Large-scale structure of the Doldrums multi-fault transform system (7-8ºN Equatorial Atlantic): preliminary results from the 45th expedition of the R/V A.N. Strakhov

Abstract

The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transformdomains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs).Among these megatransforms, the Doldrums system (7-8oN) is arguably the less studied, although it may be considered the most magmatically active. Newgeophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allowto reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled withextensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate thatthe Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward thecentral part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminaryinterpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regimeor the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections.We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphereby tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, andthe exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys