Deformed basement and cover rocks of the St. Jonsfjorden area further define the character of Spitsbergen's Tertiary fold and thrust belt. Three major thrusts comprise a folded, ca N060° E-directed, foreland propagating thust stack, with likely additional subsurface thrust faults. The folded Vegardbreen-Robertsonfjellet Thrust is the most significant fault in the complex and re-emerges in the core of a disrupted synclinal structure in Triassic strata. Shortening across the area indicated from line length restoration is of the order of 13 km. Differences in stratigraphy between and within thrust sheets indicate the thrust stack initiated along the edge of a westward deepening Carbonbiferous basin with a fault-step margin and represents basin inversion. Early structures related to basin formation had a major control on later thrust sheet evolution and the complexity of inversion structures reflects the initial complexity of the basin geometry. Such basin inversion could characterize the length of the fold and thrust belt. The St. Jonsfjorden structures are primarily contractional, and together with areas farther southeast in the fold-thrust belt, indicate shortening took place perpendicular to the fold belt across much of the length of the evolving margin.
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