During Deep Sea Drilling Project (DSDP) Leg 21, Cenozoic and latest Cretaceous sediments were recovered at Site 208 on the Lord Howe Rise, Southwest Pacific. We provide new biostratigraphic, magnetostratigraphic and chemostratigraphic data from Site 208 to constrain the stratigraphy around the Cretaceous-Paleogene (K-Pg) boundary and to determine the depth of the K-Pg boundary more precisely. Biostratigraphic data from calcareous nannofossils indicate a near-continuous succession of sediments from the mid-Maastrichtian (Late Cretaceous) to lowermost Thanetian (Paleocene) at depths of 540−590 m below seafloor (mbsf). The biostratigraphic data suggest that the K-Pg boundary corresponds to a siliceous claystone at the base of an interval of silicified sediments (576.0−576.8 mbsf). Carbonate carbon isotopic composition (δ13Ccarb) reveals a negative shift across this interval, which is consistent with global patterns of δ13C across the K-Pg boundary. Osmium concentration and Os isotopic composition (187Os/188Os) can also be used to identify the K-Pg boundary interval, as it is marked by a peak in Os concentration and a drop in 187Os/188Os values to 0.12−0.15, both of which are the result of the Chicxulub impact event. Our 187Os/188Os data show trends similar to those of coeval global seawater with the lowest value of 0.12−0.16 in the siliceous claystone (576.8 mbsf). However, the concentration of Os is low (<80 pg g−1) in this sample, which suggests that this siliceous claystone was deposited around the K-Pg boundary but may not include the boundary itself. Although the sedimentary record across the K-Pg interval at Site 208 may not be completely continuous, it nevertheless captures a time interval that is close to the Chicxulub impact event.

Continental rifting and breakup of eastern Gondwana during the Cretaceous separated northern Zealandia from eastern Australia, but the processes leading to this highly extended and largely submerged block of continental crust are unknown. We acquired and processed multichannel seismic reflection data across northern Zealandia and examine the stratigraphy of the Middleton Basin. We identified a two-phase formation process for the basin, as evidenced by an unconformity separating two postrift units. After initial basin formation and slow deposition of the lower postrift unit, deposition rates within the Middleton Basin rapidly increased in response to the latest stage of subsidence and to create the modern basin. We propose a tectonic model wherein the Middleton Basin initiated through oceanic spreading and the subsequent postrift subsidence of the newly created oceanic lithosphere was delayed due to thermal buoyancy associated with nearby oceanic spreading in the Tasman Basin. Our results provide new constraints on rifting and breakup processes of wide, magma-poor, and asymmetric margins and indicate that multiple regions of weak lithosphere may have influenced the breakup. Plain Language Summary Tens of millions of years ago, an elongate continental fragment now known as Zealandia broke away from eastern Australia. Because most of Zealandia is underwater in a remote part of the southwest Pacific, little is known about the processes that led to its submergence and isolation. We have used a common subsurface imaging technique based on sound waves bouncing off deep rock layers to map sediments below the seafloor that record the geological events leading to the formation of Zealandia. Our focus is on the Middleton Basin, an elongate trough between two continental blocks that has a uniquely flat seafloor and is filled with up to 3.5 km of sediments. Our results suggest that the geological history of the Middleton Basin initially involved formation of a new, narrow ocean between two continents-like the Red Sea today. Ocean formation then jumped further to the west, ultimately resulting in the wide and deep Tasman Sea. This stop-start formation of two new oceans during the fragmentation of a continent is uncommon and highlights Zealandia's role in revealing the intriguing geological history of Earth's continents.

During the Late Cretaceous, the fragmentation of eastern Gondwana led to the formation of the narrow eastern Australian margin and the wide Lord Howe Rise during the opening of the oceanic Tasman Basin. To provide crustal-scale constraints on this margin, a 680-km-long, east-west oriented refraction transect with 100 ocean bottom seismometers was acquired from the Tasman Basin to the Lord Howe Rise. After traveltime tomographic inversion of the first refracted arrivals and reflected arrivals from the Moho, the final P wave velocity model reveals strong variations in crustal thickness. The Tasman Basin is floored by a two-layered and 7-km-thick oceanic crust. To the east, the Middleton Basin separates the Dampier Ridge, with 16-km-thick continental crust, from the Lord Howe Rise, where the extended continental crust is 20-23km thick. Below the Middleton Basin, Moho reflections are recorded at the base of 7-km-thick crust. The velocity gradient of this two-layer crust suggests an oceanic origin for the Middleton Basin. Our results show no clear evidence for mantle exhumation or a sizable igneous intrusion within three separate and relatively narrow (<70km) necking zones. The northwestern Zealandia margin thus appears to be magma poor and, despite the considerable width of this margin (>1,000km), the lack of evidence for mantle exhumation, the evidence for oceanic crust under the Middleton Basin, and the narrow necking zones combine to suggest that northern Zealandia is not a hyperextended margin.

An International Ocean Discovery Program (IODP) workshop was held at Sydney University, Australia, from 13 to 16 June 2017 and was attended by 97 scientists from 12 countries. The aim of the workshop was to investigate future drilling opportunities in the eastern Indian Ocean, southwestern Pacific Ocean, and the Indian and Pacific sectors of the Southern Ocean. The overlying regional sedimentary strata are underexplored relative to their Northern Hemisphere counterparts, and thus the role of the Southern Hemisphere in past global environmental change is poorly constrained. A total of 23 proposal ideas were discussed, with ∼ 12 of these deemed mature enough for active proposal development or awaiting scheduled site survey cruises. Of the remaining 11 proposals, key regions were identified where fundamental hypotheses are testable by drilling, but either site surveys are required or hypotheses need further development. Refinements are anticipated based upon regional IODP drilling in 2017/2018, analysis of recently collected site survey data, and the development of site survey proposals. We hope and expect that this workshop will lead to a new phase of scientific ocean drilling in the Australasian region in the early 2020s.

The Houtman Sub-basin is an under-explored region of Australia’s continental margin. It is located at the transition between the non-volcanic margin of the northern Perth Basin and the volcanic province of the Wallaby Plateau and lies adjacent to the Wallaby-Zenith Transform Margin (WZTM). In 2014, Geoscience Australia acquired new 2D seismic data (3300 km) across the northern Houtman Sub-basin to better image deep crustal structures in this frontier province. Interpretation reveals that this depocentre contains up to 19 km of sediments and regional correlation of the seismic stratigraphy across the northern Perth Basin suggests this includes up to 16 km of Permian—Cretaceous succession. However, the depth and nature of the crystalline basement, the total crustal thickness as well as the extent and distribution of Seaward Dipping Reflector Sequences (SDR) and intra-basinal volcanics associated with development of the Wallaby Plateau volcanic province and the WZTM remain poorly constrained. An integrated geological and geophysical study, based on available seismic and potential field data was undertaken to aid the structural interpretation of the deep crust and Moho in order to better define the basin’s crustal architecture. In addition, the transition between non-volcanic and volcanic margin segments was delineated and, in conjunction with the regional seismic interpretations, better understanding of the timing, distribution, and magnitude of multiple basin forming events was gained. The Ocean-Continent Transition (OCT) shows along strike and dip variations from extended and hyperextended (<5 km thick) continental crust beneath the main Permian depocentre to a zone of volcanic SDRs located outboard. Continental thinning and stretching phases occurred during both the Permian and Late Jurassic extensional phases. Volcanic margin development began in the Early Cretaceous, immediately prior to the separation of Greater India and Australia, suggesting that the volcanic margin experienced a phase of hyperextension before the magmatic break-up. Structural inheritance played an important role in basin development. It is likely that Early Permian graben formation was influenced by rheological contrasts in the underlying Proterozoic basement. The distribution of Permian rifts in turn further localised strain during Jurassic—Early Cretaceous rifting, strongly influencing the location and style of rifted margin development during Valanginian continental break-up. 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During the Late Cretaceous, the eastern Australian margin rifted to form the Lord Howe Rise continental ribbon. To understand the history of this region since rifting we use reflection seismic data collected in 2016 onboard R/V Kairei. We focus on processing and interpreting a regional ~900-km-long east-west oriented seismic reflection profile at 27.2°S. The seismic data were processed through pre-stack depth migration and interpretation shows the structure and evolution of this margin. The profile covers the oceanic Tasman Basin through the continental Lord Howe Rise. Sediment-filled depressions are found within the Tasman Basin and likely relate to early transform faulting with later deposition. The Lord Howe Rise is largely made up of syn-rift and post-rift sedimentary sequences in multiple structurally controlled basins. Two additional features are found between these regions, the Dampier Ridge and the Middleton Basin. The Dampier Ridge has a sharp, probably transform, boundary against the eastern edge of the Tasman Basin. Within the ridge are multiple rift basins up to 3 km deep that are comparable in size and structure to those found on the Lord Howe Rise. Between the Dampier Ridge and the Tasman Basin is the Middleton Basin which contains well-stratified sediments that are up to ~3.5 km thick. Stratal relationships indicate that the Middleton Basin formed during a post-rift event with large amounts of subsidence. Deep reflections beneath this basin reveal mantle at a shallow depth. The results have important implications for the evolution of the margin from initial rifting, opening of the Tasman Basin, and subsequent deformational processes. SCG71-01 JpGU-AGU Joint Meeting 2017

The Lord Howe Rise is a ribbon of submerged and extended continental crust that separated from Australia during the Late Cretaceous. The Lord Howe Rise is remote and concealed beneath the Tasman Sea in water depths of 1000–3000 m, therefore current knowledge of Lord Howe Rise geology is based on sparse shallow ( 100-million-year geological, tectonic and climatic history of the region. To this end, Geoscience Australia and JAMSTEC are leading an international effort to drill a deep stratigraphic well through a Lord Howe Rise rift basin that will core Cretaceous and older sediments and potentially basement rocks. This deep riser drilling will extend to a depth of up to about 2500 m below the seafloor. Two shallow, non-riser holes may also be drilled up to ~500 m below the seafloor into basement horst blocks. A proposal for drilling using the JAMSTEC drilling vessel CHIKYU was submitted to the International Ocean Discovery Program (IODP) in October 2015 (Proposal 871-CPP) and was rated “excellent” by the IODP Science Evaluation Panel in January 2017. The objectives outlined in this IODP proposal are to: 1) define the role and importance of continental crustal ribbons, like the Lord Howe Rise, in plate tectonic cycles and continental evolution; 2) recover new high-latitude biomarker and micropaleontology data in the southwest Pacific to better constrain Cretaceous paleoclimate and linked changes in ocean biogeochemistry, and; 3) test fundamental evolutionary concepts for sub-seafloor microbial life over a 100-million-year timeframe. The deep stratigraphic drilling is planned for 2019 or 2020, subject to funding approval. Preparations for drilling include a seismic survey conducted in the first half of 2016 that acquired 2D seismic reflection and refraction data along an east–west transect across the Lord Howe Rise to map regional crustal structure and 2D seismic reflection data at the prospective drill sites. Results from this survey helped to better constrain depth to and character of the basement beneath the proposed drill sites and suggest that the crust beneath the Lord Howe Rise is about 20 km thick. Initial velocity models also provide evidence of crustal segmentation linked to lineaments that align with fracture zones in the Tasman Sea oceanic crust. A second survey in late 2017 will acquire the geotechnical data necessary to successfully drill a deep stratigraphic well. This detailed site survey will also acquire high-resolution seabed and shallow sub-seafloor data, shallow sediment cores (up to 20 m below-seafloor) and underwater video.