Pangea and the Lower Mantle

Abstract

We show that the peripheral Pangea subduction zone closely followed a polar great circle. We relate it to the band of faster-than-average velocities in lowermost mantle. Both structures have an axis of symmetry in the equatorial plane. Assuming geologically long-term stationarity of the deep mantle structure, we propose to use the axis of symmetry of Pangea to define an absolute reference frame. This reference frame is close to the slab remnants and NNR frames of reference but disagrees with hot spot-based frames. We apply this model to the last 400 Myr. We show that a hemispheric supercontinent appeared as early as 400 Ma. However, at 400 Ma, the axis of symmetry was situated quite far south and progressively migrated within the equatorial plane that it reached at 300 Ma. From 300 to 110-100 Ma, it maintained its position within the equatorial plane. We propose that the stationarity of Pangea within a single hemisphere surrounded by subduction zones led to thermal isolation of the underlying asthenosphere and consequent heating as well as a large accumulation of hot plume material. We discuss some important implications of our analysis concerning the proposition that the succession of supercontinents and dispersed continents is controlled by an alternation from a degree 1 to a degree 2 planform.

Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

“…Two large regions of low S wave speeds, LLSVPs, one beneath the Pacific and the other beneath Africa, dominate the structure of the lower mantle (Figures 2, 3, 5, and 6). As discussed above, during much of Mesozoic time Pangea seems to have overlain the LLSVP beneath Africa (Chase & Sprowl, 1983;Le Pichon et al, 2019;Le Pichon & Huchon, 1983, which suggests that the LLSVPs are long lived features, an inference that Busse (1983) associated with the low order of the convective planform, and as many have subsequently argued (e.g., Ballmer et al, 2017;Deschamps et al, 2011;Gonnermann et al, 2002;Heyn et al, 2018;Trønnes et al, 2019). Dziewonski et al (2010) suggested that these features might be fixed in place by their ensuring that one of the principal components of the moment of inertia tensor lies along the equator.…”

“…Some pointed out that hot spots seemed to preferentially overlie the regions of low speeds and geoid highs, especially after masses of subducting slabs of lithosphere were taken into account (e.g., Anderson, ; Cazenave et al, ; Chase, , Crough & Jurdy, ). Chase and Sprowl () and Le Pichon and Huchon (, ); Le Pichon et al, ] reconstructed continents to their relative positions at ~125 Ma (though with little difference among reconstructions between 100 and 200 Ma); to determine past longitudes, they either assumed hot spots to be approximately fixed or they kept Africa essentially in its past position. In such reconstructions, Pangea occupied one hemisphere centered on one of the regions of large geoid highs in the Degree‐2 pattern.…”

“…In such reconstructions, Pangea occupied one hemisphere centered on one of the regions of large geoid highs in the Degree‐2 pattern. Chase and Sprowl () and Le Pichon and Huchon (, ); Le Pichon et al, ] suggested that flow in the lower mantle had communicated with the plates above and had held Pangea in place for more than 100 Myr. These authors inferred that flow in the lower mantle was “weakly coupled” to that in the upper mantle, with lateral temperature gradients in the lower mantle requiring convergence and downwelling near the margins of Pangea, where subduction zones surrounded the supercontinent.…”

“…A problem posed by the long lifetime of the LLSVP blobs or piles is how do they affect surface geology, such as the relationship of the LLSVP beneath Africa to Pangea discussed by Chase and Sprowl (1983) and Le Pichon & Huchon, 1983Le Pichon et al, 2019]. In a view that differs slightly from that of Le Pichon et al (2019), I imagine that energetic plumes offer "blow torches" that help break continental plates apart, and this accounts for the breakup of Pangea. Then, how did Pangea find itself over the African LLSVP and protected for hundreds of million years?…”

Ice, Fire, or Fizzle: The Climate Footprint of Earth's Supercontinental Cycles

“…Figure 5 and Lenardic et al, 2011;Robin et al, 2007;Thayalan et al, 2006). Plate reconstructions and geological data suggest that subduction zones probably encircled Rodinia and to a lesser extent Pangea by the time these supercontinents were each formed (e.g., East et al, 2019;Evans, 2009;Lee et al, 2013;Le Pichon et al, 2019;Li et al, 2013;Müller et al, 2016). Depending on the continuity, geometry, and age of subducting slabs, such a planform can have varied consequences for the character and extent of mantle thermal mixing.…”