CRUSTAL STRUCTURE OF NORTH-ATLANTIC FRACTURE-ZONES

Seismic studies have established that large-offset transforms along th e slow spreading Mid-Atlantic Ridge exhibit anomalous crustal structur es that fall well outside the range typically associated with oceanic crust. Seismically, fracture zone crust in the North Atlantic is extre mely heterogeneous in both thickness and internal structure. It is fre quently quite thin (<1-2 km thick) and is characterized by low compres sional wave velocities and the absence of a normal seismic layer 3. A more gradual crustal thinning can extend up to several tens of kilomet ers from these fracture zones. Anomalously thin crust has also been in ferred from both seismic and gravity studies at smaller ridge axis dis continuities along the Mid-Atlantic Ridge. The geological nature of th e seismically anomalous crust found within Atlantic fracture zones, an d how this crust forms, are still controversial. One interpretation co nsistent with available seismic observations is that the crust within North Atlantic fracture zones consists of a thin, intensely fractured, and hydrothermally altered basaltic section overlying ultra-mafics th at are extensively serpentinized in places. Variations in apparent sei smic crustal thickness along fracture zones may reflect different degr ees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust. The existence of a thinner crustal sec tion in fracture zones can be explained by a reduced magma supply with in a broad region near ridge offsets due to the three-dimensional natu re of upwelling beneath a segmented spreading center and by tectonic d ismemberment of the crust by large-scale detachment faults that form p referentially In the cold. brittle lithosphere near the ends of segmen ts.