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Strike-slip Faulting (strike-slip + faulting)
Selected AbstractsThe source process of the 2001 July 26 Skyros Island (Greece) earthquakeGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004Zafeiria Roumelioti SUMMARY The spatial and temporal distribution of slip during the 2001 July 26 Skyros (Greece) earthquake Moment magnitude (M 6.5) is investigated using broadband data recorded at regional distances. The applied method involves estimation of the source time functions of the examined event through an empirical Green's function approach and inversion of their shapes to estimate kinematic source parameters. Our test inversions to statistically identify the fault plane, together with the distribution of aftershocks clearly indicate sinistral strike-slip faulting. In view of the fact that the Skyros epicentre lies near the western termination of the dextral strike-slip North Anatolian Fault (NAF) into the Aegean Sea, this sinistral strike-slip motion, for the first time instrumentally identified, has great tectonic significance. The best values searched through the inversion are 0.7 s for the rise time, and 2.4 km s,1 for the rupture velocity. Most of the slip appears to be concentrated in a relatively small area around the hypocentre, while a smaller slip patch was found at relatively large depth (18,24 km). At least two of the large aftershocks following the main event also occurred at the deeper part of the fault. Smaller amounts of slip are distributed in a wider area with dimensions similar to those inferred from the aftershock distribution studies and the empirical relations applicable to Greece. [source] The 1933 Ms= 7.3 Baffin Bay earthquake: strike-slip faulting along the northeastern Canadian passive marginGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Allison L. Bent Summary The 1933 November 20 (Ms= 7.3) Baffin Bay earthquake is one of the largest instrumentally recorded passive margin earthquakes. Analysis of seismograms of this earthquake shows strong evidence for strike-slip faulting, which contrasts with the generally accepted belief that Baffin Bay is dominated by thrust faulting. The best-fitting solution consists of a large strike-slip subevent (strike 172°, dip 82°, rake 6°) followed by two smaller oblique-thrust subevents (strike 190°, dip 30°, rake 62°). All subevents occur at a depth of about 10 km. An instrumental moment magnitude of 7.4 was determined. Preliminary analysis of subsequent large (magnitude , 6.0) earthquakes in Baffin Bay finds additional evidence for strike-slip faulting in the region. The results for Baffin Bay, together with those for other passive margin earthquakes, suggest strike-slip faulting may be more prevalent in these regions than was previously believed. [source] On strike-slip faulting in layered mediaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Maurizio Bonafede Summary We study the effects of structural inhomogeneities on the stress and displacement fields induced by strike-slip faults in layered media. An elastic medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space characterized by different elastic parameters. Shear cracks with assigned stress drop are employed as mathematical models of strike-slip faults, which are assumed to be vertical and planar. If the crack is entirely embedded within the lower medium (case A), a Cauchy-kernel integral equation is obtained, which is solved by employing an expansion of the dislocation density in Chebyshev polynomials. If the crack is within the lower medium but it terminates at the interface (case B), a generalized Cauchy singularity appears in the integral kernel. This singularity affects the singular behaviour of the dislocation density at the crack tip touching the interface. Finally, the case of a crack crossing the interface is considered (case C). The crack is split into two interacting sections, each placed in a homogeneous medium and both open at the interface. Two coupled generalized Cauchy equations are obtained and solved for the dislocation density distribution of each crack section. An asymptotic study near the intersection between the crack and the interface shows that the dislocation densities for each crack section are bounded at the interface, where a jump discontinuity is present. As a corollary, the stress drop must be discontinuous at the interface, with a jump proportional to the rigidity contrast between the adjoining media. This finding is shown to have important implications for the development of geometrical complexities within transform fault zones: planar strike-slip faults cutting across layer discontinuities with arbitrary stress drop values are shown to be admissible only if the interface between different layers becomes unwelded during the earthquake at the crack/interface junction. Planar strike-slip faulting may take place only in mature transform zones, where a repetitive earthquake cycle has already developed, if the rheology is perfectly elastic. Otherwise, the fault cannot be planar: we infer that strike-slip faulting at depth is plausibly accompanied by en-echelon surface breaks in a shallow sedimentary layer (where the stress drop is lower than prescribed by the discontinuity condition), while ductile deformation (or steady sliding) at depth may be accommodated by multiple fault branching or by antithetic faulting in the upper brittle layer (endowed with lower rigidity but higher stress). [source] Analyses of the stress field in southeastern France from earthquake focal mechanismsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001Emmanuel Baroux Summary Owing to the apparent deformation field heterogeneity, the stress regimes around the Provence block, from the fronts of the Massif Central and Alpine range up to the Ligurian Sea, have not been well defined. To improve the understanding of the SE France stress field, we determine new earthquake focal mechanisms and compute the present-day stress states by inversion of the 89 available focal mechanisms around the Provence domain, including 17 new ones calculated in the current study. This study provides evidence of six distinct deformation domains around the Provence block, with different tectonic regimes. On a regional scale, we identify three zones characterized by significantly different stress regimes: a western one affected by an extensional stress (normal faulting) regime; a southeastern one characterized by a compressional stress (reverse to strike-slip faulting) regime with NNW- to WNW-trending ,1; and a northeastern one, namely the Digne nappe front, marked by a NE-trending compression. Note that the Digne nappe back domain is controlled by an extensional regime that is deforming the western Alpine core. This extensional regime could be a response to buoyancy forces related to the Alpine high topography. The stress regimes in the southeast of the Argentera Massif and around the Durance fault are consistent with a coherent NNW-trending ,1, implying a left-lateral component of the active reverse oblique slip of the Moyenne Durance Fault. In the Rhone Valley, an E-trending extension characterizes the tectonic regime, implying a normal component of the present-day N,^mes fault displacement. This study provides evidence for short-scale variation of the stress states, which arises from abrupt changes in the boundary force influences on upper crustal fragments (blocks). These spatial stress changes around the Provence block result from the coeval influence of forces applied at both its extremities, namely in the northeast the Alpine front push, and in the southeast the northward African plate drift. In addition to these boundary forces, the mantle plume under the Massif Central influences the western block boundary. [source] Hydraulic pathways in the crystalline rock of the KTBGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Günter Zimmermann Fracture systems and fluid pathways must be analysed in order to understand the dynamical processes in the upper crust. Various deterministic as well as stochastic fracture networks in the depth section of the Franconian Lineament (6900 to 7140 m), which appears as a brittle ductile shear zone and prominent seismic reflector, were modelled to simulate the hydraulic situation at the two boreholes of the Continental Deep Drilling Program (KTB). They led to estimations of the hydraulic permeability in crystalline rock. The geometrical parameters of the fractures, such as fracture locations and orientations, were determined from structural borehole measurements, which create an image of the borehole wall. The selection of potentially open fractures was decided according to the stress field. Only fractures with the dip direction (azimuth) of the fracture plane perpendicular to the maximum horizontal stress field were assumed to be open. The motivation for this assumption is the fact that the maximum horizontal stress is higher than the vertical stress from the formation, indicating that the state of stress is a strike-slip faulting. Therefore, the probability of open fractures due to this particular stress field at the KTB sites is enhanced. Length scales for fracture apertures and extensions were stochastically varied and calibrated by hydraulic experiments. The mean fracture aperture was estimated to be 25 ,m, assuming an exponential distribution, with corresponding permeability in the range of 10,16 m2. Similar results were also obtained for log-normal and normal distributions, with a variation of permeability of the order of a factor of 2. The influence of the fracture length on permeability of the stochastic networks was also studied. Decreasing the fracture length beyond a specific threshold of 10 m led to networks with vanishing connectivity and hence vanishing permeability. Therefore, we assume a mean fracture length exceeding the threshold of 10 m as a necessary assumption for a macroscopic hydraulically active fracture system at the KTB site. The calculated porosity due to the fracture network is of the order of 10,3 per cent, which at first sight contradicts the estimated matrix porosity of 1 to 2 per cent from borehole measurements and core measurements. It can be concluded from these results, however, that if the fluid transport is due to a macroscopic fracture system, only very low porosity is needed for hydraulic flow with permeabilities up to several 10,16 m2, and hence the contribution of matrix porosity to the hydraulic transport is of a subordinate nature. [source] Transpressional tectonics of the Mineoka Ophiolite Belt in a trench,trench,trench-type triple junction, Boso Peninsula, JapanISLAND ARC, Issue 4 2005Ryota Mori Abstract Structures developed in metamorphic and plutonic blocks that occur as knockers in the Mineoka Ophiolite Belt in the Boso Peninsula, central Japan, were analyzed. The aim was to understand the incorporation processes of blocks of metamorphic and plutonic rocks with an arc signature into the serpentinite mélange of the Mineoka Ophiolite Belt in relation to changes in metamorphic conditions during emplacement. Several stages of deformation during retrogressive metamorphism were identified: the first faulting stage had two substage shearing events (mylonitization) under ductile conditions inside the crystalline blocks in relatively deeper levels; and the second stage had brittle faulting and brecciation along the boundaries between the host serpentinite bodies in relatively shallower levels (zeolite facies). The first deformation occurred during uplift before emplacement. The blocks were intensively sheared by the first deformation event, and developed numerous shear planes with spacing of a few centimeters. The displacement and width of each shear plane were a few centimeters and a few millimeters, respectively, at most. In contrast, the fault zone of the second shearing stage reached a few meters in width and developed during emplacement of the Mineoka Ophiolite. Both stages occurred under a right-lateral transpressional regime, in which thrust-faulting was associated with strike-slip faulting. Such displacement on an outcrop scale is consistent with the present tectonics of the Mineoka Belt. This implies that the same tectonic stress has been operating in the Boso trench,trench,trench-type triple junction area in the northwest corner of the Pacific since the emplacement of the Mineoka Ophiolite. The Mineoka Ophiolite Belt must have worked as a forearc sliver fault during the formation of a Neogene accretionary prism further south. [source] THE EVOLUTION OF A MODEL TRAP IN THE CENTRAL APENNINES, ITALY: FRACTURE PATTERNS, FAULT REACTIVATION AND DEVELOPMENT OF CATACLASTIC ROCKS IN CARBONATES AT THE NARNI ANTICLINEJOURNAL OF PETROLEUM GEOLOGY, Issue 2 2001F. Storti Recent hydrocarbon discoveries in the Southern Apennines of Italy have focussed attention on the importance of studying fracturing and cataclasis in carbonate rocks because of their fundamental impact on reservoir permeability and connectivity. The Narni Anticline in the central Apennines consists of a stack of easterly-verging carbonate thrust sheets compartmentalized by extensional and strike-slip fault zones. The structure provides afield analogue for studying the evolution of superimposed fold- and fault-related fractures in carbonate reservoir rocks. The fracture pattern at the Narni Anticline developed as a result of three mechanisms: (a) layer-parallel shortening predating folding and faulting; (b) thrust-related folding and further thrust breakthrough; and (c) extensional and strike-slip faulting. Along-strike (longitudinal) fractures developed during progressive rollover fault-propagation folding, and their intensity depends on the precise structural position within the fold: fracture intensity is high in the forelimb and low in the crest. The 3-D architecture of the mechanical anisotropy associated with thrusting, folding, and related fracturing constrained the location and geometry of subsequent extensional and strike-slip faulting. The superimposition in damage zones of a fault-related cleavage on the pre-existing fracture pattern, which is associated with layer-parallel shortening and thrust-related folding, resulted in rock fragmentation and comminution, and the development of cataclastic bands. The evolution of fracturing in the Narni Anticline, its role in constraining thrust breakthrough trajectories and the location of extensional and strike-slip faults, and the final development of low-permeability cataclastic bands, will be relevant to studies of known oilfields in the Southern Apennines, as well as for future exploration. [source] Carbonate sedimentation in a starved pull-apart basin, Middle to Late Devonian, southern Guilin, South ChinaBASIN RESEARCH, Issue 2 2001D. Chen ABSTRACT Geological mapping and sedimentological investigations in the Guilin region, South China, have revealed a spindle- to rhomb-shaped basin filled with Devonian shallow- to deep-water carbonates. This Yangshuo Basin is interpreted as a pull-apart basin created through secondary, synthetic strike-slip faulting induced by major NNE,SSW-trending, sinistral strike-slip fault zones. These fault zones were initially reactivated along intracontinental basement faults in the course of northward migration of the South China continent. The nearly N,S-trending margins of the Yangshuo Basin, approximately coinciding with the strike of regional fault zones, were related to the master strike-slip faults; the NW,SE-trending margins were related to parallel, oblique-slip extensional faults. Nine depositional sequences recognized in Givetian through Frasnian strata can be grouped into three sequence sets (Sequences 1,2, 3,5 and 6,9), reflecting three major phases of basin evolution. During basin nucleation, most basin margins were dominated by stromatoporoid biostromes and bioherms, upon a low-gradient shelf. Only at the steep, fault-controlled, eastern margin were thick stromatoporoid reefs developed. The subsequent progressive offset and pull-apart of the master strike-slip faults during the late Givetian intensified the differential subsidence and produced a spindle-shaped basin. The accelerated subsidence of the basin centre led to sediment starvation, reduced current circulation and increased environmental stress, leading to the extensive development of microbial buildups on platform margins and laminites in the basin centre. Stromatoporoid reefs only survived along the windward, eastern margin for a short time. The architectures of the basin margins varied from aggradation (or slightly backstepping) in windward positions (eastern and northern margins) to moderate progradation in leeward positions. A relay ramp was present in the north-west corner between the northern oblique fault zone and the proximal part of the western master fault. In the latest Givetian (corresponding to the top of Sequence 5), a sudden subsidence of the basin induced by further offset of the strike-slip faults was accompanied by the rapid uplift of surrounding carbonate platforms, causing considerable platform-margin collapse, slope erosion, basin deepening and the demise of the microbialites. Afterwards, stromatoporoid reefs were only locally restored on topographic highs along the windward margin. However, a subsequent, more intense basin subsidence in the early Frasnian (top of Sequence 6), which was accompanied by a further sharp uplift of platforms, caused more profound slope erosion and platform backstepping. Poor circulation and oxygen-depleted waters in the now much deeper basin centre led to the deposition of chert, with silica supplied by hydrothermal fluids through deep-seated faults. Two ,subdeeps' were diagonally arranged in the distal parts of the master faults, and the relay ramp was destroyed. At this time, all basin margins except the western one evolved into erosional types with gullies through which granular platform sediments were transported by gravity flows to the basin. This situation persisted into the latest Frasnian. This case history shows that the carbonate platform architecture and evolution in a pull-apart basin were not only strongly controlled by the tectonic activity, but also influenced by the oceanographic setting (i.e. windward vs. leeward) and environmental factors. [source] Dating of the Karakorum Strike-slip FaultACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 1 2001ZHOU Yong Abstract This paper mainly discusses the timing of the Karakorum strike-slip fault, and gives a brief introduction of its structures, offset, and deformational style. This fault strikes NNW-SSE. Asymmetrical folds, stretching lineation, S-C fabrics, feldspar and quartz s,-porphyroclasts, domino structure, shear cleavages and faults in the fault zone are products of tectonic movements. They all indicate a dextral slip sense of faulting. Mylonitic bands are widely developed along this fault. Phengite appears, indicating rather high deformational pressure. Geochronological data indicate that the Karakorum strike-slip faulting occurred from 6.88±0.36 to 8.75±0.25 Ma. The cumulative displacement from Muztag Ata to Muji is about 135 km. [source] | ||||||||||