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Comments on “The crustal structure of the Balearic Sea” — in light of deep-sea drilling in the Mediterranean
Tectonophysics,
20 (1973)
0 Elsevier Scientific
COMMENTS
303-306
Publishing
Company,
Amsterdam
ON “THE CRUSTAL
- IN LIGHT OF DEEP-SEA
- Printed
STRUCTURE
DRILLING
in The Netherlands
OF THE BALEARIC
SEA”
IN THE MEDITERRANEAN’
K.J. HSU and W.B.F. RYAN Geologisches Institut Eidgeniissische Technische Hochschule Ziirich, Ziirich (Switzerland) Lamont-Doherty Geological Observatory, Palisades, N. Y. (U.S.A.) (Received
December
18, 197 1)
ABSTRACT Hsii, K.J. and Ryan, W.B.F., 1973. Comments on The crustal structure of the Balearic Sea - in light of deep sea drilling in the Mediterranean. In: S. Mueller (Editor), The Structure of the Earth’s Crust, based on Seismic Data. Tectonophysics, 20 (l-4): 303-306. Supplementary to the paper by K. Him on The crustal structure of the Balearic Sea, some results are reported from the deep-sea drilling cruise of the “Glomar Challenger”.
INTRODUCTION
The results of the France-German Basin are significant
geophysical research project ANNA in the Balearic
to our understanding
of the geologic history of the western Mediterra-
nean. During the Deep-Sea Drilling Cruise, Leg XIII, to the Mediterranean,
August-October,
1970, four sites (totalling 10 holes) were drilled in the Balearic basin and Valencia trough regions. This note includes a discussion of our drilling data and our geological conclusions, which are relevant to the interpretations Dr. Hinz.
BALEARIC
of the excellent seismic investigations
presented by
BASIN HOLES
Regional framework Site 124, 38”52.4’N, 04”59.7’E, at 2709 m depth, is located on the southern margin of the Balearic rise some 150 m above the Balearic abyssal plain. The site was targeted on the northwestern flank of a buried linear basement ridge. Two major seismic reflectors were recognized: The Mediterranean reflector at about 0.4 set and the Basement reflector at about 0.9 set subbottom. The ANNA III profile traversed a section considerably to the west of Site 124, as shown by the fig. 1 of Hinz. A geologically equivalent position on the profile would be approximately 20 km north of Position A (see Hinz, fig. 2).
K.J.HSti AND W.B.F.RYAN
304
Site 134,SO” 12’N. 07”18’E. is located at 2864 m depth, on the eastern edge of the Balearic abyssal plain. Six holes were drilled at this site along a 900 m east--west profile above the western flank of a buried basement
ridge. This site is situated on the eastern side
of the Balearic basin and thus cannot be given an equivalent
position on the ANNA III pro-
file. Buried basement ridges at both Sites 124 and 134 give no magnetic anomalies. Those linear structures mark, respectively, the northwestern and the eastern boundaries of the Balearic abyssat plain, and are believed to be similar features. We were not able to reach the basement in Hole 124. However, the sedimentary cover at Site 134 is thinner and we were able to sample basement in five of the six holes there. Sedimentary
covers
At Site 124, the top of a strong reflector, identified as the M-reflector*, was encountered at 359 m subbottom. This reflector is underlain by anhydritic and dolomitic sediments, with a few intercalated marine oozes and non-marine diatomites. At Site 134, the M-reflector was penetrated at 350 m subbottom, where halite was sampled. The sediments above the M-reflector in the Balearic basin are Quaterna~y and Pliocene marine sediments, including both pelagic oozes and terrigenous elastics. The sediments just below the M-reflector are the Messinian evaporite. We have penetrated only 60 m into the anhydrite
at Site 134, and some 10 m into the halite at Site 134. Further penetration
was
not possible because of mechanical and other difficulties. However, we believe that the Balearic evaporites in the abyssal plain province could be a few kilometers thick, as the equivalent
formation
in Sicily includes more than 1 km of halite (see Ogniben,
1957).
On the basis of our drilling results, we agree with Hinz that his Layer 2 on the ANNA III profile represents Quaternary
and Pliocene sediments. The top of his Layer 3 could be cor-
related to the top of our M-reflectors. The uppermost
part of the Layer 3 is certainly
the
Upper Miocene (Messinian) evaporite. Our drilling technique did not permit us to penetrate through the Layer 3 in the middie of the Balearic abyssal plain. On the basis of regional geology considerations, we believe that the Balearic basin had its origin through the counterclockwise
rotation
of Sardinia and
Corsica which probably started in Early Miocene, namely in Aquitanian or Burdigahan time (see Ryan et al., in preparation). If so, the whole of the Layer 3 in ANNA III profile, some 3.5 km thick, would be Miocene and the bulk could be the Messinian halite. Basement The basement at the eastern margin of the Bale&c basin was sampled at depth greater than 3000 m subsea. Drilling showed that the fineear basement ridge represented a tilted
* Not the Mohorovi&Cdiscontinuity (Ed.).
CRUSTAL STRUCTURE OF THE BALEARIC
305
BASIN - COMMENTS
fault block. Slates and graywackes, probably Paleozoic in age, were sampled on the upthrown block of the graben faults. The drilling results confirm the interpretation that the thickened underlain
portion
by Hinz
of the Layer 4 (north of Position A on ANNA III profile) is
by a sialic crust.
Our drill-results
indicated
that the edge of continental
blocks bounding
the Balearic has
subsided more than 3000 m since the early Miocene. However, the sialic rocks were sampled from the upthrown the downthrown
side of the Balearic boundary
faults; we have not been able to sample
side. The regional geological evidence favors the interpretation
that the
central Balearic owed its origin to rifting and should thus be underlain by an oceanic crust (see Ryan et al., 1973). We are pleased to learn that the seismic data by Hinz indicate a 5 km thick oceanic crust for the central Balearic basin.
VALENCIA TROUGH SITES
Regional framework Site 122, 40”2687’N, 02”37,46’E, 2146 m depth. was drilled on the flank of a basement high, which is characterized by a highly positive magnetic anomaly. The hole encountered the M-reflector at 160 m subbottom and was bottomed in gypsum two meters deeper. The hole was terminated because of mechanical difficulties. Site 123,40”37,83’N, 02”50.27’E, 2290 m depth, was drilled almost immediately above the top of the basement high mentioned above. The acoustic basement at this site consists of thick, dacitic ash deposit. The basement is overlain at 273 m subbottom Lower Pliocene graded beds. The ash deposit, has been dated by radiometric methods at 22 m.y. Sedimentary
Cover
The Quaternary
and Pliocene sediments
meters thick. They obviously correspond
of the Valencia trough region are a few hundred to the Layer 2 on the ANNA II profile (fig. 5 of
Hinz). The nature of the pre-Pliocene
sedimentary
sequence has been revealed by a study of a
gravel in Hole 122. This Late Miocene gravel has only four components - andesitic basalts, limestones, gypsum, and shallow-water shells. No quartz, no feldspar, no lithic fragments o? metamorphic rocks, nor of plutonic igneous rocks were found. While the basalt apparently belongs to the igneous series deposited on the flanks of composite volcanoes, the other three components should represent the materials that constitute the Layer 3 on the ANNA 11 profile. The gypsum belongs to a Messinian gypsiferous series. The limestones are pelagic oozes of Middle Miocene age. These scant data thus suggest that the Layer 3 here. like that in the central Balearic, is also largely Miocene.
K.J. HSii AND W.B.F. RYAN
306
In Hole 123 we penetrated
an ash deposit more than 120 m. The presence of such a thick
vesicular deposit, the notable absence of marine faunas in the ash, and the welding-together of glass fragments and associated minerals indicate that this ash represents the flank deposit of a once-subaerial
comppsite volcano. In addition,
the andesitic basalts found in the
gravel discussed above is alkalic and quite different in their major and trace-element corn. positions from the oceanic tholeiites (Ryan et al., 1973). The ash and the vesicular basalts apparently constitute the top of the acoustic basement in the central Valencia trough region. Similar volcanic rocks have been reported from the Betic Cordillera of Spain. Available evidence suggests that the Valencia trough volcanics were the product of andesitic volcanism behind an island arc, relating to the subduction
of a lithospheric
plate. This
interpretation is in agreement with that by Hinz, who postulated a north-dipping subduction zone under the Balearic Island. We believe, however, that the compressional tectonics and under-thrusting took place in Early Tertiary and culminated in Early Miocene some 20 m.y. ago. It has been suggested that the movement by a fragment of the subducted lithospheric plate may have been the cause of the Spanish Deep Earthquake (D. McKenzie, personal communication, 1971). The subduction zone is no longer active after the Early Miocene. Since then, the rotation
of Corso-Sardinia
block led to the rifting, and to the Neogene graben-
tectonics of the Balearic basin.
REFERENCES Ogniben, L., 1957. Petrografia della serie solfifera si&ana.Mem. Des. Carta Geol. d’ltalia, v.33, 273 PP. Ryan, W.B.F., Hsii, K.J., Nesteroff, W.D., Pautot, C., Wezel, W.C., Lort, J.M., Cita, M.B., Maync, W., Stradner, H., Dumitrica, P., 1973. Initial Reports of the Deep-Sea Drilling Project, vol. XIII. National Science Foundation, Washington, DC., 1347.~~.
20 (1973)
0 Elsevier Scientific
COMMENTS
303-306
Publishing
Company,
Amsterdam
ON “THE CRUSTAL
- IN LIGHT OF DEEP-SEA
- Printed
STRUCTURE
DRILLING
in The Netherlands
OF THE BALEARIC
SEA”
IN THE MEDITERRANEAN’
K.J. HSU and W.B.F. RYAN Geologisches Institut Eidgeniissische Technische Hochschule Ziirich, Ziirich (Switzerland) Lamont-Doherty Geological Observatory, Palisades, N. Y. (U.S.A.) (Received
December
18, 197 1)
ABSTRACT Hsii, K.J. and Ryan, W.B.F., 1973. Comments on The crustal structure of the Balearic Sea - in light of deep sea drilling in the Mediterranean. In: S. Mueller (Editor), The Structure of the Earth’s Crust, based on Seismic Data. Tectonophysics, 20 (l-4): 303-306. Supplementary to the paper by K. Him on The crustal structure of the Balearic Sea, some results are reported from the deep-sea drilling cruise of the “Glomar Challenger”.
INTRODUCTION
The results of the France-German Basin are significant
geophysical research project ANNA in the Balearic
to our understanding
of the geologic history of the western Mediterra-
nean. During the Deep-Sea Drilling Cruise, Leg XIII, to the Mediterranean,
August-October,
1970, four sites (totalling 10 holes) were drilled in the Balearic basin and Valencia trough regions. This note includes a discussion of our drilling data and our geological conclusions, which are relevant to the interpretations Dr. Hinz.
BALEARIC
of the excellent seismic investigations
presented by
BASIN HOLES
Regional framework Site 124, 38”52.4’N, 04”59.7’E, at 2709 m depth, is located on the southern margin of the Balearic rise some 150 m above the Balearic abyssal plain. The site was targeted on the northwestern flank of a buried linear basement ridge. Two major seismic reflectors were recognized: The Mediterranean reflector at about 0.4 set and the Basement reflector at about 0.9 set subbottom. The ANNA III profile traversed a section considerably to the west of Site 124, as shown by the fig. 1 of Hinz. A geologically equivalent position on the profile would be approximately 20 km north of Position A (see Hinz, fig. 2).
K.J.HSti AND W.B.F.RYAN
304
Site 134,SO” 12’N. 07”18’E. is located at 2864 m depth, on the eastern edge of the Balearic abyssal plain. Six holes were drilled at this site along a 900 m east--west profile above the western flank of a buried basement
ridge. This site is situated on the eastern side
of the Balearic basin and thus cannot be given an equivalent
position on the ANNA III pro-
file. Buried basement ridges at both Sites 124 and 134 give no magnetic anomalies. Those linear structures mark, respectively, the northwestern and the eastern boundaries of the Balearic abyssat plain, and are believed to be similar features. We were not able to reach the basement in Hole 124. However, the sedimentary cover at Site 134 is thinner and we were able to sample basement in five of the six holes there. Sedimentary
covers
At Site 124, the top of a strong reflector, identified as the M-reflector*, was encountered at 359 m subbottom. This reflector is underlain by anhydritic and dolomitic sediments, with a few intercalated marine oozes and non-marine diatomites. At Site 134, the M-reflector was penetrated at 350 m subbottom, where halite was sampled. The sediments above the M-reflector in the Balearic basin are Quaterna~y and Pliocene marine sediments, including both pelagic oozes and terrigenous elastics. The sediments just below the M-reflector are the Messinian evaporite. We have penetrated only 60 m into the anhydrite
at Site 134, and some 10 m into the halite at Site 134. Further penetration
was
not possible because of mechanical and other difficulties. However, we believe that the Balearic evaporites in the abyssal plain province could be a few kilometers thick, as the equivalent
formation
in Sicily includes more than 1 km of halite (see Ogniben,
1957).
On the basis of our drilling results, we agree with Hinz that his Layer 2 on the ANNA III profile represents Quaternary
and Pliocene sediments. The top of his Layer 3 could be cor-
related to the top of our M-reflectors. The uppermost
part of the Layer 3 is certainly
the
Upper Miocene (Messinian) evaporite. Our drilling technique did not permit us to penetrate through the Layer 3 in the middie of the Balearic abyssal plain. On the basis of regional geology considerations, we believe that the Balearic basin had its origin through the counterclockwise
rotation
of Sardinia and
Corsica which probably started in Early Miocene, namely in Aquitanian or Burdigahan time (see Ryan et al., in preparation). If so, the whole of the Layer 3 in ANNA III profile, some 3.5 km thick, would be Miocene and the bulk could be the Messinian halite. Basement The basement at the eastern margin of the Bale&c basin was sampled at depth greater than 3000 m subsea. Drilling showed that the fineear basement ridge represented a tilted
* Not the Mohorovi&Cdiscontinuity (Ed.).
CRUSTAL STRUCTURE OF THE BALEARIC
305
BASIN - COMMENTS
fault block. Slates and graywackes, probably Paleozoic in age, were sampled on the upthrown block of the graben faults. The drilling results confirm the interpretation that the thickened underlain
portion
by Hinz
of the Layer 4 (north of Position A on ANNA III profile) is
by a sialic crust.
Our drill-results
indicated
that the edge of continental
blocks bounding
the Balearic has
subsided more than 3000 m since the early Miocene. However, the sialic rocks were sampled from the upthrown the downthrown
side of the Balearic boundary
faults; we have not been able to sample
side. The regional geological evidence favors the interpretation
that the
central Balearic owed its origin to rifting and should thus be underlain by an oceanic crust (see Ryan et al., 1973). We are pleased to learn that the seismic data by Hinz indicate a 5 km thick oceanic crust for the central Balearic basin.
VALENCIA TROUGH SITES
Regional framework Site 122, 40”2687’N, 02”37,46’E, 2146 m depth. was drilled on the flank of a basement high, which is characterized by a highly positive magnetic anomaly. The hole encountered the M-reflector at 160 m subbottom and was bottomed in gypsum two meters deeper. The hole was terminated because of mechanical difficulties. Site 123,40”37,83’N, 02”50.27’E, 2290 m depth, was drilled almost immediately above the top of the basement high mentioned above. The acoustic basement at this site consists of thick, dacitic ash deposit. The basement is overlain at 273 m subbottom Lower Pliocene graded beds. The ash deposit, has been dated by radiometric methods at 22 m.y. Sedimentary
Cover
The Quaternary
and Pliocene sediments
meters thick. They obviously correspond
of the Valencia trough region are a few hundred to the Layer 2 on the ANNA II profile (fig. 5 of
Hinz). The nature of the pre-Pliocene
sedimentary
sequence has been revealed by a study of a
gravel in Hole 122. This Late Miocene gravel has only four components - andesitic basalts, limestones, gypsum, and shallow-water shells. No quartz, no feldspar, no lithic fragments o? metamorphic rocks, nor of plutonic igneous rocks were found. While the basalt apparently belongs to the igneous series deposited on the flanks of composite volcanoes, the other three components should represent the materials that constitute the Layer 3 on the ANNA 11 profile. The gypsum belongs to a Messinian gypsiferous series. The limestones are pelagic oozes of Middle Miocene age. These scant data thus suggest that the Layer 3 here. like that in the central Balearic, is also largely Miocene.
K.J. HSii AND W.B.F. RYAN
306
In Hole 123 we penetrated
an ash deposit more than 120 m. The presence of such a thick
vesicular deposit, the notable absence of marine faunas in the ash, and the welding-together of glass fragments and associated minerals indicate that this ash represents the flank deposit of a once-subaerial
comppsite volcano. In addition,
the andesitic basalts found in the
gravel discussed above is alkalic and quite different in their major and trace-element corn. positions from the oceanic tholeiites (Ryan et al., 1973). The ash and the vesicular basalts apparently constitute the top of the acoustic basement in the central Valencia trough region. Similar volcanic rocks have been reported from the Betic Cordillera of Spain. Available evidence suggests that the Valencia trough volcanics were the product of andesitic volcanism behind an island arc, relating to the subduction
of a lithospheric
plate. This
interpretation is in agreement with that by Hinz, who postulated a north-dipping subduction zone under the Balearic Island. We believe, however, that the compressional tectonics and under-thrusting took place in Early Tertiary and culminated in Early Miocene some 20 m.y. ago. It has been suggested that the movement by a fragment of the subducted lithospheric plate may have been the cause of the Spanish Deep Earthquake (D. McKenzie, personal communication, 1971). The subduction zone is no longer active after the Early Miocene. Since then, the rotation
of Corso-Sardinia
block led to the rifting, and to the Neogene graben-
tectonics of the Balearic basin.
REFERENCES Ogniben, L., 1957. Petrografia della serie solfifera si&ana.Mem. Des. Carta Geol. d’ltalia, v.33, 273 PP. Ryan, W.B.F., Hsii, K.J., Nesteroff, W.D., Pautot, C., Wezel, W.C., Lort, J.M., Cita, M.B., Maync, W., Stradner, H., Dumitrica, P., 1973. Initial Reports of the Deep-Sea Drilling Project, vol. XIII. National Science Foundation, Washington, DC., 1347.~~.