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AAPG Bulletin

Abstract


Volume: 39 (1955)

Issue: 10. (October)

First Page: 2038

Last Page: 2052

Title: Folding of Unmetamorphosed Strata Superjacent to Massive Basement Rocks

Author(s): Frank S. Hudson (2)

Abstract:

Folding without faulting of strata resting unconformably on massive granite was recognized and described many years ago. Credible theories for the formation of folds of this sort by tangential compressive forces were proposed about 25 years ago, but, if one may judge from recorded geology, these theories are not acceptable to most geologists. Otherwise there would not be the widely held view that tangential forces had no role in the formation of the Plains type of anticline.

Four areas are described where the crystalline basement, unconformably subjacent to beds which have been folded by tangential compressive forces, has been partly denuded of its cover of strata, thus affording the opportunity to compare the shape of the basement surface with the structure of the overlying beds. Massive granite forms the basement in two of these areas, whereas, in the third, folds are present above both schist and granite and, in the fourth, the bedrock is a complex of gneisses, schists, and massive intrusive rocks.

The following conclusions are presented. 1. When a mass of strata and its unconformably subjacent basement of massive crystalline rock are subjected to tangential compression, folds may form in which the strata are not faulted and in which the surface of the basement conforms in shape to the structure of the overlying beds. 2. The prime requisite for such folding is the weakening of the basement rocks, prior to the deposition of the strata, through the formation of zones of crushing, systems of fractures or minor faults, or a combination of two or more of these. 3. Other conditions being equal, the more intense the preparatory crushing or fracturing, the more severe will be the subsequent folding. 4. When there have been recurrent deformations during the deposition of strata on a mass ve basement that has been insufficiently weakened to preclude faulting of the basal strata during compressional deformation, younger beds may fold without faulting during a late compressional episode. Folds of this sort may be more severe than those of strata on a basement of foliated crystalline rock, when the two have like orogenic histories. 5. In accord with Powers (1931), the Mid-Continent folds of the Plains type appear to be, in large part, the result of tangential compression.

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