Paper No. 7
Presentation Time: 3:05 PM


WALDRON, John W.F., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G2E3, Canada, SCHOFIELD, D.I., British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom, THOMAS, C.W., British Geological Survey, Murchison House, West Mains Road, Keyworth, Edinburgh, EH93LA, Scotland and MURPHY, J. Brendan, Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada,

In the opening of the Iapetus Ocean, numerous continental fragments were split from diverging Laurentia, Amazonia, and Baltica. The classification of these fragments as peri-Laurentian or peri-Gondwanan is based on the presence or absence of late Neoproterozoic 'pan-African' basement, but faunal provinciality, sediment provenance, and tectonic position have all provided evidence for the origin of individual slices.

The margin of Laurentia underwent rifting from ~615 Ma or before, to ~515 Ma, based on the age of the breakup unconformity in Newfoundland. In most reconstructions a Neoproterozoic 'early' Iapetan rift to the east of peri-Laurentian microcontinents was superseded by a western, early Cambrian rift, between the fragments and the Laurentian margin. Analogies with modern oceans suggests that this history would have substantially separated the fragments from Laurentia.

This scenario is confirmed by the Taconian/Grampian collisional history. Ages of metamorphism and cross-cutting plutons, together with isotopic data, all show that these offshore blocks were deformed and metamorphosed around 490 Ma while the Laurentian margin was still undergoing passive thermal subsidence and shelf sedimentation free of arc influence. The earliest stages of Taconian/Grampian orogenesis therefore took place well offshore in the Iapetus Ocean. Collision of the Laurentian margin did not begin until the Middle Ordovician, and the initiation of a foredeep on the former shelf began around 466 Ma.

Peri-Gondwanan fragments were also involved in localized, possibly oblique collisions from 490 Ma onwards suggesting multiple arcs and transform faults in the central Iapetus. The earliest peri-Gondwanan fragments began to arrive at the now active Laurentian margin by ~450 Ma. Although these fragments show late Neoproterozic tectonim, characteristic of peri-Gondwana, occasional 1 Ga detrital and inherited zircons suggest an source in eastern Laurentia or western Amazonia. It is therefore likely that both peri-Gondwanan and peri-Laurentian microcontinents originated in a complex mosaic during Iapetan rifting, close to the triple junction that separated Laurentia, Amazonia, and Baltica.