DATE AND ORIGIN OF FLUID FLOW EVENTS ALONG THE MOINE THRUST, NORTHERN SCOTLAND

Faults can act as barriers to flow or they can be conduits for flow thereby controlling the distribution of diagenetic alteration and mineralization. The influence of faults on fluid flow, however, is still poorly understood. In this study, paleomagnetic and geochemical methods were used to date and determine the origin of fluids, which migrated along the Moine Thrust Zone, Scotland. Samples were collected from in and around the fault zone in several units. Altered Durness Limestone (Cambrian), Torridonian Sandstone (Precambrian), and Quartzites (Cambrian) in and near the fault zone contain a magnetization with southerly declinations and moderate negative inclinations. This magnetization resides in hematite based on rock magnetic results. Some samples of the Torridonian Sandstone also contain an older primary magnetization or an early remagnetization. A regional fold test indicates the southerly magnetization is post tilting and the pole position (Lat: 54.7°, Long: 157.8°) suggests it is late Paleozoic to Triassic in age. On the Isle of Skye to the south, the Torridonian Sandstone and Lewisian Gneiss (Precambrian) in the fault zone contain a Tertiary magnetization. Petrographic and geochemical studies indicate that hydrothermal fluids have altered the rocks in the fault zone. The magnetizations are interpreted as chemical remanent magnetizations (CRMs) that were acquired when fluids migrated along the fault and precipitated hematite. On Skye, the CRM is the result of hydrothermal fluids, associated with Tertiary intrusions, moving along the faults. The Late Paleozoic-Triassic CRM and associated alteration in the Moine Thrust Zone are direct evidence for post-orogenic activity, in which the thrusts vented excess heat during regional crustal extension.