W. L. Pickles

Geological and geobotanical studies of Long Valley Caldera, CA, USA utilizing new 5m hyperspectral imagery

In May of 1989, a six month-long small magnitude earthquake swarm began beneath the Pleistocene-aged dacitic cumulovolcano Mammoth Mountain. The following year, increased mortality of trees in the Horseshoe Lake region was observed. Their deaths were initially attributed to the Sierran drought of the 1980s. In 1994 however, soil gas measurements made by the USGS confirmed that the kills were due to asphyxiation of the vegetation via the presence of 30-96 % CO/sub 2/ in ground around the volcano. Physiological changes in vegetation due to negative inputs into the ecological system such as anomalously high levels of magmatic CO/sub 2/, can be seen spectrally. With this phenomena in mind, as well as many other unanswered geological and geobotanical questions, seven lines of hyperspectral 5-meter HyMap data were flown over Long Valley Caldera located in eastern California on September 7, 1999. HyMap imagery provides the impetus to address geobotanical questions such as where the tree kills are currently located at Mammoth and other locales around the caldera as well as whether incipient kills can be identified. The study site of the Horseshoe Lake tree kills serves as a focus to the initial analyses ofthis extensive HyMap dataset due to both the tree kills geologically compelling origins and its status as a serious volcanic geohazard.

LiDAR and hyperspectral analysis of mineral alteration and faulting on the west side of the Humboldt Range, Nevada

In order to evaluate the setting of the Humboldt–Rye Patch geothermal field, we carried out a program of hyperspectral and light detection and ranging (LiDAR) imaging of the Humboldt River basin to test (1) whether fault patterns, surface mineral alteration, and mud volcanoes in the Humboldt–Rye Patch district offer the potential for additional geothermal exploration sites; (2) whether mud diapirism in this region could be caused by seismic shaking; and (3) whether significant improvements in exploration can be made using these remote-sensing tools in addition to the more traditional techniques. In the southern (Rye Patch) region, a set of faults cuts the surface of the alluvial fans, and several faults cut shorelines of Lake Lahontan. These shorelines lie at an elevation of 1290 m, which corresponds with the elevation of the Lake 12,500 ± 500 yr ago. We find no signs of surface mineral alteration in the Rye Patch area in spite of the existence of these faults and known alteration at depth. Farther north, in the Humboldt House region, we find abundant evidence of alteration products, including siliceous sinter, carbonate, montmorillonite, hematite, and jarosite. This alteration is widespread, and corresponds to young faulting in only one location. The LiDAR data show at least two mud volcanoes and a large field of low-carbonate mounds. Some of these (apparently) diapiric features may have been associated with seismicity, and both active and paleoseismic events would have been sufficiently close and energetic to have initiated liquefaction in this region. Such liquefaction events would have been more likely, however, during the high stands of Lake Lahontan, when the ground would have been saturated, consistent with reported ages on rocks correlated with the carbonate mounds. We propose further geothermal exploration based on these results.

Chemical and structural dynamics of a geothermal system

In an ongoing project to relate surface hydrothermal alteration to structurally controlled geothermal aquifers, we mapped a 16 km swath of the eastern front of the Stillwater Range using Hyperspectral fault and mineral mapping techniques. The Dixie Valley Fault system produces a large fractured aquifer heating Pleistocene aged groundwater to a temperature of 285/spl deg/ C at 5-6 km. Periodically over the last several thousand years, seismic events have facilitated flow of heated fluids to the surface, leaving a rich history of hydrothermal alteration in the Stillwater Mountains. At Dixie Hot Springs, the potentiometric surface of the aquifer intersects the surface, and 75/spl deg/ C waters flow into the valley. We find a high concentration of alunite, kaolinite, and dickite on the exposed fault surface directly adjacent to a series of active fumaroles on the range front fault. This assemblage of minerals implies interaction with water temperatures in excess of 200/spl deg/ C. Field spectra support the location of the high temperature mineralization. Fault mapping using a digital elevation model in combination with mineral lineation and Held studies show that complex fault interactions in this region are improving permeability in the region leading to unconfined fluid flow to the surface. Seismic studies conducted 10 km to the south, at Dixie Hot Springs, show that the range front fault dips 25-30/spl deg/ to the southeast [R. E. Abbott et al. (2001)]. At Dixie Meadows the fault dips 35/spl deg/ southeast showing that this region is part of the low angle normal fault system that produced the Dixie Valley Earthquake in 1954 (M=6.8). We conclude that this unusually low angle faulting may have been accommodated by the presence of heated fluids increasing pore pressure within the fault zone. We also find that younger synthetic faulting is occurring at more typical high angles. In an effort to present these findings visually, we created a cross-section, illustrating our interpretation of the subsurface structure and the hypothesized locations of increased permeability. The success of these methods at Dixie Meadows will greatly improve our understanding of other Basin and Range geothermal systems.

Remote sensing analysis of structure and geothermal potential of the Humboldt Block, Nevada

Fluid flow along normal faults has created much of the geothermal energy that is currently being exploited in the Basin & Range. We used remote sensing (HyMap, ASTER) data and field-based methods in the Humboldt Block of the northwest Basin & Range to map fault zones and the surface distribution of minerals associated with hydrothermal fluid flow. The Humboldt Block lies on the Battle Mountain High heat flow area (>100 m/spl bsol/V/m/sup 2/), and has very high shallow water temperatures of around 200/spl deg/ F. This area has undergone large amounts of extension from Oligocene to the present, accommodated along large, range-bounding normal faults. The western flank of the Humboldt Range is bounded by a normal fault that trends N-NE, dips W, and brings into contact Mesozoic sedimentary and volcanic rocks with Quaternary deposits. The structural setting, high heat flow, and high shallow water temperatures suggest significant geothermal potential for the Humboldt Block. We carried out the remote sensing study in two stages. We created fault maps by overlaying ASTER data onto a DEM, and mapping lithologic changes and structurally controlled lineations that were distinct from expected topographic patterns. This revealed two distinct fault patterns: N-NE trending faults and NW-SE trending faults. We then used HyMap (hyperspectral) data to identify and map minerals associated with hydrothermal fluid flow and their relation to regional structure. Sinter was mapped in the Humboldt River Valley along two distinct N-NE trends. Kaolinite was mapped in the Humboldt Range along the range front fault, and NW-SE trends.