A Symmetric Solution for a Rift Paradox

Numerical experiments reproduce the fundamental architecture of magma-poor margins such as the Iberian or Alpine margins if the lithosphere has a particular vertical and horizontal stratification: (1) A thin weak mid-crustal layer is sandwiched between brittle upper crust and strong lower crust. (2) The entire lithosphere has a local thermal weakness in the center of the model. With such a stratification, the upper crust collapses over the weak mid crustal channel into the rift center where the lower crust and upper mantle undergo localized necking. The resulting margins show (1) the array of tilted upper crustal blocks resting directly on top of exhumed mantle at the distal continental margin, (2) the mid crustal high-strain zone with a top to the ocean sense of shear at the base of the tilted blocks (S-reflector), (3) the new ocean floor as a low angle normal fault at the tip of the continent, and (4) faulting in the brittle upper crust indicating less stretching than the bulk thinning of the crust. Based on the first three observation structural geologists proposed asymmetric rifting at the Iberian margins with these margins representing the footwall of a lithospheric detachment. However, observation (4) lead to the recognition of the same margins as upper plate margins by hydrocarbon geologists who focused on the subsidence history. Our model reproduces all observations in a rift geometry that is symmetric at a lithospheric scale. Only in the final breakup phase with the crust being thinned down to less than ten kilometers faulting in the crust localizes and a detachment fault develops. In our experiments the lower crust has to be strong in order to reproduce observations from magma-poor margins. With weak lower crust an upper crustal detachment fault may form in an early stage of rifting and a rift geometry asymmetric at a lithospheric scale can develop, if this detachment fault is not located directly above the necking site in the upper mantle. However, lower crust easily flows into the evolving rift depression and the resulting architecture of margins is very different from the one observed at the Iberian and Alpine margins. In experiments with weak lower crust, the lower plate margin shows a broad area of exhumed lower crust and neither margin displays a tilted-block array or a structure corresponding to the S-reflector.