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Kinematic History of a Salient-recess Junction Explored through a Combined Approach of Field Data and Analog Sandbox Modeling
Chapters
Summary August 5th, 2016
Kinematic histories of fold-thrust belts are typically based on careful examinations of high-grade metamorphic rocks within a salient. We provide a novel method of understanding fold-thrust belts by examining salient-recess junctions. We analyze the oft-ignored upper crustal rocks using a combined approach of detailed fault analysis with experimental sandbox modeling.
Transcript
The overall goal of this experiment is to track the deformation of thrust sheets as they move over oblique ramps. This method can help answer key questions in the field, such as how thrust sheets evolve in complex settings. The main advantages of this technique are that it is simple, inexpensive, and can be compared directly to the field data.
This video shows how to use a push-block sandbox to replicate micro-scale faults found in the field. Field identification of micro-scale faults is covered in the text protocol, as is the general construction of the sandbox. To load the sandbox, use standard play sand, which is relatively homogeneous, with an average grain size of 5 milimeters.
Prepare the sand by dying half of it. First, fill a 5 gallon bucket a quarter full. Then mix in enough black food coloring to make the sand a uniform dark green, which is easily distinguishable from the sand's original color.
Upon drying the sand, as described in the text protocol, load the box with alternating layers of colored and uncolored sand, and ensure that each layer's leveled after it is added. The thicknesses of the sand pack that provides the clearest and most reproducible results should be arrived at empirically. This example shows a 3.5 centimeter sand pack with alternating colors every 6 centimeters.
Next, gently press a plastic mesh composed of half inch of half inch squares onto the sand to produce a grid indentation. Then, insert square cross pins two inches apart throughout the sand. Next, mount a camera to view the entire sandbox.
Now, push the sand with the crank-driven push-block to recreate shortening seen in the field. Move the push-block slowly enough that changes in the sand can be carefully documented. The speed should not affect the results.
While pushing the sand, track the deformation by observing the shape changes of the squares, and track the amount of transport and vertical rotation by observing the motion of the pins. Repeat this process until the formed structures resemble those preserved in nature. Once the sandbox results mimic those preserved in nature, collect sample data.
First, remove all cross pins from the sand. Then, to make samples, separate in epoxy portions of the deformed sand. This requires pre-cut sheet metal dividers.
The bottom edge of the dividers must be made to match the angle of the ramp where the sample is to be taken and must fully extend the length of the ramp. The divider must also be taller than the sand pack. Before using a divider, cover it with painter's tape for easy cleanup.
One end of the divider must be closer to control the flow of the epoxy, and the other end of the divider must be open to minimized disturbances to the sand pack. Next, fasten the divider using 1/4 inch by four inch screws, sheets with 3/8 inch diameter aluminum tubing. In this case, two dividers are used.
One is placed on the oblique ramp, the other covers the frontal oblique ramp junction. Now, pour warm epoxy over the sand within the divider until no more epoxy can be absorbed, so the sand is fully saturated and will not come apart. After several days of drying, slide the dividers out of the sandbox, then lift the epoxied samples out from the surrounding sand.
Using a rock saw, cut the samples perpendicularly and parallel to the strike of the ramps. Then, using permanent marker, highlight the bedding, folds, and faults within the exposed slices. A field study found four adjacent regions, each with mesoscale faults.
The faults preserve a deformation fabric, which was penetrative and homogeneous at the mesoscale. The fault patterns were also unique within each region, which supported the macroscale assumption that an oblique ramp underlies regions two and three, and suggested that the conjugate-conjugate fault analysis was reliable. Using a sandbox model, a break was formed that was comparable to the position and orientation of the boundary between regions two and three on the macroscale maps.
This supported the notion that the break in the overlying thrust sheet may have formed via a complex interaction of an eastward-moving thrust sheet over an oblique ramp. Analysis of epoxied sandbox samples was also supportive. Samples taken from the frontal ramp region accommodated transport to the east, while samples from the oblique ramp region accommodated transport to the southeast.
After watching this video, you should have a good understanding of how to build, run, and modify sandbox models to replicate a specific field area. Once mastered, this technique can be done in one week if performed properly. After its development, this technique paved the way for researchers in the field of structural geology to explore the usefulness of fault data and how these data can be further interpreted with the aid of models.
Thanks for watching and good luck with your models.
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