In the center of the frontal wedge of the Hikurangi subduction zone, New Zealand, Mahia Peninsula and its submarine continuation, Lachlan Ridge, are being uplifted and folded above an active landward- dipping thrust-fault complex that is 80 km long. High-quality marine seismic reflection profiles reveal complex deformation of a Cretaceous to Holocene sedimentary section and enable a detailed analysis of the stratigraphy, structural evolution, deformation rates, and future earthquake potential. The structural analysis is facilitated by uplifted marine terraces on Mahia Peninsula and by 14 submarine unconformities in the hanging-wall sequence, five of which are correlated across the eroded crest of Lachlan Ridge and into the footwall basin. The ages of the unconformities are determined by seismic ties to an offshore exploration well, onshore outcrops on the peninsula, and seabed samples dated by pollen, coccolith nannoflora, and foraminifera biostratigraphy. Nine regional Quaternary unconformities, which developed in response to eustatic fluctuations in sea level and are not older than ca. 1 Ma, are correlated with oxygen isotope stages in equatorial Pacific Ocean Drilling Project core 677.
The ages of fault-growth strata and progressive restorations of deformed stratigraphy indicate that Lachlan Ridge developed during three phases of deformation since subduction of the Pacific plate commenced beneath the Australian plate in the early Miocene. These include an initial phase of thrust faulting, a subsequent phase dominated by extensional faulting, and the current, mainly Pleistocene to Holocene phase of structural inversion, reactivated listric thrust faulting, and folding. Early to middle Miocene thrusts in the deeper core of the complex developed out of sequence, by sequentially stepping up into the hanging-wall section, creating an imbricate fan with emerging thrust tips now buried within the forelimb basin. In the middle Miocene–early Pliocene, listric extensional faults developed in the active thrust wedge—possibly as a result of substantial relief on the subducted Pacific plate—and controlled the development of Lachlan Basin to the west of the ridge.
The principal active thrust is the Lachlan fault, a listric extensional detachment reactivated to accommodate thrust movement and consisting of at least three right-stepping segments. Depth-converted seismic profiles indicate that the fault dips westward at 15°–20°, 6–8 km beneath the western flank of Lachlan Ridge, and steepens to 55°–70° or even steeper in the upper 1–2 km of section beneath the eastern flank. Syninversion Pleistocene fault-growth strata on both flanks of the associated anticline provide an exceptional record of progressive fold-limb rotation resulting from the listric-fault geometry. A geometric analysis of the fault-growth strata and deformed terraces was used to derive a maximum dip-slip displacement rate of as fast as 3.0–6.5 mm/yr. The implied shortening rate of 2.6–6.3 mm/yr represents ∼8%–20% of the total 31 mm/yr of orthogonal plate convergence across this part of the upper plate of the Hikurangi subduction zone.
The top of the pre–fault-growth Paleogene section reveals as much as 5.8 ± 1.5 km of vertical separation in the north, decreasing to ∼30%–50% of this value in the south. Temporal (103–106 yr) and spatial (103–104 m of strike length) variations in vertical-deformation rate have occurred during the past 1 m.y.; maximum rates occurred in the Holocene and middle Quaternary. A long-term increase in vertical- separation rate on all segments during the Pleistocene largely reflects a change in thrust kinematics associated with structural inversion. The relatively greater increase in uplift rate on the northern part of the fault during the past 1 m.y. could be related to the possibility that a subducted seamount lies >10 km beneath the peninsula.
Estimates of earthquake source parameters, incorporating paleoseismic uplift data from Mahia Peninsula, indicate a potential moment magnitude of up to Mw 7.6–8.0 for an earthquake that ruptures all three segments of the Lachlan fault. The average recurrence interval for such events is estimated to be 615–2333 yr, which is consistent with a mean recurrence interval of 1062 yr for four late Holocene earthquakes. Thus, the uplift and folding of Mahia Peninsula and Lachlan Ridge results from coseismic displacements on a major listric thrust fault that ruptures the upper plate frequently in association with large-magnitude earthquakes.