CONSTRAINING THE TECTONIC EVOLUTION OF MAJOR EXTENSIONAL FAULT SYSTEMS IN THE AEGEAN (GREECE), USING THERMOCHRONOLOGY. EXAMPLE OF NAXOS/PAROS EXTENSIONAL FAULT SYSTEM

Rates of tectonic processes are of fundamental importance for understanding deformation of the lithosphere. We have examined the slip history of major extensional fault systems exposed in the Cyclades.

The unusually complete Naxos/Paros extensional fault system (NPEFS) consists of a thick lower-crustal ductile shear zone grading upwards into a thin brittle detachment. It presents an ideal opportunity to constrain slip rates from deep-crustal to near-surface conditions by comparing published slip rates for the ductile crust with new thermochronologic constraints on slip rates in the brittle regime. Based on apatite and zircon fission track (AFT and ZFT) and (U-Th)/He (ZHe and AHe) dating we recognize an increase in the slip rate across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8±0.8 to 9.7±0.8 Ma and 11.2±1.6 to 8.2±1.2 Ma while ZHe and AHe ages are between 10.4±0.4 to 9.2±0.3 Ma and 10.7±1.0 to 8.9±0.6 Ma. On Paros, ZFT and AFT ages range from 13.1±1.4 Ma to 11.1±1.0 Ma and 12.7±2.8 Ma to 10.5±2.0 Ma while the ZHe ages are slightly younger between 8.3±0.4 Ma and 9.8±0.3 Ma. These anomalously young ZHe ages might be explained by U-enriched rims observed in zircon fission-track mounts. Therefore, using a traditional F_T correction assuming homogeneous U and Th distribution would lead to an underestimation of the F_T correction and consequently of our ZHe ages. Nevertheless, all ages consistently decrease northwards in the direction of hangingwall transport. Published slip rates of ~5-6 km Myr^-1 constrain the speed of the NPEFS in the ductile regime between 16-12 Ma on Naxos. When integrated with our new data it seems that the slip rate increased from ~5-6 km Myr^-1 to ~8-9 km Myr^-1. Based on published modeling work we propose that the brittle/ductile transition is a main storage of elastic energy and a local heat source.

The other data obtained in the Cyclades demonstrated that the main period of detachment faulting and magmatism activity are between ~15-10 Ma without specific pattern of the timing of extension. This intimate relationship between arc-related magmatism and detachments has been aided by high thermal gradients and extensional stresses (subduction zone retreat). Excepted on Naxos, Miocene normal faulting did not cause much exhumation (15-10%) but were the primary agent for opening Aegean Sea.