SEISMIC-REFLECTION PROFILES ACROSS THE CENTRAL KAPUSKASING UPLIFT

The central Superior Province is transected by the intracratonic Kapus kasing uplift, which contains rocks exhumed from 30 to 35 km paleodept h. As part of the Lithoprobe Kapuskasing transect, approximately 52 km of 16 s seismic reflection data were collected in the central segment of the uplift along three profiles that traverse the northern Groundh og River block, the bounding Saganash Lake fault, and the eastern Val Rita block. The seismic sections have the following characteristics in common: (i) a complexly reflective uppermost portion (<1 s) limiting correlation of reflective zones and surface features; (ii) numerous su bhorizontal, east- and west-dipping reflection zones; and (iii) a sign ificant reduction in reflectivity beyond the refraction-defined Moho ( similar to 14 s). Beneath the Groundhog River block a series of straig ht, west-dipping (similar to 20 degrees) reflection zones between 2 an d 10 s is underlain by subhorizontal reflections in the lower crust. A cross the Saganash Lake fault, the Val Rita block is characterized by a maze of discontinuous, curvilinear reflections with general easterly dip down to 8-10 s, below which west-dipping events are prominent. A north-south cross profile reveals a highly reflective crust with domin antly horizontal reflection geometry below the Saganash Lake metavolca nic belt, and a steep truncation of reflection zones down to at least 7 s, which correlates with the surface trace of the Nansen Creek fault . This fault resembles well-known strike-slip faults in intraplate set tings. The Saganash Lake fault, variably interpreted as a west-side-do wn normal fault with up to 15 km of throw or a major strike-slip zone, may be visible as a west-dipping, weakly reflective zone steeply trun cating east-dipping reflections and becoming listric at depth. This in terpretation accords with surface geological observations and gravity models for the structural geometry of the region in which the Groundho g River block is a thin thrust sheet of granulite perched on Abitibi b elt rocks and truncated on the west by the crustal-scale Saganash Lake fault. Alternatively, the fault could be a seismically unresolved maj or transcurrent structure juxtaposing blocks with disparate reflection patterns in the upper 8 s. Limited amounts of late strike-slip motion have been inferred from various geophysical studies.