The Formation of Seaward-dipping Reflectors in Volcanic Margins: Insights from High-resolution Visco-elasto-plastic Geodynamic Models with Extrusive Surface Processes

Erik Arthur Kneller

Seismic reflection data from volcanic margins reveal thick packages of seaward-dipping reflectors (SDRs) that are commonly interpreted as buried subaerial lava flows. This study presents the first systematic investigation of SDR formation using high-resolution visco-elasto-plastic geodynamic models that incorporate strain weakening and couple melt generation to a surface-processes model that simulates sediment transport and extrusive lava-flow emplacement using a cellular automata approach. The models show that ridgeward-dipping, downward-concave lava-flow packages, which become SDRs once a margin is flooded, naturally result from syn-kinematic subaerial basaltic lava-flow emplacement at an active spreading center above anomalously hot asthenosphere. Near-horizontal lava-flow packages are emplaced and bisected by extension and frictional-plastic deformation at the ridge axis into conjugate ridgeward-dipping packages with dips that increase in the down-dip direction. Lava-flow packages are subsequently modified by additional plastic deformation and rotation driven by progressive burial by younger flows, continued spreading, lithospheric density changes associated with thermal cooling and partial-melt solidification, and volcanic-load-induced ductile thinning of the gabbroic magmatic crust.
The models identify several extensional mechanisms capable of producing SDR-like geometries, 
including normal faulting within volcanic edifices above the ridge axis, syn-kinematic emplacement of subaerial lava above listric normal faults that detach into ductile gabbroic crust, and syn-kinematic lava emplacement above triangular horsts formed within newly accreted gabbroic crust at the ridge axis. The dominant mechanism, together with the detailed geometry of lava-flow packages and associated magmatic crustal structures, is sensitive to the presence or absence of melt damage above partially molten asthenospheric domains, the magnitude of the asthenospheric thermal anomaly, the spreading rate, the gabbroic crustal rheology, and the distribution and timing of volcanic loads that drive ductile deformation of the magmatic crust. 
The results further demonstrate that reproducing the key geometric features of conceptual SDR 
models based on seismic reflection data requires a probabilistic melt-damage weakening mechanism above zones of melt focusing. These features include symmetric, downward-concave lava-flow packages with relatively small dip variations in their up-dip sections, progressively increasing dips in the down-dip direction, and relatively low-relief gabbroic basement. This melt-damage mechanism approximates the rheological weakening effects of channelized melt networks and dike injection within the lithosphere. The models also show that volcanic-load induced ductile deformation of the gabbroic crust can modify the geometry of dipping lava-flow packages and promote asymmetric accretion of gabbroic crust.