Neotectonics of interior Alaska and the late Quaternary slip rate along the Denali fault system

Abstract

The neotectonics of southern Alaska (USA) are characterized by a several hundred kilometers-wide zone of dextral transpressional that spans the Alaska Range. The Denali fault system is the largest active strike-slip fault system in interior Alaska, and it produced a M w 7.9 earthquake in 2002. To evaluate the late Quaternary slip rate on the Denali fault system, we collected samples for cosmogenic surface exposure dating from surfaces offset by the fault system. This study includes data from 107 samples at 19 sites, including 7 sites we previously reported, as well as an estimated slip rate at another site. We utilize the interpreted surface ages to provide estimated slip rates. These new slip rate data confirm that the highest late Quaternary slip rate is ~13 mm/yr on the central Denali fault near its intersection with the eastern Denali and the Totschunda faults, with decreasing slip rate both to the east and west. The slip rate decreases westward along the central and western parts of the Denali fault system to 5 mm/yr over a length of ~575 km. An additional site on the eastern Denali fault near Kluane Lake, Yukon, implies a slip rate of ~2 mm/yr, based on geological considerations. The Totschunda fault has a maximum slip rate of ~9 mm/yr. The Denali fault system is transpressional and there are active thrust faults on both the north and south sides of it. We explore four geometric models for southern Alaska tectonics to explain the slip rates along the Denali fault system and the active fault geometries: rotation, indentation, extrusion, and a combination of the three. We conclude that all three end-member models have strengths and shortcomings, and a combination of rotation, indentation, and extrusion best explains the slip rate observations.

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

“…Slip rates along the northern section of the EDFZ and Totshunda fault sum to approximately equal the CDFZ slip rate, suggesting these two faults act in conjunction to accommodate plate boundary strain (Matmon et al, 2006). South of the EDFZ-Totshunda intersection and near our study area, Quaternary dextral slip rates further decrease to~< 1-2 mm/year on the EDFZ, while the Totshunda Fault accommodates~15 mm/year of dextral slip (Figure 1a; Haeussler et al, 2017;Marechal et al, 2018).…”

“…Present-day activity on the Denali fault is recorded by GPS data and abundant seismicity, including a 2002 M w 7.9 earthquake that ruptured the CDFZ, Susitna Glacier, and Totshunda faults (Eberhart-Phillips et al, 2003;Elliott et al, 2010;Fisher et al, 2004;Freymueller et al, 2008). Quaternary dextral slip rates peak at~13-14 mm/year along the CDFZ and decrease both westward and eastward (Haeussler et al, 2017;Marechal et al, 2018;Matmon et al, 2006). The westward decrease in slip rate may reflect partitioning of the slip budget along thrust faults oriented perpendicular to Yakutat microplate convergence, although the exact mechanisms of strain accommodation are debated (e.g., Haeussler et al, 2017).…”

“…The apatite (U-Th)/He (He), apatite fission-track (FT), zircon He, and zircon FT thermochronometers record cooling Plafker and Berg (1994). Available Quaternary slip rates are for dextral slip in mm/yr, with exception of DRF which gives horizontal shortening assuming range of fault dips (Haeussler et al, 2017;Marechal et al, 2018;Matmon et al, 2006). Mountain range abbreviations (south-north): STE = St. Elias Mountains; WM = Wrangell Mountains; KR = Kluane Ranges; EAR = Eastern Alaska Range; CAR = Central Alaska Range; WAR = Western Alaska Range.…”

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

“…Shortening at the southern Alaska margin takes up ~75% of the geodetically inferred ~50‐mm/year Yakutat convergence velocity (Elliott et al, ). The southern Alaska lithospheric block transmits the remaining ~25% several hundred kilometers inland to the Denali Fault, where Late Pleistocene average rates of ~13‐m/kyr dextral slip account for the residual plate velocity near the Totschunda Fault junction, but decrease westward to ~5 m/kyr over ~350 km along strike (Haeussler et al, ; Matmon et al, ; Mériaux et al, ; Figure a).…”

“…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, ). Absent direct constraints, Late Pleistocene rates of southern and central Alaska deformation off the Denali Fault remain unquantified, leaving unclear the far‐field distribution of strain related to the Yakutat microplate collision (e.g., Bemis et al, ; Haeussler, Matmon, et al, ).…”