Structure of the crust and upper mantle at Goban Spur, Southwest of the British Isles — some implications for margin studies

Abstract

Goban Spur is a marginal plateau on a sediment-starved passive continental margin. Recent detailed geophysical investigations on and around the spur reveal four interesting aspects of passive margin structure. 1. (1) A well-exposed continent-ocean boundary, whose shallow structure may be very similar to that of the former passive margins of the Tethys ocean. The ease with which the continent—ocean boundary can be detected geophysically greatly depends on the extent of rifting-phase igneous activity however. A ridge-generated isostatic gravity anomaly, as discussed by Rabinowitz and La Brecque (1977), appears to mark the boundary in this area, but the composition of the ridge and its lateral extent are not established. 2. (2) Transverse structures (faults) that generate small offsets in the continent—ocean boundary and extented from continental into oceanic crust. These may be widespread features of margin basements. 3. (3) A zone of very thin continental crust above a mantle that is normal or of density only very slightly less than normal. Areas of very thin continental crust must be able to originate by mechanisms that do not require the anomalous, low-density upper mantle discussed by Van der Linden (1977) in his “continental spreading” hypothesis. Other thinning mechanisms (with the amount of crustal thinning as a percentage of the original crustal thickness) are crustal stretching (c. 10% per 10° rotation of fault blocks), thermal expansion and supracrustal erosion (up to ca. 15%), lateral creep of continental crust (unlimited?) and upward migration of a basalt-eclogite phase change at the Moho (c. 1% per 1 km sediment blanket). 4. (4) A possible change in upper mantle density across the margin from about 3.40 g/cm 3 beneath continental areas to about 3.36 g/cm 3 beneath oceanic areas may satisfy the regional gravity data. A global change in upper mantle density across passive margins may be expected from physical and chemical considerations. However, it is doubtful that such a change would be easily detected and a change may not always be present because of local variations in upper mantle properties.