Conductivity Structure of Crust and Upper Mantle Beneath the Northern Tibetan Plateau: Results of Super‐Wide Band Magnetotelluric Sounding

Abstract

To study the conductivity structure of crust and upper mantle as well as the thermal regime of lithosphere beneath the northern Tibetan Plateau, the project INDEPTH (III)‐MT completed super‐broadband magnetotelluric (MT) sounding along Dêqên‐Longwei Cuo (line 500) and Nagqu‐Golmud (line 600) in northern and central Tibet in 1998 and 1999. The result shows that the conductivity structure of crust and upper mantle is quite different across the Kunlun Mountains. North to the mountains the crust and upper mantle is relatively highly resistive. South to the mountains, the conductivity structure of crust and upper mantle is of obvious stratification as described following. The upper crust is dominated by discontinuous high‐resistivity bodies with intercalated low‐resistivity anomalies. And the electric structure of the upper crust in NS direction looks like complicated with discontinuous and block‐shaped features. Meanwhile the intermediate and lower crust are characterized by large‐scale high conductivity anomalies, indicated by separated high‐conductivity bodies of big sizes and differing shapes with resistivity less than 4m. Beneath the Bangong‐Nujiang and Jinshajiang River sutures, the high‐conductivity bodies in the crust tend to extend toward the upper mantle, implying existence of a low‐resistivity conduit between crust and mantle. Based on the observed electric structure of crust and mantle beneath the northern and central Tibetan Plateau, we infer that there are extensive partial melt and thermal fluids there like southern Tibet. Their origin is associated with thermal exchange between crust and mantle beneath the Bangong‐Nujiang and Jinshajiang River sutures which occurred in their own conduits. The thermal exchange conduit below the Bangong‐Nujiang River suture was formed earlier than that beneath the Jinshajiang River suture. Therefore the thermal activity of crust and mantle beneath the study area began from south and west, then spread toward north and east, leading to the current heat flow distribution in middle and northern Tibet which becomes larger from west to east and from south to north.

Numerical Simulation and Discussion on the Mechanism of Postseismic Deformation After Kunlun Ms8.1 Earthquake

“…Also, the value of η is lower in some regions, such as 5.0 × 10 17 Pa·s in Taiwan [25] and Sumatra [47] . On the other hand, geomagnetism and geo-electric measurements indicated that there may be partial melting material in southern of the eastern Kunlun fault [17,18] , which supports the rationality of our best-fitting results.…”

“…GPS results revealed that the differential movement between the two sides of the fault was about −10 ∼ 12 mm/a, which was consistent with the sliprate from geology since Holocene (∼ 12 ± 2 mm/a) [9∼12] . From the geophysical observation, there are many differences in physical behavior between the Qiangtang block and the Qaidam basin, such as velocity [13∼15] , density [13] , anisotropy [16] , electromagnetism [17,18] , geothermic characters. Just as the differences of physical behavior, the geodynamic environment exhibits distinctness in this region.…”

Conductivity structure and rheological property of lithosphere in Southern Tibet inferred from super-broadband magnetotelluric sounding

“…By 1997, three super-broadband (320-0.00005 Hz) MT profiles were completed, which are Yatung-Xoggola (line 100), Taktse-Bamucuo (line 200), and Xoggola-Damshung (line 300) [2][3][4]. The subsequent INDETH (III)-MT in 1998 and 1999 completed another two profiles, i.e., Dechen-Longweico (line 500) and NakchuGolmud (line 600) [5][6][7].…”

Electric Structure of Crust and Upper Mantle Beneath the Bangong‐Nujiang Suture in Tibet from Magnetotelluric Sounding

“…As stated in Section 4, there exist extensive high-conductivity layers in middle and lower crust in the Tibetan Plateau. There have been many discussions on their causes and many lines of evidence suggest they are associated with temperature in crust [27] .…”