Geophysics, Geodynamics and Tectonics.
Continental Tectonics - From Fault Breccias to Collision Zones
Charting how and why the continents deform is fundamental to understanding the Earth. The geometries of deformation structures strongly influence where petroleum is stored in the Earth and where mineral deposits form. Traditional field-based observation of these structures remains invaluable in evolving new ideas and testing existing hypotheses. Field observation on scales from centimetres to kilometres can be extended downwards by lab-based microscopy and upwards by regional comparisons. Examples of recent projects on several different scales are given below
- Reassessment of the timing and field relations of the Acadian deformation, 400 million years ago, usually attributed to the closure of the Iapetus Ocean between England and Scotland. We have shown that this deformation post-dates closure by 20 million years, and that the intervening period was dominated not by crustal shortening but by crustal extension, sedimentation and magma production.
- Reconstruction of the fault geometry, kinematics and fault rocks along the Dent Fault Zone in northwest England. This fault zone underwent both vertical and lateral slip about 300 million years ago. The way in which these displacements are partitioned between different faults in the zone provides an instructive model for fault zones in general. The way in which permeability created by fault breccia formation was then rapidly resealed has important lessons for petroleum and mineralization systems.
- Reinterpretation of fault breccias and vein fills along Variscan dilational fault zones in South Wales. These zones clearly show that wide open fissures can form along faults at depths up to a few kilometres, acting as high permeability valves to fluids in the uppermost crust.
- Establishment of a revised non-genetic classification of fault breccias, which is being widely used.
The description and diagnosis of fault breccias forms one ongoing research direction. Another direction involves (with Jack Soper and Andrew Miles) the age and tectonic context of the Trans-Suture Suite of Caledonian granites, particularly the Shap granite.
Older Publications by Dr Nigel Woodcock