Anatomy of an oceanic megatransform: geochemical variability of the oceanic lithosphere of the Doldrums Fracture Zone (7-8ºN, Mid Atlantic Ridge

In the Equatorial Atlantic Ocean, the slow-spreading Mid Atlantic Ridge (MAR) is segmented by the motion of several transform systems. Some of these fracture domains stand out from the common type of oceanic transform faults for width of the deformed area, which has a lens shape bounded by two principal transform faults, displacing the ridge axis with an offset up to 30 Ma. The morphology of these ‘megatransforms’ better reminds that of the continental strike slips (i.e. Saint. Andreas fault), which, being developed in cold regime, facilitate the wide diffusion of the brittle deformations within the tick lithosphere. Such large-scale fracture zones are unexpected at Mid Ocean Ridges, mainly because the lithosphere tends to reduce its thickness approaching the ridge axis, where the lithosphere-asthenosphere boundary is few kilometres to the surface; hence their origin is still enigmatic. The active strike-slip tectonic and the formation transverse and median ridges into these regions allow the exposure of large amounts of deep-seated rocks, such as mantle peridotites and gabbros, which can be directly compared to the basalts erupted on the seafloor. Hence, the spatial association of extrusive and deep rocks represents a unique change to gain information on the origin of these oceanic domains. In this framework, the present project seeks to use petrology and geochemistry of mantle and basalts from the Doldrums Fracture Zone and adjoining segments to understand the role of mantle heterogeneity in promoting segmentation in the Equatorial sector of the Mid Atlantic Ridge. On November 2019, I took part to the 45° expedition of the R/V Akademik Nikolaj Strakhov (S45) in the Atlantic Ocean at the Doldrums FZ. Geophysical and bathymetry data were acquired, covering an area of 29000 km2 and 12 dredges were deployed collecting gabbros, peridotites and pillow basalts. Following our new data we defined the Doldrums Fracture Zone as a megatransfrom having a lens-shaped, 110 km-wide region of intense deformation characterized by four intratransform ridge segments (ITR), which are bounded by five active transform faults. New petrological and geochemical data on 19 peridotites and 100 basalt samples, also including samples from past expeditions in the same area (Expeditions R/V A.N. Strakhov n 6, 9 and 22 and Expedition R/V Ioffe 10) were acquired in the framework of this joint PhD project. Major and trace element compositions of mineral phases and glasses were acquired at University of Pavia and IGG-CNR; Sr-Nd-Hf-Pb radiogenic isotopes were acquired at the Westfälische Wilhelms-Universität of Münster.