Geophysical Investigation of Recent Faulting in the East Gardnerville Basin Weston Thelen 1, Jim B. Scott 1, Matthew Clark 1, Craig M.

Transcription

1 Geophysical Investigation of Recent Faulting in the East Gardnerville Basin Weston Thelen 1, Jim B. Scott 1, Matthew Clark 1, Craig M. depolo 2 and John N. Louie 1 1. Seismological Laboratory, University of Nevada, Mail Stop 174, Reno, NV Nevada Bureau of Mines and Geology, Mail Stop 178, Reno, Nevada Contact: Weston Thelen wethelen@seismo.unr.edu

2 ABSTRACT Using three simple shallow geophysical methods, we have characterized small active faults near the surface in the east Gardnerville Basin, Nevada. Shallow seismic reflection, total-field ground magnetics and apparent resistivity from small Wenner arrays identify poorly understood features and, in some cases, their three-dimensional geometry. Faulting in the area is associated with zones slightly elevated magnetic susceptibility, and in one case, a lower susceptibility accompanied by high resistivity, suggesting silicification. Saturated sediments, producing an apparent-resistivity decrease across discontinuities, may be associated with impermeable clay-rich fault traces. At one locality, seismic imaging confirms faulted stratigraphy underlying both resistivity and magnetic anomalies. Our results contribute to understanding the tectonic history of this section of the Walker Lane and the evolution of the Gardnerville Basin. The presence of active faulting in the east Gardnerville Basin could significantly alter local earthquake hazard analyses. Additionally, the three techniques we employ in this study may be applied in other areas of the Great Basin to characterize ambiguous features in poorly understood areas. Keywords paleoseismology, faults, geophysics, magnetic exploration, resistivity, reflection

3 INTRODUCTION The east Gardnerville Basin is situated in the transition zone between the Sierra Nevada mountain range and the Basin and Range province (Fig. 1). To the west, the Carson Range rises above the Genoa Fault, the most active fault zone in western Nevada (Ramelli et al., 1999). In the eastern part of the basin lies a 14 km-wide fault zone adjacent to the Pine Nut Mountains (Maurer, 1984; Bell and Hoffard, 1990). The Gardnerville Basin represents one of the largest and deepest basins in the western Basin and Range Province (Trexler et al., 2000). Understanding the tectonic evolution of the valley is important in understanding the Tertiary and Quaternary evolution of the western Basin and Range. depolo et al. (2000) geologically mapped the east Gardnerville Basin, finding an unusually large number of short north-trending, east-dipping normal faults (Fig. 2A). Bell and Hoffard (1990) suggest the fault zone may be due to antithetic faulting associated with the west-dipping range front fault of the Pine Nut Mountains. Muntean (2001) attributes the faulting to an accommodation zone associated with the maximum offset of the Genoa Fault across the valley. In 1988, Bell et al. (1989) reported 1-kmlong extensional cracks along the east Gardnerville Basin fault zone, citing fault creep in the area. Bell and Hoffard (1990) also suggested, from exploratory trenching, that the latest fault displacement in the east Gardnerville Basin occurred in the mid- to late- Holocene. Our field area is located approximately 15 km to the northeast of Gardnerville along Buckeye Canyon Road (Fig. 1). Our surveys crossed 10 mapped faults north of the Buckeye Creek drainage and 6 mapped faults south of the drainage (Fig. 2A).

4 Morphologic scarps in the area, cutting Quaternary alluvium, are up to 30 meters in height. The purpose of this study is to establish geophysical evidence for faulting in the east Gardnerville Basin in the vicinity of the Buckeye Creek drainage. This study is interested in the signature of both mapped faults and unmapped faults alike. After confirmation of a discontinuity with multiple techniques, a secondary goal is to attempt to geophysically evaluate the displacement of the fault, and any other characteristics of the fault zone such as mineralization and fault timing that may produce a geophysical signal. Such characterizations will be important in describing the geologic history of the basin and in seismic hazard analyses of the area. METHODS Two magnetic transects about 4 km long, one on each side of the Buckeye drainage, were surveyed in parallel, west-northwest trending lines to identify potentially major geophysical features in the area (Fig. 2). The magnetic lines crossed the majority of the many faults in the east Gardnerville zone that extend into the Buckeye Creek area. The three largest of the magnetic anomalies were then surveyed with Wenner resistivity arrays over distances of 0.1 to 1.0 km. Identifying the largest magnetic and resistivity anomalies, we chose a site for a short (150-m-long) seismic-reflection survey. Magnetic Survey The magnetic survey deployed Scintrex ENVI and Scintrex MP-2 magnetometers as rover instruments, and a Scintrex ENVI magnetometer as the base station. Each transect was between 3 km and 4 km long with measurements taken at 50 m intervals (by handheld GPS traverse). Across large anomalies, we re-sampled at 12.5 m intervals.

5 Rover values were linearly corrected based on recorded observations at the base station by the rover instruments. Several sources of error may be present in our magnetic readings. Diurnal errors stem from approximating the diurnal magnetic variation with a line, across periods between base measurements of up to four hours. A base station malfunction prevented corrections at finer intervals. Observations of measurements by the rover instruments at the base station suggest errors of up to 10.5 nt. Errors also originate from magnetic microbursts that originate in the Ionosphere. In the base station record, the data suggest that microbursts contributed up to 8.5 nt to the overall error calculation. Resistivity Survey Our resistivity surveys crossed two magnetic anomalies on the south line and one anomaly on the north line. We used an L and R Instruments SN-110 Mini-Res directcurrent resistivity meter. At each magnetic anomaly we surveyed the apparent resistivity with lines of repeated Wenner arrays. A Wenner array configuration locates both the source and receiver electrodes a uniform distance a apart (Telford et al., 1990). In this study, we used an a spacing of 20 m, giving us a depth penetration of m. At each anomaly we also collected an approximately perpendicular resistivity array measurement, in order to reduce the effect of buried pipes or wires in our analysis. At each anomaly, measurements were collected perpendicular to the mapped strike of the anomaly until a local baseline was found. In this paper, apparent resistivity is plotted against the location of the center of the array. The apparent resistivity is found by the equation, ρ a = 2πaΔV /I

6 where ρ a is the apparent resistivity, a is the spacing between the electrodes, V is the voltage across the potential electrodes, and I is the current through the current electrodes (Telford et al., 1990). Seismic Reflection Survey The seismic reflection profile crossed the largest of the magnetic and resistivity anomalies, at Waypoint 76. A Bison Instruments Galileo-21 recording instrument was used with a 48-channel array. We deployed 6-m-long arrays of six 100 Hz reflection geophones for each channel, at 3 m spacing creating a 3-geophone overlap. Sledgehammer blows to a steel plate initiated the recording cycle synchronously for each of five hammer blows at every third station. Twenty hammer blows were also recorded at each end of the line. A band-pass filter and a dip filter (Hale and Claerbout, 1983) were used before stacking to reduce coherent noise. In order to pick the stacking velocities, we inspected the results of 100 m/s stacking-velocity intervals from m/s. We then binned common depth points at 3-m intervals that reduced to a common depth-point stack, which was then Stolt-migrated using the picked stacking velocities. The Stolt-migrated image was further dip-filtered to a cutoff slope of 15 samples per trace. RESULTS All three techniques employed in the field produced anomalies near the location of mapped faults (Fig. 2). Since each method gives insight to different properties of the shallow geology at different depths, using all three measurements together provides for a more robust analysis of the geologic setting.

7 The magnetic surveys revealed a multitude of anomalies on both the north and south lines (Fig. 2). On the north line, at Waypoint 229, there is a magnetic anomaly of approximately -30 nt over a horizontal distance of 50 m. Coincident with the apex of the anomaly is a north-trending gully about 20 m deep. Forward modeling of the magnetic anomaly at Waypoint 229 used the GM-SYS program from Northwest Geophysical Associates, and is shown in Figure 3. The best fitting model (lowest rms error) suggests a shallow, zero-magnetic-susceptibility wedge within sediments having a magnetic susceptibility of An offset layer with a normal, down to the east separation of about 20 m also fits the data reasonably well, if the layer has a magnetic susceptibility above the background. Attempts to fit the anomaly with a prism of elevated susceptibility failed to fit the data within a reasonable amount of error. At Waypoint 229, our resistivity profiling produced a 32% apparent resistivity increase over the same area as the magnetic low (Fig. 3). The resistivity anomaly, paired with the magnetic anomaly discussed above, favors the low susceptibility wedge over the offset layer model. Magnetic data at Waypoint 318 also shows a magnetic anomaly of approximately -25 nt. This anomaly is asymmetric and may represent two separate structures. Attempts at forward modeling with GM-SYS alone suggests two alternative structural models (Fig. 4). Both models have similar root mean square (rms) errors even though they represent two disparate geologic phenomena. The first model suggests two magnetized prisms with susceptibilities of The second model proposes a layer that has been offset by two normal faults down to the west, with a magnetic susceptibility of The major discrepancy in the models is in the western feature. In both models the western feature is deeper than the eastern feature, however in the first model, the

8 feature is an east-dipping prism while in the second model, the feature represents a westdipping normal-separation fault (Fig. 4). Surface morphology gives no insight to the equivalence. Resistivity profiling produces a resistivity high and low, from west to east, near the same locations as the magnetic anomalies (Fig. 4). Because of the limited depth penetration of the resistivity array, we favor the magnetized prisms (Fig. 4C). The modeled offset layer is too deep to be detected by our Wenner arrays, at 50 m, while the mineralized zone model is at a depth that is detectable by both methods. The anomalies at Waypoint 318 are defined by the fewest number of points of any feature reported in this paper. Waypoint 76 has the largest magnetic anomaly of our surveys, approximately 98 nt. The area modeled (Fig. 5) was best represented using four independent features. A large area was modeled in order to take into account the effect that one anomaly may have on another. The preferred model proposes four east-dipping magnetized prisms for each of the four anomalies, each with a susceptibility of An offset magnetized-layer model, similar to Figure 4, could not fit to the observed data with an acceptable amount of error. No surface expression is present in the immediate area however; a prominent east-dipping escarpment exists in the canyon wall immediately to the north of Waypoint 76, which strikes into the area of the magnetic anomaly. The apparent resistivity data shows a gradual decline to a minimum at approximately the same location as the magnetic low (Fig. 5). The apparent resistivity survey assesses the same area as the magnetic survey. The apparent resistivity rises abruptly to the east of the minimum. The anomaly shows a total drop in apparent resistivity of 100 ohm-m. The magnitude of the anomaly suggests a shallow feature is

9 present near the location of Waypoint 76. The apparent resistivity at Waypoint 76 (Fig. 5) shows a similar anomaly to that of Waypoint 318 (Fig. 4). Due to the magnitudes of the magnetic and resistivity anomalies at the site near Waypoint 76, a shallow seismic reflection survey was completed over a portion of the area (Fig. 5, 6). The alluvium-bedrock contact is evident in the section and appears to show a fault scarp with normal separation down to the east. The fault trace is buried under the active alluvium of the Buckeye Creek drainage. Using stacking velocities of 400 m/s for the shallow alluvium, the depth to the upper (west) reflector is 8.24 m while the depth to the lower (east) reflector is 13.7 m. Underneath the prominent interface at the base of the active alluvium, older west-dipping sediments are present. The underlying rock is likely the west-dipping Tertiary sedimentary Sunrise Pass Formation described by Muntean (2001). The seismic section loses resolution below 30 m depth. DISCUSSION Several studies in the east Gardnerville Basin have recognized faults based on air photo mapping and exploratory trenching (Bell et al., 1989; Bell and Hoffard, 1990; depolo et al., 2000; Muntean, 2001). Given the combination of magnetic, resistivity and seismic data, we believe we have found geophysical evidence for the existence of normal faulting in the east Gardnerville Basin. A study by Shields et al. (1998) used a nearly identical combination of geophysical measurements to locate strands of the Pahrump Valley fault zone in southern Nevada. They identified the largest-magnitude resistivity and magnetic anomalies with the most recently active strands of the fault.

10 At Waypoint 229, the combination of a resistivity high, combined with a magnetic low, suggests that a very shallow structure beneath the surface is present (~1 m deep). One material that possesses low magnetism and high resistivity is quartz (Telford et al., 1990). The structure may be a wedge of partially silicified material, possibly created by deposition of quartz by shallow groundwater that has been collected behind an impermeable zone of fault gouge. No mapped faults cross or project into the area near Waypoint 229. The modeled east dip of this anomaly may coincide with modeled structures in the west of the Waypoint 76 section, however confirmation of this relationship would require more detailed data. At Waypoint 318, two slightly magnetized zones facing each other at approximately 15 m depth are modeled. The drop of apparent resistivity to the east of the eastern anomaly could represent the presence of groundwater backed up behind an impermeable fault zone. This may indicate a saturated area in the zone between the magnetized areas. The location of projected faults into the area of interest to not correspond well with the modeled location of faults, however the presence of pervasive faulting in the area may confuse modeling or mapping and contribute to the discrepancy between observed and predicted faulting. At Waypoint 76, we have strong evidence for at least one normal-separation fault. Seismic evidence suggests offset of an erosional surface of about 5.5 m, buried 10 m below the current ground surface. Our magnetic data suggests that some very weak magnetized zones are present at slightly deeper depths, between 10 and 20 m. One mechanism for the occurrence of a magnetized zone may be through the deposition of iron oxides by water, whose transport has been facilitated by fault gouge or fracturing

11 near the fault plane. A similar mechanism was proposed for the Pahrump Valley fault by Shields et al. (1998). The resistivity data in the area suggests a shallower feature than indicated by either the magnetic or seismic data. The resistivity anomaly was the largest encountered in the area. The simplest explanation for a broad change in apparent resistivity is the saturation of the shallow sediments near the survey point by groundwater (Telford et al., 1990). If the anomaly is due to water backed up behind an impermeable layer, then shallow sediments of the most Recent age are likely offset, as well as the Tertiary sediments below. The zone of higher saturation would likely be to the east of the fault, as Buckeye Creek runs east to west, and the groundwater gradient likely follows the topography, high on the east. An impermeable fault zone can easily be created by exposing clay to brittle deformation inside a fault zone. The fault imaged in this survey appears to be the most active fault in the survey area due to the large electrical and magnetic anomalies. Many normal faults project into the area near Waypoint 76. The locations of modeled faults correspond very well with the location of projected faults in this area. The separation of the faults (down to the west or down to the east) is somewhat ambiguous in the area (Fig. 2). Interacting faults to the north of Waypoint 76 show mainly down to the east faults and, accordingly, agree with our seismic and magnetic data. To the east of Waypoint 76, the modeled fault agrees with surficial morphology. To the west, the modeled faults do not agree with the surface morphology. The cause of the discrepancy is the subject of further research.

12 CONCLUSION Combining magnetics, apparent resistivity and seismic data, we find geophysical evidence for normal faulting in the east Gardnerville Basin. At all three zones where the methods were concentrated, evidence is present that suggests discontinuities in Recent sediments. These discontinuities appear to coincide with mapped active faults strands. This combination of geophysical methods may be used throughout the Great Basin to find the location, geometry and geologic conditions of hidden or questionable faults. The confirmation of active faulting assists in the understanding of the evolution of the Gardnerville Basin. The presence of active faulting in the east Gardnerville Basin has pronounced effects on regional seismic hazard analysis. Recent faulting located away from the predominant Genoa fault, on the west edge of the basin, creates a broader geographic distribution of potential earthquake sources. Our study provides a preliminary analysis for a more comprehensive seismic hazard study in the future.

13 ACKNOWLEDGEMENTS A special thanks goes to those who have helped in the completion of this report: Shane Smith for his participation in field work; Drs. Gary Oppliger and Robert Karlin who supplied the magnetic instruments; and Ron Petersen for loaning the resistivity instrument. Drs. Pat Cashman and Jim Trexler also helped in evaluating the seismic data.

14 REFERENCES Bell, J.W. and Hoffard, J.L., 1990, Late Quaternary tectonic setting for a possible fault creep event in the Pine Nut Mountains Area, western Nevada: Geological Society of America Abstracts with Programs, v. 22, p. 7. Bell, J.W., Ramelli, A.R., and depolo C.M., 1989, Extensional cracking along an active normal fault: A case for creep on a Basin and Range Fault?: Seismological Research Letters, v. 60, p. 30. depolo, C.M., Ramelli, A.R., and Muntean, T., 2000, Preliminary geologic map of the Gardnerville 7.5 minute Quadrangle, Douglas County, Nevada: Nevada Bureau of Mines and Geology, 1:24000 scale, 1 sheet. Hale, D., and Claerbout, J.F., 1983, Butterworth dip filters: Geophysics, v. 48, p Maurer, D.K.,1984, Gravity survey and depth to bedrock in Carson Valley, Nevada- California: U. S. Geological Survey Water-Resources Investigation Report, no , 20 pp. Muntean, T.W., 2001, Evolution and Stratigraphy of the Neogene Sunrise Pass Formation of the Gardnerville Basin, Douglas County, Nevada [Master s Thesis]: University of Nevada, Reno, 223 pp.

15 Raines, G. L., Sawatzky, D. L., and Conners, K. A., 1996, Cenozoic faults (geographic projection): U.S. Geological Survey Great Basin Geoscience Database, DDS-41. Ramelli, A.R., 1999, Large magnitude, late Holocene earthquakes on the Genoa fault, west-central Nevada and eastern California: Bulletin of the Seismological Society of America, v. 89, p Shields, G., Allander, K., Brigham, R., Crosbie, R., Trimble L., Sleeman, M., Tucker, R., Zhan, H., and Louie, J. N., 1998, Shallow geophysical survey across the Pahrump Valley fault zone, California-Nevada border: Bulletin of the Seismological Society of America, v. 88, p Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990, Applied Geophysics: Cambridge, Cambridge University Press, ed. 2, 770 pp. Trexler, J.H., Cashman, P.H., Henry, C.D., Muntean, T., Schwartz, K., Tenbrink, A., Faulds, J.E., Perkins, M., and Kelly, T., 2000, Neogene basins in western Nevada document the tectonic history of the Sierra Nevada-Basin and Range transition zone for the last 12 Ma, in Lageson, D.R., et al., eds., Great Basin and Sierra Nevada: Boulder, Colorado, Geological Society of America Field Guide 2, p

16 FIGURE CAPTIONS Figure 1: Location map and regional tectonic setting of the Buckeye drainage. Note the multitude of mapped faults (dark lines, Raines et al., 1996) on the east side of the Gardnerville Basin. CR= Carson Range, PN= Pine Nut Mountains, GF= Genoa Fault, BC= Buckeye Creek (field site). Figure 2: (A) Fault map of our field area with the magnetic measurement locations plotted (depolo et al., 2000). White symbols with black outlines are the locations of the magnetic measurements. Dark lines are quaternary faults with balls marking the downdropped side of a normal fault. Dotted lines represent where faults are inferred. (B) The total field ground magnetic measurements are plotted according to longitude on the north and south transects. In both figures, the three areas of interest are shown. Figure 3: (A) Apparent resistivity data for the area near Waypoint 229. (B) Results of forward modeling of the magnetic data. Dark circles are total field ground magnetic measurements around Waypoint 229. The thick gray line represents the results of forward modeling. The magnetics scale is relative to nt. Note the resistivity maximum occurs coincident with the magnetics minimum. (C) The best-fit model shows a large non-magnetic zone. With no vertical exaggeration, the bottom boundary of the non-magnetized zone dips at nearly 45 degrees. The calculated rms error for this model is 7.6. No other trial models were able to fit the magnetic data within an acceptable amount of error. All three graphs are at the same horizontal scale. No mapped faults intersect or project into this profile.

17 Figure 4: (A) Apparent resistivity data for the area near Waypoint 318. Note the apparent resistivity low near Waypoint 318 (0 distance). (B) The total field ground magnetic measurements for the area and the results of forward modeling around Waypoint 318. Dark circles represent magnetic measurements. The magnetic scale is relative to nt. The dotted gray line is the result of forward modeling with the model shown in C. The solid gray line is the result of forward modeling with the model shown in D. (C) Model showing the two magnetized zones of a magnetic susceptibility of (D) The mineralized layer model was also modeled with a magnetic susceptibility of Root mean square errors for the top and bottom layers were 7.0 and 7.40, respectively. The area around both models has no magnetic susceptibility. Mapped faults, as they are projected into the area of interest, are shown with dotted lines. All panels are at the same horizontal scale. Figure 5: (A) Apparent resistivity data for the area surrounding Waypoint 76. Note the gradual decline in resistivity with increasing distance east. (B) Results of forward modeling of the magnetic data near Waypoint 76. The magnetic scale is relative to nt. The solid line is the model fit while the dark circles are the measured data. (C) The final model used in forward modeling. The calculated root mean square error for the model is 7.3. The area around the mineralized prisms are modeled with no magnetic susceptibility. The light gray box shows the extent of the seismic section. Mapped faults, as they are projected into the area of interest, are shown with dotted lines. All panels have the same horizontal scale.

18 Figure 6: Stacked and migrated seismic section of Waypoint 76 showing a prominent boundary between Quaternary alluvium and west-dipping sediments. The depth to the boundary on the west side of the section is ~8 m and the depth to the boundary on the east is ~14 m using a stacking velocity of 400 m/s. Imaging suggests a fault dip of about 26 degrees east. The morphology of the boundary matches that of normal fault scarps immediately north of Waypoint 76.

19 Carson City PN CR GF BC Gardnerville km Figure 1

20 A '00" '00" 39 00'00" Waypoint '00" Waypoint 229 Waypoint Kilometers B 40 North Transect 0 Waypoint 229 South Transect Waypoint Waypoint Figure 2 Longitude (-119+decimal degrees)

21 A 110 Apparent Resistivity (ohm-m) B Magnetics (nt) C Elevation (meters) West Sediments s=5.5e-4 Non-magnetic Zone s=0 East 1450 V.E.= Distance from Waypoint 229 (meters) Figure 3

22 A Apparent Resistivity (ohm-m) B Magnetics (nt) C Elevation (meters) West Projection of faults mapped by Depolo et al., 2000 Magnetized Zones s=0.001 East 1450 D Elevation (meters) West Projection of faults mapped by Depolo et al., 2000 Magnetized Layer s=0.001 East V.E.= Distance from Waypoint 318 (meters) Figure 4

23 A 140 Apparent Resistivity (ohm-m) B Magnetics (nt) C Projection of faults mapped by Depolo et al., 2000 Elevation (meters) West Magnetized Zones s=1.5e-5 East Figure V.E.=2 Distance from Waypoint 76 (meters)

24 Distance (m) From Waypoint Projection of fault mapped by Depolo et al Quaternary Alluvium 50 ~ 8 m Time (ms) West Dipping Sediments ~ 14 m ~ 3x vertical exaggeration Figure 6