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Subsidence of continental margins
Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
SUBSIDENCE OF CONTINENTAL MARGINS R-E. SHERIDAN Department of Geology, University of Delaware, Newark, Del. (U.S.A.) (Received November 21, 1968)
The possibility of the subsidence of the Blake-Bahama Basin is discussed. Restoration of this basin to its earlier shallow position indicates that it can be compared with several similar structures presentlyfoundat shallow depths. This comparison may provide clues to the processes involved in the subsidence. INTRODUCTION It has long been obvious that the marginal areas of many of the continents have subsided and tilted seaward accumulating the thick sediments of
coastal basins (Drake et al., 1959). Along some margins where sedimentation has not accompanied subsidence, the ocean appears to have expanded at the expe.nse of the continents and vast marine basins were formed with thinned crusts (Beloussov and Kosminskaya, 1968). The accommodation of these thick sediments and deep marginal basins requires significant crustal changes. The exact nature of these crustal changes is still a debatable subject, but common to the ideas which have been proposed is the conversion of lighter materials to denser ones under the areas of subsidence. Processes which change the density beneath subsiding basins may include a sort of erosion of the base of the crust by moving currents in the upper mantle, as advocated by Van Bemmelen (1968). Another process suggested by Beloussqv (1960) is the igneous assimilation of the granitic crust by the upwelling of superheated basaltic magma from within the mantle. Another suggestion was put forward by Ringwood and Green (1966) in which the accumulating weight of volcanics along the already deepened margins leads to a phase change to denser eclogite beneath the continental rise eugeosyncline. The dense eclogite then produces drastic subsidence which eventually leads to erogenic folding and intrusion of the marginal areas. Bess (1955) has attributed the vertical movements of both oceanic and continental crusts to density changes related to serpentine-olivine reactions. Part of the confusion about these proposals for subsidence along continental margins is the lack of observational data to establish the geological history of the subsidence and to establish which crustal layers are actually altered and when in the sequence of subsidence they are altered, It is not quite clear whether the margins subside because of crustal changes or that the crustal changes take place because of the subsidence. Tectonophysics, 7(3) (1969) 219-229
219
W
IO/KM
8
21
0 r-
oc
Structure section across Sheridan et al., 1966).
L
200
Lo
I
c
the continental
margin
east of Florida
at about 29ON. Vertical
exaggeration
EVIDENCE
FORSUBSIDENCE
An equation of the subsidence of the conti~en~l margin east of Florida is useful because of the large amount of data available. Deep wells have penetrated a continuous sequence of shallow water carbonates and evaporites to depths of greater than 5 km under the Florida Platform and the Bahamas. From seismic data, a similar section is interpreted to exist under the Blake Plateau (Sheridan et a~.,l~6), (Fig.1). The thick sequence of shallow water carbonates of the Blake Plateau crop out along the Blake escarpment where they have been sampled by dredging (Heezen and Sheridan, 19661, (Fig.2). In the Florida and Bahama wells and along the Blake Escarpment,‘the shallow water limestones of earliest Cretaceous age (Neoeomian) are found at depths similar to the ocean floor to the east.
W
BLAKE
PLATEAU
BLAKE-BAHAMA
BASIN
E
100 G
0
---r----~-“------r----
, GRAY
FORAM. LUTlTE
A*167-21
TAN CALC
-A-167-25
, PLfOCENE
LUTlTE.
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EOCENE
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LUTITE,
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3
APTiAN
APTIAN
4
4
VERTICAL
EXAGGERATION
5 X
5
5
0
5
IO
15
20
25
35
30
KILOMETERS
Fig.2. Outcrops on the Blake escarpment at 29ON. Dashed arrows are data extrapolated large distances along strike. (After Heezen and Sheridan, 1966.)
Tectonophysics, ?(3)(1969)219-229
221
Although it seems clear that the Blake Plateau, Florida, Bahamas area subsided at least 5 km since the earliest Cretaceous, the history of the adjacent ocean floor is less clear. There is evidence, based on deep-seacoring, seismic profiler data, and deep-ocean drilling (Joides) in the oceanic basins east of the Blake-Bahama area, which reveal that the sediments of reflector @ (Ewing et al., 1966) are deep-water facies of earliest Cretaceous (Neocomienf and Late Jurassic age. Windiseh et al. (1968} reported a core from this layer as containing reworked fragments of Neocomian shallow water limestone which appear to have been washed into deeper-water sediments from the Bahama escarpment. Berggren et al. (1968) reported that a Joides drill core penetrated Late Jurassic chert and hard limestone of deep water facies at a depth of 5.6 km in horizon&. Tracing reflector p from the areas where these cores were taken into the Blake-Bahama Basin indicates that horizon @ corresponds to the top of a 3.64-km/set velocity refractor. This velocity appears to be characteristic of horizon/3 which is only observed near to the continental margin (Houtz et al., 1968). The depth to horizon 6 and the 3.64-km/see layer in the Blake-Bahama Basin is 5.80 km (Fig.1). If the BlakeBahama Basin subsided in the same manner as the Florida, Bahamas, Blake Plateau area without a significant post-Jurassic fault or flexure between these areas, this would imply that the Blake-Bahama Basin was only 800 m deep in Neocomian and earlier times. The seismic data reveal that the sediments of horizon /3 and younger age are flat-lying right up to the Blake escarpment with little of no evidence of shearing or differential movement (Windisch et a1.,1968). The seismic data then do not imply the presence of a post-Jurassic fault at the base of the escarpment. If the ocean floor of the Blake-Bahama Basin was only 800 m deep in Neocomian and earlier times, the sedimentary facies might stillbe dominated by planktonic faunas and floras in a non-elastic environment. It cannot be certain, therefore, that the deeper-water planktonic facies of Upper Jurassic age recovered in the Joides drilling off the Bahama escarpment was not deposited on this 800 m deep ocean floor. The Blake-Bahama Basin is interesting because of its typical oceanic depths and its atypical oceanic seismic-structure. It is unlike the true oceanic basins in that the deepest refractor observed on normal length seismic profiles has a velocity of 7.49 km/set (Sheridan et a1.,1966) (Fig.1). This velocity is intermediate between those of the normal oceanic crust ($.2-7.2 km/set) and normal mantle (7.8-8.5 km/see). Such ambiguous velocities are commonly observed near other continental margins (Fig.J), (Drake and Nafe,1968). The Blake-Bahama Basin is also unlike the major part of the oceanic areas since it lacks the short wavelength magnetic anomalies produced by the spreading sea floor. There is a very sharp boundary east of the Blake-Bahama Basin beyond which the typical “rough” oceanic magnetic anomalies occur, whereas the magnetic anomaly pattern of the basin is relatively smooth (Heirtzler and Hayeq1967). This magnetically “quief’ zone has been interpreted by Heirtzler and Hayes (1967) to be an area of normal ocean floor spreading during the Permian when there was no mixed magnetic polarities. It has also been interpreted by Zietz et a1.(1967) as an area where the upper crust is a thick sedimentary section resting on a normal oceanic crust. On the other hand, Brakl et al.@9681
222
Tectonophysics, 7(3) (1969) 219-229
l Continental I,“”
0
marqins.
) IOOOm depth
1
Treryhes
-k
12
2
I
3
4
I
5
6
7
6
0
Fig.3. Compressional wave velocity vs. depth beneath the sea floor for all refraction data (up to 1965) on the continental margins deeper than 1,000 m. (After Drake and Nafe, 1968.) Tectonophysics,
7(3) (1969) 219-229
223
OBSERVED RESTORED BLARE-SAWMA SASIN STRUCTURE 0
I
ANOMALOUS OBSERVED NEWFOUND1 .AND SHELF I I
CONTINENTAL STRUCTURES IE D SEA IVREA ZONI r
z
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i
;,.. .,.;;.. :: .‘..‘:., .:’ . . . . . .” _ ‘...‘:‘, :’ ..‘...: .;: ..:;..., ‘. ‘.’ : :1’.::,:..:f,. ‘/6.6e:
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/’ I\‘\’ ; 1, 1.’ ,;,, L; I,,>\ ‘,.I.” ; I-; ?‘\ ;,
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+ 7.49+
Fig.4. Restored Blake-Bahama Basin seismic structure of end of Jurassic-beginning of Cretaceous time compared to observed anomalous continental structures. Note that nearly 700 m of Cr&aceous-Recent sediments should be removed from this restored Blake-Bahama Basin. interpret the broad linear anomalies which extend from the BlakeBahama Basin to north of Cape Hatteras as being due to reversely magnetized basement blocks at depths of about 8 km. This implies the structure may be similar to normal oceanic areas but with a deeper basement. In light of the existing data, the interpretations can now be made that the structure of the Blake-Bahama Basin has subsided at least 5 km since earliest Cretaceous and that the section of the upper crust consists mostly Qf marine sedimentary rocks of pre-Cretaceous age which extend to about 8 km depths. The observed seismic-structure in the Blake-Bahama Basin (Fig.4) shows the sedimentary layers (1.82, 2.84, and 3.64 km/set) are about 3 km thick over a 6.65-km/set crystalline basement (Sheridan et a1.,1966). The intermediate velocity layer, 7.49 km/set, is observed at a depth of 11.75 km. RESTORATIONOF SUBSIDEDCRUST Returning now to the earlier discussion on subsidence, the history and observed structure of the Blake-Bahama Basin may provide some insight into the crustal changes involved in its. subsidence. Upon restoring the 224
Tectonophysics, 7(3) (1969) 219-229
seismic-structure section to its near sea-level position at the end of Jurassic and beginning of Cretaceous (Fig.4), the shallow 6.75-km depth to the 7.49-km/see material is the most imposing problem. If such a section existed at such a depth, it would imply that this part of the restored continent was made up of a largely basic and ultrabasic crust. Also, depending on the existence and depth of a mantle discontinuity, it is probable that such a structure would be out of isostatic adjustment. It could be interpreted more conveniently that the 7.49-km/set material never did exist at this shallow depth in the restored continent, but that it was formed during or after subsidence. This would imply that the restored continent was a more normal structure. This Iate stage transformation of normal continental crust might be accomplished by subcrustal erosion, basification, deserpentinization or eclogite phase changes already mentioned, However, it may be that the restored continental structure was actually basic and ultrabasic in nature even to a shallow depth. To believe this, one would expect to find examples of largely basicand ultrabasic crusts occurring in continental areas with velocities of about 7.4 km/set observed at shallow depths, At present a few such areas are known (Fig.4). One is the Newfoundland area in the Canadian Appalachians (Sheridan and Drake,1968), another is the Ivrea zone in the Italian Alps (Fuchs et a1.,1963), and another is the Red Sea rift (Drake and Girdler, 1964). The comparison of the restored Blake-Bahama section to those of Newfoundland shelf areas, the Ivrea zone, and the Red Sea rift is remarkable (Fig.4). The common factors relating the Newfoundland area, the Ivrea zone, and the Red Sea rift are that these areas are within zones of major mountain belts or rift systems having extensive intrusions of basic and ultrabasic rocks. These areas have significant positive Bouguer anomalies indicating a lack of isostatic adjustment. The denser crust in these areas should have a tendency to sink. If the comparison of any of these Newfoundland, Ivrea or Red Sea sections with the restored Blake-Bahama section is valid, it can be speculated that the structure beneath the sediments and sedimentary rocks of the Blake-Bahama Basin includes a highly metamorphosed and intruded basement which is intruded extensively by ultrabasics derived from a deeper, denser subbasement. The structure may have once been involved in an extensive orogeny or taphrogeny before the subsidence and deposition of the sediments of the basement. From this discussion the sequence of events in the subsidence of the Blake-Bahama Basin might resemble: (1) An orogeny or taphrogeny ‘in an extensive area along what was the old North American continental margin probably during pre-Jurassic times. (2) Extensive emplacement of ultrabasic intrusions and the development of the 7.4-km/set layer at a shallow depth. (3) Possible uplift and erosion of the metamorphosed basement. (4) Beginning of subsidence of the unstable denser crust by Jurassic with the accumulation of sediments at a rate nearly equal to subsidence. (5) Continued subsidence and very little deposition of sediments since earliest Cretaceous. This resulted in the increasing water depths to more than 5 km.
Tectonophysics, 7(3) (1969) 219-229
225
OmR
AREAS OF SUBSIDENCE
The connection between the 7.4-km/‘sec material, erogenic and taphrogenie belts, and subsiding continental margings is not limited to the specific examples discussed above, but appears to be more common. The relationship of erogenic belts to the margins of continents is very well known and consistent for the Cenozoic, Mesozoic, and Paleozoic systems. This may even be so for the Precambrian. Summarizing the worldwide crustal velocity data, Drake and Nafe (1968) (Fig.5) find that the peculiar 7.4-km/see material is commonly observed in erogenic and taphrogenic belts of Paleozoic and younger age. This type of material is apparently absent from the older stable shields forming the nuclei of the continents. The common observation of the 7.4-km/see material in areas of the continental margin allows the suggestion of a transitional sequence from stable continents to erogenic or taphrogenic belts to subsided margins and perhaps to deep ocean basins. The absence of these anomalous ‘7.4-km/set velocities in most deep basins implies the removal of this material for the sequence mentioned above to be possible. The removal of the bottom of the 7.4-km/set material and the development of a shallower mantle is also required for subsiding margins. From the world data the mantle must shoal from 30-35 km depths to as shallow as 12 km beneath the thinned crusts of the margins. The transition of 7.4-km/set material to something with a velocity of
(After 226
Fig.5. Summarized velocity-depth Drake and Nafe, 1968.)
data for all crustal
Tectonophysics,
provinces.
i’(3) (1969) 219-229
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Fig.6. Velocity-depth data for some deep ocean basins where subsidence is suspected. (After Drake and Nafe, 1968.) Tectonophysics,
7(3) (1969) 219-229
227
about 8.0-km/see at such shallow depths of 12 km requires a reaction that can occur at relatively low pressures. Deserpentinization reactions are such phase changes (Bowen and Tuttle,1949). Some of these reactions occur at temperatures ofe340-36O’-‘C implying that thermal conditions in the areas of these transitions at IZ-km depths were other than normal. These high temperatures at such shallow depth may be explained if the subsiding marginal block is influenced by a higher than normal geothermal gradient. In areas of Mesozoic-Cenozoic erogenic and taphrogenic belts the mean heat flow is anomalously high, (MacDonald, 1965), and temperatures may reach near 360°C at I2 km depths under such tectonic areas. If the 7.4-km/set material subsides within this thermal regime, critical temperatures for deserpentinization may be found, Although this is speculation, there appear to be chemical reactions that are possible which might allow the removal of the transitory 7.4-km/see material. Besides the areas of the continental margin, which may have been pre-existing shallow areas since foundered, there are other deep ocean basins which may have similar origins (Menard,I967). The deep Gulf of Mexico is underlain by salt domes (Worzel et al., 1968) which formed as diapirs from a salt layer of probably shallow water origin; and the Caribbean Sea is underlain by Eocene shallow water limestones in some areas (Fox et a1.,1968). These areas have crustal velocity distributions very similar to the deep continental margins (Drake and Nafe,1968) (Fig.G), and fit into the above hypothesis. The Gulf of Mexico and Caribbean are also surrounded by or are along the extension of erogenic serpentine belts. Similarly, the western Mediterranean, by virtue of the presence of the 7.4-km/see material and the neighboring erogenic serpentine belts, may be a deep basin of subsided continental structures. Certainly there is good geological evidence for the presence of a previous landmass in this area (Klemme.1958). ACKNOWLEDGEMENTS
Much of the evidence and data on the continental margins has been reported by workr rs of Lamont Geological Observatory. Their excellent work is referenced extensively in this discussion paper. Discussions with these researchers at Lamont were very helpful, especially those with N.T. Edgar, C. Windisch, J.E. Nafe and C,L. Drake. Although these workers have read this discussion paper, the conclusions and remarks made are not necessarily in agreement with their views. REFERENCES Beloussov, V.V., 1966. Development of the earth and tectogenesis. J. Geophys. Res., 65: 4127-4146, Beloussov, V.V. and Kosminskaya, I.P., 1968. Structure and development of the transition zones between the continents and oceans. Can. 3. Earth Sci., 5: 1011-1026. Berggren, W-A., Bukry, D., Fischer, A.G., Passagno, EA., Slade, M. and Lyle, 1968. Deep-sea Drilling-project-preliminary results, 1. Paleontology and lithology of the samples recovered, Geol. Sot. Am., Progr. Ann. Meeting, Mexico City, p.23. 228
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Bowen, N.L. and Tuttle, O.F., 1949. The system MgO-SiO,-H-0, Geol, Sot, Am., ‘ ‘ Bull., 60: 439-460. Brakl, J., Clay, C.S. and Rona, P.A,, 1968. Interpretation of a magnetic anomaly on the Continental rise off Cape Hatteras: J. Geophvs. Res., 73: 5313-5315. Drake, C.L., Ewing, M. and Sutton,-G.H., 1959. Continental Margins and Geosynclines. The east coast’of North America north of Cape Hatteras. In: L.H. Ahrens, F. Press, K. Rankama and SK. Runcorn (Editors), Physics and Chemistry of the Earth. Pergamon Press, London, pp.llO-198. Drake, C.L. and Girdler, R.W., 1964. A geophysical study of the Red Sea. Geophys. J., 8 (5): 473-495. Drake, C.L. and Nafe, J.E., 1968. The transition from ocean to continent from seismic refraction data. In: L. Knopoff, C.L. Drake and P. Hart (Editors), The Crust and Upper-Mantle of the Pacific Area-Geophys. Monograph., 12: 174-186. Ewing, J., Worzel, J.L., Ewing, M. and Windisch, C., 1966. Ages of horizon A and the oldest Atlantic sediments. Science, 154: 1125-1132. Fox, P.J., Ruddiman, W., Heezen, B.C. and Ryan, W.B.F., f968. Mesozoic igneous oceanic crust from the Caribbean. Geol. Sot, Am., Progr. Ann. Meeting, Mexico City, p. 101. Fuchs, K., Mueller, S., Peterschmitt, E., Rothe, J.P., Stein, A., and Strobach, K., 1963. Krustenstrukturen der Westalpen nach refraktionsseismischen Messungen. Gerlands Beitr. Geophys. 72: 146-169. Heezen, B.C. and Sheridan, R.E., 1966. Lower Cretaceous rocks (Neocomian-Albian) dredged from Blake escarpment. Science, 154: 1644-1647. Heirtzler, J.R. and Hayes, D.E., 1967. Magnetic boundaries in the North Atlantic. Science, 157: 185-187. Hess, H.H., 1955. Serpentines, orogeny, and epeirogeny. In: A. Poldervaart (Editor). Crust of the Earth-Geol. Sot. Am., Spec. Papers, 62: 391-408. Ho&z, R,, Ewing, J. and LePichon, X., 1968, Velocities of deepsea sediments from sonobuoy data. J. Geophys. Res., ‘73: 2615-2642. Klemme, H.D., 1958. Regional geology of Circe-Mediterr~ean region, Bull. Am. Assoc. Petrol. Geologists, 42: 477-512. MacDonald, G.J.F., 1965. Geophysical deductions from observations of heat flow. In: W.H.K. Lee (Editor), Terrestrial Heat Flow - Geophys. Monograph., 8: 191-210. Menard, H.W., 1967. Crust under small ocean basins. J. Geophys, Res,, 72: 3061-3088. Ringwood, A.E. and Green, D.H., 1966. An experimental investigation of the gabbro-eclogite transformation and geophysical implications. Tectonophysics, 3 (5): 383-427. Sheridan, R.E., Drake, C.L., Nafe, J.E. and Hennion, J., 1966. Seismic refraction study of continental margin east of Florida. Bull. Am. Assoc. Petrol. Geologi&s, 50: 1972-1991. Sheridan, R.E. and Drake, CL., 1968. Seaward extension of the Canadian Appalachians. Can. J. Earth Sei., 5 (1): 337-373. Van Bemmelen, R.W., 1966. On mega-undations: A new model for the earth’s evolution. Tectonophysics, 3 (2): 83-127. Windisch, C.L.,Leyden, R.J., Worzel, J.L., Saito, T. and Ewing, J., 1968. Investigation of Horizon-& Science, 162: 1473-1479. Worzel, J.L., Leyden, R. and Ewing, M., 1968. Newly discovered diapirs in the GulfofMexico. Bull. Am. Assoc. Petrol. Geologists, 52: 1194-1203. Zietz, I., Taylor, P.T. and Dennis, L.S., 1967. Geologic implications of aeromagnetic data for the eastern continental margin of the United States, Geol. Sot. Am. Progr. Ann. Meeting, New Orleans. ~~‘247.
Tectonophysics,
7(3) (1969) 219-229
229
SUBSIDENCE OF CONTINENTAL MARGINS R-E. SHERIDAN Department of Geology, University of Delaware, Newark, Del. (U.S.A.) (Received November 21, 1968)
The possibility of the subsidence of the Blake-Bahama Basin is discussed. Restoration of this basin to its earlier shallow position indicates that it can be compared with several similar structures presentlyfoundat shallow depths. This comparison may provide clues to the processes involved in the subsidence. INTRODUCTION It has long been obvious that the marginal areas of many of the continents have subsided and tilted seaward accumulating the thick sediments of
coastal basins (Drake et al., 1959). Along some margins where sedimentation has not accompanied subsidence, the ocean appears to have expanded at the expe.nse of the continents and vast marine basins were formed with thinned crusts (Beloussov and Kosminskaya, 1968). The accommodation of these thick sediments and deep marginal basins requires significant crustal changes. The exact nature of these crustal changes is still a debatable subject, but common to the ideas which have been proposed is the conversion of lighter materials to denser ones under the areas of subsidence. Processes which change the density beneath subsiding basins may include a sort of erosion of the base of the crust by moving currents in the upper mantle, as advocated by Van Bemmelen (1968). Another process suggested by Beloussqv (1960) is the igneous assimilation of the granitic crust by the upwelling of superheated basaltic magma from within the mantle. Another suggestion was put forward by Ringwood and Green (1966) in which the accumulating weight of volcanics along the already deepened margins leads to a phase change to denser eclogite beneath the continental rise eugeosyncline. The dense eclogite then produces drastic subsidence which eventually leads to erogenic folding and intrusion of the marginal areas. Bess (1955) has attributed the vertical movements of both oceanic and continental crusts to density changes related to serpentine-olivine reactions. Part of the confusion about these proposals for subsidence along continental margins is the lack of observational data to establish the geological history of the subsidence and to establish which crustal layers are actually altered and when in the sequence of subsidence they are altered, It is not quite clear whether the margins subside because of crustal changes or that the crustal changes take place because of the subsidence. Tectonophysics, 7(3) (1969) 219-229
219
W
IO/KM
8
21
0 r-
oc
Structure section across Sheridan et al., 1966).
L
200
Lo
I
c
the continental
margin
east of Florida
at about 29ON. Vertical
exaggeration
EVIDENCE
FORSUBSIDENCE
An equation of the subsidence of the conti~en~l margin east of Florida is useful because of the large amount of data available. Deep wells have penetrated a continuous sequence of shallow water carbonates and evaporites to depths of greater than 5 km under the Florida Platform and the Bahamas. From seismic data, a similar section is interpreted to exist under the Blake Plateau (Sheridan et a~.,l~6), (Fig.1). The thick sequence of shallow water carbonates of the Blake Plateau crop out along the Blake escarpment where they have been sampled by dredging (Heezen and Sheridan, 19661, (Fig.2). In the Florida and Bahama wells and along the Blake Escarpment,‘the shallow water limestones of earliest Cretaceous age (Neoeomian) are found at depths similar to the ocean floor to the east.
W
BLAKE
PLATEAU
BLAKE-BAHAMA
BASIN
E
100 G
0
---r----~-“------r----
, GRAY
FORAM. LUTlTE
A*167-21
TAN CALC
-A-167-25
, PLfOCENE
LUTlTE.
GREEN
EOCENE
PYRITIC
LUTITE,
CENOMANIAN
2
2 -I%-2 E-966-7
GRAY
LUT1TE
, OLIGOCENE
GRAY B TAN
ALGIAN
CALCtCUTITE.
:: c” # E-966-9
2” Y
GG‘tTtC
CACCARENITE,
NEOCOHIAN-
3
APTiAN
APTIAN
4
4
VERTICAL
EXAGGERATION
5 X
5
5
0
5
IO
15
20
25
35
30
KILOMETERS
Fig.2. Outcrops on the Blake escarpment at 29ON. Dashed arrows are data extrapolated large distances along strike. (After Heezen and Sheridan, 1966.)
Tectonophysics, ?(3)(1969)219-229
221
Although it seems clear that the Blake Plateau, Florida, Bahamas area subsided at least 5 km since the earliest Cretaceous, the history of the adjacent ocean floor is less clear. There is evidence, based on deep-seacoring, seismic profiler data, and deep-ocean drilling (Joides) in the oceanic basins east of the Blake-Bahama area, which reveal that the sediments of reflector @ (Ewing et al., 1966) are deep-water facies of earliest Cretaceous (Neocomienf and Late Jurassic age. Windiseh et al. (1968} reported a core from this layer as containing reworked fragments of Neocomian shallow water limestone which appear to have been washed into deeper-water sediments from the Bahama escarpment. Berggren et al. (1968) reported that a Joides drill core penetrated Late Jurassic chert and hard limestone of deep water facies at a depth of 5.6 km in horizon&. Tracing reflector p from the areas where these cores were taken into the Blake-Bahama Basin indicates that horizon @ corresponds to the top of a 3.64-km/set velocity refractor. This velocity appears to be characteristic of horizon/3 which is only observed near to the continental margin (Houtz et al., 1968). The depth to horizon 6 and the 3.64-km/see layer in the Blake-Bahama Basin is 5.80 km (Fig.1). If the BlakeBahama Basin subsided in the same manner as the Florida, Bahamas, Blake Plateau area without a significant post-Jurassic fault or flexure between these areas, this would imply that the Blake-Bahama Basin was only 800 m deep in Neocomian and earlier times. The seismic data reveal that the sediments of horizon /3 and younger age are flat-lying right up to the Blake escarpment with little of no evidence of shearing or differential movement (Windisch et a1.,1968). The seismic data then do not imply the presence of a post-Jurassic fault at the base of the escarpment. If the ocean floor of the Blake-Bahama Basin was only 800 m deep in Neocomian and earlier times, the sedimentary facies might stillbe dominated by planktonic faunas and floras in a non-elastic environment. It cannot be certain, therefore, that the deeper-water planktonic facies of Upper Jurassic age recovered in the Joides drilling off the Bahama escarpment was not deposited on this 800 m deep ocean floor. The Blake-Bahama Basin is interesting because of its typical oceanic depths and its atypical oceanic seismic-structure. It is unlike the true oceanic basins in that the deepest refractor observed on normal length seismic profiles has a velocity of 7.49 km/set (Sheridan et a1.,1966) (Fig.1). This velocity is intermediate between those of the normal oceanic crust ($.2-7.2 km/set) and normal mantle (7.8-8.5 km/see). Such ambiguous velocities are commonly observed near other continental margins (Fig.J), (Drake and Nafe,1968). The Blake-Bahama Basin is also unlike the major part of the oceanic areas since it lacks the short wavelength magnetic anomalies produced by the spreading sea floor. There is a very sharp boundary east of the Blake-Bahama Basin beyond which the typical “rough” oceanic magnetic anomalies occur, whereas the magnetic anomaly pattern of the basin is relatively smooth (Heirtzler and Hayeq1967). This magnetically “quief’ zone has been interpreted by Heirtzler and Hayes (1967) to be an area of normal ocean floor spreading during the Permian when there was no mixed magnetic polarities. It has also been interpreted by Zietz et a1.(1967) as an area where the upper crust is a thick sedimentary section resting on a normal oceanic crust. On the other hand, Brakl et al.@9681
222
Tectonophysics, 7(3) (1969) 219-229
l Continental I,“”
0
marqins.
) IOOOm depth
1
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-k
12
2
I
3
4
I
5
6
7
6
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Fig.3. Compressional wave velocity vs. depth beneath the sea floor for all refraction data (up to 1965) on the continental margins deeper than 1,000 m. (After Drake and Nafe, 1968.) Tectonophysics,
7(3) (1969) 219-229
223
OBSERVED RESTORED BLARE-SAWMA SASIN STRUCTURE 0
I
ANOMALOUS OBSERVED NEWFOUND1 .AND SHELF I I
CONTINENTAL STRUCTURES IE D SEA IVREA ZONI r
z
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i
;,.. .,.;;.. :: .‘..‘:., .:’ . . . . . .” _ ‘...‘:‘, :’ ..‘...: .;: ..:;..., ‘. ‘.’ : :1’.::,:..:f,. ‘/6.6e:
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/’ I\‘\’ ; 1, 1.’ ,;,, L; I,,>\ ‘,.I.” ; I-; ?‘\ ;,
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Fig.4. Restored Blake-Bahama Basin seismic structure of end of Jurassic-beginning of Cretaceous time compared to observed anomalous continental structures. Note that nearly 700 m of Cr&aceous-Recent sediments should be removed from this restored Blake-Bahama Basin. interpret the broad linear anomalies which extend from the BlakeBahama Basin to north of Cape Hatteras as being due to reversely magnetized basement blocks at depths of about 8 km. This implies the structure may be similar to normal oceanic areas but with a deeper basement. In light of the existing data, the interpretations can now be made that the structure of the Blake-Bahama Basin has subsided at least 5 km since earliest Cretaceous and that the section of the upper crust consists mostly Qf marine sedimentary rocks of pre-Cretaceous age which extend to about 8 km depths. The observed seismic-structure in the Blake-Bahama Basin (Fig.4) shows the sedimentary layers (1.82, 2.84, and 3.64 km/set) are about 3 km thick over a 6.65-km/set crystalline basement (Sheridan et a1.,1966). The intermediate velocity layer, 7.49 km/set, is observed at a depth of 11.75 km. RESTORATIONOF SUBSIDEDCRUST Returning now to the earlier discussion on subsidence, the history and observed structure of the Blake-Bahama Basin may provide some insight into the crustal changes involved in its. subsidence. Upon restoring the 224
Tectonophysics, 7(3) (1969) 219-229
seismic-structure section to its near sea-level position at the end of Jurassic and beginning of Cretaceous (Fig.4), the shallow 6.75-km depth to the 7.49-km/see material is the most imposing problem. If such a section existed at such a depth, it would imply that this part of the restored continent was made up of a largely basic and ultrabasic crust. Also, depending on the existence and depth of a mantle discontinuity, it is probable that such a structure would be out of isostatic adjustment. It could be interpreted more conveniently that the 7.49-km/set material never did exist at this shallow depth in the restored continent, but that it was formed during or after subsidence. This would imply that the restored continent was a more normal structure. This Iate stage transformation of normal continental crust might be accomplished by subcrustal erosion, basification, deserpentinization or eclogite phase changes already mentioned, However, it may be that the restored continental structure was actually basic and ultrabasic in nature even to a shallow depth. To believe this, one would expect to find examples of largely basicand ultrabasic crusts occurring in continental areas with velocities of about 7.4 km/set observed at shallow depths, At present a few such areas are known (Fig.4). One is the Newfoundland area in the Canadian Appalachians (Sheridan and Drake,1968), another is the Ivrea zone in the Italian Alps (Fuchs et a1.,1963), and another is the Red Sea rift (Drake and Girdler, 1964). The comparison of the restored Blake-Bahama section to those of Newfoundland shelf areas, the Ivrea zone, and the Red Sea rift is remarkable (Fig.4). The common factors relating the Newfoundland area, the Ivrea zone, and the Red Sea rift are that these areas are within zones of major mountain belts or rift systems having extensive intrusions of basic and ultrabasic rocks. These areas have significant positive Bouguer anomalies indicating a lack of isostatic adjustment. The denser crust in these areas should have a tendency to sink. If the comparison of any of these Newfoundland, Ivrea or Red Sea sections with the restored Blake-Bahama section is valid, it can be speculated that the structure beneath the sediments and sedimentary rocks of the Blake-Bahama Basin includes a highly metamorphosed and intruded basement which is intruded extensively by ultrabasics derived from a deeper, denser subbasement. The structure may have once been involved in an extensive orogeny or taphrogeny before the subsidence and deposition of the sediments of the basement. From this discussion the sequence of events in the subsidence of the Blake-Bahama Basin might resemble: (1) An orogeny or taphrogeny ‘in an extensive area along what was the old North American continental margin probably during pre-Jurassic times. (2) Extensive emplacement of ultrabasic intrusions and the development of the 7.4-km/set layer at a shallow depth. (3) Possible uplift and erosion of the metamorphosed basement. (4) Beginning of subsidence of the unstable denser crust by Jurassic with the accumulation of sediments at a rate nearly equal to subsidence. (5) Continued subsidence and very little deposition of sediments since earliest Cretaceous. This resulted in the increasing water depths to more than 5 km.
Tectonophysics, 7(3) (1969) 219-229
225
OmR
AREAS OF SUBSIDENCE
The connection between the 7.4-km/‘sec material, erogenic and taphrogenie belts, and subsiding continental margings is not limited to the specific examples discussed above, but appears to be more common. The relationship of erogenic belts to the margins of continents is very well known and consistent for the Cenozoic, Mesozoic, and Paleozoic systems. This may even be so for the Precambrian. Summarizing the worldwide crustal velocity data, Drake and Nafe (1968) (Fig.5) find that the peculiar 7.4-km/see material is commonly observed in erogenic and taphrogenic belts of Paleozoic and younger age. This type of material is apparently absent from the older stable shields forming the nuclei of the continents. The common observation of the 7.4-km/see material in areas of the continental margin allows the suggestion of a transitional sequence from stable continents to erogenic or taphrogenic belts to subsided margins and perhaps to deep ocean basins. The absence of these anomalous ‘7.4-km/set velocities in most deep basins implies the removal of this material for the sequence mentioned above to be possible. The removal of the bottom of the 7.4-km/set material and the development of a shallower mantle is also required for subsiding margins. From the world data the mantle must shoal from 30-35 km depths to as shallow as 12 km beneath the thinned crusts of the margins. The transition of 7.4-km/set material to something with a velocity of
(After 226
Fig.5. Summarized velocity-depth Drake and Nafe, 1968.)
data for all crustal
Tectonophysics,
provinces.
i’(3) (1969) 219-229
I
‘\ _..x*o --\,L \ -00
-2-
\O ‘.\
-3
* \ \
-4-
. .
-5
.
\.
\X
-6-
.
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. Ix
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-8-
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-9
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-I3
.
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-15
-16
-17
-18
-19 km/set 2
3
I
Fig.6. Velocity-depth data for some deep ocean basins where subsidence is suspected. (After Drake and Nafe, 1968.) Tectonophysics,
7(3) (1969) 219-229
227
about 8.0-km/see at such shallow depths of 12 km requires a reaction that can occur at relatively low pressures. Deserpentinization reactions are such phase changes (Bowen and Tuttle,1949). Some of these reactions occur at temperatures ofe340-36O’-‘C implying that thermal conditions in the areas of these transitions at IZ-km depths were other than normal. These high temperatures at such shallow depth may be explained if the subsiding marginal block is influenced by a higher than normal geothermal gradient. In areas of Mesozoic-Cenozoic erogenic and taphrogenic belts the mean heat flow is anomalously high, (MacDonald, 1965), and temperatures may reach near 360°C at I2 km depths under such tectonic areas. If the 7.4-km/set material subsides within this thermal regime, critical temperatures for deserpentinization may be found, Although this is speculation, there appear to be chemical reactions that are possible which might allow the removal of the transitory 7.4-km/see material. Besides the areas of the continental margin, which may have been pre-existing shallow areas since foundered, there are other deep ocean basins which may have similar origins (Menard,I967). The deep Gulf of Mexico is underlain by salt domes (Worzel et al., 1968) which formed as diapirs from a salt layer of probably shallow water origin; and the Caribbean Sea is underlain by Eocene shallow water limestones in some areas (Fox et a1.,1968). These areas have crustal velocity distributions very similar to the deep continental margins (Drake and Nafe,1968) (Fig.G), and fit into the above hypothesis. The Gulf of Mexico and Caribbean are also surrounded by or are along the extension of erogenic serpentine belts. Similarly, the western Mediterranean, by virtue of the presence of the 7.4-km/see material and the neighboring erogenic serpentine belts, may be a deep basin of subsided continental structures. Certainly there is good geological evidence for the presence of a previous landmass in this area (Klemme.1958). ACKNOWLEDGEMENTS
Much of the evidence and data on the continental margins has been reported by workr rs of Lamont Geological Observatory. Their excellent work is referenced extensively in this discussion paper. Discussions with these researchers at Lamont were very helpful, especially those with N.T. Edgar, C. Windisch, J.E. Nafe and C,L. Drake. Although these workers have read this discussion paper, the conclusions and remarks made are not necessarily in agreement with their views. REFERENCES Beloussov, V.V., 1966. Development of the earth and tectogenesis. J. Geophys. Res., 65: 4127-4146, Beloussov, V.V. and Kosminskaya, I.P., 1968. Structure and development of the transition zones between the continents and oceans. Can. 3. Earth Sci., 5: 1011-1026. Berggren, W-A., Bukry, D., Fischer, A.G., Passagno, EA., Slade, M. and Lyle, 1968. Deep-sea Drilling-project-preliminary results, 1. Paleontology and lithology of the samples recovered, Geol. Sot. Am., Progr. Ann. Meeting, Mexico City, p.23. 228
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Bowen, N.L. and Tuttle, O.F., 1949. The system MgO-SiO,-H-0, Geol, Sot, Am., ‘ ‘ Bull., 60: 439-460. Brakl, J., Clay, C.S. and Rona, P.A,, 1968. Interpretation of a magnetic anomaly on the Continental rise off Cape Hatteras: J. Geophvs. Res., 73: 5313-5315. Drake, C.L., Ewing, M. and Sutton,-G.H., 1959. Continental Margins and Geosynclines. The east coast’of North America north of Cape Hatteras. In: L.H. Ahrens, F. Press, K. Rankama and SK. Runcorn (Editors), Physics and Chemistry of the Earth. Pergamon Press, London, pp.llO-198. Drake, C.L. and Girdler, R.W., 1964. A geophysical study of the Red Sea. Geophys. J., 8 (5): 473-495. Drake, C.L. and Nafe, J.E., 1968. The transition from ocean to continent from seismic refraction data. In: L. Knopoff, C.L. Drake and P. Hart (Editors), The Crust and Upper-Mantle of the Pacific Area-Geophys. Monograph., 12: 174-186. Ewing, J., Worzel, J.L., Ewing, M. and Windisch, C., 1966. Ages of horizon A and the oldest Atlantic sediments. Science, 154: 1125-1132. Fox, P.J., Ruddiman, W., Heezen, B.C. and Ryan, W.B.F., f968. Mesozoic igneous oceanic crust from the Caribbean. Geol. Sot, Am., Progr. Ann. Meeting, Mexico City, p. 101. Fuchs, K., Mueller, S., Peterschmitt, E., Rothe, J.P., Stein, A., and Strobach, K., 1963. Krustenstrukturen der Westalpen nach refraktionsseismischen Messungen. Gerlands Beitr. Geophys. 72: 146-169. Heezen, B.C. and Sheridan, R.E., 1966. Lower Cretaceous rocks (Neocomian-Albian) dredged from Blake escarpment. Science, 154: 1644-1647. Heirtzler, J.R. and Hayes, D.E., 1967. Magnetic boundaries in the North Atlantic. Science, 157: 185-187. Hess, H.H., 1955. Serpentines, orogeny, and epeirogeny. In: A. Poldervaart (Editor). Crust of the Earth-Geol. Sot. Am., Spec. Papers, 62: 391-408. Ho&z, R,, Ewing, J. and LePichon, X., 1968, Velocities of deepsea sediments from sonobuoy data. J. Geophys. Res., ‘73: 2615-2642. Klemme, H.D., 1958. Regional geology of Circe-Mediterr~ean region, Bull. Am. Assoc. Petrol. Geologists, 42: 477-512. MacDonald, G.J.F., 1965. Geophysical deductions from observations of heat flow. In: W.H.K. Lee (Editor), Terrestrial Heat Flow - Geophys. Monograph., 8: 191-210. Menard, H.W., 1967. Crust under small ocean basins. J. Geophys, Res,, 72: 3061-3088. Ringwood, A.E. and Green, D.H., 1966. An experimental investigation of the gabbro-eclogite transformation and geophysical implications. Tectonophysics, 3 (5): 383-427. Sheridan, R.E., Drake, C.L., Nafe, J.E. and Hennion, J., 1966. Seismic refraction study of continental margin east of Florida. Bull. Am. Assoc. Petrol. Geologi&s, 50: 1972-1991. Sheridan, R.E. and Drake, CL., 1968. Seaward extension of the Canadian Appalachians. Can. J. Earth Sei., 5 (1): 337-373. Van Bemmelen, R.W., 1966. On mega-undations: A new model for the earth’s evolution. Tectonophysics, 3 (2): 83-127. Windisch, C.L.,Leyden, R.J., Worzel, J.L., Saito, T. and Ewing, J., 1968. Investigation of Horizon-& Science, 162: 1473-1479. Worzel, J.L., Leyden, R. and Ewing, M., 1968. Newly discovered diapirs in the GulfofMexico. Bull. Am. Assoc. Petrol. Geologists, 52: 1194-1203. Zietz, I., Taylor, P.T. and Dennis, L.S., 1967. Geologic implications of aeromagnetic data for the eastern continental margin of the United States, Geol. Sot. Am. Progr. Ann. Meeting, New Orleans. ~~‘247.
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