Geometry and structural evolution of ultra-high-pressure and high-pressure rocks from the Dora-Maira massif, Western Alps, Italy

Abstract

The crystalline nappes of the Dora-Maira massif, Western Alps, essentially made of continental material from the upper crust, show petrological 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 porphyroclasts, in a retrograde, greenschist-facies regional deformation fabric. The 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. Shear sense markers indicate a W-verging overthrust mechanism. The UHP and HP relic structures arc of variable nature. The coexistence of equant and inequant, either symmetric or asymmetric fabrics, indicates that the deformation at UHP-HP conditions was strongly heterogeneous and partitioned. This is also supported by the local preservation of Hercynian, magmatic fabrics. The UHP and HP deformation involved, at least locally, rotational components, although less intensive than during the latter retrograde stage. The regional structural evolution is envisaged as follows: (i) the Eoalpine subducted crust was subdivided into lenticular bodies surrounded by UHP-HP shear zones. The main part of the exhumation processes remains unconstrained due to the sparseness and late rotation of the UHP-HP structural relics; conflicting models are possible depending on the interpretation of the early sense of movement (normal vs reverse) along the faults that limit the lens-shaped units; and (ii) the late, heterogeneous, regional greenschist deformation can be attributed to the Eocene collapse of the Alpine orogenic wedge.