Abstract: Quaternary Geologic Processes Associated with Geothermal Energy

L. T. Grose

Numerous geophysical and geochemical methods are being used in the search for high-temperature geothermal-energy sources. Geologic guidance in the main has been general and, where specific, has been influenced strongly by the occurrence of thermal springs. Individual goethermal convection systems are rarely more than 1 m.y. old and many are probably less than 100,000 years old. Because geologic processes of significant heat generation and entrapment are mostly Quaternary, knowledge of Quaternary crustal evolution in geothermal regions is essential for successful application and interpretation of geophysical and geochemical data. Most other energy and mineral deposits are not so young nor so transient.

The direct source of heat for many geothermal systems is believed to be magma at a few kilometers depth. New exploration efforts, being pursued mainly by the U.S. Geological Survey, include research on (1) geology of shallow magma masses, their volume, shape, composition, heat content, cooling or heating rates, age, and evolution, and (2) geophysical methods to locate them specifically (i.e., seismic, electrical, and others).

Heat from natural flow and/or igneous masses is trapped beneath thick sedimentary and/or volcanic layers of low conductivity and low permeability. Where this thermal insulation occurs in regions of tectonic expansion and high heat flow, many local hidden convection systems may be present. The geologist has the opportunity, through knowledge of Quaternary processes, to discover areas favorable for the application of several specific geophysical and geochemical exploration methods.

Localization of economically recoverable heat (comprising the individual geothermal prospect) is controlled by convection. Convection is permitted and controlled by those fracturing zones (among other conditions) which continually are kept open to fluid flow by recurrent faulting and rifting. Many hot spring systems, including some which are being exploited commercially, are 10,000 to 100,000 years old. They may represent local ephemeral leakages from much larger heat reservoirs that have been building up for 1 m.y. to 10 m.y. Exploration for Quaternary, historic, and current faulting (seismicity), tectonic-extension directions, recurrence intervals, etc., should aid in the location of convection systems in various stages of evolution--systems for which there are no surface manifestat ons at the present time.

Studies of Quaternary tectonics, volcanism, and sedimentation will increase the success of conventional geothermal exploration methods.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado