Foreland folding

Publisher Website
Google Scholar
Abstract
Summary: A study of foreland fold zones indicates that the cover of sedimentary rocks plays a passive role and accommodates itself to movements of the basement beneath. Lateral pressure exerted along a sedimentary cover from a thrust mountain front, such as is implied by the Abscherung theory of Buxtorf to explain the Jura Mountains structures, is not regarded as probable. Mountain building is the consequence of contraction of the interior of the earth and crustal compression from this cause has been dominant throughout revealed geological time. Examples of compression structures in shield areas far distant from active mountain fronts, as in Nigeria, are described. The Gulf of Suez and the Dead Sea rift valleys show evidence of both compressional and tensional features, but the former is the more important. A crystalline basement complex can flexure and form the cores of anticlines with, in some cases, little or no faulting. Oil exploration borings in Venezuela, Sumatra and elsewhere have reached basement rocks in the core of anticlines and have shown that the active principle is movement of the basement and not sheared, rootless folding in the sedimentary cover produced by lateral pressure. Examples of different types of foreland folding are described. The Jura Mountains structures are ascribed to steep folding or fault-slices in the basement below, the sedimentary cover having become detached as a result of the lubricating effect of the Triassic salt and folded into an independent pattern. The South-West Persian mountain belt is taken as an example of simple folding with long anticlines of up to 250 miles in individual length. Assam is described as an example of imbric folding— strongly thrust-faulted anticlines arising from thrust-folds in the basement below. The great flexure on the south flank of the Shillong plateau. between the Himalayan front and the Assam ranges, shows how an apparently rigid crystalline complex can be bent into a steep flexure and an erosion surface down-warped to a depth of about 30,000 feet. Clay diapir anticlines in Trinidad are the surface expression of steep folding below a cover of soft incompetent clays, with high-pressure water and gas acting as agents to promote the upward extrusion of the clay core. Salt diapir anticlines, as distinct from circular salt plugs, also owe their origin to compressional movements in their substratum. The degree to which basement rocks have been faulted, folded or flexed in some foreland zones is in marked contrast to more rigid behaviour in others. The great thrust-anticlines along the Rocky Mountain front in Alberta, such as Turner Valley, are the consequence of fault-slices in the basement. This complex zone is thrust on to the sedimentary cover of an unyielding foreland zone extending for hundreds of miles to the eventual outcrop of the basement rocks.

This publication has 4 references indexed in Scilit: