IASPEI - Picture Gallery

25.01.2015 Views
IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS017 Oral Presentation 2367 Mantle contributions to elevation in China continent: results of thermal isostasy analysis Mr. Shan Bin Key Laboratory of Dynamic Geodesy Institute of Geodesy and Geophysics, CAS IASPEI Xiong Xiong The failure of simple isostasy models which involve crustal density and crustal thickness alone, to explain the surprising observations of high elevation over thin crust and low elevation over thick crust in some tectonic blocks of China continent, suggests that the contribution of lithosphereic mantle for isostasy should be incorporated. Thermal isostasy, which is based on the fact that a change in temperature results in a change in volume and therefore in density, should be taken into account. In this study, using mass balance model, we conducted thermal isostasy analysis to separate the contributions of crust and mantle to surface elevation. Based on 723 heatflow measurements in China mainland and additional data from the global heatflow dataset, we divided China mainland and its adjacent area into nineteen tectonic units to exhibit overall variations of heat flow pattern. The contribution of crust to elevation was calculated with crustal structure and related parameters which were derived from seismological studies. Introducing temperatures of Moho and thickness of lithospheric mantle, and applying mass balance model, we calculated the contribution to elevation from the buoyancy of lithosphere mantle. The results suggest that the elevation of most tectonic units could be explained well by integrating the contributions from crust and lithosphereic mantle. For instance, the elevations of the Cuxiong and Tarim basins, 1900m and 1230m, are far from the predictions by Airy isostasy model, which are ~2275m and ~2050m, respectively. By assigning Moho temperatures of Cuxiong and Tarim basins, 900oC and 500oC, we obtained the results that the elevations were reduced by the mantle lithosphere contribution of ~-320m for the Cuxiong and ~-980m for the Tarim, resulting in a combined elevation of ~1955m for the Cuxiong and ~1070m for the Tarim, respectively. Therefore, after integrating the two contributions together, we found the elevation difference calculated by the mass balance model between the two tectonic units fitted well with the observed one. The contribution of lithospheric mantle can be understood in the way that high temperatures in the upper mantle reduce density by thermal expansion and compensate for thin high density crust, and vice versa. The compensation status was also in good agreement with the results of vertical crustal movement derived from geodetic studies. Our study suggests that mantle contribution to lithosphere isostasy is so important that it should be taken into accounted in isostasy analysis. Furthermore, since mantle contribution is estimated from lithosphere thickness and average temperature of lithospheric mantle, thermal isostasy analysis provides an effective way to constrain roughly the temperature of Moho in the areas where geothermal observations are not available. Keywords: thermal isostasy, elevation, china

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS017 Oral Presentation 2368 Thermal evolution and stability of the cratonic lithosphere Prof. Jean-Claude Mareschal GEOTOP-UQAM-McGill University of Quebec at Montreal Claire Perry, Claude Jaupart The Archean geological record indicates elevated temperatures in the lower crust while the preservation of Archean features in the mantle requires mantle temperatures to remain sufficiently low. Archean provinces are presently characterized by low heat flow, with an average of 41 mW m2, i.e. less than the global continental average (56mW m2). The range of regionally averaged heat flow values in Archean Provinces (18-54mW m2) is narrower than in younger terranes. But there are important variations in crustal heat production between different Archean Provinces. At the end of the Archean, when crustal heat production was double the present, surface heat flow varied over a range (45-90 mW m2) as wide as that presently observed in Paleozoic Provinces. For the present-day vertical distribution of radioelements, high heat production during the Archean is insufficient to account for elevated lower crustal temperatures. High temperature-low pressure metamorphism conditions require additional heat input, for example by emplacement of large volumes of basaltic melts, crustal thickening or higher concentrations of radio-elements in the lower crust than at present. In the Archean, with a crust thicker than 40km or with the radio-elements uniformly distributed throughout a 40km thick crust, the lower crust was near melting and, with an effective viscosity 1019 Pa s, it could not sustain the stress due to crustal thickening. Long-term crustal stability requires the enrichment of the upper crust in radioelements through melt extraction from the lower crust. After root emplacement, thermal conditions in cratons remained far from equilibrium for 1-2Gyr. Depending on the mechanism of root formation, temperature at 150km might only be 150K higher than present, implying that the lithospheric mantle remained sufficiently cold and strong to preserve Archean features. In thick continental lithosphere, temperatures in the crust and deep in the continental root are effectively decoupled for a long time Keywords: heatflow, craton

Page 1 and 2: IASPEI INTERNATIONAL ASSOCIATION OF

Page 3 and 4: IUGG XXIV General Assembly July 2-1


























































































































































































































































































































































Page 695: IUGG XXIV General Assembly July 2-1































Page 759: Published and Distributed by USMA 2

iaspei

tsunami

seismic

earthquake

physics

seismology

assembly

iugg

perugia

xxiv

IASPEI - Picture Gallery

IASPEI - Picture Gallery ... View more IASPEI - Picture Gallery

Delete template?

Save as template ?

IASPEI - Picture Gallery IASPEI - Picture Gallery