GRAVITY AND MAGNETIC IMAGES OF THE PROTEROZOIC TRANS-HUDSON OROGEN, CANADIAN SHIELD: IMPLICATIONS FOR INTERNAL STRUCTURE AND PLATE TECTONIC MODELS

Gravity and magnetic images suggest that the Proterozoic Trans-Hudson orogen extends from South Dakota to northern Hudson Bay. Although largely buried over the 2800 km distance, a broad tract of the orogen, strike-length ~ 500 km, is exposed in the central Canadian Shield, and sizeable areas are mapped near the eastern shore of Hudson Bay. Trends of gravity and magnetic anomalies define internal domains and structural fabric, and boundaries of the orogen. A single domain is proposed to underlie much of Hudson Bay. It is characterized by N to NE trends (like most other domains) and is separated and dextrally offset from domains to the south by a narrow WNW-trending domain containing WNW trends. Such trends are anomalous within the orogen and may have an ancestry in transform faulting within a spreading Trans-Hudson ocean. A conspicuous aspect of the orogen is the similarity in the shapes of its western and eastern margins, leading to speculation that it developed within a major ocean that first spread and then closed. Recognizing the details of this process is complicated by isotopic evidence indicating the presence of both Archean and Proterozoic juvenile crust within the orogen. Seismic evidence indicates that the Archean components attain crustal-scale thicknesses. Can their presence be explained by unilateral subduction associated with continent-continent convergence, in which case one margin of the orogen represents a suture zone and the other marks the limit of reactivation of existing Archean crust (induced by the underriding plate)? Reactivation of the South America Plate over a distance of 700 km by the shallow-dipping, subducting Nazca plate provides a modern analogue. Or, do the Archean components represent structural basement to thin-skinned Proterozoic crust developed in a series of island arcs that were eventually squeezed between two converging continents? Possible geometrical relationships of plate interactions within the orogen may be observed in the pattern of the plate tectonic mosaic in eastern Indonesia, specifically the area of the Banda Sea and the Philippines provides a good example. Analogues for subduction in the Banda Sea, transform faults between Sulawesi and New Guinea, and tectonic relationships in the Philippines region may be recognized in parts of the Trans-Hudson orogen.