MOIST and the continuity of crustal reflector geometry along the Caledonian-Appalachian orogen

Abstract

New aspects of the deep structure of the Scottish Caledonides are revealed by the Moine and Outer Isles deep seismic reflection traverse (MOIST). The Caledonian foreland is underlain by an easterly-dipping, strongly reflecting surface cutting through the Moho and traceable to more than 45 km depth. Thrusts within the foreland basement and Caledonian orogen have been reactivated as normal faults bounding half-grabens filled with sediments of late Palaeozoic to Mesozoic age. The Moine Thrust, which carries rocks of the orogen over Lewisian foreland, dips at 20–25° to the E on MOIST, and is either: (a) the westernmost of a series of easterly-dipping reflections (thrusts) which flatten or terminate at 17–20 km depth, or (b) a thrust lying further E which structurally overlies these easterly-dipping reflectors. By comparison with easterly-dipping reflections on COCORP lines in the northern and southern Appalachians the latter interpretation is preferred, implying that many of the easterly-dipping reflections on MOIST correspond to off-shelf metasedimentary rocks, with slivers of basement, stacked and imbricated against the Lewisian basement edge. Regional seismic refraction and conductivity data from northern Scotland suggest that regions of the Caledonian orogen have lower mid-crustal velocity gradients and higher conductivities than the foreland, which might be explained by this interpretation of the easterly-dipping reflections. Despite similarities in crustal reflector geometries, there are important differences between the MOIST and COCORP lines, including (1) a remarkably continuous, relatively horizontal Moho seen on MOIST data at about 25 km depth, and (2) the sedimentary basins offshore from northern Scotland which have formed by reactivation under crustal extension of the easterly-dipping thrusts. Furthermore, it is probable that the easterly-dipping reflectors in these areas of the MOIST and COCORP lines were formed at different times in the early Palaeozoic.

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…A thick-skinned style is in effect confirmed by seismic profiling of MOIST [151] where the upper crust contains several reflections, which are interpreted as thrust faults that sole into a horizontal reflection band at a depth of around 20 km. However it is not clear yet if these imbricate thrusts are in the hanging wall of the Moine thrust -in which case the Moine thrust would root at around 20 km depth -or if these imbricate thrusts are in the footwall of the Moine thrust.…”

Acceleration of slip motion in deep extensions of seismogenic faults in and below the seismogenic region

“…As indicated in Section 5 of this paper, such a material will not localise during deformation unless softening occurs. Localisation in exposed lower crustal materials is ubiquitous (see for instance, Ramsay, 1974) and seismic profiling of the lower crust commonly reveals localisation of deformation (see for instance, Brewer and Smythe, 1984). This means that we need to understand softening mechanisms in viscous materials or address the issue that perhaps lower crustal materials also have a plastic component to their constitutive behaviour so that localisation controlled by yield-surface corner behaviour (see Rudniki and Rice, 1975) or other yield associated behaviour (see Vermeer and de Borst, 1984) can be incorporated into realistic models of the Earth's crust.…”

Introduction: the deformation of continental crust and Mike Coward's impact on its understanding

“…Thus Enfield & Coward (1987) showed that the West Orkney basin preferentially reactivated the Caledonian deformation fabric, as imaged in the MOIST deep seismic data (e.g. Brewer & Smythe 1984). This was not simply reactivation of pre-existing faults.…”