Wet and dry model granulates under mechanical load : a confocal microscopy study

Granular matter, also known as bulk solids, consists of discrete particles with sizes between micrometers and meters. They are present in many industrial applications as well as daily life, like in food processing, pharmaceutics or in the oil and mining industry. When handling granular matter the bulk solids are stored, mixed, conveyed or filtered. These techniques are based on observations in macroscopic experiments, i.e. rheological examinations of the bulk properties. Despite the amply investigations of bulk mechanics, the relation between single particle motion and macroscopic behavior is still not well understood. For exploring the microscopic properties on a single particle level, 3D imaging techniques are required.rnThe objective of this work was the investigation of single particle motions in a bulk system in 3D under an external mechanical load, i.e. compression and shear. During the mechanical load the structural and dynamical properties of these systems were examined with confocal microscopy. Therefor new granular model systems in the wet and dry state were designed and prepared. As the particles are solid bodies, their motion is described by six degrees of freedom. To explore their entire motion with all degrees of freedom, a technique to visualize the rotation of spherical micrometer sized particles in 3D was developed. rnOne of the foci during this dissertation was a model system for dry cohesive granular matter. In such systems the particle motion during a compression of the granular matter was investigated. In general the rotation of single particles was the more sensitive parameter compared to the translation. In regions with large structural changes the rotation had an earlier onset than the translation. In granular systems under shear, shear dilatation and shear zone formation were observed. Globally the granular sediments showed a shear behavior, which was known already from classical shear experiments, for example with Jenike cells. Locally the shear zone formation was enhanced, when near the applied load a pre-diluted region existed. In regions with constant volume fraction a mixing between the different particle layers occurred. In particular an exchange of particles between the current flowing region and the non-flowing region was observed. rnThe second focus was on model systems for wet granular matter, where an additional binding liquid is added to the particle suspension. To examine the 3D structure of the binding liquid on the micrometer scale independently from the particles, a second illumination and detection beam path was implemented. In shear and compression experiments of wet clusters and bulk systems completely different dynamics compared to dry cohesive models systems occured. In a Pickering emulsion-like system large structural changes predominantly occurred in the local environment of binding liquid droplets. These large local structural changes were due to an energy interplay between the energy stored in the binding droplet during its deformation and the binding energy of particles at the droplet interface. rnConfocal microscopy in combination with nanoindentation gave new insights into the single particle motions and dynamics of granular systems under a mechanical load. These novel experimental results can help to improve the understanding of the relationship between bulk properties of granular matter, such as volume fraction or yield stress and the dynamics on a single particle level.rnrn

Evidence for a Frozen Bed, Byrd Glacier, Antarctica

Ice thickness, computed within the fjord region of Byrd Glacier on the assumptions that Byrd Glacier is in mass-balance equilibrium and that ice velocity is entirely due to basal sliding, are on average 400 m less than measured ice thicknesses along a radio-echo profile. We consider four explanations for these differences: (1) active glacier ice is separated from a zone of stagnant ice near the base of the glacier by a shear zone at depth; (2) basal melting rates are some 8 m/yr; (3) internal shear occurs with no basal sliding in much of the region above the grounding zone; or (4) internal creep and basal sliding contribute to the flow velocity in varying proportions above the grounding zone. Large gradients of surface strain rate seem to invalidate the first explanation. Computed values of basal shear stress (140 to 200 kPa) provide insufficient frictional heat to melt the ice demanded by the second explanation. Both the third and fourth explanations were examined by making simplifying assumptions that prevented a truly quantitative evaluation of their merit. Nevertheless, there is no escaping the qualitative conclusion that internal shear contributes strongly to surface velocities measured on Byrd Glacier, as is postulated in both these explanations.

The Basement of the Deccan Traps and Its Madagascar Connection: Constraints from Xenoliths

Paleogeographic reconstructions of India and Madagascar before their late Cretaceous rifting juxtapose the Antongil Block of Madagascar against the Deccan Traps of India, indicating that the Western Dharwar Craton extends below the Deccan lavas. Some recent studies have suggested that the South Maharashtra Shear Zone along the northern Konkan coast of India limits the northern extent of the Western Dharwar Craton, implying that the craton does not extend below the Deccan Traps, raising a question mark on paleogeographic reconstructions of India and Madagascar. The continuity of the Western Dharwar Craton north of the South Maharashtra Shear Zone below the Deccan Traps—or its lack thereof—is critical for validating tectonic models correlating Madagascar with India. In this study, zircons in tonalitic basement xenoliths hosted in Deccan Trap dykes were dated in situ, using the U-Pb isotope system. The data furnish U-Pb ages that define three populations at 2527 ± 6, 2456 ± 6, and 2379 ± 9 Ma. The 2527 ± 6 Ma ages correspond to the igneous crystallization of the tonalites, whereas the 2456 ± 6 and 2379 ± 9 Ma ages date metamorphic overprints. The results help to establish for the first time that the basement is a part of the Neoarchean granitoid suite of the Western Dharwar Craton, which extends northward up to at least Talvade in central and Kihim beach in the western Deccan. By implication, the South Maharashtra Shear Zone cannot be the northern limit of the Western Dharwar Craton. The granitoids are correlated with the Neoarchean felsic intrusions (2.57–2.49) of the Masaola suite in the Antongil Block of Madagascar, supporting the existence of a Neoarchean Greater Dharwar Craton comprising the Western Dharwar Craton and the Antongil-Masora Block.

Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen

The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.

Mineralogy and structural specification of ODP Leg 125 peridotite samples

Abundant serpentinite seamounts are found along the outer high of the Mariana forearc at the top of the inner slope of the trench. One of them, Conical Seamount, was drilled at Sites 778, 779, and 780 during Leg 125. The rocks recovered at Holes 779A and 780C, respectively, on the flanks and at the summit of the seamount, include moderately serpentinized depleted harzburgites and some dunites. These rocks exhibit evidence of resorption of the orthopyroxene, when present, and the local presence of very calcic-rich diopside in veins oblique to the main high-temperature foliation of the rock. The peridotites, initially well-foliated with locally poikiloblastic textures, show overprints of a two-stage deformation history: (1) a high-temperature (>1000°C), low-stress (0.02 GPa), homogeneous deformation that has led to the present Porphyroclastic textures displayed by the rocks and (2) heterogeneous ductile shearing at a much higher stress (0.05 GPa). This heterogeneous shearing probably describes a single tectonic event because it began at high temperatures, producing dynamic recrystallization of olivine in the shear zone, and ended at low temperatures in the stability field of chlorite and serpentine. In a few samples, olivine shows evidence of quasi-hydrostatic recrystallization at a very high temperature. Here, we propose that this recrystallization was related to fluid/magma percolation, a process that can also account for the resorption of the orthopyroxene and for the late crystallization of diopside veins in the rock. The impregnation by fluid or magma, development of the main high-temperature, low-stress deformation, and subsequent migration recrystallization of olivine probably occurred in a mantle fragment involved in the arc formation. In addition, this mantle has preserved structures that may have formed earlier in the oceanic lithosphere upon which the arc formed. Heterogeneous ductile shear zones in the peridotites may have developed during uplift. The "cold" deformation may have taken place during diapiric rise of hot mantle that underwent subsequent serpentinization or gliding along normal faults associated with the extension of the eastern margin of the forearc.

Seismic velocities and elastic properties of gabbroic rocks from ODP Hole 118-735B

The nearly continuous recovery of 0.5 km of generally fresh, layer 3 gabbroic rocks at Hole 735B, especially near the bottom of the section, presents scientists an unusual opportunity to study the detailed elastic properties of the lower oceanic crust. Extending compressional-wave and density shipboard measurements at room pressure, Vp and Vs were measured at pressures from 20 to 200 MPa using the pulse transmission method. All of the rocks exhibit significant increases in velocity with increasing pressure up to about 150 MPa, a feature attributed to the closing of microcrack porosity. Measured velocities reflect the mineralogical makeup and microstructures acquired during the tectonic history of Hole 735B. Most of the undeformed and unaltered gabbros are approximately 65:35 plagioclase/clinopyroxene rocks plus olivine or oxide minerals, and the observed densities and velocities are fully consistent with the Voigt-Reuss-Hill (VRH) averages of the component minerals and their proportions. Depending on their olivine content, the predominant olivine gabbros at 200 MPa have average Vp = 7.1 ± 0.2 km/s, Vs = 3.9 ± 0.1 km/s, and grain densities of 2.95 ± 0.5 g/cm3. The less abundant iron-titanium (Fe-Ti) oxide gabbros average Vp = 6.75 ± 0.15 km/s, Vs = 3.70 ± 0.1 km/s, and grain densities of 3.22 ± 0.05 g/cm3, reflecting the higher densities and lower velocities of oxide minerals compared to olivine. About 30% of the core is plastically deformed, and the densities and directionally averaged velocities of these shear-zone tectonites are generally consistent with those of the gabbros, their protoliths. Three sets of observations indicate that the shear-zone metagabbros are elastically anisotropic: (1) directional variations in Vp, both vertical and horizontal and with respect to foliation and lineation; (2) discrepancies among Vp values for the horizontal cores and the VRH averages of the component minerals and their mineral proportions, suggesting preferred crystallographic orientations of anisotropic minerals; and (3) variations of Vs of up to 7%, with polarization directions parallel and perpendicular to foliation. Optical inspection of thin sections of the same samples indicates that plagioclase feldspar, clinopyroxene, and amphibole typically display crystallographic-preferred orientations, and this, plus the elastic anisotropy of these minerals, suggests that preferred orientations are responsible for much of the observed anisotropy, particularly at high pressure. Alteration tends to be localized to brittle faults and brecciated zones, and typical alteration minerals are amphibole and secondary plagioclase, which do not significantly change the velocity-density relationships.

Mineral composition and 40Ar-39Ar data of samples from Garnet Point, Aurora Peak, Correll Nunatak, Penguin Point and Horn Bluffs, East Antarctica

George V Land (Antarctica) includes the boundary between Late Archean-Paleoproterozoic metamorphic terrains of the East Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross-Delamerian Orogen. This therefore represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschist-facies conditions. 40Ar-39Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age for ductile deformation under greenschist-facies conditions is 1502 ± 9 Ma and reveal no evidence of reactivation processes linked to the Ross Orogeny. 40Ar-39Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ~1.7 Ga old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east of Ninnis Glacier (~180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths in Cambro-Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica 40Ar-39Ar ages from ~530 to 640 Ma and a minimum age of 518 ± 5 Ma. This pattern compares remarkably well with those previously obtained for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.

The evolution of high-temperature mylonitic microfabrics: evidence from simple shearing of a quartz analogue (norcamphor)

Plane strain simple shearing of norcamphor (C7H10O) in a see-through deformation rig to a shear strain of γ = 10.5 at a homologous temperature of Th = 0.81 yields a microfabric similar to that of quartz in amphibolite facies mylonite. Synkinematic analysis of the norcamphor microfabric reveals that the development of a steady-state texture is linked to changes in the relative activities of several grain-scale mechanisms. Three stages of textural and microstructural evolution are distinguished: (1) rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears. A symmetrical c-axis girdle reflects localized pure shear extension along the main microshear set (Sa) oblique to the bulk shear zone boundary (abbreviated as SZB); (2) progressive rotation of the microshears into parallelism with the SZB increases the component of simple shear on the Sa microshears. Grain-boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide. This is associated with a textural transition towards two stable c-axis point maxima whose skeletal outline is oblique with respect to the Sa microshears and the SZB; and (3) at high shear strains (γ > 8), the microstructure, texture and mechanism assemblage are strain invariant, but strain continues to partition into rotating sets of microshears. Steady state is therefore a dynamic, heterogeneous condition involving the cyclic nucleation, growth and consumption of grains.

Inherited structural controls on fault geometry, architecture and hydrothermal activity: an example from Grimsel Pass, Switzerland

Exhumed faults hosting hydrothermal systems provide direct insight into relationships between faulting and fluid flow, which in turn are valuable for making hydrogeological predictions in blind settings. The Grimsel Breccia Fault (Aar massif, Central Swiss Alps) is a late Neogene, exhumed dextral strike-slip fault with a maximum displacement of 25–45 m, and is associated with both fossil and active hydrothermal circulation. We mapped the fault system and modelled it in three dimensions, using the distinctive hydrothermal mineralisation as well as active thermal fluid discharge (the highest elevation documented in the Alps) to reveal the structural controls on fluid pathway extent and morphology. With progressive uplift and cooling, brittle deformation inherited the mylonitic shear zone network at Grimsel Pass; preconditioning fault geometry into segmented brittle reactivations of ductile shear zones and brittle inter-shear zone linkages. We describe ‘pipe’-like, vertically oriented fluid pathways: (1) within brittle fault linkage zones and (2) through alongstrike- restricted segments of formerly ductile shear zones reactivated by brittle deformation. In both cases, low-permeability mylonitic shear zones that escaped brittle reactivation provide important hydraulic seals. These observations show that fluid flow along brittle fault planes is not planar, but rather highly channelised into sub-vertical flow domains, with important implications for the exploration and exploitation of geothermal energy.

Las mineralizaciones auríferas de Galicia Occidental en la Zona de Cizalla Busto-Limideiro (A Coruña)

Las mineralizaciones de oro de Galicia Occidental ligadas a grandes estructuras comienzan a ser interpretadas de manera más rigurosa, al mismo tiempo que la continuada elevación de las cotizaciones del oro han hecho más atractiva la reactivación de áreas hasta ahora casi olvidadas. El propósito de esta investigación participa de las razones anteriores. La zona de cizalla Busto Limideiro se inscribe en una de estas estructuras, la cizalla Santa Comba-Punta Langosteira, con la que se relacionan un importante número de yacimientos auríferos entre los que destacan Corcoesto y Monte Piñor. Por su situación geológica, estos indicios se enmarcan en el Macizo Ibérico, en la Zona de Galicia Tras-os-Montes. Este trabajo aporta más conocimiento sobre un modelo general del oro de los yacimientos del noroeste español, integrándolo en los yacimientos de oro orogénico del Varisco europeo. Este estudio ha supuesto un importante esfuerzo investigador centrado sobre de los procesos genéticos que han dado lugar a las mineralizaciones auríferas. Los estudios petrográficos, metalogénicos, geoquímicos, microtemométricos y el análisis de isótopos han permitido establecer un modelo general y otros más específicos sobre la existencia y condiciones de formación en la concentración del oro mineralógico. De esta forma se ha caracterizado su mineralogía, quimismo, edad, y naturaleza de los paleofluidos que han intervinieron en su formación. De los resultados obtenidos se destaca que estas mineralizaciones se encuentran fuertemente ligadas a las estructuras y transformaciones físicas y químicas de las rocas afectadas por la banda de cizalla. Derivada de esta situación, se comprende la disposición espacial, a diversas escalas, con estructuras claramente en la transición del dominio dúctil a frágil. Además, resulta evidente, a escala macro y microscópica, el emplazamiento preferente del oro en estructuras de deformación miloníticas. Los estudios anteriores posibilitan formalizar un modelo de génesis para el yacimiento de Monte Piñor que resulta compatible con el establecido por Groves (1998), e incluirlas en un mismo contexto, como yacimientos de oro orogénico mesotermales, válido para el conjunto de todos los de la zona de cizalla Busto Limideiro. El estudio no omite la consideración geoestadística de este yacimiento, justificando la elección de direcciones de anisotropía explicadas por consideraciones de carácter estructural. Con ello, buscando el lado práctico desde el punto de vista minero, esta investigación propone líneas generales de aplicación de actuación con sentido de exploración, contribuyendo a la puesta en valor del conocimiento de las mineralizaciones de oro del noroeste español. ABSTRACT Old and nearly forgotten gold mines from Western Galicia are currently attractive to be recovered due to both, the new strict interpretation of gold mineralization linked to Variscan large structures and the increasing gold prices. The shear zone Busto Limiderio is relevant as a part of a Variscan large structure, Santa Comba-Punta Langosteira, which is related with a high number of gold deposits as Corcoesto and Monte Piñor. For its geological situation, these signs are part of the Iberian Massif in the area of Galicia Tras-os-Montes. Hence, the main aim of this study is to provide additional and important knowledge of Busto Limiderio about the general model of gold deposits in the Spanish northwest, integrating them in the orogenic gold deposits of European Variscan, enabling to analyze the economic recovery possibility of this area. This thesis has been focused on the genetic processes that have led to the gold mineralization. Petrographic studies, metalogenic, geochemical, microthermometric and isotope analysis have allowed to establish a general and specific models about the existence and training conditions in the concentration of mineralogical gold. Consequently, the mineralogy, chemical characteristics, age, and nature of paleofluids that have participated in its formation have been characterized. The results showed that Busto Limiderio gold mineralization are strongly linked to the structures and physical and chemical transformations of affected rocks by the shear band. The mineralization spatial distribution at various scales is due to the link with the large structures in the ductile-to-brittle transition domain. Furthermore, this factor determine at macro and microscopic scale, the preferred location of the gold structures mylonitic deformation. Previous studies have permitted to establish a model of genesis for the site of Monte Piñor which is compatible with the established by Groves (1998), and also to include them in the same context as mesothermal orogenic gold deposits, valid throughout all of Busto Limideiro shear zone. This study does not omit consideration of the geostatistical deposit, justifying the choice of directions of anisotropy explained by structural considerations. This thesis concludes with general action guidelines for mining exploration, contributing to the enhancement of knowledge of Spanish gold mineralization northwest.

Tasas de deformación GPS en la cuenca del Bajo Segura (Cordillera Bética oriental)

En este trabajo hemos cuantificado las tasas de deformación actual de la cuenca del Bajo Segura (NE del corredor de cizalla de la Bética oriental), a partir del análisis de una red GPS con 11 vértices geodésicos. Se han analizado los datos de cuatro campañas GPS entre junio de 1999 y enero de 2013, que han sido procesados con la versión 6.2 del software GIPSY-OASIS. Este software utiliza la técnica de posicionamiento puntual de precisión conocido por las siglas PPP. Se observa un acortamiento ~N-S en toda la cuenca de mayor magnitud en el sur, en la zona de falla del Bajo Segura, con valores que varían de oeste a este entre 0,73 y 0,24 mm/año. En el borde septentrional de la cuenca, en la zona de falla de Crevillente, los valores de acortamiento N-S son menores. Sin embargo, en esta falla se ha observado un movimiento lateral sinistrorso que, en la componente E-O, varía entre 0,44 y 0,75 mm/año.

Magnetic anisotropy reveals Neogene tectonic overprint in highly strained carbonate mylonites from the Morcles nappe, Switzerland

The anisotropy of magnetic susceptibility (AMS) has been measured with low- and high-field methods, in deformed carbonate rocks along the Morcles nappe shear zone (Helvetic Alps). High-field measurements at room temperature and 77 K enable the separation of the ferrimagnetic, paramagnetic and diamagnetic anisotropy. The ferrimagnetic sub-fabric is generally insignificant in these rocks, contributing less than 10% to the total AMS. AMS results for both the separated diamagnetic and paramagnetic subfabrics are consistent with the regional shear movement in the late-stage formation of the Helvetic nappes, as seen in the Morcles nappe, whose inverted limb indicate shear displacement towards the northwest. The diamagnetic anisotropy correlates well quantitatively with the calculated magnetic anisotropy based on the calcite texture. There is a gradational change in the degree of anisotropy related to the strain gradient along the shear zone. A more complex magnetic fabric, resulting from partial overprinting due to displacement along the Simplon–Rhône fault, is evident at one site near the root zone of the nappe. Partial overprinting of the magnetic fabric appears to have taken place in two locations farther up the shear zone as well. This late phase deformation is associated with recent exhumation of the Mont Blanc and Belledonne external massifs and orogen parallel extension, and is reflected by the AMS. Rocks with bulk susceptibility ∼0 SI, and simple mineral compositions are ideal for low temperature high-field torque, as this method helps to enhance the paramagnetic susceptibility and anisotropy, which may otherwise be masked by the mixed magnetic contributions of the composite magnetic fabric.

An insight into the breakup of Gondwana: Identifying events through low-temperature thermochronology from the basement rocks of Madagascar

Fission track analysis was applied to the Precambrian suites of Madagascar in order to identify the lower-temperature cooling histories and their relationships to the Phanerozoic events that affected the island. Apatite ages range from 431 to 68 Ma, and zircon ages range from 452 to 238 Ma. Thermochronologically, the island can be divided into a southern, central, and northern region each with a subdivision on an east-west basis. The southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest-southeast striking Ranotsara Shear Zone (RSZ). The change in apparent ages over the RSZ is indicative of later reactivation along younger brittle faults. The central region has the oldest ages of the island and has a diffuse contact to the third region northward. Along the entire western margin of the Precambrian basement initial Paleozoic exhumation was followed by heating (burial by sediments) during Jurassic and Cretaceous times. A decrease in ages along the eastern margin from 119 to 68 Ma coincides with the predicted positions of the Marion hot spot after effects of erosion are considered. On the other hand, these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder. The eastern part of the central region has remained very stable since at least Devonian times, undergoing only long-term very slow exhumation at rates of 1–5 m/Myr.

Partitioning of deformation within a subduction channel during exhumation of high-pressure rocks: a case study from the Western Alps

The metamorphic belt of the Western Alps was subjected to widespread extensional tectonism at the end of the Eocene (ca. 45-35 Ma). Extension was accommodated by hinterland-directed movements along gently inclined extensional shear zones, which facilitated rapid exhumation of high-pressure and ultra-high-pressure rocks. This deformation resulted in a normal metamorphic sequence. Extension in the inner parts of the Western Alps was coeval with shortening at the front of the belt (foreland-directed thrusts), which took place during decompression, and emplaced higher grade metamorphic units over lower grade metamorphic rocks, thus forming an inverse metamorphic sequence. Two mechanisms for this extensional episode are discussed: (1) collapse of an overthickened lithosphere, and (2) internal readjustments within the orogenic wedge due to subduction channel dynamics. We favour the latter mechanism because it can account for the development of the observed inverse and normal metamorphic sequences along foreland-directed thrusts and hinterland-directed detachments, respectively. This hypothesis is supported by published structural, metamorphic and geochronological data from four geological transects through the Western Alps. This study also emphasizes the importance of post-shearing deformation (e.g. horizontal buckling versus vertical flattening), which can modify the distribution of hinterland- and foreland-directed shear zones in orogenic belts. (c) 2006 Elsevier Ltd. All rights reserved.

Sismicidade e esforços tectônicos na zona sísmica Acaraú, Nordeste do Brasil

Inside the Borborema Province the Northwestern Ceará (NC) is one of the most seismic active regions. There are reports of an earthquake occurred in 1810 in the Granja town. On January, 2008 the seismic activity in NC has increased and it was deployed a seismographic network with 11 digital stations. In 2009, another earthquake sequence began and it was deployed another seismographic network in the Santana do Acaraú town with 6 stations. This thesis presents the results obtained by analyzing the data recorded in these two networks. The epicentral areas are located near the northeastern part of the Transbrasiliano Lineament, a shear zone with NE-SW-trending that cuts the study area. The hypocenters are located between 1km and 8km. The strike-slip focal mechanisms were found, which is predominant in the Borborema Province. An integration of seismological, geological and geophysical data was performed and it show that the seismogenic faults found are oriented in the same direction to the local brittle structures observed in field and magnetic lineaments. The SHmax (maximum compressional stress) direction in NC was estimated using an inversion of seven focal mechanisms. The horizontal maximum compression stress (σ1 = 300°) with orientation NW-SE and extension (σ3 = 210°) with NE-SW and σ2 vertical. These results are consistent with results of previous studies. The seismic activity recorded in NC is not related to a possible reactivation of the Transbrasiliano Lineament, by now.