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Volcano spacing, fractures, and thickness of the lithosphere — A reply
162
M.C.W. BAKER/P.R. VOGT
20%
15% lo%
g ~- 5%
h 2'5 spacIf/g {kin)
30
3'5
40
45
5b
20%, 15%. > lo%
~'o Spacing (km)
Fig. 3. Histograms of distances between adjacent volcanic centres. A. Centres with youthful morphology. B. Centres of all ages.
separate, distinct volcanic centres can be identified. Almost all are large, radially symmetrical cones with central craters. Many have clearly visible lava flows. Twin cones were counted as single centres and linear clusters of overlapping vents were omitted from spacing measurements. The inset to Fig. 1 shows the distribution in a sub-area with a high density of centres. The subregular arrangement is strongly suggestive of control by two sets of fractures; one set parallel with the
regional trend of the volcanic belt and the other set at a large angle to this direction. Elsewhere, the arrangement is more irregular. The variation in density of volcanic centres over the whole area is shown quantitatively in Fig. 2. The distances between adjacent volcanic centres is depicted in Fig. 3 by means of histograms. For morphologically youthful centres, the mean spacing is 13 kin; that for centres of all ages is 10 km. The oldest centres are probably about 6 m.y. [2] and the youngest are 0.2 m.y. old. The characteristic spacing measured is in marked contrast to that of 75 km quoted by Vogt for the central Andes. Therefore, in this area, there is a very large discrepancy between volcano spacings and lithospheric thickness. I believe that the probable explanation for this is that in the central Andes, the fractures which locate volcanic centres are produced by the intrusion of large batholith complexes at relatively shallow depths [3], rather than by tectonic events beneath the lithosphere. Hence, the distribution and spacing of fractures in the crust above the batholith will be controlled by the size, distribution and depth of intrusion of individual plutons comprising the batholith. In particular, fracture spacing is controlled by the thickness of the crust above the batholith, not the thickness of the lithosphere.
References 1 P.R. Vogt, Volcano spacing, fractures and thickness of the lithosphere, Earth Planet. Sci. Lett. 21 (1974) 235. 2 P.W. Francis and C.C. Rundle (1974) in press. 3 E.J. Cobbing and W.S. Pitcher, The coastal batholith of Central Peru, J. Geol. Soc. Lond. 128 (1972) 421.
VOLCANO SPACING, F R A C T U R E S , AND THICKNESS OF THE LITHOSPHERE - A R E P L Y PETER R. VOGT U.S. Naval Oceanographic Office, Washington, D.C. (USA) Received July 23, 1974
Baker's discovery [1 ] that there is considerably larger number of major Pliocene/Recent volcanoes in
the central Andes than shown on the Soviet tectonic chart [2] I had used [3] is interesting and significant.
VOLCANO SPACING, FRACTURES, AND THICKNESS OF THE LITHOSPHERE
~lt may be that where large batholiths have intruded the lithosphere, the typical 50-100 km spacing of major volcanoes is overprinted by a second-order spacing of 10-13 km. This second-order spacing and sub-regular distribution [ 1] would imply, as Baker suggests, that the spacing is a measure of the thickness of "plate" above the batholiths. Alternatively, the central Andes lithosphere is more thoroughly fractured than typical. From Baker's discussion it is apparent that he does not accept the existence of a "first-order" spacing of magnitude 75 km [2,3] in the central Andes. Yet, his Fig. 2 shows that there are volcano density maxima at about the spacing I suggested, a spacing that would roughly equal lithosphere thickness. There are roughly 10 such maxima in Fig. 2. Perhaps these maxima reflect major batholiths emplaced from sub-litho-
163
spheric depths and controlled by fractures in the lithosphere. The actual volcanoes as located by Baker in Fig. 1 would then be controlled by the rigid roof above these batholiths, as he suggests. Detailed investigations of other volcanic areas are needed to verify such a two-tiered spacing system for volcanoes.
References 1 M. Baker, Volcano spacing, fractures, and thickness of lithospherre - a discussion, Earth Planet, Sci. Lett. 23 (1974) 161, this issue. 2 Anonymous, Tectonic Map of the Pacific Segment of the Earth (Geol. Inst. of Oceanology and the Academy of Sciences of the U.S.S.R., Moscow, 1970). 3 P.R. Vogt, Volcano spacing, fractures, and thickness of the lithosphere, Earth Planet. Sci. Lett. 21 (1974) 235.
M.C.W. BAKER/P.R. VOGT
20%
15% lo%
g ~- 5%
h 2'5 spacIf/g {kin)
30
3'5
40
45
5b
20%, 15%. > lo%
~'o Spacing (km)
Fig. 3. Histograms of distances between adjacent volcanic centres. A. Centres with youthful morphology. B. Centres of all ages.
separate, distinct volcanic centres can be identified. Almost all are large, radially symmetrical cones with central craters. Many have clearly visible lava flows. Twin cones were counted as single centres and linear clusters of overlapping vents were omitted from spacing measurements. The inset to Fig. 1 shows the distribution in a sub-area with a high density of centres. The subregular arrangement is strongly suggestive of control by two sets of fractures; one set parallel with the
regional trend of the volcanic belt and the other set at a large angle to this direction. Elsewhere, the arrangement is more irregular. The variation in density of volcanic centres over the whole area is shown quantitatively in Fig. 2. The distances between adjacent volcanic centres is depicted in Fig. 3 by means of histograms. For morphologically youthful centres, the mean spacing is 13 kin; that for centres of all ages is 10 km. The oldest centres are probably about 6 m.y. [2] and the youngest are 0.2 m.y. old. The characteristic spacing measured is in marked contrast to that of 75 km quoted by Vogt for the central Andes. Therefore, in this area, there is a very large discrepancy between volcano spacings and lithospheric thickness. I believe that the probable explanation for this is that in the central Andes, the fractures which locate volcanic centres are produced by the intrusion of large batholith complexes at relatively shallow depths [3], rather than by tectonic events beneath the lithosphere. Hence, the distribution and spacing of fractures in the crust above the batholith will be controlled by the size, distribution and depth of intrusion of individual plutons comprising the batholith. In particular, fracture spacing is controlled by the thickness of the crust above the batholith, not the thickness of the lithosphere.
References 1 P.R. Vogt, Volcano spacing, fractures and thickness of the lithosphere, Earth Planet. Sci. Lett. 21 (1974) 235. 2 P.W. Francis and C.C. Rundle (1974) in press. 3 E.J. Cobbing and W.S. Pitcher, The coastal batholith of Central Peru, J. Geol. Soc. Lond. 128 (1972) 421.
VOLCANO SPACING, F R A C T U R E S , AND THICKNESS OF THE LITHOSPHERE - A R E P L Y PETER R. VOGT U.S. Naval Oceanographic Office, Washington, D.C. (USA) Received July 23, 1974
Baker's discovery [1 ] that there is considerably larger number of major Pliocene/Recent volcanoes in
the central Andes than shown on the Soviet tectonic chart [2] I had used [3] is interesting and significant.
VOLCANO SPACING, FRACTURES, AND THICKNESS OF THE LITHOSPHERE
~lt may be that where large batholiths have intruded the lithosphere, the typical 50-100 km spacing of major volcanoes is overprinted by a second-order spacing of 10-13 km. This second-order spacing and sub-regular distribution [ 1] would imply, as Baker suggests, that the spacing is a measure of the thickness of "plate" above the batholiths. Alternatively, the central Andes lithosphere is more thoroughly fractured than typical. From Baker's discussion it is apparent that he does not accept the existence of a "first-order" spacing of magnitude 75 km [2,3] in the central Andes. Yet, his Fig. 2 shows that there are volcano density maxima at about the spacing I suggested, a spacing that would roughly equal lithosphere thickness. There are roughly 10 such maxima in Fig. 2. Perhaps these maxima reflect major batholiths emplaced from sub-litho-
163
spheric depths and controlled by fractures in the lithosphere. The actual volcanoes as located by Baker in Fig. 1 would then be controlled by the rigid roof above these batholiths, as he suggests. Detailed investigations of other volcanic areas are needed to verify such a two-tiered spacing system for volcanoes.
References 1 M. Baker, Volcano spacing, fractures, and thickness of lithospherre - a discussion, Earth Planet, Sci. Lett. 23 (1974) 161, this issue. 2 Anonymous, Tectonic Map of the Pacific Segment of the Earth (Geol. Inst. of Oceanology and the Academy of Sciences of the U.S.S.R., Moscow, 1970). 3 P.R. Vogt, Volcano spacing, fractures, and thickness of the lithosphere, Earth Planet. Sci. Lett. 21 (1974) 235.