Paleogene igneous underplating and subsidence anomalies in the Rockall-Faeroe-Shetland area

Crustal thickening caused by igneous underplating related to continental break-up in the northeast Atlantic has been invoked as the cause of the apparent lack of thermal subsidence seen in many Mesozoic basins in the northwest European area. In this study, we examine a set of commercial wells in the Rockall-Faeroe-Shetland region and calculate tectonic subsidence histories. The rate of post-rift basement subsidence during the Cretaceous-Cenozoic, determined from backstripping calculations, always implies a higher degree of extension than the total basement subsidence, assuming uniform extension. Furthermore, the size of this discrepancy increases northward along the Rockall Trough, and westwards in the northern North Sea towards Scotland. We interpret this depth anomaly to reflect the effect of igneous underplating, rethickening the crust following Mesozoic and Cenozoic extension. Late Paleocene extension is identified in subsidence reconstructions throughout the west of Britain area, and is greatest in the Rockall Trough. Assuming that the underplating is gabbroic in composition, the subsidence depth anomalies indicate a regional pattern of 2-3 km underplated between the Erris Trough and Porcupine Basin, increasing to 4-5 km in the West Lewis Basin and Hebrides Shelf area. In the Faeroe-Shetland Basin underplating is 1.0-1.5 km in the east, and increases to 4-5 km in the west. The data support the hypothesis of a major asthenospheric thermal anomaly centered under the Faeroes in the Paleocene. Dynamic uplift of the region is often difficult to identify but is locally seen to develop rapidly around 63 Ma, implying rapid emplacement of the mantle anomaly. The intrusion of gabbroic bodies to the base of the crust in the late Paleocene may have had an impact on the thermal maturation of petroleum source rocks in the region. However, thermal maturity indicators show smaller peak heatflows than predicted from simple thermal models, suggesting that underplating was a gradual process, achieved through the accretion of several small, rapidly cooled bodies. (C) 1998 Elsevier Science Ltd. All rights reserved.