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Surface-Wave Tomography of the Emeishan Large https://doi .org/10.1130/G49055.1 Manuscript received 21 December 2020 Revised manuscript received 15 March 2021 Manuscript accepted 15 March 2021 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 17 May 2021 Surface-wave tomography of the Emeishan large igneous province (China): Magma storage system, hidden hotspot track, and its impact on the Capitanian mass extinction Yiduo Liu1*, Lun Li2,3*†, Jolante van Wijk4§, Aibing Li1 and Yuanyuan V. Fu5 1 Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA 2 Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, China 3 Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, Guangdong 510999, China 4 Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA 5 Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting, China Earthquake Administration, Beijing 100036, China ABSTRACT candidate for having caused the global biota cri- Large igneous provinces (LIPs) are commonly associated with mass extinctions. However, sis in the mid-Capitanian (ca. 262 Ma) (Wignall the precise relations between LIPs and their impacts on biodiversity is enigmatic, given that et al., 2009; Bond et al., 2020), which preceded they can be asynchronous. It has been proposed that the environmental impacts are primar- the major volcanic activity of the ELIP (260– ily related to sill emplacement. Therefore, the structure of LIPs’ magma storage system is 257 Ma) (Shellnutt et al., 2012). We attempt to critical because it dictates the occurrence and timing of mass extinction. We use surface-wave characterize the ELIP’s magma storage system tomography to image the lithosphere under the Permian Emeishan large igneous province and explain the unusual asynchrony. (ELIP) in southwestern China. We find a northeast-trending zone of high shear-wave veloc- ity (Vs) and negative radial anisotropy (Vsv > Vsh; v and h are vertically and horizontally GEOLOGICAL BACKGROUND polarized S waves, respectively) in the crust and lithosphere. We rule out the possibilities of The ELIP is located in the western Yangtze rifting or orogenesis to explain these seismic characteristics and interpret the seismic anomaly craton of the South China block (Figs. 1A and as a mafic-ultramafic, dike-dominated magma storage system of the ELIP. We further pro- 1B). It comprises mainly tholeiitic continental pose that the anomaly represents a hidden hotspot track that was emplaced before the ELIP flood basalts with a lesser amount of picrites, eruption. A zone of higher velocity but less-negative radial anisotropy, on the hotspot track lamproites, gabbros, pyroxenites, and other but to the northeast of the eruption center in the Panxi region, reflects an elevated propor- lithologies (e.g., Chung and Jahn, 1995; Xu tion of sills emplaced at the incipient stage of the ELIP. Liberation of poisonous gases by the et al., 2004). These rocks were mainly derived early sill intrusions explains why the mid-Capitanian global biota crisis preceded the peak from the sublithospheric mantle (Chung and ELIP eruption by 2–3 m.y. Jahn, 1995; Xu et al., 2004; Zhou et al., 2005). The southwestern corner of the ELIP is offset INTRODUCTION the major phase, leaving the exact relationship left-laterally by the Cenozoic Ailao Shan–Red Large igneous provinces (LIPs) are charac- between LIPs and mass extinctions enigmatic. River shear zone and is poorly exposed. The terized by rapid emplacement of primarily mafic It has been proposed that the structure of larger, better-preserved northeastern part, cover- magma in the lithosphere and volcanic eruptions magma storage systems of LIPs, particularly sill ing at least ∼3 × 105 km2, is divided into three forming plateau basalts in an area >105 km2 intrusions, is a key to understanding this relation concentric zones (inner, intermediate, and outer) (Bryan and Ernst, 2008). Many LIPs have been (Svensen et al., 2009). Modern hotspots, such that represent a decrease in flood basalt thick- associated with abrupt environmental catastro- as Yellowstone (northwestern United States), ness outward (He et al., 2003; Xu et al., 2004). phes and mass-extinction events (Sobolev et al., display the magma storage system of active LIPs Before the ELIP eruption, the Maokou Forma- 2011; Ernst, 2014). While LIP-related extinc- (Jiang et al., 2018). However, they do not pro- tion that was deposited on the Yangtze carbonate tions generally occurred during or shortly after vide a direct constraint on the timing of mass platform underwent denudation and karstifica- the major volcanic phase of the corresponding extinction. Ancient LIPs, by contrast, potentially tion (He et al., 2003; Xiao et al., 2016). The LIPs (Bond and Wignall, 2014), some preceded preserve a complete record and thus offer an denudation began in the northeastern Sichuan opportunity for studying their structures and Basin and propagated southwestward in the mid- *These authors contributed equally to this work. assessing the environmental impacts. to late Guadalupian (Hu et al., 2012). A map of †E-mail: [email protected] We studied the seismic structure of the the remnant thickness of the Maokou Formation §Current address: Los Alamos National Labora- Emeishan large igneous province (ELIP) in shows a northeast-trending zone of anomalously tory, Earth and Environmental Sciences Division, Los southwestern China using a new surface-wave thin strata extending from the Sichuan Basin to Alamos, New Mexico 87545, USA tomography model. The ELIP is, so far, the only the ELIP center (Fig. 1C). CITATION: Liu, Y., et al., 2021, Surface-wave tomography of the Emeishan large igneous province (China): Magma storage system, hidden hotspot track, and its impact on the Capitanian mass extinction: Geology, v. 49, p. 1032–1037, https://doi.org/10.1130/G49055.1 1032 www.gsapubs.org | Volume 49 | Number 9 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/9/1032/5387131/g49055.1.pdf by guest on 30 September 2021 AB C Figure 1. Geologic map of the Emeishan large igneous province (ELIP) region in southwestern China. (A) Index map. IC—Indochina; IN—India plate; NC—North China block; SC—South China block; TP—Tibetan Plateau. (B) Map of the ELIP, showing eruption center, uplift center, flood basalts exposed on the surface and drilled in wells, buried volcanic craters, basaltic dikes, and borders of the inner (I), intermediate (II), and outer (III) zones. ASRRSZ—Ailao Shan–Red River shear zone; LMST—Longmen Shan thrust belt; XXF—Xianshuihe-Xiaojiang fault. (C) Map of remnant thickness of the Maokou Formation that was deposited on the Yangtze carbonate platform before the ELIP. 160 and 250 m contours are dashed. See Table S1 for data sources (see footnote 1). Paleomagnetic studies show an equatorial ing hexagonal anisotropy with a vertical sym- low shear-wave velocities. We explain this as a paleolatitude for the ELIP at the peak eruption metry axis (Babuska and Cara, 1991). Radial result of subhorizontal ductile deformation in time and an overall clockwise rotation of ∼27° anisotropy may be induced by lattice preferred the middle and lower crust due to the lateral of the South China block since 260 Ma (Huang orientation of minerals such as mica, olivine, expansion of the Tibetan Plateau. In the litho- et al., 2018). Magmatic underplating occurred at and pyroxene (Silver and Chan, 1991; Shapiro spheric mantle below 80 km, positive radial the Moho depth in the inner zone of the ELIP, as et al., 2004) or by shape preferred orientation anisotropy exists ubiquitously. Similar positive evidenced by high density, high seismic veloc- of structures such as fractures, foliation, and radial anisotropy is present in other cratons (Net- ity, high ratios of compressional to shear-wave magma (Silver and Chan, 1988; Emmermann tles and Dziewoński, 2008). This largely results velocity (Vp/Vs), and thickened crust (Chen and Lauterjung, 1997). from the lattice preferred orientation of olivine et al., 2015). A pronounced northeast-trending Our shear-wave velocity model (Figs. 2 and in horizontal planes. positive residual gravity anomaly extends from 3; Figs. S12–S14 in the Supplemental Mate- The most intriguing phenomenon is a north- the ELIP center to the Sichuan Basin and has rial) shows high velocity at 15–220 km depths east-trending coherent zone of negative radial been attributed to ELIP intrusions (Deng et al., beneath the Yangtze craton east of the Xians- anisotropy (Vsv > Vsh) paired with high shear- 2014). huihe-Xiaojiang fault and a low-velocity zone wave velocity under the inner and intermediate in the crust of the southeastern Tibetan Plateau. zones of the ELIP (Figs. 2 and 3). It appears as METHOD AND RESULTS These results agree with those of previous stud- a broad zone in the shallow crust (0–15 km) and We developed a radially anisotropic shear- ies: high-velocity anomaly dominates the Yang- can be traced down to ∼80 km depth. Such a wave velocity model of the lithosphere under tze craton due to the cold and thick cratonic zone of reduced radial anisotropy is still detect- the ELIP region from Rayleigh wave and Love lithosphere, and the low-velocity zone is related able down to 120 km depth (Fig. 2; Fig. S15). wave phase velocity at periods of 8–167 s (see to the thickened, partially molten Tibetan crust The amplitude of the negative radial anisotropy the Supplemental Material1). Rayleigh and (Huang et al., 2010). is mostly ∼−2%, but is as strong as −4% in the Love waves are primarily sensitive to vertically Radial anisotropy results show lateral and lower crust under the Sichuan Basin. A similar (Vsv) and horizontally (Vsh) polarized shear- vertical variations in the crust and mantle body of negative radial anisotropy was imaged, wave velocities, respectively.
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