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Structural Data for MetPetDB

Purpose of including structural data

Strain is manifest in nearly all scales. While quantifying strain is of importance to structural geologists, equally important is analysis of the three-dimensional variability of strain. Structural analysis, on all scales, relates genetically to the grains that define deformational fabrics and lineations, their crystallization history, and the conditions under which the grew or recyrstallized. Therefore, MetPetDB shall include structural data as inherently linked to the metamorphic history of a sample or subsample.

Format of sturctural data

The majority of field geologists studying the tectonics of metamorphic terranes make measurements of the fabric elements in a metamorphic rock. These elements are approximated as being planar or linear. Each fabric element includes:

  1. Classification (linear or planar)
  2. Orientation in space (strike/dip, plunge/trend, etc.)
  3. Location in space (Lat/Long of sample).
  4. Defining element (mineral(s) such as "biotite-defined foliation" or "staurolite mineral lineation"

And some may include:

  1. Other categorization: e.g. stretching lineation, mylonitic fabric
  2. Other type of information: shear sense, shear sense indicators (sigma, delta, etc.)
  3. Information about fold orientation.
  4. Generation of fabric (ie. S0, S1 etc.)

Data Model for structural data

The existing data model for metamorphic rocks and their geochemical data is based on spatially located samples from which multiple subsamples can be derived. Hence, sample and subsample information is linked to a latitude/longitude spatial location. Linear and planar fabric elements on a sample scale can be either uniform or variable within the sample (e.g. a sample/subsample may include a folded foliation). Detailed, intra-sample variation is beyond the scope of MetPetDB, and therefore will not be stored in a defined manner. We suggest that users may record intra-sample structural variability in the open text description or comment field for a sample/subsample.

Fields for Structural Data

Linked to each sample are these fields for sturctural data, shown in the table. Under construction

Structural measurements are included in the database as real numbers to preserve calculation accuracy. These data are displayed as rounded to the nearest integer.

Preserved Sedimentary Structures:

While some metamorphic rocks may preserve sedimentary structures, this is not related to the metamorphic history or the rock. This information can be critical to the interpretation of whether a metasedimentary rock or sequence is upright or structurally overturned. We encourage a user to include these data, and we suggest the "other" category with a comment as to the type of structure.

Linking structural data to subsamples and images

Structural data included in MetPetDB (unless otherwise noted) is assumed to be uniform in orientation on the intra-sample scale. Subsamples of type "thin section" and "polished thin section", and their images, correspond to planar "slices" of rock and therfore have a distinct orientation in space that may be approximated by a plane. A thin section that is cut normal to a foliation and parallel to a lineation may therefore be oriented in space by knowing this relationship, as well as whether the subsample or image is upright or inverted. To explain this, think of the thin section oriented in space as it was in the original outcrop. Is the glass slide under the rock "slice" or above it? For perfectly vertical thin sections, a "facing" would be used, ie. "east-facing" for a north-south vertical thin section.

This information allows the spatial orientation of a thin section to be defined by the type of cut (e.g. foliation normal lineation paralell) from a sample of known uniform fabric orientation. Now, one additional piece of information is required to know the spatial orientation of a thin section/image. While the plane of the thin section is defined by the type of cut and the sample fabric orientation, the rotation of this thin section about an axis normal to the planar cut is required as well. This information allows a rectangular image, such as a thin section scan or X-ray map, to be oriented in space.

These pieces of information are linked only to a subsample or an image; and, given the sample's structural data are included in the database, can be used to determine the orientation of the subsample or image.

Steps to including structural data:

  1. Add data to existing table of sample information.
  2. Add necessary data to link thin section type subsamples and images to structural sample data to determine thin section/image orientation...
  3. Add map plotting capabilities for structural data
  4. Add stereographic projection plotting capability of group of sample data.

Plotting structural data:

A future endeavor will be to allow for plotting of sets of sturctural data on a map or in a stereographic projection.

Brittle structural information

The relationship of brittle structures to the chemistry and P–T conditions of their formation is clearly different from that of ductile structures. While brittle structures are of importance to the tectonic/structural history of a particular rock in a particular location, they do not form under conditions associated with metamorphism or metamrophic rocks and hence are not included in a database of metamorphic rocks and geochemistry.