Growth histories of contractional structures at the southern end of New Zealand's Hikurangi forearc basin have been analysed for the last c. 10 m.y. Growth data are from outcrop and seismic-reflection profiles that contain syntectonic strata and angular unconformities, and from deformed fluvial terrace surfaces. Deformation is described for up to eight intervals of time, spanning c. 12 000 yr to 5 m.y., the ages of which were determined by biostratigraphy and tephrochronology. Reverse faults and related asymmetric folds, which strike parallel to the subduction margin and verge troughwards, experienced variable rates of shortening through time. The current period of deformation commenced at c. 1.8 Ma with displacement rates of c. 0.1-0.7 mm/yr on the main faults (i.e., Martinborough, Huangarua, and Mangaopari Faults). Before this time there were periods of accelerated deformation during the mid Pliocene (c. 3.4-2.4 Ma) and latest Miocene (c. 8.0-6.0 Ma). Therefore, shortening since 10 Ma accumulated mainly during three periods of 1-2 m.y., with structures active in the Quaternary forming in the late Miocene or earlier. Local intervals of accelerated deformation are coincident with the timing of intervals of uplift and faulting along much of the emergent forearc and cannot be attributed to local transfer of displacements between faults. Instead, these intervals of deformation appear to reflect regional changes in the kinematics of the upper plate. These changes could arise due to margin-normal migration of strain to regions outside the forearc basin or may indicate temporal variations in the dynamics of subduction.

The Awatere Fault is one of four principal strikeslip faults within the obliquely convergent plate boundary zone in northeastern South Island, New Zealand. The eastern section of the fault last ruptured in 1848, in the M w c. 7.5 Marlborough earthquake, when surface rupture was traced by first-hand accounts from the South Island's east coast inland to Barefell Pass (c. 110 km). We excavated two paleoseismic trenches near Upcot Saddle on the eastern section of the fault, c. 70 km inland from the coast. Interpretation of crosscutting relationships between sediments exposed in these trenches, in association with 21 radiocarbon dates, suggest at least 5, and possibly 6, events have ruptured this part of the Awatere Fault in the last 5600 yr. These events include a young rupture (<300 cal. yr BP) that is presumed to be the surface rupture of the 1848 Marlborough earthquake. The Upcot Saddle rupture history is combined with paleoseismic data from two previous trenches, located c. 55 km to the northeast at Lake Jasper, to infer a chronology of 9-10 events between AD 1848 and 8610 cal. yr BP on the eastern section. These 9-10 events define a mean recurrence interval of 820-950 cal. yr on that fault, which is within error of the mean recurrence interval calculated using available data on late Quaternary slip rates (6 ± 2 mm/yr) and mean coseismic slip during the 1848 earthquake (5.3 ±1.6 m). The intervals between individual paleoearthquakes on the eastern section are non-uniform, and range from 220-620 to 1540-2120 yr.

A NNE-SSW-trending linear scarp, trenched in Whitemans Valley near Upper Hutt, proved to be a fault scarp upthrown to the west. The trench, in the steepest part of the 8-9 m high fault scarp, revealed two reverse fault planes dipping at c. 45° to the west. Sediments displaced by the faults include weathered fan gravel, three loess/paleosol couplets including the Porewan, Ratan and Ohakean loesses, Kawakawa Tephra (22.6 ka), and topsoil. The fan gravel represents the youngest alluvial sedimentation at the site and, at c. 80 000 yr (ka), it forms the oldest displaced unit seen in the trench. With the exception of the topsoil, each unit in the trench has been displaced vertically 1.4-2.1 m (equivalent to true dip-slip displacement of c. 3 m), which we interpret as a single-rupture displacement. A possible dextral displacement component is suggested by greater displacement of younger units in the trench than older units and possible dextral displacement of a stream channel. The rupture cuts and deforms a soil B horizon developed in the Ohakean loess, which is considered to be c. 10 ka or younger, but the topsoil is undeformed. Displacement on the fault strands in the trench is insufficient to account for the total vertical displacement represented by the 8-9 m scarp height, indicating the existence of a further fault strand(s) immediately to the west of the trench. The vertical displacement of c. 2 m observed in the trench is our best estimate of displacement for the most recent surface rupture earthquake, and total vertical displacement on the scarp may represent four or five events. Ifso, rupture recurrence interval is estimated at 15-20 000 yr. The c. 3 m, single event, dip-slip displacement is probably a minimum net single-event displacement. On the basis of the minimum net slip of c. 3 m per event, and the mappable length of c. 20 km, surface rupture of the Whitemans Valley Fault is thought to result in M 7.0 ± 0.3 earthquakes. The Whitemans Valley Fault is just one of conceivably 10 similar active "second order" faults in the Wellington region, suggesting that collectively these faults may make a noticeable contribution to the region's seismic hazard.