Takemi Ishihara

Cenozoic motion between East and West Antarctica

The West Antarctic rift system is the result of late Mesozoic and Cenozoic extension between East and West Antarctica, and represents one of the largest active continental rift systems on Earth. But the timing and magnitude of the plate motions leading to the development of this rift system remain poorly known, because of a lack of magnetic anomaly and fracture zone constraints on seafloor spreading. Here we report on magnetic data, gravity data and swath bathymetry collected in several areas of the south Tasman Sea and northern Ross Sea. These results enable us to calculate mid-Cenozoic rotation parameters for East and West Antarctica. These rotations show that there was roughly 180 km of separation in the western Ross Sea embayment in Eocene and Oligocene time. This episode of extension provides a tectonic setting for several significant Cenozoic tectonic events in the Ross Sea embayment including the uplift of the Transantarctic Mountains and the deposition of large thicknesses of Oligocene sediments. Inclusion of this East–West Antarctic motion in the plate circuit linking the Australia, Antarctic and Pacific plates removes a puzzling gap between the Lord Howe rise and Campbell plateau found in previous early Tertiary reconstructions of the New Zealand region. Determination of this East–West Antarctic motion also resolves a long standing controversy regarding the contribution of deformation in this region to the global plate circuit linking the Pacific to the rest of the world.

A new global marine magnetic anomaly data set

The Geophysical Data System (GEODAS) stores more than 20 million magnetic measurements acquired over oceans and seas since the 1950s. Usually, the original total field (TF) and magnetic anomaly values are both stored. The anomaly results from the subtraction of the core and external magnetic field estimates from TF values. The most recent International Geomagnetic Reference Field models available at the time of the surveys were used to estimate the core field component (these models were revised later). External fields were estimated from magnetic observatory data. However, most of the measurements were not corrected for the external fields. Here we use comprehensive models to properly remove the core and external magnetic fields from all original TF measurements stored in the GEODAS. Besides, a track-by-track analysis of each data is necessary mainly to correct or to remove many shifted values as well as to reduce the noise in some track lines. Two additional processes are applied to obtain a data set coherent over the world. It includes an adjustment of long-wavelength magnetic anomalies using the National Geophysical Data Center (NGDC) -720 model, plus a line leveling method which mainly reduced some inconsistencies between different surveys. The root mean square of the crossover differences was reduced from 179.6 to 35.9 nT. Comparisons of magnetic anomaly maps before and after our treatment also highlight an improvement in the quality and the coherence of the data set. This study will serve to build a new World Digital Magnetic Anomaly Map.

ADMAP — A Digital Magnetic Anomaly Map of the Antarctic

For a number of years the multi-national ADMAP working group has been compiling near surface and satellite magnetic data in the region south of 60° S. By the end of 2000, a 5 km grid of magnetic anomalies was produced for the entire region. The map readily portrays the first-order magnetic differences between oceanic and continental regions. The magnetic anomaly pattern over the continent reflects many phases of geological history whilst that over the abyssal plains of the surrounding oceans is dominated mostly by patterns of linear seafloor spreading anomalies and fracture zones. The Antarctic compilation reveals terranes of varying ages, including Proterozoic-Archaean cratons, Proterozoic-Palaeozoic mobile belts, Palaeozoic-Cenozoic magmatic arc systems and other important crustal features. The map delineates intra-continental rifts and major rifts along the Antarctic continental margin, the regional extent of plutons and volcanics, such as the Ferrar dolerites and Kirkpatrick basalts. The magnetic anomaly map of the Antarctic together with other geological and geophysical information provides new perspectives on the break-up of Gondwana and Rodinia evolution.

Development of a hybrid gravimeter system onboard an underwater vehicle

We are developing a hybrid gravity survey system using a gravimeter and a gravity gradiometer to survey sub-seafloor density structure. This paper describes the development of a gravimeter with forced gimbals for this purpose. The aimed precision is about 0.1 mgal, one order better than sea surface gravimetry. After a successful feasibility study by using a Scintrex CG-3M gravimeter under a condition of pitching and rolling as well as heaving, we are developing a new underwater gravimeter system with a gravity sensor by Micro-G LaCoste. The design, performance, and future tasks are described.

Magnetic images of the Sumatra region crust

Magnetic images of the region surrounding the Great Sumatra earthquake and tsunami of 26 December 2004 provide a current and historical view of subduction in the region. A quasi-static far-field image (Figure la) clearly shows the compressional boundary between the Eurasian and Indo-Australian plates. Along the fault rupture, magnetic crustal thicknesses increase markedly in an easterly and northeasterly direction. Both the island arc and the subducting slab are magnetic, and the subducting slab is diving into the mantle at a steep angle, further increasing the magnetic thickness. To the east, between Singapore and the south coast of the island of Borneo, a previously unrecognized first-order feature parallels the currently active subduction zone. Like the present subduction zone, the feature is characterized by a two-fold to three-fold increase in magnetic thickness in a northeasterly direction. While the features origin is enigmatic, it probably reflects the past history of subduction in the region.

Gravity gradiometer implemented in AUV for detection of seafloor massive sulfides

Gravity surveys are useful for profiling underground density structures. We propose a hybrid gravity survey method using gravimeters and gravity gradiometers to detect submarine ore deposits, such as massive sulfides buried below the seafloor. This paper describes a gravity gradiometer, which can be implemented in an AUV (Autonomous Underwater Vehicle) for this purpose. We developed a new gravity gradiometer comprising of two vertically separated accelerometers with astatic reference pendulums, whose motions are precisely detected by optical sensors. Two identical pendulums are installed in a forced gimbal to reduce the rotational motion of the underwater vehicle. The Laboratory measurements show that the noise level of the gravity gradiometer is estimated to be 7 E (1 E = 1 × 10-9 /s2), with which typical submarine ore deposits can be detected using the gravity gradiometer implemented in an AUV. The gravity gradiometer is housed in a pressure-proof vessel made of titanium alloy, 500 mm in diameter and 700 mm in height, and the vessel is implemented in an AUV. We plan a sea trial in September, 2012, in Sagami Bay, off the coast of Shizuoka Prefecture in Japan.

A gravity gradiometer to search for submarine ore deposits

Gravity surveys are useful in profiling the underground density structure. We propose a hybrid gravity survey method using gravimeters and gravity gradiometers to detect submarine ore deposits. This paper describes the development of a gravity gradiometer for this purpose. The required resolution is estimated to be finer than approximately 10 E (= 1 × 10−8 /s2), considering typical dimensions of submarine ore deposits and survey altitude from the seafloor. To attain the required resolution, we newly developed a gravity gradiometer comprising two vertically-separated accelerometers with astatic reference pendulums. Laboratory measurements showed that the gravity gradiometer attained the required resolution. Its design, performance, and future tasks are described.

A new leveling method without the direct use of crossover data and its application in marine magnetic surveys: weighted spatial averaging and temporal filtering

The author has developed a new leveling method for use with magnetic survey data, which consists of adjusting each measurement using the weighted spatial average of its neighboring data and subsequent temporal filtering. There are two key parameters in the method: the ‘weight distance’ represents the characteristic distance of the weight function and the ‘filtering width’ represents the full width of the Gaussian filtering function on the time series. This new method was applied to three examples of actual marine survey data. Leveling using optimum values of these two parameters for each example was found to significantly reduce the standard deviations of crossover differences by one third to one fifth of the values before leveling. The obtained time series of correction values for each example had a good correlation with the magnetic observatory data obtained relatively close to the survey areas, thus validating this new leveling method.