Stefan A. Revets

NW Australian intraplate seismicity and stress regime

[1] The interrelations between stress orientation, regional deformation, and seismic activity in the context of intraplate seismicity continue to pose difficult questions. Stress models of the Australian continent are largely based on nonseismic information. Our study of 26 microearthquakes in NW Australia shows that they are good indicators of the seismogenic stress field. By inverting focal mechanisms calculated from the events we obtained deviatoric stress tensors in full agreement with the known in-situ stress field. The stress tensors corroborate the elastic modeling of the continent. The methods used here have widespread applicability in determining crustal stress field parameters in regions where in-situ stress data are sparse or absent.

THE GENUS ROTORBINELLA BANDY, 1944 AND ITS CLASSIFICATION

The study of the type series of Rotorbinella colliculus Bandy, 1944, shows the presence of coverplates and thus confirms the rotaliacean nature of Rotorbinella . Notwithstanding preservational defects, Rotorbinella can now be distinguished consistently from the many genera with which it has been confused in the past.

ON THE ROTALIINE NATURE OF GAVELINOPSIS HOFKER, 1951

The discovery of coverplates sealing off the umbilical parts of foramina in the type species of Gavelinopsis suggests a reclassification of the genus from the Rosalinidae to the Ammoniinae. Inhomogeneities in the ultrastructure of the coverplate raises doubts about the phylogenetic homology of this anatomical feature. The uncertainty of the taxonomic value of this character increases the fragility of the suprageneric taxonomic framework of the Rotaliina.

THE GENUS ALBERTINOPSIS, GEN. NOV. AND THE RELATIONS BETWEEN GAVELINELLIDAE AND ROSALINIDAE

A new genus, Albertinopsis is proposed to accommodate the unusual species Discorbis norrisi Mellon and Wall, 1956, originally described from Albian deposits in Alberta, Canada. The new genus is classified in the Rosalinidae, thereby extending the fossil record of this family into the Albian. This extension further complicates the attempts to understand the origin and diversification of the rotaliine foraminifera, which appears to have taken place in the Lower Cretaceous.

THE SYSTEMATIC POSITION OF CONORBINA BROTZEN, 1936

Study of the type material of Conorbina marginata, the type species of Conorbina, reveals the presence of a small open umbilicus and of umbilical folia associated with the aperture. The observation of these characters forces a reclassification of the genus into the Rosalinidae and the lapse into synonymy of the family taxon Conorbinidae. The dissolution of the Conorbinidae and the extension of the fossil record of the Rosalinidae into the Lower Cretaceous further complicates any attempt towards reconstructing the phylogeny of the rotaline Foraminifera.