AAPG Middle East Region Geoscience Technology Workshop, Rift Basin Evolution and Exploration

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Magma-Poor Rifted Margins: The Alpine Tethys Perspective

Abstract

This contribution focuses on remnants of fossil conjugate rifted margins resting inside the European Alps. The rifted margin system of the Alps preserves evidence for multistage magma-poor rifting that, from Middle Jurassic onwards, led to the opening of the “Piedmont Ligurian Ocean”. This latter developed through a process of exhumation of sub-continental mantle along extensional detachment faults. During the last decades, the Alps have been considered as a key area to understand the evolution of modern magma-poor rift systems. The quality of good outcrops and their relatively easy access contributed to decipher the evolution of proximal and distal domains of the Tethys margins and their relationships and evolution in time and space. In this contribution we summarize the latest efforts to unravel the characterization and evolution of the Tethys margins. Proximal, distal and exhumed domains of the Alpine Tethys margins are described using field examples and discussed using interpreted sections from modern margins. Key-structures of rifting as necking zones and detachment faults, that are usually visible at the seismic scale only, are approached with field geology. Observations on the sedimentary record suggest that from an initial stage of widespread extension that generated large half-graben basins, the system focused deformation into a narrow domain, the distal margin, characterized by peculiar basins with different sedimentary architecture and thermal history. While the proximal margin basins were mainly filled by pelagic carbonate muds shed by the neighboring platforms, the distal basins accumulated a huge amount of coarse clastics generated by high angle normal faults rooted in the basement of the distal domain. Exhumation of subcontinental mantle follows, leading to a widening though in which complex tectonics is locally active, that brought to the formation of new, smaller half-graben basins onto the exhumed domain. Moreover, studies made on the stratigraphy of the Alpine Tethys basins and from present-day seismic data, led researchers to consider strongly asymmetric margins during the final stage of rifting and to recognize Upper and Lower plates. We will discuss this architecture in the light of the recent hypothesis of a flip in the rifting geometry, similar to that observed in the Atlantic margins, that can be observed in the Alps. Finally, the aim of our presentation is to review existing literature data and to present new research on the Alpine Tethys margins that may shed light and enhance the knowledge on magma-poor rifting dynamics.