This thesis combines a range of petrological, geochemical, and geophysical techniques to investigate the properties, behaviour, and evolution of the Precambrian lithosphere.

In Chapter One, I summarise some of the key open questions regarding lithospheric evolution and the development of strength contrasts, and how they will be addressed in this thesis.

Chapter Two investigates the feasibility of ‘sagduction’, which is a tectonic process that has been proposed to occur during Earth’s early history. I focus on a case-study from north-west Scotland, a region where Archean sagduction has been suggested. I integrate results from fieldwork, phase equilibria modelling, and numerical modelling to show that sagduction was unlikely to be feasible. Furthermore, I develop a method that allows the feasibility of sagduction to be assessed at other sites. I conclude that sagduction is unlikely to be an important Archean tectonic process.

Chapters Three and Four focus on understanding how strength contrasts develop and persist in the lithosphere. I consider a region in England and Wales, known as the ‘Midlands Microcraton’. The microcraton formed in the late Precambrian and has behaved as a relatively strong block compared to its surroundings, displaying ‘craton-like’ properties.

In Chapter Three, I integrate results from fieldwork, phase equilibria modelling, and geochronology. I demonstrate that the microcraton experienced a syn- to post- subduction metamorphic event. Using geotherm modelling, I infer that this event led to partial melting in the mid-to-lower crust, resulting in the development of the microcraton as a strong region.

In Chapter Four, I compare the characteristics of the mantle on and off the microcraton using mantle xenoliths. I find that England and Wales are underlain by non-cratonic lithosphere that records a complex history of melting and metasomatic processes, confirming the untypical mode of formation of the strong crust proposed in Chapter Three. I then use the Phanerozoic deformation record to show that the microcraton has persisted as an (anisotropic) strong region throughout the Phanerozoic, and has exerted a strong influence on Britain’s geological evolution throughout this period.

Although focused on Britain, the results in this thesis provide insights into the general concepts that control the rheology, deformation, and evolution of the lithosphere.