The Long Valley/Mono Basin Volcanic Complex in eastern California: Status of present knowledge and future research needs

The Long Valley/Mono Basin volcanic complex in eastern California is one of the few major silicic systems in western North America that have exhibited volcanic activity so recently that they may still be potentially active. Such complexes are typically associated with highly developed convective hydrothermal systems and hot spring activity at the surface. Because of the ongoing interest among the scientific community in understanding the intercoupling of shallow hydrothermal reservoirs with magmatic heat sources at depth in the crust, these volcanic complexes have been studied for many years by workers interested in a variety of geological problems. Among these are metallogenesis, the mitigation of volcanic and earthquake hazards, and the development of conventional and nonconventional forms of geothermal energy. The status of some of this work, as it relates to the Long Valley/Mono Basin volcanic complex, is reviewed by way of a progress report. Although a great deal of exciting research is presently underway, much still needs to be done to adequately characterize the tectono‐magmatic elements of this volcanic system. One research technique that has not been exploited is scientific drilling to intermediate depths (2–3 km). A carefully structured research program in which scientific drilling is closely integrated with surface geological and geophysical field studies would help clarify many of the present issues surrounding this volcanic complex. For example, while many workers feel that hydrothermal magma systems are prime subjects for the study of metallogenesis, there is little evidence in existing borehole data for such processes at shallow depths beneath Long Valley. This may be because volcanism is too recent (Inyo craters and domes have C 14 ages younger than 1000 yr). Mineralization is usually associated with older, mature (solidified?) magmatic systems. We may have, in Long Valley, an opportunity for defining the initial conditions associated with the evolution of these deposits. Long Valley caldera is the only member of this class of volcanic systems for which a quantitative caldera‐scale hydrothermal model has been proposed. In addition, this region offers a unique opportunity in volcanic and earthquake hazards research. Recent tectonic deformation, seismicity patterns, and the reactivation of fumarolic activity caused the USGS to issue, on May 25, 1982, a notice that a potential volcanic hazard exists for the southwestern segment of Long Valley caldera. If, as has been proposed, magma has intruded the upper crust of this area, surface geophysics and limited borehole observations may be employed to monitor tectonic and magmatic activity associated with such a phenomenon. Finally, a surprising result of commercial drilling for geothermal energy in this area is the failure to detect directly high‐temperature hydrothermal reservoirs. Their presence can be inferred from other studies—particularly those using geochemical indicators in thermal fluids. In addition, present evidence supports the possibility of molten magma at depths as shallow as 6–8 km. Thus future surface geophysical and geological studies, in conjunction with intermediate depth drilling, might help understand the nature of the presently undetected (though inferred) high‐temperature hydrothermal reservoir and perhaps provide some clues as to how it is coupled to the possible magma body at depth.