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High Geologic Slip Rates since Early Pleistocene Initiation of the San Jacinto and San Felipe Fault Zones in the San Andreas Fault System
Geological Society of America Special Paper (2010)
The San Jacinto right-lateral strike-slip fault zone is crucial for understanding
plate-boundary dynamics, regional slip partitioning, and seismic hazards within the
San Andreas fault system of southern California, yet its age of initiation and long-term
average slip rate are controversial. This synthesis of prior and new detailed studies
in the western Salton Trough documents initiation of structural segments of the San
Jacinto fault zone at or slightly before the 1.07-Ma base of the Jaramillo subchron.
The dextral faults changed again after ca. 0.5–0.6 Ma with creation of new fault segments
and folds. There were major and widespread basinal changes in the early Pleistocene
when these new faults cut across the older West Salton detachment fault. We
mapped and analyzed the complex fault mesh, identifi ed structural segment boundaries
along the Clark, Coyote Creek, and San Felipe fault zones, documented linkages
between the major dextral faults, identifi ed previously unknown active strands of the
Coyote Creek fault 5 and 8 km NE and SW of its central strands, and showed that
prior analyses of these fault zones oversimplify their complexity. The Clark fault is a
zone of widely distributed faulting and folding SE of the Santa Rosa Mountains and
unequivocally continues 20–25 km SE of its previously inferred termination point to
the San Felipe Hills. There the Clark fault zone has been deforming basinal deposits
at an average dextral slip rate of 10.2 +6.9/ 3.3 mm/yr for ~0.5–0.6 m.y.
Five new estimates of displacement are developed here using offset successions
of crystalline rocks, distinctive marker beds in the late Cenozoic basin fi ll, analysis
of strike-slip–related fault-bend folds, quantifi cation of strain in folds at the tips of
dextral faults, and gravity, magnetic, and geomorphic data sets. Together these show
far greater right slip across the Clark fault than across either the San Felipe or Coyote
Creek faults, despite the Clark fault becoming “hidden” in basinal deposits at its SE
end as strain disperses onto a myriad of smaller faults, strike-slip ramps and fl ats,
transrotational systems of cross faults with strongly domain patterns, and a variety
of fault-fold sets. Together the Clark and Buck Ridge–Santa Rosa faults accumulated
~16.8 +3.7/ 6.0 km of right separation in their lifetime near Clark Lake. The Coyote
Ridge segment of the Coyote Creek fault accumulated ~3.5 ± 1.3 km since roughly 0.8–
0.9 Ma. The San Felipe fault accumulated between 4 and 12.4 km (~6.5 km preferred)
of right slip on its central strands in the past 1.1–1.3 Ma at Yaqui and Pinyon ridges.
Combining the estimates of displacement with ages of fault initiation indicates a
lifetime geologic slip rate of 20.1 +6.4/ 9.8 mm/yr across the San Jacinto fault zone
(sum of Clark, Buck Ridge, and Coyote Creek faults) and about ~5.4 +5.9/ 1.4 mm/yr
across the San Felipe fault zone at Yaqui and Pinyon ridges. The NW Coyote Creek
fault has a lifetime slip rate of ~4.1 +1.9/ 2.1 mm/yr, which is a quarter of that across the
Clark fault (16.0 +4.5/ 9.8 mm/yr) nearby. The San Felipe fault zone is not generally
regarded as an active fault in the region, yet its lifetime slip rate exceeds those of the
central and southern Elsinore and the Coyote Creek fault zones. The apparent lower
slip rates across the San Felipe fault in the Holocene may refl ect the transfer of strain
to adjacent faults in order to bypass a contractional bend and step at Yaqui Ridge.
The San Felipe, Coyote Creek, and Clark faults all show evidence of major structural
adjustments after ca. 0.6–0.5 Ma, and redistribution of strain onto new rightand
left-lateral faults and folds far removed from the older central fault strands.
Active faults shifted their locus and main central strands by as much as 13 km in
the middle Pleistocene. These changes modify the entire upper crust and were not
localized in the thin sedimentary basin fi ll, which is only a few kilometers thick in
most of the western Salton Trough. Steep microseismic alignments are well developed
beneath most of the larger active faults and penetrate basement to the base of the
seismogenic crust at 10–14 km.
We hypothesize that the major structural and kinematic adjustments at ca. 0.5–
0.6 Ma resulted in major changes in slip rate within the San Jacinto and San Felipefault zones that are likely to explain the inconsistent slip rates determined from geologic
(1–0.5 m.y.; this study), paleoseismic, and geodetic studies over different time
intervals. The natural evolution of complex fault zones, cross faults, block rotation,
and interactions within their broad damage zones might explain all the documented
and implied temporal and spatial variation in slip rates. Co-variation of slip rates
among the San Jacinto, San Felipe, and San Andreas faults, while possible, is not
required by the available data.
Together the San Jacinto and San Felipe fault zones have accommodated
~25.5 mm/yr since their inception in early Pleistocene time, and were therefore slightly
faster than the southern San Andreas fault during the same time interval. If the
westward transfer of plate motion continues in southern California, the southern San
Andreas fault in the Salton Trough may change from being the main plate boundary
fault to defi ning the eastern margin of the growing Sierra Nevada microplate, as
implied by other workers.
  • High geologic slip rate,
  • Pleistocene initiation,
  • San andreas fault
Publication Date
January 1, 2010
Citation Information
Janecke, S.U., Dorsey, R.J., Forand, D., Steely, A.N., Kirby, S.M., Lutz, A.T., Housen, B.A., Belgarde, B., Langenheim, V.E., and Rittenour, T.M, 2010, High Geologic Slip Rates since Early Pleistocene Initiation of the San Jacinto and San Felipe Fault Zones in the San Andreas Fault System: Southern California, USA: Geological Society of America Special Paper 475, 48 p., doi: 10.1130/2010.2475. isbn 9780813724751