Direct geological evidence for oceanic detachment faulting: The Mid-Atlantic Ridge, 15°45′N

Abstract

From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreadingdirection-parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.

Strain localization on an oceanic detachment fault system, Atlantis Massif, 30°N, Mid‐Atlantic Ridge

“…Deformed gabbros at Atlantis Bank [Cannat, 1991;Miranda et al, 2002], and the MARK area [Agar andLloyd, 1997, Cannat et al, 1997] indicate strain localization at amphibolite grade followed by low-temperature deformation. A study of a core complex at 15°N on the MidAtlantic Ridge indicates strain localization in peridotite at greenschist-and subgreenschist-grade conditions and no high-temperature deformation [MacLeod et al, 2002;Escartín et al, 2003]. Deformation textures at Atlantis Massif indicate that strain is initially localized into gabbro and peridotite at granulite and amphibolite grade over a wide zone, then into a narrow (100 m wide) fault in metaperidotite at greenschist and subgreenschist grade.…”

Widespread active detachment faulting and core complex formation near 13° N on the Mid-Atlantic Ridge

“…Direct evidence for such faults has only recently come from bathymetry 1,2 , and seafloor sampling and drilling 4,5,8 . The faults are corrugated parallel to the spreading direction and cap smooth topographic highs, termed oceanic core complexes, where deepseated rocks such as gabbros and serpentinized peridotites are exposed 4,5,9,10 .…”

“…Direct evidence for such faults has only recently come from bathymetry 1,2 , and seafloor sampling and drilling 4,5,8 . The faults are corrugated parallel to the spreading direction and cap smooth topographic highs, termed oceanic core complexes, where deepseated rocks such as gabbros and serpentinized peridotites are exposed 4,5,9,10 . Most core complexes have been identified towards the ends of spreading segments where magma supply appears to be low, but in places they extend for tens of kilometers parallel to the axis 4,5,[11][12][13] .…”

“…The faults are corrugated parallel to the spreading direction and cap smooth topographic highs, termed oceanic core complexes, where deepseated rocks such as gabbros and serpentinized peridotites are exposed 4,5,9,10 . Most core complexes have been identified towards the ends of spreading segments where magma supply appears to be low, but in places they extend for tens of kilometers parallel to the axis 4,5,[11][12][13] . In parts of some segments, extension by low-angle faulting may have accounted for >50% of the total extension by spreading 3,10,14 .…”

“…In well-explored oceanic core complexes the existence of a corrugated surface is correlated with detachment faulting and exposures of deep-seated rocks [1][2][3][4]6 . Consequently, corrugations have been used to identify other core complexes in regions with limited data 2 .…”