EXHUMATION AND PROBABLE EXTENSION IN THE HELGELAND NAPPE COMPLEX, UPPERMOST ALLOCHTHON, CENTRAL NORWAY

In the Caledonides of north-central Norway, the Uppermost Allochthon includes the Helgeland Nappe Complex (HNC) and the ~481-430 Ma Bindal Batholith (BB). Within the Velfjord-Tosen area, the HNC can be divided into lower and upper nappes consisting of Proterozoic partially migmatitic gneiss, calc-silicate, and marble, and a middle nappe consisting of Ordovician ophiolite sequences and unconformably overlying medium- to low-grade metasandstones, metaconglomerates, and metacalcareous rocks. Amalgamation of the HCN occurred prior to emplacement of the 448 Ma Andalshatten pluton that is interpreted to truncate thrust faults associated with nappe formation. Our ongoing petrologic and structural studies in the Velfjord region of the BB indicate that magmas were emplaced into partially migmatitic rocks of the lower nappe at pressures of 700-800 MPa and were subsequently isothermally exhumed to 400-500 MPa while the host rocks were still partially molten. No previously recognized structures can adequately explain this exhumation history. However, we note that the juxtaposition of lower-grade (and younger) metamorphic rocks of the middle nappe above higher-grade (and older) metamorphic rocks of the lower nappe is more compatible with extension than contraction and nappe emplacement. New mapping along the boundary between the lower and middle nappes in the area has recognized the presence of asymmetric folds and outcrop-scale asymmetric structures (S/C, porphyroclasts) that suggest top to the southeast sense of shear along shallowly east-dipping shear surfaces. Therefore, we suggest that exhumation of the BB may have involved large amounts of extension along the boundary between the lower and middle nappes in the region and potentially other shear zones within the HCN, and that extension may have begun prior to 448 Ma. If these interpretations are correct, then extensional deformation in this region represents the oldest locally recognized phase of extension in the Caledonides.