PRELIMINARY INTERPRETATION OF THE LAURENTIAN RIFTED MARGIN ON THE ST. LAWRENCE PROMONTORY

Neoproterozoic to Early Cambrian rocks on the St. Lawrence promontory (StL) in western Newfoundland record protracted continental rifting related to the final breakup of Rodinia and opening of the Iapetus Ocean. While the 2-D map trace of the Laurentian rifted margin is generally established, relatively few studies characterize the 3-D tectonic framework. The distribution and thickness of synrift successions in western Newfoundland suggest a distinction between upper- and lower-plate configurations along the rift on the StL where synrift deposits have two contrasting expressions: a shallow-water platform (Labrador Gp) and a deep-water slope succession (Curling Gp and Maiden Point Fm).

In two areas on the StL, Port au Port peninsula to the south and the western side of the Long Range inlier on the Great Northern peninsula, the synrift succession consists of siliciclastic rocks (upper Labrador Gp), which were deposited in predominantly shallow-water environments. There, Early Cambrian rift-to-drift arkosic sandstones of the Bradore Fm (Labrador Gp) lie on basement and the entire synrift succession is regionally thin (<750 m), suggesting an upper-plate configuration.

In two other areas on the StL, northeast of Port au Port near Humber Arm and on the northern tip of the Great Northern Peninsula near Hare Bay, deeper water shelf-edge, shelf-break, and slope deposits constitute the synrift and post-rift stratigraphy. In these areas, allochthonous slices of the Laurentian slope contain thick synrift successions that are structurally interleaved with rift-related volcanic suites. Complexly deformed Early Cambrian synrift slope deposits of the Curling Gp locally exceed 1800 m in thickness near Humber Arm; to the north around Hare Bay, the allochthonous rift-related Early Cambrian Maiden Point Fm is ~2000 m thick. These relationships suggest lower-plate geometries for the areas around Humber Arm and Hare Bay.

The stratigraphic transitions between the upper- and lower-plate rift segments on the StL are abrupt along strike (<15km) and coincide with sharp, NNW-trending linear anomalies on regional Bouguer gravity maps. These linear anomalies (located near Serpentine Lake, Bonne Bay, and Hare Bay) are interpreted as rift-related transform faults that separate upper- and lower-plate domains of the rifted margin.