What drives Alpine Tethys opening? Clues from the review of geological data and model predictions

Abstract

Permo‐Triassic remnants (300–220 Ma) of high‐temperature metamorphism associated with large gabbro bodies occur in the Alps and indicate a high thermal regime compatible with lithospheric thinning. During the Late Triassic–Early Jurassic, an extensional tectonics leads to the break‐up of Pangea continental lithosphere and the opening of Alpine Tethys Ocean (170–160 Ma), as testified by the ophiolites outcropping in the Central–Western Alps and Apennines. We revise geological data from the Permian to Jurassic of the Alps and Northern Apennines, focusing on continental and oceanic basement rocks, and predictions of existing numerical models of post‐collisional extension of continental lithosphere and successive rifting and oceanization. The aim is to test whether the transition from the Permo‐Triassic extensional tectonics to the Jurassic opening of Alpine Tethys occurred. We enforce the interpretation that a forced extension of 2 cm/year of the post‐collisional lithosphere results in a thermal state compatible with the Permo‐Triassic high‐temperature event suggested by pressure and temperature conditions of metamorphic rocks and widespread igneous activity. Extensional or transtensional tectonics is also in agreement with the generalized subsidence indicated by the deposition of sedimentary successions with deepening upward facies occurred in the Alps from the Permian to Jurassic. Furthermore, a rifting developed on a thermally perturbed lithosphere agrees with a hyperextended configuration of the Alpine Tethys rifting and with the duration of the extension necessary to the oceanization. The review supports the interpretation of Alpine Tethys opening developed on a lithosphere characterized by a thermo‐mechanical configuration inherited by the post‐Variscan extension which affected Pangea during the Permian and Triassic. Therefore, a long‐lasting period of active extension can be envisaged for the breaking of Pangea supercontinent, starting from the unrooting of the Variscan belts, followed by the Permo‐Triassic thermal high, and ending with the crustal break‐up and the formation of the Alpine Tethys Ocean.

3D reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the Alpine subduction system: the example of Mt. Mucrone (Sesia–Lanzo Zone, Western Alps)

“…A pre-Alpine polyphasic metamorphic evolution, from granulite-to amphibolite-and greenschist-facies conditions, is still preserved in marbles, metapelites, metagranitoids, and meta-basics (Compagnoni et al 1977;Lardeaux et al 1982b;Castelli 1991;Lardeaux and Spalla 1991;Rebay and Spalla 2001;Roda et al 2019a): the pre-Alpine T-climax has been constrained at T = 730-830 °C and P = 0.7-0.9 GPa (Lardeaux and Spalla 1991). Granulite-and amphibolitefacies imprints have been interpreted as the result of an extension-related uplift of a portion of the Variscan crust, occurred in Permian-Triassic times during the lithospheric thinning leading to the Tethys opening (e.g.…”

“…Granulite-and amphibolitefacies imprints have been interpreted as the result of an extension-related uplift of a portion of the Variscan crust, occurred in Permian-Triassic times during the lithospheric thinning leading to the Tethys opening (e.g. Lardeaux and Spalla 1991;Marotta et al 2009;Roda et al 2019a). This pre-Alpine tectono-metamorphic evolution is preserved in the metamorphic complexes forming the SLZ: the Eclogitic Micaschist Complex (EMC), the Gneiss Minuti Complex (GMC), the II Dioritic-Kinzigitic Zone (IIDK), and the Rocca Canavese Thrust Sheets (RCT) (e.g.…”

Integrating X-Ray Computed Tomography With Chemical Imaging to Quantify Mineral Re-crystallization From Granulite to Eclogite Metamorphism in the Western Italian Alps (Sesia-Lanzo Zone)

“…The pre-alpine stage has been constrained at T = 730-830 • C P = 0.7-0.9 GPa for acid and basic granulites surfacing in other localities of the SLZ (Lardeaux and Spalla, 1991). Pre-D1 has been interpreted as the result of an extension-related uplift of a portion of the Variscan crust; this extensional tectonics is inferred to be of Permian-Triassic time, during the lithospheric thinning leading to the Tethys opening (e.g., Lardeaux and Spalla, 1991;Marotta et al, 2009;Roda et al, 2018b).…”

How many subductions in the Variscan orogeny? Insights from numerical models

“…Most of the pre-Alpine continental lithosphere recycled during the Alpine subduction shows a pre-Mesozoic metamorphic evolution compatible with the evolution of the European Variscan belt (von Raumer et al, 2003;Spalla and Marotta, 2007;Spiess et al, 2010;Spalla et al, 2014;Roda et al, 2018a). von Raumer et al (2003) suggested that the present day Alpine domains (Helvetic, Penninic, Austroalpine and Southalpine) were probably located along the northern margin of Gondwana.…”

“…Pre-Alpine HP metaophiolite remnants described in Helvetic to Austroalpine domains (e.g. Miller and Thöni, 1995;Guillot et al, 1998;Nussbaum et al, 1998;Spalla et al, 2014;Roda et al, 2018a) indicate that segments of the Variscan suture zone, incorporating the records of oceanic lithosphere subduction, were included in the Alpine belt. Oldest ages of Variscan HP metamorphic imprints range from Silurian to Middle-Devonian (437-387 Ma) and HP-UHP rocks display ages up to Upper Missisipian (~330 Ma) (e.g.…”