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.
Highlights
The type and involved mechanisms of fracturing in volcaniclastic sequences, such as the Bishop Tuff, depend on rock properties governed by the degree of welding of the rock [1], with densely-welded tuff accommodating brittle deformation via discrete fracturing, and poorly or non-welded tuffs frequently displaying tabular, porosity-reducing deformation bands (e.g., [2,3,4])
We observe columnar joints to form rosette structures adjoining vertical conjugate joints in cross-section (Figure 3a). These observations all serve as lines of evidence that many of these joints behaved as fumarolic pathways in the Bishop Tuff
The synthetic joints are bisected by σ1 generated from the stress inversion of strike-slip fault surfaces observed in the field (Figure 8c). These findings indicate that the synthetic joint set shows similar orientations as the set of mineralized joints observed in the field, and that joints of those orientations are compatible with the kinematic record of strike-slip faulting preserved in the Bishop Tuff
Summary
The type and involved mechanisms of fracturing in volcaniclastic sequences, such as the Bishop Tuff, depend on rock properties governed by the degree of welding of the rock [1], with densely-welded tuff accommodating brittle deformation via discrete fracturing, and poorly or non-welded tuffs frequently displaying tabular, porosity-reducing deformation bands (e.g., [2,3,4]) Both types of fractures are known to affect subsurface fluid flow, as they either form pathways for or perturb fluids. We document and analyze a fracture network consisting of several sets of joints and faults in the Bishop Tuff and study its relation to hydrothermal fluid migration associated with most recent supervolcanic eruption of the Long Valley Caldera, CA, USA. Characterizations of fracture flow in such neotectonic and active volcanic settings contribute toward forecasting potential hazards and allow tying ancient tectonic and volcanic activity to observed physical or compositional aspects of mineralized fractures
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