THE DYNAMICS OF PLATE TECTONICS AND MANTLE FLOW: FROM LOCAL TO GLOBAL SCALES

Understanding the dynamics of plate motions has long been a challenge since global models have never been able to incorporate the essential physics of tectonic plates. Plate motion/mantle flow models have been limited through the use of simplified rheologies, artificial boundary conditions, or parameterized edge forces. We describe a new generation of global mantle flow models predicting the motion of plates with individual plate margins resolved down to a scale of 1 km, allowing for the bending of subducting slabs and deformation of plate boundaries and their interiors. These models have been forged by advances in the scalability of adaptive mesh refinement methods on massively parallel computers. The global flow models have been constrained through observations over large and small scales: Euler poles of plates, internal plate deformation (including micro-plate motion), and state of stress from seismic focal mechanisms in Benioff zones. Back-arc extension and slab roll back (including the rapidly deforming Tonga-Kermadec and New Hebrides subduction zones) are emergent consequences of slab descent in the upper mantle. The cold thermal anomalies within the lower mantle are often coupled into oceanic plates through narrow high viscosity slabs, speeding up or slowing down oceanic plates. The calculations show that viscous dissipation within the bending lithosphere at trenches amounts to only ~5-20% of the total dissipation through the entire lithosphere and mantle. The models are consistent with detailed constraints on the state of stress and strain rate from deep focus earthquakes. Models that best fit plateness criteria and plate motion data have strong slabs that have high stresses. The regions containing the Mw 8.3 Bolivia and Mw 7.6 Tonga 1994 events are considered in detail. Modeled stress orientations match stress patterns from earthquake focal mechanisms. A yield stress of at least 100 MPa is required to fit plate motions and matches the minimum stress requirement obtained from the stress drop for the Bolivia 1994 deep focus event. The minimum strain rate determined from seismic moment release in the Tonga slab provides an upper limit on the strength in the slab. We describe how geodynamics could unfold in the next few years as the methods are pushed into the time domain while being linked to geological observations.