Quartz recrystallization regimes, c-axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone (Ribeira Shear Zone, SE Brazil)

The crystal-plastic behavior of quartz mylonites from the Ribeira Shear Zone (SE Brazil), a major strike-slip structure that was active during a prograde metamorphic phase related to the Neoproterozoic Brasiliano-Pan African Orogeny, was investigated using a multi-method approach. Geothermobarometry results indicate deformational conditions ranging from similar to 300 to similar to 630 degrees C and 500-700 MPa. A strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone. Bulging recrystallization (BLG) dominates within the chlorite zone between 300 and 410 degrees C, subgrain rotation recrystallization (SGR) operates within the biotite zone from 410 to 520 degrees C, and grain boundary migration recrystallization (GBM) dominates in the garnet zone above 520 degrees C. The development of quartz c-axis textures is mainly governed by temperature and dynamic recrystallization mechanisms. Textures from BLG zone mylonites are characterized by maxima around Z; SGR zone mylonites display single girdles or asymmetric type I crossed girdles; and GBM zone mylonites comprise maxima around Y and intermediate between X and Z. The scarcity or absence of water-bearing fluid inclusions in quartz mylonites from the SGR and GBM zones, which are dominated by carbonic inclusions, suggests water-deficient conditions, whereas BLG zone mylonites are dominated by water-bearing inclusions. This evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature. No effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected, and little influence on c-axis texture development was observed. Most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history, recording a decompression in the range of 300-500 MPa, while microstructural reequilibration effects related to the prograde metamorphism are largely preserved. Fluid inclusion microstructures and densities from two SGR zone samples preserved evidence for a near isothermal compression within the interior of the Ribeira Shear Zone during the prograde metamorphism. (C) 2009 Elsevier B.V. All rights reserved.

Cambrian mafic to felsic magmatism and its connections with transcurrent shear zones of the Borborema Province (NE Brazil): Implications for the late assembly of the West Gondwana

New U-Pb (SHRIMP) and (40)Ar/(39)Ar isotopic data of igneous rocks and mylonites of the Borborema Province (NE Brazil) show that a wide range of tectonothermal events affected the province during the transition from the Precambrian to the Cambrian. Concordant zircon U-Pb data constrained the crystallization age of mafic stocks, mafic to felsic dikes and granite batholiths between 548 and 533 Ma. These bodies were emplaced in a regional strain field combining extension and dextral shearing. The ductile shear deformation overprinted an older basement fabric to develop a low- to medium metamorphic grade vertical mylonite belt that cut the province in the E-W direction. Magnetic fabrics of the Cambrian batholiths determined by anisotropy of magnetic susceptibility are consistent with syntectonic emplacement. The magmatic pulses and shear deformation would have supplied enough heat to reset the synkinematic micas of mylonites to yield (40)Ar/(39)Ar plateau cooling ages between ca. 550 and 510 Ma. These results provide evidence that emplacement of Early Cambrian mafic and felsic magmas were accompanied by regional-scale shear deformations, probably in the consequence of late collisions along the West Gondwana margin. (C) 2010 Published by Elsevier B.V.

Conflicting structural and geochronological data from the Ibituruna quartz-syenite (SE Brazil): Effect of protracted ""hot"" orogeny and slow cooling rate?

The Ibituruna quartz-syenite was emplaced as a sill in the Ribeira-Aracuai Neoproterozoic belt (Southeastern Brazil) during the last stages of the Gondwana supercontinent amalgamation. We have measured the Anisotropy of Magnetic Susceptibility (AMS) in samples from the Ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric. A large magnetic anisotropy, dominantly due to magnetite, and a consistent magnetic fabric have been determined over the entire Ibituruna massif. The magmatic foliation and lineation are strikingly parallel to the solid-state mylonitic foliation and lineation measured in the country-rock. Altogether, these observations suggest that the Ibituruna sill was emplaced during the high temperature (similar to 750 degrees C) regional deformation and was deformed before full solidification coherently with its country-rock. Unexpectedly, geochronological data suggest a rather different conclusion. LA-ICP-MS and SHRIMP ages of zircons from the Ibituruna quartz-syenite are in the range 530-535 Ma and LA-ICP-MS ages of zircons and monazites from synkinematic leucocratic veins in the country-rocks suggest a crystallization at similar to 570-580 Ma, i.e., an HT deformation >35My older than the emplacement of the Ibituruna quartz-syenite. Conclusions from the structural and the geochronological studies are therefore conflicting. A possible explanation arises from (40)Ar-(39)Ar thermochronology. We have dated amphiboles from the quartz-syenite, and amphiboles and biotites from the country-rock. Together with the ages of monazites and zircons in the country-rock, (40)Ar-(39)Ar mineral ages suggest a very low cooling rate: <3 degrees C/My between 570 and similar to 500 Ma and similar to 5 degrees C/My between 500 and 460 Ma. Assuming a protracted regional deformation consistent over tens of My, under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time. This may be a characteristic of slow cooling ""hot orogens"" that rocks deformed at significantly different periods during the orogeny, but under roughly unchanged temperature conditions, may display almost indiscernible microstructure and fabric. (C) 2009 Elsevier B.V. All rights reserved.

Functional and Structural Connectivity Between the Perigenual Anterior Cingulate and Amygdala in Bipolar Disorder

Objective: Abnormalities in the morphology and function of two gray matter structures central to emotional processing, the perigenual anterior cingulate cortex (pACC) and amygdala, have consistently been reported in bipolar disorder (BD). Evidence implicates abnormalities in their connectivity in BD. This study investigates the potential disruptions in pACC-amygdala functional connectivity and associated abnormalities in white matter that provides structural connections between the two brain regions in BD. Methods: Thirty-three individuals with BD and 31 healthy comparison subjects (HC) participated in a scanning session during which functional magnetic resonance imaging (fMRI) during processing of face stimuli and diffusion tensor imaging (DTI) were performed. The strength of pACC-amygdala functional connections was compared between BD and HC groups, and associations between these functional connectivity measures from the fMRI scans and regional fractional anisotropy (FA) from the DTI scans were assessed. Results: Functional connectivity was decreased between the pACC and amygdala in the BD group compared with HC group, during the processing of fearful and happy faces (p < .005). Moreover, a significant positive association between pACC-amygdala functional coupling and FA in ventrofrontal white matter, including the region of the uncinate fasciculus, was identified (p < .005). Conclusion: This study provides evidence for abnormalities in pACC-amygdala functional connectivity during emotional processing in BD. The significant association between pACC-amygdala functional connectivity and the structural integrity of white matter that contains pACC-amygdala connections suggest that disruptions in white matter connectivity may contribute to disturbances in the coordinated responses of the pACC and amygdala during emotional processing in BD.

Matrix Representation of the Stationary Measure for the Multispecies TASEP

In this work we construct the stationary measure of the N species totally asymmetric simple exclusion process in a matrix product formulation. We make the connection between the matrix product formulation and the queueing theory picture of Ferrari and Martin. In particular, in the standard representation, the matrices act on the space of queue lengths. For N > 2 the matrices in fact become tensor products of elements of quadratic algebras. This enables us to give a purely algebraic proof of the stationary measure which we present for N=3.

Representation type of Jordan algebras

The problem of classification of Jordan bit-nodules over (non-semisimple) finite dimensional Jordan algebras with respect to their representation type is considered. The notions of diagram of a Jordan algebra and of Jordan tensor algebra of a bimodule are introduced and a mapping Qui is constructed which associates to the diagram of a Jordan algebra J the quiver of its universal associative enveloping algebra S(J). The main results are concerned with Jordan algebras of semi-matrix type, that is, algebras whose semi-simple component is a direct sum of Jordan matrix algebras. In this case, criterion of finiteness and tameness for one-sided representations are obtained, in terms of diagram and mapping Qui, for Jordan tensor algebras and for algebras with radical square equals to 0. (c) 2010 Elsevier Inc. All rights reserved.

Hamiltonian constraint analysis of vector field theories with spontaneous Lorentz symmetry breaking

Recent investigations of various quantum-gravity theories have revealed a variety of possible mechanisms that lead to Lorentz violation. One of the more elegant of these mechanisms is known as Spontaneous Lorentz Symmetry Breaking (SLSB), where a vector or tensor field acquires a nonzero vacuum expectation value. As a consequence of this symmetry breaking, massless Nambu-Goldstone modes appear with properties similar to the photon in Electromagnetism. This thesis considers the most general class of vector field theories that exhibit spontaneous Lorentz violation-known as bumblebee models-and examines their candidacy as potential alternative explanations of E&M, offering the possibility that Einstein-Maxwell theory could emerge as a result of SLSB rather than of local U(1) gauge invariance. With this aim we employ Dirac's Hamiltonian Constraint Analysis procedure to examine the constraint structures and degrees of freedom inherent in three candidate bumblebee models, each with a different potential function, and compare these results to those of Electromagnetism. We find that none of these models share similar constraint structures to that of E&M, and that the number of degrees of freedom for each model exceeds that of Electromagnetism by at least two, pointing to the potential existence of massive modes or propagating ghost modes in the bumblebee theories.

Vector Theory of Spontaneous Lorentz Violation

Classical electromagnetism predicts two massless propagating modes, which are known as the two polarizations of the photon. On the other hand, if the Lorentz symmetry of classical electromagnetism is spontaneously broken, the new theory will still have two massless Nambu-Goldstone modes resembling the photon. If the Lorentz symmetry is broken by a bumblebee potential that allows for excitations out of the minimum, then massive modes arise. Furthermore, in curved spacetime, such massive modes will be created through a process other than the usual Higgs mechanism because of the dependence of the bumblebee potential on both the vector field and the metric tensor. Also, it is found that these massive modes do not propagate due to the extra constraints.