Supercontinent cycles

Phanerozoic history of the earth can be related to the assembly and breakup of the Pangea supercontinent. Most recent workers have adopted this long-term platetectonic cycle as the basis for hypotheses of the earth’s dynamics. The formation of a supercontinent creates a thermal blanket that inhibits convective release of radiogenic heat from the mantle. Changes in the rotation of the Earth’s core and in the convective patterns in the mantle may be either the cause or the consequence of these surface events, which also appear to be linked to changes in the earth’s magnetic field. Development of the thermal blanket may be the cause of the eventual breakup of the supercontinent, following establishment of a new pattern of mantle convection. The formation of the thermal blanket beneath a super continent leads to heating and regional epeirogenic uplift on a continental scale. The uplift rate would be 5–10 m/million years, and could persist for 100 million years, resulting in an uplift of 500 m to 1 km. It has been argued that dynamic mantle uplift is extremely long-lived. It generates a positive geoid anomaly that survives for 108 years. There is existence of a large positive anomaly beneath Africa. The position of this anomaly beneath a reconstruction of the Pangea supercontinent. The correspondence is remarkably close, and confirms that Africa was at the centre of Pangea. Africa has undergone anomalous uplift since the early Tertiary. This is too late to have been caused directly by the heating effect, which would have taken place in the late Paleozoic or early Mesozoic following continental assembly. However, it is possible that the uplift relates to intraplate compressive stress generated by the opening of oceans virtually all around the continent. Dispersing continental fragments tend to migrate toward geoid lows, where mantle temperatures are lower, and where relative sea-levels will rise, leading to extensive platform flooding. The total length of rifting continental margins and of seafloor spreading centres increases during the breakup of a supercontinent and may be accompanied by increased rates of oceanic crust generation, and active subduction, plutonism, and arc volcanism on the outer, convergent plate margins of the dispersing fragments, as suggested by the westward drift of the Americas since the Triassic, and the consequent subduction of tens of thousands of kilometres of paleo-Pacific (Panthalassa) oceanic crust. Spreading rates are episodic, reflecting the structure and behaviour of the mantle convection cells that drive them. Major eustatic transgressions occur because of the displacement of ocean waters by thermally elevated young oceanic crust and active spreading centers in the new Atlantic-type oceans. During the first 50–100 million years after initiation of breakup of a supercontinent, the global average age of oceanic crust decreases because of the active development of Atlantic-type oceans. This will lead to a rise in sealevel, without any change in global average spreading rate. A simple isostatic model to quantify the effects of changing age of the earth’s oceanic crust, simplifying the earth to a two-ocean system, with an opening Atlantic-type ocean replacing a Pacific-type ocean undergoing consumption. This effect alone can account for a change in sealevel of∼100 m over about 120 million years. It has been suggested that the average rate of spreading slows at times of continental assembly, at a time of ridge reordering following major continental collision and suturing events. Collision results in crustal shortening and thickening, which has the effect of increasing the ocean-basin volume. Therefore, at the end of a supercontinent assembly cycle, large areas of old, and therefore cool, and subsided oceanic crust will underlie the world’s oceans. All these effects lead to enlargement of the world’s ocean basins. Times of low sea-level might therefore be expected to correlate with, or follow, major suturing episodes. The effects of sea-level change on climates, sedimentation and biogenesis. Although eustatic sea level is predicted to fall during continental assembly, a synchronous rise in relative sea-level in intracratonic basins and continental margins throughout North America during the middle Paleozoic (Late Devonian-mid Mississippian), at which time it is postulated that the late Proterozoic supercontinent was dispersing, and Pangea was assembling the regional rise in sea level to synchronous enhanced subsidence, and argued that this could not have been caused directly by plate-margin processes. Many of the basins are beyond the flexural reach of the continental margin orogenies that were underway at the time, and some of the data were derived from areas undergoing continental extension where no thermal event has been documented that could explain the timing or rate of subsidence. The synchronous nature of the subsidence calls for a continental-scale process, and the cause was intraplate compressive stress resulting from the movement of the North American plate over a region of mantle down welling during supercontinent dispersion. 

Subsidence over down welling mantle is an expression of “dynamic topography”. It may be responsible for regional changes in relative sea level over time scales of tens of thousands to tens of millions of years. In the case of the North American sea-level rise in the Middle Paleozoic, subsidence would have been enhanced by the increase in crustal density overlying a cool downwelling current (the dynamic topography effect). It is not yet clear how much this effect contributed to the overall relative sea-level rise. There is an ambiguity in attributing causes to long-term sea-level changes. Mantle convection leads to generation of dynamic topographies, which are reflected in the stratigraphic record by their continental-scale effects on relative sealevels. However, the same processes lead to changes in the global average rate of sea-floor spreading, which affect the volume of the ocean basins, and thereby generate eustatic sea-level changes. During times of supercontinent fragmentation, in areas of mantle downwelling, these two processes will be in phase, and therefore additive, which makes it difficult to separate and quantify their effects.
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