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Recent models propose that the exhumation of high-pressure rocks occurs by means of return flow inside a low-viscosity channel of serpentinite situated between the plates. To test this hypothesis, we investigated a serpentinite melange in the Western Alps, which contains exotic mafic and metasedimentary tectonic blocks, recording heterogeneous metamorphic evolutions and variable high-pressure ages. The peak metamorphic conditions range from eclogite- to garnet-blueschist-facies. The structural evidence and the pressure-temperature paths of the different blocks suggest coupling between blocks and matrix, at least in the blueschist facies. [sup.39]Ar-[sup.40]Ar dating indicates eclogite-facies peak at ca. 43 Ma and blueschist-facies peak at ca. 43 and 40 Ma in different blocks, respectively. These data point to diachronous metamorphic paths resulting from independent tectonic evolutions of the different slices. We therefore propose that this melange formed during exhumation of subducted rocks equilibrated at different depths inside a subduction channel. This mechanism can be extended to other serpentinite melanges in the Alps and other orogens (e.g., the Cyclades, the Coast Ranges of California) for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure-temperature paths can be expected with further investigations. Keywords: subduction channel; tectonic melange; Western Alps; high-pressure, [sup.39]Ar-[sup.40]Ar geochronology

We present new apatite fission-track (AFT) data from the central western Alps that confirm the synchronicity and high cooling rates during Latest Miocene-Pliocene final exhumation of the External Crystalline Massifs but also provide evidence for diachronous Neogene evolution along and across the internal arc. To the SE of the Pelvoux Massif, across the front of the internal arc (Penninic Frontal Thrust), the jump in AFT ages (c. 22 Ma) and in final cooling rates is significantly larger than further north. This difference results from reversal of movement along a major Oligocene thrust. In its hanging wall, the western Brianconnais Zone provides a mean AFT age of c. 27 Ma, which is older than further north. Early cooling in the southern Brianconnais Zone would result from rapid erosion of the compressional tan structure built during the Oligocene. Across the entire Brianconnais and Piemont nappe stack, with the exception of the Dora-Maira Massif, AFT ages young eastward and span the entire Miocene, a period during which this structure underwent extension. Further north a reverse gradient with ages younging northwestward has been described, prompting the question of the asymmetry of the internal western Alpine arc during its late-stage tectonic and morphological evolution.

[1] The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65-80 Ma) subduction to different depths at high-pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0-1.2, 1.0-1.5, and 1.5-2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal-scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E-W trending mylonitic shear zone under retrograde blueschist- to greenschist-facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15-20 km occurred in the footwall of a greenschist-facies, top-SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb-Sr mineral ages constrain D2 to have occurred at about 60-65 Ma and D3 at about 45-55 Ma. Thus top-SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro-Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra-HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo-Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature-dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high-stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low-stress subduction of the Liguro-Piemont oceanic lithosphere. doi: 10.1029/2005TC001927.