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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.

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Seismic reflection and outcrop data from the onshore Hikurangi forearc reveal the styles and history of deformation for a c. 3000 km 2 region between Dannevirke and Hawke' s Bay. The data cover the forearc basin, including its western and eastern boundaries, and delineate folds and faults in a late Miocene-Recent sedimentary sequence. Five seismic horizons, including the basement/Neogene cover unconformity (variable age), base Waipipian (3.7 Ma), base Mangapanian (3.2 Ma), base Nukumaruan (2.6 Ma), and base Castlecliffian (1.6 Ma) were identified using outcrop and well ties. These horizons were traced across the study region to provide information on the geometry, spatial distribution, and timing of structures. To the west, the rangefront fault is predominantly reverse and separates uplifted Torlesse basement of the axial ranges from Neogene forearc basin sediments. Structures within the forearc basin are dominated by north-northeast-striking reverse faults and associated asymmetric folds which parallel the subduction margin and often have sinuous traces. Faults are planar to depths of at least 1-2 km and typically dip at 30-80°NW (most often at 40-70°). Many faults in the basin terminate northwards and fault-normal spacings decrease from 2-8 km in the Dannevirke area to c. 20 km near Hastings. Angular unconformities and syntectonic strata constrain the timing of deformation on fault/fold pairs. Faults within the forearc basin were active over two main periods, at c. ?3.1-2.5 Maandc. 1.6 Ma-Recent. Active fault traces are confined to the edges of the forearc basin. In the west, the Mohaka and Ruahine Faults have mainly recent right-lateral offsets, but interpretation of a seismic reflection line which crosses the Mohaka Fault indicates minimal (<300 m) right-lateral displacement over the last 3 m.y., and lateral slip may not significantly predate the late Quaternary. In the east, a zone of reverse faults (including the Longlands, Poukawa, Tukituki, and Oruawharo Faults) began forming ate. 1 Ma and remains active. Margin-normal shortening across the forearc basin and basin-bounding structures ranges up to 5 mm/yr (120- *

[1] Differences in Pn speeds within continental lithosphere off the west coast of South Island, New Zealand, require anisotropy of at least 10 ± 3%. These data concur with SKS splitting and are inferred to show high strain of mantle lithosphere over a zone >200 km wide. An active source seismic experiment yields Pn speeds of 8.6 and 7.7 km/s along nearly perpendicular lines. The higher speed is sub-parallel to the polarization of the faster quasi-S wave measured from SKS splitting. Most of the observed $2 s of splitting in South Island can be accommodated in mantle lithosphere 150 km thick, if we assume a P to S anisotropy ratio of 1.4. The large magnitude of Pn anisotropy shows that anisotropy must occur in the relatively cold uppermost part of the mantle lithosphere and raises the possibility that dynamic recrystallization occurs in cold ($500 -600°C) olivine at geologic strain rates.INDEX TERMS: 8120 Tectonophysics: Dynamics of lithosphere and mantle-general; 7218 Seismology: Lithosphere and upper mantle; 5199 Physical

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