Abstract: Geologic Criteria for Identifying Regions Underlain by Buried Thrust Faults: Preliminary Report

Janine Weber-Band, D. L. Jones, T. V. McEvilly

Detailed structural geological investigations of two areas in California: the Sacramento Delta and the San Fernando Valley, have revealed a set of related geologic and geomorphic features we interpret to be associated with active buried thrust faults. These characteristics can be used to identify other areas underlain by active thrusts. The Northridge earthquake of 1994 established the presence of an active thrust beneath the San Fernando Valley. We compare structural and stratigraphic evidence from the Northridge area with a similar area in the Central Coast Ranges, the Sacramento Delta region where extensive seismic reflection profiles (from BASIX and Calcrust projects), subsurface well data and analysis of seismicity establish the presence of active blind thrusts as we l.

Characteristic features found in areas underlain by active thrusts include 1) an asymmetric basin filled with post-Miocene sediment, 2) folds in Quaternary strata, 3) uplifts bounded by high-angle reverse or thrust faults (pop-up structures) on the steep side of the basin, 4) stratigraphic evidence for tectonic extension causing subsidence of the basin prior to tilting and folding, 5) proximity to a major strike-slip fault on one or both sides of the basin axis. Together, these structures describe features expected in an upper crustal block as it shortens over a deep-seated thrust.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California