Geophysical constraints on the stratigraphy, structure and tectonic evolution of the Late Devonian/Carboniferous Moncton Subbasin, New Brunswick

摘要

A synthesis of public domain seismic reflection, gravity, magnetic, borehole and map data reveals that the Moncton Subbasin evolved as series of linked half-grabens which formed by oblique extension beginning in Late Devonian time. Transpressional wrench faulting resulted in deformation, localized uplift and erosion in the Early Carboniferous (end of Tournaisian); episodes of strike-slip faulting punctuated a broad sag-style subsidence in the Late Carboniferous. Quantitative estimates of net extension and subsidence rates have been derived by balancing interpreted seismic cross-sections and making seismic-unit thickness and map measurements; these suggest that north-south oblique extension on the order of 10 km accommodated subsidence along the Caledonia-Clover Hill Fault zone in the Late Devonian and Tournaisian. The subsidence rate averaged at least 120 mm/10$sp3$ yr and was greatest during the late Tournaisian.;The distribution of Avalon zone basement units beneath the Carboniferous cover is inferred from an analysis of gravity and magnetic data in concert with borehole and map observations. The Brookville terrane is interpreted to lie tectonically emplaced over the younger Caledonia terrane, and to extend beneath Carboniferous cover as far north as the Northumberland Strait.;Seismic stratigraphic analysis of the terrestrial sediments of the Horton Group in the Elgin/Portage Vale area reveals alluvial fan and fan-delta seismic stratigraphic units in a previously undescribed Late Devonian/Tournaisian depocentre. The Albert Formation oil shale member coincides with a maximum flooding surface and is mappable seismically in the thick lacustrine succession. Seismic isopachs of progressively younger Lower Carboniferous sequences migrate to the southwest in a manner consistent with the oblique extension model of basin evolution.;Taken as a whole, these observations suggest the pull-apart model of the Maritimes Basin should be re-evaluated. The early, extensional phase of basin evolution is consistent with the gravitational collapse of the Acadian orogen.