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ABSTRACT: Trap Geometries Associated with Normal Fault and Thrust Fault Interactions: Examples from California Basins

Jay Namson, Thom Davis

Normal faulting associated with the formation of restricted sedimentary basins is often affected by a subsequent phase of compression leading to the interaction between normal faults, thrust faults, and thrust-related folds. California basins provide excellent examples of the trap geometries that develop from these interactions because (1) there is extensive surface and subsurface data, (2) the Miocene and early Pliocene normal faults have 2-5 km of structural relief, and (3) the late Pliocene and Quaternary shortening has produced significant thrust faults and anticlinoria.

The normal faults can be cut and translated by thrusts resulting in trap styles that include faulted fold crests and limbs in the hanging wall sheet. Traps in the subthrust footwall are related to normal fault-related folds and fault blocks.

Another common structural style involves large thrust-related fault-propagation folds along the basin-bounding normal faults. The normal faults are often rotated to a reverse fault geometry and in some instances are reactivated as reverse faults during compression. Prospective trap geometries are faulted fold crests and limbs that occur on both the upthrown and downthrown sides of the normal faults. Most of the oil fields of the Los Angeles basin, southern Ventura basin, southern San Joaquin basin, and Orcutt-Casmalia trend in the Santa Maria basin can be attributed to this trap style, highlighting the prospectiveness of traps in compressional structures formed along normal-faulted basin margins.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990