HR: 1330h
AN: V12B-0594    [PDF]
TI: Formation and Significance of the Greenland-Iceland-Faores Ridge: Importance of Crustal Flow.
AU: * Jones, S M
EM: stephen.jones@tcd.ie
AF: Trinity College, Dublin, Department of Geology, Trinity COllege,, Dublin, 2 Ireland
AU: Maclennan, J
EM: maclenna@ipgp.jussieu.fr
AF: Laboratoire des Geosciences Marines, Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris, 75005 France
AB: The Greenland-Iceland-Faroes Ridge (GIFR) is regarded as a classic hotspot track, built by interaction between the Iceland Plume and the Mid-Atlantic Ridge. However, the GIFR-plume link remains poorly understood beyond this general concept because of uncertainty over the mantle, crustal and plate tectonic processes that govern GIFR morphology via crustal thickness. The main mantle processes controling crustal thickness are thought to be plate-driven vs plume-driven upwelling and variations in asthenosphere temperature and composition. The most important tectonic events are the ridge jumps that progressively relocate the spreading axis within the GIFR eastwards relative to the Mid-Atlantic Ridge. Here, we argue that lower crustal flow is an equally important process that governs many major morphological features of the GIFR. Theoretical and experimental studies suggest that when crust is hotter than about $750^\circ$C, variations in crustal thickness drive ductile flow within a channel bounded by the brittle upper crust and the top of the mantle. Controlled source and earthquake seismology results show that Icelandic crust varies in thickness between 20 and 40km, and suggest that the $750^\circ$C isotherm lies at a depth of a few km beneath the spreading axes and no deeper than 15km across Iceland. Conditions are therefore ideal for crustal flow, and several observations strongly suggest that it occurs. First, there is topographic asymmetry about both the Reykjanes and Kolbeinsey Ridges where they intersect the GIFR. Secondly, axis-parallel variation in crustal thickness differs markedly between zero-age crust within the Eastern Volcanic Zone and off-axis crust adjacent within 100km. Thirdly, spreading ridge segments are linked by zones of en-echelon fractures rather than by `hard' transform faults. Lower crustal flow can also explain the sharp topographic step bounding the Iceland Shelf, the roughly circular plateau of average diameter $\sim 600$km encompassing Iceland itself. This sharp egde corresponds neither to the seismic low velocity anomaly of diameter 100--200km beneath SE Iceland, often interpreted as a plume conduit, nor to regional anomalies in topography, gravity, crustal thickness and geochemistry, which have diameters of $\sim 2000$km. However, a topographic step similar to the Iceland Shelf edge is predicted to develop when relatively large crustal thickness variations drive viscous flow into a relatively thin lower crustal channel. An important consequence of crustal flow is that the crustal `memory' of ridge-plume interaction has likely been erased in all parts of the GIFR except the active spreading ridges. Hence crustal thickness maps of the GIFR can be used neither to derive a plume flux history, nor to test in detail whether the present-day Iceland Plume centre is fixed relative to other hotspots.
DE: 8122 Dynamics, gravity and tectonics
DE: 8150 Plate boundary--general (3040)
DE: 8159 Rheology--crust and lithosphere
DE: 9325 Atlantic Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting