Slip history and the role of the Agua Blanca fault in the tectonics of the North American–Pacific plate boundary of southern California, USA and Baja California, Mexico

Abstract

The Agua Blanca fault (ABF) is a west-northwest–trending oblique dextralnormal fault that defines the southern boundary of the Big Bend domain (BBD) of the Pacific–North American plate margin and the northern limit to the rigid Baja California microplate. Our geologic and geodetic studies demonstrate that finite slip on the ABF reaches a maximum of ~11 km of nearly pure dextral strike slip in central portions of the fault, whereas the magnitude of displacement decreases and the proportion of extension increases in the sections to both the east and west. To the east, the ABF appears to die out before crossing into the San Pedro Mártir fault, with slip transferred onto a series of more northerlytrending, dip-slip faults. To the west, the ABF bifurcates to form the Santo Tomás fault (STF) in the Valle Santo Tomás section, where we measure ~5 km of dextral offset for the ABF and ~3 km estimated offset on the STF. And we report a measurement of ~7 km of dextral offset on the ABF in the Punta Banda section. Small offset faults proximal to the ABF likely accommodate additional dextral shear in the western sections of the fault. The STF in the Valle Santo Tomás section and the ABF in the Punta Banda section exhibit 0.58 and 0.65 km of extensional heave, or ~7% and 10% of the total displacement in each section, respectively. Block modeling based on geodetic data agrees well with geologic determinations of slip direction and reveals near perfect alignment of the central ABF with the relative block motion vectors and increased proportions of fault-perpendicular extension to both the east (3%–10%) and west (5%–13%). Based on our new estimates of the total offsets combined with existing slip rates, the ABF likely initiated between 3.3 and 1.5 Ma. This age range overlaps with those reported for other faults within the area of the BBD southwest of the San Andreas fault. The ABF has a more westerly orientation than the transpressional restraining-bend segment of the San Andreas, yet it accommodates transtensional shearing. This requires a reevaluation of the processes that control transpression and transtension within the BBD. INTRODUCTION The North American–Pacific plate boundary through the United States and Mexico is composed of a complex system of faults whose kinematics and geographic distributions allow for a subdivision into three domains (Fig. 1; Fletcher et al., 2014). From north to south, these include the San Andreas, Big Bend, and Gulf of California domains. The San Andreas domain includes the San Andreas fault (SAF) along with an anastomosing network of faults following the central California coast and includes domains of partitioned plate motion in the Walker Lane, Great Basin, and Rio Grande rift. The Big Bend domain (BBD), located between the Garlock fault and the Agua Blanca fault (ABF), is composed of a network of faults stretching across a ~500-km-wide region with the SAF at its approximate center. The faults of the Mojave section of the Eastern California shear zone (ECSZ) are present to the northeast, and a network of faults identified by Legg et al. (1991) as the Southern California shear zone (SCSZ) is present to the southwest of the SAF. The SCSZ predominantly consists of northwest-trending dextral strike-slip faults that initiated in the Quaternary (e.g., Steely et al., 2009; Janecke et al., 2010; Dorsey et al., 2012). The Gulf of California domain, located to the south of the ABF, includes a suite of en echelon transform faults with short spreading centers in the steps. Unlike the two northern domains, the vast majority of the plate boundary shear (~90%; Plattner et al., 2007, 2009) is confined to these transform faults. The remaining plate motion in this domain is accommodated by active faults west of the Baja California microplate (Plattner et al., 2007, 2009). The ABF defines the northern limit of the Baja California microplate and the southern limit of the BBD. It is a unique structure within the North American–Pacific plate boundary zone of southern and Baja California in that it is a predominantly dextral strike-slip fault (e.g., Allen et al., 1960 and this study) with a trace oriented at a high angle (276° to 302°) to the regional maximum horizontal compressive stress (Fig. 1; Townend and Zoback, 2004). Its strike is, GEOSPHERE GEOSPHERE; v. 15, no. 1 https://doi.org/10.1130/GES01670.1 12 figures; 4 tables CORRESPONDENCE: wetmore @usf .edu CITATION: Wetmore, P.H., Malservisi, R., Fletcher, J.M., Alsleben, H., Wilson, J., Callihan, S., Springer, A., González-Yajimovich, O., and Gold, P.O., 2019, Slip history and the role of the Agua Blanca fault in the tectonics of the North American–Pacific plate boundary of southern California, USA and Baja California, Mexico: Geosphere, v. 15, no. 1, p. 119–145, https:// doi .org /10 .1130 /GES01670.1. Science Editor: Raymond M. Russo Associate Editor: Jose Miguel Hurtado Received 11 January 2018 Revision received 5 June 2018 Accepted 3 October 2018 Published online 5 December 2018