Permian‐Triassic high thermal regime in the Alps: Result of late Variscan collapse or continental rifting? Validation by numerical modeling

Abstract

[1] The Permian-Triassic igneous activity, associated with regional scale deformation developed under high-temperature/low-pressure (HT-LP) metamorphic conditions and widely recorded in the pre-Alpine crust of the European Alps, can result from late orogenic collapse of a collisional belt or from lithospheric thinning leading to a continental rifting process. In order to reduce this ambiguity, we use a two-dimensional finite element model to give new insights on the sequence of mechanisms operating during active ocean-continent convergence, followed by continental collision and pure gravitational evolution and on the regional geodynamic interpretation of the Paleozoic-Mesozoic evolution of the Alpine area. The modeling predictions are compared with the PT climax conditions of Variscan and Permian-Triassic metamorphism affecting the continental crust of the Helvetic to Southalpine domains. The good agreement between model predictions and natural data realized for the early to Neovariscan evolution indicates that during Paleozoic, the pre-Alpine crust of the Alps was part of an active ocean-continent convergence margin and an intracontinental suture zone. Furthermore, modeling results support the interpretation envisaging a Permian-Triassic lithospheric extension as responsible for the HT metamorphism and related intense igneous activity. This evolution was precursor of the Mesozoic oceanization, during which the tectonic units, coupled and accreted during the Variscan subduction and collision, were separated to form the two passive European and Adriatic continental margins.

2-D numerical study of hydrated wedge dynamics from subduction to post-collisional phases

“…Furthermore, the REF models (Appendix B) lack a short-wavelength convective flow in the mantle wedge area and show no evidence of the inclusion of recycled subducted material, which prevents exhumation during subduction (e.g. Marotta & Spalla 2007;Spalla & Marotta 2007). The temperature in the subducted lithosphere is colder for higher velocities of subduction because the slabs heats up by conduction and cold sections of the slab reach greater depths than if they were moving with a slower velocity.…”

“…Velocities of 3 and 8 cm yr −1 have been chosen to describe slower and faster subduction zones (observed respectively in portions of Central and in South America and the Philippines, Cruciani et al 2005;Lallemand et al 2005), respectively. The velocity of 5 cm yr −1 has been chosen to represent a moderate velocity convergence system, which is consistent with the model described by Marotta & Spalla (2007). To facilitate the subduction of the oceanic lithosphere, the same velocity is also fixed along a 45 • dipping plane that extends from the trench to a depth of 100 km.…”

“…Since the reference model (REF) has been already extensively described in a previous papers (Marotta & Spalla 2007), the upgrades of the REF model and its thermomechanical evolution are synthesized in Appendix B.…”

“…In this scenario, both crustal thickening and heat supplied from radiogenic decay are mainly driven by the burial of the continental crust of the lower plate during collision (e.g. Marotta & Spalla 2007;Spalla & Marotta 2007). Additional heat can be supplied by shear heating, affecting the thermal state of subduction zones and, thus, the rheology of the system (e.g.…”

“…In ocean-continent sub-Q3 Q4 duction systems, low viscosity hydrated mantle facilitates the exhumation and underplating of subducted oceanic and continental material at the base of the crust of the upper plate (Billen & Gurnis 2001;Hirth & Kohlstedt 2003;Billen 2008;Hirth & Guillot 2013;Nagaya et al 2016). When hydration is not considered, a dry and stiff mantle buttress characterizes the wedge area and strongly affects the thermomechanical regime during the subduction stage and subsequent continental collision (Marotta & Spalla 2007;Spalla & Marotta 2007;Gerya et al 2008;Meda et al 2010). In this scenario, both crustal thickening and heat supplied from radiogenic decay are mainly driven by the burial of the continental crust of the lower plate during collision (e.g.…”