Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)

Abstract

Microbathymetry data, in situ observations, and sampling along the 13°20′N and 13°20′N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high‐angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the along‐extension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension‐parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 13°20′N OCC, and gabbro and peridotite at 13°30′N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 13°30′N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 13°20′N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution.

Pervasive silicification and hanging wall overplating along the 13°20′N oceanic detachment fault (Mid‐Atlantic Ridge)

“…These detachment‐dominated corridors show a thin crust attributed to low magma supply relative to the more magmatic 14°N area [ Smith et al ., ; Mallows and Searle , ]. Here the oceanic lithosphere is associated with significant extension and tectonic rotation along these large fault scarps [ MacLeod et al ., ; Schouten et al ., ], in addition to several active hydrothermal fields, such as Logachev, Ashadze, or Semyenov, among others [ Beltenev et al ., ; Fouquet et al ., ; Escartín et al ., ].…”

“…This OCC exhibits a domal structure ∼5 km wide along axis that rises ∼1 km above the nearby axial valley. The detailed morphology of this detachment demonstrates that it is well preserved and unfaulted (Figure b) [ Escartín et al ., ]. This OCC has been extensively studied in recent years with near‐bottom, high‐resolution sonar mapping [ MacLeod et al ., ; Mallows and Searle , ], in addition to camera tows and sampling [ Cherkashev et al ., ].…”

“…This OCC has been extensively studied in recent years with near‐bottom, high‐resolution sonar mapping [ MacLeod et al ., ; Mallows and Searle , ], in addition to camera tows and sampling [ Cherkashev et al ., ]. In 2013, we conducted the ODEMAR cruise to collect near‐bottom multibeam bathymetry, imagery, and samples, and main geological observations and detachment‐related processes are detailed elsewhere [ Escartín et al ., ]. The detachment fault plane is exposed at the seafloor as it emerges from the rift valley floor at an angle of ∼15° along the hanging wall cutoff (termination).…”

“…The fault flattens progressively at ∼3 km off axis, where it summits ∼900 m above the hanging wall cutoff. The microbathymetric corrugations, parallel to the spreading direction, have amplitudes of <20 m extending up to 2 km along‐extension [ Escartín et al ., ]. Further off‐axis, between the corrugated surface and the detachment breakaway ridge (footwall cutoff, Figure b), the seafloor relief is elevated and rougher, corresponding to the chaotic terrain as described for this OCC [ Escartín et al ., ].…”

On the Development of Shear Surface Roughness

“…This new type of mechanical model for the origin of slickensides has significant implications in interpreting the roughness characteristics of slip surfaces in specific settings, which do not show any imprints of abrasion, plowing, or synkinematic mineral growth, as elaborately discussed by Means (). For example, several studies have shown that the corrugated topography on slip surface results from plastic strain localization in the slip zones (Escartín et al, ; Parnell‐Turner et al, ). Our mechanical model suggests that such linear irregularities can also develop as a consequence of mechanical instabilities in the ductile slip zones.…”