Fault rock evolution and fluid flow in sedimentary basins

Abstract

Structural studies have been undertaken in two extensional fault regimes associated with post-Caledonian basin-forming events in northern Scotland. A combination of detailed mapping and microstructural analysis has revealed the deformation processes and mechanisms involved in fault rock evolution and fluid flow associated with extensional faulting in upper crustal conditions. Intrabasinal fault rock evolution has been investigated in the Orcadian Basin, NE Scotland, which developed in Old Red Sandstone (ORS) times, soon after cessation of the Caledonian Orogeny. High pore fluid pressures developed in lower Middle ORS lacustrine facies sediments as a result of overpressuring due to rapid subsidence in the early stages of basin evolution. This facilitated gravity-driven movement of sediments in the hangingwalls of tilted half-grabens, resulting in the development of bedding parallel detachment horizons. These horizons contain shear sense indicators showing displacement to the W-WNW, whilst normal faults which detach onto these horizons show NW-SE extension directions. Microstructures indicate that displacement within the bedding parallel detachment horizons was accommodated by independent particulate flow processes in weakly lithified sediments. The Scapa Fault System was active in upper Middle ORS to Upper ORS times during deposition of the fluvial Scapa Sandstone. Microstructures in the Scapa Sandstone in the hangingwall of the North Scapa Fault indicate that this early faulting led to extreme grain size reduction by a combination of grain boundary and transgranular fracture processes. The cataclasis, together with subsequent precipitation of illite cement up to one metre from the fault plane resulted in the sealing of the fault early in the diagenetic history of the sediment. Subsequent uplift of the Orcadian Basin, most probably during Carboniferous times, resulted in a range of inversion geometries. In the lower Middle ORS lacustrine facies rocks, thrusts exploited the bedding parallel detachment horizons, and folds and reverse faults developed as a result of buttressing against the earlier normal faults. The presence of vein arrays associated with these later reverse faults suggests the existence of high pore fluid pressures. Bitumen in these veins indicates the mobility of hydrocarbons at the time of deformation. The North Scapa Fault was reactivated in a sinistral, oblique-slip sense during the inversion event. Fracture arrays and narrow cataclastic zones outside the previously developed sealed domain provided pathways for the migration of mature hydrocarbons. The East Scapa Fault reactivated in a reverse sense, and also contains fault rocks which record the presence of hydrocarbons at this time. Permo-Carboniferous dykes on Orkney are deformed during later dextral movements on the Great Glen fault system, which further reactivated the East Scapa Fault in a (dextral) transtensional sense. The development of fault rocks along the East Scapa Fault at this time is complex and heterogeneous, and is dependent on fault geometry and kinematics. Basin-margin faults exposed on the NW Scottish Mainland are most probably related to extension during evolution of the Minch Basin to the west of Scotland. The steeply-dipping extensional faults cut through Caledonian thrust sheets in Sango Bay, Durness. The resulting cataclastic deformation in a quartzite with an originally mylonitic microstructure has allowed assessment of the influence of initial microstructure on the cataclastic grain size reduction processes. The evolution of the fault rocks in terms of clast size, and clast/matrix ratios is not a simple function of displacement magnitude on the faults. Detalied microstructural investigation in the quartzite thrust sheet reveals a range of cataclastic fault rocks, from clast dominated microbreccias to matrix dominated ultracataclasites. The recrystallised grain size and the sub-grain size in the original mylonite appear to control the development of the fine-grained matrix in the microbreccias and cataclasites by locating fracture along grain and sub-grain boundaries. Further grain size reduction generating the ultracataclasites and the finer-grained matrix zones in the microbreccias is dominated by transgranular fracturing. The host rock clasts present in the fault zones in the quartzite show a significant increase in dislocation density indicating that a component of low temperature crystal plasticity is associated with the faulting. In addition, the fault rocks show evidence of partial cementation by the growth of quartz and carbonate cements. This emphasises the importance of fluids during healing of the fault zone