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, EOO3), B (pole: S24, E340) and C (pole: S10, 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 Caledouian 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 period.

Early Carboniferous Unroofing in Western Norway: A Tale of Alkali Feldspar Thermochronology

“…This activity was followed by block faulting in central England and Scotland and thick Lower Carboniferous sedimentary basin fill, as well as initiation of alkaline and tholeiitic volcanism of Visean-Tournaisian age (Midland Valley region) (Francis 1983;Cameron and Stephenson 1985;Gatliff et al 1994). Plutonic activity on Shetland of Early Carboniferous age (Torsvik et al 1989) appears to have followed closely on folding of Devonian strata in the ORS. Similarly, in Svalbard, Early Carboniferous faulting and basin formation succeeded Late Devonian folding of ORS sediments (Harland et al 1984;Bergh et al 1997).…”

The British Siluro-Devonian palaeofield, the Great Glen Fault and analytical methods in palaeomagnetism: comments on paper by K. M. Storetvedtet al.

“…(a). We also include the results of other studies yielding comparable directions(Table 2b).It becomes apparent that in assessing the 'Mid-Palaeozoic' palaeofield of Europe,Storetvedt et al (1990a) lend support to palaeomagnetic data (shallow inclinations with declinations N-S or NNE-SSW) for which there are no reliable tests of remanence age(Storetvedt & Petersen 1972;Storetvedt & Carmichael 1979;, 1985Storetvedt & Meland 1985; Storetvedt & Otters 1988), or for which the remanence proves to be secondary (e.g Torsvik et al 1989a;Storetvedt et al 1990a…”