Re-evaluating the South Cyclades Shear Zone, Ios, Greece

Abstract

Metamorphic core complexes (MCC) are large-scale extensional structures that exhume mid- to lower- crustal rocks beneath low-angle detachment systems and provide important insight into the processes of crustal deformation, metamorphism, and tectonic exhumation. The island of Ios, Greece, has long been regarded as a classic example of an MCC, where exhumation of the Cycladic Basement (CB) has traditionally been attributed to Miocene displacement along the so-called South Cyclades Shear Zone (SCSZ). However, the structural evolution of the island remains controversial due to the preservation of opposing kinematic indicators, and complex overprinting relationships. This study re-evaluates the tectonic evolution of Ios through detailed field mapping, structural analysis, petrography, and in-situ white mica ⁸⁷Rb/⁸⁷Sr geochronology. Emphasis is placed on the relationships between folding, foliation development, shearing, and the nature of the contact between the Cycladic Basement and Cycladic Blueschist Unit (CBU). Field observations demonstrate that both units preserve a pervasive schistose foliation (S₂) that is cut by a regionally developed axial planar cleavage (S₃), which overprints mineral assemblages and mylonitized fabrics. Opposing top-to-north and top-to-south kinematic indicators occur within the same lithologies, structural domains, and are commonly preserved with folded fabrics. These relationships suggest that many apparent kinematic reversals result from structural rotation during folding rather than recording discrete episodes of opposing shear. White mica in-situ ⁸⁷Rb/⁸⁷Sr geochronology from both units suggests a similar exhumation history, where retrograde related recrystallization occurred between c. 30-20 Ma, matching similar ages previously published for the island. The CB-CBU contact lacks a consistent strain gradient, metamorphic discontinuity, and structural continuity expected of a crustal-scale detachment zone. Instead, it is interpreted as a folded lithological boundary that shared a common tectonometamorphic and exhumation history. A younger top-to-north Headland Shear Zone locally overprints earlier fabrics and represents the latest recognized phase of ductile deformation on the island. Collectively, these observations challenge traditional interpretations of Ios as a classical metamorphic core complex and highlight the importance of structural inheritance, polyphase deformation, and fold-related modification of kinematic indicators when reconstructing the evolution of extended continental crust.