Seismic Refraction Evidence for a Basement Ridge Between the Derbyshire Dome and the W of Charnwood Forest

$Seismic Refraction Evidence for a Basement Ridge Between the Derbyshire Dome and the W of Charnwood Forest$

J. geol. Soc. London, Vol. 138, 1981, pp. 653-659, 7 figs, 2 tables. Printed in Northern Ireland.

Seismic refraction evidence for a basement ridge between the Derbyshire Dome and the W of Charnwood Forest

D. N. Whitcornbe & P. K. H. Maguire

SUMMARY:Results from a 40-kmseismic refraction profile crossing a series of positive gravity anomalies extending across the postulated mouthof the Widmerpool Gulf are reported. A refractor with a similar velocity to the Precambrian rocks of Charnwood Forest dips away from Charnwoodto the southern flank of the Derbyshire Dome to a depth of 2 km.This refractor is faulted into horst and graben structures. The horst blocks correlatewith the positive gravity anomalies. A northern boundary fault to the Precambrian outcrops of Charnwood is defined.

This profile was the third in a series of seismic refrac- proved Ordovician mudstonesat a depth of 1.8 km tion experimentscarried out by the University of beneath CarboniferousLimestone (Dunham 1973). Leicester to investigate the contact of the Precambrian The Ashby borehole BH5 (Fig. l), sunk by the IGS in rocks of Charwood Forest with the surrounding 1978, reachedpre-Tremadoc strata of probable Mesozoic sediments, and to define the nature of the basement to these sediments. The results of the first two profiles have been reported in Whitcombe & Maguire 1981. This northern profile extendedfrom Charnwood Forest to the southern edge of the Derby- 381 shire Dome, crossing apostulated basement ridge (Maroof 1973) associated with a series of positive Dome gravity anomaliesextending across the proposed mouth of the Widmerpool Gulf (Falcon & Kent 1960). 36 .E PEAK CLLIDON \ CALDO . Geology and previous .-- -_ geophysical work 3L . Widmorpool Solsmic Gulf The position of the seismic profile, quarry shot points, \ Profile and relevant boreholes, in relation to Charnwood For- est and the Derbyshire Dome is shown in Fig. 1. Fig. 2 32 shows the simplified surface geology of the area crossed by the seismic profile and the positions of the individual recording stations. The oldest rocks in the areaare the ‘Charnian’ late Precambrian rocks of Charnwood Forekt Charnwood Forest, folded into a south easterly asym- 301 I I I 1 metrical plunging anticline (Watts 1947) and consi- L0 0 L20 L L0 L60 EASTING dered to have a faulted northern boundary (Moseley 1979). An analysis of the velocity structure of these BOREHOLES m BH1 - Eyam, BH2 - Woodale, rocks gave a P wave velocity of 5.65 km/s forthe BH3 - Caldon Low, BHL-Ropton. olderBlackbrook ‘Series’ (Whitcombe & Maguire I BH5 - Ashby 1980). An underlying basement refractor with a veloc- QUARRIES A ity of c. 6.4 km/shas been mapped at a depth of FAULTS F1 - Boothorpc, F2 - Thrlngstonr, >2 km beneath Charnwood Forest. Seismic refraction F3- New Brook Volley. FL - Worthlngton profiles radiating to the SW and ENE of Charnwood CARBONIFEROUSLIMESTONE OUTCROPS 0 age D2DI age ago Forest indicated refracting horizons with P wave vel- a.. ocities of 5.76 and 5.64 kmls, respectively (Whit- . Older C2 S1 ago Wldmorpool foclos combe & Maguire 1981), considered to be associated with Charnian type basement. The velocity structure FIG. 1. Seismicprofile, quarry shotpoints and of the upper crust to the N of Buxton has been defined boreholes in the study area, showing the relation- by the LISPB experiment (Bamford et al. 1977). The ship of CharnwoodForest to the Derbyshire Eyamborehole to the E of Buxton (BH1 in Fig. 1) Dome.

0016-7649/81/1100653%02.00 @ 1981 The Geological Society

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L20 c30 L L0 L50 Contoursin rnlllloals EASTING Prccambrion m Widmerpoolfocies Carboniferous Limestone 8 MillstoneGrit. m CorbonifcrousLimestone H Cool Measures m Perrno -Triassic 0 Post - Jurossic Igneousrocks

BAL: Ballidonquarry M : MiddlePeokquorry C : Cloud Hill quarryW: Whitwick quorry B : BardonHill quorry CL1 to CL3Corboniforous Llrnestone inlters

FIG. 2. Geological map of the study area showing the seismic recording site locations.

Cambrianage at a depth of 174111 beneath Coal Measures. The Caldon Low borehole, BH3, situated in the Weaver Hills on the SW edge of the Derbyshire Domeand also sunk by the IGS in 1978, reached L20 L30 LLO L50 EASTING sandstone of either Carboniferous or Devonian age at BAL . Bollidon quarry.M Middle Peak quarry a depth of 365 m. Old Red Sandstone underlies the C ’ Cloud Hillquarry. W: Whltwtckquorry. Coal Measures of the Staffordshire Coalfield (Hains & B ’ Bordon Hill quarry. Horton 1969). The Woodale borehole BH2 (Fig. 1) reached volcanic rocks at a depth of 312 m;these were G’, G’ onornollosdiscussed In text originallycorrelated with the PrecambrianCharnian FIG. 3. Bougueranomaly map of thestudy area rocks(Cope 1949). However, Le Bas(1972) consi- showing the seismic recording site locations.

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G’ and G’. Modelling of theseanomalies indicates at all the other stations were placed in shallow pits dug into Mesozoic thicknesses of 200-700 m under the centres soil. of each anomaly, with pre-Carboniferous basement at Quarry blasts from the 10 quarries (Fig.1) were used in the depths of 1.0-1.7 km on their flanks (Maroof 1973). analysis. Good quality signals could generally be obtainedup The anomalies could bedue to either a smoothly to a distance of c. 35 km from the various quarries, but good signalswere recorded at c. 60 km fromTunstead quarry. undulating basement or upfaulted blocks. The latter Table 1 gives the number of traveltime observations re- interpretation was favoured for anomaly G’. The steep corded from each shot point. The unfltered analogue tape contours of this anomaly correlate with the positions recordingswere replayed directly onto paper records. The of the Hercynian Boothorpe Fault and the New Brook relative arrival times of the signals could in most cases be Valley Fault (Fig. 1). The Worthington and Thring- picked to accuracies of better than 10ms. stone faults also occur in this area (Fig. l), but from the Bouguer anomaly map (Fig. 3) do not appear to be major structuralfeatures. Several Carboniferous Data analysis Limestone inliers also occur in this structurally com- plex area, and may be considered in 3 groups (CL1- CL3, Fig. 2). The first group (CL1) lie outside the limits The method of the Widmerpool gulf and also to the SW of the New A fuller description of the method used forthe Brook Valley fault and are therefore located on the analysis is given in Whitcombe & Maguire (1981). In postulatedhorst block. The secondgroup (CL2) are brief, the reduced travel times were calculated using situated nearthe Worthingtonfault and have large reducing velocities similar to the expected basement dips, unlike the CL1inliers. The.beds of the CL3inlier, refractor velocity. Changes in the reduced travel times situated immediately to the NW of the Precambrian between adjacent stations are a good first approxima- outcrops, are almost horizontal. tion tothe changes in recordingstation time-terms between these stations. A study of the reduced travel Data collection times therefore reveals the approximate topography of the basement refractor (Whitcombe & Maguire 1979). Theseismic profile was dog-legged to avoid the city of Arrivals which donot originate from the basement Derby. The profile was positioned to pass through Cloud Hill refractor may be easily identified from a study of the quarry which is situated ina group CL2 Carboniferous Limes- reduced travel times, and excluded from the data set tone inlier, and which acted as a shotpoint for the experiment. The profile crossed the Repton borehole, BH4, (Fig. 1) used todetermine a leastsquares velocity estimate (station 24), and terminatedat Ballidon quarry in the S of the using the time-termmethod (Willmore & Bancroft Derbyshire Dome. Because of the large number of stations 1960). requiredfor this seismic experiment it wasnecessary to undertake the profile in sections. It later proved necessary to supplementthe data byreoccupying 8 of theoriginal 39 The reduced travel times stations, and continuously recording for a further 3 weeks. Each station consisted of a short period Willmore Mk I11 In view of the basement velocities obtained in the seismometer. The signals were recorded on two ‘Geostore’ Charnwood area (-5.6 km/s) and in the Buxton area analogue tape recorders. The Rugby MSF radio clock was by the LISPBexperiment (-5.8 km/s), it was ex- recorded as the time base for the experiment. Stations 35-39 pected that a similar velocity refractor would be weresituated on or near outcropping Precambrian rocks. defined beneath this seismic profile. The reducing Station 34 was situated near the Grace Dieu Carboniferous velocities were therefore set to 5.6, 5.7 and 5.8 km-’ Limestone inlier. Station 1 was situated on outcropping Car- boniferous Limestone at Ballidon quarry. The seismometers and the reduced travel times calculated. These values were then plotted against the position of the recording stationalong the seismic profile. Examples of these TABLE1: reduced travel time plots for the Tunstead, Ballidon, Cloud Hill and Cliffe Hill quarry blasts are given in Quarry shotpoint No. observations of Figs 4-6. The refractortopographies compiled by Tunstead 25 Tunstead theseplots are consistent with the total data set. A Dowlow 10 study of the reduced travel time plots indicates that Middle Peak Middle 8 data can be explained by a three layermodel. The Ballidon 15 Ballidon majority of first arrival data originates from a base- Caldon 12 ment refractor with a velocity of c. 5.6-5.8 kmls. Cloud Hill 20 Hill Cloud Shots from the N and S of the profile imply mutually Whitwick 4 consistent basement topographies when the reducing Bardon Hill Bardon 10 velocity is set in the range 5.6-5.8 km/s. This base- Cliffe Hill Cliffe 9 Mountsorrel 8 ment refractor is referred to as the a, refractor to be consistent with the LISPB experiment.

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BALLIDONCHARNWOOD CHARNWOOO BALLIDON 0 0 0 0 0 L 06 0 TUNSTEAD VR = 5-6kmk V, = 5 6 kmls 0 7 BALLIDON 0 5 10 15 20 25 30 35 &O L5 50 0 5 10 K 20 25 3530 L0 45 50 DISTANCE OF STATION FROM BALLIDON IKml

1 1 1 1 - 1 E -1 0 VR = 5 6 km15 TUNSTEAD EOL VR i S 2 kmh c-o 2 0 5 BALLIDON 0 5 10 15 20 25 30 35 L0 L5 50 W VI 0 5 10 15 20 25 3530 L0 L5 50 Y DISTANCE OF STATION FROM BALLIDON lKml

,. . , , . , , . . . B-1 8 ,.;;-;--.- 2-1 7

-1 5 vR =S 8kmls -1 L TUNSTEAD 0.7 0 5 10 15 20 25 30 35 L0 L5 50 V, :S 6kmlr 0 .B BALLIDON 0 5 l0 15 20 25 30 35 L0 L5 50

FIG. 5. As for Fig. 4, Ballidon quarry. -0 1 00 01 2' I1 02

TUNSTEAD VI =6 2kmIs 0 5 10 15 20 25 30 35 L0 L5 50 06 v, = 5 6 kmlr CLOUD HILL FIG. 4. Reduced travel times at various recording 07...I...... I stations for particular reducing velocities for Tun- 0 5 10 15 20 25 30 35 L0 L5 Y) stead quarry. DISTANCE OFSTATION FmM BALLIDON l Km I

-OIF ' ' ' ' ' ' ' ' ' '4

The first arrivals recordedfrom the Ballidon and Middle Peakquarry blasts, N of station 13, have 03 apparent velocities of 5 km/sand are not consistent ;0 L CLOUD HILL V, i 5 6 kmls with arrivalsoriginating from the a. refractor.They fl 0 5 10 15 20 25 Y) 35 L0 L5 50 Ln are interpreted as direct wave arrivals through a thick W sedimentary or a, layer overlying the a. refractor. E There is no evidence for such a thick, high velocity, a, layer atthe southern end of the profile. The first arrivals from CloudHill into the adjacent stationshave anapparent velocity of c. 4.5 km/sand are quickly 03 vR i 5 6kmls overtaken by a, headwaves. L CLOUD HILL The first arrivals from Tunstead quarry recorded to 0 5 10 15 20 25 30 35 L0 L5 50 the S of approximately station 20, are inconsistent wiith arrivals originating from the a. basement refrac- -O3& -0-O3& 2 tot. They have ahigher apparent velocity thanthe Ballidon arrivals, andare interpreted as originating -:;l01 from a deeper, higher velocity or a, layer, beneath the 02 v, = 5 ' 6 kmlr a. basement. A velocity estimate is not possible for CLIFFE HILL this layer because the data is unreversed. 0 5 10 15 20 25 30 35 L0 (5 50 The reduced travel time plots are particularly useful FIG. 6. As for Fig. 4, CloudHill quarry and for resolving the short wavelength structure. Sudden (lowermost) Cliffe Hill.

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discontinuities in the compiled a, basementtopog- TABLE 2: The time terms (S) for solutions 2, 3 and 4 raphy areinterpreted as geological faults andare defined in the vicinity of stations 14, 19, 27and 34 Solution no. 2 3 4 (Fig. 4). A graben is defined betweenstations 27 and34. Station no. 1 0.25 0.20 0.24 The a, basement dips steeply to the N between sta- 5 0.24 0.20 0.23 tions 28 and 24 and then appears to be approximately 6 0.24 0.20 0.24 7 0.24 0.20 0.24 horizontal up to station 22. A basement horst block is 8 0.24 0.20 0.23 resolved between stations 15 and 19. There appears to 9 0.30 0.26 0.30 be a basement low region to the N of station 15, but it 12 0.26 0.21 0.21 is not clear from thedata if this region is fault- 13 0.30 0.25 0.30 bounded to the N, or whether the basement slopes 14 0.29 0.24 0.29 gently to shallower depths beneath the northern sta- 15 0.24 0.19 0.24 tions. 16 0.20 0.17 0.21 The long wavelength structure implied by the re- 17 0.17 0.14 0.18 duced travel time data is of an a, refractor dipping to 18 0.15 0.12 0.16 19 0.23 0.19 0.24 the N from Charnwood Forest showing no sign of 24 0.39 0.34 0.40 rising to shallow depths beneath the southern flank of 25 0.31 0.26 0.33 the Derbyshire Dome as predicted by Maroof (1973). 26 0.18 0.13 0.19 Thisresult is consistent with the direct wave first 27 0.14 0.08 0.19 arrivals being observed at greater distances at the 28 0.25 0.08 0.16 northern endof the seismic line than at the southern. 29 0.25 0.19 0.27 30 0.25 0.21 0.27 31 0.15 0.12 0.14 Time-term analysis 32 0.09 0.09 0.08 33 0.11 0.10 0.10 Due to the‘bittiness’ of the datawhich was obtained 34 -0.01 0.01 -0.02 from a total of 21 shots into not more than 9 recording 35 -0.03 0.00 -0.03 stations at a time, and due to the non-linearity of the 36 -0.05 0.00 -0.04 profile, the data is suited to the time-term method of 37 -0.03 0.00 -0.05 analysis. 38 -0.04 0.00 -0.03 A time-term solution (1) of all the data considered 39 -0.04 0.00 -0.04 to originate from the a. refractor gave a velocity of Cloud Hill 0.14 0.09 0.13 5.58 km/s. The solution had a variance approximately Tunstead 0.14 0.10 0.25 0.18 68 times greater than could be explained by the esti- Dowlow 0.04 0.12 Bardon 0.17 -0.01 0.04 mated observational errors, giving an F ratio of about Whitwick 0.06 0.00 0.06 68. If the solution was a perfect fit to the data the F Cliffe Hill 0.08 0.00 0.04 ratio would be approximately 1. In order to try to reduce the bad fit between the model anddata, asubset of the data was defined omitting Cloud Hill quarry arrivals into stations 7 and 9, which were two observations of dubious quality. cambrian rocks, aresult to be expected if the Pre- Reversed travel time observations to stationsas- cambrian rocks constitute the a, refractor. The solu- sociated with rapidly changing time-termswere also tion (3) time-terms for Tunstead and Dowlow quarries omitted. The time-term solution (2) (Table 2) of this are consistent with the LISPB depth to the a, refractor revised data set gave a velocity of 5.68 km/s with a beneath Buxton. The Cloud Hill, Bardon, Whitwick much improved F ratio of 13. A thirdsolution (3) and Cliffe Hill time-terms as defined by solution (3) (Table 2) was carried out allowing for a linear increase are similar to the values determined by previous work in the velocity of the refractor with depth. This gave a for these quarries (Whitcombe & Maguire, 1980). refractor velocity of 5.64+0.092 km/s with a slightly It has not been possible to reduce the F ratio below improved F ratio of about 12. A fourth solution (4) the value of 12 obtainedfrom solution (3). Lateral (Table 2)was carried out constraining the basement variations in the velocity of the a, basement refractor refractor velocity to that found by the LISPB experi- would contribute to such a high F ratio (Whitcombe & ment for the a, layer, namely V = 5.8+0.03z km/s. Maguire 1979). However, there is no evidence in the This gave a slightly worse ratio of approximately 13. reduced travel time diagrams for lateral variations in As well as producing the best fitting model, solution the basement velocity, of the size observed by the (3) gives the mostreasonable time-terms. It can be seismic profiles described by Whitcombe & Maguire seen from Table 2 that this solution gives zero time- 1981. High F ratios are expected for refractors with a terms for stations 35-39, situated on outcropping Pre- topographic wavelength less thanthe length of the

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BALLIDON CHARNWOOD

1 2 34 5 6 8 1011l2 13 IS 16 17 18 19 20 21 22 23 2425 26 18 29 30 31 32 343537 39 GO 0,-- ! . ’: ! .’ ! : ! ! ’: : : ::I : ::! ; E 0-5

I1 .o OS 1 .5 z 2-0 Fault p25 “0 Valley Fault E3 0 V= 5 6L+O 092 (kms-’)

I l I 1 I I 1 I I I 0 -5 10 15 20 25 30 35 L0 L5 50 DISTANCE FROM BALLIDON (km)

FIG.7. Depth section to the a, refractor.

seismic line. This is clearly the case for this seismic tute this 1.8 km thick section. All these rocks would be profile. An iterative time-term solution would help to expected to have velocities of 5 km/s or less within reduce the F ratio. However, an iterative solution has this depth range (Faust 1951; Bullerwell 1967; Whit- not been carried out for the following reasons: (i) It is combe & Maguire 1981). considered thatthe control on the velocities of the The velocity depth models for this northern seismic sediments is too poor; (2) It is unlikely that iteration profile andthe LISPB seismic line are compatible. would better define the a, refractor velocity since this Both have an upper layer with a velocity of c. 5 kmls. parameter is little influenced by short wavelength Both have a basement refractor at a depth of c. 2 km basement topography (Whitcornbe & Maguire 1979). with similar velocities. However, the geological model The time-terms from solution (3) relating to the a, for the northern seismic profile is somewhat at var- basementrefractor havebeen converted intodepth iance with the geological interpretation of the LISPB estimates (Fig. 7). It has been assumed that the mean velocity depth model. The LISPB a, refractor was velocity of the a, sedimentarylayer varies smoothly considered to be Carboniferous Limestone and possi- from 5.0 km/s atthe northern end of the line to bly OldRed Sandstone, andthe a. refractorpost- 4.5 km/s at the southern. It is important to note that ulated to be Lower Palaeozoic. At a depth of 10 km a this depth section doesnot containany additional deepera, refractor was considered to be‘pre- information about the shape of the refractor, to that Caledonian basement’. resolved from the reduced travel times. It is possible that the a, refractor mapped by the northern seismic line is a Lower Palaeozoic horizon Geological interpretation that pinches out to theN of Charnwood Forest. If such and diocussion a horizon hada velocity of 5.6 km/s it would be seismically indistinguishable from the Charnian rocks. The velocity determined for the a, refractor from the Such alayer could bethe pre-Tremadoc Cambrian northern seismic line is similar to the velocities found strata reached by BH5. While it is accepted that the a. forthe Precambrianrocks of Charnwood Forest refractor may be partly Lower Palaeozoic, the a, re- (Whitcombe & Maguire 1980), and also to the base- fractor beneaththe northern profile must contain a ment velocities beneath the two seismic profiles de- considerable thickness (c. 1.8 km) of Lower Palaeozoic scribed by Whitcombe & Maguire 1981. Because of rocks. The LISPB a, refractor may be (in part) Pre- this the a, refractor is interpreted as Charnian, imply- cambrian, similar to the metasediments and metavol- ing that Precambrian basement exists at a depth of canics of Charnwood Forest. 2 km beneath the southern flank of the Derbyshire Dome.As Maroof (1973)predicted 130-300m of ACKNOWLEDGMENTS.Wewould like to thank Dr M. A. Carboniferous Limestone in this area, this implies a Khan and the technical staff of the Department of Geology, 1.8 km thickness of Lower Palaeozoic rocks beneath University of Leicester, for considerable assistance with the fieldwork; and Drs T. D. Ford, R. J. King and M. J. Le Bas the CarboniferousLimestone and above the ‘Char- for invaluable advice on the geology of Central England. The nian’ typebasement. It is considered that Old Red recordingequipment used was on loan from the Natural Sandstone, Ordovicianmudstones (proved in the Environment Research Council (NERC) seismic equipment Eyam borehole), and possibly Stockingford Shales pool. One of us (DNW) was supported by a NERC research (similar to those found in the Nuneaton area) consiti- studentship.

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BAMFORD,D., NUNN,K., PRODEHL,C. & JACOB,B. 1977. 39, 71-86. LISPB-111. Upper crustal structure of northern Britain. MAROOF,S. I. 1973. Geophysical investigations ofthe Car- J. geol. Soc. London, 133, 481-8. boniferous and Precambrian . of the EastMid- BULLERWELL,N. 1967. In: STUBBLEFIELD,C. J. & BULLER- lands of England. Thesis, Ph.D., Univ. Leicester (un- WELL,W. Discussion following some results of a recent publ.). Geological Survey boring in Huntingdonshire. Proc. MOSELEY,J. 1979. The geology of the late Precambrian rocks Geol. Soc. London, 1637, 38-9. of Charnwood Forest, Leicestershire. Thesis, PhD, Univ. COPE, F. W. 1949.Comments on the WoodaleBorehole. Leicester (unpubl.). Proc. Geol. Soc. London, 1446, 24. WATTS, W. W. 1947. Geology of the Ancient Rocks of Cham- DUNHAM,K. C. 1973. A recent deep borehole near Eyam, wood Forest, Leicestershire. Leics. Lit. Phil. Soc., 160 pp. Derbyshire. Nature, Phys. Sci. 241, 84-5. WHITCOMBE,D. N. & MAGUIRE,P. K. H. 1979.The re- FALCON,N. L. & KENT, P. E. 1960. Geological results of sponse of the time-termmethod to simulated crustal petroleum exploration in Britain 1945-1957. Mem. geol. structures. Bull. seim. Soc. Am. 69, 1455-75. Soc. London, 2. - & - (1980).An analysis of the velocity structure FAUST,L. T. 1951. Seismic velocity as a function of depth of thePrecambrian rocks of CharnwoodForest. and geological time. Geophysics, 16, 192-206. Geophys. J. R. astron. Soc. 63. GEORGE, T. N. 1963. Tectonics and palaeogeography in - & - 1981. A seismic refraction investigation of the northern England. Sci. Progr. 51, 32-59. Charnianbasement and graniticintrusions flanking HAINS, B. A. & HORTON,A. 1969. Central England (3rd. Charnwood Forest. J. geol. Soc. London, 138. ed.). British Regional Geology. I.G.S., H.M.S.O. WILLMORE,P. L. & BANCROFT,A. M. 1960 The time-term LE BAS, M. J. 1972. Caledonian igneous rocks beneath approach to refraction seismology. Geophys. J. R. astron. central andeastern England. Proc. Yorkshiregeol. Soc. SOC.3, 419-32.

Received 8 August 1980. D. N. WHITCOMBE,Dept. of Earth Sciences, University of Leeds, Leeds LS2 9JT. P. K. H. MAGUIRE,Dept. of Geology. The University, Leicester LE1 7RH.

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