Orogen-parallel tectonic transport in the Variscan belt of northeastern Sardinia (Italy): implications for the exhumation of medium-pressure metamorphic rocks

Abstract

A transpressive crustal-scale dextral shear zone is documented in the Variscan Basement of northeastern Sardinia. It indicates the presence of a shear deformation parallel to the belt overprinting previous D1 structures related to nappe stacking and top-to-the-S and -SW thrusting. The L2 stretching lineation points to an orogen-parallel stretching and to a general change in the tectonic transport from D1 to D2. Phase D1 developed during initial frontal collision, whereas the D2 deformation was characterized by dextral shearing during the increasing curvature of the Ibero-Armorican arc. Transpressional deformation developed in a regime of decreasing pressure. It caused telescoping of the Barrovian isograds and the exhumation of the low- to medium-grade metamorphic rocks. In this sector of the Variscan belt, exhumation is due to continuing compression with an increasing component of horizontal displacement. The overall change of the shortening direction in a large sector of an orogenic belt, with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse.

Reply to discussion by Elter and Padovano of ‘Deformation during exhumation of medium‐ and high‐grade metamorphic rocks in the Variscan chain in northern Sardinia (Italy)’

“…This model (Corsini and Rolland, 2009, figure 5), cited by Elter and Padovano to deny the frontal collision, refers to the D2 deformation phase dated at 310-290 Ma (see figure caption), which is younger with respect to the frontal collision during the previous D1 tectonic phase. During this time (D2 at 320-300 Ma) Sardinia was involved in orogen-parallel transpressional tectonics (see Corsini and Rolland, 2009, figure 4) as previously proposed by Carosi and Palmeri (2002), Carosi and Oggiano (2002), Iacopini et al (2008) and Frassi et al (2009).…”

“…The reconstruction proposed by Carosi et al (2009) is based on the documentation of an older D1 stretching lineation testifying to an orogen-perpendicular displacement in both the western and eastern transects in Sardinia (Carosi and Palmeri, 2002;Carosi and Oggiano, 2002;Montomoli, 2002;Iacopini et al, 2008). The D2 stretching lineation orientation and sense of shear clearly show a switch from a down-dip attitude in D1 to a sub-horizontal and orogenparallel displacement in D2 at nearly 320-310 Ma (Carosi and Palmeri, 2002;Carosi and Oggiano, 2002;Montomoli, 2002;Carosi et al, 2004;Di Vincenzo et al, 2004;Iacopini et al, 2008). We would like to stress that we have described this structural setting accurately based on field and structural work along all the transects in northern Sardinia ranging from Nurra peninsula-Asinara island in the NW to Gallura and Anglona in northern-central portion and Baronie to the East.…”

Deformation during exhumation of medium‐ and high‐grade metamorphic rocks in the Variscan chain in northern Sardinia (Italy)

“…According to Carosi and Palmeri (2002) the dextral shear belt (PAL) documented in the northern segment of Sardinia could be related to the indentation of the Ibero-Armorican arc or to an oblique continental collision. The change from sinistral (with top-to-the NW movement) to dextral shear sense in the study area implies a more complex kinematic evolution, probably related both to the original irregular margins of the continental crustal blocks involved and/or to a change in the direction of convergence after the initial collisional event.…”

“…In this paper, we examine a system of regional-scale shear zones in northern Sardinia (in Anglona and SW Gallura regions; Figures 1 and 2), known to be a system of dextral strike-slip shear zones developed in Late Variscan times ). Carosi and Palmeri (2002), Carosi and Oggiano (2002), Carosi et al (2005) and Iacopini et al (2008) documented the presence of a wide dextral transpressional belt (the Posada-Asinara Line, PAL: Cappelli et al 1992) active around 315-320 Ma (Di . Moreover, we document in the strongly foliated boudins of metabasite have been documented, which generally preserved a complex metamorphic evolution from eclogitic, granulitic, amphibolitic to greenschist facies (Ricci and Sabatini 1978;Ghezzo et al 1979;Cappelli et al 1992;Oggiano and Di Pisa 1992;Franceschelli et al 1982Franceschelli et al , 2005 with references therein; Cortesogno et al 2004;Palmeri et al 2004;Giacomini et al 2005Giacomini et al , 2006.…”

“…Recent structural and kinematic studies, carried out in the western (Nurra region and Asinara Island: Carosi and Oggiano 2002;Carosi et al 2004;Iacopini et al 2008) and eastern transects (Baronie region: Carosi and Palmeri 2002;Di Vincenzo et al 2004) across the MGMC and HGMC highlight the presence of a major dextral postcollisional transpression regime with a top-to-the NW sense of shear. During this regime, a large amount of medium-and high-grade metamorphic rocks were exhumed by the compressive post-collisional processes (D2 and D3 deformation phases), whereas extensional tectonics were confined to the closing stages of orogenesis at upper crustal levels.…”