Gravity and Magnetic Signatures of Different Types of Spreading in the Atlantic: Characterisation of Ocean-Continent Transition

Magmatic accretion and tectonic extension have been recognised as the driving forces that forms the oceanic crust at mid-ocean ridges. At slow-spreading ridges, as the melt supply falls below a critical level, the plate separation is accommodated by long-lived detachment faulting rather than the typical magmatic accretion. The detachment fault accommodates exhumation of lower-crust and upper-mantle rocks to the ocean floor, forming domed structures known as Oceanic Core Complexes (OCCs). These domed structures are commonly found at one side of the spreading axis, indicating the occurrence of asymmetric spreading, as opposed to the symmetric fault-bounded abyssal hills commonly found over magmatic crust. Meanwhile, parts of the ultra-slow-spreading ridges are completely devoid of magmatism, where large detachment faults form continually at both axis flanks to facilitate the plate separation. At passive continental margins, these crustal morphologies are not recognisable by shipboard multibeam bathymetry data, as they have been buried by sediments deposited from the continental crust. Hence, this study aims to classify the types of oceanic crust based on the gravity and magnetic characteristics observed over the spreading axis by: (1) characterising the different types of spreading by quantifying parameters observed in shipboard multibeam bathymetry of an active slow-spreading ridge; (2) assessing and improving established gravity and magnetic data enhancement techniques to characterise and classify crustal types of a slow-spreading ridge, and; (3) applying the assessed enhancement techniques to the available gravity and magnetic data over a passive continental margin.

A novel automatic terrain classification technique, namely the slope-weighted eccentricity (SWE) is established based on the parameterisation of the shape, directionality, and curvature of the ocean floor where shipboard multibeam bathymetry data has been made available. Investigation by means of gravity and magnetic anomalies are also conducted to investigate spreading evolution and the crustal thickness variation. Crustal thickness is computed from the isostatic mantle Bouguer anomaly (IMBA), a type of gravity anomaly developed in this study by removing the gravity effects observed within the water-crust and crust-mantle interfaces. The evolution of the alternating spreading modes is then identified by comparing the SWE number and computed crustal thickness over time through the interpreted magnetic chrons. The crustal thickness computation a well as a number of existing gravity and magnetic data enhancement techniques are also applied to a larger set of data over the Labrador Basin, where a composite of field magnetic surveys is made available. Consistent with the recognised characteristics of ultra-slow-spreading ridge morphology, a significant area of thin crust is identified across the basin, where upper-mantle rocks are likely to be exhumed through large detachment faulting and went through a high degree of serpentinisation.

This thesis has contributed to the establishment of a new grid-based interpretation technique that is tested and ready to be applied to shipboard multibeam bathymetry at various areas, as well as testing and applying several existing gravity and magnetic interpretation techniques to identify and characterise crustal structures.