A New P‐Wave Tomographic Model (CAP22) for North America: Implications for the Subduction and Cratonic Metasomatic Modification History of Western Canada and Alaska

Abstract

Our understanding of the present‐day state and evolution of the Canadian and Alaskan mantle is hindered by a lack of absolute P‐wavespeed constraints that provide complementary sensitivity to composition in conjunction with existing S‐wavespeed models. Consequently, cratonic modification, orogenic history of western North America and complexities within the Alaskan Proto‐Pacific subduction system remain enigmatic. One challenge concerns the difficulties in extracting absolute arrival‐time measurements from often‐noisy data recorded by temporary seismograph networks required to fill gaps in continental and global databases. Using the Absolute Arrival‐time Recovery Method (AARM), we extract >180,000 new absolute arrival‐time residuals from seismograph stations across Canada and Alaska and combine these data with USArray and global arrival‐time data from the contiguous US and Alaska. We develop a new absolute P‐wavespeed tomographic model, CAP22, spanning North America that significantly improves resolution in Canada and Alaska over previous models. Slow wavespeeds below the Canadian Cordillera sharply abut fast wavespeeds of the continental interior at the Rocky Mountain Trench in southwest Canada. Slow wavespeeds below the Mackenzie Mountains continue farther inland in northwest Canada, indicating Proterozoic‐Archean metasomatism of the Slave craton. Inherited tectonic lineaments colocated with this north‐south wavespeed boundary suggest that both the crust and mantle may control Cordilleran orogenic processes. In Alaska, fast upper mantle wavespeeds below the Wrangell Volcanic Field favor a conventional subduction related mechanism for volcanism. Finally, seismic evidence for the subducted Kula and Yukon slabs indicate tectonic reconstructions of western North America may require revision.

The Upper‐Mantle Structure Beneath Alaska Imaged by Teleseismic S‐Wave Reverberations

“…All results show elevated “410,” slightly depressed “660,” and thus thick MTZ in central Alaska (near 64°N, 150°W in Figure 6), suggesting that the Pacific slab has entered the MTZ in this region. This is consistent with recent tomography models (e.g., Boyce et al., 2023; Gou et al., 2019; Jiang et al., 2018; Martin‐Short et al., 2018). Furthermore, the results of applying the appropriate corrections based on the MS18 and J18 models exhibit little topography on the “660” (Van Stiphout et al., 2019) in central Alaska, indicating that the slab has probably not penetrated into the lower mantle.…”

“…(2017) and Boyce et al. (2023) found that the NE‐SW‐striking high‐velocity feature extends as deep as 600‐km depth, but the feature in the latter study is more concentrated in southwestern Alaska in the lower MTZ, whereas Gou et al. (2019) observed that the anomalies weaken significantly beyond 500‐km depth.…”

“…Instead, the thin MTZ could be caused by upwelling of hot material from greater depths. The presence of a low‐velocity anomaly below the high‐velocity slab, as observed in some tomography models (Boyce et al., 2023; Gou et al., 2019; Jiang et al., 2018), provides evidence for this interpretation, although further investigations are needed to confirm this hypothesis. By combining multiple types of observations, we conclude that the NE‐SW‐striking Pacific slab is likely bent or segmented, with the northeastern section beneath central Alaska having entered the upper MTZ but not the lower MTZ and the southwestern section beneath the Alaska Peninsula not yet reaching the MTZ.…”

“…Among P-wave tomography studies, Qi et al (2007) found high-V P anomalies within the MTZ beneath western Alaska, with a gap separating the upper part of the anomaly, which was interpreted as the Pacific plate, and the lower part, which was suggested to be the remnants of the Kula Plate. Both Burdick et al (2017) and Boyce et al (2023) found that the NE-SW-striking high-velocity feature extends as deep as 600-km depth, but the feature in the latter study is more concentrated in southwestern Alaska in the lower MTZ, whereas Gou et al (2019) observed that the anomalies weaken significantly beyond 500-km depth.…”

“…Recent P and S wave tomography models (Gou et al., 2019; Jiang et al., 2018; Martin‐Short et al., 2018) show a steeply subducting slab in this region, but the main high‐velocity anomaly does not extend beyond 410‐km depth in most studies, significantly shallower than in central Alaska (Figure S6 in Supporting Information S1). The latest P‐wave model (Boyce et al., 2023) revealed that the high‐velocity anomalies from central to southwestern Alaska are broadly continuous above 400‐km depth but become less continuous in the MTZ below the Alaska Peninsula and the Aleutian volcanic arc, where weak low‐velocity anomalies are imaged at 400‐km depth. Given the thin MTZ and the depth extent of the high‐velocity anomalies, the slab is unlikely to have entered the MTZ beneath southwestern Alaska.…”