Waldien et al. (2021) present new bedrock geologic mapping, U-Pb geochronology, and 40Ar/39Ar thermochronology from the eastern Alaska Range in south-central Alaska to determine the burial and exhumation history of metamorphic rocks associated with the Alaska Range suture zone, interpret the history of faults responsible for the burial and exhumation of the metamorphic rocks, and speculate on the relative importance of the Alaska Range suture zone and related structures during Cenozoic reactivation. They also propose that ultramafic rocks in their Ann Creek map area in south-central Alaska (herein referred to as the “Ann Creek ultramafic complex”) correlate with the Pyroxenite Creek ultramafic complex in southwestern Yukon, and that this correlation is “consistent with other estimates of >400 km” of offset on the Denali fault. However, despite Waldien et al.'s (2021) claim that the purportedly offset ultramafic rocks are “similar” and that characteristics of the Ann Creek ultramafic complex “make a strong case” for a faulted portion of an Alaska-type ultramafic intrusion, their paper gives short shrift in describing the Pyroxenite Creek ultramafic complex and in discussing previous estimates of displacement on the Denali fault. In Addition, Waldien et al. (2021) are either unaware of or ignore several key references of the Pyroxenite Creek ultramafic complex and estimates of displacement on the Denali fault. As a result, Waldien et al.'s (2021) claim of a “correlation” between allegedly offset ultramafic rocks is suspect, and their reference to “other estimates of >400 km” of offset on the Denali fault is incorrect, or at the very least misleading.

Waldien et al. (2021, their Fig. 1) conducted bedrock mapping immediately west of the Denali fault in the Ann Creek and the Broxson Gulch map areas in south-central Alaska: the Ann Creek area is located closest to the Denali fault, whereas the Broxson Gulch area is located several km farther west. Ultramafic rocks occur within a unit referred to as the Clearwater metasediments (Waldien et al., 2021).

The Clearwater metasediments comprise a heterogeneous unit of metagreywacke, graphite-muscovite-chlorite phyllitic schist, porphyroblastic pyrite-graphite phyllite, argillaceous slate, and albite-actinolite-epidote-chlorite schist that was subject to greenschist facies metamorphism (Waldien et al., 2021). The metasedimentary rocks are dominated by a detrital zircon U-Pb age population of 160–150 Ma with inferred maximum depositional ages ranging from 156 to 151 (Waldien et al., 2021). Waldien et al. (2021) suggest that the Clearwater metasediments formed part of a Mesozoic flysch basin that collapsed during north-to-south accretion of the Wrangel composite terrane (also referred to as the Insular terrane) with the western margin of North America, and that the metasedimentary rocks were sourced from the Talkeetna arc (ca. 205–155 Ma; Onstott et al., 1989; Pálfy et al., 1999; Rioux et al., 2003, 2007) and Chitina arc (ca. 160–140 Ma; Plafker et al., 1989; Nokleberg et al., 1994; Roeske et al., 1991, 2003), which were emplaced along the outboard margin of the Wrangellia composite terrane.

In the Broxson Gulch area, the Clearwater metasedimentary rocks are locally interfoliated with metamorphosed ultramafic rocks. The ultramafic rocks include serpentinite, amphibolite, and minor amounts of talc-chlorite schist, as well as an unfoliated, coarse-grained amphibolite (Waldien et al., 2021). In the Ann Creek map area, pegmatitic hornblende-biotite pyroxenite (the Ann Creek ultramafic complex) occurs along the northern margin of a weakly foliated granodiorite that intrudes the metasediments (Waldien et al., 2021). Waldien et al. (2021) report a zircon U-Pb date of 102.4 ± 1.1 Ma for the granodiorite, and reference Nokleberg et al. (1992) in reporting a K-Ar date of 91.9 ± 2.8 Ma from amphibole and a K-Ar date of 97.7 ± 2.9 Ma. from pyroxenite. No other information is provided for the Ann Creek ultramafic complex, and yet it is these rocks that Waldien et al. (2021) suggest are the offset portion of the Pyroxenite Creek ultramafic complex that occurs in the Dezadeash Formation in southwest Yukon. In addition, no other information is provided for the associated granodiorite, although Waldien et al. (2021) equate the age of this intrusion with that of the Shorty Creek pluton in southwest Yukon.

Waldien et al. (2021) suggest that ultramafic and intermediate rocks throughout southeastern Alaska and southwest Yukon range from 115 to 100 Ma, and use a secondary reference (i.e., Eisbacher, 1976) in describing the Pyroxenite Creek ultramafic complex and Shorty Creek pluton in southwest Yukon. Eisbacher (1976), however, references Sturrock (1975) in describing the Pyroxenite Creek ultramafic complex as a coarse-grained intrusion comprising concentrically arranged olivine pyroxenite, magnetite pyroxenite, and hornblende pyroxenite, and in referring to a hornblende K-Ar date of 118 Ma and a biotite K-Ar date of 107 Ma from the ultramafic rocks. Eisbacher (1976) also reports a biotite K-Ar date of 106 Ma for the Shorty Creek pluton. In addition, Eisbacher (1976) suggests that the Pyroxenite Creek ultramafic complex is related to the Chisana arc (ca. 120–105 Ma; Trop et al., 2002; Short et al., 2005) that was emplaced along the inboard margin of the Wrangellia composite terrane.

Although Sturrock (1975) is an unpublished B.Sc. thesis that may be difficult to obtain (perhaps through interlibrary loan?), Waldien et al. (2021) could have performed a Google search using “Pyroxenite Creek ultramafic” and “Sturrock” to reveal two key references: Sturrock et al. (1980) and Dodds and Campbell (1988). Sturrock et al. (1980) provides an updated summary of the Pyroxenite Creek ultramafic complex, describing it as comprising a western core of fine-grained magnetite pyroxenite (∼1/3 of the total core), an eastern core of coarse-grained olivine pyroxenite (∼2/3 of the total core), and a thin rim of hornblende pyroxenite, all variably intruded by gabbro and diorite. Sturrock et al. (1980) also report a hornblende K-Ar date of 124 ± 4 Ma, a biotite K-Ar date of 113 ± 4 Ma, and a biotite Rb-Sr date of 116 ± 4 Ma for the ultramafic rocks. Dodds and Campbell (1988) present an updated summary of K-Ar ages from southwestern Yukon, referencing the ages reported by Sturrock et al. (1980), and also report a biotite K-Ar date of 106 ± 4 Ma for the Shorty Creek pluton. In addition, Dodds and Campbell (1988) provide an interpretation of the intrusive rocks, suggesting that the Pyroxenite Creek ultramafic complex and Shorty Creek pluton are part of the 117–106 Ma Kluane Ranges plutonic suite that represents the roots of the Chisana magmatic arc (Fig. 1). The Kluane Ranges plutonic event of 117–106 Ma is not quite the same as the 115–100 Ma generalization proffered by Waldien et al. (2021) for ultramafic and intermediate rocks throughout southeastern Alaska and southwest Yukon.

Waldien et al. (2021) reference Nokleberg et al. (1985) in referring to “other estimates of >400 km” offset on the Denali fault. However, Nokleberg et al. (1985, p. 1261) states “These striking similarities provide strong evidence that the Maclaren terrane has been offset along the Denali fault ∼400 km from the Kluane Schist and the Ruby Range batholith in the southwestern Yukon Territory, as previously suggested by Forbes and others (1973[a]), Smith and others (1974), and Smith (1981).” The estimated ∼400 km displacement by previous workers is repeated by Nokleberg et al. (1985, p. 1264), and the only estimate of >400 km is in fact Nokleberg et al. (1985, p. 1251 and p. 1263) who suggest a minimum offset of ∼400 km! Note that Forbes et al. (1973a, 1973b), Smith et al. (1974), Turner et al. (1974), and Smith (1981) all suggest that the estimated offset on the Denali fault is ∼400 km, and not >400 km.

Two key references pertaining to the amount of displacement on the Denali fault apparently unknown or ignored by Waldien et al. (2021) include Lanphere (1978) and Lowey (1998). Lanphere (1978) presents a summary of the displacement on the Denali fault, and notes that Eisbacher's (1976) estimate of 300 km, based on displaced flysch strata (i.e., the Dezadeash Formation in southwest Yukon and the Nutzotin Mountains sequence in southeastern Alaska), represents a minimum amount of offset, whereas Forbes et al.'s (1973a, 1973b), Smith et al.'s (1974), and Smith's (1981) estimate of 400 km, based on displaced metamorphic assemblages (i.e., the Kluane Schist in southwestern Yukon and the Maclaren Schist in southeastern Alaska), represents a maximum amount of offset. Lowey (1998) summarizes evidence for various estimates of 0–400 km of offset on the Denali fault and one estimate of 450 km of offset. It is curious that Waldien et al. (2021) did not provide an estimate of the amount of offset for their supposedly displaced ultramafic rocks. However, using “Movable Type Scripts” (available at https://www.movable-type.co.uk/scripts/latlong.html, accessed 20 April 2021), a displacement of ∼500 km was obtained for the distance separating the Anne Creek ultramafic complex and the Pyroxenite Creek ultramafic complex. Note that Movable Type Scripts is based on the “haversine” formula that calculates the great-circle distance (i.e., the shortest distance over the earth's surface) between two points using latitude and longitude. The obtained 500 km offset for the ultramafic rocks on the Denali fault is in excess of any of the estimates summarized by Lowey (1998) or estimates commonly quoted in the literature.

According to Waldien et al. (2021, p. 706), “The proximity of the hornblende-biotite-pyroxenite in the Ann Creek map area to the Denali fault suggests that it may be an offset portion of the Pyroxenite Creek ultramafic body, which intrudes the Dezadeash formation and is cut by the Denali fault in southwestern Yukon Territory (Eisbacher, 1976). Such correlation is consistent with other estimates of >400 km of Cenozoic dextral separation on the Denali fault (Nokleberg et al., 1985).” Although determining the amount of offset on the Denali fault was not mentioned by Waldien et al. (2021) as one of the primary purposes of their paper, the fact that they make such a presumptuous estimate for the displacement on the Denali fault warrants the following conclusions.

  1. Waldien et al.'s (2021) claim that the Ann Creek ultramafic assemblage in southeastern Alaska and the Pyroxenite Creek ultramafic assemblage in southwestern Yukon are “similar” and “make a strong case” for their being the same dismembered and displaced intrusive body is not supported by the meager data they present, nor is it supported by Sturrock et al. (1980) and Dodds and Campbell (1988), key references that Waldien et al. (2021) were either unaware of or omitted.

  2. Waldien et al.'s (2021) speculation that the alleged offset ultramafic rock “is consistent with other estimates of >400 km of Cenozoic dextral separation on the Denali fault” is incorrect, or at the very least misleading, and is not supported by Nokleberg et al. (1985) that they reference, nor is it supported by Lanphere (1978) and Lowey (1998), further key references that Waldien et al. (2021) were either unaware of or omitted.

  3. In making such a presumptuous statement that the alleged offset is “consistent with other estimates,” it is indefensible that Waldien et al. (2021) did not offer any argument as to why offset ultramafic rocks provide a more accurate estimate of the displacement on the Denali fault, compared to offset metamorphic assemblages (i.e., the Maclaren Schist and Kluane Schist as suggested by Forbes et al., 1973a, 1973b, Smith et al., 1974, and Smith 1981), offset sedimentary strata (i.e., the Nutzotin Mountains sequence and Dezadeash Formation as proposed by Eisbacher, 1976), or offset carbonate megaboulders that occur in the Nutzotin Mountains sequence and Dezadeash Formation (as determined by Lowey, 1998).

Science Editor: Rob Strachan
Gold Open Access: This paper is published under the terms of the CC-BY license.