The structural geology of north Hadeland

The Cambro-Middle Ordovician sequence of north Hadeland is deformed by numerous, hinterland-dipping imbricate faults. The imbricates fan off a low-angle detachment that separates deformed Lower Palaeozoic rocks from undeformed Precambrian basement and a thin veneer of Cambrian shales. Deformation evolved by an early layer-parallel shortening event indicated by thrust-wedging and pressuredissolution cleavage followed by the main imbrication and folding event. A balanced cross-section through north Hadeland shows an estimated shortening of 60% in a NNW-SSE direction. The deformation style in north Hadeland contrasts sharply with broad folds in Middle Ordovician-Silurian rocks in south Hadeland. Hence a bed-parallel detachment in Middle Ordovician shales between the two areas needs to be inferred.