The Pangea problem

A recent study describes the problem plate tectonics theory has with itself in assembling the supercontinent Pangea.  Here are excerpts.

"Although the existence of Pangea is one of the cornerstones of geology, the mechanisms responsible for its amalgamation are poorly understood."

"The formation of Pangea is primarly recorded in the origin and evolution of the Paleozoic oceans (Iapetus, Rheic, Prototethys) between Laurentia (ancestral North America), Baltica (northwestern Europe), and northern Gondwana (South America-West Africa)."  These oceans had to close for the various parts to come together to make Pangea.

"There is a broad consensus that sinking of cold lithosphere (slab pull) provides 90% of the force needed to drive plate tectonics and is indirectly responsible for upwelling beneath mid-ocean ridges."  "However, if these geodynamic arguments are applied to the early Paleozoic, they fail to predict the formation of Pangea in the correct configuration."

Subduction in the wrong direction
"By the early Paleozoic, Laurentia, Baltica, and Gondwana had separated, resulting in the opening of the Iapetus and Rheic Oceans."  "However,... the dispersing continents should have migrated toward the subduction zones in the exterior paleo-Pacific Ocean [Panthalassa], and it is this ocean that should have closed.  Instead, a wealth of geologic evidence indicates that subduction commenced in the relatively newly formed interior (Iapetus and Rheic) oceans located between Laurentia, Baltica, and Gondwana, and that Pangea assembled as the interior ocean closed."

"Subduction in the paleo-Pacific Ocean [Panthalassa] was clearly well established ~80 million years before subduction commenced in the Iapetus Ocean and before the Rheic Ocean opened."  "Slab pull forces associated with this subduction should have resulted in the migration of the dispersing continents toward the paleo-Pacific subduction zones."  "However, this motion reversed itself when subduction commenced in the relatively newly formed Iapetus and Rheic Oceans between Laurentia, Baltica, and Gondwana."  "Therefore, subduction was not only initiated within the new interior oceans, but the rates of subduction of this relatively young Paleozoic lithosphere must also have overpowered those of the already well-established subduction zones within the exterior (paleo-Pacific) [Panthalassa] ocean."

Next problem: initiating subduction
"A key problem that needs to be addressed in resolving this conundrum is the geodynamic scenario that facilitated the initiation of subduction in the Iapetus and Rheic Oceans and generated sufficient slab pull to drive the dispersing continental fragments toward the Pangea configuration.  Subduction initiation is controversial, although it is generally accepted that initiation at passive margins is mechanically difficult because of the flexural rigidity of the oceanic lithosphere.  No simple modern analogues for such a process have been documented."  They speculate that compression across old transform faults started subduction, although the source of compression and why these and not other transform faults subducted is not addressed.

But the new subduction is too weak
Even so, "subduction zones initiated in this manner cannot pull the large overriding plates toward them, and so cannot be considered a mechanism that began the assembly of Pangea."

They are forced to resort to mantle plumes.
"This conundrum suggests that top-down [plate tectonic] geodynamic forces can be overridden by other forces.  Assuming that continents migrate from geoid highs to geoid lows, the reversal of their movement in the middle Paleozoic implies that the geoid lows became geoid highs.  The mechanisms for this are unclear.  However, one possibility is that the descent of subducted slabs in the paleo-Pacific [Panthalassa] Ocean to the core-mantle boundary thermally insulated that boundary, leading to the ponding of hot mantle material that built up and rose toward the surface as a superplume."  "Such an event would result in a geoid high and enhanced seafloor spreading in the paleo-Pacific [Panthalassa] Ocean that could have been responsible for the reversal in the direction of motion of the continents."  "An Ordovician superplume has been proposed... to account for the period's high sea levels, high paleotemperatures, abundant black shales, and strong biological activity."

The idea of mantle plumes, once almost universally accepted, has become highly controversial.  See

Murphy, J. Brendan, R. Damian Nance. September 2008. The Pangea conundrum. Geology, Vol. 36, No. 9, pp. 703-706.

John Michael Fischer, 2008