COMBINING FIELDWORK AND SEISMIC INTERPRETATIONS TO ELUCIDATE THE ALONG-STRIKE VARIATION IN STRUCTURE OF THE TETON ANTICLINE, SAWTOOTH RANGE, MONTANA

The Teton Anticline has long been considered the classic example of buckle fold geometry, particularly by fracture modellers. However, a field study of the Teton Canyon area indicates that both the Teton Anticline and the adjacent Little Teton Anticline are detachment folds, whose geometry changes significantly along strike. In the north of the mapped area, the Teton Anticline and Little Teton Anticline are incipient box folds with broad rounded hinges and variable wavelengths controlled by the depth to the detachment horizons. Teton Anticline becomes asymmetric when traced several km to the south, as the forelimb becomes oversteepened. Little Teton Anticline also shows some asymmetry before dying out in the south of the mapped area, as the oversteepening of Teton Anticline becomes more pronounced.

The change in surface geometry of the Teton Anticline implies that there is a corresponding change in the associated subsurface thrust faults. This is confirmed by a series of seismic lines across the two fold structures. Displacement on the major fault beneath the folds in the north is transferred in the south, onto a thrust to the hinterland which splays off the same detachment horizon. In addition, to the south of the Teton Canyon, this major fault is deformed by repetition of the Devonian shale in the footwall. The spatial development of the subsurface complexity coincides with the location of a basement fault zone and may therefore be a result of pre-existing basement topography that acted as a buttress which localised the thrusting and associated folding in the overlying units.

The two folds mark the deformation front of the fold-thrust belt at this point in the Sawtooth Range. To the north, the deformation front is a classic triangle zone (e.g. the Alberta Triangle Zone) and to the south, a classic broad zone of deformation above a basal detachment (e.g. the Sun River Canyon). In the Teton Canyon region, the deformation front is transitional in character between these two styles.