Oceanic lithosphere is continuously generated at mid-ocean ridges and destroyed at subduction zones. Both mid-ocean ridges and subduction zones are essential components of the plate tectonic cycle, without which all tectonic, magmatic, and associated surface processes would not exist. Subduction is therefore a fundamental geodynamic process that has shaped our planet throughout geological times, but scientists still know very little about it. This is due to the lack of modern examples of subduction initiation, and the logistical problems associated with accessing the locations where subduction zones formed (typically within deep ocean basins).
An alternative way to study subduction initiation is by analysing rocks that formed above an emerging subduction zone and, for this reason, were able to record the tectonic and magmatic effects of subduction initiation. Those rocks, known as ophiolites, are today exposed in well-known regions. The Semail ophiolite in Oman is the most intact and best exposed ophiolite on Earth, which offers the opportunity to investigate subduction initiation processes in great detail.
This project aims to examine the most problematic aspect of subduction initiation, which is the deformation induced by this process within the upper plate (i.e., the plate that is above the down-going plate). According to previous studies (van Hinsbergen et al., 2015), during the formation of a new subduction zone, oceanic lithosphere in the upper plate may attenuate by as much as 30 km, indicating either extensive stretching or other processes able to remove a substantial portion of the upper plate lithospheric mantle. Whether the upper plate is experiencing thinning during subduction initiation, and if this is the case, whether this is occurring by tectonic extension or mechanical removal of the lithospheric mantle remains unknown. Ophiolites may therefore hold the key to answer these questions.
The aim of this project is to analyse using both magnetic and structural geological methods the deformation of the lowermost section of the Oman ophiolite (i.e., mantle rocks) to determine if, where, and how the ophiolite (i.e., the upper plate) experienced thinning. This project will therefore involve field work, sampling, and lab work (described in more detail in the next section).
Methodology:
This project will first involve fieldwork in the Oman ophiolite, where the PhD candidate will carry out sampling and structural geological observations within the mantle section at several localities. Sampling and field observations will be carried out throughout the 7-km thick mantle section of the Oman ophiolite. The collected oriented samples will be cut into standard paleomagnetic specimens and then the internal fabric (i.e., the preferred alignment of minerals) will be measured using various rock magnetic techniques, including the in-phase/out-of-phase anisotropy of magnetic susceptibility (AMS), the anisotropy of anhysteretic remanent magnetization (AARM), and electron backscattered diffraction (EBSD). These rock magnetic analyses, in combination with the field structural geological data will reveal variations in deformation patterns throughout the mantle section of the Oman ophiolite.
Training and skills:
Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.
The PhD candidate will be trained on various aspects of the project from paleomagnetic sampling and structural geological measurements in the field, to rock magnetic analyses in the paleomagnetic laboratory. Rock magnetic analyses include the measurement of AMS and AARM using appropriate instruments (kappabridges and magnetometers). Magnetic mineralogy characterisation is another component of the project for which the PhD candidate will receive adequate training by the supervisors. EBSD analyses may be carried out in collaboration with external partners or entirely performed by external laboratories.
Partners and collaboration (including CASE):
Collaboration with external partners for EBSD analysis will need to be established. Alternatively, EBSD analyses will be performed by external laboratories.
COVID-19 Resilience of the Project:
If there is a complete lockdown then fieldwork cannot proceed and we will have to consider how much theoretical background reading is worthwhile. Depending on the nature of any covid related restrictions, the project field programme can adapt to work locally (i.e., within the Lizard ophiolite in Cornwall) and accommodate social distancing, hygiene, avoid public transport and wearing of face covering. Fieldwork otherwise does not require close working and so can proceed under most restrictions provided training can be accomplished. Most training can be done remotely and if training is curtailed we can make use of remote field supervision using remote mobile wifi.
Possible timeline:
Year 1: Background theory, paleomagnetic theory, fieldwork planning, field sampling training, initial field campaign, preliminary magnetic analyses.
Year 2: Magnetic analyses and characterisation experiment design, second fieldwork (if needed), structural modelling and analyses.
Year 3: Completion of magnetic analyses and consolidation of magnetic properties (publication on this), EBSD analyses.
Please email potential supervisor Dr Maffione ([Email Address Removed]) for more information.
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