Periodic Reporting for period 1 - THIN (Lithosphere THINning During Subduction Initiation)
Reporting period: 2020-09-01 to 2022-08-31
The formation of new subduction zones is a fundamental process of the solid Earth that controls the motion of the tectonic plates, and therefore the formation of volcanoes and earthquakes around the globe. Despite its importance, this process is still poorly understood. The THIN project, as originally submitted in September 2020, aimed at studying the deformation in the mantle section of ophiolites to reconstruct the nature, style, and kinematics of the tectonic processes associated with the formation of a new subduction zone. This main aim, was achievable by using a set of techniques such as: paleomagnetism, magnetic fabric, field structural geology, and an innovative magnetite geochronology method.
Having the project started soon after the outbreak of the COVID-19 pandemic, the primary fieldwork in the Sultanate of Oman could not be carried out due to travel restrictions imposed by the Oman authorities to non-residents. Because of this, the original project was substantially revised in terms of main objectives and study area. The alternative project that have been carried out in the end, addressed another key question of global plate tectonics: what happens when a spreading ridge ceases its activity? The revised THIN project aimed at studying the deformation of the upper mantle and lower crustal section of the Troodos Ophiolite of Cyprus to understand the tectonic processes associated with the demise of a spreading ridge.
The topics investigated by the revised THIN project are relevant to the society, as ore deposits are typically extracted from ophiolites for economic exploitation. Ore deposits, primarily copper and chrome, are formed in the oceanic crust and mantle, respectively, during the accretion of new crust at a spreading ridge. Ophiolites are pieces of oceanic lithosphere that have been uplifted above sea level. Hence, understanding how a spreading ridge preserved in an ophiolite like Troodos ends its activity and is eventually disrupted by tectonic processes is fundamental to predict if, where, and how much ore deposits were formed at that specific spreading ridge.
Having the project started soon after the outbreak of the COVID-19 pandemic, the primary fieldwork in the Sultanate of Oman could not be carried out due to travel restrictions imposed by the Oman authorities to non-residents. Because of this, the original project was substantially revised in terms of main objectives and study area. The alternative project that have been carried out in the end, addressed another key question of global plate tectonics: what happens when a spreading ridge ceases its activity? The revised THIN project aimed at studying the deformation of the upper mantle and lower crustal section of the Troodos Ophiolite of Cyprus to understand the tectonic processes associated with the demise of a spreading ridge.
The topics investigated by the revised THIN project are relevant to the society, as ore deposits are typically extracted from ophiolites for economic exploitation. Ore deposits, primarily copper and chrome, are formed in the oceanic crust and mantle, respectively, during the accretion of new crust at a spreading ridge. Ophiolites are pieces of oceanic lithosphere that have been uplifted above sea level. Hence, understanding how a spreading ridge preserved in an ophiolite like Troodos ends its activity and is eventually disrupted by tectonic processes is fundamental to predict if, where, and how much ore deposits were formed at that specific spreading ridge.
Two field trips were carried out in the Troodos Ophiolite in November 2020 and September 2021, and rock samples were collected from both the mantle (peridotite) and the lower crust (gabbros) of the ophiolite at 21 sites. Paleomagnetic and rock magnetic analyses were performed on these samples, and the results showed a series of tectonic rotations that have been interpreted as related to the demise of the Solea fossil spreading ridge preserved in the Troodos Ophiolite. Magnetic fabric analyses were also performed on gabbros samples to help restoring these rocks to their original position and to calculate more accurately the paleomagnetically-detected rotations. This allowed us to reconstruct the style and kinematics of the tectonic processes operating when a spreading ridge ceases its activity. In particular, we inferred that during the end of magmatic activity at a spreading ridge, the region immediately adjacent Ito the spreading axis, missing its dynamic support from hot molten mantle underneath, collapses while the overall plate divergence is slowing down but still active. This results into a set of historic faults that thin and dismember the lower crustal and upper mantle section of the ophiolite. These results have been presented so far at the European Geoscience Union (EGU) General Assembly in Vienna in May 2022 (poster presentation), and at a local conference in Newfoundland (Canada) in July 2022 (oral presentation). We are now writing two papers containing the results of the THIN project, one about the magnetic fabric in the gabbros, and one about the tectonic rotations. We aim to publish these two papers of open access journals to maximise their impact on the scientific community. As soon as these manuscripts have been submitted we will also upload the main datasets to open access repositories (figshare).
The results of the THIN project have triggered a number of novel ideas regarding the final stage of the evolution of a spreading ridge, and those ideas has been applied more recently on a new study developed by the project supervisor in the Bey of Islands ophiolite (Canada). In this respect, the results of the THIN project go well beyond its specific aims, as they will help the scientific community to understand better a process that has only been speculated to exist so far. The THIN project, in fact, provided the quantitative proof for the existence of tectonic dismemberment during the demise of a spreading ridge. This understanding can be employed within the broader field of economic geology. In fact, understanding how a oceanic spreading ridge ends its magmatic activity, will inform on what, where and how much economically exploitable ore deposits (mainly copper and chrome) may be present at a specific ophiolite (i.e. relics of ancient oceanic lithosphere and spreading ridges exposed above sea level).