INTRODUCTION

The South Anyui suture zone (SASZ) is a key tectonic boundary for paleogeographic reconstructions of the Arctic region prior to the opening of the Amerasia basin. Sokolov et al. (2009, p. 201) noted that “many aspects of the geological setup of the [South Anyui suture] remain poorly understood and there are dissenting viewpoints on the following issues”, and then listed the following: the location of the boundaries of the SASZ, the origin and age of the oceanic basin, its tectonic history, and the timing of the onset and termination of collision. Thus, it is not surprising that workers in this remote area with poor exposure (Fig. 1) will come to differing conclusions about the significance of the rocks and structures of the region.

It has been difficult for geologists outside of Russia to understand the geology of the SASZ because of the limited availability of published material in English. Some data referred to by Sokolov (2017) in his Comment on our paper (Amato et al., 2015) remain unpublished in the form of a Ph.D. thesis at the University of Moscow (Bondarenko, 2004). Dr. Sokolov has been publishing for years on the geology of the region (e.g., Sokolov et al., 2002, 2009, 2015), but many questions remain concerning the geology of the region.

Our paper (Amato et al., 2015) had the following goals: (1) to provide a concise overview of this geologically complex region highlighting the known versus speculative aspects of existing models, relying on easily obtained published works; (2) to publish new data on the depositional age and provenance of key sedimentary units using detrital zircon ages, sedimentology, and point-counting data from sandstones; (3) to publish new data on the ages of igneous rocks in the SASZ, as well as from volcaniclastic rocks that help establish the age of volcanism; (4) to evaluate structural relationships, a challenge given the poor outcrop (particularly of major faults) and locally intense deformation; (5) to interpret a seismic reflection line that passes through the field area; and (6) to present a tectonic model that incorporates our new data, existing published data, and a GPlates (www.gplates.org) animation. We have never claimed to have resolved all of the SASZ controversies and appreciate that Sokolov (2017) has highlighted some remaining uncertainties. We would like to point out that we all are interested in reaching a better understanding of this complex area that is an important piece of the tectonic puzzle in the Arctic region. Below we respond to the some of his major concerns and highlight existing problems in the region that can be evaluated with additional data.

ACCURACY OF PUBLISHED GEOLOGIC MAPS

Sokolov (2017) writes that we used outdated geologic maps as the basis of our compilation map shown in figure 4 of Amato et al. (2015) and that we should have cited the 1:200,000-scale maps of Shekhovtsov and Glotov (2000, 2001). These citations refer to the explanatory notes to the maps rather than the maps themselves. The citations to the maps are Shekhovtsov (2000a, 2000b). We note that Sokolov et al. (2002; their figure 2) also did not reference these newer maps, and Sokolov et al. (2009) referenced the newer maps but did not provide sufficient detail in their geologic map (their figure 3, published at 1:7,500,000 scale) to demonstrate any key differences between the older and newer geologic maps of the region.

We were aware of these newer maps of the area, and we cited them five times in our original contribution. However, it is worth noting that the new maps do not cover the majority of our study area nor the area that is questioned by Dr. Sokolov in his comment. The most important quadrangles for our study are Q-58-IX and Q-58-X state geological survey maps, not sheets XI, XII, XVII, and XVIII that were updated and mentioned by Sokolov (2017). Figure 2 shows a simplified geologic map of our study area and the area shown in more detail in Figure 3 is indicated. The main structural difference between the older maps (Yegorov, 1962; Dovgal, 1964; Gulevitch, 1968) and newer maps of Shekhovtsov and Glotov (2000, 2001) is that many of the contacts that were interpreted as depositional on the older map (Dovgal, 1964) are shown as faults in the new map (Shekhovtsov and Glotov, 2000, 2001). This is reasonable, given the structural complexity of the SASZ, but we know from our field experience that virtually none of these faults are directly observable. The new map should show them as “inferred” with the appropriate symbol. The existence, or non-existence, of the minor faults does not affect our interpretations. The major fault that Shekhovtsov (2000a) showed in the northeastern portion of their map corresponds with our thrust contact between the Arctic Alaska–Chukotka microplate and the SASZ, so in that respect we are in agreement.

ARE THERE TRIASSIC ROCKS IN THE SASZ?

Dr. Sokolov disputes our interpretation that the majority of the sedimentary rocks in the SASZ are Jurassic–Early Cretaceous, which contradicts some previous maps and studies. Based on our study, two samples collected from the major sedimentary unit mapped as Triassic contain a statistically significant population of Jurassic zircons (samples 02An-01 and 02An-04). We did not sample this unit everywhere, but it is worth noting that in the text accompanying the original geologic map, Dovgal (1979, p. 20) wrote:

“Carnian strata crop out in the upper part of the Monni River; these were not studied in detail because of poor exposures, particularly in this area. The described unit of Carnian strata is characterized by single discoveries of Halobia cf. charlyana Mojs., and Otapiria sp. indet. (determined by V.P. Kinasov) which were found in the vicinity of Volcano Mount. The rest of the area where Carnian strata are shown does not contain any fossil remnants” (translation by V. Akinin; emphasis ours).

Dr. Sokolov noted in his Comment that “Triassic units with rare Late Triassic faunal assemblages are present in the Glubokaya, Uyamkanda, Moni, and Ustieva river basins”. However, a close reading of the published geologic maps does not show any faunal symbols on sheets Q-58-IX, Q-58-X, and Q-58-II in the Uyamkanda, Moni, and Glubokaya river valleys. A belt of rocks located in the SASZ was interpreted in the older map (Dovgal, 1964) as being Early Cretaceous. This interpretation was supported by fossil localities (circled on Fig. 3). On the newer maps of Shekhovtsov and Glotov (2000, 2001), part of this belt has been reinterpreted as being of Triassic age. The fossil localities from the old map are absent in the new map, and no new fossil data are shown to support this new interpretation. It is unclear what data were used to make this new interpretation.

There are two fossil localities (Dovgal, 1964) on the right bank of the Orlovka River (for location, see Amato et al., 2015, our figure 4), but subsequent workers (e.g., Lychagin, 1985, 1991b) were unable to reproduce these results. Further, another map covering the region in question (Malysheva, 1999) interpreted the same unit that was previously mapped as Triassic as being Jurassic, consistent with our results. Thus, even before our detrital zircon study (Amato et al., 2015), there was significant controversy over the extent of Triassic rocks in the SASZ.

In the newer geologic map explanation, it was noted that:

“The age of the Triassic Ustieva formation is based on poor fauna discoveries. Only in the western part of the map (Q-58-XI, XII) on the right bank of the Ustieva River there was collected rare Otapiria sp. indet., whereas outside of the map in the Uyamkanda River basin there was collected Monotis ex gr. scutiformis (Tell.) and M. sabaicalica (Kipar). Triassic conodont remnants were collected in cherty concretions in the upper part of Yakoveem creek (near Stadukhino). According to V.A. Aristov, these conodonts dated the age of the strata as older than Early Jurassic. The age of the Ustieva formation was determined as early to late Norian” (in the map explanation of Shekhovtsov and Glotov, 2000; translation by V. Akinin).

Thus it seems as though the assigned age for many of the early Late Triassic units is speculative, and we do not consider these sparse faunal occurrences to be necessarily definitive.

It is true that our samples 02An-32 and 02An-10 contain Triassic zircons and no Jurassic zircons, but these are maximum depositional ages and the rocks could be younger. In addition, these samples could have been from Triassic blocks (i.e., olistostromes) within Jurassic strata. Another explanation is that instead of the unit being entirely Triassic, there could be a succession ranging from Triassic to Jurassic. Our interpretation that these rocks were largely Jurassic comes from the unambiguous Jurassic detrital zircon ages, rather than the sparse fossil localities.

The maps by Shekhovtsov (2000a, 2000b) do not contain any new radiometric dates, unlike our contribution. The minimal outcrop in the region, along with extremely limited fossil localities, emphasizes the importance of detrital zircon studies like ours in which the maximum depositional age of the strata can be determined. Thus we stand by our interpretation of the new U-Pb zircon ages from strata in this unit. Future work should focus on additional sampling for U-Pb detrital zircon dating in the Mesozoic sedimentary units within the SASZ in order to resolve remaining uncertainties. Regardless of the presence or absence of Triassic strata in the SASZ, this sedimentary sequence is distinct from the large swaths of passive-margin strata north of the SASZ in the Chukotka fold belt extending from the Keperveem-Bilibino region northwards (Tuchkova et al., 2009; Amato et al., 2015, our figure 4).

SIGNIFICANCE AND AGE OF THE VURGUVEEM COMPLEX

Dr. Sokolov disputes our interpretation of the mafic-ultramafic Vurguveem complex and writes that it cannot be Jurassic arc basement. These ultramafic rocks were not a focus of our study, but this comment misrepresents our interpretations. On page 6 of our original paper (Amato et al., 2015) we cited Ganelin and Silantyev (2008), who interpreted the geochemistry of these rocks as consistent with arc magmatism, but we did not say that the Vurguveem had to be Jurassic. On our figure 4 (Amato et al., 2015) map, we cited the age as Permian(?). We also cited the Sokolov et al. (2009) Carboniferous argon dates from this complex (possibly the same argon data reported for sample GK-983-4 in Ganelin et al. [2013] and Sokolov et al. [2015]), even though these dates were obtained from “secondary amphibole from an amphibolized gabbronorite” (p. 10). So we were well aware of the postulated late Paleozoic age for this complex and agree (as we did in our original contribution) that this complex is not related to Jurassic arc basement. We did not study the bounding rocks for this complex and thus cannot comment on whether they are Paleozoic or Mesozoic.

SUBDUCTION POLARITY AND THE NUTESYN ARC

Dr. Sokolov is correct in that we give a lot of weight to the deep crustal seismic data in our crustal-scale model. After all, seismic reflections are one of the few ways we have of elucidating deep structure. The details of the structural history of the region are complicated, but we have used an Occam’s razor approach to interpreting the seismic section. The most impressive feature on the seismic line (Amato et al., 2015, our figure 13) is a major north-dipping panel of reflectors that we interpret as a south-vergent megathrust that is the bounding fault between the SASZ and the Chukotka microcontinent. The key observations are that the seismic fabric of the South Anyui zone is dipping at a relatively low angle and that it is clearly north dipping. Those are simple observations that need to be explained. We believe the best interpretation is that the SASZ was predominantly a south-vergent thrust system. Sokolov (2017) notes that thrust faults “dip both to the north and south” and that “collision-related deformation…was south vergent”. Others are welcome to interpret these data differently, but we do not see any evidence for major south-dipping, north-vergent structures on this line.

Dr. Sokolov states: “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.” Frankly, we do not understand this comment. Does he mean that the original north-vergent fabric has been rotated about a horizontal axis in such a way that it now appears to be south vergent? It is always possible to come up with a convoluted explanation to account for simple observations, but we prefer the most straightforward interpretation. However, this interpretation does require a re-evaluation of the earlier hypothesis.

If one accepts the north-dipping seismic fabric as being real, as we did in our publication, it follows that, at the crustal scale, the Chukotka microcontinent was thrust over the SASZ, and that the SASZ, in turn, was thrust over the terranes that now lie to the south. Retro-deforming the crustal-scale cross-section (as we have done in figure 14 of Amato et al. [2015]) leads to a north-dipping subduction zone prior to closure of the South Anyui ocean. It also happens that there is a belt of Jurassic volcanic arc rocks within the SASZ. Natal’in (1984) called them the Nutesyn arc. Sokolov et al. (2002) called them the Kulpolney arc. Because these arc rocks are now imbricated in the SASZ, their original stratigraphic relationships are unclear. We show them in our tectonic model as forming above the north-dipping subduction zone, much in the same way that they were shown in Sokolov et al. (2002), but we are the first to admit that given the limited data available on these rocks, this is somewhat speculative.

Sokolov (2017) stated that the Franke (2008) seismic line shows reflection data that counters our interpretation of vergence in the SASZ. It is true that this line shows south-dipping reflectors, but not only was it collected >700 km to the northeast of our study area (Amato et al., 2015, our figure 1), it does not cross the SASZ. We addressed this as follows (p. 1553): “The structures observed in the 2DV line are the retrowedge of the collision zone, while the structures imaged by Franke et al. (2008) are the prowedge. The structures imaged by Franke et al. (2008) are along strike of north-vergent structures observed on land in the northern part of the Chukotka fold belt.” Thus, the presence of north-vergent structures does not negate our interpretation because orogenic belts are commonly doubly vergent (e.g., Willett et al., 1993). We suggest that the seismic data collected in our study area and directly across the SASZ provide a better constraint on the dip of the subduction zone that closed during the collision.

We do, however, agree with Dr. Sokolov that strike-slip faulting is likely to have been important. In fact, it was Natal’in (1984) who first documented northwest-striking strike-slip faults in SASZ. In our paper we wrote (p. 1554) “Previous workers (Sokolov et al., 2002) have emphasized the role of strike-slip deformation during the late stages of the SASZ. This interpretation is entirely possible given the oblique plate motions involved in the closure, but our data do not address this question.” In fact, our plate model (Amato et al., 2015, our animation 1) clearly showed the importance of dextral shearing. The seismic data cannot reveal these structures because of their steep inclination, and demonstrating their existence unambiguously with field data is also difficult given the quality of exposure and our imperfect understanding of the stratigraphy. In Amato et al. (2015) we have emphasized what is observable in the seismic and field data. If the area was subsequently modified by dextral strike-slip deformation, the original dominant crustal fabric as revealed on the 2DV seismic line was not apparently significantly disturbed.

TIMING OF CLOSURE OF THE SOUTH ANYUI OCEAN

Sokolov (2017) takes issue with the comment that the timing of the closure was described as “constrained only broadly”. However, it appears that we both agree that the closure preceded deposition of the overlapping sedimentary rocks of the Albian-Cenomanian Okhotsk-Chukotka volcanic belt and the Aptian-Albian Ainakhkurgen basin. It also occurred before emplacement of the 117–109 Ma intrusions. These constraints do not come from Bondarenko (2004) or Sokolov et al. (2009, 2015), but from the ages of the sedimentary rocks noted by Dovgal (1964) and the ages of the cross-cutting intrusions (Miller et al., 2009; Katkov et al., 2010; Amato et al., 2015).

CONCLUSIONS

Suture zones are some of the most tectonically complex regions on Earth, and those that are better exposed and more well studied also remain controversial (e.g., Burke et al., 1977; Meng and Zhang, 1999). We agree with Sokolov et al. (2015, p. 3) that “incomplete knowledge became a cause of different and often controversial viewpoints on the structure and evolution history of this region.” We also agree that this is a critical region for understanding the Mesozoic history of the Arctic region. Our contribution (Amato et al., 2015) was an attempt to evaluate the age and deformational history of the key units across the SASZ and come up with a tectonic model that is consistent with the seismic line that was obtained from the region after our field work was completed. Future workers can use both Amato et al. (2015) and the contributions of Sokolov et al. (2002, 2009, 2015) as guides to focus on the remaining uncertainties, which are numerous but, we believe, not as numerous as they were before our study.

ACKNOWLEDGMENTS

Co-author Marianna Tuchkova declined to participate in this Reply. Co-author Alexander Salnikov was invited to participate but did not respond. We thank them for their contributions to the original publication. We appreciate the comments and suggestions of Geosphere Science Editor Ray Russo.