Window (geology)
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A tectonic window (or Fenster (lit. "window" in German)) is a geologic structure formed by erosion or normal faulting on a thrust system. In such a system the rock mass (hanging wall block) that has been transported by movement along the thrust is called a nappe. When erosion or normal faulting produces a hole in the nappe where the underlying autochthonous (i.e. un-transported) rocks crop out this is called a window.
Windows can be almost any size, from a couple of metres to hundreds of kilometres.
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Look up autochthonous in Wiktionary, the free dictionary.