Thursday, April 13, 2017

Oceanic Crustal Thickness Since The Breakup Of Pangea

Of interest:

Decrease in oceanic crustal thickness since the breakup of Pangaea - Harm J. A. Van Avendonk, Joshua K. Davis, Jennifer L. Harding and Lawrence A. Lawver

Earth’s mantle has cooled by 6–11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15–20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.

Continental insulation refers to the idea that an unbroken continental crust such as that provided by a supercontinent may act as a blanket resulting in a slow build up of heat over tens to hundreds of millions of years in the underlying mantle. Eventual continental breakup will lead to enhanced magmatism and thicker ocean crust along these previously insulated regions.

The Pangaean paleogeography of the Triassic (252 million to 201 million years ago) is depicted in the map below. The distribution of continents is lopsided covering the sites of the future Atlantic and Indian Oceans.

 Source: Paleobiology Navigator

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