Geochemical and petrographic variations in pseudotachylyte along the Outer Hebrides Fault Zone, Scotland

Abstract

The islands of the Outer Hebrides, Scotland host a c. 200 km long fault zone that was active between 1100 and 250 Ma, during which time a belt of fault rock several kilometres wide was created. The fault zone hosts crush breccias, phyllonite, mylonite and pseudotachylyte. The latter unit is often found in major fault zones and within meteorite impact structures. Although pseudotachylyte is widely believed to be a product of frictional melting within a fault zone, there is still controversy over the specific mechanisms of formation. Pseudotachylyte was collected from the southern block of the Outer Hebrides Fault Zone. A geochemical mixing model was applied to reconstruct the geochemistry of the pseudotachylyte matrix using the locally available lithologies. The results show that the pseudotachylyte could have been derived from the local country rock, although some mixing was necessary to provide accurate reconstructions, suggesting a degree of mixing between melts derived from different protoliths. Petrographic observations identified additional characteristics of melt transportation, including transported clasts, aligned fragments, melt segregation and injection of the melt into tensile fractures. The latter probably served as the transport mechanism, drawing melt into the low-pressure tensile structures that developed adjacent to the melt-forming zones within the fault plane. Supplementary Material: Sample site co-ordinates and electron microprobe settings are available at: https://doi.org/10.6084/m9.figshare.c.4567931