The unroofing history of Naxos and Paros: Constraints and questions from thermochronology and thermal modeling

Abstract

The Naxos/Paros extensional fault system (NPEFS) accommodated a significant portion of Miocene extension in the Aegean. The exposure is unusually complete, consisting of a thick lower-crustal ductile shear zone grading upwards into a thin, brittle detachment. Thermochronometric systems with closure temperatures documenting passage of the footwall through the brittle (apatite and zircon fission-track and (U-Th)/He) and ductile (hornblende K-Ar) zones provide important constraints on the unroofing history of this system. Brittle-zone slip rates based on fission-track data are generally in the 6–8 km/Myr range spanning times from 13–8 Ma, whereas inferred ductile-zone slip rates average ~5 km/Myr from 16–12 Ma. We used finite element thermal modeling to test the hypothesis that Naxos and Paros unroofed simultaneously and at similar rates, from ~16 Ma to ~8 Ma, with the only difference being a ~12 Ma granodiorite intrusion on Naxos. We found that a simplified model with uniform detachment fault dip (30°) and slip rate (6.5 km/Myr.) adequately explains all of the fission-track and some zircon (U-Th)/He data, although other (U-Th)/He data are problematic. The data are somewhat more fully reproduced if there was some acceleration during the latter stages of unroofing, but large uncertainties in slip rates preclude a firm conclusion. The thermal model also provides a reasonable mechanism for unifying inferred brittle-zone slip rates with slower ductile-zone ones, as the latter may have been rendered artificially low due to isotherm advance during the early stages of detachment faulting. At the same time, the modeling exercise forces a thorough assessment of the thermal budget of the crust before and during the initiation of detachment faulting. Other data have suggested that unroofing began as early as 20 Ma, and that early decompression was associated with heating. Insofar as unroofing is only in itself a cooling mechanism for the footwall, the source of this additional heat, and what happened to it, is a matter requiring further investigation before a complete picture of extension on the NPEFS can be formulated.