Uplift and denudation of the central Alaska Range: A case study in the use of apatite fission track thermochronology to determine absolute uplift parameters

Abstract

Apatite fission track thermochronology (AFTT) on granitic samples collected in the central Alaska Range in conjunction with geologic constraints from basins to the north (Nenana Basin) and south (Cook Inlet) of the range is used to constrain the timing, amount, rate, and pattern of surface uplift, rock uplift, and denudation since the late Miocene. The conversion from a thermal frame of reference (apatite fission track data) to an absolute frame of reference (with respect to mean sea level), which requires constraining the paleoland surface elevation, the paleomean annual temperature, and the paleogeothermal gradient, is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone (PAZ). Apatite ages at Denali (Mount McKinley) range from 16 Ma near the summit (∼6 km elevation) to 4 Ma at ∼2 km elevation. A distinctive break in slope in the apatite age profile at an elevation of 4.5 km, also marked by a change in confined track length distributions, marks the base of an exhumed apatite PAZ. Rock uplift and denudation are greatest at Denali, decreasing southward away from the McKinley strand of the Denali fault system as shown by progressively older apatite ages (7–35 Ma) from a suite of samples along the Kahiltna Glacier. A correlative decrease in topography occurs southward from the fault. The central Alaska Range lies within an arc defined by the Denali fault, with the highest peaks (including Denali) concentrated at the arc apex. Patterns of rock uplift and denudation within the central Alaska Range mimic topography. Between early and late Miocene, and possibly earlier, the central Alaska Range was most likely an area of relative tectonic and thermal stability. Rock uplift, denudation, and mean surface uplift of the Denali region began by the Late Miocene (∼5–6 Ma), being ∼8.5 km, ∼5.7 km, and ∼2.8 km, respectively, at average rates of ∼1.5 km/m.y., ∼1 km/m.y., and ∼0.5 km/m.y. The amount of rock uplift, denudation, and surface uplift decreases to ∼3 km, ∼2 km, and ∼1 km at Little Switzerland, some 45 km south of the Denali fault. We conclude that the topographic and rock uplift patterns of the central Alaska Range, the shape and proximity of the McKinley strand of the Denali Fault to these patterns, the timing of the onset of rock uplift and denudation at ∼5–6 Ma, and a significant change in relative plate motion between North America and the Pacific plates circa 5.6 Ma are all inherently related.

Exumação tectônica e reativação de paleolineamentos no Arco de Ponta Grossa: termocronologia por traços de fissão em apatitas

“…A correlação entre idades de traços de fissão e altitude tem sido amplamente utilizada para reconstruir taxas de exumação tectônica em diversos ambientes geotectônicos (Fitzgerald et al 1995, House et al 1998, Braun 2002. No Brasil, em especial na região sudeste, Gallagher et al (1995), Hackspacher et al (2003) e Franco-Magalhães (2009) utilizaram esta análise para estimar e reconstruir as taxas de exumação tectônica do relevo em diversas áreas geológicas.…”

A linear inversion method to infer exhumation rates in space and time from thermochronometric data

“…2. Apatite fission track ages from Mt Denali (Fitzgerald et al, 1995) plotted as a function of eleon. There is a break in slope at 6 Ma, interpreted as a response to an increase in exhumation rate.…”

“…6. Exhumation rates inferred from age data from (Fitzgerald et al, 1995) weighted average of the exhumation rates evaluated at the data locations. Here the relative weights are defined by the separation distances between the data locations and the point of interest and the correlation function imposed in the inversion.…”

“…We use published apatite fission track (AFT) data from samples collected within the footwall of the Denali Fault, taken from Fitzgerald et al (1995) (Fig. 2).…”

“…If the samples exhumed together at a constant rate through a steady temperature field, the ages will show a linear relationship with elevation. The slope of this age-elevation relationship (AER) is equal to the exhumation rate and changes in slope can be interpreted as changes in exhumation rate through time (e.g., Wagner and Reimer, 1972;Wagner et al, 1979;Fitzgerald and Gleadow, 1988;Fitzgerald et al, 1995).…”