David V. Wiltschko

Microfracturing, paleostress and the growth of faults

Abstract We have analyzed the microfractures in samples taken near one thrust fault and five normal faults of known displacement to test the dependence of microfracture density and distribution on fault slip. We have also recorded the orientation of microfractures as a function of distance from the associated fault surface. In order to control as many parameters as possible, shallow faults with similar lithologies were chosen. They showed the same kind of deformation, namely, microfracturing without block rotation, recrystallization, extensive granulation or crystal plasticity. The results of over 8000 microfracture measurements suggest that maximum microfracture density is independent of net slip. We interpret this relationship as indicating that the preponderance of microfracturing occurs in proximity to the propagating fault tip. The angular relationship between the shear plane and microfractures was commonly found to be between 5° and 20°, significantly less than the ≈30° expected between the far-field maximum principal stress and the shear surface. This low angle between microfracture orientation and shear plane orientation is predicted for a local stress field associated with a propagating mode II fault tip. Other possible explanations for the small angle for tensional microfractures and the shear surface are: (1) the effective confining pressure was very low; or (2) shear strength at failure was much larger than expected.

Duplex style and triangle zone formation: insights from physical modeling

Duplexes are a common feature in thrust belts at many scales. Their geometries vary significantly from antiformal stacks with significant forethrusting in the cover (e.g. southern Pyrennes, Spain) to triangle zones (e.g. foreland Canadian Rockies) to low-displacement individually spaced ramp-anticlines (e.g. Sub-Andean thrust belt, Bolivia). We present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style. The models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers. The strength of the decollements in the models is a function of the shortening rate applied to the model. The relative strength of the decollements and surrounding rocks affects the development of active- or passive-roof duplexes (triangle zones). It also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps, which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp-anticlines. Model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions. The presence of relatively strong decollements promotes local underthrusting of the cover, individual ramp-anticlines, internal deformation of thrust sheets, low early layer-parallel shortening, and sequential towards-the-foreland propagation of structures. Weak decollements promote forethrusting of the cover, antiformal stacks, coeval growth of structures, and low internal strain, with the exception of significant early layer-parallel shortening. No underthrusting at a regional scale occurred in any model.

Crystallization pressure versus “crack seal” as the mechanism for banded veins

Banded fibrous veins are often pointed to as evidence for episodic crack opening driven by oscillations in fluid pressure or bulk strain. Advances in understanding the geochemistry of precipitation, data on veins, and experiments suggest that pressure due to growing crystals may be an alternate explanation for many observations on these types of veins. We propose that some veins originate at sites of precipitation and then propagate due to the pressure exerted by the crystal growth. As materials precipitate, the vein walls are pushed apart. The resulting veins have shapes typical for mode I cracks except that, mechanically, crystallization pressure replaces the role of internal fluid pressure in their propagation. A nonzero remote differential stress serves to align veins. Mechanical and geochemical considerations suggest that this process will be most important in fine-grained rocks such as greenschist-grade pelites where diffusion from sites of dissolution rather than advection is the dominant mass-transport process. Veins may owe their orientation to tectonism, but their initiation and growth are due to processes that supersaturate the pore fluid. Veins formed by this mechanism involve cracking to the extent that precipitation forces an original flaw to extend during precipitation. Cyclic quartz-mica bands may indicate geochemical self-organization at the vein wall driven by pressure-solution-enhanced supersaturation in the pore fluid and nonlinear precipitation kinetics at the vein wall.

Kinematic models of deformation at an oblique ramp

Kinematic models for the deformation of hanging wall material moving over a footwall oblique ramp are developed by considering two end members of assumed mechanical behaviour, vertical shear and layer-parallel shear. In the former case, material is sheared vertically and displacements remain within the tectonic transport plane; the deformation is accommodated by thinning of the hangingwall over the ramp. In the later case, material is deflected out of the transport plane such that the pitch angle of the hangingwall particle path in the plane of the oblique ramp is equal to the angle between the transport direction and the strike of the oblique ramp. As a result, shear strains above the oblique ramp are non-zero in both the transport and transport-normal planes. The deflection and transport-normal shear strains are a minimum for the special cases of pure frontal and lateral ramps, and maximum at an intermediate orientation, depending on oblique ramp dip. Fault-bend folds are similar in most respects for both vertical shear and layer-parallel shear mechanisms. At frontal ramp — oblique ramp intersections, synformal or antiformal multiple bends in the footwall generate, respectively, second order hangingwall synclines or anticlines, which terminate along strike into simple fault-bend folds. For the layer-parallel shear mechanism along the pure oblique ramp, deflected hangingwall material passes through the transport plane, conserving area and volume. At the rearward intersection zone (concave toward the transport direction), hangingwall material diverges resulting in local strike-parallel extension. This extension may be a mechanism for the generation of transverse faults (or ‘tear faults’) in the hangingwall. At the forward intersection zone (convex toward the transport direction), displacement paths converge resulting in local strike-parallel shortening. The attitude of the oblique ramp and the amount of displacement significantly affect the map geometry and magnitude of lateral strains.

An experimentally verified model for calcite precipitation in veins

Calcite is commonly found as a vein-filling mineral in rocks. However, the factors controlling its deposition are complex and not well understood in quantitative terms. In order to advance our understanding of the processes involved, we have refined the model for calcium carbonate mass transport in subsurface carbonate rocks of Morse and Mackenzie (1993) and developed a new experimental technique to test it. This technique uses a flow-through reactor that simulates a vein opening. Agreement was observed between model predictions and experimental observations for the deposition of calcite in synthetic veins. The influences of surface area to solution volume ratio, solution saturation state with respect to calcite, and flow velocity were well predicted by the model. The model predicts that in order to have a fairly uniform deposition of calcite within a vein, solution flow must be quite rapid (tens to thousands of cm h−1) or the solution must be only slightly supersaturated with respect to calcite. A low degree of solution supersaturation demands what may be unreasonably large volumes of solution flow to achieve vein filling for the vein configuration we have studied.

Possible effects of pre-existing basement topography on thrust fault ramping

Finite-element models show that one way in which thrust ramps may arise is through the mechanical interaction between basement and overlying sediments. In the simplest case, shear coupling between a planar basement-sediment causes the differential stresses in the sediments to die out with depth and distance from the applied load. For such cases, curved thrust faults may result if the strength of the rock is exceeded. Basement topography may also affect the location and shape of ramps by acting as a stress concentrator, by producing a stress shadow and by changing principal stress orientations. Modeling suggests that whether or not these basement topographic features cause ramping will depend on the height and angularity of the feature as well as the rock types that overlie it. Under the assumption of linear elasticity and for given boundary conditions, the Poissons ratio plays an important role in determining the orientation and magnitude of the principal stresses. Calculations using experimentally measured Poissons ratios predict that the earliest maximum compressive stress directions should be nearly vertical in the more cratonward portions of thrust belts. However, the stress directions which are inferred to have occurred earliest in this part of thrust belts are nearly horizontal. This suggests that non-elastic or ductile processes have an effect on the propagation of thrust faults.

Stresses beneath a ramping thrust sheet

Abstract There is a systematic progression of both the timing of motion of thrust sheets and the spacing of thrust ramps during the development of some thrust-dominated foreland fold-and-thrust belts. On the basis of these observations, we suggest that a regional mechanical process that controls frontal ramp formation may dominate local effects such as stratigraphic pinchouts and interaction with basement. To investigate this process we have developed elastic, plane strain models of the state of stress in the footwall of the last formed, or most forward thrust ramp. These models include the load of an overriding thrust sheet and a synorogenic sedimentary cover. The effect of emplacing a thrust sheet or developing a synorogenic sedimentary cover above an incipient thrust sheet is to stabilize the footwall toward the hinterland with respect to failure, and thereby move failure more to the foreland. Failure is localized where there is a favorable trade-off between the spatial rates at which mean stress and differential stress decrease toward the foreland. The models also predict that frontal ramps nucleate well above the basal decollement and propagate upward and downward eventually linking with the regional decollement. Fault trajectories predicted by using a Coulomb criterion are subhorizontal in the hinterland portion of the model and curve upward to the foreland resulting in a curved ‘flat and ramp’ geometry. These analyses show that each successive thrust sheet would be thinner, as well as shorter, than the previous thrust sheet. Based on these results, we propose a model for the formation of the first ‘thin-skinned’ thrust sheet in the external portions of orogenic belts. The synorogenic sediments that form during uplift of the internal zones of mountain belts will cover the preorogenic strata to the foreland. As deformation progresses to the foreland, and deformation becomes more shallow and more brittle in nature, the preorogenic strata will interact with the synorogenic strata and failure will occur creating the first ‘brittle’ thrust sheet. The location of this first thrust will be controlled by the mechanisms outlined in our models. To the foreland of this point, each sucessive thrust fault will localize as a result of the combined effects of an overriding thrust sheet and synorogenic deposits.

Fault controlled sequential vein dilation: competition between slip and precipitation rates in the Austin Chalk, Texas

Abstract Multi-layered calcite veins in a dilatant jog of a left-stepping, left-slipping shallowly buried fault segment are composed of alternating millimeter- to submillimeter-thick calcite veinlets and host lithons forming a coarse ‘crack–seal’ texture. The grain fabrics in calcite veinlets are mostly equant or irregular, suggesting face-controlled grain growth in a fluid-filled cavity. The relatively thick veinlets can be developed by progressive fault slip and veinlet opening simultaneously with calcite precipitation under low effective stress. Continuous changes in the oxygen isotopic compositions of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed from the footwall to the hanging wall. There is no particular evidence that these veins represent excursions in fluid pressure or instantaneous fracture opening related to episodic fault slip; the fracture formation and filling cycle could have taken place along a continuously slipping fault contained within a porous rock with normal fluid pressure.

Interaction of basement uplift and thin-skinned thrusting, Moxa arch and the Western Overthrust Belt, Wyoming: A hypothesis

Immediately north of Big Piney in western Wyoming, the Moxa arch is modeled as emplaced during the Late Cretaceous along an east-dipping, low-angle thrust (Moxa thrust) that has Precambrian basement and Paleozoic and younger cover rocks in the hanging wall. The west-verging Moxa thrust cut up-section from the basement-cover contact and flattened to the west along a detachment in Lower Triassic rocks (Thaynes detachment). During motion on the Moxa thrust, the leading edge of the hanging wall wedged westward along the Thaynes detachment, peeling back Triassic and younger rocks and thrusting them relatively eastward along the ancestral Prospect thrust. The 5.3 km of westward movement along the Moxa thrust was matched by eastward movement along the west-dipping ancestral Prospect thrust. The ancestral Prospect thrust moved an additional 5.1 km during the late Paleocene when thrust-belt deformation progressed eastward to the Moxa arch. It appears that in the Snider Basin area, the Prospect thrust does not share a ramp with the Darby thrust and that the emplacement of the Moxa arch and Prospect thrust determined the position of the later and more westerly Darby thrust.

Décollement folding as a mechanism for thrust‐ramp spacing

The hypothesis that early formed decollement folds localize thrust ramps, explaining the regular spacing of major thrust ramps observed in many mountain belts, is examined. The model for folding instability consists of a rigid basement, a weak decollement zone, and a stiff thrust sheet. The model incorporates the effects of topographic stresses, and syntectonic erosion and redeposition and uses a linearized power law rheology for the thrust sheet. This approximation is valid for small angles of fold limb dip (<15°) and is appropriate for analysis of the wave-length selection of folds at the onset of deformation. The spacing of major thrust ramps and the stratigraphic thicknesses of the decollement zone and thrust sheet from the Wyoming portion of the Sevier fold-and-thrust belt are used to test the model. The ratio of the fold dominant wavelength to thrust sheet thickness, Ld/h2, increases with the ratio, R, of the viscosity of the stiff layer to that of the decollement zone and decreases with the power law stress exponent, n2. The fold amplification factor increases with both n2 and R. As the topographic decay ratio, D, increases, Ld/h2 increases at fixed n2 and R. Acceptable models for the Absaroka and Darby thrust sheets have n2 ≥ 3. Only when R < 1000 and significant erosion and redeposition of sediments accompany the folding of the thrust sheet (D ≥ 103) does the required R fall below a reasonable limit. Thus a model in which decollement folding is accompanied by syntectonic erosion and redeposition precedes thrusting is consistent with the available observational constraints.