Upward extrusion and subsequent transpression as a possible mechanism for the exhumation of HP/LT rocks in Evia Island (Aegean Sea, Greece)

Abstract

Structural, kinematic and strain-path analyses were used to elucidate how strain was accommodated at deep tectonic levels during the exhumation of high-pressure/low-temperature (HP/LT) units of the Attico-Cycladic-Massif (ACM), which are exposed on Evia Island (West Aegean Sea, Greece). These analyses are combined with data from the structural evolution of the overlying non-metamorphic belt (Pelagonian) to provide new insights into the orogenic evolution of this region. According to the proposed model, the exhumation of the HP/LT-rocks in the Evia area occurred under a mechanism which includes upward extrusion and subsequent transpression. In the study area, a continent–continent collision began during the Eocene involving the subduction of the protolith of the Evia Blueschist Unit (EBU) beneath the Pelagonian microcontinent. Continued compression and progressive underthrusting of the Almyropotamos passive continental margin through Oligocene resulted in the successive east-directed ductile extrusion of the allochthonous EBU, derived from the deepest underthrust crustal parts. Therefore, at the Oligocene/Miocene boundary the EBU emplaced tectonically over the Almyropotamos Unit (AU) and the latter underwent a mild HP-metamorphism. During this extrusion process, the EBU underwent intensive deformation under plane strain conditions, which partitioned into two homogeneous domains: (a) a root zone characterized by pure shear dominated deformation, and (b) a frontal flat-lying domain where deformation includes a high simple shear component. Starting at the early Miocene and extending into the Middle Miocene the nappe pile was caught in transpression, which led to the development of the Pelagonian Fault. Dextral transpressional shearing along this major fault caused further eduction and doming of the HP-units, juxtaposing these against weakly metamorphosed rocks of the uppermost tectonic unit (Pelagonian). Transpression occurred under continuous cooling and transferred into localized, distinct deformation zones close to the Pelagonian Fault. Strain-path analyses show an increasing component of flattening strain within these shear zones.