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 northerly-trending, 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. GEOSPHERE GEOSPHERE; v. 15, no. 1

Quantifying rates of “rifting while drifting” in the southern Gulf of California: The role of the southern Baja California microplate and its eastern boundary zone

“…GPS data were processed at the University of South Florida using GYPSY/OASIS II, Release 6.4, software and non-net-rotation satellite orbit and clock files provided by the Jet Propulsion Laboratory (JPL; Zumberge et al, 1997). The analysis followed the description of Wetmore et al (2019) and Malservisi et al (2015), with the daily solutions aligned to IGb08 (Rebischung et al, 2012). At the processing time (December, 2018), the JPL reprocessing of the orbits for ITRF2014 did not go earlier than 2000.…”

Time‐Invariant Late Quaternary Slip Rates Along the Agua Blanca Fault, Northern Baja California, Mexico

“…The Laguna Salada Fault and faults within the Sierra El Mayor‐Sierra Cucapah (Axen et al, 1999; Axen & Fletcher, 1998; Fletcher et al, 2014; Fletcher & Spelz, 2009) are southern extensions of the Elsinore Fault (Suarez‐Vidal et al, 1991) and accommodate at least an additional ~2–3 mm/a (Mueller & Rockwell, 1995) immediately west of the Cerro Prieto Fault. Farther west, roughly ~7–8 mm/a of slip is accommodated by the Agua Blanca and San Miguel‐Vallecitos Faults across the Peninsular Ranges (Allen et al, 1960; Bennett et al, 1996; Dixon et al, 2002; Hirabayashi et al, 1996; Rockwell et al, 1993; Wetmore et al, 2018). Slip from these faults in part feeds into faults off the Pacific coast of northern Baja and southern California (Legg, 1991), which also accommodate a combined 7–8 mm/a of dextral slip (Larson, 1993; Platt & Becker, 2010).…”

“…Disrupted Quaternary landforms unambiguously demonstrate a history of Late Pleistocene and Holocene dominantly dextral surface displacement along the ABF (Figure 2), although historically it has been nearly devoid of microseismicity (Frez et al, 2000; Frez & González, 1991; Gonzalez & Suárez, 1984). The ABF has accommodated 7–11 km of total dextral slip (Allen et al, 1960; Wetmore et al, 2018), and if existing slip rate estimates are more or less representative of the long‐term rate, slip on the ABF would have commenced ca. 2–3 Ma (Wetmore et al, 2018).…”

“…The ABF has accommodated 7–11 km of total dextral slip (Allen et al, 1960; Wetmore et al, 2018), and if existing slip rate estimates are more or less representative of the long‐term rate, slip on the ABF would have commenced ca. 2–3 Ma (Wetmore et al, 2018). Although minor secondary fault traces paralleling the ABF are evident from vegetation lineaments and subtle offsets, no major structures diverge from the main trace except in the Valle Santo Tomas near the western end of the fault where the Maximinos and Santo Tomas Faults branch off and follow a more westerly path south of Punta Banda Ridge (Figure 1b).…”

“…At its eastern end, the ABF intersects, but does not visibly cross, the Main Gulf Escarpment, which separates the Peninsular Ranges from the Gulf of California (Axen, 1995), and south of the ABF intersection is defined by the active Sierra San Pedro Martir Fault (SSPMF) (Section S1; Brown, 1978; O'Connor & Chase, 1989). Slip is transferred between the ABF and SSPMF by a distributed network of faults that can be identified by vegetation lineaments and subtle tectonic geomorphology across the Valle de la Trinidad and Valle San Matias south of the ABF (Figure 1; Section S2) (Hilinski, 1988; Wetmore et al, 2018).…”