ALONG-STRIKE VARIATIONS IN THE DEFORMED LAURENTIAN MARGIN IN THE NORTHERN APPALACHIANS: ROLE OF INHERITED MARGIN GEOMETRY AND COLLIDING ARCS

The Laurentian margin preserved within the Northern Appalachian-Caledonide Orogen, is one of Earth's best studied ancient continental margins. However, an orogen-wide synthesis of the margin’s variability has been difficult to achieve. To achieve orogen-scale correlations, Ediacaran to Ordovician biostratigraphic and detrital zircon provenance data are placed on a common timescale.

Contrasts in detrital zircon populations from Cambrian rift rocks in Newfoundland indicate two distinct provenance domains: A proximal western succession, dominated by detritus from local Grenville basement, and a distal eastern succession with Paleoproterozoic and Archean sources like the Laurentian margin in Greenland and Scotland. South of Newfoundland, the margin is dominated by Mesoproterozoic detritus and Paleoproterozoic grains are rare or absent. These contrasts are consistent with deposition on an irregular margin with a major NW-striking transfer fault at the promontory-embayment transition, blocking sediment transport.

Microcontinents, rifted from Laurentia, were deformed during earliest stages of Taconian arc-continent collision. Deformation was strongly diachronous as was subsequent closure of the Taconic Seaway. Diachronous subduction polarity reversal, occurring first in Newfoundland (~460 Ma) and later in the Québec Embayment (~450 Ma), resulted in simultaneous westward and eastward subduction at different places along the margin leading to a unique hybrid basin (pro and retro-arc) in Newfoundland, analogous to the current position of the Akimeugah Basin on the northern Australian Plate.

All major deformation episodes affecting the northern Laurentian margin were diachronous. This resulted from: the irregularity of the Laurentian margin; the distribution of off-margin microcontinents; and/or the geometry of colliding arcs and microcontinents.