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

Abstract

The Anglona and SW Gallura regions represent key places to investigate the tectonic evolution of medium- and high-grade metamorphic rocks cropping out in northern Sardinia (Italy). From south to north we distinguish two different metamorphic complexes recording similar deformation histories but different metamorphic evolution: the Medium Grade Metamorphic Complex (MGMC) and the High Grade Metamorphic Complex (HGMC). After the initial collisional stage (D1 deformation phase), both complexes were affected by three contractional deformational phases (D2, D3 and D4) followed by later extensional tectonics. The D2 deformation phase was the most significant event producing an important deformation partitioning that produced localized shearing and folding domains at the boundary between the two metamorphic complexes. We highlight the presence of two previously undocumented systems of shear belts with different kinematics but analogous orientation in the axial zone of Sardinia. They became active at the boundary between the MGMC and HGMC from the beginning of D2. They formed a transpressive regime responsible for the exhumation of the medium- and high-grade metamorphic rocks, and overall represent a change from orthogonal to orogen-parallel tectonic transport

A balanced foreland–hinterland deformation model for the Southern Variscan belt of Sardinia, Italy

“…Accordingly, the former SSW-directed thrusts (early-D1) are best preserved in the nappe zone and within the greenschist belt, where they are partly buried by large W-directed tectonic units (Carmignani et al, 1978;Conti and Patta, 1998;Conti et al, 2001;Funedda, 2009). Instead, at the lower greenschist-amphibolite facies levels (inner zone), the evolution of the D1 phase leads, in many cases, to a generalized reactivation of early orogen-parallel thrusts within NNW-SSE left lateral transpressive shear zones which obliterate the SSW-directed structures almost entirely (Carosi and Oggiano, 2002;Carosi et al, 2009 Conti et al, 1999), which refolded the older structures. Equivalent upright folds have also been documented in the metamorphic complexes of the hinterland zone, indicating the onset of exhumation (Carosi and Oggiano, 2002;Casini and Oggiano, 2008).…”

“…Based on thermobarometry from the synkinematic mineral assemblages in metapelites and quartz/feldspathic paragneisses, Frassi (2006) and Casini et al (In press) suggest a temperature range between 580-6358C, with pressure of the order of 0.7-0.9 GPa for the D1 deformation in the GBSZ, rapidly followed by decompression at 500-5208C and $ 0.4 GPa. A strain gradient can be easily recognized in the field over a distance of about 2 km in the MGMC, from the garnet-to kyanite-in isograds (Carosi et al, 2009) close to the phyllonitic belt. This represents the inferred zone of maximum strain.…”

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)’

“…It is possible that migmatization continued after D1 during the decompression of the high-grade rocks as suggested by Cruciani et al (2008) and Carosi et al (2009). Carosi and Palmeri (2002) stressed the occurrence of a prograde D1 phase characterized by increasing P and T, during which medium-grade metamorphic rocks in NE Sardinia reached a pressure of nearly 10 kbar.…”

“…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).…”

“…The paper by Carosi et al (2009) reports many original structural data from the central-northern portion of the Variscan basement of Sardinia highlighting the occurrence of sinistral and dextral shear zones. The final interpretation also takes into account a huge amount of structural, kinematic, petrological and geochronological data recorded over the last ten years by the research group in Pisa across several transects in northern Sardinia.…”