Abstract

A new structural model is presented for the giant Western Gneiss Region (WGR) in western Norway, a part of the Baltica continental margin that was subducted to eclogite-facies conditions during the Silurian–Devonian Caledonian (Scandian) orogeny. The WGR is divided into three tectonostratigraphic subdivisions that are composed largely of felsic orthogneiss, and they are separated from each other by regional shear zones. From structurally lowest to highest, these subdivisions are the Western Gneiss Complex, the Fjordane Complex, and the Tafjord Complex. The Western Gneiss Complex is mainly exposed across the southern extent of the WGR, and it is inferred to reappear again in the north. The Fjordane Complex occupies a shallowly east-plunging synclinorium through the center of the WGR; it is separated from the Western Gneiss Complex by the Sandane shear zone. The Tafjord Complex lies above the Fjordane Complex, across the east-dipping Geiranger shear zone. Both the Sandane and Geiranger shear zones contain top-to-the-foreland shear indicators; the Sandane shear zone is also overprinted toward the western coast by later extensional shear (top-to-the-hinterland). These gneiss sheets resulted from progressive delamination of the frontal and upper sections of the WGR during Scandian collision and subduction from ca. 430 to ca. 405 Ma. Combining the structural model with existing pressure-temperature and geochronological data indicates: (1) the Tafjord Complex likely detached relatively early and stagnated beneath the overlying orogenic pile while the Baltica slab continued sliding underneath; (2) the Fjordane Complex detached later in the eclogite facies, and also likely stalled at these conditions; and (3) the Western Gneiss Complex continued subducting at eclogite-facies depths for perhaps another 5 m.y. before it began to exhume at ca. 405 Ma.

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