GeoScienceWorld
Volume

Analogue and Numerical Modelling of Crustal-Scale Processes

Edited by S. J. H. Buiter and G. Schreurs

Abstract

The crust of the Earth records the deformational processes of the inner Earth and the influence of the overlying atmosphere. The state of the Earth’s crust at any time is therefore the result of internal and external processes, which occur on different time and spatial scales. In recent years important steps forward in the understanding of such complex processes have been made by integrating theory and observations with experimental and computer models. This volume presents state-of-the-art analogue and numerical models of processes that alter the Earth’s crust. It shows the application of models in a broad range of geological problems with careful documentation of the modelling approach used. This volume contains contributions on analogue and numerical sandbox models, models of orogenic processes, models of sedimentary basins, models of surface processes and deformation, and models of faults and fluid flow.

    1. Page 1
      Abstract

      We report a direct comparison of scaled analogue experiments to test the reproducibility of model results among ten different experimental modelling laboratories. We present results for two experiments: a brittle thrust wedge experiment and a brittle-viscous extension experiment. The experimental set-up, the model construction technique, the viscous material and the base and wall properties were prescribed. However, each laboratory used its own frictional analogue material and experimental apparatus. Comparison of results for the shortening experiment highlights large differences in model evolution that may have resulted from (1) differences in boundary conditions (indenter or basal-pull models), (2) differences in model widths, (3) location of observation (for example, sidewall versus centre of model), (4) material properties, (5) base and sidewall frictional properties, and (6) differences in set-up technique of individual experimenters. Six laboratories carried out the shortening experiment with a mobile wall. The overall evolution of their models is broadly similar, with the development of a thrust wedge characterized by forward thrust propagation and by back thrusting. However, significant variations are observed in spacing between thrusts, their dip angles, number of forward thrusts and back thrusts, and surface slopes. The structural evolution of the brittle-viscous extension experiments is similar to a high degree. Faulting initiates in the brittle layers above the viscous layer in close vicinity to the basal velocity discontinuity. Measurements of fault dip angles and fault spacing vary among laboratories. Comparison of experimental results indicates an encouraging overall agreement in model evolution, but also highlights important variations in the geometry and evolution of the resulting structures that may be induced by differences in modelling materials, model dimensions, experimental set-ups and observation location.

    2. Page 29
      Abstract

      We report results of a study comparing numerical models of sandbox-type experiments. Two experimental designs were examined: (1) A brittle shortening experiment in which a thrust wedge is built in material of alternating frictional strength; and (2) an extension experiment in which a weak, basal viscous layer affects normal fault localization and propagation in overlying brittle materials. Eight different numerical codes, both commereiai and academic, were tested against each other. Our results show that: (1) The overall evolution of all numerical codes is broadly similar. (2) Shortening is accommodated by in-sequence forward propagation of thrusts. The surface slope of the thrust wedge is within the stable field predicted by critical taper theory. (3) Details of thrust spacing, dip angle and number of thrusts vary between different codes for the shortening experiment. (4) Shear zones initiate at the velocity discontinuity in the extension experiment. The asymmetric evolution of the models is similar for all numerical codes. (5) Resolution affects strain localization and the number of shear zones that develop in strain-softening brittle material. (6) The variability between numerical codes is greater for the shortening than the extension experiment.

      Comparison to equivalent analogue experiments shows that the overall dynamic evolution of the numerical and analogue models is similar, in spite of the difficulty of achieving an exact representation of the analogue conditions with a numerical model. We find that the degree of variability between individual numerical results is about the same as between individual analogue models. Differences among and between numerical and analogue results are found in predictions of location, spacing and dip angle of shear zones. Our results show that numerical models using different solution techniques can to first order successfully reproduce structures observed in analogue sandbox experiments. The comparisons serve to highlight robust features in tectonic modelling of thrust wedges and brittle-viscous extension.

    1. Page 65
      Abstract

      The influence of pre-existing thrusts on the development of later normal faults was investigated using scaled laboratory analogue models. Experiments consisted of a phase of shortening followed by extension at variable angles of obliquity (a) to the shortening direction. Results suggest that the angle a has a major influence on the surface fault pattern and on the interaction between shortening-related structures and later extensional structures. Three different modes of interactions were identified depending upon the extension kinematics. (1) For orthogonal extension = 0°), shortening-related fold and thrust structures strongly influence the development of normal faults: graben structures nucleate within anticlines and the normal faults reactívate thrusts at depth (branching at depth mode of interaction). (2) For highly oblique extension > 45°), shortening-related structures exert no influence on normal faults as extension-related steeply-dipping faults (characterized by an oblique component of movement) displace early thrusts (no interaction mode). (3) For intermediate obliquity angles (a = 15°, 30°), an intermediate mode ofinter-action characterizes the experiments, where the no interaction and branching at depth modes coexist in different regions of models. Modelling results can be used to infer regional extension directions as is shown for the Northern Appenines (Italy).

    2. Page 79
      Abstract

      The post-accretionary deformation of wide, hot orogens is characterized by pure-shear or transpressional shortening of relatively weak lithosphere (the orogen) between converging stronger blocks (the vice). We report on a series of analogue vice models and compare the resulting three-dimensional strain fields and surface topographics to equivalent two-dimensional numerical experiments. In the analogue models a rheologically stratified (frictional/viscous) weak orogenic lithosphere overlying a viscous asthenosphere is squeezed between converging strong lithospheric blocks. Ductile lower crust and mantle in the weak lithosphere is free to flow laterally, parallel to the orogen. The Argand number describes the model dynamics and strongly controls both the orogenic relief and the degree of lower crustal orogen parallel stretching in the analogue models. Cross sections of numerical and analogue experiments display consistent geometrics in which upper crustal deformation is characterized by upright folding compared to apparently decoupled horizontal strains in the lower crust. The relative buoyancy and degree of orogen parallel flow in the lower crust of the analogue models has a dramatic influence on three-dimensional strain fields and the kinematics of upper crustal curvilinear shear zones. The analogue and numerical results demonstrate the importance of three-dimensional effects in determining the structure of natural orogens and compare favourably to field and geophysical observations of large hot orogens in the geological record.

    3. Page 105
      Abstract

      We present finite-element models that investigate the relative importance of both trenchward motion of the upper plate and interplate coupling for the development of topography at convergent margins. Commonly, the role of a trenchward moving continental plate for the growth of topography is neglected in both modelling and field studies. Instead, forces exerted by the downgoing plate on the continental plate as well as interplate coupling are thought to be responsible for the deformation of the upper plate. Our model set-up includes an oceanic plate, which is in contact with a continental plate along a frictional plate interface and driven by slab pull. Both lithospheres have an elasto-visco-plastic rheology. The models demónstrate that friction along the plate interface can only lead to a high topography if the upper plate is moving toward the trench. Without such a trenchward advance, no high topography is generated, as the upper plate subsides owing to the drag exerted by the subducting plate. Increasing the coefficient of friction only amplifies the drag and increases the amount of subsidence. Our findings imply that trenchward motion of the continental plate plays a key role for the development of mountain beits at convergent margins; subduction of an oceanic plate even with high interplate coupling cannot explain the formation of Andean-type orogens.

    4. Page 117
      Abstract

      Results of scaled sandbox models, containing three viscous layers located at different geographic and stratigraphic Levels simulating three evaporitic units in the South Pyrenean Triangle Zone, and interpreted field data are presented here to explain structural variation and kinematics in shortened areas containing multiple weak horizons acting as detachments. In the Southern Pyrenean Triangle Zone, the Beuda, Cardona and Barbastro thrust fronts have similar geometric features to those developed in the models, suggesting that they could have formed and evolved in a similar way. These deformation fronts are not always perpendicular to the regional shortening direction. Instead, their direction is governed by the initial pinch-out of the viscous horizons. Model results show that triangle zones form when: (1) deformation is transferred to weak horizons located at higher stratigraphic levels, and (2) the deformation front reaches the pinch-out of the weak horizons. Model results also show that the rheology of the detachment horizons controls the geometry of the deformation front. Weak detachments (Cardona Formation, and pure silicone in the models) promote folding and back-vergent structures, and thus formation of triangle zones at the deformation front, irrespective of the location of the thrust front relative to the pinch-out of the viscous detachment. However, over strong (more viscous) detachments (Barbastro and Beuda formations, and impure silicone in the models), folds that eventually evolve to thrusts are dominant. In such cases, backthrusts form only at the pinch-out of the detachment layer. In cases where no viscous detachment is present, no backthrusts form, and therefore the thrust front does not develop a triangle zone geometry. Instead, a foreland-vergent piggyback sequence of thrusts forms. Model results show that the stratigraphic level of a detachment governs size, geometry and spacing of the imbricates formed above it.

    5. Page 135
      Abstract

      This paper presents the results of an analogue modelling study on the reactivation of Riedel shears generated by basement-induced sinistral strike-slip faulting. It is based on a natural example in the Sierra de Albarracín, Iberian Range (Spain). The area has a polyphase deformation history, defined by the Variscan and Alpine orogenies. Late Variscan deformation was concentrated in a wide NW–SE shear zone with accompanying kilometre-scale Ε-W Riedel shears, which divided the Palaeozoic basement into large fault blocks. Alpine reactivation resulted in differential movements on the Riedel shears, as evidenced by a NW–SE chain of Palaeozoic inliers surrounded by a Mesozoic cover that generally shows minor deformations except near the Ε-W Riedel shears, where strata locally appear in near-vertical to overturned position.

      Sandbox analogue modelling was applied to improve insight into the structural history. It focused on the kinematics of spontaneously developed en echelon Riedel shears, reactivated in a rotated stress field. Sand with a controlled added strength was used to form Riedel shears in a first deformation phase to act as weak zones for a second phase.

      The modelling showed that in the first deformation phase large pop-up structures developed between the Riedel shears in a basement-induced sinistral strike-slip zone. Later reactivation in the Ν060Έ and Ν135Έ shortening directions was taken up respectively by sinistral-reverse and dextral-reverse shear along the pre-existing Riedel shears, but only if the sand on one side of the fault zone was allowed to move freely along the other. Scissor faulting along the Riedel shears with their complex 3D-geometry increased the height of the up-squeezed blocks. For experiments with fixed boundaries and no oil-water emulsion layer between the base plate and sand pack, thrusting at the backstop occurred rather than reactivation of the Riedel shears. This approach provided robust insights on the 4D development of the Sierra de Albarracín area.

    6. Page 153
      Abstract

      In the Archaean, the combination of warmer continental geotherm with a lighter sub-continental lithospheric mantle suggests that gravitational forces played a more significant role in continental lithospheric deformation. To test this hypothesis, we compare the evolution of the deformation and the regional state of stress in ‘Archaean-like’ and ‘Phanerozoic-like’ lithospheres submitted to the same boundary conditions in a triaxial stress-field with imposed convergence in one direction. For plausible physical parameters, thickening of normal to cold Phanerozoic lithospheres produces relatively weak buoyancy forces, either extensional or compressional. In contrast, for Archaean continental lithospheres, or for anomalously warm Phanerozoic lithospheres, lateral gravitationally-driven flow prevents significant thickening. This conclusion is broadly consistent with: (1) the relative homogeneity of the erosional level now exposed at the surface of Archaean cratons, (2) the sub-aerial conditions that prevailed during the emplacement of up to 20 km of greenstone cover, (3) the relatively rare occurrence in the Archaean record of voluminous detrital sediments, (4) the near absence of significant tectonic, metamorphic and magmatic age gradients across Archaean cratons, (5) the relative homogeneity of strain across large areas, and (6) the ubiquitous presence of crustal-scale strike slip faults in many Late Archaean cratons.

    7. Page 169
      Abstract

      Active accretionary prisms at subduction margins generally include a horizontal detachment, décollement, within the sedimentary pile. The décollement, and its extension to undeformed regions (i.e., proto-décollement), corresponds to a layer of high fluid pressure. The deformation of the prisms, including such an anomalous layer, can be modelled and examined using analogue experiments and numerical simulations. Both these methods approximate the material under deformation as an assembly of partides (grains). The décollement layer is found to be best modelled by intercalating a layer with smaller internal frictional coefficient than the surrounding materials corresponding to the sediments. Our analogue experiments with dry sand and microglass beads reproduce structural geometry similar to that of interpreted seismic profiles at the toe of the prisms. Thrust faults originate from the horizontal beads layer and propagate upward with a constant angle of about 30°. Each of the fault bends produces a series of minor back thrusts. A particle image velocimetry (PIV) analysis revealed that the fault activity is characterized by intermittent reactivation and segmentation. The numerical simulations based on the distinct element method (DEM) were performed with similar kinematic settings and material properties as the analogue experiments. The numerical simulation results not only reproduce similar geometries as in the analogue experiments, but also show that the particle assembly experiences temporai variations in the deformation velocity and stress field as deformation propagates. This might be related to stick-slip motion of the frictional fault surfaces, which is a common feature of faulting during accretionary processes at subduction margins.

    1. Page 185
      Abstract

      Structural geometries, faults and their movement histories, together with the petrophysical properties of flow units, are some of the major controls on hydrocarbon migration pathways within sedimentary basins. Currently, structural restoration, fault-seal analysis and hydrocarbon migration are treated as separate approaches to investigating basin geohistory and petroleum systems. Each of these separate modelling approaches in their own fields is advanced and sophisticated but they are not compatible with each other. Lack of integration produces incorrect palaeogeometries in basin models and inaccurate migration pathways.

      A combined structural restoration and fault-seal analysis technique, integrated with fast hydrocarbon migration pathway modelling code based on invasion percolation (IP) methods, is described. These modelling methods are used to develop a 4D basin modelling workflow in which evolving basin geohistories and geometries form an integral part of the analysis of hydrocarbon migration and trapping. By combining structural restoration and 3D fault-seal analysis it is possible to investigate the evolution of structurally complex traps through time. Integration of these techniques with a numerically fast migration pathway modelling technique allows hydrocarbon migration pathways and accumulations to be modelled through the evolution of the basin with time. Additionally, the effects of uncertainties in structural geometry, fault seal or any of the model input parameters can be explored using a risk-driven approach to modelling.

      These methods are demonstrated using synthetic, computer generated, 3D models and a well-constrained model of the Moab Fault, Utah, USA. Comparison of modelled structural geometries, fault-seal properties and predicted trapped hydrocarbons with outcrop data is used to validate the integrated modelling approach. The validated techniques are then applied to a seismically derived, 3D model from the southern North Sea, UK, to demonstrate how an integrated, risk-driven approach to modelling allows the effects of uncertainties in the distribution of hydrocarbon accumulations to be investigated.

    2. Page 213
      Abstract

      We used analogue models to study the fault evolution produced by extension through a heterogeneous crust. In the experiments, the heterogeneous crust consisted of a gently dipping silicone layer surrounded by brittle material. The viscous silicone level simulates a weak, upper crustal nappe stack that formed during a previous phase of shortening. X-ray scanner facilities allowed us to acquire 3D images of the experimental models at regular time invervals and hence to study the fault pattern development and the location of the main depocenters during rifting. The experimental results show that the inherited weak nappe stack acts as a décollement and localizes deformation. In the early stages of extension a system of conjugate high-angle normal faults initiates close to the upper tip of the gently dipping silicone layer near the free surface and propagates upwards, resulting in an initial symmetrical graben configuration. Further extension results in (1) a progressive asymmetry of the rifted zone, due to migration of its right margin down the nappe, (2) a shift of the main depocentre downward along the décollement, and (3) the simultaneous activity of several normal faults within the rifted zone. When the pre-existing silicone layer is oblique to the extension, the normal faults develop in an en echelon array, with a strike intermediate between the azimuth of the gently dipping silicone layer and the extension direction. The experiments also show how rheological differences between areas with potential intracrustal weak layers and adjacent domains without décollement level can lead to significant differences in fault pattern, dimension and orientation of the rifted zone. Complete asymmetry of a rift and switches in fault dip direction between adjacent domains can be explained by the presence of pre-existing upper crustal heterogeneities.

    3. Page 233
      Abstract

      The Gulf of Corinth is a young (1 Ma) active rift currently extending N00, which displays significant contrasts in structural style along strike. A possible explanation for these variations is the presence of the Phyllades nappe in the basement of the western part of the Gulf. Previous 2D thermo-mechanical models have shown that a strong strength contrast between this metamorphic unit and the rest of the basement can explain the kinematics and the spacing of the faults in the western part. The rift, however, displays a wide variety of 3D features (e.g., en echelon faulting, N30 transverse normal faults) that cannot be taken into account using 2D modelling. To obtain 3D insights into the role of an inherited dipping weakness zone, analogue (sand and PDMS) experiments based on the results of the 2D numerical thermo-mechanical model have been performed. The ana-logue models show that a 30° discrepancy between the dipping direction of the weak nappe and the direction of extension leads to the formation of en echelon and N30 striking normal faults as observed in the Gulf of Corinth. However, fault spacing and graben width completely misfit both the data and the results of the thermo-mechanical models on which the analogue experiments were based. In order to understand those differences, numerical mechanical benchmarks of the analogue experiments have been run to test different factors (3D lateral displacements, values of the elastic parameters and bottom boundary conditions) that could have affected the dynamics of the analogue model. This approach highlights, for our case study, that the misfits are mostly related to the lack of isostatic compensation at the base of the analogue experiments.

    4. Page 253
      Abstract

      We use both analogue and numerical experiments to study the inversion by shortening of a symmetric sedimentary basin. The combination of the two modelling techniques uses the strengths of each method to provide insight into basin-inversion processes. The experiments start with a pre-existing basin filled, in part, with weak layers simulating weak sedimentary rocks. Both footwall and hanging wall can deform freely. The physical properties of the materials used in the analogue experiments (sand and microbeads) and the numerical experiments are appropriately scaled to represent upper crustal rocks. We present a systematic study of the effects of basin infill, basin width and basin location and a sensitivity analysis to understand the effects of the boundary conditions. The results of both methods show that the graben fill accommoda tes most shortening. Weak layers play an important role in localising shortening with limited reactivation of pre-existing (but weakened) faults. In general, forward thrusts and back thrusts nucleate at the lateral contrast of strong and weak materials and cut across the graben-bounding faults. Weak basal detachments are required to transfer shortening to the basin region. The overall evolution of the analogue and numerical models is encouragingly similar.

    5. Page 271
      Abstract

      Although lithospheric modelling has provided extraordinary insights into the processes that shape the continental crust, considerable uncertainty surrounds the basic rheology that governs behaviour at geological timescales. In part, this is because it has proved difficult to identify the geological observations that might discriminate, or unify, models of lithospheric rheology. In particular, the relative strength of lower crust and upper mantle remains a contentious aspect of continental lithospheric rheology. We show that various models for lower crustal rheology may produce distinct patterns of inversion in extensional sedimentary basins, consistent with some of the observed natural variability of inversion styles. Inversion of basin interiors, as is common in European Mesozoic basins, is favoured by a lithospheric rheology more sensitive to lateral thermal structure than to changes in the depth of the Moho, consistent with there being little strength contrast between the lower crust and upper mantle in these settings. In contrast, inversion of basin margins, particularly involving basinward verging structures, is consistent with a rheological sensitivity to the depth of Moho as would apply for a lower crust much weaker than the upper mantle. We use an example from central Australia to demonstrate this latter response, together with thermochronologic data that suggests that a relatively weak lower crust in this setting may reflect abnormally high geothermal gradients.

    6. Page 285
      Abstract

      We use geometric and experimental models to study the development of extensional fault-bend folds. The geometric models show that fault shape, fault displacement, and patterns of aggradation/erosion profoundly affect the distribution of growth beds, the magnitude and direction of dip of pregrowth and growth beds, and the location and dip of the outer limit of folding in pregrowth and growth beds. Complex structural and stratigraphic patterns develop if the rate of aggradation/erosion relative to the rate of fault displacement changes through time. The experimental models (with dry sand and wet clay) show that several deformational styles can accommodate extensional fault-bend folding. In sand models, a few, relatively major, secondary antithetic normal faults accommodate most hanging wall deformation. Pregrowth layers, although faulted, remain flat. The effective shear direction parallels the antithetic normal faults, and the shear angle is about 60°-65°. In clay models, numerous, relatively minor, secondary normal faults (antithetic and synthetic) and cataclastic flow accommodate most hanging wall deformation. The deformed pregrowth and growth layers dip gently toward the main fault. The effective shear angle (35Q-50°) is considerably less than the dip of the antithetic normal faults. In the sand models and geometric models with a large shear angle (60°), more displacement occurs on the main normal fault and the hanging wall collapses in a relatively narrow zone. In the clay models and geometric models with a small shear angle (35°), less displacement occurs on the main normal fault. Instead, the hanging wall stretches substantially and collapses in a relatively wide zone.

    1. Page 307
      Abstract

      I present a brief summary of recent advances in the field of computaţional geomorphology and various attempts to couple numerical models of landscape evolution to models of crustal/lithospheric deformation. The most commonly used formulations for the various physical processes at play during surface erosion, transport and deposition are presented, as well as an outline of how they have been incorporated in a variety of numerical schemes. I also explain how the coupling between erosion and tectonics has been performed under various simplifying assumptions. Determining the rate constants for each of the proposed landforming mechanisms remains a difficult challenge that has recently been helped by the advent of new low temperature thermochronometers and exposure dating by cosmogenic radionuclides. I demonstrate how the information contained in the relationship between age and elevation can be used to provide constraints on the ‘age’ of a landscape, as well as how important rate information can be extracted from various datasets by using simple modelling techniques. This paper demonstrates why the field of computaţional geomorphology needs to harmonizē the various parameterizations (often the legacy of empirical relationships derived from observations at the human scale), quantitative estimates of the value of the numerous rate parameters and improvement of the numerical techniques.

    2. Page 327
      Abstract

      We review results from laboratory-scale modelling of erosion and relief dynamics under variable uplift and rainfall rates. Under constant values of these forcing parameters an experimental landscape typically evolves towards a steady-state between uplift and erosion, and we show how the geometry of the steady-state landscape adjusts to the rates of uplift and rainfall. The comparison between these laboratory-scale landscapes and the natural ones is not straightforward because contrary to analogue modelling in tectonics, natural conditions of relief evolution cannot be downscaled to the laboratory without any scale distortions. Laboratory-scale modelling in geomorphology is therefore only experimental, not analogue. Despite these limitations, experimental models may be used to provide physical tests for numerical models and they give insights into first-order behaviours and directions for future research.

    3. Page 341
      Abstract

      The interplay between tectonics and erosion has a predominant control on the evolution of the morphology of mountain belts. Here we investigate the modalities of deformation in Central Nepal on a c. 100 ka time scale in response to tectonic and externai forcings, through the use of a finite-element thermomechanical model coupled with an integrative denudation formulation that accounts for fluvial incision and hillslope land-sliding. We study the complex coupling existing between tectonics and erosion, with special emphasis on the influences of rock strength and rainfall distributions. Our results underline the key role played by lithologie variations in the elevation of both rivers and mean topogra-phy. We show that the location of the Main Frontal Thrust is mainly controlied by the low erodability of the unconsolidated sandstone in the Siwaliks Hills. As previously suspected (Burbank et al. 2003), our simulations demonstrate that the pattern of uplift in Nepal is mainly dependent on both erodability and fault geometry, rather than on rainfall distribution.

    1. Page 359
      Abstract

      Modelling of sediment compaction requires that the rate limiting processes are understood. The compaction of uncemented sediments at relatively shallow burial depths should be modelled as a function of effective stress following soil mechanical principles and using experimental compaction data for calibration. In siliceous rocks chemical compaction is dominant at depths greater than 2–3 km (80–100°C). Chemical compaction should be modelled as a function of the temperature history and the mineralogical and textural composition of the sediments. The rate of chemical compaction for siliceous sediments is to a large extent a function of the quartz cementation, which is an exponenţial function of temperature, while the effective stress plays a minor role. In the case of carbonate sediments the kinetics of precipitation of cement is much faster and the effective stress is more important than temperature.

      The magnitude and distribution of effective in situ stresses is a complex function of external tectonic stresses, gravitaţional forces and fluid pressures. Sediments undergo mechanical compaction when subjected to high effective stress and are much more compressible than basement rocks. Chemical compaction also results in a reduction in rock volume and this has a strong feedback on the in situ stresses. If the horizontal stress is greater than the vertical stress, both mechanical compaction and chemical compaction will also occur in the horizontal direction, thus relaxing in situ stresses unless there is significant basin shortening. Calculations show that relatively large in situ stress anomalies (10 MPa) may be relaxed in 5–10 ka by chemical compaction during basin subsidence. Chemical compaction may also continue during uplift; it is fundamentally different from mechanical compaction and must be modelled separately.

    2. Page 381
      Abstract

      Rotational behaviour and deformation around multiple faults was investigated in analogue experiments using a linear viscous matrix material under simple shear boundary conditions. Previous analogue and numerical studies have shown that, for single faults, characteristic deformation geometries are produced in initially straight marker lines parallel to the shear zone boundary (flanking structures). Observations from several natural shear zones suggest that not only single faults, but often several parallel or conjugate fault planes are subjected to progressive shear resulting in distinctive deflection geometries. If the distance between faults is on the order of their length, or less, then the perturbation flow fields interfere and coalescence, and finite deflection structures develop that are distinctly different from those around single fractures. In particular, coeval contractional and extensional geometries may develop across conjugate faults, although for bulk simple shear the total length of marker lines parallel to the shear zone boundary cannot change. This advises caution in inferring shear-zone parallel contraction or extension from secondary slip surfaces. In contrast to single flanking structures, conjugate flanking structure systems occurring in natural shear zones are reliable shear sense indicators due to their triclinic symmetry.

    3. Page 397
      Abstract

      Using an elastic dislocation model, we incorporate a historical earthquake catalog, mapped Marmara Sea fault traces, and fault slip distributions for the 1999 Izmit earthquake inferred from InSAR and GPS data to determine various stress change scenarios crucial for evaluating future earthquake potential in the eastern Marmara Sea. We have tested six plausible past rupture configurations arising from the uncertainty in the location of the western termination of 1999 Izmit earthquake rupture and the location of the 1963 Yalova earthquake rupture. Coulomb stresses calculated are increased on the Princes’ Islands, Çinarcik, and Armutlu fault segments in each case. In four of the six plausible configurations of previous ruptures, the Çinarcik fault receives the greatest average stress change. In one other configuration, the average stress increase on the Princes’ Islands fault is greatest. In another, the stress changes on the Çinarcik and Princes’ Islands faults are comparable. Moreover, we show that rupture initiating on either the Princes’ Islands or Armutlu faults would be favoured to propagate onto the Central Marmara, or Imrali fault, respectively, based on the favourable geometries of the respective fault intersections. Rupture initiating on the Çinarcik fault, however would be limited to a much shorter length based on its mapped western termination. Therefore, while the earthquake-induced stress changes may, in most cases, be greatest on the Çinarcik fault, an earthquake initiating on this fault segment may produce a shorter cumulative rupture compared to rupture initiated on the two other major eastern Marmara Sea fault segments. These results are encouraging for the use of geomechanical modelling tools in addressing uncertainties inherent in most geological and geophysical data applied to earthquake-related problems.

    4. Page 415
      Abstract

      The West Carpathian thrustbelt advanced northeastwards over the European Platform. Its thrust sheets comprise sediments of the Early Cretaceous rifts that evolved on a passive margin of the European Platform, the Late Cretaceous–Paleocene basins formed by rift inversion, and the Eocene-Oligocene flexural basin. Geochemical analyses established a clear link between pooled oils in the foreland and the Oligocene Menilite Formation inside the thrustbelt. In order to understand the driving forces for this oil migration scenario, finite-element models of fault-propagation and fold-bend folds are used to study the mean stress distribution in the thrust sheets and the foreland. Mean stress has a profound control on the pore fluid pressure through the relationship affected by sediment porosity, and sediment skeleton and fluid compressibilities. Modelling results suggest that only fault-propagation folds are capable of generating foreland-directed mean stress gradients as they are characterized by a large foreland area of decreased mean stress, by coupled increased/decreased mean stress areas on advancing/receding sides of the ramp tip, and an overall mean stress decrease inside the thrust sheet in the direction towards the foreland. This interpretation is in accordance with the dominant fold-and-thrust style in the Western Carpathians inferred from balanced cross-section restoration. It shows that frontal fault-propagation folding was active during the late Oligocene–Early Miocene, providing an effective tectonic driving force for hydrocarbon migration from source rocks inside the thrustbelt towards reservoirs in the foreland.

    5. Page 429
      Abstract

      The San Andreas Fault system is a complex tectonic ensemble that accommodates most of the relative plate motion between the Pacific and the North American plates. The structure and rheological properties of the faults vary along the plate boundary and lead to the distribution of deformation that we observe today. In order to learn more about the mechanical behaviour of such a fault system, a model of the northern California fault system is built, constrained by heat flow data, GPS and palaeoseismological measurements of slip rates (on the San Andreas, the Maacama and Bartlett Springfaults), and stress orientations. Our basic assumption is that the upper crust has a high frictional strength and that major faults represent weak zones with a lower effective friction. Several combinations of effective fault frictions on the three major faults of the system in the model are tested. We find that slight variations of the effective friction angle on one of the three active strands lead to an important redistribution of slip rates through the system. If present in nature, this fault behaviour could explain why fault slip rates vary in time, as suggested by slip rate variations over geological scales in intracontinental fault systems.

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