Abstract. New marine geophysical data along the Macquarie Ridge Complex, the AustraliaPacific plate boundary south of New Zealand, illuminate regional neotectonics. We identify tectonic spreading fabric and fracture zones and precisely locate the Australia-Pacific plate boundary along the Macquarie Ridge Complex. We interpret a-•5-10 km wide Macquarie Fault Zone between the two plates along a bathymetric high that extends nearly the entire length of the Australia-Pacific plate boundary south of New Zealand. We conclude that this is the active Australia-Pacific strike-slip plate boundary. Arcuate fracture zones become asymptotic as they approach the plate boundary. A broad zone of less intense deformation associated with the plate boundary extends -50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike-slip deformation, although teleseismic evidence overwhelmingly indicates strike-slip motion along the entire surveyed boundary today. The McDougall and southernmost Puysegur segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia-Pacific crest in the region. Our conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex addresses the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.

[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.

International audienceNew high-resolution marine data acquired aboard R/V Le Suroît was used to map active normal faults offshore Montserrat in greater detail. The main faults of the Montserrat-Havers fault zone have cumulative scarps up to 200 m high, and offset sedimentary layers by hundreds of meters. They are arranged in a right-stepping, en echelon, trans-tensional array, which confirms that they accommodate the left-lateral component of motion resulting from slip partitioning of oblique convergence along the volcanic arc. They cut across Montserrat's recent volcanic complex. Faulting and fissuring exerted control on the position of andesitic domes, which are aligned along the N110°E average fault trend. The ≈10 km-long fault segments that cross the island could produce damaging, M ≈ 6 events comparable to the shallow, 16 March 1985, Mw∼6.3 earthquake that ruptured a submarine, N140°E striking, left-lateral fault near Redonda

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 (4 0 km) steepsided cross section, and the axial strike-slip fault separates a shallow (125-625 m), flattopped 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 kni and normal fault throws increase along its outer wall, indicating greater flexure of the downgoing plate.

Abstract.Swath

The upper plate deformation pattern reflects the mechanical behavior of subduction zones. Here we focus on the consequences of the entrance of a buoyant bank into the Caribbean subduction zone during the Eocene by studying the oldest exposed rocks belonging to the Lesser Antilles volcanic arc. Using a novel geochronological data set, we show that the volcanic arc activity on the island of St. Barthelemy spanned over the mid‐Eocene to early Miocene with a westward migration of the tectono‐volcanic activity, which is comparable to what has already been observed on other volcanic islands in the Lesser Antilles. The kinematics analysis allows us to identify a switch in the stress field from pure to radial extension at the Oligo‐Miocene hinge with a subhorizontal σ3 that has a mean trend of N20°. A three‐step restoration of the regional deformation indicates that this switch from pure parallel‐to‐the‐trench extension to radial extension may reflect a strain partitioning initiation affecting the upper Caribbean Plate in response to trench bending that followed the entrance of the Bahamas Bank into the subduction zone. We show that the northern end of the Lesser Antilles arc shows a tectono‐volcanic evolution which is similar to the southern one. The north‐south dichotomy in the perpendicular‐to‐the‐trench extension, 15% in the north versus 30% in the south, may reflect different slab ends that are highly curved to the north (restraining the extension in the upper plate) versus a tear to the south (allowing a larger amount of extension within the upper plate).

Among the seismic surveys carried out in the framework of the EU -THALES WAS RIGHT project in the Lesser Antilles subduction zone, the SISMANTILLES II cruise of N/O ATALANTE (IFREMER, PI M. Laigle) collected 3 375 km of multi-channel reflection seismics with its 4.5 km long, 360 channels streamer.This survey focuses on the updip portion of the contact zone between the forearc and oceanic crusts, a proxy of the updip limit of the sismogenic portion of the subduction megathrust fault. The geometry of the survey has been designed based on the results of a preliminary SISMANTILLES cruise with N/O NADIR (IFREMER). It consists in a grid of profiles comprising 7 north-south strike-lines (300 km long and spaced by 15 km) crossed by 12 dip-lines (150 km long and spaced by 25 km), with an Ocean Bottom Seismometer network (OBS) deployed on the nodes of this MCS grid.We present the 12 dip-line transects spaced at about 25 km from each other and sampling a 280 km long segment of the subduction, from offshore Martinique Island in the south up to offshore Antigua Island in the north. They have all been processed on board with CGG-Veritas Geovecteur and Geocluster softwares up to post-stack time-migration with constant water velocity. Some profiles have been reprocessed at IFM-GEOMAR (Kiel, Germany) in the frame of a EU-TMR project with pre-stack depth migration (PSDM) processing after deconvolution and multiple attenuation and will be presented instead.The 12 dip-line transects reveal the trenchward-dipping forearc basement, the transition between the forearc sedimentary domain and the accretionary prism, as well as the arcward-dipping decollement and oceanic crust. The forearc basement can be followed beneath the 4 westernmost crossing strike-lines, reaching distances of 160-190 km from the volcanic arc, and up to 5 s twt beneath the sea-bottom reflection. In the northern half, together with the previous survey, 4 dip-lines reached out over the deformation front of the accretionary wedge over the incoming Atlantic lithosphere of the North American plate. The downgoing decollement and oceanic crust are imaged from the deformation front over a distance of approximately 75-80 km, and the signal can be followed down to the sea-bottom multiple, 6-7 s twt beneath the sea-bottom reflection west to the easternmost crossing strike-line (∼ 12-15 km depth).A first-order result is the tremendous along-strike variations in the forearc domain of its basement topography and basin thickness, as well as in the frontal part of the accretionary domain of the decollement and oceanic crust topography. A second first-order result is that these dip-lines reveal images that illustrate different stages of the upper-plate deformation induced by the oblique subduction of the two WNW-ESE aseismic ridges (topographical highs): the Barracuda ridge in the northern part, previously identified by the first survey to prolongate beyond the deformation front beneath the frontal accretionary wedge, and now also the Tiburon ridge in the southern part. Here, the P...

The Grenada Basin is bounded to the east by the active Lesser Antilles Arc, to the west by the north-south trending Aves Ridge, commonly described as a Cretaceous-Paleocene remnant of the "Great Arc of the Caribbean" (Burke, 1988), and to the south by the transpressional plate boundary with South America (Figure 1). This setting led previous authors to propose various models for the origin of the Grenada Basin, most of them assuming the basin to be at least partly floored by oceanic crust that was formed during the