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Tectonophysics 508 (2011) 85–95
Contents lists available at ScienceDirect
Tectonophysics
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o
Seismic structure of the upper mantle along the long-range
PNE profiles — rheological implication
N.I. Pavlenkova
Institute of Physics of the Earth RAN, Moscow, Russian Federation
a r t i c l e
i n f o
Article history:
Received 6 January 2009
Received in revised form 8 November 2010
Accepted 17 November 2010
Available online 25 November 2010
Keywords:
Upper mantle
Seismic stady
Rheology
Northern Eurasia
a b s t r a c t
The long-range seismic profiling made in Russia with Peaceful Nuclear Explosions reveals structural
regularities in the continental upper mantle which characterize its rheological stratification. The latter is
difficult to describe in the form of the asthenosphere–lithosphere system because the ‘thermal’ asthenosphere
(partly melted and lower velocity zone) was not traced by these studies. The rheological stratification follows
from the regular change of horizontal heterogeneity which determines three layers of different plasticity. The
layers are divided by the seismic boundaries N and L at a depth of around 100 and 200 km. The boundaries are
not simple discontinuities, but heterogeneous (thin layering) zones. Beneath the N boundary the block
structure typical of the upper brittle part of the lithosphere disappears and low-velocity layers are often
observed. At the L boundary the Q factor decreases and the upper mantle structure changes showing the
isostatic equilibrium at this level (the latter coincides with the bottom of the ‘thermal’ lithosphere). It does not
seem to be a coincidence that the xenoliths are also originated around depths of 100 and 200 km. A possible
explanation of all the data is a concentration of mantle fluids at these critical depths. The fluids change the
mechanical properties of the matter and initiate partly melting. The matter flow along the weak zones results
in the origin of the corresponding seismic boundaries (the weak zones).
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The deep seismic studies are important for understanding the
evolution of different tectonic units and interaction between the crust
and upper mantle. In order to understand this interaction both crust
and upper mantle structures have to be studied in detail. Such studies
have recently become possible due to long-range seismic profiles
carried out with large chemical and Peaceful Nuclear Explosions
(PNE) in the Northern Eurasia (Fig. 1).The PNE studies were made by
the GEON Centre of the USSR Ministry of Geology (now Ministry of
Natural Resources of Russia). It was a large program of seismic
profiling carried out in 1970–1980s in order to study comprehensively
the structures of the upper mantle and the mantle transition zone to a
depth of 700 km (Egorkin and Chernyshov, 1983; Benz et al., 1992).
The profiles with a total length of more than 20,000 km include 25
PNEs and many large chemical explosions. The seismic studies were
made with 3-component magnetic type recordings at a large number
of stations positioned on profiles with an interval of 10 km. The shot
intervals between the chemical explosions were 100–150 km,
between PNEs in average 800–1000 km. The length of the profiles
varies from 1500 to 3200 km. The chemical explosions provided
recordings up to 300–600 km offsets, the PNEs — up to 3200 km.
E-mail address: [email protected].
0040-1951/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2010.11.007
The profiles cross several large tectonic units of the Northern
Eurasia: the East European and the Siberian cratons of the Archean–
Proterozoic ages, the Urals Paleozoic orogenic belt, the West-Siberian
and Timan-Pechora plates of the Caledonian–Hercynian ages (Fig. 1).
The tectonic units differ in geological history, in crustal structure and
geophysical fields: the heat flow is 40–50 mW/m2 in the East
European-Craton, 30–40 mW/m2 in the Siberian Craton and in the
Urals, and 50–60 mW/m2 in the West-Siberian and Timan-Pechora
Plates (Pollack et al., 1993; Artemieva and Mooney, 2001).
The interpretation of the PNE long-range profile data was made at
first by GEON Centre (Egorkin and Chernyshov, 1983; Pavlenkova and
Yegorkin, 1983; Egorkin et al., 1987; Egorkin, 1999). During 1990s the
PNE records were digitised and the data became available for
international groups (Cipar et al., 1993; Mechie et al., 1993; Priestly
et al., 1994; Ryberg et al., 1995; Thybo et al., 1997; Morozov et al.,
1999; Nielsen et al., 1999, 2002; Pavlenkova et al., 2002). General
analysis of the data and their comparison with the other long-range
seismic profiles were made in Fuchs (1997).
During the last years the experimental data from all the long-range
profiles carried out in the Northern Eurasia (Fig. 1), were compared
and they were processed in terms of a unified approach, and combined
solutions for both chemical and nuclear explosions were obtained
(Pavlenkova, 1996; Pavlenkova and Ushakov, 2005; Pavlenkova, 2006;
Pavlenkova and Pavlenkova, 2006, 2008). This allowed to refine the
general structure of the upper mantle and to make all resulting models
match at intersection points of the profiles, thereby increasing the
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Fig. 1. Map showing the location of the long-range seismic profiles and main tectonic structures in the Northern Eurasia: the East European old platform, the Siberian Craton (SC),
West-Siberian (WS) and Timan-Pechora (T-P) young platforms, the Urals and the Vilyui Basin (VB). Letters indicate locations of the Peace Nuclear Explosions (PNE): Q, Q1, Q2, and
Q3 along the Quartz profile, C1, C2, C3, and C4 along the Craton profile and so on (Pavlenkova and Pavlenkova, 2006).
reliability of the models. This combined processing helped to recognize
the common features of the crust and upper mantle structure
throughout the whole investigated area, in particular, to trace the
reference boundaries.
The present paper continues the PNE data analysis and its key
questions are the following:
What are the general changes of the upper mantle velocity
structure and how do they characterize its rheological properties?
What are the origins of the regional seismic boundaries in the
upper mantle and what is their role in the lithosphere rheology?
2. The observed wave fields and velocity models of the upper mantle
Records from nuclear sources obtained from the super-long
seismic profiles reflect a complex pattern of the wave fields with
sharp variations in apparent velocities and amplitudes of mantle
waves recorded as first arrivals and as high amplitude secondary
arrivals (Figs. 2–4). However, some common features of the wave
fields are noteworthy.
The group of the upper mantle waves, observed at distances of
200–2000 km as first and secondary arrivals with apparent velocities
of 8–9 km/s, consists of few waves. The first wave Pn with a recording
interval of 200–700 km is often characterized by small amplitudes
and is indiscernible in records. Its intensity rapidly decays with the
distance from a source, the attenuation coefficient is of 0.003–
0.045 km−1. The apparent velocities vary from 7.8 to 8.4 km/s,
reflecting a complex fragmented structure of the mantle directly
under the Moho boundary.
The wave Pn and the next waves PN have often step-like traveltimes (Figs. 2 and 4) because of the alteration of layers with high and
low velocities in the uppermost mantle. This suggests the existence of
a velocity inversion zone or a decrease in the velocity gradient at a
depth of about 100 km.
The waves PN is recorded mainly at distances of 800–1500 km, and
its apparent velocities increase with the source–receiver distance
from 8.2–8.4 to 8.4–8.6 km/s. At first these waves were identified on
numerous shorter seismic profiles and were associated with the
reflector N located at a depth of 80–100 km (Pavlenkova, 1996). H.
Thybo has shown that this boundary at a depth of about 100 km is of a
global significance and called it as 8° boundary (Thybo and Perchuć,
1997; Thybo, 2006). Detailed analysis of wave fields on all long-range
profiles on the territory of Russia (Pavlenkova and Pavlenkova, 2006)
showed that the N wave group is generally divided into two waves PN1
and PN2; i.e., we may state that two closely spaced boundaries N1 and
N2 with close average velocities are present.
The PL wave is observed at the first arrivals at distances of 1500–
1800 km. It has a higher apparent velocity (8.6–8.7 km/s) than PN
waves. At the first arrivals this wave is weak, whereas at the later ones
its intensity increases at offsets of 800–1300 km (Figs. 2 and 4).
Sometimes, the records of PL continue the multi-phase record of PN2;
therefore, these waves are difficult to separate (Figs. 2 and 3). The PL
waves are refractions/reflections from depths of about 200 km. They
can be related to the L boundary, known in seismology as the
Lehmann boundary (Lehmann, 1959; Dziewonski and Anderson,
1981; Hales, 1991). Very often the PL waves strongly attenuate at
distances of 1300–1500 km (Fig. 3), which can be due to decreasing of
the velocity gradient under this boundary.
The PH waves are recorded mainly as secondary arrivals at
distances of 1700–2200 km. It is wave P350 in Thybo et al., 1997).
Due to the long many phase records of these waves it is difficult to
determine the depths of the correspondent reflector; we labelled the
wave as PH.
The stable and intense seismic records were obtained at the first
and secondary arrivals at offsets of 1500–3000 km (Fig. 2), and they
are the waves from the upper–lower mantle transition zone. The most
distinct waves are well known in seismology P410, P520 and P680 waves
(the wave index marks the average depth of corresponding boundary,
km). These waves differ significantly in apparent velocity (Va): P410
(Va = 10 km/s), P520 (Va = 10.5 km/s), and P680 (Va = 11 km/s).
The times and apparent velocities of the identified waves vary
along the profiles, implying that the velocity structure is horizontally
heterogeneous. None of the discovered reference boundaries forms a
continuous horizon: they are separate reflectors differing significantly
in reflection properties. In all the cases these boundaries are not of the
first order discontinuities. Both the first and secondary waves from
these boundaries are complicate multi-phase groups with long coda
(Figs. 2–4). As shown in Fig. 5, such many phase groups are typical of
the high reflective zones with alternating of the high- and lowvelocity layers. Average velocity contrasts are not high at the
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Fig. 2. Record-section of the upper mantle waves from the PNE C1 with the calculated travel-times (a) and the calculated rays (b) for the velocity model along the Craton profile
(Figs. 1 and 6). The reduction velocity is 8.7 km/s, t is time, d is distance from PNE. Several mantle waves may be traced in the records: Pn — refraction in the uppermost mantle
(apparent velocities Va = 8.0–8.5 km/s), PN1 (Va = 8.2–8.5 km/s) is the reflection/refraction from the seismic boundaries N1 (boundary velocity Vb = 8.35 km/s), PN2 (Va = 8.3–
8.6 km/s) from the boundary N2 (Vb = 8.4 km/s), PL (Va = 8.6–8.8 km/s) from the boundary L (Vb = 8.5 km/s) and PH (Va = 8.8–9.0 km/s) from the boundary H (Vb = 8.6 km/s).
P410, are the waves from the top of the upper–lower mantle transition zone T1 (Vb = 9.4 km/s) at a depth about 410 km; P520 — from the internal boundary T2 (Vb = 9.8 km/s) at a
depth about 520 km and P680 — from the transition zone bottom T3 (Vb = 10.6 km/s) at a depth about 680 km. All the upper mantle waves have long codas and may be separated
only at the first arrivals; their apparent velocities differ from the model velocities due to the Earth's surface curvature.
boundaries but they are high within the thin layering. The comprehensive velocity modelling for such lamellas and other heterogeneous zones
are given in Perchuć and Thybo (1996), Nielsen et al. (2002), and Thybo
et al. (2003). Egorkin et al. (1987) and Morozov et al. (1999) show these
boundaries as thin low-velocity layers.
In Figs. 6 and 7, as examples, the velocity cross-sections are
presented along the longest Craton and Quartz profiles (Fig. 1). The
other cross-sections are published in Pavlenkova and Pavlenkova
(2006).
The velocity models were constructed for all profiles with ray
tracing using the S83d program by Červeny and Pšenčik (1983). The
ray tracing was carried out in two ways: for some typical PNEs there
are 1-D solutions with the spherical surface of the Earth, and the 2-D
solutions with the plane surface were obtained for all the profiles
(Figs. 2–4 and 8–11). The velocity values obtained by the 2-D
modeling were corrected for the surface curvature after the
comparison them with the 1-D models. The corrections are negligible
down to a depth of 100 km but they reach 0.6–0.7 km/s at the bottom
of the upper mantle and at the top of the mantle transition zone (T1
boundary).
A common basic velocity model was used for the modeling along
all the profiles of Fig. 1. This model has five layers in the upper mantle
and the velocities increase linearly in the layers in the following
ranges: 8.0–8.3, 8.35–8.4, 8.4–8.5, 8.5–8.6 and 8.6–8.7 km/s. (The
layer velocities are lower than the corresponding wave apparent
velocities due to the Earth surface curvature). The layers are divided
by the boundaries N1, N2, L and H. The modeling included travel-time
data from both PNEs and chemical explosions. The ray tracing was
carried out with the constant angle between the rays. In that case the
point density on the travel-time curves characterizes the refracted
wave amplitudes and it is not necessary to calculate the synthetic
seismograms (Figs. 2–4, 8–11).
The 2-D models confirm that the upper mantle is characterized by
the strong horizontal inhomogeneity (Figs. 6 and 7). It is manifested
by the changes of seismic velocities, by the degree and nature of
layering and by the relief of seismic boundaries. The main feature of
the upper mantle structure along all the profiles is the most significant
heterogeneity in the upper 100 km where the velocity changes occur
not gradually but very often suddenly, indicating a block structure.
The blocks correlate well with the large tectonic domains. Higher
velocities (over 8.2 km/s) are characteristic of the East European and
Siberian cratons, while smaller values (8.0–8.1 km/s) are observed
beneath the Timan-Pechora and West-Siberian young plates. The
close correlation is also determined between the uppermost mantle
velocities and the heat flow. In the areas with low heat flow (30–
40 mW/m2) the velocities are higher (the Siberian Craton), the higher
heat flow regions (50–60 mW/m2, the West-Siberian and Pechora
plates) are correlated with the lower mantle velocities.
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Fig. 3. Record-section of the upper mantle waves from the PNE C2 with the calculated travel-times (a) and the calculated rays (b) for the velocity model along the Craton profile
(Figs. 1 and 6). Legend is in Fig. 2.
At a depth of 100–120 km a low-velocity zone is determined along
many profiles. The deeper horizontal changes of the velocities are
smoother and a clear difference is observed in the mantle structure
only between large tectonic units: the N and L boundaries uplift
beneath the cratons. At a depth of 300 km the situation changes: the H
boundary depth decreases beneath the Siberian craton.
The seismic cross-sections, constructed along all the profiles of
Fig. 1, were used to compile a 3-D upper mantle model (Pavlenkova
and Pavlenkova, 2006). The model confirms the regularities of the
mantle structure observed along the Quartz and Craton profiles.
Beneath the Moho the velocities change from 8.0–8.2 km/s in the
West Siberia to 8.3–8.4 km/s in some blocks of the Siberian Craton and
of the Urals. The lowest velocities (8.0–8.1 km/s) are characteristic of
the central part of the West Siberia and for the Timan-Pechora Plate.
At a depth of 100 km the velocity distribution looks otherwise. The
most local high-velocity anomalies disappear and only two large
anomalies are observed in the depth maps of the N1, N2 and L
boundaries: lower velocities in the central part of the West Siberia and
the higher velocities in the Siberian Craton. The depth map of the H
boundary reveals the opposite pictures: greater depths beneath the
eastern part of the craton (330 km) and the depths of 300 km beneath
the West-Siberian platform.
At the previous interpretation of the Quartz and Craton profiles
(Mechie et al., 1993; Ryberg et al., 1995; Egorkin at al., 1987; Morozov
et al., 1999; Nielsen et al., 1999, 2002; Nielsen, Thybo, 2006) the similar
generalized velocity models were determined. They show the same
upper mantle velocity difference between the East European Platform,
the West-Siberian plate and the Siberian Craton and the low-velocity
layer at a depth of 100 km. But the models differ in details: in the
numbers and depths of the determined boundaries and in their structure.
No data were obtained on the mirror correlation between the L and H
boundaries. Our interpretation of all the profiles using the PNEs and large
chemical explosions together gave also a possibility to study the more
detailed horizontal heterogeneity of the uppermost mantle.
3. Seismic evidence and interpretation in terms of the upper
mantle rheological properties
The rheological properties of the upper mantle are usually
described in the form of the lithosphere–asthenosphere system. The
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Fig. 4. Record-section of the upper mantle waves from the PNE Q2 (the observation to the north–east) with the calculated travel-times (a) and the calculated rays (b) for the velocity
model along the Quartz profile (Figs. 1 and 7). Legend is in Fig. 2.
definition of the asthenosphere, however, becomes increasingly
blurred with time. An important ambiguity is well illustrated by
comparing the depths of the lithosphere–asthenosphere boundary
obtained by different geophysical methods. The “thermal” asthenosphere determined from heat flow data was defined as the zone of
partial melting (Pollack et al., 1993; Rudnick et al., 1998; Artemieva
and Mooney, 2001). The depths of the “thermal” asthenosphere vary
from 200–300 km beneath the continents to 80–100 km beneath the
oceans. The geoelectrical definition of the asthenosphere is identical
to the thermal one because the electrical conductivity is very sensitive
to the melting. In the oceans the “thermal” and geoelectrical
asthenosphere really coincide. But beneath the continents the higher
conductivity layers are determined at shallower depths, for instance,
at a depth of 100 km (Jones, 1992; Kovtun et al., 1994).
Seismic studies define the asthenosphere as a zone of the lowvelocity and low Q factor. In the active tectonic regions such zones are
really often observed. Beneath the Siberian Craton the “thermal”
asthenosphere was supposed at depths of 250–300 km (Artemieva
and Mooney, 2001). The long-range seismic profiling, however, has
not discovered any low-velocity zone at these depths, only the Q
factor decreases at this depth (Egorkin and Kun, 1978; Egorkin, 1995).
During the last years new methods have been developed to
determine the temperature regime of the upper mantle using
geophysical and petrophysical data (Sobolev et al., 1997; Goes et al.,
2000; Cammarano et al., 2003; Kuskov et al., 2006; Kronrod et al.,
2010). The temperature profiles are obtained from absolute values of
velocities, taking into account phase transformations, anharmonicity,
and anelastic effects. Based on the chemical thermodynamic methods,
the chemical composition, temperature, density, and seismic wave
velocities a new petrophysical model is constructed for the Siberian
Craton lithosphere (Kronrod et al., 2010). The bottom of the craton
lithosphere was determined at the crossing of the temperature profile
with the adiabat at a depth of 275 km. These data agree with our
generalized velocity model (Fig. 12).
The velocity model of Fig. 12 characterizes the large platform areas
of the Northern Eurasia and it differs from the IASP91 model (Kennet
and Engdahl, 1991) constructed mainly for the tectonically active
regions of the world. In Siberia the velocities are higher in the
uppermost part of the mantle and it is understandable for the cold
cratonic region. But it is not easy to explain why in the low part of the
upper mantle the Siberian velocities are lower than in the IASP91
model. As it is shown in Kuskov et al. (2006), the inversion of the
LASP9I model with a fixed bulk composition of garnet peridotites (the
primitive mantle material) leads to a temperature inversion at depths
of 200–250 km, which is physically meaningless. It is supposed that
the temperature inversion can be removed by gradual fertilization of
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Fig. 5. Comparison of the synthetic seismograms for the two types of the seismic boundaries: (1) the simple first order boundary and (2) the thin layering (lamella) zone.
the mantle with depth. In this case, the lithosphere should be
stratified in chemical composition.
The PNE studies reveal also some structural properties of the
continental upper mantle which may characterize its rheology. A
strong change of the rheology may be supposed at depths of 70–
120 km at the N1 boundary. The most important feature of the
boundary is that it divides the lithosphere in two portions with
different inner structures. Above the N1 boundary the sub-Moho
Fig. 6. Velocity cross-section along the Craton profile (the letters show locations of the PNEs, Fig. 1). The seismic boundaries: M is the Moho, N1, N2, L, and H are the upper mantle
basic boundaries, and T1 is the top of the upper/lower mantle transition zone (the velocities increase linearly between the boundaries). 1 — seismic boundary with the boundary
velocity (km/s); 2 — reflector generating the high amplitude waves; 3 — lower velocity zone; 4 — high-velocity block; and 5 — zone of the higher inhomogeneity.
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Fig. 7. Velocity cross-section along the Quartz profile (the letters show locations of the PNEs, Fig.1). Legend is in Fig. 6.
lithosphere has a complex block structure (Figs. 6 and 7), beneath the
boundary the blocks disappear. In many regions this boundary
underlines low-velocity layers. These characteristics of the N1
boundary (change of the upper mantle general heterogeneity and
existence of the lower velocity zones) indicate that it may be a bottom
of the brittle part of the lithosphere (the mechanical but the “thermal”
lithosphere, Fig. 12). Beneath this boundary the lithosphere material
is more plastic and cannot preserve its own inhomogeneity.
The change of the rheology, which may be interpreted as the
lithosphere bottom, is visible beneath the L boundary. As mentioned
above in many record-sections the waves from this boundary
attenuate and the low-velocity gradient may be proposed beneath
the boundary. The Q factor which has been determined from the
mantle wave spectrums also decreases at depths of 200–250 km
(Egorkin and Kun, 1978; Egorkin, 1995). Finally, the increasing of the
plasticity below these depths follows from structural features of the L
and H boundaries. H boundary has a form reverse to L boundary
(Fig. 6), and it means that the matter between the boundaries can flow
to create the isostatic equilibrium of the upper mantle.
Thus, the observed structure of the continental upper mantle is
difficult to present in the traditional lithosphere–asthenosphere
system. The model shows the absence of a distinct low-velocity
layer at a depth of 200–250 km which could be related to the partly
melting zone. In contrary, three layers of different plasticity are
determined in the upper mantle, their bottoms are at depths of
around 100 (the bottom of the brittle part of the lithosphere) and of
200 km (the lithosphere bottom). Beneath the lithosphere, the solid
state matter of higher plasticity is proposed (Fig. 12).
The seismic boundaries between the three layers of the different
plasticity are not simple discontinuities. They are complicated
heterogeneous zones like the zone 2 in Fig. 5. As it was mentioned
above. A. Egorkin (1999) and Morozov et al. (1999) preferred to
Fig. 8. Comparison of the observed and calculated travel-times for SP C3 (Craton profile, Fig. 6). Legend is in Fig. 2.
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Fig. 9. Comparison of the observed and calculated travel-times for SP C4 (Craton profile, Fig. 6). Legend is in Fig. 2.
present these boundaries as the low-velocity layers. A question arises,
what is the origin of such boundaries in the upper mantle?
4. Nature of the upper mantle seismic boundaries and
low-velocity zones
In general, the existence of the regional seismic boundaries and
low-velocity zones in the mantle lithosphere is an unexpected result
because it looks unrealistic to find the regular and strong enough
velocity contrasts and velocity inversions in the upper mantle of the
old platforms. No phase transitions were revealed at the depths where
the boundaries are traced (Griffin et al., 1998, 1999; Kukkonen et al.,
2003).
However, the large explosion experiment and seismological data
show that these boundaries may have a global significance because
they were found in many different regions of the world. Recording of
the Early Rise explosion indicated that beneath the central and
eastern North America in the depth interval from about 94–107 km
there is evidence for a low-velocity channel (Masse, 1973). H. Thybo
(2006) has determined the boundary and the low-velocity zone at a
depth of about 100 km from the Early Rise explosion, from the longrange Fennolora profile in Fennoscandia Shield and from the PNE
profiles in Siberia (Thybo and Perchuć, 1997; Pontevivo and Thybo,
2006). He considers this boundary as a global 8° boundary.
Velocity models derived from the refraction profile in northern
Australia (Leven et al., 1981) include a pronounced high-velocity zone
at a depth of around 200–250 km. Conventional petrological
Fig. 10. Comparison of the observed and calculated travel-times for SP Q1 (Quartz profile, Fig. 7). Legend is in Fig. 2.
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Fig. 11. Comparison of the observed and calculated travel-times for SP Q3 (Quartz profile, Fig. 7) Legend is in Fig. 2.
interpretation in terms of mineralogical or compositional changes
cannot explain this feature. Calculated velocities for the garnet
pyrolite model (using a derived geoterm), fit the velocities above
and below this high-velocity zone, but cannot account for the highvelocity (8.6 km/s) zone itself. This future is explained in terms of
velocity anisotropy.
In Dey et al. (1993) two composed record-sections for the
northwestern Australia show a significant attenuation zone for Swaves beneath 210 km.
A.L. Hales (1991) made the summary that the explosion data from
the Nevada Test Site, GNOME and Early Rise are all consistent with
Lehmann's suggestion that there is a discontinuity at a depth of about
Fig. 12. Generalized velocity models and the qualitative rheological characteristic of the
upper mantle of the Northern Eurasia. The velocity models: (1) for the Siberian Craton,
and (2) IASP91model.
200 km. He notes that the only reasonable explanation for the
discontinuity is that it represents the termination of a zone of partial
melting.
In the Tibetan Plateau the seismic boundaries were revealed at
depths of 80 and 100 km (they were named as H and G boundaries)
and the L′ and L boundaries at depths of 200 and 300 km (Revenaugh
and Sipkin, 1994).
A thin low-velocity layer was determined from the explosion
experiment in the Pacific upper mantle at a depth of about 100 km
(Asada and Shimamura, 1976, 1979).
Comparison of the seismic data with other geophysical and
geological data reveals some other characteristics of these complicate
mantle boundaries. In many regions they are characterized by higher
electrical conductivity (Jones, 1992; Kovtun et al., 1994) favoring the
existence of fluids at a depth interval of 100–150 km.
Another specific feature of these complicated layering zones is
obtained from the xenoliths data. The generalization of the data all
over the world (Gordienko and Usenko, 2007) shows that the most
part of the xenoliths comes from the depths around 100, 150 and
200 km (Walter, 1998; Griffin et al., 1999; Ionov et al., 2002; Bell et al.,
2003; Hearn, 2004) which are close to the average depths of the
regional seismic boundaries N1, N2 and L. Solov'eva et al. (1989) note
that xenoliths from the Siberian Platform kimberlites taken from the
depths of these seismic boundaries, have indications of film melting.
Comparison of the deep earthquake distribution with depth
indicates some additional peculiarity of the N and L boundaries. In
different tectonic regions, inside the continents and in the continental
margins, most earthquakes are located at depths of around 100 and
200 km where we determine these regional seismic boundaries. For
instance, the earthquake distribution in the Tian-Shan and Pamir
region (Luck and Yunga, 1988) shows two maximums at depths of
100 and 200 km (Fig. 13a). Distribution of the deep earthquakes for
the Kamchatka region (Tarakanov and Levii, 1967) shows the same
picture: most earthquakes are located at depths around 100 and
200 km (Fig. 13b). Moreover the geophysical models of the western
margin of the American continents, for instance, the data of the
international projects CINCA and ANCORP, show the same pictures in
the earthquake distribution in the Benioff zones: two clusters of
seismicity occur at 90–110 km and 190–250 km depth (Schurr et al.,
1999; ANCORP Working Group, 2003).
The correlation of the xenolith origin depth and the earthquake
clusters with the regional mantle boundaries (thin layering zones)
Author's personal copy
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N.I. Pavlenkova / Tectonophysics 508 (2011) 85–95
The determined upper mantle weak zones can have a great effect
on all dynamic processes. Together with deep faults they form a
channel system for the mantle fluids and matter transportation.
During tectonic activation the weak layers may be transformed in
asthenolites by partial melting and provoke plume tectonics. The
weak zones may play an important role in the horizontal displacement of the lithosphere blocks and in the formation of tectonic
structures.
6. Conclusion
Fig. 13. Distribution of the earthquake number (n) with depth for: (a) 5 year intervals
of observations the Middle Asia region (Luck and Yunga, 1988) and (b) KurilKamchatka continental margin (Tarakanov and Levii, 1967).
could not be an accidental correlation and it shows that the depths of
the regional boundaries are critical depths where some regular
transformations of the matter are happen. The most realistic answer is
that these reflective boundaries appear due to the concentration of
mantle fluids and corresponding matter transformation (Pavlenkova,
1996).
The problems of the deep fluids and the Earth's degassing are well
investigated in Russia. The studies based on the H and He flow
measurements and on the laboratory experiments of the fluid
transportation through the mantle rocks at high pressure and
temperature and of the chemical transformation of the rocks at the
fluids presence (Letnikov, 2000; Dmitrievsky and Valjaev, 2008). The
laboratory experiments confirm the possibility of the hydrocarbon
detonation at 70 km depth (Karpov et al., 1998).
As suggested by Gilat and Vol (2005) “H- and He-release from core
solutions and incorporating in H-He and other chemical compounds
and following gradual decomposition due to decompression are
accompanied by intense energy release. The practically infinite energy
source for earthquakes will be the explosive chain reaction of the
decomposition, triggered by decompression within the fault zone”.
Thus, it may be proposed that the upper mantle boundaries appear
at first not as petrological boundaries but as the physical ones. The
physical boundaries most generally represented are sharp changes in
the physical or mechanical properties of the material which is
homogeneous in composition and in the degree of metamorphism
(e.g. changes in porosity, fissuring, or fluid content, transition from
solid state to liquid through creep, film and partial melting, transition
into the state of true plasticity and other physical transformations).
Such kind of changes cannot produce sharp seismic boundaries by
themselves but they may cause the formation of such boundaries. An
increase or a decrease in porosity is invariably followed by a change in
fluid content, which might cause the beginning of different physicochemical matter transformation (Spenser and Nur, 1976; Kern, 1982),
such as new degrees of metamorphism, and stimulate partial melting
and mobility of material at a relatively low temperature. The matter
flow along these weak zones results in the origin of the plane seismic
boundaries and in the formation of the anisotropic high-velocity
intermediate layers. The deep earthquakes which are concentrated
around the depths of 100 and 200 km may be also a result of deep
fluid decompression and detonation at these critical PT levels.
The long-range seismic profiling made in Russia with Peace
Nuclear Explosions shows that the upper mantle is clearly stratified.
Besides the velocity layering the regional reflecting boundaries are
traced in the large area of the Northern Eurasia.
The regular change of the upper mantle horizontal inhomogeneity
determines three main layers which are divided by the high
reflectivity boundaries at depths of around 100 and 200 km. Beneath
the 100 km depth the block structure typical of the uppermost mantle
disappears and low-velocity layers are often observed. At 200 km
depth the Q factor decreases and the upper mantle structure is
changed showing the isostatic equilibrium at this depth (the
boundary depressions in the lower part of the mantle correspond to
the uplifts of the boundaries above 200 km). The reflective boundaries
at depths of 100 and 200 km are 10–20 km thick zones with the
alternation of high and low velocities in inner layering. Above the
boundary at a depth of 100 km a low-velocity zone is often observed.
The change of these three main layer structural properties may be
interpreted by the change of the mantle matter plasticity: the upper
layer is the brittle part of the lithosphere, the second layer is the
ductile lithosphere and the third layer is the solid state matter
asthenosphere (Fig. 12). The layers are divided by the high reflectivity
boundaries at a depth of about 100 and 200 km. The partly melted
matter may be suggested in the low-velocity and high reflectivity
zones from the electromagnetic and xenolith data.
Acknowledgments
The author thanks G.A. Pavlenkova, A.V. Egorkin, H. Thybo, J.J. Cipar
and K. Priestly for the many years of cooperation in the interpretation
of the PNE data. Many thanks also to Marek Grad and other reviewers
for the critical comments and important corrections to the manuscript. These studies were possible with the support of the Russian
Fund of Fundamental Investigation (RFFI), the grant 09-05-00238.
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