Neoproterozoic and post-Caledonian exhumation and shallow faulting in NW Finnmark from K–Ar dating and p ∕ T analysis of fault rocks

Well-preserved fault gouge along brittle faults in Paleoproterozoic, volcano-sedimentary rocks of the Raipas Group exposed in the Alta-Kvaenangen tectonic window in northern Norway yielded latest Mesoproterozoic (ca. 1050 ± 15 Ma) to mid Neoproterozoic (ca. 825–810 ± 18 Ma) K/Ar ages. Pressure-temperature estimates from microtextural and mineralogy analyses of fault-rocks indicate that brittle faulting may have initiated at depth of 5–10 km during the opening of the Asgard Sea in the latest Mesoproterozoic-early Neoproterozoic (ca. 1050–945 Ma), and continued with a phase of shallow faulting during to the opening of the Iapetus Ocean-AEgir Sea and the initial breakup of Rodinia in the mid Neoproterozoic (ca. 825–810 Ma). The predominance and preservation of synkinematic smectite and subsidiary illite in cohesive and non-cohesive fault-rocks indicate that Paleoproterozoic basement rocks of the Alta-Kvaenangen tectonic window remained at shallow crustal levels (< 3.5 km) and were not reactivated since mid Neoproterozoic times. Slow exhumation rate estimates for the early-mid Neoproterozoic (ca 10–75 m per Ma) suggest a period of tectonic quiescence between the opening of the Asgard Sea and the breakup of Rodinia. In the Paleozoic, basement rocks in NW Finnmark were overthrusted by Caledonian nappes along low-angle thrust detachments during the closing of the Iapetus Ocean-AEgir Sea. K/Ar dating of non-cohesive fault-rocks and microtexture-mineralogy of cohesive fault-rock truncating Caledonian nappe units show that brittle (reverse) faulting potentially initiated along low-angle Caledonian thrusts during the latest stages of the Caledonian Orogeny in the Silurian (ca. 425 Ma) and was accompanied by epidote/chlorite-rich, stilpnomelane-bearing cataclasite (type 1) indicative of a faulting depth of 10–16 km. Caledonian thrusts were inverted (e.g. Talvik fault) and later truncated by high-angle normal faults (e.g. Langfjord-Vargsund fault) during subsequent, late Paleozoic, collapse-related widespread extension in the Late Devonian-early Carboniferous (ca. 375–325 Ma). This faulting period was accompanied by quartz- (type 2), calcite- (type 3) and laumontite-rich cataclasites (type 4), which crosscutting relationships indicate a progressive exhumation of Caledonian rocks to zeolite facies conditions (i.e. depth of 2–8 km). An ultimate period of minor faulting occurred in the late Carboniferous-mid Permian (315–265 Ma) and exhumed Caledonian rocks to shallow depth 1–3.5 km. Alternatively, late Carboniferous (?) – early/mid Permian K/Ar ages may reflect late Paleozoic weathering of the margin. Exhumation rates estimates indicate rapid Silurian-early Carboniferous exhumation and slow exhumation in the late Carboniferous-mid Permian, supporting decreasing faulting activity from the mid-Carboniferous. NW Finnmark remained tectonically quiet in the Mesozoic-Cenozoic.