Thin-skinned Models for Undergraduate Teaching Labs - Flour Structures
Dan Davis, Stony Brook University
Analog modeling of thin-skinned tectonics can be useful in teaching as well as in research. Unfortunately, such modeling is typically not well suited for use in undergraduate teaching laboratories in structural geology. Typical 'sandbox' models take considerable time and effort to set up and are difficult to exhume in order to analyze the structures that have formed.
We present here a very simple and inexpensive way for an undergraduate lab class to explore the effects of a variety of geometries and mechanical parameters in thin-skinned deformation. The experiments use only easily obtained materials: flour, sandpaper, graphite, transparency sheets, and athletic field marker chalk. In each experiment, students place flour with colored marker beds within a large shallow box (a geologic sample drawer is ideal), and they set up the model using sieves, brushes, and wooden blocks.
Although the model scaling is not perfect (scaled cohesion is about an order of magnitude high), students can easily model many aspects of thin-skinned deformation. Because the experiments are so easy to set up, a typical lab class can be split into a half dozen or more separate experiments, each with different boundary conditions and set up by separate groups of 2 or 3 students. For example, the effects of basal detachment strength on wedge taper, horizontal strain, and on thrust density and vergence can be modeled simply by placing either sandpaper or graphite on the base of the drawer. A horizontal sheet of wood or Plexiglas in front of the pushing block allows modeling a forearc with a double accretionary wedge, outer-are high, and forearc basin. Along-strike contrasts in the model can be used to generate flower structures and wrench tectonics. A block of wood glued to the base of the box in the 'foreland' and can be used to mimic the effects of basement structure.
Football field `hash marks' placed at 10 cm intervals allow for easy calculation of strain distribution at any time during or after the experiment. The model can be excavated at any angle using a plastic photocopier transparency sheet. Thin layers of colored field marker chalk between flour layers make it possible to reveal many internal details of the fold-thrust belts that have been generated, including along-strike structures.
These 'flour experiments' have proved to be a fun, inexpensive, and highly effective way to teach about thin-skinned deformation. The poster will be accompanied by suggested lesson plans.