Earthquakes at Collisional Plate Boundaries: an Overview of Himalayan

Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure Earthquakes at collisional plate boundaries: an overview of Himalayan tectonics and seismicity Introduction Seismicity on Earth is concentrated along plate boundaries, tectonically active regions subjected to a range of stress-regimes leading to violent slip events. This paper will focus on seismicity along converging plate-boundaries, and more specifically on seismicity along the continental collision that led to the formation of the 1400 km long Himalayan mountain belt in the Cenozoic. This paper will provide an overview of the main geological features of the Himalayas and their seismic implications; it offers a comprehensive summary of seismicity induced by the continental collision, and is thus meant to be used as a spring for more in depth study of the collision zone. The geological setting associated with the Indo- Asian collision will be discussed at first, from the early separation of India from Gondwana, to the post- orogeny state of the mountain chain, after which the discussion will shift to the main mechanisms that cause seismicity in the mountain range. This will include a discussion of the fault configuration and separation of the mountain chain into different lithological units by researchers. I will then consider the case of the 2015 Ghorka earthquake to illustrate seismic distribution along the two converging plates (seismic gaps) and discuss the ways this earthquake can be used –or not… – in seismic prediction. Finally, seismicity that takes place in India outside of the main Himalayan belt, and the possible mechanisms involved in their triggering, will be considered. Geological context of the Himalayan mountain chain Continental collision zones are inherently complex regions, both from a lithological and from a tectonic point of view. They bear the geological history of the old craton prior to the collision, as well as 1 Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure the evidence of more recent tectonic changes related to the collision event –ongoing tectonic changes in the case of the Himalayas. Another active continental collision today is that occurring between the Arabian plate and Iran, which has given rise to the Zagros mountain chain. The formation of the Zagros is closely related to that of the Himalayas, as both were induced by the closure of the Tethys Ocean which started in the end of the Mesozoic. In fact, these two orogenic locations bear resemblances both in the topography resulting from the collision –rise of the Tibetan Plateau on one side and that of the Iranian Plateau on the other – and in the seismic patterns recorded along the mountain chains (Hatzfeld et al. 2010). It thus appears that common features can be drawn from different continental collision settings, and that seismicity observed in the Himalayas is not unique to this site. The scale of the collision in the Himalayas however exceeds by far that in any other similar setting, which motivates this paper. The complexity of the Himalayan orogeny is recorded in the lithology of the long mountain belt that separates Northern India from the Tibetan Plateau. The changes brought to the old Indian craton during collision include the initial formation of an accretionary prism, the uplift and denudation of the Asian crust, metamorphism and anatexis of the rocks caught in the suture zone. The Himalayas are made up of an amalgamation of continental arc rocks, S-type granitic plutons, metamorphosed uplifted oceanic sediments (Harrison et al 2000), that all record a specific step that led to the eventual formation of the main mountain chain which can be seen today. Figure 1 Figure 1 from Shanker at al. 2010: the different lithological units that make up the main mountain chain. The purple formations correspond to Neogene sediments, the brown to highly metamorphosed plutonic bodies and the green ones to uplifted Tethyan sediments. 2 Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure The Himalayan orogeny started with the Indian plate detaching from supercontinent Gondwana 70 Ma ago, and moving North at an initial rate of 15-25cm/a, which is a record in plate tectonic motion. Based on a subsequent decrease in the rate of advance of the Indian plate recorded in magnetic anomalies of the Indian Ocean, the Indo-Asian collision has been fixed at 50 Ma. This decrease in the relative rate of convergence of the two plates is believed to correspond to increased kinematic resistance between the plates, as the buoyant Indian margin collided with Asia after the subduction of the oceanic portion of the Indian crust under Asian continental crust (Harrison et al. 2000). Stratigraphic evidence has also been used to constrain a lower limit (youngest age) for the Indo-Asian collision, as around 52 Ma an abrupt change from marine to terrestrial depositional environment is recorded in northeastern India (Harrison et al. 2000). The Himalayan orogeny is not restricted to the main mountain chain that spans most of Nepal, Bhutan and Northern India, but rather extends more than 600 km north from the Higher Himalayas (where the Everest is located) in the form of the Tibetan Plateau. Indeed, the Indo-Asian collision not only led to the formation of the mountain belt and its associated faults, but further led to the uplift of the Tibetan Plateau, which is composed of terranes, such as the Qiangtang and Lhasa ones, that were accreted to Asia before the onset of the collision (Harrison et al. 2000). The mechanism behind the uplift of the Tibetan Plateau, although debated, is believed to be the subduction of a slab of Greater Indian lower crust under the Asian continent (DeCelles et al. 2002). Although continental buoyancy usually stalls subduction shortly after collision, plate velocities were merely reduced in the case of the Himalayas when the Indian margin impinged on Asia; the subduction of the Greater Indian continent continues to this day (Capitano et al. 2010). It would be interesting to compare this situation to other continental collision zones along the Alpine-Himalayan chain to determine the nature of the driving forces behind the convergence. The unusual rate of advance of the plate has been postulated as a possible cause of the continuing subduction, as well as the higher density of the lower Indian crust, whose lighter upper part got scraped off as the Himalayan front, leaving a readily subductable denser lower crust behind (Capitano et al. 2010). 3 Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure The Tibetan plateau, just like the Himalayas, is still active at present, and the East-West extension that the Plateau is undergoing results in the formation of normal faults that have led to great earthquakes in the past (DeCelles et al. 2002). This paper will focus on seismicity at the forefront of the Himalayan chain, at the contact of the two plates, and on the associated faulting. The figure below illustrates the distribution of stress along the Himalayas, and clearly shows how thrusting transitions into normal faulting as one draws deeper into Tibet. The stress-regime shifts from compressional to extensional, which is explained by the effect of the subducted lower-crust slab, which forces Tibet to extend along an East-West axis (DeCelles et al. 2002). Figure 2 Figure 2 from Shanker et al. 2011: this figure displays the change in tectonic regime that occurs on a path from the Lower Himalayas all the way to the Tibetan Plateau, currently undergoing extension An experiment by Toussaint et al. 2004 investigates the tectonic evolution of the Himalayas and its associated crustal deformation. The authors show how the age relationships of different formations within the mountain belt allow to make estimations as to the sequence of tectonic events and faulting styles that shaped the Himalayan collisional boundary. For example, strongly metamorphosed Tethys- aged sediments that used to separate the northern margin of India from the active margin of southern Eurasia can now be found north of the high Himalayas, which indicates an early uplift of the sediment- wedge caught between the converging plates. According to the model developed by Toussaint et al., the 4 Olivia Dagnaud 260633356 EPSC 330: Earthquakes and Earth structure main fault that governs the Himalayan system formed at a relatively early stage, following deformation at the suture zone. It was subsequently followed by exhumation of the lower crust of the Asian plate combined with the subduction of the Indian crust. The last stage of collision, which follows activity at the major thrust fault, consists in frontal accretion of a large wedge, a process that continues to this day. At present, it has been determined that not only India is penetrating into Asia, but it is also slowly rotating anti-clockwise (Sella et al. 2002). The rotation has notably been recorded in the province of Baluchistan, in Pakistan, where left-lateral strike-slip faults at a rate of 42 mm/a have been measured (Sella et al. 2002). Tectonic configuration Competing hypotheses suggest that the Himalayan topography is sustained and plate convergence is accommodated either predominantly on the main plate boundary fault or more broadly across multiple smaller thrust faults (Elliott et al. 2016). The main plate boundary fault, also called the Main Himalayan Thrust (or MHT), is the main basal decollement into which all minor faults sole. There is much debate however about the structure and geometry of this decollement surface. Within the fault-system, the largest faults that converge toward the Main Himalayan decollement are namely, from South to North, the Main Boundary Thrust, the Main Central Thrust and the South Tibetan detachment. The North-dipping Main Boundary Thrust marks the contact between the Lesser Himalayan Formations and the underlying Miocene-Pleistocene Siwalik Formations, which are the youngest and least metamorphosed beds in the mountain chain, and which are still actively underthrust (An Yin et al.

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