Thermal Evolution of Hyper-Extended Rifted Margins: Insights from ⁴⁰Ar/³⁹Ar Thermochronologic Investigations of the Fossil Alpine Tethys Margins Preserved in the Alps

Beltrando, Marco ¹; Manatschal, Gianreto ²
(1) Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche, Torino, Italy. (2) CGS-EOST, Université Louis Pasteur, Strasbourg, France.

Recent studies show that over 50% of the present-day distal rifted margins are characterized by a complex architecture, consisting of (1)a necking zone, where continental crust thins from ca. 30 km to ca. 10 km, (2) a wide area of thinned crust and (3) the so-called Ocean-Continent Transition Zone. While the large-scale architecture of distal rifted margins is being increasingly understood thanks to extensive geophysical investigations, at present relatively little is known about the lithological composition and thermal evolution of the hyper-extended continental basement. Understanding such thermal evolution is pivotal to reconstruct the temperature history of the overlying sediments.

Orogenic belts provide unique access to fossil distal margins. This study focuses on the Western Alps, which sample both the European and Adriatic hyper-extended margins of the Jurassic Western Tethys. In the Western Alps, despite pervasive orogeny-related deformation and metamorphism, several present-day tectonometamorphic units can be restored to the pre-orogenic margin architecture based on the preservation of primary relationships between mantle rocks, continental basement and syn- to post-rift sediments.

Pre-Alpine biotites have been selected from European- and Adriatic-derived continental basement units in order to constrain their rift-related thermal evolution with ⁴⁰Ar/³⁹Ar geochronology. Samples were chosen among granitoids that were formed during late-orogenic lithospheric thinning that widely affected this area in the Late Permian, ca. 50 Myrs prior to the onset of rifting. Petrological studies allowed to determine the pre-rift depth of the rocks that were later exhumed during the rifting stage. Preliminary results indicate that biotite thermochronometers were widely reset at shallow crustal levels in the distal margins at ca. 190-170 Ma, prior to breakup, which occurred at ca. 165 Ma.

These results suggest re-heating of the crustal rocks forming the most distal parts of the Alpine Tethys margins during rifting. Re-heating may be explained by thinning and thermal erosion of the lithospheric mantle during rifting and is compatible with the observed subsidence history of the Briançonnais domain. The potential re-heating of basement during rifting has important implications for the maturation of the overlying sedimentary sequence and for thermal modeling of petroleum systems in hyper-extended rifted margins.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.