Tektonostratigrafi och strukturgeologi i Ålkatj-Tielmamassivet, norra svenska Kaledoniderna

Sammanfattning: The bedrock of the Ålkatj-Tielma massif in the central part of the Sarek National Park in the northern Scandinavian Mountains, has never before been investigated. This massif was mapped during the summer of 1997 and the bedrock consists entirely of Caledonian nappes. The lowermost unit in the study area is the Akkajaure Nappe Complex (ANC), consisting of variably deformed thrust sheets of Precambrian crystalline rocks, dominated by syenite and gabbro, with a conspicuous anorthosite component, and subordinate thin covers of sedimentary rocks. Overlying the ANC is Skarja Nappe, consisting of monotonous and strongly deformed garnet mica schist with very subordinate occurrences of amphibolite boudins. Contained within the Skarja Nappe is the herein defined Skårva unit, which is less than 50 m thick and predominantly consists of metamorphosed plutonic rocks and some sedimentary rocks of comparatively low metamorphic grade. Stratigraphically on top of the Skarja Nappe rests the Mikka Nappe, dominated by strongly deformed mica schist and quartzite, with locally abundant boudins of garnet amphibolite. The topmost tectonic unit in the study area is the Sarektjåkkå Nappe, dominated by diabase and garnet amphibolite with subordinate mica schist and marble. Structurally, the southern part of the Ålkatj-Tielma massif is characterized by the dome-like Tielma Culmination (TC) in the ANC. The northern and western border of the TC constitute frontal and oblique ramps. The TC may thus have formed as a result of lateral stacking of thrust sheets due to converging thrust directions. As a consequence, the overlying nappes form a large recumbent fold - the Jåkåtjkaska Fold (JF) - with a more than 500 m thick overturned lower limb, in which the entire tectonic stratigraphy above the ANC is inverted. At the lateral ramp, the JF was stretched and downfolded, perhaps synchronously with the formation of the TC.