Deja Vu

As we entered the Taconic Mountains on US 4 in Vermont, something didn’t look right, or it looked too familiar to be correct. It took a while to realized what was wrong with Fig. 1.

Figure 1. Road cut along US highway 4, near Rutland, Vermont. This is Precambrian (1000 to 500 Ma) schist with strong foliation dipping to the west.

These are the mountains for which the Taconic Orogeny (550-440 Ma) was named. They were deposited as long ago as a billion years in a shallow sea (e.g. Sea of Japan) and then buried, before being compressed and heated, finally being pushed onto the porto-north America continent by 440 Ma. During this long period of metamorphism, the clay minerals comprising the bulk of the sediments recrystallized into mica (mostly muscovite), a platy mineral that creates both a sheen and a fissile texture, the tendency to flake apart (Fig. 2).

Figure 2. Close-up of Fig. 1, showing the glistening caused by alignment of platy muscovite in the sun (upper center), and fissile texture caused by the same alignment and weathering as water works its way between mineral grains. Image width is about 6 feet.
Figure 3. Close-up of the foliation surface of the rocks seen in Figs. 1 and 2. Note the linear ripple-like texture, which may be remnant from the original sediment (note the surfaces seen in the last post), or a coincidence. The brightness is caused by aligned muscovite crystals. (Image is 2 feet across.)

The Taconic Mountains are the remnants of a mass of metamorphic rock that was pushed over younger, less-altered rocks in this region. This occurs along low-angle thrust faults when the rocks are buried less deeply, so that they break rather than fold like putty. Speaking of ductile deformation, we saw plenty of evidence of that in the White River‘s exposed bed (Figs. 4-6).

Figure 4. Photo of White River near the village of South Royalton, VT, showing exposures of Precambrian schist and gravel bars. The following photos were taken on this outcrop.
Figure 5. Detail of orthoclase (pinkish area) and albite (whitish) feldspar minerals squeezed out of the original muddy sediment during metamorphism. These minerals may have originally been present as lenses of sandy sediments or be the product of remineralization, which includes a component of concentrating incompatible elements. Very little quartz was present.
Figure 6. Image of nearly vertical foliation (i.e. layering) of schist in White River bed. This broad area of irregular feldspar and quartz may have been a large sand lens (e.g. a flood deposit) in the original sediments.

This post is titled “Deja Vu” because we saw schist with a similar composition and orientation in the Potomac River, more than 500 miles to the south, in a band tens of miles across, centered on Great Falls, Virginia. Such a broad distribution tells us that a vast mountain belt eroded about one billion years ago, and then its erosional remains were buried so deep that they nearly melted. The subsequent collision was no laughing matter. I have been using Japan as an analogue for the Taconic Orogeny for two reasons: (1) Honshu, the largest Japanese Island is about 800 miles long and it is depositing vast quantities of mud into the Sea of Japan; (2) using a modern analogue demonstrates that mountain building is a slow process, barely noticed by the inhabitants of island arcs destined to be smashed onto the continents facing them.

Rocks like those seen in this post are already buried beneath the Japan Sea and deformation has no-doubt begun. We just have to wait 400 million years for them to come out of the oven…

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  1. What Goes Up… | Timothy R. Keen - July 4, 2022
  2. Recap… | Timothy R. Keen - July 5, 2022

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