245 Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part I

We welcome contributions inspired by the diverse research of John Platt in structural geology and tectonics showing how the rock record can be used to illuminate the fundamental processes of crustal deformation and mountain building.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217D

Multiscale Transects Illuminate the Shallow-to-Deep Crustal Architecture of the Western Transverse Ranges Mountain Belt

Paper number: 245-11
Program: Technical Sessions
Time: 10:40 AM – 10:55 AM

Authors

Abstract

The ~100km wide Transverse Ranges, straddling the Big Bend of the San Andreas fault in Southern California, are both data-rich and complex, presenting opportunities and challenges. The current era of transpressive tectonics since ~6Ma is characterized by M6-7+ earthquakes on thrust faults that build the topographic mountain belt in addition to the major San Andreas and other strike-slip events. This current architecture inherits a more complex pre-history and structure that includes ~90° clockwise transtensional rotation since 18 Ma of the western Transverse Ranges. The network of major active thrust and strike-slip faults in the upper crust is well known from decades of earthquake hazards research, but how they extend downward through the crust is still elusive because many faults are not yet sufficiently illuminated by seismicity, given the long recurrence intervals. Here we show how modern full-waveform and ambient-noise anisotropic Vs and Vp tomographies image velocity boundaries and velocity inversions that, when combined with seismicity and upper crustal data, illuminate major deeper structure of the Transverse Ranges.

We constructed new multiscale transects of the western Transverse Ranges by integrating data from the surface to the Moho. In the upper crust, we used the exceptional Dibblee Foundation maps, plus petroleum well data and SCEC three-dimensional community fault models. We focus here on the downward deep crustal extent of two well-documented active faults that bound the deep Ventura Basin [1] the San Cayetano thrust at the northern edge, which has experienced M7+ earthquakes and shows ~10mm/yr shortening and [2] the Oakridge fault at the southern edge. We observe a regionally extensive ~20° north-dipping ~15-km-thick low-velocity channel extending north of the Ventura Basin to the base of the crust with velocities ranging from sedimentary at shallow depths to metasedimentary near the Moho. The roof thrust of the channel, below overlying basement velocities, projects upward to the San Cayetano thrust and downward to intersect and perhaps truncate the San Andreas fault, where abundant thrust mechanisms are observed. The floor thrust is seismically illuminated, including the Fillmore swarm. A very high velocity middle and lower crust underlies the Santa Monica Mountains south of the Ventura Basin and shows a steep 60-70° N-dipping edge extending to the Moho, which is apparently the deep Oakridge fault bounding the Ventura rift basin.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413756

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