Geologic and kinematic constraints on Late Cretaceous to mid Eocene plate boundaries in the southwest Pacific

• Published in: Earth-Science Reviews
• Main Authors: Matthews, KJ, Williams, SE, Whittaker, JM, Muller, RD, Seton, M, Clarke, GL
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier Science Bv 2015
• Subjects: Earth Sciences; Geology; Tectonics; Antarctic; Indian; Pacific; Tonga; Antarc*
• Online Access: https://doi.org/10.1016/j.earscirev.2014.10.008; http://ecite.utas.edu.au/96549

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 howto implement a robust southwest Pacific plate circuit. It is agreed that the southwest Pacific comprisedthree 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 theWest Antarctic Rift System (WARS) and between the Lord Howe Rise (LHR) and Pacific. Relevantgeologic and kinematic data from the region are reviewed to better constrain its plate motion history duringthis period, and determine the time-dependent evolution of the southwest Pacific regional plate circuit. Amodel 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 beenproposed. 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, < 20 km of convergence witha component of strike-slip motion. Subduction must therefore have been primarily driven by the negativebuoyancy of the slab, or perhaps forced trench retreat due to orogenic collapse. We propose that at leasttwo plate boundaries separated the Pacific plate from the LHR during this time, however, as there was littleto no motion between these plates then a plate circuit which treats the Pacific plate and LHR as a singleplate (Australian circuit) will produce similar kinematic results to a circuit which leaves their relativemotion unconstrained and treats them as separate plates (Antarctic circuit). Prior to 74 Ma the reliability ofmagnetic anomalies from southwest Pacific spreading systems is questionable and it is difficult to properlytest alternative plate circuits. After 55 Ma we advocate using an Antarctic plate circuit as the Australianplate circuit models that were tested predict significant net compression in the WARS, for which evidence isabsent. Our preferred model makes testable predictions, such as burial of an arc beneath the Tonga andVitiaz ridges, and Late Cretaceous to Eocene slabs in the mantle beneath the southwest Pacific, both ofwhich can be investigated by future work. These predictions are particularly important for testing the earlier85-55 Ma phase of the model, which is largely underpinned by ages and interpretations of South LoyaltyBasin crust obducted onto New Caledonia, rather than an extinct arc or arc-related rocks.