Structural setting of a transpressive shear zone: insights from geological mapping, quartz petrofabric and kinematic vorticity analysis in NE Sardinia (Italy)

Riccardo Graziani,Rodolfo Carosi,Chiara Montomoli,Salvatore Iaccarino,Laura Nania,Luca Menegon

doi:10.1017/s0016756820000138

Abstract

AbstractThe Posada–Asinara Line is a crustal-scale transpressive shear zone affecting the Variscan basement in northern Sardinia during Late Carboniferous time. We investigated a structural transect of the Posada–Asinara Line (Baronie) with the aid of geological mapping and structural analysis. N-verging F2 isoclinal folds with associated mylonitic foliation (S2) are the main deformation features developed during the Posada–Asinara Line activity (D2). The mineral assemblages and microstructures suggest that the Posada–Asinara Line was affected by a retrograde metamorphic path. This is also confirmed by quartz microstructures, where subgrain rotation recrystallization superimposes on grain boundary migration recrystallization. Crystallographic preferred orientation data, obtained using electron backscatter diffraction, allowed analysis of quartz slip systems and estimation of the deformation temperature, vorticity of flow and rheological parameters (flow stress and strain rate) during the Posada–Asinara Line activity. Quartz deformation temperatures of 400 ± 50 °C have been estimated along a transect perpendicular to the Posada–Asinara Line, in agreement with the syn-kinematic post-metamorphic peak mineral assemblages and the late microstructures of quartz. The D2 phase can be subdivided in two events: an early D2early phase, related to the metamorphic peak and low kinematic vorticity (pure shear dominated), and a late D2late phase characterized by a lower metamorphic grade and an increased kinematic vorticity (simple shear dominated). Palaeopiezometry and strain rate estimates associated with the D2late deformation event showed an intensity gradient increasing towards the core of the shear zone. The D2early deformation developed under peak temperature conditions, while the D2late event was active at shallower structural levels.

Highlights

Collisional type orogens are often characterized by the presence of crustal-scale shear zones driving and affecting the tectono-metamorphic evolution of the inner portion of the belts (Fossen & Cavalcante, 2017)
Syn-collisional transpression has been investigated in first-order regional-scale shear zones within the main European crystalline basements such as the South-Armorican Shear Zone in Brittany (Gébelin et al 2009); the central sector of the Maures-Tanneron massif in southern France (Corsini & Rolland, 2009; Schneider et al 2014); the Ferrier-Mollieres shear zone in the Argentera– Mercantour Massif (Carosi et al 2016; Simonetti et al 2018); and the Posada–Asinara Line (PAL) in northern Sardinia (Carosi & Palmeri, 2002; Carosi & Oggiano, 2002; Carosi et al 2005, 2009; Iacopini et al 2008; Frassi et al 2009; Cruciani et al 2015)
In this work we investigated an area in NE Sardinia (Fig. 1) where a portion of the PAL, one of the first-order transpressive shear zones of the Variscan Belt in southern Europe, crops out (Corsini & Rolland, 2009; Carosi et al 2015; Simonetti et al 2018)

Summary

Introduction

Collisional type orogens are often characterized by the presence of crustal-scale shear zones driving and affecting the tectono-metamorphic evolution of the inner portion of the belts (Fossen & Cavalcante, 2017). Syn-collisional transpression has been investigated in first-order regional-scale shear zones within the main European crystalline basements such as the South-Armorican Shear Zone in Brittany (Gébelin et al 2009); the central sector of the Maures-Tanneron massif in southern France (Corsini & Rolland, 2009; Schneider et al 2014); the Ferrier-Mollieres shear zone in the Argentera– Mercantour Massif (Carosi et al 2016; Simonetti et al 2018); and the Posada–Asinara Line (PAL) in northern Sardinia (Carosi & Palmeri, 2002; Carosi & Oggiano, 2002; Carosi et al 2005, 2009; Iacopini et al 2008; Frassi et al 2009; Cruciani et al 2015) In these areas, a major role of transpressive structures related to the exhumation of high- and medium-grade metamorphic complexes has been recognized, well before the onset of the typical post-collisional extensional events (Turrillot et al 2011; Schneider et al 2014). Combining our structural and petrofabric data with the existing published dataset, we propose an updated model of the PAL tectonic evolution and its role in the exhumation/extrusion of metamorphic complexes in orogenic belts

Geological setting

Deformation history and structural analysis

EBSD analysis and quartz petrofabric

Discussion

Conclusions