T41A-1924
Evolution of the Vening Meinesz Fault Zone, Northern New Zealand Continental Margin
The linear northern margin of New Zealand from the Norfolk Ridge to the Havre Trough is a partial transform, strike-slip fault system, active during Early Miocene opening of the Norfolk and South Fiji back-arc basins. It was intimately associated with arc volcanism in the Northland Arc, Northland Plateau and Three Kings Ridge (TKR). Differences in displacement vectors in the Vening Meinesz Fault Zone (VMFZ) through time are revealed in (1) the seismic stratigraphy of the Northland Plateau and Reinga Basin, (2) the detailed bathymetry of the Norfolk Basin and (3) regional satellite gravity trends. SW Pacific models place the TKR and Northland Plateau adjacent to the Norfolk Ridge-New Zealand margin in the very early Miocene, followed by lateral displacement along the VMFZ. The details of that displacement are investigated here. The Reinga Basin is on the continental side of the VMFZ and the Northland Plateau is on the oceanic side. Initially, the Northland Plateau lay beside and seaward of the Reinga Basin. In both features, the stratigraphy records Early Miocene extension or transtension, followed by strike-slip, and finally a late phase of contraction or reversal with transpression. The late phase uplifted much of the Wanganella and Reinga ridges but has yet to be explained. Swath bathymetry shows that the initial extensional vector of the Norfolk Basin was N95°, shifting the TKR and Northland volcanic plateau eastwards by probably less than 100 km. The effect on the NZ margin was oblique extension in the Northland Plateau, with uplift of at least one metamorphic core complex, and formation of half grabens where the Reinga and Wanganella ridges now stand. The subsequent extension and spreading vector in the Norfolk Basin was N115°. This motion completed the 250 km displacement of the TKR and Northland Plateau by strike slip, fully displacing the Northland volcanic plateau to lie beside and seaward of the onland Northland arc. At the eastern end of the VMFZ, where the fault zone approached the developing North Island-Colville Ridge-Pacific plate boundary, faults were formed orthogonally and obliquely to the VMFZ trend. The combination of strains on the VMFZ has left a visible tripartite morphology in the bathymetry - a left stepping Reinga escarpment, a straight northwestern shelf edge, and a crenelated southeastern shelf edge.
T41A-1925
PyLith: A Finite-Element Code for Modeling Quasi-Static and Dynamic Crustal Deformation
We have developed open-source finite-element software for 2-D and 3-D
dynamic and quasi-static modeling of crustal deformation. This
software, PyLith (current release is version 1.3), combines the
quasi-static viscoelastic modeling functionality of PyLith 0.8 and its
predecessors (LithoMop and Tecton) and the wave propagation modeling
functionality of EqSim. The target applications contain spatial scales
ranging from tens of meters to hundreds of kilometers with temporal
scales for dynamic modeling ranging from milliseconds to minutes and
temporal scales for quasi-static modeling ranging from minutes to
hundreds of years. PyLith is part of the NSF funded Computational
Infrastructure for Geodynamics (CIG) and runs on a wide variety of
platforms (laptops, workstations, and Beowulf clusters). It uses a
suite of general, parallel, graph data structures called Sieve for
storing and manipulating finite-element meshes. This permits use of a
variety of 2-D and 3-D cell types including triangles, quadrilaterals,
hexahedra, and tetrahedra. Current features include kinematic fault
ruptures, Dirichlet (displacement or velocity), Neumann (traction),
and absorbing boundary conditions, linear elastic, generalized
Maxwell, and Maxwell linear viscoelastic materials, gravitational body
forces, and automatic time step selection for quasi-static
problems. Future releases will add dynamic fault interface conditions
(employing fault constitutive models), additional viscoelastic and
viscoplastic materials, and automated calculation of suites of Green's
functions. We also plan to extend PyLith to allow coupling multiple
simultaneous simulations. For example, this could include (1) coupling
an interseismic deformation simulation to a spontaneous earthquake
rupture simulation (each using subsets of the software), (2) coupling
a spontaneous earthquake rupture simulation to a global wave
propagation simulation, or (3) coupling a short-term crustal
deformation simulation to a mantle convection simulation and an
orogenesis and basin formation simulation.
http://www.geodynamics.org/cig/software/packages/short/pylith/
T41A-1926
Unraveling faulting in a complex earthquake sequence in the South Iceland Seismic Zone.
The South Iceland Seismic Zone (SISZ) is an E-W transform zone, where the relative spreading of the North American and Eurasian plates across southern Iceland is accommodated by motion on many parallel N-S right-lateral strike slip faults, rather than left-lateral motion on a single E-W through going fault. Historically, earthquake sequences with main shocks reaching M7 have occurred in the SISZ, many initiating in the eastern part of the zone with subsequent events further west. A magnitude 6.3 earthquake occurred in the western part of the SISZ on May29, 2008. Aftershock locations and global centroid-moment-tensor solutions indicate rupture on at least two parallel N-S faults. The rupture on the second fault, located about 4 km west of the initial event,appears to have initiated less than one second after the main shock,suggesting dynamic triggering. The May 2008 earthquakes are a continuation of the June 2000 sequence, when two Mw=6.5 events struck the eastern and central part of the SISZ. The June 2000 main shocks ruptured two parallel N-S faults, spaced about 17 km apart, occurring about 3 1/2 days apart. Here, we present a geodetic and seismic study of the May 2008 earthquakes based on continuous and annual GPS measurements, as well as InSAR and aftershock locations. The GPS network was surveyed in April, a month before the events and remeasured immediately after. Maximum coseismic displacements of about 15 cm (horizontal) were recorded at the closest continuous GPS stations on each side of the two faults. We also measured continuously at about 20 GPS benchmarks for more than a month after the event. A small transient (about 1 cm) was recorded during the first 10 days following the earthquake. This transient motion does not appear to be caused by poro-elastic rebound due to pressure changes in the ground water system, as was observed following the June 2000 earthquakes. The aftershocks lineate at least two N-S structures as well as an E-W conjugate fault. Preliminary modeling of GPS and InSAR data indicate that most of the surface deformation can be explain with strike-slip at shallow depth on two N-S structures, with little or no slip on the E-W lineament. Our first estimate using a layered-elastic Earth model indicates slip of about 2 m at shallow depth. The relatively high ratio of slip to fault length was also evident in the June 2000 events. Such high stress drop events indicate that the SISZ is a young and immature fault zone.
T41A-1927
Tracing of Paleoseismic Events in the Eastern Mediterranean: U-series Dating and Stable Isotope Studies of Syntectonic Carbonate Veins
We analysed syntectonic carbonate-filled fissures and veins from three key localities in the eastern Mediterranean, including the East Anatolian Fault Zone (EAFZ) in SW Turkey, and the Dead Sea Fault Zone (DSFZ) in northern and southern Israel. Fracture mineralization formed in two ways. Carbonates were precipitated from earthquake-mobilized fluids in syntectonic fissures, or from carbonate-enriched meteoric water in opening-mode fractures. By studying the isotopic records and ages of mineralized fractures, one may differentiate between these two mechanisms, which in turn may contribute to the understanding of paleoseismic activity along these major plate-boundary faults. Field observations indicate that all of the fissures and veins are related to fault-zone deformation along the adjacent plate boundary transforms. We observed fault planes with slickenlines and syntectonic fissure and vein structures filled with carbonate material in the central EAFZ, in SW Turkey. In the DSFZ in northern Israel, a carbonate-filled vein system strikes ~ E-W. The veins are 2 to 60 cm thick and several meters to >20 m high. They are characterized by vertical bands of calcite crystals aligned parallel to vein walls and show no evidence of shearing, thus, reflecting ~N-S extension. We also sampled carbonate minerals from tension gashes and other asymmetrical features from the DSFZ in southern Israel. Our U-Th dating results from the three localities sampled along the DSFZ and EAFZ systems indicate that active deformation occurred between 450 ka and 17 ka ago. The maximum opening rate of veins in northern Israel is on the order of 0.45 mm/ka, and likely represents an extensional response to convergent strike-slip motion along the sinistral Dead Sea fault. Results from the stable isotope analyses (δ18O and δ13C) indicate that water-rock interaction occurred at various temperatures and/or involved different types of fluids. Petrographic observations of calcite-filled veins in samples from the EAFZ site, together with depletion in δ18O and δ13C values, suggest that high-pressure, CO2-rich fluids were responsible for carbonate precipitation at elevated temperatures. In contrast, in the DSFZ in northern Israel, the oxygen isotopic composition is indicative of meteoric water involvement, and δ18O and δ13C values are similar in composition to speleothems from the nearby 'Peqiin Cave'. This suggests that meteoric water containing CO2 from oxidation of Mediterranean-type vegetation is the source of fluids at this site. In the DSFZ in southern Israel, the carbon isotopic composition is indicative of marine limestone values, suggesting that the fluid composition is buffered by the limestone host rock.
T41A-1928
Field Mapping of Geologic Structures Bounding Little Lake Valley Correlate With Multiple Seismogenic Clusters Within the Maacama Fault Zone, Northern California
Analysis of microseismicity, combined with geologic and geomorphic mapping, suggests that the Maacama Fault zone adjacent to Little Lake Valley (LLV) consists of at least two active seismogenic zones, each being 2-3 km thick. In contrast, paleoseismic studies of the faults in and around LLV are restricted to closely spaced active strands on the west side of the valley, which are interpreted to be the dominant expression of the Maacama Fault. Mapping on either side of the identified active faults show that LLV is bounded by broad zones of deformation. The eastern deformation zone includes intensely faulted and brecciated rock, and linear bodies of serpentinite with associated talc deposits. Opposing bedding dips are mapped across the shear zone, along with geomorphic features that include a fish-hooked drainage, beheaded streams, and mass-wasting deposits that may be seismically induced. The fault locations interpreted from geomorphic evidence are also supported by shallow resistivity sections that outline discontinuities in the subsurface along the eastern edge of LLV. The western deformation zone includes a calc-silicate deposit that ties to a distinct linear valley with evidence of slumping on adjacent slopes. Cluster analysis of microseismicity beneath LLV reveals a ~2-km-thick tabular seismogenic zone that projects directly along the mapped zone of deformation on the west side of the valley, and a ~3-km-thick tabular seismogenic zone that projects along the mapped zone of deformation on the east side of the valley, suggesting that both zones are active. Overall, the available geologic, geomorphic, and microseismic data, are consistent with the interpretation that the Maacama Fault at LLV comprises at least two, 2-3-km-thick active seismogenic zones, rather than a single active fault strand.
T41A-1929
Non-Vertical Dips of the Southern San Andreas Fault and their Relationships to Mantle Velocities, Crustal Tectonics, and Earthquake Hazard
The San Andreas Fault (SAF) in southern California is in most places non-vertical, based on seismic- imaging, potential-field, earthquake-aftershock, and selected microseismicity studies of the crust. The dip of the SAF changes from SW (55-75 degrees) near the Big Bend to NE (10-70 degrees) southeastward of the eastern San Gabriel Mountains, forming a crude propeller shape. The uncertainty of most of the dip observations is about 5-10 degrees. To examine the geometry of the fault surface, we have developed a three-dimensional model of a dipping SAF, extending from Parkfield in central California to the SAF's southern termination at the Salton Sea. Knowledge about the dip of the SAF is important for estimating shaking potential of scenario major earthquakes and for calculating geodetic deformation. In sections across the SAF, P-wave tomographic images of the mantle beneath southern California (Kohler et al., 2003) suggest that the plate boundary extends into the mantle and is continuous with the SAF in the crust. The dip of the plate boundary appears to steepen in the mantle. Seismicity sections across the locked part of the SAF, from Indio towards the northwest, reveal different seismicity regimes on either side of our model SAF surface, but do not delineate the fault itself. These differences include changes in the maximum depth of the seismogenic zone and in the abundance of seismicity over the past ~20 years. The distinct seismicity regimes may reflect changes in physical properties and/or stress state of the crust on either side of the SAF at seismogenic depths. Mantle velocities southwest of this projected plate boundary, within the Pacific Plate, are relatively high and constitute the well documented upper-mantle high-velocity body of the Transverse Ranges. This relationship is similar, in some ways, to that between the Alpine Fault of New Zealand and its underlying mantle, and suggests that in both California and New Zealand, Pacific lithospheric mantle is downwelling along the plate boundary (Fuis et al., 2007).
T41A-1930
Re-investigation of the Geometry and Slip Distribution of the 1931 Fuyun Surface Rupture, northwest China
We have been re-investigating the details of fault geometry, and amount and distribution of slip of the rupture of the M=8.0 Fuyun earthquake of 1931, northwest China, to reveal the size and scaling relations of worldfs largest fault segment. We have mapped the entire strands of the rupture with the assistance based on high-resolution satellite images, and made reliable offset measurements at over one thousand points in the field. The rupture extends for 160 km, and is predominant in right lateral displacement with normal component in the north and reverse component in the south. The rupture divided into five segmentsCseparated from each other by 1 to 4-km-long gap, restraining step-over of 6-km-long, and terminates at a 5-15 km-long and 4-5 km-wide releasing jog at the south end,. Northern two segments are 15 to 20 km-long each, and characterized by significant normal-slip component. Each of central to southern three segments is 37 to 47 km-long strand having a maximum displacement of 9 to 10 m in right-lateral. The scaling relation between the maximum displacement and the fault length for those 15-47km-long segments are approximately 1:3000- 5000. The rupture is characterized by the nesting segment-structure in geometry, and the jogs between those segments show similarity in plane-view geometry. Each of segments is sub-divided into 2-7 km-long sub- segments by step-overs or bends of fault strand. Although jogs between major segments and minor sub- segments shows large variability in length and width, the vertical displacement in the step-over has small variability, and seems to be a function of the amount of fault slip near the jog.
T41A-1931
Mechanisms of Tectonic Uplift in the Santa Cruz Mountains, CA
GPS velocity measurements in the San Francisco Bay Area limit transpression across the region to < 1.2 mm/yr, suggesting that contraction and uplift are accommodated by conjugate strike slip faults that strike sub-parallel to the plate motion vector or within restraining bends where changes in fault geometry result in localized contraction and uplift. The Santa Cruz Mountains (SCM) restraining bend along the San Andreas Fault (SAF) undergoes a local change in orientation by approximately 8-10 degrees. While active seismicity and surface velocities suggest that the restraining bend accommodates the geodetically observed contraction in this area, low temperature thermochronology, uplifted marine terraces, and uplifted Pliocene marine units indicate that contraction and uplift may be more uniformly distributed than can be explained exclusively by the restraining bend. Thus, it is unclear to what degree the geologic strain accommodated throughout this region results from plate-normal contraction versus restraining-bend tectonics. To determine the relative roles of these two factors, we measured cosmogenic radionuclide erosion rates from four basins flanking the southwestern SCM, whose topography results from deformation associated with the SAF. The southernmost basin is seated within the heart of the SCM restraining bend, while basins farther north are located progressively farther away from the bend. Thus, if restraining-bend tectonics dominate the deformation of the area, we expect rapid erosion rates to be focused in these areas, while more uniform rock uplift and erosion rates may be indicative of regional transpression. Erosion rates from basins within the restraining bend are about 0.53 mm/yr, twice as high as those north of the bend (0.24 - 0.31 mm/yr). The clear spatial association of high erosion rates with the restraining bend suggests that at least half of the observed uplift and erosion may result from restraining-bend deformation. Accordingly, relatively uniform erosion rates north of the bend may result from either rock uplift that is driven by isostatic unloading of the upper crust or limited regional transpression.
T41A-1932
Active Tectonics in the Central Chilean Andes: 3D Tomography Based on the Aftershock Sequence of the 28 August 2004 Shallow Crustal Earthquake
Most of the seismological research in the Andes has been mainly oriented to the detection and understanding of the seismicity associated with megathrust earthquakes that characterize the subduction environment that governs the Andean tectonics. However, deployments of temporary networks have allowed the detection of intense crustal seismicity beneath the Chilean forearc-arc region. The temporary seismic network deployed along the Las Leñas and Pangal river valleys (34°25'S), between January and May 2004 permitted to better constrain the abundant shallow intra-continental seismicity previously detected in that region. Although most of the seismicity is randomly distributed in the region, several microearthquakes occur along the trace of the major El Fierro fault-system. This system is well recognized between 33°30' and 35°15'S and is located at or close to the eastern contact between Mesozoic and Cenozoic deposits in the Principal Cordillera and, locally, below active volcanoes, being considered to have participated in the extension and tectonic inversion of a widely extended (>600 km long) Cenozoic basin along the Principal Cordillera. Further south, at 35°S, a Mw=6.5 strike-slip shallow earthquake occurred on August 28, 2004, near of the headwater of the Teno river, close to the Planchon volcano. A 3D detailed Vp and Vs velocities determination was obtained along the 2004 earthquake aftershock area. The aftershocks are distributed along one branch of the El Fierro fault system, with a NNE-SSW direction and depths lower than 15 km. The rupture zone coincides with a sharp contrast in Vp and Vs, also in coincidence with the presence of hydrothermal fluids, gypsum diapers and the volcanic arc, suggesting rheological contrast controlling deformation. At the surface, this zone present an intense contractive deformation produced during the Neogene, which differs from what can be observed in other regions. Present day deformation related to seismicity has no deformation related at the surface, maybe because of large landsliding that could hide surface rupture. However, the presence of these mass wasting phenomena suggests that rupture propagation to the surface is more diffusive, being accommodated by a wide microfracturing and thus not showing appreciable slip. Such a kind of features has been also observed in northern Chile and near Santiago. Both situations differ from what has been commonly assumed for crustal deformation, and therefore they should be studied critically. One alternative to explain this kind of ruptures could be related to the fact that fluid and heat in this zone are larger than in other crustal fault systems. The relation between fluid, heat and seismicity along the Andes is one of the main goals of the ACT-18 PBCT project.
T41A-1933
Examining the Evolution of the Peninsula Segment of the San Andreas Fault, Northern California, Using a 4-D Geologic Model
Retrodeformation of a three-dimensional geologic model allows us to explore the tectonic evolution of the Peninsula segment of the San Andreas Fault and adjacent rock bodies in the San Francisco Bay area. By using geological constraints to quantitatively retrodeform specific surfaces (e.g. unfolding paleohorizontal horizons, removing fault slip), we evaluate the geometric evolution of rock bodies and faults in the study volume and effectively create a four-dimensional model of the geology. The three-dimensional map is divided into fault-bounded blocks and subdivided into lithologic units. Surface geologic mapping provides the foundation for the model. Structural analysis and well data allow extrapolation to a few kilometers depth. Geometries of active faults are inferred from double-difference relocated earthquake hypocenters. Gravity and magnetic data provide constraints on the geometries of low density Cenozoic deposits on denser basement, highly magnetic marker units, and adjacent faults. Existing seismic refraction profiles constrain the geometries of rock bodies with different seismic velocities. Together these datasets and others allow us to construct a model of first-order geologic features in the upper ~15 km of the crust. Major features in the model include the active San Andreas Fault surface; the Pilarcitos Fault, an abandoned strand of the San Andreas; an active NE-vergent fold and thrust belt located E of the San Andreas Fault; regional relief on the basement surface; and several Cenozoic syntectonic basins. Retrodeformation of these features requires constraints from all available datasets (structure, geochronology, paleontology, etc.). Construction of the three-dimensional model and retrodeformation scenarios are non-unique, but significant insights follow from restricting the range of possible geologic histories. For example, we use the model to investigate how the crust responded to migration of the principal slip surface from the Pilarcitos Fault to the modern San Andreas Fault trace between ~5 and ~3 Ma. This example demonstrates how our approach allows us to begin holistic evaluation of deformation over geological time scales in complex tectonic settings.
T41A-1934
Active Strike-Slip Faulting in the Inner Continental Borderland, Southern California: Results From New High-Resolution Seismic Reflection Data
The inner Continental Borderland offshore of southern California accommodates about 7 mm/yr of slip between the North American and Pacific plates. Nearly half of this total has previously been thought to be taken up on the Palos Verdes (PV) and Coronado Bank (CB) fault zones, which have been modeled as a single, continuous fault zone in recent seismic hazard assessments for southern California. Although these faults lie roughly on strike with each other, a connection between these faults has not been clearly demonstrated. Newly acquired high-resolution seismic reflection data indicate that the PV fault terminates southwest of Lasuen Knoll in a horsetail splay that becomes progressively buried to the south. The lack of a connection between the PV and CB fault zones implies that a significant amount of slip must be taken up elsewhere in the inner Continental Borderland. Two other significant offshore faults, the San Diego Trough (SDT) and San Pedro Basin (SPB) fault zones, lie about 10-15 km southwest of and sub parallel to the trace of the PV and CB faults. The SDT fault zone extends from south of the Mexican border near Punta Santo Tomas for about 150 km northward to near Crespi Knoll. The SPB fault zone extends northward from off Santa Catalina Island to near Point Dume. The new seismic reflection data reveal a previously unmapped but apparently active fault zone along strike and in the area between the known strands of the SDT and the SPB fault zones. This newly recognized fault links the SDT and SPB faults, forming a continuous, active fault zone that extends about 250 km along the inner Continental Borderland. Although there are no slip rate data available for this fault zone, its overall length, continuity, and active character suggest that a significant portion of the plate motion that occurs offshore is accommodated along the SDT-SPB fault zone, which may pose a more significant seismic hazard than previously recognized.
T41A-1935
Characterization of a Strike-Slip, Serpentine-Rich Fault System in the Twin Sisters Ultramafic Massif, Washington
The Twin Sisters ultramafic massif, Washington, USA, consists dominantly of ultramafic rocks with a regionally consistent foliation (~165, 70°E) characterized by high-temperature (mantle) fabrics. This foliation is crosscut by both a series of EW-trending serpentine faults and locally a major ENE-trending (070, 80°W) system of serpentine faults. The ENE-trending fault system locally crosscuts the EW-trending faults suggesting it is younger. We have mapped structures in a 500 m long segment of the ENE-trending fault system, corresponding to ~150 m of vertical relief. The fault system consists of a series of subparallel fault zones offset to the right or left and ranging from a few meters to 10 m apart. Each fault zone is 1-3 m thick, typically with a pair of discrete, nearly parallel, bounding faults that are coated by either mm-thick serpentine slickenfibers or cm- thick layers of foliated serpentine. Steps on slickenfibers and structures in the foliated serpentine are consistent with sinistral, strike-slip movement. The bounding faults are linked by anastomozing splay faults and bands of foliated serpentine that separate lenticular masses of fine-grained, foliated serpentine and lens-shaped lithons. Some lithons retain mineral composition and fabric comparable to that observed in the main body of the ultramafic away from the fault zone. Others are heavily serpentinized but retain evidence for the high temperature foliation. Of particular interest is the occurrence of dunite mylonites, not typical in the main body of the ultramafic, locally within and adjacent to the deformation zone branches. The serpentine faults are consistent with low to moderate temperature and lower pressure deformation typical of mid- to upper-crustal conditions. These structures are likely to have formed during the emplacement of the Twin Sisters ultramafic body. The occurrence of dunite mylonite only in the vicinity of this fault suggests that either (1) the serpentine-dominated deformation initiated along a zone of pre-existing higher-temperature shear, or (2) a deformation zone that initiated during the high-temperature deformation of the Twin Sisters massif continued to accrue slip as the body cooled.
T41A-1936
Kinematic analysis of present-day strain and rotation within a large restraining bend: The Lebanese Restraining Bend along the Dead Sea Transform
Restraining bends along strike-slip faults are typically characterized by relative structural and kinematic complexity, owing to the obliquity of plate motion with respect to the structural trend. One example of a large restraining bend is the Lebanese Restraining Bend (LRB), a ~180 km long bend in the left-lateral Dead Sea Transform System. Compared with other large restraining bends, such as the Big Bend along the San Andreas Fault, the LRB has a relatively simple geological structure consisting of two large strike-slip faults (the through-going Yammouneh fault and the terminating Serghaya fault) and several smaller faults. Owing to the less complicated structure, the LRB presents an opportunity to study basic kinematics of deformation within restraining bends, in general. This study examines the present-day deformation associated with the LRB based on Global Positioning System (GPS) measurements from Lebanon, Syria, and adjacent areas. The GPS network in Lebanon consists of 23 survey-mode sites and 1 continuous site spanning 2002 2008, and GPS measurements in Syria span 2000 2008. 1-sigma uncertainties on most sites are less than 0.65 mm/yr. Application of elastic dislocation models suggests slip rates of 3.8 4.5 mm/yr for the Yammouneh fault and 1.0 1.5 mm/yr along the Serghaya fault, which are consistent with geological estimates. The relatively dense GPS network permits calculating velocity gradients that can be decomposed into infinitesimal rotations (vorticity), as well as infinitesimal strains. The results suggest counter clockwise rotations of 0.5 1.8 degrees/MA within the restraining bend. For this analysis, the vorticity can be decomposed into contributions from long-term fault slip (shear strain related to slip rate) and vertical axis rotations the latter can be compared with long-term rotations from paleomagnetic studies. These kinematic constraints can be applied to testing proposed models for restraining bend kinematics involving continuous deformation and right block rotation.
T41A-1937
CONSTRAINING THE GEOMETRY AND TECTONIC EVOLUTION OF THE MANEADERO BASIN THROUGH COMBINED GEOPHYSICAL AND STRUCTURAL DATA SETS
Maneadero basin, located 5 km south of Ensenada, Mexico, is bound to the south by the dextral Agua Blanca Fault (ABF). The ABF is the southwestern-most in a network of faults transferring plate boundary strain out of the Gulf of California and around the bend in the San Andreas in southern California. The basin is about 60km2 with an axis that trends WNW, subparallel to the ABF. The WNW-trending, western ABF juxtaposes the basin and the Bahia Todos Santos to the northwest, with the 1000m high Punta Banda Ridge. The goals of this study were to constrain the geometry of the Maneadero basin and the distribution of faults bounding and within the basin, as well as to provide constraints on the kinematic evolution of the basin. A combined geophysical (gravity and magnetic) and structural study were employed to address these issues. Gravity data define an anomaly of about 60 mGal across the basin and onto the adjacent up-thrown block, and can clearly identify two gravity highs separated by an intervening gravity low. The highs, irrespective of elevation, are offset by approximately 7km of dextral strike slip. This is consistent with the results of field mapping conducted during the summer 2008 that identifies a Cretaceous intrusion, located just beyond the southeast corner of the Maneadero Basin which is cut by the ABF, which is also displaced by approximately 7km of slip on the ABF. Gravity data also allow for the estimation of a dip-slip component of displacement across the ABF. As expected, the gravity low appears in the middle of the basin, which is shown to have sediment thicknesses of approximately 1km near the ABF and gradually shallowing northward. There is also a slight gradient that decreases in magnitude from southeast to northwest along the axis of the basin. A density contrast of 0.37 g/cm3 between the basin fill and outcropping Punta Banda Ridge, as described by Florez et al (2004), was used to generate this model. A normal slip component is also documented by Rockwell et al (1989) through a study of uplifted marine terraces on Punta Banda. They documented 260m of uplift since 840ka. The apparent 740m deficit might be explained by strike-slip displacement of the west-sloping coastal margin or the normal component of slip has taken place over a longer time frame than represented by the terraces.
T41A-1938
New GPS constraints on the kinematics of the southern Dead Sea Fault System
The southern Dead Sea Fault System (DSFS) traces ~400 km from the Gulf of Aqaba in the south to the southern end of Lebanese Restraining Bend along the DSFS. The general structure involves two main segments, the Wadia Araba fault and the Jordan Valley fault, that control the Dead Sea pull-apart basin. This study assesses the present-day kinematics along the southern DSFS as expressed by present-day deformation. This study combines survey-mode (SGPS) and continuous GPS (CGPS) measurements from Jordan with other available GPS data to assess possible kinematic variations along the southern DSFS. The GPS network in Jordan consists of 15 SGPS sites that have been measured four times over a span of more than three years (2005 2008), along with two CGPS stations that have operated for more than 2 years. Preliminary velocities for SGPS sites yield uncertainties of approximately 1 mm/yr, and the CGPS sites yield uncertainties less than 0.8 mm/yr. Velocity patterns are generally consistent with locked faults accumulating strain. 1-D and 2-D Elastic dislocation models suggest slip rates of 3.8 4.6 mm/yr and 4.0 4.9 mm/yr for the Wadi Araba and Jordan Valley segments, respectively. These geodetically-based slip rates compare well with late Quaternary estimates based on faulted landforms. In addition to elastic models, the spatial coverage of GPS sites permits calculating velocity gradients and assessing infinitesimal strains and rotations along the fault, and within the Dead Sea pull-apart basin. Comparing the strain patterns with more detailed structural maps of the Dead Sea basin provides a means of assessing the kinematics involved in transferring displacement across a large releasing fault step. Furthermore, the rates of strain accumulation provide valuable constraints for assessing the earthquake hazard along the southern Dead Sea fault.
T41A-1939
Strong Rupture and Postseismic Response of the 8 June, 2008 Dextral Strike-Slip Earthquake in Western Peloponnese, Greece
On 8 June 2008, a strong rupturing MW 6.3 crustal earthquake occurred in western Peloponnese, Greece. Interestingly, the dextral strike-slip event was initially reported to be much larger (Ml 6.5-6.7), with significant strong shaking causing widespread damage to un-/poorly reinforced houses in nearby communities. The earthquake ruptured to the NNE aligning with other recent strike-slip events regionally. Interestingly, no positive evidence of surface rupture exists for this event, and no geologically identifiable dextral offsets have been observed. These suggest that this event is part of a sequence acting to build a newly-developed transform fault responsible for accommodating a portion of the ~30mm/yr of Aegean- Eurasian plate motion. Following the methodology of Newman and Okal [1998] we used waveforms for over 150 teleseismic stations to determine the seismic energy, finding it to be extremely energetic (1015 J; Me 7.1) and having an Energy/Moment Ratio of 10-3.6, about 10 times higher than the global average. This result helps explain the locally large magnitude determinations. Additionally, the high energy release suggests a very large stress drop, probably from relatively large coseismic slip. Between 24 and 72 hours of the initial rupture, we installed a local network of 9 continuous GPS sites surrounding the rupture zone, and maintained their operation for approximately three months. Surprisingly little observable deformation was found in either JPL-GIPSY processed daily solutions, or in MIT-GAMIT 15- second kinematic differential solutions, suggesting little afterslip occurred. It is likely that this event is end- member example of the previously observed anti-correlation between coseismic and postseismic slip [Marone, 1998] along with events like the 1992 Landers Earthquake, and contrasting the 1966 and 2004 Parkfield Earthquakes along the mature San Andreas Fault, where afterslip is comparable to coseismic slip [Freed, 2007]. While attempts to determine coseismic rupture from ENVISAT SAR images has thus far been problematic due to large areas of decorrelation, it is possible that little coseismic surface deformation occurred due to the dampening effect of the ~2 km thick layer of weak Flysch above the rupture zone.
T41A-1940
Seismic Investigations of Lithospheric Transitions between the Northern and Southern Australian Cratons (BILBY)
We aim to determine the nature of the transition at lithospheric depths between the northern and southern Australian cratons. What are the controlling factors of the regions with anomalously slow velocities beneath the central Australian intercratonic suture zones? Do these intercratonic transitions propagate with depth and, if so, in what manner? To answer these questions, 25 broadband seismic stations will be deployed in August-September 2008 and will remain operational for approximately one year. Much of the Australian continent is an amalgamation of several smaller cratons and multiple orogenic events. The transitions between any two cratonic regions, however, do not necessarily reflect the same processes. The transition between the Yilgarn and Pilbara cratons through the Capricorn Orogen is consistent with cool lithosphere and high seismic wavespeeds extending to substantial depth. In Central Australia there are lowered seismic wavespeeds in the upper mantle down to at least 75 km depth, yet with low attenuation. Yet by 125 km depth the whole zone is covered by fast wavespeeds. Such behavior is difficult to reconcile with a purely thermal influence. The experiment configuration is designed specifically to connect the Gawler Craton in the south to components of the North Australian Craton and to examine the lithosphere in the region influenced by the Alice Springs Orogeny (400-300 Ma). A combination of rigorous analysis tools, such as joint velocity and attenuation tomography coupled with receiver functions, will help to provide a comprehensive understanding of the amalgamation of continental cratons and the associated intercratonic transitions. In addition, a significant amount of information will be added to the present understanding of intercratonic suture belts and further constraints on the Australian lithospheric structure and overall continental amalgamation will be realized.
T41A-1941
Rates of Extension Along the Fish Lake Valley Fault and Transtensional Deformation in the Eastern California Shear Zone
New geologic slip rates from the Fish Lake Valley fault (FLVF) provide constraints on the kinematic relationships amongst the major faults in this complex part of the eastern California shear zone (ECSZ). Right-lateral shear accommodates most slip on the NW-striking faults, but fault segments that strike approximately North also have an extensional component. Here, we present slip rates for the extensional component of slip of the FLVF based on cosmogenic 10Be geochronology and analysis of airborne laser swath mapping (ASLM) data (1 m horizontal resolution and decimeter vertical accuracy). We are studying four offset locations along the FLVF (Furnace Creek, Wildhorse Creek, Perry Aiken Creek, and Indian Creek, from south to north). Previous studies report cosmogenic 10Be ages from the offset Furnace Creek and Indian Creek alluvial fans that suggest deposition at ~94 ka and ~71 ka, respectively. The cumulative vertical displacements at these two locations range from 25 m at Furnace Creek to 75 m at Indian Creek, implying a vertical component of slip from 0.3 to 1.1 mm/yr. The average vertical component of oblique slip along the FLVF at Wildhorse Creek is 42 m and 85 m at the Perry Aiken Creek site. Determining the age of the offset fans of the latter two locations by dating recently collected cosmogenic geochronology samples will further enable us to resolve the extension rate across the FLVF. The slip rates we have obtained thus far are generally in agreement with previous estimates based on alluvial fan morphology, soil development, and theodolite surveys. Assuming a 60° dip for the fault planes, we calculate late Pleistocene FLVF extension rates between 0.2 and 0.6 mm/yr. Comparison of these rates with geodetic measurements of ~1 mm/yr of extension across the ECSZ north of the Garlock fault indicate that as much as half of the east-west extension in this region is accommodated along the FLVF. Our data also imply an increase in late Pleistocene extension rates from south to north, opposite the trend of the dextral slip rate along the FLVF. This discrepancy, as proposed by others, can be explained by an extensional transfer zone in northern Fish Lake Valley accommodated by vertical axis clockwise block rotation between FLVF and the Walker Lake to the north and east, and by westward transfer of slip onto the Saline Valley-Hunter Mountain-Panamint Valley fault system.
T41A-1942
Evaluating Temporal Variations in Fault Slip-Rate and Fault Interaction in the Eastern California Shear Zone
Delineating spatiotemporal patterns of strain accumulation and release within plate boundaries remains fundamental to our understanding of the dynamics of active crustal deformation. The timescales at which active strain varies or remains constant for individual fault systems, however, are often poorly resolved. The origin of large-magnitude strain transients in the Eastern California shear zone remains enigmatic and underpins the importance of quantifying active deformation at multiple geologic timescales along this tectonic boundary. Here, we focus on the Late Pleistocene Holocene record of slip on the NW-striking Little Lake fault zone, one of the primary structures responsible for transferring Pacific-North American plate motion between the northern Mojave Desert and the east side of the Sierra Nevada block north of the Garlock fault. Discrepancies between geologic and geodetically determined rates of motion along the Little Lake fault zone in the China Lake-Indian Wells Valley area suggest a potentially complex temporal history of slip on this structure with some slip stepping eastward onto structures bounding the west side of the Coso Range. Preliminary reconstruction of a slip-rate history on the Little Lake fault from multiple generations of displaced Quaternary geomorphic features suggests potential variation in fault-slip rates at timescales of 104- 105 years. Two paleochannel margins on a basalt strath in the Little Lake spillway represent the youngest of these features. Each margin exhibits ~30 m of right-lateral displacement and suggests a minimum slip rate of ~1.4 mm/yr during Holocene-Late Pleistocene time. Additionally, a prominent fluvial escarpment or terrace riser along the east side of Little Lake wash is offset at least ~150 to 700 m, depending on how the initial geometry of this feature is reconstructed. Pending geochronologic constraints on the age of this feature, such an offset potentially suggests higher rates of slip averaged over longer timescales.
T41A-1943
Spatial and temporal constancy of seismic strain release along the Death Valley-Fish Lake Valley fault and Pacific-North America plate boundary strain distribution
The Death Valley-Fish Lake Valley fault (DV-FLVF) is the most active fault system along the Pacific-North America plate boundary east of the San Andreas fault. Recent work in the region suggests the late Pleistocene slip rate decreases from approximately 4.5 mm/yr along the central part of the DV-FLVF in northern Death Valley to approximately 2.5 mm/yr at the northern end of the DV-FLVF in Fish Lake Valley. This decrease in slip rate is at odds with observations at the latitude of northern Death Valley, which show late Pleistocene rates of deformation across the eastern California shear zone are coincident with short- terms rates of dextral shear determined from GPS data. Here, we report on alluvial fan offsets from three additional sites along the DV-FLVF to help determine the distribution of strain and the temporal and spatial constancy of deformation in the northern eastern California shear zone. From south to north these sites are: Cucomongo Canyon, Leidy Creek, and Indian Creek. We generated digital elevation models with 1 m horizontal resolution and decimeter vertical accuracy of the offsets from an airborne laser swath mapping (ALSM) survey along the fault zone. Right lateral displacements in offset late Pleistocene alluvial fans at Cucomongo Canyon and Leidy Creek are approximately 190 m and 50 m, respectively. A Holocene fan deposit at Indian Creek displays a dextral offset of approximately 15 m. Sixteen surface boulder samples were collected from the Leidy Creek and Indian Creek alluvial fans and five samples were collected from a depth profile at the Cucomongo Canyon fan for cosmogenic Beryllium-10 geochronology. These samples will help establish the age of the alluvial fans and when combined with the measured offsets from the ALSM data will provide slip rates over a variety of temporal and spatial scales. Ultimately, these rates will add to a growing number of such data, which are helping to elucidate how strain is distributed along this important, evolving segment of the Pacific-North America plate boundary.
T41A-1944
Wide-Angle Seismic Experiment Across the Oeste Fault Zone, Central Andes, Northern Chile.
From December 6-21, 2007, we conducted a 3-component, radio-telemetric, seismic survey along a ~ 15-km wide E-W transect in the Central Andes, at a latitude of ~ 22.41° S, centered north of the city of Calama (68.9° W), Chile. The study area is sandwiched between the Central Depression in the west and the Andean Western Cordillera of Chile. Recording stations, nominally spaced at intervals of either 125 or 250 m collected up to 3.5 s of refracted seismic arrivals at maximum source-receiver offsets exceeding 15 km. Ten shothole sources, spaced 2-6 km apart focused energy on the shallow (0-3 km), crustal, Paleogene-age structures. Preliminary, tomographic inversions of refracted first arrivals show the top of a shallow (< 1km), high- velocity (VP, ~5 km/s) crust, deepening sharply eastward to at least 2 km. At the surface, this central basement step correlates to a regionally extensive (> 600 km), strike-slip fault zone known as the Oeste fault. Turning ray densities suggest the base of the overlying velocity gradient unit (VP, 2-4 km/s) dips inwardly from both east and west directions toward the Oeste fault to depths of almost 1 km. Plate reorganization commencing at least by the latter half of the Oligocene led from oblique to more orthogonal convergence between the South American and the Nazca (Farallon) Plates. We interpret previously mapped, older, minor faults as being generated within the right-lateral, orogen-parallel, Oeste strike-slip fault zone, and postdated by Neogene, N-S striking thrust faults. In this context we also interpret that the spatial distribution of velocity units requires an period of extensional activity that may (1) postdate the transpressional strike slip fault activity of the Neogene, (2) be related to a later releasing bend through the translation and interaction of rigid blocks hidden at depth or even (3) be the consequence of inelastic failure from the result of flexural loading.
T41A-1945
Do transform faults parallel plate motion?
A central principle of plate tectonics is that relative plate motion is parallel to transform faults. Several workers have convincingly argued, however, that transform fault valleys widen with age due to horizontal thermal contraction of the lithosphere (Collette 1974; Roest et al 1986 ; Sandwell 1986). If so, then the transform fault zone, which is the locus of active strike-slip faulting in a transform fault, is not parallel to the direction of plate motion. It is also affected by the ridge-parallel contraction of the lithosphere and is biased by a predictable amount. Here we apply a recent model for horizontal contraction of oceanic lithosphere as a function of age (Kumar & Gordon 2008) to calculate this bias as a function of offset, spreading rate, and ridge length, with slightly different formulations for crenelate and stepping mid-ocean ridge segments. The bias causes right-slipping transform faults to be counter-clockwise of the true plate motion direction while left-slipping transform faults are clockwise of the true plate motion direction. The bias is larger for longer ridge segments, smaller offsets, and slower spreading. The bias ranges in magnitude from about 0.1 degree to 1.5 degrees for stepping boundaries and 0.2 degree to 3 degrees for crenelate boundaries. Application of the bias correction to NUVEL-1 transform fault azimuths about the Rodrigues, Juan Fernandez, Galapagos, and Bouvet triple junctions improves the closure of the triple junction in the first three cases but makes it slightly worse in the fourth case. Additional applications will be presented.
T41A-1946
Effects of Heterogeneous Permeability on Surface Heat Flow Near Parkfield, CA
Surface heat flow near Parkfield, CA exhibits substantial scatter that is not observed in other portions of the California data set, with differences as large 15 mW/m2 over lateral distances of 5-10 km. This scatter has been an important limitation on interpretations of regional heat flow in terms of geodynamic processes, but to date has not been explained. Several processes have been hypothesized to explain the scatter, including conductive refraction, topographic refraction, and advection of heat by groundwater flow. Previous studies designed to investigate the effects of groundwater flow have focused primarily on the role of advection in perturbing a thermal anomaly associated with frictional heat production along the San Andreas Fault (SAF); these studies have considered only simple permeability architectures for the crust, and the results do not readily explain the observed scatter in the data. Notably, in the California Coast Ranges, distinct lithologic units with widely varying permeabilities have been juxtaposed by Mesozoic subduction and displacement along the fault, suggesting that complex groundwater flow paths are likely. Here, we test the hypothesis that heat advection through an upper crust characterized by heterogeneous permeability can generate the magnitude and spatial characteristics of the scatter in the heat flow data set. We created a 2D coupled groundwater flow-heat transport model along a cross-section perpendicular to the SAF, constrained by data from geologic maps, wells, and geophysical surveys, in order to explore and quantify the relationships between topography, permeability, and simulated near-surface heat flow. We assign a constant temperature and atmospheric pressure at the topographic surface; the lateral model boundaries correspond to regional groundwater divides and are designated as no-flow for both heat and fluid flow, and we prescribe a constant heat flux of 78 mW/m2 and a no-flow hydrologic boundary at the model base 10 km beneath sea level. We evaluate the role of the 1-3 km thick package of Tertiary sediments overlying basement by assigning an impermeable basement and considering sediment permeabilities from 10-15 - 10-20 m2. We also consider the effects of the fault zone itself, and model it as a low permeability barrier, a high permeability conduit, and as a combined conduit-barrier. The standard deviation in heat flow ranges from 16 mW/m2 to 0 mW/m2 for sediment permeabilities of 10-15 m2 to 10-20 m2, respectively. This is ~60% lower than the standard deviation predicted by models with homogeneous permeability. Additionally, including the fault zone itself in simulations results in significant localized variations in heat flow. For example, a high permeability damage zone results in heat flow extremes confined to the width of the damage zone, whereas a high permeability damage zone with a 200 m wide central barrier (combined conduit-barrier) reduces these extremes. Based on our results, we suggest that heat advection by groundwater flow mediated by heterogeneous permeability in the upper crust is one plausible mechanism for generating the observed scatter in heat flow.
T41A-1947
Fault strength in a strike-slip setting from finite element models: California, USA
The strength of secondary faults within plate-boundary zones and that of master faults like the San Andreas
has been controversial for decades. Here we use a global finite element code with a variable-resolution grid
and mantle-derived driving forces to determine whether the effective friction μast on the San
Andreas fault is high (μast = 0.6 - 1), intermediate (μast = 0.3 - 0.1) or low (μast
< 0.1), whether a single value of μast can be used for all mapped faults within the region, and
whether weakening of the ductile lower crust directly below faults is important. We compare our model results
with existing data on fault slip-rates, GPS velocity field, stress field, and depth of earthquakes.
This comparison indicates that all faults are weak (μast ≤ 0.1), and that slip-dependent
weakening is important. All viable models show that weakening of major faults in the lower crust is necessary
to avoid excessive weakening in the brittle crust and therefore unrealistic depths to the brittle-ductile
transition. The strongest faults in California have μast in the range 0.1-0.03. The San Andreas fault
is a very weak fault among weak faults, with μast values of about 0.01. Our success in modeling this
region also shows that a global code with appropriate grid-refinement and mantle-driving forces can
reproduce the tectonics of local areas while being driven by global mantle circulation models.
http://homepage.mac.com/scarena/webpage/images/GlobeTopo.mp4
T41A-1948
Using Mesozoic to Cenozoic Tectonostratigraphy to Constrain Neogene Slip of the Rodgers Creek - Maacama Fault System, Northern Coast Ranges, California
We use mapped terranes of the Jurassic Coast Range Ophiolite (CRO) and overlying Great Valley Sequence (GVS) and details of their boundary geometries determined from their aeromagnetic expressions to constrain 20-24 km of long-term displacement on the active Rodgers Creek-Maacama Fault system. These terranes occupy roof and floor thrust zones of the eastwardly wedged Franciscan Central and Coastal belts. Along the west side of the Great Valley, the CRO and basal GVS include the Del Puerto terrane, distinguished by keratophyric lava and silicic tuff in the upper part of the CRO, and the Elder Creek terrane, distinguished by a coarse CRO-derived mafic breccia in the basal GVS. Farther west, the CRO and GVS are more disrupted and slivers of these rocks are peeled-up and obliquely translated along Eocene to early Miocene roof and floor thrusts separating the Franciscan Central and Coastal belt wedges. Offsets in distribution of these CRO and GVS terrane slivers across the Rodgers Creek-Maacama Fault system constrain its long-term strike-slip history. West of this fault system the CRO consists of Del Puerto terrane, except for a section of Elder Creek terrane near Hopland. East of the Maacama Fault and northwest of Lake Berryessa, only Elder Creek terrane is interleaved with the Franciscan Complex. The Maacama Fault truncates the west side of a WNW-trending, SW-dipping section of Elder Creek terrane at Geyser Peak near Geyserville. The Geyser Peak section is right-laterally separated 20-22 km along the Maacama Fault from the truncated east side of the similarly oriented Hopland section. Offset aeromagnetic anomalies associated with the CRO and Central belt permit slightly larger offset of 24-28 km, and a 3.2 Ma tuff of the Sonoma Volcanics is displaced 17-24 km. This control suggests a maximum dextral offset of 20-24 km for the Maacama Fault since 3-4 Ma. An earlier Miocene and older vertical displacement history for the Maacama Fault related to thrusting in the roof of the Coastal belt wedge, is suggested by the absence of the Coastal belt in the upper few km east of the Rodgers Creek-Maacama Fault system, and a significantly thinned Central belt to the west.
T41A-1949
Active Orogen Parallel Strike-Slip Faulting in the Lower Dolpa Region, Northwest Nepal: Implications for Expansion of the Himalayan Arc
We present preliminary results from geologic mapping and remote sensing in northwest Nepal that reveals the geometry and kinematics of a significant right-oblique slip fault we term the Tibrikot fault system. The Tibrikot fault system strikes approximately N70° W and a continuous well-exposed segment of the fault extends for approximately 40 km. The trace of the Tibrikot fault system lies in between the Thakkola graben to the east and Gurla-Mandhata fault system to the west, cutting obliquely across Greater Himalayan crystalline rocks and the Lesser Himalayan sequence. Where exposed, the Tibrikot fault is linear in regions of high relief suggesting it has a sub-vertical dip. Geologic mapping shows that deformation is mainly accommodated by right-slip motion with minor normal sense-of-shear. Brittle fault fabrics include fault striations and tool marks consistent with right-oblique normal slip, and are sub-parallel to the strike of the Himalayan orogen. Disrupted and right laterally offset landforms observed along the Tibrikot fault include fault scarps, river channels, terrace surfaces, and mountain fronts, all consistent with right-slip motion suggesting the Tibrikot fault system cuts across the Main Central Thrust at depth in the Lower Dolpa region of Nepal. Although our preliminary results do not demonstrate that the Tibrikot fault directly strikes into the Thakkola graben or Gurla Mandhata fault system, they may be linked via distributed right-stepping en echelon arrays of right-slip faults based on previous results of regional geologic mapping. If correct, this implies that transtensional structures within the Tibetan plateau are communicating with active strike-slip structures in the High Himalaya. We suggest that the function of this relationship serves to accommodate E- W stretching of the Himalayan thrust wedge driven by outward radial growth of the Himalayan arc.
T41A-1950
Frequent surface rupturing earthquakes along the Carrizo section of the San Andreas Fault since A.D. 1250.
Paleoseismological investigations of the San Andreas Fault (SAF) in the Carrizo Plain have greatly influenced general models of fault behavior and our understanding of seismic hazard in southern California. Interpretations from seven new excavations across the SAF at the Bidart Fan site in the Carrizo Plain contradict the widely accepted hypothesis that this section of the fault ruptures relatively infrequently and only during large earthquakes with large (~8 m) offsets. Our new paleoseismic data indicate that the Carrizo section of the southern SAF has ruptured six times since ~A.D. 1250. The most recent earthquake, event A, was the 1857 Fort Tejon earthquake. The penultimate earthquake, event B, occurred sometime after A.D. 1620 and not sometime between A.D. 1405 and A.D. 1510, as previously thought. Four earthquakes, events C, D, E and F, occurred between A.D. 1250 and A.D. 1640. Our findings are similar to the new results from the Frazier Mountain site (worked conducted by Scharer and colleagues about 100 km southeast), which indicate 4-5 earthquakes since A.D. 1400. These new data from the northern section of the southern SAF indicate that since about A.D. 1250, the Carrizo section has failed more regularly and more often than previously thought. Additional paleoseismological investigations are needed to expand the record of the past earthquakes and determine the slip associated with each. This information will better constrain the past SAF rupture patternan essential element in the assessment of its future behavior.
T41A-1951
Kinematically Coupled Strike-Slip and Normal Faults in the Lake Mead Strike-Slip Fault System, Southeast Nevada
The Lake Mead fault system consists of a ~95 km long, northeast-trending zone of strike-slip faults of
Miocene age that accommodate a total left-lateral offset of 20-65 km. We use a combination of detailed field
mapping and numerical modeling to show that a previously unnamed left-lateral strike-slip segment of the
Lake Mead fault system and a dense cluster of dominantly west-dipping normal faults acted in concert to
accommodate regional left-lateral offset. We suggest that the strike-slip fault that we refer to as the Pinto
Ridge fault: (1) was kinematically related to other faults of the Lake Mead fault system; (2) was responsible
for the creation of the normal fault cluster at Pinto Ridge; and (3) utilized these normal faults as linking
structures between separate strike-slip fault segments to create a longer, through-going fault. Results from
numerical models demonstrate that the observed location and curving strike patterns of the normal fault
cluster is consistent with the faults having formed as secondary structures as the result of the perturbed
stress field around the slipping Pinto Ridge fault. Comparison of mechanical efficiency of various normal fault
geometries within extending terranes suggests that the observed west dip of normal faults reflects a west-
dipping anisotropy at depth, such as a detachment. The apparent terminations of numerous strike-slip faults
of the Lake Mead fault system into west-dipping normal faults suggest that a west-dipping detachment may
be regionally coherent.
http://www.people.umass.edu/stmarsha/nevada.html
T41A-1952
Moderately High Holocene Rate of Slip on the Western Garlock Fault Favors a Conjugate Fault Model
The precise tectonic role of the left-lateral Garlock fault in southern California has been controversial. Three tectonic models that have been previously proposed in the literature yield significantly different predictions for the slip rate, history, orientation and total bedrock offset as a function of distance along strike. In an effort to test these models, I present the first slip-rate estimate for the western Garlock fault that is constrained by radiocarbon dating. The site is located 18 km northeast of the town of Mojave, and 3.5 km southwest of the point where Lone Tree Canyon crosses the fault. A channel (referred to here as Clark Wash) incised into a latest Pleistocene alluvial fan has been left-laterally offset at least 66 ± 6 m and no more than 100 meters across the western Garlock fault, indicating a left-lateral slip rate of 7.6 mm/yr (95 % confidence interval of 5.3-10.7 mm/yr) using bracketing dendrochronologically calibrated radiocarbon dates. Vertical slip has been negligible on the Garlock fault at Clark Wash during the Holocene. A few hundred meters to the north, however, the westward continuation of the central Garlock fault strand appears to be expressed as an active normal fault at the southern range front of the Sierra Nevada with a Holocene to latest Pleistocene dip-slip rate of 0.4-0.7 mm/yr. The relatively high left-lateral slip rate determined here indicates that the western segment of the Garlock fault shows a similar rate of Holocene movement to the central Garlock fault. The moderately high rate of motion on the western Garlock fault is most consistent with a model in which the Garlock fault (or at least the western Garlock fault) acts as a conjugate shear to the San Andreas system. Models in which left-slip on the Garlock fault accommodates extension north of the fault are problematic because present-day (geodetic) deformation reveals little or no Garlock-parallel extension north of the Garlock fault, and older strain markers yield an extension direction that is more closely parallel to some of the right-lateral faults north of the Garlock than to any segment of the Garlock fault. The western Garlock fault, in particular, is at a high angle (about 45 degrees) to the long-term extension direction north of the fault. Similarly, block rotation in the northeastern Mojave Desert, while important, cannot be the primary explanation for left slip on the western and central Garlock fault, because the east-west striking, left-lateral faults that accommodate clockwise rotation in the northeastern Mojave Desert are only capable of explaining left-slip on the easternmost one-third or less of the Garlock fault. Some other mechanism is necessary to explain the relatively high slip rate that we have measured on the western Garlock fault and to explain the large bedrock offsets that extend westward beyond the region of block rotation. The conjugate model for the Garlock fault seems to be the only model capable of explaining the moderately high slip rate reported here for the western Garlock fault. It may also provide a useful view point for investigating why the rates of left-lateral strain accumulation along the Garlock fault (measured geodetically) are significantly lower than the rates of left-lateral Holocene strain release along the fault. If the Garlock fault is a conjugate fault within the broader, northwest-trending system of right-lateral shear, should one expect to ever see left-lateral elastic strain accumulation along it, or might left-lateral strain release along the fault be driven by the stresses caused by northwest-trending, right-lateral strain accumulation (as has been previously suggested by Savage and others, 2001)?
T41A-1953
Fault Mechanics and Active Strain Along the Garlock Fault in SE California
We report here results from new geologic mapping along a 38 km segment of the Garlock Fault (GF) between US 395 and the Slate Range, and an 8 km segment at the northern terminus of the Blackwater- Calico fault (BCF) in the Lava Mountains. This study area lies within the ENE-striking central segment of the sinistral GF. NNW-striking faults of the dextral Eastern California shear zone approach the GF, but do not offset it: exact mechanisms of strain transfer across the GF from the Mojave Desert to the Basin and Range is enigmatic. Field mapping reveals that the GF is complex with numerous sub-parallel strands both north and south of the mapped fault. Holocene slip on the GF is dominantly sinistral, but a major zone to the north adjacent to the bedrock of the southern Slate Range is dip-slip. The mapped portion of the northern BCF is expressed as a bedrock scarp and does not cut Holocene sediments. Significant N-S shortening is superimposed along the GF adjacent to the southern Slate Range, in the Christmas Canyon area, and the Lava Mountains. Pliocene- Pleistocene sediments are uplifted and deformed into E-W open to chevron folds in the Christmas Canyon and Slate Range areas. Cretaceous quartz-monzonite and overlying Miocene strata are deformed by similar structures in the northern Lava Mountains. In general, areas of topographic uplifts are disjointed and spatially restrictive in comparison to the more continuous GF and the BCF. These observations suggest several possibilities for the region. (1) Active slip on the GF and the Eastern California shear zone are driven by a single, Mojave-wide stress field with sigma-1 oriented roughly N-S. (2) The GF may be a weak zone in the lithosphere and crust with sigma-1 oriented nearly perpendicular to strike as evidenced by ENE- to East-trending fold hinges in Pliocene-Pleistocene sediments. (3) The continuous trace of the GF rupture through the 38-km-long study area suggests that it, at least locally, poses a mechanical boundary to the localization of NW-directed shear of the Eastern California shear zone. (4) N-S shortening (off-fault deformation) is spatially variable and is genetically related to strain on the GF and the Eastern California shear zone.
T41A-1954
The Development of the Hayward-Rodgers Creek-Maacama Plate Boundary Fault System
The plate boundary corridor extending from the San Andreas and Hayward faults in central California to the nascent fault system of the northern Maacama fault is developed in response to a fundamental plate boundary change from subduction to translation with the passage of the Mendocino triple junction (MTJ). How this primary plate boundary structure develops in the wake of the MTJ is not clear. The recently acquired GeoEarthScope LiDAR along this plate boundary corridor affords a fresh perspective on the distribution and geometry of active fault segments, as expressed in landscape topography. Using this high-resolution image of the fault system, we are evaluating the manner in which fault segments form, lengthen and coalesce. The resultant dataset of fault segment length, distribution and geometry can be compared to the space-time evolution of the plate boundary in an effort to test competing hypotheses for the localization of shear in a nascent fault system. In particular we are assessing whether this fundamental plate boundary fault system develops by systematic northward propagation in the wake of the MTJ or a more complex pattern of localized patches of fault development coalesce producing a more punctuated development to the faults.
T41A-1955
Paleoseismic interpretation and a preliminary geologic slip rate for the Parkfield segment of the San Andreas Fault
The Parkfield segment of the San Andreas Fault (SAF) is the northwest terminus of the great 1857 Fort Tejón earthquake. Slip budgets accounting for creep and moderate magnitude earthquakes suggest a slip deficit of 5 m along the extent of the 1857 rupture and a smaller deficit waning into the Parkfield segment. However, a geologic slip-rate for the Parkfield segment has not been established and it is unclear how slip is partitioned between the SAF and nearby sub-parallel faults. In 2007, we excavated three trenches at the Miller's Field paleoseismic site. This effort followed a similar paleoseismic campaign in 2004. Here we present new radiocarbon dating from our previous excavations across a sag pond (MST04) and a pressure ridge (PT04) and interpretations of the new paleoseismic exposures on these same geomorphic features (MST07 and PT07). We also opened two pilot trenches along the Southwest Fracture Zone (SWFZ). The Miller's field paleoseismic site is a Holocene terrace of the Little Cholame Creek. The exposed stratigraphy is overbank deposits of sand and silt. Some units are separated by thin charcoal-rich horizons that we interpret to have settled out during the waning of paleofloods. Along the SAF scarp, several buried soil horizons are deformed by the fault zone and clay-rich layers have accumulated within a sag pond. Deformation within the sag is partitioned among several zones of faulting with multiple splays. Many splays extend to the surface of the trenches and some are oriented obliquely to the trend of the SAF, consistent with formation from en-echelon surface cracking similar to the 2004 M6 Parkfield earthquake rupture. Interpreting ground rupturing paleoearthquakes at Parkfield must be cautionary because the SAF is creeping and numerous moderate magnitude earthquakes have occurred there historically. Upward terminating offsets could be formed on creeping fault splays. Despite this caution, we did observe three upward terminating offsets in the MST04 trench. This is weak evidence for two events between 898 A.D. and 1483 A.D. and one event prior to 898 A.D. Also two sets of upward terminating offsets and three lobes of reworked material (possibly fissures or colluvial wedges) within the PT exposures provide weak evidence for five ground rupturing events prior to 1857. However, we did not observe any geomorphic or stratigraphic evidence for a multi-meter rupture extending this far northwest in 1857. New radiocarbon dates allow us to estimate the sag pond began to form between 898 A.D. and 1036 A.D. Using the sag pond scarp formation as an initiation time for the channel that is apparently offset 17 to 24 meters across the MST site, we estimate a minimum slip rate of 16 to 25 mm/yr across a 10 meter aperture of the SAF. A slip rate of ~20 mm/yr for the main SAF at Parkfield indicates that another 10-15 mm/yr are accommodated by structures in a wider aperture of the fault zone. The SWFZ could be responsible for a significant percentage of this slip, but probably not all of it because other splays have experienced slip in the most recent Parkfield earthquakes and nearby faults such as the Buzzard Canyon and White Canyon faults display tectonic signals in their geomorphology.
T41A-1956
Evidence for Discrete and Distributed Deformation Accommodated by Bookshelf Faulting From the Central Sierra Nevada, California.
Evidence for right-lateral strike-slip faulting along the eastern Sierran Nevada has been identified by a number of workers and these offsets at least partially occurred during late Cretaceous arc plutonism. This evidence includes large magnitude dextral offsets across subvertical ~ north striking faults of the Sr 0.706 line, Paleozoic metasedimentary rocks, and the Independence dike swarm. A portion of this offset occurred across a 110 m thick cataclastic fault zone in the West Pinnacle Creek drainage in the east-central Sierra Nevada (approximately 3 km of dextral offset). The zone of discrete cataclastic faulting is bordered by a region of distributed deformation (i.e., penetrative at map scale) characterized by small-scale cataclastic to protomylonitic sinistral and dextral shear planes within the Turret Peak, Lake Edison, and Lamarck plutons of the Cretaceous John Muir Intrusive Suite. The distributed damage zone east of the West Pinnacle Creek fault (WPCF) was accommodated primarily by antithetic south-southwest striking left-lateral shear planes. Distributed deformation to the west of the discrete zone was accommodated primarily by synthetic north- northeast-striking right-lateral shear planes. Deformation in the distributed region is characterized by a relatively simple architecture of low displacement strike-slip shear fractures. Average shear strain in the antithetic distributed zone was calculated by measuring centimeter to meter-scale offsets of planar markers (e.g., dikes, enclaves, and modal layers) across macroscopic shear planes (n= 71) along a 0.25 km traverse in the Lake Edison pluton (es = 0.052). The distributed zone deformation is manifested strongly within 3 kilometers of the master fault but dies out quickly beyond this distance. Total displacement within the distributed damage zone (0.194 km) is equal to ~6% of the displacement along the WPCF (~3 km). The strain within the damage zone was focused onto pre-existing joints within the surrounding plutons and the distribution and orientation of these joints controlled the relative sense of motion across the shear planes within the distributed zones (i.e. synthetic dextral slip across north trending joints and antithetic sinistral slip along southwest striking joints).