Common usage of the word collapse suggests falling, contracting, “to fall
into ... a flattened form by loss of rigidity or support.” (OED). Perhaps
geologic application of the word should be reserved for similar
situations. Gravitational or orogenic collapse seems to me a loss of
elevation, at an orogenic scale, facilitated by horizontal extension.
Extension does occur during convergence in Tibet (and incidentally also
along the entire length of Taiwan, e.g. Crespi et al., 1996, “Synorogenic
extension and exhumation of the Taiwan hinterland”), but its not “orogenic
collapse” in my mind because the region is still propped up. Peter Clift
makes an important point that syn-orogenic extension, like orogenic
collapse, is also extension and is also flow driven by a positive GPE
anomaly. But to me the notion of “gravitational collapse” becomes
indistinct from “syn-orogenic extension” if used so broadly.
Steve Kidder
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