John Suppe

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.

Rates of folding and faulting determined from growth strata

Many upper crustal folds on the scale of 1–10 km in compressive mountain belts grow by kink-band migration as a result of fault-bend, fault-propagation, or box folding. One or both kink-band boundaries sweep through the rock as the kink bands widen during fold growth. The kink bands typically have a constant width in pregrowth strata, which are strata that existed before deformation, whereas we predict — and observe on seismic lines — an upward decrease in kink-band width within the stratigraphic sequence deposited during fault slip and associated fold growth -here called growth strata. In fact this growth stratigraphic sequence provides a complete, decipherable record of the kinematics of deformation, much in the same way that sea-floor magnetic anomalies provide a decipherable record of plate kinematics. It is the continual addition of material that provides the detailed record of motion in both cases. In the simplest folds the kink band has a constant width within the pregrowth strata, narrows upward through the growth strata, and finally has a zero width at the top of the growth stratigraphy.

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.

Origin of rollover

Rollover is the folding of hanging-wall fault blocks by bending or collapse in response to slip on nonplanar--commonly listric--normal faults. The shapes of rollover folds are controlled by a number of variables, including (1) the shape of the fault, (2) the total fault slip after a bed is deposited, (3) the direction of relative particle motion in hanging-wall collapse, (4) the history of sedimentation rate relative to fault slip rate, and (5) compaction. The importance and role of each of these variables is illustrated by a two-dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments. In this modeling, an active axial surface, oriented parallel to the direction of relative p rticle motion in hanging-wall collapse, emanates from each fault bend and is the instantaneous locus of folding. The quantitative correctness of this theory of rollover is tested by modeling natural structures from the Gulf of Mexico for which both fault shape and fold shape are known from high-quality seismic and well sections. The direction of hanging-wall collapse commonly is in the antithetic or synthetic normal-fault or Coulomb-shear orientations, although sliding along weak bedding planes also is an important collapse mechanism in some regions. Collapse is in the antithetic-shear direction for concave fault bends and in the synthetic-shear direction for convex bends. These collapse directions can be observed directly in some high-quality seismic images as axial surfaces emanating from fault bends. The shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate, as well as the total slip after a bed is deposited. The crests of classic Gulf Coast rollovers are growth axial surfaces, along which are abrupt changes in sedimentation rate within the growth st atigraphic interval. These changes are produced by deformation of the sediment-water interface along active axial surfaces. Compaction can substantially modify the relationship between fault shape and rollover shape; however, under certain common conditions, the history of compaction can be neglected if the folding is modeled in the compacted state.

Earthquake hazards of active blind‐thrust faults under the central Los Angeles basin, California

We document several blind-thrust faults under the Los Angeles basin that, if active and seismogenic, are capable of generating large earthquakes (M = 6.3 to 7.3). Pliocene to Quaternary growth folds imaged in seismic reflection profiles record the existence, size, and slip rates of these blind faults. The growth structures have shapes characteristic of fault-bend folds above blind thrusts, as demonstrated by balanced kinematic models, geologic cross sections, and axial-surface maps. We interpret the Compton-Los Alamitos trend as a growth fold above the Compton ramp, which extends along strike from west Los Angeles to at least the Santa Ana River. The Compton thrust is part of a larger fault system, including a decollement and ramps beneath the Elysian Park and Palos Verdes trends. The Cienegas and Coyote Hills growth folds overlie additional blind thrusts in the Elysian Park trend that are not closely linked to the Compton ramp. Analysis of folded Pliocene to Quaternary strata yields slip rates of 1.4 ± 0.4 mm/yr on the Compton thrust and 1.7 ± 0.4 mm/yr on a ramp beneath the Elysian Park trend. Assuming that slip is released in large earthquakes, we estimate magnitudes of 6.3 to 6.8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps. Multiple-segment ruptures could yield larger earthquakes (M = 6.9 to 7.3). Relations among magnitude, coseismic displacement, and slip rate yield an average recurrence interval of 380 years for single-segment earthquakes and a range of 400 to 1300 years for multiple-segment events. If these newly documented blind thrust faults are active, they will contribute substantially to the seismic hazards in Los Angeles because of their locations directly beneath the metropolitan area.

Bed-by-bed fold growth by kink-band migration: Sant llorenç de Morunys, eastern Pyrenees

Ahstract<rowth strata deposited over and against the flank of the Sant Llorenc de Morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth. We use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding. The progressive restorations, as well as balanced forward modeling, show that the last few hundred meters of fold growth were dominated by kink-band migration of a sort that is typical of much faultrelated folding. The kink-band migration has produced complex anticlinal hinge-zone geometry, including segmented fold hinges linked by disconformities and unconformities, which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved-hinge kink-band migration rate. Large fluctuations in the convolution of non-steady sedimentation and deformation are demonstrated, although the absolute fluctuations in deformation and sedimentation are unknown. At a length scale of 100 m, kink-band migration with little or no deposition is interspersed with sedimentation with little or no deformation. At the length scale of 500 m, deposition ranges from 200% to 50% of uplift. 0 1997 Elsevier Science Ltd. All rights reserved.

Active faulting and growth folding in the eastern Santa Barbara Channel, California

We develop new methods to identify blind-thrust fault systems, determine fault slip rates, and estimate potential earthquake magnitudes and recurrence intervals in active fold-and-thrust belts. These methods are applied to compressive folds along the Offshore Oak Ridge and Blue Bottle trends, which overlie active blind-thrust faults in the eastern Santa Barbara Channel. These folds and their causative faults are interpreted using fault-bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data. The structures are mapped using a new technique of axial-surface mapping in seismic reflection grids, which defines three-dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike. Analysis of syntectonic (growth) sediments yields Pliocene and Quaternary fault slip rates of 1.3 mm/yr on a deep thrust (≥16 km) and 1.3 mm/yr on shallower faults (2-5 km). The combined 2.6 mm/yr slip rate represents only part of the 6 mm/yr of shortening measured by geodesy across the channel and estimated from relative Pacific-North American plate motions across the Transverse Ranges. Additional shortening is probably accommodated on other active thrusts in the western Transverse Ranges and in the northern channel along the Santa Barbara coast. Deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern California. Unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes. This analysis suggests that a ramp in the Channel Islands fault beneath the Offshore Oak Ridge trend is capable of rupturing in a M s ≥7.2 earthquake. Earthquakes of this magnitude may release ∼2 m of slip, which, when combined with the estimated slip rate (1.3 mm/yr), yields an earthquake recurrence interval of ∼1500 yr for this Channel Islands fault ramp.

Absolute fault and crustal strength from wedge tapers

The strengths of mountain belts and major faults have been notoriously difficult to constrain and there is ongoing debate over the controlling mechanisms and stress magnitudes. Here we show that the strengths of active thrust-belt wedges and their basal detachments can be directly determined from the covariation of surface slope α with detachment dip β, without strong assumptions about the specific strength-controlling mechanisms. Even a single taper measurement (α, β) can strongly constrain the set of possible wedge and detachment strengths. This theory is tested with dry sand wedges and then applied to the Niger delta thrust belt, the active Taiwan mountain belt, and the thrust that slipped in the M = 7.6 Chi-Chi earthquake. Their basal detachments are shown to be exceedingly weak, with effective coefficients of friction (0.04–0.1) that are an order of magnitude less than most laboratory friction coefficients (0.6–0.85). In contrast, these wedges are moderately strong internally, within the range of pressure-dependent strengths in deep boreholes. These results confirm the existence of exceedingly weak faults and strong crust, which raises important causal questions.

Active detachment of Taiwan illuminated by small earthquakes and its control of first-order topography

We use 110 000 small earthquakes to locate and map active faults in three dimensions within the Taiwan arc-continent collision. The structure is dominated by a nearly horizontal band of small earthquakes at ∼10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt, with other illuminated fault zones abutting the detachment zone. The zone steepens below eastern Taiwan to 30°–90° and reaches depths of 30–60 km. The three-dimensional shape of the detachment zone in relation to topography allows a new test of critical-taper wedge mechanics and suggests that the reversal of topographic slope across Taiwan is controlled by the shape of the detachment.