Glancing impacts on Earth

There is this guy who appears regularly on Twitter, trying to fill the shoes of Archimedes Plutonium, and the Expanding Earth guy, who asserts ad nauseam that the Puerto Rico back-arc basin and some structures in the Southern Andes are the product of glancing impacts by cosmic projectiles. He's a bit irritating - typically very dismissive of counter evidence, totally dismissive of plate tectonics possibly operating in "his" (I assume it's a "he" ; safe bet) area, but somewhat content that plate tectonics operate in the rest of the world. He's got a bit of and "Electric Universe" or "plasma physics" hang up too, associated with his putative impacts. He's probably also afraid of 5G telephones, and I think he's spoken approvingly of Expanding Earth ideas too.

A bog-standard, run of the Internet kook, in other words.

Well, I'm not saying that this piece's author is the same guy, or even knows of his existence, but this is the sort of thing that would greatly encourage the kook. I found this while checking for references about the potential "glancing impact" origin of Mars' Borealis (North Polar) basin.

On a Possible Giant Impact Origin for the Colorado Plateau

This is an Arxiv preprint, for a paper that was submitted to EPSL ("Earth and Planetary Science Letters" - a medium-hitter of a journal in the field) but with no mention of it actually being published. I'm not surprised.

So, firstly, it's a single-author paper. It would seem that Xiaolei Zhang hasn't managed to persuade any of his GMU (George Mason University, wherever that is) colleagues that his theory is valid. He's got a reasonable track record of publishing in "galaxy dynamics" (which he shoehorns into this paper too - confirming the identity of the author), but AFAICT, this is his only foray into geology/ planetary science. In itself, this isn't a disbarring factor, but it is a warning sign.

What is the theory? That about 750 Myr ago (in the Neoproterozoic era), the Earth suffered a "glancing blow" by an impactor to produce an astrobleme of about 640km diameter, which we now call the "Colorado Plateau". 

Aside : digging out the "area under discussion" was surprisingly hard - the author clearly thinks his audience has the same familiarity with the area that he does. There is a map - labelled as "Figure 1", but placed on P 52. That's an artefact of publishing conventions (let the journal arrange the figures into the text ; the author supplies them after the text) - annoying but not the author's fault. There's no legend on the map, which I think only shows volcanic rocks at surfaces versus "other" - which looks very single minded from a geologist's PoV.

Now, this is where the author departs from conventional cratering theory (to put it politely). Given the dimensions of the area considered "anomalous", and an estimate for how deeply the anomalies are incised into the Earth (estimate : 16km, but this seems to be derived from conventional cratering scaling laws, which this paper is rejecting for this "feature" - that point needs justification) then the author derives a simple geometrical estimate for the size of the "grazing" impactor as "Mars size". Actually, "3208km". The calculation used is the inverse of the well-known one for "distance to horizon from ships mast/ cliff, whatever". An allowance is made for drag changing the travel vector of the impactor by several degrees. But that's just about bonkers.

At this sort of scale, planets don't have significant strength. Their spherical shaoe is because the strength of the rocks is negligible compared to the hydrostatic forces due to their weight. Essentially, planets behave as strengthless drops of liquid. A contact like that would leave both objects with surfaces vibrating up and down by hundreds if not thousands of km until the energy is dissipated through most of the mass of the bodies.

It's also a very improbable contact. Even a small degree closer to head on, and the bodies would have merged, or generated so much ejecta that there would have been global secondary impacts, if not forming a moon of orbiting ejecta. And if the alignment had been 16km in the other direction (0.5% of impactor diameter) then it would have been a near miss.

It seems the geological data the astronomer is tieing onto is reporting of apparent horizontally directed shear at high levels in the crust of the Colorado plateau, combined with the Plateau's elevation. He's also relying on there being some great mystery about the so-called "Great Unconformity" observed in the Neoproterozoic of the Grand Canyon. (We have a "Great Unconformity" covering about the same Interval here in Scotland, but we don't blame it on wildly unusual events.)

The author ascribes the appreciable NW-SE elongation of the "Plateau" to the motion of the "impactor" ("grazer"?), but makes very little mention of the Sudbury structure in the Canadian Shield, which is generally accepted as being an impact structure that has been compressed on a NW-SE axis to have about twice the NE-SW dimension compared to the NW-SE dimension. The deformation of Sudbury is generally ascribed to continent-scale compression during the Grenville orogeny shortly after the impact. The more modest non-circularity of the posited Colorado Plateau structure is as easily explained by distortion since it's formation - regardless of intrinsic (e.g. mantle plume) or extrinsic (impact, the FSM's paintbrush) origin. (I emphasised elongation versus compression to avoid people thinking there was some stress field similarity - the elongation is similar, but the direction is opposite.)

Memo to astronomical dynamicists : you leave the geology alone, and I'll leave stirring the pot of star alone. OK?