The rocks of the Indian subcontinent are last seen south of the Ganges before they plunge beneath the Himalaya and the Tibetan plateau. They are next glimpsed in seismic reflection profiles deep beneath southern Tibet, yet the surface seen there has been modified by processes within the Himalaya that have consumed parts of the upper Indian crust and converted them into Himalayan rocks. The geometry of the partly dismantled Indian plate as it passes through the Himalayan process zone has hitherto eluded imaging. Here we report seismic images both of the decollement at the base of the Himalaya and of the Moho (the boundary between crust and mantle) at the base of the Indian crust. A significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths. North of the Himalaya, the lower Indian crust is characterized by a high-velocity region consistent with the formation of eclogite, a high-density material whose presence affects the dynamics of the Tibetan plateau.

Earthquakes beneath the Himalayan collision zone occur at depths between near surface and around 100 km below sea level. After relocating earthquakes with two one‐dimensional (1‐D) velocity models, we found a clear bimodal depth distribution for earthquakes in the Himalayas of eastern Nepal and the southern Tibetan Plateau and evidence that some earthquakes originate at upper mantle depths. Seismicity in Nepal shows an accumulation of earthquakes along the front of the Himalayan arc, with a seismic gap between longitudes 87.3°E and 87.7°E. Although upper crustal seismicity along the topographic front of the High Himalaya is consistent with a region of high strain accumulation associated with convergence on the Main Himalayan thrust fault, microearthquakes do not necessarily occur on this fault. Instead, they concentrate in the hanging wall. Seismic activity in the sub‐Himalaya and the Terai Plains is almost exclusively limited to the vicinity of the location of the magnitude 6.5 20 August 1988 Udayapur earthquake, with most of the earthquakes in the lower crust and the upper mantle. Clusters of earthquakes in the Lesser and High Himalayas and south Tibet (Tethyan Himalayas) mark very well defined zones of seismicity at depths between 50 and 100 km, confirming the presence of earthquakes in the upper mantle in the region of continental collision. The occurrence of earthquakes at sub‐Moho depths favors the idea that the continental upper mantle deforms by brittle processes.

Subduction of the Nazca and Caribbean Plates beneath northwestern Colombia is seen in two distinct Wadati Benioff Zones, one associated with a flat slab to the north and one associated with normal subduction south of 5.5°N. The normal subduction region is characterized by an active arc, whereas the flat slab region has no known Holocene volcanism. We analyze volcanic patterns over the past 14 Ma to show that in the mid‐Miocene a continuous arc extended up to 7°N, indicating normal subduction of the Nazca Plate all along Colombia's Pacific margin. However, by ~6 Ma, we find a complete cessation of this arc north of 3°N, indicating the presence of a far more laterally extensive flat slab than at present. Volcanism did not resume between 3°N and 6°N until after 4 Ma, consistent with lateral tearing and resteepening of the southern portion of the Colombian flat slab at that time.

[1] Variations in the seismic velocity structure of the Himalayan collision zone include significant differences between its north and south portions, with transitions in physical properties across the Greater Himalaya. We combined P-and S-wave traveltimes from a temporary broadband seismic network in eastern Nepal and southern Tibet with arrival times at the permanent station network of the Department of Mines and Geology of Nepal to determine the seismic velocity structure across the Himalaya, using local earthquake tomography and traveltimes of regional earthquakes. The P-to-S velocity ratio (Vp/Vs) structure marks the difference between the Indian Plate and the overlying materials, with the Vp/Vs ratios being high for the former and low for the latter. We also found a significant increase in the uppermost mantle seismic velocities from south to north, reaching P-wave velocities (Vp) over 8.4 km/s north of the Greater Himalaya. These high Vp values do not seem to be the result of biases due to anisotropy in the upper mantle beneath the Greater and Tethyan Himalayas. Instead, we suggest that rocks in the lower crust of the underthrusting Indian Plate undergo metamorphism to eclogite as they plunge to greater depth beneath the mountain range, explaining the high seismic velocities.Citation: Monsalve, G., A. Sheehan, C. Rowe, and S. Rajaure (2008), Seismic structure of the crust and the upper mantle beneath the Himalayas: Evidence for eclogitization of lower crustal rocks in the Indian Plate,