PRE-RIFT COMPRESSIONAL STRUCTURES AS A CONTROL ON PASSIVE MARGIN FORMATION

Passive margins are commonly separated into volcanic and non-volcanic modes, each with a distinct formation mechanism and structure. Both form the transition from continental to oceanic crust. Large amounts of geophysical data at passive margins show that the tapering continental crust is often underlain by high-velocity and density bodies (“Lower Crustal Bodies”, LCBs). A widely accepted theory of the origin of LCBs is that they were emplaced by magmatic underplating at volcanic margins. At the same time mantle serpentinization is thought to create geophysically similar structures at non-volcanic margins due to syn or post rift hydrothermal circulation.

In this study an alternative model is presented that explains the oceanic rifting process from the onset to the formation of passive margins without the requirement of magmatic underplating or in situ mantle serpentinization. Instead rifting is focussed at relict subduction and suture zones, which may inherit rheological and compositional anomalies, such as eclogitized crust or serpentinized mantle, formed by the collision. Thermo mechanical numerical modelling shows that these pre-existing bodies both contribute to the distribution of strain during extension and form structures within the lower crust. The numerical model is consistent with the observation that passive margins often show dipping sub-Moho structures that are commonly interpreted as remnants of subducted crust emplaced during pre-rift compressional phases of the Wilson cycle and shows that such structures can ‘survive’ subsequent rifting and continental break up.

Our model is a simple alternative that explains observations at passive margins and rift zones by accounting for the observation that most passive margins are sub-parallel to earlier shortening and extension events (Wilson cycles) and therefore provides a self-consistent mechanism for both the localization of rifting and origin of LCBs. The rifting process is spatially triggered and focussed by the inherited weaknesses, which at the same time produce the distinct structures of both volcanic and non-volcanic margins. The understanding of structures produced by the collapse and extension of former orogens may impose important constraints for the reconstruction of former plate motion and strain rate histories.