| Title | Crustal Thermal Regime at The Geysers/Clear Lake Area, California |
|---|---|
| Authors | Kamil Erkan, David D. Blackwell and Mark Leidig |
| Year | 2005 |
| Conference | World Geothermal Congress |
| Keywords | The Geysers, heat transfer, modeling, magma |
| Abstract | The Geysers/Clear Lake area in northern California is characterized by extensive volcanism and intrusion of bimodal magmatic products within the last 2 Ma and by the presence of one of the largest (steam dominated) geothermal systems in the world. Based on a compilation of data from over 650 heat flow sites in 100 to 600 m wells, the region is also characterized by a large area of elevated regional heat flow (an area greater than 1,600 km2). The heat flow over this large area averages 150 mW/m2, about double the already high Coast Range regional heat flow of 75 mW/m2. Temperature-depth data from numerous deep wells (average 3 km) demonstrate characteristics of the thermal regime in this magmatic/volcanic system to depths where the temperatures are 250 to 350 ?C. One of the characteristics of vapor dominated systems is their low permeability. Thus the large thermal data set and the very low permeability of the upper crust in this region allow a look at the upper and mid-crustal thermal effects of a large scale intrusive center with limited surface expression. Two dimensional forward and a 3-d inverse thermal models of the intrusions (varying both in size and frequency, based on the present day heat flow pattern and the igneous record of the last 2 M.Y.) that underlay this upper crustal system are described. The heat flow and deep temperature data are powerful constraints on the interesting parameters that are otherwise only known from fossil systems, not currently active, large, cryptic, magmatic systems. The rate of intrusion is on the order of 0.005 km3/yr which is slow enough to allow for significant cooling so that most of the upper crust is below magmatic temperatures for most of the time. This region of thermal disturbance is characterized by a negative gravity anomaly and by shallowing of the seismogenic layer. Direct seismic evidence of magma has not been identified, but nonetheless this area represents an active plutonic system in a geologic sense. |