T23G-01
Mantle Lithosphere Beneath Tibet: A Synthesis With Special References to Results From Project Hi-CLIMB
We present a synthesis regarding the current configuration of Asian and Indian mantle lithosphere beneath the Himalayan-Tibetan collision zone -- the most prominent zone of active continental collision. The following cross- sectional view is achieved by combining new results from complementary approaches, including modeling profiles of triplicate waveforms, migration of scattered wave-fields, patterns in shear-wave birefringence, travel- time tomography, regional gravity anomalies/modeling of plate flexure, and distributions of helium isotopic ratios: The leading edge of the Indian lithospheric mantle (Indian mantle front, IMF) extends about 200 km north of the Bangong-Nujiang suture in the heartland of Tibet while the southern terminus of the Asian counterpart (the mantle suture, MS) is about 100 km to the north of the Indus-Yarlong suture in southern Tibet (see Fig. 1 of the included URL) Furthermore, we carried out a preliminary reconstruction of lithospheric configuration by extrapolating current velocity of ground motions determined from GPS-based geodesy back to 15 Ma ago -- the timing of the last major episode of (potassic) magamtism that mainly occurred in the Qiangtang terrane. The results indicate that a large- scale anomaly of high P-wave speeds (Vp), currently resting just above the 660-km discontinuity under central Tibet, is likely to be associated with a cold remnant of lithospheric mantle that was convectively removed about 15 Ma ago from thickened lithosphere of the Qiangtang terrane. Meanwhile, noticeable lateral (east-west) variations are also recognized. For instance, low Vp in the lower crust/uppermost mantle just north of the Indus-Yarlong suture is not a continuous feature. Intriguingly, the anomaly of low Vp correlates with low amplitudes of P- to S conversions, indicating a gradual crust-mantle transition. These results suggest that currently channel flows in the lower crust, if any, is limited in extent. An anomalous region of null shear-wave birefringence is also apparent in the Lhasa terrane so the MS is not always well-resolved between Hi-CLIMB and Yardong-Golmud profiles. http://www.uiuc.edu/goto/pubs/63_ChenTsengDetachedLithoTibet.JGR.07.pdf
T23G-02 INVITED
Constraints on Crust and Upper Mantle Structure Beneath Tibet from Multi-scale, Finite- Frequency Travel-time Tomography
In order to illuminate the inner-works of continental collision between India and Asia, several studies have used travel-time tomography to image variations in seismic velocities under Tibet. Partly due to very sparse station distributions, global tomography has resolved only very broad features such as seismically slow crust and lithospheric mantle beneath central Tibet and long-wavelength, high-velocity anomalies underneath the southern edge of Tibet which have been attributed to under-thrusting of the Indian lithosphere (e.g., Li et al., 2006). Since 1991, data from arrays of broadband deployments in the interior of the Tibetan Plateau and the Himalayas have provided steady improvements to the resolution of crust and upper mantle structure beneath this key collision zone. From September of 2002 to August of 2005, Project Hi-CLIMB completed an N-S trending linear array of over 800 km in length across the Himalaya-Tibetan collision zone and a complementary, E-W trending regional array in southern Tibet of comparable aperture. Data from over 210 stations, spaced only 3-8 km apart in the linear array, provide for unprecedented spatial resolution in tomography. Using this new dataset, I report results of 3-D, tomographic inversion of P-wave travel-times, including P, PKIKP and PcP phases, as measured by inter-station cross-correlation of waveforms at both high- (0.3-2 Hz) and low-frequencies (0.03-0.125 Hz). In the inversion, I employed physically realistic 3-D sensitivity kernels for travel-time delays and multi-scale parameterization that facilitates spatially-varying, data-adaptive resolutions. The most robust and surprising results include two disjointed anomalies of low-velocity in the lithosphere, confined to depths of about 150 km or less near the Indus-Yarlong suture (IYS). One anomaly is about 100 km in radius, centered around 29.5°N, 86°E just north of the IYS, and the other lies farther eastward and straddles the IYS. The former anomaly clearly correlates with a localized region of gradual crust-mantle transition while the latter seems to coincide with an anomalous zone of null shear-wave birefringence. At greater depths, a broad swath of high P-wave speeds extends under much of the Lhasa terrane beyond the Bangong-Nujiang Suture (BNS), consistent with the notion that the leading edge of the Indian mantle lithosphere (¡§Greater India¡¨) extends just beyond the BNS.
T23G-03
Imaging the Crust and Upper Mantle beneath the Hi-CLIMB Seismic Array in Tibet Using Gaussian-Beam Migration of Radial Receiver Functions
A broadband, linear seismic array of over 800 km in length was deployed across the Himalayan-Tibetan collision zone as part of Project Hi-CLIMB. Overall, more than 200 seismic station locations were occupied during 2002- 2005, with an average station spacing of 3-8 km along the linear array. In this study, we use Gaussian-Beam (GB) migration of scattered P-wavefield to obtain images of crust and upper mantle beneath Tibet. The sources of illumination are teleseismic P-waves and we form seismic profiles of radial receiver-functions from groups of earthquakes, with each group restricted to a small range of azimuths and distances. The best data come from a group of 12 located to the southeast of the array in the southwestern Pacific, and another group of 5 events located to the northeast of the array in the northwestern Pacific. Prior to imaging, we first analyze azimuthal variations in both radial and transverse components of receiver- functions from individual events at selected stations. We then stack the receiver-functions for each group of earthquakes and interpolate the data to a uniform spacing of 8 km before applying the GB migration algorithm to obtain images of variations in seismic wave speeds in the crust and the upper mantle. In a previous study, we already tested the GB algorithm with full synthetic seismograms to ensure that it is robust in imaging geologic features beneath a thickened crust. Initial results show a number of interesting features throughout the thickened crust of Tibet. Moreover, broad bandwidth of the data produces images at different frequency bands up to more than 1 Hz, constraining scattering properties at multiple scales. Overall, there is stronger lateral heterogeneity or anisotropy beneath the Qiangtang terrane in the heartland of Tibet than the Lhasa terrane in southern Tibet. Upper crustal anisotropy seems particularly strong at stations within 50 km south of the surface trace of the Bangong Nujiang Suture (BNS) – a feature corroborated by azimuthal variability and/or polarity switches in the transverse component of receiver-functions. P-to-S conversions across the Moho are generally strong and continuous under much of the Lhasa terrane, corresponding to a well-defined Moho. A noticeable exception occurs about 80 km north of the Indus-Yarlong suture where an apparently gradual Moho correlates well with a curious anomaly of low P-wave speeds detected by travel-time tomography. Disturbances to the Moho are also evident in the vicinity of the BNS, apparently a lithospheric structure; and increased scattering near the Moho generally persists under much of the Qiangtang terrane to the north where the most recent episode of regional magmatism was concentrated.
T23G-04 INVITED
Discordant anomalies of P- and S-wave speeds under Tibet: a case for hydrous remnant of sub-continental lithospheric mantle
Using high-resolution, triplicate shear-waveforms recorded by broadband seismic arrays, I show that a corresponding anomaly of high S-wave speed ( VS) is absent where an anomaly of high P-wave speed ( VP) was recently recognized in the transition zone of the mantle (TZ) beneath central Tibet. A likely cause of the discrepancy between anomalies in VP and VS is a minor amount of water in nominally anhydrous polymorphs of olivine. Prior to thickening by continent-continent collision, the Tibetan lithospheric mantle was part of a mantle wedge which has been hydrated during past episodes of subduction. Hydration of the sub-continental lithospheric mantle (SCLM) provides a natural mechanism to facilitate ductile deformation, so rapid thickening of the SCLM, which would have been hindered by advection of cold materials, can take place more easily. Convective instability would then lead to removal and sinking of thickened, cold SCLM, leaving a remnant in the TZ detectable only in VP. This interpretation not only is consistent with previous reconstruction of the SCLM in Tibet based on several types of independent data, but also provides a new pathway for water to enter and be stored in the TZ.
T23G-05
A Magmatic Perspective and Zircon Hf Isotope Constraints on Tibetan Orogenesis
The continental collision between India and Asia represents the latest albeit ongoing phase of sequential geologic processes that, evolving repeatedly from the southern to northern hemispheres, involved Gondwana dispersion, Tethyan subduction and terrane amalgamation. The immense plateau, in particular its southern part, is now underlain with the thickest continental crust on Earth. When and how was such thick crust formed, which would have exerted pivotal controls to the plateau?|s formation, however, remains an issue with little consensus owing to the scarcity of reliable constraints. Here we review the pre- to postcollisional magmatic evolution in southern Tibet, and report in-situ Hf isotopes of zircons from the Gangdese batholith and associated igneous rocks, which enable us to unravel the sequential geologic processes related to Neo-Tethyan subduction and Tibetan orogeny. These zircons, crystallizing from ca. 190 to 15 Ma, possess juvenile mantle type Hf isotopes comparable to those of certain granitoids in eastern Australia derived from east Gondwana and suggesting basaltic magma underplating as a key crustal formation process in the subduction zone. Our data further indicate that in southern Tibet the crust underwent significant tectonic thickening during ca. 45 and 30 Ma, thus shedding new insights about when, and how, the Earth?|s thickest continental crust was formed. The lower part of the thickened crust consisted prevailingly of mafic lithologies, which we infer to have resulted from intense basaltic underplating and remelting that occurred during Cretaceous and mid-Eocene time owing to the Neo-Tethyan subduction, a process responsible for not only the juvenile crust production but also creation of a thermally softened lithosphere. Subsequent collision of India with Asia, therefore, caused distributed lithospheric thickening and formation of an orogenic root underneath southern Tibet. Root foundering during the Oligocene gave rise to the postcollisional magmatism, regional uplift, and onset of northward underthrusting of the Indian plate that has since played a role in forming the entire Tibetan plateau.
T23G-06
Deep Structure of the NE Tibetan Plateau: An Introduction to Project INDEPTH, Phase IV
Since 1992, INDEPTH has acquired active and passive seismic data in three major experiments from south of the High Himalaya to the Qiangtang terrane, north of the Banggong suture. Phase IV of Project INDEPTH is focussed on delineating deep crustal and mantle structure beneath the northeast margin of the Tibetan Plateau, thought by many to represent the focus of active growth of the plateau into the Asian continent. Primary scientific objectives include: i) elucidating the crustal geometry of the thrust/strike slip fault system of the Kunlun that marks the edge of the high plateau, ii) testing the controversial suggestion that the Asian continental lithosphere is underthrusting southwestward beneath the Tibetan Plateau beneath the Kunlun, iii) assessing the role of partial melting in the tectonics of NE Tibet, and contraints on postulated crustal flow. The major technical components of this year's work include: 1) Active seismic imaging with near-vertical and wide-angle techniques; 2) Passive seismic profiling using dense seismometer spacing (ca 5 km) designed to produce high resolution receiver-function images of lithospheric structure; and 3) Geological surface investigations. Additional components scheduled for next year include: 1) a high resolution gravity survey along the seismic profile, and 2) wideband MT and long-period MT ( LIMS ) measurements to investigate deep electrical conductivity along the seismic profiles. The INDEPTH IV controlled source profile extends from Qarhan Salt Lake in the center of the Qaidam basin, across the Kunlun mountains of northern Tibet to Quemalai near the Chumaer River. The INDEPTH IV passive- source profiles are collocated with the controlled source profile across the Kunlun, but includes a second high resolution profile across the Jinsha Suture on the Tibetan Plateau. The INDEPTH IV geological studies are focussed on the timing of crustal shortening and uplift of the Kunlun and adjacent Plateau. Preliminary results will be presented here and in associated INDEPTH posters
T23G-07
Controlled Source Imaging of the Kunlun Suture, Northeast Tibetan Plateau
In June of 2007, project INDEPTH IV conducted a 260 km controlled source seismic refraction/reflection experiment across the Kunlun suture in the Northeast Tibetan Plateau from the Qumarlai area to the Qaidam Basin. Approximately 950 PASSCAL "Texan" seismographs were deployed, together with a conventional 1000- channel cabled seismometer system (Sercel 408XL, 24 bit) in a fixed spread. The cabled system with 50-m geophone spacing and ~650 Texans with 100-m spacing instrumented a central 115-km reflection segment across the Kunlun fault and suture. The "reflection" profile used ~100 explosive sources, nominally of 80 kg in 30-m shotholes, at 1-km spacing. ~300 additional Texans at 650-m spacing extended the reflection recording spread 80 km north and south along the profile to acquire wide-angle reflection/refraction data, out to the critical distance for Moho reflections. Five large explosions of 2000 kg, 1500 kg, or 1000 kg detonated at roughly even increments along the line acted as principal sources for the wide-angle profile. The controlled source data will allow the determination of crustal thickness and overall velocity and reflectivity structure. Anticipated results include insights into the Tertiary evolution and modern tectonic environment of the plateau. We expect the geophysical data to provide constraints on competing theories of plateau formation (e.g., deep crustal flow vs. imbricate stacking and subduction of Asian crust) and to further discern fault geometries of strike-slip and thrust systems (e.g., whether faults penetrate the entire crust).