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

In the aftermath of Neoproterozoic Panafrican orogenies including the Cadomian orogeny, the erosion of mountain belts increased the sediment supply into circum-Gondwanan subduction systems. In response, super-large subduction-accretion complexes that encompassed the entire upper plate formed during the Cambrian and Ordovician. Cenerian basement units within and outside the European Alps were recently interpreted to represent remnants of such Cambro-Ordovician subduction-accretion complexes hosting a predominantly peraluminous magmatic arc. However, the dynamics and cratonization process of such hot complexes has not been evaluated in mechanical terms. We therefore corroborate the existing model based on petrological data by mechanical considerations and insights from active subduction zones with respect to the following three aspects of Cenerian basement units: Firstly, the lithological spectra, the rock volumes and metamorphic record indicate a crust recycling process as it occurs at an active margin, fed by a very high sediment supply, and hosting a peraluminous magmatic arc. Secondly, moderately to steeply oriented elongated sheets of plutonic bodies point to synmagmatic strike-slip faulting and margin-parallel shear indicating oblique subduction and a near-neutral stress state in the thermally weakened volcanic arc region. Thirdly, the Cenerian pressure-temperature-time (P-T-t) evolution is in line with lateral crustal growth by analogy to modern subduction-accretion systems, without producing high topography of an overthickened crust due to the low-stress conditions. The geological and mechanical constraints gained from active subduction-accretion systems are consistent with field observations and the petrology of Cenerian crystalline basements units regarding the setting of super-large cratonizing subduction-accretion complexes during anatexis and peraluminous arc magmatism.