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The Lofoten archipelago exposes Precambrian Baltic basement and Caledonian allochthonous sequences within a 1000 km long chain of gravity and magnetic highs and structural culminations along the extended, British and Norwegian continental shelf. Previous regional geophysical studies indicate that post‐Caledonian extension and development of the northern Norwegian shelf occurred during broadly defined Carboniferous‐Permian, Cretaceous‐Jurassic, and early Tertiary events. Structures related to these events are known to young westward. We report field, structural, and 40Ar/39Ar thermochronologic data from rocks exposed in Lofoten that further define the history and style of post‐Caledonian extension. The islands of southwest Lofoten also represent the most outboard exposures of Caledonian basement in northern Norway that presumably formed the middle to deep crustal core of the orogen. Metasedimentary rocks and penetratively deformed basement in Lofoten record high‐grade Silurian‐Devonian metamorphism and top‐to‐the‐east (hereinafter tops‐east) thrusting followed by episodes of Late Devonian to Early Carboniferous, tops‐west, ductile extension which progressed into oblique left‐slip movements. The structural style and timing of Silurian contraction in this area are remarkably similar to that determined for the more forelandward areas on the mainland, ∼120 km to the east, supporting the inference that distal parts of the Baltic continental margin that were once deeply subducted are presently exposed in Lofoten. The timings of post‐Devonian structural events that affected rocks in Lofoten are partially constrained by the ages of unconformities and strata known to be preserved in graben flanking the Lofoten culmination. The radiometric age and structural data presented in this study, in combination with stratigraphic constraints, suggest a westward progression through time of extensional deformation over a protracted interval of Silurian to Permian time. The latest, Permian extension in Lofoten is largely characterized by brittle structures that formed at conditions substantially less than 300°C. Compared to the exhumation history of the southern Western Gneiss Region, the depth of Caledonian, continental (A‐type) subduction and subsequent unroofing of Lofoten are of lesser magnitude, and the present erosional level remained in the middle crust for a much longer interval of time. The Permian 40Ar/39Ar mineral ages documented in this study are the youngest such ages yet identified in Scandinavia. These ages relate to episodes of deformation and cooling in response to extensional tectonic events that occurred roughly 100 m.y. after comparable effects identified on the Caledonian mainland. Our preferred explanation for the Carboniferous‐Permian radiometric ages, structural evolution, and stratigraphic data for Lofoten is that they all developed in the context of a long‐lived Cordilleran‐style metamorphic core complex.

Field observations and electron microprobe analyses indicate that pseudotachylytes discovered on the Lofoten island of Flakstadøy, north Norway, represent rare examples of deep-crustal paleoseismic faults. The pseudotachylyte occurrences are restricted to the margins of eclogite-facies shear zones that sharply cut pristine granulite-facies continental basement rocks. Generally, pseudotachylyte veins are sharply truncated by the eclogite shears, but some have been sheared and folded into them, documenting prekinematic to synkinematic injection. Textures preserved in the pseudotachylyte matrix document crystallization directly from the frictional melt; for example, dendritic garnets, similar in appearance, size, and composition to those from eclogite pseudotachylytes of the Bergen Arcs and Ålesund (Austrheim and Boundy, 1994; Lund and Austrheim, 2003), refl ect rapid (likely in terms of tens of seconds) crystallization, and distinct fi ning of grains toward the margins of the pseudotachylyte veins indicates quenching textures. Electron microprobe analysis and backscattered-electron imaging document that the pseudotachylyte matrix is composed of microlites of garnet (Gr 25-30 , Py 15-19 , and Al 54-58 ), orthopyroxene (En 61-64 ), low-Na clinopyroxene (Jd 6 ), amphibole (ferroan pargasite), with or without K-feldspar, quartz, biotite, various Fe opaques and Fe-Ti opaques, kyanite, dolomite, and calcite. The cogenetic eclogite-facies shear zones and pseudotachylytes were variably retrograded during Caledonian amphibolite-facies metamorphism. Omphacite is replaced by clusters or symplectites of low-Na clinopyroxene (Jd 6 ) and oligoclase/andesine (An 20-36 ); kyanite, orthopyroxene, Na-Ca clinopyroxene, amphi-bole, and dolomite occur as inclusions in garnet. The Flakstadøy pseudotachylytes indicate that the rocks exposed in Lofoten were rigid and resilient parts of the lower crust of an ancient continent from ca. 1.8 Ga until the Middle Ordovician. Subduction to deepercrustal levels (depths >~45 km) caused the stiff, nonreacted granulite to accommodate aseismic, steady-state fl ow in fl uid-mediated, eclogite shear zones by concomitant, brittle, seismogenic failure and pseudotachylyte formation. Later in the Middle Ordovician, these deep-crustal rocks were exhumed to middle-crustal levels, where they were retrograded under amphibolite-facies conditions. Our results help to explain how deep-crustal earthquakes form in modern continent-continent collisional zones like the Himalayas.

Isotope dilution thermal ionization mass spectrometry U-Pb dating and coupled Lu-Hf solution inductively coupled plasma mass spectrometry analyses of zircon were acquired from magmatic rocks along two transects across the Scandinavian Caledonides in the TromsOfoten region of Norway to explore possible correlations and gain insight into the evolution of far-travelled nappes within the Upper and Uppermost Allochthons. One pulse of magmatic activity was recorded at c. 489 Ma in the Tromsø Nappe. In the underlying Nakkedal Nappe, a magmatic pulse was recorded at c. 450 Ma, being contemporaneous with eclogite facies metamorphism in the area. Tonalites in the structurally underlying Lyngen and Gratangseidet ophiolitic complexes, both forming the substratum to carbonate -schist-quartzite sequences (Balsfjord and Evenes groups, respectively), yielded ages of 481 and 474 Ma. Obtained 1 Hf(t) values are, however, distinctly different and indicate a juvenile origin for the Gratangseidet tonalite (1 Hf(474) ¼ + 9.57) and the presence of Palaeoproterozoic source material for the Lyngen tonalite (

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