Download Document

Oceanic Moho and Mantle
what we learned from recent active source seismic studies
Shuichi Kodaira IFREE, JAMSTEC
Reaching the Mantle Frontier: Moho and Beyond
Sep. 9 -11, 2010, Washington, 1D.C.
Active source seismic studies in NW Pacific
after Muller et al. (1997)
The oldest oceanic crust on the Earth
2
after Nakanishi et al. (1989)
Active source seismic studies in NW Pacific
Formed near Izanagi – Farallon –
Pacific triple junction
after Nakanishi et al. (1989)
after Nakanishi et al. (1989)
Clear magnetic anomaly except for
the Jurassic magnetic quiet zone
Oceanic Moho at NW Pacific
25 km
6000 m
Simple and clear Moho as a single reflector
4
Oceanic Moho at NW Pacific
Scattered reflector
in upper crust
Unclear Moho
Clearly imaged Moho
consisting of sub-parallel reflectors
ODP801 Tivey et al. 2005
Kaneda et al. (accepted)
Marcus profile
Northwestern Pacific profile
Thick layer 2 formed by near-axis and off-axis volcanism
6
Variety of Moho (crust-mantle transition) from Oman ophiolite
Within-crust
dunite/wehrlite
Late-intrusive
dunite/wehrlite
100 m
10 m
sharp
Figure courtesy of S. Arai
diffuse
Modeling Moho reflection based on ophiolite
Marcus profile
Sheeted
wehrlite model
sinuous bands
500m
Layered gabbro
200m
planar bands
200m
Moho
wehrlite: 7.8km/s
harzburgite: 8.0km/s
dunite: 8.2km/s
sinuous bands
Modeling Moho reflection based on ophiolite
Background velocity White et al. (1992)
NW Pacific
sinuous bands
500m
planar bands
200m
sinuous bands
7.0(km/s)
8.0(km/s)
Moho
9
T – D/8 (s)
Oceanic Mantle at NW Pacific
8.5 km/s
Refraction from uppermost mantle
Apparent Vp of 8.9 km/s
OBS090
Offset (km)
Oikawa et al. 2010
0
0600
50
100
150
200
250
300
350
400
450
500
550
MTr6 Only wide-angle OBS data were acquired
Depth(km)
5
10
15
20
25
8.7
8.6
8.6
8.7
7.8
7.8
7.7
8.6
Unusually high velocity uppermost mantle
Vp ~ 8.7 km/s
NW
Northern part; clear magnetic anomaly
Reduction of uppermost mantle velocity
Vp ~ 7.8 km/s
Southern part; seamount activity
SE
10
High velocity (Vp=8.7 km/s) and strong anisotropy (~10%)
immediately below Moho
8.5 km/s
Parallel to MA
Vp = 7.9 km/s
Refraction from uppermost mantle
Apparent Vp of 8.9 km/s
Oikawa et al. 2010
0
0600
50
100
150
200
250
300
350
400
450
500
550
MTr6 Only wide-angle OBS data were acquired
Depth(km)
5
10
15
20
25
8.7
8.6
8.6
8.7
7.8
7.8
7.7
8.6
Unusually high velocity uppermost mantle
Vp ~ 8.7 km/s
NW
Northern part; clear magnetic anomaly
Reduction of uppermost mantle velocity
Vp ~ 7.8 km/s
Southern part; seamount activity
SE
11
Oceanic Mantle at NW Pacific
Parallel to MA
Perpendicular to MA
12
Seismic anisotropy in oceanic lithosphere has
been reported by many studies since Hess
(1964)
Shimamura et al. (1983)
In the NW Pacific, Vp=8.6 km/s with ~10%
anisotropy at 40 – 50 km below Moho is
detected as an average structure along the
1500 km-long profile,
but Vp and anisotropy just below Moho was
not resolved
New observation: high Vp and anisotropic layer exists immediately
below Moho in the NW Pacific fast spreading oceanic lithosphere
High velocity/anisotropic mantle and LCRs
New observation:
Lower crustal dipping reflectors
extent over the entire high Vp/ strong
anisotropy mantle
14
Eittreim et al., 1994
Ranero et al., 1995
Shape Moho and lower crustal reflectors dipping to the ridge
Lower crustal dipping reflectors have reported by previous seismic studies in the
Pacific.
New observations:
-Lower crustal dipping reflectors coexist with high velocity / anisotropic mantle
immediately below Moho
15
-Maximum dip direction of LCRs is parallel to the fast P-wave direction
Reston et al., 1999
Holtzman et al., 2005
Vp=8.6-8.7, anisotropy 7-10 %
Two models for LCRs: lithological layering and shear zone
The new observations support the model of basal shear at Moho
i.e. mantle moved faster than crust due to active mantle upwelling
Moho is petrological and mechanical boundary
16
Mantle velocity reduction toward trench
17
Hydration of uppermost mantle ?
Kerrick, 2002
Hyndman and Peacock, 2003
18
New observations
„
„
High velocity (Vp = 8.6 ~ 8.7 km/s) with strong
anisotropic (7 ~ 10 %) mantle immediately below Moho
and coexists with ridge ward lower crustal reflectors
and simple/clear Moho reflection
„
„
„
Layered Moho reflection at the magnetic quiet zone
„
„
indicating that strong share at Moho
i.e. mantle moved faster than crust due to active mantle upwelling
thick crust/mantle transition zone due to off-ridge magmatic activity
Velocity reduction (6 ~ 8 %) toward a trench from an
outer rise (due to hydration ?)
„
Need detailed Vp/Vs structure
19
Mohole candidate sites
Ildefonse et al. 2010
Draft site survey plan in 2011
Geophysical studies toward Mohole
„
1st stage:
„
„
„
2nd stage:
„
„
Seismic imaging of “typical “ oceanic crust in the
northwestern Pacific
Modeling Moho reflection based on ophiolite
Integrated geophysical study in Mohole candidate
areas
3rd stage:
„
Geophysical studies while- and post-drilling
Core – log – seismic integration
„ Long-term monitoring (geophysical / geochemical)
21
„