Transcript
Page 1:  · Web viewCoeval extensional and contractional tectonics in the overriding plate is a key observable that can help to decipher between rollback-driven back-arc extension and back-arc

The Earth’s Paleozoic/Mesozoic tectonic and paleogeographic evolution

Kara Matthews, Nicolas Flament, Dietmar Müller

Continents and sedimentary basins through time have recorded fundamental Earth

system cycles, reflecting environmental change, migration of fauna and flora and

shifting coastlines. It was originally thought that successive advances and retreats of

shallow inland seas mainly reflect global sea level variations (eustasy). However, it is

now well established that large-scale surface morphology such as the high

topography of the East African Rift, the low-lying Amazon River Basin and the

southwest to northeast tilt of the Australian continent are strongly controlled by

processes deep within the Earth. Quantifying the magnitude and time-dependence

of mantle-driven topography requires integrating geological data with coupled

models of the plate-mantle system. In turn, these models need to be validated with

observational data, such as published paleogeographic maps and paleobiology data.

The overarching aim of these projects is to understand the deep-seated driving

forces of large-scale topographic change, providing dynamic models of the Earth’s

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subduction history, deep plume sources and dynamic topography for the Paleozoic-

Mesozoic periods. The Paleozoic follows the breakup of the supercontinent Rodinia

after the end of the so-called Snowball Earth period. Throughout the early Paleozoic,

the Earth's landmass was broken up into a substantial number of continents.

Towards the end of the era, continents gathered together into the supercontinent

Pangaea. We offer two Honours projects focussed on building models for the

Paleozoic/Mesozoic Earth using the software GPlates, and using geological

observations to test geodynamic models, which predict mantle convection patterns

and surface uplift/subsidence through time:

Project 1: The evolution of proto-Atlantic/Indian ocean basins and marginal seas in

the Cambrian to Devonian

Project 2: The evolution of proto-Atlantic/Indian ocean basins and marginal seas in

the Carboniferous to Jurassic

These projects will address the following questions:

How were ocean basins, including back-arc basins, created and destroyed

between the Cambrian and Jurassic periods?

How have the fundamentally different plate tectonic configurations before and

during/after the assembly of the supercontinent Pangea affected subduction

history, the history of mid-ocean ridge system evolution, mantle convection

patterns and ultimately regional sea level fluctuations?

The projects will involve acquiring various software and database skills, including

GPlates, including spatio-temporal data mining, ArcGIS, the Generic Mapping Tools,

shell scripting, dealing with the paleobiology database, as well as learning the basics

of geodynamic modelling. These projects will prepare students both for working in

the exploration industry as well as for a research-oriented career in government

agencies or universities.

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Constructing a revised plate tectonic history for the Caribbean

Supervisors: Prof Dietmar Müller, Dr Simon Williams

The Caribbean has witnessed a complex geological evolution during the last 150 Ma.

Understanding this history, in particular tracking the locations and polarity of

subduction zones within this region, has important implications for geodynamic

modeling of the area. However, there remains much debate amongst geoscience

researchers over some of the fundamental details of this area’s plate tectonic history

– for example the polarity of subduction along the so-called ‘Great Arc’ of the

Antilles during the Cretaceous, and whether or not a large Oceanic Plateau collided

with this arc.

The model of Ross and Scotese (1988) provides a quantitative framework of how the

different plates and magmatic arc fragments evolved within this region. However, a

wealth of new geological and geophysical data have been collected during this time,

and there is a clear need to reassess these earlier models in the context of these new

data. Numerous authors propose alternative reconstruction histories in terms of

simple cartoons for small regions of study. In our case you will use GPlates (plate

tectonic analysis software developed by the Earthbyte group) to test and generate

truly quantitative models that fit within a globally consistent plate model.

The central aim of this project will be to derive revised plate reconstructions for the

Caribbean. The project will be multidisciplinary in nature – collating data from online

sources and the scientific literature that tell us about the nature and timing of

geodynamic processes occurring throughout the Caribbean – for example

magmatism, subduction, phases of extensional and compressional deformation. You

will then assimilate all these data within GPlates, and use these constraints to test

existing models of Caribbean geodynamics and generate a new set of plate

reconstructions including plate boundary locations and plate velocities. This project

will also involve the analysis of seismic tomography and geodynamic modeling

output to help validate the chosen locations of subduction.

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Tilting continents: influence of dynamic topography on relative sea level

Supervisors: Nicolas Flament, Dietmar Müller

It has long been identified that continents tilt as they drift over the convecting

mantle. Recent work has shown that mantle convection makes it impossible to

determine global sea level at a single passive margin. The aim of this project is to

estimate the relative contributions of mantle convection and global sea level change

to the waxing and waning of continental interiors by shallow seas observed in the

geological record.

Continental dynamic topography at 101 Ma (left), and its rate of change (right)

between 111 and 101 Ma, both shown in the present-day frame of reference

The project will involve analysing the dynamic topography and its rate of change

predicted by global mantle flow models (example shown on figure) and comparing

them to geological constraints. This will require the use of analytical skills, basic

scripting (in shell, python or other) and the use of various software skills, including

GPLates, the Generic Mapping Tools. Part of a large industry collaboration, this

project will prepare students both for working in the exploration industry as well as

for a research-oriented career in government agencies or universities.

Tectonic evolution of the eastern Tethys

Supervisors: Dr Simon Williams, Dr Kara Matthews

The continental margins of the Tethys Ocean have undergone a largely

uninterrupted history of sedimentation and carbonate platform build-up over the

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last 500 Myr, accentuated by episodes of rift basin formation, broad subsidence and

major transgressions in the mid-late Cretaceous. While the contribution of mantle

flow to the flooding of other continents (e.g. Cretaceous North America) has long

been established, it is still unknown to what extent flooding in Eurasia at this time, or

any other time, is due to dynamic topography. Part of the problem is a lack of

detailed plate reconstructions within a deforming plate framework for the area to

use as surface boundary conditions into numerical models. In this project, you will

construct an end-member plate kinematic model for the opening and closure of the

Eastern Tethys Ocean, which will include a detailed history of microcontinent

accretion and back-arc basin formation along the southern Eurasian margin and

rifting and basin formation along the western Australian margin. These

reconstructions will be examined in the context of paleo-geographic maps and

compared to geodynamic model output to better resolve the continental flooding

history along the northern Tethyan margin.

3D Numerical Experiments of Salt Tectonics

Supervisors: A/Prof Patrice Rey, Luke Mondy and Dr Sascha Brune

Evaporite deposits exert a very strong control on the structural evolution of rift

basins. Because of its capacity to flow at upper crustal temperatures and under small

deviatoric stresses, salt allows for the mechanical decoupling of the post-salt

sedimentary sequences, which can slide above pre-salt strata. These gravitational

décollements produce significant extensional and contractional structures, as well as

salt diapirs and salt canopies often closely related to hydrocarbon traps. Through a

series of 3D numerical experiments, the project aims to understand how the

thickness, depth and viscosity of the salt layers control the style, distribution and

magnitude of deformation. This project doesn’t require any particular computational

skills and can suit an astute student interested to develop computational skills.

Landscape connectivity through the Wilson cycle: Implication for biodiversity

Supervisors: A/Prof Patrice Rey, Guillaume Duclaux and Luke Mondy

We know that plate tectonics, through continental break-up, is a major driver for the

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evolution of species. At the scale of each continent, landscape connectivity favour

natural selection processes, which impediment to biodiversity. In the contrary,

during orogenic periods, the fragmentation of landscape favour the multiplication of

ecological niche driving species differentiation and favouring biodiversity. This

pioneering project aims at investigating how tectonic processes coupled to surface

processes interplay to control landscape connectivity. The project will involve

studying how a fragmented landscape, made of multiple drainage basins, high

mountain ranges, elevated plateaux and deep valley, evolves under the action of

erosion, sediments transport and accumulation. If time permit, the project will also

investigate the evolution of a highly connected landscape during orogenesis.

What Happens to the Continental Crust Once it Has Been Subducted?

Supervisors: Patrice Rey, Christian Teyssier (Univ. of Minneapolis) and Donna

Whitney (Univ. of Minneapolis)

Continental subduction is the process through which the continental crust gets

dragged into a subduction zone burying the continental crust deep into the diamond

stability field (>150km). The subduction of the continental crust is however limited

by the buoyancy of the crust. It is not clear what happens to the continental crust

once the maximum depth is reached. Does the crustal slab melt and rise through the

overriding mantle wedge as a large crustal diaper that flattens out at the base of the

overriding crust, or does the slab get exhumed back to the surface as a rigid buoyant

slab? Also important is the question of how the growing gravitational force related

to the subducting continental crust impacts on the converging continents.

This project aims at exploring the processes controlling the relaxation of subducted

continental crusts. The project involves 2D and 3D thermo-mechanical experiments

using Underworld. The subduction of Baltica under Laurentia (440-390 Ma) will be

used as a prime example providing first order constraints on the timing of

continental subduction as well as the PTt paths followed by exhumed high-pressure

crustal eclogites.

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The role of gravitational forces in the opening of back-arc basins: application of the

opening of the Aegean Sea

Supervisors: Patrice Rey

Slab rollback, the oceanward motion of the subducting oceanic lithosphere, is a

favourite amongst the processes leading to back-arc extension and detachment of

micro-continents from continental mainlands. A different way to interpret the

dynamic of back-arc extension involves gravitational forces that arise from the

thinning of the lithospheric mantle. A recent set of numerical experiments show that

gravitational forces can drive back-arc extension forcing slab rollback. Coeval

extensional and contractional tectonics in the overriding plate is a key observable

that can help to decipher between rollback-driven back-arc extension and back-arc

driven slab rollback.

This project aims at exploring further the link between gravitational forces and back-

arc extension and to apply this new concept to the opening of the Tasman Sea, the

Aegean Sea and the evolution of the Appennines.

How far can the lower crust flow?

Supervisor: Patrice Rey

Gravitational potential energy stored in an orogenic plateau can be strong enough to

deform the surrounding region (foreland), hence contributing to both plateau

growth and collapse. Gravity-driven channel flow from the plateau lower crust into

the foreland lower crust, called channel extrusion, has been proposed as a main

contributor to the eastward growth of the Tibetan plateau, possibly driving the lower

crustal channel as far as 2000 km in 15 myr, at an average flow velocity over 10

cm/yr. However, isostasy-driven upward flow in response to either erosion focused

on the plateau steep margins, or stretching of the plateau upper crust to produce

domical structures (metamorphic core complexes), compete with horizontal channel

flow extrusion. Using 2D and 3D thermal-mechanical modelling, this project aims at

exploring the dynamic coupling among the various flow processes that take place

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during gravitational collapse and the assess the magnitude of channel extrusion in

southeast Tibet.

Multispectral Mapping the Opalized Redox Front in the Great Artesian Basin

Supervisor: Patrice Rey, Gemma Roberts

The vast majority of precious opal is hosted in the Great Artesian Basin (GAB) in

central Australia, one of the largest intra-continental basins on Earth. Opal formed

via acidic oxidative weathering following the regression of the Eromanga Sea that

flooded central Australia between 125 and 95 Ma. This opalized horizon is now

preserved below the Tertiary unconformity. Using multispectral satellite images this

project aims i/ to construct at the scale of the Great Artesian Basin a 3D map of the

unconformity between Tertiary sedimentary rocks and the Lower-Cretaceous

formations, and ii/ to map where the opalized redox front has been preserved and

where it has been eroded. This project will be appropriate for a student willing to

become an expert in satellite image processing, and 3D surface modelling.

Control of inherited fault patterns on rift evolution

Supervisors: Sascha Brune and Patrice Rey

The architecture of continental rift systems (e.g East Africa), and passive margins

(e.g. Africa-South America, Australia-Antarctica) is controlled by plate kinematics,

rock rheology, and volcanism. This research field is of special importance to the

hydrocarbon industry, since rift basins are primary target for exploration.

Here you have the opportunity to evaluate the impact of inherited fault networks on

the evolution of rift systems and subsequent passive margin formation. Using an

established geodynamic model setup, you will study the effect of random initial fault

patterns on key rift properties: symmetry, basin depth, and margin width. This will

require basic analytical skills and some computational competence for scripting and

model visualization.

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Rate of deformation for two rift models with different random initial fault patterns

but otherwise identical properties. This illustrates the impact of the initial fault

configuration on overall rift dynamics.

Sinking slabs: Deciphering the 200 My memory of Earth’s mantle

Supervisors: Sascha Brune, Nicolas Flament, and Dietmar Müller

At subduction zones, dense lithospheric slabs are pulled into the inside of the

Earth where they sink for hundreds of millions of years towards the core mantle

boundary. The present day structure of Earth’s mantle as it is imaged through

seismic tomography contains information on the locations of ancient subduction

zones and past plate motion. When deciphering this information, the movement of

slabs within the convecting mantle of the Earth must be taken into account.

However, fundamental physical properties of the deep Earth (e.g. the viscosity

distribution) are not easily accessible.

Within this project you will apply an established numerical model of mantle

convection. The aim is to evaluate the vertical and lateral velocities of subducted

slabs for different viscosity distributions of Earth’s mantle which will give new insight

into the viscosity distribution of the deep Earth. By comparing the numerical results

to seismic tomography images (see Figure below), you will establish a link between

past plate motions and present-day mantle structure. This will be important in order

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to enhance absolute plate motion models before 100 My.

You will be trained in using the plate tectonics software GPlates, the mantle

convection model Terra, standard visualization tools (GMT and Paraview) and basic

scripting. You will acquire valuable insight into geodynamic processes that are

ultimately responsible for the formation of georesources. This knowledge base might

make the difference when you apply for positions in exploration industry or if you

start a research career at a university.

Comparing predictions of different numerical model setups (red, blue, black contours)

to present-day mantle structure from seismic tomography (in background).


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