Intracratonic indentation of the Archean Slave Province into the Early Proterozoic Thelon Tectonic Zone of the Churchill Province, northwestern Canadian Shield

Abstract

Some time between 1735 and 1840 Ma an intracratonic compression resulted in the indentation and consequent underthrusting of the eastern Thelon Tectonic Zone by the central Slave Province and part of the western Thelon Tectonic Zone. The wedge-shaped indentation involved major, transcurrent displacement on the McDonald and Bathurst fault systems. The affected region is characterized by (i) a prominent negative Bouguer gravity anomaly within the Slave Province between the faults and a smaller positive anomaly over the Thelon Tectonic Zone to the east, (ii) an easterly decreasing regional aeromagnetic field over the indenter, and (iii) an easterly increase in Proterozoic metamorphic grade. Rb–Sr dating of biotite shows a 1735 Ma "age plateau" in the eastern part of the wedge, whereas to the west the ages range between 2.0 and 2.5 Ga. Close coincidence between the margin of the 1735 Ma plateau, the metamorphic isograd pattern, and the negative gravity anomaly contours suggests a probable temporal and formative relationship between the metamorphic gradient, gravity anomaly, and the faults. Following indentation and resultant crustal thickening, isostatic rebound in the younger, weaker Thelon Tectonic Zone took place along older Thelon structures. In the older, stronger, structurally more homogeneous Slave Province, isostatic rebound was incomplete. A consequence of the indentation and rebound was crustal flexure to the east of the uplifted area, which resulted in the formation of the large structural basin, symmetrically disposed with respect to the indenting wedge, in which the originally more extensive Middle Proterozoic Thelon Formation is preserved.