Universität Tübingen
Tectonic evolution of the Tethyan Himalaya in SE Tibet deduced from magnetic fabric, structural, metamorphic and paleomagnetic data
Abstract
A multidisciplinary approach was carried out to elucidate the kinematic and metamorphic evolution of the Tethyan Himalayan sequence in SE-Tibet in order to contribute to the understanding of continental collision orogens as the Himalaya. Phyllosilicates preferred orientation interpreted from magnetic fabric studies, structural-field data, illite crystallinity, vitrinite reflectance and K/Ar dating were analyzed to characterize the deformation and metamorphic phases of the Triassic flysch sequence in the eastern Tethyan Himalaya. The second part of the thesis utilizes a paleomagnetic approach to determine the Miocene vertical-axis block rotation pattern in SE-Tibet and kinematic implications for the Himalayan Belt and southeastern Tibetan Plateau evolution. Additionally the paleomagnetic study was extended to the southern most area of the Tethyan Himalayan sequence within the Lingshi klippe in NW Bhutan. The tectonometamorphic evolution of the Triassic flysch in SE-Tibet can be summarized as follows. During the D1 phase E-W trending south facing isoclinal folds with related axial planar foliation (S1) developed. South of the locality of Qonggyai (D1 phase domain) the N-S trend and northward plunge of the magnetic lineation is in agreement with thrust emplacement towards the south (Himalayan foreland). The same studied sites indicate diagenetic to low-anchizonal metamorphism. The D1 deformation phase described here took place most probably during Eocene times. During the D2 phase continuation of N-S shortening in the flysch fold-thrust-wedge derived in a change in the fold vergence and magnetic and tectonic foliation dipping. This is generally observed in the northern studied areas east of the Cona Graben. D2 deformation is recorded through south-dipping magnetic foliation, north-vergent F2 folds and related S2 tectonic foliation. A slightly prolate magnetic ellipsoid was found in between the D1 and D2 domains recording the intersection of S1 and the subtle development of the S2 tectonic foliation. Kübler Index and vitrinite reflectance data indicate a wide range of thermal overprint from high anchizone to epizone during the D2 phase. S2 foliation development can be related to the ca. 22 Ma K/Ar ages found in the D2 domain. During the D3 phase backward propagation of the deformation led to the Great Counter backthrust generation, recorded by SSW steeply plunging magnetic lineation parallel with the stretching lineation east of Gyaca in the Great Counter backthrust zone. In the final D4 phase Middle-Late Miocene E-W extension is witnessed by ca. N-S kilometric normal faults of the Cona Graben which cross-cut all the previous structures. Paleomagnetic analyses from Cretaceous metadykes in SE-Tibet and metasedimentary formations in NW-Bhutan indicate that the main carrier of the characteristic remanence is pyrrhotite. The pyrrhotite component unblocks between 280-350°C in SE-Tibet and 270-325°C in NW-Bhutan, and shows reverse and normal polarities. It has a ca. constant declination in SE-Tibet through the D1 and D2 folds, and reveals best grouping at 18 % of unfolding in one site in the Linghsi klippe. Early to Middle Miocene thermal overprint of the studied metadykes and metasedimentary formations constrained by K/Ar method is related to the time of remanence acquisition. This event is younger than the D1 and at the end of D2 described in SE-Tibet. In the Lingshi klippe the pyrrhotite mean site directions show a small circle distribution likely related to the last large-scale folding event. A small circle method permits to better unravel the degree of vertical-axis rotation. Pyrrhotite components are ca. 0°, 20° and 37° clockwise rotated with respect to the Early to Middle Miocene references of stable India at the Nagarze, Qonggyai and Lingshi study areas, respectively. The rotation is related to Middle-Late Miocene strain partitioned between near-field E-W extension (with unclear influence of reactivation of older structures as strike-slip faults) and far-field eastward motion/extrusion of the Tibetan Plateau crust.
Author and committee
dc:creator, dc:contributor.*- Author
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- Antolín Tomas, Borja
Identifiers
dc:identifier.*- Identifier
- hdl:10900/49417