NEW INSIGHTS FROM THE 2019 RIDGECREST EARTHQUAKES INTO THE EVOLVING ACCOMMODATION OF TRANSVERSE DEXTRAL-SLIP INTO THE THROUGH-GOING SINISTRAL-SLIP GARLOCK FAULT ZONE

Frank Monastero’s research, operations, and vision for the Geothermal Program Office has contributed to a great expansion of fundamental geologic knowledge in the region surrounding Coso. The research presented here joins two of Frank’s scientific passions, the sinistral Garlock fault zone (GFZ) and the dextral faults that run through the Coso area.

The earthquakes of July 2019 led to creation of >60 km of fault scarps along the NNW-striking, dextral-slip Airport Lake fault (ALF) and continued southward along previously unknown segments through the eastern Spangler Hills. The numerous earthquake foci continue all the way to and stop abruptly at the GFZ. We have mapped previously unknown segments of the southern ALF all the way to the GFZ. The Spangler Hills are Jurassic plutonic rocks in which a set of distinctive felsic dikes strike NE and are offset ~1200 m by ALF strands. Farther south, near the Garlock fault, continuations of the ALF offset Pliocene rocks a similar amount.

The western Spangler Hills is cut by the Cerro Coso fault (CCF). The CCF offsets a Late Cretaceous(?), north-striking dextral ductile shear zone in Jurassic plutonic rocks. The main splay of the CCF has 7.4 km of dextral slip and a western splay has 1.9 km. This fault zone continues northward across Indian Wells Valley cutting Pleistocene(?) alluvium to join the Sierra Nevada frontal fault and Little Lake fault zones. A zone of previously mapped fault scarps and the 2019 scarps and earthquake foci show a east-northeast striking zone of left-lateral faulting that connects the CCF to the ALF.

The two dextral faults at either end of the Spangler Hills display very different behaviors as they close on the Garlock fault. A NNW-striking dextral fault that intersects from the north to a through-going WSW-striking sinistral fault (GFZ) should produce a compression regime in the NE corner of the intersection and a tension regime in the NW corner. The ALF intersection displays this expected behavior but the CCF displays the opposite state. We hypothesize that the CCF is an older and more evolved fault that is now being pirated by the sinistral faults along the Spangler Hills. The sinistral faults are interpreted to be connecting up to the GFZ and therefore absorbing a component of the sinistral slip of the GFZ, just as the Owl Lake fault does farther east along the Garlock fault.