Evidence for a Slow Spreading Ocean Ridge in the Southern Rockall Trough From
Satellite Gravity Inversion and Seismic Data
A.R. Chappell and N.J. Kusznir
Department of Earth and Ocean Science, University of Liverpool, 4 Brownlow Street,
Liverpool, UK L69 3GP
(arc@liverpool.ac.uk, n.kusznir@liverpool.ac.uk)
The southern Rockall Trough, located to the west of Ireland and the UK in the NE
Atlantic, has been interpreted as both a Mesozoic intra-continental rift basin (O’Reilly
1995) and a mid Cretaceous ocean basin (e.g. Roberts et al. 1980). The continental rift
hypothesis (O’Reilly 1995) requires differential stretching of the upper and lower
crust and syn-tectonic cooling to mechanically explain the formation of 5-6km thick
continental crust and allow serpentinisation of the upper-mantle. In this model
serpentinisation of the upper mantle is needed to explain low upper mantle seismic
velocities. The serpentinisation has also been required to fit gravity modelling of
seismic transects to the observed gravity (e.g. Shannon 1999).
We use satellite gravity inversion to map Moho depth and crustal thickness (Chappell
& Kusznir 2005) for the Rockall Trough area. The satellite gravity inversion is a 3D
spectral method incorporating a correction for the residual lithosphere thermal gravity
anomaly present in continental rifted margin lithosphere and oceanic lithosphere. The
gravity inversion predicts Moho depth and geometry in agreement with wide-angle
seismic estimates without invoking the extensive serpentinisation of the upper-mantle
needed by the intra-continental rift hypothesis (O’Reilly 1995).
Recent seismic modelling (Morewood 2005) suggests that the thin crust in the
southern Rockall Trough does not have the seismic layering associated with oceanic
crust formed at intermediate or fast spreading rates. Also, wide-angle seismic data
shows low upper-mantle seismic velocities are present and spatially associated with
the thin 5-6km crust (Shannon 1999). These observations are consistent with models
and observations of oceanic crust formed at slow spreading ocean ridges (Cannat
1996, Jokat 2003). Such models are based on a proportion of melt being retained in
the upper-mantle, producing low seismic velocities, and a reduced supply of melt to
the crust, resulting in thin seismic crust with some serpentinised mantle material
included. We propose that the southern Rockall Trough was formed by continental
break-up and a period of slow mid Cretaceous sea floor spreading rather than as an
intra-continental rift basin.
This work forms part of the NERC Margins iSIMM project. iSIMM investigators are
from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger
Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess
Ltd, Anadarko, Conoco-Phillips, Shell, Statoil and WesternGeco. The iSIMM team
comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, AR Chappell, J Eccles,
RJ Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, VJ
Tymms & R Spitzer.