GRAVITY ANOMALIES AT CONTINENTAL MARGINS BY J. LAMAR WORZEL AND G. LYNN SHURBET LAMONT GEOLOGICAL OBSERVATORY,* COLUMBIA UNIVERSITY, PALISADES, NEW YORK Communicated by M. Ewing, April 27, 1955 Introduction.-The oceans are the normal part of the earth's crust and the con- tinents the anomalous part, 64 per cent of the earth's surface being covered by oceans deeper than 1,000 fathoms. The M-discontinuity between the crust and the mantle lies 9-12 km. below sea level in ocean areas and 30-40 km. below sea level beneath the continents. At the continental margins the crustal thickness changes by a factor of about 5. There are few data on this transition. It is difficult to investigate by explosion seismology because of the thickness of the layers, the complex nature of the interfaces, layer slopes, changes of lithology, etc. Gravity surveys at present give the only data which cover this transition zone in detail. Observations.-Gravity observations at 104 stations were made on board U.S.S. "Tusk" at the locations shown in Figure 1. The observations were made with a Meinesz pendulum apparatus.' The data for Browne corrections were obtained with an auxiliary long-period pendulum apparatus2 loaned by Professor Vening FIG. 1 458 Downloaded by guest on September 26, 2021 VOL. 41, 1955 GEOLOGY: WORZEL AND SHURBET 459 Meinesz. Modifications in the apparatus were made and are described by Worzel and Ewing.3 Seven gravity sections are shown in Figures 2-8, the distance from a point near the shore line being given in nautical miles and kilometers. Part A of each figure shows the topographic section and the seismic data available. Part B shows the assumed structures and densities used for the gravity calculations. Part C shows a MT. DESERT SECTION, USS TUSK (SS426) Distance in Nautical Mi/es from Mt. Desert Rock /00 200 300 Distance in Kilometers from Mt Dcsert Rock o0,./.o..q,5o.3900, .30 , A- TOPOGRAPHIC 8 SE/SM/C SECT/ON Georges Bank 0r)eg see level Or,X '~~~im~~*ec ________ _____ ~~~~~water _ _ 5.8-610 ____Officer ___SEwing _r roke ei Ai i --4 -At/ontis i64 '64-65 6- ASSUMED STRWTURE 8 ENS/TIES FOR GRAVITY CALCULATI7ONS 20.19:03C :, ;, /090f- sea/eve .3 C- COAIFPUED 8 OBSERVED GRAVITY ANOMALIES -1.1-1 _ I I,- 40 I' \jA Free-Air Anomj __J_ U ,,;::T '81,5h _ iS.S.Tusk,947 :t.z Ilz-I ._ |STRUCTURE- SCTION DECk'XED FRO SE/IIC 8 GZ4AT EV/Xt2 /OpOfm Sea /ew/ol /0 L - _S i kl--,. C-1 20~~~~~~~~'W- -- ~~~~~~~~~~~~tZ~x-X~4 X Vertlcoi Exaggeration 4.i I-111Z x~~~~~~~~ DistanceDistacn-ce 'inin /9fMi/ometers from Mt. Des~ert.........Rock b 200 3b0 Distance in NaVutical Miles from Mt. Desert Rock FIG. 2 Downloaded by guest on September 26, 2021 460 GEOLOGY: WORZEL AND SHURBET PROC. N. A. S. PORTLAND SECTION, USS TUSK (SS 426) Distance in Nautical Miles fromn Cape Elizabeth Q~~~~~~0 /a200 .........300 IDistance in Kilometers from Cape Elizabeth O A - TOPOGRAPHIC 8 SEISM/C 4.57-6,OkmANtskerf | Georges Bank SECT/ONw ater See text for descript/on /0 of selsmic data oval bk __ . here__ 20 B - ASSUMED STRUiYCTURE a DENSTIES FOR GRAVITY CALCULATIONS se lew rz 0.-- I,-IIZI 4n/// </</;//////////2//84 C.,Z). D-STR IURE SECTION DEXJED FROM SEISMIC a GRAVITY EVIDENCE o-/-- / sea level O. _ /Oftfstins~~~~~~~~~~~~~~~~~~~~~~~~~-, I0 /0 1 \\ ' I \ \,- r I -2.84 - |__- .zt --.1/ _\-----9 -X%o \ / \ \ \, / \ 2 o_ 1_"3.27 _ / IYx/Ix I~x x ~I x~I} I/\\ t_-I 30_.-~ '~' Vertical__Exaggeration 4:1_ Distance in Kilometers from Cape lizabeth Distance in Nautical Miles from Cape Elizabeth _/-20i_ FIG. 3 Downloaded by guest on September 26, 2021 VOL. 41, 1955 GEOLOGY: WORZEL AND SHURBET 461 WOODS HOLE SECTION) USS TUSK(SS426) Distance in Nautical Mi/es from Gay Head 9 .. .. ........... /00 200 Distance in Kiometers from Gay HeacdI ....... Z2t 4M £ go A - TOPOGRAPHIC 5 SE/SM/C SECT/ON sea := IThIIa-= leve- enai/ /- - U00 ' _toc At 600 km 4:: C t.npuvJ/*a 4 unuisnai4F DV 'a WOer depth Is 45km ~~~~~..w nInI.- ,/n -7> ___sediment thickness is /8km Balanus -Ttlantis bomrentthickrnessissnotdetermined /P I I|94l5 4t/antis 179 15 _ I__ f_Ktzx7=n //- nrwahird DttajA"rj /- ASSUMED ST15WTJURE a DENSIES 0~~~ FOR GRAVITY CALCULATIONS SW level F-Z .IlzI 111Z 114. 30 WS W 4- _ 7t;/tII G.R 0 _-Jr_2 4 40'C-COMPUTE 8 OBSERED GRAV/TY<ANOMA/JES = -.tl t3 I.Q ,It) IR * Bouguer -2t, 4O |An7rffWGPWoolardl --jjm4reA*l- -\ - - |_rusk,-947 - r:,- 1,3., 11 (11!. 801 D-STRUCTURE SEC/ON DEEWED FROM SE/SM/C 8 GRAVITY EVI/DEAE COpOfms sea level .- , I o IF AT - I'-'',:' " 1-'.,, -, N, N , - I '- /! '%- -. t. ," o a Z - - --w L 9 ." . ,~ / I - . 2.84 , , .'1 b. -1 I % I I- -.,ll / ; . I''--,I'\ - -.1 - / / , I -I I / I I 11 XX X X .1%1.,. - " .', . .,. .,.,IK . X.X.1 -, I % , , ".. -O-J27 v x x v cz 2 -X ,7 {; ,~' ' X,ill,, x X x Vertical Exa geration 4.1 x X i v Xx 'L~9 Dis1'ance in A/ ometers rromn Goy /iad 4 ! ito 260 Distance in Nautical Mi/es from Gay Head vrs\..-,.\-efx FIG. 4 Downloaded by guest on September 26, 2021 40462~~~GJRoLOGY: WORZ19L AND Sf1 URBIR74'ucPROC. NN. A.. S. NEW YORK SECTION, USS TUSK (SS426) Distance Ifl Nautical Miles from Flire Is/and Lgt I - q ./L.. Vistance In K'llometers trom f-ire Island Q 3Q Light A- G/L/ TOPOGRAPHIC a1 SEISMIC SECTION sea level ,I/ ~~~~~~~~~~~~Iwater --- 11 5Y4i'¾?tkj~e$e q) I- -____ __ _______ -Bb/orX1s - Rh)e/nc km_ I___I____ _____ __ /2/3/948 At/on/is /64 '24 -+---------~Katz19--~ ~ 1,13 et a/UOnpub/is 8 - ASS5UMEt) STRWT/JRE a DENS/TES - ~~~flY GRAVITY CALCULATIONS o- N~~~~~~~j~latmf sea level /5. ,-2.84 30--- // -.tl ta 2%.7 IR I ..3 rz .5 Distance In Nautical Miles from Fi're Island Light FIG. 5 Downloaded by guest on September 26, 2021 VOL. 41, 1955 GEOLOGY: WORZEL AND SHURBET 463 CAPE MAY SECTION, USS TUSK(SS426) Distance in Nautical Mi/es from Seen Mile Beach °, , , , ./X7 .... .. 20p *)V/stance in Kilometers from Seven Mile Beah o .. .TOPOGRPH.a...S..E..iw SECT/ONo..i!WX....... A-' TOPOGRAPHI/C SEIMcaSECTION lee ( ,,----Z:=- .,,,,, ,I, , ,e_- ,., I " 5 42'- ---------.- _, v v tEwing et a7/ 940, 1950 ' 5V #m4fc - 'T97*w K /5 7./3 m/n. eoh _ tiantisi64#24 ~Unpubi14'red Data -__ Katz et oi. --Amos f12i6- UL.......ipubi/shed .... DJota B - ASSUMED STRUCTUZRE a DEVlS/TIES FOR GRAVITY CALCULATIONS 0v /0QO fm sea level qt 30 77U C- COMPUTED a OBSERVED GRAV/TY ANOMAL/ES .~~ ~ ~ ~ ~ .. _-. I_.............. 46 1"., 'V FA.p.IV-VI~Venina Meines II ll; 0. 100~~~~~1 I .t -~:2 0- FP-:4Anaaj____I- I__ -4C WX -- _ . J--I-0.11pe,,,An- l O 61fe.tr4ee-A r > -. -p ' I- -7FU.55 rusk, D-STRCTU/RE SECT/ON £ LEJXED FROM SESM/C a GRAV/ITY EVIDENCE IOfm see level ,--Id = : - x I- r If I v 0 1, L Al v . , ,'- -. - ? ,, 2. L 1 t, .. bvo I * x x x 'I ''\ t'IC.-'-I '- I- *2' xx MX 3 .X X X X X x x x x x +, i---7,.1>t,\.'- w_ xVertical Exaggeratfon 4 '_ 3-4 / or ) ,W { Io nnn wnn n~~ ..1 1- -11- -1 -1 Distcnce in Kilometers from---I-- Seven Mile Beach I-/00 2t Distance in Nautical Mi/es from Seven Mile Beach FIG. 6 Downloaded by guest on September 26, 2021 464 GEOLOGY: WORZEL AND SHURBET PROC. N. A. S. CAPE HENRY SECTION, USS TUSK (SS 426) Distagce In Nautical Mi/es from Cape Henry Light ,log .. 00 20,. Distance in Kilometers from //enry Cape, Light.a400 = 9 O A- TOPOGRAPHICq_'~~~~~~~~~~~~~~~~~~~~~~~Z9 , oa SE/SM/C SECT/ON s level /5-_........2~~~~~~~~~~0_ Katz eta/ 30 0- ASJMED STR//CTURE a DEVS/T/ES 0 FOR GRAVITY CALCULATIONS sea level 3-F C- COMPUTED a -0SERVED GRAVITY ANOMALIES D-STRWTURE SECT/ON DW;,FROM SE/SM/C 8 GRAVITYEY 7L /8|\v/\X,/ / z X | X s t, v / ' b / &< o -84 j, 6 vv vv v ILbSsb /.s--_ IS |I \\ /-z.......................................................................RI_XIXXv _ X _ X XX x X X .rzq) '/ X -A C/ ,~, i~, )( Vertcal Exaggeration 4/ /P-o. /00 A) 00 00 300 400 50 Distance...- .in...Kilometers from. Cape Henry Light Distance in Nautical Miles from Cape Henry Light FIG. 7 Downloaded by guest on September 26, 2021 Voi,.VoL.41,41, 1955955GEOLOGY: WORZEL AND SHURBET46465 CAPE HATTERAS SECTION, USS TUSK (SS426) ViSta Ce in Nautical Msfromn Cape Light Ditacin........~/om te/ fj1 f.qteasCap Hatterwi A- TOPOGRAPHIC a SE/SM/C SECT/ON 9finv'm sea level /0______ ~~~~~~~~~~~~706 km/sec 20 _ Z)- B6 - ASWAMED STR(XTE a DENSrITES FOR GRAV/TV CALCULATIONS 0 fpsea level 14: VL 30 -~ DV-STRUCTURE SECT/ON DEWUCED FROM SEISMIC a GRAVITYEV/DENCE _ _ 30 I X x) X I-I: ~x x Verical Exaggeration_4:1 q): lou IQ2 3002 400 500 Di'stance In K lometlers from Cope /iatteras Light b,160,it 2120 Distance In NauticGIallMies from Cape Hatteras Light FIG. 8 Downloaded by guest on September 26, 2021 466466GEOLOGY: WORZEL AND SHURBET PRoc. N. A. S. comparison of the anomaly curve computed from Part B and the observed free-air gravity anomalies.