Imaging magmatic rocks on the Faroes Margin

Abstract

Break-up of the North Atlantic in the Paleocene was accompanied by widespread magmatism, which provides a challenge both to imaging basin structure and to modelling the subsidence and development of the continental margins. The rationale for new methods of seismic acquisition and modelling are reported here, followed by an illustration of the techniques with data from the Faroes region, which lay close to the hottest part of the mantle plume at the time of continental break-up. Igneous rocks were added to the crust during continental break-up on rifted margins as extrusive lavas, as sills intruded into the subsurface, and as lower crustal intrusions or underplate. Each provide different, but tractable problems to seismic imaging. It is shown that many of these difficulties can be surmounted by using very long offsets (long streamers or two-ship methods) with a broad-band, low-frequency source, and by using dense arrays of ocean bottom seismometers to provide complementary P- and S-wave velocity control. These methods are illustrated using new results from the integrated Seismic Imaging and Modelling of Margins (iSIMM) project, which acquired regional data across the Faroes shelf and margin in summer 2002. The iSIMM profile used 85 four-component ocean bottom seismometers and a 104 litre (6340 in 3 ) airgun array for wide-angle data, with vertical arrays for far-field source signature recording. The wide-angle profiles were overshot using three single-sensor, Q-Marine streamers, one 12 km long, the other two 4 km long. A 48-gun, 167 litre (10 170 in 3 ) deep-towed, broad-band airgun array was used with the Q-marine streamers. The airgun arrays were designed and tuned to enhance the bubble pulses, with peak frequencies at 8–11 Hz. They performed well, with excellent arrivals visible to ranges beyond 120 km on the OBS, and penetration through the basalts and well into the upper mantle. Examples are shown of the new seismic data, which enable imaging of the basalt flows and underlying sediments, as well as lower-crustal igneous intrusions. This allows good constraints to be placed on the igneous distribution and volumes on the rifted margin and adjacent continental shelf.