Benoît Deffontaines

Okinawa trough backarc basin' Early tectonic and magmatic evolution

The Okinawa Trough, lying between Japan and Taiwan, is a backarc basin formed by extension within the continental lithosphere behind the Ryukyu trench-arc system. Stress directions associated with the two last extensional phases in the southwestern Okinawa Trough have been deduced from a comparison with analog modeling: the direction of extension is N 150 o for the Pleistocene phase of extension (2-0.1 Ma) and N 170 o for the late Pleistocene-Holocene phase of extension (0.1-0 Ma). The present-day Ryukyu volcanic arc, a narrow continuous feature extending from Japan to Taiwan, is located on the eastern side of the Okinawa Trough, 80-100 km above the Wadati-Benioff zone, the minimum depth for emplacement of arc magmatism. Scarce present-day backarc volcanism appears in the middle and southern Okinawa Trough within linear en echelon bathymetric depressions. A N045 o oriented seamount volcanic chain cuts across obliquely the southwestern Okinawa Trough and lies in the direct line of extension of the Gagua ridge, a N-S linear volcanic feature of the Philippine Sea plate. Associated with this extension of the Gagua ridge, a large reentrant located at the base of the Ryukyu prism, the uplift of part of the Nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross-backarc volcanism could be tied to the subduction of the Gagua ridge located there at a depth of 80-100 km beneath the backarc basin. A second area of anomalous volcanism has been identified in the middle Okinawa Trough in the ENE extension of the Daito ridge, a WNW-ESE 400-km-long volcanic feature of the Philippine Sea plate. We suggest that the Gagua and Daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere, allowing magmas with arc affinities to erupt at the seafloor. This excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts. The Okinawa Trough is consequently still in an early stage of evolving from arc type to backarc activity.

Active interseismic shallow deformation of the Pingting terraces (Longitudinal Valley – Eastern Taiwan) from UAV high-resolution topographic data combined with InSAR time series

ABSTRACT We focus herein on the location, characterization and the quantification of the most active structural feature of Taiwan: the Longitudinal Valley Fault that corresponds to the suture in between the Philippine and Eurasian Plates. In order to determine and monitor its present inter-seismic deformation, we focus on the Pingting Terraces area, situated in the South Longitudinal Valley (Eastern Taiwan). We first determine the structural geometry issued from both photo-interpretation deduced from new unmanned aerial vehicle (UAV) high-resolution Digital Terrain Model data that we acquired (34.78 km2 with 7.73 cm ground sampling distance), combined with geological field work. In order to characterize and quantify the present deformational patterns over the Pingting terraces, we used an InSAR time series Interferometry algorithm (MT-InSAR) applied to nine L-band SAR images from ALOS satellite acquired over the period 2007–2010. The unprecedented density of measurements (about 120 points per km2 for a total of 6,400 points) gives a continuous overview of the inter-seismic shallow deformation. The structural geometry combined with the mean velocity map (MT-InSAR) reveals two clear active faults situated above the scarps of the Pingting terraces and responsible for up to 7 and 20 mm/yr velocity offset along the radar line of sight. A temporal analysis of the deformation is performed with one measurement at each SAR acquisition date, giving major improvements in the characterization and quantification of the Longitudinal Valley active Fault trace.

New Structural and Geodynamic Coastal Jeffara Model (Southern Tunisia) and Engineering Implications

Thanks to its geographical position in the western Mediterranean domain, Tunisia faces, since mid-Cretaceous (Aptian/Albian time period), to the inversion of the Tethys due to the northward African plate motion toward Eurasia. The coastal Jeffara is a part of the southern zone of deformation witness of the eastward migration of Tunisia to the Mediterranean Sea. We focus herein following Perthuisot (Cartes geologiques au 1/50.000 et notices explicatives des feuilles de Houmet Essouk, Midoun, Jorf, Sidi Chamakh, 1985) and others on the neotectonic of the coastal Jeffara (southern Tunisia) and its engineering implications. Based on the results of previous studies and new evidences developed herein, we propose a new structural and geodynamic coastal Jeffara model, influenced by the continuous post lower cretaceous northward migration of northern African toward the Eurasian plates. We herein study the Digital Elevation Model (issued from SRTM), which was checked with field surveys and 2D numerous seismic profiles at depth both onshore and offshore. All data were, then, integrated within a GIS Geodatabase, which showed the coastal Jeffara, as a part of a simple N–S pull-apart model within a NW-SE right lateral transtensive major fault zone (Medenine Fault zone). Our structural, geological and geomorphological analyses prove the presence of NNW-SSE right lateral en-echelon tension gashes, off shore NW-SE aligned salt diapirs, numerous folds offsets, en-echelon folds, and so-on… that are associated with this major right lateral NW-SE transtensive major coastal Jeffara fault zone that affect the Holocene and the Villafrachian deposits. These evidences confirm the fact that the active NW-SE Jeffara faults correspond to the tectonic accident, located in the south of the Tunisian extrusion, which is active, since mid-cretaceous, as the southern branch of the eastward Sahel block Tunisian extrusion toward the free Mediterranean sea boundary. Therefore this geodynamical movement explains the presence, offshore, of small elongated NW-SE, N-S and NE-SW transtensive basins and grabens, which are interesting for petroleum exploration.

Monitoring Swelling Soils in Eastern Paris (France) Through DinSAR and PSI Interferometry: A Synthesis

Swelling soils (below S2) due to volumetric clays changes under various atmospheric dryness and wet climatic conditions, induce small topographic surface displacements and therefore affects and deforms overlying buildings. It is a major concern for societies due to both its large spatial extension and to its so expensive costs for remediation. The aim of this work is therefore to locate, characterize and to quantify those small humidity-dependant topographic displacements through new interferometric methods such as Differential Interferometry (DinSAR) as well as Persistent Scatterers Interferometry (PSI) applied to the Paris area (France). If the Differential Radar Interferometry (DinSAR) is a method that enables one to map relative surface displacements from two different radar images acquired on a specific area with a different time span. It gives a rather poor monitoring result in the eastern Paris Basin due to the paucity (versus lack) of coherence in the Paris surroundings despite they are highly affected by recent dryness hazards (last major crisis in 2003). On the contrary, most promising and interesting results are obtained through Persistent Scatterer Interferometry (PSI) that revealed so precisely the absolute surface continuous displacements through time of the topography due to the variation of geometry of the swelling soils (below S2). The latter are highly linked to seasonal climatic parameters as highlighted by the processing of more than 400 ascending and descending radar images. We then succeed to monitor the small topographic displacements through time of the eastern Paris area. In order to validate this approach, we used an home made GIS data base where we compare quaternary geology, lithology, vegetations, buildings and deformed buildings and topographic displacements acquired with interferometric studies. As a result, we realized herein that the vegetation and specifically trees are of first order for the individual building deformations due to S2. Furthermore we note that each trees species have a specific behaviour toward S2 due to both their root geometry (planimetric versus vertical geometry) and their maturity. We propose herein to calculate the potential S2 deformation due to the trees evapotranspiration by quantifiing by trees average altitude above the ground depending of species. Therefore each species of trees due to their differential evapo-transpiration have a specific «planimetric extension agressivity». By combining the geometry of the roots of the trees (surficial contrasting to vertical) and looking for both our wok in the fields and the bibliography, we were herein able to determine the planimetric extension of the « underground water pumping agressivity of each trees species » on swelling soils deformation which is directly related to evapotranspiration. This application presents high potential to better understand the swelling soils natural hazards and the induced and associated geological processes on the individual buildings.

Tunisia: A Mature Case Example of Structural Extrusion

Neotectonics is revealed by numerous approaches such as a new re-interpretation and homogeneisation of the geological data set, earthquakes, field work studies and detailed numerical geomorphic analyses of the topography. Tunisia appear to be a case example for this kind of studies as it is affected by both active tectonics highlighted by few minor earthquakes and present numerous associated faults and folds. We develop first herein basic key geomorphic processing deduced from the bibliography and new indicators developed throughout this study, processed numerically through home made geodatabase, that lead us to propose a new structural scheme of extrusion revisiting the Tunisian neotectonic setting (Deffontaines et al. 2008, Postdam GFZ). We then update the Tunisian geological mapping by improving it with the high resolution existing Digital Terrain Model and remote sensing images. Fieldwork studies lead us to better solve mapping anomalies. At least we used numerous published existing seismic profiles that validate our model. We then propose herein an eastern extrusion model of Central Tunisia due to the northward migration of African plate toward Eurasia since mid cretaceous due to the Tethys inversion.

Geotechnical Study of Urban Soil and Subsoil of Fez City (N. Morocco) and Natural Risk Mapping Using Geographic Information System (GIS)

Over the past four decades like all imperial cities of Morocco, the urban area of Fez City (Rif, Northern Morocco) has continued to grow and develop locally on dangerous grounds consisting of soft impervious clayey and marly substrate which are subject to multiple constraints and issues. These issues are made worse by rapid population growth coupled with serious administrative delays in the development and planning of Fez City. All these circumstances have generated an unstable building situation highlighted unfortunately by some tragic events. Indeed, as the statistics show, the majority of large buildings disorders may be due to incompatibility of foundation soils with the type of construction. Furthermore other types of less noticeable disorders, which represent major costs to the Moroccan society, are related to variation in the geometry of the soil such us volumetric changes of clay soils (shrink-swell phenomenon) or landslides events on the steepest slopes. These soil factors are affecting all infrastructures (e.g., buildings, roads and structures). The present research aims to identify and characterize all urban and suburban soils and lithology in the Fez City, using geotechnical and hydrogeological studies, field investigations, and exploitation of geological, hydrological and topographic data available in the area. All the required information has been sorted, analyzed and re-organized within a digital data base. Map layers and data cross-referenced with management plans using the Geographic Information System (GIS) were used to map instability risk areas according an increasing degree of hazard. Three categories of natural risk areas have been therefore identified: High risk areas (HRA); Medium risk areas (MRA); Low risk areas (LRA).

The Extrusion of South-West Taiwan: An Offshore-Onshore Synthesis

Neotectonic Interpretation of the different marine surveys (ACT, …) swath bathymetry and different onshore-offshore sismic profiles combined to classical structural fieldwork, geodetic, seismological and interferometric studies lead us to propose a global structural scheme and confirm the regional escape tectonics affecting both onshore-offshore of SW Taiwan. First, it is highlighted here the difficulty to only interprete the swath bathymetry even in the northern tip of the Manila accretionnary prism which is a rather simple geological context but affected by both (1) a strong amount of sedimentation due to the Taiwan mountain belt erosion, and (2) to the submarine erosion of the giant Penghu canyon. Second point, is the importance of the seismic interpretation in order to get the offshore bedding and structural data combined with the swath bathymetry and to the photointerpretation of the digital Terrain Model combined to the accurate geological maps to precisely delineate the blocks that is inferred to be submitted to a classical escape tectonic. Third, the precise study of the two new major structural boundaries Fangliao and Young-An structures which guides the SW Taiwan extrusion. Combined with onshore studies (e.g. interferometry (DINSAR), geodetic, seismology and field work) which gives (1) locations, characterization and quantification on the interseismic displacements and (2) lead us to modify our view of the global tectonic structures of the SW Taiwan. To conclude, it is highly recommended to combine both approach on and offshore geology in order to better understand geology and active structures in that part of the world.

Analysis of Recent Deformation in the Southern Atlas of Tunisia Using Geomorphometry

In this work, we propose to locate, characterize and quantify some topographic deformations linked to the seismotectonic context, and anthropogenic actions in the southern Atlas of Tunisia. The analysis of morphometric parameters (Residual Topography, Hypsometric Integral, Drainage anomalies, Maximum curvatures and Roughness) has revealed that three structures in the study area, J. Ben Younes, J. Bou Ramli and J. El Abiod, are distinguished by a specific morphometric and anomalous response which may reflect an important morphodynamic activity caused principally by the numerous reactivation of the Gafsa fault.