Origin of extensional core complexes: Evidence from the Mid‐Atlantic Ridge at Atlantis Fracture Zone

The contrast in geologic structure observed on opposing flanks of the Mid‐Atlantic Ridge, where it is offset by the Atlantis transform fault, illustrates how significant differences in crustal structure can result from tectonic processes that operate near the ends of slow spreading segments. New geophysical and geological data provide information on the nature of large massifs that punctuate the strips of crust formed at the inside corner of ridge‐transform intersections (RTI), as well as of the low‐relief volcanic morphology that typifies the outside corners. The geological relations mapped at the inside corner of the eastern Atlantis RTI are strikingly similar to those seen in the Basin and Range where metamorphic core complexes are unroofed through asymmetric detachment faulting. The core of the eastern RTI massif exposes deep‐seated rocks beneath a shallow‐dipping, corrugated surface which is interpreted as a fault surface. On the median valley side of the massif, this seafloor detachment is overlain by upper crustal blocks bounded by steeper fault scarps. The western side of the 15‐km‐wide massif is characterized by en echelon faults which face away from the ridge axis. Similar features are mapped at two fossil massifs that are interpreted to have formed at the inside comers of each RTI and to have rafted off‐axis as plate spreading proceeded. Analysis of new and preexisting shipboard gravity data indicates that high‐density material is not continuously emplaced at the inside corner. Rather, peaks in the gravity anomaly map are patchily distributed along the transform valley walls. The gravity highs associated with the three massifs (oceanic core complexes) in this area are not centered with respect to their morphology but are located toward their spreading axis and transform sides. Gravity modeling suggests that the western boundary of a high‐density wedge at the eastern RTI massif is steeply dipping, whereas the eastern boundary may dip about 15° toward the median valley. In contrast to the inside corners of the RTIs in our study area, the outside corner seafloor is characterized by volcanic constructions similar to those found on either side of the spreading axis at the center of the segments and inferred to be typical basaltic upper crust. Kinematic analysis at the Mid‐Atlantic Ridge‐Atlantis Transform RTI suggests that the formation of seafloor detachments may occur when the rate of extension not accommodated by magmatic input exceeds about 4 mm/yr. Isolated volcanic ridges that extend into the fracture zone domain, curving as they approach the fault trace, mark times of abundant magma supply at the segment ends. The apparent interplay between magmatic and tectonic strain accommodation at a mid‐ocean ridge, as well as the overall structure of oceanic core complexes, may provide important kinematic constraints on core complex formation and the development of shallow‐dipping detachment faults.