CORE Metadata, citation and similar papers at core.ac.uk Provided by Caltech Authors 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) and basemap 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