17 November 2018

Flow banding in pseudotachylyte, South Mountains, Arizona

Posted by Callan Bentley

Pseudotachylyte is fault glass. It’s rock that got melted due to frictional heat generated “in the heat of the moment” as an earthquake occurs, then froze before having the chance to crystallize. My friend and colleague Christie Rowe likes to call pseudotachylyte a “fossil earthquake,” and I think that’s a lovely way of thinking of it. Here’s an example I saw Thursday afternoon, in the South Mountains of Phoenix, Arizona:


The South Mountains are a metamorphic core complex: a large sort of structure that is moderately common in the mountains of southwestern Arizona, but also present in other areas of the world. These metamorphic core complexes are characterized by a dome-like blob of exhumed rock rising up, and overlying rocks slipping off the top. The rocks that pooch upward in the middle of the complex go from deep, hot conditions to shallow, colder conditions. The deformation they experience thus starts off ductile, and then transitions to brittle as they make their way up to the surface.

At Dobbins Lookout in the South Mountains, you can see this for yourself. There, a Tertiary-aged granite pluton shows mylonitic fabric with a dominantly horizontal foliation and an east-west-trending lineation, cross-cut by veins of pseudotachylyte. Here is an example:


The principle of cross-cutting relationships reminds us that when one geologic unit or structure cuts across another one, the cutter must be younger. At this site, the ductile fabric of the mylonite is transected by the veins of brittle pseudotachylyte. This shows in miniature the structural imprint of the granite’s journey upward through different deformational conditions through time.

Here is another outcrop showing the same sort of thing:

…Also annotated:

Now check this out:

Do you see what I see?

Let’s zoom in…

Note the “sideways diapir-shaped” set of concentric bands within a small pocket of pseudotachylyte –

It almost looks like a thumbprint, or a surficial deposit of mud. But it’s not! It’s part of the rock.

It struck me that this could be flow banding within the melt, recording the expansion of the earthquake-frictionally-melted rock into a ~square dilational pocket, as the block of granite to the left slid to the left, relative to the block of granite to the right, something like this:

In the annotation, the pseudotachylyte vein’s contact with the mylonitized granite is outlined in black, the concentric flow banding is traced out in white, and my interpretation of flow direction is shown with the yellow arrows.

What do you think? I thought this was very cool. I’ve seen flow banding before in igneous rocks, both felsic and mafic, volcanic and plutonic, but this was my first time seeing it in pseudotachylyte. I ran my interpretation by Steve Reynolds of Arizona State University, the geologist who mapped out these rocks and first successfully interpreted them as a metamorphic core complex, and he concurs, so therefore I know it has to be legitimate! 🙂

I first saw these rocks in January on a field trip that Steve led for the Structural Geology and Tectonics Forum, but I didn’t have nearly as much time to explore the site then as I did this past Thursday, when I was lucky enough to find myself in Phoenix with a free afternoon, and good company in the person of my former student Stephanie Sparks, now working on a PhD at ASU under the supervision of Kip Hodges. Stephanie and I saw some other cool outcrops on our field trip, and I’ll feature them in future posts later this week.