Geology topics

J.-Y. Collot

Publications and source records attributed to J.-Y. Collot.

5 recordsLinked to original sources

USGS2012

The collision zone between the North d'Entrecasteaux Ridge and the New Hebrides island arc. 2. Structure from multichannel seismic data

The d'Entrecasteaux zone (DEZ) collides with the central New Hebrides island arc and consists of two subparallel ridges that strike east-west, stand 1–2 km above the surrounding oceanic plate, and subduct obliquely (15°) northward beneath the arc. Rocks dredged from the north ridge as well as reflections evident in multichannel seismic reflection data indicate that this ridge has a volcanic origin. Crystalline volcanic rocks are common along the lower flank of the ridge, but sedimentary, probably volcaniclastic, rock caps the ridge. Seismic reflection data collected over the lower arc slope reveal that mass wasting deposits locally make up most of the accretionary wedge. These deposits appear to form discrete bodies, suggesting that mass wasting occurred episodically. Large anticlines and thrust faults having large vertical separation are not readily evident where the colliding ridge intersects the arc slope; apparently, slope rocks have low strength so that mass wasting deposits formed instead of large-relief structures. Mass wasting is thought to occur as the accretionary wedge is uplifted in response to the northward oblique subduction of the north ridge. The toe of the north ridge flank marks an abrupt transition in the lithologies that make up the footwall of the interplate decollement. Footwall lithologies change from ocean basin to volcaniclastic ridge material, and this transition probably marks a discontinuity in friction along the decollement or in rock mechanical properties because north of the transition, thrust faults deform the accretionary wedge whereas south of the transition, steep reverse faults crosscut the wedge and pierce the north flank of the ridge. This piercement means that the decollement at least locally lies within the ridge and that ridge material exotic to the New Hebrides arc may be incorporated into the accretionary wedge.

Journal of Geophysical Research↗

USGS2010

Structure of the collision zone between Bougainville guyot and the accretionary wedge of the New Hebrides Island arc, southwest Pacific

Multichannel seismic reflection data show the structure that develops within an island arc-guyot collision zone. The Bougainville guyot fills the New Hebrides trench, stands about 3 km above the abyssal ocean plain, and is capped by a broad platform that is underlain by a parallel bedded sequence, probably lagoon and reef rocks. A thick debris apron made up of unconsolidated sediment underlies the unsubducted part of the lower guyot flank and possibly the subducted flank as well. The contact zone between the arc and the north and east sides of the guyot is marked by discontinuous antiforms that include reflective, thrust faulted rocks that may have been derived from the guyot. The extent of collision deformation to the arc and guyot depends in part on the contrast in compressibility and viscosity between these features. We propose that the high-drag, subcircular guyot evolves during collision into a more streamlined shape. Streamlining may be achieved by processes like: (1) bulk deformation of the guyot by thrust faulting and sediment compaction; (2) ripping away of projecting parts of the guyot and blunting sharp edges (tectonic erosion), like that formed by the limestone cap; and (3) formation of a boundary layer, between the guyot and the arc, that is made up of water-rich, mobilized units of soft sediment. We draw an analogy between some features of glacial origin and the subducted part of a guyot. If the bulk of the guyot is low strength, especially in view of the thick debris apron, then the guyot may behave like till that was overridden by a glacier and forms a drumlin, an elongated, blunt-faced, streamlined feature. Thrust faults evident in the guyot may indicate deformation that leads toward a streamlined shape and not necessarily to accretion of guyot rocks.

Tectonics↗

USGS2010

Geologic structure of the northern New Caledonia ridge, as inferred from magnetic and gravity anomalies

Bathymetric, gravity, and magnetic data collected in the southwest Pacific Ocean over the northern New Caledonia ridge show that the main geological units known from the island of New Caledonia extend northward from this island, beneath the Grand Lagon Nord, the Grand Passage, and the d'Entrecasteaux reefs. These data support the model of tectonic evolution of the New Caledonia region proposed by Kroenke [1984]. We interpret a linear axial gravity low that extends from southern New Caledonia to the Grand Passage as evidence for the continuity of the thick pre-Permian to Jurassic core of the island. The Belep magnetic pattern, which covers the southwestern half of the Grand Lagon Nord, shows linear, high relief (1000–2000 nT) anomalies that are locally associated with a 120-mGal gravity high. These magnetic and gravity anomalies resemble anomalies measured over the west coast of New Caledonia, suggesting that the Cretaceous to Eocene basaltic complex of the coastal area is overlain by ophiolite remnants as far north as the western d'Entrecasteaux reefs. The similarity between the Belep magnetic pattern and a highly magnetic province evident 200 km southwest across the New Caledonia basin, along the Fairway ridge, indicates that volcanic rocks lie symmetrically on both sides of the New Caledonia basin. We suggest that part of these volcanic rocks were emplaced during the middle Cretaceous when the New Caledonia ridge rifted from the Australian margin. Simple gravity models of an elongated gravity high, having peak values in the range of +130 to +150 mGal, suggest that the ophiolite, which was thrust over New Caledonia during the Upper Eocene, extends along the east side of the Grand Lagon Nord and as far north as the d'Entrecasteaux reefs. Gravity and magnetic models suggest that in the area of the d'Entrecasteaux reefs, the ophiolite belt includes two subunits. The first subunit underlies the Huon-La Surprise platform and may include highly serpentinized ultramafic rocks without basaltic oceanic crust. The second subunit, which lies beneath the Guilbert ridge, may include ultramafic rocks as well as thin imbricate slices of oceanic crust. At the northern termination of the New Caledonia ridge, seismic and geopotential data evidence a major east-west trending tectonic zone that separates the basement of the New Caledonia ridge from the d'Entrecasteaux zone, an arcuate oceanic feature extending northward from the ridge. Differences in structure, geophysical signatures and morphology evident between areas north and those south of the Grand Passage, together with the nearness of the Le Noroit massif west of the Grand Passage, suggest that contemporaneously with Eocene to early Oligocene subduction along the western New Caledonia margin, an arc-ridge collision may have occurred near the northern termination of this subduction zone.

Tectonics↗

USGS1992

Geology of the d'Entrecasteaux-New Hebrides arc collision zone: Results from a deep submersible survey

During the SUBPSO1 cruise, seven submersible dives were conducted between water depths of 5350 and 900 m over the collision zone between the New Hebrides island arc and the d'Entrecasteaux Zone (DEZ). The DEZ, a topographic high on the Australian plate, encompasses the North d'Entrecasteaux Ridge (NDR) and the Bougainville guyot, both of which collide with the island-are slope. In this report we use diving observations and samples, as well as dredging results, to analyse the geology of the Bougainville guyot and the outer arc slope in the DEZ-arc collision zone, and to decipher the mechanisms of scamount subduction. These data indicate that the Bougainville guyot is a middle Eocene island arc volcano capped with reef limestones that appear to have been deposited during the Late Oligocene to Early Miocene and in Miocene-Pliocene times. This guyot possibly emerged during the Middle and Late Miocene, and started to sink in the New Hebrides trench after the Pliocene. The rocks of the New Hebrides arc slope, in the collision zone, consist primarily of Pliocene-Recent volcaniclastic rocks derived from the arc, and underlying fractured island-arc volcanic basement, possibly of Late Miocene age. However, highly sheared, Upper Oligocene to Lower Miocene nannofossil ooze and chalk are exposed at the toe of the arc slope against the northern flank of the NDR. Based on a comparison with cores collected at DSDP Site 286, the ooze and chalk can be interpreted as sediments accreted from the downgoing plate. East of the Bougainville guyot an antiform that developed in the arc slope as a consequence of the collision reveals a 500-m-thick wedge of strongly tectonized rocks, possibly accreted from the guyot or an already subducted seamount. The wedge that is overlain by less deformed volcaniclastic island-arc rocks and sediments includes imbricated layers of Late Oligocene to Early Miocene reef and micritic limestones. This wedge, which develops against the leading flank of the guyot, tends to smooth its high-drag shape. A comparison between the 500-m-thick wedge of limestones that outcrops southeast of the guyot and the absence of such a wedge over the flat top of the guyot, although the top is overthrust by island-arc rocks and sediments, can be interpreted to suggest that the wedge moves in the subduction zone with the guyot and facilitates its subduction by streamlining.

Tectonophysics↗

USGS1989

Formation of forearc basins by collision between seamounts and accretionary wedges: An example from the New Hebrides subduction zone

Seabeam data reveal two deep subcircular reentrants in the lower are slope of the New Hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin. The Malekula reentrant lies just south of the partly subducted Bougainville seamount. This proximity, as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off Japan, suggests that the Malekula reentrant formed by the collision of a seamount with the arc. An arcuate fold-thrust belt has formed across the mouth of the reentrant, forming the toe of a new accretionary wedge. The Efate reentrant may show the next stage in basin development. This reentrant lies landward of a lower-slope ridge that may have begun to form as an arcuate fold-thrust belt across the mouth of a reentrant. This belt may have grown by continued accretion at the toe of the wedge, by underplating beneath the reentrant, and by trapping of sediment shed from the island arc. These processes could result in a roughly circular forearc basin. Basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the Aleutian trenches.

Geology↗