Heterogeneous and asymmetric crustal accretion : new constraints from multibeam bathymetry and potential field data from the Rainbow area of the Mid-Atlantic Ridge (36°15'N)

Abstract

At slow-spreading mid-ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma-poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multibeam bathymetry and potential field data from the Rainbow area of the Mid-Atlantic Ridge (35°40'N–36°40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T, and the fraction of magmatic accretion, M. Estimated T values range from 0.2–0.4 on ridge segments to 0.6-0.8 at the Rainbow nontransform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high-temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep-penetrating second-generation faults.