Fault (geology)

About: Fault (geology) is a research topic. Over the lifetime, 26732 publications have been published within this topic receiving 744535 citations.

High‐velocity frictional properties of a clay‐bearing fault gouge and implications for earthquake mechanics

Abstract: [1] Frictional properties of natural kaolinite-bearing gouge samples from the Median Tectonic Line (SW Japan) have been studied using a high-velocity rotary shear apparatus, and deformed samples have been observed with optical and electron (scanning and transmission) microscopy. For a slip velocity of 1 m s−1 and normal stresses from 0.3 to 1.3 MPa, a dramatic slip-weakening behavior was observed. X-ray diffraction analysis of deformed samples and additional high-velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip. The critical slip-weakening distance Dc is of the order of 1 to 10 m. These values are extrapolated to higher normal stresses, assuming that Dc is rather a thermal parameter than a parameter related to a true characteristic length. The calculation shows that dimensionally, Dc ∝ 1/σn2, where σn is the normal stress applied on the fault. The inferred Dc values range from a few centimeters at 10 MPa normal stress to a few hundreds of microns at 100 MPa normal stress. Microscopic observations show partial amorphization and dramatic grain size reduction (down to the nanometer scale) localized in a narrow zone of about 1 to 10 μm thickness. Fracture energy Gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction, and energies of mineralogic transformations. We show that most of the fracture energy is either converted into heat or radiated energy. The geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones, which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust.

Propagation of rifting along the Arabia‐Somalia Plate Boundary: The Gulfs of Aden and Tadjoura

Abstract: The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large-scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia-Somalia relative motion vector, began about 30 Myr ago between the Error and Sharbithat ridges. It was an episodic process, with stages of rapid propagation, mostly at rates >10 cm/yr, interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the Arabia-Somalia plate. The longest pause was at the Shukra-El Sheik discontinuity (≈45°E), where the ridge tip stalled for ≈13 Myr, between ≈17 and ≈4 Ma. West of that discontinuity, rifting and spreading took place at an azimuth (≈N25°±10°E) and rate (1.2±0.3 cm/yr) different from those of the global Arabia-Somalia motion vector (≈N39°, ≈1.73 cm/yr), implying an additional component of movement (N65°±10°E, 0.7±0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra-El Sheik, the typical oceanic ridge gives way to a narrow, WSW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which result from normal faulting and fissuring along N110°–N130°E trends. All the segments step to the left southwestward, mostly through oblique transfer zones with en echelon normal faults. Only two segments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches onland, is composed of two parallel subrifts (Iboli, Ambabbo) that propagated northwestward and formed in succession. The most recent, southwestern subrift (Ambabbo) represents the current tip of the Aden ridge. We propose a mechanical model in which the large-scale propagation of the ridge followed a WSW trending zone of maximum tensile stress, while the small-scale propagation of its NW trending segments was dictated by the orientation of that stress. Oblique propagation was a consequence of passive lithospheric necking of the Arabia-Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea-Ethiopian rift, above the Afar plume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward because of confinement within the necking zone. Self-sustaining, plate-scale necking may explain why the Aden ridge did not connect with the Red Sea through Bab El Mandeb but continued straight into Afar.

GPS estimate of relative motion between the Caribbean and South American plates, and geologic implications for Trinidad and Venezuela

Abstract: Global Positioning System (GPS) data from eight sites on the Caribbean plate and five sites on the South American plate were inverted to derive an angular velocity vector describing present-day relative plate motion. Both the Caribbean and South American velocity data fit rigid-plate models to within ±1–2 mm/yr, the GPS velocity uncertainty. The Caribbean plate moves approximately due east relative to South America at a rate of ∼20 mm/yr along most of the plate boundary, significantly faster than the NUVEL-1A model prediction, but with similar azimuth. Pure wrenching is concentrated along the approximately east-striking, seismic, El Pilar fault in Venezuela. In contrast, transpression occurs along the 068°-trending Central Range (Warm Springs) fault in Trinidad, which is aseismic, possibly locked, and oblique to local plate motion.

Thin-Skinned Tectonics in the Plateau and Northwestern Valley and Ridge Provinces of the Central Appalachians

Abstract: Folds in the Appalachian Plateau and northwestern Valley and Ridge provinces of the Central Appalachians involve allochthonous Paleozoic rocks which have been transported westward along nonoutcropping low-angle detachment thrust faults. Deep wells in the Plateau and along the Appalachian Structural Front southeast of the Plateau penetrate major overthrusts which form part of the sole fault system. The great folds along the arcuate Structural Front were formed in passive response to steplike upward shearing of the detachment thrusts from a mid-Cambrian shale glide zone in the Valley and Ridge province up through competent carbonate rocks into glide zones in the Upper Ordovician or Upper Silurian beneath the Plateau. The folds evidence shortening of the stratified sequence superincumbent on the sole thrusts. Anticlines in the Plateau and in local parts of the Valley and Ridge provinces terminate or change trend abruptly in groups along west-to-northwest-trending lineaments. Some of the lineaments appear to reflect tear-faulting along the margins of semi-independently advancing thrust blocks, whereas others, along which no faulting is noted, seem to be zones at which the sole thrusts change stratigraphic position along the structural strike. The termination of sets of anticlines along the lineaments and the absence of uniformly offset counterparts across the lineaments in adjacent blocks indicate that a large part of the net subhorizontal translation of the thrust sheets occurred prior to most of the folding, very early in the deformation. Both limbs of most of the anticlines in the north-western two thirds of the folded Plateau region are broken by synclineward-dipping thrust faults at the Lower Devonian level. These thrust faults, which uplift both anticlinal flanks above a relatively depressed former axial zone, appear to flatten into bedding in evaporites of the Upper Silurian Salina group. The faults facilitate movement of material up and out of the tightening synclines onto the anticlines and are thus simply an extension of the flexural-slip folding process. Structural relief of the anticlines fault-folded in this manner is significantly reduced beneath the Salina decollement zone. The tectonic style and mode of deformation of the “folded” Central Appalachians is, thus, practically identical with that observed in the “thrust-faulted” Southern Appalachians. The only important difference between the two regions is that the upward-shearing segments, or “toes,” of thrust sheets in the Southern Appalachians have been exposed by erosion, whereas the toes of the major Central Appalachian thrust sheets are stil covered by a mile or more of stratified rocks.