POLYPHASE GLACIAL DYNAMICS IN THE EASTERN LAURENTIDE ICE SHEET - THE ROLE OF ICE STREAMS AND DISPERSAL CENTERS

Crossing patterns of glacial striae and other ice-flow indicators, as well as dispersal patterns in glacial sediments, in central and northern Québec show that the main divide of the eastern dome of the Laurentide Ice Sheet (LIS) underwent a major re-orientation during the last glaciation: at the last glacial maximum (LGM), the divide had rotated to a north-south trend from the NE-SW trend which characterized the penultimate glacial maximum (PGM). This body of ice-flow data also suggests that early glacial inception took place in northeastern Ungava and that the ensuing Ungava dome was fully developed well before its coalescence with the more southerly Mistassini dome. While flow re-orientations in the core region of the eastern LIS dome commonly reached 90°, they were generally less in the inner peripheral regions. In the outer peripheral regions of the eastern LIS, sudden and large flow pattern changes were produced by ice streams which developed episodically in the major troughs (Hudson Strait and St. Lawrence trough) leading to ice shelves bordering the North Atlantic. Limited ice-flow data also show evidence of ice stream activity prior to the LGM. Reconstructed Wisconsinan flow patterns in the eastern LIS dome show that two series of processes controlled regional ice dynamics: (1) changes in accumulation patterns in the core regions which were presumably driven by changes in cyclonic trajectories and (2) ice-stream activity at marine and glaciolacustrine margins.