EARTHS GLACIAL RECORD AND ITS TECTONIC SETTING

Glaciations have occurred episodically at different time intervals and for different durations in Earth's history. Ice covers have formed in a wide range of plate tectonic and structural settings but the bulk o f Earth's glacial record can be shown to have been deposited and prese rved in basins within extensional settings. In such basins, source are a uplift and basin subsidence fulfill the tectonic preconditions for t he initiation of glaciation and the accomodation and preservation of g laciclastic sediments. Tectonic setting, in particular subsidence rate s, also dictates the type of glaciclastic facies and facies succession s that are deposited. Many pre-Pleistocene glaciated basins commonly c ontain well-defined tectonostratigraphic successions recording the int erplay of tectonics and sedimentation; traditional climatostratigraphi c approaches involving interpretation in terms of either ice advance/r etreat cycles or glacio-eustatic sea-level change require revision. Th e direct record of continental glaciation in Earth history, in the for m of classically-recognised continental glacial landforms and ''tillit es'', is meagre; it is probable that more than 95% of the volume of pr eserved ''glacial'' strata are glacially-influenced marine deposits th at record delivery of large amounts of glaciclastic sediment to offsho re basins. This flux has been partially or completely reworked by ''no rmal'' sedimentary processes such that the record of glaciation and cl imate change is recorded in marine successions and is difficult to dec ipher. The dominant ''glacial'' facies in the rock record are subaqueo us debris flow diamictites and turbidites recording the selective pres ervation of poorly-sorted glaciclastic sediment deposited in deep wate r basins by sediment gravity flows. However, these facies are also typ ical of many non-glacial settings, especially volcanically-influenced environments; numerous Archean and. Proterozoic diamictites, described in the older literature as tillites, have no clearly established glac ial parentage. The same remarks apply to many successions of laminated and thin-bedded facies interpreted as ''varvites''. Despite suggestio ns of much lower values of solar luminosity (the weak young sun hypoth esis), the stratigraphic record Of Archean glaciations is not extensiv e and may be the result of non-preservation. However, the effects of v ery different Archean global tectonic regimes and much higher geotherm al heat flows, combined with a Venus-like atmosphere warmed by elevate d levels of CO2, cannot be ruled out. The oldest unambiguous glacial s uccession in Earth history appears to be the Early Proterozoic Gowgand a Formation of the Huronian Supergroup in Ontario; the age of this eve nt is not well-constrained but glaciation coincided with regional rift ing, and may be causally related to, oxygenation of Earth's atmosphere just after 2300 Ma. New evidence that oxygenation is tectonically, no t biologically driven, stresses the intimate relationship between plat e tectonics, evolution of the atmosphere and glaciation. Global geoche mical controls, such as elevated atmospheric CO2 levels, may be respon sible for a long mid-Proterozoic non-glacial interval after 2000 Ma th at was terminated by the Late Proterozoic glaciations just after 800 M a. A persistent theme in both Late Proterozoic and Phanerozoic glaciat ions is the adiabatic effect of tectonic uplift, either along collisio nal margins or as a result of passive margin uplifts in areas of exten ded crust, as the trigger for glaciation; the process is reinforced by global geochemical feedback, principally the drawdown of atmospheric CO2 and Milankovitch ''astronomical'' forcing but these are unlikely, by themselves, to inititiate glaciation. The same remarks apply to lat e Cenozoic glaciations. Late Proterozoic glacially-influenced strata o ccur on all seven continents and fall into two tectonostratigraphic ty pes. In the first category are thick sucessions of turbidites and mass flows deposited along active, compressional plate margins recording a protracted and complex phase of supercontinent assembly between 800 a nd 550 Ma. Local cordilleran glaciations of volcanic peaks is indicate d. Many deposits are preserved within mobile belts that record the sub duction of interior oceans now preserved as ''welds'' between differen t cratons. Discrimination between glacially-influenced and non-glacial , volcaniclastic mass flow successions continues to be problematic. Th e second tectonostratigraphic category of Late Proterozoic glacial str ata includes successions of glacially-influenced, mostly marine strata deposited along rifted, extensional plate margins. The oldest (Sturti an) glaciclastic sediments result from the break-out of Laurentia from the Late Proterozoic supercontinent starting around 750 Ma along its ''palaeo-Pacific'' margin with a later (Marinoan) phase of rifting at about 650 Ma. ''Passive margin'' uplifts and the generation of ''adiab atic'' ice covers on uplifted crustal blocks triggered widespread glac iation along the ''palaeo-Pacific'' margin of North America and in Aus tralia. A major phase of rifting along the opposite (''palaeo-Atlantic '') margin of Laurentia occurred after 650 Ma and is similarly recorde d by glaciclastic strata in basins preserved around the margins of the present day North Atlantic Ocean. Glaciation of the west African plat form after 650 Ma is closely related to collision of the West African and Guyanan cratons and uplift of the orogenic belt; the same process, involving uplift around the northern and western margins of the Afro- Arabian platform subsequently triggered Late Ordovician glaciation at about 440 Ma when the south polar region lay over North Africa. Early Silurian glaciation in Bolivia and Brazil was followed by a non-glacia l episode and renewed Late Devonian glaciation of northern Brazil and Bolivia. The latter event may have resulted from rotation of Gondwana under the South Pole combined with active orogenesis along the western margin of the supercontinent. Hercynian uplift along the western marg in of South America caused by the collision and docking of ''Chilinia' ' at about 350 Ma (Late Tournasian-Early Visean) was the starting poin t of a long Late Palaeozoic glacial record that terminated at about 25 5 Ma (Kungurian-Kazanian) in western Australia. The arrival of large l andmasses at high latitude may have been an important precondition for ice growth. Strong Namurian uplift around virtually the ent