Tertiary divergent thrust directions from partitioned transpression, Broggerhalvoya, Spitsbergen

Broggerhalvoya, located at the northwestern terminus of Spitsbergen's Terti ary fold-thrust belt, is underlain by a basement-involved thrust stack defi ned by an anomalous WNW-ESE strike direction relative to the overall NNW-SS E strike further south. Three kinematically separable thrust nappes are ide ntified: (i) a lower nappe is characterized by low-angle to bedding-paralle l imbricates, (ii) a middle nappe comprises macroscopic anticlines and sync lines, rotated imbricate fans and duplexes within Palaeozoic-Mesozoic cover strata, and (iii) an upper nappe consists of overthrust Caledonian basemen t rocks. In addition, steep N-S-striking oblique-normal faults offset the f old-thrust stack and can be traced southwards into parallelism with the For landsundet Graben and a major transcurrent fault, the Svartfjella-Eidembukt a-Daudmannsodden Lineament. A three-phase kinematic development of the napp es and bounding thrust systems is invoked: (i) an early phase of basement-i nvolved uplift and foreland thrusting of the lower and middle nappes at obl ique thrust directions varying from NW to NNE, (ii) a mid-phase out-of-sequ ence thrusting of the upper nappe towards the ENE, and (iii) a late-phase t runcation of the nappe stack by N-S-striking normal faults with subsidiary strike-slip components. Comparisons with other segments of the fold-thrust belt further south temporally link the early-phase thrusting in Broggerhalv oya to a Palaeocene-early Eocene coupled transpressional event (stage 1). T he mid-phase ENE-directed thrusting in Broggerhalvoya can be temporally cor related with the main Eocene decoupled transpressional event of central Spi tsbergen (stages 2 and 3), while the late-phase N-S-striking normal faults may correlate with Late Eocene-Oligocene faults bounding the Forlandsundet graben (stages 5 and 5). We explain the origin of the anomalous WNW-ESE str uctural trend in Broggerhalvoya as a response to differential translation a nd temporal changes in thrust/shortening directions. We also advocate varia ble degrees of coupled and decoupled orogen-normal and orogen-parallel defo rmation along western Spitsbergen. The overall result is that structures as cribed to the early NW- to NNE-directed shortening events (i.e. orogen-obli que motions) predominate in Broggerhalvoya, while ENE-directed structures a scribed to the main/late stage event (i.e. orogen-normal motions) predomina te in the south. Other causes of the anomalous WNW strike in Broggerhalvaya may be location near the termination of the Carboniferous St. Jonsfjorden Trough, and changes in orientation, thickness and/or facies types of the Ca rboniferous through Mesozoic strata.