Widespread compression associated with Eocene Tonga-Kermadec subduction initiation (original) (raw)
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Geophysical Journal International, 2022
SUMMARY Rapid onset of subduction tectonics across the western Pacific convergent margins in the early Eocene was followed by a slower phase of margin growth of the proto Tonga-Kermadec subduction system north of Zealandia during a middle Eocene phase to tectonic adjustment. We present new age constraints from International Ocean Discovery Program Expedition 371 borehole data on deformation events in northern Zealandian sediments that document the formation of the convergent margin boundary northwest of New Zealand. The deformation shows a shortening event that lasted up to 20 myr and acted over distances of ∼1000 km inboard of the evolving plate margin, just northwest of New Zealand. Multichannel seismic profiles tied to our new borehole sites show shortening occurred predominantly between 45 Ma and 35 Ma with some deformation related to slope failure continuing into the Oligocene. The termination of shortening is linked to opening of the backarc basins of the southwest Pacific and...
Journal of Geophysical Research, 2003
We first present a synthesis of the Macquarie Ridge Complex (MRC) tectonic structures as well as paleo-reconstruction models of the kinematic evolution of the Pacific-Australia plate boundary south of New Zealand, since the Eocene. We then ascertain the geodynamical conditions that preceded subduction initiation, and identify the nature and structures of the crust that first subducted, at the Puysegur subduction zone. This synthesis is used to produce a subduction initiation model for the Puysegur Region. Concomitant to inception of the Alpine Fault (ca. 23 Ma), a 150-km-wide transpressive relay zone developed along Puysegur Bank inherited structures, enabling localization of compressive deformation. Right-lateral motion at the relay zone has juxtaposed oceanic and continental crusts facilitating inception of subduction and controlling the subduction vergence. Subsequently, the Puysegur subduction zone initiated at the transpressive relay zone ca. 20 Ma. Upper and lower plate inherited structures guided and facilitated the lengthening of the subduction zone during the Neogene. The four individual segments of the MRC represent different stages of incipient subduction whose development depends on local geodynamical conditions and lithospheric heterogeneities. The example of the MRC demonstrates that subduction can initiate from an oceanic spreading center, through progressive changes in plate kinematics within a 10-15 Myr time frame.
Seismicity and state of stress within the overriding plate of the Tonga-Kermadec subduction zone
Tectonics, 2007
1] To reassess the main tectonic units and to quantify the slip partitioning within the overriding plate of the Tonga-Kermadec subduction zone, a seismotectonic study was performed using global seismicity and focal mechanisms catalogs. (1) New tectonic features were identified within the Lau Basin and the volcanic arc by remarkable shallow hypocenters alignments. (2) The Centroid Moment Tensor solutions catalog was processed in order to map the stress tensor variation in the upper plate. We found the tectonic features characterized by a diffuse seismicity are subjected to a composite stress regime and they are interpreted as diffuse immature plate boundaries controlled by the high thermal anomaly lying beneath the Lau Basin. (3) We quantified the margin-parallel rates of motion using the azimuth of the maximum compressive stress component computed within the interplate zone. The results highlight a major tectono-kinematic segmentation related to the subduction of the Louisville Seamount Chain. Citation: Bonnardot, M.-A., M. Régnier, E. Ruellan, C. Christova, and E. Tric (2007), Seismicity and state of stress within the overriding plate of the Tonga-Kermadec subduction zone, Tectonics, 26, TC5017,
Earth and Planetary Science Letters, 1997
Since the Eocene, the Pacific-Australia plate boundary south of New Zealand has evolved from a spreading system into a transform boundary. Swath data acquired in the Southeast Tasman oceanic crust, between the Macquarie Ridge complex and the Resolution Ridge system, show that the spreading fabric changes orientation southwards along the Puysegur Trench, striking successively N60"E, N85"E and N120"E. This reflects the reorganisation of the plate boundary in response to changes in relative plate motion. A comparison of these orientations with the positions of the Pacific-Australia relative poles of rotation enables us to estimate the age of STOC, where there are no identified magnetic anomalies. The youngest age of the oceanic crust is ca. 12 Ma at the south end of the Puysegur Trench. This age is consistent with spreading rates and the amount of crust generated since 31 Ma. Curved fracture zones on either side of the Macquarie Ridge complex suggest a continuous reorientation of transform faults, between 31 Ma and ca. 15 Ma. Small-scale seafloor morphology shows a 13" change of orientation in the L' Atalante Fracture Zone, that indicates incremental, rather than continuous, changes in azimuth of the transform faults. Patterns of fanning ridges indicate that periods of asymmetric spreading accompanied the spreading segment reorientations. Using swath data and plate reconstruction models we infer that between 31 and 12 Ma the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cumulative length of spreading segments, along the whole plate boundary. This indicates that, since 14-15 Ma, the plate boundary has become progressively predominantly transcurrent, allowing strike-slip motion to develop along a line of merging transform faults that connected to the intracontinental Alpine Fault.
Starkly contrasting tectonic reconstructions have been proposed for the Late Cretaceous to mid Eocene (~85– 45 Ma) evolution of the southwest Pacific, reflecting sparse and ambiguous data. Furthermore, uncertainty in the timing of and motion at plate boundaries in the region has led to controversy around how to implement a robust southwest Pacific plate circuit. It is agreed that the southwest Pacific comprised three spreading ridges during this time: in the Southeast Indian Ocean, Tasman Sea and Amundsen Sea. However, one and possibly two other plate boundaries also accommodated relative plate motions: in the West Antarctic Rift System (WARS) and between the Lord Howe Rise (LHR) and Pacific. Relevant geologic and kinematic data from the region are reviewed to better constrain its plate motion history during this period, and determine the time- dependent evolution of the southwest Pacific regional plate circuit. A model of (1) west-dipping subduction and basin opening to the east of the LHR from 85–55 Ma, and (2) initiation of northeast-dipping subduction and basin closure east of New Caledonia at ~55 Ma is supported. West-dipping subduction and basin opening were not driven by convergence, as has previously been proposed. Our plate circuit analysis suggests that between at least 74 Ma and subduction initiation at ~55 Ma there was little net relative motion between the Pacific plate and LHR, b 20 km of convergence with a component of strike-slip motion. Subduction must therefore have been primarily driven by the negative buoyancy of the slab, or perhaps forced trench retreat due to orogenic collapse. We propose that at least two plate boundaries separated the Pacific plate from the LHR during this time, however, as there was little to no motion between these plates then a plate circuit which treats the Pacific plate and LHR as a single plate (“Australian” circuit) will produce similar kinematic results to a circuit which leaves their relative motion unconstrained and treats them as separate plates (“Antarctic” circuit). Prior to 74 Ma the reliability of magnetic anomalies from southwest Pacific spreading systems is questionable and it is difficult to properly test alternative plate circuits. After 55 Ma we advocate using an Antarctic plate circuit as the Australian plate circuit models that were tested predict significant net compression in the WARS, for which evidence is absent. Our preferred model makes testable predictions, such as burial of an arc beneath the Tonga and Vitiaz ridges, and Late Cretaceous to Eocene slabs in the mantle beneath the southwest Pacific, both of which can be investigated by future work. These predictions are particularly important for testing the earlier 85–55 Ma phase of the model, which is largely underpinned by ages and interpretations of South Loyalty Basin crust obducted onto New Caledonia, rather than an extinct arc or arc-related rocks.