Lithospheric Structure, Stress, and Magmatism at the Rainbow Non-Transform Offset on the Mid-Atlantic Ridge

Abstract:

New oceanic lithosphere is formed at slow-spreading mid-ocean ridges by a combination of eruption and intrusion of magma and by tectonic exhumation and alteration of lower crustal and mantle rocks. We look at the relationship between these two processes and how their relative contributions vary at non-transform ridge-segment offsets (NTOs). Models of mantle upwelling predict magmatic input and heat flux to be relatively low at NTOs, yet many host high-temperature hydrothermal systems, which are difficult to explain without the presence of a crustal magmatic source. We analyzed newly acquired swath bathymetry, gravity and magnetic data from the MARINER experiment together with archived data from the Rainbow NTO (36º10’ N) on the Mid-Atlantic Ridge. This NTO is currently experiencing both mantle exhumation and magmatic input as evidenced by the active Rainbow high-temperature hydrothermal field. We calculate mantle Bouguer gravity anomalies and crustal magnetization to constrain the lithospheric structure and tectonic evolution of the NTO during the past ~2 Myr. The swath bathymetry data are used to map faults, extrusive volcanic terrain and tectonized blocks and show that the style of crustal accretion varies along the adjacent ridge segments. Spatial changes in the style of extensional faulting are indicative of variations in the mechanical properties and the state of stress of the lithosphere. We suggest that the availability of magma to drive hydrothermal activity at Rainbow and other similar settings is controlled not only by the thermal regime and the structure of the lithosphere but also by the effect of local stress conditions on magma migration. Models of magma migration and dyking show that changes in the direction of minimum compressive stress affect the propagation of magmatic intrusions. We argue that stress rotation can explain the formation of crustal magma chambers at NTOs despite a reduced magmatic flux. These constraints help determine the role of stress in favoring or inhibiting magma supply at NTOs and the mechanisms affecting the availability of magmatic heat to drive hydrothermal circulation.