Tectonic Analysis of the Pennsylvanian Ely‐Bird Spring Basin: Late Paleozoic Tectonism on the Southwestern Laurentia Margin and the Distal Limit of the Ancestral Rocky Mountains

Abstract

AbstractDuring Pennsylvanian to Early Permian time, much of southwestern Laurentia experienced broad deformation far from the nearest plate boundaries. The bulk of this deformation is known as the Ancestral Rocky Mountains orogeny, which consists of several basement uplifts and proximal basins north and northwest of the Ouachita‐Marathon suture. The Ely‐Bird Spring basin formed during this time, located between the classic Ancestral Rocky Mountains and the complex and poorly constrained western Laurentian margin. In this study, we used tectonic subsidence curve analyses to evaluate the tectonic style of basin subsidence in this basin and several northern Ancestral Rocky Mountains basins. The Ely‐Bird Spring and Wood River basins have convex upward and stair‐stepped tectonic subsidence curves similar to curves from migrating foreland basins. These curves, combined with sedimentological and structural data, are consistent with these basins forming due to loading from the west and northwest as part of the complex western Laurentian plate boundary, not due to classic Ancestral Rocky Mountains tectonics. The other northern Ancestral Rocky Mountains basins have much steeper and more linear subsidence curves, similar to foreland basins with fixed loads. However, the Oquirrh basin, in particular, displays a large amount of subsidence far from hypothesized tectonic drivers of deformation. A combination of transmitted stress from both the western Laurentia margin and traditional Ancestral Rocky Mountains tectonism explains the anomalously large subsidence in the Oquirrh basin. Therefore, transmitted stresses from more than one side of western and southern Laurentia are required to explain the observed pattern of deformation.