Fingerprints of necking domains at rifted margins: A review of the best-documented examples worldwide

During rifting, the continental crust is usually thinned from its initial thickness to ca. 10 km over only a few million years. The mechanisms of this so-called necking phase are incompletely understood, and yet they have major implications on the structural, thermal and isostatic evolution of rift systems. One major difficulty in studying the necking phase arises from its transient character in the framework of continental rifting, and from the subsequent extension and thermal relaxation that may overprint it. Consequently, the fingerprints of the necking process are partially dismembered and overprinted in present-day rifted margins, and hence are poorly constrained.

In this contribution, we synthesize data from the best calibrated necking domains worldwide to define general recognition criteria and identify the processes controlling the necking phase. We show that the necking domain of rifted margins usually displays the following body of evidence: (1) deformed basement directly overlain by undeformed syn-rift sediments; (2) exhumation of relatively deep continental crust; (3) syn-rift basement erosion and adjacent sandstone deposition; and (4) syn-rift and syn-tectonic/syn-kinematic shallow-water sediments directly overlain by syn-rift but post-tectonic/post-kinematic deeper-water deposits. We assert that the combination of these fingerprints cannot be explained by the classical high-angle normal fault model by itself and discuss the possible additional and/or alternative processes. We argue that extensional detachment faults can by themselves account for the combination of fingerprints listed above and are often required to accurate kinematic restoration of rift systems evolution.