Combined kinematic, structural and palaeostress (calcite twinning, fault-slip data) analyses are used to study the exhumation mechanism of the high-pressure rocks exposed on the island of Crete (southern Aegean, Greece). Our study shows that the evolution of windows in central Crete was controlled by two main contractional phases of deformation. The first phase (D1) was related to the ductile-stage of exhumation. NNW–SSE compression during D1 caused layer- and transport-parallel shortening in the upper thrust sheets, resulting in nappe stacking via low-angle thrusting. Synchronously, intracontinental subduction led to high-pressure metamorphism which, however, did not affect the most external parts of the southern Hellenides. Subsequent upward ductile extrusion of high-pressure rocks was characterized by both down-section increase of strain and up-section increase of the pure shear component. The second phase (D2) was associated with the brittle-stage of exhumation. D2 was governed by NNE–SSW compression and involved conspicuous thrust-related folding, considerable tectonic imbrication and formation of a Middle Miocene basin. The major D2-related Psiloritis Thrust cross-cuts the entire nappe pile, and its trajectory partially follows and reworks the D1-related contact between upper and lower (high-pressure) tectonic units. Eduction and doming of the Talea Window was accompanied by gravity sliding of the upper thrust sheets and by out-of-the-syncline thrusting. Late-orogenic collapse also contributed to the exhumation process. Therefore, it seems that the high-pressure rocks of central Crete were exhumed under continuous compression and that the role of extension was previously overestimated.

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Microstructural, petrofabric, strain and vorticity data from quartz-rich tectonites were used to investigate the kinematics of rock flow in the Evia and Ochi ductile thrust zones, formed during exhumation of the high-pressure nappes of the Attico-Cycladic Massif. The Evia thrust zone defines the base of the Styra nappe while the Ochi thrust zone defines the contact between the Styra and the overlying Ochi nappe. A dominant top-to-the-ENE sense of shearing along both thrust zones is indicated by several shear sense criteria. Deformation in the structurally deeper Evia thrust zone occurred under approximately plane strain conditions and was characterized by a R XZ strain ratio varying from 3 to 6. The vorticity profile above the thrust plane shows a slight down-section increase in the kinematic vorticity number (W m ) from 0.8 to 0.9, as well as the presence of local thin domains with a higher pure shear component of deformation. In the overlying Ochi thrust zone, a downward increase in W m values from 0.6 to 0.9 is detected both above and below the thrust plane. Here, rocks have been deformed in the general constrictional field with R XZ values ranging between 5 and 8. A transport-parallel elongation of 30-90% and 50-160% has been estimated for the Evia and Ochi thrust zones, respectively, implying that ENE-directed extrusive flow controlled the formation, stacking and exhumation of the Styra and Ochi nappes.

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[1] Detailed geological mapping, structural investigation and amphibole chemistry analyses in southern Evia (Aegean Sea, Greece) allow us to place new constraints on the internal structural architecture and tectonic evolution of the Cycladic Blueschists. We show that the early deformation history was related to ESE directed thrusting resulting in the stacking of the Styra and Ochi nappes, which constitute the Cycladic Blueschist unit in Evia. These early thrust movements initiated just before and proceeded at peak conditions of the Eocene high-pressure metamorphism. Subsequent constrictional deformation gave rise to E-W trending upright folding accomplished at the early exhumation stage. The main ductile-stage exhumation occurred during a single deformation phase associated with the decompression of blueschist rocks from the stability field of crossite to that of actinolite. This phase was characterized by localization of ductile deformation into a series of major, tens of meters thick, ENE directed shear zones, which cut up-section in their transport direction and restack the early thrust and fold sequence, locally bringing the structurally lower Styra nappe over the higher Ochi nappe. It is suggested that these zones operated as thrusts rather than normal sense shear zones as has been previously argued and were possibly formed during the Oligocene ENE-ward extrusion of the blueschists. Brittle-ductile NE dipping normal faulting of post-early Miocene age was probably responsible for the final exhumation of rocks.Citation: Xypolias, P., I. Iliopoulos, V. Chatzaras, and S. Kokkalas (2012), Subduction-and exhumation-related structures in the Cycladic Blueschists:

with concordia ages at 291 ± 3, 300 ± 1 Ma (Rogdia) and 286 ± 3, 300 ± 3, 313 ± 2 Ma (Bali). Both types of metasediments and their zircons are similar to those of the pre-Alpine basement and overlying Tyros Beds of eastern Crete, revealing a provenance at the southern active margin of Laurasia. Thus, in central Crete the Paleotethys suture should be situated inside the Rogdia Beds. Magmatic zircons separated from a rhyolite boulder of the lower Achlada Beds yielded a concordant U-Pb zircon age at 242 ± 2 Ma placing a maximum age for the deposition of the (meta) conglomerate from which the boulder was collected. This age is compatible with an Olenekian-early Anisian age of the underlying Vasilikon marble suggested by new findings of the foraminifera Meandrospira aff. pusilla. Both the Achlada Beds and the Vasilikon marble can be attributed to the lower Tyros Beds of eastern Crete. The Alpine deformation led to a pervasive mylonitic foliation, which is affecting most of the studied rocks. This foliation results from D2 top-to-the-north shearing, which post-dates the growth of blue amphiboles (crossite).

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