Cooling history of the St. Elias syntaxis, southeast Alaska, revealed by geochronology and thermochronology of cobble-sized glacial detritus

Abstract

We investigate the spatiotemporal evolution of exhumation in the ice-covered St. Elias syntaxis area, southeast Alaska, using multiple thermochronometers and geochronometers from cobble-sized glacial detritus. Multiple thermochronometers reveal the cooling histories from 500 to 60°C of 27 glacially transported cobbles from the two largest catchments of the syntaxis. Cobble lithologies and 21 zircon U-Pb ages (~277-31 Ma) were examined to determine sample provenance. Furthermore, eight amphibole and seven biotite 40 Ar/ 39 Ar ages (~276-16 Ma and~50-42 Ma, respectively), four zircon and six apatite (U-Th)/He ages (~35-4.8 Ma and~4.2-0.6 Ma, respectively), and four apatite fission track ages (~17-1.6 Ma) were used to reconstruct the individual cobble cooling histories. An additional four bedrock samples from the Fairweather Range yielded three biotite 40 Ar/ 39 Ar ages between~42 and 5 Ma. A compilation of published bedrock and new cobble cooling histories from the St. Elias Mountains and Fairweather Range reveals the regional Cenozoic cooling and exhumation history, emphasizing the position of the St. Elias syntaxis as a transitional zone between transpression and subduction settings. The new cobble and bedrock data indicate an onset of rapid exhumation at~5 Ma that was limited in duration (2-3 Myr) and amount (~10 km) in the syntaxial region. This study also demonstrates the usefulness of cobbles for revealing thermal histories of otherwise inaccessible regions as cobble analysis combines advantages of bedrock and detrital thermochronology.

Source to sink evaluation of sediment routing in the Gulf of Alaska and Southeast Alaska: A thermochronometric perspective

“…Collectively, these studies highlight the importance of surface processes in shaping the orogen. Erosion and exhumation are most rapid in the wet regions to the south of the drainage divide that face the coast, whereas the dry northern side erodes at much slower rates [ Berger et al ., ; Berger and Spotila , ; Enkelmann et al ., , ; Falkowski et al ., ]. In areas where surface uplift is driven by convergence and crustal thickening, feedback mechanisms between tectonic processes and erosion can develop [e.g., Beaumont et al ., , ; Tippett and Kamp , ; Willett , ; Zeitler et al ., , ; Koons et al ., ].…”

“…The consequence of the interplay between tectonic uplift and erosion is exhumation of rocks with extremely high rates (>2–5 km/Myr) from ~10 km depths [e.g., Enkelmann et al ., ]. This finding was revealed by detrital ZFT dating, yielding age populations that peak at 3–2 Ma, found in the modern sediment from the Malaspina and Hubbard Glaciers and adjacent catchments [ Enkelmann et al ., , , ; Falkowski et al ., ; Falkowski and Enkelmann , ], and by <3 Ma zircon (U‐Th)/He ages of cobbles from the Malaspina and Hubbard Glaciers [ Grabowski et al ., ; Falkowski et al ., ]. Thus, the material eroded at the St. Elias syntaxis region has a distinct thermochronometric signature that is different from the region along the coastal side, as well as the dry northern side, of the St. Elias Mountains [ Enkelmann et al ., ].…”

Late Miocene to Pleistocene Source to Sink Record of Exhumation and Sediment Routing in the Gulf of Alaska From Detrital Zircon Fission‐Track and U‐Pb Double Dating

“…Although these rocks are not very voluminous (based on modern exposures), this interpretation is supported by the extreme rapid exhumation at the core of the syntaxis region between particularly from 5 to 2 Ma (Enkelmann, Koons, et al, ). Even today, large portions of the cobble‐sized clasts transported by Seward‐Malaspina Glacier are granites, orthogneisses, and igneous mylonites (Grabowski et al, ) that yield zircon U‐Pb ages of 55–48 Ma (Falkowski et al, ). Because of the spatial and temporal correlation with our western borehole samples, we propose the St. Elias syntaxis region and the northern end of the Fairweather Fault as sediment source.…”

“…The area surrounding the indenting plate corner is known as the St. Elias syntaxis and is notable for its high elevation and local relief, and complex structures that bend around and transition from strike‐slip to thrust‐dominated deformation (dashed red circle in Figure ; Bruhn et al, ; Koons et al, ; Pavlis et al, ). The St. Elias syntaxis has localized deep‐seated, extremely rapid exhumation (>2–5 km/Myr) that started ~10–8 Ma (Enkelmann et al, , , Enkelmann, Koons, et al, ; Falkowski et al, , ; Falkowski & Enkelmann, ; Grabowski et al, ). The locus of this extreme rapid exhumation shifted southward over time as a consequence of interaction between tectonics and surface processes (Enkelmann, Koons, et al, ; Falkowski & Enkelmann, ).…”

“…The efficient interplay between tectonics and climate‐driven surface processes has built the highest coastal mountain range on earth with up to 5,000 m of local relief (Figure ; Bruhn et al, ; Enkelmann, Koons, et al, ; Enkelmann et al, ; Gulick et al, ; Pavlis et al, ). This impressive landscape is the result of voracious glacial erosion and the effective transport of sediment by tidewater glaciers into the Gulf of Alaska (Enkelmann, Koons, et al, ; Headley et al, ; Jaeger et al, ; Pavlis et al, ; Péwé, ; Spotila et al, ; Falkowski et al, ). Glaciation in the St. Elias Mountains is suggested to have started by ~6–5 Ma (Lagoe et al, ) with sediment mainly deposited on the Yakutat shelf.…”