Does a detachment level exist beneath the Ceraunius Fossae? Insights from graben mapping and lost-area balancing analysis

We took advantage of the close relationship between graben width and rheology of the involved materials (e.g., number, thickness and spacing of rheological layers, presence of mechanical discontinuities of different nature) and attempted to obtain information about the mechanical stratigraphy of the Ceraunius Fossae area (Northern Tharsis Region). The possible existence of detachment levels beneath the Ceraunius Fossae area were investigated using the lost-area balancing method and a topographic profile derived from Mars Orbiter Laser Altimeter (MOLA) data. Results suggest a marked difference in structural style between the western and eastern sectors of the study area, which is interpreted as a result of different mechanical stratigraphies. On the western sector the maximum depth reached by grabens is well localized within 1000 m from the topographic surface suggesting the existence of a detachment level, which we interpret as a weak horizon at the base of the Late Hesperian-Early Amazonian units. The ductile behaviour of this horizon could be favoured by the presence of volatile reservoir or ice. In the eastern sector of the profile (HNf and Hf units) the maximum depth reached by the grabens is scattered and does not support the existence of clearly defined detachment horizons suggesting that Noachian Fractured Rocks are mechanically homogeneous. A maximum depth, however, could be established

Crustal extension in the Ceraunius Fossae, Northern Tharsis Region, Mars

We investigated the Ceraunius Fossae area, Northern Tharsis, in order to obtain quantitative information on the tectonic extension affecting this area. Tectonic structures of the Ceraunius Fossae area have been previously described using Viking images and interpreted as extensional structures. Laser altimetry data (MOLA) can be used to quantitatively investigate these structures with a better resolution. We developed a method to obtain E-W oriented profiles (perpendicular to the main tectonic structures) with a sufficiently high resolution to analyze tectonic structures in spite of the low data density in this direction. We interpreted all the recognizable extensional structures along the profiles, and using a simplified structural model, we estimated tectonic extension along these transects. The extension calculated over the entire profiles is 36 km (e24 = (l1 - l0)/l0 = (910 km - 874 km)/874 km = 0.041) and 42 km (e26 = (730 km - 688 km)/688 km = 0.061) along profile 24 and profile 26, respectively. In the most deformed area, extension reaches the value of 22 km (emax = (l1 -l0)/l0 = (186 km - 164 km)/164 km = 0.134). Since the extension accounted by the topographic doming is negligible, a significant horizontal crustal motion is required to explain the observed extension

Modelo 3D da estrutura da falha Marquês de Pombal

A falha Marquês de Pombal (FMP) desde que foi referida por Zitellini et al. (1999) como sendo a estrutura sismo-tsunamigénica do sismo de Lisboa de 1755, foi objecto de estudo de várias campanhas oceanográficas. A correlação entre as linhas sísmicas de reflexão, obtidas em três dessas campanhas (ARRIFANO-1992, BIGSETS-1998 e TTR10-2000), que intersectam a FMP, num total de 11 linhas, acrescida pelo trabalho de conversão em profundidade e reconstituição da deformação compressiva agora apresentado, permitiu pormenorizar a geometria da estrutura da FMP. Esta falha trata-se de um cavalgamento activo, com ruptura superficial de ~60km, situado a sudoeste do Cabo de S. Vicente onde se evidencia o desvanecimento gradual da falha a sul e terminação abrupta contra uma possível falha de transferência a norte

Il Foglio 280 – Fossombrone 3D: un progetto pilota per la cartografia geologica nazionale in tre dimensioni

Understanding the geometry of structures and relationships among stratigraphic and tectonic units at the depth is one of the main challenges faced by geologists. This aim is traditionally pursued through interpreting geological and geophysical data in one or two dimensions (logs of deep wells, stratigraphic columns, cross-sections, structural maps) and performing structural analysis (cut-off lines maps, branch lines maps, restoration and balancing of crosssections). The recent development of software devoted to geological modelling in three-dimensions represents a significant resource for geologists for reconstructing and visualizing geological bodies at depth (SLAT et alii, 1996; DE DONATIS, 2001). Geological models in three-dimensions integrate different kinds of data (cross-sections, seismic profiles, logs of deep wells) more easily than traditional techniques and allow a more coherent and exhaustive representation of the reality. Furthermore, progressive insertion of data in the model makes the geologist able to verify step by step the quality of his previous interpretations and to reach better results through iterative corrections. The aim of this prototype study project is to define and test a methodology for building three-dimensional geological models based on maps on the scale of 1:50000, from the new Italian geological mapping program. Another target of this work is to test the suitability of the national geological data base for 3D reconstruction purpose (DE DONATIS et alii, 2002). The recently developed three-dimensional visualization technique is used to build a structural model of sheet 280-Fossombrone, an external area of northern Apennines, central Italy. This area is characterized by faulted anticlines and broad synclines, involving a Triassic-Paleogene succession that is detached from the underlying basement. The exhaustive knowledge of the regional and local geology together whith the availability of subsurface (well and seismic) data makes this area a good test to define a methodology for building a 3D geological map. This model has been built in two main steps. In the first step, a 2.5 dimensional geological model has been built using a digital elevation model and the new geological map of the study area at the scale of 1:50000. This 2.5 model better shows the relationships among topography, geology and main structures better than traditional 2D geological maps. During the second phase a deep model was built, integrating a large amount of sub-surface data, as commercial seismic profiles (arising from hydrocarbon explorations made by Eni - div. Agip during past decades) and deep well logs, with field data resulting from the recent mapping project. The whole 3D geological model of Sheet 280-Fossombrone allowed a better understanding of the geometry of the deep structures and the kinematics of this external part of northern Apennines. Structural and geomorphological analysis are performed on the model to improve the knowledge of several aspects of the study area