Showing papers on "Fault (geology) published in 1988"

Strike-Slip faults

Abstract: The importance of strike-slip faulting was recognized near the turn of the century, chiefly from investigations of surficial offsets associated with major earthquakes in New Zealand, Japan, and California. Extrapolation from observed horizontal displacements during single earthquakes to more abstract concepts of long-term, slow accumulation of hundreds of kilometers of horizontal translation over geologic time, however, came almost simultaneously from several parts of the world, but only after much regional geologic mapping and synthesis. Strike-slip faults are classified either as transform faults which cut the lithosphere as plate boundaries, or as transcurrent faults which are confined to the crust. Each class of faults may be subdivided further according to their plate or intraplate tectonic function. A mechanical understanding of strike-slip faults has grown out of laboratory model studies which give a theoretical basis to relate faulting to concepts of pure shear or simple shear. Conjugate sets of strike-slip faults form in pure shear, typically across the strike of a convergent orogenic belt. Fault lengths are generally less than 100 km, and displacements along them are measurable in a few to tens of kilometers. Major strike-slip faults form in regional belts of simple shear, typically parallel to orogenic belts; indeed, recognition of the role strike-slip faults play in ancient orogenic belts is becoming increasingly commonplace as regional mapping becomes more detailed and complete. The lengths and displacements of the great strike-slip faults range in the hundreds of kilometers. The position and orientation of associated folds, local domains of extension and shortening, and related fractures and faults depend on the bending or stepping geometry of the strike-slip fault or fault zone, and thus the degree of convergent or divergent strike-slip. Elongate basins, ranging from sag ponds to rhombochasms, form as result of extension in domains of divergent strike slip such as releasing bends; pull-apart basins evolve between overstepping strike-slip faults. The arrangement of strike-slip faults which bound basins is tulip-shaped in profiles normal to strike. Elongate uplifts, ranging from pressure ridges to long, low hills or small mountain ranges, form as a result of crustal shortening in zones of convergent strike slip; they are bounded by an arrangement of strike-slip faults having the profile of a palm tree. Paleoseismic investigations imply that earthquakes occur more frequently on strike-slip faults than on intraplate normal and reverse faults. Active strike-slip faults also differ from other types of faults in that they evince fault creep, which is largely a surficial phenomenon driven by elastic loading of the crust at seismogenic depths. Creep may be steady state or episodic, pre-seismic, co-seismic, or post-seismic, depending on the constitutive properties of the fault zone and the nature of the static strain field, among a number of other factors which are incompletely understood. Recent studies have identified relations between strike-slip faults and crustal delamination at or near the seismogenic zone, giving a mechanism for regional rotation and translation of crustal slabs and flakes, but how general and widespread are these phenomena, and how the mechanisms operate that drive these detachment tectonics are questions that require additional observations, data, and modeling. Several fundamental problems remain poorly understood, including the nature of formation of en echelon folds and their relation to strike-slip faulting; the effect of mechanical stratigraphy on strike-slip-fault structural styles; the thermal and stress states along transform plate boundaries; and the discrepancy between recent geological and historical fault-slip rates relative to more rapid rates of slip determined from analyses of sea-floor magnetic anomalies. Many of the concepts and problems concerning strike-slip faults are derived from nearly a century of study of the San Andreas fault and have added much information, but solutions to several remaining and new fundamental problems will come when more attention is focused on other, less well studied strike-slip faults.

The relationship between plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and Middle East

Abstract: SUMMARY This paper is concerned with the relationship between the overall motion across a zone of distributed continental deformation and the seismic moment tensors of earthquakes that occur within it. The overall deformation in the zone is described by the deformation gradient tensor L, which may be split into a symmetric part, S, and an antisymmetric part, A. S is the strain tensor, and can always be determined from the sum of the moment tensors, following the result of Kostrov (1974). A corresponds to a rigid body rotation, and is in principle unobservable seismically: the moment tensors contain no information about A, regardless of whether the ambiguity between fault and auxiliary planes is resolved. From S the integrated rates of motion normal and parallel to the zone boundary, as well as vertically, can be calculated. Of these, only the motion normal to the zone is specified by the motion across its boundaries. In general, S (and hence L) is not specified by the motion of the plates bounding the zone. Only if some a priori assumptions about L are made, can information about A be recovered from the seismic moment tensors. Otherwise A must be determined independently from paleomagnetic or geodetic measurements. These results are applied to the Mediterranean region to see whether the motion between the relatively rigid regions of central Iran, Turkey, Arabia, Africa, the Adriatic Sea and Eurasia is accommodated seismically within the upper crust of the wide deforming zones that bound these regions. In NE Iran, the North Anatolian Fault Zone and the Aegean Sea all or most of the deformation is probably taken up seismically. In the Zagros, Caucasus, Hellenic Trench and western Mediterranean probably 10 per cent or less of the upper-crustal deformation is seismic and the rest must be accommodated by creep. The Cyprus arc, the East Anatolian and Dead Sea Fault Zones have had insufficient seismicity this century for any conclusions to be drawn. The seismicity in central Italy and Yugoslavia accounts for velocities of about 2 mm yr-' normal to the deforming zones, but there is no independent estimate of the velocities on the borders of the Adriatic Sea with which to compare these. The seismicity in the Aegean region indicates very high stretching velocities (c. 60 mm yr-') and strain rates (c. 4 x s-l). These in turn require correspondingly high subduction rates (c. 100 mm yr-') in the Hellenic Trench. If these rates have been constant in time, it is unlikely that the tip of the sinking slab beneath the southern Aegean began to be subducted more than 5Ma ago. These high-stretching rates in the Aegean, if extrapolated back to the Pliocene, are compatible with observed finite strains and paleomagnetic rotations. They are also likely to have raised to a shallower depth the isotherm corresponding to the seismic-aseismic transition in the crust, perhaps accounting for the relatively shallow focal depths of normal faulting earthquakes in the Aegean compared with those in other areas of continental extension where strain rates are lower. It is possible that the reason for the dominantly aseismic deformation in the Zagros and Hellenic Trench is related to the great thickness of sediments, partly decoupled from the basement by salt, in both places. This may lead to elevated basement temperatures and inhibit upward fault propagation, thus restricting the size of seismogenic fault planes (and hence seismic moment) and causing the sedimentary cover to deform independently from the basement, partly by folding. However, this explanation has serious drawbacks and is not easily applicable to other areas, notably the western Mediterranean.

Strike-slip fault geometry in Turkey and its influence on earthquake activity

Abstract: The geometry of Turkish strike-slip faults is reviewed, showing that fault geometry plays an important role in controlling the location of large earthquake rupture segments along the fault zones. It is found that large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 degrees. It is suggested that certain geometric patterns are responsible for strain accumulation along portions of the fault zone. It is shown that fault geometry plays a role in the characteristics of earthquake behavior and that aftershocks and swarm activity are often associated with releasing areas.

Formation and evolution of strike‐slip faults, rifts, and basins during the India‐Asia Collision: An experimental approach

Abstract: The processes which have governed the formation and evolution of large Tertiary strike-slip faults during the penetration of India into eastern Asia are investigated by means of plane strain indentation experiments on layered plasticine models. The steady state deformation of plasticine obeys a power creep flow law (e˙=C(T)σn). The stress exponent (n) is between 6 and 9 at 25°C. Uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening, a process observed for strain rates ranging from 3.5×10−5 to 3.6×10−3 s−1. Fault or shear zones form after only 7–10 % bulk strain. Subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine. A series of nine plane strain indentation experiments shows the influence of boundary conditions, as well as that of the internal structure of the plasticine model on the faulting sequence. The ubiquity of strain softening in experimental deformation of a variety of rocks, as well as the widespread occurrence of shear zones in nature suggest that long-term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain. The deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision-induced strikeslip faulting in Asia, particularly to total offsets and rates of movements on the faults. The experiments simulate the evolution of the western ends of the strike-slip faults, which have probably been analogous to trench-fault-fault triple junctions. The experiments also illustrate mechanisms for the formation of extensional basins, such as the South China Sea, North China Basin, and Andaman Sea, near active continental margins. The basins, which appear to absorb terminal offsets along major strike-slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts. The existence of distinct phases of strike-slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous, non-steady state deformation of the continental lithosphere.

Flexural rotation of normal faults

Abstract: A conceptual model is proposed for the generation of low-angle normal faults in Metamorphic Core Complexes. The model is based on three assumptions: (1) the isostatic response to normal fault motion is of regional extent; (2) when a fault segment is significantly rotated from the optimum angle of slip, relative to the crustal stress field, it is replaced by a new planar fault oriented in the optimum direction; and (3) the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust. The stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations. Active low-angle normal faults are difficult to reconcile with rock mechanics theories, earthquake focal mechanism studies, and geochronologic results indicating rapid cooling of core complex rocks. The model does not require active fault slip on low-angle faults. The flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation. The anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust. This can explain the observed short wavelength of topographic response to normal fault loads. The model results in: (1) a nearly flat-lying abandoned normal fault (or “detachment”) below slices of upper plate rocks and sedimentary infill; (2) a strong contrast in metamorphic grade across the abandoned “detachment”; (3) rapid movement of lower plate rocks from midcrustal depths to shallower depths. These results are qualitatively in agreement with geologic observations for core complexes.

The brittle-plastic transition and the depth of seismic faulting

Abstract: A simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow. The two are separated by an abrupt brittle-plastic transition, which is assumed to be indicated by the lower limit of seismicity. Experimental studies, however, as well as the deformation structures of mylonites, indicate that a broad transitional field of semi-brittle behavior lies between these extremes. This is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws. For quartzofeldspathic rocks the semi-brittle field lies between T1, the onset of quartz plasticity at about 300 °C and T2, feldspar plasticity at about 450 °C. A model is presented in which the transition T1 does not correspond to a transition to bulk flow but to a change from unstable, velocity-weakening friction to stable, velocity-strengthening friction. T1 thus marks the depth limit of earthquake nucleation, but large earthquakes can propagate to a greater depth, T3, (T3

Seismological and structural evolution of strike-slip faults

Abstract: The mapped traces of strike-slip faults are commonly characterized by discontinuities that appear as steps in map-view. Here I present observations to show that the number of steps per unit length along the trace of major strike-slip fault zones in California and Turkey is a smoothly decreasing function of cumulative geological offset. When coupled with a growing body of evidence that indicates that steps in fault traces work to impede or arrest the propagation of earthquake ruptures, the apparent smoothing of fault traces with displacement is interpreted to suggest that the spatial distribution of strength properties on a fault plane is a function of cumulative geological offset. A consequence of this structural evolution is that faults may undergo a seismological evolution a well, whereby the size and frequency distribution of earthquakes is also a function of cumulative offset.

The growth of geological structures by repeated earthquakes, 1, conceptual framework

Abstract: In many places, earthquakes with similar characteristics have been shown to recur. If this is common, then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures. Following this paradigm, we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault. In these models an elastic layer containing the fault overlies a viscous half-space with a different density. Fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored. Deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion. In this paper, the parameters that control the growth of dip-slip structures are identified. We find that the flexural rigidity of the crust (or the apparent elastic thickness) provides the main control of the width of a structure. The loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault. The flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character. We find that, in general, real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities.

The structural evolution of the northern Viking Graben and its bearing upon extensional modes of basin formation

Abstract: Recent advances in the understanding of rift basin formation, coupled with the increasing public availability of seismic and well data across the northern Viking Graben (60–61°N), have enabled a detailed analysis of its development. Two rifting episodes can be recognized: ?Late Permian–early Triassic and Bathonian–Ryazanian. Major regional unconformities, thought to be primarily tectonic rather than eustatic in origin, separate and subdivide the rifting episodes. The earlier episode involved extension about a N–S axis; ensuing (Triassic–Mid-Jurassic) thermal subsidence was accommodated on steep faults. During the later episode a new NE–SW fault trend was superimposed on pre-existing patterns. Major block rotation, marking active rifting, ceased at the end of the Ryazanian. During the second post-rift episode there was a progressive migration of active faulting towards the basin margins and, as a result, a widening-with-time of the area undergoing subsidence. Asymmetric subsidence of the central part of the basin was hinged at the western margin of the Horda Platform, and accommodated to the NW on major faults within the Tampen Spur, β factors, for the second rifting episode were calculated both by relating subsidence to extension, and by measuring observed extension. Values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases, but are greater for the latter phase. Subsidence patterns are similar for both rifting and thermal subsidence episodes, so that there is vertical stacking of relatively thick sequences in the axis of the northern Viking Graben. These factors preclude the application of models involving uniform and non-uniform stretching and also preclude oblique extension; depth-dependent stretching is preferred.

Growth and persistence of Hawaiian volcanic rift zones

Abstract: Hawaiian volcanic rift zones are modeled by representing the rifts and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms. The intrusion of dikes along the axis of a rift requires a mechanism to generate the appropriate dike-trapping stress field within the prism. Possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis, faulting, and gravitational sagging of the topography. In extreme models with very steep slopes and high Poisson's ratio, corresponding to the gelatin models of rift zones by Fiske and Jackson (1972), results of finite element calculations indicate that gravity-induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the rift zone as proposed by Fiske and Jackson. However, the mechanism does not work for gently sloping flanks or a more acceptable Poisson's ratio of about 0.25. Additionally, trapping stresses in the gravity-loading and density stratification models will not persist after a few dike injection episodes. Therefore in mature Hawaiian rift zones with possibly thousands of dikes, additional processes must act to control the stresses that permit continued dike intrusion and rift persistence. It is proposed that accommodation to dike emplacement occurs by slip on deep faults, possibly of the type proposed for the 1975 Kalapana, Hawaii, earthquake. As suggested by others for this earthquake, the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments. Such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the rift axis. Other possible faulting mechanisms, such as shallow gravity slides and normal faulting of the flanks, do not appear to favor rift zone persistence. In this model the horizontal stress generated by a standing column of magma at the time of dike emplacement, the stresses in the ridges, and the fault strength are coupled. This results in a feedback between the maximum height that magma can rise along the rift, fault friction, and fault width. Through this feedback the slope of the volcano flank is controlled by the fault friction. When applied to Kilauea Volcano, the model yields an estimate for the coefficient of fault friction as high as 0.39 assuming normal hydrostatic pore fluid pressure. An implication of this model, supported by other studies, is that rift intrusion and lateral spreading could be major contributors to volcano growth.

Low-temperature deformation mechanisms and their interpretation

Abstract: Low-temperature deformation is characterized by heterogeneous strain in which the bulk of the material clearly retains its primary texture. Deformation is by grain-scale crystal plasticity, rotation, fracture, and pressure solution, and by transgranular mechanisms that crosscut numerous grains. The important low-temperature crystal-plastic features are twin lamellae, deformation bands, and undulatory extinction. Subgrain formation by recrystallization or crystal-plastic strain of more than 15% marks the upper limit of the low-temperature regime. Grain rotation may produce foliations in soft sediments or rocks. Microscopic to mesoscopic kinks and crenulations of bedding occur in soft clay and shale. Transgranular features include Luders' bands, cooling and desiccation cracks, joints, extension-fracture cleavage, clastic dikes, mineral-filled veins of several types, recrystallization/replacement veins, vein arrays, boudins, faults, stylolites, slickolites, solution cleavages that range from widely spaced to slaty and pencil cleavage. Pressure fringes form adjacent to relatively rigid grains and have fabrics analogous to those in veins. Faults include conjugate fault pairs (Andersonian faults) multiple simultaneous conjugates (Oertel faults), and Riedel shear-zone configurations. The sense of fault displacement is determined from bends, steps, trails, tails, and feather fractures. Superplasticity, especially if aided by diffusion in grain-boundary water, might be important at low temperatures. Fault textures are diagnostic of the environment of deformation but have yet to be uniquely correlated with the presence or absence of earthquakes. Riedel shears and pseudotachylite may form in earthquake source regions, although pseudotachylite is evidently rare in brittle fault zones. The best indicators of stress magnitudes are the critical' resolved shear stress for deformation twinning and the presence of tensile fractures. Strain magnitudes and stress and strain tensor orientations can be determined with a variety of methods that are based on mechanical twins, platy grain orientation, grain center distribution, and fault geometry and slip directions. Different deformation mechanism associations, expressed by the partitioning of the total strain into different mechanisms, are related to the ductility and environment of deformation. Deformation fronts separating different mechanism associations are defined on the basis of changes in the crystal-plastic component of strain.

Roughness and wear during brittle faulting

Abstract: In many natural fault systems, the thickness of gouge and breccia increases approximately linearly with displacement. In contrast, many experimental faults show non linear thickness/displacement relationships. The linear relationship for natural faults has been explained in the past using engineering models for adhesive or abrasive wear. Non linear relationships for experimental faults have not been explained. A model for wear during brittle faulting which considers the scaling of surface roughness can successfully describe the difference between wear on experimental faults and wear on natural faults. We suggest the linear relationship for natural faults results from the approximately self-similar roughness of the fault surfaces. Experimental faults do not generally follow linear relationships because the roughness of ground surfaces normally used in experimental studies scales differently than the roughness of natural rock surfaces. A simple model which assumes that the volume of wear material created is proportional to the volume of mismatch between the surfaces can explain the differences between wear on experimental faults and wear on natural faults. For ground surfaces of experimental samples, the volume of mismatch is independent of the total slip because at the largest scales these surfaces are flat. In contrast, for natural, self-similar surfaces the volume of mismatch increases with slip, because slip isolates larger and larger asperities from their original positions in the opposite surface. Natural and experimental faults evolve differently because of the difference in scaling of their respective surface roughnesses.

Surface Deformation and Shallow Dike Intrusion Processes at Inyo Craters, Long Valley, California

Abstract: The Inyo craters are the two largest of four phreatic craters that lie within a 25-km-long, 500- to 700-m-wide N-S trend of faults and fissures at the south end of the Inyo volcanic chain in eastern California The alignment of these features with dike-fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650–550 yr BP E-W extension south of south Inyo crater is ten to several tens of meters, suggesting that the dike is at least that thick To understand how the faults and fissures developed, we mapped and studied the fault and fissure pattern; used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium; and conducted physical model experiments of fault and fissure growth Results of the field studies and experiments indicate that deformation develops according to the following sequence: 1) extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike; 2) fissures form along two parallel trends at the surface on opposite sides of the dike plane, leaving a relatively unfractured region in between; 3) dip-slip movement on subsurface fractures, and linkage of these fractures with surface fissures produces inward-facing normal fault scarps that bound a nested graben above the dike Experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium This fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface If the ratio between the depth to the dike top and the distance between the outermost surface fissures at Inyo craters is within the range of ratios measured experimentally, then the depth to the dike top is between 250 m and several times that A slant hole drilled in the summer of 1987 by the Department of Energy, Office of Basic Energy Sciences, intersected three breccia bodies of possibly intrusive origin directly below the center of south Inyo crater between 550 m and 610 m below the crater rim No intact juvenile igneous rock was intersected, suggesting that the hole passed above the (magma-filled) dike top, perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results

Fault plane solutions for the 1984 Morgan Hill, California, Earthquake Sequence: Evidence for the state of stress on the Calaveras Fault

Abstract: Fault plane solutions for 946 aftershocks of the April 24, 1984, Morgan Hill, California, M6.2 earthquake reveal a pattern of complex faulting within a 10-km-wide zone surrounding the Calaveras fault. The fault plane solutions fall into three groups: strike-slip mechanisms located along the Calaveras fault with north-northwest striking dextral slip planes nearly parallel to the fault, strike-slip mechanisms located northeast of the Calaveras fault with north striking dextral slip planes, and reverse mechanisms located southwest of the Calaveras fault with northeast or southwest dipping slip planes. The average azimuth of P axes for aftershocks located on the Calaveras fault is N10°±1°E. In contrast, the average azimuths of P axes for aftershocks northeast and southwest of the Calaveras fault are N49°±7°E and N37°±3°E, respectively. By assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field, we are able to infer from the observed average slip directions an orientation for σ1, the axis of maximum horizontal compression, 63°–80° from the local strike of the Calaveras fault. Such a high-angle orientation of σ1 is incompatible with the classical Andersonian model of strike-slip faulting but is consistent with an alternate model in which a weak Calaveras fault is driven by plate motion slightly convergent with respect to the San Andreas fault system in central California. If this inferred high-angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 Morgan Hill main shock, the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions. The orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the Coulomb failure criterion. Areas with predicted increases in the Coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity. The invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 MPa. In addition to explaining a preponderance of the seismic observations of the aftershock sequence, the presence of a predominantly fault-normal compressive regional stress field accounts for the contemporary development of folds, reverse faults, and topographic relief observed throughout the central California Coast Ranges.

Development of simple strike-slip fault zones, Mount Abbot quadrangle, Sierra Nevada, California

Abstract: Simple strike-slip fault zones mark the third stage of faulting in granitic plutons in the Mount Abbot quadrangle of the Sierra Nevada of California. Deformation began with the opening of nearly vertical subparallel joints. These joints were filled mostly with epidote and chlorite, are up to a few tens of meters long, and typically are less than 1 cm wide. Next, some of these joints slipped left-laterally and became small faults. Small faults accommodated up to ∼2 m of displacement and are characterized by mylonitic fabrics and ductilely deformed quartz. Oblique fractures commonly developed near the ends of small faults and in many cases linked faults end-to-end. Simple fault zones developed as abundant oblique fractures linked small faults side-to-side. These fractures opened and were filled with chlorite, epidote, and quartz. Such fractures are scarce outside the two faults that mark the boundaries of a zone. Simple fault zones typically are 0.5-3 m wide, hundreds of meters long, and laterally displace dikes up to ∼10 m. Displacement is concentrated along the boundary faults, which are characterized by cataclastic textures and brittlely deformed quartz. The fault zones consist of noncoplanar segments a few tens of meters long that join at steps or bends. The segmentation reflects the initial joint pattern and indicates that fault zones grew in length as noncoplanar faults linked end-to-end. Away from bends, the most prominent internal fractures have straight traces and strike 20°-60° counterclockwise from the boundaries, whereas near bends they have gentle S-shaped traces and are nearly perpendicular to the boundaries. We suggest that as some faults linked to form longer structures, a "shear stress shadow" was cast over adjacent smaller faults, causing slip on them essentially to cease. In this manner, displacement progressively became localized on the longer faults and fault zones. If the regional shear strain rate remained constant during this process, then the shear strain rate across the still active faults must have increased. This may have caused cataclastic textures to develop in the boundary faults.

The 1986 Kalamata (South Peloponnesus) Earthquake: Detailed study of a normal fault, evidences for east-west extension in the Hellenic Arc

Abstract: Tectonic and seismological data collected in the field following the September 13, 1986, Kalamata earthquake (south Peloponnesus) are presented and analyzed to discuss the earthquake rupture process and the regional tectonics. The event occurred on the Kalamata normal fault whose trace was mapped with SPOT images and topographic and field observations. This fault is part of an approximately NNW-SSE en echelon system cutting through the Hellenic nappes. The fault striking N15°E on the average, with a dip of about 50°, has a minimum cumulated Quaternary throw of the order of 1 km. The measured coseismic slip is 6–18 cm over a length of 6 km. The main shock focal mechanism obtained from long-period waveform modeling (strike=201° (+10°,−20°), dip=45°±5°, rake=283° (+10°,−25°)) represents almost pure east-west extension and is in good agreement with tectonic observations. The centroid depth is constrained to 5±3 km and the seismic moment to 7.0±2.5×1017 N m. Over 700 aftershocks, located by a 16-station network installed after the earthquake for a period of 2 weeks, define two clusters separated by a “gap” of aftershock activity, from the surface to a depth of about 10 km. The main cluster, to the south, defines a 45° west dipping plane which lies on the downward extension of the fault mapped at the surface. Focal mechanisms of aftershocks on this fault plane are homogeneous and represent E-W extension as the main shock. In contrast, the majority of focal mechanisms in the uppermost part of the foot wall show more or less E-W compression, probably corresponding to postseismic stress release. The northern cluster of aftershocks is very dense and located away from the surface rupture, within a relay zone between the Kalamata and the next en echelon faults to the NW, the Thouria faults. There focal mechanisms represent extension from about N115° to N70° and N20°, corresponding mostly to fault reactivation in an area where nonrigid deformations prevail. The main shock probably initiated in this relay zone 3–4 s before the rupture front reached the main fault plane and released most of the energy there, the rupture presumably propagating southward. The focal mechanism of the Kalamata earthquake and that of the April 27, 1965, earthquake located to the northwest of Crete, as well as the regional active normal fault pattern, imply that E-W extension oblique to the Hellenic arc is presently the dominant tectonic regime. E-W stretching occurs partly on reactivated NW-SE faults parallel to the Hellenic structures but mostly on newly formed N-S normal faults across those structures. The latter faults are responsible for the apparent segmentation of the Hellenic belt from southern Peloponnesus to Crete. The existence of active E-W extension in this region implies a recent change in the tectonic regime and consequently a change in boundary conditions at the subduction zone, probably in response to the incoming margin of Africa.

Microearthquakes beneath Median Valley of Mid-Atlantic Ridge near 23°N: Tomography and tectonics

Abstract: Data from a microearthquake experiment in the median valley of the Mid-Atlantic Ridge near 23°N in 1982 are used to measure earthquake source parameters, to determine the laterally heterogeneous seismic velocity structure across the inner floor, and to develop a kinematic tectonic model for this portion of the median valley. Fifty-three microearthquakes occurred over a 10-day period beneath the median valley inner floor and eastern rift mountains. Twenty of 23 well-located inner floor epicenters define a line of activity, about 17 km long, having a strike of N25°E and located near an along-axis depression some 300–400 m deeper than surrounding regions. Earthquakes with well-resolved hypocenters generally have focal depths of 4–8 km beneath the seafloor of both the inner floor and the rift mountains; the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure. Composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45°. Normal faulting mechanisms, although poorly constrained, are also indicated for the rift mountain microearthquakes. Seismic moments, approximate fault dimensions, and average stress drops for the largest events recorded are 1019–1020 dyn cm, 200–400 m, and 1–70 bars, respectively. A twodimensional tomographic inversion of P wave travel time residuals from microearthquakes and local shots indicates a well-resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor. P wave velocities at 1–5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions. The most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column. By this view, the seismic velocity structure at the ridge axis evolves, probably by the sealing of cracks and pores, within the first few hundred thousand years of crustal accretion. Consideration of the detailed Sea Beam bathymetry in this region of the inner floor, the characteristics of large earthquakes that the region has experienced during the past 25 years, and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years.

Seismically active fold and thrust belt in the San Joaquin Valley, central California

Abstract: Structural analysis of late Cenozoic folds along the western and southern margins of the San Joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics. The folds have formed by the processes of fault-bend folding and fault-propagation folding, which commonly occur in fold and thrust belts. Our structural interpretation attributes the seismically active Coalinga and Kettleman Hills North Dome anticlines to fault-bend folding above a thrust fault(s) that steps up from a detachment within the Franciscan Assemblage to a detachment at the base of the Great Valley Group. This thrust does not reach the surface (blind thrust), and its slip is consumed in backthrusting and formation of subsurface folds under the San Joaquin Valley. Movement on the postulated thrust(s) would explain the cause of the May 2, 1983, Coalinga earthquake and the August 15, 1985, Avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes. Well-documented examples of fault-bend and fault-propagation folding also occur at Wheeler Ridge, in the San Emigdio Mountains, and at Kettleman Hills South Dome. Deformation associated with fold and thrust belts can be extremely complicated. Considerable fault movement can occur without accompanying surface rupture or shallow-level faulting. Conversely, surface rupture can occur that has no direct relationship to fault slip at depth. An example would be flexural-slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth. Consequently, traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts. Our interpretation of the structural style that developed in central California during the late Cenozoic requires that the strain associated with the transpressive motion between the Pacific and North American plates be resolved into normal and tangential components: thrust faulting and folding normal to the plate boundary and strike-slip faulting parallel to the plate boundary (San Andreas fault). The thrust faults root in a deecollement at the brittle- ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction.

Focal mechanisms and the state of stress on the San Andreas Fault in southern California

Abstract: Focal mechanisms have been determined from P wave first motion polarities for 138 small to moderate (2.6 ≤ M ≤ 4.3) earthquakes that occurred within 10 km of the surface trace of the San Andreas fault in southern California between 1978 and 1985. On the basis of these mechanisms the southern San Andreas fault has been divided into five segments with different stress regimes. Earthquakes in the Fort Tejon segment show oblique reverse sup on east-west and northwest striking faults. The Mojave segment has earthquakes with oblique reverse and right-lateral strikesup motion on northwest strikes. The San Bernardino segment has normal faulting earthquakes on north-south striking planes, while the Banning segment has reverse, strike-sup, and normal faulting events all occurring in the same area. The earthquakes in the Indio segment show strike-slip and oblique normal faulting on northwest to north-south striking planes. These focal mechanism data have been inverted to determine how the stresses acting on the San Andreas fault in southern California vary with position along strike of the fault. One of the principal stresses is vertical in all of the regions. The vertical stress is the minimum principal stress in Fort Tejon and Mojave, the intermediate principal stress in Banning and Indio, and the maximum principal stress in San Bernardino. The orientations of the horizontal principal stresses also vary between the regions. The trend of the maximum horizontal stress rotates over 35°, from N15°W at Fort Tejon to N20° at Indio. Except for the San Bernardino segment, the trend of the maximum horizontal stress is at a constant angle of about 65° to the local strike of the San Andreas fault, implying a weak fault. The largest change in the present stress state occurs at the end of the rupture zone of the 1857 Fort Tejon earthquake. It appears that the 1857 rupture ended when it propagated into an area of low stress amplitude, possibly caused by the 15° angle between the strikes of the San Jacinto and San Andreas faults. The strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation.

Triassic – Jurassic rifting and opening of the Atlantic: An overview

Abstract: Events leading to the breakup of the Pangean plate and evolution of the Atlantic passive margins are recorded in the rock record of more than 40 offshore and onshore Late Triassic – Early Jurassic synrift basins that formed on the Variscan – Alleghanian orogen. The record shows that rifting took place along low-angle detachment faults, giving rise to half-grabens along a conjugate set of lower and upper plate margins that are noteably asymmetric. The American plate was marked by a broad belt of marginal plateaus with many northeast-trending detrital basins that were linked to eachother by transfer faults and displaced by cross faults. The Moroccan plate, on the other hand, was marked by few broadly subsiding evaporite basins. Typically each half-graben on the American plate was bordered by a hinged margin and one major basin-bounding fault, which delineated the surface trace of synthetic or antithetic listric faults on a seaward-dipping detachment zone. The American plate (during the Late Triassic) was dominated by high relief with high-altitude fluvial-lacustrine basins along the western part of the orogen, and by low-relief sea-level evaporite basins proximal to the future spreading axis. During detachment faulting, in the Late Triassic – Early Jurassic, the lower plate must have been uplifted isostatically into a broad central arch that migrated seaward, as the load of the overlying upper plate continued to be reduced by erosion and listric faulting. This had the consequence of elevating Late Triassic marine strata that lay near the proto-Atlantic axis. During the Lias, these marine basins were eroded and their strata reworked and transported landward toward the onshore basins of Morocco and North America. The topographic reversal is thought to reflect the easterly migration of upwelling asthenosphere, in response to tectonic thinning along the newly forming margin. It was a time of major crustal thinning with development of the postrift unconformity (COST G-2 cores), and adiabatic decompression on the upwelling asthenosphere. Whereas the earliest melts yielded off-axis alkaline-rich volcanics (as in Morocco), subsequent melts, which were derived from later partial melt derivatives, were tholeiitic (as in the Palisades). As the upwelling asthenosphere migrated eastward in response to tectonic thinning, the ‘abandoned’ rift-stage crust cooled and subsided, thereby ushering in the drifting phase of the margin. The Moroccan plate, by contrast, was a broad region of low relief throughout most of the Triassic and Liassic. It was distinguished by few detrital basins, and almost all of these occurred along the South Atlas fracture zone, as Triassic strike-slip basins in the High Atlas. Except for the offshore Essaouira basin, which is a seaward extension of the High Atlas Argana basin, the Moroccan margin (unlike the American) consists of few documented Triassic – Liassic rift basins. Triassic rifting of the Middle Atlas (e.g. at Bab-Bou-Idir and Berkane) broke the orogen into the Oranian and Moroccan mesetas, and is manifested by a thick carbonate sequence. The majority of the intraplate basins of North African occur on the mesetas, and are nonrift; typically they contain nonclastic, marine and fresh water evaporites of Liassic and younger strata that formed in broad, shallow, drift-type basins on a generally subsiding terrane of low relief, near the very end of synrift time.

Listric extensional fault systems - results of analogue model experiments

Abstract: Analogue models are a powerful tool for investigating progressive deformation in extensional fault systems. This paper presents exciting new insights into the progressive evolution of hanging wall structures in listric extensional terranes. Analogue models, scaled to simulate deformation in a sedimentary sequence, were constructed for simple listric and ramp/flat listric extensional detachments. For each detachment geometry homogeneous sand, sand/mica and sand/clay models were used to simulate respectively, deformation of isotropic sediments, of anisotropic sediments and of sedimentary sequences with competency contrasts. Roll-over anticlines with geometrically necessary crestal collapse graben structures are characteristic of the steepening-upwards segments of listric extensional fault systems in all of our models. With progressive deformation, crestal collapse grabens show hanging wall nucleation of new faults. Variations in graben size, amount of fault rotation and throw, are dependent on detachment curvature and amount of extension. Individual faults and associated fault blocks may significantly change shape during extension. Complex and apparently conjugate fault arrays are the result of superposition of successive crestal collapse grabens. Ramp/flat listric extensional fault systems are characterized by a roll-over anticline and a crestal collapse graben system associated with each steepening-upwards segment of the detachment and a ramp zone consisting of a hanging wall syncline and a complex deformation zone with local reverse faults. The roll-over anticlines and crestal collapse graben are similar in geometry to those formed in simple listric extensional systems. The models demonstrate that the geometry of the detachments exerts a fundamental control on the evolution of hanging wall structures. Analysis of particle displacement paths for these experiments provides new insights into the mechanical development of roll-over anticlines. Two general models for deformation abo4 simple listric and ramp/flat listric extensional detachments have been erected.

Centroid Depths of Mid-Ocean Ridge Earthquakes: Dependence on Spreading Rate

Abstract: From inversions of teleseismic P and SH waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962–1983 on slowly spreading mid-ocean ridges. All events are characterized by predominantly normal faulting on planes that dip at approximately 45° and strike parallel to the local trend of the ridge axis. Centroid depths range from 1 to 6 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to a depth of 2–10 km for these earthquakes. The water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley. The maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate, and the maximum seismic moment may also decrease with increasing spreading rate. These results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region. The concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley.

The critical slip distance for seismic faulting

Abstract: Experimentally based friction laws1,2 have been found to predict virtually the entire range of observed behaviour of natural faults3,4. These laws contain a critical slip distance, L, which plays a key role in determining the degree of fault instability, the size of the zone of earthquake nucleation, the frictional breakdown width, and the proportion of pre- and post-seismic slip to co-seismic slip. In laboratory measurements L is found to be about 10−5m, but modelling results show that it must be about 10−2m if natural earthquake behaviour is to be simulated. The discovery that fault surfaces are fractal over the scale range 10−5–105 (refs 5,6), even for faults with large net slip, has confused the problem of scaling this parameter from laboratory experiments to natural faults, because fractal surfaces have no characteristic length. Here I show that geometrically unmated fractal surfaces, when in contact under a normal load, develop a characteristic length in their contact because long-wavelength apertures close under load whereas short-wavelength apertures may remain open. This critical distance may be identified with L, and calculations based on fault topography data show that at seismogenic depths it will be in the range anticipated from the earthquake modelling studies.