John A. Stamatakos

NORMAL FAULT CORRUGATION : IMPLICATIONS FOR GROWTH AND SEISMICITY OF ACTIVE NORMAL FAULTS

Abstract Large normal faults are corrugated. Corrugations appear to form from overlapping or en echelon fault arrays by two breakthrough mechanisms: lateral propagation of curved fault-tips and linkage by connecting faults. Both mechanisms include localized fault-parallel extension and eventual abandonment of relay ramps. These breakthrough mechanisms produce distinctive hanging wall and footwall geometries indicative of fault system evolution. From such geometries, we can estimate the positions of tilted relay ramps or ramp segments and ramp internal deformation in incompletely exposed or poorly imaged fault systems. We examine the evolution of normal fault corrugations at Fish Slough (California), Yucca Mountain (Nevada), and Pleasant Valley (Nevada), in the Basin and Range province. We discuss how evolution of the Pleasant Valley and Yucca Mountain systems relates to seismicity. For example, the 1915 Pleasant Valley earthquake produced four en echelon ruptures that appeared as overlapping segments of a single immature fault at depth. At Yucca Mountain, we argue that an en echelon array, which includes the Solitario Canyon and Iron Ridge faults, should be considered a single source, such that western Yucca Mountain could experience up to a M w 6.9 earthquake compared to M w 6.6 estimates for the largest individual segment.

ROLE OF A DUCTILE DECOLLEMENT IN THE DEVELOPMENT OF PULL-APART BASINS : EXPERIMENTAL RESULTS AND NATURAL EXAMPLES

Abstract Traditional models describe pull-apart basins as graben or half-graben basins with normal or normal-oblique slip master faults, analogous to Death Valley, California. Yet many pull-aparts are characterized by asymmetric basins with strike-slip master faults indicating that not all pull-apart basins conform to the simple Death Valley models. We present analogue modelling results that show developmental sequences and structural styles of pull-aparts are dramatically different when overburden rides over a ductile horizon, and that thickness of the ductile horizon exerts control on basin development. In our models, synthetic and antithetic strike-slip faults control basin geometries, while localized normal faulting and local oblique slip on strike-slip faults accommodate basin subsidence. Faults evolve from initial strike-slip to normal-oblique and normal dip slip to form a system of either isolated sub-basins in the case of thick ductile layers, or coalescing sub-basins in the case of thin ductile layers. These results demonstrate distinct differences between non-ductile and ductile decollement pull apart structures. Basin boundaries dominated by normal faults suggest a decollement within or at the base of a non-ductile layer similar to Death Valley, California. Basin bounding faults dominated by strike-slip and oblique-slip faults indicate basin formation over a ductile layer, similar to the Gulf of Elat (Aqaba) or Gulf of Paria (Venezuela and Trinidad).

Crossing Conjugate Normal Faults

Normal faults commonly develop in two oppositely dipping sets having dihedral angles of around 60o, collectively referred to as conjugate normal faults. Conjugate normal faults form at a range of scales from cm to km. Where conjugate normal faults cross each other, the faults are commonly interpreted to accommodate extension by simultaneous slip on the crossing faults. Using two-dimensional geometric modeling we show that simultaneous slip on crossing conjugate normal faults requires loss, gain, or localized redistribution of cross-sectional area. In contrast, alternating sequential slip on the crossing faults can produce crossing fault patterns without area modification in cross section. Natural examples of crossing conjugate normal faults from the Volcanic Tableland (Owens Valley, California), Bare Mountain (Nevada), and the Balcones fault zone (Texas) all indicate formation by sequential rather than simultaneous slip. We conclude that truly simultaneous activity of crossing normal faults is likely to be limited to extremely small displacements due to rate-limiting area change processes. If their associated movement is truly simultaneous, crossing normal faults are virtually unrestorable and should show evidence of significant cross-sectional area change (e.g., area increase may be indicated by salt intrusion along fault, area decrease by localized dissolution or mechanical compaction may be indicated by extreme displacement gradients at fault tips). In the absence of such evidence, even the most complicated crossing fault pattern should be restorable by sequentially working backward through the faulting sequence. In common with other structures that affect permeability and that cross at high angles, conjugate normal fault systems are likely to produce bulk permeability anisotropy in reservoir rocks that can be approximated by a prolate (elongate) permeability ellipsoid, with greatest permeability parallel with the line of intersection. Characterization of the fault pattern in a faulted reservoir provides the basis for interpreting the bulk permeability anisotropy in the reservoir, an important step in optimizing well placement. (Begin page 1544)

New model for evolution of fold and thrust belt curvature based on integrated structural and paleomagnetic results from the Pennsylvania salient

In a number of curved fold and thrust belts worldwide, such as the Cantabrian arc and the Wyoming-Idaho salient, paleomagnetic data indicate vertical axis rotations inconsistent with structural findings. This apparent conflict is especially pronounced in the Pennsylvania salient, where the paleomagnetically defined vertical axis rotations are opposite of the rotation sense indicated by structural studies. We resolve this apparent disparity by developing an integrated and kinematically admissible model that has implications for other curved mountain belts. The curvature of the Pennsylvania salient may be explained by deformation partitioning in an initially laterally tapered basin between the Cambrian-Ordovician carbonates and the overlying siliciclastic rocks. Earliest deformation occurred in the lowest strata and progressed toward the foreland and up section with time. Lateral differences in initial layer-parallel shortening in the Cambrian-Ordovician carbonates caused differential translation and rotations about a vertical axis in the cover sequence. Because of initial basin geometry, the orogenic wedge developed a lateral taper. With further shortening in the wedge and involvement of the cover rocks in the fold and thrust deformation, gravitational driving forces became more important, and the paleostress trajectories diverged in opposite directions on the two arms of the salient in response to the lateral taper.

Late Cretaceous paleogeography of Wrangellia: Paleomagnetism of the MacColl Ridge Formation, southern Alaska, revisited

Volcanic and sedimentary strata of the Late Cretaceous MacColl Ridge Formation were sampled and demagnetized to reevaluate the paleomagnetically derived paleolatitude of the allochthonous Wrangellia terrane. Characteristic directions from 15 sites representing ∼750 m of the MacColl Ridge Formation (80 Ma) reveal a reversed-polarity primary magnetization yielding a paleomagnetic pole at 126°E, 68°N, A 95 = 9°. Comparison of this pole with the Late Cretaceous reference pole for North America indicates 15° ± 8° of latitudinal displacement (northward) and 33° ± 25° of counterclockwise rotation. In contrast to previously reported low paleolatitudes (32° ± 9°N) for the MacColl Ridge Formation, these new results place the Wrangellia terrane at a moderate paleolatitude (53° ± 8°N) in the Late Cretaceous.

Extensional layer-parallel shear and normal faulting

Abstract An extensional fault system in Bare Mountain, Nevada, U.S.A., contains abundant evidence of layer-parallel shear deformation contemporaneous with faulting. Layer-parallel shear is manifest by deformation of pre-faulting fabrics and cleavage at low angles to bedding that indicate shear in the down-dip direction, perpendicular to fault-bedding intersections. Layer-parallel shear along discrete bedding planes locally offsets normal faults, and shear distributed within layers reorients block-bounding normal faults. In simple rigid block models of extension accommodated by normal faults above a low-angle detachment or decollement, extension causes faults to rotate to progressively shallower dips, while originally horizontal beds rotate to steeper dips. These rotations reorient faults out of originally optimum conditions for slip into orientations of a lower slip tendency, whereas bedding rotates to steeper dips with progressively higher slip tendency. The timing or amount of rotation before the initiation of layer-parallel shear depends on the frictional resistance to sliding or resistance to shearing within layering in fault blocks. Offset or deflection of block-bounding normal faults may cause faults to lock as extension increases. Alternatively, bedding and faults may become simultaneously active, progressively lowering dips of faults and bedding until neither is well oriented for slip, at which point new faults are required to accommodate additional extension. At Bare Mountain, early extension within the fault system was accomplished by fault slip and associated block rotation. Continued extension took place by slip along bedding within fault blocks.

QUATERNARY SLIP HISTORY OF THE BARE MOUNTAIN FAULT (NEVADA) FROM THE MORPHOLOGY AND DISTRIBUTION OF ALLUVIAL FAN DEPOSITS

Analyses of alluvial fan sedimentation around Bare Mountain, Nevada, indicate differential slip along the Bare Mountain fault during the Pleistocene and Holocene. Analyses show that the ratios of fan area to sediment source area and the degree of Holocene sedimentation at the fan heads can be correlated with dip and displacement of the fault. The well-developed bajada and Holocene deposits near the fan toes along the southwestern flank of Bare Mountain contrast with the location of Holocene deposits near the fan heads and significantly smaller individual fan lobes on the eastern flank of the mountain (adjacent to the Bare Mountain fault). In addition, ratios of fan area to source area along the eastern flank of the mountain decrease from north to south, which we interpret to be caused by increased Quaternary slip (from north to south) along the fault, with relatively little Holocene throw at Tarantula Canyon and maximum throw near Wildcat Peak. The dip of the fault also changes from north to south, with relatively shallow dip angles (45° to 50°) at Tarantula Canyon and steeper (∼ 70°) dips near Wildcat Peak. Geometric constraints dictate that, for any given increment of horizontal extension, vertical offsets will be smaller on those segments of the fault that have shallower dip angles. On the basis of these observations, we conclude that the recent slip rate of about 0.02 mm/yr derived from trenching studies in the Tarantula Canyon fan should be considered a minimum value that may not represent the slip rate of the Bare Mountain fault as a whole.

Magnetic surveys help reassess volcanic hazards at Yucca Mountain, Nevada

Natural disasters like volcanic eruptions occur infrequently, but if they occur near nuclear power plants or high-level radioactive waste repositories, local and global communities can be threatened. Ideally, such facilities should be constructed only where geologic risk is very low. n nEstimating the probabilities of such events requires a comprehensive understanding of site geology and the geologic processes operating in the site region on timescales of 104 to 107 years. In light of these requirements, geologists and geophysicists must continually improve techniques for site characterization.

Low-frequency radar sounding investigations of the North Amargosa Desert, Nevada : A potential analog of conductive subsurface environments on Mars

[1]xa0Theoretical estimates of low-frequency radar sounding performance and its potential for mapping moist subsurface interfaces in conductive environments on Mars are controversial, with predictions of ultimate penetration depth ranging from a few meters to kilometers. To address this issue, we conducted a broadband electromagnetic field survey in which we combined ground penetrating radar (GPR) operating at multiple low frequencies with the transient electromagnetic method (TEM) to investigate the dependence of radar penetration depth on ground resistivity. Surveys were performed in the frequency range 16–100 MHz at two locations on the northwest margin of the Amargosa Desert, Nevada, where numerous Mars-analog investigations have been performed. The surveys were conducted on a 20-m-high homogenous sand dune and on the flanks of a 20-m-high scoria cone and above a buried lava flow. A wet alluvial interface was located at the bottom of each structure. GPR detected the wet alluvium contact at the base of the sand dune, but failed to penetrate to the same depth at the scoria cone under similar residual moisture content. Depths of investigation for both the scoria cone and the buried lava flow were limited to approximately 10 m owing to the presence of conductive inclusions in the first few meters, which are below the radar resolution but dramatically decreased the dynamic of the radar-backscattered echoes and hence the penetration depth. Absorption models constrained by the TEM data are in good agreement with these observations. Depths of investigation varied weakly with frequency owing to substantial, frequency-independent absorption.

Numerical Simulations of Tsunami Wave Generation by Submarine and Aerial Landslides Using RANS and SPH Models

Tsunami wave generation by submarine and aerial landslides is examined in this paper. Two different two-dimensional numerical methods have been used to simulate the time histories of fluid motion, free surface deformation, shoreline movement, and wave runup from tsunami waves generated by aerial and submarine landslides. The first approach is based on the Navier-Stokes equation and the volume of fluid (VOF) method: the Reynolds Averaged Navier-Stokes (RANS)-based turbulence model simulates turbulence, and the VOF method tracks the free surface locations. The second method uses Smoothed Particle Hydrodynamics (SPH)—a numerical model based on a fully Lagrangian approach. In the current work, two-dimensional numerical simulations are carried out for a freely falling wedge representing the landslide and subsequent wave generations. Numerical simulations for the landslide-driven tsunami waves have been performed with different values of landslide material densities. Numerical results obtained from both approaches are compared with experimental data. Simulated results for both aerial and submerged landslides show the complex flow patterns in terms of the velocity field, shoreline evolution, and free-surface profiles. Flows are found to be strongly transient, rotational, and turbulent. Predicted numerical results for time histories of free-surface fluctuations and the runup/rundown at various locations are in good agreement with the available experimental data. The similarity and discrepancy between the solutions obtained by the two approaches are explored and discussed.Copyright