Lawhorne Mill High Strain Zone

On the field trip I took Tuesday with Jay Kaufman (University of Maryland) and his PhD student Kristin Miller, we were hosted by Chuck Bailey (College of William & Mary) and his students, “the Alberene Dream Team”. Though we had come for the Neoproterozoic conglomerates, Chuck made our first stop was the Lawhorne Mill High Strain Zone, a Paleozoic shear zone that deforms granitoids and mafic dikes in the central Blue Ridge basement complex.

Here’s some of the undeformed Grenvillian granite (outside the shear zone), showing lots of K-spar and plenty of blue quartz:

Zircons from this unit yield uranium/lead ages of about 1.0 billion years old, which is a common plutonic age in the Blue Ridge basement — it’s the age of the Grenville Orogeny, a Mesoproterozoic episode of mountain building that added a big swath of crust along what is today the eastern edge of North America.

Further along, we see a pronounced alignment of the feldspar megacrysts. Is this a magmatic fabric (lathe-like crystals entrained and oriented by flow in a “mushy” magma), or is it indicative of some bulk strain imparted to these rocks (i.e., it was originally equigranular and then later squeezed as a result of differential pressure during subsequent mountain building)?

A short distance away, we see evidence that lends support to the strain hypothesis – the rocks are very much sheared out into finer-grained, highly foliated rocks that scream “shear zone.” (Although Chuck, being a literal purist, insists on “high strain zone”: after all, strain is what we see nowadays in these rocks; shear is inferred to have caused it: it’s the difference between descriptive and generative nomenclature.)

Here’s a stitched composite of the next part of the outcrop on route 617, at the intersection with 623 (a pretty good look via Google Streetview, by the way). You can make this photo bigger by clicking on it:

Inside the shear zone, our granite looks rather different:

Note the moderately steep eastward dip, finer grain size, and the relative symmetry of the feldspar porphyroclasts.

In the photo below, Chuck talks to Jay about vorticity analysis, and why he thinks this shear zone was pure-shear-dominated. Some nice, mostly symmetrical boudinage appears in the background:

A smaller example of boudinage, with spiky plants in the foreground:

Here’s a contact between finer-grained, more foliated mafic material (a former dike?) and the coarser-grained, less foliated granitic host rocks:

Argon/argon cooling ages in the muscovite of the shear zone suggest the shear zone was active about 350 million years ago (Devonian), which is roughly an ‘Acadian’ age. This was somewhat surprising to me, as the Acadian Orogeny isn’t a huge part of the traditional structural interpretation of Blue Ridge rocks. I would have guessed Alleghanian (~300 to 250 Ma) myself. But when the data don’t fit your model, it’s the model that has to change, not the data! : )

I like this outcrop a lot, and need to come back sometime when there are fewer ticks, fewer pricker bushes, and less of a swamp filling the trough between the road and the rocks! If you visit it yourself, be sure to apply some Deet to your ankles. As a result of our dalliance with the LMHSZ, I’ve garnered a collection of seven lovely chigger bites. I suspect they will be keeping me itching all the way to Bozeman…

Anyhow, more to come on these rocks in the next couple of posts. Stay tuned!