R. A. Chadwick

Extension tectonics in the Wessex Basin, southern England

The Permian to Cretaceous tectonic evolution of the Wessex Basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere. The gross morphologies of its constituent structures were governed by the location of Variscan thrust and wrench faults in the upper and middle crust, which suffered extensional reactivation in tensional stress fields oriented approximately NW-SE. Several episodes of crustal extension can be resolved, in early Permian, early Triassic, early Jurassic and late Jurassic/early Cretaceous times. These were characterized by the rapid subsidence of fault-bounded basins and commonly, by erosion of adjacent upfaulted blocks. Superimposed upon the fault-controlled subsidence, dominant during periods of fault quiescence, and becoming increasingly important with time, a component of regional subsidence is considered to have a thermal origin. This suggests that crustal extension was accompanied by some form of, not necessarily uniform, lithospheric thinning. Subsidence analyses assuming local Airey isostasy give cumulative crustal extension factors of 20–28% beneath the grabens. A more reasonable assumption of regional Airey compensation indicates basinwide crustal extension of 13–17%. which is consistent with BIRPS offshore deep seismic reflection data.

Aspects of basin inversion in southern Britain

The Wessex Basin of southern England, a Permian to Cretaceous extensional basin, can be structurally divided into a set of constituent asymmetrical graben, bounded by major E–W-trending zones of en echelon syn-depositional normal faults. The graben were inverted in late Cretaceous and Tertiary times by compressive stresses oriented roughly north–south. Inversion structures fall into two related categories. Regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben-fill. Superimposed upon and geographically delimiting the regional upwarps, are roughly east-west trending linear zones of en echelon inversion structures. These coincide with the earlier graben-bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault. The linear reverse fault/monocline inversion structures were a relatively inefficient method of basin shortening. Because upper crustal faults in the region steepen upward markedly, reversal of these faults under compression resulted in a shortening discrepancy at shallow depths. Locally, particularly in south Dorset, this was overcome by bucklefolding and low-angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures. Elsewhere however, the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary-fill. This occurred preferentially in those graben containing young, poorly lithified and therefore weak sediments. Thus the early Cretaceous depocentres of the Weald and Channel basins were strongly upwarped, with axial uplifts of over 1000 m. Conversely, graben having older, more lithified sequences suffered little regional upwarp, shortening primarily by fault reversal along the linear inversion structures. The amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension.

The Sticklepath-Lustleigh fault zone: Tertiary sinistral reactivation of a Variscan dextral strike-slip fault

Dextral strike-slip movement on the Sticklepath-Lustleigh fault zone (SLFZ) is indicated by displacements of ?Permian and older rocks. Previous authors have inferred that the main dextral movement which caused these displacements was post- Permian and, noting the presence of small Tertiary pull-apart basins along the fault zone, probably of Tertiary age. However, the geometry of these early Tertiary pull-apart basins indicates sinistral rather than dextral strike-slip movement. We present an alternative model for the history of the Sticklepath-Lustleigh fault zone, summarized below: 1 Late Variscan strike-slip movement, with a total displacement of up to 10 km, produced the SLFZ and offset dextrally an earlier Variscan thrust. 2. Extensional reactivation of this thrust led to rapid Permo-Triassic subsidence in the Crediton Trough and a dextrally offset neighbour, the Hatherleigh outlier. 3. Approximately 6 km of early Tertiary sinistral movement on the SLFZ produced small pull-apart sedimentary basins, and reduced the net dextral offset across the fault. 4. In mid-Tertiary times, minor dextral movements on the SLFZ may have produced reverse faulting on the margins of the Tertiary basins.

Deep crustal structure and Carboniferous basin development within the Iapetus convergence zone, northern England

Seismic reflection data from northern England are interpreted to show a northward dipping Caledonian thrust zone lying beneath the faulted southern margin of the Northumberland Trough. The thrust zone lies upon the updip projection of a northward dipping high conductivity layer in the middle and lower crust, identified by magnetotelluric sounding. Together, these features indicate the presence of a crustal-scale Caledonian shear zone, dipping north at an average angle of 20–25°. Extensional reactivation of the middle and upper crustal parts of the shear zone, in Carboniferous times, led to development of the Northumberland Trough. Reappraisal of the BIRPS NEC seismic profile suggests the presence of the same (or similar) crustal shear zone, reactivated to form the offshore extension of the Northumberland Trough. The shear zone probably formed as a major compressive or transpressive feature during late Caledonian (Acadian) continental collision. In the lower crust, the shear zone lies close to, or may actually represent, the Iapetus suture. At middle to upper crustal levels the shear zone lies wholly within Avalonian (English) crust, well to the south of the (faunally determined) Iapetus suture. Westward extrapolation of the shear zone indicates that it may crop out within the northern Lake District, perhaps as the Causey Pike and associated thrusts. The faunal suture is likely to subcrop north of the Lake District and Alston Block, beneath the central part of the Northumberland Trough.

The structure and evolution of the Northumberland Trough from new seismic reflection data and its bearing on modes of continental extension

An 18 km long N–S seismic reflection profile has been acquired across the faulted boundary between the Alston Block and the Northumberland Trough. A NW–SE geological cross-section has been constructed across the trough by integrating this line with other data, and a Tricentrol seismic profile confirms the suitability of this model. The trough has a markedly asymmetric form, with a thickness of more than 4.2 km of Dinantian strata adjacent to its faulted southern margin. The present day surface faults (Stublick, Ninety Fathom) are related to Variscan and later inversion and transpression, and do not everywhere correspond precisely to the earlier syn-depositional normal faults. The basin is believed to have formed in response to reactivation of the Iapetus Suture in a dominantly N–S extensional stress field. Basin evolution has been analysed by fault restorations and subsidence modelling. The fault restorations indicate an upper crustal extension factor of 1.15 to 1.19. Subsidence modelling indicates a whole crustal extension factor of 1.30, with similar sub-crustal lithospheric extension. It is possible that the difference between the inferred extension factors is due to non-uniform (increasing downwards) lithospheric extension, but the uncertainties inherent in both methods are such that this cannot yet be confirmed.

Lower crustal heterogeneity beneath Britain from deep seismic reflection data

450 km of deep seismic reflection data have been acquired over the UK landmass. A qualitative assessment of lower crustal reflectivity (based on the amplitude and continuity of deep crustal events) has been made. In the cratonic area of central England the lower crust is generally poorly reflective, while in the foldbelt terrains of southern, eastern and northern England it is more strongly reflective. Beneath the Variscides of southern England an important distinction can be made between poorly reflective ‘foreland-type’ lower crust lying beneath the Variscan Front thrust and strongly reflective ‘orogenic’ lower crust farther south. A brief review is given of hypotheses for the origin of lower crustal reflectivity, and the relationship of the latter to crustal tectonic regime, both in the UK and elsewhere. It is concluded that the types of lower crustal heterogeneity required to produce the observed reflections are more easily generated in orogenic provinces (fold/thrust/metamorphic belts) than in areas which have undergone only low to moderate crustal extension (beneath typical sedimentary basins). The generation of granites by partial melting within the lower crust, and processes of metamorphic differentiation and recrystallization which accompany orogenic activity, are likely to play a significant role in the development of such heterogeneities. In addition, post-orogenic processes of magmatic underplating and low-viscosity ordering may also have played a part.

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

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.

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

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.

The Maryport fault: the post-Caledonian tectonic history of southern Britain in microcosm

The Maryport fault forms part of the faulted southern margin of the Northumberland-Solway Basin. Seismic reflection data reveal a complex history of repeated fault reactivation. Large oblique-normal displacements on the fault controlled early Carboniferous extensional basin development. Subsequently, Variscan basin inversion was accompanied by partial reversal of the fault. In Permian and Mesozoic times, renewed oblique-normal displacements strongly influenced basin subsidence. Further partial reverse displacement occurred as the Solway Basin was again inverted, probably in mid-Tertiary times in response to Alpine compression. The Maryport fault is most unusual, therefore, in providing direct evidence of the principal post-Caledonian tectonic events in southern Britain.