An 40Ar/39Ar thermochronology of the Ofoten-Troms region: Implications for terrane amalgamation and extensional collapse of the northern Scandinavian Caledonides

Abstract

Fifteen Ar-40/Ar-39 cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the Ofoten nappe stack of northern Norway. Eight cooling ages on hornblende range from 425 to 394 Ma and seven muscovite ages, from the same or nearby outcrops as the hornblendes, range from 400 to 373 Ma. These data are compared with Ar-40/Ar-39 ages from over a large part of the northern Caledonides to evaluate regional mineral cooling patterns. Results indicate that (1) Scandian (Silurian-Devonian) metamorphism was predominant; (2) most of the nappes investigated contain some vestige of pre-Scandian tectonism and/or metamorphism; (3) hornblende and muscovite cooling ages are progressively younger to the west and south, which suggests a hinged-to-the-east mineral cooling pattern; and (4) a late, out-of-sequence thrust is the only disruption of this cooling pattern. Synmetamorphic amalgamation of the nappes resulted from Scandian A type subduction. The hinged-to-the-east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west clipping, crustal-scale extensional fault, located somewhere west of the present Norwegian coast, during late synorogenic gravitational collapse. The late out-of-sequence fault formed contemporaneously with uplift in the hinterland, implying a kinematic and temporal connection with east directed contractional faulting in the foreland.