Potential Role of Dikes in Damaging Rock to Support Hydrothermal Fluid Flow, Surprise Valley California, USA: Implications for Geothermal Development

Abstract

Geothermal energy potential depends on locating highly porous and permeable zones that support fluid flow to extract heat. Hot springs in the playa of Surprise Valley, CA are distributed along gaps and bends in magnetic anomalies interpreted as sub-cropping mafic dikes (Glen et al., 2013). In addition to these dikes in the Valley Playa, dikes outcrop in the Hays Canyon Range (HCR) that defines the eastern margin of the valley. Dikes in the HCR have two distinct attitudes (1) N-S striking dikes (~180) that dip 60oW, and (2) NNW-SSE striking dikes (~330) that dip 85oE. Both attitudes are spatially associated with locally high fracture density and minor hydrothermal alteration that may have formed from dike emplacement. This study tests whether the distribution of hot springs can be explained by elastic distortions around an array of opening dikes that promotes localized dilation to support a network of open secondary structures focusing fluid flow to supply the hot springs. This is done through two mechanical model experiments which use boundary elements in an elastic half-space. The first model is a sensitivity study investigating the role of dike dimensions and position in the density stratigraphy on their opening. Field analysis constrains the strike-length, thickness, and the upper tip position of dikes, although height is unknown. The model reproduced the mapped dike-length (4000 m) and thickness (2.0 m) with a dike-height of 60000m and a magma density of 2500 kg/m3 which is consistent with mafic dikes. The second model applies the dike dimensions and calculates the resulting stress state and fracture potential around an array of dikes conforming to both attitudes of the dike array mapped magnetic anomalies and informed by from field results of dike orientation in the Hays Canyon Range. Simulations of the N-S trend predict regions of enhanced Coulomb stress and tension that promote fracture formation and opening near dike tips where segments are isolated and where two closely spaced dike segments underlap. Conversely, compression is enhanced along the dike walls and where the segments closely overlap. The NNW-SSE trending array of dikes predict increased Coulomb stress and tension at similar locations in the array, but with more extreme values. Thus, the NNW-SSE dike array geometry better matches areas of enhanced fracturing with locations with active hot springs (as well as regions of enhanced compression with their absence) than the geometry of the N-S dike array.