Mingming Jiang

Lithospheric and upper mantle structure of the northeastern Tibetan Plateau

[1]xa0We 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 70xa0km in northern Tibet. In addition, we observe a 20xa0km Moho offset beneath the northern margin of the Kunlun Mountains, a 10xa0km 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 80xa0km depth which dips ∼10° to the north, reaching ∼120xa0km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20xa0km 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 ∼10xa0km 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.

Presence of an intralithospheric discontinuity in the central and western North China Craton: Implications for destruction of the craton

Detailed knowledge of lithospheric structure is essential for understanding the long-term evolution and dynamics of continents. We present an image of lithospheric structure across the central and western North China Craton (NCC), derived using S and P receiver functions from a dense seismic array. A negative velocity discontinuity is identified at ∼80–100 km depth within the thick lithosphere (∼160–200 km), similar to that observed in many other cratonic regions and roughly at the same depth as the base of the lithosphere in the eastern NCC. The intralithospheric discontinuity may indicate an ancient, mechanically weak layer within the overall strong cratonic lithosphere, and probably also existed beneath the eastern NCC before the Mesozoic. The presence of such a weak layer could have facilitated simultaneous lithospheric modification at the base and the middle of the lithosphere in the eastern NCC, especially under the strong influence of the Mesozoic Pacific subduction, leading to the severe lithospheric thinning and destruction recorded in this region. The weak layer probably did not strongly affect the stability and evolution of the central and western NCC and other cratonic regions where effects from plate boundary processes were weak. Our seismic images, integrated with geological data, provide new insights into structural heterogeneities in the subcontinental lithospheric mantle and their roles in the dynamic evolution of continents.

Indian mantle corner flow at southern Tibet revealed by shear wave splitting measurements

[1]xa0We constrain anistropic seismic structure in the southernmost Tibet and the Tethyan Himalays using SKS and SKKS phases recorded from a temporary seismic network, operated from July 2004 to August 2005 in conjunction with the Hi-Climb project. Shear wave splitting is not detected at 16 stations, most of which are located near and to the north of the Indus-Yalong suture (IYS). For the first time anisotropy with an N-S fast direction and 0.4 ∼ 1 s delay times is observed to the region about 50 km south of the IYS. This weak anisotropy correlates with the flat part of the subducting Indian lithosphere and could be caused by northward asthenospheric flow and shear at the base of the Indian lithosphere. The null measurements in the vicinity of the IYS are most likely the result of a vertical asthenospheric flow, the corner flow induced by the subvertical subduction of the Indian lithosphere just south of the Bangong-Nujiang suture (BNS). Observations of the systematic changes in mantle anisotropy from southern Tibet to the central Tibet provide evidence for different mantle flow fields between the Indian and the Eurasian asthenospheric mantle, which are separated by a subvertical Indian lithosphere at the BNS.

Regional earthquakes in northern Tibetan Plateau: Implications for lithospheric strength in Tibet

[1]xa0A total of 400 regional earthquakes were located in northern Tibetan Plateau from data recorded by INDEPTH-IV and PKU Eastern Kunlun arrays from May 2007 to June 2009. The distribution of these earthquakes is compatible with a continuously deforming Tibetan lithosphere. Most earthquakes occur at a depth range of 0-15 km, but no event is deeper than 30 km. This observation strongly supports the existence of a hot and weak lower crust beneath the northern Tibet. The crustal seismogenic zone appears slightly thicker beneath the northern Tibet than in the southern plateau, possibly reflecting a difference in the rheological (dry vs. wet) structure of the crust. The absence of lower crustal and uppermost mantle earthquakes in northern Tibet is consistent with a localized asthenospheric upwelling under the Qiangtang and Songpan-Ganze terranes. Finally, the lack of mantle earthquakes should be fully addressed in any models of subduction in northern Tibet.

Crustal structures across the western Weihe Graben, North China: Implications for extrusion tectonics at the northeast margin of Tibetan Plateau

The stable Ordos Plateau, extensional Weihe Graben, and Qinling orogenic belt are located at the northeast margin of the Tibetan Plateau. They have been thought to play different roles in the eastward expanding of the Tibetan Plateau. Peking University deployed a linear seismic array across the western end of the Weihe Graben to investigate the crustal structures of the tectonic provinces of this structure. Receiver function analyses revealed low-to-moderate Poissons ratios and anticorrelations between Poissons ratios and topography beneath the Qinling Orogen. These features may indicate a tectonic thickening of the felsic upper crust by folding and thrusting within the Qinling Orogen. We observed a strong horizontal negative signal at the midcrust beneath the Ordos Plateau which may indicate a low-velocity zone. This observation would suggest the stable cratonic Ordos Plateau had been modified due to the compression between the Tibetan Plateau and the Ordos Plateau. We also observed an abrupt 4u2009km Moho offset across the Weihe Fault, changing from ~44u2009km beneath the Ordos Plateau to ~40u2009km beneath the Qinling Orogen. We conclude that the Weihe Fault is a lithosphere-scale fault/shear zone, which extends into the upper mantle beneath the Weihe Graben. It acts as the major boundary separating the stable Ordos Plateau and the active Qinling Orogen.

PERMANENT GENETIC RESOURCES: Development and characterization of 14 polymorphic microsatellite loci in the endangered tree Euptelea pleiospermum (Eupteleaceae)

Fourteen polymorphic microsatellite loci were developed and characterized for the endangered and tertiary relict tree, Euptelea pleiospermum. A genomic DNA enrichment protocol was used to isolate microsatellite loci and polymorphism was explored using 32 individuals from one natural population. The observed number of alleles ranged from two to nine. The ranges of observed and expected heterozygosities were 0.25–1.00 and 0.22–0.85, respectively. These microsatellite markers provide powerful tools for the ongoing conservation genetic studies of E. pleiospermum.

Crustal S-velocity structure and radial anisotropy beneath the southern part of central and western North China Craton and the adjacent Qilian Orogenic Belt from ambient noise tomography

The crustal S-velocity structure and radial anisotropy along a dense linear portable seismic array with 64 broadband seismic stations were investigated from ambient noise tomography with about one-year-long ambient noise recordings. The array transverses the southern part of the central North China Craton (CNCC) and western NCC (WNCC) from east to west and reaches the adjacent Qilian Orogenic Belt (QOB). The phase velocity structures of Rayleigh waves at 5–35 s and Love waves at 5–30 s were measured. The crustal S-velocity structures (Vsv and Vsh) were constructed from the dispersion data (Rayleigh and Love waves, respectively) from point-wise linear inversion with prior information of the Moho depth and average crustal Vp/Vs ratio. The radial anisotropy along the profile was calculated based on the discrepancies between Vsv and Vsh as 2×(Vsh−Vsv)/(Vsh+Vsv). The results show distinct structural variations in the three major tectonic units. The crustal architecture in the southern CNCC is complicated and featured with wide-distributed low-velocity zones (LVZs), which may be a reflection of crustal modification resulting from Mesozoic-Cenozoic tectonics and magmatic activities. The pronounced positive radial anisotropy in the lower-lowermost crust beneath the Shanxi-Shaanxi Rift and the neighboring areas could be attributed to the underplating of mantle mafic-ultramafic materials during the Mesozoic-Cenozoic tectonic activation. In southern Ordos, the overall weak lateral velocity variations, relative high velocity and large-scale positive radial anisotropy in mid-lower crust probably suggest that the current crustal structure has preserved its Precambrian tectonic characteristics. The low-velocity westward-dipping sedimentary strata in the Ordos Block could be attributed to the Phanerozoic whole-basin tilting and the uneven erosion since late Cretaceous. Integrated with previous studies, the systematic comparison of crustal architecture was made between the southern and northern part of CNCC-WNCC. The similarities and differences may have a relation with the tectonic events and deformation histories experienced before and after the Paleoproterozoic amalgamation of the NCC. The nearly flat mid-crustal LVZ beneath the southern QOB weakens gradually as it extends to the east, which is a feature probably associated with crustal vertical superpositionand ductile shear deformation under the intensive compressional regime due to the northeastward growth and expansion of the Tibetan Plateau.

Limited southward underthrusting of the Asian lithosphere and material extrusion beneath the northeastern margin of Tibet, inferred from teleseismic Rayleigh wave tomography

The northeastern margin of Tibet is of key importance in understanding the uplift and expansion of Tibetan Plateau. In this research, we perform Rayleigh wave tomography from 20xa0s to 167xa0s using the data recorded by China Digital Seismic Network and several portable seismic arrays in Northeastern Tibet and adjacent areas. Our resultant 3-D shear wave velocity model exhibits strong lateral variations in the lithosphere. High shear wave velocities are observed in the mantle of the cratonic Ordos and Yangtze Blocks, as well as the East Qinling Orogen. The Qilian Orogen is underlain by a ∼120xa0km thick lithosphere of intermediate velocities and a low-velocity layer of asthenospheric mantle below the depth of 120xa0km. Low velocities are characteristic in the middle-to-lower crust of the Songpan-Ganzi Terrane and the West Qinling Orogen, possibly representing the crustal flow penetrating the Kunlun Fault. An uppermost mantle low-velocity zone is found in the same location, indicating either the asthenosphere upwelling or the transitional crust-mantle boundary layer. Our observations have two tectonic implications. First, the large-scale eastward Tibetan material extrusion into Eastern China through the East Qinling Orogen is unlikely to be an ongoing process above 200xa0km depth; second, the southward subduction of the North China Craton toward Northeastern Tibet may not have fully developed in this region. Furthermore, we propose that the difference in the lithospheric strength between the Qaidam Block and the West Qinling Orogen is a critical element to control the tectonics in the northeastern margin of Tibet.