The San Gabriel Faults, Newhall Area, Los Angles County, California

Henry Walrond

The San Gabriel fault, extending from the eastern Los Angeles County line north westerly to Frazier Mountain in Ventura County, is considered by many to be a major strike-slip system, possibly ancestral to the San Andreas, with. a cumulative right separation of up to 60 kilometers. The two northern most segments of the system, the Palomas Canyon and Newhall faults, share a common trend but otherwise appear to be genetically unrelated.

A series of structural cross-sections located between the Placerita and the Honor Rancho oil fields show the Newhall segment to be an easterly dipping Pleistocene reverse fault that makes a pronounced westerly swing in the Honor Rancho vicinity (effecting in part the northerly closure of the South East Honor Rancho pool) before apparently terminating into a zone of steep and overturned beds farther west.

The easterly dipping, Palomas Canyon segment which is exposed on and forms the west flank of the Ridge Basin is pre-Pliocene in age with apparent normal displacement and, depending on the investigator, has right lateral separation ranging from three to sixty kilometers.

How far the Palomas segment extends south into the subsurface of the Newhall basin is uncertain, but a Palomas-type fault beneath the Newhall reverse fault is indicated just north of the Placerita oil field based on sub-surface well control.

The recognition of these two dissimilar faults not only has economic implications, but should also further the understanding of this complex and controversial structural system.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California