Cenozoic tectonic evolution of the Beaufort-Mackenzie fold and thrust belt (Arctic Canada): from orogenic shortening to gravity spreading

The Beaufort-Mackenzie fold and thrust belt in Arctic Canada corresponds to the offshore part of the Cordilleran orogen in Yukon and Alaska. The main folds are associated with thrusts that root in a basal décollement at depths of ca. 12-15 km. Deformation mainly occurred prior to the deposition of the Richards sequence (late Eocene) and subsequent folding that progressed basinward and continued episodically to the present accounts for < 10% of the total shortening. In the shelf region, normal faults dipping either seaward or landward are located on the apex of anticlines. Detailed analysis on seismic sections of the variations of throw (T) and expansion index (Ei) with depth (z) indicates that despite the clear spatial relationship with anticlines, normal faults initiated after fold development. Normal faulting migrated seaward during the Oligocene to Pliocene (and possibly later) along with the shelf edge that resulted from the progradation of thick sedimentary wedges of clastic sediments. This strongly suggests that normal faulting is not related to tectonics (i.e., linked with the building of the Cordillera), but to gravity spreading due to high sedimentary loading in the Mackenzie delta; i.e. main depocenter for sediments originating from the northeastern Cordillera since the latest Cretaceous. The superimposition of extensional structures on an existing fold and thrust belt shows the complex interplay in time and space between far-field stresses linked with plate-dynamics and stresses associated with sedimentary loading in a delta setting.