Katharina Hochmuth

Playing jigsaw with Large Igneous Provinces—A plate tectonic reconstruction of Ontong Java Nui, West Pacific

Ontong Java Nui is a Cretaceous large igneous province (LIP), which was rifted apart into various smaller plateaus shortly after its emplacement around 125 Ma in the central Pacific. It incorporated the Ontong Java Plateau, the Hikurangi Plateau and the Manihiki Plateau as well as multiple smaller fragments, which have been subducted. Its size has been estimated to be approximately 0.8% of the Earth’s surface. A volcanic edifice of this size has potentially had a great impact on the environment such as its CO2 release. The break-up of the “Super”-LIP is poorly constrained, because the break-up and subsequent seafloor spreading occurred within the Cretaceous Quiet Period. The Manihiki Plateau is presumably the centerpiece of this “Super”-LIP and shows by its margins and internal fragmentation that its tectonic and volcanic activity is related to the break-up of Ontong Java Nui. By incorporating two new seismic refraction/wide-angle reflection lines across two of the main sub-plateaus of the Manihiki Plateau, we can classify the break-up modes of the individual margins of the Manihiki Plateau. The Western Plateaus experienced crustal stretching due to the westward motion of the Ontong Java Plateau. The High Plateau shows sharp strike-slip movements at its eastern boundary towards an earlier part of Ontong Java Nui, which is has been subducted, and a rifted margin with a strong volcanic overprint at its southern edges towards the Hikurangi Plateau. These observations allow us a re-examination of the conjugate margins of the Hikurangi Plateau and the Ontong Java Plateau. The repositioning of the different plateaus leads to the conclusion that Ontong Java Nui was larger (~1.2% of the Earth’s surface at emplacement) than previously anticipated. We use these finding to improve the plate tectonic reconstruction of the Cretaceous Pacific and to illuminate the role of the LIPs within the plate tectonic circuit in the western and central Pacific.

Glaciomarine sedimentation dynamics of the Abbot glacial trough of the Amundsen Sea Embayment shelf, West Antarctica

Abstract Sedimentary sequences of the continental shelf of the eastern Amundsen Sea Embayment in West Antarctica represent records of past outlet glaciers and ice streams. The former flow of ice streams was channelled through glacial troughs, which now form large bathymetric depressions. We therefore selected one of the largest troughs, the Abbot glacial trough in the outer shelf, to analyse its glacial depositional and erosional history, based on horizon-stratigraphy derived from seismic data. Several basement highs channellized the delivery of sediment and controlled the grounded ice sheet in early glacial periods. Both pre-glacial and full glacial seismic facies were identified. Glacially transported and deposited sediments extended the shelf break by 75 km from the pre-glacial shelf-edge. The main Abbot glacial trough contains sediment from confluent ice flows of the Pine Island/Thwaites, Cosgrove and Abbot Glacier systems, as well as smaller contributions from local ice streams emanating from Thurston Island. Sherman Island of Peacock Sound played an important role in the dynamics of the Abbot Glacier by dividing the ice flow into two ice streams, which interfered with the main glacial sediment transport paths from the south. This study contributes to an understanding of the formation of the Amundsen Sea shelf and the extent of past ice sheet advances.

Collision of Manihiki Plateau fragments to accretional margins of northern Andes and Antarctic Peninsula

The Manihiki Plateau, a large igneous province (LIP), was emplaced in the Early Cretaceous as a single plateau together with the Ontong Java Plateau and the Hikurangi Plateau. Additional to the present Manihiki Plateau, fragments to its northeast and east have been formed. Plate kinematic reconstructions suggest the capturing of these fragments by the Farallon Plate and the Phoenix Plate, respectively. By tracing these fragments, we report a Paleocene collision of the northeastern Manihiki Plateau fragment with the northern South American craton. The northern Andes exhibit multiple terranes of LIP origin. We infer, based on geophysical, petrological, and geochemical data, that the Pinon formation consists of crustal units of the former Manihiki Plateau. An Early Cretaceous collision of the eastern Manihiki Plateau fragment is reconstructed for West Antarctica. The subduction of this fragment in the Palmer Land region can be associated with the so-called Palmer Land Event and a flattening of the subduction slab. By reconstructing the dispersal of the fragments of the Manihiki Plateau, we provide a deeper insight in the possible subduction scenarios and buildup of the accretional margins of the northern Andes and the Antarctic Peninsula.