Cholame and Carrizo SAF field trip stops

Suggestions from Ramon Arrowsmith, Lisa Grant, and Dallas Rhodes.

Introduction to the Cholame and Carrizo segments of the SAF

In order to determine the size and extent of past ruptures along the south-central San Andreas Fault (SAF) and formulate and test hypotheses of fault segmentation, we need more information about the behavior of the Cholame segment of the SAF. Paleoseismic data will permit us to infer the timing of previous earthquakes, and by correlation with events inferred for the Carrizo and Mojave segments we may determine rupture extent in events along the SAF and whether or not the ruptures have characteristic extent and slip distributions. As the 1994 Working Group pointed out, the gaps in data along the Cholame and northwestern Carrizo segments make interpretation of slip per event and recurrence intervals difficult. There are large gaps in paleoseismic sites along the south-central SAF. The Watertank site is about 3 km NW of Highway 46 [Sims, 1987]. The next paleoseismic sites are on the Carrizo Plain at Phelan Creeks [Sims, 1994; Sims et al., 1989], Phelan Fan [Grant and Sieh, 1993], and the Bidart site [Grant and Sieh, 1994], about 70 km SE of Highway 46. Within those ~73 km, there are several sites for paleoseismic investigations.

Suggested field trip stops

The two sites at which we have spent the most time in reconnaisance are the Bitterwater Canyon and Still Lake sites. We have not excavated at either. Note that you can see the location of these sites and other features along the SAF in this area in the figure at: http://www.public.asu.edu/~arrows/cpgeo.gif.

Bitterwater Canyon--Top choice

The Bitterwater Canyon site is located on a fluvial terrace in a wide portion of the typical gorge-like Bitterwater Canyon (see http://www.public.asu.edu/~arrows/stereos.gif for stereo airphotos of this site and Still Lake). The deposits there are interfingering fluvial deposits from Bitterwater Creek and alluvial deposits from the drainage to the NE. The aerial photographs and our field mapping indicate that the SAF formed a 0.5 to 1 m high moletrack along here in 1857 (see http://www.public.asu.edu/~arrows/BitterDetail.html for a detailed look at the site). Portions of the mole track have been buried by the alluvial material. If previous ruptures were similar, the distortion and vertical displacement of the ground surface may be preserved by the burial from the fluvial and alluvial fan materials. We expect that the relatively low relief surface may permit the growth of grasses and peats that should have been buried by the relatively rapid sedimentation. The deposition of alluvial material should be rapid enough to minimize the bioturbation that would probably be seen on other portions of the terrace surface.

Still Lake--second Cholame choice

The Still Lake site is a depression within a right step along the SAF (see http://www.public.asu.edu/~arrows/stereos.gif for stereo airphotos). Its lowest portion contains evaporites and salt tolerant plants indicating that it is a sag pond. John Sims and colleagues surveyed the site as a potential paleoseismic candidate. Arrowsmith and Rhodes reconnoitered it in November 1995, and produced a map of the principal landforms and fault traces using Sims' topographic data (see http://www.public.asu.edu/~arrows/StillDetail.html for a detailed look at the site). We also mapped a 2 km reach of the SAF centered on Still Lake in summer 1997.

Carrizo Plain site

We don't know what Charlie Rubin has in mind for the Carrizo Plain, but one site that we have considered in the past is in the Northern Elkhorn Hills (note its location on the overview figure and the stereo views and detail in http://www.public.asu.edu/~arrows/NEH.html). An event chronology from this site could permit us to determine the extent of events that rupture the Cholame segment and possibly the central Carrizo sites. Do ruptures only occur on the Cholame segment? Do they get much further southeast than the Central Carrizo sites [Grant and Sieh, 1993; Grant and Sieh, 1994]? Or do they rupture through the southeastern Carrizo Plain and into the Big Bend? This site probably has tens of cm of vertical offset (southwest side up) in each SAF rupture. The vertical offset is accommodating uplift of the Dragon's Back pressure ridge (this is the southeasternmost portion of the Dragon's Back; [Arrowsmith, 1995; Arrowsmith and Rhodes, 1992]). Therefore, we expect to observe the formation of a colluvial wedge with each offset event and the low relief of the site should permit the healthy growth of grasses that would be buried and preserved after each event.

Investigation of historic land surveys

In 1855 and 1856, James E. Freeman surveyed Township and Range lines in the region of the Carrizo Plain and northward. Grant and Donnellan, 1994 recovered original monuments from that survey spanning the SAF in the Carrizo Plain near Wallace Creek and determined that the 1857 slip along the SAF between those monuments was 11 ą 2.5 m. While most of the monuments have been moved, destroyed, or otherwise lost, the possibility of recovering a few more along the Cholame segment is conceivable. We have found several in the field and Lisa Grant has collected an extensive history of numerous monuments. A stop in the northern Carrizo Plain or Cholame segment could include a visit to one of these corners and a discussion of this promising dataset. Have a look at one of the corners at http://www.public.asu.edu/~arrows/corner.jpeg. In that photo, Ramon is sitting adjacent to a group of cobbles at T28SR18E section 36 NW corner. One has been carved and was probably added in the 1890s survey (?). In the center of the cobbles is a black steel cap inscribed by the Kern County surveyor in 1978.

References

Arrowsmith, J.R., Coupled tectonic deformation and geomorphic degradation along the San Andreas Fault system, PhD. Dissertation, 347 p. thesis, Stanford University, 1995.
Arrowsmith, R., and D.D. Rhodes, Kinematics, structure, and degradation of pressure ridges along the San Andreas fault in the Carrizo Plain, San Luis Obispo County, California, Geological Society of America Abstracts with Programs, 24 (5), 3, 1992.
1994 Working group on the probabilities or future large earthquakes in Southern California, Seismic hazards in Southern California: probable earthquakes, 1994 to 2024., Bulletin of the seismological society of America, 85 (2), 379-439, 1995.
Grant, L.B., and A. Donnellan, 1855 and 1991 surveys of the San Andreas Fault; implications for fault mechanics, Bulletin of the Seismological Society of America, 84 (2), 241-246, 1994.
Grant, L.B., and K.E. Sieh, Stratigraphic evidence for several meters of dextral slip on the San Andreas fault during the 1857 earthquake in the Carrizo Plain, Bulletin of the Seismological Society of America, 83, 619-635, 1993.
Grant, L.B., and K.E. Sieh, Paleoseismic evidence of clustered earthquakes on the San Andreas Fault in the Carrizo Plain, California, Journal of Geophysical Research, 99, 6819-6841, 1994.
Lienkaemper, J.J., and T.A. Sturm, Reconstruction of a channel offset in 1857(?) by the San Andreas Fault near Cholame, California, Bulletin of the Seismological Society of America, 79 (3), 901-909, 1989.
Schwartz, D.P., and K.J. Coppersmith, Fault behavior and characteristic earthquakes; examples from the Wasatch and San Andreas fault zones, Journal of geophysical research, 89, 5681-5698, 1984.
Sieh, K.E., Central California foreshocks of the great 1857 earthquake, Bulletin of the Seismological Society of America, 68, 1731-1749, 1978a.
Sieh, K.E., Slip along the San Andreas fault associated with the great 1957 earthquake, Bulletin of the Seismological Society of America, 68, 1421-1448, 1978b.
Sieh, K.E., and R.H. Jahns, Holocene activity of the San Andreas Fault at Wallace Creek, California, Geological Society of America Bulletin, 95, 883-896, 1984.
Sims, J.D., Late Holocene slip rate along the San Andreas Fault near Cholame, California, in Geological Society of America Abstracts with Programs, pp. 451, 1987.
Sims, J.D., Stream channel offset and abandonment and a 200-year average recurrence interval of earthquakes on the San Andreas fault at Phelan Creeks, Carrizo Plain, California, in Proceedings of the workshop on paleoseismology, US Geological Survey Open-File Report 94-568, edited by C.S. Prentice, D.P. Schwartz, and R.S. Yeats, pp. 170-172, 1994.
Sims, J.D., T. Ito, J.C. Hamilton, and D.B. Meier, Late Holocene record of earthquakes and slip along the San Andreas fault in excavations on the Carrizo Plain, Central California, EOS, Transactions of the American Geophysical Union, 70, 1349, 1989.
The Working Group on California Earthquake Probabilities, (WGCEP), Probabilities of large earthquakes occurring in California along the San Andreas fault; USGS Open File Report 88-398, 62 pp., 1988.