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Subduction Polarity in Ancient Arcs: a Call to Integrate Geology And COMMENT & REPLY Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY Terry L. Pavlis, Dept. of Geological Sciences, University of Texas, El Paso, Texas 79968, USA; Jeffrey M. Amato, Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA; Jeffrey M. Trop, Dept. of Geology and Environmental Geosciences, Bucknell University, Lewisburg, Pennsylvania 17837, USA; Kenneth D. Ridgway, Dept. of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, Indiana 47907, USA; Sarah M. Roeske, Earth and Planetary Sciences Dept., University of California, Davis, California 95616, USA; and George E. Gehrels, Dept. of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA INTRODUCTION drafts of the paper. However, when we problem is not solved, which underscores We welcome the opportunity to discuss the realized that experts in this field were our conclusion that resolving ancient sub- views expressed by Sigloch and Mihalynuk in actively debating the topic, we reduced our duction polarity problems requires a con- their Comment (Sigloch and Mihalynuk, discussion to a short paragraph. In their certed collaborative effort between the geo- 2020; referred to here as SM) on Pavlis et al. Comment, SM reiterate parts of their model logic and geophysical communities. (2019) because it provides an opportunity to but emphasize a consensus in the geophysi- A foundation of Sigloch and Mihalynuk’s elaborate on the criteria for determining sub- cal community that deep mantle tomo- (2013, 2017) interpretations is that plate duction polarity, an important problem in the graphic anomalies are subduction zone rem- reconstructions restore North America to a North American Cordillera and for tectonic nants. We agree there is a consensus that position too far east for a continuous east- reconstructions in general. However, we some upper-mantle anomalies can be tracked dipping subduction system to generate the believe SM have disregarded extensive geo- up to existing subducting slabs and locally deep anomaly. We suggest this reasoning logic data in an attempt to support their model, lower-mantle anomalies exist and can appear has two pitfalls: (1) plate reconstructions for underscoring our original point that tectonic to be continuous with upper mantle anoma- the Cenozoic are well-constrained, but it models need to accommodate geologic obser- lies. However, we disagree with extending becomes increasingly suspect in deeper vations. We suggest that their perspective that statement to the interpretation of what time because of difficulties with relative hot arises from two assumptions: (1) that the geo- the deep anomalies represent and, in particu- spot motions (e.g., Tarduno et al., 2009); and physically imaged slab walls indicate ancient lar, their relationship to the positions of (2) if the margin was not strictly Andean, trench position; and (2) that because plate ancient subduction zones. As we noted in our east-dipping subduction in an offshore arc reconstructions place North America east of original paper (Pavlis et al., 2019), interpre- system could equally well have produced imaged slab walls, North America must have tations of the tomographic images presented the anomaly if the ocean basin was large been east of the trench. From these assump- in Sigloch and Mihalynuk (2013, 2017) have enough. Note that we agree with SM that in tions they subsequently disregard, or attempt been the subject of debate (e.g., Liu, 2014; our paper the argument using plate recon- to explain away, key geologic relationships Sun et al., 2017). We have concerns that the structions of van de Meer et al. (2010) countering this model. However, it is only a caveats expressed by Foulger et al. (2013) involved somewhat circular reasoning, but model, and some of their dismissal of existing about the interpretation of tomographic conceding this point does not eliminate data apparently comes from a misunderstand- images have not been adequately addressed either of these serious pitfalls. ing of geologic relationships and, importantly, by the geophysics community. Molnar (2019) the timing of these relationships. We readily reviewed results from mantle convection TIME ISSUES acknowledge some of the ambiguities in the models, tomographic models, and mantle In their Comment, SM call on tectonic geologic record, but any integrated model evolution models and concluded that whole- events that are reasonable but infer that the must address the available observations. Dis- mantle convection models are almost cer- events occurred at times inconsistent with parate views of five key relationships are dis- tainly wrong. He instead argued for a revi- published geologic data. This was, in fact, cussed in this paper. sion of two-layered convection with a one of our greatest concerns with the SM lower-upper mantle boundary in the depth model and the “ribbon continent” models INTERPRETATION OF DEEP range of 1000–1500 km. The importance of we cited (e.g., Johnston, 2008). There are MANTLE TOMOGRAPHIC these models is clear in the context of this several examples, but here we cite two. ANOMALIES AND PLATE discussion; if Molnar (2019) is correct, using First, west-dipping subduction with a col- RECONSTRUCTION deep-mantle anomalies to constrain plate liding arc is not a new concept for Cordille- This topic is a key part of the story, and we motion is meaningless, and this entire dis- ran tectonics (as SM noted), nor is the con- devoted several pages to this issue in early cussion is moot. Thus, the deep mantle cept of a Mesozoic archipelago analogous to GSA Today, v. 30, https://doi.org/10.1130/GSATG465Y.1. Copyright 2020, The Geological Society of America, CC-BY-NC. e51 the southwest Pacific (e.g., Silver and Smith, eastward subduction as the Andean-style accretion in the accretionary complex as evi- 1983; Blakey and Ranney, 2018). The prob- model, just located further west” and that Fig- dence of continuous subduction. However, lem is where and when west-dipping subduc- ure 1 in Pavlis et al. (2019) misrepresents there is clear evidence globally that most tion occurred. SM argue for long-lived, con- SM’s portrayal of this eastward subduction arcs, particularly oceanic arcs like the Talk- tinuous, west-directed subduction that would zone. Although Sigloch and Mihalynuk (2013) eetna arc, which forms part of the WCT/INS, still be active in the Late Cretaceous (SM, show east-dipping subduction along a portion undergo episodes of either subduction ero- their fig. 1A), but no geologic evidence for of WCT/INS in the Cretaceous, Figure 1C in sion or non-accretion (von Huene and Scholl, this polarity exists beneath the Wrangellia Pavlis et al. (2019) is an accurate representa- 1991; Clift and Vannucchi, 2004). The fact Composite/Insular (WCT/INS) belt and none tion of Figure 4B of Sigloch and Mihalynuk that the oldest part of the Talkeetna arc over- exists beneath the Intermontane/Yukon- (2017), which shows only west-dipping sub- laps in age with the oldest blueschist facies Tanana that is younger than early Jurassic duction along the inboard margin of WCT/ metamorphic rocks on the oceanward (west- (and even that is debated; e.g., compare INS. Despite this confusion, however, even if ern) side of the arc (Fig. 2), and that blue- Dusel-Bacon et al., 2015, with Mihalynuk et they now accept eastward subduction west of schist facies rocks are preserved intermit- al., 1994). There is abundant evidence for a WCT/INS, it is clear in Sigloch and Mihaly- tently along strike for hundreds of kilometers Permian arc collision that involved west-dip- nuk’s (2017) original model that they rejected and span >20 m.y. of crystallization ages, is a ping subduction (e.g., closure of the Slide eastward subduction for this boundary over compelling case for the blueschists recording Mountain ocean; Dusel-Bacon et al., 2006; large swaths of geologic time. initiation and early stages of subduction Beranek and Mortensen, 2011) and our Fig- The subduction polarity in southern beneath WCT/INS in the latest Triassic– ure 1A (a more accurate version of Pavlis et Alaska was controversial in the past (e.g., early Jurassic (Roeske et al., 1989). As we al., 2019, fig. 1A) shows the Brooks Range in Reed and Lanphere, 1974; Hudson, 1979; stated in our original paper, the gap in time northern Alaska forming via outward (away Reed et al., 1983; Wallace et al., 1989; Decker between the blueschist facies rocks and the from continent) subduction-collision with an et al., 1994; Plafker and Berg, 1994), but the more outboard part of the accretionary com- arc. However, the timing of this event is sig- basis of the controversy has diminished as plex coincides very clearly with the migra- nificantly older than closure of the ocean more data have accumulated. Most of us tion of arc magmatism away from the trench basin between the WCT/INS belt and North have worked on this problem from a variety (Fig. 2). Thus, we interpreted this preserva- America, and no continuity exists between of perspectives, including structural/petro- tion gap in the accretionary complex as arcs of the WCT/INS and those recording logic studies, stratigraphic studies, and igne- resulting from subduction erosion (Amato et closure of the Angayucham Ocean. Thus, we ous/detrital geochronology. We admit there al., 2013). The next phase of accretion (Fig. 2) show these as separate subduction systems. are ambiguities arising from an incomplete was a mélange with maximum depositional A second example is the statement in SM’s geologic record. Nonetheless, as we empha- ages ranging from ca. 170–150 Ma (Amato et Comment that we prefer an “always-Andean” sized in our original paper, there is strong al., 2013), a second blueschist assemblage model; this misrepresents our view of the Cor- upper-plate geologic evidence for all of the with a maximum depositional age (MDA) of dillera over time.
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