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

Highlights

• We present three geophysical profiles along the southern Chatham Rise margin.

• Structural variations relate to Hikurangi Plateau collision and Zealandia rifting.

• SE Chatham Terrace represents a broad COT (thin and modified continental crust).

• The southern Chatham Rise margin is a unique hybrid-rifted margin.

• Initial rifting was passive, but upwelling mantle affected the Chatham Rise margin.

Abstract

Passive continental margins are commonly classified as magma-poor and magma-rich types. Related breakup processes are often associated with far-field tectonic stresses or upwelling mantle plumes. The Chatham Rise east off New Zealand records a sequence of Late Cretaceous tectonic events, which include subduction and collision of the oceanic Hikurangi Plateau to subsequent continental rifting and breakup. The mechanisms triggering the change in tectonic forces are poorly understood but address open questions regarding the formation of passive margins. We acquired wide-angle seismic reflection/refraction, multi-channel seismic and potential field data along three profiles crossing the southern Chatham Rise margin and SE Chatham Terrace to the oceanic crust in order to image and understand the crustal structure and breakup mechanisms. Variations in crustal thickness along the highly faulted Chatham Rise are most likely related to the collision with the Hikurangi Plateau. Our data indicate that the SE Chatham Terrace represents a broad continent-ocean transition zone (COTZ), which we interpret to consist of very thin continental crust affected by magmatic activity. Along the southern Chatham Rise margin, features of both, magma-poor and magma-rich rifted margins are present. We suggest that passive rifting initiated at 105–100 Ma related to slab dynamics after the Hikurangi Plateau collision. We revise the onset of seafloor spreading south of the eastern Chatham Rise to ~88 Ma from the extent of our inferred COTZ. Geographically extensive, but low-volume intraplate magmatism affected the margin at 85–79 Ma. We suggest that this magmatism and the onset of seafloor spreading are a response to upwelling mantle through a slab window after 90 Ma. After 85 Ma, spreading segments became connected leading to the final separation of Zealandia from Antarctica. We interpret the southern Chatham Rise margin as a unique hybrid margin whose tectonic history was influenced by passive continental rifting and mantle upwelling.

Introduction

The breakup of supercontinents such as Gondwana is often associated with a change from lithospheric convergence (i.e., subduction activity and orogeny) to lithospheric divergence (i.e., crustal thinning prior to seafloor spreading). The mechanisms controlling the polarity of tectonic forces play a key role in the Wilson cycle (Dewey and Burke, 1974), but are poorly understood. Options include triggering by mantle dynamics leading to rising plumes and result in active and ‘hot’ rifting or, alternatively, by tectonic forces through far-field processes with significantly less or without any magmatic activity, which lead to passive and ‘cold’ rifting.

The Cretaceous collision of the Hikurangi Plateau with Zealandia (Fig. 1A) is interpreted to have initiated the end of subduction activity and compression at the East Gondwana margin (e.g., Davy, 2014; Davy et al., 2008). At approximately the same time intracontinental rifting was initiated, which led to the separation of Zealandia and Antarctica (e.g., Mortimer et al., 2016; Tulloch et al., 2009b). While the Late Cretaceous to Cenozoic seafloor spreading history between Zealandia and Antarctica is relatively well studied (e.g., Eagles et al., 2004a; Wobbe et al., 2012; Wright et al., 2016), the early rifting history between both conjugate margins, in particular those of Chatham Rise and Amundsen Sea/eastern Marie Byrd Land sector (Fig. 1B), remains poorly understood. The temporal overlap of the Hikurangi Plateau collision, subduction cessation and onset of rifting raises the following questions: Did the former Hikurangi Plateau subduction influence or initiate the early rifting? Or were collision and crustal extension two independent causal processes in the highly complex continental area of the Chatham Rise?

Knowledge about the nature and crustal structure is essential for any sound reconstruction of the margin evolution and the role of subduction cessation and breakup at the former East Gondwana subduction zone. Is the southern Chatham Rise margin a volcanic-rifted type margin with seaward dipping reflectors indicating excessive emplacement of magma triggered by an upwelling plume (Storey et al., 1999; Weaver et al., 1994)? Or was the breakup at the southern Chatham Rise margin non-volcanically driven by tectonic forces like transtensional movements along the West Wishbone Ridge (Fig. 2A; Barrett et al., 2018; Davy, 2014)? Contrasting paleotectonic scenarios are possible depending on whether oceanic, stretched continental crust and/or exhumed mantle is assumed to underlie the SE Chatham Terrace (Figs. 1A and 2A), an area of seafloor south of the Chatham Rise, where water depth is shallower than the abyssal oceanic crust and hosts abundant seamounts and guyots. For a fundamental understanding of the driving forces of the breakup it is also essential to know details on the exact timing for the first formation of oceanic crust and the amount of volcanism related to the breakup.

In this study we attempt to answer these questions for the particular situation of the Chatham Rise east of the Chatham Islands and SE Chatham Terrace. We acquired three 330–485 km long seismic wide-angle reflection and refraction profiles during the RV Sonne cruise SO246 in 2016 (Gohl and Werner, 2016). Additionally, multi-channel seismic (MCS) reflection and potential field (gravity, magnetic) data were collected along these profiles. Here, we present the results of P-wave velocity and density forward modelling together with an interpretation of the MCS reflection data. Subsequently, we define crustal thicknesses and specify the nature of the southern Chatham Rise margin. Our study explores potential processes governing the cessation of subduction along the East Gondwana active margin and the earliest East Gondwana breakup along the southern Chatham Rise margin, i.e., the transformation of a formerly active subduction margin to a passive rifted margin. Our findings also have implications for the geological and tectonic evolution of other, once nearby, continental areas like the Bounty Trough and rift basins east of the South Island as well as the conjugate Marie Byrd Land margin of West Antarctica.