Seismic P-wave tomography in eastern Tibet: Formation of the rifts

Abstract

For better studying the relationship between the rifts and deep structure, a detailed P-wave velocity structure under eastern Tibet has been modeled using 4767 arrival times from 169 teleseismic events recorded by 51 portable stations. In horizontal slices through the model, a prominent low-velocity anomaly was detected under the rifts from the surface to a depth of ~250 km; this extends to a depth of ~400 km in the vertical slice. This low-velocity anomaly is interpreted as an upper mantle upwelling. The observations made provide seismic evidence for the formation of north-south trending rifts. East of the low-velocity anomaly, a clear high-velocity anomaly is found between depths of 40 and 200 km. Due to its shallow depth, we suggest that it consists of materials from an ancient continental closure rather than the Indian Plate. From depths of 250 to 400 km, a high-velocity anomaly appears to the south of the Jiali Fault. This anomaly may correspond to the northern edge of the Indian Plate that detached from the surface under the Himalayan block. We suggest that the Indian Plate underthrusts no further than the Jiali Fault in eastern Tibet. The Tibetan Plateau is the world's highest altitude area and a very important region for the geosciences. Since the plateau was formed by the collision of the Indian and Eurasian plates about 50 Ma ago [1] and the subsequent postcollisional intra-continental deformation, strong structural distortion, lithospheric thickening, uplift of the plateau and development of the Himalayan mountains has occurred in this area. Although the Indus-Tsangpo, Bangong-Nujiang and Jinsha River sutures provide some evidence of Tethyan evolution, the precise relationships between the north-south trending structures of southern Tibet and the deep structure of the region are still not clear. Are they grabens [2] or deep fractures? Undoubtedly, these north-south trending structures are the most prominent surface feature in southern Tibet, but their formation mechanism and their relationships with shallow and deep structure [3] are not obvious. Resolving these problems will provide a way to better study the formation and evolution of the Tibetan Plateau.To study the uplift mechanism of the Tibetan Plateau, international exploration projects such as the INDEPTH, Hi-CLIMB and ANTILOPE projects have been undertaken. These projects have provided extensive seismic data sets that can be used to investigate deep structure in this area [4][5][6][7].In the last 20 years, much seismic research has been carried out in the Tibetan Plateau facilitated by the rapid development of seismometer technology. Surface wave investigations suggest that the entire plateau is underlain by a relatively cold lithospheric mantle [8]. Applications of travel time techniques, Zhou [9] have shown that the Indian Plate underthrusts the whole plateau. Other research has found that relatively low S-wave velocities occur in the upper mantle of central and northern Tibet [10,11]. The work of Chen [12] supports the existence of strong ...

Convergence of the Indian and Eurasian plates under eastern Tibet revealed by seismic tomography

“…In Figure 5a, a low‐V anomaly, D, is located beneath the Cona rift, and two high‐V zones, which are interpreted as the subducted and delaminated slabs, are imaged. This result is consistent with the finite‐frequency tomography [ Ren and Shen , 2008] and a recent travel‐time tomography [ Zhang et al , 2011]. Ni et al [1989]suggested that the Burmese microplate descends eastward beneath the Eurasian plate while the Indian plate underthrusts northward.…”

“…At greater depths (Figures 4f–4i), two distinct high‐V anomalies are visible around 92°E and 100°E. The western one is related to the subducting Indian slab [ Ren and Shen , 2008; Zhang et al , 2011], while the eastern one is associated with the subducting Burmese plate [ Huang and Zhao , 2006; Li and van der Hilst , 2010; Li et al , 2008].…”

Tomographic imaging of the underthrusting Indian slab and mantle upwelling beneath central Tibet

“…The former low-V zone is also visible in finite-frequency tomography (Liang et al, 2011), and its shallower part is consistent with Pn wave tomography (Liang and Song, 2006). The low-V belt also coincides well with a similar feature in southeastern Tibet (Ren and Shen, 2008;Zhang et al, 2011), which was interpreted as an upwelling induced by delaminated lithospheric mantle. The different velocity structures under these rifts may reflect different kinds of the rifts or different episodes in the origin of the rifts, though our tomography only shows the current images of the structures (Fig.…”

“…Normal faulting is likely to initiate when convective removal of the thickened lower lithosphere takes place (Molnar et al, 1993). Other explanations include a combination of eastward Tibetan extrusion and oblique convergence between the Indian and Eurasian plates (Tapponnier et al, 2001), and mantle upwelling induced by the delaminated lithosphere (Ren and Shen, 2008;Zhang et al, 2011).…”

Formation of Rifts in Central Tibet: Insight From P Wave Radial Anisotropy

“…Geochronologic dating suggested that younger rocks in the rift region of eastern Tibet are possibly produced by the decompression melting of a metasomatically depleted mantle (Wang et al, ). Seismic tomography detected a tilted low‐velocity (low‐V) zone extending to ~300‐km depth beneath the Cona rift to the east of the Pumuqu Xianza rift (PXR), which was interpreted as mantle upwelling due to continental delamination (Ren & Shen, ; Zhang et al, ). Williams et al () suggested that the temporal association of crustal extension and magmatism was related to lithospheric erosion associated with slab break‐off and/or convective removal in southern Tibet.…”