Bernard Pelletier

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.

Seismotectonics and present-day relative plate motions in the Tonga-Lau and Kermadec-Havre region

Abstract An updated compilation of shallow seismicity in the Tonga-Kermadec arc and back-arc region, including especially 304 focal mechanisms from centroid moment tensor solutions files, provides new insights on the present-day tectonics and allows the relative motions between the Pacific, Indo-Australian and Tonga-Kermadec plates to be quantified. Azimuths of slip vectors along the trench change at 19°S. In the southern domain (35-19°S) the mean azimuth ranges from N280°E to N285°E, while in the northern domain it trends N276–277°E. This 19°S boundary also exits in the back-arc domain. South of 19°S, from 35°S to 20°S the direction of back-arc extension is respectively N135°E, N122°E and N111°E within the southern Havre trough, northern Havre trough and southern Lau basin. In the southern Lau basin the N111°E opening direction is compatible with the trend of the newly mapped Valu Fa ridge. North of 19°S, in the northern Lau basin (16–18°S), a N93°E trending crustal extension occurs along the N135°E Peggy ridge and along an inferred N5–10°E spreading zone. Between these two extensional features a fourth plate, the northern Lau microplate, is present. At 18–19°S a ridge-ridge-ridge triple junction, accommodated by N95°-115°E right-lateral strike-slip fracture zones, is proposed at the complex transitional zone between the southern and the northern parts of the Lau basin. The Fiji-North Tonga fracture zone which bounds northward the Lau back-arc basin is composed of two segments: a N95°E eastern segment extending from the northern end of the Tonga trench to 178°30′W, and a N75°E western segment lying north of the Fiji platform. A ridge-fracture-fracture triple junction is inferred at 178°30′W–15°45′S between the northern end of the Peggy ridge and the two segments of the Fiji-North Tonga fracture zone. Using the model RM-2 of Minster and Jordan (1978) and the directions of back-arc spreading and convergence at the trench, relative plate motions are estimated. The extension rates in the back-arc domain increase northward, and are respectively 0.8, 2.1 and 8 cm/y at 33°S, 28°S and 24°S. Farther north parallelism between directions of different motions does not allow the calculation of any accurate result. Therefore north of 24°S we used a spreading rate of 8 cm/y which corresponds to the 24°S result and also to the value deduced from magnetic anomalies. The consumption rates along the Tonga-Kermadec trench are respectively 7.2, 9.6, 16.4, 17.1 and 17.8 cm/y at 33°S, 28°S, 24°S, 20°S and 17°S. Both the back-arc opening rate and the consumption rate of the Pacific plate sharply increase between the northern Havre trough and the southern Lau basin. This limit coincides with the Louisville ridge-trench junction. Along the N95°E eastern segment of the Fiji-North Tonga fracture zone, a pure left-lateral strike-slip motion, parallel to the N93°E opening tectonics within the northern Lau basin occurs. In contrast along the N75°E western segment of the Fiji-North Tonga fracture zone, a left-lateral strike-slip motion of 7 cm/y should be accompanied by a N135°E extensional motion of 3.5 cm/y.

Geodynamic evolution of the Taiwan-Luzon-Mindoro belt since the late eocene

Abstract The structural framework of the Taiwan-Luzon-Mindoro belt (or festoon) is described, following three major transects: the Luzon transect with active subduction and active island arc; the Taiwan transect with active collision; the Mindoro transect with active subduction and inactive collision. Based on this geological study and on available geophysical data, a model for the geodynamic evolution of this portion of the Philippine Sea and Eurasia Plates boundary is proposed in a succession of reconstructions between the Late Eocene and the Present. The major geodynamic events are: 1. (1) beginning of the opening of the South China Sea (S.C.S.) in Lower Oligocene times, contemporaneous with obduction of the Zambales and Angat ophiolites on Luzon. 2. (2) subduction of a Mesozoic (?) oceanic basin along the proto-Manila trench from the Upper Oligocene to the Lower Miocene. 3. (3) obduction of the South China Sea oceanic crust onto the Chinese and Reed Bank—Calamian passive margins in Middle Miocene time (14–15 Ma) related to a major kinematic reorganization (end of opening of the S.C.S.). 4. (4) beginning of collision between the Luzon microblock and the two margins of the S.C.S. in the Upper Miocene (~ 7 Ma); collision is still active in Taiwan whereas it stopped in Mindoro during the Pliocene.

The North Fiji Basin Geology, Structure, and Geodynamic Evolution

As the result of intensive studies conducted by U.S., French, and Japanese scientific teams, the North Fiji Basin ridge, poorly known 10 years ago, is one of the most exhaustively investigated ridge axes of the world’s oceans. Today, a ridge segment more than 800 km long and 100 km wide has been fully mapped with the Sea Beam and Furono echo sounders. This ridge axis shows four main segments characterized by the same morphostructural aspect and limits that characterize mid-oceanic ridges. Along the whole length of the axis, a water-column sample has been taken every 20 km and rock samples every 10 km. Different types of hydrothermal activity have been discovered and explored either during the Nautile cruise in 1989 or during the Shinkai 6500 cruise in 1992. The most famous site is the “White Lady,” located around 17°S; it is characterized by 285°C shimmering hot water, which is very poor in metallic elements, expelled through an anhydrite chimney. This water probably represents the low salinity end-member resulting from phase separation in the deep levels of the oceanic crust. Other active sites have been observed all along the axis showing different characteristics such as low-temperature diffusion zones. Even though some parts of the North Fiji Basin remain poorly investigated, the newly acquired data from the ridge axis and from the eastern and northwestern parts allow us to develop a new tectonic model of basin evolution since its creation 12 m.y. ago.

Middle miocene deduction and late miocene beginning of collision registered in the hengchun peninsula: Geodynamic implications for the evolution of Taiwan

Abstract Taiwan is located in the axis of the Manila Trench. It results from an oblique collision between the northernmost part of the Luzon arc and the Chinese passive margin. This active collision follows the subduction of the Oligocene-Miocene oceanic crust of the South China Sea along the Manila Trench. The tectonized Chinese margin emerged in the Hengchun peninsula (South Taiwan). Gentle folds which are delineated by the Quaternary reefal limestones demonstrate Recent deformations. These folds deformed a thick detrital sequence of Miocene age (Ssuchung Chi series) which was previously strongly folded and thrust westward (axis NS-N20) upon the Renting melange of Latest Miocene age. These main deformations, sealed by the Middle Pliocene, are the evidence for the onset of collision in this part of Taiwan at the end of the Miocene. Because of its obliquity, the collision started already in the northern part of Taiwan during the Late Miocene (6-7-8 Ma ?). The Ssuchung Chi series, a sequence of proximal turbidites, has contained, since the Middle Miocene (NN 6~13 Ma), fragments of an Oligocene to Lower Miocene oceanic crust. This ophiolitic material is very similar to the East Taiwan Ophiolite of the Coastal Range. It originated most probably from a slice of South China Sea crust obducted in Middle Miocene times (13–14 Ma) upon the Chinese margin (North of the Hengchun peninsula). This obduction occurred 7 to 8 Ma before the beginning of collision. These results make it possible to propose an evolutionary model for Taiwan from the Oligocene to the Recent, with the different phases of a collision between a volcanic arc and a passive margin.

Comparing the role of absolute sea-level rise and vertical tectonic motions in coastal flooding, Torres Islands (Vanuatu)

Since the late 1990s, rising sea levels around the Torres Islands (north Vanuatu, southwest Pacific) have caused strong local and international concern. In 2002–2004, a village was displaced due to increasing sea incursions, and in 2005 a United Nations Environment Programme press release referred to the displaced village as perhaps the world’s first climate change “refugees.” We show here that vertical motions of the Torres Islands themselves dominate the apparent sea-level rise observed on the islands. From 1997 to 2009, the absolute sea level rose by 150 + /-20 mm. But GPS data reveal that the islands subsided by 117 + /-30 mm over the same time period, almost doubling the apparent gradual sea-level rise. Moreover, large earthquakes that occurred just before and after this period caused several hundreds of mm of sudden vertical motion, generating larger apparent sea-level changes than those observed during the entire intervening period. Our results show that vertical ground motions must be accounted for when evaluating sea-level change hazards in active tectonic regions. These data are needed to help communities and governments understand environmental changes and make the best decisions for their future.

Newly identified segments of the Pacific–Australia plate boundary along the North Fiji transform zone

Abstract The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific–Australia plate boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise (March 2000) on board the R/V L’Atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100° has been mapped over a distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature. The uplifted Futuna–Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna spreading center trending 20–30° is composed of a series of en echelon left-stepping spreading segments. It represents a 200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones. The three active extensional zones have an ENE–WSW to NNE–SSW orientation, while compressive features along the Futuna transform fault are NW–SE to NNW–SSE oriented, in accordance with the present-day left-lateral transform motion along this part of the Pacific–Australia plate boundary.

Morphostructure and magnetic fabric of the northwestern North Fiji Basin

Four successive spreading phases are distinguished in the northwestern part of the North Fiji Basin. After an initial NE-SW opening, a N-S spreading phase took place, up to the northwesternmost tip of the basin, along the South Pandora, Tikopia and 9°30 Ridges. The N-S spreading phase in the northern North Fiji Basin was followed by an E-W opening phase along the central North Fiji Basin axis. A triple junction was probably active during an intermediate stage between the two phases. E-W spreading underwent a reorganisation that induced the functioning of the 16°40′S triple junction and the development of the E-W trending Hazel Holme Extensional Zone from the active central spreading axis to the southern tip of the New Hebrides Back-Arc Troughs. Active extension also occurs along the E-W Santa Cruz Trough which crosscuts the arc platform at the northern end of the N-S trending Back-Arc Troughs. The existence of the Back-Arc Troughs is mainly due to the construction of the 400 km-long volcanic Duff Ridge which trapped a piece of the old North Fiji Basin oceanic crust.

Seabeam and seismic reflection imaging of the tectonic regime of the Andean continental margin off Peru (4°S to 10°S)

Abstract Marine geophysical surveys employing Seabeam, multi- and single-channel seismic reflection, gravity and magnetic instruments were conducted at two locations along the continental slope of the Peru Trench during the Seaperc cruise of the R/V “Jean Charcot” in July 1986. These areas are centered around 5°30′S and 9°30′S off the coastal towns of Paita and Chimbote respectively. These data indicate that (1) the continental slope off Peru consists of three distinct morpho-structural domains (from west to east are the lower, middle and upper slopes) instead of just two as previously reported; (2) the middle slope has the characteristics of a zone of tectonic collapse at the front of a gently flexured upper slope; (3) the upper half of the lower slope appears to represent the product of mass wasting; (4) thrusting at the foot of the margin produces a continuous morphologic feature representing a deformation front where the products of mass-wasting are overprinted by a compressional tectonic fabric; (5) a change in the tectonic regime from tensional to compressional occurs at the mid-slope-lower slope boundary, the accretionary prism being restricted to the very base of the lower slope in the Paita area. The Andean margin off Peru is an “extensional active margin” or a “collapsing active margin” developing a subordinated accretionary complex induced by massive collapse of the middle slope area.

A coral‐based reconstruction of sea surface salinity at Sabine Bank, Vanuatu from 1842 to 2007 CE

Climate variability associated with the El Nino Southern Oscillation (ENSO) results in large sea-surface temperature (SST) and sea-surface salinity (SSS) anomalies in many regions of the tropical Pacific Ocean. We investigate interannual changes in SSS driven by ENSO in the southwestern Pacific at Sabine Bank, Vanuatu (SBV, 166.04°E, 15.94°S) using monthly variations in coralδ18O from 1842 to 2007 CE. We develop and apply a coral δ18O-SSS transfer function, which is assessed using a calibration-verification exercise (1970-2007 CE). The 165-year reconstructed SSS record contains a prominent trend toward freshening from 1842 to 2007 CE; mean SSS for 1842-1872 CE is 35.46 ± 0.28 psu, which contrasts with a mean value of 34.85 ± 0.31 psu for 1977-2007 CE, with a freshening trend during the latter part of the 20th century that is not unprecedented with respect to the overall record. Variance in the record is concentrated in the interannual (42%) and interdecadal (29%) bands. The SBV-SSS record matches well with a similarly reconstructed SSS time series at Malo Channel, Vanuatu, which is located ∼120 km to the east of SBV. This regional signal is likely driven by ENSO-related changes in the SPCZ and interdecadal changes in surface water advection. The pattern of interdecadal variability at SBV agrees reasonably well with coral records of interdecadal variability from Fiji and Tonga, especially in the pre-1940 portions of the records, further evidence for the regional extent of the salinity signal at Sabine Bank, Vanuatu.