T21 Dynamics of Gneiss Domes, Core Complexes, and Orogenic Plateaux

Sunday, 4 November 2012
8:00 AM - 12:00 PM
207D

KINEMATIC AND PETROLOGIC CONSTRAINTS ON THE EVOLUTION OF THE LHAGOI KANGRI GNEISS DOME, SOUTHERN TIBET

Paper number: T21-10
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

The North Himalayan gneiss domes are a series of features exposed in southern Tibet along the transition between the High Himalayas and the Tibetan Plateau that preserve key structural and metamorphic aspects of Himalayan mid-crustal evolution. Several models have been proposed for the evolution of these structures, and each model entails a specific tectonic history for the orogen. Characteristics shared by the North Himalayan gneiss domes include: (1) a core of gneiss, granite, and migmatite that is overlain by a metasedimentary sequence capped by unmetamorphosed sediments; (2) Barrovian metamorphism with grade increasing concentrically toward the core; and (3) at least two deformation events, the second of which is recorded by a well-defined, approximately north-south stretching lineation that transitions to mylonitic fabric adjacent to the core-cover contact. The Lhagoi Kangri gneiss dome located ~100 km northeast of Mt. Everest is one such dome for which relatively little data are available. Preliminary structural and petrographic analyses suggest that Lhagoi Kangri shares several similar features to the adjacent Mabja Dome.

The Lhagoi Kangri gneiss dome is a 25 x 40 km ellipse with a roughly east-west oriented long axis. The core is composed of orthogneiss and leucogranite and is intruded on its western margin by the Gyaco La granite. The cover rocks grade from staurolite-mica schist at the structurally lowest positions to phyllitic sandstone at the structurally highest positions. The highest structural levels exhibit km-scale, north- and south-vergent folds that are successively tighter with structural depth (D1). On the north side of the dome, deeper structural levels contain isoclinal, steeply SE-dipping folds (F1) with a penetrative axial planar fabric (S1). At moderate structural levels, D2 is recorded by refolded F1 folds and a crenulation cleavage (S2). L1x2 intersection lineations are subparallel to stretching lineations (Lm2) that trend NNE and SSW on the north and south sides of the domes, respectively. Near the core-cover contact penetrative mylonitic foliation defines D2. On the north side of the dome, shear bands and boudinaged veins record top-down-north shear sense, and on the south, garnet porphyroblasts exhibit asymmetric tails that record dominantly top-down-south shear sense.