COMPLEX STRUCTURE ALONG A MESOZOIC SEA-FLOOR SPREADING RIDGE - BIRPS DEEP SEISMIC-REFLECTION, CAPE-VERDE ABYSSAL-PLAIN

As part of an intensive study of a small area of oceanic lithosphere, the British Institutions Reflection Profiling Syndicate (BIRPS) acquir ed closely spaced deep-seismic-reflection profiles over the Early Cret aceous crust of the Cape Verde abyssal plain off West Africa. The surv ey consisted of profiles spaced at 4 km arranged into strike lines par allel to the old sea-floor spreading axis ('isochron' profiles) and or thogonal dip lines oriented in the original direction of spreading ('f low' profiles). A large-capacity, well-tuned airgun source and very qu iet shooting: conditions ensured a high signal-to-noise ratio for deep reflection. Devising a strategy for mitigating contamination from 'wr ap-around' multiples arriving from previous shots enabled us to use th e minimum possible shot-point interval (50 m) allowed for collecting l ong (18 s) records. Data processing was oriented towards a medium with low root-mean-square velocity, steeply dipping structure, and pervasi ve low apparent velocity noise from diffraction at the top of the igne ous crust. The contrast between the isochron and flew profiles is stri king. Isochron profiles are typically highly reflective throughout the igneous crust, consisting of bright, bidirectionally dipping reflecti on sets that extend in places from the top of the igneous basement dow n to the interpreted Moho reflection. These reflections do not offset intracrustal or top-basement structure and thus are not interpreted as faults: an igneous intrusive origin seems more likely. Flow profiles are more sparsely reflective but show individual steeply dipping refle ctions best developed in the upper igneous crust, continuing down in p laces to the Moho. Dipping reflections on the flow profiles are interp reted as major normal faults since they are clearly associated with of fsets of the top of the basement as well as truncation of horizontal r eflections within the igneous crust. The dominant dip of these reflect ions is to the west towards the spreading ridge axis. Reflections from the vicinity of the Moho, while well developed in some places, are no t particularly prominent across the survey area. Moho reflections appe ar to show a different structural relation to crustal features on the isochron and flow profiles: on isochron profiles, dipping reflections occasionally flatten out into, and may merge with, the Moho reflection ; on flow profiles, as dipping crustal reflections approach the Moho r eflection, they are usually abruptly cut off by it without extending d eeper. This survey shows how oceanic crustal structure can vary rapidl y over relatively smalt areas, provides convincing evidence that a str ucturally complex fabric dominates oceanic igneous crust, and gives a conclusive observation of faults that penetrate the entire oceanic cru st.