Cretaceous intracontinental rifting at the southern Chatham Rise margin and initialisation of seafloor spreading between Zealandia and Antarctica

Extent and Cessation of the Mid‐Cretaceous Hikurangi Plateau Underthrusting: Impact on Global Plate Tectonics and the Submarine Chatham Rise

“…While the P‐wave velocities within layer lc1 decrease slightly toward the southern termination of the profile (6.3–6.8 km/s), gravity modeling suggests a gentle southward increase of densities in layer lc1 similar to the southward‐increasing densities in the overlying layers uc1 and uc2 (Figure 2b). We find that the ranges of P‐wave velocities and densities in layer lc1 are the same as those observed in the lower crust of the Chatham Rise along the seismic refraction profiles east of the Chatham Islands (AWI‐20160100 and AWI‐20160200; Riefstahl et al, 2020). The base of layer lc1 is defined by continuous high‐amplitude reflections (P lc2 P; Figures 4 and 5) between 70 and 240 km along profile.…”

“…In this study, we define P‐wave velocities higher than 7.9–8.0 km/s as the mantle, which is clearly separated from the Hikurangi Plateau and Chatham Rise crust by a distinct P m P (Moho) reflection. This is similar to the eastern and thinner Chatham Rise (Figure 6), where only the Phoenix Plate, and not Hikurangi Plateau, was not subducted (Barrett et al, 2018; Riefstahl et al, 2020). We do not rule out that the Hikurangi Plateau may be thicker below the North and South Islands than beneath the Chatham Rise, as the western part of the Hikurangi Plateau was closer to the Ontong Java Nui eruption center (Hochmuth et al, 2015).…”

“…Furthermore, we observe model layer lc1 up to 14 km thick at 150–180 km along profile and thins out in southward toward the Bounty Trough, where it is less than 8 km thickness (Figure 3a). A lower crustal thickness of up to 14 km and P‐wave velocities of 6.3–6.8 km/s are largely similar to the lower crust further east of the Chatham Islands (Riefstahl et al, 2020). Accordingly, we interpret layer lc1 as lower continental crust of the ancient Chatham accretionary wedge, which most likely consists of amphibolite facies gneiss with southward‐increasing metamorphic grade (Figure 6a).…”

“…P‐wave velocities and densities are also in the range of magmatic mafic underplating. Mafic underplating of much less thickness (only 2 km) has been interpreted further west along AWI‐20160200 at the eastern Chatham Rise (Figure 6b; Riefstahl et al, 2020). The location crosses the central part of the Chatham Rise, and we would only expect magmatic underplating in areas where the crust is much thinner.…”

“…Slab tearing created a pathway for mantle upwelling, which affected the evolving southern Zealandia rifted margin around 85 Ma (Hoernle et al, 2020; Riefstahl et al, 2020; Tulloch et al, 2019). Alkaline magmatism occurred on the Chatham Islands between 86 and 79 Ma (Panter et al, 2006) and at several seamounts further to the south and west of the Chatham Islands (Hoernle et al, 2020; Homrighausen et al, 2018; Mortimer, Campbell, & Moerhuis, 2019).…”