Center-to-center analysis and flow fabric characterization in ash-flow tuffs |
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| Authors: | Sheila J Seaman Michael L Williams |
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| Institution: | (1) Department of Geology, Colgate University, Hamilton, NY, USA;(2) Department of Geology and Geography, University of Massachusetts, Amherst, MA, USA |
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| Abstract: | The center-to-center method of strain analysis can be used to estimate flow lineation in high-silica ash-flow tuffs. It can be used as an alternative or supplement to other techniques for flow lineation identification, such as the examination of flow textures in thin sections and the measurement of anisotropy of magnetic susceptibility. The center-to-center method is a modification of a technique described by Fry (1979) and by Ramsay and Huber (1983) for determination of finite strain based on the spacing of particles within a deformed rock. In the present study, application of the method to an anticlustered array of phenocrysts in the flattening plane of an ash-flow tuff produces an ellipse with center to edge distances representative of the minimum distance between centers of phenocrysts in all directions within the flattening plane. The long axis of the ellipse corresponds to the maximum axis of finite strain; this direction is suggested to correspond to the flow lineation. The orientation of the stretching lineation was chosen both by eye and by least-squares analysis from center-to-center plots. The calculated orientation of the long ellipse axis can be varied by choice of a maximum distance between digitized objects which are included in the calculation. Comparison is made between late-stage flow lineations identified using the center-to-center method, and the AMS (anisotropy of magnetic susceptibility) method on samples of the high-silica Oligocene Bloodgood Canyon Tuff from the Mogollon-Datil volcanic field of southwestern New Mexico. Flow lineations based on center-to-center analyses of flattening plane-parallel rock slabs and thin sections agree well with AMS-derived flow lineations on most samples from which high-quality AMS lineations were obtained. Center-to-center analuses from flattening plane-perpendicular, lineation-parallel planes of ash-flow tuff produce ellipses inclined from 15° to 85° to the flattening plane, despite compaction of the ash, which should cause angles of inclination to be very low. The inclined ellipses may result from heterogeneities in grain size and distribution of phenocrysts in vertical sections of tuff, or from fragmentation of phenocrysts which occurred during the final stages of emplacement and compression. Center-to-center analyses on rock slabs rather than thin sections helps to avoid the effects of either textural heterogeneity and fragmentation of phenocrysts. With flow lineation identified by center-to-center analysis, petrographic examination of thin sections cut perpendicular to the flattening plane and parallel to the flow lineation allow for the identification of flow direction. |
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