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Group 2 – SoSAFE | Poster Abstracts
reasons, SGP has come to be known as a structural knot in the SAF—a knot that plays an enigmatic
role in scenario earthquakes.
From oldest to youngest, the SAF zone in SGP consists of four discrete strands, each having its own
movement history period: (1) the Wilson Creek strand (~5 to ~4.0 Ma, ~40 km); (2) the Mission
Creek strand, the major SAF strand in terms of right-slip (~90 km) but not the site of active faulting
since ~1.0 Ma; (3) the Mill Creek strand, locus of mid to late Quaternary right-slip (~8-10 km); and
(4) the San Bernardino strand (~2-3 km), the modern trace of the SAF that approaches SGP from the
northwest via Burro Flats but loses its geologic expression before it reaches the core of SGP. Two
young SAF strands approach SGP from the Coachella Valley (CV): (a) the Banning strand and (b)
the Garnet Hill strand; both are aligned with the San Bernardino strand, but they do not meet it at
the surface. Instead, the SGP Fault zone intervenes, forming a transverse contractional system that
obscures any Holocene right-slip faults that once may have formed a throughgoing SAF. No
tectonogeomorphic evidence exists for a throughgoing earthquake rupture on the SAF in the SGP
region in the last several thousand years, despite a generally similar chronology of
paleoearthquakes in the CV and the Cajon Pass region on either side of the Pass.
Ongoing fault-history and landscape studies suggest that the longer Quaternary history of rightslip
in the SGP region may be comparable to the Holocene history—that is, slip throughout the SAF
zone may have been considerably lower in SGP relative to the CV. Especially at the mouth of
Mission Creek, slip on the combined Mill Creek and Mission Creek strands may have been quite
low, with Quaternary deposits as old as 0.5 Ma displaced no more than 8-10 km. Matti and Morton
(1993) used these data to propose that Quaternary slip on the SAF has largely stepped left across
SGP and onto the San Jacinto Fault, beginning as recently as 1.2 Ma. Of interest to SoSAFE: is the
Holocene San Gorgonio Pass knot in the SAF tying up or untying?
2-047
UNCERTAINTIES IN SLIP RATE ESTIMATES FOR THE MISSION CREEK STRAND
OF THE SOUTHERN SAN ANDREAS FAULT AT BISKRA PALMS OASIS Behr WM,
Rood DH, Fletcher KE, Guzman NE, Finkel R, Hanks TC, Hudnut KW, Kendrick KJ, Platt JP,
Sharp WD, Weldon RJ, and Yule JD
This study focuses on uncertainties in geologic slip rate estimates on the southern San Andreas
fault where it offsets an alluvial fan (T2) at Biskra Palms Oasis in southern California. We provide
new estimates of the amount of offset of the T2 fan aided by trench excavations, and new
cosmogenic 10-Be age determinations from the tops of 12 boulders on the fan surface. We present
three alternative offset models: a minimum, maximum, and preferred of 660 m, 980 m, and 770 m,
respectively; and we assign an age of 50 ± 5 ka from the 10-Be data, which is significantly older
than previously recognized, but is consistent with ages from U-series on pedogenic carbonate from
T2 described in a companion presentation by Fletcher et al. (2008). These new constraints lead to a
range of slip rates of between 12 and 22 mm/yr with a preferred estimate of 15.6 mm/yr for the
Mission Creek strand of the southern San Andreas fault. We also place these results in the context
of discrepancies between geologic and geodetic slip rates. Previous studies suggested that the
geologic and geodetic slip rates at Biskra Palms were incompatible. We find, however, that
considerable uncertainty affects both the geologic and geodetic slip rate estimates, such that if a
real discrepancy between these rates exists for the southern San Andreas fault at Biskra Palms, it
cannot be demonstrated with available data.
2008 SCEC Annual Meeting | 163