Enhanced Mantle Upwelling/Melting Caused Segment Propagation, Oceanic Core Complex Die Off, and the Death of a Transform Fault: The Mid‐Atlantic Ridge at 21.5°N

Abstract

Crustal structure provides the key to understand the interplay of magmatism and tectonism, while oceanic crust is constructed at Mid‐Ocean Ridges (MORs). At slow spreading rates, magmatic processes dominate central areas of MOR segments, whereas segment ends are highly tectonized. The TAMMAR segment at the Mid‐Atlantic Ridge (MAR) between 21°25′N and 22°N is a magmatically active segment. At ~4.5 Ma this segment started to propagate south, causing the termination of the transform fault at 21°40′N. This stopped long‐lived detachment faulting and caused the migration of the ridge offset to the south. Here a segment center with a high magmatic budget has replaced a transform fault region with limited magma supply. We present results from seismic refraction profiles that mapped the crustal structure across the ridge crest of the TAMMAR segment. Seismic data yield crustal structure changes at the segment center as a function of melt supply. Seismic Layer 3 underwent profound changes in thickness and became rapidly thicker ~5 Ma. This correlates with the observed “Bull's Eye” gravimetric anomaly in that region. Our observations support a temporal change from thick lithosphere with oceanic core complex formation and transform faulting to thin lithosphere with focused mantle upwelling and segment growth. Temporal changes in crustal construction are connected to variations in the underlying mantle. We propose that there is a link between the neighboring segments at a larger scale within the asthenosphere, to form a long, highly magmatically active macrosegment, here called the TAMMAR‐Kane Macrosegment