Magnetic fabric of granitoids from Southern Corsica and Northern Sardinia and implications for Late Hercynian tectonic setting

Abstract

Anisotropy of magnetic susceptibility (AMS) data (180 sites) from Hercynian granitoids from Southern Corsica (France) and Northern Sardinia (Italy) provide a high-density net of structural information for this part of the European Hercynian belt. Two main groups of granitoids, defined according to petrography and relative chronology, provide contrasting AMS results. For both groups, shallowly dipping magnetic foliations and lineations suggest that these granitoids were emplaced as large-scale shallowly dipping sheets or open funnel-shaped structures at a high structural level. In non-leucocratic granitoids (305–300 Ma), foliation and lineation trajectory maps reveal antiforms and synforms with a N130 axial trace. Intersection of the foliation planes indicates a N120 low-plunging ‘fold’ axis, revealing a N120 maximum stretching direction. Slightly younger leucocratic granitoids (295–285 Ma), bounded by late Hercynian faults, display N50 lineations parallel to the elongated shapes of these intrusions. This structural pattern is undisturbed by the faults, and suggests an intrusion in decakilometre-scale tensional fractures indicating a N140 instantaneous stretching direction compatible with the stretching direction evidenced in non-leucocratic granitoids. A younger c . N20-oriented Permian dyke swarm confirms the permanence of this stretching direction over at least 35 Ma and indicates a coaxial strain regime. To integrate these new data within a broader context in the Hercynian belt, Corsica has to be back-rotated clockwise by 70° to its Permian position, which indicates a north to N30 stretching direction for the 305–285 Ma interval, compatible with the post-collisional strain and stress fields of the French Massif Central at about the same period.

Crustal exhumation during ongoing compression in the Variscan Maures-Tanneron Massif, France-Geological and thermo-mechanical aspects

“…290 Ma, e.g. Gattacceca et al, 2004), and that it is oriented orthogonal to the direction of convergence. The second possible exhumation process invokes transpression (e.g.…”

“…The system cannot be seen as 2D anymore. From 300 Ma (phase D4), NS extension occurs, with emplacement of U3 alkaline plutons in EW tensile fracture orthogonal to the previous lateral flow, suggesting a progressive transition from orogenic thinning to Permian rifting (see Appendix A, Gattacceca et al, 2004;Casini et al, 2015).…”

“…Rosenberg et al, 2007;Schmalholz et al, 2014) spread out towards these more compliant felsic domains, initiating strike-slip motion and heterogeneous structures that drove, ultimately, to the thinning of the orogen and locally to Permian rifting (eg. Gattacceca et al, 2004;Casini et al, 2015).…”

Evolution of the Corsica–Sardinia Batholith and late-orogenic shearing of the Variscides

“…1), nearly parallel to the orogenic zonation and to the transpressional D2 structures (Carmignani et al, 1994;Carosi and Oggiano, 2002;Casini and Funedda, 2014;Casini et al, 2010). Although there is only poor evidence for post-magmatic deformation, U2 plutons have a generally well-developed flat magmatic foliation and a gently plunging lineation almost parallel to the metamorphic fabric of country rocks Gattacceca et al, 2004). Most micro-granular mafic enclaves in U2 rocks are highly stretched along the magmatic fabric and have a tonalitic to granodioritic composition with biotite as the dominant Fe/Mg-bearing phase .…”

Structural map of Variscan northern Sardinia (Italy)

“…This recognized structural control motivated geologists to improve their knowledge about the geometry and timing of different plutons in relation to major orogenic events. Work has included detailed structural maps of Variscan batholiths that have been coupled to high-resolution geochronological studies in the French Massif Central (Faure, Cocherie, Mézème, Charles, & Rossi, 2010;Gébelin et al, 2009), External Crystalline Massifs on the western Alps (Rubatto, Ferrando, Compagnoni, & Lombardo, 2010), the Pyrenean region (Liesa et al, 2011;Vilà et al, 2010) and the Corsica-Sardinia block (Casini, Cuccuru, Maino, Oggiano, & Tiepolo, 2012;Gattacceca et al, 2004;Gébelin et al, 2009;Rossi & Cocherie, 1991). However, recent work on the Sardinian part of the Corsica-Sardinia Batholith have focused on petrological aspects related to the origin of melts (Cruciani, Franceschelli, Jung, Puxeddu, & Utzeri, 2008;Ferré & Leake, 2001;Gaggero, Oggiano, Buzzi, Slejko, & Cortesogno, 2007;Macera, Di Pisa, & Gasperini, 2011).…”