Print this pageHas the San Gabriel Fault Been 
Offset
?
Jack R. Sheehan
The San Gabriel fault (SGF) in southern California is a right-lateral,
strike-slip fault extending for 85 mi in an arcuate, southwestward-bowing curve
from near the San Andreas fault at Frazier Mountain to its intersection with the
left-lateral San Antonio Canyon fault (SACF) in the eastern San Gabriel
Mountains. Termination of the SGF at the presently active SACF is abrupt and
prompts the question "Has the San Gabriel Fault been offset?" Tectonic and
geometric relationships in the area suggest that the SGF has been offset
approximately 6 mi in a left-lateral sense and that the offset continuation of
the SGF, across the SACF, is the right-lateral, strike-slip San Jacinto fault
(SJF), which also terminates at the SACF.
Reversing the left-lateral movement on the SACF to rejoin the offset
ends of
the SGF and SJF reveals a fault trace that is remarkably similar in geometry and
movement (and perhaps in tectonic history), to the trace of the San Andreas
fault through the southern part of the San Bernardino Mountains. The
relationship of the Sierra Madre-Cucamonga fault system to the restored SGF-SJF
fault is strikingly similar to the relationship of the Banning fault to the
Mission Creek-Mill Creek portion of the San Andreas fault.
Structural relations suggest that the San Gabriel-San Jacinto system predates the San Andreas fault in the eastern San Gabriel Mountains and that continuing movement on the SACF is currently affecting the trace of the San Andreas fault in the Cajon Pass area.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.