Evolution and kinematics of a giant fossil landslide mass transport complex off the west coast of North Island, New Zealand

Submarine landslides pose significant risks to offshore infrastructure, such as seafloor telecommunication cables and oil and gas pipelines. To address geohazards associated with mass transport processes, it is crucial to understand the origin and behaviour of ancient mass transport complexes (MTCs). This study investigates the evolutionary stages and kinematics of a giant fossil MTC in the Taranaki Basin, off the West Coast of North Island, New Zealand. The submarine landslide occurred during the Pleistocene, covering an area of ~ 21,856 km² and evacuating 3,713 km³ of sediment in a NW direction. The landslide has been mapped in this study in greater detail, using a regional grid of 2D seismic reflection lines, allowing us to define its extent more accurately.

The MTC consists of four distinct failure events (A-D), each characterized by distinct headwall, translational, and toe domains. MTC A, B, C, and D span areas of 16,512 km², 2,318 km², 1,287 km² and 1,277 km² respectively. The MTC A is characterized by disintegrated extensional blocks and debris flow with an extensive runout of 328 km. MTC D is a frontally emergent slide complex with a shorter runout of 55 km. Both MTC A and MTC D are slope-attached failures, and mobilised 700 to 900 meters thick sediments near the headscarp region, whereas MTC B and MTC D mobilized 100-200 m thick sediments downslope.

A 3D prestack depth migrated seismic volume provides insight into the internal architecture of the MTC D. It is a faulted coherent slide block, which features thrusts, pop-up blocks and fault inversion zone, located behind a frontal ramp. The basal shear plane lies within a turbidite layer, sandwiched between two pre-existing MTCs. 3D seismic analysis reveals that, during sliding, part of the underlying older MTC was eroded and remobilized, due to shear softening, and was incorporated into the overlying MTC D. The remobilized MTC above the basal shear plane shows linear zones of thinning and stratal welding, where fault blocks became attached to the basal shear plane, creating high-friction pinning areas that inhibited further translation. Slide cessation is evidenced by transformation of earlier extensional faults into thrusting, stratal folding, and formation of backthrust.

In our study, we document for the first time the complex interaction between an older MTC and a more recent submarine landslide, highlighting its role in halting the slide. The insights gained from the study have important implications for geohazard assessments, emphasizing the need to account for the interplay between older and newer MTCs to better constrain the risk of submarine landslides.