THE GRAVITY-FIELD OF EASTERN HUDSON-BAY - EVIDENCE FOR A FLEXTURAL ORIGIN FOR THE HUDSON-BAY (NASTAPOKA) ARC

The Nastapoka Arc, on the east coast of Hudson Bay, is almost perfectly circular and closely follows the unconformable contact between Archean Superior province gneisses and shallowly seaward dipping early Proterozoic Nastapoka Group supracrustals. Its form may be explained by flexing of the lithosphere due to loading by thrust sheets from the Trans-Hudson orogen to the west. Such flexing produces a peripheral bulge east of the coast which should be reflected in a positive gravity anomaly there. Gravity data averaged around the arc show such a positive anomaly, although its precise amplitude is uncertain because of local variation in the gravity field. Lithospheric flexure of the arc due to loading can be modeled by superposition of disc-shaped loads centered on the arc. For a lithospheric Young's modulus of 1.0 x 10(11) N m-2 and Poisson's ratio 0.25, the present geometry of the arc can be reproduced only by reducing the elastic thickness to 20 +/- 2 km. The peripheral bulge predicted for the model yields a gravity anomaly very similar to that observed if it is fully imaged at the Moho. The absence of a regional negative gravity anomaly over the arc, despite a great thickness of transported rocks present at its center, requires that the crust in the region was unusually thin before thrust emplacement. The unusually low elastic thickness required for the model may reflect highly fractured crust. Both features are consistent with the former presence of a rifted continental margin in the region, as predicted for many models of the Trans-Hudson orogen.