Stratigraphy and Tectonics of the Continental Norfolk Ridge, SW Pacific Ocean.
Résumé
The Norfolk Ridge is a north-south trending ca. 1000 x 80 km bathymetric high located along the easternmost edge of the Zealandia continent at ca. 1 km water-depth. It lies in a key tectonic position which marks the boundary between Neogene subduction-related arcs and backarc basins to the east (e.g. 3-4 km deep Norfolk Basin) and Mesozoic to Paleogene continental basins to the west (e.g. 2-3 km deep New Caledonia and Reinga basins). The ridge is emergent in New Caledonia and New Zealand where obduction is known to have occurred in the late Paleogene to early Neogene. It is also hypothesized to be proximal to where the Tonga Kermadec Subduction initiated. Yet, the structure, geology and stratigraphy of the Norfolk Ridge remain largely unknown. New geophysical and geological data from the TECTA and VESPA voyages acquired in 2015 onboard RV L’Atalante reveal the structural style and stratigraphy of the ridge. It is composed of a thick sedimentary succession perched between the Norfolk and New Caledonia basins. Two main seismic units are observed. The lower unit (NR1) is thick (> 2 s twt), tilted east towards the Norfolk Basin and its top is marked by a major erosional unconformity. The upper unit (NR2) is thinner (< 0.5 s twt), likely composed of hemipelagic sediments affected by deep sea currents; and hence it drapes the ridge unevenly, exposing NR1 on the seafloor in many places. Some VESPA rock dredges were made accross the unconformity and sampled Oligocene shallow water carbonates. Volcanoes are prominent in several sectors of the ridge and post-date the erosional unconformity. Both the eastern and western margins of the ridge are affected by normal faulting that also seemingly post-dates the erosional event. No evidence of contractional deformation is observed on the ridge, except very locally close to New Caledonia and within Reinga Basin where contraction is widespread. Collectively these new observations indicate the ridge underwent important vertical motions (>1 km) that in the Eocene-Oligocene, but – surprisingly – without evidences for contraction. These events were closely followed by, or synchronous with, a volcanic episode and normal faulting that formed the ridge’s present-day morphology. This calls into question the modalities of the induced subduction initiation model that is proposed for the region.