Dilatant normal faulting in jointed cohesive rocks: a physical model study

Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity are not well understood. We present an analogue modeling study using cohesive powder with pre-formed joint sets in the upper layer, varying the angle between joints and a rigid basement fault. We analyze interpreted map-view photographs at maximum displacement for damage zone width, number of connected joints, number of secondary fractures, degree of segmentation and area fraction of massively dilatant fractures. Particle imaging velocimetry provides insight into the deformation history of the experiments and illustrates the localization pattern of fault segments. Results show that with increasing angle between joint-set and basement-fault strike the number of secondary fractures and the number of connected joints increase, while the area fraction of massively dilatant fractures shows only a minor increase. Models without pre-existing joints show far lower area fractions of massively dilatant fractures while forming distinctly more secondary fractures

Evolution of a highly dilatant fault zone in the grabens of Canyonlands National Park, Utah, USA - Integrating fieldwork, ground-penetrating radar and airborne imagery analysis

Abstract. The grabens of Canyonlands National Park are a young and active system of sub-parallel, arcuate grabens, whose evolution is the result of salt movement in the subsurface and a slight regional tilt of the faulted strata. We present results of ground-penetrating radar (GPR) surveys in combination with field observations and analysis of high-resolution airborne imagery. GPR data show intense faulting of the Quaternary sediments at the flat graben floors, implying a more complex fault structure than visible at the surface. Direct measurements of heave and throw at several locations to infer fault dips at depth, combined with observations of primary joint surfaces in the upper 100 m, suggest a highly dilatant fault geometry. Sinkholes observed in the field as well as in airborne imagery give insights in local dilatancy and show where water and sediments are transported underground. Based on correlations of paleosols observed in outcrops and GPR profiles, we argue that either the grabens in Canyonlands National Park are older than previously assumed or that sedimentation rates were much higher in the Pleistocene. This is the final version of the article. It first appeared from Copernicus Publications via http://dx.doi.org/10.5194/se-6-839-2015 The supplement related to this article is available online at http://dx.doi.org/10.5194/se-6-839-2015-supplemen

Late burial to early tectonic quartz veins in the periphery of the High-Ardenne slate belt (Rursee, North Eifel, Germany)

A detailed structural mapping and geometrical analysis of distinct bedding-(sub)perpendicular and bedding-parallel quartz veins has been performed in the northeastern part of the High-Ardenne slate belt (Rursee, North Eifel, Germany), with the aim to reconstruct the local fracturing/veining history. The structural relationship of these two types of veins as well as their relationship with cleavage, folds and faults allows attributing a pre- to early-Variscan age to these veins. The first type of veins is oriented (sub)perpendicular to bedding and consists of several, mutual cross-cutting generations, which clearly predate Variscan deformation. The second type of veins, bedding-parallel veins, post-dates the bedding-(sub)perpendicular veins and reflects bedding-parallel thrusting at the onset of Variscan deformation, predating folding. Subsequently, during progressive Variscan compression both types of veins were passively folded within characteristic, NW-vergent, overturned folds. Locally, due to flexural slip folding, reactivation along the bedding-parallel veins may have taken place.status: publishe

Mullions in the High-Ardenne Slate Belt (Belgium): numerical model and parameter sensitivity analysis

In the High-Ardenne Slate Belt (Belgium), pre-existing layer-perpendicular quartz veins in psammite layers acted as mechanical boundaries due to the difference in competence between vein quartz and psammite. This caused the formation of mullions during layer-parallel shortening. In this paper, the process of mullion formation is modelled using finite-element techniques. A parameter sensitivity analysis of the mullion model demonstrates that the stress exponent of psammite, the total horizontal shortening of the psammite layers, the initial aspect ratio of the psammite segments between the veins and the competence contrast between psammite and vein quartz are the controlling parameters for the shape of the mullions. Our results suggest that the morphology of the mullions can be used to constrain the rheology of psammite deforming in the middle crust. Moreover, the parameter sensitivity analysis illustrates the range of layer-parallel shortening/extension structures associated with layer-perpendicular quartz veins that can be expected in nature (e.g. dogbones, inverted mullions) and creates future perspective for the use of the model as a paleorheological gauge for these structures. (C) 2004 Elsevier Ltd. All rights reserved.status: publishe

Early vein generations in the High-Ardenne slate belt (Belgium, Germany): the earliest manifestations of the Variscan orogeny?

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