From crustal structure to plate kinematics : the role of Large Igneous Provinces in the Pacific Ocean

Abstract

The bathymetry of the Pacific Ocean is dominated by three Large Igneous Provinces (LIP): the Ontong Java Plateau, the Hikurangi Plateau and the Manihiki Plateau. Taylor (2006) proposed their joined emplacement as one "Super"-LIP Ontong Java Nui in the early Cretaceous. Petro- logical and geochemical data point to this scenario, but geophysical evidence is sparse. To evaluate the hypothesis of Ontong Java Nui, refraction/wide-angle reflection seismic data was collected in 2012, during the RV Sonne cruise So-224 across the two main sub-provinces of the Manihiki Plateau. The modeling and interpretation of P-wave velocity, S-wave velocity and density profiles across the Manihiki Plateau along with available seismic reflection data aims to enhance our understanding of the crustal structure of the Manihiki Plateau and improve the plate kinematic reconstruction of the western Pacific region during the Cretaceous. If the hypothesis is correct, the Manihiki Plateau exposes break-up margins to all other LIPs of Ontong Java Nui. The Manihiki Plateau itself is fragmented into multiple sub-provinces. The two largest sub-provinces, the High Plateau and the Western Plateaus have been studied in- tensively in this experiment. The crustal structure of the High Plateau is comparable to other LIPs with a high velocity zone (P-wave velocities >7.3 km/s) in the lower crust and a basaltic to gabbroic crust. The crustal thickness is 20 km. Secondary magmatic phases are strong on the High Plateau expressed in multiple volcanic centers. The Danger Islands Troughs are a series of pull-apart basins, which separate the High Plateau from the Western Plateaus. These plateaus have been subject to massive tectonic deformation such as the gradual decrease in crustal thickness from 17.3 km in the East to 9.1 km in the West. Secondary volcanism is limited to fracture zones and low volume seamount volcanism. Since the crustal structure of the Western Plateaus points to a joined emplacement, the "Super"- LIP can be reassembled. The data also provides further evidence for a eastern and a northeastern continuation of the Manihiki Plateau by the sudden termination of the high velocity zone in the lower crust towards the East. It has been accounted for subducted LIP-parts, the rotation of LIP fragments such as the Hikurangi Plateau, crustal stretching invoked during the break-up and the crust emplaced during secondary magmatic stages. This calculates to an approximated initial size of Ontong Java Nui of 1.1. % of the Earth's s surface. Based on this information I reconstructed the "Super"-LIP Ontong Java Nui and modeled its break-up during the Cretaceous Normal Superchron. The initial emplacement of Ontong Java Nui can be explained by the interaction of a mantle plume with the Pacific-Phoenix ridge resulting in different crustal thicknesses throughout the plateau. I modeled the motion of different fragments of Ontong Java Nui using mapped fracture zones, traces of former plate boundaries and refined kinematic rotation poles of the western Pacific. Paleogene and Neogene intraplate tectonic activity occurred within the Ellice Basin between the Ontong Java Plateau and the Manihki Plateau and on the Manihiki Plateau itself. The eastern and northeastern fragments of the Manihiki Plateau have been captured by the Phoenix and Farallon Plate, respectively. The eastern fragment subducted analog to the southern Hikurangi Plateau at the eastern Gondwana margin in today's s Bellingshausen Sea and Palmer Land region during the Mid-Cretaceous, possibly flattening the slab of the subduction zone. The northeastern fragment collided with the South American craton during the Paleocene. In this amagmatic trench setting, oceanic terranes were accreted to the craton building up today's s northern Andes. The Pinà oà n formation of Colombia and Ecuador is a possible candidate to be a remnant of the lower crust of the Manihiki Plateau.