10. UNDERWAY OBSERVATIONS LEG 24, GLOMAR CHALLENGER1

PART I: DJIBOUTI TO SEYCHELLES ISLANDS

Elizabeth T. Bunce, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

INTRODUCTION

The underway geophysical observations made during Leg24 between Djibouti, French Territory of the Afars and theIssas, and Mauritius presented here include the reducedbathymetric and magnetic anomaly profiles (Figures la-6a)and photographic reductions of the seismic reflection pro-files (Figures lb-6b). The observations are grouped by geo-graphic area: the Gulf of Aden, the Somali Basin, and theMascarene Plateau-Central Indian Ridge-Chagos/LaccadiveRidge. The procedures used in reduction of the first groupof data are those in standard use at the Deep Sea DrillingProject where both original records and the results of thecruise may be consulted.

The reflection profiles at vertical exaggeration of about15:1 are reproduced at a scale that enables reasonableexamination and are thus expanded horizontally in com-parison with the magnetic-bathymetric profiles.

GULF OF ADEN

The lack of site surveys in the Gulf of Aden necessitatedreconnaissance tracks in the three site areas to insure thatthe final selections met the specifications of the AdvisoryPanel.

The track from Djibouti to Site 231, located in theHalf-Degree Square of Laughton et al. (1970), is generallyeast-southeast, almost normal to the bathymetric contours(Laughton) and south of the West Sheba Ridge structure.The magnetic profiles over the first 500 miles show minorexcursions superimposed on broader but also small-scaleanomalies; the latter are probably generated in the acousticbasement structure observed on the seismic profile. Thebroad feature between 0 and 80 nmi may be part of theTadjura Trench structure, since the track cuts across itssouthern margin. The reflection profile here shows at least1 sec of sediment and at 75 nmi what appears to be a deep(1.5 sec) "valley" occurs. Between 200 and 300 nmi bothmagnetic and seismic character change; the former totypically oceanic type, the latter to a southeast-dippingsequence of sediment-filled flat-floored basins confined bybasement ridges. Basement is first detected near 80 nmi andis observed continuously from there to Site 231. Theuppermost sequence contains layered sediments alongalmost the entire track. Except over the initial 75 to 100nmi, all these exhibit some degree of deformation, partly

Contribution No. 3231 of the Woods Hole Oceanographic Instit-ution.

the result of uplift by basement structure, partly inde-pendent of such activity (see the section from 150 to 250nmi, for example). Tectonically related structure is notsurprising, since the area lies south of and parallels WestSheba Ridge, the spreading center. Structures not related tobasement might perhaps be attributed to current activity.

Site 231 to Sites 232 and 233: from Half-Degree Squareto the Alula-Fartak Trench. From Site 231 the track runsnorth a short distance before turning northeast, semiparallelto the West Sheba Ridge structure which it crosses normalto the axis in one of the offset regions. The approach to thetrench was made from south of the planned site locationsto enable several enroute crossings to reconnoiter bothmargins and both sites prior to drilling.

The magnetic information in general resembles thatobserved between Djibouti and Site 231, at least as far asthe Ridge where the magnetic signature of the medianvalley is rather broad, with peak-to-peak amplitude ofabout 1000 gammas. The remainder of this profile isextremely subdued, with minor anomalies (about 200gammas) observed over the eastern margin of the trench.

On the reflection profile the acoustic basement is tracedapparently continuously to the western margin of thetrench. (The qualification "apparently" is based on thedrilling results and has been elaborated upon elsewhere inthis volume.) Both basement and overlying sedimentschange appearance near the ridge. Basement relief issubdued in the two basins crossed, in comparison with thatof the bordering (enclosing) structures; in both basins thesediments ponded above basement are layered at the topand acoustically transparent beneath. The layering ter-minates abruptly against a bordering basement rise wherethe upper surface of the transparent material that coversthe lower rise shows deformation, possibly the result ofcompression. There are no visible sediments in the medianvalley, although they extend thinly to within 300 fathomsof the top of the east-flanking ridge and somewhat higheron the back slope of the lower western ridge. From here tothe Alula-Fartak Trench, the semitransparent sedimentsagain are deformed and are not necessarily conformablewith the underlying basement structures; in this theyresemble the appearance of sediments on the flanks of theMid-Atlantic Ridge.

The two margins of the trench differ in seismic structureas well as magnetic character. On the west the sediments arelayered and somewhat deformed, on the east they aresemitransparent, with no visible evidence of deformation.Faint reflectors possibly representing thin layers appear inthe semitransparent material (near 0300, 1000, 10 May).

591

E. T. BUNCE

The acoustic basement of the eastern margin is a strongerreflector than that on the west, where there is also the hintof an even deeper reflector about 0.75 sec below sea floor(1300,10 May and site location).

GULF OF ADEN-NORTHWEST SOMALI BASIN

From the Alula-Fartak Trench to the next site (234)which lies on a ridge bordering the western side of theSomali Basin, the track is southward, roughly on longitude52°E, in the Gulf of Aden and between Socotra and theHorn of Africa, thence trending generally south-southwesterly along the abyssal plain.

The magnetic profile is significant chiefly for its infor-mative contribution on the quiet-zone character of thenorthwest Somali Basin. The small amplitude anomalies inthe Gulf of Aden terminate abruptly at the rise betweenAfrica and Socotra (between miles 800 and 850), with thisdistinctive topographic feature showing only minormagnetic activity that also ceases midway down the southslope (mile 990) toward the abyssal plain.

The seismic profiles show the Gulf of Aden sediments tothicken toward the African coast and be deformed wherethey flank the Socotra Rise (mile 800). Except for a centralsediment pocket (850 nmi), the Continental Rise appears tobe acoustically opaque. From this point to the flat-lyingplain the track crosses the features at an oblique angle, thusexaggerating the apparent effects of slumping, incision, andreflector truncation. Basement (basaltic) cannot bedetected beneath the abyssal plain. A distinctive reflector1.25 sec beneath the sea floor appears suddenly at 8.0 sec(1700, 18 May). This feature has been observed on otherprofiles across the area, and it may be a continuous zone ofreflectors traceable throughout the basin. Sampling thisreflector was a primary target of drilling at Site 234.

The west-east reflection profile between Sites 234 and235 that crosses both the basin and Chain Ridge is typicalof profiles in this region. Basement is not observed beneaththe thick sedimentary western section, yet the structure ofthe deeper horizons is obvious. The records on either sideof Chain Ridge show all the features significant to thesesites' selections: the slightly deeper sea floor on the east(left); the similarity of the reverberant uppermost (0.25sec) sediment sequences; the basement structure obvious onthe east, but too deep for detection with this profilingequipment to the west; and the distinctive reflector visibleabove basement on the east and only a trifle deeper than itspossible analog on the west. The apparent abrupt trunca-tion of the uppermost sediment sequence against thetransparent outcrop on Chain Ridge (east) is probably dueto track angle. Other crossings made more normal to theridge lineation in this area are not so dramatic. On the

magnetic profile very small-scale excursions occur over theridge topography, otherwise it is quiet.

Between Sites 235 and 236 the track is southeastwardthrough the Somali Basin. The magnetic anomalies reflectthe abrupt change in both sea floor and subbottomstructure that occurs east of Chain Ridge. Somewhatfarther to the southeast the abyssal plain terminates equallyabruptly where it is ponded between basement rises(1700-1900, 26 May). Beyond these the thinner sediments(less than 0.5 sec) are acoustically semitransparent; somelayering being observed where ponding occurs betweentopographic highs. The basement resembles unusually low-relief oceanic layer 2 with some hills rising above the seafloor.

The profile from Site 236 southwest to the Seychellesshould cross the hypothesized oceanic basalt-Seychellesgranite transition or demarcation. This particular section,however, because of the inability to "see" deeper, onlyserves to raise further speculation.

An arguable and still unanswerable question is raised bythe similarity in appearance of the peaks observed at 1500and 2200, respectively (record of 1 June). That at 1500 canbe interpreted as a continuation of the ocean-acousticbasement material observed and drilled at Site 236 where inits upper portion it was tholeiitic basalt. The hill at 2200rises from an apparently (seismically) invisible source butdoes closely resemble the earlier peak. Between the twothere appear to be hints or traces appearing as fainthyperbolic echoes. Thus, oceanic basement material mayexist very near the almost vertical rise of the platform toform a constraint on the extent of the Seychelles granite.

REFERENCELaughton, A. S., Whitmarsh, R. B., Jones, M. T., 1970. The

evolution of the Gulf of Aden: Phil. Trans. Roy. Soc.London, Series A, v. 267, p. 227-266.

FIGURES

To facilitate correlation of significant features the rec-ords have been annotated (date, time, and distance alongthe track), along the bottom of each profile. GMT time,marked hourly and half-hourly, is annotated at 5 hour inter-vals, cumulative distance at intervals of 100 nautical miles,and course and course changes are also indicated. All ofthese records were made at a 10 second source repetitionrate; the depth scale is indicated in one-second intervals oftwo-way travel time. Subbottom depth (sediment thickness)can be estimated assuming 2 km/sec velocity (or 1 km for 1sec of travel time). Vertical exaggeration is about 15:1.

The records are continuous, each one reading from rightto left.

592

E. T. BUNCE

1000.318 300 250

NAUTICAL MILES

200 150 100 50

500.-4-

- 5 0 0 .

-1000.

4000.

O ,— i—

HOURS

s

Figure la. Djibouti to Half-Degree Square, Site 231 (magneticanomaly).

594

UNDERWAY OBSERVATIONS: PART I

DJIBOUTI TO SITE 231

j " ' ' ' " ' ' it '-164° 055° 270°-225°

I 318 300-

Figure lb. Djibouti to Half-Degree Square, Site 231 (seismic profile).

NAUT.-MILES

595

E. T. BUNCE

1000 719 700 650

NAUTICAL MILESSITES 231 t o 232 and 233

600 550 500 450

HOURS

400 350 318

Figure 2a. Site 231 to Alula-Fartak Trench, Sites 232 and 233 (magneticanomaly).

596

UNDERWAY OBSERVATIONS: PART I

-062°—

— 500-

I ' I \ ' I ' I ' I10 ,

057° 070° —

1 I ' I ' I ' I I0

-065°

9 MAY 18 MAY

I ' I20 TIME

346°C/C

3 4 8 NAUT.i l ü MILES

ii i i i i1935 19 18 1710 15

iC/C— 285°105°192'i l l|~ 13 MAY i l l

719.4 704 703.5 700-

I I I10

300°

1

600-

1

10 MAY

)

11

1

MAY

' 1 ' 1 • 120 1 TIME

NAUT.MILES

Figure 2b. Site 231 to Alula-Fartak Trench, Sites 232 and 233 (seismic profile).

597

E. T. BUNCE

"– 2000. -

3000. -

NAUTICAL MILESnoo 1050 750 721

HOURS

Figure 3a. Site 233 to Somali Basin, Site 234 (magnetic anomaly).

598

SITE 233 TO SITE 234

UNDERWAY OBSERVATIONS: PART I

SEC

-3t; V **ß*#

I I I I . I . I . I . I . ' I , I ,

17 MAY u 16 MAY

I ' I ' I ' I • i ' I I ' I • I • I I I ' I ' I ' I20 15 . 1 0 7 TIMEI

•184° -194° C/CNAUT.

— 721 MILES

" ' " I " ' ' i•••—^•WI

18 MAY ° 17 MAYI I

•197° 195°

5 19 MAY ° 18 MAYI I I ! I I

-093o3°194°335o 128° 220°I

20°

I ' I '10 TIME

-C/CNAUT.

-MILES

I ' 115

-c/cNAUT.

-MILES

Figure 3b. Site 233 to Somali Basin, Site 234 (seismic profile).

599

E. T. BUNCE

NAUTICAL MILES1500 1450 1400 1372

HOURS

Figure 4a. Site 234 to Site 235 (magneticanomaly).

600

UNDERWAY OBSERVATIONS: PART I

SITE 234 TO SITE 235

22 MAY

E l l I"M 513.0 1500

' I ' I ' I '

I-137°

1 I I ' I ' I20 1906 TIME

137° C / C

NAUT.-1372.7 MILES

Figure 4b. Site 234 to Site 235 (seismic profile).

601

E. T. BUNCE

1943 19001000.1 ~~T~

1850NAUTICAL MILES

1800 1750 1700' 1650 1600 1555

3500.

4000.

i i i i i i i I i i i i i i i i i - i i I I I i 1 ' i i

Figure 5a. Site 235 to Site 236 (magnetic anomaly).

602

SITE 235 TO SITE 236

UNDERWAY OBSERVATIONS: PART I

I ' [10

i ' i

-1700-

1 I5

1

]2790°-

1

0MAY 26

1 1 ' 1

MAY

i | i i | i | i

20 1219 TIME

-139° 149°C/CNAUT.

1514.2 MILES

J \«i -•rr- '

0900

ON SITE 2361943.8

28 MAY 27 MAY

Figure 5b. Site 235 to Site 236 (seismic profile).

i • i20

C/CMILESDATE

603

E. T. BUNCE

2100NAUTICAL MILES2050 2000

HOURS

Figure 6a. Site 236 to Mahé (magneticanomaly).

604

SITE 236 TO MAHE I S . . SEYCHELLES

UNDERWAY OBSERVATIONS: PART I

2 JUNE 1 JUNEI

- 2 2 2 ° -

Figure 6b. Site 236 to Mahé (seismic profile).

-229° 242° 115° C/CI NAUT.

605