Mapping Mylonites in Arizona
Evan Strickland
My undergraduate honors research was a study of the nature of deformation of a footwall corrugation in the Buckskin-Rawhide metamorphic core complex, west-central Arizona. This footwall corrugation is expressed at the earth’s surface as a NW-SE trending, linear mountain range known as the Little Buckskin Mountains, which is ~10 km long. To be able to determine why this footwall corrugation exists would yield an explanation as to why this particular mountain range exists as well!
So what is a footwall corrugation? And what about the nature of the footwall itself? And what’s a metamorphic core complex? To explain these I will start with the latter. Metamorphic core complexes are features of large magnitude, crustal extension. They were originally discovered in the western U.S., within the Basin and Range province. Metamorphic core complexes form when extension is accommodated by a regional scale, low angle (<30 degrees), normal fault known as a “detachment” fault. Tens of kilometers of extension are accommodated by these faults, which results in the juxtaposition of upper crustal rock against mid-crustal rock (~15 km depth). The two “blocks” of rock that are moved passed each other along the detachment fault are the footwall, beneath the detachment, and the hanging wall, above the detachment (just like any normal fault). The Little Buckskin Mountains is made of mid-crustal, crystalline footwall rocks (as it is a footwall corrugation) that were mylonitized during extension in the mid-Tertiary. Mylonitization occurs within the top of the ductile, mid-crustal footwall block as the brittle, upper crustal hanging wall block is displaced across it. The top of the ductile footwall block undergoes ductile shearing, which results in the alignment of platy minerals yielding mylonitic foliations, and in the stretching of quartz grains, yielding stretching lineations. This is what is meant by a rock being mylonitized, it deveolopes mylonitic foliations and stretching lineations.
So back to the original question of what a footwall corrugation is, and the purpose of its investigation. The detachment fault of most metamorphic core complexes is not planar, but is undulating. These undulations form extension-parallel ridges and valleys within the detachment fault itself, yielding these same ridges and valleys, or “corrugations”, on the surface of the footwall (thus, footwall corrugations). When erosion cuts down into the footwall corrugations, the less resistant upper crustal hanging wall rock is more easily removed, leaving behind a linear topographic high of more resistant mylonitic footwall rock. Footwall corrugations are thus expressed as linear mountains ranges, like the Little Buckskin Mountains. The nature of these corrugations is still controversial. Many models have been created to explain their origin, including corrugations as folds, mega-boudins, or an original geometry of the detachment fault. My research was to study the nature of deformation of the Little Buckskin Mountains footwall corrugation to see if I could infer anything about its formation.
During the spring break of 2009, I spent seven days collecting structural data at the Little Buckskin Mountains with John Singleton, a PhD candidate at the University of Texas who designed this awesome project. We measured many features including mylonitic foliations, stretching lineations, epidote veins, faults, folds, and boudins. I made geologic maps of the Little Buckskin Mountains with this information, and constructed three cross-sections perpendicular to the length of the mountains range. We confirmed that the Little Buckskins Mountains is an antiform that is very well defined by mylonitic foliations. This was already determined by previous workers. However, because we collected a denser package of measurements I was able to calculate a more precise attitude of the antiform axis using a stereonet and determined that the axis is statistically exactly the same at either ends of the antiform. Amazing! I then separated the stretching lineations and epidote veins on opposite sides of the antiform axis. When plotted on a stereonet, there is a clear statistical difference in their orientations on opposite sides of the antiform axis of ~15-20 degrees trend and strike. This suggests that these features were rotated about the antiform axis, and thus the antiform is not an original feature but formed after the lineations and epidote veins. Using my cross-sections, I chose average limb dips of the antiform to rotate to horizontal. When the limbs of the antiform are rotated to horizontal, the lineations and epidote veins on opposite sides of the antiform axis are reoriented into parallelism. This further suggests that the surface of the footwall was originally flat when these features formed within it, and was later arched, producing the antiform.
Using the power of structural analysis, I was able to determine that the antiform is a late-stage feature that formed after the lineations and eipdote veins. But what is the nature of its formation? An important feature we found, it turns out, are numerous meter-scale upright folds that parallel the orientation of the main antiform, which is also upright. Also, as seen in cross-section, there are large reversals of foliations on the limbs of the antiform that look like upright folds as well. Perhaps the most important feature we found was evidence that some of the meter-scale upright folds formed by flexural slip along the mylonitic layering, which basically means they are brittle folds. This is significant because the antiform is likely a brittle feature because it formed after the epidote veins did (which are basically extension fractures). I therefore interpreted the meter-scale upright folds, and larger reversals of foliations on the limbs of the antiform, as parasitic folds. In the end, I interpret the Little Buckskin Mountains antiform (footwall corrugation) as a fold, forming from extension-perpendicular shortening. Restoring the foliations in my cross-sections to horizontal, I calculated ~10% horizontal shortening as being responsible for the formation of this antiform. Besides possibly the footwall corrugations within west-central Arizona, no other evidence for post-Tertiary crustal shortening in the region has been found. This research, and the research that John Singleton is currently conducting, may spark the search for further evidence of shortening within this region.
In the end, this project was amazing! I was able to exercise many skills in geology, including collecting measurements and observations in the field, constructing geologic maps and cross-sections, analyzing structural data with stereonets, and I even cut my own thin-sections that I used to describe mineralogies and sense of shear within the mylonites. Plus, I exercised the greatest skill in geology…putting it all together!