Jointed Virgelle

One of the stops my Rockies students and I made this summer was a dinosaur paleontology tour through the Two Medicine Dinosaur Center in Bynum, Montana. The folks there are very accommodating, and at my request gave the class a bit of stratigraphic context for the dinosaur fossils. For instance, we visited the geologic formation which underlies the dinosaur-bearing Two Medicine Formation: it’s a beach sandstone called the Virgelle Formation. The Virgelle was deposited along the shore of the Western Interior Seaway, a Cretaceous-aged transgression of seawater onto the North American continent.

While our guide Corey discussed the primary structures that showed the unit to be “beachy” to my students, I got distracted by this outcrop:

virgelle_crackedField notebook for scale (long side 18cm).

So what’s so great about this? It struck me as a nice little demonstration of the relationship between stress directions and joint orientations. σ1 is our maximum principal stress direction (i.e., the direction of greatest stress), in this case caused by acceleration due to the force of gravity. σ2 is perpendicular to the screen of your computer (and the plane of the photograph): that is the intermediate principal stress direction. σ3 is our minimum principal stress direction (weakest stress), in this case pushing in from the sides (atmospheric pressure only, no overlying rock weight):

virgelle_cracked_2

By definition, σ1 is greater than σ3.

So we have a low-level confining stress paired up with the differential stress imparted by the heavy rock pushing down on the slab of sandstone beneath it. As long as that difference in stresses is greater than the strength of this weakly lithified Virgelle sandstone, then the rock will break, and the orientation of those breaks will be ~parallel to σ1, and ~perpendicular to the extension direction, σ3:

virgelle_cracked_3

You’ll also note that the bedding planes in the Virgelle sandstone are planes of weakness, accommodating the extension by allowing blocks of sandstone to slip sideways over what amount to small-scale “detachment faults” (low-angle, upper block sliding downward relative to lower block).

So does an understanding of these stress directions and the resulting structures’ orientation do us any good beyond this one lone slab of fractured sandstone?

Indeed it does. Keeping in mind that we are rotating our perspective from horizontal (“side view”) to vertical (“bird’s eye view”), consider the following map of central Asia:

baikal_ext_sigma

As the Indian subcontinent impacts the Eurasian continent, it moves towards the northeast. This results not only in the northwest-southeast-trending Himalayan mountain front at the site of impact, but also in extensional faulting further into the heart of the continent. Down-dropped blocks of crust in desert areas show up as northeast-southwest-striking rift valleys, but in wetter areas, those low-lying cracks fill with water, and show up to us as linear lakes.

baikal_ext_lakes

Lake Baikal in Russia is a famous example of this, but Mongolia’s Lake Hovsgol is a smaller version of the same thing. The lakes are oriented with their long axis ~parallel to the σ1 direction, as they have been opened up due to stretching in the σ3 direction.

Caveat blog-reader: The kinematics and dynamics of central Asia are actually a lot more complicated than this simplistic picture I’ve painted. My main point in drawing the parallel between the two examples is that outcrop-scale structures can serve as analogues that can help us understand regional-scale processes.

Snowy décollement

Earlier in the month, during the big snowstorms, my window got plastered with snow. This snow formed a vertical layer which then deformed under the influence of gravity. Looking at it through the glass, I was struck by how it could serve as a miniature analogue for the deformation typical of a mountain belt.

Let’s start our discussion by taking a look at an iPhone photograph of the snow:

So here’s what I notice about this (vertically-oriented) photo:

The big sheet of snow is sliding downward over the face of the glass. This surface of slip is thus analogous to a low-angle thrust fault. Here, the maximum principal stress (known as σ1 to structural geologists) is gravity. The minimum principal stress (σ3) is perpendicular to the window, and the intermediate principal stress (σ2) is horizontal, parallel to the bottom edge of the window (i.e., left-to-right). As deformation proceeds, the snow slab folds up on itself and pooches outward in the area of least stress (σ3); away from the surface of the window.

As the snow layer moves downward, it creates a major fold which thickens the snow in a big line perpendicular to gravity, parallel to σ2. Along the vertical part of the window frame, the snow sheet has detached in a vertically-oriented fracture (i.e., parallel to σ1). Oblique to both σ1 and σ2 is a series of smaller folds with diagonal axes.

We can see a similar pattern in this map of the Himalayan mountain belt:

Note that the map* is oriented with north at the bottom, and south at the top, so as to be able to better compared it to my window. Note the broad arc of the Himalayan mountain front (~parallel to the Nepali border) which is perpendicular to the motion of India relative to Eurasia. The minimum principal stress direction (σ3) is vertical, which is why the mountains grow upwards (and the crust thickens downwards into the mantle, too, making the Himalayan mountain belt the site of the thickest crust on the planet). Along the edge of the impactor (analogous to our snow sheet), for instance in northern Burma, we see the same “splay” of folds with axes perpendicular to the the India-Eurasia convergence vector. The crust there is not as thickened.

Though a gooey slab of snow on my window isn’t a perfect analogue for Himalayan mountain-building, we can see some similarities in gross morphology — structural similarities that are fundamentally tied to the orientation of the principal stress directions.

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* Modified by me from a Google Maps “terrain” view.