![]() ![]() To alleviate this difficulty, the anomaly is reduced to the pole (RTP), where both the magnetization and geomagnetic field vectors are vertical and the anomaly located above its causative body. The resulting magnetic anomaly is generally made of two lobes and is difficult to interpret. When the basaltic oceanic crust cools, its magnetic minerals acquire a remanent magnetization aligned with the ambient geomagnetic field. We combine the magnetic response of hydrothermal mounds at different altitudes above the seafloor with high-resolution bathymetric data to constrain the geology, tilt and conduit inclination, unveiling the detailed tectonics of the TAG segment (See “Methods”). An additional parameter to be considered is the possible inclination of the hydrothermal conduits. This mismatch is either interpreted as a consequence of the tectonic tilt of a homogeneous underlying block 16 or of a composite, heterogeneous geology of the basement. The magnetic signature of these hydrothermal sites is not always centered above the features. Hydrothermal massive sulfide mounds usually form prominent bathymetric features associated with a strong magnetic signature reflecting the geological setting, and are therefore easy to identify 8, 9, 10, 11, 12, 13, 14, 15. ![]() Seismic reflection data were also acquired using airgun shots and a deep-towed multichannel seismic streamer. Due to its importance as a host to massive sulfide deposits, the slow-spreading TAG Segment (26°N, Mid-Atlantic Ridge, spreading rate 23.2 mm/y) was surveyed in 2016 during cruise M127 of German R/V Meteor by the Autonomous Underwater Vehicle (AUV) Abyss to collect high-resolution, near-seafloor magnetic and bathymetric data. As a result, slow-spreading seafloor exposes variably faulted and tilted crustal blocks, dykes, lower oceanic crust and mantle rocks, comprising oceanic core complexes (OCCs) 6, 7. Unlike fast-spreading centers associated with intense volcanic activity, slow-spreading ridges are characterized by lower magma flux 1 and tectonic extension, often accommodated by long-lived detachments faults 2, 3, 4, 5. Thermal and magmatic variations control the structure and morphology of the seafloor above the subhorizontal detachment surface, occasionally leading to relocating the detachment. The deep subvertical detachment fault roots on the plate boundary, marked by a thermal anomaly and transient magma bodies. Our study reveals that the presence and evolution of oceanic core complexes play a key role in triggering block movements. Here, we use near-seafloor magnetic and bathymetric data and seismic profiles collected over the TAG Segment of the Mid-Atlantic Ridge to constrain the tectonic evolution of these blocks. These blocks undergo highly variable evolution, such as tilts or dissection by renewed tectonic extension, depending on their positions with respect to the spreading axis, core complexes, detachment or transform faults. Spreading processes associated with slow-spreading ridges are a complex interplay of volcanic accretion and tectonic dismemberment of the oceanic crust, resulting in an irregular seafloor morphology made up of blocks created by episodes of intense volcanic activity or tectonic deformation.
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