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Illuminating Ultra-Slow Spreading Centres: a seismic approach to the Cayman Trough

Periodic Reporting for period 1 - CAYMAN (Illuminating Ultra-Slow Spreading Centres: a seismic approach to the Cayman Trough)

Reporting period: 2019-09-01 to 2021-08-31

About 25% of the Earth’s mid-ocean ridges spread at ultraslow rates of less than 20 mm/yr. However, most of these ultraslow spreading ridges are located in geographically remote areas, which hamper investigation. Consequently, how the crust forms and ages at such spreading centres, which traditional models predict to be magma-starved and cold, remains poorly understood. Recent studies of the ultraslow Mid-Cayman Spreading Centre (MCSC) in the Caribbean Sea, have observed the deepest and hottest black smoker hydrothermal systems on Earth, and off-axis medium-temperature venting with exhumed lower crustal and upper mantle and volcanic lithologies juxtaposed by detachment-style faulting. In CAYMAN we will establish the lithospheric context of these observations that contradict the predictions of traditional models, and test recently developed models of oceanic crustal formation at the slowest of spreading rates. We will use for the first time closely-spaced stations for a high-resolution subsurface mapping of seismic velocities to study the temporal and spatial interplay between magmatic accretion and magmatic tectonic extension, and the controls on and relationship between faulting and hydrothermal activity. The project scope is centred on the growing body of evidence that crust accreted at slower spreading rates does not form by a simple process of symmetric, magmatic accretion as traditional models predict. Instead tectonic extension accommodated by large-offset detachment faults, along which water percolates and by which the lower crust and the serpentined upper mantle are exhumed, appears to play a significant role. CAYMAN is an innovative project which results provided the first 3D image of the crust of an ultra-slow spreading centre.

The main objectives of CAYMAN project are:
O1) To test the hypothesis that properties should change significantly from the centre of the Mid-Cayman spreading axis towards the segment ends, using the seismic crustal structure to characterise the mantle melting process along the MCSC and estimate the relative importance of mantle temperature, upwelling rate and magma plumbing for crustal accretion.
O2) To test widely discussed hypotheses and their alternatives for the OCCs formation –Cannat et al. [2006] end-members– mapping crustal velocity to characterise the physical properties and structure of the crust and uppermost mantle over the MCSC.
O3) To characterise phases of predominantly magmatic accretion versus phases of amagmatic accretion, by means of temporal variations in crustal thickness and spreading rate.
In order to scrutinise the structures of interest and achieve CAYMAN objectives, I have worked with active-source Wide-Angle seismic data from CAYSEIS project, including P01, P02 and P03 profiles and offline stations along the MCSC (Fig. 1). The introduction of the offline stations provided a 3D model of the MCSC ridge to determine the along-axis variability in the structure and thickness of the crust accreted at the MCSC (O1). This dataset is also designed to distinguish between the different models of OCC formation by imaging the structure of the crust at Mt. Dent (Fig. 1), determining any asymmetry between the western and eastern ridge flanks, and determining off-axis continuity and any periodicity in OCC formation (O2). High-resolution images of the crust are used to determine the interplay between tectonic and magmatic construction processes (O3). A high-resolution image is fundamental to characterise phases of magmatic accretion, due to the limit size expected for the magmatic intrusions in this magma-starved setting.
The main results achieved during CAYMAN are 1) a 3D model of the crust and mantle structure on the Cayman Trough area and 2) a high-resolution image of the crust along the MCSC axis. These results have been achieved using state-of-the-art tomographic methods, including 2D and 3D tomographic inversion.
The first results of the 3D image of the crust have been presented at national and international conferences. The high-resolution image obtained along the MCSC has been used to relocate the seismicity in the area, and contribute to a broader analysis of the seismogenic structure of ultra-slow spreading centres worldwide. The innovative methodology applied (3D tomographic inversion of real datasets), the further analysis of the 3D image of the crust and the insights of the seismogenic structure of ultra-slow spreading regions will have an impact on the scientific community.
During CAYMAN, I have learnt and implemented a bleeding-edge technique barely used: the 3D tomographic inversion of real data. This is the first time that the tomo3D code is used to perform the tomographic inversion of real datasets. Being trained in this methodology suppose a quality step for my career, as it is a unique skill. Together with my previous knowledge about Multichannel seismic data, these skills configure an excellent geophysical profile.
CAYMAN study area. Black lines: WAS profiles, white and green dots: OBH/S, red dots: earthquakes