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Geological Society of America Bulletin, 2015

Fault-propagation folds are common structures that accommodate crustal shortening in various compressional settings worldwide. Motivated by the wide range of geometries observed for fault-propagation folds, we investigated the role played by mechanics in the variations observed for this structural class. Detailed structural measurements of a series of 15 fault-propagation folds from the Niger Delta, Argentina, and southeastern Asia reveal several relationships between aspects of the structural geometries. We found that the decrease in displacement updip along the fault is well approximated by a linear trend that has a relatively consistent slope, and that this gradient remains constant for increasing total displacement. This suggests that the faults propagate self-similarly, consistent with a range of kinematic models that have been used to describe them. Additionally, we observed that uplift has contributions both from rigid translation along a dipping fault and folding, and that the values observed for many natural structures lie between those predicted by the trishear and kink-style models, such as fixed-axis and constant-thickness fault-propagation folding. Finally, we found that fault-propagation folds exhibit a range of fault dips, with many structures having fault dips coincident with those characteristic of fault-bend folds, while another group is characterized by significantly higher fault dips. By developing a series of discreteelement mechanical models, we found that mechanical layering plays a first-order role in the development of different styles of faultpropagation folding. Homogeneous materials produce trishear-like fault-propagation folds, while strongly layered materials produce structures more similar to the kink-style kinematic models. Comparison with the observations from natural structures indicates that these mechanical models reproduce the observed trends, and that most natural structures fall between these two styles of models. This suggests that trishear and the kink-style (fixed-axis and constant-thickness) faultpropagation folding models may be thought of as end members on a continuum of possible fault-propagation folding geometries that are largely dictated by the degree of mechanical layer anisotropy in the stratigraphy. Finally, we suggest that fault steepening in highly anisotropic models may develop due to strain localization in tightly folded structural forelimbs.

Journal of Structural Geology, 2015

Flexural slip folds are distinctive of mixed continuous-discontinuous deformation in the upper crust, as folding is accommodated by continuous bending of layers and localized, discontinuous slip along layer interfaces. The mechanism of localized, layer-parallel slip and the stress and fluid pressure conditions at which flexural slip occurs are therefore distinctive of shear localization during distributed deformation. In the Prince Albert Formation mudstone sequence of the Karoo Basin, the foreland basin to the Cape Fold Belt, folds are well developed and associated with incrementally developed bedding-parallel quartz veins with slickenfibers oriented perpendicular to fold hinge lines, locally cross-cutting axial planar cleavage, and showing hanging wall motion toward the fold hinge. Bedding-parallel slickenfiber-coated veins dip at angles from 18 to 83 , implying that late increments of bedding-parallel shear occurred along unfavorably oriented planes. The local presence of tensile veins, in mutually cross-cutting relationship with bedding-parallel, slickenfiber-coated veins, indicate local fluid pressures in excess of the least compressive stress.

Journal of Structural Geology, 2006

Fault-propagation folding is a common folding mechanism in thrust-and-fold belts and accretionary prisms. Several geometrical models relating the fold shape to the ramp shape have been proposed. In all these models, ramps always emanate from a basal fault and propagate upwards. We have developed a new kinematic and geometric model of fault-propagation folding, named double-edge fault-propagation folding. The model simulates folding at thrust ramps as a function of their nucleation site and propagation history within the folded multilayer. The fold shape depends on the initial length and location of the ramp, its dip, and the S/P ratio (i.e. incremental ramp slip versus propagation) of both the upper and lower ramp tips. This solution increases the geometrical flexibility of fault-propagation folding reducing, for example, the direct dependence between the backlimb dip and the ramp dip, as double-edge fault-propagation folding is characterised by a backlimb panel not necessary parallel to the ramp. Non-parallelism between the ramp and the backlimb is commonly observed in thrustrelated anticlines, within fold-and-thrust belts and accretionary prisms. The excess layer-parallel shear imposed by the development of double-edge fault-propagation folding can be easily accommodated by discrete faulting and/or penetrative deformation. The dependence of the fold shape on the fault behaviour provides a tool for including the role of mechanical stratigraphy and environmental conditions of deformation into kinematic models. Natural examples of anticlines that could be modelled by double-edge fault-propagation are presented. q

The development of new reverse faults and related folds is strongly controlled by the mechanical characteristics of the host rocks. In this study we analyze the impact of a specific kind of anisotropy, i.e. thin mechanical and frictional discontinuities, in affecting the development of reverse faults and of the associated folds using physical scaled models. We perform analog modeling introducing one or two initially horizontal, thin discontinuities above an initially blind fault dipping at 30 in one case, and 45 in another, and then compare the results with those obtained from a fully isotropic model. The experimental results show that the occurrence of thin discontinuities affects both the development and the propagation of new faults and the shape of the associated folds. New faults 1) accelerate or decelerate their propagation depending on the location of the tips with respect to the discontinuities, 2) cross the discontinuities at a characteristic angle (~90), and 3) produce folds with different shapes, resulting not only from the dip of the new faults but also from their non-linear propagation history. Our results may have direct impact on future kinematic models, especially those aimed to reconstruct the tectonic history of faults that developed in layered rocks or in regions affected by pre-existing faults.

Journal of Structural Geology, 2005

We investigated the effects of fault-related folding mechanisms along a single fault that is pinned on both ends, upsection and downsection. Using a numerical model we produced flanking structures under plane strain transtension for the whole range between simple shear, general shear and pure shear, with layer-parallel shortening parallel and shear zone widening normal to the shear zone boundaries. Under these boundary conditions, contractional flanking folds with thrusting kinematics are the structures that are most likely to form and s-type flanking folds develop stable orientations. Comparison with natural examples reveals that contractional flanking structures occur from the outcrop scale within ductile shear zones, where they can be used as kinematic indicators in special cases, up to the mesoscopic scale within fold and thrust belts. The fundamental differences of our model to existing fault-related fold models like fault-propagation folds, fault-bend folds or break-thrust folds are: (1) the fault does not necessarily maintain a stable orientation but may rotate during progressive development; (2) the drag can change from reverse to normal along the fault; (3) the displacement along the fault has its maximum in the centre of the fault and decreases in both directions, downsection and upsection towards fixed fault tips.

Journal of Geophysical Research, 2007

We analyze the kinematics of fault tip folding at the front of a fold-and-thrust wedge using a sandbox experiment. The analog model consists of sand layers intercalated with low-friction glass bead layers, deposited in a glass-sided experimental device and with a total thickness h = 4.8 cm. A computerized mobile backstop induces progressive horizontal shortening of the sand layers and therefore thrust fault propagation. Active deformation at the tip of the forward propagating basal décollement is monitored along the cross section with a high-resolution CCD camera, and the displacement field between pairs of images is measured from the optical flow technique. In the early stage, when cumulative shortening is less than about h/10, slip along the décollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium. In this stage of detachment tip folding, horizontal displacements decrease linearly with distance toward the foreland. Vertical displacements reflect a nearly symmetrical mode of folding, with displacements varying linearly between relatively well defined axial surfaces. When the cumulative slip on the décollement exceeds about h/10, deformation tends to localize on a few discrete shear bands at the front of the system, until shortening exceeds h/8 and deformation gets fully localized on a single emergent frontal ramp. The fault geometry subsequently evolves to a sigmoid shape and the hanging wall deforms by simple shear as it overthrusts the flat ramp system. As long as strain localization is not fully established, the sand layers experience a combination of thickening and horizontal shortening, which induces gradual limb rotation. The observed kinematics can be reduced to simple analytical expressions that can be used to restore fault tip folds, relate finite deformation to incremental folding, and derive shortening rates from deformed geomorphic markers or growth strata.

Journal of Structural Geology, 2012

Several conceptual approaches have been proposed to account for the development of fault-propagation folds whose geometry and kinematics depend on the amount of displacement along a basal decollement level, the ramp angle and the slip to propagation ratio. Among these, the variable interlimb angle model of is able to explain open and close natural folds but its application is limited because the fold geometry and bed thickness evolution rely on imposed parameters that cannot be measured directly. Here, we use the ramp and the interlimb angles as input data to develop a forward fold model that accounts for thickness variations in the forelimb. The relationship between the fold amplitude and fold wavelength is subsequently applied to construct balanced geological cross-sections from surface parameters only and to propose a kinematic restoration of the folding through time. The model can catter for a wide variety of folds, reconstruct the deep architecture of anticlines and deduce the kinematic evolution of the folding with time. We consider three natural examples to validate the variable interlimb angle model. Along-strike thickness variation in the forelimb of the Turner Valley anticline in the Alberta foothills of Canada precisely corresponds to the theoretical values proposed by our model. Reconstruction at depth of the Alima anticline in the southern Tunisian Atlas implies that the decollement level is localised in the Triassic-Liassic series, as highlighted by seismic imaging. The kinematic reconstruction of the Ucero anticline in the Spanish Castilian mountains is also in agreement with the fold geometry derived from two cross-sections. The variable interlimb angle model predicts that the fault-propagation fold can be symmetric, normal asymmetric (with a greater dip value in the forelimb than in the backlimb), or reverse asymmetric (with greater dip in the backlimb) depending on the shortening amount.

Nature, 2009

Geological and geophysical evidence suggests that some crustal faults are weak 1-6 compared to laboratory measurements of frictional strength 7 . Explanations for fault weakness include the presence of weak minerals 4 , high fluid pressures within the fault core and dynamic processes such as normal stress reduction 10 , acoustic fluidization 11 or extreme weakening at high slip velocity . Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions 15 and weak mineral phases are not present in sufficient abundance to satisfy weak fault models 16 , so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro-and nanostructural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths 4,17-19 suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

Journal of Structural Geology, 2001

In fault-related folds that form by axial surface migration, rocks undergo deformation as they pass through axial surfaces. The distribution and intensity of deformation in these structures has been impacted by the history of axial surface migration. Upon fold initiation, unique dip panels develop, each with a characteristic deformation intensity, depending on their history. During fold growth, rocks that pass through axial surfaces are transported between dip panels and accumulate additional deformation. By tracking the pattern of axial surface migration in model folds, we predict the distribution of relative deformation intensity in simple-step, parallel fault-bend and fault-propagation anticlines. In both cases the deformation is partitioned into unique domains we call deformation panels. For a given rheology of the folded multilayer, deformation intensity will be homogeneously distributed in each deformation panel. Fold limbs are always deformed. The flat crests of faultpropagation anticlines are always undeformed. Two asymmetric deformation panels develop in fault-propagation folds above ramp angles exceeding 29Њ. For lower ramp angles, an additional, more intensely-deformed panel develops at the transition between the crest and the forelimb. Deformation in the flat crests of fault-bend anticlines occurs when fault displacement exceeds the length of the footwall ramp, but is never found immediately hinterland of the crest to forelimb transition. In environments dominated by brittle deformation, our models may serve as a first-order approximation of the distribution of fractures in fault-related folds. ᭧