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Transition from continental collision to tectonic escape? A geophysical perspective on lateral expansion of the northern Tibetan Plateau
© Deng et al.; licensee Springer. 2014
Received: 4 November 2013
Accepted: 25 March 2014
Published: 16 April 2014
A number of tectonic models have been proposed for the Tibetan Plateau, which origin, however, remains poorly understood. In this study, investigations of the shear wave velocity (Vs) and density (ρ) structures of the crust and upper mantle evidenced three remarkable features: (1) There are variations in Vs and ρ of the metasomatic mantle wedge in the hanging wall of the subduction beneath different tectonic blocks of Tibet, which may be inferred as related to the dehydration of the downgoing slab. (2) Sections depicting gravitational potential energy suggest that the subducted lithosphere is less dense than the ambient rocks, and thus, being buoyant, it cannot be driven by gravitational slab pull. The subduction process can be inferred by the faster SW-ward motion of Eurasia relative to India as indicated by the plate motions relative to the mantle. An opposite NE-ward mantle flow can be inferred beneath the Himalaya system, deviating E and SE-ward toward China along the tectonic equator. (3) The variation in the thickness of the metasomatic mantle wedge suggests that the leading edge of the subducting Indian slab reaches the Bangoin-Nujiang suture (BNS), and the metasomatic mantle wedge overlaps with a region with poor Sn-wave propagation in north Tibet. The metasomatic layer, north of the BNS, deforms in the E-W direction to accommodate lithosphere shortening in south Tibet.
Structural modeling from seismological tomography and gravity inversion
Based on the Rayleigh wave dispersion analysis, Zhang (2010) studied the three-dimensional (3D) velocity structure of the lithosphere-asthenosphere system (including that of the crust) in the Qinghai-Tibet Plateau and its adjacent areas (see also Zhang et al. 2014). With the purpose of constructing a robust inversion of the data, we used the program library GRAV3D, which is a suite of algorithms for inverting gravity data gathered over a 3D construction of the earth (Li and Oldenburg 1996,1998). The subsurface volume is modeled as a set of cuboidal elements each with ρ contrast. The inverse problem involves estimating the ρ contrasts of all the cuboidal elements based upon the available measurements of the Bouguer anomalies, and the inverse problem is solved as an optimization problem with the simultaneous goals of (1) minimizing the objective function on the model and (2) generating synthetic data that match observations to within a degree of misfit consistent with the statistics of the available data. The definition of length scales for smoothness controls the degree to which either of these two goals dominates. This is a crucial step that allows the user to incorporate a priori geophysical or geological information into the inversion. Explicit a priori information may also take the form of the upper and lower bounds on the ρ contrast in any element.
This approach has been extensively applied to upper crustal problems; however, applications to the lower crust and upper mantle have been limited (e.g., Brandmayr et al. 2011; Welford and Hall 2007; Welford et al. 2010). Refined mesh design and sensible regularization procedures permit reasonable resolution of the Moho surface, whilst allowing variations within the crust and mantle to be modeled as part of the same procedure (Welford and Hall 2007; Welford et al. 2010). The full 3D nature of this approach, along with the requirement of a large number of cells in the vertical direction, means that this method is more computationally demanding compared to other methods. Therefore, it is natural to use the reliable part of the crustal inversion as a constraint for the inversion involving the mantle down to 350 km (e.g., see Mueller and Panza 1986). To obtain the 3D ρ structure, we first constructed an a priori ρ model by using a literature conversion relation between Vs and ρ (Feng et al. 1986; Zhao et al. 2004; Deng et al. 2013) for the crust and PREM model (Dziewonski and Anderson 1981) for the upper mantle to a depth of 350 km. Subsequently, we applied a perturbation to the model in order to not only match the observed Bouguer gravity, but also to match the range of ρ since we use the range of Vs as the limiting condition (the Vs model is defined with its uncertainty range).
In section A-A′ (Figure 3b), the crust thickens gradually from about 40 km south of the Main Boundary Thrust (MBT) to about 80 km beneath the boundary between Tibet and the Tarim Basin. A very interesting feature is the thinning of the asthenosphere layer from about 80 to 90 km south of the Jinshajiang suture (abbreviated as JS in Figure 2 and hereafter), while, northward, the metasomatic lid appears and thickens up to 100 km beneath the north segment of the section.
Along section B-B′ (in Figure 3c), the crust and mantle structures are similar to those seen in section A-A′. The major difference between these two sections is the Moho depth, as beneath JS, an abrupt change appears in the Moho depth (Figure 3c).
Along section C-C′ (Figure 3d), the pattern of the crustal thickness is very different from that along sections A-A′ and B-B′ since the crustal thickness is relatively constant between the IYS and the north of the JS (Figure 3d). Along C-C′, the asthenosphere thins northward at the IYS while it exhibits a constant thickness to the south, and the metasomatic lid (and mantle wedge) begins to exist between the BNS and JS (the boundary between the south and north sections of the Qiangtang Block). The crustal structure beneath section D-D′ is similar to that of section C-C′, but the metasomatic lid appears south of the BNS in section D-D′ (Figure 3e). Along section E-E′ (Figure 3f), south of the JS, an abrupt change appears in the Moho depth, and the metasomatic lid begins to appear beneath the boundary between south and north Qiangtang. Along section F-F′ (Figure 3g), the abrupt change appears in the Moho depth beneath the boundary between the southern and northern sections of the Qiangtang Block, and the size of the metasomatic lid is significantly reduced with respect to that shown in the previous sections. A similar situation is seen along section G-G′ (Figure 3h), even if the metasomatic lid has almost ceased to exist (Figure 3h).
Figure 4 shows distinctive patterns in the density differences between the south and north sections of the BNS. Along profiles A-A′ (Figure 4a) and B-B′ (Figure 4b), ρ ranges from 2.4 to 2.9 g/cm3 in the crust, and the layer with ρ in the range of 2.8 to 2.9 g/cm3 thickens to about 70 km to the north (to about 30°N). These results may be consistent with the speculation that the crust of the Indian Plate penetrated the whole of the Tibetan Plateau in west Tibet (Li et al. 2008; Zhou and Murphy 2005). Along transects C-C′ (Figure 4c), D-D′ (Figure 4d), and (E-E′) (Figure 4e), we clearly observe that the convergence between the Indian and Eurasian plates thickens the crust. These results suggest that the leading edge of the injected Indian crust may be located along the BNS in east Tibet (Zhang et al. 2011).
Gravitational potential energy structure
Results and discussion
The collision of the Indian and Asian plates is accompanied by a low convergence-shortening ratio (about 1.2) since the convergence is partitioned both in the subduction and shortening, and the subduction is relatively slower than the ongoing contraction in the upper and lower plates (Doglioni et al. 2007; Gong and Chen 2014). This type of setting wherein the subduction hinge converges relative to the upper plate generates a forebelt synthetic to the subduction and a conjugate retrobelt (Doglioni et al. 2007). The Himalayan orogeny represents the coalescence of several oceanic and continental subduction zones (e.g., Yin and Harrison 2000), and the Tibetan Plateau is the area separating the active forebelt and the retrobelt, where the uplift is constrained by the isostatic readjustment due to either the convective removal of the lithosphere or the achieved critical taper of the orogenic wedge whose mechanical evolution needs to occur laterally (e.g., Molnar et al. 1993; Dahlen 1990). The direction of the Himalaya subduction is along the trend of the tectonic equator that deviates from NE to SE moving from India to China as indicated both by plate motions in the last 50 Ma and the GPS data in the no-net rotation and net rotation reference frames (Crespi et al. 2007). Therefore, the W-E extension in the Tibetan Plateau is compatible with the global flow of plates, and it may be related to the back-arc extension operating along the western margin of the Pacific realm and not necessarily to tectonic escape.
The investigations of the seismic velocity and the density structures support the subduction of the Indian lithosphere, dragged NE-ward by the mantle flow. The slab should contribute to the metasomatism of the overlying mantle wedge (Lid and LVZ) north of the BNS.
We would like to thank Gan, WJ, for providing the GPS data. The Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB03010700) and the National Natural Science Foundation of China (41021063, 41374064) supported this research. We also acknowledge support from the Italian PRIN projects 2008, 2011, PNRA projects 2004/2.7-2.8, 2009/A2.17.
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