Fault Shape Research Articles

Multibend fault-bend folding

This paper shows how slightly complex but angular fault shapes with only a few bends lead to broadly curved and/or highly complex folds. A common perception exists that simple, angular fold geometries are a necessary consequence of the normal assumptions of fault-bend fold theory — namely angular fault-bends, straight faults and flexural slip. To the contrary, it is shown here that exact application of these low-level assumptions produces great complexity from simple fault shapes. This complexity is caused by the combination of (1) generation of new axial surfaces by displacement of hangingwall cut-offs past successive fault bends and (2) fragmentation of axial surfaces by mutual interference. The result is entwined arrays of fold axial surfaces that produce quasi-curved and/or complex folds from discrete angular fault bends. Fold complexity grows in a highly non-linear way. In the most extreme case the number of fold axial-surface segments grows as the fourth power of the number of fault bends. This paper presents examples of the immense variety of complex fold geometries that are thus generated.

Three-dimensional geometry, strain rates and basement deformation mechanisms of thrust-bend folding

Models for thrust-bend folding of an isotropic medium are used to predict initial basement thrust sheet geometries and sub-surface thrust fault shapes from final basement thrust sheet structure. Predicted strains and strain rates from these models are compared with data on deformation fabrics in an example of a basement thrustbend fold in order to characterise the deformation response to thrust-bend folding. The Glencoul thrust sheet in the Moine Thrust Zone of north-west Scotland is restored to an initial thrust sheet geometry. Spatial and orientation distribution data of syn-emplacement fractures and cataclastic fault zones from within the Glencoul thrust sheet are then compared with the strain and strain rate histories predicted by thrust-bend folding models. A different set of cataclastic fault seams is demonstrated to have been generated at each frontal thrust bend. Cataclastic failure is restricted to portions of the thrust sheet that have moved over frontal bends with smaller radii of curvature. From model thrust-bend geometries and an assumed slip rate of 1 x 10 −10 ms −1, estimated minimum (critical) strain rates required for fracture failure of the Lewisian basement are 10 −11 to 10 −14 s −1 for shear strain rates and 10 −12 to 10 −15 s −1 for extensional strain rates.

Stress distribution and failure in anisotropic rock near a bend on a weak fault

Large transform faults, thin‐skinned thrust faults, and listric normal faults often contain restraining and releasing bends that can alter the state of stress in the adjacent crust during faulting episodes. Crustal rock commonly has anisotropic mechanical properties due to the presence of sedimentary layering, subsidiary fractures and faults, schistosity, or other foliation. We present an analytical solution for the stress distribution in anisotropic rock produced by sliding on a wavy, frictionless surface. The frictionless surface represents the limiting case of a weak fault. The fault shape treated is either a sinusoid, or a periodic array of isolated restraining and releasing bends. The rheological behavior of the rock is that of an incompressible linear viscous fluid with an orthotropic anisotropy characterized by a greater viscosity for shortening and extension than for shear in the principal directions of anisotropy. Our results illustrate that stress and flow associated with a bend in a fault will extend to much greater distances from the fault when the medium is anisotropic. The magnitude of the stress perturbation increases with degree of anisotropy and decreases with radius of curvature of the fault surface. Principal stress directions tend to align parallel to the principal directions of anisotropy except in the immediate region of the fault bend. Mean stress and maximum shear stress magnitudes vary along the fault in a cellular manner, with multiple maxima near a bend. Stress concentrations emanate from the bend and are elongate in directions parallel to the principal directions of anisotropy. For isotropic rock, locations and orientations of shear failure near restraining bends are distinctly different from those near releasing bends. In contrast, with application of an anisotropic failure criterion, as the magnitude of anisotropy increases, the patterns of shear failure at a restraining and a releasing bend become increasingly similar. The model may help explain field observations from dip‐slip and strike‐slip regimes that indicate a complex stress history and local stress reorientation adjacent to a bend in a fault.

Low‐angle crustal ramp and basin geometry in the Gulf of Lion passive margin: Oligocene‐Aquitanian Vistrenque graben, SE France

With more than 4000 m of Oligocene‐Aquitanian sediments, the Vistrenque graben (SE France) is the deepest synrift depocenter of the Gulf of Lion passive margin, NW Mediterranean. Detailed analysis of industrial seismic reflection profiles and borehole data show that the steep Nîmes fault, which bounds the graben to the NW, forms at depth a low‐angle (25°) crustal ramp. Along‐strike changes of hangingwall geometry allow us to infer along‐strike changes of fault shape: A rollover structure and divergent Oligocene‐Aquitanian basin fill are associated with a listric geometry of the fault in the southern part of the graben, while a pseudo‐rollover and compensation graben result from a two‐segments planar geometry of the fault in the northern part. Preexisting structures inherited from Mesozoic extension and Late Cretaceous‐Eocene Pyrenean thickening controlled the location of the Nîmes fault and the transfer zones which divide the graben into different compartments. Since both hangingwall and fault profile are well constrained, restoration techniques can be used to estimate the prerift topography. The Vistrenque graben was formed close to sealevel, but at the front of a > 1 km‐high elevated area resulting from the Pyrenean orogeny. In the studied transect, the Nîmes fault formed the landward (NW) boundary of the basement faulted domain of the margin. Extensional deformation was restricted to this domain during most of the rifting interval. Small amounts of extension were transmitted landward to Mesozoic cover décollement rooted in the Nîmes fault, only during short episodes, probably resulting from gravitational instability during margin collapse. The Nîmes low‐angle crustal ramp, as well as the other crustal ramps of the margin of similar orientation, are probably newly formed extensional structures rather than reactivated Pyrenean thrusts. Their activation at a low‐angle may have been allowed by crustal weakening resulting from the previous Pyrenean thickening. Upper crustal extension corresponding to the graben formation was transmitted basinward through an intracrustal detachment, or/and distributed in the lower crust across the margin. In contrast to the more stretched areas of the margin which do not display thick synrift series due to their initial high surface elevation, the Vistrenque basin fill records the whole rifting episode because of its location at the front of the Pyrenean orogen.

Three-dimensional variations in extensional fault shape and basin form: The Cache Valley basin, eastern Basin and Range province, United States

Seismic reflection profiles, drill-hole data, and geologic maps delimit the form of normal faults and Tertiary sedimentary rocks in the southern half of the Cache Valley basin in northern Utah. Dips of faults and sedimentary rocks were estimated from time-migrated reflection profiles by using the stacking velocities for the data. At the southern end of the basin, the East Cache fault zone is listric; it shows 50°W dips near the surface and ≈20°W dips at depth. The fault zone has been the site of ≈5.6 km of net dip slip. Tertiary rocks dip 18°E–25°E and exhibit a rollover geometry above the fault zone. No faults are interpreted on the western side of the basin. In the central part of the basin in Utah, the East Cache fault is a single fault that dips at least 45°W near the surface and is the site of 4.5–6.4 km of net dip slip. Here, the basin is bounded to the west by the West Cache fault, which is the site of at least 1 km of net slip that increases northward to 2 km of net slip. Slip on the East Cache fault resulted in planar, east-dipping, older Tertiary rocks near the bottom of the basin. Younger Tertiary strata, with southwest, west, and northwest dips, reflect complex tilting due to slip on the West and East Cache faults. Anticlines in the Tertiary basin-fill deposits are present in the central part of the basin and may reflect changes in normal-fault geometry at depth. Northward, dip slip on the East Cache fault zone decreases to 2.5 km. The basin is broad, shallow, and filled with nearly flat lying Tertiary rocks. This area, near the north-south midpoint of the basin, is bounded by the West and East Cache faults, but slip on the West Cache fault appears to diminish northward. A north-trending reflection profile tied to both drill-hole data and the east-trending seismic profiles indicates that the basin is deeper in the southern end. The along-strike changes in fault geometry, the amount of fault-slip, the subsurface form of the basin-filling sedimentary rocks, and the form of the basin indicate a complex history of faulting and deposition during its formation. This study and other recent ones from the Basin and Range province indicate that such complexities may be typical of many Tertiary basins in the region.

Three-dimensional analyses of slip distributions on normal fault arrays with consequences for fault scaling

Many fault arrays consist of echelon segments. Field data on ancient and active faults indicate that such segmented geometries have a pronounced effect on the distribution of fault slip. Outcrop measurements of slip on arrays of fault segments show that: (i) the point of maximum fault slip generally is not located at the centre of a fault segment; (ii) displacement gradients steepen towards the adjacent fault for underlapping faults; and (iii) displacement gradients become more gentle near the tips of overlapping faults. Numerical analyses suggest that mechanical interaction between neighbouring faults may cause such asymmetrical slip distributions. This interaction occurs through local perturbation of the stress field, and does not require the faults to be connected. For normal faults, the degree of fault interaction, and hence the degree of asymmetry in the slip distribution, increases with increasing fault height and fault overlap and with decreasing fault spacing. The slip magnitude along a discontinuous fault array can be nearly equal to that of a single larger continuous fault provided the segments overlap with small spacing. Fault interaction increases the ratio between fault slip and fault length, especially for closely spaced, overlapping faults. Slip-to-length ratios also depend on the three-dimensional fault shape. For normal faults, the slip-to-length ratio increases with increasing fault height. The effects of fault interaction and three-dimensional fault shape together can lead to more than one order of magnitude variation in slip-to-length ratio for the simple case of a single slip event in a homogeneous isotropic rock. One should expect greater variation for the more complex conditions found in nature. Two-dimensional fault scaling models can not represent this behaviour.

Numerical modelling of the effects of fault slip on fluid flow around extensional faults

Fluid flow and deformation in regions of fractured rock around extensional faults have been modelled using distinct element methods (UDEC code). The basic methodology is described in terms of a simple model of a planar normal fault zone, at the Earth's surface. The model is then modified to simulate deformation at greater depths and to investigate irregularities in fault shape (including dilational and anti-dilational fault jogs). The results obtained show that the deformation of a faulted region resulted in significant variation in fracture dilation (porosity), stress distribution, fluid pressure and fluid flow. The geometry of models and the applied boundary conditions had important effects on deformation and fluid flow. At shallow depth, dilation and fluid flow occurred both in the fault zone and the hangingwall, with little change in the footwall. At greater depth, the higher compressive stresses tended to close all fractures, except within the fault zone where the shear displacements caused local dilation. The presence of anti-dilational bends reduced the dilation and fluid flow in the fault zone, but promoted greater deformation in parts of the hangingwall. The dynamic response of fracture aperture, pore pressure and shear displacement to fault slip was also studied. The modelled results are in reasonable correspondence with observed natural examples and have practical significance for evaluating fluid flow and deformation in regions which exhibit normal faulting.

Application of an Inverse Method for Calculating Three-Dimensional Fault Geometries and Slip Vectors, Nun River Field, Nigeria

A general, automatic method for determining the three-dimensional geometry of a normal fault of any shape and size is applied to a three-dimensional seismic reflection data set from the Nun River field, Nigeria. In addition to calculating fault geometry, the method also automatically retrieves the extension direction (i.e., the horizontal component of the slip vector) without requiring any previous information about either the fault shape or the extension direction. Solutions are found by minimizing the misfit between sets of faults that are calculated from the observed geometries of two or more hanging-wall beds. In the example discussed here, the predicted fault surface is in excellent agreement with the shape of the seismically imaged fault. Although the calculated ext nsion direction is oblique to the average strike of the fault, the value of this parameter is not well resolved. Our approach differs markedly from standard section-balancing models in two important ways. First, we do not assume that the extension direction is known, and second, the use of inverse theory ensures that formal confidence bounds can be determined for calculated fault geometries. This ability has important implications for a range of geological problems encountered at both exploration and production scales. In particular, once the three-dimensional displacement field has been constrained, the difficult but important problem of three-dimensional palinspastic restoration of hanging-wall structures becomes tractable.

Geometric Analysis of Faulted Rollovers: Contrasting Styles of Extensional Growth Folding in the Gulf of Mexico and Offshore Trinidad: ABSTRACT

Rapid sedimentation rates combined with the presences of seaward-dipping detachment levels in both the U.S. Gulf of Mexico (GOM) and the Columbus Basin (Offshore Trinidad) have facilitated the development of large extensional growth faults and their associated hanging-wall rollover folds. While detachment in the GOM is often associated with salt and/or overpresurred shale, shale is believed to be responsible for detachment and translation of large structural blocks within the Columbus Basin. Although, gravity-driven extension seems to be the primary mechanism driving structural development in both regions, the detailed structural geometries and deformational mechanisms present seem to be quite different. In the Columbus Basin, extensional rollovers are generally dominated by the presence of pervasive synthetic faults. Restoration of these structures suggests that these synthetic faults are accomodating the bulk hanging-wall deformation as the hanging-wall slides basinwards above an underlying listric fault surface. This synthetic simple shear deformation is not typically observed in Gulf of Mexico rollovers. In contrast, although synthetic faults are present, often observed in Gulf of Mexico rollovers. In contrast, although synthetic faults are present, often spatially associated with the master growth fault, antithetic or mixed antithetic/synthetic faulting dominates the crest of the structures. An antithetic simple shear deformationmore » is borne out by simple geometric modeling and restoration. The synthetic faulting seem in Trinidadian rollovers to be influenced by the presence of important vertical pressure seals (major unconformities) that may act as internal detachment levels within the larger folds. Synthetic faults are often observed to sole or die downwards into these intervals. Generally, lateral closure of rollover anticlines in the GOM is controlled by the underlying scoop-like shape of the major listric growth faults.« less

An inverse method for determining parameters for folded structures

General analytic solutions are presented here to an equation relating two angles describing fault shape, and two angles describing fold shape. The analytic solutions provide any of the four angles when the other three are given.

Normal Faults and Their Hanging-Wall Deformation: An Experimental Study

We have used clay models to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault. The experimental results show that fault shape influences the style of secondary faulting and folding. Mostly antithetic normal faults form above concave-upward fault bends, whereas mostly synthetic normal faults form above low-angle fault segments and convex-upward fault bends. Beds dip toward the master normal fault above concave-upward fault bends and away from the master normal fault above low-angle fault segments and convex-upward fault bends. Generally, secondary faulting and folding are youngest at fault bends and become progressively older past fault bends. Hanging-wall deformation patterns differ significantly when a basal plastic sheet imposes a constant-magnitude displacement distribution on the master normal fault. In models without a plastic sheet, numerous secondary normal faults form in the hanging wall of the master normal fault. Most secondary normal faults propagate upward and, consequently, have greater displacement at depth. In models with a plastic sheet, few visible secondary normal faults develop. Most of these faults propagate downward and, consequently, have less displacement at depth. Hanging-wall folding is wider and bedding dips are gentler in models without a plastic sheet than in identical models with a plastic sheet. The observed particle paths, displacement distributions, bedding dips, and orientations of the principal-strain axes in our physical models with and without a basal plastic sheet are compatible with the assumption that homogeneous, inclined simple shear accommodates the hanging-wall deformation. Not all of our modeling observations, however, are consistent with this assumption. Specifically, the observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous, inclined simple shear as the hanging-wall deformation mechanism.

General Description of the Complex Faulting Process and Some Empirical Relations in Seismology.

A theory of the complex faulting process of large earthquakes has been developed. The faulting process is characterized by a deterministic source of coherent rupture on a fault plane as well as by a stochastic source of incoherent ruptures in fault heterogeneous areas. The theory provides an earthquake source spectrum of the complex faulting process in a general manner: Theoretical representations of strong-motion acceleration, total power of acceleration, seismic energy, seismic-wave energy, surface-wave magnitude and body-wave magnitude are derived in terms of deterministic and stochastic source parameters. A low-frequency approximation of the source spectrum provides the description of the deterministic source by parameters of seismic moment, fault length, fault width, fault dislocation, stress drop, dislocation velocity, and rupture propagation velocity. Meanwhile, a high-frequency approximation provides the description of the stochastic source by patch corner frequency, cutoff frequency, variance of stress drop, and variance of dislocation velocity. The present theory of the complex faulting process is constrained empirically against observations. It is shown that empirical relations between seismic moment, fault dimension, seismic energy, and earthquake magnitude are the manifestation of similarity rules among source parameters of the complex faulting process: The geometrical similarity is for the fault shape, the kinematical similarity is for the dislocation and rupture velocities, the dynamical similarity is for the average stress drop, and a new similarity rule, stochastic similarity is derived for the stochastic source parameters. The complex faulting process thus far constrained fully explains the general properties of large earthquakes along subduction zones.

Pliocene-Pleistocene Structural Characteristics of Central Offshore Louisiana with Emphasis on Growth-Fault Interplay with Salt Tectonics, Gulf of Mexico: ABSTRACT

ABSTRACT Interpretation of seismic and well data in central offshore Louisiana reveals that normal faults, salt structures, and intrasalt basins are pervasive and that growth faulting, salt movement, and sedimentation are interrelated processes. Normal faults are either down-to-the-basin growth faults, which dominate the area, or counter-regional growth faults. Fault shape varies along strike in plan view with arcuate down-to-the-basin growth faults characteristic of the western part of the area, and long linear down-to-the-basin growth faults characteristic of the central region. A combination of down-to-the-basin growth faults and counter-regional growth faults characterizes the eastern region. These fault groups usually become progressively younger basinward and coincide with paleoshelf breaks. Listric faults sole onto salt structures or merge with regional detachment surfaces that are believed to be related to once extensive but now largely evacuated salt sheets. A conceptual reconstruction of cross sections shows the interplay of growth faulting with salt movement and sedimentation and suggests that salt withdrawal was due to sediment loading, which in turn caused the growth faulting. That is, progradational loading initiates allochthonous salt sheet withdrawal and progressive seaward development of growth faults. The formation of growth faults depresses the sedimentation surface and provides the accommodation space for further sediment loading. Further sediment loading enhances salt withdrawal and growth faulting.

Finite-element analyses of thrust tectonics: Computer simulation of detachment phase and development of thrust faults

The mechanics of thrust fault formation in rectangular and wedge-shaped, homogeneous thrust blocks are examined using finite-element techniques incorporating elasto-plastic material behaviour. Compression is simulated by applying an incremental end displacement parallel to the top of the horizontal or inclined basement. Shear zones (or faults) can form where the material experiences plastic strain softening. These models simulate that thrusts initiate in sequences that are “pushed from behind”, by some tectonic force, over a decollement horizon. In all models, the formation of thrusts is preceded by the formation of a basal slip plane near the base of the block or in a weak substratum. The front of this slip plane does not control the location of thrusts formed subsequently. In the rectangular block, a thrust forms at the pushed end. In the tapered wedge, drained as well as undrained, a fault system composed of a thrust and a backthrust forms. It is argued that the geometry of the block is the main controlling feature for the formation of thrusts within the tapered wedge. The shape of reverse faults depends on the magnitude of shear stresses acting along the basal slip plane. The higher the shear stress, the more pronounced the curvature. In the drained tapered wedge, the intersection of the backthrust with the basal slip plane appears to control the location of the thrust. Under undrained conditions, the backthrust forms subsequent to the thrust and acts only as an accommodation feature.

Stress inversion methods: are they based on faulty assumptions?

Stress inversion methods employed by structural geologists for estimating a regional stress tensor from populations of faults containing slickenlines rely on the basic assumption that slip on each fault plane occurs in the direction of maximum resolved regional shear stress. This premise ignores directional differences in fault compliance caused by fault shape, the Earth's surface or frictional anisotropy of the fault itself. It is also assumed that the regional stress field is homogeneous in space and time. Thus, perturbations in the local stress field caused by such things as material heterogeneities near the fault and mechanical interaction with nearby faults are not considered. Regional stresses may exercise dominant control on the slip direction; however local factors may perturb this field. We show how differences in fault compliance and local stress perturbations can result in a measureable difference between the direction of resolved shear stress and the direction of fault slip. Numerical modeling of common fault geometries in an elastic half space provides a means for evaluating the magnitude of this difference. We illustrate a few examples of geological circumstances under which the inversion techniques should be reliable, and a few where errors related to violations of the basic assumptions exceed those inherent to the data gathering and inverse techniques.

Structural variety and tectonic evolution of strike‐slip basins related to the Philippine Fault System, northern Luzon, Philippines

The northern part of the Philippine fault zone, in Luzon, corresponds to a complex braided system of left‐lateral strike‐slip faults. The NW oriented main active branch, which emerges from the Philippine Sea, splits into an array of north striking splays responsible for the tectonic evolution of the Central Cordillera. This complex fault pattern has favored local stress field variations. Strike‐slip basins have evolved in this framework along or in the vicinity of the main splays of the Philippine fault. Apart from the classical pull‐apart tectonics on a releasing fault termination, overlap, or bend, we describe other mechanisms such as the strike‐slip tilting or the warping of a strip limited by two strike‐slip faults. The strike‐slip basins are good recorders of the evolution of the Philippine fault system. Those located along the north striking cordilleran faults individualized in late early Pliocene to Pleistocene time when the present‐day Philippine fault initiated, but their main control is the fault shape acquired in upper middle Miocene time. The recent tectonic evolution of the fault system is best recorded in central Luzon, where the active basins trace an asymmetrical V shape, with the longest branch trending NE parallel to the East Luzon Trough, and the shortest one trending NW related to the Philippine fault. Both the fault pattern and the basin distribution demonstrate the influence of the Benham Rise in the tectonic evolution of Luzon. The structural setting is interpreted as the result of an early Miocene collision event between the Benham Rise and the eastern margin of Luzon, and subsequent inception of the NW striking strand of the Philippine fault. The present locations of the basins result from the interaction between the structural heritage and the present‐day regional plate motion.

Experiments on Faulting in Zones of Distributed Wrenching

Analog model experiments were performed to investigate faulting in zones of distributed wrenching at basin and crustal scale. Three-dimensional fault shape and fault evolution were analyzed by x-ray-computed tomography. Stratified models were constructed with brittle analog materials (sand and glass powder) overlying a thin layer of viscous polydimethyl siloxane. Distributed wrenching is dominantly accommodated by synthetic strike-slip faults (R-shears) at low strain stages. They strike at about 20[degrees] to the shear direction. Antithetic strike-slip faults (R[prime]-shears) are rare and strike at 70-80[degrees]. Synthetic and antithetic Riede are subvertical and extend down to the base of the brittle layers. En echelon-arranged R-shears create push-up zones in the area of overlap. With increasing deformation, lower angle synthetic (R[sub L]) and lower angle antithetic faults (R[prime][sub L]) form. these secondary structures are confined mostly in between previously formed major R-shears. Newly formed R[prime][sub L]-shears strike at progressively lower angles to the shear direction with increasing wrenching and document local stress field modifications. In experiments with unconfined transversal borders, R[prime] and R[prime][sub L]-shears undergo considerable rotations about vertical axes. Space problems created by rotation of antithetic shears in between major R-shears are solved by upbulging of the free surface near their intersection. This resultsmore » in sigmoidal-shaped antithetic faults (plan view) with a dip-slip component and a dip direction that changes along strike. Possible structural traps are represented by push-up zones and areas of positive vertical relief near the intersection of antithetic and synthetic faults. There is good agreement between our models and natural examples of major wrench zones where block rotations have been documented by paleomagnetic,geodetic and seismic methods.« less

Origin of Rollover (1)

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.

Structural and Stratigraphic Evolution of the East Java Sea, Indonesia

Abstract The East Java Sea displays excellent examples of Miocene to Present-Day structural inversion of a Palaeogene extensional/transtensional basin system. The structural geometry of these inversions has been analysed by a series of regional cross-sections which demonstrate significant lateral variation in structural style. This variation is interpreted to be a function of the fault shape and linkage which evolved during the extensional phase of basin development. Prediction of fault geometry has been aided by analogue-modelling of inverted extensional basins. Driving mechanisms of structural inversion have been assessed by modelling the motion of the Eurasian, Indian Ocean, Australian and Pacific plates during the Neogene. The evolution of inversion within the East Java Sea is interpreted to be a function of the propagating collision of the Australian Plate with the Sunda Arc, and the resultant variation in relative convergence rate and direction of the subducting Indian Ocean Plate and the S.E. Eurasian Plate. Stratigraphic evolution is strongly influenced by relative sea level fluctuations, principally eustatic, within overall control of tectonic processes, particularly inversion. The sediment fill is mainly fine-grained with alternating carbonate-dominated and clastics-dominated cycles. The use of wireline logs, seismic data and biostratigraphic data allows a detailed sequence stratigraphy to be developed, with both regionally and locally recognisable highstand, lowstand and transgressive systems. A well-imaged progradational carbonate platform margin exhibits late-stage growth and subsequent inundation which are used to calibrate locally derived relative sea-level curves.

Faulting geometry and slip from co-seismic elevation changes: The 18 October 1989, Loma Prieta, California, earthquake

Abstract Leveling surveys conducted before and after the Loma Prieta earthquake provide observations of the co-seismic elevation changes. These data are used to determine the faulting geometry and distribution of slip, considering planar, listric, and negatively listric fault shapes. Both the planar and nonplanar models produce elevation changes consistent with the observations. Most of the observed elevation changes can be modeled with a rupture surface that extends from 4- to 15-km depth. If the rupture surface is planar, the observations require 2.4 m of right-lateral strike slip and 1.7 m of reverse slip on a 34-km-long plane that dips 60°SW. The best-fitting model faults lie above and to the southwest of the aftershock zone. A significantly better fit to the observations is obtained when these fault geometries are allowed to have two rake values, with a larger thrust component northwest of the epicenter and a larger strike-slip component southeast of the epicenter. When a low-modulus layer over a half-space is used for consistency with the seismic P -wave velocity structure, the fault deepens, coming within 3 km of the hypocenter, but still locates several kilometers southwest of most aftershocks.