Lithospheric structure north of Scotland--II. Poisson's ratios and waveform modelling

Abstract

SUMMARY A 200 by 50 km grid of near-normal-incidence seismic reflection data and a 350 km wide-angle seismic line have been acquired across the Flannan and W-mantle reflectors north of Scotland. A number of geological models have been published to explain the origin of these reflections; we examine these models by considering the physical properties of the lithosphere in this region. We present P- and S-wave velocity models for the lithosphere obtained using ray-based inversion schemes. We show full-wavefield synthetic P-wave data generated using a 2-D finite difference code to confirm that our velocity model can reproduce the amplitude behaviour of the observed mantle arrivals. P- and S-wave velocities beneath the Moho give a Poisson’s ratio of 0.25t0.01, which, together with evidence from local mantle xenoliths, suggests a normal peridotitic upper mantle. We observe both normal-incidence and wide-angle reflections from the mantle reflectors. Modelling and inversion of these reflection events show that the Flannan and W-reflectors represent at least a 20 per cent increase in acoustic impedance within the mantle, and that the region of elevated acoustic impedance has a thickness of at least 3 km. A mafic eclogite is the only common mantle rock type that can have an acoustic impedance significantly higher than peridotite. The physical properties of the upper mantle north of Scotland thus suggest that the Flannan and W-reflectors represent reflection events from the tops of slabs of eclogite. A weak event is interpreted as a wideangle reflection from the base of one of these slabs. A slab thickness of 6.0t1.0 km with a Poisson’s ratio of 0.29t0.02 was used to model this phase. The high Poisson’s ratio within the slab gives further support to the interpretation that these slabs are eclogitic. The lateral extent of these mantle reflection events and the thickness of the slab are most readily explained as oceanic crust that was metamorphosed to eclogite when subducted beneath the existing continent. We conclude that the eclogitic slabs are relicts of oceanic crust that remained within the lithosphere when subduction halted.