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

Abstract

Fault slip rates inform models of strain accumulation and release, which over geologic time may vary or remain constant depending on factors like structural complexity, fault strength, deformation rates, and proximity to other faults. In this study, we present a Late Pleistocene-Holocene slip history based on four new geologic slip rates for the Agua Blanca Fault (ABF), which transfers Pacific-North American dextral plate boundary motion across the Peninsular Ranges of northern Baja California. Time-averaged slip rates from three sites are 2.8 + 0.8/−0.6 mm/a since~65.1 ka, 3.0 + 1.4/−0.8 mm/a since~21.8 ka, 3.2 + 1.0/−0.6 mm/a since~12.5 ka, and 3.5 + 5.1/−2.0 mm/a since~1.4 ka; however, the actual slip rate may be closer to 4 mm/a when off-fault slip and age interpretation uncertainties are considered. Significantly, although the ABF has more in common in terms of length, net offset, and slip rate with known variable slip rate faults, the most straightforward age and offset interpretations for the ABF suggest constant slip rates over~10 kyr time scales. As with other constant slip rate faults, comparable neighboring faults that might modulate the ABF slip rate are absent, suggesting that fault interaction, or lack thereof, may be a more significant factor controlling fault behavior on this and potentially other faults. The new rates indicate that the ABF accommodates at least half of total slip across the Peninsular Ranges, clarifying strain partitioning for seismic forecasting models that previously lacked modern geologic slip rate constraints for this domain of the plate boundary.

Recency of Faulting and Subsurface Architecture of the San Diego Bay Pull-Apart Basin, California, USA

“…The Agua Blanca fault cuts across the Baja Peninsula and transfers ∼2-8 mm/yr of slip into the faults of ICB (Legg et al, 1991;Rockwell et al, 1993;Dixon et al, 2002;Gold et al, 2020). Within the ICB, the Descanso and Coronado Bank faults are strands of the Agua Blanca fault zone that help carry a portion of this slip northward, potentially partitioning it into the Rose Canyon fault through the San Diego Bay pullapart basin (Figure 1; Legg, 1985;Legg et al, 1991).…”

Behavior of Amagmatic Orogenic Geothermal Systems: Insights From the Agua Blanca Fault, Baja California, Mexico

“…From southeast to northwest, these are Valle Trinidad, Cañon Dolores, Valle Agua Blanca, Valle Santo Tomas, and the coastal plains flanking the Punta Banda Peninsula (Figure 2b). Geologic and geodetic studies in these valleys indicate consistent slip rates of 2–4 mm year −1 over ∼10 Kyr time scales (Gold et al., 2020; Wetmore et al., 2019). However, these valleys exhibit different amounts of strike‐slip along different sections.…”

“…We have therefore undertaken a geochemical, geophysical and geological study of orogenic geothermal systems along the Agua Blanca Fault (ABF) in Ensenada, Baja California, Mexico. We chose this topographically rugged area because it exhibits three favorable features: (a) seven geothermal systems are strung out along a ∼90 km stretch of the fault, permitting the quantitative correlation of hydraulic head gradients with the physicochemical properties of the springs; (b) water discharge temperatures vary along the fault from 37 to 102°C, the latter being worldwide the hottest and therefore most prospective amagmatic system to our knowledge; (c) the host fault is active and the rates and magnitudes of tectonic extension along its segments have been quantified (Gold et al., 2020; Wetmore et al., 2019), allowing qualitative assessment of permeability variations along the fault.…”