Linear ridge groups: Evidence for tensional cracking in the Pacific Plate

A new class of oceanic bathymetric feature, discovered in 1987, consists of ell-echelon groups of linear ridges. There are now at least three such set s of ridges known, and their remarkable similarity supports the notion that tl-ley result from a widespread tectonic process taking place in the inter ior of plates. In two of these major linear ridge groups, the Crossgrain an d Puka Puka ridges, numerous morphologic features indicate that they origin ated as tension cracks. The form of the individual volcanic structures that make up the ridges correlates with the! degree of tension that formed them , and the similar orientation of all ridges in a group indicates that the s tress that formed them is relatively widespread geographically and temporal ly. The ridges show a characteristic sequence of development, beginning wit h a swath of small volcanoes followed by larger domical volcanoes where the ridge will eventually develop. This paper examines proposed mechanisms of formation and concludes that no single source of tension in the lithosphere could plausibly have caused both the Crossgrain and Puka Puka ridges. The similarity of the orientations of all the groups of en-echelon linear ridge s on the Pacific plate suggests that the individual ridges form normal to a least compressive stress direction that is geographically variable in the crust. Parallel ridges constrained to a narrow band then result in an en-ec helon arrangement, though some closely spaced, synchronously formed sets of two or three ridge segments may have: influenced each other's form.