133 research outputs found
Glossary of normal faults
No full textIncreased interest in normal faults and extended terranes has led to the development of an increasingly complex terminology. The most important terms are defined in this paper, with original references being given wherever possible, along with examples of current usage
Large lateral ramps in the Eocene Valkyr shear zone: extensional ductile faulting controlled by plutonism in southern British Columbia: discussion
No full textThe nature and tectonic significance of fault-zone weakening: An introduction
No full textFault zones control the location, architecture and evolution of a broad range of geological features, act as conduits for the focused migration of economically important fluids and, as most seismicity is associated with active faults, they also constitute one of the most important global geological hazards. In general, the repeated localization of displacements along faults and shear zones, often over very long time scales, strongly suggests that they are weak relative to their surrounding wall rocks. Geophysical observations from plate boundary faults such as the San Andreas fault additionally suggest that this fault zone is weak in an absolute sense, although this remains a controversial issue. Out understanding of fault-zone structure and mechanical behavior derive from three main sources of information: (1) Studies of natural fault zones and their deformation products (fault rocks); (2) seismological and neotectonic studies of currently active natural fault systems; (3) laboratory-based deformation experiments using rocks or rock-analogue materials. These provide us with a basic understanding of brittle faulting in the upper crust of the Earth where the stress state is limited by the frictional strength of networks of faults under the prevailing fluid-pressure conditions, under the long-term loading conditions typical of geological fault zones, poortly understood phenomena sucha as subcritical crack growth in fracture process zones are likely to be a major importance in controlling both fault growth and strength. Grain-size reduction in highly strained fault rocks produced in the plastic-viscous and deeper parts of frictional regime can lead to changes in deformation mechanisms and relative weakening that can account for the localization of deformation and repeated reactivation of crustal faults. Our understanding the interactions between deformation mechanism, metamorphic processes and the flow of chemically active fluids is a key area for future study. An improved understanding of how fault- or shear-zone linkages, stregth and microstructure evolve over large changes in finite strain will ultimately lead to the development of geologically more realistic numerical models of lithosphere depormation that incorporate displacements concentrated into narrow, weaker fault zones
Fault rock evolution and fluid flow in sedimentary basins
No full textStructural 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
Convergence - related 'dynamic spreading' in a mid-crustal ductile thrust zone: a possible orogenic wedge model.
No full textDeformation in an accretionary melange, Alexander Island, Antarctica
No full textAlexander Island contains several belts of melange in a wide accretionary complex. One melange belt in the northwest of the island incorporates both oceanic and trench-fill material. It evolved by many different deformation mechanisms: dispersed independent particulate flow (IPF) and limited cataclasis at shallow levels; and diffusion mass transfer (DMT) and limited crystal plastic processes at deeper levels. Fluid pressures may have risen due to the subduction of young hot oceanic crust, which probably affected the structural evolution of the region by controlling the strength of the decollement and hence the taper of the accretionary prism
Fault Seal Prediction – Uncertainities in Petrophysical Properties and Calibration of Methodologies
No full textSeismic attribute maps; application to structural interpretation and fault seal analysis in the North Sea Basin
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