MAGNETOSTRATIGRAPHIC DATING OF THE UPLIFTED ATOLL OF MARE - GEODYNAMICS OF THE LOYALTY RIDGE, SW PACIFIC

The Loyalty Islands (SW Pacific) are uplifted as they are progressivel y affected by the lithospheric flexure of the Australian plate, before its subduction under the New Hebrides Are. These geodynamic changes a re constrained by magnetostratigraphicdly dating two sections from Mar e Island, where mineral extractions coupled with rock-magnetic experim ents suggest that the magnetic remanence is mostly carried by a mixtur e of single-domain to multidomain magnetite/maghemite. With the help o f faunal determinations and radiometric dating, the sequences of polar ity reversals, correlated to the geomagnetic polarity timescale, range from the top of Chron C4n (late Miocene) to the top of the Gauss Chro n (late Pliocene). This new chronostratigraphy refines the timing of t wo distinctive carbonate units (rhodolith platform/coral reefs) separa ted by a hardground whose transition is known to coincide approximatel y with a regional event around the Miocene/Pliocene boundary. The magn etostsatigraphic dating indicates that the hardground represents about a 1.9 m.y. hiatus and suggests variable sedimentation rates ranging f rom 4.7 to 65.4 m/m.y. during the atoll construction. The lithospheric bulge seems to have influenced the evolution of Mare Island some 3.1 m.y. ago, leading to a diachronous emersion of the northeast and south west rim of the atoll with a mean uplift rate of the order of 4 cm/kyr .