Temporal and spatial cyclicity of accretion at slow-spreading ridges-evidence from the Reykjanes Ridge

Abstract

SUMMARY A unifying model of oceanic crustal development at slow spreading rates is presented in which accretion follows a cyclic pattern of magmatic construction and tectonic destruction, controlled by along-axis variation in melt supply and coupled to along-axis variation in spreading rate and across-axis asymmetry in spreading. This study focuses on the Reykjanes Ridge, Mid-Atlantic Ridge south of Iceland, which is divided along its entire length into numerous axial volcanic ridges (AVR). Five adjacent AVRs have been analysed, located between 57 ◦ 30 � N and 58 ◦ 30 � N and south of any strong Iceland hotspot influence. The seabed morphology of each AVR is investigated using sidescan sonar data to determine relative age and eruptive history. Along-axis gravity profiles for each AVR are modelled relative to a seismically derived crustal reference model, to reveal the underlying crustal thickness and density structure. Correlating these models with seabed features, crustal structure, ridge segment morphology and relative ages, a model of cyclic ridge segmentation is developed in which accretion results in adjacent AVRs with a range of crustal features which, when viewed collectively, reveal that second-order segments on the Reykjanes Ridge have an along-axis length of ∼70 km and comprise several adjacent AVRs which, in turn, reflect the pattern of third-order segmentation. Tectono-magmatic accretion is shown to operate on the scale of individual AVRs, as well as on the scale of the second-order segment as a whole.