Fault Rock Analysis in the Chemenuevi-Sacramento Low-Angle Normal Fault System: Evidence of Lanf Slip Mechanisms

Jennifer Goyette
Department of Geology and Geophysics, University of Wyoming, Laramie, WY;
[email protected]

Previous studies of LANFs provide compelling field, seismic, and laboratory evidence supporting their existence in southeastern California (John & Foster, 1993), northern Mexico (Axen et al., 1999), the Alps (Manatschal, 1999), Papua New Guinea (Abers, 2001), etc. Skeptics argue that LANFs may be mega-landslide deposits, sedimentary deposits, nonconformities, high-angle normal faults that have rotated to more shallow dips, reactivated thrust faults, or that if LANFs do in fact exist, they operate aseismically (see review by Axen, 2007). Strong evidence has been presented supporting the initiation and slip of LANFs at low dip angles through field observations, geo- and thermochronometric studies (John & Foster, 1993), gravity and seismic studies, microstructural studies, numerical modeling, etc (see review by Axen, 2007). Documented veins of pseudotachylyte with subhorizontal generating surfaces (John, pers. comm.) and recorded large magnitude seismic events (Abers, 2001) provide evidence of paleoseismicity and active seismogenic slip on LANFs. Despite this evidence, the LANF controversy continues due to a poor understanding of initiation and slip mechanisms capable of such large-scale displacement at low angles of dip within the seismogenic regime.

Fault rock fabrics record slip mechanisms (Schmid, 1982, 2007, Sammis et al., 1986) and evidence of secondary geochemical processes that may have altered primary petrophysical properties masking evidence of fault initiation and slip. Fault zone permeability is influenced by these secondary diagenetic processes, which in turn, influences the fluid connectivity of hydrocarbon, groundwater, and potential CO₂ reservoirs. LANFs are recognized as fundamental structures associated with continental extension (both continental rifts and rifted margins). Identifying deformation mechanisms associated with LANFs and their compositional and structural evolution in both space and time will increase our understanding of LANF fault mechanics and fault zone permeability, allowing for more accurate predictions of fault seal.

AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid