THREE-DIMENSIONAL RECONSTRUCTION OF THE LEWIS-ELDORADO-HOADLEY THRUST SLAB AT THE CONCLUSION OF PALEOCENE THRUSTING, MONTANA CORDILLERA

A series of balanced cross-sections across the Lewis-Eldorado-Hoadley (LEH) thrust slab and the fringing Rocky Mountain fold-thrust belt of northern Montana permit application of the critical wedge model to ancient thrust systems. Restoration of the sections to their configurations at about 60 Ma determines the surface slope and decollement dip of the thrust wedge at the conclusion of thrusting. The restorations are constrained to a depth of 25 km by the coherent internal stratigraphy of the slab and its fringing thrust belt, augmented by thermo-geochronometry of the thrust system and provenance of erosional products delivered to the foreland basin. Post-thrusting isostatic rebound, erosion, and extension provide deep exposures that help define the internal geometry of the thrust slab. The Mesoproterozoic Belt-Purcell Supergroup forms the foundation of the thrust slab. The well-layered supergroup tapers in thickness from ~20-km at the back end of the slab to zero at its leading edge, providing a natural form for a tapered thrust wedge. The supergroup had great structural strength at the time of thrusting, because the beds were in their brittle field, were quartz-rich, laced with diabase sills, and had been significantly dehydrated by Mesoproterozoic burial metamorphism. Two-km thick Paleozoic carbonate-platform and 5-7 km thick Mesozoic foreland-basin packages overlay the supergroup during thrusting. Erosional products in the foreland basin indicate that the foreland subsided isostatically in advance of the thrust system so that erosion did not breech the Phanerozoic cover of the slab until after the conclusion of thrusting. Thermo-geochronological data from the slab and isostatic considerations control a structure-contour map of the decollement surface, an isopach map of the slab thickness, and a contour map of the surface topography upon termination of thrusting. The maps indicate that the thrust taper and decollement dip, largely controlled by the shape of the Belt-Purcell wedge, increased southward in concert with decreasing displacement of the slab. Structural contours of the foreland basin, restored for post-thrusting rebound, show that it narrows southeastward and plunges out at the pole of rotation of the slab near Helena.