Partitioning of slip between the southern San Andreas (SAF) and San Jacinto
(SJF) faults is significant for understanding earthquake behavior, seismic hazards,
fault zone evolution, and crustal interactions associated with ongoing reorganization
of the plate boundary. However, long-term (>10 kyr) slip rates on these faults
are poorly known due to limited data and difficulty of estimating ages of alluvial
surfaces using soil chronology. Even at sites where surface ages are well known,
erosion and deformation of Pleistocene alluvial features can complicate the
calculation of fault offset and slip rates. This problem has influenced previous
interpretations of an offset late Pleistocene alluvial fan at Biskra Palms,
Coachella Valley segment of the SAF. Strike-slip offset of the fan was estimated
at ~700 m by Keller et al. (1982). Van der Woerd et al. (2001) obtained an exposure
age of 30.1 +/- 2.4 ka from Be and Al isotopes, and used the 700 m offset to
calculate a slip rate of 23.3 +/- 3.5 mm/yr. This result is suspect because
the original measurement of offset assumes a linear fan margin oriented perpendicular
to two strike-slip fault strands, but low-sun-angle aerial photography shows
that the original fan margin is curved and trends at an oblique angle to the
faults. A detailed reconstruction of the fan margin indicates total offset of
~450 +/- 40 m, and the revised slip rate is ~15 +/- 3 mm/y, significantly slower
than previous estimates. This is a maximum rate because (1) in the absence of
inheritance complications, exposure ages provide a minimum estimate of surface
age, and (2) cross-fault shortening may have produced some apparent right-lateral
separation. A rate of 15 +/- 3 mm/yr at Biskra Palms is consistent with measured
rates of Pleistocene shortening in San Gorgonio Pass and San Bernadino Mts,
and 15-20 mm/yr slip rate on the SJF, as presented in other studies. Modern
seismicity and tectonic geomorphology provide further evidence that slip is
transferred southward from the SAF to the SJF across the San Bernadino basin,
a large transtensional basin bounded on the SE by active normal faults. These
observations support a model in which the SJF was initiated at ~1.5 Ma in response
to crustal shortening in the San Gorgonio restraining bend (Morton and Matti,
1993), but large gaps in existing data highlight the need for new work in this
region.
Pleistocene sedimentary rocks in the Borrego Badlands (BB) of southern California
were deposited between the Clark (CF) and Coyote Creek (CCF) strands of the
San Jacinto fault zone (SJFZ) during fault-zone evolution (Bartholomew, 1970;
Pettinga, 1991). The Ocotillo Formation overlies the Borrego Formation along
an abrupt but conformable contact estimated to be ~1.1 to 1.2 Ma (Remeika and
Beske-Diehl, 1996; this study), and has a composite thickness of ~630 m. The
Ocotillo Fm includes the 759-ka Bishop ash ~200 m above its base, and is overlain
along a low-angle unconformity by the ~350- to 450-ka Fonts Point Sandstone
(Ryter, 2002).
Two conformable members are recognized in the Ocotillo Fm. The lower member
contains two sub-members that fine upward from sandy conglomerate to siltstone,
and show lateral increase in thickness and grain size to the SW (sub-mbr 1)
and NE (sub-mbr 2). Coarse-grained facies represent distal alluvial fans and
desert washes that prograded from the NE and SW margins of the basin, passing
laterally into lacustrine siltstone and claystone in the central BB. The upper
member coarsens eastward from pebbly sandstone to cobble-boulder conglomerate
(debris-flow facies) derived from the Santa Rosa Mountains NE of the CF. Clast
counts record appearance of Pliocene Diablo Formation clasts in the western
BB (lower sub-mbr 1), and an up-section increase in mylonite, marble, and Diablo
clasts in the eastern BB (lower sub-mbr 2 and upper mbr). Clast imbrications
indicate transport toward the central BB from the NE and SW basin margins, with
some evidence for axial transport to the S and SE.
The data are consistent with a model for growth of a pull-apart basin due to
transfer of slip from the CF to the SE CCF. Elevated topography NE and SW of
the basin was produced by oblique slip on the CF and CCF, accompanied by slight
tilting toward the faulted basin margins. Progressive clockwise rotation of
the basin away from Coyote Mountain during deposition of the Ocotillo Fm (Scheuing,
1991) is compatible with a pull-apart basin geometry and will be tested with
new paleomagnetic data. Abrupt progradation of upper-member coarse detritus
into a lacustrine depocenter (represented by the lower mbr) records reorganization
of the SJFZ at ~600 to 700 ka and appears to coincide with initiation of the
NW CCF (Dorsey, 2002).
Published in: Barth, A. (ed.), 2002, Contributions to Crustal Evolution of the Southwest United States: Boulder, Co. GSA Special Paper 365, p. 251-269.
The Coyote Creek fault is a major strand of the San Jacinto fault zone (SJFZ) in southern California. Pleistocene sediments and sedimentary rocks exposed in the lower Coyote Creek area preserve a record of surface deformation, stream reorganization, and erosion that resulted from initiation and slip on the Coyote Badlands strand of the Coyote Creek fault. A well exposed section of conglomerate and sandstone contains the 760-ka Bishop Ash and reveals: (1) complete reversal of paleocurrents from northwest-directed (opposed to modern drainages) to southeast-directed (consistent with modern drainages); (2) fanning dips and a progressive unconformity bounded by the Coyote Creek and Box Canyon faults; (3) a thick gravel unit (Qg) that caps the fanning-dip section and accumulated between about 700 and 600 ka; and (4) post-Qg offset and deep erosion of the entire section. The fanning dips and reversal of paleocurrents are interpreted to record initiation of the Box Canyon and Coyote Creek faults by dip-slip (normal) displacement beginning at 750 ka. Strike-slip offset of Qg is equal to total offset on the Coyote Creek fault in the study area (about 6 km), indicating that strike-slip motion on the fault began after deposition of Qg, after ~600±100 ka. This gives a time-averaged slip rate in Coyote Creek of approximately 10 mm/yr. Two alternative models for Pleistocene fault evolution are considered: (1) prior to ~600 ka, the Clark and southern Coyote Creek faults were connected via a releasing bend that produced a pull-apart basin in the Borrego Badlands, and initiation of the Coyote Badlands strand at 600 ka represents northwestward propagation of the Coyote Creek fault; or (2) the Coyote Creek fault was initiated along most or all of its length at ~600 ka, establishing the modern link to plate-boundary faults in the Imperial Valley. Existing data are equivocal on this question.
(manuscript in preparation)
Present-day geomorphology and Quaternary stratigraphy in the Peninsular Ranges
portion of the San Jacinto fault zone provide a record of topographic disequilibrium
and ongoing adjustments related to strike-slip faulting. Surficial processes
include headward erosion by knick-point retreat in crystalline bedrock, stream
captures that are propagating northwest along a complex fault valley, rapid
dissection of Pleistocene sediments, and generation of large landslides. Deeply
eroded Pleistocene sediments include sandy conglomerate, arkosic sandstone,
pebbly sandstone and mudstone that display channeling, cross-bedding, fining-up
intervals, paleosols, and overall west-directed paleocurrents. The age of these
sediments is constrained by the 760-ka Bishop Ash in two locations. Reconstruction
of paleogeography and major faults shows that Pleistocene sediments accumulated
in a low-gradient stream system that was sourced in the high San Jacinto and
Santa Rosa Mountains and flowed west across deeply weathered (saprolitic) crystalline
rocks toward the Pacific coast. Cahuilla Creek in Anza Valley is a modern remnant
of this west-flowing stream network that is about to be captured by streams
that drain to the southeast and northwest along the fault zone. Initiation of
the San Jacinto fault in late Pliocene or early Pleistocene time is interpreted
to have breached a formerly continuous regional drainage divide and created
a steep new fault valley that now connects the high Peninsular Ranges to the
Salton Trough. This produced a base-level drop of ~1000 m and initiated a wave
of propagating stream captures that are still active today.
Channel profiles of tributaries to the main fault valley reveal systematic along-strike
variations from (1) strongly convex-up profiles in the northwest to (2) a large
(ca. 15 km2) landslide complex in the central zone and (3) concave-up profiles
in the southeast, with concavity decreasing to the southeast. A time-for-space
interpretation of the channel profiles supports a conceptual model for landscape
evolution in which (1) initial headward erosion produces slope oversteepening
and instability, (2) large deep-seated landslides reduce potential energy and
set a template for the future drainage network, and (3) post-landslide erosion
reduces topographic irregularities and channels evolve toward a graded stream
network. This model is similar to one proposed by Hovius et al. (1998) for pre-steady-state
evolution of young mountain belts, except that bedrock incision due to base-level
fall plays a central role in the generation of landslides. This process may
be common in seismically active areas where the cumulative effects of horizontal
and vertical fault displacements can produce profound slope instabilities.
(2001 AGU Fall Meeting Abstract)
Recent study of Pleistocene stratigraphy and geomorphology in the San Jacinto fault zone (SJFZ) provides new constraints on the timing of fault initiation and long-term slip rates. The Clark fault (CF) and Coyote Creek fault (CCF) are the main strands in the central fault zone. In the Coyote Badlands 240 m of alluvial gravel (Qg) overlies the Thermal Canyon Ash (740 ka), and the top of Qg is estimated to be 500-700 ka. Offset of Qg is equal to total offset on the CCF (~6 km), giving a slip rate of about 10 mm/yr since inception at ~600 ka. The CF is sub-parallel to the CCF for 30 km and reveals abundant fresh scarps and evidence for active deformation, suggesting that slip on the CF is at least as fast as the CCF. Age of initiation of the CF is ~1.5 Ma based on the geomorphology of propagating stream captures and the age of gravel (Ocotillo Cgl; 1.2 Ma at its base) which prograded into the Salton Trough from a large fault valley and catchment system. Offset on the CF is 14-15 km and average slip rate is ~10 mm/yr. These data support a new model for evolution of the SJFZ: (1) slip on the CF from about 1.5 to 0.6 Ma at 10 mm/yr; and (2) initiation of the CCF at ~600 ka and acceleration of the whole SJFZ to 15-20 mm/yr, continuing to the present.
This slip history has significant implications for the tectonic evolution of the San Andreas fault system (SAF). I infer that initiation of the CF at ~1.5 Ma resulted from oblique collision between the high San Jacinto Mts and a pre-existing thrust belt in the southern San Bernadino Mts (SBM). Rapid thrusting, uplift and gravel deposition in the northern SBM occurred at about the same time (Meisling and Weldon, 1989) in response to this event. Oblique collision caused slowing of SAF slip through San Gorgonio Pass and partial transfer of slip to the SJFZ on the west and the eastern California shear zone on the east. The CF terminates in the Salton Trough and does not link to other faults to the SE; thus the SJFZ was not established as a major plate-boundary structure until initiation of the CCF at ~600 ka produced the modern connection to faults in the Imperial Valley. Although previous models minimize the role of the SJFZ in the regional slip budget, available geologic data support the model proposed here. It appears that more data are needed to resolve the competing models.
Return to San Jacinto Fault Zone