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Le Roux-Mallouf R.,Montpellier University | Godard V.,Aix - Marseille University | Cattin R.,Montpellier University | Ferry M.,Montpellier University | And 8 more authors.
Geophysical Research Letters | Year: 2015

The Main Himalayan Thrust (MHT) is the source of great earthquakes that have been documented along the range. Its geometry is a key parameter that influences accommodation of tectonic loading and earthquake magnitudes along the Himalayan Arc. Although seismic images are available for both the western and the central part of the range, this geometry remains poorly constrained for the Bhutanese Himalayas. Here we address this issue using a 10Be cosmogenic nuclides denudation transect across western Bhutan. We observe a wide low denudation rate domain between 50km and 110km from the front followed by a strong northward increase. Using a joint inversion of denudation rates, GPS data, and Holocene uplift rates, we interpret this pattern as a consequence of a flat-ramp transition along the MHT. Compared to central Nepal and Sikkim, this location of the ramp suggests a wider décollement, with implications for greater seismogenic potential of the MHT in western Bhutan. Key Points Assessment of denudation rates in Himalayas Robust constrain on the location of the flat-ramp transition of the MHT Evidence of a strong lateral variation in the MHT geometry ©2015. American Geophysical Union. All Rights Reserved.

Vernant P.,Montpellier University | Bilham R.,University of Colorado at Boulder | Szeliga W.,Central Washington University | Drupka D.,Seismology and Geophysics Division | And 6 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2014

GPS data reveal that the Brahmaputra Valley has broken from the Indian Plate and rotates clockwise relative to India about a point a few hundred kilometers west of the Shillong Plateau. The GPS velocity vectors define two distinct blocks separated by the Kopili fault upon which 2-3mm/yr of dextral slip is observed: the Shillong block between longitudes 89 and 93°E rotating clockwise at 1.15°/Myr and the Assam block from 93.5°E to 97°E rotating at 1.13°/Myr. These two blocks are more than 120km wide in a north-south sense, but they extend locally a similar distance beneath the Himalaya and Tibet. A result of these rotations is that convergence across the Himalaya east of Sikkim decreases in velocity eastward from 18 to 12mm/yr and convergence between the Shillong Plateau and Bangladesh across the Dauki fault increases from 3mm/yr in the west to >8mm/yr in the east. This fast convergence rate is inconsistent with inferred geological uplift rates on the plateau (if a 45°N dip is assumed for the Dauki fault) unless clockwise rotation of the Shillong block has increased substantially in the past 4-8Myr. Such acceleration is consistent with the reported recent slowing in the convergence rate across the Bhutan Himalaya. The current slip potential near Bhutan, based on present-day convergence rates and assuming no great earthquake since 1713 A.D., is now ~5.4m, similar to the slip reported from alluvial terraces that offsets across the Main Himalayan Thrust and sufficient to sustain a Mw = 8.0 earthquake in this area. © 2014. American Geophysical Union.

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