TRANSTENSIONAL RIFTING: FROM PULL-APART BASINS TO SHEARED MARGINS

We use three-dimensional crustal models of pull-apart basin formation to study the formation and evolution of pull-apart basins, and their implications for the structure of sheared margins. Sheared margins are formed by oblique extension of continental lithosphere. The seafloor spreading segments that are formed after breakup are short, separated by long transform faults. This configuration reflects the discontinuous master strike-slip fault setting where pull-apart grabens develop at fault stepovers during the continental extension stage. During this stage, the geometry of the strike-slip fault discontinuity controls the structure and evolution of pull-apart basins. After breakup, adjacent segments of sheared margins are observed to be structurally very different; narrow and wide margins for example are found in adjacent segments.

Our models show how the shape and structure of the continental pull-apart basins depend on the geometry of the strike-slip discontinuity. The models also show that a situation favorable for continental breakup develops when the segments of the strike-slip faults overlap, and tensional stresses rotate to become perpendicular to the strike of the master faults. Based on these and previous modeling results and observations we propose a schematic model for sheared margin formation. The schematic model describes how the structure of a sheared margin is affected by the geometry of the pre-breakup strike-slip fault system and pull-apart basin evolution. The model offers an explanation for the variety in margin structure that is observed in the Gulf of California and at other sheared margins. It supports the transtensional shearing model for the formation of the Gulf of California, whereby the short spreading segments developed from pull-apart grabens that formed at a discontinuous fault system. The continent-ocean transition of the mature equatorial Atlantic margins is shaped as a zigzag pattern, with transform and rifted margin segments alternating. This typical sheared margin feature can be explained by our schematic model, which shows that these margins developed from pull-apart basins in an en echelon configuration.