Stratigraphic record of continental breakup, offshore NW Australia—Discussion

Abstract

Reeve et al. (2022) address the stratigraphic record of continental breakup by focusing on a set of stratigraphic unconformities from a proxima l sector of the NW Australian continenta l margin, inboard from the Exmouth Plateau. They suggest that such unconfor mities can potentially document a well-defined three-stage process: end of the synrift phase, formatio n of a wide continent-ocea n transitio n zone (COTZ) and generatio n of 'true' Penrose-type oceanic crust. We counterargue that continental breakup is a protracted event that can only be understood via seismic-and chronostratigraphic correlations of strata, and their composing sequences, across and along rifted margins. Tying proxima l stratigraphic unconfor mities to magnetic anomalies outboard from the study area in Reeve et al. ( 2022) is This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as

Reply to Alves et al. (2022) discussion on “Stratigraphic record of continental breakup, offshoreNWAustralia” by Reeve et al. (2022)

“…From these findings, we suggested that the three unconformities may have formed in response to (1) localisation of magma‐assisted rifting, linked to COTZ development and/or seafloor spreading, between 134.98 and 133.74 Ma (Intra‐Valanginian Unconformity); (2) generation of magmatic crust in COTZs between ~134 and 133 Ma (Top Valanginian Unconformity); and (3) full continental lithospheric breakup between ~132.5 and 131 Ma (Intra‐Hauterivian Unconformity) (Reeve et al, 2022). Note that contrary to the claim made by Alves et al (2022), we did not suggest that the end of the syn‐rift phase was defined by the formation of a single unconformity. Instead, we recognised that breakup was “represented by multiple unconformities [and inherently the surrounding strata] reflecting a complex history of uplift and subsidence during the transition from continental rifting to seafloor spreading” (Reeve et al, 2022).…”

“…We agree with Alves et al (2022) that lithospheric breakup in the Argo Abyssal Plain may also have instigated formation of a substantially older, margin‐wide unconformity in the Oxfordian, although exploring this was beyond the scope of Reeve et al (2022). Our interpretation that the Intra‐Valanginian Unconformity formed due to localisation of magma‐assisted rifting (Reeve et al, 2022) is also consistent with the suggestion of Alves et al (2022) that it marks lithospheric breakup in the Cuvier Abyssal Plain.…”

“…Of the three unconformities we mapped and classified as angular or simply disconformable, we acknowledge they could correspond to correlative conformities (i.e. surfaces that mark no deposition hiatus) away from our borehole constraints and/or in areas of little or no uplift (Alves et al, 2022). Critically, previous work (e.g.…”

“…Alves et al, 2020; Gong et al, 2019; Monteleone et al, 2019; Pérez‐Gussinyé et al, 2020; Peron‐Pinvidic et al, 2019; Soares et al, 2012). However, it is important to note that the statement by Alves et al (2022) at least partly emanates from analyses of the Iberia margin, where the proximal domain was situated >100–350 km from the locus of breakup (e.g. Alves & Cunha, 2018; Soares et al, 2012).…”

“…Based on the data presented in Reeve et al (2022), previous studies, and their own work on other continental margins, Alves et al (2022) offer their own interpretation of continental breakup offshore NW Australia, involving: (1) a phase of lithospheric breakup, implied mantle exhumation, and seafloor spreading along the Argo Abyssal Plain in the Oxfordian (~156 Ma), which produced a margin‐wide Lithospheric Breakup Surface (i.e. an unconformity; Marshall & Lang, 2013); (2) formation of the Intra‐Valanginian Unconformity in response to lithospheric breakup in the Cuvier Abyssal Plain, and the implied transition from continental rifting to mantle exhumation, and eventually seafloor spreading; (3) deposition of a conformable, net‐regressive sedimentary sequence with no time gap at our proposed Top Valanginian Unconformity; and (4) full continental breakup in the Aptian (i.e.…”