THE IAPETAN RIFTED MARGIN IN THE NORTHERN APPALACHIANS: A LOW-ANGLE DETACHMENT MODEL APPLIED FROM SOUTH TO NORTH

Neoproterozoic-Cambrian rifting and opening of the Iapetus Ocean produced a set of promontories and embayments on the Laurentian margin defined by northeast-trending rifts offset by northwest-trending transforms. Work in the southern Appalachians portrays the Iapetan rift in the form of an asymmetric low-angle detachment rift system that includes specific rift structures (i.e., upper-plate rifts, lower-plate rifts, transforms). However, such a model has yet to be applied to the Iapetan rifted margin in the northern Appalachians.

In southern Quebec, rift and passive-margin deposits are thin, mainly shallow-water facies that are limited in distribution. There, rift volcanics have a geochemistry indicating derivation from thick continental crust. These data suggest an upper-plate rift in the southern Quebec embayment. Southwestward into Vermont, Iapetan rift and passive-margin deposits thicken abruptly, indicating a transform margin separating the upper-plate Quebec rift from the proposed lower-plate Vermont rift. North along strike from southern Quebec, rift and passive-margin deposits on Gaspé Peninsula abruptly change from shallow- to deep-water facies, suggesting a transform margin aligned with the Saguenay graben. On Gaspé, rift and passive-margin slope deposits are more widely distributed; and the geochemistry of rift volcanics indicates derivation from highly attenuated crust, suggesting a lower-plate rift setting in the northern Quebec embayment. The northern termination of the Quebec embayment is the Sept Iles transform, which offsets the Quebec rift by ~500 km from the St. Lawrence promontory. Along the St. Lawrence promontory both in southern Newfoundland and farther north along the west flank of the Long Range inlier, thin Early Cambrian shallow-marine shelf deposits indicate an upper-plate rift setting. In between near Humber Arm and also at Hare Bay on the northern end of the promontory, synrift deposits preserved in Taconic allochthons are thicker and likely as old as the Neoproterozoic, indicating a lower-plate rift setting. The stratigraphic transitions between the upper- and lower-plate rift segments on the St. Lawrence promontory are abrupt and coincide with sharp northwest-trending linear anomalies on regional Bouguer gravity maps, suggesting transform faults.