Exploration Geophysics - Volume 24, Issue 2, 1993

Palaeomagnetism has a useful role to play in constraining models of the drift history of terranes along Australia’s active northern margin, in the collision zone between the Australian plate, and the Pacific and Southeast Asian plates. This is demonstrated in a study of the Kemum Terrane of the Bird’s Head, Irian Jaya. The results demonstrate that large-scale clockwise rotation in the Neogene and fixed position models can be ruled out, and support derivation of the Kemum Terrane from the northeastern margin of the Australian craton. The study also highlights that palaeomagnetic input is a two-way process: a well-defined reference Apparent Polar Wander Path (APWP) is as equally important as well-determined results.

A palaeomagnetic and magnetostratigraphic study has been carried out on lower Carnian and Norian rocks (Late Triassic) from four different sections of two different units of the Upper Antalya Nappes in the Taurus region of southwestern Turkey, the Bakirli Dag unit and the Kaymaz Dag unit. The origin of these nappes is uncertain and under debate. This study provides well defined directions of probable primary origin and good magnetostratigraphic correlations between the different sections, which allows us to propose a palaeogeographic interpretation. The Bakirli Dag unit of the Upper Antalya Nappes were located at a palaeolatitude of 6.9°N±1.5° during the early Carnian and at 10.2°N±1.0° during the latest Carnian-middle Norian. This slight northward movement is accompanied by a large clockwise rotation (29.5°±6.7°) which may correspond to an important breakup-phase of the north African margin during middle-late Carnian time. The palaeomagnetic results from the Kavur Tepe section of the Kaymaz Dag unit, previously considered to belong also to the Upper Antalya Nappes, show a palaeolatitude of 17.4°S±1.1° during Norian time. This result surprisingly indicates that this unit was located in the vicinity of the northern Indian margin during the Late Triassic.

Palaeomagnetic and rock magnetic properties have been determined for sediment cores collected during ODP Leg 133 from the continental slope adjacent to the Great Barrier Reef. The magnetic remanence properties of the sediments are poor, with a pervasive viscous overprint that obscures most reversal boundaries. However, an inter-core magnetic susceptibility stratigraphy has been established that permits some refinement of the existing biostratigraphy. The quality of the palaeomagnetic remanence record relates to the residence time of magnetic particles in the corrosive, near-surface, sulphate-reducing zone. Magnetic susceptibility variations appear to reflect glacio-eustatic sea level cycles, and show a strong inverse correlation with both δ¹⁸O and carbonate content. For the last several glacial maxima there are marked changes In magnetic properties, largely grain size-related, with sharp peaks in susceptibility and a reversal of the δ¹⁸O-susceptibility phase relationship. One possible explanation can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop during times of low sea level and become reworked during transgressions. Changes in the pattern of susceptibility variation occur at approximately 0.8 Ma and 0.4 Ma. Boundaries at these ages are seen in seismic profiles, they occur in the δ¹⁸O frequency spectrum, and there is a rise in sea surface temperature at ~0.4 Ma. We suggest that these boundaries relate through palaeoclimatic and palaeoceanographic changes to stages in evolution of the Great Barrier Reef.