A NEW STRUCTURAL MODEL OF FLUID UPWELLING IN AN ACTIVE TRANSPRESSIONAL FOLD HINGE ALONG THE STRIKE-SLIP BARTLETT SPRINGS FAULT ZONE, NORTHERN CALIFORNIA COAST RANGES

The structural control of fluid pathways on strike-slip faults is an important influence on fault strength evolution and natural resources such as geothermal, yet most of the detailed structural work describing zones of fluid upwelling have focused on extensional fault settings, limiting application of such information in strike-slip systems. Here I present a new structural model for fluid transport in a zone of active transpression along the strike-slip Bartlett Springs fault (BSF) in the northern California Coast ranges. Fluid geochemistry results suggest only limited fluid circulation occurs on the BSF and that the primary conduit for deeper fluid upwelling is located 7.5 km east of the fault in the hinge of the active Wilbur Springs antiform. Previous Quaternary and bedrock fault mapping show no clear structural control on hot spring locations within the BSF zone and may require future mapping efforts. Stress inversions, fold geometry, and uplifted and offset Quaternary stream terrace deposits suggest the Wilbur Springs antiform is experiencing active transpression and uplift. A network of steeply dipping, oblique normal faults, within the hinge of the antiform are oriented at a high angle to the strike of the BSF and collocated with the highest temperature hot springs. A static stress fault model reveals these faults have elevated dilational tendencies suggesting they are statically propped open, and their three-dimensional interconnected geometries allow fluid flow from depth. This work provides a new structural model of fluid circulation along active transpressional strike-slip fault zones.