On the Character of Thrust Faults With Particular Reference to the Basal Tongues

Abstract

ABSTRACT Some detailed observations made along thrust faults are reported in this paper. The following areas were visited: Canton Glarus, Switzerland; Las Vegas area, Nevada; Glacier Park area, Montana; southern Canadian Rockies west of Calgary, Alberta. The following observations seem to have universal validity: 1. The contact is usually sharp and unimpressive in view of the great amount of displacement. 2. Structures which have been named 'tongues' seem to be common. They are features where material from the overridden sequence is seemingly injected as a tongue into the base of the overthrust plate. 3. Secondary (or splay) thrusts are common. 4. Coalescence of tongues may produce pseudo-boudins. 5. Minor folding and faulting can usually be observed in both the thrust plate and the underlying rocks. The intensity of such deformations is normally comparatively weak, at least in view of the large displacements these thrust plates have undergone, and such internal distortions are strongly affected by the gross lithology of the rocks involved. 6. Late deformations, particularly by normal faulting, are present on many thrust plates. They should be recognized for what they are: post-thrusting features completely unrelated to the emplacment of the thrust plates. This paper is an observational note only and no attempt is made to give a complete explanation of the structural significance of the various features. Particular emphasis is placed on the basal tongue and the associated pseudo-boudins structures. Both have received little attention in the literature to date and both are at present not fully understood.