For the sake of analogy, let's take the Chixclub impactor and move it a mountain range akin to the modern Himalayas. Obviously we're dealing with an extinction level event here so I'm not concerned with the effects on life, and only loosely interested in the atmospheric effects. My primary concern is with the effects of the impact on the surrounding geology.

I'm mostly looking for information on how the area would look a few thousand years out - would the nature of the area better preserve a crater? Or would the ejecta fill the hollow and leave a sort of semi-flat (but still high altitude) plateau?

Feel free to ignore this, as I don't want to overburden the question, but does the result change if the impactor is at a lower speed (e.g. an orbiting body forced to deorbit)?

edit: per the clarification request below, here is a link to a spot on Earth most analogous to the environment I am imagining. A fairly wide stretch of a range of high peaks of marble and schist (et al) fairly well removed from an ocean. The impact's 'epicenter' would be surrounded by mountains on all sides.

Well, pre-impact. 'After the impact' is part of the answer I'm seeking.

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    $\begingroup$ Welcome to the site ngonStrafe, when you have a few minutes, please take the tour and read up in our help center about how we work: How to Ask. Others may post comments to help refine the question, good first post though. +1 $\endgroup$ May 26 '19 at 17:58
  • $\begingroup$ Help us out by giving us a longitude and latitude linked to Google maps showing the exact mountain range you'd like us to use as an example. I ask for this because a generic list is difficult and potentially unproductive, while explaining what would happen to that exact range (the composition and situation for which would be well known) would allow us to go into detail that I believe would be more helpful to you. $\endgroup$ May 26 '19 at 18:33
  • $\begingroup$ Thanks for the link! +1 And that's a great spot. Deep ravine surrounded by mountains. High altitude, few trees, probably tons of basalt. I'm hoping Arkenstein XII shows up. This sounds like his kind of question! $\endgroup$ May 26 '19 at 19:14

The crust in that region is about 80 km thick, and there is no ocean around to fill the hole immediately after the impact. This means there would be no exposed magma interacting with water to cause additional explosions.

After the impact there would be some rivers slowly pouring into the crater, filling it with water and sediments over the passing of millennia.

Until the basin was filled by sediments you would have a fairly circular lake, surrounded by a ridge of mountains.

After a fair amount of time the movement of the Indian subcontinent against the Eurasian plate would probably distort the circular shape and the depth profile of the crater, but it would take more than some millennia.

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    $\begingroup$ +1 for this answer. It covers all the salient points bar one. High mountain ranges experience far more erosion compared to flat ground near sea level, so there's a lot of sediment being created and transported. A crater in a mountain range is likely to fill up reasonably rapidly (thousands to tens of thousands of years). $\endgroup$ May 26 '19 at 20:21
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    $\begingroup$ Crater diameter is about 15 times impactor diameter at least at sizes up to 1 km diameter. Depth is about 1/4 diameter or about 4 times impactor diameter. So a 10 km object will create a 40 km deep crater. This is deep enough that plastic flow will be significant, and the rock hot enough to make steam. $\endgroup$ May 26 '19 at 20:22

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