James Mechie

Partially molten middle crust beneath southern Tibet : Synthesis of project INDEPTH results

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

Lithospheric and upper mantle structure of southern Tibet from a seismological passive source experiment

Fifteen seismic stations were operated with about 20-km spacing in southern Tibet across the Zangbo suture (the collision zone between India and Asia) between May and October 1994 as part of the International Deep Profiling of Tibet and the Himalaya project (INDEPTH II) for wide-angle recording of the controlled source experiment and for passive earthquake recording. In addition, a dense deployment (4-km spacing) of stations within the German Depth Profiling of Tibet and the Himalayas (GEDEPTH) project also recorded a number of teleseismic earthquakes. The third data source used in this study is the records of the permanent broadband station at Lhasa. Teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from P-to-S converted phases. Important results are as follows. The Moho at 70–80 km and a second discontinuity at 50–60 km depth are observed over the entire profile south and north of the Zangbo suture. The data from the GEDEPTH dense array enable the detection of inclined structures penetrating the crust at the Zangbo suture. A pronounced low-velocity zone exists north of the Zangbo suture at about 10–20 km depth. The locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model IASP91 [Kennett, 1991] over a large region of the Himalaya and southern Tibet.

Seismic polarization anisotropy beneath the central Tibetan Plateau

SKS and SKKS shear waves recorded on the INDEPTH III seismic array deployed in central Tibet during 1998–1999 have been analyzed for the direction and extent of seismic polarization anisotropy. The 400-km-long NNW trending array extended south to north, from the central Lhasa terrane, across the Karakoram-Jiali fault system and Banggong-Nujiang suture to the central Qiangtang terrane. Substantial splitting with delay times from 1 to 2 s, and fast directions varying from E-W to NE-SW, was observed for stations in the Qiangtang terrane and northernmost Lhasa terrane. No detectable splitting was observed for stations located farther south in the central Lhasa terrane. The change in shear wave splitting characteristics occurs at 32°N, approximately coincident with the transcurrent Karakoram-Jiali fault system but ∼40 km south of the surface trace of the Banggong-Nujiang suture. This location is also near the southernmost edge of a region of high Sn attenuation and low upper mantle velocities found in previous studies. The transition between no measured splitting and strong anisotropy (2.2 s delay time) is exceptionally sharp (≤15 km), suggesting a large crustal contribution to the measured splitting. The E-W to NE-SW fast directions are broadly similar to the fast directions observed farther east along the Yadong-Golmud highway, suggesting that no large-scale change in anisotropic properties occurs in the east-west direction. However, in detail, fast directions and delay times vary over lateral distances of ∼100 km in both the N-S and E-W direction by as much as 40° and 0.5–1 s, respectively. The onset of measurable splitting at 32°N most likely marks the northern limit of the underthrusting Indian lithosphere, which is characterized by negligible polarization anisotropy. Taken in conjunction with decades of geophysical and geological observations in Tibet, the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern Tibet is being squeezed and sheared between the advancing Indian lithosphere to the south and the Tsaidam and Tarim lithospheres to the north and west, resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability. In northern Tibet, crustal deformation clearly follows this large-scale deformation pattern.

INDEPTH III seismic data: From surface observations to deep crustal processes in Tibet

[1]xa0During the summer of 1998, active-source seismic data were collected along a transect running 400 km NNW-SSE across the central Tibetan Plateau as the third phase of project INDEPTH (International Deep Profiling of Tibet and the Himalaya). The transect extends northward from the central Lhasa block, across the Jurassic Bangong-Nujiang Suture (BNS) at 89.5°E, to the central Qiangtang block. A seismic velocity model for the transect to ∼25 km depth produced by inversion of P wave first arrivals on ∼3000 traces shows (1) a ∼50-km-wide region of low velocity (at least 5% less than surrounding velocities) extending to the base of the model at the BNS; (2) sedimentary cover for the southern Qiangtang block that is ∼3.5 km thick; (3) a distinct interface between sedimentary cover and Qiangtang basement or underplated Jurassic melange in the central Qiangtang block; and (4) evidence that the Bangoin granite extends to a depth of at least 15 km. The BNS has little geophysical signature, and appears unrelated to the ∼5 km northward shallowing of the Moho which is associated with the BNS in central Tibet. Geophysical data along the main INDEPTH III transect show little evidence for widespread crustal fluids, in contrast to the seismic “bright spots” found in southern Tibet and to magnetotelluric evidence of fluid accumulations in eastern Tibet. A comparison between the global average and Tibetan velocity-depth functions offers constraints for models of plateau uplift and crustal thickening. Taken together with the weak geophysical signature of the BNS, these velocity-depth functions suggest that convergence has been accommodated largely through pure-shear thickening accompanied by removal of lower crustal material by lateral escape, likely via ductile flow. Although we cannot resolve the details, we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of Tibet as a series of accreted terranes.

Precise temperature estimation in the Tibetan crust from seismic detection of the α-β quartz transition

In the deep crust, temperature, which is among the key pa- rameters controlling lithospheric dynamics, is inferred by extrap- olation from the surface using several assumptions that may fail in regions of active tectonics and fluid migration. In the rare case that temperatures of 700 8C or higher are exceeded in the upper and middle continental crust composed of quartz-rich felsic rocks, the a-b quartz transition (ABQT) will occur, generating a mea- surable seismic signature and offering the possibility for precisely estimating temperature from the known ABQT phase diagram. Here it is shown that all expected seismic features of the ABQT are met by the boundary between the upper and middle crust be- low the INDEPTH III profile in central Tibet. This finding implies that a temperature of 700 8C is achieved at a depth of 18 km under the southern Qiangtang block, which agrees with the depth to the top of a high electrical conductivity anomaly, likely representing partially melted crust. To the south in the northern Lhasa block, the ABQT is at 32 km depth, corresponding to a temperature of 800 8C. It thus appears that this seismic boundary representing the ABQT is the result of recent geologic processes rather than being a lithologic boundary.

Injection of Tibetan crust beneath the south Qaidam Basin: Evidence from INDEPTH IV wide‐angle seismic data

[1]xa0The International Deep Profiling of Tibet and the Himalaya Phase IV (INDEPTH IV) active source seismic profile in northeast Tibet extends 270 km roughly north-south across the Songpan-Ganzi terrane, the predominantly strike-slip North Kunlun Fault (along the Kunlun suture), the East Kunlun Mountains, and the south Qaidam Basin. Refraction, reflection, and gravity modeling provide constraints on the velocity and density structure down to the Moho. The central Qaidam Basin resembles average continental crust, whereas the Songpan-Ganzi terrane and East Kunlun Mountains exhibit thickened, lower-velocity crust also characteristic of southern Tibet. The crustal thickness changes from 70 km beneath the Songpan-Ganzi terrane and East Kunlun Mountains to 50 km beneath the Qaidam Basin. This jump in crustal thickness is located ∼100 km north of the North Kunlun Fault and ∼45 km north of the southern Kunlun-Qaidam boundary, farther north than previously suggested, ruling out a Moho step caused by a crustal-penetrating North Kunlun Fault. The Qaidam Moho is underlain by crustal velocity material (6.8–7.1 km/s) for ∼45 km near the crustal thickness transition. The southernmost 10 km of the Qaidam Moho are underlain by a 70 km reflector that continues to the south as the Tibetan Moho. The apparently overlapping crustal material may represent Songpan-Ganzi lower crust underthrusting or flowing northward beneath the Qaidam Basin Moho. Thus the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust.

Crustal Root Beneath the Urals: Wide-Angle Seismic Evidence

Wide-angle reflection and refraction data acquired as part of the URSEIS 95 geophysical experiment across the southern Uralide orogen provide evidence for a 12- to 15-kilometer-thick crustal root, yielding a total crustal thickness of 55 to 58 kilometers. Strong reflections from the Mohorovičić discontinuity (Moho) at relatively small precritical distances suggest that the crust-mantle transition beneath the crustal root is a sharp feature. The derived P- and S-wave velocity models constrain key physical properties of the crust, including the depth of the mafic rocks of the Magnitogorsk volcanic arc and the existence of a lower crustal zone of possible basic rock enrichment beneath the East Uralian zone.

Two‐dimensional velocity models of the Nazca Plate subduction zone between 19.5°S and 25°S from wide‐angle seismic measurements during the CINCA95 project

The interpretation of seismic refraction/wide-angle reflection data from the 1995 Crustal Investigations off- and on-shore Nazca/Central Andes (CINCA95) project has resulted in the derivation of nine E-W two-dimensional (2-D) velocity cross sections for the region between the Peru-Chile trench and the coast between 19.5°S and 25°S, with three of the cross sections extending a farther 100–200 km inland. These sections define the major lithospheric structures of the upper South American plate and the lower Nazca plate down to uppermost mantle depths of 30–60 km beneath this part of the present-day forearc region. In addition to showing the Nazca plate subducting at an increasing angle of 9–25° down to 30–50 km depth near the coast, these cross sections show a portion of the Moho dipping eastward from 43–50 km near the coast to 55–64 km up to about 240 km inland. Owing to a gap of almost 50 km in the data coverage of the Moho near the coast, it is uncertain from this data set alone whether the Moho defined east of the coast should be correlated with the lower oceanic Nazca plate or the upper continental South American plate. However, a comparison with other seismological data suggests that the Moho identified here east of the coast defines the base of the continental crust of the upper South American plate immediately behind the downgoing Nazca plate. Thus the hypothesis of Delouis et al. [1996] that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported. The cross sections also show between 20°S and 22.5°S, a boundary extending from upper crustal levels down to 25–30 km depth subparallel to and 5–10 km above the top surface of the subducting Nazca plate. The discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate.

Seismic wide-angle constraints on the crust of the southern Urals

A wide-angle seismic reflection/refraction data set was acquired during spring 1995 across the southern Urals to characterize the lithosphere beneath this Paleozoic orogen. The wide-angle reflectivity features a strong frequency dependence. While the lower crustal reflectivity is in the range of 6–15 Hz, the PmP is characterized by frequencies below 6 Hz. After detailed frequency filtering, the seismic phases constrain a new average P wave velocity crustal model that consists of an upper layer of 5.0–6.0 km/s, which correlates with the surface geology; 5–7 km depths at which the velocities increase to 6.2–6.3 km/s; 10–30 km depths at which, on average, the crust is characterized by velocities of 6.6 km/s; and finally, the lower crust, from 30–35 km down to the Moho, which has velocities ranging from 6.8 to 7.4 km/s. Two different S wave velocity models, one for the N-S and one for the E-W, were derived from the analysis of the horizontal component recordings. Crustal sections of Poissons ratio and anisotropy were calculated from the velocity models. The Poissons ratio increases in the lower crust at both sides of the root zone. A localized 2–3% anisotropy zone is imaged within the lower crust beneath the terranes east of the root. This feature is supported by time differences in the SmS phase and by the particle motion diagrams, which reveal two polarized directions of motion. Velocities are higher in the central part of the orogen than for the Siberian and eastern plates. These seismic recordings support a 50–56 km crustal thickness beneath the central part of the orogen in contrast to Moho depths of ≈ 45 km documented at the edges of the transect. The lateral variation of the PmP phase in frequency content and in waveform can be taken as evidence of different genetic origins of the Moho in the southern Urals.

Detection of southward intracontinental subduction of Tibetan lithosphere along the Bangong-Nujiang suture by P-to-S converted waves

Teleseismic primary (P) to secondary (S) converted waves recorded on the INDEPTH III seismic array have been used to detect lithospheric-scale deformation structures of the central Tibetan Plateau from the central Lhasa terrane to the central Qiangtang terrane. A south-dipping crustal converter is seen from the upper crust near the 500-km-long metamorphic core complex exposures in the Qiangtang terrane to the lower crust near the Bangong-Nujiang suture. At deeper depths, a southeast-dipping mantle converter is seen extending from ∼50 km north of the Bangong- Nujiang suture at the depth of the Moho, to a depth of ∼180 km, ∼100 km south of the Bangong-Nujiang suture. We found the observations to be most consistent with a model of lithospheric deformation involving (1) southward subduction of the Tibetan lithospheric mantle along the Bangong-Nujiang suture and (2) thickening of the central Tibetan crust through a thick-skinned, crustal accretionary thrust-wedge structure in response to the India9s collision with Asia.