Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

Abstract

A grid of 32 across-axis and five axis-parallel multichannel seismic (MCS) reflection profiles were acquired at an axial volcanic ridge (AVR) segment at 57° 45'N, 32° 35'W on the slow-spreading Reykjanes Ridge, Mid-Atlantic Ridge, to determine the along-axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure, the along-axis continuity and segmentation of melt supply to the crust, the development of faulting and the thickness of oceanic layer 2A. Seismic reflection profiles acquired at mid-ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events. In this paper, we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed MCS profiles. The 37 MCS profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2, to the axial magmatic system and to the faults which dismember each AVR as it ages through its tectono-magmatic life cycle and which form the median valley walls. The layer 2A event can be mapped around the entirety of the survey area between 0.1 and 0.5 s two-way traveltime below the seabed, being thickest at AVR centres, and thinning both off-axis and along-axis towards AVR tips. Both AVR-parallel and ridge-parallel trends are observed, with the pattern of on-axis layer 2A thickness variation preserved beneath relict AVRs which are rafted off-axis largely intact. Each active AVR is underlain by a mid-crustal melt lens reflection extending almost along its entire length. Similar reflection events are observed beneath the offset basins between adjacent AVRs. These are interpreted as new AVRs at the start of their life cycle, developing centrally within the median valley. The east–west spacings of relict AVRs and offset basins is ∼5–7 km, corresponding to a life span of the order of 0.5–0.7 Myr, during which AVRs appear to undergo multiple 20–60 Kyr tectono-magmatic cycles.