John H. Shaw1
(1) Harvard University, Cambridge, MA
Abstract: Structural model of thrust inversion of low-angle normal fault systems: The Oceanside - San Diego faults, Inner California Borderland, USA
Using 3D structural modeling techniques and seismic reflection data, we describe the geometry and complex reactivation history of an active blind-thrust system in the Inner California Borderland. The Oceanside - San Diego thrust system is a pair of inverted, low-angle normal faults that are overlain by both extensional and contractional fault-related folds. These fold trends are the offshore extensions of prolific hydrocarbon traps in the southern Los Angeles basin.
The Oceanside - San Diego fault system accommodated Neogene rifting associated with the opening of the California Borderlands. These structures were reactivated during Pliocene transpression as thrust faults. The two deformational phases are evident in seismic profiles as complex arrays of extensional rollovers, and as contractional fault-related folds that deform the Pliocene and younger sedimentary cover. Growth strata and prominent unconformities record the complex kinematic evolution of these structures.
We present a 3D structural model of this inverted fault system developed using GOCAD. The model integrates seismic reflection data, a complex velocity volume, bathymetry, earthquakes, and surficial neotectonic information. We suggest that the Oceanside - San Diego thrust system is active and that it caused the 1986 Oceanside (Ml 5.3) earthquake, based on a correlation of precisely relocated seismicity, mapped fault planes, and uplifted marine terraces. Moreover, this study illustrates how modern, 3D structural modeling techniques can be used to define the evolution of hydrocarbon traps, and their influence on hydrocarbon charge pathways, in complex inversion systems.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana