THEMED ISSUE: Geologic Evolution of the Alaska Range and Environs

Analysis of the late Miocene to Holocene McCallum sedimentary basin, located along the south side of the eastern Denali fault system, provides a better understanding of strike-slip basin evolution, timing of displacement on the Denali fault, and tectonics of the southern Alaska convergent margin. Analysis of the McCallum basin utilizing measured stratigraphic sections, lithofacies analyses, and 40Ar/39Ar tephra ages documented a 564-m-thick, two-member stratigraphy. Fine-grained, lacustrine-dominated environments characterized deposition of the lower member, and coarse-grained, stream-dominated alluvial-fan environments characterized deposition of the upper member. The 40Ar/39Ar dating of tephras indicated that the lower member was deposited from 6.1 to 5.0 Ma, and the upper member was deposited from 5.0 to 3.8 Ma. Our stratigraphic analysis of the McCallum basin illuminates the development of a composite strike-slip basin, with the deposition of the lower member occurring along a transtensional fault section, and deposition of the upper member occurring along a transpressional fault section. This change in depositional and tectonic settings is interpreted to reflect ~79–90 km of transport of the basin along the Denali fault system based on Pleistocene–Holocene slip rates. Previous studies of the timing of Cenozoic displacement on the Denali fault system utilizing sedimentary records emphasized a Paleogene component; our findings, however, also require a significant Neogene component. Neogene strike-slip displacement and basin development along the Denali fault system were broadly coeval with development of high topography and related clastic wedges across southern Alaska in response to flat slab subduction of the Yakutat microplate. ■ INTRODUCTION The southern Alaska convergent margin was formed by subduction of oceanic crust and accompanying island arcs, spreading ridges, and oceanic plateaus since at least the Jurassic, and this process continues to the present (Fig. 1A; Plafker and Berg, 1994; Nokleberg et al., 1994; Ridgway et al., 2007a). In response to this subduction, the upper plate of this convergent margin contains a rich record of Mesozoic to Cenozoic deformation, exhumation, and basin development (Trop and Ridgway, 2007). Oligocene to ongoing flat slab subduction of the Yakutat microplate, an oceanic plateau, is the most recent phase of this ongoing process (Fig. 1A; Elliott et al., 2010; Finzel et al., 2011; Worthington et al., 2012). Continental strike-slip faults distributed throughout southern and central Alaska accommodate a component of Yakutat–North America convergence, with the Denali fault being the largest active strike-slip system (Fig. 1A; Haeussler et al., 2017a). In the Alaska Range, late Oligocene to present-day deformation, exhumation, and basin formation along the Denali fault system have been identified in previous studies (Fitzgerald et al., 1993, 1995; Ridgway et al., 2002, 2007b; Trop and Ridgway, 2007; Benowitz et al., 2011). An understudied part of the sedimentary record of deformation in the Alaska Range is the thrust systems and related sedimentary basins located along the south side of the central and eastern Denali fault system (e.g., Fitzgerald et al., 2014; Riccio et al., 2014; Haeussler et al., 2017b; Trop et al., 2019). Our study focused on the Neogene strata of the McCallum formation in the McCallum basin, located 10 km south of the Denali fault in the eastern Alaska Range and presently exposed mainly in the footwall of a southwest-verging thrust system (McCallum Creek fault labeled on Fig. 1A). Neogene strata of the McCallum basin provide an exceptional record of depositional and tectonic processes along the Denali fault system because these strata are deformed, are well exposed, and contain abundant tephras that allow for establishment of a chronostratigraphic framework. Our data sets Downloaded from http://pubs.geoscienceworld. org/gsa/geosphere/article-pdf/doi/10.1130/GES02435.1/5541529/ges02435.pdf by guest on 10 February 2022 2 Allen et al. | Sedimentary record of the evolution of a translated strike-slip basin along the Denali fault system GEOSPHERE | Volume 17 | Number X Research Paper