Abstract

The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U-Th)/He dating we observe variable slip rates 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 and (U-Th)/He zircon and apatite ages are between 10.4 +/- 0.4 to 9.2 +/- 0.3 Ma and 10.7 +/- 1.0 to 8.9 +/- 0.6 Ma, respectively. 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 (U-Tb)/He zircon ages are slightly younger between 8.3 +/- 0.4 Ma and 9.8 +/- 0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of similar to 6-8 km Myr(-1) on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at similar to 12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of similar to 50 km between similar to 16 and 8 Ma. (c) 2005 Elsevier B.V. All rights reserved.