On the origin and the tectonic implications of magnetic overprinting of the Old Red Sandstone, Shetland

Abstract

SUMMARY The magnetic signature of the Old Red Sandstone (ORS) and post-orogenic plutonics on Shetland is influenced by three principal components, A (pole: S51,E003), B (pole: S24,E340) and C (pole: SlO,E003). As evidenced from negative fold tests, A is secondary, and most likely thermochemical in origin, presumably associated with hydrothermal fluids circulating in faults and crush zones subsequent to Permian-Early Triassic extensional reactivation of older Caledonian fault structures. From the ORS, the B component also can be proven to represent a magnetic overprint. The secondary nature of component B, and the fact that it reasonably can be correlated on both sides of the Walls Boundary Fault, commonly assumed to be the continuation of the Great Glen Fault, argues against recent suggestions of mega-shearing within the Great Glen Fault system. The precise time of acquisition of B is uncertain, but we consider a lower Carboniferous age synchronous with late-post orogenic plutonic activity (334-358 Ma) to be most likely. This implies that the B component carried by the plutonic rocks may represent a primary cooling event. C is exclusively carried out by Middle Devonian andesites and basalts from the Esha Ness Peninsula. It is evidently of post-fold origin, and we relate this earliest magnetic overprinting to Middle-Upper Devonian (Svalbardian) tectonism which affected the North Atlantic domain during this