AN AR-40 AR-39 THERMOCHRONOLOGY OF THE OFOTEN-TROMS REGION - IMPLICATIONS FOR TERRANE AMALGAMATION AND EXTENSIONAL COLLAPSE OF THE NORTHERNSCANDINAVIAN CALEDONIDES/

Fifteen Ar-40/Ar-39 cooling ages are reported for metamorphic hornblen de and muscovite from far traveled terranes constituting the Ofoten na ppe stack of northern Norway. Eight cooling ages on hornblende range f rom 425 to 394 Ma and seven muscovite ages, from the same or nearby ou tcrops as the hornblendes, range from 400 to 373 Ma. These data are co mpared with Ar-40/Ar-39 ages from over a large part of the northern Ca ledonides to evaluate regional mineral cooling patterns. Results indic ate that (1) Scandian (Silurian-Devonian) metamorphism was predominant ; (2) most of the nappes investigated contain some vestige of pre-Scan dian tectonism and/or metamorphism; (3) hornblende and muscovite cooli ng ages are progressively younger to the west and south, which suggest s a hinged-to-the-east mineral cooling pattern; and (4) a late, out-of -sequence thrust is the only disruption of this cooling pattern. Synme tamorphic amalgamation of the nappes resulted from Scandian A type sub duction. The hinged-to-the-east mineral cooling pattern implies isosta tic adjustment and exhumation of the footwall of a west clipping, crus tal-scale extensional fault, located somewhere west of the present Nor wegian coast, during late synorogenic gravitational collapse. The late out-of-sequence fault formed contemporaneously with uplift in the hin terland, implying a kinematic and temporal connection with east direct ed contractional faulting in the foreland.