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 of 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. 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 ~1.6 km by ALF strands. 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), which offsets a 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 approach the GFZ. A NNW-striking dextral fault that intersects from the north to a 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 that is now being pirated by the sinistral faults along the Spangler Hills. The sinistral faults connect to the GFZ and thus absorb a component of the sinistral slip of the GFZ. The ALF cuts the 19 km displacement Marine Gate fault, a synthetic splay of the Garlock fault that is now partially reactivated as a dip slip fault since ~3.3 Ma. These are critical observations to the evolution of the GFZ.