Project Abstract
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We are investigating the interaction of the SE tip of the large and very active Clark strand of the San Jacinto fault and the crossing left-lateral faults SE of its tip at the surface (Fig. 1).
Janecke hypothesized that the Clark fault does not terminate NW of the crossing fault arrays of the Extra and Elmore Ranch fault arrays (Fig. 2), but instead continues at depth beneath the crossing faults, has a complex mutually interfering relationship at the surface and/or produces enough distributed strain to rotate the crossing faults in a clockwise sense. Ten weeks of fieldwork and ongoing, in-depth analysis of map, structural and published geophysical datasets, supports our hypotheses. Although the Extra and Elmore Ranch fault arrays are more active, preserve many Holocene scarps and are more continuous than dextral faults at the surface, there is also strong evidence for a subsurface connection between the Clark fault zone and the Superstition Hills fault in diverse data sets. The most impressive surface manifestation of the Clark fault SE of the San Felipe Hills is the 40° to 60° reorientation of active left-lateral and left-normal faults from northerly strikes to ENE strikes within a 12-15 km wide domain above the buried trace of the Clark fault zone. A 70 km long by 15 km wide uplift of young basin fill is centered on the Clark-Superstitions Hills fault zone, and has so much activity and relief that it traps Holocene sediment on its upstream side. Aligned small earthquakes, InSAR data, and a large step in the basement-cover contact further delineate the Clark fault at depth. |
