The Role of the Western Laurentia Plate Margin in the Ancestral Rocky Mountains Orogeny: Insights From Tectonic Subsidence Analysis

Abstract

During Pennsylvanian time, much of southwestern Laurentia experienced broad deformation far from the nearest plate boundaries. Much of this deformation is grouped into the Ancestral Rocky Mountains orogeny, consisting of numerous northwest-trending basement uplifts and proximal basins that formed north and northwest of the Ouachita-Marathon suture. These basins initiated in the early Pennsylvanian, reaching peak subsidence during the early to middle Pennsylvanian. Farther west, the Ely-Bird Spring Basin of Nevada formed during the latest Mississippian to early Pennsylvanian, forming between the Ancestral Rocky Mountains and the complex western Laurentian margin. The detailed tectonics of the western margin are poorly-constrained, but in the north consisted of a down-to-the-west subduction zone to the east of the Havallah Basin, a transtensional basin with local sea-floor spreading. Farther south, the margin transitioned to a sinistral transform margin.

This study utilized tectonic subsidence curve analyses to evaluate the tectonic style of basin subsidence in the Ely-Bird Spring Basin and several northern Ancestral Rocky Mountains basins. The western-most basins (Ely-Bird Spring and Wood River) have convex-upward and stair-stepped tectonic subsidence curves similar to curves from foreland basins. The Ely-Bird Spring is dominantly a carbonate shelf, with increasing and coarsening clastic input to the north and west. This basin also experienced mid-Pennsylvanian folding and local thrust faulting that increased in severity to the north and west. These data are consistent with the Ely-Bird Spring Basin forming due to loading from the northwest as a distal part of the western Laurentian plate boundary, suggesting contraction east of the Havallah Basin. The Wood River Basin has similarly been interpreted to have formed due to loading from the west. Published subsidence curves from the Oquirrh, Paradox, and Eagle Basins are much steeper and more linear, similar to foreland basins with fixed loads. The amount of subsidence in these basins is high, considering their distance from the hypothesized Ancestral Rocky Mountains tectonic drivers. The proximity to the western margin combined with anomalously large magnitude subsidence suggests transmitted stresses from the western Laurentian margin combined with transmitted stresses from the south and/or southwest to play a role in the northern part of the Ancestral Rocky Mountains orogen.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018