MULTIPLE CRETACEOUS THROUGH TERTIARY TECTONIC EVENTS AND THE RESULTING STRUCTURAL COMPLEXITY ALONG THE LEWIS AND CLARK LINE IN WESTERN MONTANA

The Lewis and Clark Line (LCL) is a wide, complex, WNW-striking zone of faults and folds that transects the NNW structural grain of western Montana and northern Idaho. Recent mapping along a 150 km segment of the LCL in westernmost Montana reveals important age relationships and explains some of the structural diversity along the zone. The LCL postdates and forms the southern boundary of the NNW-striking, 100 Ma Libby thrust belt and related folds of the Purcell Anticlinorium. NNE-SSW shortening and sinistral shear along the LCL at about 80 Ma rotated the older NNW-trending folds in a left lateral sense and superimposed WNW-trending folds and steeply dipping faults and ductile shear zones. At the deeper structural levels exposed along the LCL's westernmost part, this transpressive deformation generated steep, contemporaneous reverse and normal faults, tight folds, and steeply dipping, WNW-trending, mylonitic shear zones with downdip lineations. Farther east along the LCL, higher structural levels are preserved, and the folds open, the faults flatten, and the shear foliation dies out. Subsequent Tertiary extension superimposed kink folds, normal faults with various geometries, and dextral strike-slip faults. Many of these Tertiary structures reactivated the older transpressive features. Facies and thickness changes in the Mesoproterozoic Belt strata across this portion of the LCL suggest that the structure and geometry of the Belt Basin controlled the location of the LCL. However, the exposed structures that define the LCL are all Cretaceous or younger. A westerly jog in the Late Cretaceous Idaho suture zone parallels the LCL to its south, and may be related to the 80 Ma transpressive event.