The Ryukyu trench-arc system can be divided into two types according to its subduction model. The normal subduction in the
northern part of the Philippine Sea plate creates a hinge sedimentary wedge with large deformation at the collision front,
while the oblique subduction in the southern part gives rise to a smaller accretion with small deformation than that in the
northern part. The mechanisms that cause distinction between these two types have been analysed and calculated by using gravity
data based on the lithosphere rheology and the stress state of the lithosphere in the subduction boundary. The two types of
subduction model are associated with the internal extension in the southern Okinawa Trough and the small extension in the
northern part. The difference of the stress state between the two types of subduction model is also manifested in other tectonic
features, such as topography, volcanic activity and crust movement. Modeling bathymetric and gravity data from this area suggests
that the oblique subduction of low angle, together with smooth geometry of the overlying plate crust, results in small stress
released on the south of the trench by the subduction plate. The intraplate faults in the southern Okinawa Trough behind the
trench stand in surplus intensive stress. On the other hand, the normal subduction of high angle, together with strong undulation
geometry of the overlying crust, results in more intensive stress released in the northern Ryukyu Trench than that in the
south. The intraplate faults in the northern Okinawa Trough behind the northern Ryukyu Trench stand in small stress. 相似文献
The Schlinig fault at the western border of theÖtztal nappe (Eastern Alps), previously interpreted as a west-directed thrust, actually represents a Late Cretaceous, top-SE to -ESE normal fault, as indicated by sense-of-shear criteria found within cataclasites and greenschist-facies mylonites. Normal faulting postdated and offset an earlier, Cretaceous-age, west-directed thrust at the base of theÖtztal nappe. Shape fabric and crystallographic preferred orientation in completely recrystallized quartz layers in a mylonite from the Schlinig fault record a combination of (1) top-east-southeast simple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directed shortening during the Early Tertiary, also recorded by open folds on the outcrop and map scale. Offset of the basal thrust of theÖtztal nappe across the Schlinig fault indicates a normal displacement of 17 km. The fault was initiated with a dip angle of 10° to 15° (low-angle normal fault). Domino-style extension of the competent Late Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting.
The Schlinig fault belongs to a system of east- to southeast-dipping normal faults which accommodated severe stretching of the Alpine orogen during the Late Cretaceous. The slip direction of extensional faults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip sense is reversed and the normal faults are slightly steeper than the thrusts. In the western Austroalpine nappes, extension started at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of Austroalpine Gosau basins with fluviatile to deep-marine sediments. West-directed rollback of an east-dipping Piemont-Ligurian subduction zone is proposed to have caused this stretching in the upper plate. 相似文献
The Aegean Sea is a broad area of submerged continental crust undergoing active extension to varying degrees. A combined near-normal incidence and wide-angle seismic recording programme was conducted in the western Aegean Sea in 1993, with the principal objective of testing the popular hypothesis that lower crustal deformation (particularily extension) is expressed as a seismically “layered lower crust” (LLC). Across the southern margin of the Cretan trough (i.e. North Cretan offshore margin), a LLC was indicated by wide-angle arrivals that was not apparent on either the coincident near-normal-in-cidence profile or on older low-frequency refraction records. North of the northern margin of the Cretan Trough, beneath the Cyclades, a domain of strong reflectivity is recorded from the middle to lower crust. Here, the near-normal incidence sections also show this typical LLC reflectivity. On the wide-angle sections, a distinct interface is suggested in addition, at a larger depth than that previously assumed for the Moho discontinuity. The structural images and interpretations derived from the new seismic data so far do not clearly support either a pure-shear crustal stretching or an asymmetric simple-shear extension model for the Aegean Sea. Our results appear to be consistent with a tectonic model, where middle crust mobilised by flow coincides spatially with upper crust that has been thinned by active extension of an orogenically thickened crust and expressed near the surface as an exhumed metamorphic core complex. 相似文献
Most of the extension fractures located in the Ethiopian rift are related to rift dynamics, and they have consistently similar orientations for hundreds of kilometers. The orientation of extension fracture gives the local extension direction which is perpendicular to its horns (end segments of an extension fracture). We have established clear geometrical relationships between tectonics and volcanism in the Main Ethiopian Rift by interpreting high-resolution images obtained from Landsat-TM and SPOT satellites and by quantitatively analyzing the geometry of extension fractures, elongated vents and linear volcanic clusters. Applying the relationships obtained, we show that extension fractures in the rift served as channels for magma rising to the surface, and that extension fractures underlie most of the elongated volcanic vents and linear volcanic clusters. The geometry of extension fractures beneath volcanic edifices can be deduced from the shape of elongated vents and pattern of linear volcanic clusters rooted on them. Utilizing the orientations of extension fractures directly observed and those inferred from elongated volcanic vents and linear clusters, we found that the extension direction of the Main Ethiopian Rift is northwest–southeast and that the direction has been rotated clockwise for about 20° in the time interval 2.83 to 0.023 Ma. The Recent axis of rift opening is oriented N40° and located closer to the southeastern escarpment giving an asymmetric geometry to the Main Ethiopian Rift (MER). 相似文献