Deepwater Fold‐and‐Thrust Belt Along New Caledonia's Western Margin: Relation to Post‐obduction Vertical Motions

Abstract

Classically, deepwater fold‐and‐thrust belts are classified in two main types, depending if they result from near‐ or far‐field stresses and the understanding of their driving and triggering mechanism is poorly known. We present a geophysical data set off the western margin of New Caledonia (SW Pacific) that reveals deformed structures of a deepwater fold‐and‐thrust belt that we interpret as a near‐field gravity‐driven system, which is not located at a rifted passive margin. The main factor triggering deformation is inferred to be oversteepening of the margin slope by postobduction isostatic rebound. Onshore erosion of abnormally dense obducted material, combined with sediment loading in the adjacent basin, has induced vertical motions that have caused oversteepening of the margin. Detailed morphobathymetric, seismic stratigraphic, and structural analysis reveals that the fold‐and‐thrust belt extends 200 km along the margin, and 50 km into the New Caledonia Trough. Deformation is rooted at depths greater than 5 km beneath the seafloor, affects an area of 3,500 km2, and involves a sediment volume of approximately 13,000 km3. This deformed belt is organized into an imbricate fan system of faults, and one out‐of‐sequence thrust fault affects the seabed. The thrust faults are deeply rooted in the basin along a low‐angle floor thrust and connected to New Caledonia Island along a major detachment. This study not only provides a better knowledge of the New Caledonia margin but also provides new insight into the mechanisms that trigger deepwater fold‐and‐thrust belts.

Recent advances in New Caledonian biogeography

“…Collot et al . () mapped the fault, and described it as ‘a major structural break’ (p. 2119). They also referred to the debate surrounding its exact nature and history.…”

Chapter 7 Post-obduction evolution of New Caledonia

“…Detailed seismic analysis reveals that the basal décollement of the DWFTB is progressively warped upward toward the western margin of New Caledonia. The dip reaches as high as 10 ± 2° to the SW along the margin, revealing that the inner part of the structure rises toward the margin of New Caledonia (Collot et al, 2017).…”

“…Following Late Eocene obduction (cf. chapter 5, this memoir), post-obduction isostasic rebound (Moretti & Turcotte 1985;Lagabrielle et al 2005;Chardon et al 2008;Sevin et al 2014;Collot et al 2017) has led to the deposition of a c. 5 km-thick sedimentary sequence in the adjacent New Caledonia Trough and Loyalty Basin (Fig. 21a and b).…”

“…The front of the deformed structure is therefore interlayered into the Oligocene to Holocene sedimentary pile deposited above the Eocene reflector. A Miocene age has been proposed for most of the sediments, whose deformation would have started during Late Miocene (Collot et al, 2017).…”

“…An active margin origin has been proposed, the structure being interpreted as an accretionary prism, deeply rooted along a palaeosubduction interface, plunging below Grande Terre (Cluzel et al 2001). Conversely, a passive margin origin related to important differential vertical movements following the latest-Eocene obduction, and therefore a gravity-driven process has been proposed (Collot et al 2017).…”