Magnetic properties and modelling: Lithoprobe seismic reflection line 7C, southern Kisseynew Gneiss Belt, Trans-Hudson Orogen

Abstract

The Missi Suite, comprising metamorphosed, magnetite-bearing molasse deposits, runs west-northwest across the southern part of the Kisseynew Gneiss Belt in the Trans-Hudson Orogen. It flanks an internal structural boundary, north of which lies the migmatitic core of the Kisseynew Gneiss Belt, formed largely of arc-derived metasediments assigned to the Burntwood Suite. Lithoprobe reflection seismic images crossing the boundary indicate north-facing, homoclinal structures in which the Missi and Burntwood suites are structurally interlayered. The strong magnetic signature associated with the Missi Suite allows an independent assessment of the seismic models, through magnetic modelling constrained by rock magnetic properties. Along Lithoprobe line 7C, the pattern of a prominent magnetic high over the Missi Suite and a subdued magnetic field over the Burntwood Suite is reflected in values of magnetic susceptibilities, demonstrating a close link between magnetic anomalies and surface geology. Measurements of natural remanent magnetization provide an additional critical constraint for modelling. In the seismic model, gently folded north-dipping layers become progressively less steep at depth, eventually becoming subhorizontal. The magnetic model indicates that layers dip at approximately 45° to a depth of about 7 km, a picture supported by steep reflectors in the seismic data themselves and in down-plunge structural modelling. It is proposed that the layers, which probably contain layer-parallel imbricate thrusts, are rooted in a subhorizontal décollement, interpreted from the seismic image, at a depth of about 10 km. The magnetic model is consistent with the presence of a ramp-like structure developed along the ancient margin of the Kisseynew basin.