Antonio M. Casas

Analogue models of transpressive systems

This paper presents three series of analogue models of transpressional deformation in a brittle/ductile system: (1) simple transpression, (2) transpression combined with erosion of uplifted areas of the hanging wall above the deformation front, and (3) transpression combined with erosion of the hanging wall and sedimentation at the foot of uplifted zones. In each series of experiments, different convergence angles α, from 0° (pure wrenching) to 90° (pure thrusting) were applied to the models. Results show a sharp contrast between structures formed at α≤15° (wrench-dominated transpression) and α≥30° (thrust-dominated transpression). For a low convergence/strike-slip ratio (0°≤α≤15°), the deformation is localized and structures are typical of a strike-slip regime (R and Y faults). For higher convergence angles (30°≤α≤90°), the deformation is similar for all models, with an elongate asymmetric uplift showing fault-propagation-fold geometries and flanked by thrust-wrench faults. Fault dips also show a significant change from more than 70° for α≤15° to less than 40° for α>30°. For α≥30°, the geometry of the main faults at the borders of the uplift zone is modified by P faults. In experiments with erosion and sedimentation, and thrust-dominated transpression, new faults with increasing dips form during progressive deformation, branching on the main fault at the base of the model.

Influence of syntectonic sedimentation on thrust geometry. Field examples from the Iberian Chain (Spain) and analogue modelling

Abstract Steep thrusts are usually interpreted as oblique-slip thrusts or inverted normal faults. However, recent analogical and numerical models have emphasised the influence of surface mass-transfer phenomena on the structural evolution of compressive systems. This research points to sedimentation and erosion during deformation as an additional explanation for the origin of steeply dipping thrusts. The present study uses both field observations and analogue modelling to attempt to isolate critical parameters of syntectonic sedimentation that might control the geometry of the upper part of thrust systems. A field study of thrust systems bounding two compressive intermountain Tertiary basins of the Iberian Chain is first briefly presented. We describe the surface geometry of thrusts surrounding the Montalban Basin and the Alto Tajo Syncline at the vicinity of deposits of Oligocene–Early Miocene alluvial fans at the footwall of faults. Field observations suggest that synthrusting sedimentation should influence the structure of thrusts. Indeed, the faults are steeper and splitted at the edge of the syntectonic deposits. Effects of sedimentation rate on footwall of thrusts, and of its change along fault strike are further investigated on two-layer brittle-ductile analogue models submitted to compression and syntectonic sediment supply. Two series of experiments were made corresponding to two end-members of depositional geometries. In the first series, the sedimentation was homogeneously distributed on both sides of the relief developed above the thrust front. In the second series, deposits were localised on a particular area of the footwall of thrust front. In all experiments, the sedimentation rate controls the number and the dip of faults. For low sedimentation rates, a single low-angle thrust develops; whereas for high sedimentation rates, a series of steeper dipping thrust is observed. In experiments with changing sedimentation rate along fault strike, the thrust geometry varies behind the areas with the thickest sediment pile.

Role of lateral thickness variations on the development of oblique structures at the Western end of the South Pyrenean Central Unit

Abstract The Montsec unit is one of the most important detached South-verging nappes within the South Pyrenean Central Unit (SPCU, Southern Pyrenees). A N–S cross-section of its Western sector, based on seismic reflection profiles, shows a hangingwall ramp geometry in Mesozoic strata, overlain by a syntectonic series of Lower Eocene sediments with growth geometry. The geometry of growth strata constrains the age of its movement between the Paleocene and the Middle Eocene. The geometry of the Western, oblique ramp of the South Pyrenean Central Unit is defined by a series of N–S folds, in some cases associated with underlying West-verging thrusts, as indicated by seismic reflection profiles and field data. In this paper, we propose that the geometry of the thrust wedge of Mesozoic units, progressively thinning from East to West, strongly contributed to constrain the location and geometry of the Western termination of the Montsec thrust. The hypothesis proposed is checked by a series of experimental wedges developed in a sandpack with lateral and three-dimensional thickness variations. Oblique structures form as thrusting progresses at the tip of the sand wedge.

LINDENS: a program for lineament length and density analysis

Analysis of lineaments from satellite images normally includes the determination of their orientation and density. The spatial variation in the orientation and/or number of lineaments must be obtained by means of a network of cells, the lineaments included in each cell being analysed separately. The program presented in this work, LINDENS, allows the density of lineaments (number of lineaments per km2 and length of lineaments per km2) to be estimated. It also provides a tool for classifying the lineaments contained in different cells, so that their orientation can be represented in frequency histograms and/or rose diagrams. The input file must contain the planar coordinates of the beginning and end of each lineament. The density analysis is done by creating a network of square cells, and counting the number of lineaments that are contained within each cell, that have one of their ends within the cell or that cross-cut the cell boundary. The lengths of lineaments are then calculated. To obtain a representative density map the cell size must be fixed according to: (1) the average lineament length; (2) the distance between the lineaments; and (3) the boundaries of zones with low densities due to lithology or outcrop features. An example from the Neogene Duero Basin (Northern Spain) is provided to test the reliability of the density maps obtained with different cell sizes.

Chronostratigraphy of the Boltaña anticline and the Ainsa Basin (southern Pyrenees)

The Ainsa Basin (south-central Pyrenees) is an exceptional example of a syntectonic foreland basin where structures oblique to the Pyrenean trend are well preserved. The absence of a complete chronostratigraphic frame motivated us to perform a detailed magnetostratigraphic study in the Ainsa Basin, including the shallow-marine–continental transition. Three sections covering almost the entire Eocene sedimentary fill were sampled (Ara, Coscollar, and Mondot), covering 2450 m of the sedimentary pile, with more than 1000 demagnetized specimens (sample spacing of 2.7 m). Data from previous magnetostratigraphic studies (Eripol section) allow us to complete the Eocene record with an 840-m-long profile within the Bartonian–Priabonian interval overlying the sampled sequence. Characteristic remanent magnetization (ChRM) directions were effectively isolated between 300 and 500 °C. Magnetite is the main magnetic carrier, with variable iron sulfide content and occasional hematite. Fold tests indicate a prefolding magnetization. Biostratigraphy based on shallow benthic foraminifera (15 localities) and a locality with abundant charophyte gyrogonites help to constrain the local magnetostratigraphic record (20 reversals) between the early Ilerdian and the middle Priabonian. A global correlation based on a magnetostratigraphic composite section derived from this work allows us to propose a complete chronostratigraphic frame for the Ainsa Basin infill between 55 and 45 Ma. Accumulation rates range from 2 to 53 cm/k.y., responding to retrogradational to progradational features during the early Lutetian, and a progressive increase from the middle Lutetian onward related to overall continentalization of the basin. Westward migration of subsidence is associated with progressively younger synsedimentary structures nucleating westward of the older ones in the South Pyrenean Basin.

Influence of along-strike pre-orogenic sedimentary tapering on the internal architecture of experimental thrust wedges

The occurrence of along-strike thickness variations in pretectonic sedimentary packages is expected to influence the structural architecture of doubly verging thrust wedges. To test this hypothesis, we used laboratory sandbox experiments. Model results show that longitudinal tapering of pretectonic sediments causes a great complexity in the internal tectonic fabric of the wedge, particularly in the pro-wedge, dominated by highly segmented, curvilinear thrusts. The along-strike variation of the mode by which the same amount of bulk contraction is accommodated in different regions of the orogen produces the obliquity of the deformation fronts in both the pro-wedge and the retro-wedge. Comparison with the overall architecture of the Lesser Antilles and Manila accretionary systems validates our experimental results.

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.

Quaternary reactivation of flexural-slip folds by diapiric activity: example from the western Ebro Basin (Spain)

In the western sector of the Ebro Basin two types of structures deform the Quaternary terraces and pediments developed by the Ebro River and its tributaries: (1) folds up to 10 km long in the lower levels of fluvial terraces and (2) normal listric faults that produce tilting and rollover anticlines of the Quaternary deposits. Both types of structures are linked to the geometrical and lithological features of the Tertiary beds that underlie the Quaternary deposits. Quaternary folds, whose axes are parallel to the strike of the Tertiary beds, are the result of reactivation of Tertiary large-scale (60 km long) folds due to diapirism of their gypsum cores, where the gypsum units reach a maximum thickness of 3 000 m. This reactivation produced flexural-slip in some beds on the limbs of the folds, bringing about the folding of the overlying Quaternary terraces. The mechanism of Quaternary folding involves layer-parallel shear in alternating Tertiary units and folding linked to detachments and reverse layer-parallel faults. Normal listric faults are widespread throughout the studied area. They are partly parallel to low dipping Tertiary beds and are the result of a NNE-SSW tectonic extension, compatible with minor structures and focal mechanisms of recent earthquakes. The relationship between the two kinds of Quaternary structures indicates that diapirism of the gypsum cores of the anticlines was activated by extensional tectonics.

Widespread Cretaceous secondary magnetization in the High Atlas (Morocco). A common origin for the Cretaceous remagnetizations in the western Tethys

In this paper, we present the first palaeomagnetic data (51 sites) for Mesozoic (Lower–Middle Jurassic) sediments of the Moroccan Central High Atlas and address the study of a widespread remagnetization. The remagnetization is characterized by a very stable component with systematic normal polarity, carried by magnetite. The relationship between the magnetic properties and location within the basin suggests that the acquisition of the remagnetization is controlled by basin geometry. Fold-tests indicate that the overprint acquisition is syn-folding in some structures but clearly predates the Tertiary compressional stage. Using the small circle intersection method we have calculated the remagnetization direction (D = 336.4°, I = 29.2°). Comparison with the global apparent polar wander path indicates that the remagnetization was acquired during the Late Cretaceous (probably Cenomanian). Considering both the basinal confinement of remagnetization and the connection with other remagnetization events in the western Tethys, we propose a scenario explaining widespread remagnetizations in the region, concerning both basin-scale conditions mainly related to sediment thickness and a regional-scale thermal event acting as catalyst of remagnetizations in those sedimentary basins that satisfy the basin-scale conditions.

Magnetic prospection as an efficient tool for doline detection: a case study in the central Ebro Basin (northern Spain)

Abstract The presence of alluvial dolines in the Ebro Basin causes problems to both agricultural and urban areas. At present, new urbanization of former farming areas requires new tools to detect karst zones and so diminish the hazard linked to collapses. In the surroundings of Zaragoza, dolines (developed mainly on Quaternary alluvial terraces covering a Tertiary gypsum substratum) are commonly filled with alluvial deposits, agricultural soils, urban debris, etc. Measurements of magnetic susceptibility show a remarkable contrast between host rocks and cavity fillings, demonstrating the value of magnetic surveying. A field test was made in a recently collapsed (September 2003) doline filled currently with urban debris. A magnetic survey was carried out following a 130 m2 grid, with 1–10 m spacing between profiles. A proton magnetometer with gradiometer was utilized, and the total field intensity and gradient measurements were taken. The magnetic survey demonstrated a strong anomaly with a dipole defined by more than 650 nT and a gradient of about 100 nT m−1. The 2.5-dimensional (2.5D) modelling of the magnetic anomaly fits well with the known geometrical data. Two other dolines (that are not clearly defined at the surface) were also detected during the survey. These results validate the starting hypothesis and open a new research approach to the problem. The magnetic survey output allows the construction of realistic geological models.