The Biscayarhalvøya Mega Shear Zone: A 420 Ma Old Dextral Shear Zone Affecting the Richarddalen Complex of NW Svalbard

Friedhelm Henjes-Kunst

The Caledonian basement of Svalbard consists of crustal blocks or terranes juxtaposed along major shear zones in late Caledonian times. The Richarddalen Complex on Biskayarhalvøya (NW Svalbard) represents an exotic fragment within the NW composite terrane. It consists of N-S trending gneisses, mica schists, marbles, and amphibolites with eclogitic relicts, tectonically aligned with the likely Grenville-age metasedimentary Biskayerfonna Group. Formation of this assemblage is traditionally interpreted in terms of Caledonian crustal stacking. Structural analyses from western Richarddalen indicate a dominant subvertical to steeply W-dipping penetrative foliation (at least S2) with a pronounced subhorizontal stretching lineation L2. The rocks are affected by cm- to dm-scale folding with axes parallel to L2, which is intensified where the foliation becomes mylonitic. Amphibolite bodies with penetrative shearing along the margins and distinct shear planes in the interior form σ-like shapes indicating dextral shear parallel to L2, as do widely distributed shear bands and σ clasts in the metasedimentary rocks. All intensely foliated rocks are interpreted as platy mylonites. Previous attempts to date the HP metamorphism of eclogites in the complex yielded ages of 650-450 Ma of unclear geological significance. Reinvestigation of zircons of an eclogitic metagabbro reveals a two-stage history: Single grain TIMS and in-situ LA ICP-MS dating gave concordant ages close to 650 Ma. In-situ dating of zircon rims yielded an average of 646 ± 10 Ma; for the cores, a 10-20 Ma older age is assumed. These results suggest a late Neoproterozoic age of the protolith of the eclogite closely followed by HP metamorphism. LA ICP-MS zircon dating of a migmatitic mylonite close to the eclogite yielded concordant Mesoproterozoic to early Paleozoic ages suggesting its origin from an early Paleozoic sediment. The youngest zircon ages form a coherent group at 422 ± 5 Ma interpreted to date regional metamorphism and shearing. LA ICP-MS zircon dating of two metasedimentary rocks of the Biskayerfonna Group yielded concordant ages of c. 1050 Ma and older. These lithologies can be closely correlated with the metasedimentary Krossfjorden Group W of the Raudfjorden Fault Zone (also within the NW composite terrane). Our data suggest that the Richarddalen Complex was affected by major dextral shear at c. 420 Ma bringing together slices of rocks of different and in part exotic origin. We interpret the Biskayarhalvøya Mega Shear Zone (BMSZ) as a regionally important late Caledonian structure. This does not necessarily exclude that dextral shearing was preceded by W-E contraction during an earlier stage of the Caledonian Orogeny. However, many of the contractional structures may have been overprinted or destroyed by the dextral BMSZ.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013