The internal structure of an oceanic core complex: An integrated analysis of oriented borehole imagery from IODP Hole U1309D (Atlantis Massif)

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Abstract

[1] Oceanic core complexes at slow-spreading ridges represent the uplifted footwalls of large-offset ‘detachment’ faults that initiate at steep dips, and rotate to flatten via a ‘rolling hinge’ mechanism in response to flexural unloading. A key question is whether oceanic core complex development is accommodated entirely by displacement on the detachment fault zone, or if significant internal deformation of the footwall occurs during flexure and rotation. We investigate this by constraining the internal architecture of the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N) using Formation MicroScanner borehole wall images and cores from the 1416 m-deep Integrated Ocean Drilling Program Hole U1309D. Two distinct sets of structures are observed. N-S-striking, E-dipping structures dominating the upper 385 m are interpreted as a brittle to semi-brittle zone of fracturing in the footwall. Structures with this geometry occur down to 750 m below seafloor, suggesting that the detachment damage zone extends deep into the footwall. The nature of this deformation is, however, enigmatic: several cataclastic shear zones with reverse geometry in their current orientations may be rotated extensional faults or relate to shortening at the base of the flexing beam of a very weak footwall. By contrast, E-W-striking, N- and S-dipping structures dominate the lowermost kilometer of the borehole. They likely represent conjugate fractures formed in the hanging wall of a late, E-W normal fault zone, separating gabbroic rocks of the central dome of Atlantis Massif from serpentinized peridotite to the south, responsible for post-detachment uplift of the southern margin of the massif.

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