Skip to main content

Exhumation mechanisms of deep crust and formation of metamorphic domes in orogens: a Synergy between naturalistic and numerical approaches

Periodic Reporting for period 2 - DOMES (Exhumation mechanisms of deep crust and formation of metamorphic domes in orogens: a Synergybetween naturalistic and numerical approaches)

Reporting period: 2020-08-01 to 2021-07-31

• What is the problem/issue being addressed?
The main issue addressed by this project is i) to understand the geology and exhumation history of metamorphic domes in order to give fundamental insights on the thermo- mechanical behavior of the deep crust and its relationship with the upper crust, and ii) decipher the behavior of this partially molten crust as it is fundamental to understand the concentration of incompatible elements in the accessible crust, including critical minerals, whose significance to our modern economies is growing.

• Why is it important for society?
As the world shifts away from fossil fuels, we will need to produce enormous numbers of wind turbines, solar panels, electric vehicles and batteries. Demand for the raw materials needed to build them will skyrocket. This includes common industrial metals such as steel and copper, but also less familiar minerals such as the lithium used in rechargeable batteries and the rare earth elements used in the powerful magnets required by wind turbines and electric cars. Production of many of these critical minerals has grow enormously over the past decade with no sign of slowing down.
Understanding the thermal and mechanical behavior of the continental crust, the large-scale geodynamic processes that make the crust accessible and the mineral-scale processes that concentrate metals is essential to locate and explore for the key mineral resources that will fuel our emerging low-carbon societies.

• What are the overall objectives?

Objectives 1 and 2 of the project are dedicated to improving the knowledge of natural metamorphic domes in which the deep crust is brought to the Earth’s surface. We explore the geometries of metamorphic domes (for example the Entia dome in Centra Australia) as well as the pressure-temperature-time history of particles that prone to record the tectono-metamorphic evolution of domes.
Objectives 3 and 4 of the project give fundamental insights on the thermo- mechanical behaviour of the deep crust through inversion of field data and thermo-mechanical 2D numerical modelling.
The main conclusion of the numerical experiments performed highlights the role of metamorphic reactions (like granulitisation and partial melting) in controlling the long-term stability of crustal roots as well as the formation and preservation of high to ultra-high temperature terranes. In addition, the economic potential in Rare Earth Elements that may be located in these pegmatite-bearing terranes is reinforced as well.
Objectives 1 and 2 are met with the completion of WP1/M1 and WP2/M2. It has been partly met in the outgoing phase as field work took place as planned in the early stages of the project (month 6). Exploitation of the collected dataset Nr 4 (incl. 62 rock samples) generated during a field trip from 11 to 25 July 2019 in the Entia gneiss dome (ED) has been performed in the incoming phase (month 20 to 30). The petrological and geochemical results highlight the presence of four types of pegmatites with various potential in Rare Earth Elements and have been presented to the most popular Geochemistry Conference (Goldschmidt Virtual, 5-9/07/21).

Objectives 3 and 4 have been fully met by an excellent progression in WP3/M3. The researcher has delivered three datasets. Dataset 1 is a first series of forty-five 2D fully-coupled thermo-mechanical models that have been performed from February to May 2019 with Underworld (www.underworldcode.org). Dataset 1 has been published in Geology, the highest impact-factor journal in her discipline. Dataset 2 consists in a second series of hundred and seven 2D fully-coupled thermo-mechanical models have been performed from September 2019 to July 2021with Underworld (www.underworldcode.org). Dataset 2 is under review in the high impact factor journal Geology. Data set 3 is a Python post-processing script written by the researcher between May 2019 and June 2021. It can be used for extracting Pressure-temperature-paths from numerical experiments. Dataset 3 belongs to Work-package 3 (WP3) “decipher the major thermo-mechanical parameters operating during dome formation” and is essential to compare predicted particle paths with those extracted from natural rocks (as used in the manuscript under review with Dataset 2). These numerical experiments highlight the role of metamorphic reactions in controlling the long-term stability of crustal roots as well as the formation and preservation of high to ultra-high temperature terranes. In addition, the economic potential in Rare Earth Elements that may be located in these pegmatite-bearing terranes is reinforced as well.
In addition, during the incoming phase, the transfer of knowledge from Sydney University to Montpellier University has been a success with one article in the Géo-Infos Magazine, one technical presentation, one scientific presentation and a final two day workshop (17-18 June 2021) that has been very well perceived by the participating colleagues. Finally, we have installed the open source code I have learnt to use in Sydney (www.underworldcode.org) on the regional supercomputer (MESO@LR https://meso-lr.umontpellier.fr). This novel, innovative and open source numerical tool is now available to any colleague interested at Montpellier University.
Dr Cenki-Tok’s contribution to understanding the thermo-mechanical state of the Earth’s crust has been significant as attested by the high impact journal she has published her results, within the first year of the project. And she is confident that, with the current dataset she is working on, she will publish at least another high impact article (currently in review for Geology).

The project has a major impact on the researcher ability to disseminate her results and ideas to a larger audience. In 30 months, the researcher has published 5 peer-reviewed scientific articles (including 2 in Geology, highest impact factor journal in her field) and 6 additional are still under revision. She has given 23 presentations (incl. 6 invited and 5 keynotes) in workshops, conferences and University seminars and has contributed to 11 outreach activities for the general public. No website has been developed for the project.

Finally, this project has had a significant impact on the researcher’s career during the lifespan of the project and in the future as well as on the researcher’s positioning and visibility in her field. As an example, she has been promoted (in Sept 2020) at a national level to “Maître de Conférences Hors Classe” (only the top 7% of academics were promoted at a national level in 2020). The researcher will be in excellent position to apply for an ERC Advanced Grant in the near future.

An unexpected scientific outcome to this project is that the researcher has managed, thanks to the excellent mentoring at Sydney Uni, to bridge the gap between small- and large-scale disciplines that are inherent to her discipline. For example, based on Dataset 1 and 2 (large scale geodynamic dataset of numerical models aiming at understanding the thermal evolution of the crust), she is preparing an additional dataset of numerical models aiming at unravelling the formation of rare-metal-bearing pegmatites, that will be directly confronted to outcomes of Dataset 4 (rocks samples collected in the field at month 6). Unlocking this issue will be a major advance in exploring for the Rare Earth Elements (REE) essential to the magnets powering the new technologies.
Crustal thickness heterogeneities can be explained by the presence of strong garnet-pyroxene rocks