Cainozoic Palaeomagnetism and Tectonics of the Marlborough Region, South Island, New Zealand

Abstract

Palaeomagnetic data have been collected from Cainozoic rocks throughout the Marlborough region in order to obtain a revised chronology of two important successions of marine strata at Blind River and Upton Brook and to determine the tectonic rotational history of the Marlborough region. The lithologies sampled were often weakly magnetised and contained multiple components of remanence. Temporal and spatial limitations of strata suitable for palaeomagnetic analysis prevents the determination of a regional rotational history for the entire Cainozoic. Data presented in this thesis are mainly from the Lower Awatere Valley where a thick succession of strata of Late Miocene to Early Pliocene age (Awatere Group) have proved most suitable for palaeomagnetic analysis. Detailed stratigraphic mapping in the Awatere Group has been undertaken to enable palaeomagnetic correlation across the Lower Awatere Valley. Ferrimagnetic iron sulphide minerals appear to be the major contributors to magnetic remanence in sediments of the Awatere Group. These minerals appear to have formed by diagenetic dissolution of detrital iron-titanium oxide minerals during burial, ca 1 - 2 ky after deposition. A reliable post-depositional remanent magnetisation has been identified in 85% of specimens collected for magnetostratigraphic analysis. Magnetostratigraphic results indicate that sedimentation rates at Blind River and Upton Brook averaged about 0.5 m/ky. A revised chronology of Late Miocene to Early Pliocene events at Blind River and Upton Brook has been determined from new magnetostratigraphic results. A coherent chronology from these two sections indicates that two synchronous low sea level and cool palaeoclimatic events occurred in the Late Miocene Kapitean Stage. This inference is supported by other well known coeval successions in the New Zealand region. The New Zealand chronology supports recent overseas determinations which indicate a two stage "Messinian salinity crisis" and a 4.8 Ma age for the Miocene-Pliocene boundary. Data from New Zealand, the Mediterranean and DSDP cores throughout the world's oceans suggest that these latest Miocene events are due to glacio-eustatic lowering of sea level. Tectonic rotation of the Marlborough region is well constrained for the period 8-4 Ma. No regional rotation is recorded before 4 Ma. Rapid regional rotation at rates of 7-8°/Ma appears to have begun at ca 4 Ma. Tectonic rotations in the Hikurangi margin are attributed to progressive changes in orientation of the underlying subducted Pacific plate. Accelerated rotation in the Hikurangi margin since 4 Ma may be related to a change in absolute motion of the Pacific plate between Anomalies 2A and 3 (3.4 - 3.9 Ma).