GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 208-10
Presentation Time: 3:30 PM


ROBINSON, Peter1, HOLLOCHER, Kurt2, ROBERTS, David1, HARPER, David A.T.3 and BRUTON, David L.4, (1)Geological Survey of Norway, Trondheim, NO-7491, Norway, (2)Geology Department, Union College, 807 Union St, Schenectady, NY 12308, (3)Department of Earth Sciences, Durum University, Durham, DH1 3LE, United Kingdom, (4)Naturhistorisk Museum (Geology), University of Oslo, Postboks 1172 Blindern, Oslo, NO-0318, Norway,

Earliest Mid-Ordovician (Dapingian) fossils (~470 Ma) occur above unconformities in geographically different parts of the Upper Allochthon of the Mid-Norway Caledonides. Those in serpentinite conglomerate unconformably on an eastern ultramafic belt at Otta are of the cold-water Celtic Province. Those in limestones, etc., unconformably on Cambrian to earliest Ordovician ophiolite at Hølonda are characteristic of the Laurentian Toquima-Table Head warm-water fauna. Paleolatitude estimates suggest 34oS for Otta, 4oS for Hølonda, a separation of 3353 km (present distance between belts, 100 km). Similar faunas occur in the northern Appalachians. The Shin Brook Formation in the Popelogan Arc, eastern Maine, unconformably on the Cambrian Grand Pitch Formation, contains the Celtic cold-water fauna. Both fossils and unconformity are characteristic of Ganderia, rifted from Amazonia and underlying the volcanic arc of the Taconian collision with Laurentia. Paleolatitude estimates are 48oS for Shin Brook, 21oS for the Laurentian margin near Quebec, a separation distance of 3055 km (present distance between these belts, 210 km).


Using paleo-estimates, present distances (100, 210 km), and estimates of post-Taconian convergence (200,100 km), one can estimate convergence distances between the localities in the time between deposition on the Ganderian and Laurentian margins (470 Ma) and the end of Taconian convergence (450 or 445 Ma); 3053 km for Norway, 2845 for Maine-Quebec. The shortest distance, longest interval give a rate 11.4 cm/year; the longest distance, shortest interval give 15.3/year, both fast relative to many modern rates but less than for the Tonga Arc at 24 cm/year.


Shortly after ophiolite obduction onto the Laurentian margin in Mid Norway, polarity shifted, so by ~470 Ma, Iapetus was subducting beneath Laurentia. This polarity persisted after Taconian collision into the Silurian-Devonian collision with Baltica. By contrast, in New England, continental arc magmatism from SE subduction below Ganderia persisted until polarity change near the end of Taconian collision. This difference leads to a scene ~460 Ma, when an intermediate part of Iapetus contained a convergent transform fault, normal or oblique to the continental margins, that migrated with time from Mid Norway (470 Ma) to New England (445 Ma).