The Outer Hebrides Fault Zone (OHFZ) is an east-dipping, major crustal structure extending >170 km along the eastern edge of the Outer Hebrides, Scotland. Opinions on the age of the structure mostly range from early Proterozoic to mid-Paleozoic. In order to better understand the evolution of this structure, we have focused on fault rocks, particularly pseudotachylyte, and dated samples from nine different localities along the length of the OHFZ.

Pseudotachylytes studied were obtained from fault veins varying from a few mm to several m thick. Nearly all are dendritic to coarsely microlitic, mostly with varying ratios of amphibole (less commonly pyroxene) to intermediate plagioclase, with accessory magnetite. We have analyzed mg-scale samples using 40Ar-39Ar laser step heating. In general, the following patterns are present in the age spectra. Low temperature steps show minor Ar loss in a few samples, suggesting slight Paleozoic resetting. Most of the samples show intermediate temperature steps that are plateau-like to slightly climbing, with ages of 0.9-1.1 Ga. High temperature steps, particularly in the last 30% of 39Ar, show excess Ar in most samples.

Collectively, these results are consistent with formation of nearly all samples at ca. 1 Ga. Only one sample suggests a late Proterozoic or very early Paleozoic age, whereas two or three samples hint at Paleozoic(?) Ar loss. These results suggest all or nearly all of the analyzed pseudotachylytes are Grenvillian, and thus while reactivation is an important part of the history of the OHFZ, the major strands of the OHFZ were active at about 1 Ga. This contrasts with the currently favored Caledonian (Paleozoic) age for the OHFZ.