Van S. Mount

New Evidence on the State of Stress of the San Andreas Fault System

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at ∼2-kilometer depth on planes parallel to the San Andreas fault.

State of stress near the San Andreas fault: Implications for wrench tectonics

Borehole elongations or breakouts in central California show that the direction of regional maximum horizontal stress is nearly perpendicular to the San Andreas fault and to the axes of young thrust-related anticlines. This observation resolves much of the controversy over shear-stress magnitude in the crust and around the San Andreas fault specifically. A low shear stress of 10–20 MPa (100–200 bar) or less on the San Andreas fault, suggested by heat-flow and seismic observations, is compatible with a high regional deviatoric stress (100 MPa, 1 kbar) when the observed principal stress directions are considered. Therefore, the San Andreas fault is a nearly frictionless interface, which causes the transpressive plate motion to be decoupled into a low-stress strike-slip component and a high-stress compressive component. These observations suggest that standard concepts of transpressive wrench tectonics—which envisage drag on a high-friction fault—are wrong. The thrust structures are largely decoupled from the strike-slip fault.

Present-day stress orientations adjacent to active strike-slip faults: California and Sumatra

Present-day stress directions interpreted from well bore breakouts adjacent to two crastal-scale, active, strike-slip faults (the San Andreas fault in California and the Great Sumatran fault in Sumatra) indicate that the maximum horizontal stress direction (SH) is oriented at a high angle (70°–90°) to both faults. The regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to SH and are therefore in the thrust-fault orientation. These observations indicate a decoupling of the strike-slip and compressional components of the deformation within these broadly transpressive zones. Borehole breakouts in 118 wells in western California indicate a regionally consistent stress pattern with SH generally oriented NE-SW, nearly perpendicular (80°–90°) to the strike of the San Andreas fault. The orientation of SH nearly perpendicular to the San Andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the Coast and Transverse Ranges indicating active compressional deformation, fault plane solutions for recent dip-slip-style earthquakes, principal stress directions determined from inversion of earthquake focal mechanisms, and induced hydraulic fracture orientations. A stress trajectory map for western California is computed using an iterative statistical algorithm in which observed directional data, such as breakout directions, are used to obtain a model regional stress field. Analysis of well bore breakouts in 25 wells within the central and southern oil districts of Sumatra indicates that the regionally defined SH adjacent to the active Great Sumatran strike-slip fault is oriented at a high angle (70°–80°) to the fault This orientation of SH is consistent regionally with geologic stress indicators and focal mechanisms of dip-slip earthquakes. Preliminary analysis of the stress field in the vicinity of the Philippine and Alpine faults suggests SH is also oriented at a high angle to these active strike-slip faults. Similarly, the minimum horizontal stress Sh is oriented at a high angle to the the Kane and Dead Sea transforms. The observation of SH and Sh in the vicinity of active strike-slip faults being oriented nearly perpendicular and parallel to the faults suggests that large, crustal-scale strike-slip faults may, in general, be inherently weak surfaces.

Foreland Basement-Involved Structures

Basement-involved structures commonly occur as long, irregular chains of uplifts in foreland basins. These structures commonly contain significant hydrocarbon accumulations, with most major fields located on the broad crests of these structures. The search for complex traps in deeper targets and in subthrust structures requires an improved understanding of the geometry and evolution of these structures. Characteristic features of basement structures include deformation zones within the sedimentary cover that dissipate significant fault slip, and gently dipping frontlimbs and backlimbs. Fault slip in the basement is usually accommodated in the cover by a triangular, widening-upward deformation zone on the forelimb, with the nature of deformation controlled primarily by the mechanical stratigraphy. If the cover contains interlayered competent and incompetent units, the incompetent units are characterized by significant penetrative deformation, whereas the competent units are faulted after a relatively small amount of penetrative deformation. Depending on the competency contrast between the basement and cover, the nature of basement, and the physical conditions of deformation, the deformation zone may also propagate downward into the basement. Gently dipping backlimb and frontlimb panels are related to movement of the hanging wall over synclinal and anticlinal bends in the major fault, respectively. Many basement faults are characterized by a number of synclinal fault bends within the basement, which result in long and gently dipping backlimbs. Forelimb panels are related to anticlinal bends that typically occur at the basement-cover interface, as well as at one or more locations in the sedimentary cover. Case studies of well-constrained examples of structures from the Bighorn and Uinta basins and the Central Basin platform, demonstrate the development of these characteristic features and their strong dependence on the mechanical stratigraphy. These models and case studies will be useful in interpreting foreland basement structures in areas with poor or limited data.

A Forward Modeling Strategy for Balancing Cross Sections (1)

A strategy to balance cross sections of complex structures is documented and illustrated by the interpretation of a compressional structure in the deep-water Gulf of Mexico. The strategy is applicable to structures formed in sedimentary rocks under low temperatures in both compressional and extensional environments, and involves the comparison of the observed structure with simple, balanced, forward models. Forward models generated using fault-related fold theory help in understanding the processes and kinematics involved in the deformation. Further, forward models are completely constrained and easy to balance, whereas it is difficult to balance data. Therefore, forward models are useful in evaluating ideas without completely solving the structure. Models are constructed assuming parallel behavior (preservation of layer thickness, no net distortion where layers are horizontal, and conservation of bed length). Unmetamorphosed, sedimentary rocks are generally observed to deform obeying the assumptions of parallel behavior in field, map, well, and seismic data.

Map-view interference of monoclinal folds

Abstract Outcrop observations and laboratory experiments show that many small chevron folds form by interference of monoclinal kink bands in multilayer buckling. Kink-band interference has also been proposed for some map-scale folds. Furthermore, fault-related folding provides additional mechanisms of monoclinal fold generation, other than buckling, and thus makes kink-band interference all the more conceptually plausible as a significant large-scale process. In this paper we document several relatively simple examples of map-scale monoclinal fold interference, including three interfering monoclines of the Colorado Plateau, a seismically-imaged example in the Perdido foldbelt of the Gulf of Mexico, and a more complex example from seismic mapping in the Santa Barbara Channel, California. Kink-band interference has normally been analyzed in cross section. Here we emphasize the map-view phenomena and present a simple balanced three-dimensional model of the interference geometry, treating the monoclines as two independent kink bands, which does not depend of the kink-band folding mechanism. This model predicts the shape of the jog produced by crossing monoclines and is used to help evaluate the role of interference in the map-view geometry of our examples. The documentation of these simple examples supports the concept that more complex monoclinal fold interference could be a significant phenomenon in the upper crust.

Seismic structural analysis of deep-water Perdido foldbelt, Alaminos Canyon, northwest Gulf of Mexico

The Perdido foldbelt is located at the base of the continental slope in the northwestern Gulf of Mexico. Seismic data in the Atwater Canyon region, southwest of the Sigsbee escarpment, indicate that the Perdido foldbelt is at least 80 km wide and consists of large (8 km wide) flat-topped anticlines involving predominantly the 6-km thick Jurassic through Paleogene deep Gulf stratigraphic section. Deformation of Holocene sediments indicates active compression. All 62 Perdido fold-belt lease blocks acquired in OCS Sale 112 (August 1987) are located in water deeper than 1.2 km (4000 ft); seven are in record water depths over 3 km (10,000 ft). Process-based structural concepts and interpretation techniques developed in overthrust belts have been applied to compressive structures observed in seismic data from the Perdido foldbelt. Fault-bend fold concepts are used to interpret fold shapes, which tightly constrain the fault geometry at depth. Interpreted fault geometry beneath the Perdido foldbelt consists primarily of long flats and short low-angle (< 25/degree/) ramps. Three types of fault-related folds (fault-bend, fault-propagation, and box folds), as well as interference between structures, are imaged in the Perdido foldbelt.