What controls soft and hard linkages within rift zones? Insights from analogue models.

Structures within rift zones exhibit two types of interaction: relay ramps or accommodation zones (soft linkages) and transfer faults (hard linkages). Analogue experiments have been performed to understand the conditions leading to these types of interaction. Sand (brittle crust analogue) overlies two adjacent silicone (ductile crust analogue) layers with different viscosity, in order to simulate crustal portions with differential percentage of extension De. The experiments show a distinct behaviour as a function of De. For De < 21%, extensional structures interact forming relay ramps subparallel to the rift trend, that is soft linkages; for De > 21%, the interaction occurs by means of transfer faults subparallel to the extension direction, that is hard linkages. A comparison with rift zones shows a consistent behaviour. Soft linkages characterize narrow rifts and oceanic ridges, where the mean percentage of extension is low (e < 16%). Conversely, hard linkages are usually found in extensional settings (passive margins, wide rifts, back-arc basins) with significant stretching (e > 39%), where the rift can locally achieve differential extension De > 21%.