Dynamics of large and hot subduction-accretion complexes: insights from fossil and active systems

The dynamics of subduction zones are influenced by the sediment supply to the trench, which determines whether the margin is erosive (low supply) or accretive (high supply). At present, trench-fill thicknesses ≥1 km promote accretion, which can lead to the construction of a crustal-scale accretionary complex. Here we discuss the dynamics of very large subduction-accretion complexes that encompass the entire upper plate. The formation of such large complexes due to high sediment supply to the periphery of Gondwana during the Cambrian and Ordovician has been inferred from Cenerian basement units within and outside the Alps. The units record Ordovician metamorphism and anatexis of metagreywackes and metapelites that host elongated sheets of peraluminous orthogneisses and banded amphibolites, indicating a peraluminous magmatic arc situated in a large, cratonized subduction-accretion complex. However, the formation and cratonization of such hot complexes has not been evaluated in mechanical terms and the relevance of some field constraints, including a prevalence of strike slip faulting and lack of late- to post-Cenerian uplift, have remained uncertain. We therefore corroborate the existing model by mechanical considerations and insights from active systems. We discuss the requirements for the formation of hot subduction-accretion complexes and address how high sediment supply affect the upper-plate stress conditions, fault kinematics, rock uplift, and development of topography.