Correspondence: Numerical modelling of the PERM anomaly and the Emeishan large igneous province

Abstract

Large igneous provinces (LIPs) are the result of catastrophic melting in the upper mantle, and by reconstructing their positions over the past 300 Myr it has been shown that most LIPs—including the 260 Myrs old Emeishan LIP in South China—probably originated from plumes at the edges of two large low-velocity regions in the lowermost mantle. In a recent article published in Nature Communications, Flament et al.1 presented a remarkable new view on the origin of the Emeishan LIP (based on numerical modelling) and we discuss here why we do not agree with their interpretation. The fundamental observation that most LIPs—when reconstructed—overlie the edges of two low-velocity regions near the core-mantle boundary (Fig. 1a) was first reported by Burke & Torsvik2. Two equatorial and antipodal regions argued to be the most probable sources of the mantle plumes that generated the LIPs were dubbed large low shear-wave velocity provinces (LLSVPs) by Garnero et al.3 and later TUZO (beneath Africa) and JASON (beneath the Pacific) by Burke4. Burke and co-authors argue that plumes mainly form at the margins of these LLSVPs, which have remained approximately stable through time. It was also noted, more than 10 years ago5 that the Siberian Traps (~252 Ma) when reconstructed overlie a smaller anomaly in the lower mantle which appears as a north-eastern arm of TUZO in many tomographic models (Fig. 1a), but is argued to represent a separate anomaly by Lekic et al.6 This anomaly was dubbed PERM and appears more isolated when using seismic voting-maps; as an example, we show voting-map contour 46 in Fig. 1b, i.e. four seismic models show slower than average velocities in the lower mantle (1000–2800 km) beneath these regions. Open in a separate window Fig. 1 Reconstruction of large igneous provinces. a Reconstruction of 26 large igneous provinces (LIPs, 31–297 Ma) using a hybrid reference frame9 and draped on the s10 mean tomographic model10. The plume generation zone (PGZ, thick red line) in this model corresponds to the 0.9% slow contour and the zero contours are shown as thinner black lines. LIPs with red symbols reconstructed with moving and fixed hotspot reference frames, while those with green symbols use a true polar wander-corrected palaeomagnetic reference frame. But the reconstruction of the 260 Ma Emeishan LIP (ELIP) is an exception: Although Pangea was amalgamated at 260 Ma (b), the supercontinent did not include South China, which is therefore without longitudinal constraints. The ELIP is on the South China block, and palaeomagnetic results position it at latitudes around 4° S (c); if ELIP had erupted above a PGZ, there are several possible longitudinal locations where the line of latitude crossed the PGZ at that time. Pangea covered TUZO (b), leaving only the options related to JASON, and the reconstruction with ELIP above the western margin of Jason at ~134° E, is a realistic alternative. One should also note that net true polar wander was zero between 250 and 260 Ma11. b Pangea reconstruction at 260 Ma7, 12 with plate boundaries and draped on seismic voting-map contour 4 in the lower mantle6. Here we only show the reconstructed location of the Siberian Traps (SIB, erupting ~8 million years after the reconstruction) and the ELIP that erupted at the equator and linked to the margin of JASON. The red star is where Flament et al.1 initiate the PERM anomaly at 190 Ma (linked to the much older ELIP!) and later shifted westwards to its current location. A Annamia (Indo-China), M-O Mongol-Okhotsk Ocean, NC North China. c Palaeomagnetically derived palaeolatitudes from the Siberian Traps (10 studies) and Emeishan volcanics (ELIP, 8 studies) with 95% confidence bars which form the basis for reconstructing South China at that time11, 12. d Detailed 260 Ma reconstruction of South China with plate boundaries7 and draped with Guadalupian (272–260 Ma) and Lopingian (260–252 Ma) coal/swamp occurrences13 that verify tropical (equatorial) humid conditions during the eruption of ELIP