Basement Structure beneath the Manicouagan Impact Melt Sheet

The rocks beneath the impact melt sheet at Manicouagan predominantly comprise high-grade metamorphic rocks of the Grenville Province that last underwent regional metamorphism at 1 Ga. Minor volumes of Ordovician limestones and shales of the Trenton Group also occur as clasts and as in situ footwall between the melt unit and the underlying Grenvillian basement. The superheated melt sheet has imparted a thermal metamorphic overprint on the subjacent rocks, but this is dependent on the clast content of the impact melt. Where clast contents are high (impact-melt breccias), the thermal overprint is less intense and more limited. Clast-free impact melts appear to have retained their heat longer, and so crystallized larger grains and heated the basement to higher temperatures and for greater distances from the contact. The contact between the melt sheet and basement is not straightforward. In many outcrops, particularly those peripheral to the melt sheet, there is an intervening basal suevite layer that comprises high-temperature, melt-bearing polymict breccias. These rocks have typically undergone flow and may have travelled considerable distances along the crater walls (i.e., they may be considered allochthonous breccias). This is distinct from the less-displaced underlying basement, which can be fractured and brecciated, but which is essentially in place (autochthonous breccias). Shock effects are also manifest in the footwall, and this presentation will discuss the interplay between early shock-generated modifications and later superimposed thermal effects from the overlying melt sheet.