EPISODIC HOT SPRING ACTIVITY AND PALEOSEISMOLOGY OF THE DIXIE VALLEY FAULT IN NORTHERN DIXIE VALLEY, NEVADA

A series of fossil spring deposits are exposed along the Dixie Valley fault just south of the high-temperature (240°C) Dixie Valley geothermal field, and at the northern endpoint of surface ruptures that are Holocene in age. These deposits are composed of both travertine and hot spring sinter that have trapped pollen and other plant material during their formation. The mineralogy and texture of the siliceous sinters is consistent with their age. One modern-age sinter consists of vitreous, opaline silica or "geyserite" that likely formed from actively spouting eruptions of boiling fluids along the fault. Radiocarbon dates on the organic material in two other opaline sinters indicate ages of 2.2 and 3.4 ka. These young sinters are composed of amorphous opal-A and paracrystalline opal-CT that have not transformed (or aged) to quartz. Banded travertine composed of calcite, dolomite, hematite, and barite may represent deposition from a warm spring at about 5.0 ka. Older quartz-rich hot spring sinters occur interbedded with pluvial Lake Dixie (11-12 ka) beach gravels along the fault zone.

Paleoseismic studies identify several large magnitude events in the central Dixie Valley area that may be related to the episodic spring activity. Two of these are historic, the 1954 M6.8 Dixie Valley and the 1915 M7.7 Pleasant Valley earthquakes. Large Holocene and Pleistocene earthquakes include The Gap Event (3.7- 2.2 ka) and an older Holocene event, the West Stillwater event (<5.6 ka) and another late Pleistocene paleoseismic event (~12- 35 ka). Based on the similarity of ages between the spring deposits and local seismic events, we suggest that: 1) the 2.0 to 3.4 ka sinters may have initiated with The Gap event; 2) the modern geyserite may be associated with either the 1915 Pleasant Valley or the 1954 Dixie Valley earthquakes; and 3) that warm spring activity at about 5 ka may be associated with the M~7 earthquake on the west side of the Stillwater Range. Changes in hydrologic activity along the Dixie Valley fault appear to reflect variations in the local static stress field during episodes of seismic activity. Dixie Valley geothermal waters are estimated to be 12-14 ky, hence these warm and hot spring deposits represent surficial discharges of the modern geothermal system.