The formation of seaward-dipping reflectors in volcanic margins: Insights from high-resolution visco-elasto-plastic geodynamic models with extrusive surface processes

Seismic reflection data from volcanic margins show thick packages of seaward-dipping reflectors (SDRs) that are commonly interpreted as buried subaerial lava flows. The origins of SDRs remain debated with proposed mechanisms including (1) syn-kinematic extrusion of lava flows on extended continental crust, (2) progressive rotation of subaerial lava flows due to volcanic loading and magmatic spreading and (3) syn-kinematic emplacement of lava flows on mobile gabbroic basement. This study presents the first systematic investigation of SDR formation using high-resolution visco-elasto-plastic geodynamic models with melt processes coupled to a surface processes model that includes sediment transport and extrusive lava-flow emplacement based on a cellular automata approach. These numerical experiments demonstrate that the typical frictional-plastic strain-weakening model commonly used in geodynamic models does not generate the symmetric, seaward-dipping lava flows and low-relief gabbroic basement structures interpreted from seismic reflection data. Instead, these models generate large off-axis extensional faults in thick volcanic packages and create large graben where thick syn-kinematic lava flows accumulate, driving ductile deformation and the formation of high-relief (> 2 km) ridges in the underlying hot, accreting gabbroic crust. The models presented in this work also demonstrate that reproducing observed seaward-dipping lava-flow geometries and low-relief gabbroic basement structures requires an additional melt-damage weakening mechanism above zones of melt focusing that approximates the effects of channelized melt networks and dike injection on lithospheric rheology. This melt-damage model probabilistically reduces friction coefficients and cohesion in the melt-extraction zone and produces seaward-dipping geometries by stabilizing the spreading axis and promoting more widely distributed subaerial flows that undergo rotation due to spreading-induced separation and burial by younger lava-flow packages. Finally, it is shown that the detailed geometry of lava-flow packages is controlled by the duration of the inter-eruption period with longer inter-eruption periods leading to a larger eruption volume that fills in axial depressions, builds up relatively flat axial plateaus, and produces more symmetric seaward-dipping geometries with smoother upward convexity.