François Bache

Widespread compression associated with Eocene Tonga-Kermadec subduction initiation

Eocene onset of subduction in the western Pacific was accompanied by a global reorganization of tectonic plates and a change in Pacific plate motion relative to hotspots during the period 52–43 Ma. We present seismic-reflection and rock sample data from the Tasman Sea that demonstrate that there was a period of widespread Eocene continental and oceanic compressional plate failure after 53–48 Ma that lasted until at least 37–34 Ma. We call this the Tectonic Event of the Cenozoic in the Tasman Area (TECTA). Its compressional nature is different from coeval tensile stresses and back-arc opening after 50 Ma in the Izu-Bonin-Mariana region. Our observations imply that spatial and temporal patterns of stress evolution during western Pacific Eocene subduction initiation were more varied than previously recognized. The evolving Eocene geometry of plates and boundaries played an important role in determining regional differences in stress state.

Stratigraphy of Reinga and Aotea basins, NW New Zealand: constraints from dredge samples on regional correlations and reservoir character

ABSTRACT Sandstone, mudstone and limestone samples dredged in the Reinga and Aotea basins, NW New Zealand during voyage TAN1312 provide age and lithological constraints on the Cretaceous–Neogene succession. A total of 46 micropaleontology and 7 macropaleontology samples were examined along with 84 thin-sectioned petrographical samples. Some were examined by X-ray diffraction and porosity-permeability analyses. Late Cretaceous sandstones are dominated by feldspathic and lithofeldspathic compositions, with mixed granitic plutoniclastic and volcaniclastic provenance; a comparison with Pakawau Group of Taranaki Basin is appropriate. Late Cretaceous–Paleogene mudstones are widespread and display close petrographical and age similarities to the Whangai Formation facies of other parts of New Zealand and fine-grained carbonate facies of the Weber and Amuri formations of eastern North and South Islands, respectively. Cretaceous limestone and Paleogene sandstone were not recovered. Carbonates and mudstones dominate the Neogene succession of Reinga and Aotea basins; rare Neogene sandstones have feldspatholithic compositions and resemble Waitemata Group sandstones of the northern North Island. In terms of petroleum prospectivity, Cretaceous sandstones represent a potential reservoir facies but are lithic with low permeability.

Use of ancient wave-ravinement surfaces to determine palaeogeography and vertical crustal movements around New Zealand

Wave-ravinement (shallow marine erosion) surfaces are formed during landwards migration of the shoreline due to rising relative sea level. They can be preserved if subsequently buried by sediment, and are easily identifiable on seismic reflection data. We present two examples from New Zealands north-western offshore frontier region where these surfaces, despite a diachronous nature, represent sequence stratigraphic and geomorphic markers that are used to constrain regional palaeogeography and tectonic history. Thermal subsidence that followed the break-up of Gondwana led to the landwards migration of shorelines across the then-emergent Challenger Plateau and formation of a prominent Late Cretaceous–Eocene wave-ravinement surface. In the Reinga Basin, subsidence of uplifted land areas that had previously emerged during Cenozoic initiation of Tonga–Kermadec subduction was similarly accompanied by formation of transgressive wave-ravinement surfaces. Wave-ravinement surfaces serve as useful proxies for palaeo-sea level and, as the above two examples show, they are powerful tools for characterising regional deformation, uplift and subsidence histories and tectonic influence on relative base level fluctuations (changes in shoreline position). The study of these and other remarkable palaeo-sea-level markers around New Zealand and in the circum-Pacific region provides a different perspective on constraining vertical crustal movements associated with major tectonic events.

Exploration of the south-eastern part of the Frontier Amadeus Basin, Northern Territory, Australia

The Neoproterozoic to Late Paleozoic-aged Amadeus Basin is a large (~170 000 km2) east–west-trending basin, bounded to the south by the Musgrave Province and to the north by the Arunta Block of the Northern Territory. Commercial oil and gas production is established in the northern part of the basin but the southern part is still a frontier exploration area. Vintage and new seismic reflection data have been used with well data along the south-eastern Amadeus Basin to construct a new structural and depositional model. Three major phases of deformation controlling deposition have been identified. The first phase is characterised by a SW–NE trending structural fabric and is thought to be older than the deposition of the first sediments identified above basement (Heavitree and Bitter Springs formations). The second phase corresponds to the Petermann Orogeny (580–540 Ma) and trends in a NW–SE orientation. The third phase is the Alice Springs Orogeny (450–300 Ma) and is oriented W–E to WNW–ESE in this part of the basin. This tectono-stratigraphic model involving three distinct phases of deformation potentially explains several critical observations: the lack of Heavitree reservoir at Mt Kitty-1, limited salt movements before the Petermann Orogeny (~300 Ma after its deposition) and salt-involved structures that can be either capped by the Petermann Unconformity and overlying Cambrian to Devonian sediments, or can reach the present day surface. Finally, this model, along with availability of good quality seismic data, opens new perspectives for the hydrocarbon exploration of the Amadeus Basin. Each of the tectonic phases impacts the primary petroleum system and underpins play-based exploration.