S. John Caskey

Paleoseismic transect across the northern Great Basin

[1] The relationship of strain accumulation to strain release over different timescales provides insight to the dynamics, structural development, and spatial and temporal pattern of earthquake recurrence in regions of active tectonics. The Great Basin physiographic province of the western United States is one of the Earth’s broadest regions of ongoing continental extension, encompassing an area reaching 800 km in width between the Sierra Nevada to the west and Wasatch mountains to the east. We present observations arising from excavations, scarp profiling, optically stimulated luminescence, and radiocarbon dating to place limits on the late Pleistocene paleoseismic history of faults bounding eight ranges across the interior of the northern Great Basin, specifically, the Shawave, Hot Springs, Humboldt, Sonoma, Shoshone, Tuscarora, Dry Hills, and Pequop ranges. Combining the observations with similar previously published studies within and at the margins of the Great Basin yields a transect that extends eastward across the basin between the 40th and 41st parallels. The sum of observations provides a picture of the patterns and rates of earthquake recurrence over the region during the last 20–45 kyr that may be compared to patterns of contemporary seismicity and recently reported measures of strain accumulation across the area using GPS. The recurrence rate of large surface rupture paleoearthquakes along ranges at the margins of the Great Basin is systematically greater than observed along ranges in the interior. The pattern is similar to seismological and geodetic measurements that show levels of background seismicity and strain accumulation are also concentrated along the margins of the Great Basin. An east-west extension rate across the interior of the Great Basin on the order of 1/2 mm yr 1 (strain rate of 1 nstrain yr 1 ) over the last 20–45 kyr is estimated by summing the record of paleoseismic displacements across the 400 km breadth of the transect, as compared to 2m m yr 1 of strain accumulation indicated by a recently reported analysis of a collinear GPS survey. The comparison is hindered by significant uncertainties coupled to the geologic rate estimate. The transect also crosses the northern limit of the central Nevada seismic belt. The central Nevada seismic belt is defined by a north-northeast trending alignment of historical surface rupture earthquakes, increased levels of background seismicity, and strain accumulation rates greater than observed elsewhere in the interior of the Great Basin. The reported recurrence rate of late Pleistocene surface rupture earthquakes within the central Nevada seismic belt is also generally greater than observed along our transect. The observations when taken together suggest that the characteristics of strain release observed historically within the central Nevada seismic belt have been operative over the latest Pleistocene and that the apparently greater rates of strain accumulation and release in the central Nevada seismic belt are diminished or less localized in regions to the north and east. Thus, while the historical alignment of surface ruptures that defines the central Nevada seismic belt remains a unique clustering of earthquakes in time and space, the likelihood of the cluster at its observed location appears greater than would be expected to the north or eastward in the interior of the Great Basin.

Pattern and Rates of Faulting in the Central Nevada Seismic Belt, and Paleoseismic Evidence for Prior Beltlike Behavior

The central Nevada seismic belt (CNSB) is a concentration of historical (1915-1932-1954) surface faulting in the western Basin and Range province, forming a linear, nearly continuous 300-km-long rupture zone. Previous results are integrated in this study with new paleoseismic and exploratory trenching data from the historical zones to look for evidence of older, similar beltlike patterns or elevated slip rates that could indicate whether the CNSB is a zone of focused, long-term crustal strain. The data show that the continuous rupture belt produced by the seven earthquakes occurring between 1915 and 1954 is unique in the available paleoseismic record. At the 1954 Fairview Peak fault, the lack of prehistorical faulting in deposits containing the Wilson Creek bed 19 tephra eliminates the possibility of an identical seismic belt in the past 35.4 ka. Our studies also show that the faults have net slip rates ranging from a low of 0.09 mm/yr on the Fairview Peak fault to a high of 0.7 mm/yr on the 1932 Cedar Mountain fault. These are considered moderate to low rates that are similar to most late Quaternary faults in the western Basin and Range province. A space-time comparison shows that the paleoseismic histories for these multiple rupture zones are diverse, and the number and timing of events in each of the zones indicate that there is little evidence for older contemporaneous ruptures of these same faults. Based on these results we reach several conclusions regarding the longer term (≈Holocene) behavior of the CNSB. Although paleoseismic data preclude an older identical rupture belt among the historical zones, consideration of associated Holocene faults within the greater CNSB region indicates that several similar, but not identical, beltlike rupture patterns are plausible during the past 13 ka, although each requires seismic gaps in the along-strike pattern. Although long-term strain (represented by density of young faults) does appear to increase from east to west into the CNSB, the slip-rate data demonstrate that the CNSB is not a belt of concentrated or elevated crustal strain compared with areas that extend west to the Sierra Nevada. The increase in the distribution of Holocene fault activity from east to west into the CNSB is consistent with a marked increase in the 1992-2002 GPS velocity field at the latitude of the 1954 rupture sequence. However, a comparison of the geologic rates across the belt at this same latitude indicates that the extension rates (0.59-1.37 mm/yr) are systematically lower than both the campaign and continuous GPS rates (2.20-3.13 mm/yr) by factors of 2-5. These discrepancies may be due to postseismic strain, or to some form of off-fault deformation. We conclude that the results of our study of fault behavior in the CNSB best support the belt migration model proposed by Wallace (1987) for the western Basin and Range province in which temporal tectonic pulses are believed to migrate regionally, activating different beltlike combinations of late Quaternary faults in an as yet unknown pattern of migration.

Geophysical confirmation of low-angle normal slip on the historically active Dixie Valley fault, Nevada

The December 16, 1954, Dixie Valley earthquake (Ms = 6.8) followed the nearby Fairview Peak earthquake (Ms = 7.2) by 4 min, 20 s. Waveforms from the Fairview Peak event contaminate those from the Dixie Valley event, making accurate fault plane solutions impossible. A recent geologic study of surface rupture characteristics in southern Dixie Valley suggests that the Dixie Valley fault is low angle (<30°) along a significant portion of the 1954 rupture. To extend these observations into the subsurface, we conducted a seismic reflection and gravity experiment. Our results show that a portion of the Dixie Valley ruptures occurred along a fault dipping 25° to 30°. As such, the Dixie Valley event may represent the first large, low-angle normal earthquake on land recorded historically. Our high-resolution seismic reflection profile images the rupture plane from 5 to 50 m depth. Medium-resolution reflections, as well as refraction velocities, show a smoothly dipping fault plane from 50 to 500 m depth. Stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth. Gravity profiles conservatively constrain maximum basin depth and define overall geometry. Extension along the low-angle section may have occurred in two phases during the Cenozoic. Current fault motion postdates a 13 to 15 Ma basalt, imaged in the hanging wall, and inherits from a fault formed during an earlier extensional pulse, concentrated at 24.2 to 24.4 Ma. The earlier extension suggests extraordinary slip rates as high as 18 mm/yr, resulting in the formation of the low-angle fault break. Sections of the Dixie Valley fault where there is no evidence for current low-angle slip correlate well with areas where no pre-15 Ma slip has been documented.

Historic Surface Faulting and Paleoseismicity in the Area of the 1954 Rainbow Mountain-Stillwater Earthquake Sequence, Central Nevada

The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 August 1954. More than 50 field measurements of surface offsets constrain the distribution of slip along the discontinuous and distrib- uted rupture zone that formed during the earthquake sequence. Vertical offsets reach a maximum of 0.8 m with the average vertical offset being 0.2 m. In contrast to original reports, we see evidence for a right-lateral component of slip along portions of the rupture zone, including offset stream channels (0.5-1.0 m), left-stepping en echelon scarps, and a well-preserved, 100-m-long mole track. The right-slip com- ponent is consistent with focal plane solutions for the events and recent geodetic results. Previously unmapped surface ruptures now extend the known rupture length of the sequence by 25 km to a total of 70 km. Surface ruptures along the previously unmapped Fourmile Flat fault are subparallel to and form a 10-km left step to the southeast of the Rainbow Mountain fault. Event locations and anecdotal information indicate that the Fourmile Flat ruptures represent minor, primary surface rupture associated with the large 6 July aftershock, triggered 11 hr after the initial 6 July Rainbow Mountain event. The paleoseismic histories of the Rainbow Mountain and Fourmile Flat faults, as recorded in natural and trench exposures, are different although both faults experi- enced three post 15-ka surface rupturing events, including 1954. Bracketing ages for triultimate events on both faults do not overlap. However, constraints on the penultimate event for the Rainbow Mountain and triultimate event for the Fourmile Flat fault do overlap slightly, allowing the possibility that they may have ruptured close in time as in 1954. The Holocene slip rate for the Fourmile Flat fault (0.40 mm/yr) is similar to the post-latest Pleistocene rate for the Rainbow Mountain fault (0.20-0.46 mm/yr) even though the total length of the Fourmile flat (10 km) is much shorter than the overall length of the Rainbow Mountain rupture zone (60 km), indicating that even minor faults can be important for assessing regional strain rates and patterns.

Mesozoic deformation in the Nevada Test Site and vicinity: Implications for the structural framework of the Cordilleran Fold and thrust belt and tertiary extension north of Las Vegas Valley

Detailed studies in the CP Hills and Mine Mountain area of the Nevada Test Site (NTS), together with analysis of published maps and cross sections and a reconnaissance of regional structural relations, indicate that the CP thrust of Barnes and Poole [1968] actually comprises two separate, oppositely verging Mesozoic thrust systems: (1) the west vergent CP thrust, which is well exposed in the CP Hills and at Mine Mountain, and (2) the east vergent Belted Range thrust located northwest of Yucca Flat. Regional structural relations indicate that the CP thrust forms part of a narrow sigmoidal belt of west vergent folding and thrusting traceable for over 180 km along strike. The Belted Range thrust represents earlier Mesozoic deformation that was probably related to the Last Chance thrust system in southeastern California, as suggested by earlier workers. A reconstruction of the pre-Tertiary geometry of the Cordilleran fold and thrust belt in the region between the NTS and the Las Vegas Range bears a close resemblance to other regions of the Cordillera and suggests that west vergent deformation developed in the hinterland of a part of the Sevier fold and thrust belt characterized by substantial structural relief. Reconstruction of the fold and thrust belt also suggests that previous estimates of upper crustal Tertiary extension north of the Las Vegas Valley shear zone (e.g., 80% [Guth, 1981]) are 2 or 3 times too large.

Low-temperature thermochronology of the Black and Panamint mountains, Death Valley, California: Implications for geodynamic controls on Cenozoic intraplate strain

We use apatite and zircon (U-Th)/He thermochronometry to evaluate space-time patterns and tectonic drivers of Miocene to Pliocene deformation within the Death Valley area, eastern California. Zircon He ages from the footwall of the Amargosa–Black Mountains detachment in the Black Mountains record continuous cooling and exhumation from 9 to 3 Ma. Thermal modeling of data from the central Black Mountains suggests that this cooling took place during two intervals: a period of rapid footwall exhumation from 10 to 6 Ma, followed by slower (<5 mm/yr) exhumation since 6 Ma. Cumulative exhumation is estimated to be 10–16 km. Paleodepth reconstruction of cooling ages from the footwall of the Panamint-Emigrant detachment, in the central Panamint Range, also show two periods of cooling. Zircons record late Miocene cooling, whereas apatite He ages show punctuated exhumation at ca. 4 Ma. The results suggest the Panamint Range experienced a minimum of 7.2 km of exhumation since ca. 12 Ma. The new data, when evaluated within the context of published fault timing data, suggest that the transition from Basin and Range extension to dextral transtension is spatially and temporally distinct, beginning at ca. 11–8 Ma in ranges to the east and north of the Black Mountains and migrating westward into eastern Death Valley at 6 Ma. Initiation of dextral transtension was coincident with a major change in plate-boundary relative motion vectors. Data from Panamint Range and several ranges to the west of Death Valley indicate transtension initiated over a large area at ca. 3–4 Ma, coeval with proposed lithospheric delamination in the central and southern Sierra Nevada Range. Our results suggest that the transition from extension to dextral transtension may reflect an evolution in tectonic drivers, from plate-boundary kinematics to intraplate lithospheric delamination.

Recency of Faulting and Neotechtonic Framework in the Dixie Valley Geothermal Field and Other Geothermal Fields of the Basin and Range

We studied the role that earthquake faults play in redistributing stresses within in the earths crust near geothermal fields. The geographic foci of our study were the sites of geothermal plants in Dixie Valley, Beowawe, and Bradys Hot Springs, Nevada. Our initial results show that the past history of earthquakes has redistributed stresses at these 3 sites in a manner to open and maintain fluid pathways critical for geothermal development. The approach developed here during our pilot study provides an inexpensive approach to (1) better define the best locations to site geothermal wells within known geothermal fields and (2) to define the location of yet discovered geothermal fields which are not manifest at the surface by active geothermal springs. More specifically, our investigation shows that induced stress concentrations at the endpoints of normal fault ruptures appear to promote favorable conditions for hydrothermal activity in two ways. We conclude that an understanding of the spatial distribution of active faults and the past history of earthquakes on those faults be incorporated as a standard tool in geothermal exploration and in the siting of future boreholes in existing geothermal fields.