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Teleseismic receiver function study and its application in Tibet and the Central Andes


Yuan,  Xiaohui
2.4 Seismology, 2.0 Physics of the Earth, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;
Scientific Technical Report STR, Deutsches GeoForschungsZentrum;

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Yuan, X. (2000): Teleseismic receiver function study and its application in Tibet and the Central Andes, PhD Thesis, (Scientific Technical Report STR ; 00/10), Potsdam : Deutsches GeoForschungsZentrum GFZ.

Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_227758
Receiver function analysis is routinely used to isolates P-to-S converted waves from a complex of earthquake recordings s as to explore crustal and upper mantle structures and to infer possible geodynamic processes within the Earth. In the last several years the number of deployments of portable seismic arrays has been greatly increased. The conventional receiver function method, which stacks receiver functions at a single station, is not suitable for such a large amount of data. In this thesis modifications of the receiver function method have been made. Techniques of reflection seismology have been introduced into the receiver function analysis. Modified receiver function method has been successfully applied to the seismological data acquired in Tibet and the Central Andes. In these two Earth"s largest and highest plateaus, data of many available seismic broadband and short-period experiments have been collected. In Tibet, data of the INDEPTH II and GEDEPTH I experiments in southern Tibet and the PASSCAL 91/92 experiment across the central Tibetan Plateau have been combined. A Total number of about 50 stations were distributed roughly in a NNE directed profile. More than 900 receiver functions have been obtained. In the Central Andes, more than 200 stations have been deployed within the experiments of PISCO, CINCA, ANCORP, PUNA and KDS of the project of the SFB 267, and the BANJO and SEDA broadband arrays of the PASSCAL experiments. More than 640 teleseismic receiver functions have been obtained. Results are summarized in the following. (1) Crustal thicknesses under the two plateaus are reliably determined by Teleseismic receiver functions. P-to-S converted waves at the Moho are clearly seen under the Tibetan plateau and under Central Andean plateau. In southern Tibet the Moho is 75-80 km deep. In northern Tibet it becomes shallower to a depth of 55-60 km. In the Central Andes, the continental Moho is 65-70 km deep beneath the Andean Plateau (it appears to be 15 km shallower beneath Puna than beneath Altiplano). The Moho abruptly reduces its depth beneath the eastern edge of the Eastern Cordillera (65-64.5°W) and remains 45-50 km depth in the Sub Andes. Further east there is another abrupt reduction of Moho depth between the Sub Andes and the Chaco Plain. The Moho is 30-35 km beneath the Chaco Plain. (2) Evidence of crustal-scale underthrusting is found in Tibet as well as in the Andes. The INDEPTH data clearly show an intra-crustal phase at a depth of 50-60 km in the southern Tibet. This conversion boundary is probably the evidence of the underthrust Indian crust. In the Andean data a more than 300 km west-dipping intra-crustal converter evidently marks the boundary of the underthrust Brazilian shield crust. This boundary exists across the entire Altiplano and Puna plateau from 20 km depth below the Eastern Cordillera to 40 km depth below the Western Cordillera and the Precordillera. In both plateaus, most of the thickened crust, if not all, can be attributed to the crustal-scale underthrusting. (3) Plate boundaries are found to a depth of about 250 km between the Indian and the Asian lithospheric mantle and to a depth of about 120 km between the Nazca plate and the South American plate. However, the nature of these boundaries is different. In the Central Andes, the plate boundary is interpreted as the oceanic Moho of the Nazca plate, above which a 10 km layer of oceanic crust with lower seismic velocity suggests that the gabbroic rocks do not completely transform to eclogite until a depth of 120 km. Most of the intermediate depth seismicity stops at the same depth, suggesting a relation with phase transformation. In Tibet the observed plate boundary of the two lithosperic mantles probably reflects the temperature difference between the two plates. The cold Indian mantle is subducted under the warm Asian mantle. The Temperature difference can be as high as 500.700 ° resulting in large seismic velocity contrast. (4) Interesting variations have been found in the upper mantle discontinuities which are related to the plate collision and subduction processes. In Tibet, the 410 km discontinuity is clearly seen in its globally average depth in the south, and is disturbed and becomes complicated in the north. The 660 km discontinuity is continuously displayed throughout the Tibetan profile. Similarly, in the Central Andes, the 410 km discontinuity is not imaged coherently, which is obviously attributed to the subduction complexity of the phase transformation of the mantle rocks. It is interesting to see that the 660 km discontinuity is depressed by about 30-40 km in the region of the cold Nazca slab, which corresponds to a temperature reduction of 300-600° within the slab.