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Excerpts from Jim Conrad's
Naturalist Newsletter
from the September 14, 2007 Newsletter issued Issued from Sierra Gorda Biosphere Reserve Headquarters in Jalpan, QUERÉTARO, MÉXICO
ROADCUT FAULT ALONG THE RESERVOIR
As you might expect of limestone rocks that used to constitute a calcium-rich, muddy ocean floor but now outcrop in a mountain-valley roadcut, rock strata around here display a lot of fracturing, tilting and faulting. The other day I walked past the fault seen above.
The fault line runs from one side of the picture to the other. Notice that layers below the fault line seem to lie horizontally but above the line layers not only dip steeply to the left but also have been twisted relative to the lower ones. Also observe the highly fractured, crumbly nature of the stone at the fault line itself. This is where rock shattered and crumbled as the two rock face ground against one another.
At http://www.iris.edu/gifs/animations/faults.htm you can see simple animations demonstrating the movements of four kinds of fault.
Even with the help of that page I don't really know what kind of fault is shown in the picture. Did the slippage take place because of pressure applied to both sides, or because the two sides were being pulled apart? Even though it looks like the strata above the fault line scraped over the stable lower strata, we can't assume that the lower horizontal layers were horizontal when the fault occurred. Often around here you see layers that once were horizontal (they all started out horizontal) but now are vertical. In some large roadcuts you even see strata so curved that on one end or the other the older strata has to lie atop the younger. Of course a basic premise of geology is that the deeper you go, the older the sediment becomes, and limestone once was sediment.
Whatever kind of fault this is, it must have fractured a very long time ago, since now our part of Mexico is quiet, geologically speaking, people saying that they never feel the temblors that are so common farther south.