Description: This study describes a peculiar, yet common type of fracture showing a staircase trajectory, which forms in rocks with moderately weak planar anisotropies. The staircase fracture trajectory is given by alternating fracture segments oriented parallel to (LaP) or at an angle (ramp) with respect to the lamination/layering. The analyses has been accomplished on travertines, which are continental microbial/hydrothermal deposits having a typical poorly stratified yet strongly laminated texture. In these rocks, porosity and permeability have a high across-lamination variability and are mostly controlled by an interconnected and locally corroded array of permeable layers, fractures, and faults. Structural analysis integrated with analytical modeling provided a conceptual model of staircase fracture localization as a function of the travertine lamination dip. Lamination-parallel fracture segments localize within the porous laminae, mostly at the interface with tight laminae. Ramp-type fracture segments cut the lamination, connecting lamination parallel segments. Two types of staircase fracture can be modeled. The first group develops in subhorizontal to gently dipping deposits (lamination dip 〈 30°) corresponding to low-energy depositional environments. The second group relates to staircase fractures developed in moderately to steeply dipping laminations (lamination dip 〉30°) and corresponds to high-energy environments. Major discoveries of hydrocarbon have been recently made in continental (lacustrine) microbial carbonates in the Brazilian South Atlantic margin, some of which exhibit a texture similar to those usually observed in travertines. Understanding of the lacustrine carbonates is still at an early stage. Given that in modern rift settings, vent-related thermal (travertine) and nonthermal (tufa) carbonates are a major component, the proposed conceptual model of staircase fracture localization contributes to the preparation of a model for the potential occurrence of high-permeability pathways in hydrocarbon and geothermal microbial reservoirs.

Description: A recent focus of major international exploration in East Antarctica has been aimed at revealing its bedrock topography and imaging its tectonic architecture and evolution. Here we present the tectonic interpretation of regional-scale lineaments revealed by the Radarsat mosaic of Antarctica on the ice sheet surface in the Vostok–Dome C–Adventure Basin region. These lineaments appear in the radar backscatter textures as alignments of marked tonal variations with lengths of tens to hundreds of kilometers and were identified using an automated methodology. We explore the origin scenarios for the ice sheet surface lineaments by comparing their azimuthal trends and spatial distribution with the main morphotectonic features of the bedrock. Azimuthal analysis reveals that lineaments cluster around two preferential directions interpreted as structural or tectonic domains. These show strong correlations with azimuths of tectonic fabrics in the bedrock. The main lineament domain parallels the morphotectonic features of the study area, namely the Adventure Basin and the Concordia and Aurora Trenches. The second lineament set corresponds to the mean orientation of the Lake Vostok depression. The spatial analyses of the two lineament domains strengthen our findings and interpretations. Comparisons with wind and ice flow directions exclude their influence on the identified lineament pattern. Results reveal the tectonic origin of the lineament domains, and demonstrate the method’s usefulness as a tool for tectonic studies of regions characterized by thick covers. These regions include other areas of the East Antarctic craton such as the Gamburtsev Subglacial Mountains, as well as deserts or surfaces of other planets.