Foreland- and hinterland-verging structures in fold-and-thrust belt: an example from the Variscan foreland of Sardinia

Abstract

In the Variscan foreland of SW-Sardinia\ud (Western Mediterranean sea), close to the leading edge of\ud the nappe zone, nappe emplacement caused folding and\ud repetition of stratigraphic successions, km-scale offset of\ud stratigraphic boundaries and an extensive brittle-ductile\ud shear zone. Thrusts assumed a significant role, accommodating\ud a progressive change of shortening direction and\ud forming complicated thrust triangle zones. During thrust\ud emplacement of the nappes, strong penetrative deformation\ud affected rocks beneath the basal thrust of the nappe stack\ud and produced coeval structures with both foreland-directed\ud and hinterland-directed (backthrusting) shear sense. Crosscutting\ud and overprinting relationships clearly show that the\ud shortening direction changed progressively from N–S to E–\ud W, producing in sequence: (1) E–W trending open folds\ud contemporaneous with early nappe emplacement in the\ud nearby nappe zone; (2) recumbent, quasi-isoclinal folds\ud with axial plane foliation and widespread, ‘‘top-towardsthe-\ud SW’’, penetrative shearing; (3) N–S trending folds with\ud axial plane foliation, contemporaneous with late nappe\ud emplacement; (4) backthrusts and related asymmetrical\ud folds developed during the final stages of shortening,\ud postdating foreland-verging structures. Structures at (3)\ud and (4) occurred during the same tectonic transport ‘‘toptowards-\ud the-E’’ of the nappe zone over the foreland. The\ud several generations of folds, thrusts, and foliations with\ud different orientations developed, result in a complex finite\ud structural architecture, not completely explicable by the\ud theoretical model proposed up to date

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

“…This reflects SW-to Wdirected shear under sub-greenschist facies conditions (Funedda, 2009). Based on a thermometric analysis of synkinematic mineral assemblages from low-grade metapelites, Lüneburg (1995) calculated a temperature range of 250-3008C for the D1 deformation in the western area of the foreland.…”

“…The W-directed tectonics (late-D1) is most intense in the inner zone in particular. 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.…”

“…In most of the samples, discontinuous layers made up of quartz porphyroclasts are embayed within a matrix rich in muscovite, poorly deformed quartz grains and opaque minerals, thereby defining a cleavage-parallel banding which resembles primary compositional layering (Funedda, 2009). Although muscovite is apparently of detrital origin, as indicated by the lack of significant internal deformation apart from kinking (Figure 3b), crystals appear to have been passively rotated towards the cleavage plane, forming a strong shape preferred orientation (SPO).…”

Structural and metallogenic map of late Variscan Arbus Pluton (SW Sardinia, Italy)

“…This structure is the foremost W-directed thrust which separates an allochthonous pile of lower greenschist units from a para-authoctonous foreland affected by very lowgrade metamorphism (Funedda, 2009). Shallow crustal emplacement levels (2-4 km) are constrained by: (i) the occurrence of andalusite-cordierite-biotite contact aureole, the Al-in hornblende barometer in granodiorites (Anderson & Smith, 1995), which yield values close to about 1 Kb (Secchi et al, 1991).…”

“…The magmatic activity lasted between 340 and 285 Ma (Cocherie, Rossi, Fanning, & Guerrot, 2005;Paquette, Ménot, Pin, & Orsini, 2003). According to Cocherie et al (2005), building the batholith involved three main episodes corresponding to three different magmatic suites: (i) the oldest Mg-K suite (U1) formed between 340 and 336 Ma, (ii) the calcalkaline suite (U2) between 320 and 290 Ma (Casini, Cuccuru, Maino, et al, 2015;Casini, Cuccuru, Maino, Oggiano, & Tiepolo, 2012) and (iii) a late-Variscan composite suite (U3) including alkaline and layered tholeitic bodies mostly emplaced around 285 Ma Gaggero, Oggiano, Buzzi, Slejko, & Cortesogno, 2007;Paquette et al, 2003). Despite the recent improvement of geochronological and geochemical data, the knowledge of the Sardinia portion of the batholith is still far from complete, as large areas, they lack detailed geological and structural field mapping.…”