Shear Coupling as a Trigger Mechanism for Slope Failures in a Turbidity Current Dominated Slope

Turbidity currents and slope failures are common processes in subaqueous settings worldwide. Their deposits, turbidites and mass-transport complexes (MTCs), constitute some of the most important components of sedimentary basin infill. We integrate high-resolution 3D seismic reflection data covering c. 3000 km², 2D seismic data spanning 40,000 km², and two industry wells from the Taranaki Basin, NW New Zealand, to investigate the preconditioning and emplacement of slope failures in a turbidity current dominated slope setting. In study area, the post-Miocene succession contain a ~300 m thick, laterally continuous interval of cyclic steps that dominates the slope region, indicating that supercritical turbidity currents were the prevailing depositional process. Within this succession, we found at least eight seismically imaged MTCs (MTC-1 to MTC-8), which together account for more than 70% of the total volume within the turbidity current dominated interval.

MTC-6 is the largest one spans more than 1200 km² in area. It is overlain by MTC-7 and underlain by the pre-existing MTC-2 above late Miocene unconformity (~7 Ma). Internally, MTC-6 is characterized by large normal faults in the headwall zone, contractional thrusts in the toe zone, NNW-dipping longitudinal shear bands and widely distributed pockmarks in the proximal zone. MTC-6 contains giant extensional blocks (450-550 m high, 0.5-4 km long), contractional pressure ridges (250-450 m high, 0.2-1.3 km long), and vertical fluid conduits that intersect both the base and top surfaces of the MTC. However, these blocks exhibit limited horizontal transport distances (less than 10 km) and internally preserve well-defined cyclic-step bedforms that can be correlated from the toe to the headwall region.

We suggest that rapid aggradation and repeated grain-size sorting induced by supercritical turbidity currents promoted underconsolidation and inefficient drainage, leading to localised excess pore-pressure build-up between the MTC-2 and the base of the cyclic steps interval. This ultimately established a mechanically weak zone that preconditioned the subsequent emplacement of MTC-6. We attribute the triggering of MTC-6 to shear coupling with subsequent MTC-7. During emplacement of MTC-7, additional loading and basal traction generated stress perturbations that were transmitted downward and preferentially localised within the preconditioned weak zone of the cyclic steps interval, inducing transient excess pore pressure. Inefficient drainage further sustained this overpressure, reducing effective stress and allowing the weak zone to reach a critical failure threshold. Subsequently, the localised overpressure redistributed via fluid migration along the weak horizon, promoting shear-rupture propagation and enlarging the failure scale. However, because the shear-coupling perturbation imparted by the MTC-7 was limited in magnitude and duration, the transmitted basal shear stress was insufficient to sustain dynamic weakening, and the associated overpressure weakening likely decayed during subsequent drainage, thereby preventing long-distance transport.

Our results indicate that turbidity current dominated slope settings may be inherently prone to repetitive slope failures. Newly emplaced MTCs can cause remobilization of underlying thick-bedded turbidite successions through shear coupling. This mechanism may represent a previously underappreciated control on multi-phase slope instability in submarine sedimentary systems.