The 2019 Stephan Mueller Medal is awarded to Serge Lallemand for fundamental analyses of the tectonics of convergent plate margins through careful combinations of natural observations with experiments.
Serge Lallemand has fundamentally impacted our understanding of tectonics processes at convergent plate margins by carefully considering the system from Earth’s surface to the mantle, paying specific attention to processes at the plate interface. His approach is original and powerful as it builds on direct observation of many aspects of convergent plate margins in regions such as Japan, Taiwan and South America. He has combined observations from marine geology and geophysics, often obtained in one of many oceanographic cruises he was involved in, with insights from analogue modelling. With this approach Lallemand has made significant advances in our understanding of the mechanisms of tectonic erosion of the upper plate through roughness of the subducting plate.
Unique insights into the behaviour of oceanic subduction zones came from the systematic studies that Lallemand undertook into the geometrical and kinematic parameters that characterise subduction. Surprisingly, the statistical analysis disproved classic correlations, such as between slab dip angle and slab age, instead pointing to relations of slab dip to, among others, back-arc stress and absolute motion of the overriding plate. The database (published in 2005) continues to provide test values for many analogue and numerical studies. Lallemand and collaborators also developed a tool for mapping subduction zones (SubMap). This dataset includes data on for example bathymetry, seismology, gravity and plate kinematics.
His work on plate coupling and the role of subducting asperities naturally evolved into studies on the nucleation of large earthquakes. Over the last years, Lallemand has addressed seismic hazards in the Taiwan collision zone, developing offshore campaigns with seismic experiments and using marine palaeo-seismology. He has recently proposed new reconstructions of the tectonic evolution of this region that include both onshore and offshore observations.
A significant strength of Lallemand’s career lies in his unselfish efforts in international collaborations, notably with Taiwan, and education. He is curious himself and triggers curiosity in the early career scientists he collaborates with. Not in the least, he is enthusiastic and drives new research. All these aspects make Serge Lallemand a very worthy recipient of the 2019 Stephan Mueller Medal.