Joint inversion of receiver functions and magnetotelluric data:Application to crustal and mantle structure beneath central Namche Barwa,eastern Himalayan syntaxis
PENG Miao1,2,3,TAN Han-Dong1,2,JIANG Mei3,WANG Wei1,3,LI Qing-Qing3,ZHANG Li-Shu3 1 School of Geophysics and Information Technology,China University of Geosciences,Beijing 100083,China 2 State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences, Beijing 100083,China 3 State Key Laboratory for Continental Tectonics and Dynamics,Institute of Geology,Chinese Academy of Geological Science, Beijing 100037,China
Using tele-seismograms from four stations(Langga station,Bengga station,Zhibai station and Lage station) for about five months in the central Namche Barwa,eastern Himalayan syntaxis in Tibet,we calculated receiver functions by time-domain iterative deconvolution technique and selected high-quality receiver functions with similar epicentral distances and back azimuths and then stacked them.Magnetotelluric apparent resistivity and phase curves are obtained by Rhoplus analysis of Berdichevskiy invariant of impedance tensors.The average receiver function and apparent resistivity and phase data from the same station are jointly inverted for one-dimensional Earth to infer lithospheric structure.Using genetic algorithm,we analyzed the compatibility between the magnetotelluric data and the seismic data by trade-off plots.The comparison of predicted data and measured data manifests that they coincide on both data sets.The results demonstrate:(1) In the mid-upper crust,a high resistivity,high velocity layer(9~14 km thickness) overlays a low resistivity and low velocity layer which may be related to aqueous fluids and partial melts.(2) There exists a highly conductive crust-mantle transition layer(thickness 20 km) with the S wave velocity of ~4 km/s in the lower crust,and also exists the lithosphere-asthenosphere boundary deeper than 130~150 km in the upper mantle.(3) A possible interpretation about the high resistivity and high velocity layer in the upper crust is the migmatitic amphibolite facies(Duoxiongla Formation),while the low resistivity,low velocity layer beneath the Zhibai station is associated with partial melting.And high-pressure granulite facies may originate from petrological transformation of mafic rocks in the crust-mantle transition layer or from mantle-derived magma underplating.