Aspects of Basin Inversion in Southern Britain

Journal of the Geological Society, London, Vol. 150, 1993, pp. 311-322, 14 figs. Printed in Northern Ireland

Aspects of basin inversion in southern Britain

R.A.CHADWICK British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

Abstract: 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 buckle- folding 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-fll. 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 1000m. 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 term 'basin inversion', synonymous with the term greatly improved understanding of basin inversion, although 'positive structural inversion' as used by Glennie & Boegner even now, inversion structures are commonly misinterpreted (1981) and Harding (1985), can be most simply defined as as wrench faults (Harding 1985 discusses this). Recent 'the preferential tectonic uplift of an earlier structural thematic publications reflect this increased awareness of depression or basin'. For extensional basins, the definition basin inversion (e.g. Ziegler 1987b; Cooper & Williams may be further refined: basin inversion causes the reversal 1989). of the subsidence patterns of an extensional sedimentary This paper will discuss some aspects of basin inversion basin in response to compressive tectonic stresses. Basin with particular reference to the Wessex Basin of southern inversion generally involves uplift of the basin floor and England, which, with its dense network of seismic lines and deformation of the basin-fill, as the throw on the ample surface exposure, has many excellent examples of basin-controlling normal faults is partially or totally reversed inversion-related structures. (Ziegler 1987a). Other forms of basin uplift, such as those driven by thermal effects, halokinesis or associated with crustal extension such as footwall-uplift, are not inversion The Wessex Basin: summary of structural evolution phenomena in the strict sense. A diagnostic feature of true The post-Variscan sedimentary basins of southern Britain basin inversion is that existing intra-basin structural lows are formed during a protracted period of extension-related uplifted preferentially compared to intra-basin structural subsidence which affected the northwest European con- highs. tinental shelf for much of Permian to Tertiary times (e.g. Historically, basin inversion has long been recognized. Cheadle et al. 1987). The Wessex Basin covers an onshore Lamplugh (1920), examining borehole data from southern area of about 20000 km a (Fig. la) with at least a similar area England, deduced that Mesozoic sediments thickened beneath the English Channel to the south. The basin rests markedly towards the centre of the Weald, and suggested upon Palaeozoic basement rocks (Smith 1985) which were that the outcropping Weald anticline was superimposed deformed during the late Carboniferous Variscan Orogeny upon a deep-seated syncline. The early workers also by thrust- and wrench-related structures. The sedimentary- speculated upon possible mechanisms of basin inversion, fill of the Wessex Basin is of Permian to Tertiary age and Lamplugh (1920) proposing isostatic rebound and Arkell locally exceeds 3 km in thickness (Fig. lb), but is more (1933) stressing the importance of compressive tectonic typically about 2 km thick. forces. The three-dimensional structural architecture of the Despite these early advances, until recently, basin Wessex Basin reflects the tectonic process dominant in its inversion has remained a poorly described and understood formation: that of crustal extension. Subsidence patterns phenomenon, possibly because the true nature of inversion were strongly controlled by E-W-trending linear zones of structures is difficult to discern from field evidence alone. major en echelon normal faults (Fig. lb). The fault zones The advent of seismic reflection profiling has enabled a delineate a system of asymmetrical graben with intervening 311

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Fig. 1. (a) Simplified surface geology of the Wessex Basin. LB, London Basin; WA, Weald anticline; HB, Hampshire Basin; CH, Channel High. (b) Contours (metres x 100) or top Variscan Basement beneath the Wessex Basin. LP, London Platform; WB, Weald Basin; PB, Pewsey Basin; DB, Dorset Basin; HDH, Hampshire-Dieppe High; CB, Channel Basin; LPF, London Platform faults; WPF, Wardour-Portsdown faults; PWF, Portland-Wight faults.

highs which constitute the principal subsurface structural Mid- to late Cretaceous times (Aptian and younger) saw elements of the basin. the cessation of active crustal extension and the In marked contrast, the dominant surface features of the establishment of regional unfaulted subsidence (Figs 2b and basin originated during a post-extensional period of crustal 3b), presumably associated with thermal relaxation effects shortening and basin inversion. Surface structural depres- inherited from the earlier lithospheric extension (Chadwick sions (the London and Hampshire basins) are superimposed 1986). Marine sandstones and shales of mid-Cretaceous upon underlying horst blocks (the London Platform and (Aptian and Albian) age pass upward into the thick, upper Hampshire-Dieppe High), whilst surface structural highs Cretaceous carbonate succession of the Chalk. It is this (the Weald anticline and the Channel High) are initially fiat and unfaulted post-extensional sequence which superimposed upon earlier basins (the Weald and Channel so well preserves the subsequent inversion features and basins). This nearly exact superimposition of compressional enables their detailed morphologies to be understood. features upon underlying extensional features (compare Fig. la and b) exemplifies perfectly the principles of structural Late Cretaceous and Tertiary basin inversion inversion (Cooper et al. 1989). General aspects of the structural inversion of the Wessex Basin are discussed in Stoneley (1982), Whittaker (1985) Permian to Cretaceous basin subsidence and Lake & Karner (1987). The sequence of events which The extensional history of the Wessex basin is described in accompanied basin inversion is summarized below. detail elsewhere (Stoneley 1982; Chadwick 1986; Karner et The end of Cretaceous times heralded important tectonic al. 1987; Jenkyns & Senior 1991), but can be summarized by changes in the Wessex Basin. Regional subsidence the restored cross-sections of Figs 2 and 3. From Permian to continued into early Tertiary times, and considerable early Cretaceous times, pulses of upper crustal extension thicknesses of shallow marine and freshwater elastic (probably accompanied by some form of lithospheric sediments accumulated. However, the growing influence of thinning) led to widespread basin subsidence with the tectonic inversion caused patterns of sedimentation to differ development of a system of asymmetrical graben (Figs 2a markedly from those of the Mesozoic era. This change is and 3a). The sedimentary fill comprises a Permo-Triassic exemplified by the preservation of Tertiary strata in succession of argillaceous continental red-beds, Jurassic grey synclines (the London and Hampshire basins, Fig. la) which marine shales with minor limestones and sandstones and a directly overlie Mesozoic extensional features of opposite lower Cretaceous (Berriasian to Barremian) brackish to structural polarity (the London Platform and the freshwater elastic sequence, arenaceous at the base but Hampshire-Dieppe High, Fig. lb). fining markedly upwards. The timing of the onset of basin inversion and the

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Fig. 3. Section C-D through the Wessex Basin (see Fig. 1). (a) Restored to an end-early Cretaceous (Barremian) datum. (b) Restored to an end-Cretaceous datum. (c) Idealized restoration for mid-Tertiary (Miocene) times, assuming no erosion. PLP, Pewsey-London Platform structures; WP, Wardour-Portsdown structures; PW, Portland-Wight structures; CB, Channel Basin; HDH, Hampshire-Dieppe High; WB, Weald Basin; LP, London Platform; CH, Channel High; HB, Hampshire Basin; WA, Weald anticline; LB, London Basin.

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precise degree to which it influenced early Tertiary patterns 2000 3000 4000 1 I sonic velocity ms" of sedimentation are uncertain. Some authors, using sedimentological evidence (Wooldridge 1926; Plint 1982; Edwards & Freshney 1987), suggested that compressive upwarping commenced at about the beginning of Tertiary times, forming embryonic precursors of the Weald • 0 • 500- Anticline, the Channel High (which acted as a sediment source) and the London and Hampshire synclines, the latter acting preferentially as depositional basins. Evidence of even earlier minor inversion movements lies within the

Chalk sequence. Backstripped subsidence history plots 1000- 0 0 (Chadwick 1985a), show that, by late Cretaceous times, basement beneath the Weald Basin had begun to subside E d: more slowly than basement beneath the London Platform. An alternative view (King 1.981) is that throughout early Tertiary times, the Weald anticline had not yet become a 1500- topographically significant feature and was covered by a Z~ E "" 900m lower Tertiary sequence only slightly thinner than that of the London Platform. Thus, according to King, the present-day preservation of Tertiary strata within the Fig. 5. Sonic velocities of argillaceous rocks from two boreholes in the Wessex Basin. Open circles, Sandhills Borehole; solid circles, London Basin (and by implication the Hampshire Basin) is Detention borehole (Fig. 4 gives borehole locations). entirely a consequence of post-depositional (Miocene) inversion. The balance of evidence suggests that minor inversion faulting and the form of its basal surface broadly reflects the movements affected the Wessex Basin from late Cretaceous effects of subsequent basin inversion. Where the base Chalk times through to the early Tertiary, probably co-eval with surface has been eroded, for example within the Weald the 'Pyrenean' phase of deformation, as lberia was caught anticline (Fig. la), seismically determined thickness trends up in the convergence of Africa relative to Europe (Coward were used to estimate the amount of erosion and hence & Dietrich 1989). However the major inversion episode did uplift. Extrapolated uplift estimates were augmented by a not take place until the Miocene, corresponding to the main depth of burial study based upon analysis of borehole logs. Alpine deformation events as continental collision occurred Sonic velocities of argillaceous strata were analysed by a between Africa and Europe. method similar to that described by Magara (1976). The method assumes that argillaceous rocks which have been Geometry of the inversion structures deeply buried and subsequently uplifted, are overcompacted The structural inversion of the Wessex Basin is summarized relative to a normal burial trend. Thus abnormally high in Fig. 4, which illustrates the principal inversion structures. sonic velocities are encountered at shallow depths. This is These fall broadly into two related categories: regional basin illustrated by measurements from the Detention Borehole in upwarps and linear inversion structures. the central part of the Weald Anticline and the Sandhills Borehole in the Hampshire Basin (Fig. 5). Lithological variability produces considerable scatter in the data, but the Regional basin upwarps overall compaction trends are clearly discernible. Argilla- There are sites of regional upwarp, several tens of ceous rocks from Detention have considerably higher sonic kilometres wide, which overlie the extensional graben. velocities for a given depth than those from Sandhills, the Figure 4 shows contours of the total (late Cretaceous to depth offset of the two compaction trends being indicative of Miocene) relative uplift. Uplift patterns were estimated by the extra erosion which has occurred at the former locality mapping the base of the Upper Cretaceous sequence (the (about 900 m). It is noteworthy that this figure is somewhat Chalk). Deposition of the Chalk post-dated extensional less than the relative uplift at Detention compared to

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Fig. 4. Principal inversion structures of the Wessex Basin. Contours (metres) ,~v "~ ~--~----1000~,.,)~c~ A MONOCLINE 1~~ ~q,~ show late Cretaceous to mid-Tertiary _ _ " -0- ANT, ,,N relative regional uplift. Abbreviations as REVERSE FAULT for Figs 2 & 3, plus SF, Sticklepath- REGIONAL UPWARP Lustleigh fault; CF, Watchet- Cothelstone-Hatch fault; D, Detention X REGIONAL DOWNWARP borehole; S, Sandhills borehole.

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Sandhills as indicated by the map (Fig. 4). This is because the Wessex Basin, particularly within the Weald Anticline the sonic velocities record only that uplift which has (Butler & Pullen 1990), where erosion has exposed reverse occurred since the sediments attained their maximum depth faulting at the surface (Figs la and 4). of burial (i.e. immediately prior to the main Miocene The relationship between the regional upwarps and the uplift). Minor relative uplift of the Weald which occurred discrete linear inversion zones of flexuring and fault reversal earlier, during late Cretaceous and early Tertiary times, is illustrated by a seismic line which crosses the important while sediments were still being deposited (see above), Portland-Wight structures where two en echelon monocl- cannot be recognized with this method. ines overlap (Fig. 7). The north-facing monoclines separate Two major upwarps (the Channel High and the Weald the Channel High (the inverted Channel Basin) from the anticline) have axial relative uplifts in excess of 1000 m (in Hampshire Basin (the inverted Hampshire-Dieppe High), substantial agreement with the estimates of Ebukanson & and overlie discrete zones of fault reversal. Figure 8 Kinghorn 1986 and Butler & Pullen 1990). The former is illustrates these features in greater detail. Structures I1 and markedly asymmetrical in profile with its northern limb I2 have similar overall geometry. The shallower beds are steeper than the southern, the latter less so (Figs 2c and 3c). folded into north-facing monoclinal flexures, whose inflexion Similar structures are found offshore, for example in the points lie upon the upward prolongation of an underlying south Celtic Sea Basin, where upper Cretaceous and reversed normal fault. At greater depths, the fault itself is Tertiary sediments were warped upwards above an earlier present and the amount of flexuring decreases markedly, faulted extensional basin (Fig. 6). In contrast, other graben, passing downwards into roughly planar bed geometry for example the Dorset and Pewsey basins (Fig. 2a) do not (commensurate with the subplanar fault). Net displacement show significant regional upwarp. across the structures changes from down-to-the-north at shallow depths, to down-to-the-south at deeper structural levels, where the original normal displacement of the fault Linear inversion structures exceeds the subsequent reverse movement. Farther west, The regional upwarps are geographically delimited by the the highest structural levels of these inversion features are second type of inversion structure. These comprise linear exposed at outcrop, with monoclinal flexuring much in E-W-trending structural zones of en echelon faulted evidence along the south Dorset coast (Arkell 1947). monoclinal or anticlinal flexures which are invariably related The Wardour-Portsdown fault zone provides another to the partial reversal of underlying basin-controlling normal good example of fault reversal. A seismic profile through faults. In the Wessex Basin, three such major inversion the Mere fault (Fig. 9) shows that the sense of throw zones are found (Fig. 4), from north to south these are: changes down the fault. At the surface, the fault has a (a) the Pewsey-London Platform inversions which reverse displacement, down-to-the north, Middle to Upper separate the London Basin from the Weald anticline Cretaceous strata being thrown against Upper Jurassic beds. (and include the well-known Hog's Back fold) and The precise displacement at this level is difficult to ascertain, overlie the Mesozoic London Platform normal faults due to erosion of the southern upthrown (hanging-wall) (Figs lb and 4); fault-block, but it is certainly considerable. At greater (b) the Wardour-Portsdown inversions which lie be- depths the reverse displacement decreases to about 90 ms tween the Weald anticline and the Hampshire Basin (about 120 m) at the intra-Jurassic reflectors R1 and R2, and and overlie the Mesozoic Wardour-Portsdown to virtually zero at the base of the Jurassic. Within the normal faults (Figs lb and 4); Permo-Triassic sequence the fault shows normal down-to- (c) the Portland-Wight inversions which separate the the south displacement. These different struc~,~ral levels are Hampshire Basin from the Channel High, and exposed at surface along the fault strike (Fig. 10). In the overlie the Mesozoic Portland-Wight normal faults east, the Mere Fault crops out as a down-to-the-north (Figs lb and 4). reverse fault, throwing Upper Cretaceous strata (Chalk) Similar, but smaller, structures are present elsewhere in against Upper Jurassic beds. Westwards along the fault,

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Fig. 7. Seismic line offshore southern England (Fig. la for location). Profile crosses the Portland-Wight structures, with simplified geological interpretation.

progressively older rocks crop out and the reverse northwards-facing monoclinal flexure, as even higher displacement decreases steadily, such that middle Jurassic structural levels are exposed. beds are present on both sides of the fault with little net A similar structural situation can be discerned at the displacement. Farther west still, the throw on the fault outcropping western end of the Portland-Wight faults (Fig. switches to normal down-to-the-south, with Lower Jurassic la). There, several faults show normal down-to-the-south strata downthrown against Permo-Triassic rocks. East of the displacement of Jurassic strata, but pass eastwards (and area shown in Fig. 10, the Mere Fault passes into a stratigraphically upwards) into down-to-the-north, reverse

Fig. 8. Migrated enlargement of part of Fig. 7, showing detail of the Portland-Wight inversion structures.

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Fig. 9. Seismic profile across the Mere Fault (Wardour-Portsdown fault zone) which shows both normal and reverse 0 1KM senses of displacement (R1 and R2 are I I intra-Jurassic reflectors).

faults and flexures in Cretaceous beds (e.g. the the dominant factor in determining the surface displace- Abbotsbury-Ridgeway faults, Arkell 1947). ments observed on such a fault, where the amounts of This type of outcrop pattern where the displacement on normal and subsequent reverse displacements may well be a fault changes from normal to reverse along its length is roughly uniform along its length, is the exposed structural commonly misinterpreted as indicating some form of level. complex 'scissors' type of movement on the fault. In reality Inversion structures more complex than simple reverse

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tion, tending to thin towards the controlling reversed fault, as opposed to the original extensional situation / where sediments thickened towards the major fault / / (see Fig. 8). / It is evident that although the monoclinal flexures arose / / from the process of regional compression, they formed / primarily by reversal of an underlying fault, and as such, are not classical compressional folds. Indeed the steep limbs of the monoclines are characterized by considerable stretching and thinning. This was confirmed at outcrop by Bevan (1985), who, working on the Portland-Wight structures, described conjugate sets of extensional mesofractures in the steep (locally overturned) limbs of the monoclines. He concluded that they are the accommodation products of stretching which occurred as the monocline developed by drape over major, deep-seated reverse faults. Fig. 11. Schematic block diagram illustrating development of two inversion structures offset by a transfer zone. (a) Prior to shortening. (b) After shortening by reversal of normal faults. Fault reactivation It is well established that inversion of the Wessex Basin was accomplished by regional compression (e.g. Stoneley 1982; fault/monocline features are encountered in the Wessex Chadwick 1985b; Lake & Karner 1987). The way in which Basin. This is particularly the case in south Dorset, where the main basin-controlling faults were reactivated during the western parts of the Portland-Wight inversion structure compression is discussed below. comprise steep-sided anticlines and periclines (House 1961). Seismic data show local tight anticlinal folds whose steeper (north-facing) limbs are sometimes cut by low-angle reverse Reactivation of E- W-trending faults faults, for example the Chaldon Herring and Poxwell The largest of the linear inversion features within the anticlines (Stoneley 1982). The possibility of further Wessex Basin is the E-W-trending Portland-Wight structural complexity is offered by the presence of salt units structure. This and a much smaller opposite-facing inversion within the Permo-Triassic sequence which may have acted as structure are well illustrated by Fig. 12 (the seismic data detachment surfaces. This local development of tight cannot resolve internal details of the large inversion anticlinal folds, rather than simple monoclinal flexuring is structure due to steep dips, but its overall geometry is discussed further below. well-constrained by outcrop and borehole data). The two In plan view, the three principal linear inversion zones of inversion structures have important properties of symmetry, the Wessex Basin comprise groups of faults or flexures in that the amount of inversion is closely related to the showing en echelon offsets (Fig. 4). This configuration is a original normal displacement of each controlling fault, the consequence of the reactivation of underlying normal faults large (Portland-Wight) normal fault showing much greater which were already offset in a similar manner (Fig. lb; see reversal than the smaller one. This strongly suggests that the Peacock & Sanderson 1991 for detailed discussion of en mechanism of inversion was kinematically similar (though of echelon fault offsets). There is a complementary relationship opposite polarity) to that of the original extension, in that between neighbouring offset inversion structures; as the the same fault network was utilised with individual faults amount of inversion (reversal) on one structure dies out, the being allocated similar proportions of displacement. Given inversion on its offset companion increases, such that the that extension on these faults was probably dominantly total inversion (crustal shortening) across the structures is dip-slip, it is likely that shortening on the faults was also roughly constant (Fig. 11). The transitional region between approximately dip-slip, indicating roughly N-S compression. two offset inversion structures is analogous to a 'transfer ramp' between offset thrusts (Dahlstrom, 1970). A consequence of this typical en echelon configuration is the Reactivation of NW-SE-trending faults tendency for inversion structures to form closed periclinal In addition to reactivating the major E-W-trending normal features, rather than open-ended monoclinal or anticlinal faults in a dominantly dip-slip sense, inversion of the folds. Wessex Basin also caused important strike-slip or The char,tzteristic geometrical properties of the linear transpressive motions along major NW-SE-trending wrench inversion structures are summarized below: faults. These faults crop out in the western part of the (a) flexuring in the shallow section, passes gradually Wessex Basin (Fig. 4) and are probably sub-vertical downwards into planar bed geometry (see Fig. 8); deeply-penetrating crustal fractures formed during the latter (b) reverse displacements in the shallow section pass stages of the Variscan orogeny. The movement history of downwards into normal displacements, provided that these faults can be discerned by analysing fault displace- the amount of reverse movement is intermediate ments in rocks of different ages and also by examining the between the maximum (deep) and minimum development of small 'pull-apart' basins which formed at (shallow) displacements of the earlier normal fault kink points on the faults. The Sticklepath-Lustleigh fault (see Fig. 9); system (Fig. 4) is particularly revealing in the latter respect. (c) the uppermost beds of the faulted sequence in the In early Tertiary times, small, but deep, sedimentary basins, hangingwall block undergo considerable deforma- rhomboid in plan, formed at left-stepping kinks in the fault,

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.. '

! (" i o.o -:--_:~--:--:::-:-:::t-:--:-----:-:----:-:--'~l..: •" :.. ~ ~ :.

i: }-J.i ,.. Z:," ...... ' ...... "

-1.0

Fig. 12. Seismic line across the Portland-Wight structure (PW), with geological interpretation. Note also the much smaller south-facing inversion monocline.

presumably during an episode of sinistral transtension cally in Fig. 13 which shows flat-lying post-extensional strata (HoUoway & Chadwick 1986). In mid-Tertiary times and overlying a normal fault which can have either constant dip probably co-eval with the main Alpine basin inversions (planar), or can steepen upwards (Fig. 13a). Shortening of farther east, N-S oriented compressive stresses produced the section with reverse reactivation of the planar fault dextral transpression along the fault. This tended to close produces upward movement of its hangingwall block (Fig. the earlier pull-apart basins, with reverse faulting along their 13b). The diagram is probably not wholly realistic in terms margins (Bristow & Hughes 1971). Similar dextral of detailed strain distributions (for example the footwall displacement along the Watchet-Cothelstone-Hatch fault block is assumed to remain undeformed), but in broad terms system (Fig. 4 and Whittaker 1972) produced sites of its geometrical properties are valid. The amount of localized uplift at left-stepping kinks. One such feature is shortening at shallow depths associated with this inversion the Compton Valence dome, previously suggested as having structure is related to the amplitude of the inversion a halokinetic origin (Falcon & Kent 1950). These dextral monocline and the dip of the underlying reversed fault: displacements are consistent with the N-S compression indicated by reversal of the E-W faults, but NNW-SSE or shortening = S~ = dl - dz = A/tan a~. even NW-SE compression cannot be ruled out. Thus, the steeper the fault, the greater the uplift produced by a given amount of shortening and the smaller the amount A mechanism for whole basin inversion of shortening for a given uplift. Consequently, reversal of The foregoing discussions address the development of steep faults is a relatively inefficient shortening mechanism. individual inversion structures but do not satisfactorily This is exemplified by Fig. 13c which shows the explain the mechanisms by which the whole basin inverted. consequences of reversing a fault which steepens upwards. Of particular importance is the relationship between the As the hangingwall block is uplifted, it becomes buttressed linear inversion zones and regional basin upwarps. against the steepening fault surface which results in a As discussed above, the linear inversion zones are not corresponding decrease in the amount of shortening at classical compressional folds. They are the consequence of shallow depths. A shortening discrepancy thus develops, displacements with a dominantly vertical component, shortening of the shallow section (dl - d3) being significantly associated with fault reversal. This is illustrated schemati- less than the causative shortening at greater depths (Si).

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anticlines and periclines crop out (see above), but is not common elsewhere in the Wessex Basin. In south Dorset the tight folds commonly form hangingwall anticlines above low-angle reverse faults (Ridd 1973; Stoneley 1982) which cut the steep limbs of the folds and thereby assist shortening still further. It may be that the presence of salt units in the ..7 Q Permo-Triassic sequence in this part of the basin promotes the development of low-angle detachments and the associated tight folding. An alternative way of overcoming the shortening discrepancy across the linear inversion structures is by bulk compression of the basin sedimentary-fill. This probably takes place by a combination of pure-shear shortening and dz-~ localised minor reverse faulting and produces regional I upwarp of the basin depocentre as the basin shortens with ---T conservation of cross-sectional area. Bulk shortening and : A regional upwarp of the basin-fill can take place without significant reversal of the major basin-margin faults (e.g. Fig. 6). To evaluate the relative importance of shortening by I fault reversal and shortening by regional upwarp in the Wessex Basin, it is necessary to consider matters on a larger scale. The constituent asymmetrical graben of the Wessex b Basin are thought to lie within the hangingwall blocks of reactivated major thrusts in the Variscan basement. These low-angle thrust-faults dip to the south, typically at about 25 ° (Chadwick et al. 1983). This situation is illustrated schematically in Fig. 14a, which shows faulted graben A sediments overlain by an unfaulted post-extensional Si= dl-dz - tancx sequence. The graben lies above a deeply-penetrating basement thrust, having formed by extensional reactivation of the thrust (Chadwick et al. 1983). The principal

,d T.-- d 3 ., i T El

C ~Si I i Sr 4 D'Si9 I ,. I I ~ .... ~..., ..-.' Fig. 13. Schematic illustration of an inversion monocline formed by reversal of an underlying normal fault. (a) Prior to section shortening. Flat-lying unfaulted strata lying above either a planar fault (solid line) or a curved fault, steepening upwards (dotted line). b (b) After section shortening by reversal of planar fault. (c) After section shortening by reversal of curved fault.

~,"~ S = Si_i.Sr In order to preserve bed-length balance in the graben, r" this shortening discrepancy must be overcome in some way. Fig. 14. Schematic illustration of asymmetric graben inversion by One mechanism involves the development of compressional reactivation of upper-crustal thrust. (a) Prior to upper crustal anticlinal buckle-folds along the inversion structure, within shortening. (b) After shortening. Graben shortens by reversal of the hangingwall block. This type of folding occurs along the margin fault and bulk shortening of the sedimentary fill, with Portland-Wight structure in south Dorset, where tight regional upwarp.

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graben-bounding normal fault, relatively steep at shallow C-D. Shortening factors (shortened section length divided depths, gradually becomes less steep downwards to coalesce by original section length) can thus be estimated at about with the thrust at depth. This is geometrically analogous, 0.98. Drawbacks of this method, namely the fact that the though on a larger scale, to the situation illustrated in Fig. basin-controlling faults and underlying thrusts may never 13c. flatten to horizontal at depth, and its disregard of shortening Horizontal compression of the upper crust is initiated at associated with regional upwarp, tend to be self-cancelling. depth by reverse reactivation of the basement thrust. This The estimated shortening factor may therefore be reverse displacement is translated upwards along the reasonably accurate, and is in reasonable agreement with an basin-controlling normal fault (Fig. 14b). However as the uplift isostasy model for basin inversion (Chadwick 1985b) fault becomes much steeper upwards (dips increase from which gave a shortening factor of 0.97. about 25 ° on the thrust, to typically 60-70 ° or more in the The amount of basin shortening, assumed from the shallower sediments), horizontal shortening at depth is above to be about 2%, is considerably less therefore than transformed progressively into predominantly vertical the amount of crustal extension which accompanied basin motion in the shallow section, the fault becoming development, estimated at 10-15% (Chadwick 1986). This correspondingly less effective at taking up the shortening. is consistent with the fact that inversion accomplished The graben thus becomes buttressed against its steep relatively minor modifications to the gross extensional boundary fault. Shortening may then preferentially occur by geometry of the basin, with locally significant, but overall bulk compression of the graben-fill, to produce regional relatively minor, destruction (erosion) of the basin-fill. upwarp. Thus the total shortening of the basement (S) is equal to the sum of shortening across the inversion structure Conclusions (Si) and shortening by regional upwarp (St). Within the Wessex Basin, those graben which contain The structural history of the Wessex Basin was characterized thick sequences of relatively young sediments appear to be by a protracted period of basin subsidence, subsequently particularly prone to regional upwarp. Thus, the Channel overprinted by tectonic inversion. The inversion occurred in and Weald basins with thick Upper Jurassic and Lower pulses, commencing in late Cretaceous times and culminat- Cretaceous sequences were strongly upwarped (Figs 2, 3 and ing markedly in the mid-Tertiary. Inversion utilized 4). Conversely, the Dorset and Pewsey basins with thick pre-existing lines of weakness, with reversal of earlier accumulations of Permo-Triassic strata did not suffer basin-controlling normal faults and regional upwarp of regional upwarp, inversion of the latter being accomplished depocentres, particularly those filled with young, poorly- primarily by reversal of the Pewsey-London Platform faults lithified sediments. E-W-trending inversion structures (Fig. 2). A similar pattern has been noted elsewhere, for suffered dominantly dip-slip reverse displacements, whereas example in the southern North Sea, where Lower NW-SE-trending fractures showed dextral transpressive Cretaceous depocentres were preferentially uplifted displacements. This suggests that the main mid-Tertiary (Gowers & Saeboe 1985). compressive stress-field was oriented roughly N-S, pre- It seems that the relative importance of fault reversal sumably associated with Alpine continental collision in and regional upwarp as means of inverting the Wessex Basin southern Europe. depended upon their relative efficiency as basin-shortening From these observations a more general conclusion can agents. Graben such as the Weald and Channel basins, with be drawn. The mode of basin inversion depends upon the considerable thicknesses of young, poorly lithified and geometry of the basin margin faults and the mechanical therefore relatively weak sediments in the shallow section, properties of the basin-fill. Basins with steep margin faults were more susceptible to shortening by regional upwarp and weak poorly-lithified sedimentary-fills (particularly in than graben containing older, more lithified sequences. It is the shallow section where shortening by fault reversal is noteworthy also that regional upwarp of the Weald Basin inefficient) will tend to shorten by regional upwarp. was most severe over its central and eastern parts (Fig. 4). Conversely, basins with moderately-dipping margin faults, Here, its northern bounding faults (the eastern end of the and strong, lithified sedimentary-fills will tend to shorten by Pewsey-London Platform faults) are relatively poorly reversal of the margin faults. defined and suffered only minor reversal, thus inversion was Permission to publish seismic reflection data was kindly given by necessarily accomplished primarily by bulk shortening of the GSI Ltd (Figs 7 & 8), Kelt UK Ltd and the Carless operated basin-fill. Wessex/ISE Group (Fig. 9) and BP Exploration Ltd (Fig. 12). Quantification of the crustal shortening associated with BIRPS seismic data was provided by the British Institutions basin inversion is difficult to estimate, firstly because its Reflection Profiling Syndicate, Cambridge. Thanks are also due to effects usually merely modify the dominant extensional an anonymous referee and J. R. Andrews whose comments led to basin architecture and secondly because inversion structures substantial improvement in the manuscript. This paper is published are commonly poorly preserved due to erosion. The with permission of the Director, British Geological Survey simplest method of estimating crustal shortening (Stoneley (NERC). 1982) assumes that all the shortening is taken up in the individual linear inversion structures and that the reactivated (reversed) faults have listric profiles, becoming References horizontal at depth. In this situation the reverse ARKELL, W. J. 1933. The Jurassic System in Great Britain. Oxford University displacement (amplitude) of each linear inversion structure Press, London. is equal to the causative basement shortening at depth. -- 1947. Geology of the country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain, HMSO. Examination of Figs 2 and 3 reveals a total of 2.0 km of BEVAN, T. G. 1985. A reinterpretation of fault systems in the Upper reverse displacement on the inversion structures of section Cretaceous rocks of the Dorset coast, England. Proceedings of the A-B, and 2.1 km of total reverse displacement along section Geologists' Association, 96, 337-342.

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Received 28 January 1992; revised typescript accepted 11 September 1992.

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