AUSTRALIAN-ANTARCTIC CONTINENTAL MARGIN EVOLUTION
Supervisors: Dietmar Müller and Patrice Rey
Recently, an extraordinary $60 million marine geophysical imaging campaign around Australia has been completed by Geoscience Australia within the Law of the Sea project. A unique seismic, magnetic and gravity data set imaged several continental margins, which once formed single continental rifts. They are the southern Australian margin and the Lord Howe Rise hydrocarbon exploration frontier areas, and their conjugate margins, including Antarctica. Using these data and our 2D/3D Particle-In-Cell software ELLIPSIS, we propose to test controversial hypotheses on margin evolution by modelling fluid convection of the Earth's mantle, crustal deformation, basin formation and sedimentation as a single system.
Our aim is to address several fundamental questions on margin evolution:
1. What are the process controlling the style of continental rifting, and the subsequent subsidence, sedimentation and resources along the continental margins?
2. Some margin pairs are symmetric, while others are extremely asymmetric. Which are the crucial rheological parameters that lead to observed styles of deformation?
3. What is the importance of successive rift phases with differing plate divergence rates and directions on margin evolution?
4. What role does mantle temperature play, i.e. contrasting the Great Australian Bight, which formed over a cool subducted sinking plate in the mantle, with southern margin segments further east or west?
5. Under which conditions does large-scale exhumation of the mantle either occur, on one or on both conjugate margins? What is the origin of the enigmatic "Diamantina Zone" southwest of Australia, and the wide margins of the Great Australian Bight and its Antarctic counterpart? How are conjugate peridotite/serpentinite ridges formed?
FROM GRAVITATIONAL COLLAPSE TO CONTINENTAL RIFTING
Supervisors: Patrice Rey and Dietmar Müller
The problem
Lithospheric mantle thinning, following thermal erosion, mechanical erosion, or delamination, can lead to gravitational extensional stresses. In a subduction context, this means that a continental plate can be under extensional stresses despite over-riding a subduction zone. For many tectonicists, gravitational collapse is the process through which an orogenic continental crust recovers a normal thickness. In fact, there is a threshold in the magnitude in lithospheric thinning beyond which gravitational collapse may lead to active continental rifting.
The Lord Howe Rise is a 400 to 600km wide piece of continental crust that extends from west of New Zealand to southwest of New Caledonia. This micro-continent, now lying 1000m below sea level, was detached from eastern Australia during the break-up of eastern Gondwana from ca. 85 to 52 Ma. An intriguing fact is that the initiation of the continental rifting, which led to the opening of the Tasman Sea, occurred shortly after a massive phase of extensional collapse that destroyed a mountain belt involving the eastern margin of Australia, New Zealand and New Caledonia. This leads to the hypothesis that gravitational collapse may have controlled the initiation and dynamic of continental rifting and the formation of the Tasman Sea, that the Lord Howe Rise, and the separation New Zealand from mainland Australia.
The objective
To document the thermo-mechanical links between continental rifting and gravitational collapse of subduction-related mountain belts using the Tasman Sea as a key example. This project involves 2D thermo-mechanical modelling using Ellipsis and SNARK. Numerical experiments will be ground truthed using data from the Tasman Sea and metamorphic core complexes in New Zealand.
Candidates from Geosciences will have the opportunity to add, if they wish, a field component aiming at developing their skills in the analysis of high-grade terranes.
MANTLE GEODYNAMICS, PLATE TECTONICS AND GLOBAL CHANGES
Supervisors: Patrice Rey and Dietmar Müller
The problem
In the first couple of billion years (4.54 to 2.5 Ga), the Earth differentiated into a concentrically layered thermo-mechanical and chemical system, able to sustain a dynamic biosphere. This differentiation did not follow a linear and smooth path, but rather a chaotic storyline involving sudden crises that punctuated periods of relative quietness. In this context, the time window 2.7 ± 0.08 Ga stands as the most dramatic in the Earth's history. Up to 80% of the present-day continental lithosphere was created before the end of the Archaean (2.5 Ga), two thirds of which was formed in between 2.78 and 2.68 Ga. At the end of that tumultuous period of creation and differentiation, the Earth had developed its main envelopes, its geodynamics was clearly under the influence of plate-tectonics processes, its atmosphere was on its path toward oxygenation, and the biosphere had evolved to support the first Eukaryote. The global changes that took place in the Late Archaean were the prelude to the birth of modern Earth. The geological database for that time period, together with numerical codes used to model thermo-mechanical and chemical processes, have progressed to the point where inversion modelling approaches can be used to constrain the succession of events and feedback processes that have affected the endogenic (core, lower and upper mantle, lithosphere) and exogenic (hydrosphere, atmosphere, biosphere) envelopes of the Earth.
The objective
Based on numerical inversion of geological observables, we propose to take on the challenge to constrain the succession of events that impacted on the internal organization and composition of the Earth's envelopes at 2.7 ± 0.08 Ga. In particular, we want to document and model the interplay between mantle geodynamics, plate differentiation and growth, plate tectonics, and surface processes that presided over the birth of modern Earth.
The Projects
Our objective will be achieved through the following projects which focus on endogenic processes (Project 1), and exogenic processes (Project 2): Project 1/ To model continental lithospheric plate creation and growth at 2.7 ± 0.08 Ga through coupling of mantle convection/plume modelling with chemical and heat transfer modelling. This will allow us to understand feedback processes induced by: i/ the sudden stabilization of larges volumes of continental lithosphere, and ii/ the sudden dehydration of large volumes of convective mantle. Project 2/ a-To constrain the volume of greenhouse gases released into the atmosphere at 2.7±0.8 Ga and assess its impact on global warming, increased weathering and erosion, and enhanced crustal recycling. b- To constrain the rate at which CO2 was pumped out of the atmosphere through enhanced weathering, and to assess the consequences on the oxygenation of the Earth's atmosphere. This project specifically tackles the pathway through which endogenic and exogenic envelopes interact. Candidates from Geosciences will have the opportunity to add, if they wish, a field component aiming at developing their skills in the analysis of high-grade terranes.
Candidates from Geosciences will have the opportunity to add, if they wish, a field component aiming at developing their skills in the analysis of high-grade terranes.
