Elsevier

Tectonophysics

Volume 488, Issues 1–4, 5 June 2010, Pages 87-109
Tectonophysics

Neogene structural evolution of the Sierra San Felipe, Baja California: Evidence for proto-gulf transtension in the Gulf Extensional Province?

https://doi.org/10.1016/j.tecto.2009.09.026Get rights and content

Abstract

The Sierra San Felipe, located in the Gulf Extensional Province of northeastern Baja California, experienced a complex deformation history of integrated normal and strike-slip faulting, block rotations and extension-parallel folding as a result of Neogene transtensional plate margin shear between the Pacific and North American plates. The eastern range-front of the Sierra San Felipe is defined by three left-stepping, en-echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones. The Las Cuevitas, Santa Rosa and Huatamote detachments comprise NE- to SE-dipping, moderate- to low-angle normal faults that accommodate between ~ 1.5 and 9 km of broadly E- to SE-directed extension and show strong along-strike displacement gradients. Hanging wall half-grabens are infilled with northwest tilted Miocene–Pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement. Stratigraphic relationships indicate that faulting on the Las Cuevitas and Santa Rosa detachment faults initiated synchronously as a kinematically linked fault system before ~ 7 Ma (~ 9–8 Ma based on the timing of footwall exhumation), during the so-called ‘proto-gulf’ phase of rifting. Paleostress calculations suggest a transtensional stress regime with NE- to SE-directed extension and permutating vertical to N–S trending, subhorizontal contraction. Fault kinematics and paleostress orientations of the fault array do not vary through time, but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments. Our data indicate that clockwise vertical-axis block rotations and constrictional folding were an integral part of the deformation history in the Sierra San Felipe since rifting began in the late Miocene, and may have played an important role in facilitating the transfer of deformation between the Main Gulf Escarpment and offshore faults in the Gulf of California. These observations support the hypothesis that middle Miocene to present oblique-divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the Gulf Extensional Province.

Introduction

As a rare example of a youthful oceanic basin, the Gulf of California presents unique insights into the transition from continental rifting to oceanic spreading and the formation of passive margins. Neogene opening of the Gulf of California initiated in response to a major plate boundary reorganisation that followed the demise of the subduction zone along the western margin of Baja California. Incipient interaction between the Pacific and North American plates induced deformation in the continental borderland west of Baja California as well as in the Gulf Extensional Province, a region of highly extended continental crust surrounding and encompassing the Gulf of California (e.g. Karig and Jensky, 1972, Spencer and Normark, 1979, Hausback, 1984, Lonsdale, 1989, Stock and Hodges, 1989, Lonsdale, 1991, Fletcher et al., 2007).

Present-day relative plate motion between the Pacific and North American plates is largely accommodated by the en-echelon array of right-stepping transform faults and nascent oceanic spreading centres in the Gulf of California (Fig. 1). Plate motion models coupled with geodetic observations suggest that the transfer of the Baja California microplate from the North American to the Pacific plate was a gradual process, beginning at ~ 12 Ma (Stock and Hodges, 1989) and continuing until at least ~ 1 Ma ago (DeMets, 1995, DeMets and Dixon, 1999), although transfer may still be ongoing (Dixon et al., 2000, Fletcher and Mungúia, 2000, Michaud et al., 2004).

The capture of the Baja California microplate traditionally has been thought to have occurred in two distinct kinematic phases (e.g. Hausback, 1984, Stock and Hodges, 1989): (1) an early (~ 12–6 Ma) proto-gulf phase when plate margin shearing was kinematically partitioned into transform faulting west of Baja California and orthogonal rifting in a NE–SW direction in the Gulf Extensional Province (Fig. 1a); and (2) a later phase (~ 6 Ma to present) of integrated transtensional shearing within the Gulf of California (Fig. 1b). Although no dynamic explanation has been proposed for this kinematic model, an early period of orthogonal rifting most likely would have been related to gravitational collapse of over-thickened crust in the Gulf Extensional Province.

The long-held view of a two-stage tectonic evolution of the Gulf of California has recently been challenged in light of new geophysical and geochronological data. Fletcher et al. (2007) documented the existence of a major normal fault named the Santa Margarita–San Lazaro fault that extends 400–500 km along the length of the continental shelf west of Baja California, which demonstrates the transtensional nature of Neogene shearing in this deformation belt. Using regional piercing points, they further demonstrated that the magnitude of dextral strike-slip displacement west of Baja California is significantly less than previously assumed, which requires additional strike-slip motion in the Gulf Extensional Province in order to satisfy the finite displacement estimates of global plate-circuit motion models. Therefore, Fletcher et al. (2007) proposed an alternative kinematic model (Fig. 1c, d) of integrated transtensional deformation on both sides of the Baja California microplate since the onset of shearing between the Pacific and North American plates in the middle Miocene. In this model, the distribution and kinematics of shearing is more strongly controlled by lithospheric weaknesses than possible gravitational instabilities in the continental crust of the Gulf Extensional Province (Fletcher et al., 2007).

The two models differ primarily in the proposed kinematics of faulting during the early stages of Neogene extension (Fig. 1a, c). Numerous structural studies in Baja California have documented multiple phases of faulting with distinct kinematics in the Gulf Extensional Province that were interpreted to represent a clockwise rotation of the extension direction as the Gulf Extensional Province supposedly changed from NE-directed proto-gulf rifting to the modern transtensional stress regime (e.g. Angelier et al., 1981, Zanchi, 1994, Umhoefer et al., 2002). In fact, fault-slip data still provide the only direct observational data that is compatible with the two-phase rifting model.

In this study, we present a detailed structural and kinematic analysis of a complex fault network on the rifted margin of Baja California. The Sierra San Felipe, located in the northern Gulf Extensional Province (Figs. 2 and 3), contains key structural relationships to characterise the kinematics of rifting along the margin of Baja California. Virtually every major class of faults is exposed in a complex array that cuts rocks ranging in age from Cretaceous to early Pleistocene, which provides a unique opportunity to evaluate spatial and temporal variations in fault kinematics across the rifted margin of Baja California. Our data demonstrate multiple directions of extension that are tempting to interpret according to a multi-phase tectonic model. However, we argue that the data fit equally well or better with a kinematic model of progressive transtensional shearing, which involves highly three-dimensional strain and a combination of vertical- and horizontal-axis block rotations.

Section snippets

Main Gulf Escarpment

The Main Gulf Escarpment has been described as the break-away fault of continental rifting in the Gulf Extensional Province (Gastil et al., 1975). At the latitude of the Sierra San Felipe, the escarpment is defined by the east-dipping Sierra San Pedro Mártir fault, which produces a 1000–2500 m high topographic escarpment that extends ~ 100 km along strike (Figs. 2 and 3). The San Pedro Mártir fault accommodates ~ 5 km of vertical offset across its central segments (Hamilton, 1971, Gastil et al., 1975

Basement and pre-late Miocene strata

The basement of the Sierra San Felipe consists of Paleozoic to Mesozoic metamorphic rocks (Pbs) that have been intruded by large volumes of granodioritic to tonalitic batholiths (Kg) during the Cretaceous. Apatite fission track results document that post-emplacement cooling and final unroofing of the uppermost basement occurred at ~ 45–35 Ma (Seiler, 2009). Erosion produced a late Eocene(?) to Oligocene nonconformity above a gently undulating topography with roughly ENE–WSW oriented drainages (

Timing of extension

A maximum age for the initiation of extension on the Las Cuevitas detachment is provided by the 19.4 Ma olivine basalt (Tmb1) at the base of the stratigraphy in the Llano El Moreno basin, as well as an undated welded rhyolite tuff that we correlate to the ~12.6 Ma Tuff of San Felipe (Tsf; Table 1). The syn-tectonic sequence of the basin also includes a well-dated sequence of latest Miocene to Pliocene marine rocks (Tpm), which indicate that faulting on the Las Cuevitas detachment must have

Structural analysis of the Sierra San Felipe fault array

Reconnaissance interpretation of aerial photographs and existing geological maps (Andersen, 1973, Bryant, 1986, Black, 2004) provided preliminary information to guide our field work. On the basis of detailed geological mapping on aerial photographs, rectified from Google Earth™ at a scale ranging from 1:7000 to 1:25,000, as well as standard stratigraphic and structural field methods, we characterised the Neogene fault array of the Sierra San Felipe and identified structures of regional

Methodology

Kinematic analysis and paleostress calculations, based on a total of 501 fault-slip measurements, were carried out separately for all fault segments. Each fault-slip datum consists of the measured fault plane, movement direction, shear sense (where possible), and a quality factor assessing the reliability of the datum. Separation into geographical areas was necessary because the main structures of the study area represent a complex fault array with diverse kinematics on variably oriented fault

Temporal and spatial analysis of fault kinematics

Paleostress orientations vary significantly within the Sierra San Felipe fault array, and these variations are crucial to evaluate the tectonic models for the evolution of the Gulf of California. Fig. 12 shows that the fault population can be divided into two groups of faults with broadly ENE and SE-trending extension axes. Similar patterns of (E)NE- and SE-directed extension have been recognised elsewhere in the Gulf Extensional Province, where the two extension directions have been

Observations from fault kinematics

Paleostress axes from all of the faults in the study area clearly demonstrate the constrictional nature of strain accommodated by the fault network of the Sierra San Felipe (Fig. 12). Constrictional strain is characterised by prolate (cigar-shaped) finite strain ellipsoids, which is produced by radial shortening perpendicular to a dominant extension direction. Both kinematic subsets with NE and NW oriented extension axes exhibit a well-developed great circle distribution of shortening axes (

Rotational nature of faulting

The rotations of tectonic blocks about both vertical and horizontal axes are well documented in the San Felipe fault array and they help explain much of the complexity of the fault kinematics. The large hanging wall cut-off angles (~ 55–60°) of pre-extensional strata in the Llano El Moreno and Santa Rosa basins show that the detachment faults initiated as steep faults and were rotated to their present orientations during progressive slip. The southern segments of both the Las Cuevitas and Santa

Transfer of onshore deformation into the Gulf of California

Much of the previous work in the Gulf Extensional Province of northern Baja California has focused on understanding the nature of strain transfer from the southern San Pedro Mártir fault onto offshore structures in the Gulf of California (Dokka and Merriam, 1982, Stock and Hodges, 1990, Nagy and Stock, 2000, Nagy, 2000, Stock, 2000). The southward decreasing displacement on the San Pedro Mártir fault, from ~ 5 km in the centre (Hamilton, 1971, Gastil et al., 1975) to < 800 m in the south (Stock and

Conclusions

Neogene rifting in the Sierra San Felipe was dominated by integrated normal and strike-slip faulting on three major detachment faults that control the eastern margins of the Sierras San Felipe and Santa Rosa. The faults are arranged in a left-stepping en-echelon configuration and are linked by accommodation and transfer zones. The Santa Rosa and Las Cuevitas detachments each accommodated a major southward-increasing displacement gradient with up to ~ 7.5–9 km of broadly E- to SE-directed

Acknowledgements

This research was partially funded by ARC DP0665127 to AJWG and BPK, and by grants CONACYT #81463 grants to and NSF EAR-0739017 Sub-Award 08-004375-01 JMF. Additional support was provided to CS by MIFRS, MIRS and MATS scholarships from the University of Melbourne and the David Hay Memorial Fund. The author is especially grateful to Yvonne Kunz for invaluable support and assistance in the field. Ramón Mendoza-Borunda, Arturo Martín-Barajas and the staff at CICESE are thanked for their help and

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