GeologyScience RepositoryThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.http://repository.geologyscience.ru:802022-09-25T00:50:25Z2022-09-25T00:50:25ZMICROSTRUCTURE AND LATTICE PREFERRED ORIENTATION OF CALCITE MYLONITES AT THE BASE OF THE SOUTHERN URALS ACCRETIONARY PRISMFernandez F.J.Brown D.Alvarez-Marron J.Prior D.J.Perez-Estaun A.https://repository.geologyscience.ru/handle/123456789/387402022-09-24T04:26:15Z2004-01-01T00:00:00ZMICROSTRUCTURE AND LATTICE PREFERRED ORIENTATION OF CALCITE MYLONITES AT THE BASE OF THE SOUTHERN URALS ACCRETIONARY PRISM
Fernandez F.J.; Brown D.; Alvarez-Marron J.; Prior D.J.; Perez-Estaun A.
Emplacement of the southern Urals accretionary complex onto the East European Craton involved off-scraping of shallow marine limestones from the continental margin and their underplating to the base of the accretionary complex during the final stages of arc-continent collision. These limestones underwent localized ductile to semi-brittle deformation at upper-crustal levels, at temperatures close to 200 C. Field observations, macroscopic fabric elements and crystallographic preferred orientations indicate a heterogeneous distribution of the deformation. Measurements from twinned calcite grains indicate that peak differential stress conditions during deformation attained c. 230 40 MPa. These differential stress conditions are further supported by the small size of both recrystallized and twinned calcite grains. Twinning and intracrystalline slip were the major contributors to the bulk deformation and lattice preferred orientation (LPO) formation. LPO evolved with progressive strain, from an initial constrictional fabric that is consistent with the macroscopic mineral lineation as the dominant macroscopic fabric element to a c-axis maximum normal to foliation as the rock underwent dynamic recrystallization and grain refinement during progressive simple shear. [PUBLICATION ABSTRACT]
2004-01-01T00:00:00ZCLASSIFICATION OF REFOLD STRUCTURESGrasemann B.Wiesmayr G.Draganits E.Fusseis F.https://repository.geologyscience.ru/handle/123456789/387412022-09-24T04:26:15Z2004-01-01T00:00:00ZCLASSIFICATION OF REFOLD STRUCTURES
Grasemann B.; Wiesmayr G.; Draganits E.; Fusseis F.
Structural geology textbooks distinguish among four end members of three-dimensional refold structures established from their two-dimensional interference patterns. Here it is shown that six different end members of three-dimensional refold structures exist. These end members can be described by a reduced direction cosines matrix L*. The classical types 1-3 are extended to have three new counterparts types 0!sub 1!-0!sub 3!, which are derived by 90(o) rotation of the superposed fold around its fold axis. The matrix L* can be used to characterize the angles between the two fold generations in a simple triangle plot illustrating the six end members and even any intermediate refold structure. [PUBLICATION ABSTRACT]
2004-01-01T00:00:00ZA METHOD FOR CALCULATING EFFECTIVE BULK COMPOSITION MODIFICATION DUE TO CRYSTAL FRACTIONATION IN GARNET-BEARING SCHIST: IMPLICATIONS FOR ISOPLETH THERMOBAROMETRYEvans T.P.https://repository.geologyscience.ru/handle/123456789/387392022-09-24T04:26:15Z2004-01-01T00:00:00ZA METHOD FOR CALCULATING EFFECTIVE BULK COMPOSITION MODIFICATION DUE TO CRYSTAL FRACTIONATION IN GARNET-BEARING SCHIST: IMPLICATIONS FOR ISOPLETH THERMOBAROMETRY
Evans T.P.
Quantitative P -T path determination in metamorphic rocks is commonly based on the variation in composition of growth-zoned garnet. However, some component of growth zoning in garnet is necessarily the result of an effective bulk composition change within the rock that has been generated by crystal fractionation of components into the core of garnet. Therefore, any quantitative calculation of the P -T regime of garnet growth should be completed using an accurate assessment of the composition of the chemical system from which garnet is growing. Consequently, a method for calculating the extent of crystal fractionation that provides a means of estimating the composition of the unfractionated rock at any stage during garnet growth is developed. The method presented here applies a Rayleigh fractionation model based on measured Mn content of garnet to generate composition v. modal proportion curves for garnet, and uses those curves to estimate the vectors of crystal fractionation. The technique is tested by calculating the precision of the equilibrium between three garnet compositional variables within the chemical system determined to be appropriate for each of a series of microprobe analyses from garnet. Application of the fractionation calculations in conjunction with the P -T estimates based on intersecting compositional isopleths provides a means of calculating P -T conditions of garnet growth that is based on individual point-analyses on a garnet grain. Such spatially precise and easily obtainable P -T data allow for detailed parallel studies of the microstructural, the P -T, and the chemical evolution of metamorphosed pelites. This method provides a means of studying the dynamics of orogenic systems at a resolution that was previously unattainable.
2004-01-01T00:00:00ZGLOBAL P AND PP TRAVELTIME TOMOGRAPHY: RAYS VERSUS WAVESMontelli R.Nolet G.Masters G.Dahlen F.A.Hung S.H.https://repository.geologyscience.ru/handle/123456789/387372022-09-24T04:26:14Z2004-01-01T00:00:00ZGLOBAL P AND PP TRAVELTIME TOMOGRAPHY: RAYS VERSUS WAVES
Montelli R.; Nolet G.; Masters G.; Dahlen F.A.; Hung S.H.
This paper presents a comparison of ray-theoretical and finite-frequency traveltime tomography for compressional waves. Our data set consists of 86 405 long-period P and PP-P traveltimes measured by cross-correlation. The traveltime of a finite-frequency wave is sensitive to anomalies in a hollow banana-shaped region surrounding the unperturbed ray path, with the sensitivity being zero on the ray. Because of the minimax nature of the surface-reflected PP wave, its sensitivity is more complicated. We compute the 3-D traveltime sensitivity efficiently by using the paraxial approximation in conjunction with ray theory and the Born approximation. We compare tomographic models with the same χ2 fit for both ray theory and finite-frequency analysis. Depending on the depth and size of the anomaly, the amplitudes of the velocity perturbations in the finite-frequency tomographic images are 30-50 per cent larger than in the corresponding ray-theoretical images, demonstrating that wave front healing cannot be neglected when interpreting long-period seismic waves. The images obtained provide clear evidence that a limited number of hotspots are fed by plumes originating in the lower mantle.
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