THE RELATIONSHIP BETWEEN BASIN AND MARGIN THERMAL EVOLUTION ASSESSED BY FISSION-TRACK THERMOCHRONOLOGY - AN APPLICATION TO OFFSHORE SOUTHERN NORWAY

We use a quantitative model of apatite fission track (AFT) annealing t o constrain the thermal evolution of a sedimentary basin and its margi n. Apatites deposited in a basin contain several types of information. Provided temperatures remained below approximate to 70 degrees C, the y retain much of their provenance thermal signatures and mainly record the thermal evolution of their source area. Above 70 degrees C, the f ission tracks in apatite rapidly fade, reflecting the thermal evolutio n of the basin. Therefore, downhole AFT dates in a well section can in principle be used to assess both the provenance detail (from shallow/ cool samples) and the subsequent thermal history in the basin (from th e deeper samples). We apply this concept to the south Norwegian offsho re and onshore using AFT and ZFT (zircon fission track) data; the latt er constrain maximum palaeotemperatures and provide additional provena nce information. AFT and ZFT data from three offshore wells in the nor thern North Sea are shown to contain a record of palaeogeographical an d tectonic evolution, closely associated with the Norwegian basement. ZFT data from Middle Triassic sediments suggest a Permian volcanic sou rce. Modelling of AFT data from Jurassic sediments presently residing at temperatures below 70 degrees C indicate rapid cooling during the L ate Triassic to Early Jurassic, similar to onshore AFT data. During th e Cretaceous minor sediment supply was derived from the Norwegian base ment, as evidenced by ZFT ages that do not correlate to the onshore, s uggesting that parts of southern Norway were covered with sediments at this time. At the end of the Palaeogene and during the Neogene, the s outh Norwegian basement again became a major source of clastics. AFT a nd ZFT data indicate that all wells are presently at maximum temperatu res. No significant (> 500 m) erosion events are indicated in the thre e wells since the Jurassic. AFT data have not yet effectively equilibr ated to present-day temperatures as nonzero fission track ages are mai ntained in sediments currently at temperatures of > 120 degrees C. Thi s implies that the present-day thermal regime has only recently been i nstalled. Probable causes include rapid subsidence and an increase in the geothermal gradient during the last 5 Myr.