I looked at the image and annotated it based on what it most likely is:
- In black is the formation itself.
- In blue is a depression underneath it.
- In grey is part of the boundary of what you believed to be the disappeared half.
- In green is a ridge to the north.
- In red are several craters.
Here's a more likely hypothesis for what this actually is.
The blue oval contains a slight depression. It could be a number of things, including an old crater, a basin, or the result of the collapse of part of the surface. I'm actually a little bothered by the idea of it as an impact crater. We can see from the shadows on the western walls that it is only a little shallow, especially compared to other similarly-sized impact craters in the area. It's possible that it's a crater that was filled in with sediment.
If it's a natural basin, it could once have been a Martian lake - although this is quite speculative. It could also be the remains of a pit crater (there are many pit craters in the area), formed possibly from the cave-in of a series of lava tubes. Again, this is also speculative, and the depression is quite large, about 10 miles long on its major axis.
If one of these options is correct, the depression is not very mysterious. You've identified the grey part of it as the location of the other half of the formation, partly because it appears to be roughly the same size. I find this unlikely, for a couple reasons:
- The ridges look similar in height to the edge of the depression on the other side of the formation, indicating that it is actually a part of that feature.
- Half of an outcropping does not simply vanish by itself. I know of no mechanism for this to happen. Erosion could take its toll over time, and perhaps that caused weathering of the formation, but it couldn't move it.
I think it would be clearer that this is actually part of the depression if the ridge enclosed in green were not there.
So if we assume that the outcropping is all that there even was of the object, we simply have to explain why it's so square. This really isn't that mysterious. There are plenty of other mesas in this cartographical area of Mars, designated the Elysium quadrangle by the USGS for mapping purposes. Here are some examples, which are similarly shaped:
Finally, we have the three large craters. They aren't too mysterious; craters are normal on Mars, and there are plenty, of varying sizes, in the Elysium quadrangle. The one reason the smallest stands out is because it is near the center of the northeast edge of the mesa. I'm reasonably certain this is just a coincidence.
Here's my hypothetical and likely not entirely correct timeline for this formation:
- The large basin forms, from either a crater or some other process.
- The mesa and ridge form, possibly as a result of the erosion of some larger feature.
- Craters form through impacts. One happens to be on the mesa itself.
I think your identification is simply a case of pareidolia, which is the human tendency to find shapes and patterns where none really exist. A well-known Martian example is the Face of Mars.