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

Recent volcanism, high heat flow (4 HFU or 167 mW/m(2)), and high conductiv e geothermal gradients (up to 120 degreesC/km) indicate that heat from a sh allow silicic intrusion in. the Clear Lake region is largely being dissipat ed by conduction. The Geysers area has the highest heat flow in the region, consistent with the presence of shallow convective heat transport within t he vapor-dominated geothermal system. Thermal modeling of the Clear Lake ma gmatic-hydrothermal system based on petrologic and geophysical constraints provides a test of petrologic models, and yields insight into the relations hips between observed thermal gradient and magma chamber size, abundance, a nd emplacement history in the crust. A user-interactive two-dimensional (2- D) numerical model allowing for complex host rocks and multiple emplacement s of magma was developed to simulate conductive and convective heat transpo rt around magma bodies using a finite-difference approach. Conductive model s that are broadly consistent with the petrologic history and observed ther mal gradients of the Mt. Konocti and Borax Lake areas imply a combination o f high background gradients, shallow magma bodies (roofs at 3-4 km), and re cent shallow intrusion not represented by eruption. Models that include zon es of convective heat transport directly above magma bodies and/or along ov erlying 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 th e Clear Lake region. Consideration of the entire Clear Lake magmatic system , including intrusive equivalents, leads us to conclude that: (1) emplaceme nt of numerous small and shallow silicic magma bodies occurred over essenti ally 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 u pper crust at Clear Lake was erupted; (3) high conductive thermal gradients are enhanced locally by fault-controlled zones of convective heat (geother mal fluid) transport; and (4) except for the Mt. Hannah and possibly the Bo rax Lake area, most of the silicic magma present in the upper crust has sol idified or nearly solidified. These bodies are currently difficult to disti nguish from surrounding hot basement rocks dominated by graywacke using geo physical 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 c onventional geothermal power production. Modeling results suggest the possi bility that granitic bodies similar to The Geysers felsite may underlie muc h of the Clear Lake region at shallow depths (3-6 km). This is significant because future HDR reservoirs could potentially be sited in granitoid pluto ns rather than in structurally complex Franciscan basement rocks. (C) 2001 CNR. Published by Elsevier Science Ltd. All rights reserved.