Post-Eocene extensional tectonics in Southern New Caledonia (SW Pacific): Insights from onshore fault analysis and offshore seismic data

Abstract

Ductile to brittle extensional deformation following thrusting of the peridotites nappe during the Upper Eocene has been shown to play a major role in the Tertiary tectonic evolution of the northern part of the main island of New Caledonia and its eastern and western margins. In this study, we provide new tectonic data from southern New Caledonia that allow to better constrain the tectonic evolution of the southern part of the main island. We present a kinematic analysis of faults and striations obtained mainly from exposures of sedimentary rocks in the region of Nouméa with complements from measurements made farther north at Népoui within post-obduction Middle-Miocene deposits. We also present additional results of an interpretation of seismic lines from the lagoon south of the Nouméa Peninsula which provide constraints on the current tectonic regime of southern New Caledonia. Extensional faults in the Nouméa region have been studied within terranes of various ages including pre- and syn-obduction deposits and ophiolites. Hence, we demonstrate that important extensional events have affected the southern part of the New Caledonia block after the obduction of the peridotite nappe. The direction of maximum extension is variable at the scale of the region. Both high angle and low angle normal faults are present and block rotation is observed at some localities. This suggests that detachments accommodating significant displacements are cutting through the sedimentary pile. The average final strain pattern of the region can be regarded as the results of a multidirectional flattening, a hypothesis consistent with vertical uplift associated with regional extension. These results are in good agreement with conclusions of earlier workers showing late extensional evolution of the ophiolites along the chain and generalized extension in northern New Caledonia. The extensional faulting event in southern New Caledonia started after the obduction of the ophiolite, probably during the Oligocene. Extension was still active after the Middle Miocene, as shown by normal faulting in the Népoui conglomerates. Quaternary neotectonic features are observed in the lagoon and correspond to groups of short segments of vertical faults having orientation of N90, N40–50, N0–10 and minor N140, delineating subsiding areas. These faults are reactivated features with orientations corresponding to trends of coastlines and other main morphological features of the island. This study enhances the strong influence of normal-faulting neotectonics and recent tectonics on the current landscape evolution of southern New Caledonia. We infer that these tectonic processes played an important role during the weathering of the peridotites and subsequent development of the lateritic Ni-bearing layers as they increase the penetration of ground waters within the actively expending peridotite basement.