Constraints on deformation conditions and the origin of oceanic detachments: The Mid‐Atlantic Ridge core complex at 15°45′N

Abstract

Deformed rocks sampled from a corrugated detachment fault surface near the Mid‐Atlantic Ridge (15°45′N) constrain the conditions of deformation and strain localization. Samples recovered in situ record deformation restricted to the cold (shallow) lithosphere (greenschist facies), with no evidence for significant high‐temperature deformation either at the fault zone or in the footwall near it. High‐temperature deformation (∼720–750°C) is observed only at two sites, and cannot be directly linked to the detachment. Detachment faulting was coeval with dyke intrusions that cross cut it, as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones, and by the presence of chill margins in diabase against fault rock. Basalts are very scarce and restricted to clasts in breccias, with no evidence of pillows or extrusive structures. Gabbros crop out along mass‐wasted and fault scarps structurally below the detachment. Footwall rocks show little or no deformation, due to strain localization along a narrow shear zone (<200 m) with fluid flow, as required to form talc‐ and amphibole schists after an ultramafic protolith. We speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time. Our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold (shallow) lithosphere is applicable to other corrugated surfaces along slow‐ and intermediate‐spreading ridges. These observations preclude detachment models rooting in melt‐rich zones (i.e., Atlantis Bank, Southwest Indian Ridge) or recording high‐temperature deformation. We infer that oceanic detachment faults (1) localize strain at T < 500–300°C, (2) persist during active magmatism, and (3) root at shallow rheological boundaries, such as a melt‐rich zone or magma chamber (“hot” detachments) or an alteration front (“cold” detachments).