From passive margins to orogens

This excursion deals with the evolution of the ophiolite-bearing crust of the Western Alps from Jurassic rifting to Eocene-Oligocene (ultra-)high-pressure metamorphism. The main topics of this excursion are: (1) the architecture of (U)HP units; (2) multi-stage deformation during subduction/exhumation; (3) the geodynamic significance of continental basement juxtaposition with serpentinized peridotites and syn- to post-rift sediments, which is common to several alpine (U)HP ophiolites.
During the first day, at the Petit St. Bernard pass, we will show that continental basement, serpentinized subcontinental mantle and metasediments preserve pre-Alpine rift-related relationships, despite having undergone multiple phases of alpine deformation and metamorphism at P≥1.5GPa. Similar relationships are typical of Zones of Exhumed Subcontinental Mantle
(ZESM), observed along present-day magma-poor rifted margins, where  sub-continental mantle is exhumed at the basin floor through the activity of low-angle detachment faults. Fault activity may locally result in sampling of hangingwall material,
including continental basement, which then resides on the fault plane as an “extensional allochthon”. These findings indicate that the presence of serpentinized ultramafics alongside continental basement and syn- to post-rift sediments, which is common to several alpine metamorphic terrains, is not necessarily related to complex subduction dynamics, but it may also be inherited from the pre-subduction history.
During the second day, a visit to the Lago di Cignana ultra-high pressure unit will give us the opportunity to discuss the tectono-metamorphic evolution of early-post rift metasediments and metabasalts in the light of the recent finding of microdiamond inclusions in garnet from Mn-bearing quartzites. Multi-stage exhumationrelated shuffling of the tectono-metamorphic pile, together with late-stage folding, prevents the recognition of coherent sections of Jurassic crust of the kind observed during the first day.

DOI 10.3301/GFT.2014.01

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