Syn- to post-orogenic exhumation of metamorphic nappes: Structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)

Abstract

The Lavrion peninsula is located along the western boundary of the Attic-Cycladic metamorphic complex in the internal zone of the Hellenic orogenic belt. The nappe stack is well exposed and made, from top to bottom, of (i) a non-metamorphic upper unit composed of an ophiolitic melange, (ii) a middle unit mainly composed of the Lavrion schists in blueschist facies, (iii) and a basal unit mainly composed of the Kamariza schists affected by pervasive retrogression of the blueschist facies metamorphism in greenschist facies. The middle unit is characterized by a relatively steep-dipping foliation associated with isoclinal folds of weakly organized axial orientation. This foliation is transposed into a shallow-dipping foliation bearing a N-S trending lineation. The degree of transposition increases with structural depth and is particularly marked at the transition from the middle to the basal unit across a low-angle mylonitic to cataclastic detachment. The blueschist facies foliation of the Lavrion schists (middle unit) is underlined by high pressure phengite intergrown with chlorite. The Kamariza schists (basal unit) contains relics of the blueschist mineral paragenesis but is dominated by intermediate pressure phengite also intergrown with chlorite and locally with biotite. Electron probe micro-analyzer chemical mapping combined with inverse thermodynamic modeling (local multi-equilibrium) reveals distinct pressure–temperature conditions of crystallization of phengite and chlorite assemblages as a function of their structural, microstructural and microtextural positions. The middle unit is characterized by two metamorphic conditions grading from high pressure (M1, 9–13kbar) to lower pressure (M2, 6–9kbar) at a constant temperature of ca. 315°C. The basal unit has preserved a first set of HP/LT conditions (M1–2, 8–11kbar, 300°C) partially to totally transposed-retrogressed into a lower pressure mineral assemblage (M3, 5–8.5kbar) associated with a slight but significant increase in temperature (∼350°C).These structural, petrologic and thermobarometric data document the tectonic evolution from construction to destruction of the Hellenic orogenic belt. The steep-dipping blueschist facies foliation, preferentially preserved at high structural level in the middle unit, is considered to reflect tectonic accretion marked by successive burial (D1M1) and syn-orogenic exhumation (D2M2) without thermal relaxation. The transposition-retrogression of the blueschist facies mineral assemblage into a shallow-dipping greenschist facies foliation associated with an increase in temperature corresponds to lateral flow of the thermally relaxed nappe stack (D3M3). The development of a low-angle detachment, accommodating post-orogenic exhumation of the orogenic root, is attributed to gravitational collapse of the Hellenic belt.