Changing exhumation patterns during Cenozoic growth and glaciation of the Alaska Range: Insights from detrital thermochronology and geochronology

Abstract

Cenozoic growth of the Alaska Range created the highest topography in North America, but the space‐time pattern and drivers of exhumation are poorly constrained. We analyzed U/Pb and fission‐track double dates of detrital zircon and apatite grains from 12 catchments that span a 450 km length of the Alaska Range to illuminate the timing and extent of exhumation during different periods. U/Pb ages indicate a dominant Late Cretaceous to Oligocene plutonic provenance for the detrital grains, with only a small percentage of grains recycled from the Mesozoic and Paleozoic sedimentary cover. Fission‐track ages record exhumation during Alaska Range growth and incision and reveal three distinctive patterns. First, initial Oligocene exhumation was focused in the central Alaska Range at ~30 Ma and expanded outward along the entire length of the range until 18 Ma. Oligocene exhumation, coeval with initial Yakutat microplate collision >600 km to the southeast, suggests a far‐field response to collision that was localized by the Denali Fault within a weak Mesozoic suture zone. Second, the variable timing of middle to late Miocene exhumation suggests independently evolving histories influenced by local structures. Time‐transgressive cooling ages suggest successive rock uplift and erosion of Mounts Foraker (12 Ma) through Denali (6 Ma) as crust was advected through a restraining bend in the Denali Fault and indicate a long‐term slip rate ~4 mm/yr. Third, Pliocene exhumation is synchronous (3.7–2.7 Ma) along the length of the Alaska Range but only occurs in high‐relief, glacier‐covered catchments. Pliocene exhumation may record an acceleration in glacial incision that was coincident with the onset of Northern Hemisphere glaciation.

Thermotectonic History of the Kluane Ranges and Evolution of the Eastern Denali Fault Zone in Southwestern Yukon, Canada

“…In the central Alaska Range, rapid cooling at ~6 Ma is documented by apatite FT data from the Denali massif and potentially indicates a change in Pacific Plate convergence direction (Fitzgerald et al, ). Other workers suggest that rapid Neogene cooling of rocks along the CDFZ and other structures is the result of advection of crust through the large restraining bend that characterizes the CDFZ and attendant large‐scale contractional deformation and vertical rock uplift (Figure , inset; Fitzgerald et al, ; Riccio et al, ; Lease et al, ). Bedrock apatite He and detrital apatite FT dates from high‐relief, glaciated catchments adjacent to the CDFZ record accelerated Pleistocene cooling and exhumation as the result of glacial erosion (Benowitz et al, ; Lease, ; Lease et al, ).…”

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…FT and (U–Th)/He data can be acquired on crystals previously analyzed for U–Pb geochronology, aided by analytical advances of laser ablation and noble gas mass spectrometry analyses. This approach includes “double” U–Pb and FT or He analysis of apatite or zircon (Campbell et al, ; Carter & Moss, ; Fosdick et al, ; Lease, Haeussler, & O'Sullivan, ; Odlum & Stockli, ; Rahl et al, ; Reiners et al, ; Ternois et al, ; Thomson et al, ) or U–Pb, FT, and (U–Th)/He “triple” dating (e.g., Carrapa et al, ; Danišík, ; Danišík et al, ). Integration of data from two or more thermochronometric systems with different temperature sensitivities in the same mineral permits a more detailed reconstruction of the thermal history in these settings.…”

Miocene‐Recent sediment flux in the south‐central Alaskan fore‐arc basin governed by flat‐slab subduction

“…Although we have only two Susitna river samples from the northern region, they are nearly identical and provide a self‐consistent signature for that region. The similarity of the age signatures implies that erosion in the lower Susitna River catchment is limited and outpaced by the high erosion that occurs in the upper portion of the catchment within the central Alaska Range [ Lease et al ., ]. The five samples from the eastern region display some variability, as expected by the unique geology of each individual watershed.…”

“…The age distributions from the northern region match other detrital zircon U‐Pb ages from modern rivers in that region; ages suggest that most of the sediment is derived from plutonic rocks in the headwaters of the watersheds (central Alaska Range), with very little recycled from the Paleozoic and Mesozoic strata [ Finzel et al ., ; Lease et al ., ]. Furthermore, the lack of Jurassic and older grains in our Susitna River samples suggests that there is very little input from the widespread Jurassic igneous rocks composing the northern Talkeetna Mountains (Figure ).…”