Susanne U. Janecke¹

(1) Utah State University, Logan, UT

ABSTRACT: Insights into Processes of Continental Extension from Field Studies in the Northern and Rocky Mountain Basin and Range Province

Integrated analyses of basin structure, facies and provenance data, geochronology, biostratigraphy, and geophysics shed light on processes of continental extension at shallow crustal levels. The stratigraphic and structural relationships within individual rift basins in SW Montana range from simple to complex. Complex rift basins exhibit extensional folds in orthogonal sets, and multiple distinct phases of rifting with highly variable extension directions over time. Up to 90° changes in extension direction occurred in short time periods. Extension folds exhibit a range of geometries, altered the facies patterns and thickness of synrift deposits in some basins. Some extensional folds postdate deposition of most of the synrift deposits. Some basins are fold prone, whereas others contain only basin-scale synclinal warps. The highest rates of extension and largest normal faults develop early in the history of normal faulting, and the largest normal faults collapse structural culminations formed during prior regional shortening in the Sevier fold and thrust belt. Areas adjacent to culminations exhibit lesser extensional strains. Preexisting structural weaknesses exert a strong influence on the geometry of extensional deformation. The stratigraphic architectures of rift basins above low-angle and listric normal faults do not differ significantly from those of basins bounded by steep to moderately dipping normal faults. The principal basin forming and sedimentation event is often followed by a separate (?) deformational event which faults, tilts, and folds the early rift deposits. Sediment is rarely preserved from this "break-up" deformational event, either due to non-deposition or subsequent erosion. Evidence for normal faulting during deposition of the early rift deposits consists of rock avalanche deposits, and coarse clastic material shed from the footwall. Yet, map-scale and outcrop-scale growth relationships are usually cryptic, and many basin-fills appear conformable.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado