Clockwise rotation of the Brahmaputra Valley relative to India: Tectonic convergence in the eastern Himalaya, Naga Hills, and Shillong Plateau

Abstract

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 2-3 mm/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 120 km 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 ≈12 mm/yr and convergence between the Shillong Plateau and Bangladesh across the Dauki fault increases from 3 mm/yr in the west to >8 mm/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 4-8 Myr. 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~5.4 m, similar to the slip reported from alluvial terraces that offsets across the Main Himalayan Thrust and sufficient to sustain a M w ≥ 8.0 earthquake in this area.

Present‐day crustal thinning in the southern and northern Tibetan Plateau revealed by GPS measurements

“…The data set of He et al [2013] contains 16 campaign-mode sites located in a localized dense network across central segment of Altyn Tagh Fault from the Tarim Basin to northern Tibet and measured twice in the period of 2009-2011. In addition, we show published velocities for surrounding regions (Figure 1): from the Nepal Himalaya [e.g., Banerjee et al, 2008;Bettinelli et al, 2006;Bilham et al, 1997;Feldl and Bilham, 2006], northwest Himalaya [Banerjee and Bürgmann, 2002], Pamir-Hindu Kush [Ischuk et al, 2013;Mohadjer et al, 2010], Kashmir and Ladakh Himalaya [e.g., Jade et al, 2004;Schiffman et al, 2013], eastern Himalaya, Naga Hills, and Shillong Plateau [Devachandra et al, 2014;Vernant et al, 2014].…”

Chapter 2 Active tectonics of Myanmar and the Andaman Sea

“…This increase is mirrored by a decrease in shortening rate across the Himalaya (Vernant et al 2014). Geological uplift rates have been estimated at 0.7-1.4 mm a −1 in the eastern part of the fault (Clark & Bilham 2008), where the GPS constraints suggest a shortening rate of 5-7 mm a −1 (Vernant et al 2014).…”

“…This increase is mirrored by a decrease in shortening rate across the Himalaya (Vernant et al 2014). Geological uplift rates have been estimated at 0.7-1.4 mm a −1 in the eastern part of the fault (Clark & Bilham 2008), where the GPS constraints suggest a shortening rate of 5-7 mm a −1 (Vernant et al 2014). These values are irreconcilable, unless (1) the Dauki Fault has an exceptionally low dip, in which case it would intersect the Oldham Fault in a physically unrealistic way (Bilham & England 2001), and contradict the recent receiver function analysis that suggests its dip to be c. 30° ; (2) the geodetic rates are increased relative to the long-term shortening rate due to long-lasting post-seismic deformation associated with the 1897 earthquake; or (3) the shortening rate has increased dramatically over the plateau's history, perhaps because of the evolving stress state as the Plateau approached the Himalayan Arc (Najman et al 2016).…”

“…A recent GPS survey of NE India has shown that the horizontal shortening rate across the Shillong Plateau is 3-6 mm a −1 , increasing to the east (Vernant et al 2014). This increase is mirrored by a decrease in shortening rate across the Himalaya (Vernant et al 2014).…”

“…Earthquakes and GPS data west of Myanmar. GPS velocities relative to stable India taken from Maurin et al (2010), Gahalaut et al (2013) and Vernant et al (2014) and converted to a consistent reference from Vernant et al (2014). Coloured dots indicate earthquake locations from Stork et al (2008).…”