Stéphanie Duchêne

Two-phase orogenic convergence in the external and internal SW Alps

The NW–SE-trending sector of the SW Alps includes the Dora Maira massif where Tertiary eclogites record ultrahigh pressures and rapid exhumation. Along a NE–SW crustal cross-section (Italy–France) compiled pressure–temperature–time data in internal zones are correlated with Tertiary stratigraphy in external zones to reconstruct orogen evolution, revealing a coherent two-phase convergence history. During the first, subduction–accretion phase (Eocene, 55–34 Ma) rapid north–south plate convergence caused the subduction and exhumation of high-pressure and ultrahigh-pressure (UHP) rocks in a steady-state subduction channel. This coincided with the north to NNW migration of an underfilled flexural basin across the European foreland. Nappe stacking within the subduction channel did not create significant relief, implying that primarily subduction forces generated this flexural basin. From 34 Ma onward, the second, collisional phase was characterized by slower NW–SE plate convergence. The internal units of the SW Alps underwent considerable anticlockwise rotation as they became involved in a NW–SE-oriented sinistral transpression zone between the European and Adriatic plates. To the north of the orogen the North Alpine Foreland Basin became overfilled as a result of high sediment supply from increasing orogen relief. In contrast, in SE France active flexure of the European plate appears to have ceased and sedimentation became limited to small thrust-sheet-top basins created by continuing gentle NE–SW shortening. Internal units were exhumed slowly from depths of c. 20 km, principally by erosion. In the SW Alps, the transition between these two phases was marked by the rapid subduction and exhumation of the Dora Maira UHP unit. Assuming lithostatic pressure, this unit would have been exhumed from 100 km depth, requiring a rate that exceeds that generated by plate convergence. Therefore, either exhumation was accelerated by additional stresses (locally generated by transpression, slab breakoff or high density contrasts) or, more controversially, the ultrahigh pressure occurred at a considerably shallower depth as a result of local overpressure.

Pressure-Temperature-time Evolution of Metamorphic Rocks from Naxos (Cyclades, Greece): Constraints from Thermobarometry and Rb/Sr dating

The Pressure-Temperature-time paths of metapelites sampled on an east-west transect across the structural dome of Naxos (Greece) have been reconstructed on the basis of new geothermobarometric data and Rb/Sr dating, as well as previously published data. One sample from an intermediate structural level records pressure and temperature conditions of 10 kbar, 500°C, corresponding to its exhumation in a highpressure/low-temperature (HP/LT) setting. The corresponding Rb/Sr exhumation age is 29.3±1.3(2s) Ma. Toward the center of the dome, metamorphic assemblages record an increase in peak-temperature and corresponding pressure (from 500 to 700°C, and from 5 to 8 kbar), in a medium-pressure/medium-temperature (MP/MT) metamorphic field gradient. Whole-rock, muscovite and biotite on two samples from deep structural levels define ages of 5.2 and 7.0 Ma, whereas garnets fall outside the isochrons and retain earlier less radiogenic signatures. Rb/Sr data on these two samples demonstrate open system behaviour coeval with, or subsequent to MP/MT metamorphism. The interpretation of the Pressure-Temperature-time paths indicates a time span of 15-20 Ma for dome formation. It also suggests that the HP/LT to MP/MT transition is due to i) heating of deeper parts of the dome through magma injection or ii) either homogeneous (75 %) or localized thinning during dome formation.

Cenozoic tectonic evolution of Naxos Island through a multi-faceted approach of fission-track analysis

Abstract New zircon and apatite fission-track ages obtained on samples from all lithotectonic units exposed on Naxos Island are presented. Zircon ages of the exhumed metamorphic rocks range from 25.2 to 9.3 Ma and from 13.0 to 6.4 Ma for apatite. Zircon track-length analysis distinguishes partial overprinting of an earlier event (M1) in the south. Northwards no overprint is seen and the ages there represent rapid exhumation since c. 12 Ma. Both zircon and apatite ages are slightly older toward the north of the island probably due to variation of the geotherm in the proximity of the fault. Zircon fission-track ages of the granodiorite range from 13.7 to 12.2 Ma are statistically identical to previously determined U–Pb ages. Apatite fission-track ages however, yield a younging trend from south to north from 12.9 to 9.0 Ma. This could be due to differential depth of emplacement and/or to differential exhumation during tectonic unroofing by a top-to-the north detachment. Fission-track ages on detrital grains in Lower Miocene sediments indicate a source not identified within the present outcropping rocks of Naxos. Ages on boulders and grains in the Middle to Upper Miocene sediments point to rapid erosion until about 8.5–7 Ma.

Sm–Nd disequilibrium in high-pressure, low-temperature Himalayan and Alpine rocks

In order to decipher the causes of Sm–Nd isotopic disequilibrium in high-pressure, low-temperature rocks, Sm–Nd isotopic analyses were carried out on minerals from four Himalayan (Tso Morari unit) and four Alpine (Dora-Maira, Monte Viso, Sesia Lanzo) eclogitic rocks of different lithologies and different intensities of post-eclogitic metamorphism. In most of these