Paper No. 8
Presentation Time: 10:15 AM
LAYERED FAULT ROCKS ALONG THE WEST SALTON DETACHMENT FAULT (WSDF), SOUTHERN CALIFORNIA: A RECORD OF DOZENS OF PALEOSEISMIC CYCLES
Dozens of cataclasite layers (CLs) in the upper footwall of the late Miocene(?)-Pleistocene WSDF in the W fork of Nolina Canyon are interpreted to record dozens of earthquake cycles. The CL body is ≤5 m thick, >50 m long, and occurs where footwall splays join the WSDF, similar to a multilayer pseudotachylyte body found below the WSDF in Nude Wash. The body is folded into an open antiform cut by an axis-parallel gully. CLs are a few mm to ~20 cm thick, up to several m long, and are separated by thin (1-4 mm), recessive ultracataclasite layers that record shear between layers. The top ~30-50 cm of CLs are parallel to the sharp, continuous upper contact, which is ornamented with ultracataclasite. Many CLs contain clasts of similar ultracataclasite. Overlying granodiorite is strongly cataclastically foliated near the contact, with shear strain decreasing upward over ~1-3 m (into breccia), as shown by foliation intensity and orientation and by thinned, drag-folded, and brecciated aplite dikes. CLs were derived from the basal granodiorite, slice by slice, by listric upper-plate primary Reidel shears that flatten into (or a few cm above) the top contact. The pre-existing foliation is destroyed in the upper CLs, suggesting rapid (co-seismic), randomizing cataclasis, but many layers display a weak brittle foliation that we think formed subsequently during slow (interseismic) shear and compaction. Structurally downward, CLs are increasingly contorted by sharp to gradational (cataclastic flow), normal-separation, domino-style or listric faults, many of which flatten up and/or down between CLs, and by open to isoclinal folds. Thus, deeper layers are older. The base of the CLs is a sharp fault against underlying granodiorite but this fault is cut by small steeper faults, indicating abandonment while the top contact continued to slip. Cataclasite injection veins also occur: one has finer grained margins consistent with cataclastic-flow sorting during injection; another includes contorted pre-existing layering, indicating some cohesion before injection. Sub- to euhedral calcic zeolite in pores is the only cement, but bulk composition is not changed from the protolith, indicating that fluid interactions were minor and local, and that mechanical processes dominated CL formation. No evidence for glass or melt was found.