No abstract

The determination of in situ stress states is vital in understanding the behavior of faults and subsequent seismogenesis of accretionary prisms. In this paper, a high-quality 3-D seismic volume is used to map the depth of the extensional-compressional decoupling (ECD) boundary in the accretionary prism of Nankai, with the prior knowledge that strike-slip and compressional stresses occur deeper than 1250 m below seafloor (mbsf) in the Kumano Basin, changing to extension toward the seafloor. A total of 1108 faults from the accretionary prism are analyzed to estimate paleostresses via fault inversion and slip tendency techniques. A key result in this paper is that the ECD boundary can be used as a proxy to identify active structures on accretionary prisms as its depth depends on (a) local tectonic uplift in areas adjacent to active faults and (b) the thickness of sediment accumulated above active thrust anticlines. The depth of the ECD boundary ranges from 0 to 650 mbsf, being notably shallower than in the Kumano Basin. In Nankai, frontal regions of the imbricate-thrust zone, and the megasplay fault zone, reveal the shallower ECD depths and correlate with the regions where faulting is most active. As a corollary, this work confirms that estimates of stress state variability based on the analysis of 3-D seismic data are vital to understand the behavior of faults and potential seismogenic regions on convergent margins.

No abstract

We investigated pinnacle features at the base of late Oligocene-Miocene isolated carbonate buildups using three-dimensional seismic and borehole data from the Browse Basin, Northwest Australia. Brightened seismic reflections, dim spots, and other evidence of fluid accumulation occur below most pinnacle features. An important observation is that all pinnacles generated topography on successive late Oligocene-Miocene paleoseafloors, therefore forming preferential zones for the settlement of reef-building organisms by raising the paleo-seafloor into the photic zone. Their height ranges from 31 m to 174 m, for a volume varying from 33 km 3 to 11,105 km 3 . Most of the pinnacles, however, are less than 2000 km 3 in volume and present heights of 61-80 m. As a result of this work, pinnacles are explained as the first patch reefs formed in association with mud volcanoes or methanogenic carbonates, and they are considered as precluding the growth of the larger isolated carbonate buildups. We postulate that pinnacle features above fluidflow conduits demonstrate a valid seep-reef relationship, and we propose them to be refined diagnostic features for understanding fluid flow through geological time.