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EVOLUTION OF DIKES IN SURPRISE VALLEY, CALIFORNIA: RELATIONSHIP AMONG DIKES AND MAGMATISM AND BASIN AND RANGE TECTONICS
Krzystek, Colin M.
Krzystek, Colin M.
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2025-05
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Geology
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https://doi.org/10.34944/8k5q-mj97
Abstract
Extension of crust occurs through the complex interplay of faulting and dike intrusion that both thin and lengthen the crust. In Surprise Valley, CA dikes coeval with extension display characteristics of both brittle and ductile fracture. These modes are evident from dike geometry in detailed outcrop maps and models of recently collected high-resolution gravity and magnetic data. Many brittle dikes are up to 1-3 m thick with aspect ratios of 500-1000 with sharp dike tips at both the upper and lateral tips. Other, typically larger dikes are 25 m thick and have aspect ratios of about 10-20 with round dike tips where the radius of curvature approaches the dike opening and is ~8-10% of dike length indicating ductile fracture. In several other cases dikes occur within normal faults; these dikes are up to 10 m thick with relatively high aspect ratios of 10-100 that might represent the accumulation of multiple injections. These disparate geometries occur within hundreds of meters of each other in the same host rock, developing ~8-3 Ma at depths of 1300 to 300 m during basin extension. The three types define a conceptual model for dike evolution during extension of Surprise Valley. Initially, dikes propagate upward as brittle fractures. Where heat loss was rapid or upward propagation slow, increasing viscosity eventually arrests upward growth driven by dike inlet pressure and buoyancy causing the dike to freeze. This result is promoted in thin dikes with large aspect ratios and thus high surface area and is associated with smaller grain sizes and minor contact metamorphism. Sustained flow in a rapidly propagating dike maintains temperature in the magma and significantly warms the host rock promoting a transition to ductile opening. Larger apertures in this scenario reduce viscous resistance to flow that helps sustain magma flow and high temperature in positive feedback. This result is associated with larger grain sizes and more intense and extensive contact metamorphism. Alternatively, faults act as pathways that capture some dikes and concentrate dike flow, resulting in local accumulations of magma. To examine the geometries of these dikes further, potential field data was collected and used to model the subsurface.
The magnetic signature of the ductile opening mode dikes and dikes captured by faults match extensive magnetic anomalies in the basin of Surprise Valley associated with active hydrothermal outflow. Both areas define segmented dike structure subparallel to the long axis of the basin. This relationship indicates dikes are both an element of basin tectonics and that they influence fluid circulation. Unlike in the valley, dikes in the Hays Canyon Range lack active or fossil hydrothermal features and instead show local contact metamorphism and jointing. This implies that the dikes are themselves not the heat source of the hydrothermal system, but only one element of the geothermal system that contributes to directing fluid flow from a deeper heat source to the surface.
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