Kinematics and Age of Syn‐Intrusive Detachment Faulting in the Southern Alps: Evidence for Early Permian Crustal Extension and Implications for the Pangea A Versus B Controversy

Abstract

Permian basin formation and magmatism in the Southern Alps of Italy have been interpreted as expressions of a WSW-ENE-trending, dextral megashear zone transforming Early Permian Pangea B into Late Permian Pangea A between~285 and 265 Ma. In an alternative model, basin formation and magmatism resulted from N-S crustal extension. To characterize Permian tectonics, we studied the Grassi Detachment Fault, a low-angle extensional fault in the central Southern Alps. The footwall forms a metamorphic core complex affected by upward-increasing, top-to-the-southeast mylonitization. Two granitoid intrusions occur in the core complex, the synmylonitic Val Biandino Quartz Diorite and the postmylonitic Valle San Biagio Granite. U-Pb zircon dating yielded crystallization ages of 289.1 ± 4.5 Ma for the former and 286.8 ± 4.9 Ma for the latter. Consequently, detachment-related mylonitic shearing took place during the Early Permian and ended at~288 Ma, but kinematically coherent brittle faulting continued. Considering 30°anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally~N-S. Even though a dextral continental wrench system has long been regarded as a viable model at regional scale, the local kinematic evidence is inconsistent with this and, rather, supports N-S extensional tectonics. Based on a compilation of >200 U-Pb zircon ages, we discuss the evolution and tectonic framework of Late Carboniferous to Permian magmatism in the Alps.

The evolution of the Sesia Zone (Western Alps) from Carboniferous to Cretaceous: insights from zircon and allanite geochronology

“…The obtained age overlaps with the crystallization age of an Ivozio mafic rock of 355 ± 9 Ma reported by Rubatto ( 1998 ) and with the intrusion age of the metagabbro body of Cima di Bonze (350 ± 4 Ma, Rubatto et al 1999 ). Carboniferous magmatism of a similar age has been reported in other portions of the Variscan chain, as extensively reported by Ballèvre et al ( 2018 ) and Pohl et al ( 2018 ). In the nearby Ivrea Zone, which also represents a portion of middle-to-lower Adriatic crust that escaped Alpine subduction, a crystallization age of 355 ± 6 Ma was obtained for an igneous felsic granulite (U–Pb in zircon, Vavra et al 1996 ) (Fig.…”

Tectono-sedimentary evolution of a rift system controlled by Permian post-orogenic extension and metamorphic core complex formation (Bidarray Basin and Ursuya dome, Western Pyrenees)

“…Our results suggest that the crust throughout the Pyrenees (the foreland of the Variscan belt) stayed hot and partially molten during early and middle Permian extension, as in other massifs in the previously collapsed hinterland of the Variscan belt, such as Corsica-Sardinia (e.g., Rossi et al, 2015;Gaggero et al, 2017), or in the Alps (e.g., Pohl et al, 2018). In this scenario, strain was vertically partitioned within the crust, with a homogeneously and longitudinally flowing lower crust and an upper crust affected by widespread N20°E normal faults and longitudinal strike-slip faults.…”

Transpressive Deformation in the Southern European Variscan Belt: New Insights From the Aiguilles Rouges Massif (Western Alps)

“…Geochronological data from pre-Alpine shear zones in the Western Alps point to an older deformation still related to the Variscan orogeny (Pohl et al, 2018;, suggesting that activity on the EVSZ is not related to post-Variscan events. A detailed structural and geochronological analysis of the shear zones in the Variscan basement of the Western Alps is therefore necessary to address this problem.…”