Response of unconfined turbidity currents orthogonally to en-echelon segmented folds: Insights from numerical modelling

Deep-water fold and thrust belts (DWFTBs) exert a first-order control on turbidity current dynamics and sediment dispersal, yet the influence of segmented folds on flow behavior and resultant deposition remains poorly constrained. This study integrates structural modeling and computational fluid dynamics (CFD) to investigate the hydraulic behavior and depositional patterns of unconfined turbidity currents interacting with en-echelon segmented folds. Upon encountering segmented folds, unconfined turbidity currents decelerate and thicken due to reverse underflow and overflow, generating low-frequency hydraulic jumps with low-amplitude fluctuations of Froude number () and height-suppressed Kelvin-Helmholtz (K-H) waves upstream of folds. Concurrently, volumetrically small and dilute currents spill through fold segment boundaries, forming high-frequency hydraulic jumps with high-amplitude fluctuations ofand enhanced K-H waves downstream of folds. Upstream hydraulic jumps dominate the preferential sediment deposition into three distinct depozones: (ⅰ) strike-parallel depozones upslope of backlimbs from reverse overflow, (ⅱ) strike-parallel elliptical depozones along the base of fold backlimbs from reverse underflow and overflow, and (ⅲ) localized transverse depozones on fold backlimbs from waning overspill. The influence of segmented folds on overrunning currents is determined by segment boundary relief, governed by across-strike distance (AcD) and evolutionary stages. En-echelon folds with low linkage ratios at early-stage of linkage facilitate flow transfer through low-relief segment boundaries. At late-stage of linkage, sediment transport across folds depends on AcD: large AcD maintains a flow pathway through the segment boundary, whereas small AcD blocks flow. As en-echelon folds evolve, reduced along-strike distance (AlD) diminishes sediment deposition. The depocentre migrates from the base of upslope fold backlimb to that of downslope fold as AcD decreases. Since en-echelon segmented folds are common features in DWFTBs, and in salt- or shale-influenced basins, these findings highlight the critical role of segmented boundaries and their linkage processes in controlling turbidity current hydrodynamics and turbiditic sedimentation.