RAMP-FLAT GEOMETRY WITHIN THE CENTRAL KAPUSKASING UPLIFT - EVIDENCE FROM POTENTIAL-FIELD MODELING RESULTS

Potential field data constrained by seismic reflection, seismic refraction, geologic, and physical-properties data have been used to study the deep crustal structure of the central Kapuskasing uplift. The Val Rita block is interpreted as a thrust sheet of granulites detached at mid-crustal depths and uplifted along a ramp producing an arched, double granulite layer. A thin thrust sheet of southward-thickening granulites best describes the Groundhog River block, whereas simple thrusting along a ramp characterizes the Chapleau block. From this interpretation, a sequential evolution of the Kapuskasing uplift is proposed. At about 2450 Ma, northwest-southeast compression (sigma(1)) resulted in variable thrusting along the Ivanhoe Lake fault zone. This was followed by a period of normal faulting, resulting in the juxtaposition of high- and low-pressure rocks across the Lepage and Saganash Lake faults, with possible truncation of the Val Rita block arch. Further rotation of sigma(1) to an east-west direction resulted in the reactivation of the Lepage and Saganash Lake faults as dextral transcurrent or transpressional faults accentuating the Val Rita block as an en echelon fold, and distorting the Matachewan dykes. Later (about 2140 Ma), rotation of sigma(1) into a southwest-northeast direction brought transpression to a halt and a period of intermittent dyke and carbonatite intrusion was initiated, which lasted to about 1200 Ma or just prior to the Grenville orogeny.