Symmetric alternative to asymmetric rifting models

Figure 1. Schematic cross sections through (A) Galicia Margin west of Iberian Peninsula (Boillot et al., 1995) and (B) Jurassic margin of Apulian continent toward Alpine Tethys (Manatschal and Bernoulli, 1999). Inset in B displays present location of margin in Swiss Alps and paleoposition of section in Jurassic time. Abbreviations in A: lcc—lower continental crust; man— mantle; S—S-reflector; sed—sedimentary rocks; ucc—upper continental crust. Tectonic units in B that incorporate corresponding positions (color coded) at rifted margin: Ber—Bernina nappe system (large horst developed during early stage of rifting); Err—Err nappe (distal continental margin); Mal—Malenco unit (lower crust and adjacent upper mantle); Pla—Platta nappe (oceanic lithosphere). ABSTRACT We describe the first numerical simulation of continental rifting that reproduces the three major structures observed at magmapoor margins such as the Galicia Margin west of the Iberian Peninsula or the Apulia Margin in the Alps: (1) Distal continental margins consist of fault-bounded blocks separated by oceanwarddipping normal faults. (2) At the tip of the continent, lower crust is scarcely preserved or absent, and upper crust directly overlies exhumed mantle. (3) The base of the rotated crustal blocks is a prominent seismic reflector and represents a high strain zone with a top-to-the-ocean sense of shear. In our model, these structures do not reflect asymmetric rift geometry at a lithospheric scale. Instead, they derive from upper-crustal collapse over a mid-crustal shear zone into the rift center and are present on both sides of the rift axis. The model has a horizontal weak zone in the middle crust on top of strong lower crust and a localized vertical zone of thermal weakness in the rift center. We hypothesize that the development of a thermal perturbation and associated strain localization in the deeper lithosphere may cause the transition from widely distributed faulting and crustal thinning to constricted faulting directed toward a well-defined rift center.