María Asunción Soriano

Alluvial dolines in the central Ebro basin, Spain: a spatial and developmental hazard analysis

Abstract Alluvial dolines are abundant in Quaternary terraces and pediments overlying Neogene gypsum in the Zaragoza area (central Ebro basin). Spatial analysis and experimental simulation show that sulphate content in the groundwater, grain size of the detrital cover, topography of the Tertiary/Quaternary boundary, annual variation of the water table and thickness of the detrital cover are the main factors controlling their development. Taking into account these variables a theoretical spatial hazard model has been elaborated, expressed as a mathematical equation and a hazard map. Both experiments and field data show a high correspondance between two basic mechanisms of evacuation and subsidence (dragging slow subsidence and collapse of cavities) and two basic types of morphology and internal structure of dolines (basin doline — funnel structure and well doline — vault structures, respectively). A genetic classification of dolines and some evolutionary patterns based on these criteria are developed. Historical changes observed in doline distribution, as well as field surveys of urban damage and microtopographic profiles allow us to evaluate the present day activity of dolines. Local subsidence rates measured in urban areas range from 2.5 to 10 cm/year, although doline generation and reactivation in the whole area tend to be compensated by filling by human activity.

The role of tectonic inheritance in the development of recent fracture systems, Duero Basin, Spain

A lineament analysis of the Duero Basin (north Spain) suggests that cover rocks have been influenced by a previously fractured basement in sediment cover with little deformation. The Duero Basin is covered with horizontal Neogene rocks (mainly sandstones, shales and limestones) with a total outcropping area of about 50 000 km2 and a maximum thickness of 300 m. The only structures found within the Neogene are map-scale monoclines near the basin margins, and joints and faults, most of them without significant displacement. From the analysis of a Landsat Thematic Mapper (TM) scene, lineaments were mapped at the eastern half of the Duero Basin. The orientation frequency of lineaments shows an absolute maximum NE–SW to ENE–WSW, with several sub-maxima oriented E–W, NW–SE and WNW–ESE. These fracture directions controlled most of the present-day fluvial network. Within the Palaeozoic and Mesozoic rocks cropping out at the basin margins the orientation of lineaments corresponds with that of mappable faults, particularly in the Palaeozoic basement. The zones with maximum density of lineaments are associated with map-scale WNW–ESE thrusts and folds located below the horizontal Neogene. The origin of the main fracture systems in the Neogene rocks of the Duero Basin appears to be controlled by older structures, namely the NE–SW faults that cross-cut the granitic and gneissic basement of the Duero Basin and its southern and western margins. These faults are late Variscan (probably Permian) in origin and were reactivated during the Mesozoic and the Cenozoic. Their activity in Miocene and post-Miocene times is related to strike-slip and extensional movements linked to the recent intraplate stress field in the Iberian Peninsula.

An example of a comparison between Thematic Mapper and radar images in the central Ebro basin

By analysing Landsat TM and ERS-1 SAR images, thousands of lineaments have been mapped within the Neogene materials of the central Ebro basin (north-east Spain). Comparison with data from previous field surveys proved that most of them are normal faults. The main trend represented in both systems is NW-SE. However in SAR other trends that correspond with smaller fractures appear to have certain significance. This fact can be a consequence of the physical characteristics of radar systems: when a surface has a consistent orientation and reflectivity it can be detected even if it is only a few centimetres high. Lineaments are also identified in zones where Quaternary deposits and farming activities prevent the recognition of fractures by conventional geological fieldwork. The study of TM and SAR images has improved geological knowledge of this area.

Interacting tectonic faulting, karst subsidence, diapirism and continental sedimentation in Pleistocene deposits of the central Ebro Basin (Spain)

During Early, as proposed by the International commission on stratigraphy Pleistocene times, interacting fluvial and aeolian processes constructed wide alluvial plains over an evaporite-dominated Miocene substratum in the central Ebro Basin. An exceptional site where these deposits show faults, folds, diapirs, karst structures and unconformities has been studied in detail. Analysis of particular structures demonstrates the interaction by that time of tectonic faulting, diapirism, karstification and sedimentation in an area where deformation was traditionally linked to the presence of underlying evaporites, without proposing any precise mechanism. Multiple approaches (sedimentology, structural geology and geophysics) have been used in order to discriminate the origin of each type of structure as well as to understand the interaction between different processes. Numerous normal faults and fractures of variable size are consistent with the regional stress field. Pleistocene deposits are pierced by diapirs of Miocene evaporites and disrupted by karst structures with different geometries (tubular, funnel and vault), both partially controlled by tectonics. The example described is proposed as an analogue model that could successfully illustrate evolution patterns of basins of potential interest for petroleum geology where similar processes have actuated, resulting in complex stratigraphical architectures.

Morphotectonics of the Concud Fault (Iberian Chain, Spain): Comparing Geomorphic and Geologic Indices of Activity of an Intraplate Extensional Fault

Abstract The results of geomorphic analysis of the Concud fault-generated mountain front (central Iberian Chain, Spain) are introduced into classifications of fault activity proposed by previous authors, and compared with slip rates calculated from geologic markers. The Concud fault is an extensional structure active since the mid Pliocene times. It gives rise to a 60 to 120 m high mountain front, where footwall rocks belonging to the Triassic and Jurassic (north-western sector) and Miocene (south-eastern sector) crop out. Conspicuous triangular facets are preserved on Jurassic rocks of the central sector, while short, generally non-incised alluvial fans make the piedmont. The value of the Mountain-front sinuosity index is Smf = 1.24 for the whole mountain front (1.17 and 1.32, respectively, for both segments showing distinct footwall lithology), as obtained by the most conservative procedure. Average valley floor width/height ratios calculated for seventeen gullies crossing the fault are Vf = 0.30 (250 m upstream from the fault trace) and Vf = 0.22 (500 m upstream). These geomorphic indices, together with qualitative features of the escarpment and piedmont landscape, indicate ‘moderate’ to ‘rapid’ fault activity. The range of slip rates estimated from such morphotectonic classification (0.03 to 0.5 mm/y) encloses the range calculated from offset Late Pliocene and Pleistocene stratigraphic markers (0.07 to 0.33 mm/y). Nevertheless, the highest potential slip rate (0.5 mm/y) clearly represents an overestimate: the mountain front could give the impression of an anomalously high level of activity owing to episodic rejuvenation caused by base level drop.