GABELMAN, JOHN W., John W. Gabelman and Associates, Inc., Danville, CA ;and EDWARD B. TOWNE, Jr., Towne Exploration Co., San Francisco, CA

ABSTRACT: The Geysers: A Distinct Rare Tectono Magmatic Type of Geothermal Resource?

Geothermal resources typically are associated with felsic magmas in volcanic fields, and are water-dominated because intrusion fosters fracturing. Volcanics are uncommon in the California Coast Ranges Province which contains The Geysers magma(?)-heated dry steam field. The Geysers is marginal, but not directly relatable, to the Clear Lake volcanic field. Although a felsite stock(?) underlies the steam field, indicated shallow (4 km) magma is several kilometers away. Coast Range volcanism occurred after subduction was replaced by dextral wrenching along and near the San Andreas transform joining the Rivera and Mendocino triple junctions at the North American-Pacific plate boundary. The steam field lies between two wrenches considerably east of the transform. Steam is trapped in a stock ork reservoir in graywacke texturally strengthened and sealed by moderate pressure-metamorphism and silicification. Geysers uniqueness is attributable to its singular tectono-magmatic history. Others have described the termination of Corda Plate subduction at the northwestwardly-migrating Mendocino Junction, allowing asthenosphere to rise into the resulting void to the south. We believe asthenospheric magma underplated the void lid and penetrated the crust only at major structural intersections, forming a step-series of passive volcanic fields. The Geysers reservoir is in a local transtensional domain within a transpressive arch in the obtuse angle of an intersection. Trapped mafic magma can melt crust to generate viscous felsic magma even less prone to rise. Access to much deep magma by limited water can generate and preserve steam to be stored in overlying or adjacent sealed fracture reservoirs not directly associated withany volcanic body or wrench.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.