HP/UHP rocks in the Western Alps: acquiring new (robust) data, constraining old (fashionable) models
Résumé
The potential exhumation mechanisms of high-pressure rocks in subduction/collision belts is a longstanding
question, that may be answered combining a wealth of sophisticated techniques. Geological mapping,
combined with geophysical investigations, reveal the first-order geometrical features of the stacked units.
In the Western Alps, the nappe stack has been ‘ordered’ by E. Argand and co-workers since more than one
century. Two major additions to the proposed geometry include (i) the recognition of the gravimetric (positive)
anomaly associated to the Ivrea Geophysical Body, thought to reflect the presence of the Adriatic mantle at
low depth, and (ii) the investigation of discontinuities in terms of P-T conditions and age between stacked
units. The first point is of major importance for understanding the crustal-scale reworking of the nappe stack
(backfolding/backthrusting) during Adria indentation, while the second offers clues to the succession of events
at depth before collision.
HP/UHP rocks in oceanic and continental rocks have in common four characteristics:
• They reveal a gradient of P-T conditions consistent with low to very low geothermal gradients (5 to 10°C/
km) along the former subduction zone.
• The ‘subduction gradient’ has been destroyed during exhumation. The latter is best interpreted as due to the
upward displacement of an extrusion wedge, bounded by (north-westward) propagating ductile thrusts at
the base and (south-eastward) ‘normal shear sense’ ductile faults at the top.
• Exhumation of HP/UHP rocks has taken place shortly after peak burial, i.e., before collision, with the
implication that the processes acting during exhumation may have been severely overprinted during
collision.
• Erosion only contributes significantly to the exhumation during the final stages of the history, when collision
is proceeding.
New discoveries of UHP rocks (Manzotti et al., this meeting) and progress in P-T conditions, age and timing
of HP/UHP metamorphism (Nosenzo et al., this meeting) will be used to constrain a geometrical-kinematical
model. Strengths and weaknesses of the model will be discussed.