Structure of the continental margin in the region of Goban Spur, southwest approaches.

Abstract

Approximately 1100 km of multichannel seismic reflection profiles from the Goban Spur region, S W of Ireland, which belong to the Department of Energy U K., have been re-examined. Interpretation of this data reveals the presence of two main seismic sequences, viz., 'Syn-rift' and 'Post-rift'. The former occurs in fault bounded sedimentary basins which are wedge-shaped in cross-section, and the latter drapes over the basement topography. The divergent pattern of seismic reflections from the strata within the basins suggests that the processes of continental rifting and deposition were contemporaneous. The younger post-rift seismic sequences are characterised by parallel to subparallel seismic reflections, and they appear not to have been affected by any major tectonic activity during their geological history. The faults bounding the basins are 'regarded' as normal and listric in form, indicating extension and rotation of the basement blocks. They appear to show a predominant northwesterly trend with downthrown side towards the ocean basin. Correlation of the seismic profiles with DSDP Drill Sites 549 and 550 through ID synthetic seismograms has made it possible to date the syn-rift and post-rift seismic sequences. The available data does not allow a very accurate assesment of the age of the break-up unconformity, nonetheless, it seems to be marked by the boundary between Aptian and Albian. The timing and appearance of the deep seismic reflections seen on the Continental Margin profiles suggest that they are lower crustal reflections similar to the ones reported from the Southwestern Approaches Basin by BIRPS. However, the observed time thickness of the band of these reflections is much smaller than that seen over the Southwest Approaches. Using the synthetic seismogram program, their causes have been reviewed. It has been shown that they can occur from very thin alternating low and high velocity laminae. It has also been shown that in a set of these alternating laminae, the seismic wave sees only a single low and a single high velocity zone rather than an individual lamina, and that the maximum reflected amplitude occurs if the total laminae thickness equals lambda/4. Furthermore, a change in the character of the deep reflections with azimuth suggests that they are probably arising from extensional features developed in relatively competent layers within the lower crust during the stretching phase. Two dimensional gravity modelling along the seismic profiles, using the free-air gravity data, has produced models for crustal structure across the continental margin and has thus provided a measure of the whole crustal extension values. From the geometry of the rotated fault blocks, the depth to detachment and superficial extension beta values have been calculated which appear to be appreciably lower than the whole crustal thinning values, thus suggesting that the extension in the lower part of the crystalline crust may be greater than in the upper part. It is implied from the depth to detachment analysis that the reflections occurring at 9 sec (twt) mark a major change in the rheological properties within the crust. Furthermore, the deeper 10 sec (twt) reflections may mark the Moho. Also, it has been found that the rotational planar faults over Porcupine Abyssal Plain do not seem to penetrate through the gravity deduced Moho.