The structure of the Neath disturbance between Bryniau Gleision and Glynneath, South Wales

Abstract

Summary The Neath disturbance is one of a number of compression belts which occur on the northern limb of the South Wales syncline, and caledonoid directions of which are at variance with the normal armoricanoid trend characteristic of South Wales structures. The main structural elements of the disturbance are (1) impersistent, caledonoid folds; (2) charnoid shear-faults which separate areas of contrasting fold pattern; (3) normal charnoid faults, some of which do not break the continuity of the folds; (4) a caledonoid, sinistral, shear-fault—the Dinas fault; (5) a caledonoid, normal fault—the Coed-Hir fault. In general, the caledonoid folds and accompanying charnoid shear-faults appear to have been formed before the Dinas fault. But local reversals of this general sequence and the partial or complete overlap of the phases indicate that they belong to a single period of earth-movement, believed to be of Armorican age. Subsequent vertical movements resulted in the formation of the Coed-Hir fault and some charnoid faults, and occurred also along the Dinas fault and many of the existing charnoid shear-faults. The Neath disturbance is a product of the resolution of a major Armorican force from the south-south-west into a compressional force directed towards the north-north-west and a sinistral shearing force along the Dinas fault. Such a resolution was probably due to a control exerted by the sub-Devonian foundation. A similar control was being exerted during late Avonian times along part of the disturbed area.

Jointing and minor tectonics of the Vale of Glamorgan between Ogmore‐By‐Sea and Lavernock Point, South Wales

“…Perhaps the lack of basement control in this area can be ascribed to the absence of active sub-Triassic lineaments in the Carboniferous floor, as this would contrast strongly with the narrow active zones of the Neath Disturbance (Owen 1954) and the reactivated basement faults of Hodgson (1965b p. 947). However, it is intriguing to recall that basement control was considered effective in the north-west of England (Moseley and Ahmad 1967 p. 78-79) where vertical movement of old blocks formed an essential part of the tectonic picture.…”

“…Perhaps the lack of basement control in this area can be ascribed to the absence of active sub-Triassic lineaments in the Carboniferous floor, as this would contrast strongly with the narrow active zones of the Neath Disturbance (Owen 1954) Jones and Owen 1956), yet are of limited occurrence in the coastal areas but form the ,major boundary faults of the Mesozoic area.…”

The influence of deep fractures on some areas of Ashgillian—Llandoverian sedimentation in Wales

“…Thus it is unlikely that movements affected the Lower and Middle Llandoverian but later activity may be discerned from the thickness variations of the Upper Llandoverian which at Corwen and Mynydd Cricor (Pringle & George, 1948) has an unconformable base rather at variance with its regional development. Thinning and development of non-sequences over a rise in contemporary surface caused by movements on large (often tear) faults is well known; being particularly well displayed along the Vale of Neath 'disturbance' by both Upper Visean and Namurian sequences (Owen, 1953;R. Oguike, personal communication, 1966).…”

A mountain front model for the Variscan deformation of the South Wales coalfield

“…Further evidence for such faults is based on condensed sequences in the Westphalian along the Pontypridd anticline and southeast-crop which could be either footwall or hangingwall condensed sequences resulting respectively from extensional or compressional activity on pre-existing basement faults. Owen (1954) considered the sub-Devonian basement exterted a strong influence on later deformation of the South Wales area. Hartley & Warr (1990) take up this theme and propose that pre-existing basement faults have been a major influence on basin development throughout southwest England.…”