T22A-01
Low Angle Normal Fault, Fossil or Active?
The Panamint Valley – Hunter Mountain – Saline Range (PHS) faults are, together with the Death Valley and Owens Valley faults, one of the three major fault zones within the Eastern California Shear Zone (ECSZ). The ECSZ is the most active fault system bounding the Basin and Range to the southwest with approximately 10 mm/yr of cumulative slip along strike-slip and trans-tensional segments. Previous work has identified the Panamint Valley and Saline Range faults as low angle normal faults and the Hunter Mountain as a transfer fault (Wesnousky and Jones, 1994). A debate exists whether this system is active at present time. Interferometry Synthetic Aperture Radar (InSAR) is a geodetic technique that allows measurement of ground motion at a mm/yr accuracy over large areas with a high measurement sampling. We processed a large number of data to investigate ground motion in the PHS fault system to shed light on the interseismic strain accumulation and its relation to the fault geometry. Preliminary results indicate high strain rate over the Hunter Mountain fault. The locking depth of the fault inferred from elastic modeling of interseismic strain accumulation is on the order of 4km, significantly shallower than for neighboring faults. In contrast, the long wavelength strain field across the Panamint and Saline faults indicates possibly deeper locking depths and/or shallower dip. The shallow locking depth of 4km inferred for the Hunter Mountain fault corresponds with the extension at depth of the two bounding low angle normal faults below Hunter Mountain, suggesting a control by the low angle normal fault system.
T22A-02
Detachment Faulting and Hanging-wall Segmentation in the Gulf Extensional Province, Baja California
The opening of the Gulf of California, caused by the re-localization of the Pacific -- North America plate boundary, is a premier example of an incipient passive margin. The San Felipe area in northern Baja California comprises a typical basin and range-style topography, produced by Neogene extension affecting the Gulf Extensional Province (GEP). The > 100 km long Sierra San Pedro Mártir (SSPM) escarpment is defined by an east-down normal fault system, separating the unextended western part of the peninsula from the GEP. The hanging-wall of the SSPM fault is characterized by several fault-bound blocks that have been rotated around both vertical and horizontal axes. The Las Cuevitas and the Santa Rosa detachments are two major fault systems in the Sierra San Felipe that are responsible for segmentation of the hanging-wall of the SSPM fault. The curvilinear, NE to NNW striking Las Cuevitas Detachment extends for more than 43 km and brings Neogene volcanics and sediments in contact with the Mesozoic basement of the footwall. The detachment dips at 15-50° and accommodates more than 4-6 km of east-down displacement. Faulting appears to preferentially occur at the contact of Mesozoic metamorphics to granitoid intrusives with transfer faults translating deformation between the faulted contacts. Fission track and (U-Th)/He thermochronology constrain the initiation of deformation on the Las Cuevitas detachment to the latest Miocene. The > 45 km long and equally curvilinear Santa Rosa Detachment system consists of a low-angle normal fault system (15-35° dip) with transfer zones facilitating the transfer of strain between individual segments of the detachment fault. The slip direction varies between top-to-the-east and top-to-the-south, a possible explanation for the change in strike from NW to NE to NW. The roughly 4-5 km offset of Neogene volcanic and sedimentary deposits has created the necessary space for syntectonic deposition in the Santa Rosa Basin. The onset of deformation is loosely constrained to postdate the 12.6 Ma Tuff of San Felipe, but (40Ar/39Ar), fission track, (U-Th)/He and cosmogenic nuclide dating in progress will refine the timing of deformation.
T22A-03
Transition In Style Of Miocene Volcanism And Inception And Style Of Faulting In The NW Basin And Range Province, Southern Oregon
New data from the northwest Basin and Range province in SE Oregon indicate a shift from arc-related volcanism to extension-related volcanism from the Early Miocene to the Late Miocene, respectively. New 40Ar-39Ar ages of Miocene volcanic rocks indicate two episodes of volcanism, between ~21-23 Ma and again at ~9-7 Ma. These episodes occurred prior to and during development of NW- and NNE-striking extensional faults near Abert Rim, one of the primary Basin and Range faults in southern Oregon. The Early Miocene Coleman Hills, located within the hanging wall of Abert Rim, are built of successively erupted rhyolite domes (21.79 ± 0.66 Ma, 2 σ), dacite domes (21.70 ± 0.56 Ma) and a central basaltic andesite to andesite volcano (22.02 ± 0.54 Ma). The Rabbit Hills occur in the footwall of Abert Rim and yield ages of 22.33 ± 0.24 Ma and 20.34 ± 0.36 Ma for rhyolite and 23.12 ± 0.48 Ma for basaltic andesite. Abundant dikes within Early Miocene volcanoes mimic the strike of both the NW and NNE, Late Miocene faults, yet it is not obvious that extensional faulting was active during this time. Middle Miocene volcanic activity is represented by 150-m thick section of Steens Basalt that covered ~500 m of volcanic paleotopography at the Coleman Hills. The age of the basalt (16.12 ± 0.30 Ma) and its composition indicate that it correlates to the upper part of the section in the source region at Steens Mountain, ~100 km to the east. Late Miocene volcanism near Lake Abert is bimodal. Silicic centers include: 8.79 ± 0.30 Ma Flint Ridge (dacite) and 8.58 ± 0.16 Ma Juniper Mountain (rhyolite). Mafic lavas include: 8.69 ± 0.42 Ma (basaltic andesite) and 7.71 ± 0.42 Ma (high-alumina olivine tholeiite). Chemical correlation places their source at Venator Butte, a small shield volcano. Geochemical data support attributing the Early Miocene magmatism to arc volcanism of the ancestral Cascades. The Late Miocene volcanism is intimately tied to the structural development of the NNE Abert Rim fault and NW faults. Extensional deformation was ongoing near Lake Abert by 7 Ma. Angular unconformities of ~5- 15°, related to stratigraphic separation produced by faulting, occur between ~8.7, ~7.7 and 7.0 Ma volcanic units. While both NW- and NNE-striking faults cut Late Miocene volcanic units, field documentation and partial restorations of cross-sections indicate that NW-striking faults formed ahead of NNE-striking faults near Lake Abert. In this way, the ~9-7 Ma volcano-tectonic setting of this part of Abert Rim is similar to the modern configuration of volcanism of the High Lava Plains at the northern tip of Abert Rim.
T22A-04 [WITHDRAWN]
Neogene Extension Along The Aegean Forearc Ridge As A Product Of Rapid Slab Roll-Back
Recent detailed mapping and structural observations in Kythera and South Peloponnese demonstrate that there are ductile, ductile-brittle and some brittle structures in the PQU metamorphic unit and its detachment from cover sedimentary units that indicate a significant localized NW-SE extension along the Aegean fore-arc ridge (in contrast to regional extension which is perpendicular to the arc). The youngest brittle structures, which affect latest Miocene-Pliocene sediments, indicate return to NE-SW regional extension. We recognize this localized extension of the southwestern part of the Hellenic arc in the Kythera strait as an exceptional arc-parallel extension of the fore-arc ridge in contrast to adjacent parts of the Aegean arc. Zircon FT cooling ages show exhumation of the PQU of Kythera at 13 Ma which must correspond to the along-arc extension and the passage through the brittle-ductile transition. The PQU exhumation structures exposed in Kythera and the nearby area around Neapolis on the SE Peloponnese contrast with adjacent areas (Peloponnese, and western Crete) where only arc- perpendicular ductile and brittle structures are developed. We think that the ductile to ductile-brittle extension in Kythera is related to a tectonic style change of the subduction to rapid slab roll-back/retreat and consequent expansion and along-strike extension of the fore-arc region, as well as differential rotation (about a vertical axis) and related bending caused by segmentation and differential forward motion of arc segments.
T22A-05
Evidence for Right-lateral Shear Along the Northwest Margin of the Eastern Snake River Plain, Idaho
Previous investigators have proposed that extension within the eastern Snake River Plain (ESRP) is accommodated by intrusion of dikes at a rate similar to the rate of extension in the surrounding Basin and Range. This hypothesis is primarily based on the lack of strike-slip offset along the northwest physiographic boundary of the ESRP, the lack of seismicity within the ESRP relative to the surrounding active Basin and Range, and the presence of NW-trending volcanic rift zones within the ESRP. The ESRP is a 400-km long region within the track of the Yellowstone Hotspot that extends from southern Idaho northeast into northwestern Wyoming. GPS data compiled for this study are used to test this hypothesis. Several institutions including the Idaho National Laboratory, National Geodetic Survey, Rensselaer Polytechnic Institute, and University of Utah observed GPS stations from 1994 to 2007 within the ESRP and surrounding region. The GPS velocities show the average orientation of horizontal GPS velocities in the adjacent northwest Basin and Range region is similar to the average orientation for the ESRP (N113°W vs N91°W, respectively), but the average magnitude of horizontal GPS velocities in the Basin and Range (1.4 ± 0.3 mm/yr) is less than that for the ESRP (2.2 ± 0.3 mm/yr). Additionally, the adjacent northwest Basin and Range extends at about 9 x 10-9 /yr with most of the deformation localized along three NW-trending normal faults (Lost River, Lemhi, and Beaverhead). In contrast, the ESRP extends at a rate that is an order of magnitude lower than the adjacent northwest Basin and Range and we see little indication of extension along the Great Rift or other volcanic rift zones over the 400 km length. The GPS differential motion along the region of the ESRP adjacent to the northwest Basin and Range indicates a NE-trending zone of right-lateral shear. Preliminary inversions of GPS velocities, earthquakes, faults, and volcanic features indicate this zone of right-lateral shear is located 10-20 km from the physiographic boundary between the ESRP and adjacent Basin and Range.
T22A-06
A Potential Reconciliation of Short-Term Geodetic and Long-Term Geological Strain-Rate Estimates Across the Owens Valley
Geodetic data show a distinctive pattern of modern strain-rate across the southwestern Great Basin (between 36° and 38° N). Strain rates are low in the interior of the Basin and in the Sierra Nevada block, whereas they are large immediately east of the Sierra Nevada. A compilation of GPS velocities indicates an average displacement rate of the Sierra Nevada block relative to the White/Inyo Mountain block of 4.1±0.8 mm yr-1 directed 305°±6°. This value constitutes approximately 25 percent of the total strain between the North American craton and the Sierra Nevada. A value this large is in apparent conflict with the results of paleoseismic studies that indicate slip rates on the Owens Valley Fault Zone (OVFZ), and associated dip-slip faults, are too low to accommodate this rate of divergence. It also conflicts with mechanical models of faulting that show that sufficient slip on high-angle normal faults to permit this rate of displacement would produce a much greater depth to bedrock than is actually observed in the Owens Valley. We have investigated the long-term slip rate on the OVFZ by using ground-penetrating radar to determine the margin of a lava flow at Crater Mountain that is dextrally offset by the fault, and hence the total displacement of the flow. We have used cosmogenic 36Cl to determine an eruption age of 68±12 ka, yielding a slip rate over this time that averages between 2.8 and 4.5 mm yr-1. In contrast to previous studies over much shorter time scales, this rate is in general agreement with that from geodetic measurements. We further propose that displacement on both strike-slip and normal, valley-bounding faults is integrated into relatively uniform regional transtensional strain by means of extensive low-angle faults underlying the Owens Valley. Displacement on listric valley-bounding faults permits significant extension without requiring vertical displacements in excess of observations. These new data and observations support the inference, based on short-term geodetic measurements, that much of the total strain between the Colorado Plateau and the southern Sierra Nevada is, and has been, accommodated in a relatively narrow band east of the Sierra Nevada.
T22A-07
Tectonic Setting of the Gravity Fault and Implications for Ground-Water Resources in the Death Valley Region, Nevada and California
The Amargosa trough, extending south from Crater Flat basin to the California-Nevada state line, is believed to be a transtensional basin accommodated in part by strike-slip displacement on the northwest-striking State Line fault and normal displacement on the north-striking Gravity fault. The Gravity fault, lying along the eastern margin of the Amargosa trough, was first recognized in the 1970s on the basis of correlations between gravity anomalies and a prominent spring line in Amargosa Valley. The Gravity fault causes an inflection in water-table levels, similar to other (but not all) normal faults in the area. Pools along the spring line, some of which lie within Death Valley National Park and Ash Meadows Wildlife Refuge, include endemic species potentially threatened by increasing agricultural activities in Amargosa Valley immediately to the west, where water tables are declining. Most of the springs and pools lie east of the Gravity fault, however, and it is important to understand the role that the Gravity fault plays in controlling ground-water flow. We have conducted a variety of geophysical investigations at various scales to better understand the tectonic framework of the Amargosa Desert and support new ground-water-flow models. Much of our focus has been on the tectonic interplay of the State Line, Gravity, and other faults in the area using gravity, ground-magnetic, audiomagnetotelluric (AMT), and time-domain electromagnetic (TEM) surveys. With 1250 new gravity measurements from Ash Meadows and Stewart Valley, we have developed a revised three-dimensional crustal model of the Amargosa trough constrained by well information and geologic mapping. The model predicts approximately 2 km of vertical offset on the Gravity fault but also suggests a complex structural framework. The fault is conventionally seen as a simple, down-to-the-west normal fault juxtaposing permeable pre-Tertiary carbonate rocks to the east against less permeable Tertiary sediments to the west. The new gravity inversion indicates a more complex footwall: some springs, for example, are associated with a concealed ridge or horst, with a secondary basin lying to the east. Six ground-magnetic transects across the Gravity fault using a truck- towed magnetometer show a characteristic magnetic anomaly reflecting different magnetic properties in rocks east and west of the fault. Ground-magnetic measurements, interpreted in conjunction with existing aeromagnetic data, allow us to map the shallow aspects of the Gravity fault and other faults in Ash Meadows in detail. Three TEM transects across the Gravity fault showed no strong evidence of a faulted contact, although depth of penetration may have been insufficient to reach associated resistivity contrasts. An AMT transect, however, shows a narrow zone of high resistivity directly along the Gravity fault. Although other interpretations are possible, this resistivity anomaly may reflect carbonate-rich cementation along the fault plane, possibly contributing to its influence on ground-water flow.
T22A-08
A 3000 Year Geologic Slip Rate From the Central North Anatolian fault, Turkey, Using 10Be Cosmogenic Nuclide Dating
Understanding whether strain loading and release rates on major faults are constant or not in time is one of the major, unresolved questions in active tectonics. As part of our ongoing studies of the North Anatolian fault (NAF) in Turkey, we mapped the offset of a drainage that has incised into a mid-late Holocene alluvial fan along the NAF in north central Turkey, near the villiage of Tahtakopru. The drainage (Karanlik Dere), which is displaced ~60 meters right-laterally with little vertical displacement, flows southward almost perpendicular to the east-west trace of the NAF at the site. We used cosmogenic 10Be to determine the age of the fluvial fan surface. The combination of the measured offset and ~3-3.5 ka fan surface yielded an average slip rate of 20 mm/yr. The Karanlik Dere slip rate is indistinguishable from the slightly shorter-term (~2-2.5 ka) rate we measured at our Eksik slip rate site, which is located ~200 kilometers to the west. Three different dating methods (14C, 36Cl and 10Be) revealed the same slip rate from two different sites within 200 kilometers. Our geological slip rates are indistinguishable from the geodetically measured 25±1 mm/yr rate of elastic strain accumulation across the NAF (Reilinger et al., 2006), suggesting that the rates of strain storage and release along the central NAF have been relatively constant over the past 3500 years. We attribute this constancy to the geometrical simplicity and mechanical efficiency of the North Anatolian fault where there are no other major faults that can produce large earthquakes and affect the timing of these earthquakes. We note however, that the geodetic rates fall near the high end of our longer-term geological rate ranges, leaving open the possibility that the lower crust beneath the NAF is deforming slightly faster than its long-term rate, possibly in response to some long-lived effects of the 20th century sequence of large NAF earthquakes.