José L. Simón

Geometry and distribution of regional joint sets in a non-homogeneous stress field: case study in the Ebro basin (Spain)

Abstract Three regional joint sets striking N–S, E–W and WNW–ESE affect the Tertiary rocks of the central Ebro basin. From analysis of their chronological relationships and spatial distribution, it is concluded that they correspond to two different tectonic events. The N–S set (oldest) and the E–W set (younger) are present in the southern and central sectors, while the WNW–ESE joint set predominates in the northern one. The N–S joints propagated in response to joint-normal and fluid loads under an intraplate stress field with S Hmax oriented near N–S (related to forces caused by the convergence of Africa, Iberia and Europe and rifting at the Valencia trough) during the sedimentary infilling of the basin. These joints are only present in the southern part of the area. The E–W joint set in the southern-central sector records the same fracturing event as the WNW–ESE set does in the northern one. Its orientation was modified by the presence of the older N–S set in the south, which perturbed the regional stress field. The younger WNW–ESE and E–W joint sets are interpreted as unloading joints. These propagated as a consequence of flexural uplift and exhumation related to isostatic rebound at the Pyrenees and the Ebro foreland basin. A numerical approach is used to explain the inhomogeneous distribution of the N–S joint set in terms of their absence being controlled by the depth of the water table at the time of their formation.

Diapiric deformations in the Quaternary deposits of the central Ebro Basin, Spain

From a study of 24 outcrops in Neogene and Quaternary deposits of the central Ebro Basin, a number of diapiric deformations have been recognized, two principal types being differentiated: domal or pillow structures, and piercement or intrusive structures. The former are incipient diapirs of gypsum. Piercing structures have reverse faulted contacts not caused by halokinesis; here Neogene marls are the active plastic material, contrasting with the competent behaviour of gypsum. These intrusive structures are viscous diapirs which easily pierce the non-consolidated, low strength Quaternary gravel overburden and submit it to a horizontal compressive stress. As a consequence, reverse faults and flexures develop in it. Generally normal faults and tension cracks do not appear. Underlying gypsum beds are frequently pulled up into diapirs and they constitute the structural core. Density contrast and conditions for plastic flow exist at the marl–gravel boundary. It seems to have been specially common at the time of Quaternary fluvial sedimentation, so that much deformation is synsedimentary. Diapiric phenomena have been very active during early to middle Pleistocene time, becoming weaker afterwards.

Tectonic subsidence v. erosional lowering in a controversial intramontane depression: the Jiloca basin (Iberian Chain, Spain)

The Jiloca basin is a large intramontane, NNW–SSE-trending topographical depression in which the relative role of tectonic subsidence and erosional lowering is currently a matter of discussion. Geometry and facies of the sedimentary infill at its central sector have now been characterized from compiled borehole data, which allows discussions of how the evolutionary model is constrained. The central Jiloca depression contains a Late Pliocene to Pleistocene sedimentary sequence made up of alluvial fan, pediment mantle and episodic palustrine deposits, overlying a carbonate unit that could represent an early lacustrine stage of Late Miocene–Early Pliocene age. The geometry of these units is partially controlled by NW–SE-striking normal faults. Both the morphological depression and the sedimentary basin truncate previous folds, whose traces beneath the Neogene–Quaternary infill have been interpreted from the geology of the basin margins, borehole data and hydrogeological criteria. The northern and southern sectors of the Jiloca depression are bounded by faults showing measurable hectometric-scale throws (Calamocha and Concud faults). Moreover, in the central sector, the ~ 350–400 m tectonic uplift of Sierra Palomera has been interpreted from a morphostructural reconstruction of the tilted block which separates the Teruel and Jiloca graben, being similar to the height of the Sierra Palomera mountain front. All these features are consistent with a tectonic basin developed within the framework of the Neogene–Quaternary extensional evolution of eastern Spain. In contrast, they are hardly compatible with genetic models based on erosional deepening, either topographic lowering by numerous nested Tertiary erosion pediplains, or sub-alluvial Pliocene–Quaternary karstic corrosion.

Erosion-controlled geometry of buckle fold interference

Refolding structures in the Aliaga area (Iberian Chain, Spain) show how erosion can exert a control on the geometry of buckle fold interference. Early Miocene ENE-trending buckle folds are superposed on a large NNW-SSE anticline (Eocene–Oligocene). North of Aliaga, the earlier hinge zone in the competent limestone unit controlling buckling (Urgon facies, Lower Cretaceous) constitutes a mechanical obstacle to refolding, which induces development of essentially conical type 2 interference. To the south, erosional removal of the hinge zone at that competent unit allowed the near-vertical eastern limb to behave as independent layers, being refolded into cylindrical, near-vertical-axis folds showing a snake-like map pattern. Lower Miocene conglomerates containing pebbles of Jurassic origin, whose source area was the core of the earlier NNW-SSE–trending anticline, indicate that this Jurassic core had been effectively exhumed before refolding occurred. In this way, snake-like folds constitute a new case of erosion-controlled tectonic structure.

Stress Partitioning: a Practical Concept for Analysing Boundary Conditions of Brittle Deformation

Although conjugate fault systems cannot accommodate three-dimensional strain, the notion of stress-driven fracturing based upon Mohr-Coulombs failure criterium can be reconciled with any kinematic boundary condition if a non-static scenario is considered. The concept of stress partitioning is proposed as a tool for understanding the mechanisms allowing such compatibility. It describes stress changes that occur in response to failure events, giving the appearance that the total stress field is decoupled into distinct, independent components. It can be mainly applied to biaxial horizontal loading conditions (e.g. multidirectional tension or multidirectional compression in intraplate regions). Swapping of horizontal stress axes, caused by stress drop subsequent to failure, can result in two distinct stress events and two, either tensile or shear fracture systems with nearly orthogonal strikes. Each fracture event develops according to Mohr-Coulombs and Griffiths failure criteria, but the resulting overall deformation is three-dimensional.

Alluvial sinkholes over gypsum in the Ebro basin (Spain): genesis and environmental impact

Abstract The Ebro Basin is located in the northeast of Spain. Its central part is filled with gypsum and carbonate deposits of Neogene age. Quaternary sediments overlie the Tertiary layers. Alluvial sinkholes have developed in this region, causing high economic losses. From the study of aerial photographs, from the compilation of historical cases of collapse and from data on the lithological characteristics of the Quaternary and Tertiary deposits (their structure, morphology, hydrological characteristics, etc.), natural and human factors in sinkhole development have been evaluated. Among the natural variables that may be considered are the lithology of the Tertiary deposits, the thickness and geomechanical behaviour of the Quaternary cover, the morphology of the contact between the Tertiary and Quaternary deposits, fracturing, content of sulphates in groundwater and the depth and seasonal variations of the groundwater level. The irrigation techniques of farmers and the presence of irrigation canals increa...

QUATERNARY ALLUVIAL SINKHOLES: RECORD OF ENVIRONMENTAL CONDITIONS OF KARST DEVELOPMENT, EXAMPLES FROM THE EBRO BASIN, SPAIN

The central Ebro Basin is an exceptional region for studying karstification through time and under different environmental conditions, as sinkholes have been developing since the Early Pleistocene. Knowledge of active sinkholes is complemented with research on paleosinkholes and contemporary deposits. Sedimentological, min- eralogical, geomorphological and structural approaches permit interpretation of the natural environmental conditions that favored karst in the past and the main genetic mechanisms involved. The sedimentary features of Pleistocene terraces indicate that they were deposited by a gravel braided fluvial system characterized by higher water and sediment availability than today, probably related to meltwater flows coming from glaciated source areas, mainly in the Pyrenees. Genesis of paleosinkholes was mainly linked to this high water supply. Some of them acted as small lakes where fine sediments are exceptionally well conserved to give clues about environmental conditions. The neo- formation of palygorskite and sepiolite suggests arid to semiarid climatic conditions, in agreement with the idea of cold glacial episodes. During Pleistocene times, development of sinkholes was influenced by tectonics. Currently, the genesis and evolution of numerous sinkholes are also influenced by water supplies from human activities such as irrigation or urbanization, sharply changing the nearly steady state exhibited in the past.

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.

Correlation of sedimentary units from grain-size and mineralogic analyses as a tool for constraining trench interpretations in palaeoseismology

In palaeoseismological trench studies, precise correlation of sedimentary units between fault blocks has an unquestionable value for accurately inferring the amounts of coseismic displacement and hence for assessing seismic hazard. A methodology based on laser analysis of particle size and mineralogic composition by X-ray diffraction is proposed in order to strengthen the correlation of sedimentary units in a trench excavated across the Concud Fault (central Iberian Chain, Spain). The surveyed sediments show sharp and multiple lateral facies changes, as well as inconsistent optically stimulated luminescence (OSL) ages. The results reinforce the correlation based on field inspection of lithologic and sedimentologic features. Moreover, they allow interpretation of rejuvenation of OSL ages of samples in the upthrown fault block, which has been attributed to partial erosion of sedimentary units, as evidenced by their smaller thickness and erosive boundaries. The correlated units are then used to estimate coseismic displacements for three palaeoseismic events.

Geophysical characterization of buried active faults: the Concud Fault (Iberian Chain, NE Spain)

The Concud Fault is a ~14-km-long active fault that extends close to Teruel, a city with about 35,000 inhabitants in the Iberian Range (NE Spain). It shows evidence of recurrent activity during Late Pleistocene time, posing a significant seismic hazard in an area of moderate-to-low tectonic rates. A geophysical survey was carried out along the mapped trace of the southern branch of the Concud Fault to evaluate the geophysical signature from the fault and the location of paleoseismic trenches. The survey identified a lineation of inverse magnetic dipoles at residual and vertical magnetic gradient, a local increase in apparent conductivity, and interruptions of the underground sediment structure along GPR profiles. The origin of these anomalies is due to lateral contrast between both fault blocks and the geophysical signature of Quaternary materials located above and directly south of the fault. The spatial distribution of anomalies was successfully used to locate suitable trench sites and to map non-exposed segments of the fault. The geophysical anomalies are related to the sedimentological characteristics and permeability differences of the deposits and to deformation related to fault activity. The results illustrate the usefulness of geophysics to detect and map non-exposed faults in areas of moderate-to-low tectonic activity where faults are often covered by recent pediments that obscure geological evidence of the most recent earthquakes. The results also highlight the importance of applying multiple geophysical techniques in defining the location of buried faults.