The Archean granites of the Vredefort impact structure show a high intensity of natural remanent magnetization (NRM) and a random dispersion of directions of high-coercivity components on the centimeter scale. It has been suggested that this anomalous remanence is carried by rod-shaped single-domain (SD) magnetites along planar deformation features (PDFs) in shocked quartz produced as a consequence of the impact event. To determine the carriers of this NRM, we conducted surface magnetic field observations using scanning magneto-impedance (MI) magnetic microscopy during stepwise alternating field (AF) demagnetization over a 1-mm-thick slice of Vredefort granite. We found that the stable component after demagnetization gives rise to just three strong magnetic anomalies. Progressive thinning of the scanned section and micro-Raman spectroscopy revealed that the source of these magnetic anomalies, the highly coercive remanence-carrying mineral, is an assemblage of relatively coarse-grained (1-200 mu m) magnetite in biotite, not single-domain magnetite embedded along PDF lamella.

This chapter contains sections titled: Introduction Geologic Setting of the Hole 1172D K-P Transition Methods Results Discussion Conclusions

The crustal magnetic field of Mars differs markedly from Earth’s as the Martian features are an order of magnitude stronger and are not distributed over the whole globe as on Earth, but are concentrated in a band in the southern highlands. Unless the Martian dynamo gave a surface field more than an order of magnitude stronger than the current geodynamo gives on Earth, which seems unlikely, some special process, which has not affected the terrestrial crust, operated on Mars to give the observed distribution of anomalies and to magnetize its ancient crust so efficiently. We suggest that water reacting with ancient Martian atmospheric carbon dioxide could give rise to fluids that dissolve igneous rocks in the crust and precipitate iron-rich carbonates, as observed in Martian meteorites. In the southern highlands, thermal decomposition of such iron-rich carbonates during metamorphism could give rise to plentiful single-domain magnetite and generate a potent source for the Martian crustal field. The lack of anomalies in the northern plains may result from higher water–rock ratios that prevented the formation or decomposition of iron-rich carbonates or the survival of single-domain magnetite.

One of the great stories of geoscience is how Gondwana broke up and the other southern continents drifted northward from Antarctica, which led to major changes in global climate. The recent drilling of Ocean Drilling Project (ODP) Leg 189 addressed in detail what happened as Australia drifted away from Antarctica and the Tasmanian Gateway opened. The drifting contributed to the change in global climate, from relatively warm early Cenozoic “greenhouse” conditions to late Cenozoic “icehouse” conditions. It isolated Antarctica from warm gyral surface currents from the north and provided the critical deepwater conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans.

Abstract The Jurassic-Cretaceous Kalaw redbeds of Myanmar yield a prefolding Late Cretaceous to Early Palaeogene magnetization which records c. 25–30° of clockwise (CW) rotation relative to the South China block. This corresponds to 10–15° CW relative to the Lower Cretaceous Khorat Plateau VGP. The data also show 5° of northward transport relative to the 100 Ma South China VGP or 12° relative to the Khorat Lower Cretaceous VGP. Similar CW rotations are measured in remagnetized Palaeozoic carbonates in Peninsular Thailand and Langkawi Island, Malaysia. These block motions most likely took place between the Late Cretaceous and the Late Oligocene. These and other recently published data have several implications for the extrusion tectonic model: (i) Sundaland has only rotated 25–30° CW relative to South China during the Tertiary; (ii) southeastward translation is only 300–500 km; and (iii) Sundaland is composed of smaller sub-blocks, some of which have moved northward. This is interpreted to indicate that deformation of the Sibumasu block is dominated by the oblique subduction of the Indian Ocean Plate while deformation of the Indochina block is dominated by extrusion, in turn driven by convergence between the Indian Craton and Eurasia.

We report on new developments in the long-core measurement techniques using u channels in connection with new small-access cryogenic magnetometers. With these new instruments, u-channel samples allow us to measure a wide range of magnetic parameters along entire-core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples. An additional advantage is that the sediment disturbance when sampling is significantly less when using u channels. We present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques. Examples of continuous measurements of low-field susceptibility, NRM, ARM, IRM, Hrc from North Atlantic deep-sea cores show that it is possible to generate logs of combination of parameters related to rock-magnetic characteristics of the sediments useful in palaeoceanography. In particular, the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo-environmental conditions. The fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable (or not suitable) for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity.