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.

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).

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).

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…