Middle Miocene extension in the Gulf Extensional Province, Baja California: Evidence from the southern Sierra Juarez Jeffrey Lee Department of Geology, Central Washington University, Ellensburg, Washington 98926 M. Meghan Miller } Robert Crippen Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109 Bradley Hacker Department of Geology, Stanford University, Stanford, California 93405 Jorge Ledesma Vazquez Facultad de Ciencias Marinas, Universidad Auto´noma de Baja California, Ensenada, Baja California, Me´xico 22800 ABSTRACT of oceanic rifting and even predates other rifting. Extension is an important compo- dated Miocene extension within Baja Cali- nent in the tectonic development of this New geologic mapping, structural stud- fornia. Second generation faults, which are transform plate boundary, yet the causal re- ies, and geochronology of Miocene volcanic comprised of east-west–striking strike-slip lationship between extension and plate- and sedimentary rocks in the southern Si- faults that cut first generation faults and boundary evolution is poorly understood erra Juarez, Baja California, shed light on associated northwest-striking, northeast- and the subject of debate. the extensional history of the Gulf Exten- dipping normal faults, may be related to Early tectonic models proposed initiation sional Province prior to sea-floor spreading early development of the Transpeninsular of oceanic rifting in the Gulf of California at in the Gulf of California. The southern Si- Strike-slip Province. 5–4 Ma, accompanied by the apparently in- erra Juarez is underlain by lower–middle Global plate reconstructions suggest that stantaneous transfer of Baja California from Miocene rocks including fluvial strata, in- transtensional motion between the North the North American plate to the Pacific termediate composition volcanic deposits, American and Pacific plates along the west- plate and the translation of Baja California basalt lava flows and cinder cones, and da- ern margin of Baja California began during northward at plate boundary rates (Larson cite pyroclastic deposits and lavas that non- middle Miocene time, coeval with east-west et al., 1968; Atwater, 1970; Moore and Cur- conformably overlie the Cretaceous Penin- extension in the southern Sierra Juarez. ray, 1982). This interpretation is supported sular Ranges batholith. The 40Ar/39Ar This observation supports a hypothesis that by NUVEL-1 (DeMets et al., 1990), which geochronology indicates that basaltic rocks middle Miocene transtensional plate mo- suggests oceanic rifting was initiated at 4.5 are 16.90 6 0.05 Ma and dacite pyroclastic tion was partitioned into two components: a Ma. Geophysical, sedimentary, structural, deposits are between 16.69 6 0.11 Ma and strike-slip component parallel to active and geochronologic evidence from the cir- 15.98 6 0.13 Ma. These strata were subse- faults along the western offshore margin of cum-gulf region, however, point to a period quently cut by two generations of faults. Baja California, and an extensional compo- of middle–late Miocene (ca. 16–5 Ma) First generation faults comprise a domi- nent normal to the margin, but located in ‘‘proto-gulf’’ extension prior to the onset of nant set of north-south–striking, west-dip- what is now the Gulf Extensional Province. sea-floor spreading (e.g., Karig and Jensky, ping normal faults, a secondary set of Hence, the onset of extension within the cir- 1972; Ingle, 1973, 1974; Angelier et al., 1981; north-south–striking, east-dipping normal cum-gulf region was in response to plate Curray et al., 1982; Dokka and Merriam, faults, and a lesser set of variably oriented boundary processes. 1982; Stock and Hodges, 1990; Lee and strike-slip faults. All three fault sets are Miller, 1994). temporally and spatially related and were INTRODUCTION Early workers attributed this period of produced by east-west extension. The dom- proto-gulf extension to a back-arc setting inant west-dipping faults, which are anti- Baja California, Mexico, lies within a (Karig and Jensky, 1972), although more thetic to and oblique to the east-dipping unique and complex plate tectonic setting, precise dating indicates that the cessation of Main Gulf Escarpment, may have been a where the peninsula has been transferred subduction occurred prior to the onset of precursor or an early phase accommodation from the North American plate to the Pa- extension (Mammerickx and Klitgord, 1982; zone along the escarpment. West-dipping cific plate during late Neogene time (Fig. 1). Lonsdale, 1991; Stock and Hodges, 1990; normal faults are cut by a 10.96 6 0.05 Ma The peninsula is currently riding northwest- Lee and Miller, 1994). Some have proposed dacite hypabyssal intrusion, thus bracket- ward on the Pacific plate parallel to trans- that the proto-gulf period of extension ac- ing the age of east-west extension between form faults that define much of the plate commodated Pacific–North American dis- 15.98 6 0.13 Ma and 10.96 6 0.05 Ma. boundary. Continental crust surrounding placement perpendicular to preexisting/ac- Hence, this faulting event clearly indicates a the Gulf of California records the transition tive strike-slip faults along the western period of extension that predates the onset from subduction to transform-dominated margin of Baja California (Spencer and GSA Bulletin; May 1996; v. 108; no. 5; p. 505–525; 14 figures; 3 tables. 505 LEE ET AL. Figure 1. (a) Modern tectonic setting of the Pacific–North American plate boundary in southern California, the Baja California peninsula, and the Gulf of California. Gulf Extensional Province is shown in dark shade and Transpeninsular Strike-slip Province is shown in medium shade. Active and suspected faults that accommodate components of plate motion are shown; some fault names are given in italics. Abbre- viations for areas discussed in text: BC, Bahia Concepcion; IT, Isla Tiburon; LP, La Paz; LR, Loreto; SR, Santa Rosalia; SRB, Santa Rosa Basin; STP, Sierra Las Tinaja and Sierra Las Pintas region; VC, Valle Chico. (b) Tectonic setting of the northern part of Baja California show- ing major active or recently active faults and location of study area. Normark, 1979; Hausback, 1984; Stock and 1989), (5) steepening and/or removal of the parallel to strike-slip faults along the west- Hodges, 1989). Others, noting continuity, Laramide slab (Atwater, 1989), and (6) ern margin of Baja California (i.e., the San common fault geometry, and parallel exten- gravitational collapse of overthickened crust Benito and Tosco-Abreojos faults) and the sion direction with the southern Basin and (Coney and Harms, 1984; Wernicke et al., other orthogonal to the margin along nor- Range Province, focus on a common origin 1987). mal faults located in the Gulf Extensional (Gastil, 1968; Dokka and Merriam, 1982; Global plate reconstructions for Baja Cal- Province. The partitioning of plate bound- Curray and Moore, 1984; Henry, 1989; ifornia indicate a middle–late Miocene tran- ary slip, in effect, isolated Baja California Henry and Aranda-Gomez, 1992). A num- sition from subduction to transform plate between the Pacific plate to the west and the ber of tectonic models have been proposed motion (Stock and Molnar, 1988). In this North American plate to the east. The lo- for the origin of the Basin and Range Prov- scenario, Pacific–North American relative cation of middle–late Miocene extension, ince including: (1) distributed shear related plate motion changed from parallel to far inland from the plate margin, is sug- to the onset of transform motion along the oblique (transtensional) during middle Mio- gested to result from a thermally weakened, Pacific–North American plate boundary cene time. Because there was no component recently active volcanic arc (Hausback, (Atwater, 1970), (2) migration of Mendo- of extension along the western margin of 1984; Stock and Hodges, 1989). Precise time cino triple junction (Ingersoll, 1982; Glazner Baja California (Spencer and Normark, constraints on the change in relative plate and Bartley, 1984), (3) a decrease in plate 1979; Hausback, 1984; Lonsdale, 1991), it motion and onset of extension, however, are convergence rates (Engebretson et al., 1984; has been suggested that plate boundary slip not known. On the basis of global plate re- Stock and Molnar, 1988), (4) change in plate was partitioned into two components (Haus- constructions, transtension along the plate motions (Pollitz, 1988; Stock and Hodges, back, 1984; Stock and Hodges, 1989): one boundary was initiated as early as 15–9 Ma 506 Geological Society of America Bulletin, May 1996 GULF EXTENSIONAL PROVINCE, BAJA CALIFORNIA Figure 2. SPOT image of southeastern Sierra Juarez, northeastern Baja California, Mexico. Major geomorphically defined faults, drainages, and location of study area are shown. and continued until 7–5 Ma (Stock and cannot account for the minimum geologic interplay of extension and transform motion Hodges, 1989). A fixed hotspot reference separation of Baja California from central during the late Neogene, but emphasizes frame model, however, indicates transten- Mexico, the onset of proto-gulf deformation dextral transform motion in the proto-gulf sional plate motion began at least as early as is a direct consequence of increased activity region. 17 Ma (Engebretson et al., 1985). along the proto–San Andreas fault system at With some exceptions (e.g., Henry, 1989; Humphreys and Weldon (1991), on the 17 Ma. Humphreys and Weldon’s (1991) Stock and Hodges, 1990; Lewis, 1994),