40Ar/39Ar Thermochronology of Late‐ and Postorogenic Extension in the Caledonides of North‐Central Norway

Abstract

We report 40Ar/39Ar mineral cooling data that elucidate the timing and style of Devonian late‐orogenic extensional collapse and Permian‐Triassic postorogenic continental rifting in the north‐central Norwegian Caledonides. 40Ar/39Ar mineral cooling ages from ductile extensional shear zones that have either excised or reactivated Caledonian thrust faults along the western flanks of two major basement domes indicate that late‐orogenic extensional deformation in this region occurred during the Early (ca. 409–388 Ma) and Late Devonian (ca. 371–355 Ma). These extensional pulses are consistent with events recently identified by workers investigating areas to the south of our study area. The timing and style of the Early Devonian event supports the northern continuation of the Devonian orogen‐parallel, scoop‐shaped extensional deformation documented in the central Norwegian Caledonides. Late Devonian extension, on the other hand, was orogen‐orthogonal. This Late Devonian event is similar in timing and kinematics to the extension documented in the Lofoten Archipelago due west of our study area. 40Ar/39Ar data on K‐feldspar samples from brittle extensional faults of the Vestfjord‐Vanna fault complex, which opened the major NNE‐SSW‐trending Vestfjord basin, document two pulses of E‐W‐directed postorogenic continental rifting. The first pulse of extension occurred during the Early Permian, at ca. 272 Ma, and the other during the Middle Triassic, at ca. 236 Ma. The Early Permian faults are upper‐crustal features found in the hanging wall of the Lofoten metamorphic core complex, the result of rifting the Lofoten block from the Norwegian mainland during the initial opening of the Vestfjord basin. Middle Triassic faulting occurred because of continued movement along the Vestfjord‐Vanna fault complex during further development of the Vestfjord basin. The timing and kinematics of development of the Vestfjord basin have similarities with those of major basins submerged along and exposed on the continental margins of Norway and Greenland, respectively. Permian‐Triassic extension, long recognized throughout eastern Greenland, is evidently more pronounced along the Norwegian continental margin than was previously thought. We interpret this event as marking the transition from Devonian‐Carboniferous extensional collapse to continental rifting sensu stricto, preceding the Eocene opening of the nascent Norwegian‐Greenlandian Sea.