David A. Ferrill

Slip-tendency analysis and fault reactivation

Slip-tendency analysis is a new technique that permits rapid assessment of stress states and related potential fault activity. The tendency of a surface to undergo slip in a given stress field depends on its frictional characteristics (primarily controlled by rock type) and the ratio of shear to normal stress acting on the surface, here defined as slip tendency (determined by orientation of the surface within the stress field). An interactive computer tool displays the stress tensor in terms of its associated slip-tendency distribution and the relative likelihood and direction of slip on surfaces of all orientations. The technique provides easy visualization and rapid evaluation of stress in terms of its potential for causing slip on individual faults or fault populations for use in seismic-risk and fault-rupture–risk assessment, exploration for high-risk and earthquake-prone blind faults, selection of likely earthquake focal mechanism solutions, and for use in analysis of compatibility of geologic structures.

Dilational normal faults

Abstract At low differential effective stress and with minimum principal effective stress near zero or tensile, rocks fail in several modes and with variable failure angles. Under these conditions mechanical stratigraphy exerts a significant influence on initial dip of normal faults. Less competent layers fail in shear mode along fractures that approximate the failure angle predicted by a standard rock mechanics analysis. Deformation of more competent layers, which is driven in part by interaction with the more rapidly deforming incompetent layers, produces hybrid mode failure in which failure angles are smaller than in shear mode. Analyses of small-displacement (

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.

Geologic factors controlling patterns of small‐volume basaltic volcanism: Application to a volcanic hazards assessment at Yucca Mountain, Nevada

The proposed high-level radioactive waste repository at Yucca Mountain, Nevada, is located within an active volcanic field. Probabilistic volcanic hazard models for future eruptions through the proposed repository depend heavily on our understanding of the spatial controls on volcano distribution at a variety of scales. On regional scales, Pliocene-Quaternary volcano clusters are located east of the Bare Mountain fault. Extension has resulted in large-scale crustal density contrast across the fault, and vents are restricted to low-density areas of the hanging wall. Finite element modeling indicates that this crustal density contrast can result in transient pressure changes of up to 7 MPa at 40 km depth, providing a mechanism to generate partial melts in areas where mantle rocks are already close to their solidus. On subregional scales, vent alignments, including one alignment newly recognized by ground magnetic mapping, parallel the trends of high-dilation tendency faults in the Yucca Mountain region (YMR). Forty percent of vents in the YMR are part of vent alignments that vary in length from 2 to 16 km. Locally, new geological and geophysical data show that individual vents and short vent alignments occur along and adjacent to faults, particularly at fault intersections, and left-stepping en echelon fault segments adjacent to Yucca Mountain. Conditions which formed these structures persist in the YMR today, indicating that volcanism will likely continue in the region and that the proposed repository site is within an area where future volcanism may occur. On the basis of these data the probability of volcanic disruptions of the proposed repository is estimated between 10−8/yr and 10−7/yr.

PHYSICAL ANALOG MODELING OF PULL-APART BASIN EVOLUTION

Abstract Pull-apart basins are structural depressions formed by localized extension along strike-slip fault systems, typically at releasing bends or steps in the fundamental strike-slip system. Analog modeling is used to evaluate the sequence of structural evolution of pull-apart basins and factors that control their degree of structural asymmetry and geometry. Basin evolution, internal structure and overall symmetry are investigated for oblique releasing step angles and with varying rates of displacement between brittle and ductile crust on opposing sides of the strike-slip system, while maintaining relative rates between brittle crust on opposing sides of the basin. Pull-apart basin evolution is separated into three stages: incipient, early, and mature. Incipient pull-apart basins are characterized by formation of a normal-fault bounded graben or half-graben parallel to the oblique step between main strike-slip zones. In the early stage of formation, additional normal faults form toward the basin interior from the original bounding faults, and cross-basin strike-slip faults cut diagonally across the basin interior; basin-bounding normal fault systems are characterized by lateral variations of fault throw and localized relay ramps. In the mature stage of evolution, strike-slip and normal faults join to completely bound the pull-apart basin. Analog model results indicate that displacement associated with cross-basin faults causes development of a through-going strike-slip fault that links the two main strike-slip displacement zones, ultimately resulting in a decline in normal fault activity. Asymmetric, symmetric, and hybrid pull-apart basins all follow the same overall deformation sequence just described. The asymmetry of a pull-apart basin is controlled by the degree of decoupling between brittle and ductile crust beneath the two crustal blocks in relative motion. This is modeled by maintaining a constant relative rate of motion between opposing fault blocks in all models, but varying the rate between the blocks and the fixed (model) basement (‘absolute’ rate). Models in which one side of the detachment is fixed with respect to the basement form asymmetric pull-apart basins defined by a half-graben with the master fault on the mobile side. In models where opposing sides of the fault system are equally decoupled from the basement, symmetrical pull-apart basins form, defined by horst and graben structures and master fault dominance switching sides along the length of the basin. Fault segmentation associated with relatively immature pull-apart basins may be capable of arresting earthquake rupture. Late linkage of main strike-slip zones by a cross-basin fault could extend potential rupture area, dramatically increasing the possibility for large-magnitude earthquakes.

Fault zone deformation controlled by carbonate mechanical stratigraphy, Balcones fault system, Texas

Normal faults in Cretaceous carbonates in the Balcones fault system provide important analogs for fault zone architecture and deformation in carbonate reservoirs worldwide. Mechanical layering is a fundamental control on carbonate fault zones. Relatively planar faults with low-displacement gradients develop in massive, strong, clay-poor limestones and dolomites. In less competent clay-rich strata, shale beds impede fault propagation, resulting in fault-related folding, and locally steep bedding dips. Faults in clay-poor massive limestones and dolomites tend to be steep (70 or more), whereas weaker, clay-rich limestones develop faults with shallower dips (60 or less). Fault zone rocks show evidence of cataclasis, cementation, deformation of cement by mechanical twinning and pressure solution, and multiple generations of cement with differing degrees of deformation, indicating contemporaneous cementation and fault slip. In stratigraphic sequences consisting of both competent and incompetent strata, the ratio of incompetent to competent strata by thickness is a useful guide for inferring the relative rates of fault displacement and propagation. Low displacement-to-propagation ratios associated with competent strata generate low-displacement gradients, inhibiting fault-related folding. Conversely, high displacement-to-propagation ratios associated with incompetent strata promote high-displacement gradients and fault-related folding.

Critical re-evaluation of differential stress estimates from calcite twins in coarse-grained limestone

Abstract Comparison of two techniques to estimate differential stress magnitude based on calcite twinning reveals significantly different calculated values of differential stress for limestone experimentally and naturally deformed at low temperature. This inequality may be due to fundamental differences between the rock type (marble versus limestone) and deformation temperatures (above and below 200°C) for samples used in technique calibration versus samples used in testing and application. A technique that was empirically derived from marble experimentally deformed at 200–800°C yields values of differential stress that are factors of 2 to 9 times larger than maximum differential stress measured during experimental deformation of limestone at low temperature (

Displacement gradient and deformation in normal fault systems

Abstract All fault systems contain faults with lateral (strike-parallel) displacement gradients. Lateral displacement gradients give rise to elongations parallel to fault cutoffs in either or both of the footwall and hanging wall fault blocks. We present a simple method for estimating cutoff parallel elongation based on geometric fault and fault-block elements. These elements are readily measured in the field or from maps and subsurface data, and include orientations of cutoff lines, faults, and displacement directions. Displacement gradients on overlapping normal faults produce relay ramps. Deformation within a relay ramp includes tilting, extension parallel to bounding cutoff lines, vertical axis rotation, and eventual breakthrough of the ramp. Relay ramp deformation is sensitive to fault displacement directions—oblique slip directions on the ramp bounding faults can cause contraction (restraining-step sense of displacement and overlap) or enhanced ramp extension (releasing-step sense of displacement and overlap). Relay ramp extension accommodated by brittle faulting and extension fracturing is important for locally altering porosity and permeability in fractured aquifers and reservoirs. Locally enhanced fault and extension fracture density can provide fast pathways for infiltration, percolation, and flow of groundwater, or barriers to fluid movement and can influence rock quality and stability of underground excavations. An example of strain localization in a displacement transfer zone from Yucca Mountain, Nevada, the proposed site for a high-level radioactive waste repository, is examined.

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).

Timing of basaltic volcanism along the Mesa Butte Fault in the San Francisco Volcanic Field, Arizona, from 40Ar/39Ar dates: Implications for longevity of cinder cone alignments

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