The Peters Hills basin, a Neogene wedge-top basin on the Broad Pass thrust fault, south-central Alaska

Abstract

The Neogene Peters Hills basin is a small terrestrial basin that formed along the south flank of the Alaska Range during a time in which there was regional shortening. The formation of the Peters Hills basin is consistent with it being a wedge-top basin that formed on top of the active southeast-vergent Broad Pass thrust fault. Movement along this thrust raised a ridge of Jurassic and Cretaceous metasedimentary rocks, which then trapped behind it Miocene and Pliocene sediments that were derived from the growing Alaska Range. The presence of this thrust fault is consistent with regional structural, stratigraphic, seismicity, gravity, and aeromagnetic data. The Peters Hills basin is no longer a depocenter, but if the Broad Pass thrust remains active, it would help explain the westward decrease in Quaternary slip rate along the Denali fault system, and it may constitute a seismic hazard that could produce earthquakes in the M w 7.6-7.8 range. PETERS HILLS BASIN Sedimentary basins of south-central Alaska are easily identified on an isostatic residual gravity map (Fig. 2). The largest of these is the Cook Inlet basin, which is almost 8 km deep and has active fault-cored folds (Haeussler et al.

Pace and Process of Active Folding and Fluvial Incision Across the Kantishna Hills Anticline, Central Alaska

“…Deformed river terraces and offset postglacial surfaces of unknown age provide evidence for ongoing deformation through Quaternary time with inferred rates of shortening approaching 3 m/kyr (Bemis et al, ). Assuming 13 mm/year of far‐field Yakutat convergence (Elliott et al, ; Haeussler, Matmon, et al, ), such shortening is consistent with the average 7–10‐m/kyr Denali Fault slip rates south of the thrust system (Haeussler, Saltus, et al, ). Given unconstrained rates of transpression south of the Denali Fault, however, the rate of northern Alaska Range shortening may be much lower, and remains untested by direct dating of deformed geomorphic markers.…”

“…Researchers attribute the westward decrease in slip rate to right transpression across thrust faults that splay southwest off the Denali Fault (Haeussler et al, ), and to partitioning of the north‐northwest component of southern Alaska block convergence across an array of thrust faults and folds situated north of, and parallel to, the Denali Fault (the northern Alaska Range thrust system; Bemis & Wallace, ; Haeussler, ; Mériaux et al, ; Bemis et al, , ; Haeussler, Matmon, et al, ). This deformation fits a kinematic model in which the southern Alaska block (a) rotates counterclockwise at rates equivalent to Denali Fault slip, (b) shortens internally across the right‐transpressive faults, (c) extrudes westward at a modest pace, and (d) translates north‐northwest and indents central Alaska at a rate commensurate with shortening across the northern Alaska Range thrust system (Haeussler, Matmon, et al, ).…”

Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

“…Stress has been transferred inboard of the Yakutat-Alaska trench leading to increased slip rates along the Denali fault and northwest convergence of southern Alaska across the Denali fault since ca. 30 Ma (Benowitz et al, 2012a, Lease et al, 2016Haeussler et al, 2017aHaeussler et al, , 2017b. Previous studies infer as much as ~250-450 km of dextral displacement along the eastern part of the fault system during Cretaceous-Cenozoic time (Nokleberg et al, 1985;Lowey, 1998;Roeske et al, 2012;Benowitz et al, 2012b;Riccio et al, 2014).…”

“…Exhumation in the Alaska Range also impacted sedimentary basins positioned farther south of the range. The Susitna and Cook Inlet basins record late Oligocene-Neogene contractile deformation and increased sedimentation, including sediment reflecting erosion of bedrock sources in the central and eastern Alaska Range (Finzel et al, 2015(Finzel et al, , 2016Saltus et al, 2016;Bristol et al, 2017;Haeussler et al, 2017b). Overall, the Oligocene-Miocene was a period of major topographic development (e.g., Bill et al, 2018) and regional paleodrainage reorganization across southern Alaska (Davis et al, 2015;Finzel et al, 2016;Benowitz et al, 2019).…”

Sediment sources and transport by the Kahiltna Glacier and other catchments along the south side of the Alaska Range, Alaska

“…This depth is dependent on the density of the matter being eroded. Most of the country rock being eroded within the drainage systems we sampled is composed of granite and metasedimentary rocks, all of which have a density of 2.6-2.7 g cm -3 (Haeussler et al, 2017b). At such densities, the typical depth of attenuation would be ~60 cm (attenuation depth = Λ/ρ).…”