NEW REFLECTIONS ON THE STRUCTURE AND EVOLUTION OF THE MAKKOVIKIAN-KETILIDIAN OROGEN IN LABRADOR AND SOUTHERN GREENLAND

Marine seismic reflection profiles across the Early Proterozoic Makkov ikian - Ketilidian Orogen in the Labrador Sea region suggest that it i s a doubly vergent, asymmetric orogenic belt, comparable in width to y ounger collisional orogens. A southeast dipping reflector package is c orrelated with on-land shear zones that mark the southeastern limit of exposed reworked Archean crust and is also associated with a cryptic isotopic boundary in granites, which documents a transition from ''anc ient'' to ''juvenile'' basement. This boundary is interpreted as a sut ure, along which juvenile Proterozoic crust has been juxtaposed over ( thrust over ?) the Archean craton. A major synorogenic to postorogenic plutonic terrace to the southeast has a poorly reflective upper crust but shows strong subhorizontal reflectivity in the lower crust and Mo ho regions. The southeastern part of the profile correlates with metas edimentary terranes of the Ketilidian Belt in Greenland and contains l ow-angle, northwest dipping, reflector packages suggestive of large-sc ale crustal imbrication by thrusting. At least two broad zones of refl ectivity at mantle depths (up to 16 s) are also recognized. One dips n orthwest below the Archean craton, but the most widespread mantle refl ectivity is southeast dipping in opposition to the dominant fabric in the overlying crust. These contrasting crustal and subcrustal reflecti vity patterns define a geometric ''focus'' beneath the orogenic belt a nd may provide information about subduction polarity during its develo pment. The doubly vergent reflectivity pattern resembles images from p ossibly correlative Precambrian orogenic belts such as the Penokean an d Svecofennian and also younger ''small collisional orogens'' such as the Appalachian and Pyrenean belts. It also broadly resembles some res ults of geodynamic crustal deformation models based on detachment and underthrusting of mantle lithosphere following collision and the squee zing of ''weak'' zones between rigid bounding blocks. A speculative mu ltistage model for the belt incorporates initial northward subduction beneath the craton, shifting to later southward subduction, followed b y oblique accretion of a composite are terrane and juvenile (?) contin ental block. The accreted, boner, juvenile, Proterozoic crust behaved differently than the stable, cool, Archean crust and experienced subho rizontal shearing in lower crustal and Moho regions, associated with s outheast directed imbrication of the middle and upper crust by thrusti ng. However, the present reflectivity pattern of the orogen may also i nclude elements related to post collisional extensional collapse.