New insights into the structure and evolution of the Isle of Wight Monocline

Abstract

High quality seismic reflection data acquired during hydrocarbon exploration activities provide evidence for the subsurface structure and evolution of one of England's most well known structures at outcrop: the Isle of Wight Monocline. It is generally seen as a major northerly verging monoclinal structure linked to the Purbeck Monocline to the west. The Isle of Wight Monocline is the result of the interplay between two east-west trending, southerly dipping and overlapping down-to-the-south major syndepositional normal faults that were active during Triassic and Jurassic times: the Needles and Sandown faults. The area between the two faults tips forms an easterly-dipping relay ramp, down which sequences of all ages thicken. Both of these major normal faults were inverted during Cenozoic (Miocene: Alpine) compressional events, folding the overlying post-rift sequences of early Cretaceous to early Cenozoic (Palaeogene) age. Interpretation of the seismic reflection data suggest that a previously unknown high-angle, down-to-the-north reverse fault cuts the northern limb of both anticlines forming the composite monocline and was likely to come to crop in the steeply-dipping Chalk and/or the drift-covered Cenozoic sequences. Its identification marked a period of discussions and testing of the model by detailed field mapping. The existence and location of such a fault was proved through an iterative process with the result that a reverse fault zone is now mapped along the northern limb of the northern Sandown Anticline section of the Monocline. The main reverse faults on the Brighstone and Sandown anticlines result in circa 550 m of displacement at top Chalk level. It is thought that a series of smaller footwall short-cut faults affect the Cenozoic strata to the north of the main reverse fault, producing upfaulted sections of flatter-lying Cenozoic strata. Reverse displacements and the severity of folding on the inverted faults decreases on each fault segment in a complementary fashion in the area of the relay ramp as one fault takes up the movement at the expense of the other. The swing in strike of the Chalk in the area of shallowly dipping strata between Calbourne and Garstons is a result of deformation of the post-rift sequences across the relay ramp established between the overlapping fault tips of the Needles and Sandown faults and the interaction of the folds developed at the tips of the reverse faults.

Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England

“…Folds comprise structures with inter-limb angles of ≤90 o , more open folds, broad warps, and large-scale tilting of strata; these strongly influence the geological map of south and southeast England. Phillips (1964) and Evans et al (2011) provide very good descriptions of these large scale structures. The monoclines have amplitudes and wavelengths of 0.5-2 and up to 5km, respectively; within the folds there are fractures that have been exploited in this study.…”

Spatial variability of the Purbeck–Wight Fault Zone—a long-lived tectonic element in the southern UK

“…The faults demonstrate significant structural variability along their length, relating for example to differing degrees of inversion in post-rift sequences and the development of overlapping major fault segments (e.g. Barton et al, 1998;Harvey and Stewart, 1998;Underhill and Paterson, 1998;Collier et al, 2006;Evans et al, 2011). Focussed inversion along these faults was accompanied by more distributed deformation and uplift of the intervening basins (Chadwick, 1993;Blundell, 2002;Sanderson et al, 2017).…”

“…1; Fig. 2; Stoneley, 1982;Hamblin et al, 1992;Chadwick and Evans, 2005;Evans et al, 2011). The history of study of the spectacular exposures of this World Heritage site 'Jurassic Coast', accompanied by detailed onshore and offshore mapping, and subsurface seismic interpretation, also make it one of the best understood examples of a major structure affecting both the deposition and deformation of cover sequences in the southern UK (e.g.…”