The Northwest Extension of the West Cycladic Detachment System, Attica, Greece

Abstract

In the central Aegean of Greece, metamorphic rocks have been brought to the surface along multiple detachment faults producing a series of core complexes. The detachment faults have been grouped into several detachment systems including the North and West Cycladic Detachment Systems. The timing and magnitude of displacement along individual detachment faults is variable, the faults exhumed footwall rocks with a range of metamorphic grades from different crustal depths. Geochronology (zircon (U-Th)/He and white mica ⁴⁰Ar/³⁹Ar analyses) was conducted to investigate the timing of activity along a paired detachment system exposed in the bedrock of Mt. Hymittos, Attic Peninsula, Greece. Taken together the geochronometers indicate the detachments of Hymittos were active from the late Oligocene to the late Miocene with both faults active during the middle Miocene. The ductile-to-brittle deformation of the structures, top-SSW kinematics, morphology of the massif, and metamorphic grade indicate that these detachments are extensions of the West Cycladic Detachment System. Additionally, the overlap in timing between the structures indicates that paired detachment systems can be coeval. Coeval displacement along separate detachment branches has important implications for strain partitioning within the crust and the development of detachment systems. Detrital zircon U-Pb geochronology was conducted on samples from exposures of the West Cycladic Detachment System on the Attic Peninsula. The results are compared to unpublished data from the Attic Peninsula and the Western Cyclades including Santorini. The analyses confirm the correlation of the footwall rocks of Hymittos to the Cycladic Blueschist Unit as well as evaluate several paleogeographic correlations for the Cycladic Blueschist Unit. The samples herein record significant recycling of Variscan zircon from the Cycladic Basement, strongly suggesting the proto-Cycladic Blueschist Unit was deposited atop the Cycladic Basement. The Pindos Zone of the Hellenides is the most likely paleogeographic equivalent to the Cycladic Blueschist Unit. Strengthening this correlation gives additional insight into the position of the proto-Cycladic Blueschist Unit which is important for understanding both the movement of Hellenic terranes through time, and the pre-subduction relationship(s) between the terranes that today comprise the Attic-Cycladic Complex. Finally, electron backscatter diffraction analysis was utilized to examine the mechanisms of deformation in a suite of calcitic and dolomitic marble mylonites from the detachments of Hymittos. This was used to shed light on the conditions under which these ductile-then-brittle structures deformed suggesting that dolomite deformed under lower to sub-greenschist facies conditions may be surprisingly weak when deforming by grain size sensitive processes. Dolomite is a major rock forming mineral and comprises significant parts of carbonate dominated terranes and (meta)sedimentary successions. The apparent weakness of dolomite reported herein suggests that under greenschist and sub-greenschist facies conditions dolomite may accommodate significant strain not predicted by experiments or commonly reported from the field. Together this thesis provides insight into detachment fault architecture, deformation processes and the pre-subduction dynamics and structural position of the geologic units exposed within the Attic-Cycladic Complex.