Marine magnetic anomalies over the Lord Howe Rise and the Tasman Sea: Implications for the magnetization of the lower continental crust

Abstract

Marine geophysical and geological data provide evidence for a continental origin of the Lord Howe Rise, a submarine plateau east of Australia. Our marine geomagnetic data reveal a long-wavelength anomaly associated with the western margin of the rise. Forward and inverse modeling shows that the anomalies can be best explained by strong magnetization of the lower crust of the Lord Howe Rise. A geomagnetic map of the northern Tasman Sea that includes Dampier Ridge, another elongated piece of presumably continental crust, shows positive anomalies over areas of continental origin and negative ones over the deep sea basin. This indicates higher values of the product of magnetization times thickness for the continental crust than for the oceanic crust. Three-dimensional model calculations show that the high magnetization in the lower crust of eastern Australia is limited to a maximum depth of 15–20 km. This is in accordance with geothermal results that indicate the Curie isotherm is at a shallow depth there. The anomalies of the Tasman Sea are negative due to the negative part of the edge anomaly on the seaward side of the continental margin. It is not necessary to hypothesize strong reversed magnetization in the lower oceanic crust and upper mantle.