KINEMATICS AND STRAIN DISTRIBUTION OF A THRUST-RELATED FOLD SYSTEM INTHE LEWIS THRUST PLATE, NORTHWESTERN MONTANA (USA)

In order to understand interactions between motion along thrusts and t he associated style of deformation and strain distribution in their ha ngingwalls, geologic mapping and strain measurements were conducted in an excellently exposed thrUSt-related fold system in the Lewis thrust plate, northwestern Montana. This system consists of: (1) an E-direct ed basal thrust (the Gunsight thrust) that has a flat-ramp geometry an d a slip of about 3.6 km; (2) an E-verging asymmetric anticline with i ts nearly vertical forelimb truncated by the basal thrust from below; (3) a 4-km wide fold belt, the frontal fold complex, that lies directl y in front of the E-verging anticline; (4) a W-directed bedding-parall el fault (the Mount Thompson fault) that bounds the top of the frontal fold belt and separates it from the undeformed to broadly folded stra ta above; and (5) regionally developed, W-dipping spaced cleavage. Alt hough the overall geometry of the thrust-related fold system differs f rom any previously documented fault-related folds, the E-verging antic line itself resembles geometrically a Rich-type fault-bend fold. The o bserved initial cut-off and fold interlimb angles of this anticline, h owever, cannot be explained by cross-section balancing models for the development of either a fault-bend fold or a fault-propagation fold. P ossible origins for the E-verging anticline include (1) the fold initi ated as an open fault-bend fold and tightened only later during its em placement along the basal thrust and (2) the fold started as either a fault-bend or a fault-propagation fold, but simultaneous or subsequent volume change incompatible with any balanced cross-section models alt ered its shape. Strain in the thrust-related fold system was determine d by the preferred orientation of mica and chlorite grains. The direct ion and magnitude of the post-compaction strain varies from place to p lace. Strains in the forelimb of the hangingwall anticline imply beddi ng-parallel thinning at some localities and thickening at others. This inhomogeneity may be caused by the development of thrusts and folds. Strain in the backlimb of the hangingwall anticline implies bedding-pa rallel stretching in the thrust transport direction. This could be the effect of bending as the E-verging anticline was tightened and transp orted across the basal thrust ramp. Strain measured next to the Gunsig ht thrust again indicates locally varying shortening and extension in the transport direction, perhaps in response to inhomogeneous friction on the fault or else to a history of alternating strain hardening and softening in the basal thrust zone.