Structure of the crust in the vicinity of the Banggong‐Nujiang suture in central Tibet from INDEPTH magnetotelluric data

Abstract

[1] Magnetotelluric data from a 150-km-long profile crossing the Banggong-Nujiang suture (BNS), central Tibet, acquired as part of the International Deep Profiling of Tibet and the Himalaya (INDEPTH) project, have been examined for crustal and upper mantle structure. Strike and dimensionality analyses demonstrate that regional-scale electrical structures are two-dimensional and oriented approximately parallel to surface geological strike. As seen elsewhere in Tibet, the double thickness crust is generally characterized by resistive upper crust (hundreds to thousands of ohm meters) overlying conductive middle and lower crust (tens to hundreds of ohm meters), but in detail, there are lateral variations at all levels. Regionally, a northward transition from thick ($45 km) to thin ($15 km) resistive upper crust coincides with (1) the surface trace of the BNS, (2) a prominent strand of the Karakorum-Jiali fault system, (3) northward decrease in upper mantle seismic velocities and increase in attenuation, and (4) pronounced northward onset of seismic polarization anisotropy. The latter two seismological features have been taken to mark the northern limit of Indian mantle lithosphere thrust beneath southern Tibet. On the basis of our electrical model, we speculate that (1) the resistive upper crustal root beneath the Neogene Lunpola and Duba basins was produced by crustal shortening localized along the northern edge of the Lhasa terrane; (2) the low midcrustal resistivity beneath the BNS reflects enhanced Neogene melting and/or metamorphic dewatering of relatively fertile subduction zone complex rocks; (3) observed steep upper crustal low-resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the BNS; and (4) these strike-slip and extensional fault arrays are surface manifestations of lithosphere-penetrating shear localized along the northern edge of the underthrust Indian plate.

Magnetotelluric Studies of Active Continent–Continent Collisions

“…Zhang et al (1996) describe three broadband MT profiles that extended across the Qiangtang terrane and showed that a mid-crustal conductor was present in the central Tibetan Plateau. Solon et al (2005) report a more detailed study of the INDEPTH 500-line where it crossed the Bangong-Nuijiang suture (BNS) and showed an increase in the conductivity of the mid-crustal layer to the North. Mechie et al (2004) used a combined analysis of the 500-line MT data and coincident seismic reflection data to determine the depth of the a-b quartz transition.…”

MT+, Integrating Magnetotellurics to Determine Earth Structure, Physical State, and Processes

“…Rigorous model assessment, as discussed next, must further be applied. A more detailed description of the magnetotelluric method and its development can be found in Simpson and Bahr (2005), Jiracek et al (1995), Jones (1992), and Vozoff (1991).…”

“…BNS = Bangong-Nujiang Suture. Figure reproduced fromMechie et al (2004) using a resistivity model modified afterSolon (2000) …”

Up-To-Date Geodynamics and Seismicity of Central Asia

“…Results of INDEPTH seismic and magneto-telluric soundings give indirect evidences of such process. They establish some layers of increased plasticity and partial melting of rocks in the Tibet crust at the depth 20-30 km [31,32]. The SE boundary of the Bayanhar Block coincides with the sharp step in the crust and in the whole lithosphere along the lineament of 102˚ -103˚ E [24] with intensive decreasing lithosphere thickness to the east [36].…”