Darrell W. Sims

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)

Structural framework of the Edwards Aquifer recharge zone in south-central Texas

The Edwards Aquifer, the major source of water for many communities in central Texas, is threatened by population growth and development over its recharge zone. The location of the recharge and confined zones and the flow paths of the aquifer are controlled by the structure of and deformation processes within the Balcones fault system, a major system of predominantly down-to-the-southeast normal faults. We investigate the geologic structure of the Edwards Aquifer to assess the large-scale aquifer architecture, analyze fault offset and stratigraphic juxtaposition relationships, evaluate fault-zone deformation and dissolution and fault-system architecture, and investigate fault-block deformation and scaling of small-scale (intrablock) normal faults. Characterization of fault displacement shows a pattern of aquifer thinning that is likely to influence fault-block communication and flow paths. Flow-path constriction may be exacerbated by increased fault-segment connectivity associated with large fault displacements. Also, increased fault-zone deformation associated with larger-displacement faults is likely to further influence hydrologic properties. Overall, faulting is expected to produce strong permeability anisotropy such that maximum permeability is subhorizontal and parallel to fault-bedding intersections. At all scales, aquifer permeability is either unchanged or enhanced parallel to faults and in many cases decreased perpendicular to faults.

Development of Synthetic Layer Dip Adjacent to Normal Faults

Field analyses of normal faulting illustrate that synthetic layer dip associated with normal faults is a common feature of extensional fault systems. These synthetic dip panels are developed where layers on upthrown, downthrown, or both sides of a normal fault dip toward the downthrown side of the fault. Synthetic dip panels adjacent to normal faults should be expected at some scale in all normal fault systems. In addition to faults that developed in the strata with a regional dip, five fault-related mechanisms for the development of synthetic dip are faulted monocline (fault tip-line folding), antilistric fault bend, distributed shear, shear in relay zone of vertically and/or laterally segmented faults, and fault block impingement and lateral contraction. Development of synthetic dip accommodates a component of throw by tilting or folding, thereby reducing the offset or true displacement on the related normal faults. Fault block deformation is strongly dependent on the mechanisms that produce synthetic dip panels and may influence fault zone and fault block permeability. Depending on stratigraphic and structural relationships, synthetic dip panels can produce a downthrown closure for hydrocarbon trapping, provide fluid migration and/or production communication pathways across faults, or produce barriers to fluid communication across faults.

Analog modeling of normal faulting above Middle East domes during regional extension

We study the effects of planform dome shape on fault patterns developing with and without concurrent regional extension oriented oblique to the long axis of the dome. The motivation was the need to understand fault and fracture patterns in two adjacent mature hydrocarbon fields in the Middle East: one, an elliptical dome, and one, an irregularly shaped dome. The largest faults have throws of approximately 30 m (98 ft), which is close to the resolution limit of older two-dimensional seismic reflection data. The known fault trends are not parallel to the highest transmissivity direction but could form compartment boundaries. Fault and fracture patterns developed over the modeled domes provide insight into the populations of faults and fractures that are likely to exist in the reservoirs but have been undetected because they are at or below the resolution limit of reflection seismic data. Major domal structural elements, crestal fault systems, end splay systems, and radial faults are observed in modeled domes rising both with and without concurrent regional extension. Experimental results indicate that fault and fracture patterns are influenced by the effects of dome shape, regional extension, and relative timing of uplift with respect to regional extension. The expression of particular sets of faults and fractures associated with concurrent doming and regional extension depends on the interaction among regional extension, outer arc extension over the dome, and tangential extension around the dome margins. Our results also indicate that the two adjacent natural domes possibly experienced different kinematic histories from those previously interpreted.

Physical models of grooved terrain tectonics on Ganymede

Grooved terrain on Ganymede consists of distinct areas of parallel to subparallel ridges and troughs at a variety of spatial scales. Grooved terrain has been interpreted as the product of tectonism in the form of fault-accommodated distributed lithospheric extension. We use physical analog methods to test the formation of grooved terrain by imbricate normal faulting in response to distributed extension. Faults and fault systems produced in the models are geometrically and kinematically similar to patterns inferred for some grooved terrains on Ganymede. The high degree of similarity between model structures and those observed on Ganymede indicates that rotational half-graben brittle block faulting can explain at least some tectonic resurfacing on Ganymede and that 20% extension is sufficient to form structures analogous to grooved terrain.

Extensional Fault System Evolution and Reservoir Connectivity

Sandbox analog modeling experiments provide new insights into the effects of fault geometry on reservoir connectivity. During progressive distributed extension, three phases of fault system evolution are apparent. In Phase I, geometrically simple faults nucleate rapidly at a large number of sites throughout the deforming region. This is followed by Phase II, in which faults link and increase in trace length. Phase III is characterized by a quasi-steady-state nucleation and linkage of faults. Reservoir connectivity has many components; here, we focus on fault-controlled connectivity, which can be viewed from two complementary perspectives: rock mass connectivity (continuity of rock between and around faults) and fault network connectivity. Which of these perspectives is adopted depends on whether faults cutting the reservoir act as barriers to flow (e.g., in highly porous sandstone reservoirs) or conduits for flow (e.g., in fractured carbonate reservoirs). We use two measures of fault-controlled connectivity: (1) a fault density measure derived from the number of intersections between faults and potential flow paths and (2) the ratio of the number of fault tips to the number of faults. Taken together, these characteristics convey both the transmissivity characteristics and the ultimate leakiness of the reservoir.