Vent distribution and its relation to regional and local tectonics, Hat Creek Graben Region, CA

Abstract

This dissertation focuses on volcano-tectonic interactions based on field and experimental observations. First, volcanic vent morphology and lineament geometry were extracted and mapped from high resolution drone data in a portion of the Hat Creek Graben, northern California. Second, analog experiments were carried out to understand the principal factors affecting dike propagation in the study area. Finally, experimental observations were compared with field observations to understand what parameters control dike propagation, geometry, and vent location. Two distinct magma compositions occur within the Hat Creek Graben region (HCGR), calc-alkaline basaltic andesite (CAB) and low-potassium olivine tholeiitic basalt (LKOT). Those two magma compositions occur in close spatial and temporal proximity. The CAB is characterized by higher-profile scoria cones with thick and blocky lava flows. In contrast, LKOT is characterized by spatter cones and ramparts with a lower profile and numerous pahoehoe lava flows. Detailed vent morphologies and trends relative to the graben axis are presented to characterize the effect of tectonic setting and magma viscosity on dike emplacement. The results suggest that dikes of low-viscosity magma related to the extensional setting (e.g., LKOT) produce lower cone morphology than dikes of high-viscosity magma related to the subduction setting (e.g., CAB). The results also suggest that a high-viscosity dike favors deflection toward the fault scarp, whereas a low-viscosity dike favors in-graben eruption. Dike rotation is associated with low-viscosity magma, sometimes resulting in en echelon segmentation. Geometric and dynamic parameters that govern dike propagation are evaluated in this study. Dike geometry and resulting vent location are affected by the graben's presence and by magma viscosity. Thus, experiments with different graben geometry and magma viscosity help answer fundamental questions regarding dike deflection and rotation. The experiments show that high-viscosity dikes deflect toward the fault scarp at greater depth, whereas low-viscosity dikes favor rotation in the upper crust. In addition, heterogeneity of shallow crustal layers and their interfaces control whether dikes arrest in the subsurface or erupt. Furthermore, in two-layer crustal systems, the presence of the graben on the surface, which modifies the stress field in the host material and hence controls intrusion formation, causes a transition from sill to laccolith. Comparing dike behavior in the experiments with vent morphology in HCGR explains conditions under which a dike may produce specific vent morphology. This approach can be used to understand whether a dike would propagate to the surface or arrest in the crust. In addition, vent location on the surface can be forecasted with knowledge of the regional and local stress field resulting from topography, and dike vicious pressure and overpressure