The paper by Amato et al. (2015) contains some new and interesting data on the geology of the South Anyui suture (SAS), a key element for understanding the tectonic evolution of northeast Asia, the eastern Arctic, and the Amerasian Basin. The SAS remains understudied and new U-Pb dating of detrital and magmatic zircons and geochemical and petrographic studies will be welcome by all geologists interested in Arctic geology. Unfortunately, there are significant inaccuracies in the Amato et al. (2015) presentation of the local geology; this results in erroneous geological interpretations and has major consequences for their tectonic model. In addition, Amato et al. (2015) neglected to consider alternative hypotheses related to the closure of the SAS and subduction zone polarity. These points are discussed in more detail herein.
The most recent geological maps covering the study area include the central part of the SAS, adjoining parts of Chukotka and the Alazeya-Oloy zone (sheets Q-58-XI, Q-58-XII, Q-58-XVII, and Q-58-XVIII, scale 1:200,000; Shekhovtsov and Glotov, 2000, 2001). These maps are significantly different from the older maps on which Amato et al. (2015, their figs. 3–5) based their geological sampling and tectonic interpretations. This has resulted in the following misinterpretations.
Amato et al. (2015, their figs. 2–5) claimed that there are no Triassic sediments within the SAS and that all rocks in the Uyamkanda and other river basins are Jurassic or Late Jurassic–Early Cretaceous in age. This is incorrect. Triassic units with rare Late Triassic faunal assemblages are present in the Glubokaya, Uyamkanda, Moni, and Ustieva river basins and Triassic conodonts have been found in cherty concretions near the settlement of Stadukhino (Yakoveem Creek; Shekhovtsov and Glotov, 2001). Moreover, Amato et al. (2015) confirmed the presence of Triassic rocks (their samples 02-An-32 and 02-An-10) within the SAS.
The occurrence of upper Triassic rocks is critical to the tectonic interpretation of Amato et al. (2015). Within the SAS upper Triassic rocks compose the lowermost tectonic unit and consist of turbidites that form distal facies to the north passive margin of the Chukotka microcontinent. Consequently, structural and sedimentological observations point to displacement of the tectonostratigraphic units of the SAS northward onto the Chukotka microcontinent margin (Sokolov et al., 2009, 2015).
While Amato et al. (2015) apparently support the provenance for Triassic sandstone suggested by Miller et al. (2006; it is the only reference that they cited), alternative interpretations have been proposed. Facies distributions combined with turbidite paleocurrent directions indicate that Triassic sandstones in Chukotka were derived from the north (modern coordinates; Morozov, 2001; Tuchkova et al., 2014). This is very important for paleogeographic reconstructions because the Triassic deposits of the Sverdrup Basin are also sourced from the north (Embry, 1993). The pattern of detrital zircons across Chukotka, Wrangel Island, and the Sverdrup Basin suggests that they were close to each other and shared a similar provenance (Tuchkova et al., 2014; Miller et al., 2006). Therefore, the source of Triassic sandstone was likely a topographic high situated between these regions, possibly Crockerland (Embry, 1993) or Arctida (Zonenshain et al., 1990), rather than Baltica (Miller et al., 2006).
Amato et al. (2015, their figs. 3–5) claimed that the Gromadny-Vurguveem massif is surrounded by rock units of (their) Nutesyn arc, but this is incorrect. A Jurassic oceanic basalt-chert assemblage (the Bystryanka unit) and a Late Jurassic–Early Cretaceous accretionary prism with blocks of oceanic basalt and chert (the South Gremuchinsky unit) are exposed north of the Gromadny-Vurguveem massif (Shekhovtsov and Glotov, 2000, 2001). Rock assemblages of the Bystryanka and South Gremuchinsky units are distinct from the Nutesyn arc. This is significant because Amato et al. (2015, their fig. 14C) depicted the Nutesyn arc in their geodynamic model to be marginal to the Chukotka microcontinent (after Natal’in, 1984; Zonenshain et al., 1990, and others) and separated from it by a backarc basin developed on thinned continental crust. However, there is no backarc basin fill in the region; Oxfordian–Kimmeridgian sandstones of the Chukotka microcontinent are synchronous with the Nutesyn arc and do not contain any volcanic arc material (Baranov, 1995; Vatruskina and Tuchkova, 2014). Oxfordian–Kimmeridgian volcanic rocks stratigraphically overlie the clastic rocks of the SAS. These volcanic rocks are likely associated with the intraoceanic Koranveem arc (e.g., Shekhovtsov and Glotov, 2001) or the Kulpolney arc (Sokolov et al., 2015) located within the South Anyui oceanic basin.
These (or similar) Oxfordian–Kimmeridgian island-arc rocks also occur to the northwest in the Polyarny uplift (Sizykh et al., 1977; Soloviev et al., 1979), where they stratigraphically overlie lower Carboniferous volcanic and carbonate rocks of the SAS and have a thrust tectonic contact with Triassic rocks of the Chukotka microcontinent (Soloviev et al., 1979; Bondarenko, 2004). It is unfortunate that Amato et al. (2015, their figs. 3 and 4) incorrectly represented the geology of this region as Late Jurassic sedimentary rocks. The Nutesyn arc and its correlatives (Koranveem or Kulpolney arcs) clearly cannot be formed on the edge of the Chukotka microcontinent as Amato et al. (2015) suggested.
Amato et al. (2015) argued that Jurassic rocks are also present south of the Gromadny-Vurguveem gabbroid massif (their “ultramafic complex”), but this is incorrect. Recent petrologic and geochemical data from the Gromadny-Vurguveem massif confirm its Paleozoic genesis in a suprasubduction environment (Ganelin and Silantyev, 2008; Ganelin, 2015). On its southeast margin, the Gromadny-Vurguveem massif is in unconformable depositional contact with Carboniferous volcanic-sedimentary rocks of island-arc association. Dikes that cut the roof of the Gromadny-Vurguveem massif and the Carboniferous and Permian strata yielded whole-rock Ar-Ar ages of 266.9 ± 1.6 to 264.8 ± 3 Ma (Ganelin, 2015). Contrary to the suggestion of Amato et al. (2015), the Gromadny-Vurguveem massif cannot represent upper Jurassic–lower Cretaceous oceanic crust of the SAS (e.g., Natal’in, 1984; Zonenshain et al., 1990), but most likely represents part of the older Yarakvaam island-arc terrane. Ganelin and Silantyev (2008, p. 17) suggested that the Gromadny-Vurguveem massif and the overlying late Paleozoic island-arc volcanic rocks represent a single complex and “… consider the gabbro rocks of the massif as a basement unit of the Yarakvaam terrane.”
Nevertheless, Jurassic oceanic rocks exist in the region and provide evidence of the South Anyui Ocean seafloor. Contrary to the statement by Amato et al. (2015, p. 1535) that there is “…little direct evidence for the Jurassic seafloor of the South Anyui Ocean…,” an oceanic setting for the basalt-chert assemblage of the Bystryanka unit and the offscraped blocks of the South Gremuchinsky unit is clear; the radiolaria in these cherts are Jurassic (pre-Tithonian) (Lychagin, 1997; Shekhovtsov and Glotov, 2001; Sokolov et al., 2009, 2015). Thus seafloor spreading in the South Anyui basin occurred in the Jurassic but had ceased by Tithonian time.
Much of the tectonic interpretation in Amato et al. (2015) rested on the interpretation of the 2DV seismic line, and there are two significant problems associated with this. (1) Amato et al. (2015, their fig. 4) considered all faults bounding the SAS and located within it to be south-vergent thrusts, when in fact most SAS-bounding faults are dextral strike-slip and thrust faults that dip both to the north and south (Natal’in, 1984; Bondarenko, 2004; Shekhovtsov and Glotov, 2000, 2001). South-dipping reflectors have previously been interpreted to indicate south-dipping subduction (Franke et al., 2008), which is consistent with north-vergent thrusting. Amato et al. (2015) did not consider that the 2DV seismic line, which shows clear north-dipping reflectors, could be interpreted to reflect the contacts of north-vergent tectonic sheets and thrust faults. This does not contradict structural observations that south-vergent thrusts cut north-vergent thrusts (e.g., Bondarenko, 2004) for example, as allochthonous fragments of the Yarakvaam terrane preserve north-directed displacement and all thrusts dip to the north (Sokolov et al., 2009, their fig. 5). Some reflectors in the 2DV seismic line shown in Amato et al. (2015) may have similar characteristics. (2) An additional complication is that the 2DV line does not actually go through the Aluchin ophiolite massif of the Yarakvaam terrane, but west of it, through the Oloy island-arc terrane (Byalobzhesky et al., 2007; Khanchuk, 2006). The Oloy island-arc terrane does not contain ophiolitic rocks.
These observations directly contradict the model of Amato et al. (2015), in which suprasubduction ophiolitic rocks of the Aluchin massif cannot be present in their section, and the ophiolitic rocks that are present are thrust westward (Bondarenko, 2004). Furthermore, the Gromadny-Vurguveem suprasubduction ophiolite and island-arc units overthrust the SAS to the north (Sokolov et al., 2009, 2015; Ganelin, 2015). The suprasubduction ophiolites of Yarakvaam terrane formed on the southern Siberian active continental margin of the proto–Arctic Ocean, while the northern Chukchi margin was passive in late Paleozoic–Early Cretaceous time (Khanchuk, 2006; Ganelin, 2015). Thus, northward subduction of oceanic crust of the SAS (e.g., Amato et al., 2015) is highly unlikely.
CLOSURE OF THE SAS
Amato et al. (2015) argued that the timing associated with the closure of the South Anyui suture is only broadly constrained. We here draw attention to the work of Bondarenko (2004) and Sokolov et al. (2009, 2015), in which the timing of deformational events and the final closure of SAS zone are well documented. Spreading in the proto–Arctic Ocean (South Anyui) was completed in Oxfordian–Kimmeridgian time. In Tithonian–Valanginian time the remnant South Anyui basin started to shorten and was filled with turbidite deposits. In Hauterivian–Barremian time the turbidite-filled basin was completely closed. At that time, collisional deformation was north vergent and tectonostratigraphic units of the SAS were thrust northward onto the passive margin of the Chukotka microcontinent. Direct evidence of north-vergent thrusting exists in tectonic windows where outcrops of distal turbidites related to the Chukotka microcontinent occur. A Triassic unit occurs in the lower part of the SAS (Byalobzhesky et al., 2007). Late-stage collision-related deformation affecting Hauterivian–Barremian sediments was south vergent. Aptian and Albian sediments of the Ainakhkurgen depression and Chukotka are the oldest sediments in the overlapping sequence and are synchronous with the age of postcollisional granites, 117–108 Ma (zircon U-Pb ages; Katkov et al., 2010). These data, combined with the information here on the intraoceanic (Nutesyn-Koranveem-Kulpolney) arc, suggest that subduction of the oceanic crust did not occur in a northward direction beneath the Chukotka microcontinent (Amato et al., 2015), but was instead southward beneath the active edge of Siberia and generated the Late Jurassic–Early Cretaceous Oloy volcanic belt and its accretionary prism (South Gremuchinsky unit) (Shekhovtsov and Glotov, 2000, 2001).
Research in South Anyui suture is financially supported by the Russian Science Foundation (grant 16-17-10251) and the Russian Foundation for Basic Research.