DATING OF SHEAR ZONE ACTIVITY IN THE CYCLADES, GREECE: EXAMPLES FROM THE ISLANDS OF SYROS AND ANDROS

The Attic-Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic collisional processes and extension-related exhumation. A prerequisite to develop an improved tectonic history of this area is dating of displacement that occurred along major shear zones. We report here results of a Rb–Sr and U–Pb study that focused on tectonic activity on the islands of Syros and Andros.

On Syros, a mélange comprising a wide variety of well-preserved HP rock types (e.g. eclogites, glaucophanites, jadeitites) mostly occurs in a well-defined and mappable horizon that is fault-bounded on top and at its base. Although blocks with blackwall alteration at contacts with serpentinitic rocks are common, the matrix predominantly consists of clastic metasediments. In order to further evaluate the timing of deformation in the mélange, we have studied 8 samples that represent matrix-forming metasediments from the mélange exposed in the northern part of the island. The mineral assemblages of the studied rocks correspond to epidote blueschist facies P–T conditions. Rb–Sr mineral isochrons indicate apparent ages between c. 49 Ma and c. 42 Ma. Judging from the field relationships and corresponding age pattern, we consider deformation-related and fluid-enhanced mica recrystallization as the most likely interpretation for this age variability. The youngest apparent ages were recognized in samples from block-rich zones that are closely associated with layers of altered ultramafic rocks. These parts of the mélange preferably accomodated deformation in localized domains of intense shearing. The associated transformation of serpentinites into chlorite-, talc- and actinolite-rich schists required not only deformation, but ingress of substantial amounts of fluids, documenting that such zones also represent important fluid conduits. Shearing most likely affected adjoining rock volumes consisting of schistose metasediments, creating favourable conditions for recrystallization processes in zones up to several meters in width. Non-pervasive fluid flow was strongly channelized into horizons with enhanced permeability. The age differences testify to different degrees of dynamic recrystallization and fluid-rock interaction. This study indicates that the Syros mélange experienced a major episode of localized deformation and fluid infiltration at c. 42 Ma.

On Andros, the metamorphic succession can be subdivided into two tectonic units, the Makrotantalon Unit and the Lower Unit. The Lower Unit can be correlated with the Cycladic blueschist sequences, which experienced HP/LT metamorphism during the Eocene and a greenschist-facies overprint at c. 23-21 Ma. The P–T evolution of the Makrotantalon Unit is poorly constrained, but apparently did not include a HP/LT stage. During the last metamorphic overprint greenschist facies conditions were attained. Previously published Rb–Sr data suggest that the Makrotantalon Unit experienced two periods of Cretaceous metamorphism (c. 100–90 Ma and c. 80–70 Ma) and a Miocene event (c. 21 Ma). Both units are separated by a low angle fault, which is well exposed in some outcrops along the NE coast of Andros, but more difficult to localize in the NW part of the island. Within the broader fault zone of NW Andros, a weak angular unconformity decorated with cm-thick veins of cohesive cataclasites cuts through the metasedimentary succession of the Lower Unit.

Rb–Sr geochronology was carried out on 9 samples from the Lower Unit representing clastic metasediments, calc schists and greenschists that were collected within a distance of <100m to the fault zone. In the NW part of the island, Rb-Sr phengite ages vary between 56 and 30 Ma, with the youngest apparent age representing a sample collected close to the inferred tectonic contact. In the NE area, 4 samples collected at or close to the detachment yielded a relatively narrow range of apparent ages (c. 29– 25 Ma). This age group is considered to closely approximate the time of tectonic juxtaposition and suggests that ductile activity along the detachment is older than previously assumed.

U-Pb dating of detrital zircon indicates maximum depositional ages of c. 260 Ma for the Makrotantalon Unit and of c. 170–160 Ma for the Lower Unit. These age constraints are consistent with previous interpretations suggesting an inverted tectonostratigraphy – rocks at the top of the succession are older than the structurally lower sequences – implying that the contact between both units originally represented a thrust fault. During large-scale regional extension, this contact was reactivated as a low-angle normal fault.