APATITE FISSION-TRACK RESULTS FROM IRELAND AND THE PORCUPINE BASIN AND THEIR SIGNIFICANCE FOR THE EVOLUTION OF THE NORTH-ATLANTIC

Apatite fission track analysis was applied to samples from the Irish m ainland and exploration boreholes (13/3-1, 26/26-1 and 26/28-1) in the northern Porcupine and Donegal basins to study the regional thermal a nd tectonic history of the Irish continental margin. Samples from the Irish mainland have apatite ages ranging from 301 +/- 25 to 56 +/- 5 M a with mean track lengths in the range 13.89 +/- 0.25 to 11.19 +/- 0.2 2 mum and indicate higher temperatures during the Middle Jurassic (abo ut 160-180 Ma) and during the Late Cretaceous to Early Tertiary (about 50-80 Ma). These two periods of increased palaeotemperatures correlat e with major reorganizations in plate movements and suggest that uplif t and erosion after the maximum palaeotemperatures was achieved as a c onsequence of a variation in the regional pattern of crustal stress. T he results from borehole 26/28-1 indicate that, within the resolution of the data, all samples are presently at their maximum post-depositio nal burial temperatures. To produce the mean track length results, in the range 11.24 +/- 0.49 to 11.81 +/- 0.20 mum, from borehole 26/26-1 it is necessary to invoke post-Palaeocene geothermal gradients in exce ss of 50-degrees-C/km, increased relative to the present day gradient (28-degrees-C/km). Similar increased geothermal gradients are recogniz ed during the Early Tertiary in adjacent sedimentary basins and may be explained as a result of heat advection by gravity-driven fluid flow, established in response to tectonic uplift of the basin margins.