fault systems, the Flathead fault and the Rocky Mountain trench fault system. Both of which are late Eocene to Miocene in age. However, the amount of shortening that has taken place on the thrust is not connected to Eocene extension due to the Rocky Mountain trench fault system and the Flathead fault having no influence positionally on the footwall and hanging wall cutoffs of the Lewis thrust. Instead, this transpression was replaced with transtension in the early Eocene involving east–west crustal extension and tectonic exhumation, which brought up mid-crustal metamorphic rocks to the surface to be exposed. Additionally, this transition from transpression to transtension resulted in rapid cooling of the metamorphic core complexes as they were exhumed and brought to the surface. Dextral transtension on intracontinental strike-slip faults in northeastern and southwestern British Columbia culminated with the mid Eocene extensional exhumation of midcrustal metamorphic core complexes. This leads to the basal décollement being exposed and the association with north–south faulting, dyke emplacement, and voluminous magmatism, which in turn, marked cessation of crustal shortening. Paleotemperatures and geothermal gradients indicate that the Lewis thrust sheet was thick when thrusting commenced. Duplexes Thrust faults often associate three types of structures, imbricate fan structures, ramp-flat structures, and duplex structures, all of which are seen within the Lewis thrust and the Rocky Mountain thrust and fold belt. Duplex structures are common and have been located in numerous locations along the Lewis thrust. These structures are distinct due to their structurally overlapping, lenticular stacked thrust fault slices. A prime example is seen in an area that extends from the Kootenay Pass north of the border to Marias Pass found in Montana. This section shows the Lewis Thrust following a series of bedding-parallel detachment horizons with a fairly thin stratigraphic interval near the base of the Purcell Supergroup, which is also the base of the mid-Proterozoic belt. Two windows in this section showing exposures of Upper Cretaceous strata exposed beneath the Lewis Thrust occur adjacent to the Flathead Fault. Within these windows, the Lewis thrust is folded along with overlying and underlying strata in a series of northwest-trending anticlinal culminations that extend the length of the west side of the salient. Furthermore, two distinct structural levels can be seen in this section, an upper level comprising the majority of the mass of the Lewis thrust sheet which is characterized by broad open folds in relatively undeformed rocks,