Permian detachment faulting, syntectonic magmatism and Alpine thrusting in the Orobic Alps, southern Alps, Italy

F. Pohl, Niko Froitzheim, Frank Tomaschek, Alexander Heuser, Thorsten J. Nagel, Oliver Schröder

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademic

Abstract

The Grassi Detachment Fault is an Early Permian extensional structure located in the Orobic Alps east of Lake Como (Froitzheim et al. 2008). The Grassi Detachment Fault separates the Variscan Basement in its footwall from the volcanic and sedimentary rocks of the Early Permian Collio Formation within its hanging wall. This contact is marked by a mylonitic and cataclastic layer whose textures indicate a top-to-the-southeast displacement. The footwall basement is formed by the Variscan Morbegno Gneiss and two granitic intrusions, the Val Biandino Quarz Diorite (VBQD) and the Valle Biagio Granite (VBG). The former is syntectonic with respect to the detachment, whereas for the latter, the relation to the detachment is unknown. The age of the VBQD is poorly defined as 312 Ma ± 48 Ma (Thöni et al. 1992); the VBG has not been dated. Volcanic rocks of the Collio Formation in the hanging wall may represent the extrusive part of the magmatic system. Many issues concerning this Permian extensional structure are unclear, like the exact timing of faulting, the relation between the intrusions and the shear zones, and the extension from the well-exposed part of the detachment towards west. In the southern part of the Orobic anticline, several faults and shear zones are exposed: (1) The Grassi Detachment Fault between the basement rocks and the Collio Volcanics, represented by mylonites and cataclasites with top-SE shear sense. The detachment is truncated by the unconformably overlying Late Permian Verrucano Lombardo towards the NW which may reflect the eroded culmination of a Permian metamorphic core complex. (2) The Sasso Rosso Fault, a steeply NW-dipping, brittle normal fault is found further west in the footwall between VBQD and VBG. It is sealed by the basal unconformity of the Verrucano Lombardo and therefore should also be of Early Permian age (Sciunnach, 2001). It may represent an antithetic fault with respect to the detachment, accommodating the uplift of the magmatically inflated core complex. (3) Further North a steeply SE-dipping reverse fault is found, affecting also the Late Permian Verrucano Lombardo. It is therefore an Alpine structure called the Biandino Fault. (4) Several south-directed Alpine thrusts duplicate the lithostratigraphy, including the detachment, and are related to the Orobic thrust further north. They also offset the Biandino Fault.
Original languageEnglish
Title of host publicationGeoFrankfurt 2014, Frankfurt, Germany.
Publication statusPublished - Sept 2014

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