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Rift Zone (rift + zone)

Distribution by Scientific Domains

Earth and Environmental Science	100%

Selected Abstracts

Seismotectonics of the Sinai subplate , the eastern Mediterranean region

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2003
Amos Salamon
SUMMARY We define the Sinai subplate, from a seismotectonic perspective, as a distinct component in the plate tectonics of the eastern Mediterranean region. This is based on the tectonic characteristics of a comprehensive listing of all ML, 4 recorded seismicity in the region during the 20th century, on newly calculated and recalculated fault plane mechanisms of first P -wave arrivals and on published solutions based on waveform inversion of broad-band data. The low seismicity level and scarcity of strong events in the region required a thorough search for useful data and a careful examination of the reliability of the focal solutions. We gathered all available records of first P -wave onsets from the ISS and ISC Bulletins and the local seismic networks. Altogether, we were able to calculate 48 new focal mechanisms and 33 recalculated ones of events that occurred during the years 1940,1992. With the increasing number of teleseismic and regional broad-band stations in the later years, we added 37 solutions based on teleseismic and regional waveform inversions of events that occurred during 1977,2001. These mechanisms enabled us to examine the seismotectonic character of the Sinai subplate. The strike and rake directions of the calculated mechanisms usually reflect the geometry and the large-scale type of deformation observed along the boundaries of the Sinai subplate,the Dead Sea Transform, the Cypriot Arc convergent zone and the Suez Rift. Nevertheless, along each of these boundaries we found anomalous solutions that attest to the complexity of the deformation processes along plate margins. Earthquakes along the Dead Sea Transform exhibit mainly sinistral transtension and transpression, reflecting its leaky manner and local change in the transform geometry. The presence of other unexpected mechanisms near the transform, however, reflects the heterogeneous deformation it induces around. As expected, thrust mechanisms along the Cypriot Arc mirror its convergent nature and typical curved geometry. Transtension and transpressional solutions in the eastern segment of the arc reflect the sinistral shear motion between Anatolia and Sinai there. However, shear mechanisms found between Cyprus and the Eratosthenes Seamount pose a problem regarding its collision process. Most intriguing of all are ML, 4 thrust and shear solutions found in the Gulf of Suez. They are associated with predominantly normal mechanisms within a rift zone and therefore constitute a unique phenomenon, yet to be deciphered. [source]

Crustal structure of central and northern Iceland from analysis of teleseismic receiver functions

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000
Fiona A. Darbyshire
We present results from a teleseismic receiver function study of central and northern Iceland, carried out during the period 1995,1998. Data from eight broad-band seismometers installed in the SIL network operated by the Icelandic Meteorological Office were used for analysis. Receiver functions for each station were generated from events for a wide range of backazimuths and a combination of inversion and forward modelling was used to infer the crustal structure below each station. The models generated show a considerable variation in the nature and thickness of the crust across Iceland. The thinnest crust (20,21 km) is found in the northern half of the Northern Volcanic Zone approximately 120 km north of the centre of the Iceland mantle plume. Thicker crust (24,30 km) is found elsewhere in northern and central Iceland and the thickest crust (37 km) is found close to the plume centre. Velocity,depth profiles show a distinct division of the crust into two main sections, an upper high-velocity-gradient section of thickness 2,8 km and a lower crustal section with small or zero overall velocity gradient. The thickness of the upper crust correlates with the tectonic structure of Iceland; the upper crust is thickest on the flanks of the northern and central volcanic rift zones and thinnest close to active or extinct central volcanoes. Below the Krafla central volcano in northeastern Iceland the receiver function models show a prominent low-velocity zone at 10,15 km depth with minimum shear wave velocities of 2.0,2.5 km s,1. We suggest that this feature results from the presence of partially molten sills in the lower crust. Less prominent low-velocity zones found in other regions of Iceland may arise from locally high temperatures in the crust or from acidic intrusive bodies at depth. A combination of the receiver function results and seismic refraction results constrains the crustal thickness across a large part of Iceland. Melting by passive decompression of the hot mantle below the rift zone in northern Iceland forms a crust of thickness ,20 km. In contrast, the larger crustal thickness below central Iceland probably arises from enhanced melt production due to active upwelling in the plume core. [source]

THE FIRST FOSSIL PROSCOPIIDAE (INSECTA, ORTHOPTERA, EUMASTACOIDEA) WITH COMMENTS ON THE HISTORICAL BIOGEOGRAPHY AND EVOLUTION OF THE FAMILY

PALAEONTOLOGY, Issue 2 2008
SAM W. HEADSArticle first published online: 14 MAR 200
Abstract:,Eoproscopia martilli gen. et sp. nov. is described from the Early Cretaceous (Aptian) Crato Formation Lagerstätte of Ceará State, north-east Brazil. The new taxon is assigned to the extant family Proscopiidae and represents the first occurrence of the group in the fossil record. Eoproscopia is similar to crown group proscopiids in its stick-like habitus, elongate prothorax and absence of the cryptopleuron, but differs in the presence of well-developed wings, the short head with a small, simple fastigium, the prothoracic legs being inserted near the posterior margin of the prothorax, and the absence of spines on the metathoracic tibiae. The discovery of Eoproscopia extends the geological range of the family by approximately 110 myr and confirms the presence of stem-group proscopiids in the Atlantic rift zone of South America during the Early Cretaceous. [source]

Cenozoic stratigraphy and subsidence history of the South China Sea margin in the Taiwan region

BASIN RESEARCH, Issue 4 2003
A. T. Lin
Seismic reflection profiles and well data are used to determine the Cenozoic stratigraphic and tectonic development of the northern margin of the South China Sea. In the Taiwan region, this margin evolved from a Palaeogene rift to a latest Miocene,Recent foreland basin. This evolution is related to the opening of the South China Sea and its subsequent partial closure by the Taiwan orogeny. Seismic data, together with the subsidence analysis of deep wells, show that during rifting (,58,37 Ma), lithospheric extension occurred simultaneously in discrete rift belts. These belts form a >200 km wide rift zone and are associated with a stretching factor, ,, in the range ,1.4,1.6. By ,37 Ma, the focus of rifting shifted to the present-day continent,ocean boundary off southern Taiwan, which led to continental rupture and initial seafloor spreading of the South China Sea at ,30 Ma. Intense rifting during the rift,drift transition (,37,30 Ma) may have induced a transient, small-scale mantle convection beneath the rift. The coeval crustal uplift (Oligocene uplift) of the previously rifted margin, which led to erosion and development of the breakup unconformity, was most likely caused by the induced convection. Oligocene uplift was followed by rapid, early post-breakup subsidence (,30,18 Ma) possibly as the inferred induced convection abated following initial seafloor spreading. Rapid subsidence of the inner margin is interpreted as thermally controlled subsidence, whereas rapid subsidence in the outer shelf of the outer margin was accompanied by fault activity during the interval ,30,21 Ma. This extension in the outer margin (,,1.5) is manifested in the Tainan Basin, which formed on top of the deeply eroded Mesozoic basement. During the interval ,21,12.5 Ma, the entire margin experienced broad thermal subsidence. It was not until ,12.5 Ma that rifting resumed, being especially active in the Tainan Basin (,,1.1). Rifting ceased at ,6.5 Ma due to the orogeny caused by the overthrusting of the Luzon volcanic arc. The Taiwan orogeny created a foreland basin by loading and flexing the underlying rifted margin. The foreland flexure inherited the mechanical and thermal properties of the underlying rifted margin, thereby dividing the basin into north and south segments. The north segment developed on a lithosphere where the major rift/thermal event occurred ,58,30 Ma, and this segment shows minor normal faulting related to lithospheric flexure. In contrast, the south segment developed on a lithosphere, which experienced two more recent rift/thermal events during ,30,21 and ,12.5,6.5 Ma. The basal foreland surface of the south segment is highly faulted, especially along the previous northern rifted flank, thereby creating a deeper foreland flexure that trends obliquely to the strike of the orogen. [source]