Julien Bourget

Neotectonics of the Owen Fracture Zone (NW Indian Ocean): Structural evolution of an oceanic strike-slip plate boundary

The Owen Fracture Zone is a 800 km-long fault system that accommodates the dextral strike-slip motion between India and Arabia plates. Because of slow pelagic sedimentation rates that preserve the seafloor expression of the fault since the Early Pliocene, the fault is clearly observed on bathymetric data. It is made up of a series of fault segments separated by releasing and restraining bends, including a major pull-apart basin at latitude 20°N. Some distal turbiditic channels from the Indus deep-sea fan overlap the fault system and are disturbed by its activity, thus providing landmarks to date successive stages of fault activity and structural evolution of the Owen Fracture Zone from Pliocene to Present. We determine the durability of relay structures and the timing of their evolution along the principal displacement zone, from their inception to their extinction. We observe subsidence migration in the 20°N basin, and alternate activation of fault splays in the vicinity of the Qalhat seamount. The present-day Owen Fracture Zone is the latest stage of structural evolution of the 20-Myr-old strike-slip fault system buried under Indus turbiditic deposits whose activity started at the eastern foot of the Owen Ridge when the Gulf of Aden opened. The evolution of the Owen Fracture Zone since 3-6 Myr reflects a steady state plate motion between Arabia and India, such as inferred by kinematics for the last 20 Myr period. The structural evolution of the Owen Fracture Zone since 20 Myr- including fault segments propagation and migration, pull-apart basin opening and extinction - seems to be characterized by a progressive reorganisation of the fault system, and does not require any major kinematics change.

Tectonic evolution of the northern Bonaparte Basin: impact on continental shelf architecture and sediment distribution during the Pleistocene

The Bonaparte Basin (NW Australia) forms a rare, recent example where Neogene deformation shaped a very wide platform (630 km wide) in which a mixed carbonate-silliciclastic sedimentary sequence developed. This study combines structural and stratigraphic analysis and provides new insights as to the role of tectonics in controlling platform shape and sediment distribution in wide shallow water settings. Detailed analysis of the structure and stratigraphy of the northern part of the Bonaparte Basin allowed identification of the main regimes and phases of deformation and their control on sedimentation during the Neogene. The results reveal that the distribution of Neogene sediments across the northern Bonaparte Basin is mainly controlled by flexure-induced deformation mechanisms associated locally with extensional faults and low-strain, left-lateral strike-slip. These processes ultimately shaped the geometry and sedimentary architecture of the wide continental shelf. They led to the development of two different types of tectonically induced shelf depocentres that controlled the gross distribution of Quaternary sediments. In particular, deformation processes enhanced the formation of the carbonate-dominated, ∼200 m-deep Malita intra-shelf basin. The Bonaparte Basin is a prime natural laboratory to describe the links between tectonics and sedimentation along a very large, mixed carbonate/clastic platform and could be used as a modern analogue to similar settings in the past Earths history.

A Pliocene–Quaternary analogue for ancient epeiric carbonate settings: The Malita intrashelf basin (Bonaparte Basin, northwest Australia)

During the Pliocene–Quaternary, the Bonaparte Basin is characterized by a very wide (>600 km [>370 mi]) carbonate platform and 200-km-wide (125-mi-wide) Malita intrashelf basin (ISB). Using three-dimensional and two-dimensional seismic data combined with exploration well data, this study characterizes the stratigraphic evolution of the Malita ISB during the last 3.5 m.y. Two third-order transgressive sequences can be distinguished. A late Pliocene transgression occurred over an irregular topography resulting from the flexural reactivation of the Malita graben. In the center of the ISB, carbonate aggradation resulted in the formation of isolated carbonate platforms separated by deeper water seaways and interplatform areas. Wider and more numerous carbonate platforms developed on the edges of the ISB. During the late Quaternary, renewed flexural deformation initiated a second transgressive cycle marked by higher subsidence rates in the ISB center than along its edges. High rates of accommodation creation (at third order) combined with higher-frequency (fourth-order), high-amplitude fluctuating sea levels and increased clastic input resulted in the progressive demise and burial of the carbonate platforms in the ISB center. Thus, the Pliocene–Quaternary stratigraphic architecture of the Malita ISB is strongly controlled by differential subsidence that controls spatial distribution of accommodation and ultimately platform architectures. The Malita ISB constitutes a rare recent analogue for Paleozoic and Mesozoic hydrocarbon-bearing ISBs.

Neogene oblique extensional system in the north-western Bonaparte Basin, Australia

The North-western Bonaparte Basin offers a very good opportunity to understand the nature of oblique extension system, where Neogene flexure-induced extension was superimposed obliquely to the Mesozoic rift-related structures. The Mesozoic trends strongly control the distribution and style of the younger Neogene structures, both at regional and local scale. The younger Neogene activity produced a new set of NE trending, right-stepping en echelon faults and reactivated the older faults. In addition, episodes of stratigraphic growth provide critical evidence regarding the timing of fault activity. Results demonstrate that, in the study area, main fault activity occurred in several pulses during the latest Miocene to Late Pleistocene. These episodes of fault activity correspond to recently constrained regional tectonic events i.e., the initial collision of the Australian Plate with the Banda Arc, the episodes of uplift of the Timor Island and the timing of lithospheric flexure.

Quaternary isolated carbonate build-ups in the Timor Sea (NW Australia) - Understandings and Implications

Distribution and growth history of isolated carbonate build-ups (ICBs) is controlled by complex interplay between various tectonic, eustatic, and oceanographic parameters. Quaternary ICBs in the Timor Sea (NW Australia) are located in tropical waters, and at present they form clusters of ~150 build-ups, developing 2 to 85 km from the edge of a wide continental shelf. The tectonic evolution of the Timor Sea lead to regional changes in the oceanography and flexural deformation of the NW Bonaparte Basin, which in turn had a major impact on the evolution of ICBs. Flexure-induced fault activity produced structural topography for the growth of ICBs over ‘highs’, while oceanic current through Timor Trough provided warm and nutrient-rich water. Our results demonstrate that, despite potentially good conditions for carbonate production, ICBs did not form until the Mid Pleistocene (ca. 0.582-0.8 Ma BP). This age corresponds to the onset of repeated, high-amplitude (+120 m) sea level fluctuations with rapid deglacial rises and slow falls. Thus, we infer that the NW Australia ICBs formed due to: (1) structural shaping of the margin; (2) oceanographic changes, and; most importantly, (3) onset of repeated short-term transgressions reactivating the carbonate production along isolated highs. The distribution and growth of ICBs could be useful to understand the evolution of ancient ICBs that formed along very wide shelves and epeiric seas.