Extension Tectonics in the Wessex Basin, Southern England

Journal of the Geological Society, London, Vol. 143, 1986, pp. 465-488, 25 figs, 1 table. Printed in Northern Ireland

Extension tectonics in the Wessex Basin, southern England

R. A.CHADWICK Deep Geology Research Group, British Geological Survey, Keyworth, Nottingham NG12 5GG

Abstract: ThePermian 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 characterizedby the rapid subsidence of fault-bounded basins and commonly, by erosion of adjacent upfaulted blocks. Super- imposedupon the fault-controlled subsidence, dominant during periods of fault quiescence, and becoming increasingly important with time,a component of regional subsidence is consideredto have a thermalorigin. This suggests that crustal extension was accompanied by some form of, not necessarilyuniform, lithospheric thinning. Subsidence analyses assuming local Aireyisostasy 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.

The Wessex Basin of southern England (Kent 1949) covers amplitude,discontinuous, sub-horizontal seismic reflection an onshore area of greater than 20000 km’, and occupies at events. least a similar area beneaththe English Channelto the The seismic reflection data are interpreted to show that south (Fig. 1). It lies uponbasementa comprising themajor normal faults of zone 1 lie above,and are principally sediments of Cambrian to Carboniferous age controlled by extensional reactivation of, the fractures which (Smith1985), which weredeformed by thrustrelated cut seismic zone 2. Moreover,these fractures, which structures during the late Carboniferous Variscan Orogeny. beneath southern England probably originated as Variscan Sediments within the basin, mostly of Permian to Tertiary thrusts, appear to die out downwards into seismic zone 3. age, are locally in excess of 3000 m thick and were deposited Elsewhere in the world where similar, deeply penetrating during the widespread post-Carboniferous subsidence of the thrusts are still active,they are foundgenerateto NW European continental shelf (e.g. Ziegler 1981). considerable upper crustal earthquake seismicity (Jackson & Modern seismic reflection and deep borehole data have McKenzie1983). It isbelieved that slippagealong the led to a much improved knowledge of the morphology of thrusts is accomplished by dominantly brittle stress release the constituent structures of the Wessex Basin. This, when mechanisms,involving cataclasis and frictional sliding integrated with modern theories of crustal and lithospheric (Sibson 1983). The fact that the fractures(thrusts) of seismic extension, can leadtowards a fuller understanding of zone 2 appear to die out within zone 3, coupled with the possible basin evolution mechanisms. paucity of recorded lower crustal earthquake sources (Chen -& Molnar 1983), suggests that with increasing depth, brittle modes of deformation give way to quasi-plastic behaviour, and that stress release within seismic zone 3 may take place Nature of the Lithosphere beneath southern England by dominantly ductile mechanisms. Beneath southern England the top of seismic zone 3 lies Beforeexamining the structure of southernEngland in atabout 6stwo-way traveltime (Fig. 2a), ata depth of detail, it is instructive to discuss the general nature of the 15-20 km, and is thought to coincide with the brittlelductile continentalcrust, and qualitatively, the mechanical pro- transition. The origin of theevents within zone 3 is perties of the lithospheric plate. uncertain (see Fuchs 1969; Mereu & Ojo 1981; Smithson & Deep seismic reflection profiles both onshore (Whittaker Brown 1977 and Hale & Thomson 1982 for discussion), but & Chadwick 1984) and offshore (Brewer et al. 1983), suggest itmay be significant thatthe strongest reflections and thatthe crust beneath the British Isles can be divided diffractions commonly lie near its top (Fig. 2b). Here, major vertically intothree zones based upon seismic reflection shear zones capable of producinghigh amplitude seismic character (Fig. 2). These are interpreted as structural units, returns maypass downwards intoa smaller scale more each with acharacteristic tectonic style. Theupper unit, penetrativeductile strain fabric, characterized by weaker, seismic zone 1, is composed essentially of flat-lying or gently more random events. Factors influencing the depth of this folded sedimentary rocks, in places cut by syn-depositional transition include geothermal gradient, crustal composition, normal faults, some of which have suffered later reversal. thepresence of fluids andthe tectonic strain rate. Brittle The middle unit, seismic zone 2, comprises strongly folded behaviour will prevail at relatively greater depths in areas of and cleaved ‘basement’rocks, with local intrusiveigneous low geothermal gradient, relatively basic (quartz-poor) dry bodies, in places cut by major, discrete planar or sub-planar crust, and high tectonic strain rate. Deep seismic reflection fractures or shearzones. The lower crustalunit, seismic lines acquired offshore from northern Scotland as part of the zone 3, is of uncertain structure, but is characterized by high BIRPS (British Institutions Reflection Profiling Syndicate)

465

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 466 R. A.CHADWICK

200 300 400 500 600 000

Fig. 1. Sketch map of southern England showing simplified surface geology and important locations.

0 4 km 0 5 ?m l programme, showevidence of brittle behaviour in the 0 uppermost mantle. Matthews & Him (1984) have suggested thata major, possibly Caledonian,fracture in the upper mantle passes upwardsand soles outinto a ductilelower crust. The grossmechanical properties of the lithosphere can qualitatively be related to the proximity of the geotherm to the solidus (Fig. 3). Wherethe temperature of the

TEMPERATURE ( C)

0 200 400 600 800 1000 1200 1400 I I1 11 I I

5 BRITTLE

1 DUCTILE

t t ' RELATIVELI l > ZONE 3 BRITTLE

10 ICREASINGL'1 DUCTILEl

DUCTILE

Fig. 2. The three proposedseismic reflection zones, illustrated by (a) seismic profile in southern England (courtesy of BP Exploration Fig. 3. Gross mechanical properties of the continental lithosphere Ltd); (b) line drawing of seismic profile in eastern England related to the proximity of solidus and geotherm (after Bott1971; (courtesy of RTZ Oil and Gas Ltd). Wyllie 1977; Dewey 1982).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EX TEN SION TECTONICS IN THE WESSEXEXTENSIONTHE TECTONICS IN BASIN 467

lithosphere is wellbelow the solidus,the lithosphere is Permian to Cretaceous extension tectonics of the relatively strong and brittle, conversely at depths where the Wessex Basin geotherm approaches the solidus the lithosphereis relatively weak and ductile. The response of the stronger parts of the lithosphere(particularly theupper and middle crust) to Geometrical evolution of the sedimentary basins tensional tectonic stresses plays a crucial role in controlling Figure 4ashows a map of the basementupon which the the distribution and form of extensional sedimentary basins. Permian to Cretaceous sedimentary basins developed. The Beneath southern England the young Variscan orogenic belt Variscan foldbelt is formed of rockswhich arefolded, providedimportant weaknesses in theupper and middle faulted and cleaved with an approximately E-W structural crust (and possibly the uppermost mantle) whose reactiva- trend. Several major thrust zones and wrench faults can be tioncontrolled the evolution of the Permian to Tertiary identified, both at outcrop (e.g. Dearman 1963; Leveridge et sedimentary basins. al. 1984) and in the subsurface from seismic reflection data

Thrust A Wrench fault VFT VariscanFront Thrusts WT WardourThrust ' - Thrusts PWT Purbeck-Wight . . ._,__,- !I,,; ~'- W]Variscan Foldbelt p00 zoo 300 400 ,-

WEALD BASIN

CHANNEL BASIN

S N HSection shown in Fig.8 Normal fault

-OOO 00 300 400 500 6oo

Fig. 4. (a) Postulated major structural features of the Variscan basement beneath southern England; (b) Permian to Cretaceous structural provinces in southern England.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 468 R. A. CHADWICK

(Kenolty et al. 1981, Chadwick et al. 1983). The Variscan extended bya factor B (Fig. 5b). The lower crustal unit Front thrusts in most places separate the Variscan foldbelt deforms by ductile thinning and the upper unit deforms by from the much less strongly deformed Variscan Foreland to faulting. The fault has a listric (concave upwards) geometry thenorth. Figure 4b showsa map of the Permian to and soles out into a horizontal detachment at the top of the Cretaceous extensional structural provinces which lie upon ductile layer. If eachincrement of extensionis small the the Variscan basement. South of the London Platform these hanging-wallblock will ideally deformunder gravity by provinceshave beengrouped together under theterm vertical simple shearto form arollover (Fig. 5~).In this ‘WessexBasin’ (Kent 1949; Stoneley 1982). The provinces ideal case the amount of horizontal extension is equal to the were most clearly structurally demarcated from Permian to heave of themajor listric fault. The presence of earlyCretaceous times, whena series of E-W trending inhomogeneities (such as weakbedding planes),andtor grabens and half-grabens were defined by zones of major, large increments of extension, may result in more complex mainly south facing, syn-depositional normal faults (cf. Fig. deformation of the hanging-wallblock. Discreteshear 20), which developed along the same lines as pre-existing surfacesmay develop, taking theform of normalfaults (reactivated) Variscan thrusts.This predominantly rifted antithetic to the major listric fault and substituting for part phase of subsidence was succeededinmid-Cretaceous of the flexure in the rollover (Fig. 5d). Figure 5e illustrates (Aptian) timesby a period of moreregional, unfaulted an ideal situation where the major normal fault has a planar subsidence which lasted until aboutthe end of the geometry. Antithetic faults develop as before, but rollovers Cretaceous period. are absent and the horizontal extension is equal to the sum In southernEngland the development of asymmetric of the heaves of the normal faults. A further possibility, that half-grabens can be related to themodels presented in Fig. 5. of rotational planar normal faulting, the ‘domino model’ (cf. Asimple crustal profile (Fig. 5a) comprising anisotropic Wernicke & Burchfiel 1982), doesnot seem to be widely brittle upper unit and a ductile lower unit, is horizontally applicable tothe WessexBasin, where significant block tilting can only locally be observed. If cross-sectional area is conserved, the following simple (a) BRITTLE hangmgwallblock relationship holds: A= A-d, Equation I DUCTILE where A = cross-sectional area of basin, A = horizontal extension and d, = depth to horizontal detachment. This equation is useful for calculating the depth of very shallow detachments,but is less appropriatefor mid- or lower crustal detachments, where isostasy plays a dominant rolein determining theamount of basinsubsidence (see below). Manysmall normalfaults seen at outcrop in southern Englandconform well tothe listric fault model, with rollovers, occasional antithetic faults and sets of extensional joints in the flexured hanging-wall block (Fig. 6). However, the geometry of the major basin-controlling normal faults has more in common with the planar fault model of Fig. 5e. Thesefaults are usually planar, or only slightly listric (at least to seismicallyresolvable depths)and although antithetic faults are common, large-scale rollover geometry israrely well-developed. Elsewhere in the world major normal faults have been demonstrated to havea basically planargeometry. Forexample, westernin Turkey earthquake analysis suggests that seismically active normal faults are planar at least to the base of brittle upper crust (Eyidogan & Jackson 1985). The Permianand Mesozic normal faults of southern Englanddepart further fromidealized extensional models because they formed within an anisotropic upper crust with many pre-existing structures. Comparison of Figs 4a and 4b illustrates the close correlationbetween the location of Permian and Mesozoic major normal faults and underlying earlier (Variscan) thrusts. It is likely thatextensional reactivation of these Variscan lines of weakness profoundly influenced the location and geometry of the Permian and Fig. 5. Effect of horizontal extension on a schematic two layer Mesozoic normal faults (Chadwick et al. 1983). Moreover, model (a) before extension; (a) extension produces void along when the extension vectorwas notperpendicular to the listric fracture;(c) formation of rollover in hanging-wall block;(d) olderstructures, themajor normal faults probably antithetic faulting in hanging-wallblock; (e) extension with planar experienced a significant component of strike-slip displace- normal faulting. ment (cf. Chadwick 1985~).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THE WESSEX BASIN 469

Fig. 6. Normal fault inUpper Triassic strata,exposed near Blue Anchor Point Somerset. Syn-depositional movement is demonstrated by sedimentary thickening towards the fault plane (A),with development of a slight rollover (B).

Figure illustrates7 schematic a model forthe ductile thinning.Extensional reactivation of the thrust development of Permian and Mesozoic sedimentary basins causes subsidence of the hanging-wall block, which fractures in southern England. It is assumed that the crust comprises intoa network of normal faults (Fig. 7b). The largest abrittle upper unit of Variscan basement rocks (seismic normal faults lie above the thrust. They downthrow in the zone 2), cut by a major thrust (here portrayed as planar, same direction as the thrust and at depth coalesce with it, though local ramp and flat geometry may be developed). probably in a listric manner. Large antithetic normal faults The thrust is assumed to pass downwards,at adepth of arealso formed, these may abut against the synthetic 15-20 km, into a ductile lower crustal unit (seismic zone 3), normal faults, or against the thrust itself. Smaller synthetic where it loses its identity (Fig. 7a). As the crustal section is and antithetic faults are ubiquitous, these commonly have a subjected to horizontal tension, stress build-up within the more strongly developed listric form than the (sub-planar) stronger upper unit leads to brittle failure which utilizes any majorfaults, enabling the localized development of suitably oriented line of weakness; in this case the Variscan small-scale rollovers. A consequence of thisreactivation thrust. Meanwhile, the weaker lower crustal unit extends by mechanism is thatextensional strain decreases upwards

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 470 R. A.CHADWICK

projection of the earlier profile, but the shallower parts of the thrusts, the post-Variscan sediments andthe whole southern end of the section are constrained by interpretation 0 of additional seismic reflection data. Along the section of Fig. 8, the upper crust is cut by three major Variscan thrust zones. The thrustsdip southwards and are roughly planar to a depth of at least 15 km, beneath which they may lose their identity within the lower crust. Large Permian and Mesozoic normal faults influenced basin developmentin the hanging-wallblock above the reactivated thrusts. The normal faults are mostly synthetic to the thrusts,but the antitheticBere Regis Fault, forming the southern margin of the Winterborne Kingston Trough, may owe its origin to thedevelopment of ramp and flat geometry extenmon by on theWardour Thrust. It is notablethat this fault is .’ ductile thinnmg associatedwith extensional rollover in its hanging-wall block,suggestive of listric fault geometry and a relatively shallow detachment.In contrast the other major faults,

(C) especially the Pewsey Faults, are not associatedwith significant rollover development,and appear to have roughly planar geometries with deep detachments. Itthospharic flexure regional unfwlted Detailedmapping usingseismic reflection data shows subsidence thatthe major normal faults commonlyshow lateral, ‘en echelon’ offsets of several kilometres (Fig. 4b). These offsets are not in general the result of later strike-slip faulting; they were present at the initiation of normal faulting. Analogy Fig. 7. Effect of horizontal extension on a two layer crust with a with compressional thrust terrains (Dahlstrom 1970) has led pre-existing fracture (a) before extension; (b) mechanical extension Gibbs (1984) to describe these offsets as ‘transfer faults’. In of the crust gives rifted subsidence (stippled area denotes basin southern England true transfer faults are rarely developed, infill); (c) post-extension crustal flexure gives regional subsidence. the term ‘transfer zone’ being more appropriate (Fig. sa). As the throw on one of the faults dies out, the throw on its along the thrust plane, the shallower parts of which may not offset companion increases in a complementarymanner, be significantly reactivated. Extension is commonly followed such that the total throw (and therefore the total extension) by a period of regional, unfaulted subsidence (Fig. 7c), see summed across the faults is approximately constant. Figure below. 9 demonstrates two possiblemechanisms for the develop- Implicit in Fig. 7b is the likelihood that the top of the ment of fault belts exhibiting transfer offsets. In Fig. 9b a ductilelower crust is not necessarily atthe same depth simple basementthrust is oblique tothe direction of during extension as it was during the earlier thrusting. The extension. As the thrust isreactivated normal faults form increased geothermal gradient (see below) and possible low above it, the ideal condition of fault trend perpendicular to strain rates associated with lithospheric extension may lead extension being achieved by dextral transfer offsets. In Fig. to a shallower brittle/ductile transition and no reactivation 9c the thrust plane itself is offset by dextral strike-slip faults. of the deeper roots of the older thrust. Extension causesreactivation of each individual thrust Some of the thrusts beneath southern England can be segment with the formation of a normal fault(s) above each. mapped from seismic reflection data. Chadwick et al. (1983) Transfer zones are analogous to mid-ocean ridge transform produced a N-S crustal section across the western part of faults, in that they offset the locus of extensional strain in an the WessexBasin which showed a tentative geological opposite sense to the actual sense of ground motion across interpretation to a depth of about 17 km. Whilst this profile the transfer zone (Fig. 9d). illustrated the relationship between Permian and Mesozoic normalfaults and underlying Variscan thrusts, it wasnot Crustal extension, lithospheric extension and isostasy ideally located to showany majorPermian and Mesozoic The models discussed above are a valuable geometric aid to basins. An alternativetrue-scale section, lying some the understanding of basin development, but are unrealistic 20-30 km east of the 1983 profile, is presented in Fig. 8. The becausethey neglect thefundamental implications of deeper parts of the new section are based upon an eastward isostasy.

S N

25’ Fig. 8. Crustal section across Wessex Basin (see Fig. 4), illustrating extensional reactivation of Variscan thrusts.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THEWESSEX BASIN 471

BIRPSdeep seismic reflection profiles indicatethat with its top surface at sea-level is horizontally extended by a offshore around Britain, Permian and Mesozoic sedimentary factor p. The crust is thinned by a factor of llp (by faulting basins are underlain by thinned crust and are in approximate in theupper crust and ductile creep in the lower crust), isostatic equilibrium (Warner, in press). Figure 10 illustrates replacement in the lithostatic section of light crustal material the situation when an isostatically balanced crustal section, by densermantle rocksresulting in isostatic imbalance. Isostatic equilibrium is restored by subsidence of the crust (a) and formation of a sedimentary basin (Fig. lob). If the basin attainsAirey isostatic balance the average thickness of sediments deposited upon the thinned crust is given by:

\ L/ (see Table 1 for parameters) In reality the evolution of a typical intra-continental sedimentary basin is more complex than would be expected with a simple crustal thinning mechanism. Many such basins show at least two distinct phases of subsidence. An initial rifting phase when subsidence is accompaniedby active normal faultingwithin theupper crust is followed by a secondary phase of regional, unfaulted subsidence. Animportant property of thesecondary, unfaulted phase of subsidence is that the rate of subsidence decreases exponentiallywith time in mannera similar tothe progressive subsidence of oceaniclithosphere as it moves away from a spreading ridge, cooling as it ages (Sleep 1971). McKenzie (1978) developed a hypothesis which can explain these observations, involving uniform extensional thinning of the entire lithospheric plate. McKenzie's model assumes an isostatically balanced crust as part of a lithospheric plate having lineara geothermal gradient and overlyingan isothermal asthenosphere. Instantaneousthinning of thelithosphere occurs as a result of horizontal extension that causes its surface area to increase by afactor p. Both crust andlithosphere are thinned by a factor Up, this thinningcausing elevation of the lithosphericisotherms (Fig. 1Oc). The crustal thinning causes an initial isostatically driven,fault controlled subsidence S,. Owing to the buoyancy effect of the elevated . Y isotherms, S, is less thanthe subsidence STotalcaused by thinning only the crust(Equation 11). With time,the elevatedlithospheric isotherms relaxback to their pre-extensionposition, allowing the crustal subsidence to approach STota,.This secondary, thermal relaxation subsidence (&) of is a regional nature (Fig. lOd), characterized byan absence of normalfaulting, and probably accomplished by lithospheric flexure (Watts et al. 1982). Sedimentsdeposited during the phase of thermal relaxation subsidence commonly overlap the margins of the earlier faulted basin, producing a characteristic 'steers-head' profile (Dewey 1982), illustrated in Fig. lla. The initial fault-controlledsubsidence (S,) andthe time dependent thermal relaxation subsidence (SR)canbe predicted from the extension factor p as follows:

J. Fig. 9. (a) Schematic block diagram of two faults offsetby a transfer zone, lying above a reactivated thrust; (b) Offset faults Equation 111 formed above thrust oblique to tension; (c) Offset faults formed above offset thrusts; (d) Sinistral ground movement across dextrally offsetting transferzone.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 472 R. A.CHADWICK

The total subsidence S(t), at time t Ma after extension, can be expressed:

= SI + &R(l) Thetotal subsidence S(m) attainedafter complete relaxation of the lithospheric isotherms being:

= SI + Thisquantity is identical to the subsidencepredicted by simple crustalthinning without perturbation of the lithospheric isotherms (S,,,, in Equation 11). Solution of Equations I11 and IV, using the parameter values inTable 1, allows the subsidencehistory of sediment-starved basins (filled only with sea water) formed byvarying amounts of lithosphericextension to be computed (Fig. llb).

Unconformities A common feature of extensional sedimentary basins is the presence of an unconformity betweenthe sediments deposited during the active rifting stage and the overlying sediments deposited during the thermal relaxation phase of subsidence (Fig. lla and Badley et al. 1984), This has been explained in terms of basin margin uplift caused by lateral heat flow during lithospheric extension, but a much more important effect may lie in the conflicting geometrical and isostatic properties of developing basins. The geometrical constraints upon basins forming by extension of a two layer crust are illustrated in Fig. 12. The brittle upper layer d, km thick is underlain by a ductile layer d, km thick. As the crust is extended (Fig. 12b), basins develop by normal faulting in the upper layer. If cross-sectional area is conserved then the cross-sectional area of the basins can be calculated (Fig. 5d and Equation I): X-sectional area of basins forming by rapid extension = (PL - L)dl Fig. Effect of horizontal extension on a crustal section 10. (PL - L)d, (a) Before extension; (b) Extension without involvement of sub- Average depth of these basins S, = crustal lithosphere gives fault-controlled isostatic subsidence S,,,,,; BL (c) Extension of whole lithosphere elevates isotherms, buoyancy of dl(P - 1) thermal anomaly buffering the fault-controlled subsidence to S,; S, = ~ (a) Isotherms relax slowly back to pre-extension position. Thermal P anomaly decays giving regional subsidence &. However, the average thickness of sedimentdeposited during the rifting phase is ultimately governed by the need to maintain isostatic equilibrium and is given by Equation 111. By inserting the parameters in Table 1 this simplifies to: 6.2(/3 - 1) Table 1. Physical properties S, = for sediment of density 2.0 g cm-3 P Symbol Value Equating these two expressions shows that S, is greater than Thickness of unextended lithosphere a 125 km S, for values of d, greater than 6.2 km. This is probably the casebeneath southern England where the brittle/ductile Thickness of unextended crust 'c 31.2 km

Density of crust (at 0 "C) PC 2.8 g transition is thought to lie at adepth of15-20 km(see Density of mantle (at 0 "C) P* 3.33 g cm-3 above).Therefore basinsdeveloping by a 'thick-skinned' Density of basin infill PS *1.03 g cm-3 extensional mechanism which is accompanied by some form Coefficient of thermal expansion ff 3.28 X 10-soC-' of lithospheric thinning, are likely to have an initial depth Temperature at base of lithosphere TL 1333 "C greater than that required to maintain isostatic equilibrium. Lithospheric thermal time constant r 62.8 Ma It ispossible that isostatic equilibriumis restored by syn-extensionalregional uplift (possibly by lithospheric *Thesubsidence curves of Fig. llb assumethat the basin is flexure) and consequent erosion (Fig. 12c). Erosion will be sediment-starved i.e. filled only with sea water. most severe onthe footwall (upthrown) extensional

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THEWESSEX BASIN 473

(a) -overlap overlap

0 -- 0 --initial subsidence (SI) thermal relaxation subsidence (STR) p=1.05 n.5- 0.5 - p=1 .l 0 p=1.15 -1 .o I 1.5- extension -1.5 pulse 3::::: I I I I I I 50 100 1 1000 50 50 200 250 TIME ELAPSED AFTER INSTANTANEOUS EXTENSION PULSE (MILLIONS OF YEARS)

Fig. 11. (a) Development of a sedimentary basin by initial tault-controlled subsidence (S,) followed by thermal relaxation subsidence (hR),giving ‘steer’s head’ profile;(b) theoretical crustal subsidence in sediment-starved basins formedby various amounts of lithospheric extension, solving EquationsI11 and IV with physical properties of Table 1.

fault-blocks and will continueuntil erosion and ongoing (0) 4 L > thermal relaxation subsidence return the land surface to a 6 depositional environment. BRITTLECRUST dl ISOSTATIC Analysis of subsidence within the Wessex Basin L EOUILIBRIUM The early Permian to end-Cretaceous structural evolution of DUCTILECRUST d2 southernEngland provides a good example of extension Moho related basin subsidence.Figure 13ashows the present configuration of the pre-Permianbasement surface, which although modified by Tertiary inversion (Chadwick 1985c), gives a reasonableimpression of thegeometry of the extensional basins. For convenience the subsidence history of the WessexBasin can be divided intothree parts: (i) Permo-Triassic, (ii) Jurassic to earlyCretaceous and (iii) middle tolate Cretaceous subsidence.Sediment isopachs for these three intervals are illustrated in Figs 13b, c & d. The isopach maps show the thickness of preserved sediment, which is in places considerably thinner than that originally deposited.Figures 14 and 15 illustraterestored

isopach and depth sections across the Wessex Basin. Where ISOSTATIC appropnate, allowancehas been made for erodedstrata, EOUlLlBRlUM and these sections portray original depositional (though not decompacted)sedimentary thicknesses. Noattempt has been made to restore the sections to pre-extension lengths Fig. U. Development of an extensional u~~conforrnity(a) before as major faults are shown. extension; (b) extension by a factor /3 gives basins deeper than The McKenzie model of uniform lithospheric extension required to isostatically balance the lithospheric thinning; has been applied with some success to the Central NorthSea (c) isostatic equilibrium restored by regional uplift and erosion.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 414 R. A. CHADWICK

0 0 0 -0 0 -0

h 13 U

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THE WESSEX BASIN 475

0 to0

0 0

0 0’ m

0 0’ N

n 0 W

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 476 R. A.CHADWICK

SOUTH NORTrl

Wardour Fault Pewsey Fault 0 -1000 2000- 2000

0 oo 1000 l000 2000 2000 3000 3000 4000 4000 5000 5000 A B

Fig. 14. Western Wessex Basin, cross-sections A-B (see Fig. 13). (a) restored Permo-Triassic isopachs;(b) restored Jurassic plus Lower Cretaceous isopachs; (c) restored Middle plus Upper Cretaceous isopachs;(d) depth section restored to end-Cretaceous datum.

Basin, initially by Sclater & Christie (1980) subsequently heat flow and non-uniformlithospheric strain all serve to refinedby Barton & Wood (1984). Theseworkers alterthe relative amounts of rift andthermal relaxation concentratedtheir analyses onthe post-rift thermal subsidence. relaxationphase of subsidence (h),which is well (iii) Onlypart of the subsidencehistory is analysed, developedabove theCentral Graben,and obtained whichincreases theimportance of uncertaintiessuch as lithospheric extension factors consistent with the seismically changes of sea level and errors of stratigraphic control. This determinedcrustal thinning. One advantage of restricting problem is particularly acute in the Wessex Basin, where the analysis to the thermal relaxation phase of subsidence is the thermalrelaxation sediments (especially those of post- absence of lateralvariation in sediment thickness dueto Aptian age) arein places very poorly preserved compared to faulting,enabling reasonable reconstruction of basin the earlier, dominantly rifted sequence. subsidence, even with a limited number of borehole sample In the following discussion the entire subsidence history points. This advantage islargely negated in areas withan of the basin (S, + &) will be analysed in terms of adequate coverage of seismic reflection data whichallows McKenzie's lithosphericextension model. Thisis a stable accurate basinwide thickness determination of both rift and method because the total subsidence measured over a long thermalrelaxation sediments. Moreover, analyseswhich period of time closely approaches the subsidence predicted seek to determine extension factors solely from the post-rift by the simple crustal extension model (Equation II), and is thermalrelaxation subsidence are beset by several relatively insensitive tothe effects of non-uniform or problems: non-instantaneous lithospheric extension. (i) In basins with a multi-stage extensional history it is Figure 16 showssix lithostratigraphical sequences from difficult to unravel the varioussuperimposed rift and the Wessex Basin, fivefrom the deeper graben areas and thermal relaxation phases of subsidence. one froman uplifted fault block (Fig. 13a). The (ii) Finiteperiods of extension(Cochran 1983), lateral stratigraphical sequences are based upon the isopach maps

SOUTH NORTP

Faults ' Portsdown Faults FaultsPlatform Londcn 0 -IOC)0

-2000~ -3000 S'

0 L 1000 l000Ok g (d) 2000 2000 3000 3000 4000 4000 C D

Fig. 15. Eastern Wessex Basin, cross-sections C-D (see Fig. 13). (a) restored Permo-Triassic isopachs;(b) restored Jurassic plus L~~~~ Cretaceous isopachs; (c) restored Middle plus Upper Cretaceous isopachs;(d) depth section restored to end-cretaceous datum,

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS WESSEXBASININ THE 477

of Whittaker (1985) andhave been restored to an episodes of syn-depositional normal faulting to be discerned end-Cretaceous datum byreplacing erodedsediment and the amount of fault movement in agiven interval of according to regional extrapolations anddepth of burial geological time to be estimated. studies (cf. Magara 1976). Seismic reflection data distributed in a representative To derive a crustalsubsidence history from a manner across the WessexBasin wereexamined for sedimentary sequence it is necessary to take account of the faulting. Onehundred and fifty-four normalfaults were following effects: analysed, ranging in throw from about 20 m (at the limit of (i) Sediment compaction. The restored sections of Fig. seismic resolution) togreater than 1OOOm (themajor 16were decompacted by themethod of backstripping basin-controlling faults). Fault movements occurring within (Sclater & Christie 1980), using density/depth relationships specified timeintervals were summed and the relative for argillaceous, arenaceousand carbonate lithologies amount of throw plotted against time (Fig. 19). Episodes of derived from a basinwide borehole study. The sedimentary active normal faulting occurred in Permo-Triassic,early section was thus reconstructed to thicknesses and densities Jurassic,late Jurassic and earlyCretaceous times,giving appropriate to the end of each stratigraphical interval. For relatively rapid,fault-controlled subsidence. By contrast, simplicity it was assumed that no further compaction of the late Permian, late Triassic, middle Jurassic and mid to late pre-Permianbasement took place as a consequence of Cretaceous times were periodsof fault quiescence, when the Permian to Cretaceous sedimentation. rate of subsidencedecreased approximately exponentially (ii) Sediment loading. This effect can be. nullified by with timeand the depositional area increased so that normalizing the thickness anddensity of the sedimentary sediments tended to overlap the underlying faulted sequence sequencedeposited atthe end of eachstratigraphical with ‘steer’s head’geometry. These features suggest that interval toan equivalent depth of seawater using the crustalextension was accompanied by some form of relationship lithospheric thinning (in the sense of elevatedisotherms). The application of amodified McKenzie model, involving - PS) S, = S, (Pm several finite periods of lithosphericextension, may (Pm - ~w) thereforebe of usein discussing mechanisms of basin subsidence. S, = crustalsubsidence in sediment starved basin; S, = sedimentloaded subsidence; ps = averagedensity of sedimentary sequence; pm = density of mantle material and Permo-Triassic subsidence. Crustal extension was prob- p,,, = density of sea water (1.03 g cm-3). ably initiatedat about the beginning of Permiantimes (iii) Depth of waterduring deposition. Permo-Triassic with grabenformation in the western WessexBasin sedimentation within the WessexBasin took place in a (Whittaker 1975). Thick sequences of continental red-beds largely continentalenvironment and was probablylittle were deposited (Rhys et al. 1982) which in the Exeter area affected by water depth. Jurassic and Cretaceous sediments andthe Crediton Trough (Fig. 1) were interbedded with weredeposited in water that was either shallow or of volcanic rocks radiometrically dated at 280 Ma (Miller et al. uncertain depth. Quantification of this effect is difficult and 1962;Knill 1969). Continentalsedimentation persisted the simplifying assumptionthat sedimentation kept pace throughPermian and Triassic times, basin development with subsidence has been adopted. being summarized by the isopach map and sections of Figs (iv) Changes of global sea level. The global sea level in 13b, 14a & 15a. Permo-Triassic times is not well known and, bearing in mind Important syn-depositional faulting was largely restricted to the western parts of the Wessex Basin, where extensional the continental sedimentary environment, any changes have reactivation of the Variscan Front and Wardour thrusts give beendisregarded. Sea level changes in Jurassicand Cretaceous times (Fig. 17) are of uncertain magnitude and birth tothe Pewsey, Wardourand Winterborne Kingston havealso beenignored. The bestdocumented change faults (Figs 8 & 14a). In this area very coarse-grained basal Permian deposits (conglomerates and breccias) are overlain occurred in late Cretaceous times when the sea level was by sandstones which in turn pass upwards into siltstones and probably between 107 m (Watts & Steckler 1979) and 350 m mudstones of late Permian age(Smith et al. 1974). These (Hallam 1984) higher than at present. However, any ‘extra’ are succeeded by coarsely arenaceous Lower Triassic strata sediment deposited during this period was probably eroded of the Sherwood Sandstone Group which pass upwards into at the end of the Maastrichtian, when the sea fell to a level Upper Triassic siltstones andmudstones of the Mercia similar to that at thebeginning of the Jurassic. It is therefore Mudstone Group (Warrington et al. 1980). Both the Upper hereconsidered that the total preserved Jurassic and Permianand the Upper Triassicargillaceous sequences Cretaceoussedimentary sequence is consistentwith a overlap the margins of the underlying arenaceousstrata roughly constant global sea level. (Audley-Charles 1970)with ‘steer’s head’geometry Application of the above corrections to the stratigraphi- (Holloway1985). This is clearly seen at outcrop in the cal sequences in Fig. 16 gives the crustal subsidence which Mendip Hills (Fig. 1) where the Mercia Mudstone Group wouldhave occurred at these localities hadthey been lapsagainst deformedCarboniferous basement (Green & covered only by sea water (Fig. 18). Welch1965) and in the subsurfaceon seismic reflection profiles (Holloway & Chadwick 1984). A poly-phase extension model for the Wessex Basin In the adjacent Worcester Basin (Fig. 4b) a comparable It is considered that the model most capable of explaining stratigraphic sequence, together with seismic reflection data the Permian to Cretaceous evolution of the Wessex Basin have been used to support a two stage extension model for requires several phases of crustal (possibly accompanied by the Permo-Triassic (Chadwick198%). The coarser-grained lithospheric)extension. Outcrop (e.g. Jenkyns & Senior sediments (of presumedearly Permian and early Triassic 1977) and seismic reflection dataenable several discrete ages)were interpretedto thicken across faultsand were

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 478 R. A. CHADWICK

l I l I I I 0 0 0 0 0 0 0 0 0 0 0 m 5: m 5:N m m 1 I I 1 I

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THE WESSEX BASIN 479

+above present below present -+ reactivatedPermo-Triassic features, but faulting along l 1 I previously dormant lines of weakness caused considerable l - 60 changes in patterns of deposition.Along the northern

Maastrlchtlan - 70 margin of the Wessex Basin movement on the Pewsey faults 4 caused Lower Jurassicsediments to thicken southwards - 80 intothe Pewsey Basin.This thickening wasless marked than in Permo-Triassic times, but the width of the Pewsey - 90 Fault zone increased as new faults developed to the south -100 (Fig. 14b). The LondonPlatform faults which formthe easterly continuation of the Pewsey faults, may have been -110 c C dormant in Permo-Triassic times, but by the early Jurassic 0 -120 t startedto control development of the Weald Basin (Fig. Q 15b). Similarly, the Portland-Wight faults started to move -130 E in early Jurassic times as subsidence in the Channel Basin L0 -140 9” accelerated (Figs 14b & 15b). Conversely, the Bere Regis Oxfordlan andCranborne faults which defined.a prominent Permo- -150 Triassic graben (Fig. 14a) were considerably less active in

-160 Jurassicand early Cretaceous times. It is possible that extensionalreactivation of the previously dormant -170 Portland-Wightthrust channelled strain away fromthe

-180 faults in the hanging-wall block above the Wardour thrust (Fig. 8). Pltensbachlan -190 Movement on themajor basin-controlling faults was Slnemurlan accompanied by smallerscale syn-depositional normal - 200 Henanglan___ faulting(Jenkyns & Senior 1977). Seismic reflection and borehole data suggest that much of the early Jurassic normal faultingtook place in Hettangianand Sinemurian times, Fig. 17. Generalized variation in global sea level, modified after Vail et al. (1977) and Hallam (1984). with markedthickness variations inthe Lower Lias (Whittaker 1975; Holloway & Chadwick 1984). Middle and Upper Lias beds (Pliensbachian to Toarcian) have a more uniform thicknessas faulting died out and subsidence probablydeposited aninactive extensional tectonic became increasingly controlled by thermalrelaxation. environment, with fault-scarp or rift valley topography. The ‘steer’s head’ onlap of the Lias resulted in the transgression finer-grainedstrata (of presumedlate Permian and late of successively higher beds across the previously emergent Triassic ages) were considered to have been associated with London Platform (Donovan et al. 1982). post-extension thermal relaxation subsidence and a subdued In Middle Jurassictimes (Aalenian toCallovian), topography. sediments were laid down in a shallow water environment, It is suggestedthat similara tectonic history, with dominantly in a carbonate facies. Thickness changes have extension in earlyPermian and early Triassic times (Fig. beennoted (e.g. Penn 1982) butthe amount of crustal 18a,b, c), canaccount for the lithostratigraphy of the extension was too small to be resolved in the broad analysis western Wessex Basin. presentedhere. Subsidence was rather slow and still In the more easterly partsof the Wessex Basin (Figs 15a, dominated by thermal relaxation effects as the width of the 18d & e) there is littleevidence of significant syn- Wessex Basin continuedto increase. In theMendip Hills depositional normal faulting and Permo-Triassic strata are (Fig. l), the Inferior Oolite (Bajocian) overlapsTriassic and thinor absent. In theeastern Channel Basin athin, Lower Jurassic strata to rest upon Variscan basement rocks predominantlyargillaceous sequence of presumedlate (Green & Welch1965). Seismic reflection dataindicate Triassic ’ age is present.This may represent flexural similar ‘steer’s head’ onlap against the southernflanks of the subsidence peripheral to active lithospheric extension to the London Platform (Smalley & Westbrook 1982). west. Similar upper Triassic strata of the Mercia Mudstone By Oxfordiantimes regimea of regional, flexural Group which overlapeastwards into the Weald Basin subsidencehad become established with littleor no (Holloway 1985), may have a related cause. syn-depositionalfaulting (Fig. 19). The Oxford Clay, a uniform,marine argillaceous unit, shows only gradual Jurassic and early Cretaceous subsidence (Hettungian to thicknesschanges (Holloway 1985). Itoverlapped earlier Barremian). Development of the Wessex Basin in Jurassic Permo-Triassicand Jurassic formations and overstepped and early Cretaceous times is summarized in Figs 13c, 14b & northwards across the contemporary margin of the Wessex 15b. There was probably a renewal of crustal extension in Basin onto the London Platform. It provides a good very lateTriassic (Fig. 6) and early Jurassic times which example of sedimentation controlled by thermal relaxation coincided with a transition from a dominantly continental subsidence (Figs 18a-e). Towards the end of Oxfordian and environmentto open marine conditions. Lower Jurassic during Kimmeridgian times, renewed normal faulting (Fig. sedimentsform thicka sequence of mudstonesand 19) led toaccelerated subsidence, especially south of the interbedded limestones (Hallam1975) known as the Lias (of major faults bounding the Weald and Channel basins (Fig. Hettangianto Toarcian age). Lithologically the Lias is 18a, d & e). The Jurassic trend towards an increased areaof laterallyrather uniform, but its thicknessvaries rapidly deposition continued and it is probable that the Kimmeridge across syn-depositional normal faults. Many of these faults Clay,a thick argillaceousdeposit, overlapped all earlier

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 480 CHADWICK R. A.

0 CUMULATIVE p = 1.29 200

400

600

800

1000

1200

1400 2 0 t.’ +F+ CUMULATIVE p = 1.26 -200 E -400 8 -600 z n -800 - v) -1000 m 3 -1200 v) J 0-1400 2 DORSET BASIN v) 0 l-F-i +F-+ +F+ 3 CUMULATIVE p = 1.24 U -200 0

- 400 - 600 - 800 - 1000 a - 1200

-1400 PEWSEY BASIN 111111i1r-r- 250 200 1 50 100 Ma before present Fe. 18. Crustal subsidence histories (corrected for sediment compaction and loading)of six locations within Wessex Basin, with theoretical

Mesozoic formations, to onlap against and eventually cover normal faulting (Fig. 19)led to the accumulation of thick the remainingemergent Palaeozoic massifs of southern sequences of terrigeneous clastic sediments, the Wealden England (Chadwick 198%). Beds(Allen 1981). Outside of the basinal areas, Thearea of depositionprobably reached its Jurassic contemporaneous isostatic recovery (Fig. 12) led to maximum inlate Kimmeridgian times. The Portlandian complementary uplift of thesurrounding regions. This, stage saw the initiation of a marked fall in global sea level combined with a low global sea level, triggered an important (Fig. 17), which by the beginning of the Cretaceous period, period of erosion in the WessexBasin which lasted from resulted in aconsiderably reduced area of deposition and Ryazanian to Barremian times and resulted in the so-called re-emergence of the London Platform. In early Cretaceous ‘late-CimmerianUnconformity’. This complexdepositional timesdeposition became restricted tothe Weald and hiatus goodais example of an unconformitywhich Channel basins, where crustal extension with considerable developed in an extensional tectonic environment and is a

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THE WESSEX BASIN 481

-400 0

-800 D- cn

-1400 4 WEALD. BASIN 6 h'+ CUMULATIVE p-1.13 a -200 0 - 400 (f 1 - p=' .02 - 600 800 - 1 000 /-F-----( episode of normal faulting ------late-Cimmerianeroslon 0 observedsubsidence CRANBORNE-FORDINGBRIDGE HIGH 111,1111 11ll,ll,~,"' 250 200 150 1 00 Ma before present

subsidence curves producedby episodes of uniform lithospheric extension. Local Airey isostasy assumed.

ubiquitousfeature of the NW Europeancontinental shelf and some expansion of the depositional regime. This latter (see Fyfe et al. 1981; Rawson & Riley 1982). trend is most evident in east Kent where Upper Wealden Towards the end of the early Cretaceous, in Hauterivian beds transgressed northwards from the Weald Basin on to andBarremian times, the intensity of normal faulting previously emergent parts of the London Platform (Fig. 21). decreasedmarkedly, as did the demarcationbetween the The increasingly marine aspect of Barremian sediments in rapidlysubsiding basins andthe eroded 'highs'. The the WessexBasin (Bennison & Wright1969) gave a establishment of regional,thermal relaxation subsidence, foretaste of the imminent Aptian and Albian transgressions. coupled with progressive erosion of exposed massifs, led to Theextent of late-Cimmerianerosion insouthern a considerablereduction of topographic relief. This, England can be gauged from Fig. 21, which illustrates the together witha steady rise in global sea level (Fig. 17) palaeogeology at the end of Barremian times, immediately caused an upwards fining of sedimentsin the Weald and prior to deposition of the Lower Greensand. In the Weald Channel basins(Gallois 1965; Melville & Freshney 1982) andChannel basins virtually conformablesequences of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 482 CHADWICK R. A.

- RELATIVE THROW Ryazanian to Barremian age werepreserved. Elsewhere contemporaneous erosion had cut deeply into older strata. It o 20 40 60 ao loo % I 11 I I was most severe over the London Platform where Middle

Cenornanian-Maastrlchtlan and Upper Jurassic rocks were eroded to reveal Palaeozoic =U2 &U basement, andon the Cranborne-Fordingbridge High d Apttan-Albtan whereover 400 m of Upper Jurassic sedimentswere 8 removed (Chadwick 1985b), to exposeMiddle Jurassic 2 Portlandlan-Earrernlan strata. In the westernparts of the WessexBasin early &E Cretaceouserosion exposed LowerJurassic andPermo- U 2 Klrnmerldglan $L, Triassic rocks. The late-Cimmerian unconformityisespecially well- $ E Bathonlan-Oxfordtan 7YI documentedbecause of the lithologically variable and 3 stratigraphically well-defined rocks which lie beneath it and I? Hettangtan-Baloclan the generally unfaulted overlying strata.Other unconfor- ...... 1 mities probably accompanied earlier extensional episodes in .:' I PERMO-TRIASSIC 1.'. . .:;l the Wessex Basin, mostnotably in Permo-Triassictimes. . ..., . :: ' 1 . . . . However these were cut by later faults, and their situation .. within a relatively homogeneous sequence with diachronous Fig. 19. Variation of normal fault activity withtime, deduced from facies, renders them much less suitablefor detection and seismic reflection data (not corrected for sediment compaction). description.

-0

-2

SOUTY NORTH . rnargln Eastn faults

Fig. U). Section across northern margin of Wessex Basin, (a) seismic reflection profiles (in part courtesy of RTZ Oil & Gas Ltd.); (b) simplified geological interpretation.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THE WESSEX BASIN 483

Fig. 21. Palaeogeological map of southern England atend of Barremian times

Mid LateCretaceousto subsidence (Aptian to that subsidence models which analyse the entire preserved Maastrichtian). In early Aptian times the cessation of active Jurassic and Cretaceous sequences can reasonably disregard crustal extension coincided with the end of late-Cimmerian the effects of interim sea level changes (see above). erosion and heralded a new tectonic regime characterizedby regional thermal relaxation subsidence, a virtual absence of Estimation of Permian to Cretaceous extensionfactors normal faulting (Figs 14, 15, 19) and an enormous increase Figure18 shows that if thegrabens of the Wessex Basin in thearea of deposition.Aptian and Albian strata were in local Airey isostaticequilibrium they would be transgressedacross the late-Cimmerian erosion surface, underlain by crust which had been thinned by cumulative restingunconformably upon the earlier faulted sequences Permian toCretaceous extension factors of between1.20 andoverlapping the faulted basin margins to restupon and 1.29. The horsts would require less crustal thinning to Palaeozoic rocks of the London Platform and SW England maintain Airey isostasy, with an extension factor of about (Fig.21). Thoseareas which hadundergone considerable 1.13 for the Cranborne-Fordingbridge High. In reality such recent extension (the Weald and Channel basins), continued rapid local variations in crustal thickness are unlikely. It is to subside rapidly as sediments draped and thickened across probable that Airey isostasy was attained by crustal thinning earlier faults. In the western part of the Wessex Basin minor on a more regional scale, of the order of a complete basin thickness changes inMid to Upper Cretaceous strata had width (100-200 km),individual grabens and horsts (10- NW-SE trends(Drummond 1970). These were probably 40 km across)remaining respectively under and over caused by movements on steepVariscan strike-slip faults compensated (cf. Fig. lob). (see Fig. 4a and below). If the sediment fill of the Wessex Basin is isostatically By the beginning of the Cenomanian, continuing thermal compensated by crustal thinning, the regional extensionfac- relaxation subsidence and a very high global sea level (Fig. tor can be calculated by integrating the sediment thickness 17) led to submergence of the British landmass, cut-off of over the entire basin. Figure 22a illustrates a simplified map temgeneoussediment supply and deposition of the of Permian toend-Cretaceous isopachs. Correction for remarkably pure carbonate sequence of the Chalk. decompactionand sediment loading gives theequivalent Mid to late Cretaceous ratesof sedimentation were quite depths of sea water in a sediment starved basin (Fig. 22b). high, in places more than 500 m of sediment were deposited In Fig. 22c the Wessex Basin is divided arbitrarily into four in less than 45 Ma (Fig. 13d). This was possible because of sectors.A cumulative extension factor was computedfor compactionwithin the underlying sediments. When this each sector, sufficient to produce the required Permian to effect, and that of sediment loading are corrected for (see Cretaceoussubsidence (assuming extensionalepisodes of above), the rates of tectonically driven basement subsidence similar durationto those in Fig.18). The cumulative are found to be very low (Fig. 18). extensionfactor decreases from about p = 1.17 in the Towards theend of theCretaceous period, in western part of the Wessex Basin to about = 1.13 in the Maastrichtian times, a rapid fall of global sea level (Fig. 17), east, with a basinwide extension factor of = 1.147. triggeredmajora erosional episode which strippeda substantial amount of Chalk off southern England. Erosion cut to the deepest stratigraphical levels where the Chalk was Nature of extensional stress fieLa3 depositionally thin; at least six zones of Upper Chalk were Previous workers have discussed the nature of the tectonic removed from the London Platform, but successively higher stress fields responsiblefor the Permian to Cretaceous zones were preservedtowards the centre of the Wessex development of the Wessex Basin.Stoneley (1982) Basin (Curry 1965). The end-Cretaceous regression restored considered that the major normal faults which trend roughly theglobal sea level toa height similar to that at the E-W were formed by a N-S tensionalstress field. beginning of the Jurassic period. A fortunate side effect is Drummond (1970) emphasized the importance of strike-slip

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 484 R. A.CHADWICK

40 50 60 I 1

1 I 1

STIPPLED AREA p = 1.147

230 40 50 60

Fig. 22. (a) Restored Permian to Cretaceous isopachs (depth to Variscan basement at end of Cretaceous period), contours in metres X 100; (b) Equivalent crustal subsidence in sediment-starvedbasin (metres X 100); (c) Crustal extension factors.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 EXTENSIONTECTONICS IN THE WESSEX BASIN 485

movement on the NW-SE faults of the western Wessex Basin and suggested that major E-W sinistral shear couples were a dominatingfactor in basin evolution. It is here considered that, with some refinement, Stoneley’s model is the more appropriate. Figure 23 illustrates effects of three pure-shearstress fields acting upon asystem of crustal fractures schematically similar to those beneath the Wessex Basin. A N-S tensional stress field (Fig. 23a) would impart dip-slip normal movement to the E-W fractures (Variscan thrusts) and sinistral transtension to the NW-SE fractures (Variscan wrench faults). A NW-SE tensional stress field (Fig. 23b) would impart sinistral transtension to the E-W fracturesbut would notreactivate the NW-SE fractures A (except forminor strike-slip transfermovements). A NE-SW tensionalstress field (Fig. 23c)would impart dextraltranstension tothe E-W fracturesand dip-slip motion to the NW-SE fractures. Within the Wessex Basin fault-controlled subsidence in Permian to early Cretaceous times was dominated by reactivation of E-W basement structures, with relatively minornormal movement and some strike-slip motion along the NW-SE fractures. These features probably rule out NE-SW tension and were more likely formed by Permian to early Cretaceous stress fields R havinga cumulative effect akin to a tensionalstress field oriented between N-S and E-W. Figure 24 shows crustal

Fig. 24. Crustal extension factors along 2-D transects, stippled areas denote basinal depocentres; (a) N-S transects; (b) NW-SE transects; (c) NE-SW transects.

extension factors computed from the amount of subsidence along 2-D transects. The NW-SE transects give fairly consistent /3 values of about 1.15, the N-S transects give /3 values from 1.13 to 1.17 and the NE-SW transects show a marked easterly decrease in /3 values from 1.18 to 1.11. This is because each NW-SE transect tends to sample just one depocentre,whereas some NE-SW transectssample two depocentres, others sample none. The offshore areas have not been considered in detail in this paper, and basinwide balance of strain on allNW-SE sections cannotbe demonstrated. The observed geometry however, isconsis- tent with a NW-SE tensional stress field giving pure shear,

(C) anisotropicextension with depocentres offset and com- f partmentalized by NW-SE oriented transfer faults (cf. Bally 1982). Furthersupport for NW-SE tension,at least in Permo-Triassic times, is afforded by normal fault patterns in the adjacent Worcester Basin (Chadwick 198%). In mid to late Cretaceous times subsldencewas driven by vertical isostatic forces associatedwith thermal relaxation. The Variscanfractures were not significantly reactivated, except for minoraccommodation movements which // transtenslonal rnotlon preferentially utilized the steeply dipping wrench faults and \\ strlke - sl~ptransfer rnotlor resulted in local NW-SE thickness trends. Finally, implicit in Fig. 23 is the important principle that Fig. 23. Effect of pure shear strcss fields on pre-existing (Variscan)l the gross morpnology of sedimentary basins is governed by crustal fractures beneath Wessex Basin (schematic), (a) N-S the geometry of pre-existing crustal weaknesses. The stress tension; (b) NW-SE tension; (c) NE-SW tension. field can influence the orientation of small,intra-basin

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 486 R. A. CHADWICK

faults,but usually merelydetermines the direction of sonic velocity of2.9 km/s(borehole sonic logs) and was movement on the major basin margin faults. underlain by crust thinned by an estimated factor of 1.147 (Fig. 25a). The seismic expression of this model (Fig. 25b) demonstratesthat crustal thinning beneath the basin is Discussion compensated by 'pull-down' duetothe lowvelocity The role of upper crustal extension in the development of sedirnents to give a roughly constant Moho "IT (cf. the Wessex Basin is demonstrated by the geometry of its Warner in press). Beneath the English Channel, the Moho, constituentstructures. Subsidence analysis assuming re- as defined by the base of layered lower crust (Barton et al. gional Airey isostasygives a crustal extension factorof 1.147 1984),is most clearly resolved on seismic lineSWAT 10 for the entire basin. The amount of extension which can be (BIRPS & ECORS). The southernhalf of this line was shot directly attributed to normal faulting within the upper crust above very shallow Variscan basement, whereasits northern is uncertain, because the top of Variscan basement can only portion is situated above the offshore continuation of the locally beresolved by seismic reflectiondata. Variscan Wessex Basin (Fig.25c). The Moho lies ata roughly basement isvisible in the vicinity of the Pewsey faults, constant Tw?T of about 10.5 S, though with local variations where local extension factors up to 1.25 can be discerned. due to incomplete compensation of horst and graben relief. Away from the major fault zones the amount of extension The seismic line is thereforeremarkably similar tothe seenon normal faults is lowerthan extension factors seismic model and strongly supports the concept of regional predictedfrom the subsidence. A similar discrepancy is crustal thinning beneath these basins. foundin the central North SeaBasin, where extension The relationship between extension in the upper crust, estimated from subsidence plots (Barton & Wood 1984), is thelower crust and thesub-crustal lithosphere is considerablygreater than is evidenton the seismically problematical. Analysis of anentire basin subsidence resolved faults. This may in part be due to inaccuracies of history, spanning some 230 Ma, together with deep seismic the physical properties assumed for the subsidence analyses, reflection data,enables reasonable estimation of crustal butprobably largely resultsfrom limitations of seismic extension, but distribution of strain within the sub-crustal resolution.Abetter independent estimate of crustal lithosphereremains ill-understood. In the Wessex Basin thinningcan be had from deep seismic reflectionand four episodes of crustal extension were each followed by a refractiondata. Beneath the onshore Wessex Basin the period of moreregional subsidence characterized by Mohohas not yetbeen satisfactorily imaged by normal- stratigraphical onlap. Whilst onlap cannot uniquely diagnose incidence reflection profiling (cf. Fig. 2a), but beneath the thermal perturbation of the lithosphere (it can result from English Channelthe BIRPS SWAT survey gives good simple massif erosion), its presence in both subaerial and resolution of crustal structure. At the end of the Cretaceous submarine sequences, together with the observed exponen- period the Wessex Basin was filled with sediments having an tially decreasing rates of subsidence, is strongly suggestive of averagethickness of about 1.9 km (Fig.22a), an average some form of lithospheric thinning, though not necessarily

2 kms-' (a) 9

27 2 km km 31.2

Lu Moho-

Fig. 25. (a) Schematic velocity/depth model of the Wessex Basin; (b) Seismic expression of (a); (c) Line drawing of seismic reflection profile SWAT 10 (courtesy of Mike Cheadle BIRPS).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 E X T E N SI O N TECTONICS IN THE WESSEXBASINEXTENSIONTHE TECTONICS IN 487

of auniform nature.The mantlelithosphere may have BALLY, A. W.1982. Musings over sedimentary basin evolution.Philosophical extended by fractureinthe relatively brittle shallow Transactions of the Royal Society of London, D,325-38. BARTON,P., MATI~-IEWS, HALL,D., J. & WARNER,M. 1984. Moho beneath sub-crustal zone (Fig. 3) and by ductile creepat greater theNorth Sea compared on normal incidence and wide-angle seismic depths. Isotherm elevation may have been accomplished not records. Nature, 308, 55-6. by simple extension, but by dyke intrusion as envisaged by BARTON,P. & Wood, R. 1984. Tectonic evolution of the North Sea basin: Royden et al. (1980). Lithospheric extension elsewhere may crustalstretching and subsidence. Geophysical Journal of the Royal also havehad a significant influence. Forexample, Astronomical Society, 7913, 987-1022. BENNISON,G. M. & WRIGHT,A. E. 1969. The Geological History of The lithospheric thinning beneath the North Sea was probably Bribh Isles. Edward Amold, London. responsible formost of the Permian to Cretaceous BIRPS & ECORS 1986. Deep seismic reflection profiling between England, subsidence of eastern England (Holloway 1985) and almost France and Ireland. Journal of the Geological Society,London, 143, certainlyinfluenced subsidence in the WessexBasin. 45-52. BOW, M. H. P. 1971. The Interior of the Earth. Edward Amold, London. Similarly, the peripheral effects of major rifting in the BREWER,J. A., MATTHEWS,D. H., WARNER, M.R., HALL,J., SMYTHE,D. K. western English Channel (cf. Smith1985) may also have & WHITTINGTON,J. R. 1983. BIRPS deep seismic reflection studiesof the been significant. British Caledonides. Nature, 305, 206-10. The role of eustasy in the evolution of the Wessex Basin, CHADWICK,R. A. 19850.Seismic reflection investigations into the stratigraphy and structural evolution of the Worcester Basin. Journal of particularly in the development of onlapping stratigraphical the Geological Society, London, 142, 187-202. sequences, isdiscussed by Lake et al. (1984). It is here - 19856. Upper Jurassic to Cretaceous semimentation and subsidence. In considered that global rises of sea level probably contributed WHITTAKER,A. (ed.) Atlas of onshore sedimentary basins in England and to stratigraphical onlap, especially in late Jurassic and late Wales: Post-Carboniferour Tectonics and Stratigraphy. Blackie, Glasgow. - 1985c. Cenozoic sedimentation, subsidence and tectonic inversion. In: Cretaceoustimes, but additional tectonic (thermal relaxa- WHITTAKER,A. (ed.) Atlas of Onshore Sedimentary Basins in England tion) subsidence helps to explain the following features: and Wales: Post-Carboniferous Tectonics and Stratigraphy. Blackie, (i)onlap of Upper Permianand Upper Triassic Glasgow. continental sequences; -, KENOLTY, N. & WHITTAKER,A. 1983. Crustalstructure beneath (ii) estimated global sea level rise is insufficient to cause southern England from deep seismic reflection profiles. Journal of the Geological Society, London, 140, 893-911. the observed lower Jurassiconlap (Donovan et al. 1982), CHEN,W. P. & MOLNAR,P. 1983.Focal depths of intracontinentaland even when sedimentary loading is taken into account; intraplateearthquakes and their implications for thethermal and (iii) more than 500 m of post-extension sediments were mechanicalproperties of thelithosphere. Journal of Geophysical deposited between Aptian and Maastrichtian times, plus up Research, 88, 4183-214. COCHRAN,J. R. 1983. Effects of finite rifting times on the development of to 600 m of Tertiary strata, despite no net rise of global sea sedimentary basins. Earth and Planetary Science Letters, 66, 289-302. level; CURRY,D. 1965. The Palaeogene beds of southeast England. Proceedings of (iv) rate of post-extensionsubsidence decreases ap- the Geologists’ Association, 16, 151-74. proximately exponentially with time. DAHLSTROM,C. D. A. 1970. Structural geology in the eastern margin of the Indeed,Watts (1982) considersthat the magnitude of Canadian Rocky Mountains. Bulletin of Canadian Petroleum Geology, 18/3,332-406. global sea level variations has been seriously overestimated. DEARMAN,W. R. 1963.Wrench-faulting in Cornwall and south Devon. He suggests that many onlappingsequences previously Proceedings of the Geofogists‘ Association, 14, 265-87. interpreted as signifying rises of globalsea level,were DEWEY,J. F. 1982. Platetectonics and the evolution of theBritish Isles. laid downduring periods of post-rift,thermal relaxation Journal of the Geological Society, London, M,371-412. DONOVAN,D. T., HORWN,A. & IVIMEY-COOK,H.C.1979. The subsidence which occurred roughlysynchronously world- transgression of the Lower Lias over the northern flank of the London wide. Thus it is exceedingly difficult to separate the relative Platform. Journal of the Geological Society, London, l36, 165-73. contributions of eustasy and tectonics in a basin subsidence DRUMMON~,P. V. 0. 1970. TheMid-Dorset Swell. Evidence of history. Albian-CenomanianMovements Wessex.in Proceedings of the Geologists Association, 81,679-714. EYIDOGAN,H. & JACKSON,J. 1985. A seismological study of normal faulting Thanks are due to many colleagues within BGS who constructively in the Demirci, Alasehir and Gediz earthquakes of 1%9-70 in western criticizedearly versions of themanuscript. Permission to publish Turkey: implications for the nature and geometry of deformation in the seismic reflection data was kindly given by RTZ Oil and Gas Ltd continental crust.Geophysical Journal of the Royal Astronomical Society, 81, 569-607. and by BPExploration Ltd. The seismic data for SWAT were FUCHS,K. 1%9. On the properties of deep crustal reflectors. Zeitschrifrfir provided by BIRPS(British Institutions Reflection Profiling Geophysik, 35, 133-49. Syndicate) under the authority of the Deep Geology Committee of FYFE, J. A., A~~orrs,I. & CROSBY,A. 1981. Thesubcrop of the the Natural Environment Research Council and by ECORS (Etude mid-Mesozoicunconformity in the UK area. In: ILLING,L. V. & Continentale et OcCanique par Reflexion et Refraction Sismique), HOBSON,G. D. (eds) Petroleum Geology of the Continental Shelf of an association of Institut FranGais du PCtrole, Institut d’Astronomie North-west Europe, Heyden, London, 236-44. GALLOIS,R. W.1965. British Regional Geology: The Wealden District. Her e t deet GCophysique,Elf-Aquitaine Centreand National Majesty’s Stationery Office. d’Exploitation des Oceans, whom I acknowledge with thanks. GIBBS,A. 1984. Structural evolution of extensional basin margins.Journal of This work is published with permission of the Director, British the Geological Society, London, 141, 609-20. Geological Survey. GREEN, G.W. & WELCH,F. B. A. 1965. The Geology of the Country around Wellr. Memoir of the Geological Survey of Great Britain Sheet 280. HALE,L. D. & THOMSON,G. A. 1982. The seismic reflection character of the continental Mohorovicic Discontinuity. Journal of Geophysical Research, References m,425-35. HALLAM,A. 1975. Jurassic Environments. Cambridge University Press. ALLEN,P. 1981. Pursuit of Wealdenmodels. Journal of the Geological - 1984. Pre-Quartenary sea level changes. Annual Review of Earth and Society, London, 138, 375-405. Planetary Sciences, L?, 205-43. AUDLEY-CHARLES,M. G. 1970. Triassic palaeogeography of the British Isles. HOLLOWAY,S. 1985.Permian to Upper Jurassic. In: WHITTAKER,A. (ed.) Quarterly Journal of the Geological Society, London, 126, 49-90. Atlas of Onshore Sedimentary Basins in England and Wales: BADLEY,M. E., EGEBERG, T.& NIPEN,0. 1984. Development of rift basins Post-Carboniferous Tectonics and Stratigraphy. Blackie, Glasgow. illustrated by the structural evolution of the Osberg Feature, Block 30% - & CHADWICK,R. A. 1984. The IGSBruron Borehole (Somerset, OffshoreNorway. Journal of the GeologicalSociety, London, 141, England)and its regionalstructural significance. Proceedings of the 621-7. Geologists’ Association, 95, 165-74.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021 488 CHADWICK R. A.

JACKSON,J. A. & MCKENZIE,D. P. 1983. Thegeometrical evolution of Sea Basin. Journal of Geophysical Research, 85, 3711-39. normal fault systems. Journal of Structural Geology, 5, 471-82. SILISON,R. 1983. Continentalfault structure and the shallow earthquake JENKYNS,H. C. & SENIOR,J. R. 1977. A Liassic palaeofaultfrom Dorset. source. Journal of the Geological Society, London, 140, 741-67. Geological Magazine, 114, 47-52. SLEEP,N. H. 1971. Thermal effects of the formation of Atlantic continental KENOLTY, N., CHADWICK,R. A., BLUNDELL,D. J. & BACON, M.1981. Deep margins by continentalbreakup. Geophysical Journal of the Royal seismic reflection survey over the Variscan Front of southern England. Astronomical Society, 24, 325-50. Nature, 293, 451-3. SMALLEY,S. & WESTBROOK,G. K. 1982.Geophysical evidence concerning KENT, P. E. 1949. A structurecontour map of thesurface of theburied the southern boundary of the London Platform beneath the Hog’s Back, pre-Permian rocks of England and Wales. Proceedings of the Geologists’ Surrey. Journal of the Geological Society, London, W9, 139-46. Association, 60. 87-104. SMITH,D. B., BRUNSTROM,R. G. W., MANNING,P. I., SIMPSON,S. & KNILL,D. C. 1%9. ThePermian igneous rocks of Devon. Bulletin ofthe SHOITON,F. W. 1974. A correlation of Permian rocks in the British Isles. Geological Suruey of Great Britain, 29, 115-38. Special Report of the Geological Society of London, 5. LAKE,S. D., KARNER, D. G. & DEWEY,J. F. 1984. The flexural development SMITH,N. 1985. Contours on thetop of theRe-Permian surface of the of theonshore WessexBasin inSouthern England. Continental United Kingdom (south). British Geological Suruey 250th Anniversary Extensional Tectonics Conference. University of Durham. Publication. LEVERIDGE,B. E.,HOLDER, M. T. & DAY,G. A. 1984. Thrustnappe SMITHSON,S. B. & BROWN,S. K. 1977. A model for the lower continental tectonics in the Devonian of southCornwall and the western English crust. Earth and Planetary Science Leners, 35, 134-44. Channel. In: HWN, D. H. W. & SANDERSON,D. J. (eds). Variscan STONELEY, 1982.R. The Structural development of the Wessex Basin. Journal Tectonics of the North Atlantic Region. Blackwell. of the Geological Society, London, W9, 545-52. MAGARA,K. 1976.Thickness of removed sedimentary rocks, palaeopore VAIL,P. R., MITCHUM,R. M., TOOD,R. G., WIDMIER,J. M., THOMPSON,111, pressureand palaeotemperature, southwestern part ofWest Canada S., SANGREE,J. B.,BUBB, J. N. & HATFIELD,W. G. 1977.Seismic Basin. Bulletin of the American Association of Petroleum Geologkrs, 60, stratigraphy and global changes of sea-level. In: PAYTON,C. E. (ed.), 554-65. Seismic Stratigraphy, Application to Hydrocarbon Exploration, American MATIMEWS,D. & HIRN, A.1984. Crustal thickening in the Himalayas and the Association of Petroleum Geologists’ Memoir, 26, 42-212. Caledonides. Nature, 308, 497-8. WARNER, M. R. (in press). The Moho, isostasy and deep seismic reflections. MCKENZIE,D. P. 1978. Some remarks on the development of sedimentary Geophysical Journal of the Royal Astronomical Society. basins. Earth and Planetary Science Letters, 40, 25-32. WARRINGTON,G., AUDLEY-CHARLES,M. G., ELLIOTT, R.E., EVANS,W. B., MELVILLE, R. V. & FRESHNEY,E. C. 1982. British Regional Geology: IVIMEY-COOK,H. C., KENT,P. E., ROBINSON,P. L., SHOTTON,F. W. & The Hampshire Basin and adjoining areas. HerMajesty’s Stationery TAYLOR,F. M. 1980. A correlation of Triassic rocks in the British Isles. Office. Special Report of the Geological Society of London, 13. MEVIEREU,R. F. & 010,S. B. 1981. The scattering of seismic waves through a WATTS,A. B. 1982. Tectonic subsidence, flexure and global changes in sea crust and upper mantlewith random lateral and vertical inhomogeneities. level. Nature, 297, 469-74. Physics of the Earth and Planetary Interiors, 26,233-240. -, KARNER, G. D. & STECKLER,M. S. 1982. Lithosphere flexure and the MILLER,J. A., SHIBATA,K. & MUNRO,M. 1962. The potassium-argon age of evolution of sedimentarybasins. In: KENT, P. E., BOTT,M. H. P., the lava at Killerton Park, near Exeter. Geophysical Journal, 6, 394-6. MCKENZIE,D. P. & WILLIAMS,C. A. (eds) The Evolution of Sedimentary ODM,G. S., CURRY,D., GALE,N. H. & KENNEDY, W. J. 1982. The Basins. Philosophical Transactions ofthe Royal Society of London, Phanerozoic time-scale in 1981. In: ODM, G. S. (ed.) Numerical Dating A305, 249-81. in Stratigraphy Q), 957-60, John Wiley & Sons. - & STECKLER,M. S. 1979.Subsidence and eustasy at the continental PEW, I. E. 1982.Middle Jurassic stratigraphy and correlation of the margin of eastern North America. Maurice Ewing Symposium, Series 3, Winterborne Kingston borehole, Dorset. In: RHYS, G. H., L~TT,G. K. American Geophysical Union, Washington DC, 218-234. & CALVER,M. A. (eds). The Winterhorne Kingston Borehole, Dorset, WERNICKE,B. & BURCHFIEL,B. C. 1982. Modes of extensional tectonics. England. Report of the Institute of Geological Sciences, 8113, 53-76. Journal of Structural Geology, 4, 105-15. RAWSON,P. F. & RILEY, L. A. 1982. Latest Jurassic-early Cretaceous events WH~AKER,A. 1975. Apostulated post-Hercynian rift valley system in and the ‘Late Cimmerian Unconformity’ in the North Sea Area. Bulletin southern Britain. Geological Magazine, 112, 137-49. of the American Association of Petroleum Geologkts, 66112,2628-48. -(ed.) 1985. Atlas of Onshore Sedimentary Basins in England and Wales: RHYS,G. H., LOTT, G. K. & CALVER,M. A. (eds) 1982. The Winterborne Post-Carboniferous Tectonics and Stratigraphy. Blackie, Glasgow.

, Kingston borehole, Dorset, England.Report of the Institute of Geological ~ & CHADWICK,R. A. 1984. The large-scale structure of the Earths crust Sciences, 8113. beneath southern Britain. Geological Magazine, 121, 621-4. ROYDEN, L., SCLATER,J. G. & VON HERZEN,R. P. 1980. Continental margin WYLLIE,P. J. 1977. Effects ofH,O and CO, on magma generation in the subsidenceand heat flow: important parameters information of crustand mantle. Journal of the Geological Society, London, 134, petroleumhydrocarbons. Bulletin of the AmericanAssociation of 215-36. Petroleum Geologists, 64, 173-87. ZIEGLER,P. A. 1981. Evolution of sedimentary basins in north-west Europe. SCLATER,S. G. & CHRISTIE,P. A. K. 1980. Continentalstretching: an In: ILLMG,L. V. & HOBSON,G. D. (eds) Petroleum Geologyof the explanation of the post mid-Cretaceous subsidence of the central North Continental Shelf of North-West Europe, Heyden, London, 3-39. Received 13 September 1985; revised typescript accepted 17 December 1985

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/3/465/4893198/gsjgs.143.3.0465.pdf by guest on 26 September 2021