We use 3D seismic data in a novel way to describe the three-dimensional geometry of a number of igneous bodies intruded into the upper crust as well as to define and classify sill junction relationships. Igneous intrusions were emplaced into Upper Cretaceous and Palaeocene sediments of the Faroe-Shetland Basin during the Early Palaeogene and in many cases they adopt remarkable saucer-or trough-shaped geometries that are 2-8 km in diameter and have a vertical relief of several hundred metres. Individual intrusions are interlinked and form highly interconnected sill complexes. Three geometrically distinctive classes of sill junctions are defined and illustrated with examples from seismic data. Each class implies a specific evolutionary sequence of events and these are discussed for each of the classes of junction. The class of junction often changes along the line of junction with one class evolving in space to another. This has significant implications for spatial reconstruction of sill complexes based on two-dimensional outcrop and this is illustrated with reference to an example from a 3D seismic dataset.

SUMMARY The 1167 published cooling unit (CU) palaeointensity estimates contained in the 400–10 Ma portion of the PINT global database were rigorously filtered according to accurate age determinations, palaeodirectional reliability, recognition of polarity and the method of palaeointensity acquisition. The remaining 865 estimates (group 1) were further filtered to ensure self‐consistency, reducing the data set to 425 estimates (group 2). Group 1 and 2 data were clustered into temporally and/or spatially distinct rock suites (RS) enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation (PSV). The record was segmented according to the distribution of the data, rather than using arbitrary time windows, to ensure quasi‐consistent behaviour within each segment. Differences between these segments clearly indicate that a significant long‐timescale (107 and 108 yr) variation of the mean geomagnetic poloidal field intensity (GPFI) occurred during the 400–10 Ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field. Both the mean dipole moment and its standard deviation appear to be a function of the range of values each CU may adopt at one particular time. This range is itself controlled by the variation of the maximum limit of dipole moment, while the value of the minimum limit remains relatively constant. Tentative support is provided for the recent suggestion that PSV may have been reduced during the Cretaceous normal superchron (CNS), though more data are needed in the range 120–60 Ma to confirm this. No conclusive evidence was found to support the suggestion that the GPFI record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt pTRM checks. Indeed, offsets caused by unreliable data in well‐represented parts of the record are likely to be random and cancel one another out. When GPFI variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency (RF), it is not possible to confirm whether the two parameters are anticorrelated, decoupled or related in some more complex way. However, it is clear that GPFI and RF are definitely not positively correlated as has been previously suggested. The present database documents sharp increases in GPFI around the onset times of the two recognized superchrons, itself implying an anticorrelation. The implications, for geodynamo and mantle modelling, of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly. A generic geodynamic model is proposed to explain the relationship between observed long‐term changes in GPFI and global geodynamic processes. The model predicts that changes in GPFI result from a chain of geodynamic processes extending from crust to core, beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection. Supercontinents...

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Documenting variations in the virtual dipole moment (VDM) of the geomagnetic field through geologic time has recently been the subject of considerable interest, not the least because a detailed and reliable VDM record would provide a key constraint for geodynamo modeling. Such a record would also inform the debate concerning issues related to Earth's geodynamic evolution, such as how the geomagnetic field responds to stages in supercontinent cycling. The relationship between variations in geomagnetic polarity and field intensity with lowermost mantle (LMM) processes is fundamental, but the paucity of the VDM record has focused research on the links between polarity and LMM processes. Nonetheless, authors agree on the crucial need to produce a detailed VDM record in which we can place sufficient confidence to allow these major issues to be addressed [e.g., Biggin and Thomas, 2003; Heller et al., 2002; Riisager et al., 2002; Tarduno et al., 2001; Selkin and Tauxe, 2000].

The results of rock magnetic, thermal demagnetization and Thellier palaeointensity studies are presented for two high‐level intrusions from southern New South Wales, Australia. The Gingenbullen Dolerite (GB) (34.5°S, 150.3°E) and the Gibraltar Microsyenite (GS) (34.5°S, 150.4°E) were emplaced during the second of three major phases of igneous activity that affected the southern Sydney Basin and are K/Ar dated at 172 and 178 Ma, respectively. The magnetic mineralogy of the two intrusions is different: the GB is dominated by single‐domain (SD)/pseudo‐single‐domain grains of magnetite, whereas the GS has both SD magnetite and haematite components, although the haematite component does not record a stable remanence. The GB records two opposed components of magnetization, above 100 °C, occupying distinct parts of the blocking temperature spectrum. The lower blocking temperature (LBT) component resides between 150 and 400 °C and is of normal polarity, having a mean direction of D/I = 021°/−79° (α95 = 8°, k = 63), with a corresponding VGP at Lat/Long = 54°S/137°E (dp = 12, dm = 13). This component is believed to be a TCRM or TVRM related to the initial opening of the Tasman Sea at ~90 Ma. The higher blocking temperature (HBT) ChRM component is recognized between 450 and 580 °C and is of reverse polarity, with a mean direction of D/I = 174°/+81° (α95 = 11°, k = 33) and a corresponding VGP at Lat/Long = 52°S, 153°E (dp = 17, dm = 18). The ChRM for the GS resides in the 100–450 °C region and, although the scatter is high, has a similar mean direction (D/I = 167°/+79°; α95 = 24°, k = 26) to the HBT component of the GB, with a pole position at Lat/Long = 54°S/158°E (dp = 30, dm = 31). There is no evidence of the GB LBT component in samples from the GS, suggesting that this intrusion was magnetically unaffected by the later event, which is also seen for other intrusions in the Sydney Basin. The VGPs calculated from the ChRM directions both plot on the Jurassic segment of the Australian APWP (Dunlop et al. 1997), providing further palaeomagnetic confirmation for the K/Ar ages. Thellier palaeointensity results were of variable quality, as expected for rocks of this age. The mean VDM values calculated for the ChRMs of the GB and GS were 1.3 × 1022 and 4.1 × 1022 Am2, respectively (approximately 16 and 51 per cent of the present value). On application of a cooling‐rate correction estimate, the GB result falls to a minimum of 0.87 × 1022 Am2 (approximately 11 per cent of the present‐day value), representing an overestimate of up to 56 per cent. These values, arising from a time window previously lacking in Thellier estimates, provide further confirmation of the existence of a pre‐Cenozoic dipole low. The VDM values derived from the overprint in the GB samples yield an average of 4.8 × 1022 Am2 (from a range between 44 and 81 per cent of the present‐day mean VDM) and are tentatively attributed to the Cretaceous Normal Superchron (CNS). These results may document evidence for a previously unrecognized low dipole moment during the bu...