Holocene tectonic histories for five islands in the south-central Lau group, south Pacific

Low-level (<30 m) evidence for emergence is reported for five islands along the south-central Lau Ridge, which has a well-documented history of late Cenozoic uplift. Most of the evidence is erosional, some of the lowest is emerged reef and beachrock: all can be linked to higher relative sea levels. Correlations are established between islands and most of the series of emerged shorelines found to exhibit characteristics of coseismically uplifted series elsewhere. Dates for various shorelines allow ages and recurrence intervals of coseismic-uplift events to be tentatively established for all islands. By reconciling this with contemporary sealevel changes, magnitudes of coseismic uplift are obtained for the middle and late Holocene. The islands of Moce and Oneata have experienced fewest coseismic-uplift events with small uplift magnitudes. Four coseismic-uplift events are identified for Lakeba with an average magnitude of 2.63 m and an assumed recurrence interval of 1045 years: no uplift has apparently occurred since 3800 BP. Five coseismic-uplift events are identified for Namuka-i-Lau with an average magnitude of 2.08 m and an assumed recurrence interval of 1045 years: no uplift has apparently occurred since 2360 BP. Three coseismic-uplift events occurred on Nayau, averaging 1.53 m in magnitude and recurring every 1045 years: no uplift has occurred since 2240 BP. The approach used here has implications not only for understanding tectonic history but may eventually prove a useful tool for isolating Holocene sealevel history in tectonically active areas. © 1995, Sage Publications. All rights reserved

Quaternary atoll development : new insights from the two-dimensional stratigraphic forward modelling of Mururoa island (central Pacific ocean).

International audienceKnowledge about the Quaternary evolution of mid-ocean atolls comes mainly from drilling and field observations carried out on a number of Pacific carbonate islands. However, little is known about the early to mid Pleistocene atoll development history, especially at margin and foreslope settings. Using previous field and subsurface data from Mururoa Atoll and a process-based modelling software (DIONISOS), a two-dimensional forward stratigraphic model of atoll development is proposed for the past 1·8 million years (Myr). Observational data from vertical to inclined coring, seismic and bathymetric surveys indicate that, from approximately 0·45 to 0·40 million years before present (Ma), carbonate deposition at Mururoa Atoll resulted in a series of mostly prograding reef units. The model is first constrained at the base by the shape and topography of the pre-Quaternary basement. A number of sensitivity tests were performed to define the respective influence of variant parameters. The best-fit development scenario that accounts for the overall geometry and stratigraphic architecture of the Quaternary sediment packages is obtained by using the sea-level curve by Miller et al. (2005), uniform subsidence rate of 105 m Myr−1, and carbonate production rates gradually increasing from 0·50 to 8 mm yr−1 between 1·80 Ma and the present. Additional controlling parameters include subaerial erosion (at a constant rate of 0·25 m/kyr), wave-energy and sediment-transport processes. The stratigraphic forward model predicts a succession of three distinct types of carbonate systems that have developed since the earliest Pleistocene: toe of slope systems from 1·80 Ma to about 0·80 Ma, open-platform systems from 0·80 Ma to 0·50 Ma, and framework-reef systems from about 0·50 Ma to the present. The development of these different systems is most likely to be controlled by climate and changes in sea-level cycles. During the low-amplitude 41 kyr cycle periods of the earliest Pleistocene, ambient conditions were not conducive to framework-reef growth; shallow-water carbonate sedimentation was dominantly gravity-driven, operating along the platform foreslopes only. During the Mid-Pleistocene Climate Transition, narrow, open-platform units have developed at the upper parts of the pre-Quaternary basement flanks. With the onset of the high-amplitude 100 kyr sea-level modes and climate restoration, reef frameworks started to be generated. These models from Mururoa agree with a number of previous studies suggesting that most of the true framework reefs were not initiated prior to 0·50 Ma. Mururoa Atoll is demonstrated to be a robust analogue for providing more realistic interpretations of the development history of Pacific atolls. Further modelling with three-dimensional DIONISOS could generate better predictions by taking into account hydrodynamic and transport parameters more accurately