The tectonometamorphic development of the Ardencaple Fjord area, North East Greenland Caledonides

Abstract

The Caledonian Orogen in East Greenland comprises an autochthonous basement complex overlain by a Late Proterozoic to Early Proterozoic cover sequence with a regionally extensive thrust sheet complex on top. The thrust sheet complex is further divided into three separate thrust sheets: (1) allochthonous basement comprises Early- and Meso-Proterozoic ortho- and paragneisses that are reworked at variable intensity during the Caledonian orogeny. Allochthonous basement is overlain by a (2) Mesoproterozoic supracrustal sequence comprising the Krummedal/Smallefjord Sequence. (3) The Neoproterozoic Eleonore Bay Supergroup (EBSGp) overlies the Mesoproterozoic Krummedal/Smallefjord Sequence and represents the upper of the three thrust sheets. An extensional N-striking and top-to-the-E dipping high strain zone at the base of the Krummedal/Smallefjord Sequence is well documented in the Central Fjord Region in NE Greenland. A similar zone was observed to separate allochthonous basement and the Mesoproterozoic Krummedal/Smallefjord Sequence in the Ardencaple Fjord Area, where the three overriding thrust sheets are present in a NW-SE trending synform. A study of the structural geology and metamorphic petrology of the Ardencaple Fjord area was undertaken, in order to decipher the kinematic evolution.

Foliated and lineated E-dipping tonalitic gneisses cross-cut by granitic dike swarms make up the allochthonous basement. The lower part of the overlying Smallefjord Sequence is defined by the Slamsø Shear Zone, a number of N-striking and top-to-the-E dipping ductile and brittle extensional shear zones. Granitic veins are not present in this part of the Smallefjord Sequence, as opposed to its middle- and upper-part. We have found no evidence that the granites emplaced in the Smallefjord Sequence are related to top-to-the-E shear, as proposed by some authors. The nature of the contact separating the Smallefjord Sequence and the EBSGp is overprinted by an undeformed granitic pluton, and thus remains unclear. Some authors have proposed that this contact is an extensional top-to-the-E high strain zone. If this is correct, this high strain zone must predate granite emplacement at ~428 Ma.

P-T estimates on rocks from allochthonous basement and the base of the Smallefjord Sequence show a clockwise path, which documents extensional movement along the Slamsø Shear Zone. Minimum displacement along this shear zone is 15-20 km. The allochthonous basement was exhumed from upper granulite facies at peak metamorphic conditions to amphibolite facies at post peak metamorphic conditions. The Smallefjord was at amphibolite conditions at both peak- and post peak-metamorphic conditions.