Abstract
The crustal-scale geometry of the European Alps has been explained by a classical subduction-scenario comprising thrust-and-fold-related compressional wedge tectonics and isostatic rebound. However, massive blocks of crystalline basement (External Crystalline Massifs) vertically disrupt the upper-crustal wedge. In the case of the Aar massif, top basement vertically rises for >12 km and peak metamorphic temperatures increase along an orogen-perpendicular direction from 250 °C–450 °C over horizontal distances of only <15 km (Innertkirchen-Grimselpass), suggesting exhumation of midcrustal rocks with increasing uplift component along steep vertical shear zones. Here we demonstrate that delamination of European lower crust during lithosphere mantle rollback migrates northward in time. Simultaneously, the Aar massif as giant upper crustal block extrudes by buoyancy forces, while substantial volumes of lower crust accumulate underneath. Buoyancy-driven deformation generates dense networks of steep reverse faults as major structures interconnected by secondary branches with normal fault component, dissecting the entire crust up to the surface. Owing to rollback fading, the component of vertical motion reduces and is replaced by a late stage of orogenic compression as manifest by north-directed thrusting. Buoyancy-driven vertical tectonics and modest late shortening, combined with surface erosion, result in typical topographic and metamorphic gradients, which might represent general indicators for final stages of continent-continent collisions.
Highlights
The crustal-scale geometry of the European Alps has been explained by a classical subductionscenario comprising thrust-and-fold-related compressional wedge tectonics and isostatic rebound
In the case of the Alps, which are considered as a classical compressional orogen, seismic tomography was recently used to hypothesize a rollback mechanism in the subducting European lithosphere as major orogenic process[2]
We study the Aar massif (Swiss Alps), the largest of the External Crystalline Massifs (EMC), and utilize it to document the young and severe vertical uplift[8] of an entire large-scale crustal block
Summary
Marco Herwegh 1, Alfons Berger[1], Roland Baumberger[1,2], Philip Wehrens1,2 & Edi Kissling[3]. Simultaneous collisional deformation focuses mainly on the mechanical active boundaries, where at the rheological brittle-ductile transition, between the lower and middle crust, mid crustal blocks of the Gotthard nappe and AAT (i) delaminate from the bending lithosphere, (ii) experience buoyancy and uplift along a N-directed ~30° mass flow vector, and (iii) become accreted to the Adriatic upper plate (TALP33; Fig. 4a,b and e) In this in-sequence manner, the mechanical active boundary migrates northward as a function of progressive rollback and mid-crustal delamination, benefitting from the inherited crustal extensional structure of the former European passive continental margin and its pre-existing mechanical anisotropies[10, 47]. In the case of old continent-continent collisions (e.g. Variscides, Kaledonides) one might even gain insights into the root zones (steep belts) of such vertical tectonic complexes owing to the long lasting and severe exhumation of these originally deeply buried crustal parts
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