Introduction
Hazard assessment and stress evolution of the southern San Andreas fault depend critically on a clear understanding of the structural complexity and the paleoseismicity of the San Gorgonio Pass region. The structural complexity in the region results from a 15-km left step of the San Andreas fault zone (Matti and Morton, 1993; Seeber and Armbruster, 1994), and has led to speculation that 1) propagation of ruptures might be impeded through this region or that 2) fault ruptures through the Pass might be unusually complex. Whether or not earthquakes rupture through the Pass has obvious implications with respect to magnitude scenarios and event frequency. The chief objectives of this project are to identify and characterize the active structural features, to establish a paleoseismic record of faults, and to assess the likelihood of through going rupture. Our paleoseismic work in 1998 suggests that faults in the Pass slip about as frequently as the strike-slip segments of the fault to the northwest and southeast of the Pass. These are, therefore, the first substantial data to support the hypothesis that ruptures of the San Andreas might commonly extend through the Pass, rather than terminate on either side.
Active Structural Features
Figure 1 shows the active structures recognized in San Gorgonio Pass. Rather than the narrow, discrete fault zone typical of the San Andreas fault elsewhere, these structures define a 5-7 km wide zone of distributed dextral shear on northwest-striking, left- and right-stepping strike-slip faults; northeast-striking, oblique-slip thrust faults; and north-striking normal faults. In addition, the northern margin of Burro Flats is bounded by a series of northwest-striking normal faults. Rough, seat-of-the-pants slip-rate estimates for this myriad of structures total 9-16 + 7 mm/yr of NW-SE directed strike-slip (see our 1996 Annual Report). The structure which appears to carry the greatest slip through the Pass is the San Gorgonio Pass fault near Cabezon, the site of our 1997 paleoseismic excavation (Figure 1).
Paleoseismic work at Cabezon
Trenches excavated across the San Gorgonio Pass fault near Cabezon (Figure 2) show evidence for late Holocene rupture events (Figure 3), a minimum uplift rate of ~2 mm/yr, and a minimum strike-slip rate of 5.7 + 0.8 mm/yr, directed parallel to N45W-S45E and accounting for about one third of the San Andreas slip in the Pass. Theses rates are average values for the last 1600-1860 years. The exposed faults are oriented N45W, 40-45° NE at the dextral, strike-slip site, and E-W, 25-30° N at the right-oblique thrust site, 400 meters to the northwest and depicted in Figure 2 and 3. These orientations support the interpretation that the active faults in this part of the Pass consist of left-stepping strike-slip faults connected by thrust faults (Figure 1, and Yule and Sieh, 1996).
The trenches show stratigraphic and structural evidence for a minimum of two distinct rupture events at both trench locations. Charcoal fragments are abundant from both sites. Charcoal from the trenches constrain the two most recent events to have occurred post-AD 1305-1430 and pre-AD 1655-1950 (Figure 3, 2s calendar C14 ages). Because no historic record exists for a large earthquake in San Gorgonio Pass, the two events occurred most likely prior to ~AD 1775. This equates to a maximum average recurrence of about 235 years between the two most recent events. No clear evidence exists for pre-AD 1305 events. However, older events seem likely given the 235-year recurrence for post-AD 1305 events, at least 3300 years of stratigraphic record in the trench walls, an observed increase in the vertical separation of strata with depth, fault slip which ends at the event I and II horizons, and increased fold amplitude with depth (Figure 3).
Implications of the Cabezon site paleoseismic record
A maximum average recurrence of 235 years for the last 700 years at Cabezon suggests the possibility of large San Andreas ruptures including the faults in San Gorgonio Pass. In fact, the two most recent events at Cabezon appear to have occurred during the same time period as the two most recent prehistoric events at sites on either side of the Pass, to the northwest at Pallett Creek and Wrightwood (Sieh, and others, 1989; Fumal, and others, 1993) and to the southeast at Indio (Sieh, 1996). However, the clear evidence of reworked alluvial sediments and detrital charcoal at the Cabezon site makes any correlation of these events speculative. Thus, it is critical to excavate a site where the stratigraphy will permit a more precise age determination of rupture events.
For this reason, we initiated a reconnaissance study of the Burro Flats (Figure 1) to examine the feasibility of conducting a detailed paleoseismic study here. The Burro Flats site is situated at the western end of an intramontane valley, at the western end of the structurally complex San Gorgonio Pass region. Our preliminary findings are very promising and are discussed in the following 1998 Proposal.
References
Allen, C.R., 1957, San Andreas fault zone in San Gorgonio Pass, southern California. Geol.
Soc. Am. Bull.., 69: 1101-1120.
Fumal, T.E., Pezzopane, S.K., Weldon, R.J., and Schwartz, D.P., 1993, A 100-year average
recurrence interval for the San Andreas Fault at Wrightwood, California, Science, 259, 199-203.
Matti, J.C., and Morton, D.M., 1993, Paleogeographic evolution of the San Andreas fault in
southern California: A reconstruction based on a new cross-fault correlation, in The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution, Powell, R.E., Weldon, R.J., and Matti, J.C., eds., Geol. Soc. Am. Memoir 178, 107-159.
Matti, J. C., Morton, D.M., unpublished mapping, Cabezon and Whitewater 7.5' quadrangles,
scale 1:24,000.
Seeber, L., and Armbruster, J.G., 1994, The San Andreas fault system through the Transverse
Ranges as illuminated by earthquakes. Jour. Geophys. Res., 100: 8285-8310.
Sieh, K., Stuvier, M., and Brillinger, D., 1989, A more precise chronology of earthquakes
produced by the San Andreas fault in southern California, Jour. Geophys. Res., 94: 603-623.
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Yule, D., and Sieh, K., 1996, The surface geometry and a kinematic model for the (neotectonic)
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Manuscripts in preparation
Yule, D., and Sieh, K., Active tectonic features of the San Andreas fault system in San Gorgonio
Pass, to be submitted to Tectonics.
Yule, D., Sieh, K., and Liu, J., Understanding the San Gorgonio "knot": Late Holocene
paleoseismicity, slip rates, and implications for through-going San Andreas rupture events, to be submitted to Bull. Seis. Soc. Am.
