Muawia Barazangi

Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America

A detailed study of the spatial distribution of precisely located hypocenters of South American earthquakes that occurred between lat 0° and 45°S shows that the data can be explained by the simple model of a descending oceanic plate beneath a continental plate and that the following conditions obtain: (1) The hypocenters clearly define five segments of inclined seismic zones, in each of which the zones have relatively uniform dips. The segments beneath northern and central Peru (about lat 2° to 15° S) and beneath central Chile (about lat 27° to 33° S) have very small dips (about 10°), whereas the three segments beneath southern Ecuador (about lat 0° to 2°S), beneath southern Peru and northern Chile (about lat 15° to 27°S), and beneath southern Chile (about lat 33° to 45°S) have steeper dips (25° to 30°). No clear evidence exists for further segmentation of the descending Nazca plate beneath South America. If the two flat segments are in contact with the lower boundary of the continental plate, the thickness of that plate is less than approximately 130 km. This is in marked contrast to the reports of thicknesses exceeding 300 km for the South American continental plate. (2) There is considerable seismic activity within the upper 50 km of the overriding South American plate. This seismic activity is well separated from the inclined seismic zones and probably occurs in the crustal part of the South American plate. Thus, hypocenters in South America are not evenly distributed through about a 300-km-thick zone as previously described. (3) A remarkable correlation exists between the two flat segments of the subducted Nazca plate and the absence of Quaternary volcanism on the South American plate. (4) The transition from the flat Peru segment to the steeper Chile segment is abrupt and is interpreted as a tear in the descending Nazca plate. The tear is located approximately beneath the northern limit of the Altiplano (a high plateau in the Andes), and about 200 km south of the projection of the oceanic Nazca ridge down the subduction zone. (5) A gap in seismic activity exists between depths of 320 and 525 km.

Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

An edited version of this paper was published by the American Geophysical Union. Copyright 2000, AGU. See also: http://www.agu.org/pubs/crossref/2000/2000JB900024.shtml; http://atlas.geo.cornell.edu/morocco/publications/calvert2000.htm

How flat is Tibet

High-resolution digital topography (three arc-second grid) for most of Tibet provides new information to characterize the relief of the highest and largest plateau on Earth. The arid to semiarid central and northern part of the plateau interior has low relief (average slopes of ∼; over 250 m windows) and a mean elevation of 5023 m above sea level. At moderate wavelengths of ∼m, relief is ∼or less for most of Tibet, as opposed to the much higher relief of up to 6 km on the plateau edges, where glacial and fluvial dissection is greater because of higher levels of precipitation. The only faults manifesting significant topographic relief are the relatively small scale, generally north-trending graben systems, primarily in southern Tibet, and several large-scale fault systems near the edges of Tibet. The flatness of Tibet implies that (1) there has been little deformation (especially shortening) of the uppermost crust north of the graben systems during the late Cenozoic, and (2) shallow crustal isostatic compensation has been acting to level the surface of the plateau.

Velocities and propagation characteristics of Pn and Sn beneath the Himalayan arc and Tibetan plateau: Possible evidence for underthrusting of Indian continental lithosphere beneath Tibet

New seismological observations on velocities and propagation characteristics of Pn and Sn waves beneath Tibet can be interpreted, although not uniquely, to indicate the shallow-angle underthrusting of the Greater Indian continental lithosphere beneath the Tibetan plateau. This is inferred from available geological and geophysical data and is now supported by new seismological observations. The most significant observation is that high-frequency Sn waves (shear waves that travel in the mantle lithosphere) propagate efficiently in the uppermost mantle beneath the Tibetan plateau, except beneath the north-central part of Tibet (the Chang Thang terrane). Furthermore, Sn propagates efficiently across the stable block of the Tarim basin, along the Tien Shan and Himalayan Mountains, and across the Indian shield. Apparent velocities of Pn and Sn waves that traverse the uppermost mantle beneath the Tibetan plateau are 8.42 and 4.73 km/s, respectively. These velocities are very similar to those beneath the Himalayan Mountains and the Indian shield. The efficient propagation and the velocities of Pn and Sn waves beneath Tibet are also similar to those commonly observed beneath shield and stable continental regions. These results are consistent with the underthrusting model and do not favor the class of alternative models in which the Tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of India and Asia.

The crustal structure of the East Anatolian plateau (Turkey) from receiver functions

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL018192.shtml; http://atlas.geo.cornell.edu/turkey/publications/Zor-et-al_2003.htm

Inversion tectonics and the evolution of the High Atlas Mountains, Morocco, based on a geological-geophysical transect

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 1998, AGU. See also: http://www.agu.org/pubs/crossref/1999/1998TC900015.shtml; http://atlas.geo.cornell.edu/morocco/publications/beauchamp1999.htm

Evidence for 830 years of seismic quiescence from palaeoseismology, archaeoseismology, and historical seismicity along the Dead Sea fault in Syria

An edited version of this paper was published in Earth and Planetary Science Letters by Elsevier Science. Elsevier Science retains the copyright to this paper (Copyright 2003). See also: http:\\dx.doi.org\10.1016\S0012-821X(03)00144-4; http://atlas.geo.cornell.edu/deadsea/publications/Meghraoui2003_EPSL.htm

Tectonic map and geologic evolution of Syria: The role of GIS

This paper was published in the journal The Leading Edge by the Society of Exploration Geophysicists. SEG retains the copyright to this paper. See also: http://www.edge-online.org/; http://atlas.geo.cornell.edu/syria/brew_tle_2000.html

Role of the Atlas Mountains (northwest Africa) within the African-Eurasian plate-boundary zone

This paper was published in Geology by the Geological Society of America (GSA), and GSA retains the copyright. Geological Society of America, P.O. Box 9140, Boulder, CO 80301-9140 See also: http://www.geosociety.org; http://atlas.geo.cornell.edu/morocco/publications/gomez2000Geology.htm

Late Cenozoic Evolution of the Great Basin, Western United States, as an Ensialic Interarc Basin

The Great Basin of the western United States corresponds closely to a well-defined zone of high heat flow, thin crust, and anomalously high-attenuation low-velocity upper mantle. Seismicity, of predominantly normal faulting type, occurs in two marginal zones above the lateral transitions in the upper mantle, which can be correlated also with the most recent volcanism. Petrological studies have indicated that the region was the site of calc-alkaline andesitic volcanism during the middle to late Cenozoic which changed abruptly to fundamentally basaltic volcanism in the late Cenozoic, accompanied by the beginning of major Basin-Range crustal extension. This is interpreted as a change from the island-arc-type volcanism to active interarc spreading. The latter was triggered by the termination of the early to middle Cenozoic West Coast subduction zone. The release of the compressive stress field of the sub-duction zone is considered necessary for interarc spreading. We suggest that the spreading is caused by a mantle diapir mobilized by the descending lithospheric slab. The diapir is trapped beneath the sialic crust, and flattens and spreads out laterally as it rises. This mechanism can explain the extension and the outward migration of volcanism in the Great Basin and the marginal seismic zones. The anomalous upper mantle now present beneath the Great Basin is interpreted as the remnant of this diapir.