Gravity, flexure, and the deep structure of the Grenville Front, eastern Quebec and Labrador

Abstract

The Grenville Front in eastern Quebec and southern Labrador is characterized by a large negative Bouguer gravity anomaly with flanking highs. Forward flexural modelling of this anomaly provides constraints on the deep structure of the eastern Grenville. The Grenville Front is assumed to mark the surface trace of a boundary between less dense crust to the northwest and more dense crust to the southeast, with constant dip all the way to the Moho. An end load is applied to the northwestern plate to simulate the effect of orogenic and (or) subsurface loads, but is then removed underneath a continuous plate to simulate the transient nature of the load. This simple model yields good fits to the gravity anomaly, provided the effective elastic thickness of the lower plate is less than about 30 km and the crustal boundary dips at about 20°. The deep structure of the eastern Grenville Front may therefore be dominated by a major shallow-dipping straight thrust. The applied end loads for the best-fitting models are only a few tens of kilometres southeast of the Grenville Front. This may reflect the former presence of allochthonous crust at and near the present Grenville Front in the eastern Grenville, or delamination of the lithospheric mantle of the northern plate from its overlying mantle, which could also be responsible for the low modelled elastic thicknesses. The absence of a similarly pronounced gravity anomaly in the western Grenville may reflect a lower orogenic load, as evidenced by the absence of a fold-and-thrust belt there, and a steeper dip for the crustal boundary.