Fault (geology)

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

Earthquake mechanisms in the Hellenic Trench near Crete

Abstract: SUMMARY In the Hellenic Trench south of Crete convergence between the southern Aegean Sea and Africa occurs at a rate of at least 60 mm yr-1. Previously published first motion fault plane solutions show a variety of different fault orientations and types, but are not well constrained. Furthermore, the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist. Nonetheless, the mechanisms of these earthquakes have strongly influenced views of the tectonics in the Hellenic Trench. We have improved estimates of the fault parameters and focal depths for 14 of these earthquakes, using long-period P- and SH-waveforms. The earthquake mechanisms fall into four groups: (a) normal faults with a N-S strike in the over-riding material above the subduction zone; (b) low-angle thrusts with an E-W strike at a depth of about 40 km; (c) high-angle reverse faults with the same strike but shallower focal depths than (b); (d) events within the suducting lithosphere with approximately E-W P axes. The thrusting in groups (b) and (c) is probably the mechanism by which the sediments of the Mediterranean Sea underplate and uplift Crete. These events have slip vectors in the direction 025 ± 12° which represents the convergence direction between Crete and Africa along the SW-facing boundary of the Hellenic Trench. One of the events in group (d) occurred beneath the Mediterranean Ridge and involved high-angle reverse faulting with a WNW-ESE P axis: almost perpendicular to the direction of shortening deduced from folds at the surface. The Mediterranean Sea floor in this region appears to be in a state of E-W compression, for reasons that are not clear.

Evolution of Salt-Related Structures in Compressional Settings

Abstract: Sandbox experiments analyzed by computerized X-ray tomography provide relevant models of salt-related contractional structures and improve understanding of the relative importance of the many parameters influencing structural style. In front of thin-skinned fold and thrust belts, the salt layers provide decollement surfaces, which allow the horizontal strain to propagate far toward the edge of the foreland. As shortening increases, older structures forming in front of the system can be overtaken by out-of-sequence faulting and folding. The very low friction coefficient of salt layers induces a symmetric stress system. This promotes pop-up structures rather than asymmetric thrust faults. Salt extrusions are related to former salt ridges or salt walls squeezed by compression and dragged along thrust planes or to local low-pressure zones along crestal tear faults during folding. The salt that spreads out from the fault is rapidly dissolved. The resultant surface collapse structures are progressively filled by a mixture of Recent sediments and reprecipitated evaporites. Salt pinch-outs, either depositional or structural in origin, are a major controlling factor of the deformation geometry in fold and thrust belts. They trigger, either locally or regionally, contractional structures, including folds and thrusts, in rapidly prograding passive margins deforming by gravity gliding. In this structural context, salt pinch-outs also thicken due to differential loading and gravity spreading. The structural complexity in inverted grabens or in basement-involved orogenic belts where salt is present is the outcome of many factors. The salt thickness, the preexisting extensional structures, the synsalt and postsalt rifting, and the related distribution of older salt structures and sediments all localize folds and thrusts during later contraction. The relative orientation of the former extensional structures to the younger shortening structures largely controls the style of inversion (fault reactivation versus forced folding and short-cuts). Salt is the main detachment level between the folded cover rocks and the underlying faulted basement. However, secondary detachments, which are common in the overburden, add further complexities--triangle zones in the cores of anticlines and fish-tailed periclinal terminations.

Roughness and wear during brittle faulting

Abstract: In many natural fault systems, the thickness of gouge and breccia increases approximately linearly with displacement. In contrast, many experimental faults show non linear thickness/displacement relationships. The linear relationship for natural faults has been explained in the past using engineering models for adhesive or abrasive wear. Non linear relationships for experimental faults have not been explained. A model for wear during brittle faulting which considers the scaling of surface roughness can successfully describe the difference between wear on experimental faults and wear on natural faults. We suggest the linear relationship for natural faults results from the approximately self-similar roughness of the fault surfaces. Experimental faults do not generally follow linear relationships because the roughness of ground surfaces normally used in experimental studies scales differently than the roughness of natural rock surfaces. A simple model which assumes that the volume of wear material created is proportional to the volume of mismatch between the surfaces can explain the differences between wear on experimental faults and wear on natural faults. For ground surfaces of experimental samples, the volume of mismatch is independent of the total slip because at the largest scales these surfaces are flat. In contrast, for natural, self-similar surfaces the volume of mismatch increases with slip, because slip isolates larger and larger asperities from their original positions in the opposite surface. Natural and experimental faults evolve differently because of the difference in scaling of their respective surface roughnesses.

Smectite Dehydration--Its Relation to Structural Development and Hydrocarbon Accumulation in Northern Gulf of Mexico Basin

Abstract: A comparison of clay diagenesis data obtained from a study of Tertiary shales from the Brazos-Colorado River system of Texas, the Mississippi River system of Louisiana, and the Niger River system of Nigeria illustrates significant differences in temperature intervals over which smectite diagenesis occurs. The threshold temperature required to initiate diagenesis ranges from about 160°F (71°C) in Mississippi River sediments to more than 300°F (150°C) in the Niger delta. Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partially trapped and released slowly through time. In either situation, the water can act as a vehicle for hydrocarbon migration p ovided hydrocarbons are present in a form and in sufficient quantities to be transported. Observations from the northern Gulf of Mexico basin indicate a close relation between buildup of high fluid pressure and the smectite-illite transformation process. Abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow, and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed. The depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting. Dips of some faults in Texas have been observed to change abruptly within the interval of smectite diagenesis, and some faults formed in the overpressured Miocene and younger sections become beddingplane types at depths where the temperature is near that required for thermal generation of petroleum. Although these faults may be important for fluid redistribution in shallow sandstone-shale sections, they are a minor factor in moving hydrocarbons out of shale below the faults in much of the Texas offshore area. Fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from fault trends in the sub-Tertiary section is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern Gulf of Mexico basin. This flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present. Shale tectonism is also the primary mechanism for structural development in the Tertiary section of the Niger delta; however, seismic data indicate that the rate of dip change of seaward-dipping listric growth faults is commonly less than that observed in Texas where dips as low as 10°-15° can occur at depths as shallow as 10,000-15,000 ft (3,048-4,572 m). Syndepositional faults in Nigeria are formed in sandstone-shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section. Subtle differences in structural styles in the Tertiary sections of Texas and Nigeria are probably the result of differences in clay composition of the shaly sections being deformed