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

Potential field data constrained by seismic reflection, seismic refrac tion, geologic, and physical-properties data have been used to study t he deep crustal structure of the central Kapuskasing uplift. The Val R ita block is interpreted as a thrust sheet of granulites detached at m id-crustal depths and uplifted along a ramp producing an arched, doubl e granulite layer. A thin thrust sheet of southward-thickening granuli tes best describes the Groundhog River block, whereas simple thrusting along a ramp characterizes the Chapleau block. From this interpretati on, a sequential evolution of the Kapuskasing uplift is proposed. At a bout 2450 Ma, northwest-southeast compression (sigma(1)) resulted in v ariable thrusting along the Ivanhoe Lake fault zone. This was followed by a period of normal faulting, resulting in the juxtaposition of hig h- 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 transpr essional 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 Gre nville orogeny.