DIACHRONOUS DEVELOPMENT OF THE TACONIAN PRO-ARC BASIN IN THE NORTHERN APPALACHIANS

The Anticosti Basin, largely hidden beneath the Gulf of St. Lawrence, includes foreland basin successions that record distinct tectonic loading events associated with Middle Ordovician to Early Devonian evolution of the northern Appalachian Orogen. Due to the absence of wells drilled offshore, important correlations of reflections to geologic boundaries cannot be made using well ties. By using a combination of available 2D seismic reflection, aeromagnetic and bathymetry data, it is possible to tie onshore outcrops to seismic reflections and produce a 3D model of the foreland basin successions.

Seismic isochron maps for the Ordovician foreland successions show important differences in geometry, and imply that orogenic loading varied through time in amount, distribution and location. The geometry and subsidence rates of the Middle Ordovician foreland succession imply formation in a pro-arc setting associated with loading from the Newfoundland portion of the Appalachians. These results are consistent with east-dipping subduction of the Laurentian margin beneath obducted allochthons and microcontinents. In Newfoundland, subduction polarity reversal at 460 Ma placed the Laurentian Craton on the upper plate by the Late Ordovician, implying that the Upper Ordovician Long Point Group was deposited in a retro-arc basin. However, high subsidence rates are consistent with development in a pro-arc setting and basin geometry indicates that loading of the Laurentian margin, by Taconian allochthons in Quebec, was responsible for generating the Upper Ordovician foreland basin. These observations are consistent with continued allochthon emplacement onto the downgoing Laurentian plate until at least 450 Ma in the Quebec Embayment, the age of the youngest flysch units incorporated into allochthons. Subduction polarity reversal immediately followed, as constrained by the 450 Ma age of the oldest forarc sediments unconformably overlying Taconian allochthons in the Quebec Embayment. The 10 Myr delay in polarity reversal in Quebec positioned the Long Point Group in a unique tectonic setting, a combined retro-arc and pro-arc setting. This basin setting is analogous to the current tectonic setting of the northern Australian Plate near Papua New Guinea and is a direct consequence of the irregular margin shape.