The formation of North-South Seismic Zone and Emeishan large igneous 1 province in Western China : Insight from teleseismic tomography 2 3

Abstract. Several models have been suggested to explain the earthquake mechanism of the North-South Seismic Zone (NSSZ) and the formation of the Emeishan Large Igneous Province (ELIP). In this study, I extended the study region and carried out detailed teleseismic tomography in the NSSZ and near-by regions. Results identified by this study reveal large plate-like high-velocity anomalies beneath the Songpan-Ganzi Block and the South China Block, which may be associated with large-scale lithospheric delamination, and low-velocity structures at 50–200 km depths in the western and southern parts of this study region, which imply upwelling asthenosphere induced by delamination and the absence of the rigid lithosphere there. Two high-velocity structures beneath the Sichuan Basin and the Alashan Block are revealed, which might be the lithospheric roots of these structures. These rigid lithospheric roots obstructed the eastward extrusion of the Tibetan Plateau and led to stress accumulations and releases (earthquakes) in the Longmenshan Orogenic Belt and the northern part of the NSSZ. Due to obstruction by the Sichuan Basin’s lithosphere, eastward extrusion was redirected southeastward to Yunnan in the southern part of the NSSZ, which led to stress accumulations and releases (earthquakes) along the Honghe and Xiaojiang Faults. This study provide velocity images reveal a slab-like high-velocity structure, which might be associated with the lithospheric vestige of the Paleo-Tethys Ocean that subducted beneath the ELIP, which resulted in large-scale return mantle flow or mantle upwelling and contribute to the LIP formation in early Mesozoic.



Introduction
The North-South Seismic Zone (NSSZ) is a region of high seismic hazard in China due to devastating earthquakes (Zhang et al., 2003;Deng et al., 2003), which are located in regions where multiple blocks amalgamate (Zhang, 2013;Wang et al., 2011;He et al., 2014b) (Fig. 1).The NSSZ is also a boundary between the highland in the western part and lowland in the eastern part of China and a north-south-trending gravity anomaly zone (Zhang, 2013).Given the documented historical earthquakes, more than one-third of strong earthquakes (magnitude over 7) in China have occurred in the NSSZ (Zhang et al., 2003;Deng et al., 2003).al., 2010al., , Xiao et al., 2004)).Based on the magma distribution, the ELIP is divided into 3 zones: inner zone, intermediate zone and outer zone (Ali et al., 2010;Xu et al., 2004) (Fig. 1).
Tomography has revealed prominent low-velocity layers in the middle crust under the eastern margin of the Tibetan Plateau (Li et al., 2009;Li et al., 2014); the Longmenshan Orogenic Belt is a boundary between the low-and high-velocity structures (Yang et al., 2014;Huang et al., 2015;He et al., 2019), and a Mesozoic deep process of large-scale delamination occurred in the western part of the Longmenshan Orogenic Belt (Bai et al., 2011;He et al., 2019).Receiver functions and tomography in the northeastern part of the Tibetan Plateau reveal an eastward subducted slab (Yang et al., 2014;Huang et al., 2015;He 2011;He et al., 2017a, b).Teleseismic tomography in the Longmenshan area has defined a large-scale high-velocity anomaly of plate-like appearance beneath the https://doi.org/10.5194/se-2019-119Preprint.Discussion started: 10 July 2019 c Author(s) 2019.CC BY 4.0 License.
Songpan-Ganzi Block (He et al., 2019), which is considered the delaminated rigid lithosphere of the Songpan-Ganzi Block.Receiver function and tomographic studies in the ELIP area have suggested that the ELIP was generated by upwelling asthenosphere, not an upwelling mantle plume rooted in the core-mantle boundary (He et al., 2014b, He andSantosh, 2017a).
However, recent tomographic studies indicate that large-scale low-and high-velocity anomalies cannot be well defined by relatively small-region tomography, and some important and large velocity structures should be further checked by relatively largeregion tomography (Bastow, 2012;Chen et al., 2017).The results determined by receiver functions (such as delamination and upwelling mantle) need to be supported by velocity images.Therefore, I collected abundant teleseismic data recorded by temporary and permanent seismic stations in the NSSZ and near-by region, and carried out detailed tomography.Results identified by this study not only demonstrates a large-scale highvelocity anomaly of plate-like appearance beneath the Songpan-Ganzi Block at 400-500 km depth but also finds another large-scale high-velocity anomaly under the Yangtze and Cathaysia Blocks at 300-400 km depth.Images identified by this tomography show two large low-velocity structures at 50-200 km depth in the western and southern parts of the study region, which imply large-scale upwelling asthenosphere and the absence of the rigid lithosphere in these areas, which might be associated with the large-scale delamination.

Results
At 50, 100 and 200 km depths, the Hv1 and Hv2 high-velocity structures underlie the Ordos Basin and Sichuan Basin (Fig. 2), respectively.Li et al. (2006)   At depths of 300 and 400 km, the Hv4 high-velocity anomaly underlies the Yangtze Block and Cathaysia Block (Fig. 2).Li et al. (2006) defined a similar high-velocity structure at a depth of 400 km.At depths of 400 and 500 km, the Hv3 high-velocity structure underlie the Songpan-Ganzi Block (Fig. 2).At depths of 400, 500, 600 and 700 km, the Hv5 and Hv6 high-velocity structure are located at the southeastern part and the eastern margin of the study region, respectively (Fig. 2).Huang et al. ( 2015) revealed a high-velocity anomaly at a depth of 300-700 km in the Chuandian area, and its location and scale are similar to Hv5.In the northern part of the NSSZ, the western section has a low-velocity structure (Lv1), and the eastern part has a high-velocity structure (Shv1 or Hv1) (Fig. 3). the lithospheric roots of these structures.The Shv2, Shv3 and Shv4 high-velocity anomalies are located in the upper mantle transition zone at depths of 300-700 km.The Hv6 high-velocity structure is a subducted plate-like feature tilting from east to west.
Based on its location and shape, I suggest that it is a Cenozoic subducted slab of the Pacific Plate (He and Zheng, 2018).Previous tomography indicated Hv1 and Lhv1 as well as high-velocity anomalies (Lhv2, Lhv3, Lhv4) in the upper mantle transition zone (Fig. S5) (He and Santosh, 2017b), which are consistent with this tomographic results (Fig. 3); however, a previous study did not reveal a clear lithospheric root for the Alashan Block, although the study also defined a high-velocity structure beneath the Alashan Block (Lhv1) (Fig. S5A) (He and Santosh, 2017b).defined similar images.The plate-like high-velocity anomaly (Hv3) underlies the Songpan-Ganzi Block, and a small high-velocity anomaly (Shv5) underlies the Sichuan Basin (Fig. 4), which is consistent with the previous study (Fig. S6) (He et al., 2019).In Fig. 5, Hv1 and Hv2 underlie the Ordos Basin and Sichuan Basin, respectively.A small high-velocity anomaly (Shv6) is found under the Ordos Basin (Fig. 5); however, the scale of Shv6 is larger than that of Shv5 (Fig. 4, Fig. 5), and the thickness of Hv2 is greater than that of Hv1, which is also consistent with previous results (Fig. S7) (He et al., 2019).In Fig. 6, the Hv2 high-velocity structure underlies the Sichuan Basin (Fig. 6i, j), and a large low-velocity anomaly (Lv2) is found under the Yangtze and Cathaysia Blocks (Fig. 6k, l).Large plate-like high-velocity anomalies (Hv4) underlie the southern part of the study region (Fig. 6k, l).The Lv2 low-velocity anomaly identified by this study occurs in the upper mantle and is not rooted in the lower mantle.The Hv5 high-velocity structure resembles a subducted plate, and previous tomographic studies also revealed a similar velocity structure in this area (Huang et al., 2015;Yang et al., 2014;He et al., 2017a, b) (Fig. S8).This tomography obtained new findings: (1) I define a clear high-velocity structure of subducted plate-like appearance (Hv6) in the eastern margin of this study region.(2) I reveal a lithospheric root for the Alashan Block.
(3) Two large low-velocity anomalies (Lv1 and Lv2) almost cover the eastern part and the southern part of the study region.
(4) A large low-velocity anomaly (Lv2) in the southern part of this study region occurs in the upper mantle and is not rooted in the lower mantle, which is different from previous tomographic images (He and Santosh, 2016;He and Santosh, 2017a).
(5) I not only define a large plate-like high-velocity anomaly (Hv3) beneath the Songpan-Ganzi Block but also find another large plate-like high-velocity anomaly (Hv4) under the Yangtze and Cathaysia Blocks.

Delamination, upwelling asthenosphere and earthquakes
The NSSZ is located in a multiconvergent regime that underwent multistage collision and assembly from the Paleozoic to Mesozoic, involving the Caledonian Orogeny (Xu et al., 2006), Indosinian Orogeny and Himalayan Orogeny (Replumaz et al., 2010), accompanied by crustal compression and thickening (Tapponnier et al., 2001) due to collision and amalgamation of multiple blocks during the Paleozoic to Mesozoic.
Crustal thickening led to the transformation of granulite into eclogite (or to a density increase) in the lower crust, resulting in gravity instability and triggering delamination of the lower crust/lithosphere (Kay and Kay, 1993;Rudnick, 1995;Xu et al., 2013).
Generally, delamination occurred simultaneously or after collision associated with an orogeny (Ueda et al., 2012).Delamination is also a major deep process for recycling lower crust/lithospheric mantle back into the Earth's interior, which can lead to heterogeneities in the mantle of velocity structure (Kay and Kay, 1993;Rudnick, 1995;Xu et al., 2013;He et al., 2019).
The low Vp/Vs ratio implies deep processes of lower crustal/lithospheric delamination in the northern part of the NSSZ and the ELIP (He et al., 2014a, b) (Figs.S9, S10).
Previous receiver functions indicated that the lower crustal/lithospheric component delaminated into the upper mantle transition zone in the northern part of the NSSZ and the ELIP, which led to shallowing of both the 410 and 660 km discontinuities (Fig. S11, Fig. S12) (He et al., 2014a).A large high-velocity anomaly (Hv3) 200 km thick is found under the Songpan-Ganzi Block at 400-500 km depths, and another large-scale highvelocity anomaly (Hv4) 200 km thick lies beneath the Yangtze and Cathaysia Blocks at 300-400 km depths, which may be the lower crust/lithospheric mantle delaminated into the upper mantle or mantle transition zone.In the northern part of the NSSZ, Shv2, Shv3 and Shv4 are located in the mantle transition zone, and these high-velocity anomalies may be associated with delamination of the lower crust/lithospheric mantle.Delamination can result in upwelling asthenosphere that fills the void formed by delamination (Kay and Kay, 1993).Lv1 and Lv2 are above Hv3 and Hv4 (Fig. 2), respectively.Due to their welldefined correspondence, I consider the Lv1 and Lv2 to contribute upwelling asthenosphere that filled voids formed by delamination (Hv3 and Hv4).
The large-scale low-velocity structure at 50-200 km depths in the western part of the Longmenshan Orogenic Belt and Alashan Block as well as the southern part of the Sichuan Basin implies the absence of lithospheric mantle in these areas.The hot asthenosphere directly contacts and heats the lower crust (Anderson, 2007) form a detachment surface between the lower crust and the top of the upper mantle, facilitating the easy eastward extrusion of the Tibetan Plateau.This process resulted in stress accumulations and releases (earthquakes) in the Longmenshan Orogenic Belt and the northern part of the NSSZ due to obstruction by the rigid lithosphere of the Sichuan Basin (He et al., 2019) and the Alashan Block.
In the southern part of the NSSZ, the seismicity shows that earthquakes are mainly controlled by the Honghe and Xiaojiang Faults (Xu et al., 2013).Geological studies have demonstrated that the eastward extrusion is redirected southeastward to Yunnan after obstruction by the rigid lithosphere of the Sichuan Basin (Clark and Royden, 2000;Royden et al., 2008), which may lead to stress accumulations and releases (earthquakes) in strike-slip faults such as the Honghe and Xiaojiang Faults that are not accommodated by east-west shortening along the margin of Tibet or western Sichuan and Yunnan (King, 1997).Accordingly, I consider the cause of the earthquakes in the southern part of the NSSZ to be different from those in other regions of the NSSZ.Zhang (2003) also suggested interactions among the Chuandian, Songpan-Ganzi and South China Blocks, resulting in prominent tectonic deformation and earthquakes, such as the Wenchuan earthquake of 2008.The primary source of deformation comes from the eastward extrusion of the Tibetan Plateau blocked by the rigid lithosphere of the Sichuan Basin (e.g., Royden et al., 2008;Burchfiel et al., 2008).

ELIP formation
The cause of ELIP formation is not only important for understanding the dynamic trigger of other large igneous provinces in the world but also is relevant to the current debate surrounding the mantle plume theory (He et al., 2014b;Xu et al., 2007).Recently, the contribution and role of an upwelling mantle plume in the Emeishan flood basalts have been challenged (He et al., 2014b;He and Santosh, 2017a).The dynamic uplift in response to upwelling mantle plumes is very difficult to assess in many igneous provinces (Peate and Bryan, 2008).Silver (2006) proposed that such magmatic activity was induced by stress perturbations, not by upwelling mantle plumes rooted in the coremantle boundary.Elkins-Tanton and Hager (2000) suggested that the preeruptive subsidence of the Siberian Traps flood basalts was associated with lower lithospheric delamination, which induced upwelling asthenosphere flowing into the voids formed by delamination.
Petrological and geological studies have suggested that voluminous continental flood basalts of the ELIP in SW China and northern Vietnam formed from the same upwelling mantle (Xu et al., 2004;Chung and Jahn, 1995).Northern Vietnam was located along the western part of the Honghe Fault in the Early Triassic; it was displaced several hundred kilometers to the southeast along the Ailao-Shan-Honghe Fault in the Oligo-Miocene (Ali et al., 2005).This situation implies that the ELIP was generated after the collision and amalgamation of the Indochina and South China Blocks in the Early Triassic along the Ailao-Shan-Honghe Fault-Song Ma suture.Geological investigations and receiver function studies have suggested large-scale delamination of the crust/lithosphere following the convergence between the Yangtze and North China Cratons and the North Tibetan continental blocks in the Triassic (Zhang et al., 2008;He et al., 2014c).The large-scale delamination of the lower crust/lithospheric https://doi.org/10.5194/se-2019-119Preprint.Discussion started: 10 July 2019 c Author(s) 2019.CC BY 4.0 License.mantle (Hv4) might induce large-scale upwelling asthenosphere (Lv2).At same time, the lower crust/lithospheric mantle (e.g., Hv4) delaminated into the upper mantle transition zone, dehydrated and formed plume-like mantle upwelling there (Lustrino, 2005;He et al., 2014b), which may also contribute to Lv2.Finally, the upwelling asthenosphere (Lv2) led to ELIP formation.New zircon U-Pb studies indicate that the Emeishan magmatism occurred between 257 and 260 Ma and was very short-lived (Shellnutt et al., 2012).An upwelling mantle plume is generally relatively long-lived (Pirajno, 2007).In contrast, delamination and the related upwelling of asthenosphere produce a relatively rapid event (Li S.Z. et al., 2012).
On the other hand, I define a slab-like high-velocity anomaly (Hv5) (Fig. 6), based on previous studies (Mo et al., 2001;Metcalfe, 2013), it might be a vestige of the subduction lithosphere of the Paleo-Tethys Ocean.The subducted slab can induce the return mantle flow and mantle upwelling in the mantle (Santosh et al., 2010;Zhao and Ohtani, 2009;Garfunkel, 1975), which possibly played an important role in the formation of the ELIP.
The large-scale low-velocity (Lv2) anomaly identified by this study just is above Hv5.Therefore, I suggest there is a possibility that the Lv2 might be linked to the large-scale mantle return flow induced by the subducted slab of the Paleo-Tethys Ocean lithosphere.

Figure 3 .
Figure 3. Profiles of P-wave velocity perturbations in the northern part of the NSSZ.

Figure 4 .
Figure 4. Profiles of P-wave velocity perturbations across the Longmenshan Orogenic Belt.

Figure 5 .
Figure 5. Profiles of P-wave velocity perturbations across the Ordos and Sichuan Basins.
of multiple blocks during the Paleozoic to Mesozoic, may be a major deep process in the NSSZ.I consider that Hv3 and Hv4 should represent the delamination of the lower crust/lithosphere due to block collision and amalgamation.This process might contribute https://doi.org/10.5194/se-2019-119Preprint.Discussion started: 10 July 2019 c Author(s) 2019.CC BY 4.0 License. to the upwelling of the asthenosphere (Lv1 and Lv2) to fill voids formed by delamination, such as Hv3 and Hv4.The western and southern parts of the study region are covered by two large low-velocity structures (Lv1 and Lv2) at 50-200 km depths, which show the absence of the rigid lithosphere in these areas.Eastward extrusion is obstructed not only by the lithospheric root of the Sichuan and Ordos Basins but also by the lithospheric root of the Alashan Block, which leads to stress accumulations and releases (earthquakes) in the Longmenshan area and the northern part of the NSSZ.In the southern part of the NSSZ, the eastward extrusion is redirected southeastward along strike-slip faults such as the Honghe and Xiaojiang Faults, which results in stress accumulations and releases (earthquakes) on these faults.This study also indicates that the ELIP was generated by upwelling asthenosphere due to delamination induced by the collision and assemble between the terrane in early Mesozoic and the mantle return flow generated by the subducted slab of Paleo-Tethys Oceanic lithosphere, not by an upwelling mantle plume rooted in the core-mantle boundary.