Structural Inheritance for the Laramide, Central Montana Uplift: A Wrench-Fault Tectonic Model Related to Proterozoic Orogenesis in the Foreland of the North American Cordillera

Abstract

The Central Montana uplift lies in eastern Montana adjacent to the Northern Rocky Mountains on the western edge of the Northern Great Plains Physiographic Province, and just north of the Laramide belt of the Central Rockies. The origins of this deformed region have received little attention relative to Laramide tectonism and Precambrian basement interrelations, particularly within the overall context of plate tectonic evolution of the North American Cordillera.

The Central Montana uplift is characterized at the surface by six well-defined fault zones with general trends to the WNW and NE. These include the Cat Creek, Lake Basin, and Nye-Bowler fault zones (WNW), and the Fromberg, Weldon, and Brockton-Froid fault zones (NE). Previous work attributes development of these fault zones to transcurrent motion on basement-rooted faults that deformed the sedimentary cover during Laramide orogenesis.

A Paleoproterozoic origin for these fault zones is presented herein, with the faults initially forming as conjugate shears during pure-shear convergence at the northeastern margin of the Wyoming Province. It is further proposed that the conjugate shears were re-activated as simple shears during the Laramide under similar SW­–NE stress conditions seen in the Precambrian.

Structural analysis of basement fabrics of the western Beartooth Mountains and Laramide surface structures performed for this study indicate that basement anisotropies (conjugate shears) may have guided Laramide deformation across eastern Montana. Isostatic gravity data support these interpretations. Precambrian development of conjugate shears was likely related to Trans-Hudson (1.78-1.74 Ga) orogenesis along the eastern Wyoming Province boundary. Data also indicate that these structures were re-activated as wrench faults during the Laramide, forming the Central Montana uplift.

This structural style observed in eastern Montana is also ubiquitous across Wyoming. Therefore, this style is present across the entire Wyoming Province. However, deformation in the core of the craton is likely Archean (Owl Creek and Tensleep fault zones). Basement-rooted deformation zones in northern Wyoming and Montana, and similar zones south of the Owl Creek zone are likely related to Proterozoic pure shear, also directed from SW­–NE. Thus, major deformation in basement of the entire province is related first to pure-shear triaxial stress transmitted SW­–NE across the craton during Archean convergence (2.6­–3.0 Ga), when the craton was much smaller, and then secondly, similar SW–NE-directed pure shear affected the craton in the Proterozoic (1.7­–1.9 Ga), after the entire Archean Wyoming Province had been assembled. This second event corresponds to Trans-Hudson orogenesis, and final docking of the Superior and Wyoming cratons to the NE, and Grouse Creek block to the SW.

These Precambrian convergent deformation systems were likely a fundamental tectonic control for Laramide arch/uplift formation in Wyoming and Montana. Western North Dakota and southern Saskatchewan may have developed similar basement anisotropies as a result of docking of the Superior/Wyoming cratons towards the latter stages of the Trans-Hudson orogen. This has implications, not only for the structural and tectonic evolution of the Williston Basin, but also for the formation and development of numerous oil-producing units within the basin.

AAPG Datapages/Search and Discovery Article #90374 © 2020 AAPG Rocky Mountain Section Meeting, 2020 Vision: Turn Hindsight to Foresight, Grand Junction, Colorado, September 13-15, 2020 (CANCELLED)