The RAMESSES experiment - V. Crustal accretion at axial volcanic ridge segments - A gravity study at 57°45′N on the slow spreading Reykjanes Ridge

Abstract

Presented in this paper are the results of a two-stage analysis of gravity data acquired during a multidisciplinary geophysical survey of a magmatically active axial volcanic ridge (AVR) segment located at 57°45′N on the Reykjanes Ridge, part of the slow spreading Mid-Atlantic Ridge south of Iceland. Modelling of the free-air anomaly in 2-D shows that, across-axis, the observed anomaly results largely from density and layer thickness variation in the midlower crust. Although seismic control on crustal thickness along-axis is limited, modelling also suggests that both crustal density and thickness also vary towards AVR tips. Using the 2-D modelling results as crustal reference, the residual mantle Bouguer anomaly (RMBA) is calculated to assess whether magma-related density anomalies are present in the mantle and to investigate the structure of, and relationship between, adjacent AVRs along-axis. RMBA lows are associated with both an along-ridge trend encompassing a number of adjacent AVRs and with individual, more topographically robust AVRs. Modelling of the RMBA low associated with the 57°45′N AVR further suggests that along-axis density variation is confined to the central region of this AVR and that the anomaly can largely be accounted for by density variation within Layer 3 and a degree of crustal thinning towards AVR tips. The nature of the along-axis variation in crustal density further suggests that it may result from repeated phases of magma supply to the crustal system from the mantle. Within the resolution of the RMBA, modelling does not confirm or preclude the presence of a subcrustal density anomaly associated with retention of a small percentage of melt in the mantle. However, a melt-free model for at least the top 40 km of the mantle is preferred as this is consistent with the results of modelling a coincident magnetotelluric data set. The ridge-trend charcteristics of the RMBA also suggest that magma delivery may take place along this trend, and in an episodic fashion that controls initiation of an AVR tectonomagmatic cycle. AVRs may, therefore, be created along a spreading-orthogonal direction by tapping of a recent magma injection into the ridge-trending crustal system, with flow of this magma laterally along AVR-parallel faults and fissures accommodating spreading and crustal accretion.