ESSENTIAL ROLE OF INITIAL PENETRATIVE PLANAR FABRIC IN CREATING CONDITIONS FOR “STRAIN SATURATION” DURING PROGRESSIVE OR POLYPHASE DEFORMATION (Invited Presentation)

Low-grade metamorphic assemblages commonly acquire an initial penetrative mesoscopic foliation whose fabric properties attract activation by a variety of deformation mechanisms during subsequent tectonic loading. Coaxial deformation within cores of orogens and non-coaxial shear within regional shear zones can drive transposition of original bedding (S₀) through isoclinal folding, attenuation of fold limbs, and pressure-dissolution-aided creation of penetrative axial planar foliation (S₁). Progressive and/or subsequent deformation of fine-grained QF- and M-domains within S₁ commonly produces additional mass transfer dissolution creep, expressed as crenulation cleavage (S₂) that overprints S₁. Moreover, schistose, phyllitic, and slaty S₁ fabrics are readily activated by frictional sliding, manifest in kink, flexural-slip, and interference folds. Mesoscopic structures and fabrics can proliferate to occupy almost every part of the rock assemblage. When viewed macroscopically, the distribution of these structures and fabrics constitutes ‘strain saturation’ as opposed to ‘strain localization.’

This same framing can apply to certain deformed sedimentary sequences. An example is seen in the presence of mesoscopic structures and fabrics in Thick White Limestone Beds Formation (TWLBs) of Cretaceous age in the Pindos Belt, Peloponnesos, Greece. In addition to outcrops, Classical-Period worked blocks and column drums provide essential records. Mass transfer dissolution creep during burial and compaction replaced true bedding (S₀) with pseudo-bedding bounded by closely spaced (~3–9 cm) stylolitic surfaces (S₁). During tectonic loading (beginning in late Maastrictian), pseudo-bedding accommodated layer-parallel shortening (LPS), achieved diachronously through dissolution creep along very closely spaced tectonic stylolites (S₂) and mesoscopic buckle folds (F₁). Amplification and tightening of quasi-flexural buckle folds were impeded by resistance to flexural slip offered by sutured stylolitic surfaces (S₂), and instead were achieved by intense dissolution by additive tectonic stylolitization (S₃) and by isoclinal folding (F₂). TWLB thus became saturated with strain, even before the onset of in-sequence regional thrust faulting and associated fault-propagation folding (F₃).