Marine and Land Active Source Seismic Imaging of mid Miocene to Holocene aged Faulting near Geothermal Prospects at Pyramid Lake, Nevada

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1 Marine and Land Active Source Seismic Imaging of mid Miocene to Holocene aged Faulting near Geothermal Prospects at Pyramid Lake, Nevada Amy Eisses 1,3, Annie Kell 1,3, Graham Kent 1,3, Neal Driscoll 2, Robert Karlin 3, Rob Baskin 4, John Louie 1,3, and Satish Pullammanappallil 5 1. Nevada Seismological Laboratory, University of Nevada, Reno; eissesa@gmail.com 2. Scripps Institute of Oceanography, University of California, San Diego 3. Department of Geological Sciences and Engineering, University of Nevada, Reno 4. United States Geological Survey, West Valley City, Utah 5. Optim Seismic Data Solutions, Reno Introduction The Pyramid Lake Basin is located along the eastern margin of the northern Walker Lane (see figure 1), a kinematically linked, en echelon system of left stepping, dextral northwest striking primarily strike slip and oblique slip faults (Faulds and Henry, 2008; Faulds et al., 2010). The Walker Lane combined with the eastern California Shear Zone unevenly accommodates 20% 25% of the motion between the Pacific Plate and the North American Plate (Thatcher et al., 1999; Dixon et al., 2000; Oldow et al., 2001; Bennett et al., 2003; Hammond and Thatcher, 2004), specifically the motion between the Sierra Nevada block relative to the Great Basin (Faulds et al., 2010). The Pyramid Lake fault zone lies in a critical region in the northern Walker Lane where transtension is accommodated through a complex pattern of dextral strike slip and normal faults (Faulds et al, 2005). North of the Pyramid Lake basin dextral shear is focused in a series of mostly right lateral strike slip faults near Honey Lake (Turner et al., 2008), in contrast south of Pyramid Lake to Lake Tahoe, transtension is expressed through north south striking, normal fault bounded basins with small amounts of opening or fanning to the north that is accommodated through sinistral faulting (cite). Figure 1: Regional map of the Sierran Microplate, Basin and Range Province, and the Walker Lane Belt (Unruh et al. 2003). The Pyramid Lake basin lies within the region of the Great Basin that contains the greatest concentration of known geothermal fields in the western United States (Faulds et al., 2004). GPS geodetic data shows that extension rates are greater in the northern Walker Lane than anywhere else in the Great Basin (Kreemer et al., 2009), perhaps due to the northwest termination of the Walker Lane against the Cascade arc (Faulds et al., 2006 Faulds and Henry, 2008). Faults are known major controls on geothermal systems and extensional regimes favor dilatational normal fault systems, which increase deep fluids, and geothermal activity (Pyramid Lake Paiute Tribe, 2009). The first of two main motivations for the Pyramid Lake geothermal exploration project aims to help the Paiute Tribe develop natural geothermal reservoirs. The second motivation attempts to gain a greater understanding of the tectonics and earthquake hazards in the Pyramid Lake basin and the northern Walker Lane, through advanced and economical seismic methods. 1

2 Pyramid Lake Basin Geology The Pyramid Lake fault (figure 2), which enters the basin from the south, extending along the west side of the lake, near the shoreline, changes from a dextral strike slip fault system south of the basin to include a down to the east normal component as it approaches and enters the lake. As the Pyramid Lake fault dies near mid lake, the East Pyramid Lake fault, or the Lake Range fault, becomes dominant and displays down to the west motion; thereafter, splays into a series of dextral dip slip faults on the northwest end of the lake. Strain is transferred from the Pyramid Lake fault to along the west dipping East Pyramid Lake fault, highlighting a polarity flip in fault architecture near Anahoe Island. Aster Pass is located on the northwest end of Pyramid Lake. The structure appears to control the tufa spire deposit in the area, and the geothermal system. Our investigations at Pyramid Lake are supported by the Pyramid Lake Paiute Tribe, with backing from the DOE Geothermal Technologies Program and the U.S. Department of the Interior Division of Energy and Mineral Development. Figure 2: Conceptualized Pyramid Lake fault showing CHIRP lines collected in summer Methods CHIRP Data Collection The seismic Compressed High Intensity Radar Pulse (CHIRP) designed by Edgetech SubScan was used to conduct the marine portion in this study. The CHIRP uses variable frequencies ranging from 500 Hz to 16 khz. The CHIRP profiles collected on Pyramid Lake used predominantly khz pulse with a 10 ms duration. In June 2010, the University of Nevada, Reno, the Scripps Institution of Oceanography, and the USGS, Salt Lake City collected more than 500 line kilometers of seismic CHIRP data that was then processed by the following steps: the data was converted from JSF to SEGY. Two way traveltimes were converted to depth assuming a 1500 m/s water velocity. (Dingler et al., 2009) Land Seismic Methods In the Astor Pass area, Optim SDS collected 27 kilometers of vibrator seismic acquisition in May 2010 using three heavy vibrators and 220 foot geophone spacing, and long offset recording with up to 240 channels live, TM velocity optimization from first arrivals, and 2D prestack depth migration. In total, 30 km of online profiling were completed, including Blackhawk collecting the initial line in 2006 (now Line 4) with one mini vibrator at 55 foot spacing. Lines 7 16 went through steep terrain, which prevented putting vibrators at some points. Optim SDS processed all the data (including Blackhawk s 2006 data). Three geothermal exploration wells, Aster Pass Slimhole (APS) #1, APS #2, and APS#3, were also completed by mid February

3 Results CHIRP Results The CHIRP data profiles collected span the full lake in a north south and east west orientation as shown in figure 2 by the white lines. The profiles show a combination of Figure 3: East west CHIRP profile in the north end of Pyramid Lake basin. This image shows a series of dip slip faults that could control fluid behavior. Gas is seen in the west side of the basin and a slide in the east. normal, strike slip and obliqueslip faults. These faults are Recent in age. CHIRP profiles show that the northern end of the Pyramid Lake basin has a denser fault network of normal and oblique slip motion than the rest of the lake. The eastern edge of the lake is significantly deeper than the western edge indicating high amounts of deformation on the East Pyramid Lake fault. Figure 3 shows a network of dip slip faults believed to control the geothermal fluids, which created the Needles and other tufa mounds in and around Figure 4: East West CHIRP profile directly south of Anahoe Island showing the structural architectures that control the island. The strike slip fault to the east produces the shallow channel to the east of Anahoe Island and the normal fault controls the deep basin shown. The inferred fluid path follows the dilatational fault and there is also gas to the west that could be controlled by a normal fault that the gas prevents imaging. The sediment that has filled the normal fault gives evidence of a classical growth fault. Pyramid Lake. The eastern side of the lake has a possible earthquake induced landslide imaged by previously collected bathymetry and also seen on the eastern edge of the D01L01 profile and others. Through the CHIRP lines it is apparent that Anahoe Island is the fulcrum for the polarity flip observed from the north and south within the basin. In Day06Line12 profile (figure 4) it can be seen that Anahoe Island is bounded to the west by a western dipping normal fault causing the deep basin shown. The eastern side of the island is bounded by a strike slip fault that produces the shallow channel. 3

4 CHIRP profiles that span the lake in north to the south orientation (figure 5) show the change in deformation styles between the north end of the lake and the south end which corresponds to the change in strain partitioning or a polarity flip from the predominantly strike slip motion in the south end to dip slip motion in the north end of the basin. This change in deformation style correlates to a change in basin geometry where south of Anahoe Island the basin is narrow and northwest oriented while north of the island the basin becomes wider and changes to a north south alignment. Figure 5: North south CHIRP profile on the western side of Anahoe Island highlighting the change in deformation style and basin geometry between the north end of the basin and the south. Layers south of Anahoe Island dip west while sediment layers north of the island dip towards the east. Land Seismic Results The on land seismic survey shows clear images of the structure of normal faults that likely also have a dextral component in Astor Pass (Pyramid Lake Paiute Tribe, 2009). The results display stratigraphic terminations and intersecting fault planes. Two Tertiary faults were previously known through geologic mapping of surficial expression (cite). These two dip slip faults, one east dipping shown in pink in figure 6, and one west dipping shown in blue, were imaged, sometimes directly (point A) in the seismic profiles. A new east west striking, north dipping fault, shown in green, was interpreted in three lines. The drilling logs confirmed that the wells went into Sierra granite at ~4000 foot depth. Figure 6: View from the northwest showing all three fault sets with the east dipping fault (shown in pink) directly imaged at point A. The newly discovered north dipping fault is shown in green and the west dipping fault in blue. This is consistent with faulting being Tertiary in age since the Sierra granite is the bottom of the Tertiary volcanic stratigraphic column. 4

5 Discussion and Conclusion Using marine and land seismic surveys, we have imaged two distinct phases of faulting, including early Walker Lane extension and shear as seen in mid Miocene volcanics, followed by a more recent episode of faulting, best seen in the Pyramid Lake CHIRP profiles. The most recent phase of faulting was not imaged in the land seismic data owing to the combined effect of minimal offsets (i.e., tens of meters) across stratigraphic layers and the vibrator source that was tuned to look more deeply into the crust to image the episodes of mid Miocene faulting. Results from the wells drilled at Astor Pass so far suggest that geothermal fluids are most likely moving through late Pleistocene to Holocene aged faults and not through mid Miocene aged conduits. The tufa mounds were created by the more recent episode of strikeslip and normal faulting, assumed to extend on land surrounding the basin. This observation is further evidence that the younger, dilatational faults are the conduits for the geothermal fluid in the basin. 5

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