Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

Abstract

40 Ar/ 39 Ar, apatite fi ssion-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ~33 Ma to ~18 Ma for 40 Ar/ 39 Ar biotite, ~18 Ma to ~6 Ma for K-feldspar minimum closure ages, and ~15 Ma to ~1 Ma for apatite fi ssion-track ages, and apatite (U-Th)/He cooling ages range from ~4 Ma to ~1 Ma. There has been at least ~11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40 Ar/ 39 Ar thermochronology. Variations in exhumation history along and across the strike of the fault are infl uenced by both far-fi eld effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ~22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (~past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

Persistent long-term ( c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

“…Less than about 3 Ma AFT and (U -Th/He) apatite (AHe) ages in the Eastern Alaska Range (Armstrong et al 2007;Benowitz et al 2011a), as well as active seismicity including the 2002 M7.9 Denali Fault earthquake (Eberhart-Phillips et al 2003), imply that the region is tectonically active. Deformation and uplift of the Eastern Alaska Range has recently been correlated with the Neogene Usibelli Group of the Tanana Basin, which is interpreted to contain a long-term record of a transpressional foreland basin system related to regional shortening in the Alaska Range along the Denali fault system (Ridgway et al 2007).…”

“…1) and is the site of one Ar K-feldspar minimum ages. The red age labels are new data, black age labels are from Benowitz et al (2011a) and blue age labels are from Benowitz et al (2011b). The shaded area is an approximately 20 km-wide wedge of Miocene 40 Ar/ 39 Ar K-feldspar minimum ages with the Denali Fault as the southern boundary.…”

“…We offer a simple approach to reconstructing the exhumation history of an orogen, building on the previous work of Batt et al (2004) by vertically stacking the best-fit MDD thermal histories of eight K-spar-bearing bedrock samples collected along and across the strike of an orogen (Figs 2 & 6), including two new samples (26RAP and 19BAL) and six others from Benowitz et al (2011a). The amount of cooling (inferred in terms of exhumation) recorded in each sample is complicated by differences in palaeo-depth, present-day elevation and the location of each individual sample in relation to structures (e.g.…”

“…Uranium -lead zircon emplacement ages of approximately 70 Ma were obtained from plutons in the Black Rapids Glacier, which is part of the Eastern Alaska Range (Aleinikoff et al 2000). Plutons in the Mount Nenana region of the western part of the Eastern Alaska Range are thought to have approximately 38 Ma emplacement ages based on K -Ar and 40 Ar/ 39 Ar dating of biotite and hornblende (Csejtey et al 1992;Benowitz et al 2011a). The plutons around Mount Kimball along the easternmost edge of the Eastern Alaska Range have inferred emplacement ages of about 100 Ma based on K-Ar dating of hornblende (Nokleberg et al 1992).…”

“…We use the same general geothermal gradient, approximately 30 8C/km, used in many of the exhumation studies performed in southern Alaska to calculate exhumation rate (O'Sullivan & Currie 1996;Haeussler et al 2008;McAleer et al 2009;Benowitz et al 2011a). This allows direct comparison of exhumation rates between southern Alaska regional exhumation studies.…”