Matteo Demurtas

A thirsty fault: prolonged fluid flow along the Helmsdale Fault

Matteo Demurtas

Within syn-rift basins, the juxtaposition of rift-related clastic deposits in the hanging wall of basin-bounding normal faults against crystalline basement is a recurrent structural setting where plays can be found. Here, fault-controlled fluid flow can exert a major role on fluid pathways by means of mineralization and cementation of the host rock.
The Helmsdale Fault in NE Scotland is an excellent exhumed example of a syn-rift basin bounding fault, where timing and paragenesis of fluid flow can be studies in details. The Helmsdale Fault bounds the western side of the Inner Moray Firth Basin in NE Scotland, which developed during rifting in the Late Jurassic. The hanging wall of the Helmsdale Fault consists of Late Jurassic Helmsdale Boulder Beds that are made of alternating debris flow to fault scarp deposits, whereas the footwall is composed of the Helmsdale Granite (Silurian-Devonian in age). Significant fluid flow along the Helmsdale Fault is testified by widespread calcite veining, locally making up to 5 m thick fault cores of stacked veins. A complete suite of microanalysis including optical microscopy, clumped isotope, geochronological analysis as well as fluid inclusion analysis suggests a high variability in time and space of fault-controlled fluid flow, and possibly a more complex deformation history than what previously though.