Abstract

Vertical variations in structural style and strain within seemingly coherent thrust sheets has been studied in the Sawtooth Range of northwest Montana. Variations in structural style are attributed to ductility contrasts and variable bedding characteristics within the stratigraphic section, whereas strain variations may be due to heterogeneous simple shear above a fault plane and longitudinal shortening within the thrust sheet. These processes of internal deformation are quantitatively important in palinspastic restorations of the thrust belt and predictions of subsurface geometry.

Vertical variations in structural style are classified into three "lithotectonic zones". Zone I is the Cambrian section, containing interbedded shales and limestones. Zone I is characterized by tightly spaced disharmonic folds and thrust faults at all scales, pressure solution cleavage and fracturing of the carbonates, and boudinage and pencil structure development in the shales. Zone I is clearly the most highly deformed and internally strained zone. Zone II is the Devonian-Mississippian section, dominated by massively bedded dolomites and limestones that show little evidence of internal deformation. The folding style of zone II is dramatically different from the underlying Cambrian section, and is characterized by kilometer-scale concentric and box folds. In a mechanical sense, zone II forms a rigid competent beam between zones I and III. Zone III is the Jurassic and Lower Cretaceous section, containing interbedded limestone, sandstone and shale. This interval contains closely spaced thrust faults and fold trains, but the magnitude of shortening and internal deformation is much less than that recorded in zone I.

Vertical zones of strain are more condensed and localized as compared to the lithotectonic zones. Internal strain may be divided into two categories: 1) longitudinal strain caused by layer-parallel shortening, and 2) shear strain caused by proximity to individual thrust faults. Both types are well developed in lithotectonic zone I, but are much less abundant or absent in zones II and III. Longitudinal strain is exemplified by pressure-solution cleavage fans that are axial-planar to ramp anticlines. This type of cleavage may account for 20-30% shortening. The degree of shear strain is a function of proximity to a thrust fault and the type of rocks involved. In general, shear strains are greater where limestone is thrust over limestone; bedding planes in the hanging wall are obliterated by pervasive fracturing, veining, and mesoscopic thrusting and duplexing. In contrast, limestone thrust over shale shows much less deformation in the hanging wall, although the shaley footwall can show disharmonic drag folds. Therefore, vertical strain variations are a function of both lithology and distance from the thrust fault plane.

These observations are important for several reasons. First, published cross sections of the Thrust Belt are minimum strain sections in that they show the overall style of folding and amount of fault translation (based on footwall cutoffs), but do not adequately show deformation within thrust sheets. Internal deformation is often regarded as negligible because the sedimentary rocks show no mineralogical evidence of regional or dynamic metamorphism. However, significant longitudinal and shear strains are observed in these unmetamorphosed rocks, and their magnitude should be considered in the construction of balanced cross sections and palinspastic restorations. Secondly, vertical variations in structural style are very important to recognize when attempting to interpret the subsurface style and geometry of a thrust sheet. These variations have many implications for the exploration or production geologist, including subsurface predictions of reservoir geometry, fracture patterns, and depth-continuity of structures. Thirdly, by understanding the internal fabrics of thrust sheets, we will gain insight to the mechanical processes involved with the emplacement of large overthrust sheets.