Emplacement and denudation history of the Lewis-Eldorado-Hoadley thrust slab in the northern Montana Cordillera, USA: implications for steady-state orogenic processes

The northern Montana Cordilleran mountain chain provides unusual opportunities to evaluate long-term orogenic processes in a foreland thrust belt. The thick Lewis-Eldorado-Hoadley thrust slab and an imbricated thrust wedge in its footwall dominate the structure of the region. The slab is composed of the displaced Mesoproterozoic Belt-Purcell depositional basin and overlying Phanerozoic strata, tectonically inverted into the broad, deeply dissected Purcell anticlinorium. The geometric simplicity and stratigraphic integrity of the structurally strong, thick slab constrain crustal-scale cross sections. Horizontal displacement of the slab increases from 40 km in central western Montana to 140 km at the United States-Canada border. Thermochronometers from the exhumed thrust slab and its footwall coupled with erosional products in the neighboring foreland basin measure regional denudation during Maastrichtian to middle Paleocene tectonic convergence. During the 15 my period of slab emplacement (74-59 Ma), regional denudation appears to have been minimal as the footwall subsided isostatically beneath the advancing tectonic load; erosion did not breach the Paleozoic cover of the slab. Similarities in burial and denudation patterns from central Montana to southern Canada indicate that regional erosional denudation was steady-state with respect to horizontal displacement and displacement rate. The steady-state configuration of the slab was interrupted immediately upon cessation of thrusting at 59 Ma, when major uplift and denudation of the slab and its footwall began, yoked to isostatic rebound of an autochthonous mega-thrust ramp at the depositional margin of the Belt-Purcell basin. The post-thrusting rebound bent the slab into the Purcell anticlinorium, and the flexure was accompanied by extensional faulting of the slab. Although parameters of orogenic relief are limited by its ancient nature, the northern Montana Cordillera is significant to evaluation of steady-state orogenic processes because its history is clearly divided into a syn-orogenic phase with low denudation rates and an immediate post-orogenic rebound phase with high denudation rates.