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Earthquake Occurrence Processes in the Indo-Burmese Wedge and Sagaing Fault Region

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Earthquake Occurrence Processes in the Indo-Burmese Wedge and Sagaing Fault Region Tectonophysics 524–525 (2012) 135–146 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Earthquake occurrence processes in the Indo-Burmese wedge and Sagaing fault region Bhaskar Kundu, V.K. Gahalaut ⁎ National Geophysical Research Institute (CSIR), Hyderabad, India article info abstract Article history: Earthquakes in the Indo-Burmese wedge and Sagaing fault regions occur in response to the partitioning of the Received 25 January 2011 India–Sunda motion along these two distinct boundaries. Under the accretionary wedge of the Indo-Burmese Received in revised form 11 November 2011 arc, majority of the earthquakes occur in the depth range of 30–60 km and define an eastward gently dipping Accepted 21 December 2011 seismicity trend surface that coincides with the Indian slab. The dip of the slab steepens in the east direction Available online 29 December 2011 and earthquakes occur down to a depth of 150 km, though the slab can be traced up to the 660 km discontinuity. Although these features are similar to a subduction zone, the nature of the earthquakes and Keywords: Indo-Burmese wedge our analysis of their focal mechanisms suggest that these earthquakes are of intra-slab type which occur Sagaing fault on steep plane within the Indian plate and the sense of motion implies a northward relative motion with Seismogenesis respect to the Sunda plate. Thus these earthquakes and the stress state do not support active subduction Earthquake focal mechanisms across the Indo-Burmese arc which is also consistent with the relative motion of India–Sunda plates. The Intra-slab earthquakes absence of inter-plate earthquakes, lack of evidence of the occurrence of great earthquakes in the historical records and non-seismogenic nature of the plate interface under the accretionary wedge suggest that seismic hazard due to earthquakes along the plate boundary may be relatively low. However, major intra-slab earthquakes at shallow and intermediate depths may still cause damage in the sediment filled valley regions of Manipur and Cachar in India and Chittagong and Sylhet regions of Bangladesh. In the Sagaing fault region, earthquakes occur through dextral strike slip motion along the north–south oriented plane and the stress state is consistent with the plate motion across the Sagaing fault. © 2011 Elsevier B.V. All rights reserved. 1. Introduction past when it was predominantly southeast–northwest trending. Though the precise timing of this transition is not known, it appears The Indo-Burmese wedge along with the Sagaing fault forms the to have occurred prior to about 50 Ma (Hall, 1997). However, after northern part of the northwestern Sunda arc (Chandra, 1984; Chen the collision of the India plate with the Eurasian plate, the Burma and Molnar, 1990; Curray, 2005; Fitch, 1972; Le Dain et al., 1984; plate, consisting of the Indo-Burmese wedge, Myanmar Central Nandy, 2001; Ni et al., 1989; Verma et al., 1976). The approximately Basin along with the Andaman–Sumatra arc, rotated clockwise to north–south trending convex westward Indo-Burmese wedge joins become predominantly north–south trending (Hall, 1997). The extru- the approximately east–west trending eastern Himalaya in the sion and clockwise rotation of the Burma plate in the late tertiary north (Fig. 1). The region of this junction is referred as the Eastern period created compressional structure in the Myanmar Central Himalayan Syntaxis, which is marked with complex tectonics, high basin (MCB) (Everett et al., 1990; Le Dain et al., 1984; Tapponnier exhumation and erosion rates, etc. (Zeitler et al., 2001). In the et al., 1982). Recent geochronology of the Mogok metamorphic belt south, the Indo-Burmese wedge joins with the north–south trending in Burma (Searle et al., 2007) supports that right-lateral motion on Andaman–Sumatra subduction zone. In the region of Indo-Burmese the Sagaing fault which might have initiated after 16–22 Ma. The wedge, the northward motion of about 35 mm/year of the India Burma plate appeared to have originated through three major phases. plate with respect to the Sunda plate (Maurin et al., 2010; Nielsen In the first phase, by the end of the Eocene (~35 Ma), Burma Plate col- et al., 2004; Vigny et al., 2003) is assumed to be accommodated lided with the northeast edge of the Indian plate and was dragged through slip partitioning in the Indo-Burmese arc and on the Sagaing northward as a fore arc sliver. In the Miocene (~15 Ma), this acceler- fault (Fig. 1). The plate reconstruction models suggest that subduc- ated motion led to the formation of NE–SW trending extensional tion probably occurred in the Indo-Burmese wedge in the geological basins bounded by NE–SW striking normal faults, and to the creation of Andaman sea rift (Curray, 2005). Finally in the Pliocene (~5 Ma), when the northern end of the Burma Plate collided with Asia, ⁎ Corresponding author. Tel.: +91 40 23434700. extensional deformation ceased and transpressional deformation E-mail address: [email protected] (V.K. Gahalaut). caused reverse faults, positive flower structures, inversion of normal 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.12.031 136 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146 Maurin and Rangin (2009) analyzed structures and kinematics of the Indo-Burmese wedge which defines the western margin of the Burma plate, a sliver between the India and Sunda plates (Gahalaut and Gahalaut, 2007; McCaffrey, 1992). On the basis of the age, grade of metamorphism and rock type, they classified it into outer, inner and core wedges. The outer wedge mainly consists of Tripura Fold belt and the eastern part of the Bengal basin and is made of Neogene clastic sequences. Age of the sediments in the outer wedge ranges from lower Miocene submarine deposits, upper Miocene shelfal deposits to Plio-Pleistocene fluvial deposits (Maurin and Rangin, 2009). The inner Indo-Burmese wedge is composed of Eocene flyschs affected by N–S trending strike-slip right faults, such as the Churachandpur–Mao fault, CMF, discussed later in the text. The core of the wedge is made of high-grade metamorphic rocks, tectonically imbricated with Mesozoic ophiolites and sedimentary sequences ranging from Late Triassic to Late Cretaceous (Bender, 1983). A detailed discussion on these units may be found in Maurin and Rangin (2009). In this article, we analyze the seismicity of the region and address all these issues to suggest that no subduction occurs across the Indo Burmese wedge and the earthquakes are of intra- slab type that occur within the Indian plate through reactivation of the old geologic fabric. 2. Seismicity of the region 2.1. Historical major earthquakes Fig. 1. Major tectonic features of the Sunda and Himalayan arc. There are two most notable great earthquakes, namely, the 1897 Shillong Plateau and the 1950 Assam earthquakes (Molnar, 1990; Seeber and Armbruster, 1981) that have occurred close to the Indo- faults and extensional basin (Bertrand and Rangin, 2003; Maurin and Burmese arc and Sagaing fault (Fig. 2). The 1897 Shillong Plateau Rangin, 2009). earthquake occurred primarily under the Shillong Plateau and is Presently, the motion between the India and Sunda plates is linked with the tectonics of the Shillong Plateau (Bilham and partitioned between Indo-Burmese arc and Sagaing fault. Our recent England, 2001). Thus this earthquake is considered as an intra-plate GPS measurements in the Indo-Burmese arc region (Gahalaut, V.K. earthquake which probably occurred through reverse motion on the et al., manuscript in preparation) and the available measurements south dipping steep fault. The 1950 Assam earthquake occurred in in the Sagaing fault region (Maurin et al., 2010; Vigny et al., 2003) the Arunachal Himalaya and the Eastern Himalayan Syntaxis region suggest that about 60% of the relative motion between India and and is considered as the interplate earthquake that occurred due to Sunda plates is taken up by the Sagaing fault. Both regions are the ongoing India–Eurasia convergence, part of which is accommo- characterized by earthquake occurrences. One of the major dated in the Himalaya (Molnar, 1990; Seeber and Armbruster, differences between the earthquakes in the Indo Burmese wedge 1981). Thus the two great earthquakes are probably not linked with and in the Sagaing fault is their focal depth. Earthquakes are generally the tectonics of the Indo-Burmese arc and the Sagaing fault regions. very shallow in the latter region, whereas in the former, they occur up We compiled a catalogue of major earthquakes in the Indo- to a depth of 150 km (Guzman-Speziale and Ni, 1996). Another major Burmese arc and Sagaing Fault regions (Fig. 2 and Table 1). We difference is that the earthquakes predominantly occur through strike excluded earthquakes occurring in the Shan Plateau and Red River slip motion on the Sagaing fault while in the Indo Burmese wedge, fault region as they are linked with the tectonics of the Tibet Plateau they occur through strike slip and thrust and oblique motion. In the extrusion due to the India–Eurasia convergence. Although there are a Indo Burmese wedge, it is not known whether these earthquakes are few unverifiable reports of earthquake occurrence as early as 1548 in of inter-plate or intra-plate (or intra-slab) type (Guzman-Speziale Tripura, Assam or Bangladesh (Iyengar et al., 1999; Steckler et al., and Ni, 1996). Several geological studies (arc magmatism and meta- 2008), there are large uncertainties in the earthquake location. We morphism, occurrence of ophiolitic rock sequences, surface as well find that the catalogue is probably reliable only after 1762. As many subsurface expression of fold and thrust belt structures), geophysical of these historical earthquakes are located on the basis of maximum observations (tomographic images and gravity anomaly), and plate damage, their epicentral locations are not very reliable.
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