Jean Delteil

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.

Strike-slip structure and sedimentary basins of the southern Alpine Fault, Fiordland, New Zealand

The Alpine Fault is an 850-km-long, continental dextral strike-slip fault that accommodates some 60%–90% (∼20–30 mm/yr) of the obliquely convergent motion between the Pacific and Australian Plates in South Island, New Zealand. The southern 230 km of the fault traverses the continental margin off Fiordland and intersects the subduction thrust at the northern end of the Puysegur Trench. Marine seismic reflection profiles and bathymetric data are used to evaluate the late Quaternary structure and 3 m.y. evolution of the fault and five sedimentary basins. The fault offshore is more complexly segmented than its onshore counterpart, and at sedimentary basins has substantial (>1 km) bathymetric relief. Three right-stepping and overlapping active sections are identified on the basis of structural continuity and geomorphic expression. The northern, Milford-Caswell section, is 90 km long and continuous with the southern Westland section on land. The southern, Resolution section is 150 km long and developed from initially discontinuous, right-stepping segments. These sections overlap by 35 km at the large Secretary-Nancy Basin, which is currently being dissected longitudinally by the Nancy section, resulting in a straighter, almost fully linked principal displacement zone. The average strike of the fault differs from the azimuth of Pacific-Australian Plate motion by 11°–25°. Nevertheless, on the whole, the structure, tectonic geomorphology, and lateral displacements indicate predominantly dextral strike-slip displacement. The basins have the structural characteristics of pull-apart and releasing-bend basins. They commonly initiated at step-overs in the fault and evolved by development of very oblique cross- and along-basin faults that linked the principal displacement zone, resulting in longer, through-going surface traces. Localized transpressional features are developing contemporaneously with the basins and may relate to constrictional fault geometries and/or a 10° rotation of part of the fault within the last 3 m.y., rather than to changes in plate motions. The absence of significant regional contraction across the fault indicates that obliquely convergent plate motion is strongly partitioned. The convergence normal to the boundary is accommodated largely on thrust and reverse faults offshore and onshore, to the northwest and southeast of the Alpine Fault, respectively. The position of the Alpine Fault is clearly associated with the position of inherited Eocene rift structures carried in the subducting Australian Plate. Our observations and kinematic model imply a spatial-temporal evolution where the position of the southwestern end of the fault trace is transitory on a time scale of 10 5 –10 6 years, and the fault matures toward the northeast. The data are consistent with models invoking dextral reactivation of the subducting rift structures and tearing of the Australian Plate in the approximate direction of plate convergence, and/or the loading of shear stresses and strike-slip deformation on the Alpine Fault in response to spatial differences in surface roughness and interplate coupling. Several potential earthquake scenarios on the Alpine Fault are considered, ranging from rupture of individual structural sections associated with earthquakes of magnitude M 7.0–7.8, to larger, onshore-offshore composite ruptures of up to M 8.1.

Crustal strain in the Southern Alps, France, 1948–1998

Active tectonics in the Western Alps is revealed by a moderate level of seismic activity and geological evidences for Quaternary deformation. We present new geodetic determinations of the current strain rates in the southern part of the Western Alps, that complement existing results in Provence, the Jura, and the Subalpine and Belledone massifs around Grenoble. We combined first- and second-order triangulation data collected in 1948 with global positioning system (GPS) data collected in 1998. We estimate shear strain rates of 0.1‐0.2 mrad yr’1 over distances on the order of 30 km, significantly diVerent from zero in most of the network. We obtain NW‐SE to N‐S compressive shear strain directions over most of the area, in agreement with the seismological and geological data. These geodetic estimates correspond to long-term overall N‐S to NW‐SE shortening of 2‐4 mm yr’1 over the study area, on the same order as the far-field shortening measured by continuous GPS on the Grasse‐Torino baseline (’2.0±0.5 mm yr’1), that incorporates our geodetic network. These results provide new constraints on interseismic strain in the Western Alps.

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.

Morphostructure of an incipient subduction zone along a transform plate boundary: Puysegur Ridge and Trench

Multibeam bathymetric and geophysical data reveal a major strike-slip fault that extends along the summit of the Puysegur Ridge east of the Puysegur Trench. The northward structural development of this ridge-trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m.y. At the southern end of the trench, where subduction has not yet started, the Puysegur Ridge has a narrow (<50 km) steep-sided cross section, and the axial strike-slip fault separates a shallow (125–625 m), flat-topped eastern crest from a deeper (400–1600 m) western crest; these characteristics indicate differential uplift during the initial stage of shortening. On the lower plate an incipient, 5.2-km-deep trench developed in conjunction with normal and reverse faults, suggesting strong interplate coupling across the trench. Northward, the ridge broadens linearly to 80 km wide, its western flank has locally collapsed, and the ridge summit has subsided, possibly by 1.5 km, suggesting that the interplate coupling decreases and that a Benioff zone is being formed. Concomitant to the northward ridge evolution, the trench deepens to 6.2 km and normal fault throws increase along its outer wall, indicating greater flexure of the downgoing plate.

Structural variety and tectonic evolution of strike‐slip basins related to the Philippine Fault System, northern Luzon, Philippines

The northern part of the Philippine fault zone, in Luzon, corresponds to a complex braided system of left-lateral strike-slip faults. The NW oriented main active branch, which emerges from the Philippine Sea, splits into an array of north striking splays responsible for the tectonic evolution of the Central Cordillera. This complex fault pattern has favored local stress field variations. Strike-slip basins have evolved in this framework along or in the vicinity of the main splays of the Philippine fault. Apart from the classical pull-apart tectonics on a releasing fault termination, overlap, or bend, we describe other mechanisms such as the strike-slip tilting or the warping of a strip limited by two strike-slip faults. The strike-slip basins are good recorders of the evolution of the Philippine fault system. Those located along the north striking cordilleran faults individualized in late early Pliocene to Pleistocene time when the present-day Philippine fault initiated, but their main control is the fault shape acquired in upper middle Miocene time. The recent tectonic evolution of the fault system is best recorded in central Luzon, where the active basins trace an asymmetrical V shape, with the longest branch trending NE parallel to the East Luzon Trough, and the shortest one trending NW related to the Philippine fault. Both the fault pattern and the basin distribution demonstrate the influence of the Benham Rise in the tectonic evolution of Luzon. The structural setting is interpreted as the result of an early Miocene collision event between the Benham Rise and the eastern margin of Luzon, and subsequent inception of the NW striking strand of the Philippine fault. The present locations of the basins result from the interaction between the structural heritage and the present-day regional plate motion.

Control of Permian and Triassic faults on Alpine basement deformation in the Argentera massif (external southern French Alps)

Structural investigations reveal intense and heterogeneous deformation of the sedimentary cover attached to the basement complex of the southern Argentera and Barrot massifs (southernmost External Basement Massifs of the French Alps). Permian and early Triassic syn-depositional extensional tectonics imparted a tilted block pattern to the massifs. An early Miocene first stage of Alpine compression caused pervasive cleavage. This cleavage was controlled by the former pre-existing faults but is nevertheless consistent with NNE contraction. Where regional shortening is orthogonal to the trend of pre-existing faults the pervasive deformation produced either irrotational compressional strain (where no fault inversion occurred), or rotational compressional strain involving syn-cleavage shearing (where faults with favorable paleo-dip were inverted). Where the shortening direction is oblique to the paleo-fault trends, a component of strike-slip movement may locally prevail. A 22 %, N020o directed horizontal shortening, of 11 km, has been calculated based on deformed sedimentary markers in the Permian series and parallel folds in Lower Triassic quartzite. A shallower deformation as brittle reverse faults postdates the cleavage at the southwestern tip of the Argentera Massif and accounts for 4 km of extra shortening. Both types of deformation are connected at depth to a crustal blind thrust system and the Argentera Massif is over-thrust to the south-southwest. The observed strain indicates the Argentera Massif area underwent, from earliest Miocene to Present, a NNE to N rotating compression at distance from the left-lateral southwestern boundary of the Adria block.

Influence of preexisting backstop structure on oblique tectonic accretion: The Fiordland margin (southwestern New Zealand)

The thinned continental crust of the Australian plate is being obliquely subducted beneath the South Island on the Pacific plate along the Fiordland margin of New Zealand. Morphostructural analysis reveals that the continental slope of Fiordland is cut by faults that appear to splay southwestward from the onland transpressive dextral Alpine fault system. Active strike-slip movement occurs close to the shoreline where upper canyon courses are dextrally offset relative to the fiords. The canyons, however, postdate strike-slip movement on the fault splays on the slope, indicating that the splays are no longer active. These now-inactive strike-slip faults have imparted a saw-tooth backstop geometry to the margin that controls the locus of sediment accretion in the trench. Accretion occurs as two discrete lobes, each at a left step in the backstop. The morphology and internal structure of the accretionary lobes indicate that there is partial strain partitioning between the Alpine fault and the base of the continental slope. Oblique convergence in association with a stepped backstop produces strongly curved fold axes and thrusts that abut almost orthogonally against the toe of the margin. Thus Fiordland provides a tectonic model for oblique convergence at a previously structured margin.

Geologie de la region du Djebel Thaalba, monts de la Mina (Tell oranais, Algerie)

Complex structures, nappes, Priabonian, Oligocene, Aquitanian, sandstone, autochthonous Miocene, foraminifera