THE EVOLUTION OF FOLD-PRONE SUPRADETACHMENT BASINS: EXAMPLES OF TRANSLATION AND BREAKUP FROM MONTANA AND SE IDAHO

Supradetachment basins have distinctive stratal architectures that record their evolution from early translation of an intact hanging wall to later breakup into discrete fault blocks. Seismic-reflection profiles and field relationships reveal nearly conformable bedding and subtle fanning dips in translational strata. This geometry arises from the great width of the original supradetachment basin, and limited tilting. The change from subtle fanning dips in early synrift deposits to clear fanning dips in younger synrift deposits is diagnostic of break-up. The evolution from translation to break up is also recorded by basin fill that is laterally continuous, of uniform thickness and reflects widespread lacustrine facies early in the basin's evolution, but later exhibits marked lateral changes in thickness and facies, may contain progressive unconformities, and coarsens adjacent to local fault blocks. Facies patterns in the translational deposits are asymmetric in their distribution: coarse footwall-derived clastics are localized along the basin-bounding normal fault whereas hanging wall-derived clastics or axial deposits occupy most of the basin's surface area. The conglomerates of the break-up phase are distinctive because they contain clasts recycled from older lacustrine beds of the translation phase and clasts from the deeper parts of uplifting fault blocks. This provides compelling evidence of reorganization and dismembering of the original translational basin. The geometry of supradetachment basins may be complicated by: (1) longitudinal and transverse extensional folds that partition the basins into subbasins during deposition, and in some cases fold the detachment fault; (2) lateral ramps and resultant changes in deformational style of the hanging wall, and (3) antithetic normal faults above the detachment. Transverse folds are mostly confined to the hanging wall and well-characterized examples in Montana forming above original corrugations in the underlying detachment fault. There, plunging antiformal corrugations in the detachment fault produced synclines in the hanging wall whereas synforms in the detachment fault produced anticlines in the hanging wall. The tendency of beds to dip toward a listric but strike-varying detachment fault may produce this type of fault-bend fold.