Structural Stratigraphy and Intrusive Rocks at The Geysers Geothermal Field

Abstract

The Geysers geothermal field lies within Jurassic-Cretaceous Franciscan rocks from an ancient subduction complex. The distribution of geologic units is largely controlled by low angel structures that formed in the subduction environment. Significant displacements by relatively young high angle faults are generally absent in the interior of the field. The Franciscan rocks form a sequence of tabular slabs of rock bounded by thrust faults. The thrust packets cap the main graywacke, which is the principal host rock to the steam reservoir. The packets are composed of structurally related rock assemblages of similar metamorphic grade. The thrust packets are stratigraphically continuous, with some packets traceable for mile along strike. Fault relationships between thrust packets imply multiple stages of thrust movement with intervening periods of high angle faulting. The Franciscan rocks at the Geysers were intruded during the Pleistocene by silicic magmas that form a composite batholiths. The axis of the batholiths trends northwestern and is roughly coincident with the axis of the producing steam field. This intrusive body also constitutes a reservoir host rock.