GSA: making sense of the shapes of rocks

I'm a structural geologist. And that means that when I see something like this:



or this:



... well, my first reaction is "oh, WOW!" But my second reaction is to squint a little, tilt my head to the side, and ask (usually silently, unless I'm teaching): "I wonder how that formed?"

Structural geology is a funny discipline. It has borrowed a lot of ideas from other fields (especially engineering and old-school Newtonian physics, but also from other subdisciplines in geology), and then built on them and redeveloped them for decades. And sometimes, when I teach the class, it feels like a bit of a hodge-podge of ideas. In particular, there are two somewhat different approaches to trying to understand a structure: kinematics and mechanics. Kinematics deals with the changes that a body of rock undergoes when it deforms – how does a rock of one shape change into a rock of another shape? Mechanics deals with the forces and stresses that cause a rock to change shape, and tries to predict the shapes of the final structures through the behavior of a certain material under certain stresses.

If you’re anything like I was when I read my first structural geology textbook, your eyes just crossed and you have absolutely no idea what I just said. So let me try again, but with an example.

Take, for instance, the San Rafael Swell (the second photo). The rock layers used to be flat-lying sediments. They aren't flat any more. How did they get all tilted up like that?

The simplest way to explain monoclines (like the San Rafael Swell) is to imagine two wooden blocks underneath a couple thick blankets. Push one wooden block up, and the blankets are folded. Voila... monocline, above what’s known as a blind thrust fault.

Except that there are some space problems in my little sketch, and in my pile of blankets. My cat and my four-year-old’s stuffed blue puppy could hide in my model (and there’s something a bit scary about the image of a giant stuffed blue puppy hiding somewhere within the upper Paleozoic section of the San Rafael Swell... ). So the possible explanations are limited, first, by geometry: there’s a fixed volume of rock to work with, unless something odd happens.

There have been a variety of geometric and kinematic models for folded beds above fault tips. There’s John Suppe’s fault-propagation fold model, which is shown in most structural geology textbooks:



Suppe’s models fold the rocks above the fault, but don’t fold the rocks below the fault. They’re relatively easy to work with geometrically, though – the rock layers stay the same thickness, so it’s possible to draw them by hand.

There are more complicated kinematic models, such as the trishear models that many people, including Eric Erslev and Rick Allmendinger, work with:



Trishear models recognize that rocks deform both above and below the fault, and describe the deformation as occurring in a triangular zone with its apex at the fault tip. There are a number of parameters that can be varied, which makes it possible to fit the model very nicely to real faults and folds.

We talked about trishear models in my Advanced Structural Geology class – read Eric Erslev’s paper, and played with Rick Allmendinger’s software. But when we discussed them, the students had one big question: why do the rocks behave like that? They change shape in this triangular zone... but why?

Well, I have no idea, but I suspect that the answer would lie in rock mechanics. Which I’m not very good at. The basics, though, would involve thinking about what is required to break a rock, and what stresses bend a rock, and maybe would involve calculating the energy involved. And then all those equations get put into a computer model that predicts the changes in shape from the physics, and you get... well... I’m trying to find a good image, because I know that some very talented people have worked on blind faults from a rock mechanics perspective, but I’m having trouble finding an image that I understand, let alone one that I can explain.

There was a session on integrating kinematics and mechanics at the Geological Society of America meeting last week... but it didn’t answer my questions. There were mechanically modeled structures that looked reasonable, and others that raised more questions for me than they answered.

But my conclusion, at the end, was that there’s a lot more work to be done, to reconcile the physics of rock behavior with the geometry that results.

Which is probably good news for people who want to do research in structural geology...