Intraplate earthquakes in North China - Intraplate Earthquakes - page 101

5.2.1 Geological history

Much of the basement of the North China Block belongs to the Sino-Korean craton, which
includes some of the oldest rocks on Earth (Liu et al ., 1992 ) . During the Late Archean
to Paleoproterozoic, the Western Block, which includes the Ordos Block, collided with
the Eastern Block, forming the Trans-North China Orogen, which includes the basement
of today's Shanxi graben and the Taihangshan mountains (Zhao et al ., 2005 ; Zhai and
Santosh, 2011 ) . Since then the North China basement has been a coherent craton and
remained tectonically stable until the end of the Paleozoic. Geological evidence indicates
that the lithosphere under eastern North China was more than 180 km thick in the early
Mesozoic (Griffin et al ., 1998 ; Xu et al ., 2003 ) .
During the Triassic to mid-Jurassic, the North China Block collided with the South
China Block, resulting in the Qinling-Dabie orogenic belt with ultra-high-pressure meta-
morphism. The collision modified the crustal and lithospheric architecture of the North
China Block, producing thick-skinned crustal thrusts in its southeastern part (Li, 1994 ) ,
forming major faults, such as the Tanlu fault, which cut across the entire craton. This col-
lision probably also initiated the thinning of the North China lithosphere (Menzies et al .,
2007 ) .
Most of the lithospheric thinning in North China occurred during the late Mesozoic to
early Cenozoic, accompanied by widespread extension and volcanism (Zhu et al ., 2012 b ).
Petrological and geochemical probing using the upper mantle xenolith indicates that the
lithosphere was thinned to 80 km over much of the eastern NCB and less than 60 km

thickness in some places (Menzies and Xu, 1988 ; Xu et al ., 2003 ) . This is consistent with
seismic data (Chen et al ., 2009 ) . The cause of the removal of the cratonic root under
North China remains poorly known (Zhu et al ., 2012 a ). It may have resulted from the
Mesozoic collision between the North and South China blocks, which led to delamination
or thermal erosion of a thickened and weakened lithosphere (Xu, 2001 ; Bryant et al ., 2004 ) ,
or it may be related to the ocean-ward retreat of the western Pacific plate, which induced
mantle upwelling under North China. The Cenozoic tectonism in North China may also be
linked to indentation of India with Eurasia and the induced lateral mantle flow (Liu et al .,
2004 ) .

5.2.2 Lithospheric structure

Seismic imaging shows that the eastern North China Block is underlain by broad low-
velocity mantle structures ( Figure 5.3 ) . These low-velocity structures are limited to the
upper mantle; at

660 km depth flat subducting slabs, shown as a high-velocity layer,
can be traced to the subduction zone of the Pacific plate along the eastern margins of the
Eurasian plate (Huang and Zhao, 2006 ) . The western end of the stagnant slabs extends

1500 km inland from the active trench in the western Pacific, and can be correlated with
the prominent surface topographic change between the high-standing Ordos Plateau and
the Taihangshan mountains in the west, and the lowland of the North China Plain and