CRUSTAL THINNING PROCESSES AT RIFTED CONTINENTAL MARGINS: THE KEY ROLE OF NECKING ZONES

The mechanisms controlling the transition from rifting to drifting represent a long-standing problem in Earth Sciences, questioning our understanding of continental lithosphere extension and thinning processes. Using both present-day and fossil examples of rifted margins, we aim to explore in this contribution how the crust thins, what is the effective rheological evolution of the crust during extensional processes and which structures can accommodate the extreme crustal thinning observed.

From a present-day rifted margin perspective, the first-order crustal architecture can be imaged using reflection and refraction seismic data. These profiles show the generally sharp thinning of the continental crust at necking zones. Notably these zones highlight the transition from a thick continental crust in the proximal domain to hyperthinned continental lithosphere, made either of thinned crust or exhumed mantle, in the distal domain.

Additionally, fossil analogues of rifted margins preserved in mountain belts, provide direct access to structures and deformation of the continental crust during rifting. The fossil necking zone of Adriatic continental margin, exposed in the Alps in Western Europe, preserves evidence of crustal thinning processes. Within this necking zone, upper and lower continental crust record localized deformation along major low-angle crustal scale detachment systems. The intervening middle crust documents zones of coaxial shear associated with decollement horizons. Laterally, the necking zone passes into a hyperthinned continental crust where pre-rift upper and lower continental crust were directly juxtaposed with omission of the mid-crustal levels.

Based on observations from present-day and fossil rifted margins, we propose that the necking of the continental crust results from a combination of high-strain detachment systems in both the upper and lower crust rooting in a ductile zone at mid-crustal levels. Middle crust acts as a decoupling horizon associated with pure shear flattening, partitioning the deformation between the “rheologically stronger” upper and lower crusts. These results bring new insights for understanding the process of hyperextension that is responsible for the formation of many present-day deep-water rifted margins.