HR: 09:30h
AN: T51E-07    [Abstracts]
TI: Tectonic Accretion Channel (TAC): Its Role in the Exhumation of HP Metamorphic Rocks in the Central Alps
AU: * Brouwer, F M
EM: brouwer@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, Bern, CH-3012 Switzerland
AU: Engi, M
EM: engi@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, Bern, CH-3012 Switzerland
AU: Berger, A
EM: berger@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, Bern, CH-3012 Switzerland
AB: In the Central Alps high-pressure metamorphic rocks are confined to but a few tectonic units. In the Adula nappe pressures range from about 1.25 GPa in the north, to 2.5 GPa in the south [1]. The Southern Steep Belt (SSB) is essentially composed of a high-strain zone surfacing at the contact between rocks deriving from Apulia (S) and Eurasia (N). The SSB contains a tectonic composite of ortho- and paragneisses, with widespread fragments of mafic and ultramafic composition. Many of the mafic lenses are garnet-amphibolites or eclogites, with a highly variable degree of retrogression. Our petrological studies indicate that the HP rocks in the SSB show extensive variation in metamorphic pressure. In mafic fragments, pressures retained by assemblages predating the amphibolite facies overprint range from 0.8 to 2.1 GPa, while pressure estimates for some peridotites are $>$3 GPa. Some fragments show evidence of substantial heating during decompression. Lu-Hf geochronology, in conjunction with previously published data, indicates a spread in ages obtained from the high-pressure metamorphic assemblage (63 to 36 Ma). Thermal models based on simplified kinematics produce computed PTt histories that resemble those documented in individual HP fragments [2]. The SSB, the Adula nappe, and associated tectonic slices with HP-relics are interpreted to represent an exhumed part of a Tectonic Accretion Channel (TAC, cf. [3]). This TAC is assembled of numerous, relatively small fragments, which reflect a variety of PTt-paths. The different residence times and exhumation rates reflect a protracted history of subduction and extrusion, with substantial movement of fragments with respect to their current neighbours. In the Central Alps all HP-relics known to be Tertiary in age are contained in tectonic slices now interpreted to be TAC fragments. The mechanisms responsible for the rapid extrusion of deep portions of the TAC to mid-crustal levels and their incorporation in the nappe stack remain controversial. However, it seems clear that the decom-pression history of eclogites and garnet peridotites in the Central Alps was initiated by disruption of the TAC. [1] Dale & Holland (2003) J. Metam. Geol. 21: 813. [2] Roselle et al. (2002) Amer. J. Sci. 302: 381. [3] Engi et al. (2001) Geology 29: 1143.
DE: 9335 Europe
DE: 9604 Cenozoic
DE: 8102 Continental contractional orogenic belts
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting