Older than you think: Using U-Pb calcite geochronology to better constrain basin-bounding fault reactivation, Inner Moray Firth Basin, W North Sea

Like many rift basins worldwide, the Inner Moray Firth Basin (IMFB) is bounded by major reactivated fault zones including the Helmsdale and Great Glen faults (HF, GGF). The Jurassic successions exposed onshore close to these faults at Helmsdale and Shandwick preserve folding, calcite veining and minor faulting consistent with sinistral (HF) and dextral (GGF) transtensional movements, respectively. This deformation has widely been attributed to Cenozoic post-rift fault reactivation. Onshore fieldwork and U-Pb calcite geochronology of five vein samples associated with transtensional movements along the HF and a splay of the GGF show that faulting occurred during the Early Cretaceous (c. 128-115 Ma, Barremian-Aptian), whilst the HF preserves evidence for earlier Late Jurassic sinistral movements (c. 159 Ma, Oxfordian). This demonstrates that both basin-bounding faults were substantially reactivated during the episodic NW-SE-directed Mesozoic rifting that formed the IMFB. Whilst there is good evidence for Cenozoic reactivation of the GGF offshore, the extent of such deformation along the north coast of the IMFB remains uncertain. Our findings also illustrate the importance of oblique slip reactivation processes in shaping the evolution of continental rift basins given that this deformation style may not be immediately obvious in interpretations of offshore seismic reflection data. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6708518