Damien Dhont

3-D modeling of geologic maps from surface data

Recent discoveries in earth sciences are mostly related to technologies allowing graphical representations of volumes. We present a way to produce mathematically and geometrically correct three-dimensional (3-D) geologic maps consisting of the volume and shape of all geologic features of a given area. The method is innovative in that it only uses surface information based on the combination of a standard geologic map, a satellite image, and a digital elevation model. It is based on a modeling algorithm that only uses surfaces calculated from scattered data points and that intersects them following a series of geologically sound rules. The major advantage of using such technology is that it provides the user with a way to quantify geology. To illustrate how a 3-D geologic map can be computed, we explain the steps taken to build a dummy model with simple faulting and depositional sequencing. The case study chosen to illustrate the method is the Beirut watershed (Lebanon), an area with relatively simple geology. The 3-D visualization and cross sections help in the understanding of the geometrical relationship between the different geologic features, allowing a reexamination of the tectonic history of the area during the late Mesozoic.

Three-dimensional geologic imaging and tectonic control on stratigraphic architecture: Upper Cretaceous of the Tremp Basin (south-central Pyrenees, Spain)

In the Tremp Basin area (south Pyrenean foreland, Spain), the Campanian–Maastrichtian Orcau-Vell and Santa Engracia depositional sequences onlap the western termination of the Sant Corneli anticline. The precise mapping of the different systems tracts belonging to these depositional sequences, their spatial arrangement, and the structural control of the anticline on the sedimentation still remained unclear. To accurately interpret the geometry of the depositional sequences and to determine the factors influencing the sedimentation, we have developed a method that aims to build a three-dimensional (3-D) geological picture of this area. The originality of our approach is that the 3-D map, which consists of the volume and shape of all the systems tracts, has been produced mainly from the interpretation and combination of surface data, including a mosaic of aerial photographs at 50-cm (20-in.) pixel size and a digital elevation model at 10-m (33-ft) resolution. We have additionally constrained the model by integrating bedding dip and strike data and balanced cross sections. With respect to the 2-D, the 3-D visualization and field observations reveal the structural control at different scales of the lateral propagation of a fault-propagation fold (Sant Corneli anticline) on the stratigraphic architecture. The Orcau-Vell depositional sequence was controlled by the rise of the base level and was characterized by differences in the sedimentation rates. The emplacement of a north-south–trending gravitational normal fault, located at the western tip of the Sant Corneli anticline, was coeval with the emplacement of the Santa Engracia depositional sequence. This fault resulted from the westward propagation of the Sant Corneli anticline, generating a local slope and a depression that channeled the turbidites and the Gilbert-type delta deposits of the Santa Engracia depositional sequence. Uplift of the Sant Corneli anticline may have subsequently stopped, and the area subsided, inducing a rapid rise of the base level.

Structural analysis of the Húsavík-Flatey Transform Fault and its relationships with the rift system in Northern Iceland

The Húsavík-Flatey Fault (HFF), partially exposed on land in the Tjörnes Peninsula, is a major dextral transform fault of the Tjörnes Fracture Zone which accommodates the movement between the North Volcanic Zone of Iceland and the Kolbeinsey Ridge. We present a revisited structural pattern of the HFF in this area, based on the mapping of tectonic features using satellite images, aerial photography, and field structural analysis. We show that the HFF comprises several WNW-trending fault segments that localise both strike-slip and normal movements, agreeing with a transtensional deformation pattern. Two different types of transform-rift connections were recognised: (1) the progressive bending of a WNW-ESE transform fault segment that merges into a N-S rift structure and further north (2) a typical triple junction between another WNW-trending transform fault segment and a N-S normal fault of the rift. Furthermore, two immature WNW-ESE-trending faults, with poorly expressed fault traces, have been observed in the northern part of the HFF. These observations are consistent with the successive formation of transform fault segments from south to north due to the northward development of the HFF together with the rift structures.

Comparative use of processed satellite images in remote sensing of mass movements: Lebanon as a case study

Mass movements (MM) represent a serious threat to human life and activities in most mountainous areas. However, due to the rugged nature of such terrain, it is often difficult to detect such phenomena in remote areas. Hence, satellite imagery offers many attractions for the examination of MM in such environments, especially in less developed nations in which resources are stretched and levels of environmental information limited. There is a need to ensure that the techniques and images used are effective, reliable, and cheap in terms of the amount and accuracy of data that can be extracted. Taking Lebanon as a case study, this paper compares the applicability of different satellite data sensors (Landsat TM (Thematic Mapper), IRS (Indian Remote Sensing Satellite), SPOT4 (Système Probatoire pour l’Observation de la Terre)) and preferred image‐processing techniques (False Colour Composite ‘FCC’, pan‐sharpen, principal‐component analysis ‘PCA’, Anaglyph) for the mapping of MM recognized as landslides, rock and debris falls, and earth flows. Results from the imagery have been validated by field surveys and analysis of IKONOS imagery acquired in some locations witnessing major MM during long periods. Then, levels of accuracies of detected MM from satellite imageries were plotted. This study has demonstrated that the anaglyph produced from the two panchromatic stereo‐pairs SPOT4 images remains the most effective tool setting the needed 3D properties for visual interpretation and showing a maximum accuracy level of 67%. The PCA pan‐sharpened Landsat TM‐IRS image gave better results in detecting MM, among other processing techniques, with a maximum accuracy level of 62%.

Deformation of the northeastern Venezuelan Andes. Relationships with the Caribbean overthrusts

The Merida Andes (Venezuela) formed in the middle Miocene due to oblique convergence between the South American plate and the Maracaibo block [Audemard et al., 2002] (figs. 1A and 1B). The study area corresponds to the so-called Barbacoas platform [Renz, 1960], which constitutes the northeastern termination of the belt, NE of Valera (fig. 2). It is located in the northeastern part of Trujillo block [Hervouet et al., 2001], considered as an independent block separated from the main Maracaibo block along the Valera fault.nnAccording to Stephan [1982], the N170°E-trending Caribbean compression developed in this area from late Cretaceous to Eocene. It was followed by a N105°E-trending compression older than middle Miocene, and finally by the NW-SE Andean compressional stage that lasted till now in most of the chain. However, east of El Empedrado fault, a NNE-SSW compression presently occurs that is oblique to the classical Andean stage.nnThe tectonic evolution of the Andean stage is not well understood. The Merida Andes are mainly composed of Precambrian and Paleozoic rocks. The northern part of the belt only comprises a complete and continuous Jurassic to Paleogene cover. This lithologic pattern is probably a consequence of the tectonic escape of the Maracaibo block, and more particularly of the smaller Trujillo block. The Merida and Carribean belts being close to each other, the influence of the Andean deformation on the Caribbean allochthonous must be taken into account.nnIn order to make a structural analysis at regional scale, we privileged the use of remote sensing data (Landsat, Spot and Radar images) and aerial photographs. This was complemented by structural data obtained in the field, allowing the study of geometric and chronological relationships between the tectonic structures.

Geological mapping in the zone of Chotts, Tunisia, using ALOS sensors

The three sensors onboard the Advanced Land Observing Satellite (ALOS) are the Phased Array L-band Synthetic Aperture Radar (PALSAR), the Panchromatic Remote-Sensing Instrument for Stereo Mapping (PRISM), and the Advanced Visible and Near-Infrared Radiometer type 2 (AVNIR-2). Each of them has been evaluated for the geological mapping of the Zone of Chotts, Tunisia. Both the endoreic basins and the surrounding ranges have been studied. The 10 m-resolution AVNIR-2 sensor is a good trade-off for geological mapping. The 2.5 m-resolution PRISM sensor with its stereoscopic capability is very useful for the detailed study of sedimentary layers. Copolarized radar data are relevant for the study of the flat itself but the C-band is more accurate for sensing the roughness of the finer sediments (sand veneers, evaporites, etc.). The HV polarization is suitable for identifying the presence of halophytic plants that fringe the upper part of the flat.