Brittle-ductile Coupling and Block Rotation During Rifting Revealed Through Digital Volume Correlation Analysis of a Crustal-scale Analogue Experiment

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During rifting, blocks of upper brittle crust may rotate about a vertical axis above the ductile parts of the crust below, in particular in settings where two rift segments interact in a so-called rift-pass structure, with important implications for our understanding of rift system development. However, whether such block rotation is edge-driven (induced by deformation in the brittle crust itself), or by viscous flow in the underlying ductile crust, and what role kinematic coupling between material in the brittle and ductile crust plays remains unclear. In this study, we apply new digital volume correlation (DVC) analysis on a previously presented crustal-scale analogue model simulating the evolution of a rift-pass structure. The improvements to our new DVC workflow include data preprocessing, increased vector resolution, improved postprocessing, and deformation quantification using finite stretch and rotation tensors. This enhanced workflow allows us to quantify the kinematic coupling between the brittle and viscous model layers in much higher spatial resolution, and to determine previously unrecognized differences in deformation styles between the brittle and viscous layers. Our improved DVC analysis reveals new insights into the kinematic evolution of a rotating rift-pass block forming between two interacting rift segments. We document (1) the evolution of a rift-pass block in the brittle layer, (2) its effect on the underlying viscous model layer and, (3) the kinematic coupling between the two model layers. Laterally confined by two rift segments, the rift-pass block rotates about a vertical axis and exerts a drag force on the underlying viscous layer where rift-axis parallel viscous flow is stimulated. As a result, a brittle-ductile transitional zone forms that shows increased shear with spatial and temporal variations in the degree of kinematic brittle-viscous coupling. Our DVC analysis suggests that edge-driven rift-pass block rotation locally weakens kinematic coupling, resulting in rift-axis parallel flow in the lower crust. Hence, rotating blocks in the upper crust may induce considerable amount of lower-crustal material to flow out of a 2D plane, which must be considered when estimating crustal extension from rift-axis perpendicular cross-sections.