Did the Early Earth have plate tectonics?Dr Alex Webb - University of Hong Kong (CASE)Contact email: firstname.lastname@example.org
This exciting, interdisciplinary project aims to take a new look at an old problem: When did Plate tectonics as we know it start on Earth? Answering this question has significant impact on how we interpret the evolution of our and other planets. In this project you will be part of an international research team applying modern state-of the art techniques of metamorphic petrology, structural geology, geochemistry and petrochronology to decipher the workings of Earth using field data from unique outcrops and sample suites.
The dominant tectonic processes of today’s Earth are understood within plate tectonic theory, but the early processes by which Earth’s outer margin transformed from a magma ocean to a plate tectonic planet remain uncertain. One of the first-order questions that have yet to be settled is ‘when did plate tectonics start’? Without this knowledge, it is difficult to evaluate solid Earth interactions with the early atmosphere, hydrosphere, and the origin and evolution of life.
Current models for Earth’s earliest tectonics involve plate tectonic and vertical tectonic concepts. Vertical tectonic models have involved volcanism, sub- and intra-lithospheric diapirism through gravitational instability, and delamination, and may explain the observed geology of the best-preserved low-grade ancient terranes, such as the Paleoarchean Barberton and Pilbara greenstone belts (Van Kranendonk et al. 2007). Plate tectonic models readily account for relative movement of separate terranes with diverse lithologies producing characteristic geochemical signatures and features of deformation. The Eoarchean Isua supracrustal belt of West Greenland (~3.8 Ga) is at present the first known example of such a collision zone that significantly pre-dates rifting and subduction episodes at ~3.2 Ga (Nutman et al. 2007). The interpretation of Isua as a subduction zone is based on geochemical data and structural mapping indicating accretionary duplex development and collision of 3.8 Ga and 3.7 Ga terranes (Nutman & Friend 2009; Komiya et al. 1999). Nonetheless, the restricted range of lithologies and deformation styles observed at Isua matches those seen at early Earth terranes thought to be dominated by volcanism and vertical tectonic processes (Moore & Webb 2013). So, could the plate tectonic interpretation be non-unique? If so, the first third of Earth history could be pre-plate tectonic.
Metamorphic petrology and geochemistry in combination with structural geology and geochronology, both straightforward and innovative, can readily test this hypothesis. Subduction zones are generally accompanied by metamorphic characteristics such as paired belts or metamorphic gradients and hence, establishing the metamorphic history of the few ‘glimpses” of Archean rocks we have is the key to establish whether these areas are related to plate or vertical tectonics. Surprisingly, detailed knowledge of the metamorphic history of the supracrustal belt of Isua is close to absent (Arai et al. 2014; Hayashi et al. 2000; Rollinson 2003).
This project has a headstart: during a 4-week field campaign in SW Greenland in summer 2017 we systematically mapped and sampled the famous Isua greenstone belt. Interestingly, field observations and comparison to Pilbara data suggests that the Isua greenstone belt might be plausibly formed by vertical tectonics, in contrast to the dominant interpretation of it being the first terrane to record evidence of plate tectonics. Our group is now one of very few groups in the world (<5) in possession of a unique and systematic sample collection of the ISB (located at both HKU and UoL) (Figures 1, 2).
Fig. 1 – Upper left: volcanoclastic sediments exposing compositional alternation of felsic and mafic (grt-amph±ky) layers metamorphosed to amphibolite facies conditions. Upper right: garnet porphyroplasts with pyroxene/amphibole corona textures. Lower left: strongly sheared pillow basalts. Lower right: felsic magma intruding mafic enclaves causing a partial replacement reaction to form hornblendites
Fig. 2 - Geological map of the Isua greenstone belt with sample locations from recent field campaign in summer 2017.
With this extensive set of new rock samples, it is now possible to spatially and temporally study the tectono-metamorphic history of the Isua greenstone belt to answer the following questions:
- What are the metamorphic conditions and are there spatial gradients within the belt and through time?
- How many metamorphic events can be distinguished and related to specific deformation events?
- Is it possible to couple timescales of metamorphic processes to deformation processes?
- Based on results from 1-3, was plate tectonics as we know it today active 3.8 Ga ago? Are other models likewise viable?
A broad range of analytical, and theoretical tools are being used to investigate these issues. Such records can illuminate surface compositional evolution and thereby test and develop tectono-metamorphic models applicable to the early Earth. The advent of increasingly powerful analytical tools and methods continue to offer new opportunities to learn more about this crucial, poorly understood period in Earth history.
In this project, you will work with leading scientists at the University of Leeds and the University of Hong Kong to interrogate tectono-metamorphic rock samples using integrated field-based and analytical approaches and thereby establish new understandings of early Earth tectonics, metamorphism and surface evolution. You will present your findings in leading journals, and at internal, national and international research conferences. To facilitate this work, samples systematically covering all major lithologies over the exposure of the Isua belt have been collected. Additional field-based investigation and sampling will be conducted at Pilbara (Western Australia) and/or the Isua supracrustal belt (this possibility is pending additional funding). According to your particular research interests, the studentship could involve
- Textural investigation using optical and high-end electron microscopy to constrain metamorphic reaction and their relative timing to deformation events as well as strain analysis.
- Equilibrium phase petrology calculations on constrained bulk rock compositions to relate metamorphic assemblages to chemical protolith variations.
- Conducting mineral thermobarometry to track spatial distribution of metamorphic conditions.
- Dating metamorphic minerals using various minerals and whole rocks (e.g. Sm-Nd in garnet) to relate metamorphic conditions to absolute time and compare these results with existing U-Pb age data from zircon, monazite, rutile and titanite.
- Develop diffusion models to extract timescales of cooling rates of various parts of the belt from observed mineral zonation patterns and use these results to test tectonic models of plate vs. vertical tectonics.
- In depth microstructural analysis focussing on high strain, high fluid-flux zones.
- Depending on the interest of the student, it is also possible to link results with large scale numerical models and thereby test tectonic concepts developed throughout the project.
Potential for high impact outcome
Our limited understanding of early mantle and crustal evolution represents one of the most significant problems in Earth science research. Innovative, integrated research approaches have routinely generated important results in this field over the past decade. The research plan involves testing key tectonic models and generating new data types for this field, so we anticipate the project generating several papers for submission to high impact interdisciplinary and disciplinary journals.
The student will work under the supervision of Dr Thomas Mueller, Dr Sandra Piazolo and Dr Jason Harvey within the Institute of Geophysics and Tectonics as well Dr Alex Webb at the University of Hong Kong and support by Dr Julie Hollis at the Greenland Ministry of Mineral Resources. This project provides a high level of specialist scientific training in: (i) phase petrology (ii) cutting-edge geochemical / geochronological analytical approaches (iii) diffusion chronometry (iv) microstructural analysis (v) integrated tectono-metamorphic reconstruction. Co-supervision will involve regular meetings between all partners, use of Leeds analytical facilities including the state of the art TIMS, FE-SEM, EPMA, Cohen laboratories and extended visits for the student to the University of Hong Kong and/or the NERC Isotope Geosciences Laboratory. This project is part of a larger project involving all partners and is currently including 1 PhD student and 1 postdoctoral fellow with anticipation of expanding the research team in the next 1-3 years. Therefore, the student will also be trained in working in a research team. The successful PhD student will have access to a broad spectrum of training workshops run by the Faculty, including numerical modelling, managing your degree, and preparing for your viva.
The University of Hong Kong will be supporting the project as CASE partner. Dr Alex Webb will be co-supervising the student. You will spend at least 3 months of the project at the University of Hong Kong and will have access to the local facilities. This project is closely linked to ongoing and proposed research projects at HKU enabling an effective knowledge and data exchange to optimize the outcomes of this project.
The student should have a minimum of an upper 2:1 degree (or international equivalent) and strong interest in field/analytical work as well as metamorphic, geochemical and tectonic processes. A strong background in quantitative science (maths, physics and chemistry) and curiosity for interdisciplinary research is desired. Willingness to work within a research team is essential.
Arai, T. et al., 2014. Intermediate P/T-type regional metamorphism of the Isua Supracrustal Belt, southern west Greenland: The oldest Pacific-type orogenic belt? Tectonophysics, 662, pp.22–39.
Hayashi, M. et al., 2000. Archean Regional Metamorphism of the Isua Supracrustal Belt, Southern West Greenland: Implications for a Driving Force for Archean Plate Tectonics,
Komiya, T. et al., 1999. Plate Tectonics at 3 . 8 – 3 . 7 Ga : Field Evidence from the Isua Accretionary Complex , Southern West Greenland. Chicago journals, 107(5), pp.515–554.
Van Kranendonk, M.J. et al., 2007. Review: Secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia. Terra Nova, 19(1), pp.1–38.
Moore, W.B. & Webb, A.A.G., 2013. Heat-pipe Earth. Nature, 501(7468), pp.501–505.
Nutman, A.P. et al., 2007. ∼3,850 Ma tonalites in the Nuuk region, Greenland: Geochemistry and their reworking within an Eoarchaean gneiss complex. Contributions to Mineralogy and Petrology, 154(4), pp.385–408.
Nutman, A.P. & Friend, C.R.L., 2009. New 1:20,000 scale geological maps, synthesis and history of investigation of the Isua supracrustal belt and adjacent orthogneisses, southern West Greenland: A glimpse of Eoarchaean crust formation and orogeny. Precambrian Research, 172(3–4), pp.189–211.
Rollinson, H., 2003. Metamorphic history suggested by garnet-growth chronologies in the Isua Greenstone Belt, West Greenland. Precambrian Research, 126(3–4), pp.181–196.
Related undergraduate subjects:
- Earth science
- Earth system science
- Geological science
- Natural sciences