How the Continents Deform: The Evidence From Tectonic Geodesy

Space geodesy now provides quantitative maps of the surface velocity field within tectonically active regions, supplying constraints oil the spatial distribution of deformation, the forces that drive it, and the brittle and ductile properties of continental lithosphere. Deformation is usefully described as relative motions among elastic blocks and is block-like because major faults are weaker than adjacent intact crust. Despite similarities, continental block kinematics differs from global plate tectonics: blocks are much smaller, typically similar to 100-1000 km in size; departures front block rigidity are sometimes measurable; and blocks evolve over similar to 1-10 Ma timescales, particularly near their often geometrically irregular boundaries. Quantitatively, relating deformation to the forces that drive it requires simplifying assumptions about the strength distribution in the lithosphere. If brittle/elastic crust is strongest, interactions among blocks control the deformation. If ductile lithosphere is the stronger, its flow properties determine the surface deformation, and a continuum approach is preferable.