Deformation and diagenetic histories around foreland thrust faults

Abstract

This thesis is concerned with the relationship between deformation and fluid flow along thrust zones. The study was carried out in the Vercors, French Sub-Alpine Chains foreland thrust belt. Study of the thermal alteration of organic matter within the area suggests that prior to west-north-west directed thrusting within the Vercors basin in post middle Miocene times, the rocks now exposed at the surface had not been buried beneath a large thickness of foredeep sediments and remained within the diagenetic realm. Deeper buried levels within the stratigraphy passed into the hydrocarbon generation window prior to thrusting within the Vercors basin. The rocks presently exposed at the surface also remained in the diagenetic realm during and after the thrusting which suggests that thrust sheet loading did not significantly contribute to thermal alteration of organic matter. The structures of the thrust belt may have been possible structural traps for any hydrocarbons which underwent re-migration during the thrusting. The structures have been exhumed by erosion during isostatic uplift. The Rencurel Thrust and overlying Rencurel Thrust Sheet were selected for special study as they are of regional structural importance. The thrust emplaces Urgonian limestones onto Miocene molasse sediments at present erosion levels. The thrust sheet is internally deformed by thrusts and folds. Structural data indicate that the deformation within the thrust sheet and within the Rencurel Thrust Zone occurred during one kinematically linked phase of thrusting. The Rencurel Thrust Zone itself is around 100 metres thick. The higher part of the thrust zone is composed of an array of minor faults developed within the Urgonian. These fault zones are generally less than 10cm wide and are coated in fault gouge. This array of faults is underlain by a gouge zone along the thrust contact between the Urgonian and the Miocene which is several metres thick. The gouge zones were all formed during the action of diffusive mass transfer (DMT) and cataclasis as deformation mechanisms. The wall-rocks to the gouge zones are relatively undeformed by the action of cataclasis. Cataclasis is dilatant and produces fracture porosity which increases the permeability of the fault zones whilst DMT reduces the porosity and permeability of the fault zones due to cement precipitation and pressure dissolution. Cross- cutting relationships between the microstructures indicating the action of cataclasis and DMT, suggest that the porosity and permeability of the fault rocks changed in a complex manner during the incremental deformation. This has important implications for assessing syn-kinematic fluid migration through fault zones. The fault rocks exposed at the surface today are relatively impermeable compared to undeformed wall-rocks away from the fault zone which have permeabilities comparable to those found within hydrocarbon reservoirs. The thrust zone may have been a seal in the sub-surface after the cessation of thrusting but prior to uplift and erosion. Early distributed deformation produced an array of minor faults within the Urgonian. Cataclasis had ceased along these faults before later deformation became localised along the gouge zone which exists along the thrust contact between the Urgonian and the Miocene rocks. Early deformation was accompanied by the migration through fracture porosity of pore waters which were saturated with respect to calcite and had interacted with organic matter which was being thermally altered. This fluid flow system was not connected to fluid flow higher in the stratigraphy which resulted in the precipitation of ferroan calcite within fracture porosity in the Senonian limestones. Late deformation within the thrust zone was accompanied by the migration of hydrocarbons and pore waters saturated with respect to calcite and pyrite. All the pore waters involved in migration through the active thrust zone seem to have migrated up-dip. They migrated from levels in the stratigraphy where organic metamorphism and the maturation of hydrocarbons were occurring to levels in the deformed section which have always remained within the diagenetic realm. Ferroan calcite, pyrite and traces of hydrocarbons have not been found outside the gouge zone along the thrust contact between the Urgonian and Miocene. The fracturing which occurred to open this migration pathway did not re- fracture the inactive minor faults which were impermeable at this time. Fluid migration at this time was confined to beneath the zone of impermeable minor faults in the Urgonian and did not contribute to the diagenesis of the rocks above the thrust zone. Hydrocarbons could not have entered the hanging-wall anticline above the thrust zone from this migration pathway. The fracturing at this time did not produce connected fracture networks pervasively throughout the thrust zone which suggests that the deformation may not have released large amounts of energy in the form of seismic waves.