A plume model of transient diachronous uplift at the Earth's surface

Convection in the Earth's mantle appears to be strongly time-dependent on geological time scales. However, we lack direct observations which would help constrain the temporal variation of convection on time scales of 1-10 Ma. Recently, it has been demonstrated that transient uplift events punctuated the otherwise uniform thermal subsidence of sedimentary basins which fringe the Icelandic plume. In the Faroe-Shetland basin, three-dimensional seismic reflection surveys calibrated by well logs have been used to reconstruct a similar to 55 million year old transient event. The minimum amount of uplift is 490 m, which grew and decayed within 2 Ma. This event has also been mapped 400 km further east in the North Sea basin, where peak uplift with an amplitude of 300 m occurred 0.3-1.6 Ma later. Neither observation can be explained by glacio-eustatic sea-level changes or by crustal shortening. We describe a simple fluid dynamical model which accounts for these transient and diachronous observations. In this model, we assume that the Icelandic plume was already in existence and that it had an axisymmetric geometry in which hot (e.g. 1400 degrees C) asthenospheric material flows away from a central conduit within a horizontal layer. A transient temperature anomaly introduced at the plume centre flows outward as an expanding annulus. Its geometry is calculated using radial flow between two parallel plates with a Poiseuille cross-stream velocity profile. The expanding annulus of hot asthenosphere generates transient isostatic uplift at the Earth's surface. Stratigraphic observations from both basins can be accounted for using a plume flux of 1.3 x 10(8) km(3) Ma(-1) for a layer thickness of 100 km. Plume flux is broadly consistent with that required to account for Neogene (0-20 Ma) V-shaped ridges south of Iceland, although our transient temperature anomalies are larger. We suspect that the stratigraphic expression of transient convective behaviour is common and that a careful examination of appropriate records could yield important insights. (C) 2007 Elsevier B.V. All rights reserved.