WHAT CAUSED THE MAIN OROGENIC PULSES ALONG THE CALEDONIAN – APPALACHIAN MARGIN OF LAURENTIA DURING THE PALEOZOIC?

The margin of Laurentia between Scotland and Alabama suffered four orogenic pulses, differing in intensity from place to place and differentiated in time: Ordovician (Taconic), Silurian (Salinic-Caledonian), Devonian (Acadian) and Permocarboniferous (Alleghenian-Variscan). It is likely that each of these was caused by plate convergence and collision, but it is less clear which continental elements collided with Laurentia. Candidates are, from west to east: (1) the Bronson-Hill arc and similar peri-Laurentian arc-like elements (e.g., western Dunnage Terrane), (2) the eastern Dunnage Terrane, (3) originally peri-Gondwanan elements now found in the Miramichi or Gander terranes, (4) Avalonia segments, and (5) West Gondwana. Paleogeographic considerations from structural and sedimentary geology, tectonics and paleomagnetism can help in unraveling the protracted history of the impact of these elements on Laurentia.

Emplacement of the Taconic allochthons in Newfoundland, Quebec, New York, and Pennsylvania suggests that western Dunnage arc elements were the main agents in the Ordovician pulse. In the U.K and Newfoundland, the convergence between East Avalonia and Laurentia terminated in a Late Silurian-Early Devonian collision, as epicontinental redbeds of that age indicate the elimination of Iapetus. The last collision caused the Permocarboniferous pulse from England to Alabama, resulting from collision with West Gondwana after closure of the Rheic Ocean, and the amalgamation of Pangea. Silurian and Devonian orogenic activity, however, has less clear identification of the corresponding plate interactions. High-grade metamorphism in New England, regional pluton emplacement, and the deposition of clastic aprons in Quebec and the Catskills suggest that large-scale subduction and/or the arrival of a major continental element must be invoked. Could it have been parts of West Avalonia that accreted to Laurentia much later than its East Avalonia sister elements? Or is there support for the proposal that West Gondwana may have collided obliquely with Laurentia in the Devonian, before it retreated and then collided anew in the late Paleozoic? Various spatio-temporal lines of evidence for such hypothetical scenarios will be critically explored in this presentation.