Geoffroy Lamarche

National Seismic Hazard Model for New Zealand: 2010 Update

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

The giant Ruatoria debris avalanche on the northern Hikurangi margin, New Zealand: Result of oblique seamount subduction

Despite convergent margins being unstable systems, most reports of huge submarine slope failure have come from oceanic volcanoes and passive margins. Swath bathymetry and seismic profiles of the northern Hikurangi subduction system, New Zealand, show a tapering 65–30 km wide by 65 km deep margin indentation, with a giant, 3150±630 km3, blocky, debris avalanche deposit projecting 40 km out across horizontal trench fill, and a debris flow deposit projecting over 100 km. Slide blocks are well-bedded, up to 18 km across and 1.2 km high, the largest being at the avalanche deposits leading edge. Samples dredged from them are mainly Miocene shelf calc-mudstones similar to those outcropping around the indentation. Cores from cover beds suggest that failure occurred ∼170±40 ka, possibly synchronously with a major extension collapse in the upper indentation. However, the northern part of the indentation is much older. The steep, straight northern wall is close to the direction of plate convergence and probably formed around 2.0–0.16 Ma as a large seamount subducted, leaving in its wake a deep groove obliquely across the margin and an unstable triangle of fractured rock in the 60° angle between groove and oversteepened margin front. The triangle collapsed as a blocky avalanche, leaving a scalloped southern wall and probably causing a large tsunami. Tentative calculations of compacted volumes suggest that the indentation is over 600 km3 larger than the avalanche, supporting a two-stage origin that includes subduction erosion. Since failure, convergence has carried the deposits ∼9 km back toward the margin, causing internal compression. The eventual subduction/accretion of the Ruatoria avalanche explains the scarcity of such features on active margins and perhaps the nature of olistostromes in fold belts.

From Oblique Subduction to Intra-Continental Transpression: Structures of the Southern Kermadec-Hikurangi Margin from Multibeam Bathymetry, Side-Scan Sonar and Seismic Reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.

A model of active faulting in New Zealand

Active fault traces are a surface expression of permanent deformation that accommodates the motion within and between adjacent tectonic plates. We present an updated national-scale model for active faulting in New Zealand, summarize the current understanding of fault kinematics in 15 tectonic domains, and undertake some brief kinematic analysis including comparison of fault slip rates with GPS velocities. The model contains 635 simplified faults with tabulated parameters of their attitude (dip and dip-direction) and kinematics (sense of movement and rake of slip vector), net slip rate and a quality code. Fault density and slip rates are, as expected, highest along the central plate boundary zone, but the model is undoubtedly incomplete, particularly in rapidly eroding mountainous areas and submarine areas with limited data. The active fault data presented are of value to a range of kinematic, active fault and seismic hazard studies.

Faulting and Extension Rate over the last 20,000 Years in the Offshore Whakatane Graben, New Zealand Continental Shelf.

Oblique rifting in the offshore Taupo Volcanic Zone, New Zealand, is expressed in widely distributed active normal faulting in the 20 km-wide Whakatane Graben. Active faults are identified along seafloor scarps and displacements of the post-last glacial transgressive ravinement surface ( 400 faults. We derive an extension rate at seismogenic depths (6-10 km) across the graben of 13 ± 6 mm/yr, by summing surface measurements, assuming an average crustal fault dip of 45±15o, and correcting for the discrepancies between surface and deep crustal extension estimates. Structural and kinematic data implies an extension direction 20o oblique to the rift axis, resulting in up to 4.6 ± 2.1 mm/yr of dextral motion parallel to the rift axis. The strike-slip motion is accommodated by dip-slip displacements on oblique faults in the centre of the graben, and oblique-slip faulting along the rift margins. Pure dip-slip in the graben centre represents >50% of the total slip, with the Rangitaiki Fault accommodating 25% of the total extension in the graben.

Structure of the Hanmer strike-slip basin, Hope fault, New Zealand

Hanmer basin (10 x 20 km), located in northern South Island, New Zealand, is evolving where two major segments of the dextral strike-slip Hope fault are projected to converge across a 6- to 7-km-wide releasing step-over. The structural geometry and development of Hanmer basin does not conform to traditional pull-apart basin models. The respective fault segments do not overlap but are indirectly linked along the southwest margin of the basin by an oblique normal fault. The Hope River segment terminates in an array of oblique normal faults along the northwestern basin range front, and east-west-striking normal faults on the west Hanmer Plain. Faulted Holocene alluvial-fan surfaces indicate west Hanmer basin is actively subsiding evolving under north-south extension. The Conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces, small pop-up ridges, and graben depressions. Early basin-fill sediments of Pleistocene age are being folded, elevated, and dissected as the eastern part of Hanmer basin is progressively inverted and destroyed by north-south contraction. The north margin of the basin is defined by a series of topographic steps caused by normal faulting outside of the area of the releasing step-over. These normal faults we interpret to reflect large-scale upper crustal collapse of the hanging-wall side of the Hope fault. New seismic reflection data and geologic mapping reveal a persistent longitudinal and lateral asymmetry to basin development. Four seismic stratigraphic sequences identified in the eastern sector of the basin thicken and are tilted southward, with insequence lateral onlaps occurring to the north and east, and also onto basement near the fault-controlled basin margins. The basin depocenter currently contains >1000 m of sediment adjacent to the south margin and is disrupted by faulting only at depth. In the western part of the basin, the sediment fill is thinner ( Today the rate of basin deepening under transtension at the western end is matched by its progressive inversion and destruction under transpression in the eastern sector, with the oldest basin fill now being recycled. We propose a hybrid model for Hanmer strike-slip basin, one in which geometric elements of a fault-wedge basin (downward and upward tipped, spindle-shaped ends) are combined with those of a pull-apart basin (step-over region between the major fault segments). We also conclude that changes in fault geometry (releasing and restraining bends and step-overs) at a variety of scales and over short distances control the development of the extensile and contractile parts of the basin and three-dimensional basin asymmetry. Strain partitioning is complex and cannot be related simply to local reorientation of the regional stress field.

Paleogene seafloor spreading in the southeast Tasman Sea

Magnetic anomalies 11–18 have been identified in the southeast Tasman Sea, the area of ocean crust southwest of Fiordland, New Zealand, west of the Puysegur Trench (the present plate boundary) and southeast of the older (80–58 Ma) ocean crust of the Tasman Sea basin. Structures associated with two changes in spreading direction are preserved in the area. Spreading between the Australian and Pacific plates began in the Eocene, about 40 Ma in this area, orthogonal to Cretaceous-Paleocene spreading between them in the Tasman Sea. The boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the Resolution Ridge system. Rifting appears to have propagated northeast along a fracture zone. The northern blocks of the Resolution Ridge system may be isolated fragments of continental crust of the Campbell Plateau. Between anomalies 18 and 11 (40–30 Ma) the spreading rate was about 1.5 cm/yr. Swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11, about 30 Ma, and led to a change from tension to transcurrent motion along the plate boundary. The timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data.

Normal fault growth and linkage in the Whakatane Graben, New Zealand, during the last 1.3 Myr

Determination of fault growth rates and fault network evolution at time-scales from 104 – 106 years has been hampered by a lack of a well-constrained stratigraphic succession that provides a high-fidelity record of fault development over these time periods. Here we show how seismic reflection data of different spatial resolutions can be used to constrain the linkage history and displacement rate variations of a single major fault. We present data collected in the offshore Whakatane Graben, Bay of Plenty, New Zealand, where intense normal faulting occurs as a result of active back extension. The focus of our study is the Rangitaiki Fault, a linked segmented normal fault which is the dominant active structure in the graben. The total linked fault length is c. 20 km long and has a displacement of up to 830 ± 130 m in the top 1.5 km of sediments. The fault has been actively growing for the last 1.34 ± 0.51 Ma and has developed from isolated fault segments to a fully linked fault system. Initially, the dominant process of fault growth was tip propagation, with an average and maximum displacement rates of 0.52 ± 0.18 mm yr-1 and 0.72 ± 0.23 mm yr-1 respectively. Interaction and linkage became more significant as the fault segments grew towards each other, resulting in the fault network becoming fully linked between 300 and 18 ka. Following fault segment linkages, the average displacement rate of the fault network increased by almost three-fold to 1.41 ± 0.31 mm yr-1, while the maximum displacement rate increased to 2.72 ± 0.61 mm yr-1. This is the first time that the growth rate of unlinked fault segments has been resolved, and has been shown to be slower than in the subsequent linked fault system.

Subduction initiation at a strike‐slip plate boundary: The Cenozoic Pacific‐Australian plate boundary, south of New Zealand

[1] We first present a synthesis of the Macquarie Ridge Complex (MRC) tectonic structures as well as paleo-reconstruction models of the kinematic evolution of the Pacific-Australia plate boundary south of New Zealand, since the Eocene. We then ascertain the geodynamical conditions that preceded subduction initiation, and identify the nature and structures of the crust that first subducted, at the Puysegur subduction zone. This synthesis is used to produce a subduction initiation model for the Puysegur Region. Concomitant to inception of the Alpine Fault (ca. 23 Ma), a 150-km-wide transpressive relay zone developed along Puysegur Bank inherited structures, enabling localization of compressive deformation. Right-lateral motion at the relay zone has juxtaposed oceanic and continental crusts facilitating inception of subduction and controlling the subduction vergence. Subsequently, the Puysegur subduction zone initiated at the transpressive relay zone ca. 20 Ma. Upper and lower plate inherited structures guided and facilitated the lengthening of the subduction zone during the Neogene. The four individual segments of the MRC represent different stages of incipient subduction whose development depends on local geodynamical conditions and lithospheric heterogeneities. The example of the MRC demonstrates that subduction can initiate from an oceanic spreading center, through progressive changes in plate kinematics within a 10–15 Myr time frame. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8155 Tectonophysics: Plate motions—general; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: subduction initiation, Pacific, Australia, plate motion, Cenezoic, Macquarie Ridge

Abrupt strike-slip fault to subduction transition: The Alpine Fault-Puysegur Trench connection, New Zealand

Swath bathymetry and other geophysical data collected over the Fiordland Margin, southwest of New Zealand are used to investigate the mechanism of transform-subduction transition between the Alpine Fault and the Puysegur Trench, two segments of the Pacific-Australian plate boundary. In this region the Cenozoic Southeast Tasman Basin, which obliquely underthrusts Fiordland at the Puysegur Trench, is separated from the Cretaceous Tasman Basin by the Resolution Ridge System, a major lithospheric discontinuity of the downgoing plate. Interpretation of seafloor morphology shows that the Alpine Fault extends offshore along the Fiordland coast and splits into West and East Branches. The West Branch cuts obliquely across the margin and connects sharply to the Puysegur subduction front at the northeastern tip of the Resolution Ridge System. Earthquake and seismic reflection data indicate that the West Branch is genetically controlled by downgoing plate structures associated with the Resolution Ridge System. Hence the West Branch is interpreted as the surface trace of the plate boundary segment extending between the Alpine Fault and the Puysegur Trench. We conclude that the development of the strike-slip segment of the plate boundary and its sharp transition to the Puysegur subduction are controlled by inherited structures of the Australian plate. Furthermore, according to geophysical data presented here, a tearing of the downgoing plate can be interpreted beneath the West Branch. A review of geophysical data along the region of the Alpine Fault-Hikurangi Trough, northeast New Zealand, shows a progressive transform-subduction transition that is accommodated by motion partitioning between the subduction interface and strike-slip faults. This transition is accounted for by an interplate coupling that progressively increases toward the Alpine Fault in relation with a gradual thickening of the downgoing crust. The comparison between the Fiordland and the Hikurangi strike-slip-subduction transitions show that presence of inherited downgoing plate crustal faults, properly oriented with respect to the plate motion, facilitates a sharp strike-slip-subduction transition.