A maior rede de estudos do Brasil

Grátis
794 pág.
The_Lithosphere

Pré-visualização | Página 36 de 50

the
thickness of the Proterozoic crust in Greenland may reflect the tectonic boundary between
two ancient terranes (Dahl-Jensen et al., 2003). However, later studies based on RF
analysis of the same GLATIS data indicate significantly smaller values of crustal thick-
ness in the Proterozoic terranes, 39–42 km, with little lateral variations in central
Fig. 3.52 Crustal thickness in Greenland (in km) based on receiver function analysis. Numbers – interpretations of Kumar
et al. (2007) (no brackets) and Dahl-Jensen et al. (2003) (brackets). Seismic stations are shown as boxes.
108 Seismic structure of the lithosphere
Greenland (Kumar et al., 2007). A systematic difference in the values reported in the
two studies for stations in central Greenland may suggest that the later study (with a
systematically smaller crustal thickness) mapped, rather than the Moho, the top of the
high-velocity lowermost crust, which is known to be present in regions with magmatic
underplating in the crust. Alternatively, the difference between the two studies may be
associated with strong converted phases from the ice column, since the largest discrep-
ancy between the results is observed at the stations in inner Greenland where the ice
thickness exceeds 2 km.
Lithosphere thickness in the collisional orogens of Central Asia
S-receiver functions (SRF) are a useful tool in mapping mantle discontinuities in the
depth range obscured by crustal multiples present in P-receiver functions (PRF), such as
the base of the lithosphere. Spectacular images of the upper mantle structure in Tibet, the
Himalayas, and Central Asia constrained by RF for individual stations along a seismic
profile provide 2D constraints on the dynamics of lithospheric plates in the zone of the
largest present continent–continent collision and seismic images of Eurasian plate sub-
duction (Kind et al., 2002).
In the Tien Shan, RF studies of the lithosphere structure, initiated almost two decades ago
(Kosarev et al., 1993), clearly show converted phases from two interfaces: the upper with a
positive velocity contrast is interpreted as the Moho. Different RF studies have revealed
strong lateral variations in the crustal thickness of the Tien Shan and the Tarim Basin, with
values ranging from 45 km to 65 km. The crustal thickness increases southwards to 80 km in
northwest Tibet and exhibits a c. 20 km step in Moho depth across the Altyn Tagh Fault
(Wittlinger et al., 2004).
The lower interface has a negative velocity contrast and is interpreted as the top of a
layer with reduced seismic velocities. Although this interface may not necessarily be the
lithosphere–asthenosphere boundary (LAB), in most studies it is hypothesized to be the
lithospheric base. Under this assumption, lithosphere thickness variations in the Tien Shan
region as determined from S-receiver functions are moderate, 90 km to 110 km (Oreshin
et al., 2002). A later study, based on the same approach (Kumar et al., 2005), however,
reported significant variations in the lithosphere thickness of the region: arrival times for
converted waves range from c. –10 sec to c. –30 sec, which for the iasp91 model results in
depths ranging from 90 km to almost 300 km (Fig. 3.53). The depth of the mantle interface
with a negative velocity contrast is in fairly good agreement with a lower resolution, surface
wave tomography image of the region, which shows a high velocity body at 80–180 km
depth in the south (presumably the Indian plate) and a southwards dipping low-velocity
body in the north, that was interpreted as evidence for subsidence of the rigid Tarim Basin
block of the Asian lithosphere under the Pamir and Karakoram ranges (Kumar et al., 2005).
However, such an interpretation requires further support on the depth of the lithosphere–
asthenosphere boundary since mantle discontinuity with a negative velocity contrast
may not necessarily be the base of the lithosphere but, for example, a compositional
intralithospheric boundary.
109 3.4 Receiver function (converted waves) studies
Compositional boundary within the cratonic lithospheric mantle?
The presence of a prominent interface with a 4.5% reduction in seismic velocity in a 10 km
thick layer at approximately 150 km depth has been identified beneath the Kalahari craton
by S-receiver functions (Savage and Silver, 2008). Since xenolith data as well as numerous
seismic investigations based on various techniques consistently place the lithospheric base
beneath the Kalahari craton at greater depth, 200–220 km, the converter at 150 km depth
represents an intralithospheric boundary. This boundary crosses several Archean sutures and
is unlikely to be related to the Archean tectonics. The base to the boundary corresponds
approximately to the depth where olivine grain size decreases (Fig. 3.10a). A decrease in
grain size should reduce seismic velocities (Figs. 3.6a, 3.14); however the effect is probably
too weak to explain the seismic structure of the Kaapvaal mantle. Spatial correlation of the
strongest velocity anomaly with intense Karoo volcanism suggests that the seismic
discontinuity is a compositional boundary that marks the transition from a depleted
(high-velocity) cratonic lithospheric mantle to the lower part of the lithosphere that expe-
rienced the influence of basaltic melts and metasomatic fluids, which have refertilized the
cratonic lithosphere.
Similar conclusions based on buoyancy analysis have been made for the southern parts of
the Archean–Paleoproterozoic East European (Russian) Platform that have been subsiding
since the Paleozoic (Artemieva, 2003). Voluminous basaltic magmatism associated with
the Devonian (possibly, plume-related) rifting in the Peri-Caspian Basin and in the Sarmatia
subcraton (where it has led to formation of the huge, more than 20 km deep, Dnieper–Donets
rift) could have refertilized the lower part of the cratonic lithosphere and therefore increased
Fig. 3.53 Seismic section across the India–Asia collision zone (along 75 deg longitude) from S-receiver function analysis. Top
panel – the topography along the profile. Bottom panel – depth to the Moho and to the upper mantle
discontinuity with a negative velocity contrast. Zero time corresponds to the arrival time of the direct S-phase.
Negative time-scale indicates times in front of the S arrival. Due to moveout correction the time axis is valid for a
slowness of 6.4 s/deg. Gray shadings show velocity anomalies in the crust and the upper mantle based on surface
wave tomography (dark shading – fast velocities, light shading – slow velocities). Dots are the earthquake
hypocenters within a 100 km wide zone along the line. Right panel: location map with shaded topography
(redrawn from Kumar et al., 2005).
110 Seismic structure of the lithosphere
its density and created negative compositional buoyancy. The boundary between a highly
depleted upper layer (with high seismic velocities) and a more fertile lower layer (with lower
velocities) in the cratonic lithospheric mantle can produce a seismic discontinuity with
negative velocity contrast similar to the one detected by SRF beneath the Kalahari craton
(Savage and Silver, 2008).
Similarly, the presence of a seismic discontinuity with negative velocity contrast has been
observed in other stable continental regions and can be a global feature for the Precambrian
lithosphere; the depth to the top of this discontinuity varies from 80–100 km to c. 120–
130 km (Thybo and Perchuc 1997a). A layer with reduced (c. 0 +1% as compared to the
global continental model iasp91) seismic velocity has been identified within the high-
velocity (c. +1 +3% as compared to the iasp91 model) Proterozoic lithospheric mantle of
the Baltic Shield along the FENNOLORA profile (Perchuc and Thybo, 1996), within the
Archean Karelian mantle (Bruneton et al., 2004), within the Precambrian lithospheric
mantle of the northeastern East European Platform