Deformations structures in rockfall-dammed lake sediments near Sangla: Imlications on neotectonic activity in the Sutlej region (NW India)
- Author(s)
- Erich Draganits, Bodo Bookhagen, Susanne Gier, B Grasermann, Christa Hofmann, Christoph Janda, Christian Hager
- Abstract
The Sutlej Valley forms a natural cross-section perpendicular to the general trend of the Himalayas and exposes all tectonic units of the orogen. The Main Central Thrust, which separates the Lesser Himalaya and the Higher Himalaya tectonic units, is the most prominent tectonic feature in the Indian part of the Valley. Geochronological data show that the Main Central Thrust was active during the Early Miocene. Subsequently, the thrust was folded in a prominent antiform-synform foldtrain typical for the whole Himalayan orogen and in this way exposes Lesser Himalaya rocks in large windows (e.g. Shali half-window, Larji-Kullu-Rampur window). Based on geochronological data and the fact that the Main Central Thrust is folded, it is clear that the Main Central Thrust is inactive today and therefore cannot be responsible for active tectonics in this area (e.g. thermal springs, steep near-surface thermal gradients, deformed Quaternary sediments, and seismicity). A probable candidate for triggering active tectonics in the Sutlej Valley is the still on-going out-of-sequence extrusion of a wedge-shaped metamorphic massif, the Lesser Himalayan Crystalline Sequence, between a thrust at the base, the Munsiari Thrust, and a concurrent normal fault on the top, the Karcham Normal Fault (Grasemann and Vannay 1998; Janda et al. 2001). Pleistocene fission track cooling ages (Jain et al. 2000, pers. comm. M. Rahn) in the hanging wall of the Munsiari Thrust and Pliocene ages from the Main Central Thrust probably indicate that the Munsiari Thrust represents an out-of-sequence thrust. The wedge models represent attractive solutions for the observed extensional deformation within a convergent orogen. The extensional deformation is not restricted to the base of the HHC (i.e. Karcham Normal Fault) but is typical for the whole area east of Karcham (i.e. hanging wall above the Karcham Normal Fault); brittle faults indicate an overall NE-SW to W-E extensional regime (Janda et al. 2001). Deformed Quaternary sediments have already been described in the NW Himalayas (Mohindra and Bagati, 1996). In the Baspa Valley, near Sangla, in a tectonic position in the hanging wall above the Karcham Normal Fault, lake sediments have been accumulated behind a Quaternary rockfall. Sandy to silty sediments predominate; in the deepest part of the palaeo-lake, close to the rockfall, clay-rich layers containing tree leaves and bivalves have been found. Palynological examinations of this clay horizon result in a relatively diverse flora (65 taxa) that indicates considerable warmer and more humid climate (e.g. 25 taxa of ferns and fern allies), than a less diverse flora (23 taxa) in a sample from the uppermost part of the lake fill. It is therefore suggested, that a more humid climate facilitated the rockfall, additional to the seismic activity in this area. Deformation structures are common in these lake sediments; they mainly consist of water-escape-structures and soft sediment folding. Abundant brittle faults in the lake sediments indicate Quaternary tectonic activity; their orientation indicates a NE-SW to W-E extensional regime, consistent with the orientation of normal faults found in crystalline rocks (Janda et al. 2001). Probably extension in the hanging wall could be genetically linked with the general shear extrusion of an active extruding wedge above the Munsiari Thrust.
- Organisation(s)
- Department of Palaeontology
- External organisation(s)
- Universität Potsdam, Universität Wien
- Journal
- Journal of Asian Earth Sciences
- Volume
- 19
- No. of pages
- 14
- ISSN
- 1367-9120
- Publication date
- 2001
- Austrian Fields of Science 2012
- 1051 Geology, Mineralogy
- Portal url
- https://ucris.univie.ac.at/portal/en/publications/deformations-structures-in-rockfalldammed-lake-sediments-near-sangla-imlications-on-neotectonic-activity-in-the-sutlej-region-nw-india(aaf29b2b-eb05-4100-a23d-2f3a53599520).html