The geology and formation of King's Trough, northeast Atlantic Ocean

Abstract

King's Trough comprises a 450 km long chain of roughly parallel basins and flanking ridges situated 700 km northeast of the Azores. Its formation has been variously described as: a short-lived plate boundary; a compressional boundary; or a re-orientated former bend in the Mid-Atlantic Ridge. Recent studies of seismic and gravity data concluded that the flanking ridges result from hot-spot activity which began around 55 m.y. B.P., while the trough could have been produced by contemporaneous or later rifting or shearing. The geological sampling described in this paper was planned to resolve the mode of formation of the trough. A long-range sidescan sonar survey of the region was used as control for new bathymetric and geologic maps of the area and for transponder-navigated dredging and coring operations on the flanks of the trough. The samples recovered show that basalt volcanism dated at over 52 m.y.B.P. was followed by the deposition of a chalk sequence until Miocene times. Trachyte volcanism at 32.2 m.y.B.P. was followed by a period of tuff and ash volcanism near sea level in the Late Oligocene and Miocene. A middle Miocene chalk sequence is at present exposed on the trough walls. The detailed morphology and geological sequence at King's Trough agrees best with a hypothesis which involves uplift by around 2 km of an aseismic ridge (“hot-spot trace”) at about 32 m.y. ago which was followed by intra-plate rifting at around 16–20 m.y.B.P. A number of interpretations are possible for the amounts of vertical displacement during this rifting. We favour total vertical fault movements of the order of 2.5 to 3.5 km.