Geophysical transitions across the northwest Atlantic magnetic quiet-zone border

Earth and Planetary Science Letters, 29 (1976) 435-446

435

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [41

GEOPHYSICAL TRANSITIONS ACROSS THE NORTHWEST ATLANTIC MAGNETIC QUIET-ZONE BORDER PATRICK T. TAYLOR U.S. Naval Oceanographic Office, Washington, D.C. (USA)

DAVID GREENEWALT Naval Research Laboratory, Washington, D.C. [USAJ

Received February 15, 1975 Revised version received May 20, 1975 Further revised version received September 10, 1975

Deep and surface magnetic measurements, gravity and subbottom seismic profiling data have been gathered across a part of the northwest Atlantic smooth-rough magnetic border. These data indicate that the transition involves only a change in the magnetic field, without associated gravity or subbottom topographic signature. Model studies suggest that a change in basement magnetization from 0.010 emu/cm3to 0.005 emu/cmacould account for the magnetic field change across the boundary from the rough to the smooth sectors. Various theories previously proposed to explain this magnetic boundary are discussed with respect to these newer data; reduction of basement magnetization by weathering of the proto-Atlantic Ocean floor as described by Drake et al. (1968) is the preferred explanation.

1. Introduction The magnetic field off the eastern continental margin o f the United States is relatively free of large-amplitude magnetic anomalies [ 1 - 3 ] . This region has been referred to as the magnetic smooth or quiet zone. There have been several theories proposed to account for this lack of large-amplitude anomalies [ 2 - 7 ] ; none, however, have been shown to be completely satisfactory. During Leg XI of the Deep Sea Drilling Program (DSDP), site 105 was chosen in order to obtain basement rocks beneath the quiet zone [8]. By dating the sediment contiguous with the basalt recovered at this location it was determined to be 155 m.y. old or Middle Jurassic (Oxfordian) [9]. Near-bottom magnetometer measurements were made near DSDP site 105 in 1971; these data indicated that this magnetic basement was reversely magnetized [10]. Subsequent magnetic property measurements made on the recovered DSDP basement core confirmed this reversely polarized basement [11]. A series of gravity and seismic profiles was made in

the area centered around DSDP site 105 in order to se~ whether the quiet-zone border exhibited a change in basement character or basement rock density. These gravity and seismic proffdes were measured in conjunction with additional surface and near-bottom magnetic profiles obtained during June 1973 by the U.S.N.S. "Lynch". Fig. 1 illustrates the general area of this study together with the location of the near-bottom magnetic measurements. This region covers the intersection of the Hatteras abyssal plain and the lower continental rise hills. Bathymetric depths range from over 5100 m across the abyssal plain to less than 4900 m, producing a maximum relief of over 200 m.

2. Surface magnetic data During the deep-tow operations, and while transiting within this area, a proton precession magnetometer was deployed at the sea surface. A residual magnet. ic anomaly map (Fig. 2) was derived from the total

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437

east of 69 ° west longitude to between 30 and 100 gammas west of this meridian. Anomaly half-wavelengths are 5, 6, and 10 km within the quiet-zone border, and 7, 8, and 15 km outside it. In our opinion the mean anomaly half-wavelength of 7 km for the quiet zone versus 10 km in the rough zone does not in dicate a significant change in reversal frequency. However, Barrett and Keen [15] found two narrow zones of reversed polarity in their surface magnetic field study over the quiet zone off Nova Scotia, Canada; they state that this represents a significant change in anomaly wavelength. From the data in Fig. 2 it is evident that DSDP site 105 is indeed located within the magnetic quiet

intensity measurements by removal of the International Geomagnetic Reference (IGRF, 1973.5) [12] and a field level adjustment of +250 gammas. Ship positioning was obtained from both satellite fixes and LORAN-C observations recorded automatically every ten minutes. From cross-track adjustments, the estimated navigational error was less than 2 km. Previous measurements of the regional surface magnetic field in the northwestern Atlantic [13] indicated a change from high- to lower-amplitude anomalies across the quiet-zone border, shown in the insert of Fig. 1. Due to the large width ( 2 0 0 - 3 0 0 kin) o f the quiet zone, we had to restrict our study to cover the eastern border of this region. It is evident from our data (Fig. 2) that the magnetic transition in our study area is gradual. Anomaly lineation is in an approximately north 45 ° east direction; this value is comparable with the north 40 ° east azimuth given in Vogt et al. [14]. Peak to trough amplitudes vary from 150 to 300 gammas

zone.

3. Gravity measurements During this study gravity measurements were obtained using a Lacoste-Romberg meter (S-51) mount-

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439

ly negative gravity values. This is in agreement with worldwide measurements [16] which show that the western North Atlantic is in a geoidal depression of about 20 regals. The average gravity field within our region of study is not therefore anomalous with respect to the area in general. A gravity trend of about 15 mgals appears to exist within our area of study, increasing in the eastward direction, and thus suggesting that a gravity change may be assoicated with the quiet-zone border. Subsequent to our study, however, regional gravity field data became available for a larger part of the western North Atlantic, as shown in Fig. 4. While these data show the same basic gravity trend within the area where our measurements were made, that trend does not continue along the quiet-zone border either to the north or south of our area. It does not appear, therefore, that a gravity signature is associated with the edge of the magnetic quiet zone. The short-wavelength, low-amplitude, free-air gravity anomalies presented in the profiles shown in Figs. 5 9 probably represent noise which is superimposed on the longer-wavelength signal.

4. Near-bottom magnetometer data A deep-towed magnetometer was used to study the fine structure of the magnetic field in -the vicinity of DSDP site 105; surface magnetic measurements were obtained simultaneously. The deep instrument was towed by an armored conducting cable at a speed of about 1 kt; since the ship was drifting during these measurements, track direction was dictated by wind and ocean currents. Five near-bottom magnetometer lines were measured in this area, and their locations are shown in Fig. 1. Height of the instrument above the bottom was determined by means of a 12-kHz pinger clamped 30 m above the magnetometer, while ocean depth was measured with the ship's fathometer. The magnetometer position aft of the ship was estimated from length of cable out and cable angle, and was only known to within -+200 m. As the ocean bottom in this area is relatively flat, the magnetometer height above bottom, held at a nearly constant 500 m, was known to within -+10m. Since the seismic system could not be used at the low drift speed, ship tracks were subsequently retraced to determine subbottom topog-

raphy. Two-way seismic travel time in the sediments did not exceed 1 second (Figs. 5 - 9 ) and the velocity in the sediment was less than 2 km/sec [17]; therefore, sediment thickness was less than 1 km, so that the mag netometer was not more than 1.5 km above the basement. Thus only sinusoidal anomalies with wavelength greater than 2 km would be detectable at tow depth, compared with a wavelength of 7 km for the surface tow magnetometer, assuming a magnetization of 0.005 emu/cm 3. The near-bottom magnetic field is shown in Figs. 5-9, along with surface magnetic field, free-air gravity and seismic profiler data. Upward continuation of the deep-magnetic field demonstrated that it did not differ significantly from the surface data, when account was taken of the spatial lag (2 km) between the deep and surface magnetometers.

5. Magnetic model studies In order to interpret these near-bottom magnetic data, model studies [18] were conducted in which theoretical field profiles were calculated and compared with the near-bottom measurements. The theoretical magnetic profiles were produced by a two-dimensional body, the top surface of which corresponded to the measured subbottom topography. Magnetic inclination of 60 ° was chosen to be in agreement with the material from DSDP site 105 [11 ]. The remaining parameters body thickness and intensity of magnetization, were chosen arbitrarily. Initially a constant magnetic intensity and direction were used, with two models: (1) constant depth to bottom of the magnetized layer, and (2) constant layer thickness. Both these produced poor agreement between calculated and observed fields In order to bring even the amplitude of fluctuations of the calculated field into agreement with that measured, an unrealistically high intensity of magnetization was needed. Therefore, a variation of magnetic intensity and/or direction was required for the models. It was decided to use normally and reversely magnetized blocks of material in the models, keeping the absolute value of the intensity the same in each line. Other magnetic configurations can be made to yield the same calculated field. The normally and reversely magnetized blocks, however, require the lowest intensity to produce a given anomaly size. Intensity values

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found for the material obtained from DSDP site 105 [11] indicated that low magnetic intensities should be used in the models. The layer thickness for models of lines 4, 5, 6, and 7 were kept to within +-10% of a constant 1 km. Line 3, however, varied in the thickness by a factor of-+50% ( 5 0 0 - 1 5 0 0 m) in order to obtain a better match with the observed data. To avoid large magnetic gradients in the computed field, contacts between normally and reversely magnetized blocks were inclined. This angle of inclination was arbitrarily chosen to improve the fit. The same result could be produced by a gradual transition in magneti-

zation across the region of polarity change. The models used to produce the best fit of the computed with the observed fields are shown in Fig. 10. While the agreement between the calculated and computed fields is far from perfect, ambiguity in determining the actual position of the basement makes a further refinement of the model meaningless. The magnetization used for lines 3 and 5 (0.01 and 0.007 emu/cm 3, respectively) were greater than that for lines 4, 6, and 7 (0.005 emu/cm3). The former lines are outside the quiet zone, while the latter are within it.

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Across the transition region between the magnetically rough and smooth zones we have measured the sediment thickness, gravity, surface and near-bottom magnetic fields. A systematic change in character of neither the gravity field nor the acoustic basement was detected. Apparently, the magnetic field is the only geophysical parameter which exhibits consistent change across this region. These data have some bearing on the theories previously proposed to explain the origin of this region. The survey of Barrett and Keen [15] together with our magnetic data suggest the existence of magnetic field reversals recorded by the ocean crust beneath the quiet zone. It is therefore probable that this region was not formed during a long period of uniform polarity, assuming, of course, that the linear magnetic anomalies, shown in Fig. 2, represent polarity reversal record ed by the earth's crust. There is a suggestion (Fig. 2) that the characteristic length of the normally and reversely magnetized blocks decreases toward the smooth zone. As mentioned previously, Barrett and Keen [15] have detected a decrease in anomaly wavelength across this boundary. They suggested that the quiet-zone border represents an isochron separating periods of frequent and infrequent field reversals. Poehls et al. [4] proposed a similar reason for the lack of large anomalies, however, they claimed a different mechanism. The quiet zone was attributed by these investigators [4] to a very large reduction in the rate of sea-floor spreading during the time of formation of the oceanic crust beneath the quiet zone. This shortening of the average distance between normally and reversely magnetized blocks would reduce the amplitude of the anomalies produced at the sea surface. We find the observed shortening in anomaly wavelength is insufficient to cause a significant reduction in surface anomaly amplitude. Additional closely spaced magnetic data will be required between the quiet-zone border and the continental margin in order to further test this theory. The seismic data were recorded either entirely within or entirely outside the quiet zone; with the excep-

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tion of line 6 (Fig. 1), which was the only profile to apparently cross this magnetic boundary. These data revealed a gradual landward thickening of the sediment and deepening of the seismic basement of less than 300 m (Fig. 8). However, this increase in basement depth is certainly unable to produce a significant magnetic anomaly attenuation. In addition, these seismic profiler data should show whether or not there is a change in the character (e.g. roughness or reflectivity) of the acoustic basement associated with this magnetic transition, similar to the Bermuda Discontinuity [19]. If there is such a change it is not apparent in our data.

Drake et al. [20] proposed that prior to midJurassic the borders of the western and eastern Atlantic quiet zones were contiguous and formed a proto-Atlantic Ocean; subsequently rifting occurred and the resulting ocean became an expanding zone between these two boundaries. It might be expected that such a sequence of events would be recorded by a change in either the subbottom topography or the gravity field, but this is not indicated in our measurements (Figs. 3, 4, and 5 - 9 ) . Model studies, carried out to interpret the nearbottom magnetic field (Fig. 10), indicate a decrease by a factor of two in the magnetization of the base-

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ment rock west of the quiet-zone boundary compared with that to the east. We must, however, assume that the thickness of the magnetized layer is relatively constant; if not, then this apparent change in magnetization could result from a decrease in the thickness of the magnetized oceanic crustal layer. All values o f magnetization used in our model studies were an order of magnitude greater than the

value of 2.24 × 10 24 emu/cm 3 measured on the basalt samples recovered from DSDP site 105 [11 ]. Possible explanations for this disparity in observed and model value are: (1) DSDP site 105 penetrated only the uppei 11 in of layer 2, and therefore does not constitute a representative sample of this layer; (2) from Fig. 11 we note that DSDP site 105 is near a normal-reversed polarity transition, within the quiet zone. The proxim-

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ity to this normal-reversed border may account for the lower values of magnetization. The reduction in magnetization required by this current study is consistent with Taylor et al. [3] who theorized that a reduction in basement magnetization, by means of a metamorphic process, could account for this quiet zone. The proposed decrease in magnetization of the oceanic crust in our model studies occurs in a relatively limited distance, about 40 km; it is, therefore difficult to envisage a process which alters the basement magnetization of a vast area terminating so rapidly. It has been shown that weathering can reduce magnetization [e.g. 21 and 22]. Thus, this change in magnetization across the quiet zone may reflect a significant difference in age, consistent with the Drake et al. [20] model. Until we have better information on the nature of the oceanic crust in this area we cannot specify which alteration process (e.g. weathering or oxidation) if any, may have produced the observed reduction in

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446 magnetic amplitudes. Until further sampling o f the crust landward o f D S D P site 105 is made, this t h e o r y must remain speculative.

Acknowledgments We wish to thank the officers and crew o f the U.S.N.S. " L y n c h " for their c o o p e r a t i o n during the cruise. William Osborne (University o f Delaware), Clive Hulick and Thomas Getz (Williams College), and Dr. J o h n Schlee, Frank Jennings, R o b e r t Mattick, William McNair (U.S. Geological Survey) aided in the data gathering during this study. R o b e r t Strauss (U.S. Naval Oceanographic Office) was responsible for gravity data acquisition and was joined by D. Zinzer (U.S. Naval Oceanographic Office) in the post-cruise data reduction. U.S. Naval Oceanographic Office kindly allowed us to publish the data shown in Fig. 4. Drs. H.C. Eppert, Jr. and P.R. Vogt reviewed this paper. Maureen Long aided in preparation o f this manuscript

References 1 E.R. King, I. Zietz and W.J. Dempsey, The significance of a group of aeromagnetic profiles off the eastern coast of North America, U.S. Geol. Survey Prof. Paper 424-D (1961) 299. 2 J.R. Heirtzler and D.E. Hayes, Magnetic boundaries in the North Atlantic Ocean, Science 157 (1967) 185. 3 P.T. Taylor, I. Zietz and L.S. Dennis, Geologic implications of aeromagnetic data for the eastern continental margin of the United States, Geophysics 33 (1968) 755. 4 K.A. Poehls, B.P. Luyendyk and J.R. Heirtzler, Magnetic smooth zones in the world's oceans, J. Geophys. Res. 78 (1973) 6985. 5 K.O. Emery, E. Uchupi, J.D. Phillips, C.O. Bowin, E.T. Bunce and S.T. Knott, Continental rise off eastern North America, Am. Assoc, Pet. Geol. Bull. 54 (1970) 44. 6 P.R. Vogt, C.N. Anderson, D.R. Bracey and E.D. Schneider, North Atlantic magnetic smooth zones, J. Geophys. Res. 75 (1970) 3955. 7 R.L. Larson and W.C. Pitman III, Worldwide correlations of Mesozoic magnetic anomalies and its implications, Geol. Soc. Am. Bull. 83 (1972) 3645.

8 C.D. Hollister, J.I. Ewing, D. Habib, J.C. Hathaway, Y. Lancelot, H. Luterbacher, F.J. Paulus, C.W. Poag, J.A. Wilcoson and P. Worstell, Introduction, in: Initial Reports of the Deep Sea Drilling Project, XI (1972) 5. 9 D. Habib, Dinoflagellate stratigraphy, Leg XI, Deep SeaDrilling Project, in: Initial Reports of the Deep Sea Drilling Project, XI (1972) 367. 10 P.T. Taylor and D. Greenewalt, Deep magnetic measurements in the West Atlantic quiet zone, Nature Phys. Sci. 237 (1972) 51. 11 P.T. Taylor, N.D. Watkins and D. Greenewalt, Magnetic property analysis of basalt beneath the quiet zone, E~S, Trans. Am. Geophys. Union 54 (1973) 1030. 12 A.J. Zmuda, The International Geomagnetic Reference Field 1965.0, in: World Magnetic Survey 1957- 1969, ed. A.J. Zmuda, IAGA Bull. 28 (1971) 147. 13 A.M. Einwich, A magnetic zone west of the "smooth zorn East Coast of North America, EeS, Trans. Am. Geophys. Union 53 (1972) 365. 14 P.R. Vogt, C.N. Anderson and D.R. Bracey, Mesozoic magnetic anomalies, sea-floor spreading, and geomagnetic reversals in the southwestern North Atlantic, J. Geophys. Res. 76 (1971) 4796. 15 D.L. Barrett and C.E. Keen, Lineations in the magnetic quiet zone of the northwest Atlantic, Nature 253 (1975) 423. 16 E.M. Gaposchkin, Earth's gravity field to the eighteenth degree and geocentric coordinates for 104 stations from satellite and terrestrial data, J. Geophys. Res. 79 (1974) 5377. 17 E. Schreiber, P.J. Fox and J. Peterson, Compressional sound velocities in semi-indurated sediments and basalts from Deep Sea Drilling Project, Leg 11, in: Initial Reports of the Deep Sea Drilling Project, XI (1972) 732. 18 J.R. Heirtzler, G. Peter, M. Talwani and E. Zurflueh, Magnetic anomalies caused by two-dimentional structure, their computation by digital computers, and their interpretation, Tech. Rep. Columbia Univ. 6 (1962). 19 P.R. Vogt, G.L. Johnson, T.L. Holcombe, J.G. Gilg and O.E. Avery, Episodes of sea-floor spreading recorded by the North Atlantic basement, Tectonophysics 12 (1971) 211. 20 C.L. Drake, J.I. Ewing and H. Stockard, The continental margin of the eastern United States, Can. J. Earth Sci. 5 (1968) 993. 21 M. Marshall and A. Cox, Magnetic changes in pillow basalt due to sea-floor weathering, J. Geophys. Res. 77 (1972) 6459. 22 M. Marshall and A. Cox, Magnetism of pillow basalts and their petrology, Geol. Soc. Am. Bull. 82 (1971) 537.