GEOMETRY AND STRUCTURAL EVOLUTION OF ULTRA-HIGH-PRESSURE AND HIGH-PRESSURE ROCKS FROM THE DORA-MAIRA MASSIF, WESTERN ALPS, ITALY

The crystalline nappes of the Dora-Maira massif, Western Alps, essenti ally made of continental material from the upper crust, show petrologi cal relics of an ultra-high-pressure (UHP) to high-pressure (HP, 'cold ' eclogite) Eoalpine metamorphism. They also display relics of UHP-HP structures, preserved in boudins and/or within large UHP porphyroclast s, in a retrograde, greenschist-facies regional deformation fabric. Th e greenschist-facies overprint has the character of a shallow-dipping mylonitic foliation (S(m)). bearing a penetrative stretching lineation (L(m)) which roughly parallels the axes of coeval, isoclinal folds. S hear sense markers indicate a W-verging overthrust mechanism. The UHP and HP relic structures are of variable nature. The coexistence of equ ant and inequant, either symmetric or asymmetric fabrics, indicates th at the deformation at UHP-HP conditions was strongly heterogeneous and partitioned. This is also supported by the local preservation of Herc ynian, magmatic fabrics. The UHP and HP deformation involved, at least locally, rotational components, although less intensive than during t he latter retrograde stage. The regional structural evolution is envis aged as follows: (i) the Eoalpine subducted crust was subdivided into lenticular bodies surrounded by UHP-HP shear zones. The main part of t he exhumation processes remains unconstrained due to the sparseness an d late rotation of the UHP-HP structural relics; conflicting models ar e possible depending on the interpretation of the early sense of movem ent (normal vs reverse) along the faults that limit the lens-shaped un its; and (ii) the late, heterogeneous, regional greenschist deformatio n can be attributed to the Eocene collapse of the Alpine orogenic wedg e.