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Nature of the Oceanic Lithosphere across the Equatorial Fracture Zones in the Atlantic Ocean


IPGP - Îlot Cuvier


Soutenances de thèses

salle Outremer

Fares Mehouachi

Géosciences marines (LGM)

The idea of a floating rigid Lithosphere above a ductile convective Astenosphere is a major concept in plate tectonics and defines one of the most extensive boundaries in the planet: the Lithosphere- Astenosphere Boundary (LAB). Imaging the LAB has been successfully achieved by surface tomography, receiver function and electric methods but the lack of resolution is detrimental to fully understand the location and the nature of this boundary. Here we use seismic reflection method to image the LAB: a 12 km multi-component streamer at 30 m water depth is used in conjunction with large volume air guns to produce low frequency energy capable of reaching targets up to a 100 km depth while preserving an acceptable signal to noise ratio. The LAB was initially imaged around St. Paul Fracture Zone (FZ) using a seismic processing strategy heavily focused on poststack signal enhancement. The St. Paul seismic image reveals a melt-rich sub-lithospheric low velocity channel that deepens from 72 km at 40 Myr to 88 km at 70 Myr lithospheric age and thins from 18 to 12 km respectively, while exhibiting a P-wave velocity drop of ~8.5±0.5% . The upper and lower bounds of the channel structure could be fitted with isotherms from the plate cooling model at values approximately centered on the typical value of 1300 °C classically associated with the LAB. The LAB channel has a low viscosity that would allow the mechanical decoupling between the lithosphere and the asthenosphere, yet the LAB would require a large amount of volatiles, mainly water, to preserve its sub-solidus melt content (1.4±0.8%). The total amount of water trapped inside the LAB channel is very large (>>100 ppm), yet it is surprisingly age independent despite the depth and thickness variation which suggests that a horizontal flux of volatiles originating probably from the ridge is feeding the LAB with its water content that gets focused and preserved inside the thinning LAB channel. The LAB was also potentially imaged across the Mid Atlantic Ridge (MAR) area, near the Chain transform fault using a state of the art imaging method, named OC-DMO and developed specifically to handle dipping structure in low SNR (Signal to Noise Ratio) environments. Plate boundaries and particularly Transform Faults (TF) are a unique setting in which the three dimensional temperature profile could greatly affect the depth and the location of the LAB. A relatively shallow single reflector is reported underneath the MAR which deepens on both sides of the Ridge within an age range from 0 to 6 Myr but seems to flatten in the area closest to the Chain TF at ~20 km depth from the sea-surface at an age ranging from 6 to 18 Myr. This flattening stops around the edge of the TF and a rapid increase of depth is observed from 20 km at 18 Myr to 45 km at 22 Myr. The flattening cannot be explained by the thermal effect induced by the TF, which suggests the presence of different mechanism at play and potentially a lithospheric age anomaly. This age anomaly hypothesis is further confirmed by the lack of seafloor subsidence in the same area where the flattening is occurring. The lithosphere around the MAR near the Chain TF cannot be considered as a normal oceanic lithosphere, yet seems to confirm the correlation between the depth variation of the upper bound of the LAB and the lithospheric age, already observed in St. Paul area. Multichannel, controlled source, seismic reflection imaging provides original insights on the depth, location and characteristics of the LAB, yet understanding this boundary remains still elusive and would require extensive seismic profiling to decipher its true nature.