HR: 0800h
AN: T41A-0373    [Abstracts]
TI: Extreme Crustal Thinning in a Transtensional Environment: the Example of Bay of Biscay - Western Pyrenees
AU: Jammes, S
EM: Suzon.Jammes@illite.u-strasbg.fr
AF: CGS-EOST, 1 rue Bessing, Strasbourg, 67084, France
AU: * Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: CGS-EOST, 1 rue Bessing, Strasbourg, 67084, France
AU: Lavier, L L
EM: luc@ig.utexas.edu
AF: UTGI, 10100 Burnet Rd, Austin, 78759, United States
AB: The Bay of Biscay-Western Pyrenees is the ideal natural laboratory to study extreme crustal thinning within a transtensional environment. This area underwent several rift phases with a major phase occurring in late Jurassic to early Cretaceous time, responsible for the opening of the Bay of Biscay and the formation of smaller rift basins, the Parentis and Mauléon basins, ahead of the propagating ocean. The Parentis basin shows localized crustal thinning to less than 10 km, an important asymmetry of the basin and only little evidence for normal faulting. To the north the basin is characterized by a sag geometry whereas to the south it is floored by a strong reflection. This reflection truncates tilted blocks that are associated with Lower Cretaceous sediments showing evidence for growth structures. Two wells drilled across the strong reflection and penetrated into tectonized Paleozoic meta-sediments. Based on these observations we interpret the strong reflection as a detachment structure and the overlying blocks as extensional allochthons. About 30 km further to the east, the strong reflection approaches the surface near massif du Labourd. This massif is formed by granulite facies lower crustal rocks that are separated, along a mylonitic shear zone, from upper crustal rocks including Paleozoic meta-sediments. The top of the basement is tectonized and covered by tectono-sedimentary breccias, which are sealed by Lower Cretaceous sediments. In analogy to similar observations in the Alps and Iberia, we interpret the tectono-sedimentary breccias as remnants of a top basement detachment fault that truncated an older mylonitic shear zone separating lower and upper crustal rocks. This structure might be important for the extreme crustal thinning observed in the basin. The Mauléon basin was partly reactivated during the Pyrenean compression, which has the advantage that deeper parts of the basin are directly accessible and can be studied in the field. A key observation is that serpentinized mantle peridotites, associated with ophicalcites, are overlain by sedimentary breccias composed of mantle rock, lower crust rocks and mylonites. This suggests that the mantle and lower crustal rocks were locally unroofed and reworked within the Mauléon basin. In such a context, the massive pre-rift sediments forming the mountain ridges within the Mauléon basin may be interpreted as extensional allochthons rather than remnants of classical fault bounded rift-basins, an idea that need to be tested. The stratigraphic record of extreme crustal thinning is documented by up to 900 m of Albian conglomerates reworking Paleozoic metasediments, indicating rapid subsidence in the basin simultaneous with uplift at the southern margin of the basin. Our preliminary results show evidence for extreme crustal thinning in the Parentis and Mauléon basins forming ahead of a propagating oceanic system. In contrast to orthogonal rift systems, the deformation is more localized and characteristic structures such as the H-block are not observed. Instead, complex relationships with syn- sedimentary lower Cretaceous transform faults are observed and need to be taken into account to describe the overall strain distribution. The results of this work, if confirmed, will not only allow to understand extreme crustal thinning in a 3D transtensional setting, but will also result in a reinterpretation of the Pyrenees and ultimately of the kinematics of the Iberian plate with major consequences for the tectonic evolution of the North Atlantic and Alpine realms.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting