Variations in oceanic upper crustal structure in a small area of the north-eastern Atlantic

Abstract

A detailed seismic and geophysical survey of a 50 × 50 km area of 40–50-Myr-old crust in the north-eastern Atlantic Ocean delineates the position of a minor fracture zone with an approximately east—west trend traversing normal oceanic crust. The topography of both the seafloor and the top of the volcanic basement indicate a marked structural discontinuity at the fracture zone. The maximum offset across the fracture zone reaches about 15 km in the east of the area, decreasing to almost zero in the west. Well lineated seafloor spreading magnetic anomalies are present throughout the survey area, although there are marked differences in the character and number of wiggles on either side of the fracture zone. The recorded magnetic anomalies indicate differences in the periodicity of extrusion on essentially contemporaneous crust to the north and to the south of the fracture zone suggesting that the accretionary processes were independent of one another, despite the small or negligible offset. Delay times calculated from explosive charges fired around a circle within the detailed survey area show a narrow zone, less than 7 km wide, of anomalously early arrivals beneath the fracture zone. A reversed sonobuoy refraction profde suggests that a normal seismic layer 3 is not developed within the fracture zone, but is replaced by material with higher velocities of approximately 7.3–7.6 km s−1. This high velocity layer is responsible for the early arrivals from shots above the fracture zone. Density and seismic velocity considerations allow us to rule out the presence of significant serpentine diapirs. The anomalous crust of this fracture zone is probably a primary feature resulting from accretion at the ends of the two magma intrusion systems that abut it; the very small offset does not allow us to invoke special conditions such as intrusion adjacent to older lithosphere or the effects of large scale fracturing and faulting which are likely to accompany the movement on a larger offset fracture zone. Travel time and amplitude analysis of a refraction line with closely spaced explosive sources indicates that the velocity structure of layer 2 is best modelled by a relatively low velocity (less than 3.9 km s−1), upper layer several hundred metres thick underlain by a step in velocity to about 4.5 km s−1 and a uniform velocity gradient of about 0.7 s−1 in the remaining lower portion of layer 2. The uppermost basement layer is probably a remnant of a once much lower velocity layer 2A, whilst the gradient in the rest of layer 2 is largely controlled by a downwards decrease in open crack density. Significant lateral variation in the seismic structure of layer 2 is recorded by delay time variations of up to 0.1 s on the ‘normal’ crust away from the fracture zone. This variability is probably caused by lateral changes in the structure of the uppermost few hundred metres of the volcanic basement.