Paleogeography, accretionary history, and tectonic scenario: A working hypothesis for the Ordovician and Silurian evolution of the northern Appalachians

Abstract

Paleomagnetic and tectonostratigraphic data for the northern Appalachians record Silurian closure of a major ocean, the Iapetus Ocean, that was bordered by the Laurentian craton and the Avalonian microcontinent. In Ordovician times this ocean consisted of at least two basins (Iapetus I and II) and extended from a paleolatitude of 10 to 20°S (Laurentian margin) to ca. 50°S (Avalonian margin); Gondwana was located yet farther south. Paleomagnetic data from the Middle Ordovician Robert's Arm, Chanceport, and Summerford groups in north-central Newfoundland, which represent intraoceanic arcs and ocean islands, yield paleolatitudes of 30 to 33°S. In contrast, pillow lavas of the upper part of the Late Cambrian to Lower Ordovician Moreton's Harbour Group, which are currently juxtaposed to the Chanceport Group along the Lobster Cove-Chanceport Fault, acquired their remanence at 11 °S in a marginal island arc setting. Subaerial deposits of the mid-Silurian Botwood Group that unconformably overlie marine sequences in northeastern Newfoundland yield a primary magnetization with a paleolatitude of 24°S, which is indistinguishable from the Early Silurian position of the southeast-facing Laurentian margin. Silurian closure of Iapetus is supported by the timing of thrusting and folding and by the age of angular unconformities in the Central Mobile Belt.The combined paleomagnetic and tectonostratigraphic data present a working hypothesis for the geometry and tectonic evolution of the northern Appalachians. In Early Ordovician times, a volcanic arc and a back-arc basin (Iapetus I) were located near the Laurentian margin. Following Middle Ordovician obduction of ophiolites onto the Laurentian margin when Iapetus I closed, convergence of Avalon and Laurentia by northward subduction continued until closure of Iapetus II was complete by the Late Silurian.In view of these data, the traditional subdivision of Ordovician "Taconic" and Devonian "Acadian" orogenies needs to be revised. Maintaining the terminology of orogenic phases, one either has to expand the time interval of the Acadian orogeny to include the Silurian or add an orogenic phase (Caledonian?) in between Taconic and Acadian. In either case, Early to Middle Paleozoic closure of Iapetus should be viewed in terms of a progressive deformation history with peak deformation pulses rather than temporally discrete orogenies.van der Pluijm, B. A., Johnson, R.

Tectonic implications of early silurian thrust imbrication of the northern exploits subzone, Central Newfoundland

“…1970;Harland and Gayer, 1972;McKerrow and Ziegler, 1972;Dewey et al. 1983: Harris andFettes, 1988;McKerrow, 1988;van der VOO, 1988;van der Voo rt d., I99 1; Dunning et al, 1990, 199 1;van der Pluijm et al. 1993).…”

“…Traditional models of Appalachian orogenesis have recently been challenged by paleomagnetic. paleontologic, geochronologic and structural data indicating a more complex sequence of events, in which several different arc sequences developed at the same time but in different places, and were later juxtaposed during closure of the Iapetus ocean (van der Pluijm, 1986;Dunning et al, 1991;van der Pluijm et al, 1993;de Roo and van Staal, 1994;van Staal and de Roo, 1996;Dalziel et al, 1994). In this study, we examine the structural setting of a group of volcanic rocks in the Exploits Subzone of central Newfoundland's Dunnage Zone that have proven critical in determining the ages of magmatism and the relative positions of the different volcanic sequences of central Newfoundland.…”

“…Silurian erogenic activity in the Newfoundland Appalachians includes abundant strikeslip faulting (with pull-apart basin formation). penetrative deformation, metamorphism, and plutonic and subaerial volcanic activity van der Pluijm, 1986;van der Pluijm et al, 1993;Kusky et al, 1987;Dunning et cd., 1990;Elliott et al, 1991). Late Silurian events have been referred to by some as the Salinic disturbance (Boucot, 1962, Dunning et al.…”

“…Lake Ambrose, Moreton's Harbour, Sleepy Cove-Twillingtate, and the Baie Verte Peninsula ophiolites), are thought to represent an island arc(s) and/or back-arc basins (Williams et al, 1976;Dewey et al, 1983;Bostock, 1988;Swinden, 1990;Dunning et al, 199 1) developed near the Appalachian margin of Laurentia, and colliding with this margin during the Middle Ordovician Taconic orogeny. Paleomagnetic data from these rocks (van der Voo, 1988;van der Voo et al, 1991;van der Pluijm et al, 1993) indicate a paleolatitude of circa 11' S, similar to the E-W trending Appalachian margin of Laurentia. The Robert's Arm Belt (Fig.…”

“…the Ordovician (van der Voo, 1988;van der Voo et al, 199 1;van der Pluijm et al, 1993). The Notre Dame Subzone (Figs 1 and 2) contains volcanic belts with Cambrian through Middle Ordovician ages.…”