Late Westphalian inversion structures on the Variscan foreland display wide variations in orientation that are the product of the underlying basement grain. The orientation of the major basement lineaments in relation to the direction of maximum shortening also determined the severity of deformation of the inverted basins. NE–SW-trending faults were oriented roughly perpendicular to the NW–SE to NNW–SSE direction of maximum shortening determined from thrust transport directions in the Variscan orogen to the south. As a consequence, NE–SW-trending basins in the Variscan foreland have been strongly inverted, the Silesian post-rift fill has commonly been expelled from the basins and the syn-rift sediments have been deformed by chevron folds (Dublin and Bowland Basins). In contrast, the N–S- and NW–SE-trending faults were oriented more obliquely to the direction of maximum shortening and, as a consequence, display a significant component of oblique slip. This oblique slip component is manifested as en-echelon periclines and flower structures. In general, the N–S- and NW–SE-trending basins were less strongly inverted and the Silesian post-rift fill has been retained.

Inversion tectonics of the Variscan foreland of the British Isles

Two seismic reflection lines recently shot in the western part of the Worcester Basin suggest that its western boundary is defined by major syn-depositional normal faults, downthrowing to the east and active throughout Permo-Triassic times. Farther east, other faults can be seen within the basin. These moved at various times during the Permian and Triassic and profoundly affected patterns of sedimentation. The subsidence history of the basin can be explained by two phases of lithospheric stretching, one in the Permian and the second in early Triassic times, giving a cumulative stretching factor of about 1.2. This is consistent with the regional pattern of normal faulting provided that stretching was either one-dimensional, oblique to the N–S and E–W directions, or two-dimensional.

Seismic reflection investigations into the stratigraphy and structural evolution of the Worcester Basin

A major seismic reflection transect, acquired by the British Geological Survey between 1984 and 1986, has elucidated the deep geology and structure of some well-known surface features of central England. The Worcester Basin, a Mesozoic extensional graben system, is bounded to west and east by major, sub-planar normal faults which formed by the extensional reactivation of much older structures; respectively the Malvern Axis and a similar though concealed structure forming its mirror image to the east. Within the Worcester Basin a thick Permo-Triassic sequence rests directly upon Precambrian basement rocks. To the west and east, Permo-Triassic rocks are thin or absent, but relatively thick Palaeozoic sequences are preserved above the Precambrian. These strikingly different successions, together with the fault geometry, suggest that the Worcester Basin lies within a major, inverted oblique-slip fault system or flower structure. In Palaeozoic times transpressivt stresses led to reverse faulting along the Malvern Axis and its complementary structure to the east with consequent regional uplift of the intervening tract and the formation of a positive flower structure. In Permian and early Mesozoic times, transtensional stresses caused collapse and negative inversion of the flower structure and consequent subsidence of the Worcester Basin.

Short Paper: Evidence of negative structural inversion beneath central England from new seismic reflection data

Interpretation of commercial seismic reflection data from the west of Banbury, south-central England, has provided information on the late Carboniferous structural evolution of the Oxfordshire Syncline. End-Carboniferous structures are preserved in the footwall of the main basin-bounding faults of the Permo-Triassic Worcester Graben. Restoration of major pre-Triassic normal faults has allowed reconstruction of earlier compressional structures. In contrast to the WNW-ESE trend of structures to the south, in the Variscan foreland fold-thrust belt, major faults and folds in Oxfordshire are approximately N-S trending. The structures seen to deform Westphalian D-Stephanian-aged sequences on the seismic profiles are a combination of steep (30-40° dip) fault-bounded uplifts and less dominant thin-skinned tectonics, apparently showing transport to the east. In the south, some thin-skinned structures are present, and in the north cover sequences are uplifted in a narrow zone of deformation on reactivated basement faults. The amount of shortening also changes along strike, from c. 1600 m in the south to c. 950 m in the north. Reactivation of major basement faults with differential basement shortening accounts for the main structures. Localized detachment in the young and weak sediments close to the palaeo-land surface explains the thin-skinned structures seen on some of the seismic lines. These end-Carboniferous structures in Oxfordshire and structures in the Malvern-Abberley Hills represent the structural limits of a large N-S-striking intra-Stephanian uplift in the late Variscan foreland. It is suggested that the structures formed during a discrete phase of intra-Stephanian E-W-directed regional shortening prior to the final phase of movement in the Variscan belt.

End-Carboniferous fold-thrust structures, Oxfordshire, UK: implications for the structural evolution of the late Variscan foreland of south-central England

The nineteenth-century researches into the Malvernian led to the view that the complex consisted of metamorphic rocks, but the basis for this conclusion was not founded on a broad knowledge of the characters of igneous and metamorphic rocks. Field evidence relating to their origin leads to few definite conclusions. The 9 km2 complex is well exposed in numerous large quarries, but each quarry contains a wide variety of types brought together by shearing under moderate temperature conditions and later brittle faulting. The general shearing increases in intensity from north to south, the rocks in the two southernmost blocks being mainly chlorite-schist. In the north, however, some magnesium-rich ultramafic homblendites occur, occupying small areas among diorites, tonalites and granite. The diorites and tonalites, characterised by hydrolysed andesine or oligoclase, may be classed into olivine-normative mafic types and quartz-normative leucocratic types, together forming about 70 per cent of the complex. Shearing under moderate temperatures and high water vapour pressures has produced a wide variety of schistose rocks, some showing a weak lamination, erroneously interpreted earlier as a gneissose banding. Elsewhere in the Malverns, sheared pegmatites and granite veins in diorite produce similar banded rocks. The diorites and tonalites are also magnesium-rich, being generally high in potash as well. Small areas of a lineated and weakly foliated two-mica microcline-granite of near ‘minimum-melting’ type are present, as well as granite-pegmatites and potash and soda pegmatites which cut the ultramafics, diorites and tonalites, but predate the general hydrolysis and shearing. Microdiorite dykes cut the complex after some of the shearing and hydrolysis. Faulted against the plutonic rocks is a small area of pillow lavas and felsic tuffs, the Warren House volcanics, the felsic rocks being sodic dacite and quartz-keratophyre. The volcanics are cut by dykes which are mildly alkaline in composition; these dykes are not affected by the strong shearing of the plutonic rocks.

During the general shearing and hydrolysis hornblende reacted to give chlorite and epidote, biotite was chloritised, and plagioclase became altered to muscovite. An early record of sillimanite is regarded as due to misidentification of zoisite; garnetiferous granites are present as well as one example of what may be a garnet-mica-schist of sedimentary origin.

The petrography and chemistry of the igneous complex of the Malvern Hills, England

The geological structure of the Malvern, Abberley and Ledbury Hills is described in detail. The apparently anomalous association of high- and low-angle reverse faults, vertical faults, high- and low-angle normal faults, wrench faults and isoclinal folding is interpreted as the result of upthrust tectonics related to the differential movement of basement blocks. The western boundary of the Pre-Cambrian outcrop appears to be the outcrop of an upthrust fault, with downthrow to the west, and with a vertical separation as large as 4000 ft. Upthrust faults are characterized by a strongly curved profile, and their geometry is a function of the tectonic level at which they are viewed. The curved fault-planes can be shown to plunge in such a manner that highly complex outcrop patterns emerge. The mechanics of plunging upthrust faults are discussed. All folding adjacent to the major Malvern upthrusts appears to be related to them and important variations in the type and intensity of folding can be discerned in vertical sequence. The Ledbury and Mathon folds appear to have originated as the result of major sinistral wrench movement along the Ledbury and Colwall faults respectively. The eastern boundary of the Pre-Cambrian and Palaeozoic rocks appears to be a fault in most areas, but it is unlikely that it anywhere has a vertical separation in excess of 1000 ft. The presence of the deep Worcestershire basin immediately to the east of the Pre-Cambrian outcrop suggests the existence of a major upthrust fault with downthrow to the east, now covered by Mesozoic sediments. This fault, together with the upthrust fault which forms the western boundary of the Pre-Cambrian outcrop, would have defined the Malvern horst during late Palaeozoic times.

Colin Barry Phipps

Papers

Structural geology of the Malvern, Abberley and Ledbury hills