Search

Abstract: A 700km deep seismic velocity cross-section beneath the Lhasa to Golmud transect across the Tibetan plateau is presented. In contrast to the first version of this cross-section, which comprised an 800km wide swath centred on the Lhasa to Golmud transect, due to the recent proliferation of publications concerning the mantle structure beneath Tibet, this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect. The results from the recent INDEPTH IV project indicate that the crustal thickness change from 70km beneath the Songpan–Ganzi terrane and Kunlun mountains to 54km beneath the Qaidam basin is located about 100km north of the Kunlun Fault and almost 45km north of the North Kunlun Thrust. The Qaidam basin Moho is underlain by crustal velocity material for almost 45km and the apparently overlapping crustal material may represent Songpan–Ganzi lower crust underthrusting or flowing northward beneath the Qaidam basin Moho. Thus the high Tibetan plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath the stronger Qaidam crust. Beneath the crust, high-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong–Nujiang suture. Northwards, Asian lithospheric mantle is overlain by a low-velocity, less dense, warm Tibetan plate consisting of an upper lithospheric and a lower asthenospheric part. The apparent northwards deepening of the 410 and 660km discontinuities by about 20km implies that the upper mantle beneath north Tibet is slower, less dense and warmer than under south Tibet, in agreement with the observed uppermost mantle velocities. This, in turn, could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau. [Copyright &y& Elsevier]

Abstract: We performed a teleseismic P wave tomography study using seismic events at both teleseismic and regional distances, recorded by a temporary seismic array in the Argentine Puna Plateau and adjacent regions. The tomographic images show the presence of a number of positive and negative anomalies in a depth range of 20–300km beneath the array. The most prominent of these anomalies corresponds to a low-velocity body, located in the crust, most clearly seen in the center of the array (27°S, 67°W) between the Cerro Peinado volcano, the Cerro Blanco caldera and the Farallon Negro in the east. This anomaly (southern Puna Magmatic Body) extends from the northern most part of the array and follows the line with the highest density of stations towards the south where it becomes smaller. It is flanked by high velocities on the west and the east respectively. On the west, the high velocities might be related to the subducted Nazca plate. On the northeast the high velocity block coincides with the position of the Hombre Muerto basin in the crust and could be indicating an area of lithospheric delamination where we detected a high velocity block at 100km depth on the eastern border of the Puna plateau, north of Galan. This block might be related to a delamination event in an area with a thick crust of Paleozoic metamorphic rocks at the border between Puna and Eastern Cordillera. In the center of the array the Southern Puna magmatic body is also flanked by high velocities but the most prominent region is located on the east and is interpreted as part of the Sierras Pampeanas lithosphere with high velocities. The position of the Sierras Pampeanas geological province is key in this area as it appears to limit the extension of the plateau towards the south. [Copyright &y& Elsevier]

Abstract: A set of seismological stations was deployed in the Central Andes region along a ∼600km long profile at 21°S between Chile and Bolivia and operated for a period of almost two years, from March 2002 to January 2004. Here we present the results of the tomographic inversion for P-wave velocity anomalies, based on teleseismic data recorded at the stations. The reliability of the results has been checked by a series of synthetic tests. The tomographic images show high-velocities on the west of the profile that are indicative of cold material from the fore-arc. A low-velocity anomaly is detected at the border between the fore- and the volcanic arc where the Quebrada Blanca seismic anomaly was previously described. This anomaly might be related to the presence of fluids that originate at the cluster of earthquakes at a depth of ∼100km in the subducted plate. A strong low-velocity anomaly is detected beneath the entire Altiplano plateau and part of the Eastern Cordillera, in agreement with previous receiver function results. The Brazilian Shield is thought to be responsible for the strong high-velocity anomaly underneath the Interandean and Subandean regions. [Copyright &y& Elsevier]

Abstract: Data from three temporary seismic networks were merged for tomographic inversion. Although the deployments did not coincide in time, spatial overlap was achieved by re-occupying existing sites. Travel times and t ⁎ operators of about 1600 earthquakes were inverted for 3D models of ν p, ν p/ν s and P-wave attenuation (Q p −1). All three attributes provide a consistent image of the entire subduction zone on a lithospheric scale. The tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the Western Cordillera and most of the Puna plateau, indicative of weak rheology and mostly asthenospheric mantle. In contrast, forearc and eastern foreland are characterized by high Q p values, corresponding to cold temperatures in accordance with thermal models. In the backarc, between 23°S and 24°S, a high velocity, high Q p structure beneath the Eastern Cordillera and eastern Puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process. South of this structure, the mantle is characterized by low velocities, high ν p/ν s ratios, and low Q p values. Here it is believed that lithosphere originally underlying Andean crust has already been removed. This is supported by new estimates of crustal thickness and volcanic activity. [Copyright &y& Elsevier]

Aim of the study: The aim of this study is to evaluate the results of acute gallstone pancreatitis treatment and to discuss indications in relation with the different forms of the disease.Material and method: From january 1992 to june 2001, 137 patients have been treated for an acute gallstone pancreatitis. Diagnostic criteria were given by the history, clinical examination, biochemical and radiological findings. After exclusion of patients with a systemic disease, a group of 129 patients have been enrolled in a treatment regimen with an endoscopic retrograde cholangiopancreatography (ERCP) and eventual sphincterotomy, a percutaneous US-guided cholecystostomy (PC) when necessary and an elective laparoscopic cholecystectomy.Results: ERCP has been successfully performed in 121/129 patients. A PC has been performed in 5/8 patients of the failed endoscopic procedure and in 14 with acute cholecystitis. Retrograde and percutaneous cholangiographies showed main bile duct stones in 89 patients, a dilatation of the main bile duct without stones in 26 patients and a negative finding in 6 patients. An endoscopic sphincterotomy has been performed in 117 patients. A laparoscopic cholecystectomy has been performed in 118 patients. Mortality and morbidity rates were 1.6 and 10.3%, respectively.Conclusion: ERCP and sphincterotomy seem to be indicated in all patients observed during the first 72 hours. Endoscopic treatment and percutaneous procedure make it possible to reduce at a very low rate the cases with an unfavourable course of the disease. A definitive treatment may then be performed by the way of a laparoscopic cholecystectomy. [Copyright &y& Elsevier]