![]() Smear factors modified to take into account tapering of shale smears on fault plane Gouge ratios proportion of shale in a stratigraphic interval equal to the separation Modelling gouge production in clay rich successions:ĭepends on proportion of sand to shale between the offset horizons. Gouge, unlike breccia and cataclasite, is relatively impermeable. When fault cuts more ductile rocks, typically shales, gouge may form - very fine, grained, and may have some fabric, though typically very variable in orientation. Note - formation of breccia involves a dilation, so favoured by low overall pressure, high fluid pressureīreccia zones are typically permeability conduits in mineralized areas - site of significant economic mineral deposition in hydrothermal systemsīut may be cemented with minerals - silica etc, to produce harder, impermeable rock. Because fragments are angular and are displaced, typically very porous and permeable.Ĭataclasite is sometimes used to describe fine grained material without fabric and with rather even-size fragments. Distinguish into breccia series and gouge based on presence or absence of clay in the matrixīreccia series: Megabreccia - Breccia - Microbreccia. Nature of fault rocks varies with rate and amount of slip and with the temperature and pressure, which are mainly themselves functions of depth.Īt shallow depths (<10-15 km), processes are mainly brittle: produce breccia series rocks and gouge.Ĭataclastic rocks are products of brittle fracturing - include angular fragments of wall rocks, with no distortion of individual crystals. Magnetic effect of faults is usually shown in offset, sometimes in shearing, of pre-existing magnetic features Large basement-cutting faults may show gravity effect, because they control the thickness of low-densityĭiagram of major cordilleran uplift with gravity Twiss& Moores fig 6.3 Map view of reverse faults on time-structure or depth map Map view of normal separation faults on time-structure or depth map Seismic profiles through faults typically show offset of strata, rarely show reflections from a fault itself Results of migration Twiss & Moores fig 2.15 (2 parts) Imaging faults at large scale: geophysical methodsĭiagram of stacking: reflections at different angle are combined to increase signal strengthĭiagram of migration Fig 2.3.1 Twiss&Moores Picture of real thing from N sea, contoured for slip ![]() In weakly faulted bodies of rock, where faults rarely intersect, slipped region is typically elliptical, with slip decreasing towards the edges of the slipped area. Typically both slip and separation vary over the area of a fault. Typically has components parallel to dip and parallel to strike (dip slip and strike slip componenets) Slip is a vector joining equivalent points separated by a fault. Note that separation is distinct from slip. ![]() Strike separation may be left lateral or right lateral (sinistral or dextral) Figure shows dip separation (may be classified as normal or reverse) Separation refers to the distance in some specified direction between the edges of a layer severed by the fault. Planes of brittle failure, with displacement parallel to plane much larger than perpendicular to the planeįault orientation specified by strike and dip. Fault Basics (review) *Info on Earthquake in India Friday morning Local copy of seismicity map Reading on faults: Chapter 4 of the textbook by Twiss and Moores.
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