HR: 0800h
AN: S31A-11    [Abstracts]
TI: Tectonic Geomorphology of an Active Low-Angle Normal Fault, Sierra El Mayor, Northern Baja California
AU: * Fletcher, J M
EM: jfletche@cicese.mx
AF: CICESE, Dept of Geologia PO Box 434843, San Diego, CA 912143, United States
AU: Spelz, R
EM: rspelz@cicese.mx
AF: CICESE, Dept of Geologia PO Box 434843, San Diego, CA 912143, United States
AB: Low angle normal faults (LANF) are ubiquitously distributed throughout the northern Gulf of California. They commonly bound uplifted mountain ranges and are found in numerous seismic sections in the Altar Desert and Wagner Basin (A. Martin, unpublished data). The Canada David detachment (CDD) is a spectacular example of an active LANF that controls the western mountain front of Sierra El Mayor over a strike length of 60 Km. Like most LANFs, the CDD contains two prominent antiform-synform megamullion pairs that strongly control the tectonic geomorphology of the uplifted footwall block and alluvial terraces along the range flank. Quantitative morphometric analysis along the mountain front shows that drainage basins in antiformal domains have systematically higher outlet elevations, higher gradients, greater relief, and much greater hypsometric integrals. Additionally river valleys are narrower and dominated by bedrock channels that extend nearly to the outlet, which is consistent with the fact that mountain front sinuosity is almost an order of magnitude less in the antiformal domains. A sequence of as many as 8 different regional strath terraces are preserved along the range flank and reconnaissance dating of the deposits by cosmogenic isotopes suggests that they formed during the major interglacial-to-glacial climatic transitions. Strath terraces are generally much older, and relative heights between terraces is significantly lower in synformal domains. All of these geomorphologic characteristics suggest that the synformal domains have experienced much lower rates of uplift and erosion of the footwall and likewise lower rates of sedimentation in the adjacent hanging wall basin. The lack of slip gradients on the master fault between synformal and antiformal domains suggests that the megamullions formed instead by regional buckling perpendicular to the extension direction. A Quaternary scarp array extends along the entire length of the mountain front and also shows remarkable variations with megamullions. In antiformal domains, Quaternary scarps generally lie within 100 m of the trace of the CDD. The array is generally wider and contains numerous antithetic scarps that accommodate high ratios of horizontal to vertical deformation. In contrast, the scarp array in synformal domains is well removed (3.5-10 km) from the recessed trace of the CDD and is characterized by a narrow band of synthetic scarps. Our data are consistent with a basinward migration of deformation as envisioned in the "rolling hinge" model of detachment faulting, but we conclude that such a migration occurs predominantly in synformal domains as the fault adopts a straighter configuration.
DE: 8010 Fractures and faults
DE: 8155 Plate motions: general (3040)
DE: 8164 Stresses: crust and lithosphere
DE: 8175 Tectonics and landscape evolution
DE: 8177 Tectonics and climatic interactions
SC: Seismology [S]
MN: 2007 Joint Assembly