Layer parallel shortening and cataclastic flow by fractures in the Permian Kapp Starostin Formation, Mediumfjellet, Spitsbergen
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This study presents a structural analysis, from macro- to micro-scale, of the Permian carbonates and spiculites of the Kapp Starostin Formation. The analysis was conducted to better understand the fracture development and deformation in a Fold and Thrust Belt environment, located to the Mediumfjellet that is displaying a major thrust-stack in the thin-skinned part of the Paleogene West-Spitsbergen Fold and Thrust Belt. Methods include regional mapping and cross-section construction, coupled with Lidarscan interpretations in the software Lime. These data forms a base for calculations of layer parallel shortening. Field methods include lithostratigraphically logging with additional thin section studies. Further, scanlines record fracture frequency (fractures per meter) both along strike and dip with respect to bedding, and reveal fracture orientations, terminations and fill. The Kapp Starostin Formation comprises mainly spiculitic units with some silicified carbonate and shale beds. Permian strata are deformed along four well-known major thrusts (M1-M3, and G), but the observation of a new, major structure in the hangingwall of the M2, suggests the presence of another thrust in the area, termed the M2 upper splay. The bryozoan limestone has higher fracture frequency than dolomite, brachiopod limestone, spiculite and shale, and may have more fractures in thinner beds, in contrast to the other lithologies. The fracture data shows five main fracture populations striking; (i) nearly E-W, (ii) NNW-SSE, (iii) NW-SE, (iv) NE-SW to NNE-SSW and (v) nearly N-S. The findings of this study indicate a total layer parallel shortening averaging a minimum of 64 percent/m, with highest shortening in the hinge zone of the folds. The characteristics of the fracture network suggest folding by cataclastic flow, in most layers characterized by frictional flow on fractures rather than flexural slip along layers. The fractured tight Permian rocks in Mediumfjellet makes the studied strata an analogue for fractured reservoirs in fold-thrust belts.