QUATERNARY FAULTING OF THE AVAWATZ MOUNTAIN PIEDMONT EAST OF THE INTERSECTION OF THE GARLOCK AND SOUTHERN DEATH VALLEY FAULT ZONES: EVIDENCE FOR A TAPERED-WEDGE MODEL

The Avawatz Mountains are well-known as the eastern termination of the sinistral Garlock fault zone (GFZ), where it intersects the northwest-trending, dextral southern Death Valley fault zone (SDVFZ) on the north side of the range. East of the intersection, the SDVFZ is deflected eastward along the range front, where lateral displacement on the high-angle GFZ is converted to reverse displacement on the southwest-dipping Mule Spring fault zone. Previous work has shown that middle Pleistocene and older Quaternary units along the deflected part of the fault zone, between the intersection and the Salt Spring Hills, are cut by north-vergent thrust faults and overturned folds. New mapping presented here shows that south of the Salt Spring Hills along the Avawatz range front, where the Mule Spring fault zone strikes south-southeast, the Middle Pleistocene and older units in the upper part of the piedmont are cut by east- to northeast-vergent thrust faults and folds.

The change in kinematics and geometry of the SDVFZ suggests that the interaction between the Avawatz Mountain block and the Salt Spring Hills block plays an important role in the evolving geometry of the SDVFZ and GFZ. Field relations among faults, folds, and surficial deposits along the Avawatz Mountain front south of the Salt Spring Hills show that the interaction between the faults increases in age and development from south to north along the range front, reaching a maximum at the Salt Spring Hills. The Mule Spring fault zone is segmented and each segment corresponds to a zone of propagating thrust faults in the piedmont along the range front. The age of the surfaces cut by propagating thrusts and degree of stream incision decreases to the south along the range front. These two fault zones intersect at a shallow angle and their intersection reaches a maximum between the Salt Spring Hills and the Avawatz Mountain front, suggesting that the Salt Spring Hills are a tectonic obstacle to the northward translation of the Avawatz Mountain block by the SDVFZ. Historic seismicity suggests that the Avawatz Mountains and Salt Spring Hills form a crustal scale asperity that prevents active seismicity spreading along the SDFZ from the northwest to the southeast.