A review and tectonic interpretation of the Taconian–Grampian tract between Newfoundland and Scotland: diachronous accretion of an extensive forearc–arc–backarc system to a hyperextended Laurentian margin and subsequent subduction polarity reversal

Abstract

The Taconian-Grampian tract was characterized by a diachronous collision of a north-facing oceanic arc-forearc terrane and associated backarc basins with an irregular Laurentian margin with hyperextended segments. Hyperextension produced outboard continental terranes, separated by exhumed subcontinental mantle from the inboard margin. The exhumed mantle facilitated continued subduction of the extended margin after it had entered the trench. Enhanced slab-rollback resulted in spreading in, lenticular backarc basins, which gradually transitioned along strike into extensional arcs where rollback was less. Obduction of the oceanic elements onto the irregular Laurentian margin was followed by diachronous slab breakoff and a subduction polarity reversal, such that south- and north-dipping subduction zones locally were coeval along strike. The polarity flip changed the convergence obliquity from dextral to sinistral and was accompanied by shallowing of the subducting slab near the end of the Middle Ordovician. Strike-slip movements locally juxtaposed segments where tectonic events occurred at different times, producing conflicting relationships. Slab breakoff produced punctuated magmatism, largely driven by mantle-derived melts, and drove and/or enhanced metamorphism in the overlying and enveloping crustal rocks. Boninite was generated episodically over a time span of 32 my; the oldest Cambrian phase in the LBOT and correlatives was associated with subduction initiation.

A review of the Caledonian Wilson cycle from a North Atlantic perspective

“…Following the early discovery of coesite (Smith 1984;Waine 1997) and micro-diamond (Dobrzhinetskaya 1995) in gneisses of the WGR, UHP conditions were firmly established for several provinces of the WGR during the 2000s (Fig. 2; Terry et al 2000a;Brueckner et al 2002;van Roermund et al 2002;Carswell et al 2003Carswell et al , 2006Root et al 2005;Vrijmoed et al 2006;Young et al 2007;Butler et al 2013). UHP conditions of up to 3.5 GPa and 800°C are well documented to have affected large parts of the northwestern WGR (Hacker et al 2010), whereas extreme pressure conditions of up to 5.5 GPa recorded in some unusual rocks seem to be the result of local non-lithostatic pressure conditions caused by mechanical heterogeneities (Vrijmoed et al 2010;Cutts et al 2020).…”