T41F-01
Pore-Fluid Pressures and Crustal Strength
The classic graph of crustal strength as a function of depth shows linearly increasing brittle strength above the brittle-plastic transition. This linear increase is a consequence not only of the pressure dependence of brittle strength but also an assumption that the depth-normalized pore-fluid pressure λ = Pf/ ρr gz is constant, which is perhaps only plausible in the case of hydrostatic pore-fluid pressures (λ ≈0.4). Much deep bore-hole stress data agrees with this assumption, showing the predicted linear increase in strength together with stress magnitudes that are consistent with hydrostatic pore-fluid pressures. In contrast, observed pore-fluid pressures in deeper parts of deforming clastic sedimentary basins and active plate-boundary mountain belts are commonly in excess of hydrostatic. These deforming sedimentary basins typically have pore-fluid pressures that are dominated by disequilibrium compaction, showing fully compacted sediments with hydrostatic fluid pressures at shallow depths until the fluid-retention depth zFRD is reached, below which sediments are increasingly undercompacted and overpressured. For this disequilibrium- compaction mechanism, the fractional brittle weakening (1-λ) below the fluid-retention depth is a simple function of depth (1-λ)≈0.6(zFRD/z), which directly leads to a predicted crustal-strength profile that is radically different from the classic hydrostatic profile. The brittle strength below the fluid-retention depth is predicted to be constant and equal to the strength at the fluid-retention depth. Some stress measurements in deeper parts of sedimentary basins appear consistent with this constant-strength prediction. Furthermore, observations from western Taiwan show that zFRD is fixed relative to the land surface during active uplift and erosion, therefore crustal strength should be approximately unchanged by exhumation except for cohesive effects. The full limits of the disequilibrium-compaction regime are not well know. However it is only as non- hydrostatic pore-fluid pressures decay that deformed sedimentary mountain belts are expected to show crustal strengths similar to the classic graph.
T41F-02
Thrust zone architecture and evolution in deep water sediments revealed by seismic reflection data
Commercial seismic reflection data from offshore west and SW Africa provide high resolution imagery of thrust zones. These structures have formed at the compressional toe-of-slope parts of 100s km –scale gravitational deformation systems. Two systems are contrasted; the Cretaceous gravity slide system of the Orange basin slope, and part of the active deep water thrust belt of the Niger delta (courtesy of data provided by CGG Veritas). The Orange examples involve c. 500ms TWT (c. 1 km) of sediment while the Niger examples involve about 2 km of pre-tectonic and a further 2 km of syntectonic sediment. In both examples seismic data reveal thrust ramp geometries (in 3D in the Niger case study). In the Orange examples thrust zones are narrow, with stratal reflectors retaining their amplitude up to their terminations. This indicates little heterogeneous distributed strain and the adjacent folding simply relates to fault geometry. In contrast, the Niger examples show trishear fault geometries with significant amplitude degradation into the thrusts. The thrust zones show soft relays along strike with polarity reversals. Although both case studies are formed in multilayer turbitide sand and mud systems, they differ in the extent of syn-deformation deposition. The Orange system was (locally) sediment starved so thrusts emerged onto the palaeo-seabed. The Niger system was continuously swamped by sedimentation. In these situations the thrust zones steepen upwards and do not activate upper thrust flats. Deposition exerted a profound control on fault and fold evolution, controlling thrust localization behavior in these sedimentary multilayers.
T41F-03
Testing Kinematic and Mechanical Solutions for Coulomb Wedges
We have devised a "two plate" model for studying kinematics and mechanics of a Coulomb wedge. Our apparatus consists of a 2 meter-long box, bound by two vertical glass walls and a flat internal base. Two stepper motors are used to drive mylar sheets along the base of the box, such that both sheets pass through a central slot. The mylar sheets are covered with a flat layer of dry sand, which is the deformable material that will make up the wedge. This design extends the "doubly vergent" model of Malavieille (1984) to include two "tectonic plates". One mylar sheet is pulled downward through the central slot to simulate subduction. The other mylar sheet is pulled out of the slot and rearward through the back of the box to simulate back-arc divergence. The sand above these mylar sheets deforms into a doubly-vergent wedge, with a contractional pro-wedge above the subducting mylar and an extending retro-wedge above the divergent mylar. In this way, we can produce a steady-state wedge where accretion on the proside is matched my decretion on the retroside. We use a digital camera and particle image velocimetry (PIV) to measure the velocity field within the wedge. Other studies of sand wedges have focused on the unsteady behavior of the wedge, associate with fault imbrication and accretion. The fact that our wedge maintains a steady size means that we can study the time-averaged velocity field to see how it compares with the theoretical solutions of Dahlen (1984) for flow in a homogenous wedge.
T41F-04
Constraining Fold Geometry by Application of Airborne Laser Swath Mapping at Sheep Mountain Anticline, Wyoming
Sheep Mountain anticline (SMA), Wyoming, is an asymmetric fold resulting from Laramide compression that activated an underlying basement-cored thrust fault. Excellent outcrop exposures and a wealth of data, including detailed structural maps and kinematic interpretations of five fracture sets, make SMA an ideal site for studies relating the mechanics of fold evolution to present day fold geometry and fracture patterns. One of many challenges in relating folding to fracturing arises because most fractures are below the resolution of industry standard seismic imaging. Another stems from the differences in length scales: typical fold wave lengths are 1 to 10 km whereas typical fracture trace lengths are 1 to 100 m. Therefore, new methods must be found to constrain fold geometry at sites where outcrop fracture data is available. To constrain fold geometry at SMA we have obtained an airborne laser swath mapping (ALSM) digital elevation model (DEM) from the National Center for Airborne Laser Mapping (NCALM). We present and describe the DEM, on which we have mapped bedding-plane exposures of several prominent stratigraphic horizons. We use differential geometry to characterize the curvature of these outcrop exposures, considering both principle curvatures, and classifying the surfaces into the eight different fundamental geological shapes. This new characterization of the fold shape is compared to a similar analysis based upon a structural contour map of SMA prepared using standard techniques. Future work with these data will include the construction of a refined 3D model of fold geometry by interpolation of mapped bedding exposures, mechanical analysis of folding induced stress perturbations throughout the history of structural growth, and the analysis of these stress perturbations as they relate to observed fracture sets.
T41F-05
Fault Drag Along Normal Faults in Unconsolidated Sediments
A displacement gradient along the strike of a fault plane results in the formation of fault drag in layers of the adjacent host rock. We investigated normal faults in Lower Miocene (Sarmatian-Pannonian) clastic sediments in a quarry at St. Margarethen, Burgenland, Austria, situated at the Eastern margin of the Eisenstadt Basin, a subbasin of the Vienna Basin complex. The N-S trending faults crosscut a barely lithified sequence of conglomerates, fine-grained sands and silts. These marker horizons display normal offset along the conjugate fault set, which is often, but not exclusively, restricted to the conglomerate beds. A significant amount of rotation of the faults can be inferred, as largest offsets are accumulated at the more inclined fault planes, whereas steeper faults show least displacement. Associated with increasing amount of offset, pronounced reverse drag of the faulted sedimentary layers can be observed both in footwall and hanging wall, often accommodated by re-orientation of the conglomerate pebbles. Rotation and fault linkage resulted in the formation of longer faults with varying dip angles crosscutting several conglomerate beds and the intercalated sand and silt layers. In the vicinity of the fault tips, individual pebbles are intensively cracked, which we interpret as an indicator for stress concentration at the fault tips. Comparing the geometry of the observed fault drag with results from numerical models we try to estimate the initial shape and orientation of the fault planes, as well as the amount of rotation and background strain which led to their finite geometry. Extrapolating the results to basin-scale faults, we may deduce valuable parameters for the interpretation of reflection seismic images, where structural details may be blurred or below seismic resolution.
T41F-06
Predicting Folding Sequences Based on the Maximum Rock Strength and Mechanical Equilibrium
The objective is to propose and validate simple procedures, compared to the finite-element method, to select and optimize the dominant mode of folding in fold-and-thrust belts and accretionary wedges, and to determine its stress distribution. Mechanical equilibrium as well as the constraints due to the limited rock strength of the bulk material and of major discontinuities, such as décollements, are accounted for. The first part of the proposed procedure, which is at the core of the external approach of classical limit analysis, consists in estimating the least upper bound on the tectonic force by minimisation of the internal dissipation and part of the external work. The new twist to the method is that the optimization is also done with respect to the geometry of the evolving fold. If several folding events are possible, the dominant mode is the one leading to the least upper bound. The second part of the procedure is based on the Equilibrium Element Method, which is an application of the internal approach of limit analysis. The optimum stress field, obtained by spatial discretisation of the fold, provides the best lower bound on the tectonic force. The difference between the two bounds defines an error estimate of the exact unknown tectonic force. To show the merits of the proposed procedure, its first part is applied to predict the life span of a thrust within an accretionary prism, from its onset, its development with a relief build up and its arrest because of the onset of a more favorable new thrust (Cubas et al., 2007). This life span is sensitive to the friction angles over the ramp and the décollement. It is shown how the normal sequence of thrusting in a supercritical wedge is ended with the first out-of sequence event. The second part of the procedure provides the stress state over each thrust showing that the active back thrust is a narrow fan which dip is sensitive to the friction angle over the ramp and the amount of relief build up (Souloumiac et al., 2007). The stress state is dominated by a concentration at the root of the active ramp and the presence of the back thrust. Analogue experiments with sand demonstrate the ability of the first part of the proposed procedure to predict the position and the lifetime of thrusts, the topographic evolution, as well as the value of the compressive force. The simplicity and lightness of the procedure allows to determine probability distributions of the friction angles of the décollement, the ramps, and the pristine material using an inverse problem formalism. Applied to a section at the front of Nanka'i's wedge, Southeast Japon, the two parts of the method confirm the relative weakness of the basal décollement. From the first part, we conclude that the active thrust is necessarily weaker than the incipient thrust and that the frontal section is likely inhomogeneous. The second part shows that, close to criticality, for minutes changes in the décollement friction angle, the stress concentrations, marking the onset of thrusting, can be positioned at very different locations, the details of which depend on relief irregularities.
T41F-07
Stresses and deformation around fault tips and their implications for fault rupture and propagation
Stresses and deformation around fault tips are predictors of impending fault rupture and direction of propagation. In mountain building they affect the topography of sites where Earth faults at some finite depth but the fault does not cut through the surface. Fault tips are highly damaged fracture process zones where slip builds up at a rapid pace. In this work we use various simulation techniques for quantifying stresses and deformation around fault tips, including the assumed enhanced strain, nonlinear contact mechanics, and the extended finite element methods. Enhancements of the finite element description of deformation field are critical for the contact mechanics and extended finite element methods, and to this end we use linear elastic fracture mechanics to enhance the description of the deformation field around fault tips. We investigate the impact of fault tip enhancements on the predicted magnitude and distribution of slip not only around the tip but throughout the length of the fault. Finally, we use information on stresses and deformation fields around fault tips to develop mathematical criteria for fault propagation under mode II shearing. The latter mode of deformation is critical for thrust faulting typically responsible for generating asymmetric anticlines.
T41F-08
Characterization of fracturing and fracture reactivation during folding at Raplee Ridge, UT
Folding and fracturing can be intimately related processes in which the overburden stress, the remote tectonic stress, stresses induced by underlying faults, local folding related stresses and stress perturbations due to pre- existing fractures all influence new fracture formation. As a case study, we closely examine the fracture characteristics at Raplee Ridge, a Laramide aged monocline with N-S axial trend that is exposed over ~14 km length in the Monument Upwarp of south-eastern Utah. Measurements of fracture orientations and spatial densities at ~120 locations across and around the fold indicate three distinct fracture sets based on their orientations. Abutting relationships indicate relative ages of these sets and fracture densities measured along scan lines indicate how each set varies with structural position on the fold. Additionally, ~15 outcrop-scale maps of fractures on well-exposed pavements were created to identify and document the detailed interactions of the sets and determine the initiation, propagation and evolution histories of the fractures. The maps were created by taking high resolution (12 megapixel) photos of fracture pavements and measuring locations on the pavements using a GPS receiver and a laser range-finder. The locations were used to orthorectify the photos, making them spatially accurate. Fractures were mapped by hand onto large printouts of the orthorectified photographs. The mapping documents the following five stages of fracture evolution: 1) formation of pre-folding E-W Set I fractures; 2) formation of pre-folding N-S Set II fractures; 3) left-lateral shearing of Set II fractures with formation of tail-cracks and the flow of fluids through these fractures; 4) right-lateral shearing of set I fractures creating NW-SE Set III fractures as tail-cracks of Set I fractures and as independent fractures; and 5) formation of local E-W Set IV fractures as tail-cracks from slip along Set III fractures. Stages 3) through 5) are interpreted as developing during the folding of the monocline. This case study demonstrates the necessity of understanding the geometry of pre- folding fracture sets and the mechanics of fracture reactivation in order to elucidate fold-fracture relationships.