D) Seismic Profiles & Tomography:
A variety of geophysical methods have been used to interpret lithospheric structures at depth. By far the most common method is deep seismic sounding. Interpretation of seismic travel times and amplitudes along the profile provides direct measurements of seismic velocities and depth of reflection-refraction horizons. Compiled seismic profiles recorded along Tibetan plateau indicates continent-continent collision related crustal growth and thickening reaching up to 70 km in central Tibet. Most recently the collaborative geoscience project named INDEPTH collected reflection, broadband earthquake, magnetotelluric and surface geological data. Acquisition of these data revealed high velocity lower crust of underthrusting cold Indian plate and a set of crustal reflecting horizons at depths of 15 to 20 km located north of the Yarlong-Zangbo suture (Lhasa terrane) (Nelson et al. 1996) (figure 12). These horizons mainly exhibits high amplitudes, negative polarities, strong P to S conversions and high electrical conductivity with elevated heat flow suggesting the occurrence of partial melt.
Figure 12. (Top) Selected INDEPTH data including: common-midpoint reflection profile, one dimensional shear-wave velocity
profiles and wide-angle reflection data. (Bottom) Interpreted lithospheric-scale model [Nelson et al. 1996].
Further north, the Qiangtang terrane has thicker crust (70 km). Based on inefficient Sn propagation (figure 13), low Pn velocities (figure 13), high crustal Poisson's ratio and attenuation (McNamara et al. 1994, 1997; Owen & Zandt, 1997) (figure 14), and presence of volcanism of mantle lithospheric origin, a regionally developed weak partially melted uppermost mantle was suggested to exist. Some authors also modeled its origin by involving wholescale delamination of the mantle lithosphere in the Qiangtang terrane (Molnar et al. 1993).
Figure 13. Compiled seismic data for Tibet including; Pn tomography, uppermost mantle anisotropy
(SKS Splitting) and Inefficient shear-wave (Sn) propagation [McNamara et al. 1997]
Shear coupled teleseismic P-wave study by Owens & Zandt (1997), illustrated crustal thinning by up to 20 km from south to north with increasing Poisson's Ratios. According to them, low angle Indian underplating reaches to the Banggong suture and then bends down whereas northern Tibet has a thin, hot and partially molten lithosphere with possible lateral flow developing anisotropic fabric (figure 14).
Figure 14. Interpretative north-south cross section in the Central Tibetan plateau [Owen & Zandt, 1997]
In contrast Kosarev et al.(1999) based on similar methodology (P to S conversion) observed higher northward dip for Indian plate and in the north imaged highly imbricated south dipping structure that may be the mantle lithosphere of Qiadam basin (figure 15). Fundamental difference between crustal and mantle structures indicates preserved lithosphere in the south and complicated destruction related to a higher mantle temperatures beneath northern Tibet.
Figure 15. Receiver function image of Tibetan plateau and its interpretation [Kosarev et al. 1999].
A) Migrated RF image of the north-south profile (red colors mark positive RF amplitudes and blue
colors mark negative amplitudes) B) Interpretation cartoon.
Open questions on the role of extrusion and thickening for accommodating convergence, can not be quantified unless the vertical scale of strike-slip faulting is determined. In order to answer such questions, a tomographic study across the central Altyn-Tagh fault (ATF) was done by Wittlinger et al. 1998. Results of this work showed a negative velocity perturbation that extends near the base of the moving lithospheric plates suggesting a lithosphere scale ATF (figure 16). This further supports localized strain and slip partitioning in narrow zones between large extruding blocks. Moreover, this narrow and steep velocity anomaly encountered in the upper crust may be due to water-filled cracks and chemical alteration that result in weak faulting. At last, it is worth noting that Moho offset detected from double-pulse teleseismic P wave arrival (Zhu & Helmberger, 1998) (figure 17) and anisotropy with fast axis parallel to the fault across the Kunlun fault (McNamara et al. 1994) (figure 13) may represent a similar lithospheric scale deformation and is still waiting to be explored further.
Figure 16. A) Seismic tomographic section passing across the Altyn Tagh fault (ATF) Figure 17. A north-south cross section showing crustal structure inferred from
in a WNW-ESE trend along the north edge of the Tibetan plateau (Thick lines are P wave modeling . It indicates a low-velocity lower crust in northern Tibet
boundary faults: dashed line, inferred Moho, pale yellow corresponds to Cenozoic [Zhu & Helmberger et al. 1998].
sediments and green and pink to mantle and crust, more yellow or red, respectively,
where velocity is lower in tomographic image). B) Wider scale tomographic section
with computed 900 C isotherm [Wittlinger et al. 1998].
A) Tectonics & Geology B) Seismicity C) GPS D) Seismic profiles & Tomography
E) Gravity F) Anisotropy G) Paleomagnetism H) Geochemistry I) MT studies
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