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We examined the stratigraphy of alluvial fans formed at the steep range front of the Southern Alps at Te Taho, on the north bank of the Whataroa River in central West Coast, South Island, New Zealand. The range front coincides with the Alpine Fault, an Australian‐Pacific plate boundary fault, which produces regular earthquakes. Our study of range front fans revealed aggradation at 100‐ to 300‐year intervals. Radiocarbon ages and soil residence times (SRTs) estimated by a quantitative profile development index allowed us to elucidate the characteristics of four episodes of aggradation since 1000 CE. We postulate a repeating mode of fan behaviour (fan response cycle [FRC]) linked to earthquake cycles via earthquake‐triggered landslides. FRCs are characterised by short response time (aggradation followed by incision) and a long phase when channels are entrenched and fan surfaces are stable (persistence time). Currently, the Te Taho and Whataroa River fans are in the latter phase. The four episodes of fan building we determined from an OxCal sequence model correlate to Alpine Fault earthquakes (or other subsidiary events) and support prior landscape evolution studies indicating ≥M7.5 earthquakes as the main driver of episodic sedimentation. Our findings are consistent with other historic non‐earthquake events on the West Coast but indicate faster responses than other earthquake sites in New Zealand and elsewhere where rainfall and stream gradients (the basis for stream power) are lower. Judging from the thickness of fan deposits and the short response times, we conclude that pastoral farming (current land‐use) on the fans and probably across much of the Whataroa River fan would be impossible for several decades after a major earthquake. The sustainability of regional tourism and agriculture is at risk, more so because of the vulnerability of the single through road in the region (State Highway 6). [ABSTRACT FROM AUTHOR]

Early Holocene transgressive marine deposits infilling the Pakarae River paleovalley are used to extend the paleoseismic history of the Pakarae River locality, East Coast, North Island, New Zealand, back in time prior to eustatic sea level stabilization at ∼7 calibrated (cal) ka B.P. Paleoenvironmental evolution of the Pakarae River paleovalley from 7 to 10 cal ka B.P is reconstructed using sedimentology and biostratigraphy. Two estuarine units display sudden vertical transitions to floodplain sediments implying significant marine regressions and estuary abandonment. These regressions are attributed to coseismic coastal uplift events at ∼9000 and ∼8500 cal years B.P. A third uplift between 8500 and ∼7350 cal years B.P. is inferred from a significant difference between the amount of sediment preserved and the predicted sediment thickness according to the eustatic sea level curve. This study demonstrates the utility of the analysis of transgressive deposits and their paleoenvironmental characteristics for neotectonic investigations on active coasts. [ABSTRACT FROM AUTHOR]

The accretionary margin of the Hikurangi forearc on the southeast coast of the North Island of New Zealand is part of the leading edge of the Australian plate, which is overriding the obliquely converging Pacific plate. We investigate the last 10 m.y. of deformation history of the innermost (western) quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the Eketahuna area on the east coast of the North Island. The Eketahuna area is ideal for such a study because emergence of the margin in the Quaternary has exposed a complete late Neogene rock record. This record has allowed us to chronicle the strain history. From 10 Ma to about 2.5 Ma this forearc region was the locus of subsidence and marine deposition. In the latest Pliocene this part of the margin began to shorten through folding and reverse faulting, bringing an end to basin filling. The period of shortening was brief, and by the late Pleistocene, reverse faulting had ceased and was immediately succeeded by dextral strike-slip faulting, in some cases along the same faults. Presently, the dominant strain regime in the inner quarter of the forearc is strike-slip faulting. This structural history illustrates that, over time, the pattern of strain partitioning has changed in the Hikurangi forearc. The switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m.y. may be a response to more than 10° of clockwise rotation in the southern Hikurangi forearc in Pliocene and Pleistocene time. This rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging Australian and Pacific plates in this area at the southernmost end of the Kermadec-Hikurangi subduction system. The inboard portion of this young accretionary margin is exceptionally well exposed today, probably in part because of the late Neogene subduction of relatively thick, buoyant crust of the Hikurangi-Chatham plateau. [ABSTRACT FROM AUTHOR]

A sequence of seven marine terraces of late Pleistocene age constitutes approximately 40% of the area of Mahia Peninsula in eastern North Island. The peninsula is one of the closest land areas to the Hikurangi Trough (approximately 80 km further east), where the Pacific plate is being subducted beneath the Australian plate. Terrace ages of 40, 59, 81, 106, 124, 176, and 212 ka are assigned by correlation with the marine terrace sequence of Huon Peninsula, Papua New Guinea. Paleontological data and amino acid dating of marine components of the 124 ka terrace at Mahia Peninsula provide a basis for correlation to the Huon Peninsula sequence. Loess and tephra stratigraphy constrain the ages of older and younger terraces. The marine terrace sequence is tilted to the WNW on the flank of the NNE trending Lachlan anticline. Uplift of at least 130 m of the circa 124-kyr-old shoreline indicates uplift rates of at least 1 mm/yr at the axis of the anticline. Younger marine terraces have been uplifted more rapidly (about 3 mm/yr), and there is a progressive increase in rate to the youngest Holocene shoreline. Structural contours normalized to the circa 124 ka terrace illustrate the tilt on the flank of the Lachlan anticline and on a secondary structure, the Aurora Point fold. Seismic profiling has confirmed the presence of a reverse fault (the Lachlan fault) at the eastern margin of the Lachlan anticline. The Lachlan fault is inferred to be responsible for growth of the anticline and deformation of the marine terraces. The relationship of the reverse fault to the subduction thrust, which separates the two crustal plates at about 20 km depth below Mahia Peninsula and dips about 6°-12° to the NW, is uncertain. However, the late Pleistocene and Holocene growth of the Lachlan anticline indicates substantial coupling across the plate interface, deforming the inner part of the frontal wedge of the Australian plate. [ABSTRACT FROM AUTHOR]

Mahia Peninsula is a prominent coastal landmark in eastern North Island and is the closest point of land in the North Island to the Hikurangi Trough, where the Pacific plate plunges beneath the subduction complex at the eastern margin of the Australian plate. Uplifted Holocene marine deposits of both estuarine and open beach affinities are found in many parts of the peninsula and provide the basis for Holocene tectonic characterization. Estuarine deposits record the later part of the postglacial transgression that culminated about 6500 years B.P. in New Zealand. The deposits have been differentially uplifted since that time at a rate of 2.5 ± 0.3 mm/yr in the central north coast area, decreasing to 0.7 ± 0.2 mm/yr about 6 km to the west. The coastal plain is characterised in many places by a stepped sequence of emergent shore platforms overlain by fossiliferous beach deposits. Extensive radiocarbon dating of samples from beach deposits shows that terraces in widespread parts of the peninsula are of five distinct ages. Each of the terraces is inferred to have formed in conjunction with a large prehistoric earthquake because of the stepped terrace morphology, clustering of ages on each terrace, differential uplift of terraces across the peninsula, and historic coseismic uplift events in this tectonic setting. Paleoseismic events of Mw 7.5-8.0 are estimated to have occurred approximately 250, ∼1600, ∼1900, ∼3500, and ∼4500 years B.P. Uplift distribution of the Holocene and late Pleistocene marine terraces shows that the peninsula lies on the west (gentle) flank of the active Lachlan anticline, which is cored by a major west dipping reverse fault (the Lachlan fault). Holocene active deformation at Mahia Peninsula and other coastal areas of eastern North Island is a continuation of structures developed in Pleistocene time in the landward part of the subduction complex adjacent to the Hikurangi subduction zone. [ABSTRACT FROM AUTHOR]

The Pacific plate obliquely converges with the Australian plate at latitude 39°50′S along the Hikurangi margin off the east coast of the North Island of New Zealand. An extensive and youthful subaerially exposed forearc on the east coast of the North Island in the Hawke's Bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting. Geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension, contraction, and strike-slip. The domains are elongate and trend parallel to the margin. Measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface. Deformation rates calculated for the past 1-2 m.y. for structures in all six forearc domains account for 50-70% of the margin-parallel motion required by Pacific-Australian plate convergence and about 6% of the plate motion perpendicular to the plate boundary. At the surface in the forearc, this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike-slip fault zone and an accompanying contractional fault-and-fold zone on the trenchward side. There are two such transcurrent faulting-and-contraction couplets, one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin. The small amount of shortening, relative to strike-slip, in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the subduction zone megathrust. [ABSTRACT FROM AUTHOR]

Both the South Island of New Zealand and the Transverse Ranges of California have (1) a major right-slip fault marking the plate boundary, (2) a left-stepping bend in the fault resulting in ranges and basins bounded by active reverse faults that strike 45° counterclockwise to the regional strike of the main fault, (3) a set of right-slip faults on one side of the constraining bend, with most of the total displacement concentrated On the fault stepped farthest left from the continuation of the fault on the other side of the bend, and (4) intersections between the set of faults and the main fault within the bend not present or poorly defined. The faults with most of the displacement (Alpine-Wairau, San Andreas) may pre-date the constraining bend, The reverse fault province southeast of the Alpine fault is separated from that fault by the Southern Alps, which may be underlain by crust that is too hot and too weak to sustain major reverse faults. Average reverse fault recurrence intervals in New Zealand and California are 10³ to 2 × 104 years as compared to less than 10³ years for the right-slip master fault, Seismicity is high but patchy in the Nelson-Buller region of the northern South Island, low but patchy in the Transverse Ranges, and very low in Central Otago in the southern South Island, all reverse fault provinces. Seismicity patterns in both regions, poorly related to overall late Quaternary fault distribution, correspond largely to individual rupture events on portions of Quaternary faults, and the seismic inactivity of many of these faults may simply represent quiescence between seismic faulting with long recurrence intervals. The geology and Seismicity of the three reverse-fault provinces support the suggestion of C. R. Allen [1975] that late Quaternary fault history is a better long-term guide for assessment of large-magnitude earthquake hazard than instrumental and historical Seismicity for faults with long recurrence intervals. [ABSTRACT FROM AUTHOR]