I 3725'00045 LINEAMENT ANALYSIS OF AN AEROMAGNETIC SURVEY, I WARM SPRINGS VALLEY. WASHOE COLINTYNEVADA I I I I I I I I I I I I I

Michael C.Widmer t Washoe County Department of Water Resources t Reno- Nevada October 2001 T I I

I LINEAMENT ANALYSIS OF' AN AEROMAGNETIC SURVEY, WARM SPRINGS VALLEY, WASHOE COUNTY, NEVADA

I Introduction This report discusses a lineament analysis of aeromagnetic data for Warm Springs Valley. The results can be used to further the understanding of geologic structure, particularly fault structure. I The analysis combines recent work with a previous report by Paul Hartley (1995), under contract with the Washoe County Department of Water Resources. His analysis was to determine the I depth of bedrock and the delineation of sediments within a portion of Warm Springs Valley. Airborne Geophysical Data Dighem, Inc. was contracted by Washoe County to conduct an airborne geophysical survey I (Dighem, 1994). Magnetic and electromagnetic instrumentation was installed in a Lama turbine helicopter (Geoseis Helicopters, Inc.) which flew at an average airspeed of 100 kph (62 mph) I with a magnetometer bird height of 30 meters (98 feet) above ground level. The survey consisted of 414 kilometers of traverse line (257 miles) oriented at 50'1230'to geographic north with 667 meters (2000 feet) line spacing. Two tie lines were oriented at 140'1320o to geographic ! north. The magnetic data was collected with a Picodas 3340 optically pumped cesium vapor magnetometer. The sampling rate was l0 per second with a sensitivity of 0.01nT. Navigation and positioning consisting of a Sercel NR 106 real-time differential global positioning system I with <5 meter accuracy. A Scintrex MEP-710 cesium vapor magnetometer was operated at the survey base to record diurnal variations. The base station clock was synchronized with that of the airborne system to permit subsequent removal of diurnal drift. Data processing by Dighem I Inc. consisted of corrections for diurnal variations and leveling. Data processing by Washoe County consisted of reduction to pole and 50 meters of upward continuation (Geosoft,1999). The reduction to the pole shifts asymmetric magnetic fields to a symmetrical field that aides in t determining the edges of magnetic bodies. The asymmetry being the signature of a magnetic body's geometry. Upward continuation reduces short wave length "noise".

I Results Figure I is a location map of Warm Springs showing the airborne flight lines and major roads. Figure 2 is a shaded relief map of the gridded Total Field Magnetic data that was reduced to the t pole and upward continued 50m. The range between the low and high magnetic values is 2,500 nT (51,825 to 54,357 nT). In the southern half of the figure the magnetic "highs" are well correlated with the Pah Rah Range and the magnetic "lows" with relatively thick sedimentary I basin-fill. Of particular interest is the large magnetic high west of the Pyramid Lake Highway (which bisects the map and is oriented north-south). This high is a subsurface magnetic body that appears to be associated with a mapped granitic dome located immediately to the south. t Also shown is an unmapped subsurface magnetic ridge that trends northwest across the northern portion of the figure. In Figure 3, the Total Field Magnetic contours are placed onto a Digital Elevation Model (DEM) for Warm Springs. The granitic dome is well correlated to the magnetic I high. The subsurface ridge is on strike with the edge of the south face of the Pah Rah Range and I the Winnemucca Vallev. I I I t Figure 3b shows the magnetic contours placed upon a simplified geologic map. Using this figure and Figure 2, the highest magnetic signature is associated with Tertiary basalt (Tab) in the most southwestern portion of the figures. As stated earlier, the magnetic low is associated with basin T fiIl sediments (Qal), estimated at 300 meters (1,000 feet) thick (Hartley, 1995). Relatively steep magnetic gradients are found on the north and south of these sediments that correspond to the Hartford Hill Rhyolite (Th) and Cretaceous granodiorite (Kgd), respectively. To the northwest I of the Pyramid Lake Highway the magnetic features have relatively high magnetic signatures and are fairly well defined. This may indicate thin alluvial cover. The subsurface ridge appears to be correlated with Pre-Lake Lahontan, Pleistocene gravels (Qtg) in the upper northem portion of t Figure 3b. The granodiorite and the rhyolite have similar magnetic signatures. It is unclear as to the reason for the magnetic signatures associated with the Hartford Hill Rhyolite in the upper I northern portion of the figures. Figure 4 is a color-shaded relief map of the total horizontal gradient of the magnetic data, draped over the DEM. The contours are of the Total Field Magnetic anomalies (50 nT interval) from I Figure 3. The total horizontal gradient is the maximum horizontal rate of change of the strength of the magnetic field (first horizontal derivative of the magnetic freld). It is a useful analytic tool to determine where magnetic slopes have their maxima. The edges of magnetic bodies or I structures can be inferred where the magnetic gradients reach their maxima. Figure 5 illustrates this where the steepest rate of change in the magnetic field are delineated with black lines atop of these trends. Most of these lineaments trend northwest although some trend north to northeast. I The subsurface magnetic body discussed above appears to be fairly well defined with four lineaments and the granitic dome with t'wo lineaments. The subsurface ridge is also reasonably defined. The magnetic data were also processed as analytic signal and vertical derivative, but I resulted in poor resolution of gradients. Figures 5b and 5c show these grids with the total horizontal gradient lineaments plotted.

I Figure 6 shows these "gradient lineations" on the Digital Elevation Model of Warm Springs Valley. Some of these lineations match well with topographic lineaments. For example, the subsurface ridge can be traced southeastward to apparent fault scarps in the Pah Rah Range. I Lineaments also correlate with apparent fault scarp in Winnemucca Valley. Figure 6b shows I these lineaments atop the simplified geologic map. Figure 7 are interpreted faults from Hartley's analysis (1995). These interpreted faults were derived from a lineament analysis of magnetic (three data sets filtered at low pass, high pass and I reduced to pole, respectively) and electromagnetic (three resistivity data sets at 56,000 H2,7200 Hz and 900 Hz frequency) data (DIGHEM, 1994). Particular attention was placed on lineaments within the basin-fill sediments. Figure 8 displays the interpreted faults from Hartley (1995) and I from this study along with iso-contours of fluoride data. The fluoride data was collected from private wells. This anomalous concentration of fluoride in the groundwater was hypothesized to be derived from the intrusive granodiorite dome, but also appears to be associated with the I subsurface magnetic high as outlined by the lineaments (this study). I I I I

I Conclusions The lineament analysis delineates two subsurface magnetic features. The first is a subsurface ridge that may be associated with the Warm Springs Fault, a structure of the Walker Lane Fault I Zone. The second is a subsurface magnetic feature that appears to be associated with a granodiorite dome. Some of the lineaments do correlate with mapped fault structures. The low magnetic anomaly appears to be correlated with basin fill sediments, estimated at 300 meters t thick and limited to the southern portion of the valley. This conclusion is supported by the gravity work of Gimlett (L967) showing a gravity depression in the same general area with much I higher Complete Bouguer Anomalies to the northwest. It is recommended that the interested reader review the work by Hartley (1995). It is also recommended that the gravity data from Gimlett (1967) be digitized, be combined with recent I gravity work by Washoe County and the USGS, and further processed for analysis. This could provide more insight on fault structure and alluvial thickness within the valley.

I REFERENCES Bonham, Harold and Papke, Keith, 1969. Geology and mineral deposits of Washoe and Storey Counties, Nevada. Nevada Bureau of Mines and Geology, Bulletin 70. I DIGHEM, 1994. Dighemu survey for Utility Division, Washoe County Public Works, Reno, Nevada. Consultant report prepared for Washoe County Public Works, Reno, Nevada. I Geosoft, Inc.,1999. Oasis Montaj, data processing and analysis system for earth science applications, Version 4.3. Geosoft, Inc. Toronto, Ontario. Gimlett, James. 1967. Gravity Study of Warm Springs Valley, Washoe County, Nevada. Nevada Bureau of Mines Report 15. I Hartley, Paul. 1995. Warm Springs Valley Project, Washoe County, Nevada; Geological Interpretation of Airborne Geophysical Data. Consultant report prepared for Washoe County I Department of Water Resources, Reno, Nevada I

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Figure /A. X.ratytic signal on DEM with lineaments. I )zz/ I I I I I I I I I t t I I t I I I I

I Frgure 4c. Vertrcal gradient on DEM with hneaments I t 'niq I '$ I I .t\ I t2\, I I I I I I

T I I t L\ I \- t Warm Springs Valley, Washoe County, NV T] Washoe County Dept. Water Resources I ij

I Figure 6. Total Horizontal Gradient lineaments on DEM I I I IEt I o o L I ol4Ect) 2C,C, 8FE**EprFEEeE

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I Figure 7. Interpreted basin-fill faults from Hartlev. 1995 I T

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Figure 8' Fluoride isocontours (l ppm) I and well points with lineaments and Hartley inferred faults I