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[1] Microstructural observations, mineral chemistry, and the spatial distribution of deformation fabrics recorded in outcrop samples collected from Atlantis Massif, the active inside corner high at 30°N, MidAtlantic Ridge, suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault. Deformation textures in peridotite and gabbro indicate that high-temperature (>500°C) strain occurred via crystal-plastic flow and diffusive mass transfer. Low-temperature (<400°C) shear zones containing brittle and semibrittle microboudinage textures in which tremolite, chlorite, and/or talc replace fractured serpentine or hornblende cut earlier formed high-temperature deformation fabrics in peridotite. Textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism. Gabbro is only weakly deformed below amphibolite facies (<500°C), indicating that strain was partitioned into altered peridotite at low temperature. There is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the Massif. Discontinuous high-intensity crystal-plastic deformation fabrics are found at all structural depths (0-520 m) beneath the surface, indicating that high-temperature, granulite-and amphibolite-grade deformation was not localized in a single shear zone. In contrast, semibrittle and brittle low-temperature shear zones are concentrated less than 90 m structurally beneath the surface, and the most intensely brittlely deformed samples concentrated in the upper 10 m. Localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault. The structural evolution of Atlantis Massif is therefore analogous to a continental metamorphic core complex. Strain was localized onto the fault by reaction-softening and fluid-assisted fracturing during greenschist-and subgreenschist-grade hydrothermal alteration of olivine, clinopyroxene, serpentine, and hornblende to tremolite, chlorite, and/or talc.

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[1] Drilling during ODP Leg 209, dredging, and submersible dives have delineated an anomalous stretch of the Mid-Atlantic Ridge north and south of the 15°20 0 N Fracture Zone. The seafloor here consists dominantly of mantle peridotite with gabbroic intrusions that in places is covered by a thin, discontinuous extrusive volcanic layer. Thick lithosphere (10-20 km) in this region inhibits magma from reaching shallow levels beneath the ridge axis, thereby causing plate accretion to be accommodated by extensional faulting rather than magmatism. The bathymetry and complex fault relations in the drill-core suggest that mantle denudation and spreading are accommodated by a combination of high-displacement, rolling-hinge normal faults and secondary lower-displacement normal faults. These extensional faults must also accommodate corner flow rotation (up to 90°) of the upwelling mantle within the shallow lithosphere, consistent with remnant magnetic inclinations in denuded peridotite and gabbro from Leg 209 core that indicate up to 90°of sub-Curie-temperature rotation.

We document soil and groundwater contamination caused by industrial airborne PFAS emission over an area of ∼200 km2 that has impacted over 1200 wells.