Fault growth and orthogonal shortening in transtensional supradetachment basins: Insights from the ‘Old Red’ of western Norway

Abstract

AbstractFor basins that evolve adjacent to large‐magnitude normal faults, tectonic controls on sedimentation involve isostatic back rotation of an exhuming footwall and, commonly, the evolution of kilometre‐scale extension‐parallel folds. Based on observations from classic localities in western Norway, we propose a three‐stage evolution scenario for transtensional supradetachment basins where the basins become progressively re‐arranged because of core complex exhumation and subsequent orthogonal shortening. Extension‐parallel transverse synclines initially form due to a normal displacement gradient, but when displacements accumulate beyond a certain magnitude, the hanging wall increasingly responds to core complex exhumation and the original depocentre, formed close to the original area of maximum displacement, will become inverted and dismembered above the core complex. Two new synclinal depocentres will develop along the flanks. Because these synclines form by extensional fault growth rather than by shortening, they will be associated with widening of the basin and onlap onto basement at high angles to the maximum elongation trend with overall grain‐size decrease and retrogradational stacking patterns. Further, because these synclines grow away from the evolving core complex, sedimentary units will become asymmetrically distributed inside each syncline in such a way that the oldest deposits in the syncline will be preserved on the flank most proximal to the core complex. In transtensional environments, a third evolutionary stage may involve constrictional strain where extension‐parallel folds and reverse faults produced by orthogonal shortening enhance or interact with other structures. Ultimately, initial extensional sub‐basins may become warped across extension‐parallel folds. Hanging wall deformation will be manifested in shifting accommodation patterns, with depocentres that generally migrate in the direction of the detachment fault. Accommodation patterns initially related to megafault growth may conceptually evolve into depocentres controlled by orthogonal shortening.