Fault Population Analysis Research Articles

Deformation processes in the Britannia Field, UKCS

The intensity and distribution of deformation within cores recovered from 24 Britannia Field wells has been quantified and used to assess the sub-seismic deformation present. Structural logging has been amalgamated with microstructural and petrophysical analysis of fault rocks and the results used to construct models of the fluid flow behaviour of fault zones and to evaluate the correlations between core-scale structures and the larger-scale seismically mapped fault arrays. Deformation within Britannia is controlled primarily by (1) early sediment disruption, dewatering, detachment and folding in semi-lithified sediments and (2) more long-lived clustered arrays of small faults developed within fault damage zones. Fault population analysis illustrates that seismic-based mapping alone does not provide a good platform for accurately predicting the number of sub-seismic faults present. Analysis of the effect of deformation features on fluid communication highlights the importance of small-scale faults in enhancing communication in complex and initially separate sand bodies.

The recent (upper Miocene to Quaternary) and present tectonic stress distributions in the Iberian Peninsula

A general synthesis of the recent and present stress situation and evolution in the Iberian Peninsula was obtained from microstructural and seismological analysis. The stress evolution was deduced from (1) fault population analysis (FPA) from 409 sites distributed throughout the Iberian Peninsula, (2) paleostress indicators given by 324 stations taken from the bibliography, and (3) seismic data corresponding to 161 focal mechanisms evenly spread in the studied region. The application of FPA together with the determination of stress tensors and focal mechanisms for the whole Iberian microplate has provided two main results: (1) the Iberian Peninsula is undergoing a NW‐SE oriented compression, except for the northeastern part (Pyrenees, Ebro Basin, and Iberian Chain), where it is N‐S to NE‐SW, and the Gulf of Cádiz, where it seems to be E‐W, and (2) the main trends of the stress field have remained almost constant since the upper Miocene. The analysis performed by zones suggests the presence of local heterogeneities in the stress field.

Calcite twins from microveins as indicators of deformation history

In order to investigate whether the evidence of the deformation history is preserved in the microvein calcite twinning microstructures, several families of calcite veins have been studied from weakly deformed mesozoic limestones of the western Iberian Chain (eastern Iberian Peninsula). The maximum shortening directions established with this method correspond to the previously established paleostress evolution of this chain. The reconstructed stress regimes evolve from predominantly compressional (four compressional episodes, with the maximum shortening direction oriented successively NW–SE, N–S, NE–SW and E–W) to extensional. The established orientations of maximum shortening axes are consistent with the directions of paleostrains that have been independently deduced from stylolites and fault striations in the same region. Moreover, the results clearly show that the e-twins record some paleostress orientations that are not registered in the studied area by fault population analysis. In this sense, the e-twins record older paleostress orientations than the small faults, probably because the faults were reactivated several times and the twins were not modified since their formation. The calculated values of twin strains, ∼1%; differential stress magnitudes, 62 MPa; and the existence of type I twins formed below 200°C, confirm that this region was slightly deformed under low P– T conditions.

Structure and kinematics of a foothills transect, Lago Viedma, southern Andes (49°30′S)

The western edge of Patagonia, south of 47°S, experienced a major tectonic reorganization during the Tertiary. The Chile ridge, separating Nazca from Antarctica, collided obliquely with western Tierra del Fuego at about 14 Ma and the triple point migrated northwards to its present position at about 47°S. Consequently, the southern tip of South America has passed from a Miocene context of rapid oblique convergence (ENE–WSW at about 9 cm/yr) between Nazca and South America, to a Pliocene context of slow frontal convergence (EW at about 2 cm/yr) between Antarctica and South America. The Andean foreland fold-and-thrust belt lies on the eastern side of the Patagonian Cordillera and is well exposed along the northern shore of Lago Viedma (49°30′S). Structural observations, digital mapping, subsurface data, balancing of a cross-section and kinematic analysis of fault populations provide new information on the structure of the fold-and-thrust belt, the timing and style of deformation and their relationship with Tertiary plate tectonics. Along the studied transect, synsedimentary structures show that compressional deformation began at least during the Late Cretaceous, was ongoing during the syntectonic emplacement of the Lower Miocene granitic Monte Fitz Roy pluton and continued into the Pliocene. Folds and thrusts are thick-skinned in the west, and mostly thin-skinned above a décollement in Early Cretaceous black shales in the east. Analysis of fault populations, measured within Jurassic basement and its Cretaceous cover, provides subhorizontal principal directions of shortening, striking between E–W and ENE–WSW. Compressional deformation was associated with a major component of right-lateral wrenching parallel to the Cordillera.

Seismotectonics of the Sierra Albarrana area (southern Spain). Constraints for a regional model of the Sierra Morena-Guadalquivir Basin limit

A new approach to the present day stress field of the Sierra Albarrana area (southern Spain) has been inferred from the application of fault population analysis (FPA) to a set of fault/striae pairs and a short sample of earthquakes and from the simultaneous determination of stress tensor and fault plane solutions for this last set. The first population of data concerned 226 fault and striae pairs obtained in 8 field outcrops. In their turn, seismic data correspond to 45 polarities of P-wave arrival and take-off angles for a set of source-station pairs. These data were obtained in a microseismicity survey of 30 months carried out in an area of 50 × 40 km 2. Four FPA methods have been chosen: the slip model, the right dihedral method, a 3D random search of the stress tensor and the stress inversion method. In addition, fault planes have been selected by using the symmetries established in the slip model, and the methodology of FPA has also been applied to seismic data. In this way, it has been possible to take advantage of two sources of information too small to be considered independently. The use of complementary methods based on the FPA has allowed the possible solutions for the fault planes to be constrained and has yielded a picture of the tectonic evolution of the area. The analysis of the seismic information and the comparison of the results obtained independently with FPA methods provide a reliable interpretation of the present-day stress distribution in the zone. The regional stress tensor shows a compression close to uniaxial with its compressive axis, σ y (or σ Hmax), oriented N132E and plunging 0°–19°. Focal mechanisms correspond to either reverse or normal faults, some of them with a noticeable strike-slip component. The reverse faults show a NW dip and are deeper than the normal ones. A regional deformation model including a crustal flexure with axis parallel to Guadalquivir River has also been sketched.

Determination of present-day stress tensor and neotectonic interval in the Spanish Central System and Madrid Basin, central Spain

A brittle deformation tectonic analysis was performed in central Spain (Spanish Central System and Madrid Basin) in order to decipher and understand the deformation processes that take place in a typical intracontinental zone. 1174 fault slickensides obtained in materials with ages between Late Cretaceous and Quaternary have been analyzed by means of fault population analysis methods to reconstruct paleostress tensors. Nine earthquake focal mechanisms have been determined, with magnitudes ranging between 3 and 4.1. With regard to regional structural features and sedimentary record data, the characteristics of present-day and neotectonic stress fields have been figured out, which determine the neotectonic period for this region. Thus, we have established that the intraplate zone represented by central Spain has been subjected to a stress field from the Middle Miocene until the present-day with a largest horizontal shortening direction (S HMAX) located between N130E and N160E. Finally, three paleostress maps with the main active structures are presented for: (a) Middle Miocene to Late Miocene, the period when the Spanish Central System was mainly formed, (b) Late Miocene to Quaternary, and (c) the present-day stress field, deduced from earthquake focal mechanisms.

Cenozoic dextral transpression and basin development at the southern edge of the Puna Plateau, northwestern Argentina

In northwestern Argentina, the southern edge of the Altiplano-Puna is the most remarkable positive topographic anomaly (average altitude > 3700 m) of the Andean chain. To the south are the Sierras Pampeanas, a less elevated domain made of alternating compressive basins and ranges developed during the Cenozoic. At about 27°S, the transition between the Puna and the Sierras Pampeanas coincides with a substantial narrowing of the high chain and with a change in the dip of the Pacific subduction zone (Nazca plate), from 30°E below the plateau to subhorizontal below the Sierras Pampeanas. We argue that this structural transition, which can be identified at the scale of the Andean topography, is a major dextral transpressional transfer zone, the Tucumán Transfer Zone. In this region, ENE-WSW- to E-W-directed subhorizontal shortening related to bulk convergence between the Nazca and South American plates interferes with NW-SE-directed subhorizontal shortening strains, due to lateral influence of changes in the amount of crustal thickening on intraplate kinematics. Structural observations, digital mapping, SPOT satellite images and a microtectonic analysis of fault populations provide general insights into the Neogene kinematic evolution of transpressional basins and ranges at the edge of the Puna.

The Internal Structure and Properties of Fault Damage Zones and the Impact on Sealing and Reservoir Behaviour: ABSTRACT

The concentration of sub-seismic deformation adjacent to larger faults forms a complex deformation volume or damage zone which is important to hydrocarbon migration, trapping and reservoir management. The results of a combined study of structural logging of cores, microstructural characterization and physical property measurement of deformation features preserved in cores, with analysis of fault populations from seismic and outcrop studies, are used to evaluate the properties and evolution of damage zones. The detailed internal structure of fault zones is dependent on the conditions of deformation, the lithological architecture present and the position in the fault array. Clusters of deformation within the damage zone are also surrounded by relatively undeformed volumes. Quantification of the geometry of the these zones is critical to understanding the fluid-flow and sealing properties of-the fault zone. Complete evaluation of the impact of the damage zone on reservoir properties requires knowledge of the petro-physical properties and spatial distributions of the fault rocks present. These range from quartz-rich cataclasites, developed from pure sandstones, to phyllosilicate smears developed from shales. An important class of intermediate fault rock is generated from impure sandstones. Localization of cement precipitation within the damage zone will remove the applicability of simple seal analysis basedmore » only on the host-rock lithology and displacement. Realistic prediction of the impact of faults on reservoir behavior requires integration of the detailed geometry with the petrophysical characterization of the fault rocks.« less

Analysis of fault populations in western Spitsbergen: Implications for deformation partitioning along transform margins

Mesoscale faults, expressed in the Paleogene Central basin and Cretaceous basement along the West Spitsbergen orogenic belt, are used to define the history of continental deformation associated with the development of the transform system that accompanied the opening of the Eurasian basin and the Norwegian-Greenland Sea. A fundamental change in the orientation of faults from dominantly north-south in the basal part of the basin to northeast striking in the upper units broadly correlates with a major shift in plate motion during late Paleocene–early Eocene time. The two main populations of faults are interpreted as extension fractures that were variably reactivated as shear fractures and formed in response to north-south compression followed by northeast-southwest compression. Paleostress stratigraphy, or the analysis of striated faults measured in successive stratigraphic units of the Central basin, corroborates this interpretation and allows a time sequence of paleostress tensors to be defined. Results show that a Late Cretaceous-early Paleocene north-south compression, consistent with plate motion, was followed by three relatively short-lived and regionally limited faulting events leading up to a change in plate motion during late Paleocene time (post-Chron 25). Major transpressive tectonism followed, producing northeast-trending fractures that were reactivated as dextral and sinistral faults defining a prolate-shape paleostress ellipsoid with a horizontal σ 1 . This regionally reproducible tensor, consistent with the prediction of dextral transpression along the paleotransform, was probably initiated at approximately the Paleocene-Eocene boundary. Finally, a post–early Eocene deformation, pervasively recorded in the study area by an oblate shape paleostress ellipsoid with horizontal σ 3 , is tentatively correlated with the dextral transtension that characterized the post–Chron 13 opening of the Norwegian-Greenland Sea and Eurasian basin. This study offers documentation of deformation partitioning during the evolution of a transform margin. The Late Cretaceousearliest Paleocene compression associated with the prototransform did not involve substantial partitioning, and a north-south maximum principal stress pervaded the continental crust away from the transform. In contrast, the late Paleocene–Eocene transpression reflects total partitioning of deformation along the evolved, mature transform margin. Oblique plate motion was presumably partitioned into a strikeslip zone(s) offshore, whereas the onshore Central basin recorded only the orthogonal component of oblique convergence and divergence, a pattern similar to modern orogenic belts developed in zones of oblique plate motion.

Facturación a distintas escalas y campos de esfuerzos durante la tectogenesis alpina en el área de Mosqueruela (Teruel)

The analysis of the brittle tectonic deformation on all scales in one area of the eastern Iberian Chain (Mosqueruela, Teruel) allows us to define a fracture pattern characterized by three main sets: NNE to NE, ENE and E to ESE. Brittle structures (stylolites, tension gashes, brittle-ductile shear zones and faults) show a poliphase Alpine tectonics. From the analysis of fault populations two compressive (wrench regime) phases and a multidirectional extension regime have been inferred. Their chronological evolution is the fol1owing: 1) ENE compression; 2) ESE compression; 3) nearly multidirectional NNE extension. The study of fractures using a number of methodologies at macro-, meso- and microstructural scales allows us to suggest a model of stress distribution and relationships between different structures in sorne areas. The detailed fracture mapping on a hectometric scale shows the distribution and strike variations of fractures. These changes have been interpreted as stress deflection phenomena due to major structures which, in many cases, were identified. Examples of stress deflections have been analyzed both for strike-slip and normal fault sets.

Stress deflection in a tectonic compressional field: A model for the northwestern Iberian Chain, Spain

By means of fault population analysis (using the right dihedra, y‐R diagram and Etchecopar's methods), the states of stress in a number of sites of the Cameras Massif (Northwestern Iberian Chain, Spain) and nearby areas of the Ebro Basin have been determined. These states of stress have been used as a framework in order to infer a model of the actual stress field in that area during the main stage of the Alpine compression (Oligocene‐lower Miocene). The three main compression directions obtained (NW‐SE, NNE, NE‐SW) can be explained by a single compression field with horizontal σ1 trending NNE and vertical σ2, whose primary trajectories are deflected due to (1) large‐scale strike‐slip faults or lateral ramps of thrusts or (2) frontal ramps of thrusts oblique to the main compressive axis. The reliability of this scheme has been checked by means of a two‐dimensional mathematical model based upon the finite element method and applied on a sheet whose rheological properties are similar to those of a standard limestone. This model simulates the main features of the regional stress field, and it was constructed after determining the average parameters of the possible external or “primary” field and simulating the major faults that could deflect the stress trajectories.

An application of the slip model of brittle deformation to focal mechanism analysis in three different plate tectonic situations

The “slip model” of brittle deformation ( Reches, 1983) imposes a set of constraints on the angular relationships between faults, their striations on slickensides and the type and orientation of the deformation ellipsoid. In focal mechanisms of earthquakes, these relationships can be used to determine which of the two nodal planes is the fault plane, and then to obtain the strain ellipsoid of regional tectonic deformation controlling seismicity. In this approach it is convenient to use pitch/dip diagrams ( De Vicente, 1988) and P/T diagrams ( Angelier and Mechler, 1977), both developed for fault population analysis. In this paper three regions in different plate-tectonic situations are analysed using these methods. These regions are a zone of intracontinental seismicity in the eastern Peruvian Andes, a zone of continental compressional deformation in the Iranian Plateau, and the Fenwei Graben of China. Nodal planes of focal mechanisms, published in different papers about seismicity in these regions, have been used, the following conclusions being reached. In the analysed zone of the Peruvian Andes, three regional deformation types appear, causing fault movements that range from slightly reverse strike-slip to reverse strike-slip. In addition, exclusively at depths in excess of 18 km, there is a deformation of an uniaxial shortening type. In all of these deformations the horizontal shortening axis is oriented from N87° to N93°. In the Iranian Plateau two horizontal shortening directions appear, N151° and N61°, even showing faults that imply an uniaxial deformation ellipsoid the shortening axis of which is at N61°. In the Fenwei Graben the ellipsoid of regional deformation has a stretching principal axis at N145°/150° and a shortening axis at N55°/60°, the strike-slip movement along faults being as important as the normal one.

Modes of extension at oceanic spreading centers: evidence from the Solea graben, Troodos ophiolite, Cyprus

The Solea graben in the Troodos ophiolite, Cyprus, represents a preserved Cretaceous-age axial valley formed during a brief period of amagmatic seafloor spreading. Structural analysis of all crustal levels of the graben reveals two periods of deformation; an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the Tertiary-Recent. Amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike-plutonic contact and oriented subparallel to ophiolite pseudostratigraphy. Individual fault segments are essentially planar but link to approximate listric geometries locally. Bulk structural rotation of fault blocks bounding the graben occurred about a NNW-trending axis, parallel to the present graben trend. Average dips in the sheeted complex within the central portions of the graben suggest 40–50° of structural rotation, although this is a minimum figure because present dike dips are not a simple function of rotation magnitude. Tensor analysis of fault populations associated with extensional deformation shows that σ 3 was oriented approximately perpendicular to the graben axis. Late, near-axis extensional deformation segmented the detachment fault along NNW-trending normal faults. Obduction-related uplift and doming of the Troodos ophiolite is expressed in the Solea graben by minor thrust, reverse and strike-slip faults principally developed during reactivation of pre-existing low-angle fault zones formed during seafloor deformation. Tensor analysis of this later phase of faulting shows that σ 1 was oriented approximately N—S and that σ 2 and σ 3 alternated position during the deformation. The style of deformation documented in the Solea graben supports recent speculation for Basin and Range-style detachment faulting at slow-spreading ridge crests with low magma budgets, such as parts of the Mid-Atlantic Ridge near large-offset transform faults. Natural cross-sectional exposure of the Solea graben shows how extensional structures might link at depth beneath such slow-spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large-scale (ore-forming) hydrothermal circulation cells.

A numerical method for determining the state of stress using focal mechanisms of earthquake populations: application to Tibetan teleseisms and microseismicity of Southern Peru

The numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms. This method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane. The proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes. This enables one to test the data homogeneity. Furthermore, this restricts the space where the principal stress axes have to be searched. Then, for each principal stress reference whose location is constrained by above confined zones, the R value (chosen equal to (σ 2′ − σ 1′)/(σ 3′ − σ 1′) ) is computed which fits the slip vector on each nodal plane. This permits one to select a set of preferred seismic fault planes from a set of auxiliary planes. Finally, a state of stress is computed from the preferred seismic fault plane set using the non-linear simplex method already applied to fault populations. This algorithm is constructed so as to avoid two major difficulties: misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions. This calculation does not take into account possible changes of the strike and dip of the nodal planes. However, we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress. Tibetan teleseisms and southern Peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions. This suggests that major seismic events may give a good evaluation of the regional (mean) state of stress. Focal mechanisms of the small events are also modelled by a mean stress deviator, but this differs significantly from the regional state of stress in southern Peru deduced from Quaternary faulting. This may be due to imprecise location of the small events. However, some movements are strongly different from those predicted by the computed mean stress deviator. This suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations.