SUPERSTRUCTURE OF THE WESTERN GNEISS REGION, NORWAY: A NEW PARADIGM FOR GIANT ULTRAHIGH-PRESSURE TERRANES?

Numerical models of deep continental subduction and exhumation emphasize burial of coherent crustal sections, thermal weakening, crustal sinking or buoyancy through phase transformations, partial melting, reverse flow in the subduction channel, mixing of various units, and high strain zones that bound exhuming plumes, plugs, or slabs. To what extent do these predictions satisfy constraints leveraged from detailed structural, metamorphic and geochronological studies of terrestrial HP terranes?

As one of the largest and most well exposed domains of eclogite and ultrahigh-pressure (UHP; P>2.5 GPa) rocks on Earth, the Western Gneiss Region (WGR) in the Scandinavian Caledonides is an archetype for collisional processes at the base of the crust and into the mantle lithosphere. Its great extent and structural complexity, however, has obscured the size & shape, stacking order, and deformation history of internal crustal subdivisions.

Long described as relatively monolithic entity with enigmatic traces of allochthonous rocks, campaign structural mapping reveals that the WGR comprises perhaps three or more regional subunits. The Baltica autochthon is overlain by at least two thick sheets of allochthonous continental gneiss, separated by major shear zones that contain slivers of metasedimentary rocks and mantle wedge peridotite. The regional structure of the central WGR is a gently E-plunging synform, and much of the eclogite-bearing region is composed of these allochthonous thrust sheets; the Baltica basement is only exposed in the southern and northwestern parts of the WGR. UHP rocks are found in both basement and allochthonous units, indicating juxtaposition by thrusting before deep subduction.

This new conception of the regional structure of the WGR suggests even the largest ancient or active (U)HP terranes may be composite entities. The WGR remained broadly coherent during UHP metamorphism, however, and rather than mixing internally, was exhumed with limited internal strain. In spite of burial lasting tens of millions of years, partial melting and phase transformation was also relatively restricted.