Similar dyke thickness variation across three volcanic rifts in the North Atlantic region: Implications for intrusion mechanisms

Abstract

The thicknesses of 1935 mafic dykes have been recorded through meticulous mapping across (1) the East Greenland coastal dyke swarm, (2) an extinct rift zone in SE Iceland and (3) an obducted dyke swarm segment within the Swedish Caledonides. In all three cases, the thickness of almost every dyke along well-exposed and coherent profile segments could be measured and analyzed. Statistics show that dyke thickness distributions more often are negative exponential (i.e., random) than log-normal within any given segment, with a regression's inverse exponential coefficient representing a more sophisticated average thickness. For all three dyke profiles, there is a similar decrease in average thickness from thicker dykes along the margin of the swarm to narrower dykes along its axis. Cross cutting relationships within two profiles, furthermore, suggest that the average dyke thickness decreased with time. The random thickness distribution of dykes is most likely governed by dyke initiations, releasing differential stresses at random time intervals during constant rates of plate separation. It is argued that the thickness of a dyke does not change significantly within the depth ranges that these dyke swarms are exposed, allowing systematic spatial and temporal changes in average dyke thicknesses to be related to other factors. Results are primarily related to the depth of an underlying sub-crustal magma reservoir, which progressively rose to shallower elevations beneath an active volcanic rift. As an alternative, or in conjunction with this model, stress concentrations towards the rising crest of a sub-crustal magma reservoir might increase the average frequency of randomly released differential stresses, leading to move rapid injections of thinner dyke toward swarm centres and with time. Correlating average dyke thicknesses to crustal depths, I end up with an empirical dyke thickness/height ratio of ∼ 2 × 10 − 4, yet variable thickness/length ratio in order to accommodate the elliptical surface outline of swarms.