HR: 0800h
AN: T31D-0672    [Abstracts]
TI: Orogenic superstructure behaviour and mid-crustal plastic flow in the central Nepal Himalaya
AU: * Godin, L
EM: godin@geol.queensu.ca
AF: Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L3N, Canada
AU: Kellett, D A
EM: dawn.kellett@dal.ca
AF: Department of Earth Sciences, Dalhousie University, Halifax, NS B3H3J5, Canada
AU: Larson, K P
EM: larson@geoladm.geol.queensu.ca
AF: Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L3N, Canada
AB: In the central Nepal Himalaya, the Tethyan sedimentary sequence (TSS) forms the superstructure to mid-crustal infrastructure rocks of the Greater Himalayan sequence (GHS); the top-to-the-north South Tibetan detachment system (STDS) defines their contact. North-verging folds, opposite to the main orogenic vergence, structurally dominate the TSS. Although the absolute age of this folding is unknown, structural observations and 40Ar/39Ar thermochronology indicate that it formed between 50-23 Ma, predating the dominant Miocene motion on the STDS. The GHS records a two-stage post-collisional history, marked by ca. 35 Ma burial metamorphism, followed by high-T, low-P, ca. 22 Ma metamorphism. Dominant top-to-the-south shear fabrics developed at peak temperatures at ca. 22 Ma pervasively transpose linear and planar features within the GHS. Vorticity analyses yield kinematic vorticity numbers between 0.29 and 0.80 (81–41% pure shear), with a significant amount of stretch parallel to the flow plane (34-53%). 40Ar/39Ar thermochronological data indicate that southward extrusion of the GHS terminated with cessation of movement on the STDS at 19 Ma. Our data suggest that the orogenic superstructure actively influenced the behaviour of the infrastructure in the early stages of orogenesis through fold-thrust belt formation leading to prograde 35 Ma metamorphism in the GHS. Associated melt weakening in the infrastructure allowed the initiation of southward plastic flow of the GHS, locally modifying the vergence of superstructural folds towards the north. As melt weakening in the middle crust intensified and the rheological contrast between superstructure and infrastructure increased, the upper crust decoupled from the middle crust and deformation in the upper crust temporarily ceased. By 17 Ma the extruded mid-crustal rocks cooled sufficiently to require the upper, brittle component of the STDS to become active. As cooling continued (17-14 Ma), the superstructure and underlying infrastructure (i.e., the upper crust, STDS and exhumed mid-crust) re-coupled and was subjected to localized large-scale buckling. This marked a transient stage of out-of-sequence deformation before the activation of new thrusts structurally below the GHS in late Miocene.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: 2007 Fall Meeting