During convergence between continental and oceanic tectonic plates, the continental plate usually rides over the top of the oceanic plate, causing the latter to sink (subduct) steeply (≥30°) into the mantle. This process results in a predictable arrangement of basins and deformation (i.e., mountain range formation) flanking a volcanic arc.
However, in some subduction zones, the subducted plate does not enter the mantle at a steep angle. Instead, it subducts as a nearly horizontal slab, influencing the characteristics of the overriding plate with respect to basin formation, magma evolution, and deformation. Examples of flat slabs in Peru, Chile, Mexico, and Alaska demonstrate their association with complex basin geometries, gaps in volcanic arcs, far-field mountain building, and great earthquakes.
The angle at which oceanic plates subduct thus exerts first-order control on the evolution of the overriding plate—this fact is well established. However, linking geodynamic models to observations of upper plate deformation and basin formation remains problematic. Last January, more than 60 participants gathered to discuss strategies for linking geodynamic models of flat subduction with the details of basin formation and deformation at Earth’s surface.
Presentations spanned flat-slab subduction from South America, the Laramide in western North America, and the past and present flat slab in Alaska. These presentations followed the workshop theme of connecting observational constraints from basin evolution, seismology, and volcanology with mechanism-driven hypothesis testing from 2-D and 3-D geodynamical modeling.
Participants, ranging from full professors to undergraduate students, discussed several well-established consequences of flat subduction. Perhaps the most common consequence is an inboard migration of the volcanic front associated with slab flattening, culminating in a temporary cessation of arc volcanism. Another common effect is hydration or compositional changes (metasomatism) of the materials in the upper plate.
Other forces and their consequences are less well characterized. Uncertainty remains about factors that control foreland partitioning and the locus of basement uplifts. Are they driven by end loading or basal shear of the overriding plate? Meeting participants identified the importance of distinguishing deformation due to horizontal tectonic motion stresses from deformation caused by mantle flow. Another example is the uncertainty in vertical and horizontal scales of dynamic topography, which leads to difficulty linking observations and models. Finally, spatial and temporal relationships between flat subduction and basin evolution remain controversial.
Participants identified future study sites, and they developed a collaborative context linking observations with modeling. The foremost priority is determining the scales and rates of processes associated with flat subduction, particularly dynamic subsidence. Flat slabs are associated with both thickened oceanic crust and young oceanic plates adjacent to spreading ridges, prompting the need for distinguishing between these disparate mechanisms in the rock record (Figure 1). Also, the role of preexisting structure in the overriding and subducting plate is still poorly understood.
Suggested future research targets include constructing a comprehensive comparison of modern flat-slab segments and their effect on the overriding plate, evaluating observable effects of flat subduction in a location whose history is independently well documented, and determining a robust discriminator for the inboard extent of flat subduction in the geologic record. All these challenges point to the need for a collaborative approach to investigate flat-slab subduction.
We thank John Suppe and the Center for Tectonics and Tomography at the University of Houston for financial support.
—Joel E. Saylor, Department of Earth and Atmospheric Sciences, University of Houston, Texas; Emily Finzel, Department of Earth and Environmental Sciences, The University of Iowa, Iowa City; and Margarete Jadamec, Department of Geology and Computational and Data-Enabled Science and Engineering Program, The State University of New York at Buffalo