HIMALAYAN-STYLE ESCAPE TECTONICS ALONG THE SUPERIOR PROVINCE MARGIN IN MANITOBA, CANADA

The Superior Boundary Zone northeast of Thompson, Manitoba, is an array of shear zones that separates the Neoarchean Pikwitonei Granulite Domain of the Superior Province to the southeast from terranes of the Paleoproterozoic Trans-Hudson Orogen to the northwest. While some of these shear zones were active in the Archean, they were strongly overprinted by Paleoproterozoic movement.

Paleoproterozoic movement along the Superior Boundary Zone has previously been interpreted with a promontory model, in which the Superior Province moved to the northwest to collide with rocks of the Trans-Hudson Orogen. This model implies and is consistent with sinistral, southeast-side-up movement along the 030°-trending SSW segment and dextral, southeast-side-up movement along the 050°-trending ENE segment of the proposed promontory. Detailed kinematic analysis of selected shear zones lead to the following additional observations: (1) NW-side-up dextral movement along an 050°-trending central segment of the promontory; (2) north-side-up dextral movement along a 110°-trending shear zone of the ENE segment; (3) a high dip-slip (SE-side up) and a low strike-slip (sinistral) component along an 030°-trending shear zone of the SSW segment; and (4) curvature of shear zones of the ENE segment.

We propose a tectonic escape model in which lateral extrusion and clockwise rotation occurs along the northern margin of the ‘Superior indentor’, similar to extrusion and rotation along the eastern margin of the Himalayas. In this model, the transport direction was to the WNW, as opposed to the NW in the previous promontory model, making the SSW segment the frontal thrust zone and allowing for a greater rotation and stretch of the ENE segment. Shear zones in the central and ENE segments with a north(west)-side-up component initiated as WNW-side-up back-thrusts and were subsequently rotated clockwise, so that their dextral strike-slip component increased. Extrusion along shear zones also explains uplift of granulite-grade (lower to mid crustal) segments bounded by amphibolite-grade (upper crustal) terranes. Rotation of crustal blocks and shear zones along the Superior Province margin explains the curvature of two shear zones in the ENE segment. While all shear zones are subvertical now, they may have initiated at more shallow dips.