S13D-1102
FINE-SCALE STRUCTURE OF THE MOHO FROM RECEIVER FUNCTIONS: EFFECTS OF A DEFORMING CRUST
G. Zandt, H. Gilbert, A.A. Ozacar, University of Arizona, & T. J. Owens, University of South Carolina
INTRODUCTION
NEW MOHOS IN ROOTLESS MOUNTAINS: Foundering of Eclogitic Roots
METHODOLOGIES FOR MOHO MEASUREMENTS: Receiver Functions
Crosswhite & Humphreys, Geology, v. 31, 2003
Ba
sin
Long Valley
Pds
P
vad
y
C
tle
Man
converted S
60
d
α2, β2, ρ2
38
140
Kings +
V.F.
Moho
"hole"
+
+
4
2
Section T10
+
+
+ +
+
+
+
CA
+
+
30
crust-mantle shear zone variable P-s wave arrival
+
+
+
Sierra Nevada
Big Pine V.F.
small
E
100
Moho
2
Peridotite
Mantle
melt
bands
Late
Zone
Shear
4
depth (km)
TL
10
90
20
-0.1 0
0.1 0.2 0.3
APVC
S
100
M?
30
70
50
75
slab
50
-0.1 0
60
70
80
slab
90
0.1 0.2 0.3
0
0.1 0.2 0.3
Greater Indian crust
Indian Lithosphere
200
0
100
200
Neotethyan
slab
PEL
70W
Sakaguchi et al.,in preparation, 2004
In the forearc of subduction zones, fluids released from the subducting slab can hydrate the overlying mantle. For cold mantle
(<~700°) partial to full serpentinization can reduce the shear
wavespeed such that it is nearly the same or less than the overlying crustal S-wavespeed. The resulting Moho can “disappear” or
become “inverted”. Examples from the US NW Pacific subduction zone are shown on the left and the western South American
subduction zone are shown on the upper right.
500
KS
JS
Kunlun
fault
al T
hru
Altyn Tagh
fault
st
N
e
low Pn-zon
-,
S
t
n
n
ie
Ineffic
Re
dR
Mantle
Anisotropy
150
165
180
195
-31o
ic
rkey Seism
Eastern Tu riment
Expe
1999-2001
PLATE
F
Anatolian
Plate
39º
lt
au
nF
Low crustal Vp
0
Very Low Vp & Vs
Lhasa Terrane
SSE
St.18
BNS
fabric in LVZ
HRPT
34º
Qiantang Terrane
NNW
Moho
38º
4.0
Vs
5.0
7.0
8.0
20
High Vp & Vs
40
50
60
70
80
Old Indian Moho
100
31º
Indian
Mantle
New AsianMoho
LVZ
32º
BNS
Latitude
33º
34º
N
S
~6
5k
m
initial
shear zone
N
Mid-crustal
shear zone
20
S
30
25.5 %
anisotropy
LVZ
moho
320º
final
shear zone
LVZ
MOHO
0º
80º
Station 18 (Data)
160º
240º
320º
back azimuth
Station 18 (Synthetics)
S-C
fabric
0
slow axis
symmetry
depth
fast axis
Seismic velocity (km/s)
3.0
4.0
5.0
6.0
N St. 18
N
14.4 % aniso.
10
5
free surface
delamination
S
3
Hexagonal Anisotropy
slow axis
N
S
0
Indian lower crust
Indian
mantle
240º
0
3
fast axis
symmetry
40
5
80º
160º
240º
back azimuth
320º
0º
80º
160º
240º
back azimuth
320º
S
18.2 % aniso.
Vs
Density = 2.7 (g/cc)
Vp / Vs = 1.80
E
W
5 % aniso.
30
0º
6.0
0
E
Eastern Turkey is a young continental collision zone that may be a good
analog for the early stages of the India-Asia collision, prior to crustal doubling. A recent broadband seismic experiment in the region is summarzed by
Sandvol et al., GRL, v. 30, 2003. Our analysis of data from a station within
the triangular region bounding the tectonic escape of the Anatolian plate reveals very strong anisotropy at the base of the crust, suggesting the crust is
the Anatolian plate lithosphere. This will be part of the topic of a poster on
Friday by Ozacar et al. ( T51C-0463).
30
90
160º
LLAN
PICH
Moho
Pn Moho
71o
strong RF M
weak RF M
70o
69o
68o
longitude (degrees W)
67o
66o
65o
Observed
Synthetic
Isostatic
RMS=0.25 km/s
Fromm et al., GRL, v. 31, 2004
N
Vp
5
back azimuth
PACH HEDI RINC
-64o
7.0
plunge / trend of slow-axis
moho dip / dip direction
40
4
80º
8.0
-66o
partially eclogitized
72o
9.0
ELBO NEGR
-68o
-31o
100
200
300
400
500
Distance (km)
600
700
800
41º
W
1
0º
40º
39º
6.0
Density = 2.7 (g/cc)
Vp / Vs = 1.73
10
0
HURT
0
20
40
60
80
Bitlis Suture
Arabian Plate
Seismic velocity (km/s)
3.0
0
4
Low Vp & Vs
E
-70o
4
Station HRPT (Synthetic)
1
10
AF
-72o
PICH
38º
ARABIAN PLATE
0
Strong SKS Splitting
Eurasian
Plate
NA
N
Dead Sea Fa
ult
lt
INDEPTH III
Big mountains and high plateaus are made in continental collision zones. In the process, the crustal
thickness is doubled, but how? In the Himalayas, the
upper crust from the Indian lithosphere is sheared off
and stacked into the Himalayan fold and thrust belt.
What happens to the remaining lower crust? Some
have suggested that it underplates the entire Tibetan
Plateau (DeCelles et al., Tectonics, 2002). Beneath
the Bangong suture in central Tibet, mid-crustal seismic anisotropy indicates a fabric with a bottom to the
north sense of shear, consistent with an underthrusting lower crustal slab. If correct, this model suggests
the old Indian Moho becomes a plane of delamination and a new Asian Moho forms as the lower
crustal slab emerges from the delamination zone.
40º
elev (km)
135
5
High conductivity zones
410
90
105 120
x offset (km)
Anatolian Fau
lt
Station HRPT (Data)
rF
au
Moho
300
LLAN
ELBO
Chile Argentina
counterflow
drip
s
Ea
ive
Sierras
Pampeanas
PACH HEDI RINC
asthenosphere
lia
re
ato itlis Sutu
n
B
tA
Asian
Lithosphere
km
300
90
80W-0.1
Moho
100
(km)
Very low Lg Q
North
Mediterranean Sea
INDIA
Indian cr
ust
80
alay
an F
ron
t
Songpan-Ganzi T. Kunlun-Qaidam Tarim
Qiangtang Terrane
BNS
IZS
STD
MBT
Lhasa Terrane
75
Kunlun Fau
lt
Him
Himalayan Fold-thrust Belt
60
Black Sea
ANATOLIAN
Keskin, GRL, v. 30, 2003
60
40
depth (km)
50
45
time (s)
80
LVC
40
30
time (s)
slab
PEL
15
time (s)
0
M?
0
N
lt
Fau Qiadam Basin
h
g
a
yn T
-64o
-29o
NEGR
Radial
70
Tarim Basin
-66o
Coastal High Precordillera
Region Cordillera
HURT
lt
20S
Owens Valley
Tulare Basin
Depth (km)
AN
AN
~500 km
20
depth (km)
60
A)
Sierra Nevada
Owens
Valley
60
~250 km
TIBET
30
-29o
195
MOHO
au
eF
uih
BANJO &
SEDA
180
"HOLE"
sh
50
165
We normally think of mountains as regions with thick crust, yet there are regions with mountains and no crustal roots. In the Sierra Nevada of the western US, the extraction of the great
Sierran batholith left an underlying residual of garnet pyroxenite (a type of eclogite). After
cessation of Farallon subduction, this untra-dense root started foundering in a convective instability (a “drip”). As the lower eclogitic root shears off westward from the upper granitic
body, a new Moho is formed, and the thinner crust, progressively relieved of its anchor, tilts
westward and its eastern edge rises up to form the High Sierra Nevada. A signature of this process is the presence of lower crustal seismic anisotropy, a relict feature from the intial stages
of separation of the root from its batholith.
th III
depth (km)
N
10
40
150
MOHOS IN CONTINENTAL COLLISION ZONES: Shearing and Delamination
0
300
135
-68o
CRUST
Radial
75
90
105 120
x offset (km)
Sierra Nevada
n
Xia
200250
LVC
75
Section T7
40
100
Indep
M?
30
6 Ma to recent
60
MOHO MULTIPLE
Alt
50
45
-70o
80
100
20
30
Transverse
NNA
15
Transverse
10
0
-72o
0
M
TL
TL
E
r
a
e
sh e
zon
Arabian
Plate
NNA
60
E
Moho
Depth (km)
0
S
asthenosphere
20
M
se
M
M
rpe
AN
AN
TL
zed
i
n
i
nt
Moho
E
E
Moho
"HOLE"
App. velocity
(km/s)
d
e
z
i
it
MAGMA?
MOHO
80
Early
Zone
Shear
variable
with az
40
Depth (km)
small
W
CRUST
CRUST
CRUST
CRUST
Granitic
Batholith
large
CRUST
20
Depth (km)
high-speed crust and normal
mantle – small P-s wave
depth (km)
normal curst and low-speed
mantle – small P-s wave
We normally think of mountains as regions with thick crust, yet there are regions with thick crust and no mountains. One possible explanation is the presence of eclogites which have densities that are 200-500 kg/m3 denser than
other ultramafic rocks. The eclogites could occur below the Moho where
their mantle-like wavespeeds would make them “invisible” seismically. An
example shown above is of a mountainless root across the 1.8 Ga Cheyenne
belt suture in the western US. But the eclogites could also reside above the
Moho as in the example shown below from the south-central Andes. This example will be the topic of a talk by Gilbert et al. (T43E-02, Th-1:55, rm3001).
+
0
Normal crust and mantle large P-s wave arrival
eclogite
+
Big Pine
V.F.
Sierra Nevada
NV
+
+
35 +
Moho depth
contours (km)
Zandt et al., Nature, v. 431, 2004
120
M
Big Pine
+ V.F.
+
+
118
220
+
+
36°
117
incident P
VANISHING OR INVERTED MOHOS: Serpentinization of the Forearc Mantle
F o re a rc
Visalia
119
37°N
Contact Information: George Zandt, Dept. of Geosciences, University of Arizona, [email protected]
Bostock, et al., Nature, v. 417, 2002
+
drip outline
at 150 km
depth T7
°W
116
180
This research was supported by the EAR Division of NSF. Data collection and archiving was supported by IRIS, specifically by the PASSCAL and DMC programs.We acknowledge the P.I.s,
collaborators, and graduate students who spent months in the field to collect the data, and the authors of the papers cited in this summary presentation.The first author (GZ) would like to acknowledge Professor K. Aki who first instilled in him the curioisity about what those wiggles mean, and taught him a few tools to try to figure it out. This poster is dedicated to Roberto
Fromm, an extraordinary graduate student and friend to many, who died doing what he enjoyed, in July 2004.
Brocher et al., v. 31, Geology, 2003
Fresno
BASIN & RANGE
PROVINCE
35
+
+
T10
100
Depth [km]
α1, β1, ρ1
“What do those wiggles mean?” Professor K. Aki with a group of MIT graduate students examining a seismogram, sometime in the late 1970s.
Moho
Zandt, Nature, v. 417, 2002
"
"Drip
20
og
l
c
e
Moho weak or
absent
o
Moh
rust
?
transmitted P
outline of 3-4 Ma
potassic volcanism
Long Valley
Caldera
loc
a
sub l
densice
at V
alle
y
38°
Death Valley
40
Receiver Function
Val
le
a
Gre
nge
35
Converted waves Conversion of P to S and S to P occurs at a discontinuity for nonnormal incidence. These converted waves sometimes show distinct arrivals on the
seismogram between the P and S arrivals, and may be used to determine the location
of the discontinuity. - from the Glossary of Waves in Quantitative Seismology, Aki
& Richards, 1980.
Ne
ens
Ra
118°
DA
VA
NE
rra
120°
A
Sie
Ow
and
RR
SIE
Seismology ... offers a means by which investigation of the Earth’s interior can be
carried out to the greatest depths, with resolution and accuracy higher than are attainable by any other branch of geophysics. - from the Introduction in Quantitative Seismology, Aki & Richards, 1980.
Y
LE
AL
TV
EA
GR
Andrija Mohorovicic, a Croatian seismologist, is credited with
the first estimation in 1909 of crustal thickness using the critically refracted phase Pn. The crust-mantle boundary has
become commonly known as the Moho and its depth, structure, formation, and evolution remains an important research
topic in seismology, petrology, and tectonics. Other seismic
phases sensitive to Moho depth and structure are the converted
phases Ps and Sp, and the associated 2p1s and 1p2s reverberation phases that are isolated in receiver function waveforms.
With sufficient station coverage, multiple receiver functions
can be migrated and stacked into cross-sections of the crust.
Crustal cross-sections from tectonically active regions reveal
dramatic variations in amplitude and frequency content of
Moho phases that we associate with fine-scale structure, and
possibly anisotropy at the crust-mantle boundary. Processes
directly related to these Moho structures include crustal thickening, crustal extension, crustal flow, delamination or convective removal, and eclogitization. Examples of receiver function crustal images and their tectonic implications are presented here from the western US, South American Andes, and
the Anatolian and Tibetan plateaus.
Teleseismic line
OLD MOHOS IN MOUNTAINLESS ROOTS: Eclogitization over and under the Moho
Vp
S
N
Ozacar & Zandt, GRL, v. 31, 2004
plunge / trend of
slow-axis
dip / dip direction
Andrija Mohorovicic
Andrija Mohorovicic was born on 23 January 1857 in Volosko, a coastal Istrian village near Opatija, where his father, also
named Andrija, was a blacksmith making anchors. Andrija Jr. obtained his elementary education in his home town, continued
his study in the gymnasium of a neighboring town, Rijeka, and received his higher education in mathematics and physics at the
Faculty of Philosophy in Prague in 1875.
On January 1, 1892 he became the head of the Meteorological Observatory on Gric in Zagreb, where he continued to work in
the meteorological observatory, establishing a service to all of Croatia, all the while simultaneously teaching geophysics and astronomy at the university. In 1901 he was appointed head of the complete meteorological service of Croatia and Slavonia, which
he raised to a European level in personnel and equipment. And finally, he gradually extended the activities of the observatory
to other fields of geophysics: seismology, geomagnetism and gravitation, switching his main interest toward seismology. He acquired a few seismographs that were installed before the occurrence on October 8, 1909 of the Pokuplje (Kupa Valley) earthquake with its epicentre 39 km southeast of Zagreb.
From 1893 to 1917-18 he taught subjects in the fields of geophysics and astronomy at the Faculty of Philosophy in Zagreb. He
discovered that when an earthquake occurs two waves, longitudinal and transverse, that propogate through the soil with different
velocities. Analyzing the data of seismographs from a dozen stations, Mohorovicic showed that the Earth consists of a surface
layer above an internal core. From the calculations he was able to estimate the thickness of the upper layer as 54 km. In these
studies he was the first in the world to establish, on the basis of seismic waves, a surface of velocity discontinuity that separates
the crust of the Earth from the mantle and which was named the Mohorovicic Discontinuity, also known as the Moho, in his
honour. He retired at the end of 1921, and died on December 18, 1936 and was buried at the Mirogoj Cemetery in Zagreb.
On December 19, 1936, a day after the death of Andrija Mohorovicic, the Zagreb paper Novosti published the following article: "The scientist Professor Andrija Mohorovicic, member of the Yugoslav Academy of Sciences and Arts, one of the founders of modern seismology, has died. He was a well-known and respected figure in Zagreb, and his scientific work in the field
of seismology gained him world recognition. He is today considered one of the founders of modern seismology in the world."
Sources:
• http://www.istrians.com/istria/illustri/mohorovicic/
• Skoko, Dragutin, "Andrija Mohorovicic", U.S.G.S., v36 (January 2000), p. 1-2.
• University of Zagreb, Department of Geophysics, Faculty of Science - Dragutin Skoko and Marijan Herak, Andrija Mohorovicic
• Norwegian Physical Society - Andrija Mohorovicic