ORIGINS OF CONTINENTAL LAVA FLOOD

Origins of continental lava Flood



Abstract

Under plating of the crust by basaltic melt is thought to be an important feature of many magmatic provinces (Cox, 1993). The strongest evidence that magmatic under plating has taken place comes from petro logical arguments based on the composition of basaltic rocks (Thompson, 1974; Cox, 1980; Brodieand White, 1994). The presence of high seismic velocities in the lower crust of these provinces is in accord with the petro logical arguments (Barton and White, 1995). Several lines of evidence indicate that much of the region around the British Isles underwent permanent exhumation of several kilometers during the Tertiary, and this exhumation was likely to be caused by under plating that occurred during continental breakup in the presence of the early Iceland plume (Brodie and White, 1994; Rowley and White, 1998).

Keywords

Lava, Flood, Under Plating, Earth Crust,Seismic

Origins of continental lava Flood

Introduction

The uplift has been linked to sea-level fall and the generation of sandstone bodies' offshore Scotland during the Paleogene (White and Lovell, 1997). However, numerous episodic rises in sea level that occurred over limited time periods have also been reported from the Paleogene offshore (Jones and Milton, 1994). These sea-level rises have not yet been related to under plating and their origin has not been previously explained. After reinforcing the petro logical arguments for magmatic under plating, we show that coupled uplift and subsidence episodes are expected to result from such under plating. We show that under plating alone may be responsible for a rich variety of uplift and subsidence patterns. We suggest that the episodic coupled decreases and increases in sea level recorded offshore Scotland may be the results of under plating.

Petro logical Evidence For Under plating

The compositions of many continental flood basalts have been influenced by high-pressure (; 1 GPA) fractional crystallization (Thompson, 1974; Cox, 1980). Application of recently developed geochemical techniques to a set of basalt samples from the well-studied Mull Plateau Group lavas (Kerr, 1998; Kerr et al., 1999) supports this model, and shows that substantial quantities of magmatic material were added to the lithosphere of western Scotland ca. 60 Maduring a period of 1.9 6 0.4 m.y. (Chambers and Fitton, 2000). Major element trends in basalt samples are dominantly controlled by fractional crystallization of cumulates (Fig. 1). If the initial melt contains; 15 wt% MgO (Scarrow and Cox, 1995), then samples with 5 wt% MgO have lost; 70% of their original liquid mass accumulates. If the solid products of this crystallization were added to the lithosphere over an area similar to that of the lava field (840 km2), then the approximate thickness of under plated cumulate material would be similar to that of the lava pile, 2 km (Walker, 1971). Such a thickness is an underestimate for the under plated material because it does not include melt that is entirely trapped in the lithosphere. Independent estimates of the thickness of under plated material under the British Isles range from 5 to 8 km (Brodie and White, 1994; ...