Palimpsest is a great old word. At first it meant an ancient manuscript that was erased to reuse the parchment. Often the erased words can still be read beneath the later text, giving you two documents for the price of one. Geologists, who recycle lots of words, made it an adjective—a rock whose texture is partially wiped out by a later geologic event is said to have a palimpsest texture. The name "pseudotachylite" is a palimpsest word.
Pseudotachylites: Strange Melts
The word "pseudotachylite" was coined early in the 1900s to name a peculiar glassy rock that looks sort of like tachylite but isn't. Tachylite ("TAK-a-lite") is a basaltic glass, and that's the last I'll say about that. Pseudotachylites (or pseudotachylytes) are very different, and they deserve a better name but that's the one they've got.
As geologic faults move, the rocks on both sides are slowly ground to bits. With heat and water and chemical action you get weird minerals and textures in the resulting fault gouge or fault breccia—weird names for the results, too, like cataclasite and ultramylonite. As a class, though, these are "chewed" stones that don't actually melt. On the other hand you have igneous dikes, cracks filled with magma, and that's certainly melted.
Pseudotachylites are like both at once. They're cracks filled with dark rock, extremely fine grained or even glassy. This means the rock was fully molten, then cooled very quickly. But then suspended in this glass are chunks of the local rock around the crack which did not melt. In fact, in large pseudotachylites these chunks lie on the bottom of the crack, settled like chocolate chips in pancake batter, with the dark glass on top.
Geologist John Spray showed what pseudotachylites are in a simple and ingenious experiment published in the December 1995 Geology. He took two cylinders of granite, jammed their ends together, and spun the one against the other at 2000 rpm for just 2 seconds.
At the center of the cylinders the motion between the two was fairly slow, and the ground-up mess between the cylinder faces looked like regular fault gouge. But the farther from the center, the faster the relative motion and the greater the friction. About halfway out the radius of the cylinders, he wrote, the rock melted and stretched "like warm mozzarella to form delicate glass strands." The result was an artificial pseudotachylite. Farther out still, at the edges of the cylinders, the rock shattered and sprayed the surroundings with jagged grit. That's why they do high-pressure experiments in armored chambers. (See another example of "instant lava" in this YouTube video.)
The Superfault Conjecture
Spray concluded that pseudotachylites form with extensive motion on a fault—more specifically, superfaults associated with impact craters. They're easy to understand if you picture how craters take shape.
First an impacting object—comet, meteorite, whatever—in just a couple seconds punches a big round hole in the ground many kilometers across. A few seconds after that, the walls of the hole slump into it in big arc-shaped slices, creating a much wider, shallower hole as shown by the lunar crater Copernicus. Those slices slide on their bottoms, and if you can imagine the forces involved you can see that the friction along the bottoms is enormous after just a few tens of meters of slip.
That friction alone melts the glass in pseudotachylites. The molten glass in turn lubricates the slippage. Because the melting happens for only a few seconds, maybe a few minutes at most, the big chunks never melt.
Now there is strong evidence that large subduction faults, like the one that caused the 2004 Sumatra earthquake, can melt pseudotachylites. Researcher Christen Rowe has documented clear examples from Kodiak Island, where ancient strands of the Alaskan subduction zone are exposed. The melted zones there are more than 10 centimeters thick.
But the champs of this game are the big impact-related superfaults. The Vredefort Ring, in South Africa, has especially large and famous examples. In the huge Sudbury impact site in Canada, the pseudotachylites reach a full kilometer in thickness. Let's call them superfault superbreccias.
PS: Another type of superfault was recently proposed for one of the world's strangest geologic mysteries: the gigantic Heart Mountain detachment. However, the rock type there is dolomite, which doesn't produce a melt but rather a lubricating layer of superheated, supercritical carbonate.