Eocene (46–44 Ma) Onset of Australia‐Pacific Plate Motion in the Southwest Pacific Inferred From Stratigraphy in New Caledonia and New Zealand

Abstract

The Pacific plate circuit went through a complex reorganization during the early to middle Eocene, approximately coinciding with the onset of subduction along the western Pacific margin. However, the timing and dynamics of this change in the southwest Pacific and evolution of subduction beneath the Tonga‐Kermadec Arc are not fully resolved. We present magneto‐biostratigraphic data from an early to middle Eocene sedimentary section exposed in the Koumac‐Gomen area, New Caledonia, which is an emerged portion of the Norfolk Ridge. The 260 m‐thick succession contains a transition from pelagic micrite to terrigenous‐rich calciturbidite that is observed regionally in New Caledonia and which is interpreted to represent a shift from sedimentation on a stable submarine plateau to slope formation developed under a convergent tectonic regime. The stratigraphic contact between pelagic micrite and overlying calciturbidite is not exposed, but our magnetic polarity‐based chronology constrains the age of transition to 46–44 Ma, in agreement with the 45.3 Ma age recently obtained from the Noumea area in southern New Caledonia. We integrate records from New Caledonia with recent magnetostratigraphic data from South Island, New Zealand, where marked variations in terrigenous input occurred during the early and middle Eocene. Synchronous sedimentary changes in the southwest Pacific occurred at the same time as onset of rapid seafloor spreading south of Australia and New Zealand. We infer that the underlying cause of stratigraphic change was inception of slip at a new configuration of the Australia‐Pacific plate boundary, which evolved into the Tonga‐Kermadec subduction system.

Testing the Reliability of Sedimentary Paleomagnetic Datasets for Paleogeographic Reconstructions

“…The Eocene sedimentary record of the Koumac area lies on top of the Mesozoic basement, but New Caledonia as a whole is constituted by sedimentary, metamorphic, and igneous terranes that are overlain by allochthons of sedimentary, mafic, and ultramafic nappes obducted from the late Eocene with the opening of the North Loyalty Basin (Cluzel et al, 2001, Cluzel et al, 2005, Cluzel et al, 2006, Cluzel et al, 2012Maurizot, 2011;Maurizot et al, 2020a). The paleomagnetic declination obtained by Dallanave et al (2020) is in general agreement with the tectonic constraints, indicating a postmiddle Eocene rotation of New Caledonia of about 60°CCW. The ChRM directions inclination is primary and should be usable (at a first inspection) as paleogeographic constraint.…”

“…Using simulated paleomagnetic data, we show that strain related deflection of ChRM directions promptly modify the distribution elongation expected from the TK03.GAD model at a given latitude. We apply this concept to assess the reliability of a recently published dataset from Eocene sedimentary rocks exposed in New Caledonia (Dallanave et al, 2020).…”

“…Paleomagnetic Directions and Anisotropy of Magnetic Susceptibility Dallanave et al (2020) recently published paleomagnetic data from carbonate rocks exposed in the Koumac region of northern New Caledonia. New Caledonia is the emergent part of the northernmost Norfolk Ridge, which is in turn part of a large continental mass that is submerged for more than 90%, referred to as Zealandia (Mortimer et al, 2017;Sutherland et al, 2019).…”

“…(A) Elongation vs. inclination (E/I) of the paleomagnetic distribution for the whole (red) and the filtered (blue) dataset, compared with the value expected by the TK03.GAD model; the yellow dots represent the "unstrained" condition where the elongation direction is parallel to the average declination, shown with the inclination 95% confidence angle determined by applying standard Fisher (1953) statistics; the elongation values of the original and "unstrained" steps are shown with the 90% confidence zone determined by means of 1,000 bootstrapped pseudo-dataset expected inclination calculated from the compilations of Torsvik et al (2012*) and Veevers and Li (1991**) are shown. (B) Upper projection: AMS from Sommet-Khian; k1 (red symbols), k2 (blue symbols), and k3 (gray symbols) are respectively the major, intermediate, and minor axes of the AMS, with average direction of the eigenvectors; lower projection: characteristic remanent magnetization (ChRM) directions from Sommet-Khian in insitu coordinates, together with the Fisher (1953) mean; in both projections the dashed circle is the bedding plane; data from Dallanave et al (2020). (B) Effect of the "unstraining" on the whole (red) and filtered (blue) dataset from Sommet-Khian; the yellow star indicates the condition where the elongation direction (dotted line) is parallel to the declination; the number within the diagrams indicate the matrix used (Table 4) and the associated E/I couple in panel A.…”

“…The southwest Pacific area went through a complex tectonic and paleogeographic evolution during the Eocene, related to the inception of the Tonga-Kermadec subduction system (recent reviews can be found in Cluzel et al, 2012;Collot et al, 2020;Dallanave et al, 2018b, Dallanave et al, 2020Maurizot et al, 2020a, Maurizot et al, 2020bSutherland et al, 2020). The Tonga-Kermadec subduction initiated in the middle Eocene immediately to the east of the Norfolk Ridge, the northernmost part of which is The eigenvector are oriented as the average eigenvector of the anisotropy of magnetic susceptibility (AMS), shown in Figure 8B.…”