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Major Faults (major + fault)
Selected AbstractsFirst evidence of post-seismic deformation in the central Mediterranean: Crustal viscoelastic relaxation in the area of the 1980 Irpinia earthquake (Southern Italy)GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2003G. Dalla Via SUMMARY Comparison between measured vertical displacements obtained from two levelling campaigns performed in 1981 and 1985 in the epicentral area of the 1980 Irpinia earthquake (MS= 6.9) and predictions from viscoelastic Earth models reveal the occurrence of post-seismic deformation due to stress relaxation in the ductile part of the crust. Two regions of broad uplift and subsidence, accumulated during the time interval, characterize the deformation pattern in the footwall and hangingwall of the major fault. The spatial wavelength of the deformation pattern favours relaxation occurring in the lower crust rather than in a weak upper-mantle: the uplift in the footwall explains the 30 mm of upwarping of the crust measured along the levelling line crossing the area where the fault pierces the Earth's surface. [source] Variation in peak P,T conditions across the upper contact of the UHP terrane, Dabieshan, China: gradational or abrupt?JOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2006Y. SHI Abstract The Southern Dabieshan Terrane (SDT) has previously been divided into high-pressure (HP) and ultrahigh-pressure (UHP) terranes, and its regional extent and the tectonic nature of its boundaries are hotly debated topics. In this study, an eclogite-bearing area of 100 km2 near Taihu is mapped in detail, and divided into Northern, Middle and Southern Zones on the basis of lithological characteristics. The Northern Zone consists of epidote-biotite gneiss and eclogite blocks, the Middle Zone includes granitic gneiss, biotite gneiss, eclogites and amphibolite, and the Southern Zone is composed mainly of garnet-bearing mica schist. The eclogites occur mainly as lens or blocks in the Northern and Middle Zones. The peak P,T conditions for 61 eclogite samples across the area are estimated using the Grt-Cpx Fe2+ -Mg thermometers and the Grt-Cpx-Phe barometers. The results indicate three different P,T regions: 2.82,4.09 GPa/759,942 °C in the Northern Zone, and 2.00,3.54 GPa/641,839 °C in the granitic gneiss and 1.38,2.36 GPa/535,768 °C in the biotite gneiss from the Middle Zone. Combined with the spatial distribution of eclogites across the area, the P,T values for eclogites increase continuously from the south to the north, defining a reference ,geotherm' of 5 °C km,1. However, some unreasonable apparent gradients can be established along two south,north profiles across the area, and display a P,T difference between the Northern and Middle zones. On the basis of the average P,T data for eclogites across the area, a gap of at least 0.3 GPa/20 °C exists between the Northern and Middle zones. By contrast, the P,T values of eclogites from the Middle zone show a coherent pattern with transitional characteristics from HP in the south to UHP in the north. We suggest that the SDT was a coherent slab during subduction, and was broken up by a major fault during exhumation, which was formed under UHP metamorphic conditions. [source] Extensional development of the Fundy rift basin, southeastern CanadaGEOLOGICAL JOURNAL, Issue 6 2009Martha O. Withjack Abstract The Fundy rift basin of Nova Scotia and New Brunswick, Canada, is part of the Eastern North American rift system that formed during the breakup of Pangaea. Integrated seismic-reflection, field, digital-elevation and aeromagnetic data indicate that the Fundy rift basin underwent two phases of deformation: syn-rift extension followed by post-rift basin inversion. Inversion significantly modified the geometries of the basin and its rift-related structures. In this paper, we remove the effects of inversion to examine the basin's extensional development. The basin consists of three structural subbasins: the Fundy and Chignecto subbasins are bounded by low-angle, NE-striking faults; the Minas subbasin is bounded by E- to ENE-striking faults that are steeply dipping at the surface and gently dipping at depth. Together, these linked faults form the border,fault system of the Fundy rift basin. Most major faults within the border,fault system originated as Palaeozoic contractional structures. All syn-rift units imaged on seismic profiles thicken towards the border,fault system, reflecting extensional movement from Middle Triassic (and possibly Permian) through Early Jurassic time. Intra-rift unconformities, observed on seismic profiles and in the field, indicate that uplift and erosion occurred, at least locally, during rifting. Based on seismic data alone, the displacement direction of the hanging wall of the border,fault system of the Fundy rift basin ranged from SW to SE during rifting. Field data (i.e. NE-striking igneous dykes, sediment-filled fissures and normal faults) indicate NW,SE extension during Early Jurassic time, supporting a SE-displacement direction. With a SE-displacement direction, the NE-striking border,fault zones of the Fundy and Chignecto subbasins had predominantly normal dip slip during rifting, whereas the E-striking border,fault zone of the Minas subbasin had oblique slip with left-lateral and normal components. Sequential restorations of seismic-reflection profiles (coupled with projections from onshore geology) show that the Fundy rift basin underwent 10,20,km of extension, most of which was accommodated by the border,fault system, and was considerably wider and deeper prior to basin inversion. Post-rift deformation tilted the eastern side of the basin to the northwest/north, producing significant uplift and erosion. Copyright © 2009 John Wiley & Sons, Ltd. [source] Basement controls on Acadian thrusting and fault reactivation along the southern margin of the Welsh BasinGEOLOGICAL JOURNAL, Issue 5 2009D. I. Schofield Abstract Inversion of the Lower Palaeozoic Welsh Basin during the Early to Mid-Devonian is generally thought to have been achieved by a combination of approximately co-axial shortening and transcurrent movement along major faults to produce a strongly partitioned transpressional strain. However, new field observations from Rhydwilym in southwest Wales reveal superimposed deformations which indicate that thrust tectonics operated within the Welsh Borderland Fault System (WBFS) along this segment of the basin margin. An increasing regional magnetic response towards the south suggests that contrasting depth to magnetic basement across the WBFS may have buttressed basin shortening and provided the focus for thrusting and late-Caledonian or proto-Variscan reactivation. British Geological Survey © Nerc 2009. All rights reserved. [source] Geological evolution and structural style of the Palaeozoic Tafilalt sub-basin, eastern Anti-Atlas (Morocco, North Africa)GEOLOGICAL JOURNAL, Issue 1 2008E. A. Toto Abstract The Tafilalt is one of a number of generally unexplored sub-basins in the eastern Anti-Atlas of Morocco, all of which probably underwent a similar tectono-stratigraphic evolution during the Palaeozoic Era. Analysis of over 1000,km of 2-D seismic reflection profiles, with the interpretation of ten regional seismic sections and five isopach and isobath maps, suggests a multi-phase deformation history for the Palaeozoic-aged Tafilalt sub-basins. Extensional phases were probably initiated in the Cambrian, followed by uniform thermal subsidence up to at least the end of the Silurian. Major extension and subsidence did not begin prior to Middle/Upper Devonian times. Extensional movements on the major faults bounding the basin to the north and to the south took place in synchronisation with Upper Devonian sedimentation, which provides the thickest part of the sedimentary sequence in the basin. The onset of the compressional phase in Carboniferous times is indicated by reflectors in the Carboniferous sequence progressively onlapping onto the Upper Devonian sequence. This period of compression developed folds and faults in the Upper Palaeozoic-aged strata, producing a structural style characteristic of thin-skinned fold and thrust belts. The Late Palaeozoic units are detached over a regional décollement with a northward tectonic vergence. The folds have been formed by the process of fault-propagation folding related to the thrust imbricates that ramp up-section from the décollement. Copyright © 2007 John Wiley & Sons, Ltd. [source] A Bayesian approach to estimating tectonic stress from seismological dataGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2007Richard Arnold SUMMARY Earthquakes are conspicuous manifestations of tectonic stress, but the non-linear relationships between the stresses acting on a fault plane, its frictional slip, and the ensuing seismic radiation are such that a single earthquake by itself provides little information about the ambient state of stress. Moreover, observational uncertainties and inherent ambiguities in the nodal planes of earthquake focal mechanisms preclude straightforward inferences about stress being drawn on the basis of individual focal mechanism observations. However, by assuming that each earthquake in a small volume of the crust represents a single, uniform state of stress, the combined constraints imposed on that stress by a suite of focal mechanism observations can be estimated. Here, we outline a probabilistic (Bayesian) technique for estimating tectonic stress directions from primary seismological observations. The Bayesian formulation combines a geologically motivated prior model of the state of stress with an observation model that implements the physical relationship between the stresses acting on a fault and the resultant seismological observation. We show our Bayesian formulation to be equivalent to a well-known analytical solution for a single, errorless focal mechanism observation. The new approach has the distinct advantage, however, of including (1) multiple earthquakes, (2) fault plane ambiguities, (3) observational errors and (4) any prior knowledge of the stress field. Our approach, while computationally demanding in some cases, is intended to yield reliable tectonic stress estimates that can be confidently compared with other tectonic parameters, such as seismic anisotropy and geodetic strain rate observations, and used to investigate spatial and temporal variations in stress associated with major faults and coseismic stress perturbations. [source] Regional GPS data confirm high strain accumulation prior to the 2000 June 4 Mw= 7.8 earthquake at southeast SumatraGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001G. W. Michel Summary Site velocities derived from repeated measurements in a regional GPS network in Southeast Asia help to constrain the motion of tectonic blocks as well as slip rates along major faults in the area. Using 3-D forward dislocation modelling, the influence of seismic elastic loading and unloading on the measured site motions are approximated. Results suggest that the northwestern Sunda arc is fully coupled seismogenically, whereas its eastern part along Java shows localized deformation. Higher horizontal velocity gradients than expected from the modelling of a fully coupled plate interface west of Manila in the Philippines suggest that deformation may be localized there. Assuming that geodetically derived convergence represents long-term rates, accumulated geodetic moments are compared to those derived using seismic data from 1977 to 2000 (Harvard CMT catalogue). If areas displaying localized deformation are dominated by creep processes, the largest difference between accumulated and seismically released deformation is located where the 2000 June 4 Mw = 7.8 Sumatra earthquake occurred. [source] Seismic reflection imaging of active offshore faults in the Gulf of Corinth: their seismotectonic significanceBASIN RESEARCH, Issue 4 2002A. Stefatos ABSTRACT High resolution seismic reflection surveys over one of the most active and rapidly extending regions in the world, the Gulf of Corinth, have revealed that the gulf is a complex asymmetric graben whose geometry varies significantly along its length. A detailed map of the offshore faults in the gulf shows that a major fault system of nine distinct faults limits the basin to the south. The northern Gulf appears to be undergoing regional subsidence and is affected by an antithetic major fault system consisting of eight faults. All these major faults have been active during the Quaternary. Uplifted coastlines along their footwalls, growth fault patterns and thickening of sediment strata toward the fault planes indicate that some of these offshore faults on both sides of the graben are active up to present. Our data ground-truth recent models and provides actual observations of the distribution of variable deformation rates in the Gulf of Corinth. Furthermore they suggest that the offshore faults should be taken into consideration in explaining the high extension rates and the uplift scenarios of the northern Peloponnesos coast. The observed coastal uplift appears to be the result of the cumulative effect of deformation accommodated by more than one fault and therefore, average uplift rates deduced from raised fossil shorelines, should be treated with caution when used to infer individual fault slip rates. Seismic reflection profiling is a vital tool in assessing seismic hazard and basin-formation in areas of active extension. [source] | ||||||||||