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Research Article |
1 Centre for Geodynamics, Geological Survey of Norway, NO-7491 Trondheim, Norway
2 Physics of Geological Processes, University of Oslo, NO-0316 Oslo, Norway
3 School of Geosciences, University of Witwatersrand, WITS 2050, South Africa
4 Department of Geology, University of Wisconsin, Oshkosh, WI 54901, USA
5 Department of Earth Sciences, University of California, Riverside, CA 92521, USA
6 Department of Geology, Colorado College, Colorado Springs, CO 80903, USA
*Corresponding author (e-mail: trond.torsvik{at}ngu.no)
To test whether the Tethyan Himalaya were part of the northern margin of India in the early Palaeozoic we have produced the first primary palaeomagnetic data (bedding-corrected declination 267.5°, inclination 63.0°, 
95 = 10°; pole latitude 20.2°N, longitude 28.6°E) from low metamorphic grade Ordovician red beds in the Tethyan Himalaya (Shian Formation). The palaeomagnetic data are of excellent quality, and a statistically positive fold test combined with a comparison with late Cambrian–Ordovician Gondwana poles suggests a primary hematite-bearing magnetization, acquired between 470 and 500 Ma. This is in excellent agreement with stratigraphic, faunal and provenance age estimates, and the palaeomagnetic data demonstrate that the Tethyan Himalaya must have been located in proximity to the Indian craton during early Ordovician times, and are therefore consistent with a continuous margin at that time. The Shian Formation pole overlaps with 470–500 Ma Gondwana poles, but an even better fit can be obtained by invoking a post-Ordovician clockwise rotation of 13° ± 4°. Such a rotation is similar in both sense and magnitude to clockwise rotations recorded in primary Triassic sequences as well as Palaeogene palaeomagnetic overprint data from the Tethyan Himalaya: rotations of the Tethyan Himalaya compared with cratonic India are thus probably all of post late Eocene age. Triassic and Early Ordovician data do not imply any crustal shortening between Tethyan Himalaya and cratonic India. However, in the Early Ordovician, India was rotated 90° compared with its present orientation, and any enlargement of India would not be detected by palaeomagnetic data.
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