No abstract

Glacimarine processes affect about 20% of the global ocean today, and this area expanded considerably under cyclical full-glacial conditions during the Quaternary (Fig. 1) . Many of the submarine landforms produced at the base and margin of past ice sheets remain well preserved on the seafloor in fjords and on high-latitude continental shelves after the retreat of the ice that produced them. These glacial landforms, protected from subaerial erosion and beneath wave-base and tidal currents in water that is often hundreds of metres deep, are gradually buried by both hemipelagic and glacimarine sedimentation; they may be preserved over long periods in the geological record if palaeocontinental shelves are not reworked by subsequent glacier advances or bottom currents ). This means that, first, submarine glacial landforms can be observed at or close to the modern seafloor after retreat of the last great ice sheets from their most recent Quaternary maximum about 18-20 000 years ago using swath-bathymetric mapping systems and, secondly, buried glacial landforms may also be identified and examined within glacial-sedimentary sequences from Quaternary and earlier ice ages using seismic-reflection methods.The development of multibeam echo sounding over the past two decades, coupled with high-accuracy GPS positioning, has allowed morphological mapping of the seafloor at an unprecedented level of detail. In this paper, the variety of submarine glacial landforms observed in modern, Quaternary and more ancient sediments is described. Landforms produced subglacially, those formed at and beyond marine ice-sheet margins, together with the slope processes and bottom currents likely to result in their reworking, are considered along with the sediment volumes of these landforms and the time they take to develop. This is followed by discussion of the significance of submarine glacial landforms and landform-assemblages for reconstruction of the form and flow of past ice sheets, including the former extent and flow direction of ice sheets, whether they are flowing fast as ice streams or slowly in inter-ice stream areas, the nature and rate of ice-sheet deglaciation, conditions at past ice-sheet beds, and inferences on the characteristics of the basal hydrological system and past climatic conditions.

A thermal model of transform-margin evolution, including both shear heating and lateral conduction of heat from hot oceanic to colder lithosphere, was developed to gain insight into transform-margin crustal structure. Results indicate that over 2 km of crustal uplift may occur at the fault trace for a modelled transform fault 500 km in length with spreading half-rates of 1.0 and 4.0 cm/year. This uplift decreases away from the fault over a distance of 60–80 km. The viscosity of the lower continental crust and upper mantle adjacent to the transform margin is reduced by a factor of more than 100. In response to plate motion and asthenospheric upwelling at the spreading ridge, flow of this thermally weakened material may also play a role in continental crustal thinning.Thermal model predictions are compared with geological observations and crustal structure across transform margins. In particular, we show that the geology of the Southwest Newfoundland Transform Margin, eastern Canada, and the Cape Range fracture zone, Western Australia, supports the model predictions of uplift, erosion, and crustal thinning.