During the past decade, methods of space geodesy have demonstrated that the kinematics of the intra- continental deforming zones that lie between the large global plates can be usefully described as relative motions among small elastic blocks or microplates. At the same time, kinematic models that assume a smoothly varying deformation field have been developed and applied to the same data. Both models generally fit the data comparably well and there is much debate about which approach--blocks or continuum--is 'better'. However, there is really no disagreement about the existence of crustal blocks in deforming zones and only their size and number are contentious. Therefore a perhaps more useful way of framing the debate is to examine the purpose of each modeling approach, its success in meeting that purpose, and its limitations. Continuum modeling approaches are typically a prelude to dynamic modeling of continental deformation, thus far usually using a thin viscous sheet rheology for the lithosphere. The purpose of continuum modeling is then to quantify the forces driving and resisting motions and understand their relation to the observed deformation. This approach has been notably successful in determining the relative importance of plate boundary tractions and internal buoyancy forces (gravitational potential energy) in driving intra-continental deformation, particularly in central Asia and western North America. Continental deformation is block-like because major faults are weak and block interiors are much stronger. The main purpose of simple rigid plate kinematics is to quantify the rate and sense of slip across major faults and mountain belts, with applications to active tectonics and earthquake hazard assessment. Where available, late Quaternary and Holocene fault slip rate estimates, with few (but notable) exceptions, agree with geodetically- estimated rates obtained from the block models. Where block rotations are sufficiently large, late Cenozoic rotation rates can be determined paleomagnetically and these rates commonly agree with the space geodetic estimates. Despite several similarities, continental block kinematics differs in notable ways from global plate tectonics. First, microplates are much smaller, typically ~100-1000 km in size. Departures from block rigidity are small but measurable and represent either heterogeneous internal deformation or a more complex but unresolved block structure. While major oceanic plates may persist for tens or 100s of Ma, continental microplates change and evolve over much shorter timescales, particularly near their often geometrically irregular boundaries. The depth to which discrete block structures extend is uncertain. While some major faults probably extend through the crust into the upper mantle as narrow ductile shear zones, blocks elsewhere may be at least partially decoupled from the mantle lithosphere by pervasive ductile flow of weak lower crust. Continental blocks must ultimately be subject to the same forces that drive and resist global plate motions. However, the role and importance of local forces is often evident from the observed patterns of continental block motion. These local forces include internal buoyancy due to lateral density gradients in continental lithosphere and block boundary forces such as those caused by slab roll-back, trench suction, and resistance to subduction of buoyant lithosphere. The importance of basal tractions that may drive or resist block motions is uncertain and controversial.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1209 Tectonic deformation (6924);
- 8160 Rheology: general (1236;