DYNAMICS OF FAULT SLIP AT THE INTERFACE OF THE GARLOCK FAULT WITH THE WALKER LANE BELT, SPANGLER HILLS, CALIFORNIA

The sinistral Garlock fault zone is a wide region of active deformation that accommodates transversely oriented dextral slip of the Walker Lane belt. The 2019 Ridgecrest earthquakes ruptured mostly unstudied faults within the zone of interaction. We present geologic mapping and fieldwork data of offset markers for numerous faults to examine the slip history and to interpret the mechanisms of accommodation of dextral slip into the Garlock fault.

We evaluated the total offset on the Airport Lake fault (ALF) in the Spangler Hills using Jurassic and Pliocene rocks and Pleistocene sediments. The splays of the NW-striking ALF have dextral slip of 200, 100, 150, and 600 m, going from east to west. The eastern splays have fault scarps related to the 2019 earthquakes. These splays can be followed southward to the Garlock fault, where they offset NW-dipping Pliocene rocks. The trace of the ALF across a wide valley of intervening alluvium coincides with a northward deflection of Pleistocene lake high-stand shorelines indicating uplift of the east side of the ALF. The calculated offset of the Pliocene rocks is comparable to the offset of Jurassic rocks in the Spangler Hills.

A SW-striking zone of sinistral slip occurs north of the Spangler Hills and west of the ALF. These faults have larger measured amounts of total offset of Jurassic features than the ALF, of 500 to 700 m each, for a total of >3000 m. These also cut Pliocene to Pleistocene sediments and the northern ones have fault scarps from the 2019 earthquakes. The older, inactive sinistral strands are offset by northwest striking dextral faults.

The interpreted history shows an eastward progression of dextral faulting. There is a northward migration of activity on the sinistral faults with the older segments being offset by the older dextral faults. This progression indicates that the sinistral faults are the dominant component of accommodation of the incoming dextral slip and should be reexamined for seismic hazards analysis.