The Seismic Structure of the Central Indo-Burman Subduction Zone
About the Event:
Studies of the evolution of the India-Eurasia collision have shown that the geodynamic processes associated with trench/slab rollback along the Indo Burma ranges have played a significant role in the large scale rotational deformational patterns and influenced the building of the Eastern Himalayan Syntaxis portion of the Tibetan Plateau. Furthermore, this complex tectonic system provides an excellent opportunity to study the nature of mantle deformation in response to the dynamic interaction of a possible clockwise rotation around the Eastern Himalayan Syntaxis and its relation to the subduction of the Indian plate. The crustal clockwise rotational deformation continues into northern Myanmar, accommodated by a series of East Northeast-West Southwest left-lateral strike-slip faults bounded between the Sagaing Fault and the Red River Fault. The relative contribution of crustal and mantle processes to surface clockwise rotational deformation remains debatable. GPS and fault slip data indicate that this rotational deformation extends to the Sagaing Fault, the western boundary of the Burma platelet, mantle deformation measured from shear wave splitting data deviates greatly from crustal deformation in regions south of 27°N. Characterizing the mantle flow field can lead to understand the deformation and regional tectonics of this part of plate boundary. In order to estimate the three dimensional anisotropic seismic structure as well as the geometry of the subducting Indian lithosphere, and seismic structure of the Indian-Burma plate boundary we have deployed a swath of seismic stations across central Myanmar extending from the Myanmar-India border regions to the Shan plateau.
Firstly, we have begun using both deep local and teleseismic shear waves to create a detailed map of variations in upper mantle anisotropy. This map shows dominant trench-parallel North-South fast direction. We suggest that this is due to the trench-parallel flow possibly induced by trench migration. We also observe splitting lag times gradually decrease towards the back-arc with increasing complexity in the fast directions. Local splitting from deep events show over 1s lag time suggesting a highly anisotropic mantle wedge compared to anisotropy above the slab in other subduction zones. Additionally, the relatively small difference in lag times between local and tele-seismic S wave splitting indicates that the mantle wedge above the slab is the main source of anisotropy, especially towards the back arc. Furthermore, a linear east-west fast directions trend emerges from the local splitting that is consistent with a slab tear or termination of subducting slab around 24°N. Our results also indicate a complex splitting pattern in northeast and southeast directions that may be attributed to the toroidal flow around the slab and poloidal flow below the slab respectively.
About the Speaker:
Eric Sandvol has worked on characterizing crustal and upper mantle seismic velocity structure beneath continental plateaus and mountain belts in South America, Eurasia, the Middle East and North Africa. This research has been focused at utilizing seismic waveforms to measure Moho thicknesses, crustal and upper mantle shear-wave velocities, as well as propagation efficiencies. He has more recently worked on applying surface wave tomography to broadband data sets in South America, Tibet, and eastern Turkey.