Thermal modeling of the Clear Lake magmatic-hydrothermal system, California, USA

Abstract

Recent volcanism, high heat flow (4 HFU or 167 mW/m 2), and high conductive geothermal gradients (up to 120°C/km) indicate that heat from a shallow silicic intrusion in the Clear Lake region is largely being dissipated by conduction. The Geysers area has the highest heat flow in the region, consistent with the presence of shallow convective heat transport within the vapor-dominated geothermal system. Thermal modeling of the Clear Lake magmatic-hydrothermal system based on petrologic and geophysical constraints provides a test of petrologic models, and yields insight into the relationships between observed thermal gradient and magma chamber size, abundance, and emplacement history in the crust. A user-interactive two-dimensional (2-D) numerical model allowing for complex host rocks and multiple emplacements of magma was developed to simulate conductive and convective heat transport around magma bodies using a finite-difference approach. Conductive models that are broadly consistent with the petrologic history and observed thermal gradients of the Mt. Konocti and Borax Lake areas imply a combination of high background gradients, shallow magma bodies (roofs at 3–4 km), and recent shallow intrusion not represented by eruption. Models that include zones of convective heat transport directly above magma bodies and/or along overlying fault zones allow for deeper magma bodies (roofs at 4–6 km), but do not easily account for the large areal extent of the thermal anomaly in the Clear Lake region. Consideration of the entire Clear Lake magmatic system, including intrusive equivalents, leads us to conclude that: (1) emplacement of numerous small and shallow silicic magma bodies occurred over essentially the entire region of high heat flow (about 750 km 2); (2) only a very small fraction (much less than 10%) of the silicic magma emplaced in the upper crust at Clear Lake was erupted; (3) high conductive thermal gradients are enhanced locally by fault-controlled zones of convective heat (geothermal fluid) transport; and (4) except for the Mt. Hannah and possibly the Borax Lake area, most of the silicic magma present in the upper crust has solidified or nearly solidified. These bodies are currently difficult to distinguish from surrounding hot basement rocks dominated by graywacke using geophysical methods. The Clear Lake region north of the Collayomi fault is one of the best prospects for hot dry rock (HDR) geothermal development in the US, but is unlikely to provide significant development opportunities for conventional geothermal power production. Modeling results suggest the possibility that granitic bodies similar to The Geysers felsite may underlie much of the Clear Lake region at shallow depths (3–6 km). This is significant because future HDR reservoirs could potentially be sited in granitoid plutons rather than in structurally complex Franciscan basement rocks.