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GSA TODAY | FEBRUARY 2015 with near-vertical foliation planes would be adequate to cause
splitting of up to 1.0 s. On the other hand, based on relations
between delay time �t, anisotropy magnitude A, and length of the
anisotropic path L,
A = (�t*Vs)/L
(Bonnin et al., 2010), where Vs is velocity of shear wave considered “root.” Examples include the North China Craton in East Asia, the
(here SKS), a 70–80-km-thick lithosphere with anisotropy magni-
tude A = 0.05 would produce a delay time of 1.0 s. The continent-
continent collision between the North and South China blocks,
one of the most important tectonic events in the Asian continent
during the Mesozoic, is the potential process that produced the
ENE-WSW- to E-W–trending fast polarization direction anisot-
ropy on a whole lithosphere scale. The N-S–directed collision and
crustal shortening have been reported in previous studies. Closure
of the ocean between the NCB and SCB has been suggested to
have occurred through subduction with northward polarity (e.g.,
Hsu et al., 1987). The lithofacies distribution along the southern
margin of the NCB and northern margin of the SCB is consistent
with the interpretation (Yin and Nie, 1993) that the southern edge
of the NCB was originally a smooth, E-W–trending boundary
prior to the collision, whereas the SCB had an irregular geometry
with its eastern part extending some 500 km farther to the north
than its western counterpart. Based on the study of regional
geologic setting, Yin and Nie (1993) considered the left-lateral
Tan-Lu strike-slip fault in China and the right-lateral Honam
strike-slip fault in Korea to be transform faults that accommo-
dated the northward indentation of the SCB. Triassic to Early
Jurassic deformation is widespread in the NCB north of the Sulu
belt, probably related to the continent-continent collision. In the
northeastern NCB, E-W- to ENE-WSW–trending thrusts and
folds involving Permian strata are unconformably overlain by
Jurassic strata (Geologic Map of Liaoning Province, 1989),
suggesting Triassic crustal shortening in a north-south direction.
Triassic to Early Jurassic deformation during the Indosinian
orogeny is widespread in Korea. In particular, E-W- to ENE-
WSW–trending thrusts and folds developed along and north of
the Imjingang belt (Um and Chun, 1984). The suture between the
blocks in the region east of the Tan-Lu fault trends E-W or
ENE-WSW, as inferred from the analysis of aeromagnetic anoma-
lies (Li, 1994), and extends to the Imjingang belt in the central
part of the Korean Peninsula (Yin and Nie, 1993).
As illustrated in Figure 4A, we suggest that the E-W- to
ENE-WSW–trending fast polarization direction represents
“frozen” anisotropy in the lithosphere produced by the collision
between the NCB and SCB during the Late Paleozoic or Triassic.
Thus, the convergence of the two blocks in the east is inferred to
have been in a NNW-SSE direction. Subsequently, lithosphere
shortening induced an LPO with an E-W or ENE-WSW orienta-
tion. This proposal is supported by the ENE-WSW fast direction
reported from South Korea (Kang and Shin, 2009). During the
late Mesozoic, the lithosphere of the eastern NCB lost a significant deformation beneath eastern China. Our results show a domi-
proportion of its deep mantle keel through cratonic erosion and
reactivation (e.g., Griffin et al., 1998; Santosh, 2010; Guo et al., Some individual measurements, as well as the station averages,
2013). The thinning of the lithosphere may extend southward into
Dabie and Sulu, as well as the northern part of SCB, as inferred
from the thin lithosphere in these regions as imaged by seismic
studies (Chen, 2010). Although a substantial part of the deep
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cratonic root has been removed, our results suggest that the
“fossilized” anisotropic signature is still preserved in the remnant
lithosphere beneath eastern China (Fig. 4B).
Recent studies indicate that the cratonic lithosphere in some
regions on the globe has been extensively destroyed or reactivated,
resulting in loss or modification of the refractory lithospheric
southwestern part of the Kaapvaal Craton in South Africa, the
Wyoming Craton in North America, and the Brazilian Craton in
South America, among which the eastern part of the North China
Craton is considered to be one of the best examples for wholesale
destruction of cratonic root (Zhang et al., 2013). Following
cratonization in the late Paleoproterozoic, the interior part of the
North China Craton remained largely quiescent until the
Mesozoic when extensive reactivation, erosion of the cratonic keel,
and differential destruction of the lithosphere occurred (Zhai and
Santosh, 2011, 2013). The craton was in a “superconvergent
regime” (Li et al., 2013), caught up among the southward indenta-
tion of the Siberian block following the closure of the Mongol-
Okhotsk Ocean, the collision between the North and South China
Blocks, and the oblique subduction of the paleo-Pacific plate from
the east. Our study demonstrates that despite the extensive
erosion and destruction of the cratonic root beneath eastern
China, a “fossilized” anisotropic signature is preserved in the lith-
osphere, and this has important bearing on understanding the
stability and destruction of ancient cratons.
CONCLUSIONS
Because seismic waves effectively propagate and interact with
the internal structure of lithosphere, they provide high-resolution
data for studying the structure of the crust and upper mantle, as
well as to constrain the geodynamic processes. Similar to the
reconstruction of the tectonic evolution of a region based on rock
records by geologists, seismologists try to understand the ancient
dynamic processes by exploring the structure and rock fabric in
the crust and upper mantle, which are not reset during later
tectonic events. In eastern China, the collision between the NCB
and SCB resulted in lithospheric compressional deformation and
constructed the Qinling-Dabie-Sulu orogenic belt during the Late
Paleozoic to Triassic. During the late Mesozoic, extensive cratonic
reactivation and dramatic lithospheric thinning affected the
eastern NCB. From the late Mesozoic through the Cenozoic,
deformation of the eastern Asian continent was dominated by
extensional tectonics associated with the Pacific plate subduction
along the eastern margin of Asia. Whether the deformed litho-
sphere induced by the convergence between the SCB and NCB was
destroyed or reset by these subsequent tectonic events is a topic of
wide interest.
In this study, we performed teleseismic shear wave splitting
measurements to investigate lithosphere and upper mantle
nant E-W or ENE-WSW fast direction and a delay time of 1.0 s.
are characterized by a WNW-ESE to NW-SE fast direction.
Based on fitting the fast directions and delay times as a function
of the back azimuth, the two-layer anisotropic models at three
stations show similar features. The fast direction of the upper
layer is ENE-WSW or E-W, whereas the lower layer shows a