Strain partitioning during 3D general shear in a heterogeneous low-angle shear zone: some hold strong, while the schists fall flat

General shear, wherein deformation incorporates elements of both coaxial and non-coaxial strain, is a prevalent strain regime in natural high-strain zones. In extensional tectonic settings, three-dimensional forms of general shear may enhance exhumation via additional crustal thinning (i.e., via pure shear or flattening strain components) or counteract it by inducing crustal thickening (i.e., via a constrictive strain component), without necessarily producing a conspicuous crustal-scale shear zone or fault. Schists and phyllonites demarcating a major tectonic boundary between thrust sheets on Evia in the NW Cyclades record structural evidence for general shear with a NE-directed non-coaxial component and contemporaneous flattening. The package of rock accommodating this strain is lithologically heterogeneous, comprising intercalations of carbonate-, quartz-, and phyllosilicate-dominated schist, as well as dispersed m- to hm-scale olistoliths and blocks of marble and metabasite. Flattening in these rocks is exemplified by foliation-oblique quartz ± calcite veins exhibiting pinch-and-swell or boudinage structure alongside dominant bidirectional dips perpendicular to the regional NE-SW stretching lineation. We combine in-situ 40Ar/39Ar and 87Rb/87Sr dating of white mica with quartz c-axis petrofabric analysis of the deformed quartz veins to elucidate the timing and styles of deformation recorded by these rocks. White mica provides mainly late Oligocene 40Ar/39Ar dates in samples with a single dominant foliation, whereas mica defining composite or crenulated foliations records late Eocene-early Oligocene dates, or age populations spanning the Oligocene. Some samples record dispersed Paleocene-Eocene dates older than the earliest proposed timing of metamorphism, although white mica from these rocks provides more geologically plausible early Oligocene 87Rb/87Sr dates. Vein quartz c-axis fabrics consist primarily of c-axis maxima or small-circle girdles centered about the Z-axis, with subordinate fabrics defining top-to-NE asymmetric type-I cross girdles or Y-axis maxima. Considered together with vein macro- and micro-structure, our data indicate that the deformed schists accommodated top-to-NE general shear at temperatures only slightly above 300°C, resulting in an oblate finite strain ellipsoid. Deformation over this interval produced differential transposition of earlier tectonic fabrics and structures into a sub-horizontal penetrative cleavage in the rheologically weak mica schists, whereas sections dominated by more quartzose- and carbonate-rich lithotypes display comparatively well-preserved older foliations and structures and a spaced secondary cleavage. The prevalence of late Oligocene 40Ar/39Ar dates in samples exhibiting a single, shallowly-dipping micaceous foliation implies that flattening general shear coincided with, and likely helped facilitate, exhumation. Our data indicate that unroofing may be partly facilitated by inconspicuous zones accommodating distributed inhomogeneous strain, a potentially important observation for exhumed subduction zones featuring prevalent block-in-matrix mélanges.

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