East Asia: Seismotectonics, magmatism and mantle dynamics

Abstract

In this article, we review the significant recent results of geophysical studies and discuss their implications on seismotectonics, magmatism, and mantle dynamics in East Asia. High-resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes, which may reflect magma and fluids that affected the rupture nucleation of large earthquakes. In subduction zone regions, the crustal fluids originate from the dehydration of the subducting slab. Magmatism in arc and back-arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab. The intraplate magmatism has different origins. The continental volcanoes in Northeast Asia (such as Changbai and Wudalianchi) seem to be caused by the corner flow in the big mantle wedge (BMW) above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well. The Tengchong volcano in Southwest China is possibly caused by a similar process in BMW above the subducting Burma microplate (or Indian plate). The Hainan volcano in southernmost China seems to be a hotspot fed by a lower-mantle plume associated with the Pacific and Philippine Sea slabs’ deep subduction in the east and the Indian slab’s deep subduction in the west down to the lower mantle. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab–plume interactions. Some of these issues, such as the origin of intraplate magmatism, are still controversial, and so further detailed studies are needed from now.

Introduction

The structure and tectonics of East Asia are affected by the interactions of five tectonic plates: the Eurasian, Pacific, Okhotsk, Philippine Sea and Indian plates (Fig. 1). In the east, the Pacific and Philippine Sea plates are subducting beneath the Okhotsk and Eurasian plates, causing the western Pacific islands arcs, marginal seas, and continental rift zones. In the southwest, the India–Asia collision leads to the shortening and elevation of the Tibetan plateau, causing high and great mountain ranges such as the Himalaya, Pamirs, and the Hindu-Kush mountains. The Asian continent has very complex surface topography, active crustal deformation, and intensive seismic and volcanic activities (Fig. 1, Fig. 2). Investigation of seismotectonics, volcanism, mantle dynamics and their relationships in the Asian region is very important from the viewpoints of both scientific research and mitigation of natural hazards. In this broad region, large earthquakes and volcanic eruptions take place frequently, which have caused heavy casualties and significant damages to the human society of many Asian countries, such as the 1991 explosive eruption of the Pinatubo volcano in Philippines, the 2004 Sumatra earthquake (M 9.2) in Indonesia and the huge tsunami it caused, and the 2008 Wenchuan earthquake (M 8.0) and the 2010 Yushu earthquake (M 7.1) in Southwest China. To clarify the mechanism of the earthquake generation and volcanic eruptions, it is necessary to study the detailed crust and upper mantle structure in the earthquake and volcanic areas as well the plate tectonics and mantle dynamics of the broad region which are the fundamental causes of the seismic and volcanic activities.

In East Asia, there are many intriguing geological and geophysical features which are related to the fundamental scientific problems of solid-Earth sciences, such as the origin of the intraplate volcanoes (e.g., Changbai, Tengchong, Hainan, etc.), the structure and fate of subducting slabs, the India–Asia collision and its cause of great earthquakes in and around the Tibetan Plateau, lithospheric thinning and reactivation of the North China Craton, and the origin of intracontinental rift zones (such as Lake Baikal), etc. The mantle heterogeneity and dynamics as well as the stress regime are the fundamental causes of the seismic and volcanic activities near the Earth’s surface, while earthquakes and volcanoes are important aspects of mantle dynamics. Therefore it is important and necessary to have a consistent and unified understanding of these scientific problems in the framework of global tectonics and mantle dynamics.

Structural heterogeneities in the Earth’s interior exist at various scales, which are associated with seismic and volcanic activities and other tectonic movements near the Earth’s surface. Seismic tomography is a powerful tool to detect the structural heterogeneities in Earth and it can provide crucial information to improve our understanding of the dynamic evolution of Earth. However, because of the limitation of seismic data now available in different areas and the difference in nature and features of each scientific target, multi-scale (local, regional and global) approaches of seismology and geophysics have been adopted to study the seismotectonics, volcanism and mantle dynamics (Zhao, 2009). For example, local tomography is used to determine high-resolution 3-D fine structure of the crust and uppermost mantle under a volcanic area and an earthquake source zone to detect the structural heterogeneities (including fluids and magmas) which are associated with earthquake nucleation and volcanic eruptions; regional tomography is used to image the subducting slabs and mantle plumes down to the mantle transition zone or the uppermost part of the lower mantle, in particular, the detailed structure and behavior of the subducting slabs under East Asia; while global tomography is used to determine the large-scale whole-mantle structure to clarify the fate of subducting slabs and the origin of deep mantle plumes as well as the mantle–core interactions.

In this article, we review the recent multi-scale geophysical studies and discuss their implications on the seismotectonics, volcanism, and mantle dynamics in the East Asia region. We have attempted to make a complete and balanced review on these topics, however, because of the numerous literatures on these topics made by a great number of researchers and the space is limited here, we have to introduce mainly the studies which we were involved in or are relatively familiar with.