Syncontractional extension and exhumation of deep crustal rocks in the east Greenland Caledonides

Abstract

Tectonic models for the North Atlantic Caledonides typically show Baltica being subducted below Laurentia prior to late orogenic collapse and exhumation of deep crustal rocks, conveniently explaining the high‐ and ultrahigh‐pressure provinces in the Scandinavian Caledonides. However, these models offer no tectonic explanation for the deformation and overthickening of the overriding Laurentian plate. Here we document major anatectic midcrustal ultraductile top to the (N)W shear zones in the east Greenland Caledonides that may account for this orogenic thickening. U‐Pb dating of monazite and 40Ar/39Ar dating suggests that high‐grade fabrics (∼ 435–424 Ma) and granites (∼427 and 422 Ma) involved in these zones show that these were contemporaneous with syntectonic granites involved in the Fjord Region Detachment Zone (FRDZ), a major top to the east extensional fault. More than 100 km of top to the NW overthrusting has been documented in the foreland of the orogen, yet we show that rocks in the hanging wall of the thrust were thinned by orogen‐parallel extension rather than thickened during emplacement, thereby exhuming the underlying active midcrustal thrust zones. This is documented by Late Silurian (∼419 Ma) to Early Devonian (∼401 Ma) 40Ar/39Ar‐muscovite cooling dates from rocks uplifted along major normal faults and intrusion of mafic dikes (∼ 419 Ma) into extensional faults. Folding and thrusting in the foreland probably continued well beyond this time, and underthrust high‐grade nappes probably were exhumed in the triangular zone between the thrust faults (west) and the structurally higher FRDZ (east) before final NW vergent thrust emplacement over low‐grade sedimentary rocks. Final cooling of the metamorphic core below closure temperatures for Ar diffusion in K‐teldspar occurred in the early Carboniferous. Major folds previously regarded as having formed during the main Caledonian event at ∼425 Ma may in part be up to 100 m.y. younger. Some of these folds also deform sedimentary rocks associated with collapse basins of Middle Devonian to Carboniferous age.