LETTIS, W. R.
William Lettis & Associates,
Inc., Walnut Creek, CA
Abstract: The San Gregorio/Hosgri Fault
System, California
- An Evaluation of the Style and Rate of Quaternary Deformation
The San Gregorio/Hosgri fault
system extends for a distance of 410
km along the coast of central California coming onshore at three, and possibly
four, locations: Seal Cove, Ai~o Nuevo, San Simeon, and possibly Point
Sur. The
fault
system consists of at least three distinct faults: from
north to south, the San Gregorio
fault
, the San Simeon
fault
, and the Hosgri
fault
. The San Gregorio and San Simeon faults clearly offset a sequence
of marine terraces at Seal Cove, Ano Nuevo and Point San Simeon. The offset
marine terraces and an offset archaeological site at Seal Cove demonstrate
right-lateral strike-slip displacement at a preferred rate of 6 to 8 mm/yr
along the San Gregorio
fault
and 1 to 3 mm/yr along the San Simeon
fault
.
Paleoseismic trenches also show that the faults have had multiple surface
faulting events during the Holocene. To the south, the San Simeon
fault
forms a 3- to 5-km wide right en echelon stepover with the northern Hosgri
fault
zone. Geophysical and stratigraphic data define a distinct subsiding
basin between the two faults in the stepover region. We interpret the basin
to be a pull-apart structure accommodation transfer of right slip between
the San Simeon
fault
and northern Hosgri
fault
. The decreasing slip rate,
from 6 to 8 mm/yr on the San Gregorio
fault
to 1 to 3 mm/yr on the San
Simeon and Hosgri
fault
, shows that slip rate decays southward along the
fault
system. We attribute this decay to north-northeast-directed crustal
shortening along the eastern margin of the
fault
system in the southern
Coast Ranges and Santa Maria valley area. The decay in slip and crustal
shortening appear to be the result of progressive transfer of slip across
a left-restraining stepover between the northern San Gregorio/Hosgri
fault
system and the southern Rinconada
fault
or perhaps the Carrizo Plains segment
of the San Andreas
fault
. In addition, we recognize a large restraining
bend along the San Gregorio/Hosgri
fault
system centered on the San Simeon
fault
near Piedras Blancas. The restraining bend has produced localized
crustal shortening along the
fault
zone associated with the offshore Piedras
Blancas antiform and uplift of the northern Santa Lucia Range.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California