New Insights Into Basal Slip Processes and Kinematics of a Giant Pleistocene Submarine Mass Transport Complex, West of New Zealand's North Island

Abstract Submarine landslide runout influences the catastrophic impact of sediment mobilization on seafloor infrastructure, yet the basal slip processes that control runout remain poorly understood due to limited observations. This study examines the evolution and kinematics of a giant Pleistocene Mass Transport Complex (MTC) in the Taranaki Basin, located west of New Zealand's North Island. Using a regional grid of 2D seismic data, we refined its spatial extent and identified four distinct failure sectors (A–D) exhibiting remarkable differences in runout. MTC A, the largest debris flow deposit, covers ∼16,500 km 2 with a ∼345 km runout. In contrast, MTC D is a frontally emergent slide with a shorter runout of 55 km. A 3D seismic reflection volume reveals MTC D as a coherent, internally faulted slide block showing a frontal ramp, thrusts, pop‐up structures, and inverted normal faults. The basal shear surface (BSS) of MTC D lies within a turbidite layer above an earlier MTC. During MTC D sliding, shear softening partially remobilized the underlying MTC, which was subsequently incorporated into the overlying slide block of MTC D. We propose that the remobilized material behaved like a viscous mud, migrating away from high‐pressure areas and welding the overlying faulted blocks to the BSS. The resulting high‐friction zones at the BSS effectively arrested the movement of MTC D. Our findings present a new conceptual model showing how pre‐existing MTCs can influence subsequent sliding processes. This has implications for tsunamigenic hazard assessments, as treating multi‐phase failures as single events may overestimate tsunami potential.