Implications of mesozoic recent basin development in the western inner moray firth, uk

On the Scottish mainland the Great Glen fault (GGF) displaces the Emsian to Frasnian Orcadian Old Red Sandstone (ORS) by only 25–29 km dextrally but net post–ORS dextral offsets in Shetland are much larger (120 km total). Most of the displacement (15–20 km) Permian initiation of the Inner Moray Firth Basin. It probably occurred between Frasnian cessation of Orcadian extension and accompanied transpressional inversion of the Orcadian Basin in the mid-late Carboniferous and/or possibly the late Devonian. Devono-Carboniferous transtension may also have occurred. The earlier history of the GGF includes late Caledonian sinistral motion which must have ceased by the late Emsian.

Short Paper: Devonian and later movements on the Great Glen fault system, Scotland

Synopsis Lacustrine Middle Devonian (Old Red Sandstone) sediments show climatically-controlled cycles resulting from the long-term rise and fall of lake levels in an enclosed basin. Lithologies present were deposited in deep (stratified), shallow and impermanent lakes. Varves, the common recurrence of specific lithologies and the cyclic growth of cosmoid fish scales suggest a marked seasonal climate. Fish faunas were subject both to minor seasonal controls and more profound long-term effects. When lake level was low the faunas may have died out. When the lake overflowed, population replenishment and/or exchange with neighbouring basins was possible. Consequently fish faunas show erratic distributions (involving sudden influxes and extinctions) and no undoubted phylogenetic lineages. Discrete faunal assemblages form internally correlatable “stratigraphic zones”.

Lacustrine cycles, fish ecology and stratigraphic zonation in the Middle Devonian of Caithness

Paleomagnetism is a quantitative tool for determining relative displacements and rotations of rock bodies in the geological past. During the last 20 years, paleomagnetic work in continental cratons has contributed greatly to our understanding of continental drift, but more recently, paleomagnetism has begun to be used on a more local scale, to elucidate tectonic problems in mountain belts. We have selected several geographical areas which illustrate an approach to paleomagnetism in mountain belts.

Paleomagnetism in orogenic belts

The Old Red Sandstone (Middle Devonian) Orcadian basin was formed as a consequence of extensional collapse of the Caledonian orogen. Onshore study of these collapse-basins in Orkney and Shetland provides directions of extension during basin development. The origin of folding of Old Red Sandstone sediments, that has generally been related to a Carboniferous inversion phase, is discussed: syndepositional deformation supports a Devonian age and consequently some of the folds are related to basin formation. Large-scale folding of Devonian strata results from extensional and left-lateral transcurrent faulting of the underlying basement. Spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left-lateral megashear in NW Europe during late-Caledonian extensional collapse. Later inversion (probably during the Upper Carboniferous) is characterized by E–W to NE–SW contraction. It induced reactivation of extensional faults as thrusts, development of small-scale folds and thrusts, and right lateral transcurrent movement of the major faults such as the Great Glen and Walls Boundary faults.

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Devonian extensional tectonics versus Carboniferous inversion in the northern Orcadian basin

STORETVEDT1 has argued that Devonian palaeomagnetic results from Norway and northern Scotland indicate a post-Devonian sinistral displacement along the Great Glen Fault of some 200–300 km. This has been objected to on geological grounds by Mykura2 and on palaeomagnetic grounds by Turner et al.3. Here we present a simplistic Devonian palaeographical model of the Orcadian Basin based on observation of onshore sediments. We believe the model is compatible with only minor dextral displacement along the Great Glen Fault in post-Devonian times.

Devonian palaeogeography of the Orcadian Basin and the Great Glen Fault

The validity of using Devonian palaeomagnetic data to determine a post-Devonian displacement along the Great Glen Fault is assessed and it is concluded that there is no statistically significant difference between previous magnetic vectors E and W of the fault and thus no palaeomagnetic evidence for a large sinistral displacement in post-Devonian times. New palaeomagnetic results are presented which suggest that the Devonian red sandstones in the Orcadian Basin have such a complex diagenetic and palaeomagnetic history that they will then be unsuitable for such tectonic evaluations in the foreseeable future, although palaeomagnetic dating can be used to resolve some diagenetic-sedimentary features.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/FE2FA4C95AE476D29360A419DB02A2F3/S0016756800047646a.pdf/div-class-title-a-palaeomagnetic-argument-concerning-post-devonian-displacement-along-the-great-glen-fault-div.pdf

R. Archer

Papers

Devonian palaeogeography of the Orcadian Basin and the Great Glen Fault

A palaeomagnetic argument concerning post-Devonian displacement along the Great Glen Fault