Constraining the timing of fault reactivation: Eocene coseismic slip along a late ordovician ductile shear zone (northern Victoria Land, Antarctica).

The reactivation of faults and shear zones is a widely documented process and represents a fundamental characteristic of deformation in the continental lithosphere. The Ross Sea in Antarctica mainly owes its present-day configuration to an extended period of early Paleozoic subduction-related crustal accretion and the subsequent Mesozoic-Cenozoic tectonics of the West Antarctic Rift System. It thus represents an ideal setting in which to study reactivation processes. This study uses the 40Ar-39Ar laserprobe technique in conjunction with mesostructural, microtextural, and nanotextural analyses to unravel the style and timing of a newly discovered mylonite-pseudotachylyte association from the rift shoulder in northern Victoria Land of Antarctica. Pseudotachylyte-bearing fault rocks overprint a reverse ductile shear zone developed in a Cambrian granite, within an overall transpressional right-lateral kinematic scenario. In situ 40Ar-39Ar analyses of biotite from the mylonite mainly yielded ages of 460-440 Ma, suggesting that ductile shearing occurred during the early Paleozoic orogenic cycle. In contrast, in situ data on the pseudotachylyte matrix yielded scattered and much younger ages of ca. 120-66 Ma. Younger ages of ca. 50 Ma were derived from step-heating experiments on matrix specimens obtained through microtexturally controlled microsampling. Taking into account the possible effects of analytical artifacts due to sample irradiation and of Ar loss due to the ultrafine grain of the matrix, coseismic faulting is assigned to the middle Eocene. Results indicate a minimum 390 m.y. period of tectonic quiescence and prove that brittle reactivation occurred in response to a totally different stress regime. Regionally, the study confirms a post-early Eocene age for the activation and propagation of intraplate dextral strike-slip tectonics in Victoria Land.