Timing of collision initiation and location of the Scandian orogenic suture in the Scandinavian Caledonides

The Scandinavian Caledonides represent a classical example of a deeply eroded Himalayan-style orogen formed during Baltica-Laurentia continent collision. We propose that initial contact along continental-margin promontories led to a drop in convergence rate, resulting in increased slab rollback along parts of the margin still undergoing oceanic subduction. Slab rollback caused extension of the overlying lithosphere with orogen-wide emplacement of mafic layered intrusions, ophiolite formation and bimodal magmatism at 438-434 Ma, in what immediately thereafter became the upper plate (Laurentia) in the Scandian continent-continent collision. A compilation of magmatic ages provides evidence of long-lived, Ordovician arc magmatism in units above the suture, which is essentially absent below the suture. This model provides a tight constraint on the timing of collision initiation, and provides a framework by which tectonic units comprising the Scandinavian Caledonides can be assigned a Baltican or more exotic heritage.