TECTONIC ARCHITECTURE OF AN ARC-ARC COLLISION ZONE, NEWFOUNDLAND APPALACHIANS

The Dunnage Zone of Newfoundland preserves evidence of an Ordovician arc-arc collision between the Red Indian Lake Arc, which forms part of the peri-Laurentian Annieopsquotch accretionary tract (c. 480–460 Ma), and the peri-Gondwanan Victoria Arc (c. 473–453 Ma). The coeval volcanism in the two arc systems indicates that the Iapetus Oean plate was subducting along its northern and southern margin, in a configuration that is similar to the Molucca Sea and Solomon Sea collision zones. The coeval arc systems can be differentiated on the basis of the contrasts across the Red Indian Line suture zone. These include structural and tectonic history, stratigraphy, basement characteristics, radiogenic lead in mineral deposits and fauna. The timing of the collision is best constrained by the stratigraphic contrasts between the two arc systems. During the collision, the Victoria Arc occupied a lower-plate setting and underwent subsidence, similar to the Australian active margin and Halmahera arcs in the Southwest Pacific. The timing of the subsidence is constrained by the ages of volcanic rocks in the Victoria Arc (c. 457-453 Ma) which immediately predate or are coeval with deposition of the Caradoc black shale. In contrast the Red Indian Lake Arc contains a sub-Silurian unconformity and distinct lack of Caradoc black shale suggesting uplift during the collision. The emergent peri-Laurentian terranes provided detritus into the newly created basin above the Victoria Arc. The evidence of this basin is preserved in the Badger Group, which stratigraphically overlies the peri-Gondwanan Victoria Arc but incorporated Laurentia-derived detritus. Thus the Badger Group forms a successor basin(s) over the Red Indian Line. Following the collision, subduction stepped back into an outboard basin, the Exploits-Tetagouche backarc, closing the Iapetus Ocean along the Dog Bay Line in the Silurian.