Accretion of oceanic crust under conditions of oblique spreading

Abstract

The accretion of oceanic crust under conditions of oblique spreading is considered. It is shown that deviation of the normal to the strike of mid-ocean ridge from the extension direction results in the formation of echeloned basins and ranges in the rift valley, which are separated by normal and strike-slip faults oriented at an angle to the axis of the mid-ocean ridge. The orientation of spreading ranges is determined by initial breakup and divergence of plates, whereas the within-rift structural elements are local and shallow-seated; they are formed only in the tectonically mobile rift zone. As a rule, the mid-ocean ridges with oblique spreading are not displaced along transform fracture zones, and stresses are relaxed in accommodation zones without rupture of continuity of within-rift structural elements. The structural elements related to oblique spreading can be formed in both rift and megafault zones. At the initial breakup and divergence of continental or oceanic plates with increased crust thickness, the appearance of an extension component along with shear in megafault zones gives rise to the formation of embryonic accretionary structural elements. As opening and extension increase, oblique spreading zones are formed. Various destructive and accretionary structural elements (nearly parallel extension troughs; basin and range systems oriented obliquely relative to the strike of the fault zone and the extension axis; rhomb-shaped extension basins, etc.) can coexist in different segments of the fault zone and replace one another over time. The Andrew Bain Megafault Zone in the South Atlantic started to develop as a strike-slip fault zone that separated the African and Antarctic plates. Under extension in the oceanic domain, this zone was transformed into a system of strike-slip faults divided by accretionary structures. It is suggested that the De Geer Megafault Zone in the North Atlantic, which separated Greenland and Eurasia at the initial stage of extension that followed strike-slip offset, evolved in the same way.