Geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the San Jacinto fault zone, southern California

Abstract

We examine the role of preexisting crustal structure within the Peninsular Ranges batholith on determining the location of the San Jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data. A 1000-km-long boundary within the Peninsular Ranges batholith, separating relatively mafi c, dense, and magnetic rocks of the western Peninsular Ranges batholith from the more felsic, less dense, and weakly magnetic rocks of the eastern Peninsular Ranges batholith, strikes north-northwest toward the San Jacinto fault zone. Modeling of the gravity and magnetic fi eld anomalies caused by this boundary indicates that it extends to depths of at least 20 km. The anomalies do not cross the San Jacinto fault zone, but instead trend northwesterly and coincide with the fault zone. A 75-km-long gradient in initial strontium ratios (Sr i ) in the eastern Peninsular Ranges batholith coincides with the San Jacinto fault zone. Here rocks east of the fault are characterized by Sr i greater than 0.706, indicating a source of largely continental crust, sedimentary materials, or different lithosphere. We argue that the physical property contrast produced by the Peninsular Ranges batholith boundary provided a mechanically favorable path for the San Jacinto fault zone, bypassing the San Gorgonio structural knot as slip was transferred from the San Andreas fault 1.0-1.5 Ma. Two historical M6.7 earthquakes may have nucleated along the Peninsular Ranges batholith discontinuity in San Jacinto Valley, suggesting that Peninsular Ranges batholith crustal structure may continue to affect how strain is accommodated along the San Jacinto fault zone.

Quaternary landscape evolution in the San Jacinto fault zone, Peninsular Ranges of Southern California: Transient response to strike-slip fault initiation

“…A younger age (1.0 to 1.5 Ma) has been inferred from geologic mapping and stratigraphic studies Matti and Morton, 1993;Dorsey, 2001) and morphometric analysis that suggested that the Pleistocene slip rate may be faster than 20 mm/year (Kendrick et al, 2002). Geophysical and isotopic mapping shows that the fault zone formed along preexisting faults and crustal weaknesses in the Peninsular Ranges batholith when structural complications in San Gorgonio Pass transferred slip away from the San Andreas fault (Langenheim et al, 2004).…”

Two lithospheric profiles across southern California derived from gravity and seismic data

“…Based on seismic and other data (Magistrale and Zhou, 1996;Lutter et al, 1999Lutter et al, , 2004Fuis et al, 2001Fuis et al, , 2003Fuis et al, , 2004Langenheim et al, 2000Langenheim et al, , 2004Yeats and Huftile, 1995), basin depths at the profile location are estimated to be 8-9 km for the Los Angeles basin, and 5 km for the San Fernando and San Gabriel basins. The seismic refraction model along the LARSE-1 line (Fuis et al, 2001) shows increasing velocities between 15-27 km beneath the Los Angeles basin with velocities of 6.1-6.5 km/s in the middle crust, and ∼6.7 km/s in the lower crust.…”

Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California

“…A fourth basement type, structurally isolated and exposed only in the southeastern part of the San Gabriel Mountains (Fig. 2), has been tentatively correlated with Peninsular Ranges rocks (Matti et al, 1985;May, 1989), a correlation at odds with its magnetic signature (Anderson et al, 2004;Langenheim et al, 2004). These regional basement types have distinct gravity and magnetic characters, as discussed later.…”