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

Abstract

[1] Deformed rocks sampled from a corrugated detachment fault surface near the Mid-Atlantic Ridge (15°45 0 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).

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“…Location of cored sites (RD2 and MeBo) overlain on multibeam bathymetry acquired during Expedition 357 (multibeam resolution = 50 m reoriented using borehole wall imagery demonstrate that the Atlantis Massif footwall has undergone a bulk tectonic rotation of at least 46° ± 6° around a ridge-parallel (011°-trending) axis, consistent with a "rolling hinge" flexural unloading model for the detachment fault system (Morris et al, 2009). Together, these results challenge the commonly held hypothesis that the bulk rheology of slow-spreading lithosphere is primarily controlled by cooling of a simple thermal structure and instead suggest that strain localization favored by lithologic contrasts and hydrous alteration are also important, as well as perhaps highly heterogeneous cooling around large hydrothermal systems (Boschi et al, 2006;Karson et al, 2006;Escartín et al, 2003;McCaig et al, 2010).…”

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

“…The presence of serpentinite porphyroclasts whose veining structure is cut by fibrous tremolite shear zones requires that the shear zone formed after serpentinization of peridotite. These shear zones resemble metaperidotite ''fault schists'' described by Escartín et al [2003] from an oceanic detachment fault at 15°N on the Mid-Atlantic Ridge. Semibrittle shear zones consisting of intensely altered peridotite suggest strain localization via reaction softening processes.…”

A short electromagnetic profile across the Kane Oceanic Core Complex

“…The footwall composition is debated, with models and evidence ranging from extensive serpentinite bodies, to mainly gabbro bodies capped by serpentinized rocks. The fault zone appears to be thin, possibly on the order of tens of meters or less [Escartín et al, 2003;Schroeder and John, 2004;Karson et al, 2006], and shows variable composition and degree of alteration (from talc schists to deformed mafic and ultramafic rocks). Some OCC models predict extensive serpentinite bodies while others suggest that serpentinites are essentially restricted to the fault that uplifts the gabbro footwall [Ildefonse et al, 2007].…”

“…[3] Competing models of OCC formation and evolution have been proposed, including the creation of low angle detachment faults [e.g., Cannat et al, 1997;Escartín et al, 2003;Buck et al, 2005;Tucholke et al, 2008]. In general, OCCs are thought to form when the reduction in magma supply crosses a threshold that triggers detachment formation.…”