact direction of extension is not. undated (probably Miocene) volcanicsand unconformably overlyingPliocene marine beds, capped by water-lainash, are BajaCalifomia tilted westup to 30ø on a seriesof eastdipping normal faults, and similar faults cut Quaternarydeposits (J. M. Stockand J. SierraSan Felipe and Sierra Santa Rosa. Eastof theSierra T. Smith, unpublished mapping, 1987). This suggests considerablevariability in the amountsof faultingand tilting SanFelipe, in the SantaRosa Basin, a sequence(up to 1 km affectingPliocene to Recentstrata in this regionand indicates thick)of alluvialsediments interbedded with middle to upper that post-Miocenenormal faults may be locally significantand Miocene volcanicstrata is tilted 30ø to the west abovea planar of the same orientation as late Miocene normal faults. low-anglenormal fault dipping20 ø to the eastwith 3-5 km of Dokka and Merriam [1982] observedthat Miocene stratanear normal displacement. The extension direction was Puertecitosdip 25ø to 40ø eastor westbeneath more shallowly approximatelySE [Bryant,1986]. No evidencefor progressive dipping Pliocene volcanic strata and inferred that these tilting, growthfaulting, or unconformitiesis found within the Miocene units correlated with the 7.3 to 8.3 Ma sequence section. Pliocene fanglomeratesoverlie the fault trace and describedabove. If all of the faults in the poorly mapped unconformablyoverlie the tilted Miocene strata,suggesting Puertecitosvolcanic province are inferredto dip 60ø, the area late Miocene-early Pliocene slip on this fault. A experienced10-18% extensionsince deposition of the Miocene monolithologic megabreccia of granitic rocks deposited strata(see, e.g., Figure 3 of Wernicke and Burchfiel [1982]). betweentuffs datedat 12.3+1.8 Ma and 14.9+0.9Ma [Bryant, Lower assumed dip angles would correspondto more 1986; Gastil et al., 1979] is the oldest indication of Miocene extension. Dokka and Merriam [1982] found that only a tectonism.(Uncertainties in agesare givenas + lo throughout single geometry of normal faults has been active since this this paper. All ages are conventionalK/Ar unless noted time. Many of theseNNW to northstriking normal faults cut otherwise.) A changein the provenanceof basin deposits the post-6 Ma volcanic strata [Dokka and Merriam, 1982; occurredjust above a basalthorizon dated at 8.9+1.2 Ma and Gastilet al., 1975], indicatingthe importanceof post-Miocene was interpretedby Gastil et al. [ 1979] as indicativeof normal- deformation. fault related uplift of rangesto the west. Minor ENE, NNE, Santa Rosalfa. In this region a conformablesequence of and NNW strikinghigh-angle strike-slip faults disruptall of Miocenevolcanic strata was tilted up to 45ø alonga few north the units present and are attributed to accommodationof to NNW striking, west dipping normal faults [Wilson and extension,or to conjugatestrike-slip faulting, that postdates Veytia, 1949] prior to the deposition of overlying lower the low-anglenormal fault and is still active. Pliocenemarine Pliocene [Wilson and Rocha, 1955] or uppermostMiocene stratawest of the Sierra San Felipe dip up to 45ø and contain [Sawlan and Smith, 1984] Boleo Formation. Here the tilted interbedded megabreccias of tonalire, suggesting that volcanic units yielded K/Ar dates from 19.9+0.6 Ma to significantdeformation continued locally into Pliocenetime 10.7+0.3 Ma (andesire of Sierra Santa Lucia; Sawlan and [Gastil et al., 1979]. Smith [1984]). The Main Gulf Escarpment near Tres Southern Sierra San Pedro Mfirtir and Valle Chico. Dokka Virgenes,immediately to the north,postdated 10 Ma basalt and Merriam [1982] estimatedthat extensionalfaulting began [Sawlan and Smith, 1984]. NNW striking normal faults here sometime between 17 and 9 Ma and that it was affected 10 Ma units but not 3 Ma units; however,upper approximatelyE-W directedbecause the dominantstrike of the Plioceneto Holocenealkalic volcanismin NNW trending normal faults is north-south. More detailed mappingand grabensappears to have been coeval with faulting of the geochronologyconstrain the initiation of extensionto the grabenmargins [Sawlan and Smith, 1984], suggestingmore interval between depositionof two rhyolite tuffs dated at recentactivity on someNNW strikingnormal faults. Wilson 10.85+0.16Ma (40Ar/39Artotal fusion of anorthoclase)and [1948] and Wilson and Rocha [1955] reported numerous 6.10+0.06 Ma (averageof two K/Ar ageson anorthoclase) normalfaults striking N10øW to N45øW. Thesecut Pliocene [Stock, 1989]. This time of onset of extension is based on the units and tilted them up to 15ø near Santa Rosalia. absenceof regionalunconformities in a 17-11Ma ignimbrite Pleistoceneunits in the samearea were generallyunaffected; sequence,absence of changesin thicknessof this sequence thus the age of this episode of normal faulting must be acrossthe currentstructures of the Main Gulf Escarpment,and Pliocene. A studyof fault striaein this area [Angelieret al., absenceof faults predatingthe 10.85 Ma ignimbrite[J. M. 1981] showedthat the late Miocene NNW strikingnormal Stock and K. V. Hodges, Miocene to recent structural faults were a responseto ENE oriented least horizontal Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California

principalstress (Oh) and that Pliocene motion on faultsof the Strike-slip faulting has also been important in this area. same orientation was more oblique, with the Oh direction Offset facies trends suggestsome tens of kilometers of left- oriented WNW-ESE or E-W. lateral displacementon the La Paz fault [Hausback,1984]. Bahia (70ncepci6nand Sierra de la Giganta. Near Bahia Part of this fault is still seismicallyactive [Molnar, 1973], and Concepci6n,McFall [1968] documentedNE tilting of up to geologicrelations indicate post-12 Ma normaldisplacement 45ø in upperMiocene volcanic rocks affected by NW andNE [Hausback,1984]. The timing of initial displacementalong striking normal faults. The uppermost strata in this this fault is not well knownbut may havepreceded deposition conformablesection (Ricas6n Formation of McFall [1968]) of Miocene volcanics [Hausback, 1984]. yieldeddates of 9.8_+1.6Ma and 8.0+3.7 Ma [Gastilet al., 1979]. Volcanicdeposits correlated with the entiresequence Mainland Mexico yielded ages rangingfrom 19.9_+0.6Ma to 10.7_+0.3Ma (andesiteof SierraSanta Lucia; Sawlanand Smith [1984]). CoastalSonora. Extensionalfaulting and tilting in a variety On the Concepci6npeninsula the lower part of thissequence is of directionsaffected Isla Tibur6nand adjacent Sonora between intrudedby diorite dikes strikingN70øW and N10øE and 12 and 9 Ma [Gastil and Krummenacher,1977]. Volcanic and numerousNE to ENE striking pyroclasticdikes [McFall, sedimentaryrocks 10 Ma andolder are tilted up to 60ø to the 1968]. These dikes are not observedin either the lower or eastalong a seriesof NW strikingnormal faults [Gastil et al., upperpart of the sequencewhere it is exposedelsewhere, and 1974]. Locally, on Isla Tibur6n, 19-15 Ma units are tilted they have not been dated,so their implicationsfor the late moresteeply than 13-11Ma units,suggesting that extensional Miocene stress direction cannot be assessed at present. faultingbegan between 15 and 13 Ma [Neuhauset al., 1988]. Pliocene marine strata overlie these units in angular Post-7 Ma units in coastalSonora are flat-lying, suggesting unconformity,suggesting that tilting and normal faulting that tilting andmajor extension there was mostlypre-Pliocene occurred before the end of Pliocene time. If these Pliocene in age. NE strikingfaults on Isla Tibur6ncut youngalluvium strata correlate with 3.3 Ma to 1.9 Ma strata of the Loreto and were related by Gastil and Krummenacher[1977] to embaymentto the south[McLean, 1988], the normalfaulting modern dilatation of the Gulf; faults of this strike are not predates3.3 Ma. Freshfault scarpsoccur along the N45øW recognizedfurther east in mainlandSonora. Some major faults strikingBahia Concepci6n fault zone,which may haveabout strikingN30øW to N40øW eastof Isla Tibur6nare likely to 30 km of right-lateralstrike-slip displacement [McFall, 1968]. havesignificant strike-slip displacement, although the amount West of Bahia Concepci6n,in the Sierra Giganta,normal has not been established[Gastil and Krummenacher, 1977]. faultsstriking N10øW to N35øW cut the Comond6Formation Coastal Sina10a. An area extending50 to 100 km inland andoverlying basalts [McLean et al., 1985, 1987]. This setof from the west coastof southernSinaloa, adjacent to Mazatlfin, faultsdoes not persistvery far westof the peninsulardrainage is broken into east tilted blocks boundedby NNW to NW divide, and hence we include it within the Gulf Extensional strikingnormal faults [Henry and Fredrikson, 1987]. The tilt Province. The basaltshave not been dated where they are cut of Oligoceneto middleMiocene stratareaches 60 ø in places, by the faults, but flows in similar stratigraphicpositions and extensionis estimatedto be 20-50% [C. D. Henry, Late nearbyyield agesfrom 15 to 7 Ma [McLeanet al., 1985]. No CenozoicBasin and Range structureadjacent to the Gulf of othergenerations of faultsaffect the area. As thesefaults have California, submitted to Geological Society of America only beenmapped in reconnaissance,mainly from air photo Bulletin, 1988]. This extension postdates 16.8_+0.3Ma identification,their exactdisplacement directions, dips, and the becausethis is the youngestdate obtained from a conformable amountof extensionthey representare unknown. sequenceof equallytilted volcanicstrata. Thesestrata form L0reto. About 50 km southof Bahia Concepci6n,NW to N fault-bounded basins filled by unconformably overlying strikingnormal faults eastof the peninsulardrainage divide nonmarinesediments that may be Mioceneor Pliocenein age displaceNeogene volcanics and sedimentsdown to both the [Henry and Fredrikson,1987]. UnfaultedQuaternary basalts eastand west [McLean, 1988]. One of the major faultswhich postdatethese sediments. In theabsence of furtherinformation cutsQuaternary alluvium along part of its traceoccurs at the the time of faultingis only bracketedbetween 17 and2 Ma. westernedge of a alepositionalbasin of Pliocenemarine strata Nayarit. In southernNayarit, SE of the mouthof the gulf, includingtuffs 1.9 and 3.3 Ma in age [McLean, 1988]. Dips pre-Pliocenestrata, including a regional volcanic package of Pliocenestrata are typically 10ø to 30ø. Upper Oligoceneto yielding datesfrom 21.3_+0.9Ma to 13.8_+3.0Ma, are tilted middle Miocene strata locally dip more steeply, indicating 200-30ø to the NE alongNW strikingnormal faults [Gastilet somepre-Pliocene deformation. McLean [1988] suggeststhat al., 1978]. 10-8 Ma basaltlies in angularunconformity above the major offsetson thesefaults involve pre-Pliocenestrata, the older strata [Gastil et al., 1979]. The normal faults are but the exact age of initiation of this faulting is not well overlain by, and rarely cut, Pliocene to Holocene volcanic constrainedby the existingdata. rocks. The reconnaissancenature of the studyprecluded tight La Paz. The Gulf Extensional Province is confined to a constraintson the timing of this faulting, and it is possible narrowstrip along the eastcoast of Baja CaliforniaSur (Figure that it is similar in timing and orientationto the late Miocene 1). Even within this region, Miocene units may be flat-lying faultingknown from elsewherearound the gulf. Fault-bedding and little deformed. East dipping normal faults striking relationshipsshown on interpretivecross sections by Gastilet N15øW-N35øW in the La Paz area postdatethe Comond6 al. [1978] limit the extensioncaused by this faultingto <10%. Formationand may still be active. The youngestdated unit in the Comond6 Formation here has yielded a K/Ar age of SouthernBasin and Range Province 12.5_+1.4Ma [Hausback, 1984], thus limiting the earliest possibleonset of extensionalfaulting. The latest possible The Gulf Extensional Province widens northward into the onsetis not well constrainedbecause of theabsence of younger southernBasin and Range provinceof Arizona and northern units. Hausback[1984] reporteda local angularunconformity Sonora (Figure 1). The geology of most of this part of beneath the 18-16 Ma San Juan tuff, but the regional Mexico is not well studied,and it is prematureto concludethat significanceof the correspondingdeformational event is the studiedareas adequately represent the entireextensional probablysmall. province. It is also important to note that if the Baja 0/4 Stockand Hodges:Pre-Pliocene Extension Around the Gulf of California

California peninsulais restoredto its pre-5.5 Ma position which are oriented NNW, parallel to those in the southern relative to mainland Mexico, by closure of the Gulf of Basin and Rangeprovince. Justas in the Basinand Range California by 300 km, the northern extent of the Gulf province, extension-relatedfaults in the Gulf Extensional Extensional Province in Baja California (Mexicali) lies Province include high-angle,listric, and planar low-angle adjacentto northernSonora, 250 km southof Yuma and 280 normal faults, as well as strike-slipfaults accommodating km west of Tucson (Figure 2). Thus in late Miocene time, differential motion of crustal blocks. southernArizona was not geographicallyclose to the limits of The dominantstrike of the high-anglenormal faults in the the extensionalprovince presently exposed west of the gulf. Gulf ExtensionalProvince was NNW. Becauseregional Low-angledetachment faulting in southernArizona began at mappinghas not yet revealedpiercing points on mostof these about 24-20 Ma [Glazner and Bartley, 1984]; direction of faults, the net extensiondirection is unknown. The simplest transporton thesefaults was generallyNE-SW to ENE-WSW assumptionis that dip-slip displacementdominated and hence [Wust, 1986]. YoungerMiocene sedimentsfrom SE Arizona that extension was ENE directed in late Miocene time. This containunconformities and megabreccias indicating continued directionof extensionalstrain, perpendicular to thepresent axis extensionalfaulting [Eberlyand Stanley,1978; Zoback et al., of the Gulf of California and to the late Miocene continental 1981]. A distinct post-13 Ma faulting event, forming NW margin to the west, is consistentwith observationsof fault strikingstructural troughs and associated with a majorregional striae[Angelier et al., 1981;J. M. Stockand K. V. Hodges, unconformity,ended sometime between 10.5 and6 Ma [Eberly Miocene to recentstructural development of an extensional and Stanley, 1978]. Although the 24-20 Ma extensionin accommodationzone, NE Baja California,Mexico, submitted southernArizona predatesextension in the Gulf Extensional to Journal of Structural Geology, 1988]. Without more Province,post-13 Ma extensionin southernArizona may be detailedgeologic study this direction is subjectto considerable coevalwith late Miocenecircumgulf extension; the orientation uncertainty,because extensional displacement may be oblique of structures is similar. to the dip directionof bothbedding and faults [e.g., Walker et Late Cretaceousand early Tertiary plutonsin NW Sonora al., 1986]. Although dikes have been observedin several crop out in "metamorphiccore complexes",associated with studies [e.g., McLean, 1988; McFall, 1968], dike swarms NE trendinglineations [Anderson et al., 1980] suggestingSW- suitable for estimating the direction of least horizontal NE extension[Wust, 1986]. The age of this extensionis not principalstress CSh have not beendated. Again, the simplest well constrained. This region is often included within the assumption is that CShwas oriented ENE near the Gulf southernBasin and Range physiographicprovince; note that ExtensionalProvince in late Miocene time, althoughthis the southernmostof thesecore complexeslies 200 km eastof stress direction cannot be confirmed to the level of detail Isla Tibur6n and coastal Sonora where late Miocene extension possibleto the northin the Basinand Range province [e.g., beganat about 12 Ma (Figure 1). Zoback et al., 1981]. ProtogulfMarine Sediments The CShdirection in the southernBasin and Rangeprovince was ENE-WSW in the interval 20-10 Ma and rotated to the The record of Miocene marine sediments from around the presentESE-WNW direction at about 10 Ma [Zobacket al., gulf shedsadditional light on the distributionof extendedand 1981]. The principalfaults in the southernBasin and Range subsidingregions near the gulf. The oldestknown Miocene provincestrike NW to NNW and mostlikely formedunder a marine deposits,about 13 Ma in age, are interbeddedwith pre-10 Ma ENE-WSW extensiondirection, prior to the circa 12.9+0.4 Ma volcanics and underlie 11.2_+1.3 Ma volcanics on 10 Ma rotationof the stressfield [Zobacket al., 1981]. These Isla Tibur6n [Gastil et al., 1979; Smith et al., 1985; Neuhaus faults are similarin strike to the late Miocenecircumgulf et al., 1988]. Upper Miocene marine rocks on Isla Mafia extensionalfaults. If the circumgulfextensional faults indicate Madre in the southernmostgulf (location 3M, Figure 2) an ENE-WSW directionfor C•h,it appearsthat the southern indicate a pre-8.2 Ma subsidenceevent [McCloy, 1987]. Basin and Range provinceand the circumgulfextensional Uppermost Miocene (6.0 to 5.5 Ma) diatomires occur in a province experiencedextension directions that may have Miocene-Pliocenemarine sequence near San Felipe [Boehm, differedby as muchas 45ø in late Miocenetime, despitetheir 1984], and upper Miocene strata are reportedfrom wells similar tectonic settingseast of the strike-slipPacific-North beneaththe coastalplain of westernSonora [G6mez, 1971]. Americaplate boundary. Pliocenedeposits in SW Arizonaand the SaltonTrough are underlainby uppermostMiocene marine sediments [Olmsted et AMOUNT OF EXTENSION al., 1973; Mattick et al., 1973; Ingle, 1987]. The upper MioceneHualapai limestone in the GrandWash Trough near In two areasaround the gulf with well-constrainedestimates Lake Mead, Nevada, has also been attributed to the marine for the amountof late Mioceneextension on high-anglefaults, protogulf [e.g., Blair, 1978], althougha lacustrineorigin for local valuesrange from 5% [J. M. Stockand K. V. Hodges, this unit has not been ruled out (seediscussion by Bohannon Miocene to recent structuraldevelopment of an extensional [1984]). accommodationzone, NE Baja California,Mexico, submitted The depositionaltrough represented by theseupper Miocene to Journal of Structural Geology, 1988] to 20-50% [C. D. marine sedimentsmay have been closeto the presentaxis of Henry, Late CenozoicBasin and Rangestructure adjacent to the gulf, with an outlet to the Pacific near the presentmouth the Gulf of California,submitted to GeologicalSociety of of the gulf [e.g., Winker and Kidwell, 1986; Smith, 1987]. America Bulletin, 1988]. Dips of faults and beddingon Further paleontological studies are needed to detail the publishedcross sections of otherareas may be interpretedto temporaldevelopment of this marinebasin. indicate extension values between these amounts (see discussionabove). The difficulty in obtaining accurate STYLE AND DIRECTION OF LATE MIOCENE estimatesof extensionfrom such techniques,in areas of EXTENSION detachment faulting, is illustrated by a comparisonof publishedestimates from the northern Basin and Range Post-12 Ma extension in the Gulf Extensional Province province(10% to 100%, or 75 to 325 km; see summariesby producedtopographic basins and ranges in NE BajaCalifornia Davis [1980] andWernicke et al. [1982])with a valueobtained Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California 105 fromgeologic piercing points across detachment faults further southernfragment of the Farallon plate [e.g., Dickinson and south (250-300 km, or 250%, at the latitude of Las Vegas Snyder, 1979]. South of the Rivera triple junction, [Wernickeet al., 1988]). More detailedmapping around the subductionof Farallon plate crust (the Guadalupe plate of Gulf of California is needed for an accurate assessmentof the Menard [1978] and smaller microplates that broke from it net extension there. [Lonsdale,1989b]) continued. The lengthof this subduction The presentgeographic limits of the Gulf Extensional zone decreasedas the Rivera triplejunction moved southward Province include 300-350 km of post-Miocene transform by unevenlytimed ridge deathsand microplatecapture along displacementwithin the gulf (see discussionbelow). This the Baja Californiamargin [Lonsdale, 1989b]. transformdisplacement was accompanied by diffuseextension At 13 Ma, subductionwas still occurringfrom the Vizcafno of continental crust and the formation of "transitional" crust Peninsulasouth to the tip of Baja California. This subduction within the gulf, as well as by the formationof some true stopped at about 12 Ma, when this part of the Pacific- oceaniccrust in the southerngulf. The excessarea generated Guadaluperidge stoppedspreading [Mammerickx and Klitgord, by thisdisplacement must be removedto reconstructthe latest 1982; Lonsdale,1989a,b]. The margin adjacentto the former Miocene width of the extensional province. This is Guadalupe-NorthAmerica trenchdeveloped a strike-slipfault straightforwardif the boundariesof the extensionalprovince zone (the Tosco-Abreojosfault) of the Pacific-NorthAmerica have not changedsince the end of Miocenetime, as indicated plateboundary [Spencer and Normark, 1979]. Many modelsof by the availablestructural data [e.g., J. M. Stockand K. V. gulf evolution portray this fault zone as the plate boundary Hodges, Miocene to recent structuraldevelopment of an from 12 Ma until the time that the plate boundarybegan to extensionalaccommodation zone, NE Baja California,Mexico, move into the Gulf of California. This is thought to have submittedto Journalof StructuralGeology, 1988; Hausback, occurred at about 5.5 Ma, becauseextension and subsidenceof 1984]. Then, regardlessof how thePliocene displacement was this age occurred at the mouth of the Gulf of California, partitionedwithin the Gulf ExtensionalProvince, its effecton possiblylinked to similardiffuse extension further north in the the width of the provincecan be estimatedby closingup the gulf [Currayand Moore, 1984;Lonsdale, 1989a]. About 300 edgesof the Gulf ExtensionalProvince by the 5.5 Ma (300 km of fight-lateraldisplacement on the southernSan Andreas km) reconstruction(Figure 2). If this properlyaccounts for systemand gulf transformfaults [Larson et al., 1968; Gastilet the Pliocene deformation, it indicates that at the end of al., 1981] are usuallyattributed to post-5.5Ma motionof the Miocene time the Gulf Extensional Province varied in width Baja California peninsula,on the Pacific plate, away from from .-.400 km in the northwest to --250 km at its southern end North America. (Figure2). Note thatthese boundaries include a largearea of The following reconstructionsof the Miocene plate tectonic Sonora,including the "metamorphiccore complexes",where configurationswest of Baja California (Figure 3) address the age of extension is not known. Within the Gulf constraintson threeissues: (1) the positionof the Rivera triple ExtensionalProvince, 200% EN-E extensionwould correspond junction with time, relative to Baja California; (2) the to 270 km in the north and 170 km in the south; 20% constraintson directionsof relative plate motion; and (3) the extensioncorresponds to 70 km and 40 km, respectively. orientation of the plate boundaries. These reconstructions Thesevalues merely illustrate the possiblerange in valuesof differ from previousglobal plate circuitreconstructions of the extensionthat might have occurredwithin the late Miocene Rivera triple junction [Atwater, 1970; Atwater and Molnar, Gulf Extensional Province. There is no reason to believe that 1973; Blake et al., 1978; Dickinson and Snyder, 1979; either the total amount of extension,or total percentageof Klitgord and Mammerickx, 1982; Hausback,1984] because extension,was constant throughout the extensionalprovince. they are based on updated Pacific-North America reconstructionsand uncertainties [Stock and Molnar, 1988] for TERTIARY PLATE TECTONIC SETTING OF the times of anomalies5 and 6. (These anomaliescorrespond BAJA CALIFORNIA to 10.59 Ma and 19.96 Ma, respectively,on the Decade of North American Geology (DNAG) time scaleof Berggrenet The historyof plate interactionswest of Baja Californiamay al. [1985]. This magnetic reversal time scale is used have affectedthe timing and locationof extensionwithin the throughoutthe followingdicussion.). The reconstructionsand Gulf Extensional Province. During most of Tertiary time, uncertaintiespresented here are for more closelyspaced and Baja Californiawas attached to westernNorth America, and the slightlydifferent times, in orderto take advantageof the most Farallonplate wasbeing subducted eastward beneath the Baja- informativeparts of the seafloorspreading record west of Baja Mexico margin. Volcanismrelated to this subductionoccurred California. Implicationsfor the positionof the Rivera triple in the Sierra Madre Occidental of mainland Mexico from 100- junctionare similarto the resultsof Lonsdale[1989b]. 45 Ma and 34-23 Ma [McDowell and Clabaugh, 1979] and in easternBaja Californiaand westernmostSonora from 24 to 10 ReconstructedPosition of Baja California at 5..5Ma or 8 Ma [Gastil et al., 1979; Sawlan and Smith, 1984; Hausback, 1984]. Baja California crust lay west of the In the following discussionand figures, Pacific plate crust Cenozoic volcanic arc until Miocene time; much of the east west of the presentSan Andreasfault and west of the gulf side of the Baja California peninsulacontains strata from the spreadingcenters has been reconstructedto the margin of west side of the Miocene volcanic arc. mainlandMexico by a rotationof 4.69ø about48.62øN, 75.15 ø The Pacific plate came into direct contact with North W (Figures 2 and 3). This 5.5 Ma reconstructionwas America at the latitude of California sometime after 28.5 Ma extrapolatedfrom Minsterand Jordan's [1978] RelativeMotion [Atwater, 1970] andprobably closer to 25 Ma [Atwater, 1989] 2 (RM2) angularvelocities and uncertaintiesfor the Pacific- at a point northof Baja California [Atwaterand Molnar, 1973; North America plate circuit (see Stock and Molnar [1988] for Stock and Molnar, 1988] if the Gulf of California is closedby detailsof this extrapolation.)This reconstructionremoves 290 300 km. The new Pacific-North America strike-slip plate km of displacementalong the San Andreasfault in the Salton boundaryhad a triple junction at both ends [Atwater, 1970]: troughand closesthe mouth of the Gulf of California by 308 the Mendocino (trench-transform-transform)triple junction, km along an azimuth of 127ø. The shelf edge north of the adjacentto the northernfragment of the Farallonplate, and the Marfa Magdalena rise is then aligned with the shelf edge of Rivera (ridge-trench-transform)triple junction, adjacent to the Baja CaliforniaSur at 23.7øN, 109.3øW,following the detailed 106 Stockand Hodges:Pre-Pliocene Extension Around the Gulf of California

a 55 ø N 30 ø N 25 ø N

CIFIC GUADALUPE

uncertainty reconstructed position points on Pac•fm plate

120 ø W 110ow Anomaly 6 (old edge) 20.4 Ma

55øN 30 ø N 25øN

LOR

PACIFIC

uncertainty

130 W 120øW 110øW Anomaly 5D 17.7 Ma

Fig. 3. Reconstructionsof the Pacific plate relative to thefixed North America plate in theMiocene and Pliocene, for the timesof (a) the old edgeof anomaly6, 20.45 Ma on the DNAG time scaleof Berggrenet al. [1985];(b) centerof anomaly5D, 17.74Ma; (c) centerof anomaly5C, 16.22Ma; (d) centerof anomaly5AA, 12.92Ma; (e) old edgeof anomaly5, 10.59Ma; (f) old edgeof anomaly3A, 5.89 Ma. Map is an obliqueMercator projection about44.02øN, 83.71øW, the stagepole for Pacific-NorthAmerica motion between 20 and 10 Ma [Stockand Molnar, 1988]. The Gulf of Californiais closedas in Figure2, with the 1000 fathom(1828 m) contourshown. Locations,abbreviated as in Figure2, aresquares (on North America) and triangles (on Baja California). Heavy line indicatesposition of Pacific-Riveraspreading center and is dashedwhere inferred. Position of subructionzone and Pacific-NorthAmerica strike-slip margin is schematic.Thin linesare isochrons on thePacific plate [Atwater and Severinghaus,1989]. Rotationdata and shadeduncertainty ellipses, representing 95% confidencelimits of the reconstructedposition of thePacific plate, are based on the globalplate circuit (see Stock and Molnar [1988] and discussionin text). Seetext for discussionof uncertainties.The unacceptableoverlap of Pacificplate oceanic crust ontoNorth American continental crust (Figures 3a-3e) results from assuming a rigidNorth America plate and would beremoved by includingBasin and Range extension in thereconstructions (see discussion in text). Stockand Hodges: Pre-Pliocene Extension Around the Gulf of Califomia 107

35 ø N 30 ø N 25 ø N

LOR

-:--:..:- :.-. uncer PACIFIC

130øW 120 ø W 1 10øW

Anomaly 5C 16.2 Ma

d 55 ø N 50 ø N 25 ø N

LOR

uncertainty

PACIFIC

13C W 120øW 11oow

Anomaly 5AA 12.9 Ma

_ Fig. 3 (continued) bathymetry of Mammerickx [1980] and reconstructionsof partof theplate margin should be adequatefor timesbetween Lonsdale [1989a], and is similar to the reconstructionof 20 and 5 Ma. (The presenceof a possibletranspeninsular Currayand Moore [1984]. strike-slipfault in Baja CaliforniaSur, [e.g., Crouch,1979] Thisreconstruction assumes complete rigidity of thepresent has not been conclusivelyestablished.) The continental Pacificplate (Baja California and southern California) west of marginnorth of Ensenada,including the California continental theSan Andreas fault. TheBaja California peninsula south of borderlandand the TransverseRanges, was more highly theAgua Blanca fault (near Ensenada, Figure 1), betweenthe deformedin Mioceneand Pliocene time [Doyle and Gorsline, Main GulfEscarpment and the continental borderland, has been 1977;Crouch, 1981; Hornafius et al., 1986].Thus the shape essentiallyrigid in Mioceneand younger time. Although of this part of the margin,as determinedby rigid plate Miocene extension occurred in the continental borderland reconstructions(Figure 2), is lessvalid for older times. [Normark et al., 1988], we assumethat this extensiondid not This reconstructionincludes within its uncertaintiesthe significantlyreorient the western margin of theBaja California extrapolation of the instantaneousPacific-North America peninsula. In this case the reconstructedorientation of this angularvelocity of Chase[1978]. An extrapolationusing a 108 Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California

e 55 ø N 30 ø N 25" N

LOll

..... :::::...... '-'-'-' uncertainty

PACIFIC

I 13oøw 1 20" W 110øW

Anomaly 5 (old edge) 10.6 Ma

35øN 30øN 25 ø N

.:!.: ß:..:.. uncertainty

PACIFIC

150 W 120 ø W 110øW Anomaly 3A (old edge) 5.9 Ma Fig. 3 (continued) more recent instantaneousangular velocity model known as California[Lonsdale, 1989a]. Geologicconstraints from the NUVEL-1 [DeMets et al., 1987] would closethe mouthof the Gulf ExtensionalProvince [Gastil et al., 1981] includethe gulfby onlyabout 260 km butalong the same azimuth as this separationof a pre-15 Ma conglomeratechannel with reconstruction.All of thesereconstructions result in overlap distinctivePermian clasts, found on both sidesof the Gulf of of extendedcontinental crust within the gulf becausecrust California(Figure 2). Our 5.5 Ma reconstructionappears to between the Pliocene transform faults has extendedby the overrotatethe outcrops of theconglomerate with respect to one formation of "transitional", rather than true oceanic, crust another;however, major NW strikingstrike-slip faults occur [Larsonet al., 1972; Moore, 1973]. eastof theSonoran outcrop [Gastil and Krummenacher, 1977] A value of about300-350 km for strike-slipmotion on the andmay account for someof thisdiscrepancy. Three hundred gulf transform faults seems to be constrainedwell by kilometers of motion is about 30 km less than the total displacementson majorfaults in southernCalifornia [Crowell, displacementon the SanAndreas, San Gabriel, San Jacinto, 1981] and is consistentwith the amountof oceaniccrust, and and Elsinore faults in southernCalifornia [Crowell, 1981; extendedcontinental crust, present at the mouthof theGulf of Hornafiuset al., 1986]. However,pre-5.5 Ma displacementon Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California 109

35ø N 30 ø N 25 ø N

ß GLOBAL PLATE CIRCUIT

ß HOTSPOT CIRCUIT

POINT A

POIN' 17 Ma

POINT C 0 Ma 20.0 Ma PACIFIC POINTD 10.6 Ma

130øW 120øW 1100W Fig. 4. Uncertaintiesin pastpositions of four pointson the Pacificplate relative to fixed North Americasince Miocenetime. Baja Californiahas been moved back to NorthAmerica 300 km by the reconstructiondiscussed in the text. Ellipsesare 95% confidenceregions for pointpositions at 5.5 Ma, 10.6 Ma, and 20.0 Ma from global platecircuit reconstructions [Stock and Molnar, 1988]. Trianglesare positions of samepoints at 5, 9, and 17 Ma, according to the fixed hotspot reconstructionsof Engebretsonet al. [1985]. Note that for both sets of reconstructionsthe averagedirection of motionin the interval5-10 Ma is obliqueto the trend of the continental marginwest of Baja California(the San Benito-Tosco-Abreojos fault zone). Map projectionas in Figure3.

the San Gabriel fault should not be included in the 5.5 Ma of the Miocene-Plioceneboundary (the old edgeof anomaly reconstruction. 3A, 5.9 Ma; Figure 3f) was derived,fromMinster and Jordan's The exact timing of this 300-350 km of motion is not [1978] RM2 model and uncertainties,as describedabove for constrainedwell. In the southerngulf, northof the Tamayo the 5.5 Ma fit of Baja Californiato mainlandMexico. transformfault, the spacingof anomalies0, 2 (2 Ma), and 2A In thesereconstructions the platesin the globalplate circuit (2.5 Ma to 3.4 Ma) indicatesthe formationof about 175 km (Pacific,Antarctica, Africa, andNorth America)are assumedto of oceaniccrust between the Pacific and North America plates have been rigid since 20 Ma. For 10.6 Ma and older times since3.4 Ma [Larsonet al., 1968; Klitgordand Mammerickx, (Figures3a-3e), reconstructed oceanic crust of thePacific plate 1982; DeMets et al., 1987; Lonsdale, 1989a]. South of the overlapscontinental crust of the Baja Californiaand southern Tamayo fracture zone, 300 km of seafloorformed sinceabout California margins. The amountof overlapfor sometimes 5 Ma [Mammerickxand Klitgord, 1982], but this represents exceedsthe uncertainty in thereconstructions, indicating some Pacific-Riveraspreading which was fasterthan Pacific-North problem with either the reconstructionparameters or the Americaspreading due to convergencebetween the Rivera and assumptionof rigid platesin the plate circuit. This overlap North America plates [Atwater, 1970; Larson, 1972; Molnar, alsooccurs in reconstructionsbased on the assumptionof fixed 1973; Nixon, 1982]. The additional 125-175 km of transform hotspots (e.g., Engebretson et al. [1985]; Figure 4), motion in the Gulf of California is inferred to have occurred in suggestingthat it is unlikelyto be dueto nonrigiddeformation the interval5.5-3.4 Ma [e.g., Lonsdale,1989a; Curray and in the Africa or Antarcticaplates. It is most likely due to Moore, 1984], but the time of initiation of this motion is not nonrigiddeformation of the NorthAmerica plate, considering knowndirectly and may be olderthan 5.5 Ma. The 8 Ma (or the severalhundred kilometers of Neogeneextension known earlier) initiation of motion on the San Gabriel fault [Crowell, from the Basin and Range province [Wernickeet al., 1988]. 1981]suggests that pre-3.4-Ma strike-slip displacement in the Extensionaland strike-slipdeformation here and along the Gulf of Californiamay havebeen distributed throughout the plate boundary limits the level of precision of the interval 8-3.4 Ma, perhapspartly accommodatedwithin the reconstructionsof Figure3. Despite theselimitations, some general inferences can be Gulf Extensional Province. maderegarding the positionand the lengthof the strike-slip Pacific-NorthAmerica plate boundary. At 20.4 Ma theRivera PastPosition of the Rivera Triple Junction triplejunction (RTJ) was near the reconstructedposition of Los Angeles, so subductionwas still occurringalong the Reconstructionsfor times in the Miocene (Figures3a-3e) entireBaja Californiapeninsula. The strike-slipmargin north were interpolatedfrom the reconstructionsof Stockand Molnar of this subductionzone was only about400 km long. By [1988], assumingconstant relative motion between the Pacific 17.7 Ma the RTJ could not have been much further south than and North America plates throughout this interval. Ensenada;the strike-slipmargin to the north was about750 Uncertainties in these reconstructions are at least +50 km in km long,and subduction was still occurringunder most of the latitudeand +70 km in longitudeand are estimatedto be larger Baja Californiapeninsula. Synchronous with this subduction, (+100 km) if the reconstructionwas not close in time to the between22 and 15 Ma, Neogenebasins on the continental end pointsof the interpolation.A reconstructionfor the time marginwest of BajaCalifornia experienced their major episode Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California

20 I• '''""•:•••:•••'••ssage...... '"•'':•:"'' ofRivera Triple Junction I• Earliestpossibleonset ...... •:•!:-::!!i%!!!!ii::•::i•1%•...... _ _ ofext •al faulti•9

ß TEP

VCHe.? ß MAZ Latestof extensionalpossible onset faultingSR •

o

o 500 1000 1500 Distance along Gulf of California, Km Fig. 5. Comparisonof thetime of onsetof extensionalfaulting in theGulf ExtensionalProvince with the time of passageof theRivera triple junction to thewest. Locations (abbreviated asin Figure2) areprojected parallel to the westcoast of theBaja California Peninsula (and parallel to thetrend of theSan Benito-Tosco-Abreojos faultzone) TIB andSRB are observationsof local deformation(see discussion in text) whoseregional significance has not been established. of subsidence[Normark et al., 1988]. Between 17.4 and 12.9 (SBTAFZ) after reconstructionof the Baja Peninsula to Ma, subductionceased along all of northernBaja California, mainland Mexico and are consistentwith all of the plate and the strike-slipmargin north of the Rivera triple junction motion occurring as strike-slip displacement along the grew to 1400 km in length, extending south through the SBTAFZ. However, for the period 5.5-10.6 Ma, the net southernCalifornia borderland to the Vizcainopeninsula. At azimuth of displacementof the Pacific plate varied from 12.9 Ma, subductionwas still ocurringwest of the Vizcafno 294ø+11ø to 298ø+12ø , demonstrablyat an angle to the peninsula.By 10.6 Ma, subductionhad ceasedalong most of azimuthof the SBTAFZ (Figure 6). Betweenabout 10 and 5 southernBaja California,and the former continental slope west Ma, the net plate displacementwas obliqueto the SBTAFZ in of Baja California Sur (the Tosco-Abreojosfault of Spencer an orientationthat shouldhave caused oblique extension across and Normark [1979]) was part of the 2100-km-longstrike-slip it. However,neither the Tosco-Abreojosfault nor the former margin. Thus the strike-slipmargin lengthenedby about trench extended obliquely [Spencer and Normark, 1979; 1700 km between 20 and 10 Ma, even though the relative Lonsdale, 1989b], and subsidenceof the continentalmargin displacementbetween the Pacific and North Americaplates, was minor [Normarket al., 1988]. This lack of a component west of Baja California, was only 235+100 km during this of extensionon the plate boundary,noted by Spencerand interval. Normark [ 1979] and Hausback [ 1984], indicatesthat additional A comparisonbetween the time of onset of extensional displacement,sufficient to sum to the total relative plate faultingand the time that theRivera triple junction lay westof motion, must have occurred elsewhere within the plate pointsin the (reconstructed)Gulf ExtensionalProvince (Figure boundaryzone. The plate reconstructionssuggest that the 5) shows no clear evidence for late Miocene circumgulf amount of this displacement,measured in an ENE direction extensionwhile subductionwas active to the west. There may (perpendicularto the trendof the Tosco-Abreojosfault zone), be a temporalcorrelation between the initial late Miocene varied from 160+80 km to 110+80 km along the plate circumgulfextension and the southwardmotion of the Rivera boundarywest of Baja California (Figure 6). The closest triplejunction west of Baja California,although this cannot be obviousplace for this displacementis in the Gulf Extensional confirmed by existing data. It is also possible that late Province. Miocene circumgulf extension started approximately Although the dominant extensional faults in the Gulf synchronouslythroughout the lengthof the gulf and, at least Extensional Province strike NNW, the exact direction of in someareas, considerably later than the passage of theRivera extension is not known. If we assume that this extension triple junctionto the west. direction was ENE, then the plate reconstructionsimply 160+80 km of extensionin the northernpart of the Gulf Direction and Amount of PacificrNorthAmerica Motion ExtensionalProvince, decreasing to 110+80 km of extension in the southernpart of the Gulf ExtensionalProvince. This The above reconstructionscan be usedto obtainthe average correspondsto 66% extensionin thenorth and 78% extension direction of Pacific-North America motion, and its in the south,with upperand lower limits of 150%-25%in the uncertainties,for the time intervals between reconstructions north and 300%-13% in the south. A different overall (Figure4). Sucha computationfor the intervals0-5.5 Ma, extension direction in the Gulf Extensional Province would 5.5-10.6 Ma, and 10.6 to 20 Ma (Figure 6) indicatesthat imply slightlyhigher percentages of extension.These values between 20.0 and 10.6 Ma, total Pacific-North America are within reasonablelimits of the extensionthat might have motion was 25+10 mm/yr at an azimuth varying from occurredin the Gulf ExtensionalProvince (20%-200%) andare 3030+20ø to 317ø+20ø alongthe westcoast of Baja California consistent with a kinematic model in which the Gulf Sur. These azimuths include within their uncertainties the ExtensionalProvince accommodated the "missing"component 320ø azimuth of the San Benito-Tosco-Abreojosfault zone of plate boundarymotion. A northwardincrease in absolute Stockand Hodges:Pre-Pliocene Extension Around the Gulf of California ] ] ]

50øN 25øN

10.6 to 5.5 M

fault ';60•8 km -Abreojos• rend'aulto•'''• • 110ñ80 km 120øW 110øW Fig. 6. Intervalvelocities and uncertainties for Pacificplate points west of Baja California,for the intervals20.0 to 10.6 Ma (large ellipses),10.6 to 5.5 Ma (mediumellipses, shaded) and 5.5 to 0 Ma (small ellipses). Map projectionas in Figure3. The SanBenito-Tosco-Abreojos fault zone is dotted.Vector diagrams in thelower part of the plot showthe bestfit anduncertainties in the displacementof thePacific plate relative to North Americafor the time interval10.6-5.5 Ma, the directionof the SBTAFZ, andthe amountof platemotion that could not havebeen accommodatedby strike-slipmotion along the SBTAFZ but whichmight have been accommodated within the Gulf Extensional Province.

extensionin the Gulf ExtensionalProvince is suggestedby the partitionedbetween continued strike-slip motion along the relative plate motions (Figure 6) and can be tested with Tosco-Abreojosfault zoneand extensionfurther inland. There improved geologicconstraints from the Gulf Extensional are two obviousreasons for suchkinematics: either the plate Province. boundaryat depth was not confinedto the Tosco-Abreojos Becausethese reconstructions correspond to specifictimes in fault or else a zone within the continent was closer to failure the past,the differencebetween two reconstructionsindicates by extensionthan was the plateboundary. The earlierhistory the net displacement,not the actual displacementpath, for of the strike-slipboundary as a subructionzone, and of the plate motion between these two times. The time interval Gulf ExtensionalProvince as an activevolcanic arc, may have duringwhich Pacific-North America plate motion was oblique contributedto either of thesesituations. The former arc on the to the SBTAFZ is not preciselyconstrained and could have NorthAmerica plate had been recently active, so it may have been 15 to 7 Ma or 9 to 5 Ma rather than the 10.6 to 5.5 Ma been both thermally weak and topographicallyhigh. The intervalshown by the reconstructions.More preciseconstraint subductedGuadalupe plate lay somedistance beneath Baja on this time interval awaits both closer time spacingsof California,so the attachmentof microplates(broken from the globalplate reconstructionsand more detailedchronology of Guadalupeplate) to thePacific plate as thePacific-Guadelupe the Miocene extension within the Gulf Extensional Province. ridgedied may haveprovided a mechanismfor wideningthe Pacific-NorthAmerica boundary at depth. DISCUSSION The strainpartitioning we infer for this part of the Pacific- North America plate boundary is similar in scale to the Of all the extended areas in western North America the Gulf partitioning of displacementobserved along obliquely Extensional Province shows the best constrained correlation convergentplate boundaries(Figure 7). Often, in these between the onsetof extensionoutside of the existing plate regions,subduction is not parallel to the plate motion but boundaryzone and a changein the directionof relativemotion ratherperpendicular to the plate boundary;the componentof betweenthe Pacific and North Americaplates. The onsetof strike-slipdisplacement is accommodatedon faultsparallel to extensionhere may be linkedto the transitionfrom subduction the subductionzone in the arc region [Fitch, 1972]. The to strike-sliptectonics, as inferredfor otherextensional areas similarityin scaleof thesetwo processesmay reflecttypical in western North America [Dickinson and Snyder, 1979; arc-trench distances. Glazner and Bartley, 1984]. However, geometric If this kinematicpicture applies to extensionin the Gulf considerationssuggest that the extensionreflects a resolvable ExtensionalProvince, what are the consequencesfor other changein the directionof relativeplate motion after the strike- extended areas in western North America? The northernmost slipregime was already established along part of themargin. limit of extensionattributable to plateboundary displacement The localization of this extension inland, well removed from is difficult to identify, becauselater deformationnorth of the formerplate boundary, is intriguing. The geometryof late Ensenadaprevents accurate reconstruction of the Miocene Miocene plate motion shouldhave causedoblique extension shapeof theplate boundary along the continentalmargin. If, alongthe SanBenito-Tosco-Abreojos fault, whichmight have northof Ensenada,the continental margin accommodated all of become reoriented into en echelon transform faults. This did the plate boundarydisplacement, then the Gulf Extensional not happen; instead, the plate boundarydisplacement was Provinceshould have a well-definednorthern limit bounded by 1 12 Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California

OBLIQUE CONVERGENCE OBLIQUE EXTENSION (F tch, 1972)

Oceanic plate Pacific North plate America

A v B S ! Continental plate o 2oo

A arc A' B Gulf Extensional B' • •(D Province 0__•Cea n ontinental youngi •• '•_ continentallithosphere lithosphereformer I •'"• --.--• subduction former •ithosl}here oceaniczone /-• arc i i plate boundary plate boundary zone zone Fig.7. Comparisonof partitioning of relative plate displacement in oblique strike-slip regimes [Fitch, 1972] and in the caseof the Gulf of California. Note similarityof scale. Thicknessof the lithospherebeneath Baja California andadjacent North America is not well constrained, and the direction of platemotion (heavy arrows) is schematic.

transferstructures (probably strike-slip faults) extending west localextension, computed by reconstructionof displacements acrossthe Baja California Peninsula tojoin the plate boundary on high-anglenormal faults, or inferredfrom fault-bedding in thecontinental borderland. The Agua Blanca fault, north of relationships,varies from 5% to 50%. This extensionmay Ensenada,is of a likely orientationto be sucha transfer havebegun synchronously all along the margins of thepresent structure,but it does not correspondto any structural gulfor mayhave closely followed the time of passageof the discontinuityin the extensionalprovince to the east. The Riveratriple junction and the transition to a strike-slipplate continuitybetween the Gulf ExtensionalProvince and the boundary.None of this extensionoccurred during active southernBasin and Range province suggests that late Miocene subduction. This 12-6 Ma ENE extension is the earliest Pacific-NorthAmerica displacement (manifested as extension) episodeof Mioceneextension recognized immediately adjacent alsocontinued further north. The directionof this extension to the Gulf of California; its relationshipto early to mid- neednot have been perpendicular to the plate boundary to the Miocene"metamorphic core complexes" known from southern west,as longthe sumof the extensionand the strike-slip Arizona, and similar undatedstructures in NW Sonora,is displacementequaled the total displacement between the Pacific unknown. andNorth America plates. Thus, late Miocene WNW directed Plate tectonic constraints are consistent with a plate extensionfurther north in the Basin and Range province kinematic cause for this extension, as follows. When [Zobacket al., 1981]may have been an expression of the same subductionceased west of Baja Californiaat about12 Ma, by changein directionof platemotion superposed on earlier cessationof spreadingon part of thePacific-Guadalupe ridge, extensional structures. Pacific-NorthAmerica motion was accommodatedby strike- ENE directednormal faulting continued into the Pliocene in slipmotion on faultzones parallel to, andclose to, theformer manyareas around the Gulf of California;in someareas of NE trench(the San Benito-Tosco-Abreojos fault zone). However, BajaCalifornia it continuestoday [Gastil et al., 1975;Dokka at or after about 12-10 Ma, the direction of Pacific-America and Merriam, 1982]. Thus this directionand style of motionbecame oriented oblique to thesestrike-slip fault zones extensionalfaulting persisted even after Pacific-North America andshould have caused oblique extension along them. Instead, transform motion was well establishedwithin the gulf, the plate boundarydisplacement was partitionedbetween althoughit appearsto havecaused much less extension. Its strike-slipmotion on NNW strikingfaults west of Baja occurrencetoday may be a smallerscale example of strain California, and broadly ENE directedextension on NNW partitioningin thePacific-North America boundary zone. strikingfaults east of Baja California,along the trace of the former volcanicarc. This extensioncould have displacedBaja California as much as 100-150 km WSW relative to North SUMMARY America before about 5.5 Ma. If more extension occurred to Sinceabout 12 Ma, NNW strikingnormal faults adjacent to the norththan to the south,Baja Californiawould alsohave the presentGulf of Californiahave causedextension in a beenrotated very slightly counterclockwise.During this broadlyENE direction,perpendicular to the Pacific-North extensionalfaulting, Baja Californiamay also have been Americaplate boundary and to thepresent orientation of the displacedNNW relativeto NorthAmerica by minorstrike-slip Gulf of California. In widely separatedareas the amountof faultingwithin the extensionalbelt, linkedto thepost-8 Ma Stockand Hodges: Pre-Pliocene Extension Around the Gulf of California 1 13 strike-slipmotion along the San Gabriel fault in southern Couvering,Cenozoic geochronology, G½ol, Soc, Am, Bull,, California. Thus for the period betweenabout 12 Ma and 96, 1407-1418, 1985. perhaps3.5 Ma, Baja Californiawas a rigid "microplate" Blair, W. N., Gulf of California in Lake Mead area of Arizona boundedon bothsides by displacementbetween the Pacific and and Nevada during late Miocene time, Am. Assoc. Pet. North America plates. The partitioningof displacement Geol. Bull., 62, 1159-1170, 1978. betweenthese two parallelzones of thePacific-North America Blake, M. C., Jr., R. H. Campbell,T. W. Dibblee, Jr., D. G. plateboundary changed slowly with time, until by perhaps Howell, T. H. Nilsen, W. R. Normark, J. C. Vedder, and E. about 3.5 Ma most of the plate boundary motion was A. Silver, Neogenebasin formation in relation to plate occurringeast of Baja California. The transferof the Baja tectonicevolution of San Andreasfault system,California, Californiapeninsula from the North Americaplate to the Am. Assoc. Pet. Geol. Bull. 62, 344-372, 1978. Pacificplate was a gradualprocess, which took at leastthe Boehm, M. C., An overview of the lithostratigraphy, period12-4 Ma, andperhaps is notentirely complete today. biostratigraphy,and paleoenvironments of the late Neogene Existinggeologic constraints are consistent with, but do not San Felipe marine sequence,Baja California, Mexico, in prove,this scenario. Clearly, muchbetter constraints are Geology of the Baja California Peninsula,Publ. 39, edited neededon the timing and amountof extensionin the Gulf by V. A. Frizzell, Jr., pp. 253-265, Pacific Section,Society ExtensionalProvince and on its relationshipto areasfurther of EconomicPaleontologists and Mineralogists, Bakersfield, northin the Basinand Rangeprovince. The implicationsof Calif., 1984. thiskinematic model for the dynamicsand scaleof strike-slip Bohannon,R. 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