Geologic and Kinematic consequences of major plate boundary reorganisations

Abstract

Tectonic structures on the seafloor preserve a wealth of information on seafloor spreading and plate boundary evolution, and reveal that long intervals of uniform plate motions are punctuated by episodes of relatively rapid change. These plate reorganisation events are recurrent in Earth’s history and form an integral component of plate tectonics, yet the driving mechanisms and effects of major events are insufficiently understood. In this thesis I undertake global and regional-scale investigations of plate boundary reorganisation events to better understand their geologic and kinematic consequences. I create a near-global digital tectonic fabric map of the ocean basins using satellite altimetry-derived vertical gravity gradient data, and identify intervals of widespread changes in plate motions. Kinematic observations from seafloor fabric are combined with onshore and offshore geology to investigate a postulated major plate reorganisation event at ~100 Ma and the plate boundary evolution in the southwest Pacific following this reorganisation, focusing on the Late Cretaceous to mid Eocene timeframe. I show that fracture zone indicators of plate motion changes are common in seafloor that formed during the middle of the Cretaceous Normal Superchron interval, ranging from about 120 to 83 Ma. I combine constraints from fracture zones with a detailed global compilation of oceanic and continental, tectonic and volcanic events that occurred at 110-90 Ma, and show that a major plate reorganisation event initiated between ~105-100 Ma. This event is recorded in both the oceanic and continental domains and was global in scale. In each ocean basin that preserves seafloor of mid Cretaceous age, I find evidence for changes in spreading between ~105-100 Ma. Slab pull is known to be a dominant driver of plate motion and I speculate that cessation of long-lived eastern Gondwanaland subduction is a potential driving mechanism for the event. This subduction zone constituted approximately 1/6 of the total estimated length of circum-Pacific subduction zones at this time, and I suggest that its termination had the propensity to modify the motion of plates along the margin and subsequently neighbouring plates. However, eruption of the Bouvet plume at the South American-African-Antarctic spreading ridge triple junction may have had at least a regional-scale influence on plate motions and continental tectonics. The 105-100 Ma global plate reorganisation profoundly influenced the tectonic evolution of the southwest Pacific; however due to sparse and ambiguous data there is much debate over where and when plate boundaries were active following cessation of eastern Gondwanaland subduction at this time, particularly before 45 Ma. Kinematic analyses of plate motions in the southwest Pacific using revised relative motion histories at the Australian-East Antarctic ridge and in the West Antarctic Rift System suggest the presence of a plate boundary between of the Pacific and Lord Howe Rise before 55 Ma, accommodating minor convergence (<20 km) with a possible strike-slip component. This scenario precludes subduction and slab rollback driven by strong convergence during this time, as has previously been suggested. Geologic evidence for the opening of the South Loyalty Basin to the east of New Caledonia during the mid Cretaceous-Palaeocene implies that a plate boundary must have existed in this area at this time. The results of my kinematic analysis are consistent with an existing model of trench retreat driven by collapse of the East Gondwanaland Cordillera and associated widespread extension, as this scenario can account for basin opening and slab rollback in the absence of convergence. At ~55 Ma a subduction initiation event occurred to the east of New Caledonia as supported by dyke emplacement and metamorphism in New Caledonia, and arc-type rocks dredged from the Tonga forearc. An important implication of my work on the tectonic development of the southwest Pacific and my analysis of the seafloor fabric globally relates to major plate motion changes centred at ~55 Ma. Subduction inception in the southwest Pacific at 55 Ma is coeval with other plate motion changes in the Pacific and Atlantic ocean basins and southeast Indian Ocean. This plate reorganisation in the southwest Pacific at 55 Ma may therefore form part of a much larger early Eocene Pacific-wide event. I provide observational constraints for mid Cretaceous and early Eocene reorganisations, and in order to resolve what ultimately drove these events fully dynamic numerical models are needed to quantitatively assess plate driving forces.