T56 New Perspectives on Processes of Subduction Initiation and Termination

Sunday, 22 October 2017
1:30 PM - 5:30 PM
Tahoma 5

MAGMATIC EVIDENCE FOR LOCALIZED LITHOSPHERIC DELAMINATION RESULTING FROM CONTINENTAL COLLISION IN THE PAMIR-TIBET OROGEN

Paper number: T56-14
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

The Pamir-Tibet orogen comprises several terranes (e.g., Qiangtang, South Pamir) that were accreted to southern Asia during the Mesozoic. The terranes are younger and weaker than the para-autochthonous to cratonic blocks that anchor the northern Pamir-Tibetan Plateau. The boundary between the terranes and stable lithospheric blocks, the Tanymas-Jinsha suture zone, is an important rheological discontinuity within the lithosphere. A pulse (36-42 Ma; zircon U-Pb ages) of geographically restricted (~150 km diameter ellipsoidal area) magmatism, referred to as the Vanj magmatic complex in the Pamir, is interpreted to be related to a mantle drip or small delamination event that developed along this rheological discontinuity, 10–20 Myr after subduction of oceanic lithosphere terminated and India first collided with Asia. The age, geochemistry (SiO2 = 54–75 wt. %; high-K calc-alkaline to shoshonitic; metaluminous; La/Ybn = 13–57; Mg# = 35–41), isotopic composition (εNdt = -7 to -3; 87Sr/86Sri, = 0.706–0.710; zircon εHft = -3 to +1; zircon δ18OVSMOW = 6.0–7.6 ‰), outcrop pattern, and tectonic position of the Vanj complex suggest that it is related to a series of magmatic complexes that occur in the northern Qiangtang terrane in Tibet, adjacent the Jinsha suture. The Eocene Vanj and Qiangtang magmatic complexes are collectively interpreted as nearly synchronous, discrete drips of mantle lithosphere or small delamination events that occurred for >2,500 km along strike in the Pamir-Tibet orogen. Syn-collisional magmatic products like the Vanj complex may be characteristic of continental collisional orogenesis. The results of this study support recent numerical models that suggest deformation and crustal thickening may be concentrated along rheological boundaries during continental collision and that discontinuities may help nucleate delamination events. In the case of the Vanj and Qiangtang complexes, lithospheric deformation and subsequent delamination occurred 300-600 km inboard (North) of the India-Asia suture zone.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305706

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.