Pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy. To investigate uppermost mantle anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy, and it was applied to Pn travel time data from the western United States. Results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations. A detailed resolution study examined the trade-off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage. Pn anisotropy beneath the western United States has maximum amplitudes of +0.3 km/s (7.5%) when resolved on a length scales of around 3 ø. The orientations of Pn anisotropy often correlate well with those inferred from shear-wave splitting results. This correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions. The orientation of the Pn anisotropy can be correlated with some of the tectonic processes which occur within the western United States. For example, the fast direction of Pn anisotropy parallels the northeast direction of subduction of the Juan de Fuca Plate beneath the northwest Pacific coast, suggesting that there the anisotropy is related to subduction-driven deformation. The fast direction of Pn anisotropy also parallels the strike of the San Andreas Fault system in central California, indicating that shear strains along the plate boundary may be responsible for the anisotropy there. Within the Great Basin, the Pn anisotropy varies substantially, and both partial melting and small-scale convection within the uppermost mantle could be responsible for these anisotropy variations. IntroductionMantle seismic anisotropy is a direct indicator of mantle strain history, and it constitutes an important tool in the study of the tectonics. The utility of anisotropy in tectonic studies was first made apparent by the studies of Hess [1964] and Raitt et al. [ 1969, 1971]. These early studies showed that the anisotropic fast axis within the oceanic lithosphere lies parallel to the spreading direction. Since then, many studies have used seismic anisotropy to study regional tectonics beneath both the oceans and continents [e.g., Bamford et al., 1979; Silver and Chan, 1991; Vinnik et al., 1992; Savage and Silver, 1993]. The link between mantle anisotropy and mantle tectonics is a consequence of the aggregate alignment of olivine during mantle deformation. Theoretical studies infer that the anisotropy can be attributed solely to strain and that there are consistent relationships between the mean orientation of the olivine crystal axes and axes of the the strain ellipsoid. During deformation, the olivine slow velocity axes (b axes) tend to align with the shortest strain axis, the olivine fast velocity axes (a axes) tend to align with the longest strain axis, and the olivine inte...

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Summary In the summer of 1998, project INDEPTH recorded a 400 km long NNW–SSE wide‐angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong‐Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P‐wave data reveals that (1) the crustal thickness is 65 ± 5 km beneath the line; (2) there is no 20 km step in the Moho in the vicinity of the BNS, as has been suggested to exist along‐strike to the east based on prior fan profiling; (3) a thick high‐velocity lower crustal layer is evident along the length of the profile (20–35 km thick, 6.5–7.3 km s−1); and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid‐crustal low‐velocity layer in the P‐wave data, although the data do not negate the possibility of such a layer of modest proportions. Combining the results from the INDEPTH III wide‐angle profile with other seismic results allows a cross‐section of Moho depths to be constructed across Tibet. This cross‐section shows that crustal thickness tends to decrease from south to north, with values of 70–80 km south of the middle of the Lhasa terrane, 60–70 km in the northern part of the Lhasa terrane and the Qiangtang terrane, and less than 60 km in the Qaidam basin. The overall northward thinning of the crust evident in the combined seismic observations, coupled with the essentially uniform surface elevation of the plateau south of the Qaidam basin, is supportive of the inference that northern Tibet until the Qaidam basin is underlain by somewhat thinner crust, which is isostatically supported by relatively low‐density, hot upper mantle with respect to southern Tibet.

SUMMARY Tomographic imaging of the uppermost mantle velocity in the Middle East shows normal Pn velocities (8.0–8.2 km s−1) beneath the Black Sea and the southern Caspian Sea. In contrast, low Pn velocities (<7.9 km s−1) are found beneath the Aegean Sea and much of the Turkish‐Iranian Plateau. A region of exceptionally low Pn velocities (<7.6 km s−1) beneath the Lesser Caucasus Mountains and along the Turkish‐Iranian border coincides with a region of high Sn attenuation and extensive Neogene volcanism. All these features suggest that near solidus conditions exist within the uppermost mantle beneath the Turkish‐Iranian Plateau. Such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above. the presence of a partially melted uppermost mantle weakens the lithosphere beneath the Turkish‐Iranian Plateau, thus allowing it to become the locus of deformation in the Arabian‐Eurasian collision zone.

Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a midcrustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms. Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was observed. The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.

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[1] Seismic P wave velocity variations in the uppermost mantle beneath China are tomographically imaged using Pn wave travel time data. The average velocity of the mantle lid beneath China is 8.05 km/s, with a range from À0.30 km/s to 0.35 km/s. Eastern China has slower mantle velocities and a thinner crust than western China. Different inversions performed for shorter ray path and longer ray path data sets separated the shallow and deep structure within the mantle lid. Velocities in the lower lid are no more than 0.2 km/s faster than at the top. The fastest velocities are beneath cratons, which remained relatively undeformed despite orogenic events around their margins. The Pn velocity beneath southern Tibet is faster than that beneath northern Tibet, suggesting that this contrast delineates the northern edge of the Indian lithosphere. On the southeastern margin of the Tibetan plateau, the Panxi rift, slow Pn velocity exists in the lithosphere due to the transextensional tectonic environment. In eastern China, extension of the North China Block produced a series of NNE-SSW trending rifts that cause slow mantle lid velocities. These slow velocities extend to the base of the mantle lid, suggesting that late Cenozoic Pacific subduction drove the rifting. Both high temperature and hydration may contribute to these low uppermost mantle velocities. The Shanxi rift, along the east margin of the Ordos platform, is characterized by a slow mantle velocity; in contrast, the Yinchuan rift, on the west margin, does not show any uppermost mantle velocity anomaly.

We use receiver functions calculated for data collected by the INDEPTH‐IV seismic array to image the three‐dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70 km in northern Tibet. In addition, we observe a 20 km Moho offset beneath the northern margin of the Kunlun Mountains, a 10 km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong‐Nujiang Suture at 80 km depth which dips ∼10° to the north, reaching ∼120 km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20 km depression of the 660‐km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10 km depression of the 410‐km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small‐scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau.