Fumarolic Pathways Were Structurally Controlled by a Strike-Slip Fault System Beneath the Bishop Tuff, Bishop, California

Abstract

The Volcanic Tableland, a plateau at the northern end of Owens Valley, CA, is capped by the rhyolitic Bishop Tuff. It hosts many tectonic and volcanic landforms, including hundreds of fault scarps, large joint sets, and inactive fumarolic mounds and ridges. The 1986 Chalfant Valley earthquake sequence shed light on a blind strike-slip fault system beneath the Bishop Tuff. The spatial relationships of the volcanic and tectonic structures have previously been well documented, however, the mechanisms of formation of structures and their enhancement as fumarolic pathways remain largely unexplored. We collected fault kinematic indicators, joint orientations, and documented fumarolic alterations of microcrystalline quartz in the Bishop Tuff and combined those field observations with fault response modeling to assess whether strike-slip activity played a key role in the development of fumarolic pathways. We found field evidence of dip-slip and strike-slip faulting that are consistent with the overall transtensional regional tectonics. Our modeling indicates that a blind strike-slip fault system would dilate joints in the overlying Bishop Tuff with preferred orientations that match observed orientations of joints along which fumarolic activity occurred. Our results imply that the localization of fumaroles was tectonically controlled and that fault activity in the valley floor likely initiated prior to tuff emplacement.