Programme des sessions > Recherche par auteur > Mello Sydney De Matos

The Western flank of the Rio Grande Rise (RGR), a prominent oceanic plateau in the South Atlantic, comprises a sector where deep-seated layered volcanic rocks reveal features of volcanic reactivation commonly associated with faults that convey upward fluid migration. Volcanics are covered by a sedimentary succession of circa 700 ms thick. New high-resolution multibeam bathymetry and sub-bottom profiler dataset revealed a vast field of at least 35 giant pockmarks located on the northern flank of the Western RGR (area of ~4,000 km2) at water depths of 1,100–1,700 m. Pockmarks exhibit circular to elliptical morphologies with average diameters of approximately 6,000 m (ranging from 2-8 km) and reliefs commonly exceeding 100 m. Many of them have coalesced into composite, elongated structures up to 25 km long. A striking characteristic of the field is the dominant NW-SE alignment of individual pockmarks and their coalesced “chains”, parallel to the Cruzeiro do Sul Rift, as well as to an underlying seismic recognized fault system. Slope gradient analysis indicates that the pockmarks province is situated on a gentle regional slope (typically <0.5°), which seems to rule out large-scale slope instability as their primary formation mechanism. Seismic profiles show variable internal pockmarks, suggesting episodic fluid expulsion over time. Some faults show evidence of neotectonic activity by cross-cutting pockmark rims. We conclude that; (i) the genesis of these giant pockmarks seems to be primarily controlled by fault-focused expulsion of deep-seated fluids. The occurrence of nearby mud volcanoes, morphologically and acoustically distinct yet apparently genetically-related, reinforces the interpretation of active or recent fluid flow in the region; (ii) to our knowledge, such an assembly of depression features stands as a world-class occurrence when it comes to pockmark's individual dimensions; (ii) this discovery challenges the paradigm of the RGR plateau as a quiescent feature, demonstrating that the RGR is a geodynamically active province with significant, ongoing or recent, fluid migration activity. Our findings also have profound implications for understanding intraplate tectonics, the evolution of post-hotspot tectonic provinces, fluid circulation in old crust, and the potential for associated mineral resources and ecosystems.