6. UNDERWAY SURVEYS, LEG 37

Robert J. Iuliucci and James M. Hall, Department of Geology,Dalhousie University, Halifax, Nova Scotia, Canada

andWilliam G. Melson, Department of Mineral Sciences, National Museum of Natural History,

Smithsonian Institution, Washington, D.C.

INTRODUCTION

While in transit from Recife, Brazil, to the Leg 37drill site locations on the Mid-Atlantic Ridge south-west of the Azores, and then to Dublin, Ireland, GlomarChallenger collected 7200 km of underway geophysicalprofiles, including 12-kHz depth recordings, earth'stotal field magnetometer measurements, and continu-ous seismic reflection profiles.

An index map (Figure la, b) showing the location ofthe track, is annotated in GMT hour and date. Thelarge numerals along the track locate the portions oftrack illustrated in the text. Base contours are from theBathymetric Atlas of the Atlantic, Caribbean, and Gulfof Mexico (Uchupi, 1971). The track was extractedfrom navigation information and plots prepared by theUnderway Data Processing Group, Scripps Institutionof Oceanography.

The underway geophysical data presented (Figures 2to 10) consist of photographic reproductions of originalseismic reflection records with the correspondingmagnetic anomaly profiles. Horizontal scale is GMTtime, and kilometers from the start of record collection;vertical scale is reflection time in seconds for the seismicprofiles, and gammas for the magnetic profiles.Magnetic anomalies were extracted from total fieldmeasurements following the visual fitting of a linearregional magnetic gradient over the segments in ques-tion. The vertical exaggeration for the reflection pro-files is 60 to 1 for Figures 2 to 6, and 75 to 1 for Figures7 to 10, at 18.5 km/hr (10 knots), but varies with theship's speed.

Distant ports from the Leg 37 area of interest dic-tated that the ship's track follow the quickest route toand from the drill sites and this was generally along theflanks and parallel to the axis of the Mid-AtlanticRidge, therefore definitive profiles of the ridge and sur-rounding physiographic provinces were rare. How-ever, good perpendicular crossings of the large trans-verse faults and fracture zones of the Central and NorthAtlantic were made, and it is to these, with a viewtoward future drilling site identification, that we mainlydevote our attention. The results of detailed surveyingin the vicinity of the Leg 37 sites appear elsewhere inthis volume. Underway data collected on Leg 37 notreported in this volume are available from the Deep SeaDrilling Project.

INDIVIDUAL FEATURES OF INTERESTPolitical considerations prevented collection of data

within 320 km of Brazilian territory, and consequently

the continental margin off northeastern South Americawas not profiled.

Recording began in the deep waters of the CearaAbyssal Plain (Figure 2). The track crossed to the westof the main part of the St. Paul's Fracture Zone whichhere appears as a large, up-thrust block (1930-2030 hr,7 June), marking the northern boundary of the abyssalplain succeeded to the north by a general rise in base-ment elevation of approximately a Kilometer. Themagnetic signature of the fracture zone is not pro-nounced. There is a change in the general level of themagnetic field, which is about 40 y more positive to thenorth of the prominent ridge crossed at 2000 hr than tothe south of the ridge. The magnetic topography is alsoenhanced to the north of this ridge, which is consistentwith magnetic layer 2 being about a kilometershallower than it is to the south of the ridge. It isnotable that the ridge itself does not have a magneticsignature. This could be the result ofTt being composedof a fresh essentially nonmagnetic basic or ultrabasicintrusive material.

As the Mid-Atlantic Ridge was crossed obliquelyfrom the American to the African plates (Figure 3),water depth and sediment thickness gradually de-creased until the ridge crest was crossed, 0730-1100 hr,9 June. Magnetic anomalies are attenuated in this area,even over the alternative positions of the median valley(0930, 1100, 9 June). This is likely to be a consequenceof approximately north-south geological strike andnear-horizontal magnetization. A major fracture zoneis located at not more than 10 km north of the sup-posed position of the ridge axis crossing. In fact, it isimpossible to separate with confidence strong relief dueto fracture zone tectonics from crested mountain typetopography. However, we note that the deepest rifts inthis region, regardless of their origin, are strikinglybarren of sediment, indicating that an obstruction tothe west has prevented the flow of continental sedi-ments from the Amazon River from reaching into thesevalleys and that this portion of the fault complex isprobably not linked directly to the Vema Fracture Zoneproper. Slightly further north, from 1500 hr onward,multilayered sedimentary horizons of probably ter-rigenous origin mark the western extension of the VemaFracture Zone.

The eastern extension of the Kane Fracture (Figure4) was encountered beginning at 1030 hr, 13 June. Thecrossing of a deep associated with it produced thedeepest soundings of Leg 37 (1930-2000 hr, 13 June) 8.9sec to the flat-floored, sediment-filled valley, and atleast 9.3 sec to acoustic basement. The magnetic

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R . J. IULIUCCI, J. M. HALL, W. G. MELSON

Figure la. Track chart for Leg 37 of Glomar Challenger from Recife, Brazil, to thedrill site area. Track is annotated every 6 hours GMT time and each change of day.The large numerals along the track locate the portions of the track illustrated inthe text.

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UNDERWAY SURVEYS

If*

Figure lb. Track chart for Leg 37 of Glomar Challenger from drill site area to Dublin, Ireland. Track is annotated every 6hours GMT time and each change of day. The large numerals along the track locate the portions of the track illustratedin the text.

anomaly profile consists of a series of small amplitudefeatures, which barely exceed a maximum of 2OO7 peakto peak, and which fail to show obvious correlationwith bathymetric features.

The eastern extension of the Atlantic Fracture Zone(Figure 5), crossed between 0000 and 0200 hr, 15 June,is well defined here with distinct sediment-covered, flatsteps. Providing the sections are similar on both sides ofthe fracture zone, the presence of an adequately thicksediment layer to stabilize the bottom-hole assemblyand the steplike nature of the fracture here would makethis an excellent location for a set of single bit, deeppenetration attempts. Perhaps one site could be locatedat the southern end of the upper step, another at the

northern edge of the middle step, and a third on thesouthern edge of the deepest step (arrows), enablingsucceedingly deeper penetration into basement withoutthe need for re-entry. Again magnetic anomalies areweakly developed over the fracture zone. However, incontrast with crossings of the more southerly fracturezones, anomaly sense on the Atlantis Fracture Zonecrossing correlates well with bathymetry. Thus, positiveanomalies are strongly associated with bathymetricpeaks and negative anomalies with bathymetric lows.This relationship suggests that the basement rocks inthis area are normally magnetized with fairly steep in-clination. Remanent magnetization is favored as in-duced magnetization, inclined at the dipole field value

331

R. J. IULIUCCI, J. M. HALL, W. G. MELSON

+ioo Kr

I

- 1 0 0 ?

SOUTH

f-7

0400I

342 km

0000

8 June 74

2000 1600

112 km

Figure 2. Glomar Challenger, Leg 37, seismic reflection and magnetic anomaly pro-files in vicinity of St. PauVs Fracture Zone.

of +48° for the latitude of the fracture zone crossingwould result in a less clear relationship between anoma-ly peaks and troughs and bathymetric highs and lows.

Glomar Challengers track crossed the Oceanogra-pher Fracture Zone (Figure 6) at between 1430 and1830 hr, 16 June, where it separates two ridge crest seg-ments, thus taking the track back from the Africanplate onto the American plate. The OceanographerFracture Zone here is a deep, broad rift and is the singlemost outstanding bathymetric feature crossed duringthe leg. A brief stop to test an acoustic beacon accountsfor the discontinuity of the record and the loss ofresolution on the floor of the central fracture. A verystrong negative anomaly is associated with the ridgeforming the north wall of the rift which was crossedbetween 1700 and 1830 hr, 16 June. With a peak topeak amplitude of 1045y, the magnitude of this anoma-ly was only exceeded by the central anomaly of a ridgecrest crossing in the relatively shallow water of theAzores Plateau (Figure 8). The form of the anomaly

suggests reversed remanent magnetization as thesource. Remanence inclination must be rathershallower than the value of -54° for a dipole field at thelatitude of the fracture zone. The high amplitude of theanomaly suggests a very strong intensity of magnetiza-tion. The reverse magnetization of the ridge suggeststhat it does not consist of the typical basalts of Layer 2since very strong magnetization is found only for veryyoung unoxidized basalt of the Brunhes normal polari-ty epoch (Detrick and Lynn, 1975).

The Pico Fracture Zone (Figure 7) was crossedbetween 1430 and 1630 hr, 22 July. On this crossing thefracture zone appears as a 1600-meter deep asym-metrical depression about 35 km in width. Sedimentappears to be present in two small ponds within thedepression. The ponds are on the lower slopes of themore gently inclined south wall of the depression. Themagnetic profile across the fracture zone consists of aseries of anomalies of typical amplitudes for thelatitude of the fracture zone. Individual anomalies do

332

+200 jr i -

-200 1 L_

1 I I1800

1028 km

— 3

- 4

— 5

h-6

h-7

'j•J

9 June 74

Figure 3. Glomar Challenger, Leg 37, seismic reflection and magnetic anomaly profiles in vicinity of Vema Fracture Zone.

0000 2000717 km 8 June 74 640 km

R. J. IULIUCCI, J. M. HALL, W. G. MELSON

+100 -

-100 -

- 9

i "T2772 km 1800

" I I13 June 74

I1200

I I2535 km

Figure 4. Glomar Challenger, Leg 37, seismic reflection and magnetic anomaly profiles invicinity of Kane Fracture Zone.

not correlate with bathymetric features and presumablyrepresent polarity contrasts within oceanic basement.

An oblique crossing of the Mid-Atlantic Ridge Creston the northern flanks of the Azores Plateau was madeclose to 0830 hr, 23 July (Figure 8). The thinning ofsediments towards the median valley (0830 hr) is clearlyseen, as is a notable bilateral symmetry about the me-dian valley in the bathymetry. Thus, the prominentridges at 0030 hr and 1300 hr clearly match, as prob-ably do the smaller ridge complexes centered on 0530and 1030 hr. This bilateral symmetry in bathymetricfeatures about a ridge crest is well known elsewhere andis reasonably interpreted in terms of episodic medianvalley volcanicity, with the ridges representing periodsof rapid eruption with respect to lateral spreading rate.The concept of episodic volcanicity also provides an ex-planation for the distribution of basement

paleomagnetic directions at the Leg 37 drilling sites.This section of ridge crest is marked by the largestmagnetic anomalies encountered during Leg 37. Thecentral anomaly, 1 of Figure 8, is 2030Y peak to peak,while several flanking anomalies are close to IOOO7peak to peak. These large anomaly amplitudes are theconsequence of the shallow depth of the ridge crest inthis area, the floor of the median valley having a max-imum depth of only 1430 meters. Symmetry in themagnetic anomaly pattern is not as well developed as itis in the bathymetry. Tentative identification ofanomalies 2' and 3' have been made. However, anoma-ly 1, identified by its amplitude and location over themedian valley, is very asymmetric with a deep negativetrough at the south. Several explanations can besuggested for this asymmetry, although none can betested from a single profile. The present spreading

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+100 -

- < :

1 13400 km 0600

I I15 June

I0000 3188 km- 9

Figure 5. Glomar Challenger, Leg 37, seismic reflection and magnetic anomaly profiles invicinity of Atlantis Fracture Zone.

center may strike well away from north-south or asmall transform offset, perhaps located in the deepestsouthern side of the rift valley, may bring stronglymagnetized, normally and reversely magnetized basaltsinto contact along an approximately east-west, verticalinterface.

After crossing the ridge crest, the track of GlomarChallenger extended over increasingly older oceaniccrust of the Eurasian plate. Between 0500 and 0730 hron 25 July the enigmatic Kings Trough feature wastraversed (Figure 9). On our profile, which was madenormal to the trough axis, the steep-sided depressionhas a width of close to 50 km and a maximum depthbelow the northeast wall of 2200 meters. On our sectionthe more gentle southwest wall is largely covered with aveneer of sediment while the steeper northwest wall issediment free. The ponded sediments in the deepestpart of the trough show well-developed internal reflec-tions in contrast to the single thick transparent layerdraped over the basement surface outside the trough.

The magnetic signature of the trough is complex. A25O7 positive anomaly located directly over the peak ofthe northeast wall must be a topographic effect and im-plies magnetization close to the present field directionfor the uppermost basement in the vicinity. A similartopographic effect, complicated by adjacent within-basement magnetic contrast, could explain theanomalies associated with the southwest wall.

The final feature of interest on the Leg 37 track is thecontact between oceanic and continental crust seen tothe southwest of Ireland (Figure 10). Here we see fromright to left the termination of oceanic basement, at0600 hr, 27 July, a series of apparently faulted orslumped blocks between 0600 and 0800 hr, the steepcontinental slope between 0800 and 0930 hr, and a stepuntil the continental shelf is reached at approximately1745 hr. Two small submarine canyons situated on thenortheastern edge of the step were crossed at 1600 and1630 hr. Oceanic basement is covered by about 0.4 sec(310 at 1.55 km/sec velocity) of transparent, presuma-

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R. J. IULIUCCI, J. M. HALL, W. G. MELSON

+400 I-

+300 -

+200 -

+100 -

-100 -

-200 -

-300 -

-400 L-

7 1 I2200

4050 km

I I I I I1700 1500 120016 June 74 3889 km

-6

Figure 6. Glomar Challenger, Leg 37, seismic reflection andmagnetic anomaly profiles in vicinity of OceanographerFracture Zone

bly pelagic sediment. The step is underlain by multi-layered reflecting horizons typical of sediments of con-tinental origin, showing small but distinct structures ofprobable depositional and erosional origin to the max-imum depth of penetration of about 0.7 sec. The slightrise at the seaward margin of the step bars this con-tinental material from spilling out into the ocean basin.What appears to be a landwards wedging out of amember of the step's sedimentary sequence is seen at1630 hr, 0.44 sec penetration. The magnetic profileshows an interesting relationship to bathymetry andgeological province. Rather small amplitude anomaliesof typical oceanic wavelengths occur over oceanic base-ment. These terminate with a relatively prominentanomaly, of 4OO7 peak to peak amplitude, that is exact-ly coincident with the termination of clearly un-disturbed oceanic basement at 0600 hr. The continentalslope and peripheral 40 km of the step are marked by aseries of anomalies of similar amplitude but longerwavelength than the typical oceanic anomalies. Con-tinuing towards Ireland the next 75 km of step aremarked by a gradual increase in field intensity; in termsof the total field measurements the rate of increase israther greater than the rate of increase of the dipolefield. The final 60 km of the upper step and shelf aremarked by a sequence of anomalies that are not dis-similar to the anomalies associated with oceanic crust.We interpret the spatial variation in anomaly characterin the following way:

1) Oceanic crust: Anomalies are due to polarity con-trast within continuous basic oceanic layer.

2) Contact between oceanic and continental crust at0600 hr. The prominent anomaly and change in fieldlevel is due to the sharp contrast between the mag-netization of the two types of basement.

3) Small amplitude, long wavelength anomalies from0630 to 1130 hr: It is tempting to suggest that these aredue to dispersed basic intrusions within continentalcrust, dating from the initial opening of this segment ofthe North Atlantic.

4) 75 km zone of gradual increase in field level: Athick sediment cover overlies continental basement,with and within-basement magnetic contrasts too dis-tant to be resolved.

5) Inner zone of short wavelength anomalies: Con-tinental basement has risen sufficiently near the surfacefor within-basement magnetic contrasts to be resolved.

ACKNOWLEDGMENTSWe would like to acknowledge the technical staff aboard

Glomar Challenger during DSDP Leg 37 for their expertoperation of the underway survey equipment. Robert Gers-tein was kind enough to assist with the reduction of themagnetic data.

REFERENCESDetrick, R.S. and Lynn, W.S., 1975. The origin of high

amplitude magnetic anomalies at the intersection of theJuan de Fuca Ridge and the Blanco Fracture Zone: Earth.Planet. Sci. Lett. v. 26, p. 105-113.

Uchupi, E., 1971. Bathymetric Atlas of the Atlantic Carib-bean and Gulf of Mexico, Woods Hole OceanographicInstitution, Reference No. 71-72, unpublished manu-script.

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UNDERWAY SURVEYS

+600

+400

+200

-200 -

-400 -

-600 L-

NORTHEAST

IISOUTHEAST

I I \ I5064 km 1800

I I22 July 74

— 1

— 2

1-3

LU

«•:-5

12004941 km

Figure 7. Glomar Challenger, Leg 37, seismic reflection and magneticanomaly profiles in vicinity of Pico Fracture Zone.

337

R. J. IULIUCCI, J. M. HALL, W. G. MELSON

+1400

+1200 -

+1000 -

-200 -

-400 -

-600 -

-800 L

NORTHEAST

1200 I I I0600

23 July 74

SOUTHWEST—0

— 1

I I I I T0000 5064 km

22 July 74

Figure 8. Glomar Challenger, Leg 37, seismic reflection and magnetic anomaly profiles invicinity of Ridge Crest off Azores.

338

UNDERWAY SURVEYS

+200 -

+100 -s

-100-

-2001-

NORTHEAST SOUTHEAST

12006152 km

I I I I0600

enα

UJGO

020025 July 74 5966 km

Figure 9. Glomar Challenger, Leg 37, seismic reflection and magnetic anomaly profilesin vicinity ofKing's Trough.

339

R. J. IULIUCCI, J. M. HALL, W. G. MELSON

+200 J r

-200 3T L_

NORTHEAST SOUTHEAST

I I I I I II7168 km 1600

K, ., ''ii * ,:•SΦ:••;:-j'i;k-.• • • \ • 3

• : • . • • • > . • • . •

— 0

— 1

— 2

WST~ 3

1-4 R

— 5

• • • ; : ; ;

:

1200 0800

27 July 75

0400

6903 km

Figure 10. Glomar Challenger, Leg 37, seismic reflection and magnetic anomaly profiles in vicinity ofContinental Slope at 50° North latitude.

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