Fry and Rf/ϕ strain methods constraints and fold transection mechanisms in the NW Iberian Variscides

Abstract

a b s t r a c tApúlia is a small Portuguese sector in NW of Central-Iberian Zone, that have been deformed in a noncoaxial sinistral transpressive regime during the first and main Variscan tectonic event (D 1 ). This deformation give rise to a major NWeSE anticline, where the S 1 NeS cleavage transect the inverted short NE limb; two and three-dimensional strains analysis have been done in the low metamorphic grade Ordovician quartzites of this limb using Fry and Rf/f methods. The data show that most deformation was due to intergranular deformation mechanisms. The intragranular deformation leading to the distortion of strain markers and to cleavage was very incipient and a latter event in the D 1 phase. The apparent plane strain ellipsoids (if no volume change is assumed) related to the intragranular mechanisms contrast with the more prolate strain ellipsoids related to the bulk deformation of Apúlia Quartzites. This constrictional bulk strain fabrics are characteristic of the sinistral transpressive regimes dominant in the northern sectors of the Central-Iberian Zone.

Determining finite strain: how far have we progressed?

“…This non-passive deformation can be accounted for by utilising centre-to-centre methods, which include spatial information in the estimates of strain, and provide bulk-rock strain estimates that are closer to true strain values. This has been illustrated in a range of natural settings (Meere et al, 2008, Soares andDias, 2015). Meere et al (2008) attributed non-passive deformation to the presence of a relatively incompetent clay-rich matrix, which effectively cushioned clasts from internal deformation.…”

Constraining of strain ellipsoid shape from sectional data in the Au bearing shear zone west of Iran

“…Ductile Shear zones are the widespread structure that localized strain and accommodate deformation in the lithosphere (Ramsay and Graham, 1970;Ramsay, 1980;Simpson, 1981;Shan, 2008;Fossen and Cavalcante, 2017). These zones provide valuable data about the progressive deformation history, such as mineral assemblage and microstructures, P-T conditions and shear sense indicator.…”

“…Primitive research in ductile shear zone proposed that the heterogeneous simple shear is a dominate deformation regime (Ramsay, 1980;Ramsay and Allison, 1979;Ramsay and Graham, 1970;Simpson, 1981;Watts and Williams, 1983) but recent research indicates that in most of the shear zones involve threedimensional combinations of simple and pure shear strain, such as transpression and transtension zone (Bhattacharyya and Hudleston, 2001;Montési, 2013;Liang et al, 2015;Fossen and Cavalcante, 2017;Behyari and Moghadam, 2018).…”