OROGENY, BUOYANCY TECTONICS AND PRECAMBRIAN CLIMATES

Carbon dioxide drawdown via mountain-building episodes provides an increasingly well-accepted trigger for post-Mesoproterozoic glacial intervals that span millions of years. However, the linkage between orogenesis and glaciation in the earlier Precambrian is inconsistent. Collisional tectonics and glaciation at ~2.7 Ga is consistent with orogeny-driven cooling, but the ~2.3 Ga Huronian glaciation occurs during an anorogenic interval, and the ~1.7-1.9 Ga orogenic interval lacks evidence for glaciation.

At ~80% of present solar luminosity collision of small, thick, highly buoyant Archean shield areas embedded in a buoyant oceanic lithosphere may have triggered the ~2.7 Ga glaciation. However, Paleo- and Mesoproterozoic ductile collisions between Archean shields cushioned by thinner, less buoyant Proterozoic greenstone belts and still-thick (>20 km), buoyant ocean crust were not sufficient to orogenically induce glaciation. Alternatively, a thermally domed “plateau” supercontinent at ~90% of present solar luminosity may have exposed sufficient silicate rock to stimulate the 2.3 Ga glaciation. The demise of BIFs at ~2 Ga led to iron-limited productivity in the sulfidic “Canfield” ocean that may have prevented sufficient organic carbon burial to initiate glaciation on the mostly drowned continents.

A change in the nature of ophilites indicates thinning of ocean crust and increasing density of the oceanic lithosphere that initiated slab pull at ~1 Ga. Deepened oceans could have increased global land area from <10% to >20%. Increased land area alone could potentially halve atmospheric greenhouse gas and lower average global temperature by at least 10°C. The onset of more brittle continent-continent collisions resulting from a much less buyoant oceanic lithosphere likely increased the climatic potency of orogenies. The consequent explosion in productivity and organic C burial furthered the intensity of Neoproterozoic glaciation.