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GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 5137–5141, doi:10.1002/grl.50987, 2013 Evidence of active mantle flow beneath South China Chun-Yung Wang,1 Lucy M. Flesch,2 Lijun Chang,1 and Tianyu Zheng3 Received 24 July 2013; revised 17 September 2013; accepted 23 September 2013; published 10 October 2013. [1] The India-Eurasia collision is responsible for producing the Himalayan Mountains and Tibetan plateau and has been hypothesized to have significant far field influences, including driving the Baikal rift and the eastward extrusion of South China. However, quantification of lithospheric buoyancy forces and integrated effect of tractions acting at base of the lithosphere are unable to explain the observed surface motions within South China. We present 198 new SKS shear wave splitting observations beneath South China and invert these data along with published GPS data to solve for the subasthenospheric flow field beneath South China to assess the role of small-scale convection here. We find a 15–20 mm/yr southwestward-directed mantle flow toward the Burma slab. This flow is consistent with the mantle response of slab retreat over the past 25 Ma, and counter flow due to subduction of Burma/Sunda slabs demonstrating the importance of localized mantle convection on present-day plate motions. Citation: Wang, C.-Y., L. M. Flesch, L. Chang, and T. Zheng (2013), Evidence of active mantle flow beneath South China, Geophys. Res. Lett., 40, 5137–5141, doi:10.1002/grl.50987. 1. Introduction [2] The collision between India and Eurasia has generated the highest topography on the face of the Earth, a spatially complicated surface deformation pattern, and has been posited to be responsible for far-field deformation such as the opening of the Baikal rift and extrusion of South China [Avouac and Tapponnier, 1993]. The proliferation of GPS observations over the past 15 years within this region has allowed for the detailed knowledge of how relative motions are accommodated across this broad zone of continental deformation, and thus provide constraints for quantification of the forces generating geologic features. These are generally attributed to buoyancy forces resulting from density variations within the lithosphere and the integrated effect of large-scale tractions acting at the base of tectonic plates excited from density variations within the deep mantle. Global models have quantified the relative contributions of deviatoric stresses associated lithospheric density variations and tractions acting Additional supporting information may be found in the online version of this article. 1 China Earthquake Administration, Institute of Geophysics, Beijing, China. 2 Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana, USA. 3 Chinese Academy of Sciences, Institute of Geology and Geophysics, Beijing, China. Corresponding author: L. M. Flesch, Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47906, USA. (lmfl[email protected]) ©2013. American Geophysical Union. All Rights Reserved. 0094-8276/13/10.1002/grl.50987 at the base of the lithosphere driven by deeper mantle density buoyancies [Ghosh and Holt, 2012; Lithgow-Bertelloni and Guynn, 2004]. Regionally, several authors have modeled the India-Eurasia collision accounting for the north-south convergence between the India and Eurasia balanced by the effects of gravity acting on the elevated topography; however, no study has been able to replicate both the eastward extrusion of South China and curvature of velocities around the Eastern Himalayan Syntaxis (EHS) without imposing an a priori velocity boundary condition [Flesch et al., 2001; Vergnolle et al., 2007]. Likewise, it has been shown that the northward motion of the Indian plate is connected to and driven by a large-scale deep mantle convection cell [Becker and Faccenna, 2011]; however, the induced flow on the eastern side of the Burma/ Sunda slab has yet to be related to the surface motions in South China and the EHS. The inability of either lithospheric buoyancy forces or forces associated with large-scale mantle convection to produce the observed surface motions in this region [Becker and Faccenna, 2011; Flesch et al., 2001; Ghosh and Holt, 2012; Vergnolle et al., 2007] necessitates an additional set of forces acting within the EHS and South China to generate the observed surface motions. Because the structure of the lithosphere is known through seismic studies [Li et al., 2008] and the deviatoric stress fields associated with these density variations have been quantified through lithospheric models [Flesch et al., 2001; Ghosh et al., 2009], the additional set of forces must be applied at the base of the plate from a smaller-scale convection cell not resolved in global convection models. [3] Here we present 198 new SKS observations in South China and combine them with published GPS and Quaternary fault data to invert for motion of the subasthenospheric mantle in order to assess the contribution from a smaller-scale convection cell to the surface velocity field in South China. Our model predicts a southwestward directed mantle flow field beneath South China at a rate of 15–20 mm/yr in a no-net-rotation (NNR) absolute plate motion reference frame that is consistent with the reorganization of the Burma/Sunda subduction margin and slab retreat over the past 25ma, and/or counter flow due to active subduction of Burma/Sunda slabs [Li et al., 2008; Richards et al., 2007]. This mantle flow, if coupled to South China, would generate a torque at the base of the lithosphere providing the additional driving force needed to produce observed surface motions. 2. Inversion of SKS and GPS Data [4] We determine the SKS fast axis azimuths and delay times for 198 seismic stations in South China (Figure 1a) where the fast polarization and delay times for each arrival were determined following the method of Silver and Chan [1991] and stacking procedure of Wolfe and Silver [1998] (see online supplement). Seismic anisotropy, measured through SKS shear wave splitting, develops when mantle minerals, most 5137 WANG ET AL.: ACTIVE MANTLE FLOW BENEATH SOUTH CHINA Figure 1. (a) The observed SKS splitting data used in this study [Wang et al., 2008; Flesch et al., 2005; Chang et al., 2011; Bai et al., 2009; Huang et al., 2008], plotted over seismic velocity [Huang et al., 2003]. (b) The fit of the predicted anisotropy, assuming the lithosphere is moving over a stationary asthenosphere in a no-net-rotation reference frame to the observed SKS data. Plotted in the background is the magnitude of the surface strain rate field determined from Quaternary fault slip data and GPS velocities. Data shown in Figure 1a that are located in region of surface strain rates higher than 10 × 10e 9/yr are not used in the analysis and are not plotted in Figures 1b–f. (c) The model surface velocity field in a no-net-rotation reference frame and the best fit rotation of the subasthenospeheric mantle. (d) Same as Figure 1b only for the subasthenospheric rotation shown in Figure 1c. (e) Same as Figure 1c only excluding the SKS data in the Yangtze craton (red stars) from the inversion for subasthenospheric flow. Background grid represents lithospheric thickness [An and Shi, 2006] and geologic provinces [Ren et al., 1980] are denoted by black lines. (f) Same as Figure 1d for the best fit subasthenospheric rotation shown in Figure 1e. 5138 WANG ET AL.: ACTIVE MANTLE FLOW BENEATH SOUTH CHINA Figure 2. The surface velocity field inferred from GPS and Quaternary fault slip data (black vectors) in a no-net-rotation reference frame plotted with the predicted mantle velocity field (blue vectors) in the study area of thin lithosphere. Background on the continents is lithospheric thickness [An and Shi, 2006] and in the oceans slab contours [modified from Richards et al., 2007]. noticeably olivine, undergo strain. Laboratory studies have shown, for a dry olivine, the fast axis will align with the direction of maximum shear in a simple shear regime [Zhang and Karato, 1995]. Thus, seismic anisotropy is an observation of deformation of the deeper mantle [Silver and Chan, 1991] and can be used to elucidate interaction of the deeper mantle with the Earth’s surface. Anisotropic fabric can be formed on (1) subvertical planes in the lithospheric mantle where the direction of anisotropy correlates with the surface deformation inferred from GPS observations or, (2) on horizontal planes in the asthenospheric mantle where the anisotropy represents the differential motion between the lithosphere and subasthenospheric mantle below [Silver, 1996] and measured SKS observations are most likely some combination of the two sources. Previous studies of shear wave splitting observations in the collision zone [Lavé et al., 1996; Davis et al., 1997; Holt, 2000; Flesch et al., 2005; Wang et al., 2008], which focused on Tibet and surrounding regions, identified a correlation between the surface deformation field and SKS splitting observations illustrating that deformation in the central Asian lithosphere is vertically coherent and further argued that deviatoric stresses in the lithosphere were coupled. The dramatic increase in SKS data beyond Tibet now allows for the investigation of the lateral variation in both lithospheric and lithosphere/mantle coupling throughout the India-Eurasia collision zone. [5] In order to achieve the above goal, we first determine a model strain rate and velocity field (see online supplement, Figure S1) using continuous spline functions to interpolate between observed strain rate data [Haines et al., 1998] inferred from Quaternary fault data [England and Molnar, 1997] and GPS observations (see online supplement). Following the methods of Silver and Holt [2002] and Wang et al. [2008], a joint analysis of this surface deformation field and the mantle deformation inferred from 198 new and previously published SKS data (Figure 2a) [Bai et al., 2009; Chang et al., 2011; Flesch et al., 2005; Huang et al., 2008; Wang et al., 2008] is then performed in order to assess the interaction between the surface and deeper mantle in South China. [6] Unlike Tibet where the lithosphere is thick, deformation rates are high, and anisotropy is located in the lithospheric mantle [Lavé et al., 1996; Davis et al., 1997; Holt, 2000; Flesch et al., 2005; Wang et al., 2008], South China has a thin lithosphere on the order of ~80 km [An and Shi, 2006] (Figures 1e and 1f) and deformation rates are less than 5 × 10 9/yr (Figure 1b). Thus, it is unlikely that these low rates could generate an anisotropic fabric in the thin lithospheric mantle layer in South China capable of producing delay times observed in the SKS splitting. Specifically, travel time data of the shear waves (Sg, SmS) excited by artificial sources [Carbonell et al., 2000; Satarugsa and Johnson, 2000; Readman et al., 2009] estimated less than 2% crustal anisotropy, time delay of the shear wave splitting in the upper crust is ~2.5 ms/km [Wu et al., 2008], and the average crustal thickness in the South China is ~35 km [Li et al., 2006], which corresponds to a delay time from the crust on the order of 0.10 s. For an average mantle lithospheric thickness in the South China of ~45 km [Huang et al., 2003; Priestley et al., 2006; An and Shi, 2006], with the exception of the western Yangtze craton, an anisotropy coefficient of 3% [Mainprice and Silver, 1993; Ben-Ismail et al., 2001], and the average shear wave velocity of 4.60 km/s, the estimated maximum delay time for the lithospheric mantle here is ~0.30 s, illustrating that the majority of the observed delay time (~0.60 s) must be generated within the asthenosphere. Indeed, a comparison of the predicted SKS observations from the surface deformation field with SKS data, following the method of Wang et al. [2008] used for Tibet, yields an average misfit of 41° illustrating no correlation between present-day surface deformation and anisotropy in South China. This result and above calculations argue that the anisotropy here is not located within the lithospheric mantle and must be located within the asthenospheric mantle. The observed SKS data here could also be an artifact of lithospheric fossil anisotropy from earlier deformation in South China’s long geologic history; however, the thin mantle lithosphere in most of the region would again not be able to produce the observed delay times. The Yangtze craton maybe one exception; here the lithosphere reaches thicknesses of ~200 km and fossil anisotropy present within the lithospheric mantle could generate the observed delay times. We address this possibility below. [7] Due to the above lack of evidence that the anisotropy in South China is located within the lithospheric mantle, we next explore the possibility that it is located within the asthenosphere and represents the differential motion between the lithosphere and convecting mantle below. Because the large SKS data set (Figure 1a) considered here includes 5139 WANG ET AL.: ACTIVE MANTLE FLOW BENEATH SOUTH CHINA Tibet, a region that has been shown to contain lithospheric anisotropy, we filter the existing SKS data by only using stations where the surface deformation is low and less than 10 × 10e 9/yr (Figures 1b and S1b) in order to analyze only asthenospheric anisotropy. Additionally, because the data within Yunnan exhibit the same general anisotropic trend and have the same lithospheric thickness as regions of low surface deformation, we add the SKS data in Yunnan back to the data set considered for analysis. [8] We first predict anisotropy directions under the assumption that the lithosphere is moving in an absolute reference frame over a stationary asthenospheric mantle. We rotate the continuous surface model velocity field into an NNR reference frame [Kreemer and Holt, 2001], and assume that the velocity at the surface of the lithosphere is equal to the velocity at the base of the lithosphere, to generate shear between the lithosphere and asthenosphere and predict the resulting mantle anisotropy at the splitting sites (Figure 1b). The comparison produces an average misfit between the predicted and observed anisotropy of 22°, which is larger than the 10° uncertainty of the SKS data. Additionally, there is a systematic clockwise rotation of the predicated anisotropy indicating a nonstationary motion of the deeper mantle. Therefore, we invert the surface velocity field (Figure 1c) in an NNR reference frame with the subset of SKS data to invert for a best fit rotation of the subastheospheric mantle (Figures 2c and 2d). We use the method of Silver and Holt [2002] who showed that for deforming regions a unique solution of mantle flow can be determined unlike regions of undeforming lithosphere or single splitting observations, which cannot yield mantle flow rate information or distinguish between two directions of mantle flow 180° apart. We invert for a single rotation of the subasthenosphere that produces a differential flow with the lithosphere that best fits the SKS data field (for details, see Silver and Holt [2002]). The flow field in the mantle may indeed be more completed than our inversion; however, a more complex treatment using splines or higherorder polynomials would over parameterize the solution since the SKS observations and surface velocity field only vary slightly spatially within South China. The best fit mantle rotation produces a southwestward-directed clockwise rotation of the subasthenospheric mantle at a rate of 15–20 mm/yr directed toward the Burma/Sunda subduction zones and produces an average misfit of 15°. We tested different levels of damping and an undamped inversion and all produced the same subasthenospheric velocity field. The largest misfits occur in the western Yangtze craton where the deformation rates are low, fast axis orientations are highly spatially variable, and the lithosphere is anomalously thick compared to the surrounding regions (Figures 2e and 2f). Anisotropy here is most likely fossil from past geologic processes and has been interpreted to be related to the presence of a preexisting lithospheric structure that was recorded during the continentarc-continent collision in the early Neoproterozoic following the Grenvillian subduction of oceanic crust in the late Mesoproterozoic [Charvet et al., 1996]. Therefore, it most likely does not represent present-day mantle deformation. This inference is further supported by the thicker lithosphere [An and Shi, 2006] that could produce large splitting delay times and higher seismic velocity [Huang et al., 2003] (Figure 1). However, we cannot rule out spatial variation in SKS fast directions here is caused by asthenospheric flow around the thicker Yangtze craton as has been observed in other areas [Fouch et al., 2000; Miller and Becker, 2012]. As a result, we further filter the SKS data used in the analysis to remove all splitting stations within the Yangtze craton (stars Figures 1e and 1f). The resulting inversion (Figures 1e and 1f) using the smaller subset of SKS data produces a very similar rotation of the subasthenospheric mantle as the larger data set inversion, with the exception that the mantle flow is directed slightly more southward and the average misfit is further decreased to 12°, now on the order of the uncertainty of the data. 3. Discussion [9] Seismic tomography studies identify a ~60° dipping Burma slab consistent with eastward subduction (Figure 2), independent from Indian plate subduction [Ni et al., 1989] that is currently retreating to the west [Li et al., 2008]. There is a zone of low P wave velocity anomaly below the Red River fault region visible at 100 km depth extending to 300 km [Li et al., 2008; Wang et al., 2003] that provides seismic evidence for smaller-scale convective processes in the upper mantle with warmer slower material drawn up over the Burma slab due to its retreat. [10] Furthermore, tectonic reconstructions of present-day slab geometries along the Indo-Australian plate provide a detailed history of the Burma/Sunda subduction zone [Richards et al., 2007]. The once continuous NW trending subduction margin of the Indo-Australian plate experienced a subvertical tear with the closing of the Tethyan Ocean and a subsequent clockwise rotation of the Indo-Burma subduction (Figure 2). Additionally, extension in the Andaman Sea [Raju, 2005] has been hypothesized to have developed due to a westward slab rollback/retreat of the Burma slab and southwest rollback of the Sunda slab [Richards et al., 2007]. The rotation, rollback, and retreat of the Sunda slab would draw warmer asthenospheric material southwestward toward the slab generating a small-scale convection cell. The long history of Tethys subduction has been shown to drive a mantle scale convection cell beneath India that is responsible for Indian plate motion [Becker and Faccenna, 2011]. Here we map a mantle flow beneath South China through a joint inversion of surface velocity and SKS splitting data and link it to the history of subduction of the Sunda/Burma arguing for a dynamic mantle convection cell beneath South China (Figure 2) similar to that shown to exist beneath India [Becker and Faccenna, 2011]. Additionally, if this flow is coupled to the South China lithosphere like the coupling observed beneath India [Becker and Faccenna, 2011; Ghosh and Holt, 2012], it would provide the missing force needed to drive surface motions observed in South China and around the EHS. 4. Conclusions [11] Both the original subset and smaller subset of SKS data analyzed here produce a southwestward-directed clockwise flow beneath South China toward the Burma/Sunda subduction zone at a rate of 15 mm/yr producing a differential flow of ~35 mm/yr between the lithosphere and subasthenosphere (Figure 1e). This differential shear is sufficient to generate the observed anisotropy in South China. Our predicted mantle flow is consistent with and could be excited by: (1) counter flow generated by the active subduction of the Burma [Satyabala, 1998] and Sunda slabs; and/or (2) 5140 WANG ET AL.: ACTIVE MANTLE FLOW BENEATH SOUTH CHINA westward slab roll back and retreat of the Burma and Sunda slabs [Li et al., 2008; Richards et al., 2007]. The predicted flow if coupled to the lithosphere would drive surface motions and could be the “missing” force needed to drive surface velocities in South China and around the EHS (Figure 2). These results highlight the importance of small-scale convection cells, influenced by complex subduction system of SE Asia, in the evolution of the India-Eurasia collision zone. [12] Acknowledgments. This work was improved by the helpful reviews from Karen Fischer and an anonymous reviewer. 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