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shear-wave speeds, it is necessary to estimate its value from Vp.A direct relation between Vs and Vp, therefore, is highly desir-able for many studies. This is accomplished by using empiri-cally derived Vp–Vs relations that are based on field boreholeand seismic profiling data together with laboratorymeasurements. A recent study (Brocher, 2005) has derivedempirical relations between Vp, Vs, and Poisson’s ratio thatcan be used to estimate the ratio Vp–Vs or, equivalently,Poisson’s ratio from a knowledge of Vp and rock type (i.e.,sedimentary vs. crystalline rock). The empirical and regres-sional fits are only defined for Vp between 1.5 and 8.5 km sand fit the data remarkably well (Figure 8).
Conversely, the mineralogy of the crust can be estimatedwhen both compressional-wave (Vp) and shear-wave (Vs)velocities are measured (Figures 8 and 9). The relationbetween Vp and Vs is commonly expressed by Poisson’s ratio,which varies from 0.23 to 0.32 for most minerals, but quartzhas a value of only 0.08 at room conditions (Christensen,1996). Thus, the measurement of Poisson’s ratio offers themeans of distinguishing between felsic (quartz-rich) andmafic (quartz-poor) rocks.
1.11.3.3.3 Seismic anisotropyMany minerals exhibit birefringence, which is a directionaldependence of the speed of light through the mineral. Thisphenomenon is used by petrologists to identify minerals in athin section (!1 mm) where a sample illuminated by polarizedlight is rotated under a microscope to reveal its birefringence.Likewise, elastic waves show a directional dependence in wavespeed in many minerals. Perhaps, the most prominent exampleis the mineral olivine, which is a major constituent of the uppermantle. The discrepancy between Rayleigh and Love wavespeeds was measured in the early 1960s (Anderson, 1961) andled to the recognition of seismic anisotropy in the mantle lid. Atabout the same time, laboratory measurements of metamorphicrocks demonstrated significant shear-wave anisotropy in thecrust (Christensen, 1966b). These measurements demonstratedthat seismic anisotropy is not only confined to the upper mantlebut also plays a prominent role in the crust (Figure 10). Table 4lists several key papers on the seismic properties, particularlyanisotropy, of the uppermost mantle. Additional references forlaboratory studies of the seismic properties of continental rockscan be found in Table 5.
Figure 7 Average velocity versus average density for a variety of rock types at a pressure equivalent to 20 km depth and 309 "C. Rock abbreviationsare as follows. AGR, anorthositic granulite; AMP, amphibolite; AND, andesite; BAS, basalt; BGN, biotite (tonalite) gneiss; BGR, greenschist faciesbasalt; BPP, prehnite–pumpellyite facies basalt; BZE, zeolite facies basalt; DIA, diabase; DIO, dionite; DUN, dunite; ECL, mafic eclogite; FGR, felsicgranulite; GAB, gabbro–norite–troctolite; GGN, granite gneiss; GGR, mafic garnet granulite; GRA, granite–granodiorite; HBL, hornblendite; MBL, calcitemarble; MGR, mafic granulite; MGW, metagraywacke; PGR, paragranulite; PHY, phyllite; PYX, pyroxenite; QCC, mica quartz schist; QTZ, quartzite;SER, serpentinite; SLT, slate. Reproduced from Christensen NI and MooneyWD (1995) Seismic velocity structure and the composition of the continentalcrust: a global view. Journal of Geophysical Research 100: 9761–9788.
Crust and Lithospheric Structure - Global Crustal Structure 349
5.10
8
3.67
3.06
2.69
2.45
2.27
2.14
2.03
1.94
1.87
1.81
1.76
1.71
1.67
1.638.57.56.55.54.53.52.5
λ>µ
λ=µ
λ<µ
1.50.2
0.3
Poi
sson
’s ra
tio
0.4
(1,2)0.5
Brocher et al. (1997a)
Eqn. 7,Castagna et al. (1985)
Eqn. 12,Ludwing
empirical fit
KeyAver. of crystalline rocks (Christensen,1996)
Aver. of sed. rocks (Mavko et al., 1998)
Individual lab. measurement (Calif.)
Individual lab. measurement (Non-Calif.)
Individual borehole measurement (Calif.)
USGS 30-m VSP (Boore, 2003)
Eqn. 8, Mafic line,Mafic and Calcium- rich
rocks
Vp(km s−1)
V p/V
s
Eqn.11, Brocher empirical fit
Figure 8 Poisson’s ratio as a function of Vp for common lithologies. Colored ellipses highlight measurements reported by a single reference: boldnumbers in parentheses link ellipses to similar studies. The thinner horizontal dashed line shows Poisson’s ratio of 0.25 (Vp/Vs¼1.73) commonlyassumed for the crust when the first Lame constant, l, equals the shear modulus, m. Reproduced from Brocher TM (2005) Empirical relations betweenelastic wavespeeds and density in the Earth’s crust. Bulletin of the Seismological Society of America, 95 (6): 2081–2092.
Poi
sson
’s r
atio
Vp
Vs
Felsic
Per
cent
by
volu
me
Mafic
Plagioclase
Hornblende
Ultramafic
Olivine
Min
eral
ogy
20
0
20
30
Biotite
40
QuartzK-F
eldsp
ar 50
Pyroxe
ne
60
10
40
60
80
100
2.0 0.24
0.25
0.26
0.27
0.28
0.29
Vel
ocity
(km
s–1
)
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Gra
nite
Qua
rtz
dior
ite
Dio
rite
Gab
bro
Per
idot
ite
Dun
ite
Oliv
ine
gabb
ro
Gra
nodi
orite
Figure 9 Variations in compressional-wave velocity (Vp), shear-wave velocity (Vs), and Poisson’s ratio (s) with mineral composition (Berry andMason, 1959) for common igneous rock types. Anorthite content of plagioclase feldspar is shown within the plagioclase field. Reproduced fromChristensen NI (1996) Poisson’s ratio and crustal seismology. Journal of Geophysical Research 100: 3139–3156.
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Oman : Semail Ophiolite (Nicolas, 1995)
オフィオライト:海洋地殻が陸上に衝上したものと解釈
Moholight-coloured peridotite
dark gabbro layer
26
枕状溶岩 pillow lava
(photo by fujii @ Oman in 2013)(photo by okino
@ Central Indian Ridge in 2006)
27
海洋地殻の形成• 地球における海と陸の本質的な違い
• 地球史における海の形成と海洋地殻の形成
• 観測からわかる現在の海洋地殻の特徴
• 中央海嶺ではどのようなプロセスで海洋地殻が生産されているか?
28
さまざまなテクトニックセッティングでの火山岩の組成 Volcanic rock geochemistry @ different tectonic setting
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