Field Analogues to Understand the Evolution of Hyper-Extended Ultra-Deep Water Passive Rifted Margins: the Examples of the Alpine Tethys

The study of rifted margins is incontestably undergoing a paradigm shift. The discovery of exhumed continental mantle and hyper-extended crust devoid of significant normal faulting directly overlain by shallow marine sediments, as observed in many margins, is proving fundamental in defining the controls and processes that thin the continental lithosphere. However, the development of these new concepts and their application in the hydrocarbon industry critically depends on the access to pertinent geological observations. At present, little is known about the processes controlling crustal thinning and the associated subsidence and thermal history. Having access to key outcrops is a prerequisite to rigorously test conceptual and numerical models proposed to explain hyper-extended ultra-deep water rifted margins. The aim of our presentation is to describe the rift structures and related crustal and mantle rocks and depositional systems associated with extreme crustal thinning observed in the Alpine Tethys margins. Within this ancient example the former architecture of the margin can be reconstructed. The most prominent structure is a detachment and decollement system that can be mapped from relatively unextended continental crust towards exhumed mantle into the first embryonic oceanic crust. These structures are overlain by extensional allochthons, tectono-sedimentary breccias and syn- and post-rift sediments and, further oceanwards, by MOR-basalts. In detail, these structures are complex and cannot be described as one simple low-angle detachment fault as commonly shown in the classical simple shear model. Our investigations suggest that extension at these rifted margins developed through a sequence of rift phases initiating with pure shear (e.g. stretching phase) and terminating with simple shear (e.g. exhumation phase). The most enigmatic part, still poorly understood, is the phase of deformation that links the stretching with the exhumation phase and that is responsible for extreme thinning of the crust to less than 10 km (e.g. thinning phase). In the Alps, the thinning phase is well preserved in the Bernina domain in the Eastern Central Alps, which separates the proximal from the distal margins. In the Bernina domain, rift-related pre-Alpine shear and fault zones can be mapped, that juxtapose mantle and lower crustal units against upper crustal units. Locally, sediments and extensional allochthons are observed that overlie the pre-Alpine fault zones. In our presentation, we describe the strain evolution as observed in the different parts of the margin and their relationship with mantle and sedimentary processes. These observations have major implications for the thermal evolution and consequently for the rheology and isostasy of the extending lithosphere and the survivability of syn-to post-rift petroleum systems in hyper- extended margins.