THE AMPHIBOLITE–ECLOGITE–UHP TRANSITION IN WESTERN NORWAY AND IMPLICATIONS FOR EXHUMATION TECTONICS

Various models invoked to explain the exhumation of ultrahigh-pressure (UHP) terranes generally call upon these rocks either to remain as a coherent part of the subducted lithosphere, or to delaminate from it. A basic discriminant between these two endmembers then is to examine the basal contact of a UHP terrane. If the terrane remained attached to its lithospheric substrate, a structurally continuous transition in metamorphic grade may be expected, whereas if delamination occurred, the lower contact must be demonstrably tectonic, with considerable excision of section.

In the Dora Maira and Kokchetav UHP terranes, peak pressures drop significantly across major shear zones in the footwall, favouring delamination of the UHP crust from its substrate. The transition from higher pressure rocks in the UHP Dabie Shan is not well understood--a basal decollement has been postulated but is not exposed, and seismic studies of the Dabie orogen do not reveal a clear discontinuity.

In the Nordfjord area of western Norway, UHP (>2.7 GPa) and HP (2.2–2.7 GPa) eclogites occur above and below (HP only) a major extensional detachment. The boundary zone between the UHP and HP eclogites is transitional over a 40 km distance and not coincident with the extensional detachment, and hence is likely the result of kinetics (e.g., variations in times and pressures of equilibration) or deformation predating motion along the detachment. Furthermore, the HP eclogites below the detachment diminish in abundance toward the foreland, with no recognized metamorphic or structural break. We interpret these features to indicate that the UHP and HP area of western Norway was subducted and exhumed as part of a coherent crustal (and possibly lithospheric) section and that the major extensional detachment is a later, intracrustal feature that was not responsible for significant exhumation.

Elastic rebound following slab failure is a necessary finale of continental collision and may provide a mechanism for exhuming a coherent (U)HP crustal section.