A structural study of the Nouméa Basin sediments, New Caledonia

The microcontinent of New Caledonia covers 18,580 sq km and is located in the SW Pacific, just north of the Tropic of Capricorn. Our research was based on the southwestern coast of the mainland, in the Nouméa Basin, which represents a transgressive sedimentary sequence that contains cogenetic rhyolitic lavas and less voluminous mafic and intermediate volcanic rocks. Our research area was a newly developed community, Savannah, with recently exposed outcrops allowing us to collect and interpret the structural data. The data that we collected is comprised of 139 strike and dip measurements of the sedimentary beds, as well as measurements of the faults and fractures. For accuracy, we took GPS positions of our twenty-nine sample locations. The collection of our data shows a general trend of NW-SE strike with the beds dipping towards the SW. A population of 105 strikes trend an average of 315°, a second population of 23 strikes trend an average of 062°, and a third population of 9 strikes trend an average of 008°. Faulting showed two distinctly different trends. A population of six reverse faults trends an average of 321°, and one reverse fault trends 098°. A population of three normal faults trends an average of 316° and its conjugate pair, a population of three normal faults, trends an average of 070°. The fractures trend an average of 325°. In no instance did the two orientations of faults intersect. The general trend of faulting and folding indicates shortening in a NE-SW direction. The data from our subject area suggests a shortage of 10-15% in the Nouméa Basin. We also located outcrops of basalt, which are intrusive, and have altered the surrounding sediments, and we observed volcanic ash within some of the sedimentary units. The trends of the strikes and dips, as well as the trends of faults and fractures, indicates one generation of a single force directed in the SW-NE direction, which correlates with the tectonic activity of the island, i.e. the Eocene emplacement of the Massif du Sud ophiolite.