Large Igneous Provinces linked to supercontinent assembly

Yu Wang, M. Santosh, Zhaohua Luo & Jinhua Hao

Institute of Earth Sciences, China University of Geosciences, Beijing 100083, China

wangy@cugb.edu.cn ; santosh@cc.kochi-u.ac.jp ; luozh@cugb.edu.cn ; haojh@cugb.edu.cn

This webpage is a summary of: Wang,Yu, M. Santosh, Zhaohua Luo, Jinhua Hao, Large igneous provinces linked to supercontinent assembly, J. Geodynamics, in press, 2014.

Abstract

Models for the disruption of supercontinents have considered mantle plumes as potential triggers for continental extension and the formation of large igneous provinces (LIPs). The formation of the Tarim and Emeishan LIPs on the Eurasian continent during the period 290-250 Ma broadly coincided with the timing of assembly of the supercontinent Pangea, in the absence of plumes rising up from the mantle transition zone or super-plumes from the core-mantle boundary. The formation of these LIPs was accompanied by subduction and convergence of continents and micro-continents, with no obvious relation to major continental rifting or mantle plume activity. On a global scale, the oblique, relatively local closure of the western Paleo-Tethys domain caused subhorizontal eastward asthenosphere flow which interacted with the overall northward flow that had earlier moved western Gondwana fragments north. These two major flows driving the continental blocks generated a major upwelling of the mantle. This created weak zones in the overlying lithosphere and caused the formation of LIPs at different times and in different sectors of the continental margins and interior of Eurasia.

Introduction

For the Emeishan and Tarim LIPs and Traps, many workers have invoked a mantle plume or even super-plume connection (e.g., Chung & Jahn, 1995; Xu et al., 2004; Zhang et al., 2010). However, the plume hypothesis is inconsistent with new geological, geochemical, and paleomagnetic data, as evaluated in this study. Recent investigations from the Eurasian continent (Figure 1A) show that the formation of 290-250 Ma LIPs in Siberia, Emeishan, Tarim and Lhasa may not be related to the breakup of Pangea, but may instead be related to the assembly of this supercontinent.

Figure 1: Large Igneous Provinces linked to supercontinent assembly. A: Global convergence of different plates during the 250 Ma time-interval, with specific reference to the formation of the supercontinent Pangea (based on Stampfli & Borel, 2002). T-Tarim, Y-Yangtze, NC-North China. B: Compilation of paleomagnetic data for the Siberia, Tarim, and Yangtze plates (data compiled from Cocks & Torsvik, 2007; Huang et al., 2008). Motion of the Siberia, Tarim, and Yangtze plates are from south to north and their convergence formed Pangea.

Conflicting hypotheses for LIPs: geological and paleomagnetic evidence

Regional characteristics of the Tarim and Emeishan LIPs and their time scales

The Tarim LIP formed during the period ~290-275 Ma (e.g., Zhang et al., 2010) and lies in the interior and on the northern margins of the Tarim Block. Its tectonic setting is post- or syn-orogenic, suggesting that the LIP formed during the closure of the Central Asian Ocean, after or during the Tianshan orogeny. It is clear, therefore, that from at least 290 Ma to later than 250 Ma, the Tarim Block and surrounding areas were dominated by N-S-directed compression. A rift system formed in the center of the Tarim Block and along its northern margin. The LIP contains basalt as well as some felsic volcanics. Dykes of gabbro formed mainly parallel to the rift zones rather than as giant radiating swarms. The magmas included kimberlite, rhyolite, alkaline-basalt, diabase, and gabbro, and their geochemical features suggest OIB and fore-arc material (Zhang et al., 2010).

The Emeishan LIP lies within the Yangtze Block, and erupted in the narrow time range from 257 Ma to ~ 260 Ma (Shellnutt et al., 2012). During the early stages of the LIP, rift systems formed along the western margin and in the interior of the block. Most of the basaltic eruptions took place within a N-S rift system along its western side. Meanwhile, the western side of the block experienced subduction-related island arc magmatism in the period 280-240 Ma (Jian et al., 2009; Yang et al., 2012) related to E-W directed contraction and strike-slip motion.

At the time of formation of the Emeishan LIP, the northern margin of the Yangtze Block witnessed subduction and collision of the Yangtze Block and North China Craton under conditions of N-S directed compression. At the same time rifting occurred on the southeastern side which is connected to the south China tectonic belt. The Emeishan basaltic rocks contain mega-phenocrysts of feldspar, and rhyolites occur in the bottom and upper layers. Overall, the rocks display OIB characteristics (Xiao et al., 2004) with the signature of recycled oceanic crust (Zhu et al., 2005). Some studies also reported the eruption of picrite, considered to be important evidence for a mantle plume (Zhang et al., 2006). However, more recently it has been argued that these picrites were sourced from the lithospheric mantle (Kamenetsky et al., 2012) or the asthenosphere (Hao et al., 2011). These arguments suggest the rocks are not primary mantle material, contradicting the mantle plume hypothesis argued in earlier studies (e.g., Zhang et al., 2006; Xu et al., 2004).

Convergent features revealed by polar-wandering paths and global tectonic constraints

The Yangtze Block was separated from Gondwana at around 500 Ma and subsequently moved northward (Huang et al., 2008) (Figure 1B). By 290 Ma it was situated at 30-40°N, and at 260 Ma it was located at 24°N (Figure 1B). At 500 Ma the Tarim Block was in the region of the south pole. By 280 Ma it had moved to 40°N (Huang et al., 2008), close to its present position, and collided and converged with the Kazakhstan Plate and the Siberian Plate. The Siberian Plate was located at 30-50°N from 300 Ma to 250 Ma, and there is no clear evidence for any N-S longitudinal movements (Cocks & Torsvik, 2007). All the surrounding plates or micro-blocks moved together from south to north during the period from 300 to 250 Ma (Stampfli & Borel, 2002). At around 230 Ma this collage of blocks reached its present position (Figure 1B).

Global tectonic reconstructions show that the construction of Pangea included the following phases:

• closure of the Proto-Tethys from ~>290 to 280 Ma;
• closure of the western Paleo-Tethys between Laurentia and Gondwana (concluding the formation of the Variscan orogen in western Pangea) at around 280 Ma; and
• the onset of closure of the eastern part of the Paleo-Tethys from 290 to 250 Ma.

By this time the western sector of the Paleo-Tethys was one continental mass but the eastern part contained various continental fragments that were moving northward. After 250-230 Ma the final phase of continental collision and closure of the Tethys and Neo-Tethys continued along the western side of the Yangtze Block.

Conflicts with the mantle plume hypothesis for LIPs

There are several reasons why the Tarim and Emeishan LIPs are probably not related to the postulated pair of mantle plumes:

• There are no radiating dyke swarms and no features typical of mantle head and tail magmatism in the LIPs;
• Previous reports of a 1-km domal uplift before the formation of the Emeishan LIP (He et al., 2003) are based on fold interference patterns associated with at least three phases of deformation in sedimentary layers;
• The basal conglomerate described by He et al. (2003) is in fact a fault breccia and conglomerate observed in the upper part of the Emeishan basaltic system (Wang et al., 2014);
• The kimberlites in the LIPs, interpreted to constitute evidence for a mantle plume (Wei et al., 2014), coexist with rhyolite and other felsic rocks, suggesting that they are related instead to a rift system. These rocks were emplaced over a long interval, from ~290 to 275 Ma;
• So-called “primary magma” in the Emeishan LIP (Zhang et al., 2006) was actually sourced from the lithosphere or the subcontinental lithospheric mantle, rather than the convective asthenosphere or a deep mantle plume (Kamenetsky et al., 2012);
• In the Tarim and Emeishan LIPs, only some observations are compatible with the mantle plume hypothesis such as the sudden large-scale basaltic eruptions over a short time span.

Earlier models (Morgan, 1971; Griffiths & Campbell, 1991) do not explain several features of the 290-250 Ma LIPs: their tectonic setting, the input of oceanic crust material, plate convergence, the construction of a supercontinent, the lack of evidence for any core-mantle boundary materials, and the absence of giant radiating dyke swarms. Another major problem is that during the formation of the Emeishan and Tarim LIPs there was no regional extension either before or during the basaltic eruption. The nature of the magmatism, geological evidence, paleomagnetic data and plate motion reconstructions for the period 290-250 Ma, all show that from the Late Proterozoic to the Early Triassic, the main continents, including the Siberian continent, were converging and in the Triassic the Chinese continent, especially the Yangtze and Tarim blocks and the Siberian continent, united to form a single Eurasian continent with Laurentia. This then combined with Gondwana to form the Pangean supercontinent.

Proposed asthenospheric flow and its process

Subduction-induced mantle flow has recently been suggested, and modeled by Schellart (2008, 2010), to indicate that upper mantle subduction with strong slab rollback motion generates vigorous sub-horizontal toroidal return flow in the sub-lithospheric upper mantle. More recent work shows that such sub-horizontal toroidal flow drives the motion of tectonic plates (in particular the overriding plate) and also backarc extension (Schellart & Moresi, 2013). In addition it has been shown that subduction and slab rollback are associated not only with strong horizontal flow, but also with strong upwelling in a number of locations, in particular in the region surrounding the lateral slab edges and in the distant part of the mantle wedge region, that could be responsible for intraplate magmatism (Schellart, 2010). The horizontal motion and deformation of the frontal part of a slab at the 660-km discontinuity can generate motion and deformation in the surrounding mantle (Strak & Schellart, 2014). Mantle flow can also drive motion in the surrounding lithosphere, as well as marginal strike-slip motion (e.g., Allen et al., 2006). When asthenospheric flow runs counter to that of the lithosphere, rift systems may form on the margins or interior of the plate, resulting in decompression melting of the mantle (Strak & Schellart, 2014). These new findings strongly suggest that the initial formation of mantle flow, such as during the 320-260 Ma time interval, was related to convergence at the western part of the Pangea supercontinent which would have produced asthenospheric flow from west to east.

The chain of evidence suggests that lateral asthenospheric flow would produce lithospheric-scale marginal strike-slip motion and rift systems, and may explain the following features:

Relative motion between the lithosphere and upper asthenosphere results in decoupling of the mantle and upper asthenospheric flow and global westward drift of the lithosphere (Doglioni et al., 2014).

Thus, a four-stage evolution of asthenospheric flow can be proposed (Figure 2) as follows:

• The onset of asthenospheric flow (Figure 2A);
• flow resulting in lithospheric-scale strike-slip motion (Figure 2B);
• local extension and continent or plate tearing and rifting (Figures 2C, D); and
• final convergence of asthenospheric flows moving in different directions, resulting in the formation of huge LIPs (Figure 2E).

Figure 2: Schematic model of the evolution of asthenospheric flow. (A) Initial development of asthenospheric flow. Subduction of an oceanic plate results in lateral mantle flow, parallel to the trench. (B) In response to asthenospheric flow, lithospheric-scale strike-slip faults develop in the marginal lithosphere. (C) In the case that the direction of mantle flow is parallel to the movement direction of a plate or continent the margin of the continent is torn, forming a rift system that opens parallel to the mantle flow direction, as well as local intraplate extension. These structures then channel upwelling asthenospheric magma, resulting in basaltic eruptions. (D) In the case where mantle flow is in the opposite direction to that of the plate motion a subduction system forms along with back-arc extension. (E) The convergence of mantle flow moving in two different directions results in a huge LIP, such as the Tarim and Emeishan LIPs, along the rifted or torn continent or plate.

A new geodynamic hypothesis for formation of the LIPs

The presence of picritic basalts and other geochemical features indicate that the magmas associated with some but not all LIPs were derived in part from the lithospheric or asthenospheric mantle (e.g., Kamenetsky et al., 2012). The materials might have also come from the lithosphere-asthenosphere transitional zone (e.g., Bryan & Ernst, 2008). The Emeishan and Tarim LIP magmas are generally derived from sub-lithospheric sources and recycled oceanic crustal materials in the asthenosphere, and were associated with the closure of Paleo-Tethys and its subduction under the continental margins.

Although the overall movement of Gondwana and its northern fragments was to the north, the scissor-like closure of the western part of Paleo-Tethys supposedly caused an eastward asthenospheric flow that interacted with the northward flow. During the period 290-250 Ma, the western sector of the Paleo-Tethys experienced extensive horizontal flow of asthenospheric materials in a zone ~80-410 km wide (Figure 3) in two major directions–from west to east and from south to north. The confluence of the E-W and N-S directed flows generated asthenospheric upwelling (Figure 3, the bottom panel) instead of normal sinking of subducted material into the mantle transition zone (mantle convection).

Figure 3: Schematic model showing the confluence of asthenospheric flow and upwelling leading to the formation of LIPs. The three stages of evolution shown are: (1) asthenospheric flow and confluence, (2) lines of extensional zones formed by the rapid upwelling of the mantle and interaction with the lithosphere, and (3) the rapid or slow eruption of voluminous volcanic material. The bottom panel shows convergence, the resultant horizontal flow in the asthenosphere, and the confluence of the flows.

The mechanism that controlled the movements of the converging plates is linked to horizontal or sub-horizontal mantle flow in the asthenosphere at a depth of 80-410 km. Horizontally or subhorizontally layered flow of the upper mantle changed to vertical upwelling when the two major flows from different directions intersected, and the upwelling had a significant impact on the lithosphere and its extension. At the same time, or slightly earlier, rifts or normal faults formed along the continental margin or in the plate interiors where LIPs were forming.

Our new hypothesis provides a straightforward explanation for either rapid or slow eruption of different LIPs. In this model of continental convergence driven by asthenospheric flow, different cratons or continental blocks behave differently. The Siberian craton, for example, has a thick lithospheric keel that would have been difficult to break, but once it did fracture or rift, eruptions of sublithospheric and asthenospheric magma would have taken place over a very short time. In contrast, the Tarim Block would have been relatively easily rifted, and as a consequence volcanic rocks would have been erupted over a longer period of time. The Siberian Traps were erupted over little more than 1 Myr, whereas the Tarim LIP was active for more than 15 Myr.

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