Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmdi51a..02t&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #DI51A-02
Physics
6982 Tomography And Imaging (7270, 8180), 7208 Mantle (1212, 1213, 8124), 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8137 Hotspots, Large Igneous Provinces, And Flood Basalt Volcanism, 8157 Plate Motions: Past (3040)
Scientific paper
The formation and break-up of supercontinents is a spectacular demonstration of the Earth's dynamic nature. Pangea, the best-documented supercontinent, formed at the end of the Palaeozoic era (320 Ma) and its dispersal, starting in the Early Jurassic (190 Ma), was preceded by and associated with widespread volcanic activity, much of which produced Large Igneous Provinces (LIPs), but whether any of the heat or material involved in the generation of LIP rocks comes from greater depths has remained controversial. Two antipodal Large Low Shear wave Velocity Provinces with centre of mass somewhat south of the equator (African and Pacific LLSVPs), isolated within the faster parts of the deep mantle dominate all global shear- wave tomography models. We have tested eight global models and two D" models: They all show that deep- plume sourced hotspots and most reconstructed LIPs for the last 300 million years project radially downwards to the core-mantle-boundary near the edges of the LLSVPs showing that the plumes that made those hotspots and LIPS came only from those plume generation zones. This is a robust result because it is observed in multiple reference frames, i.e. fixed/moving hotspot and palaeomagnetic frames, and in the latter case whether the effect of True Polar Wander (TPW) is considered or not. Our observations show that the LLSVPs must have remained essentially stable in their present position for the last 300 million years. LIPs have erupted since the Archean and may all have been derived from the margins of LLSVPs but whether the African and Pacific LLSVPs have remained the same throughout Earth's history is less certain although analogous structures on Mars do indicate long-term stability on that planet. Deep mantle heterogeneities and the geoid have remained very stable for the last 300 million years, and the possibility is therefore open for speculating on links to Pangea assembly. In a numerical model, Zhong et al. (2007, EPSL) argued that Pangea assembled above a major down-welling, and calculated that, following the assembly, a sub-Pangea upwelling developed relatively fast (within~50 Myr) as mantle return flow in response to circum-Pangea subduction. Collision of Gondwana and Laurussia took place during the destruction of the Rheic Ocean and parts of the Palaeotethys and bulk Pangea assembled at ca. 320 Ma. However, most of the Rheic Ocean had gone much earlier (ca. 370 Ma) and it may therefore be more appropriate in terms of mantle modelling to place "supercontinent formation" in Devonian times. In that case a large-scale thermal upwelling under Africa, and the presumed chemically distinct African LLSVP beneath it, would have existed as early as 320 Ma, so that a plume head from its edge could have impinged upon the lithosphere at the time determined for the oldest LIP we have reconstructed. In such a model, the African LLSVP should not have existed before Devonian times, because convection would have been dominated by a degree-1 mode with only one upwelling, presumably above the Pacific LLSVP. The situation becomes more obscure further back in time; TPW may have been larger during degree-1 convection, so that reconstructions in the palaeomagnetic frame are not necessarily in relation to the deep mantle. There may again have been two LLSVPs during dispersal of the previous supercontinent (Rodinia), in which case a second LLSVP may again have been approximately antipodal to the one beneath the Pacific.
Burke Korana
Smethurst Mark A.
Steinberger* Bernhard
Torsvik Trond Helge
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