greater north china initiative (gnci): cenozoic ...web.missouri.edu/~lium/pdfs/gnc-wp.pdflate...

GREATER NORTH CHINA INITIATIVE (GNCI): GREATER NORTH CHINA INITIATIVE (GNCI): CENOZOIC GEODYNAMICS, CLIMATIC CENOZOIC GEODYNAMICS, CLIMATIC

EVOLUTION, AND GEOLOGICAL HAZARDSEVOLUTION, AND GEOLOGICAL HAZARDS

A whitepaper of scientific rationale and strategic plans for cooperative research between the

IPACES and Chinese geosciences community

2005

GNCI Whitepaper - 2 -

List of the GNCI whitepaper drafting group

CHEN Yongshun Peking University FENG Xiahong Dartmouth College, USA GE Shemin University of Colorado, USA LI Zhengziang University of Western Australia, Australia LIU Mian University of Missouri-Columbia, USA NIU Yaoling University of Durham, UK SHEN Zhengkang State Seismological Bureau, China SONG Xiaodong University of Illinois-Urbana Champaign, USA WANY Yang Florida State University, USA WU Zhongliang State Seismological Bureau, China YIN An University of California-Los Angles, USA ZHANG Youxue University of Michigan-Ann Arbor, USA ZHAO Dapeng Ehime University, Japan ZHAO Meixun Dartmouth College, USA Special Acknowledgement: This document is produced from extensive discussion at a number of NCP (North China Project) workshops. We thank the National Science Foundation of China (NSFC) and Drs. Chai Yucheng, Yao Yupeng, and Yu Sheng for their support and their enthusiasm in this endeavor. Many colleagues in China contributed significantly to this document in various aspects of its development. In particular, we thank Chen Yong, Chen Xiaofei, Chen Bin, Gao Rui, He Jiankuan, Lu Huafu, Li Yanxin, Liu Qiyuan, Liu Futian, Ren Jianye, Shi Yaolin, Wei Wenbao, Wu Fuyuan, Wang Chunrong, Wang Lianshu, Wang Chengshan, Xu Yigang, Xu Xiwei, Zan Shaoxian, Zhang Peizhen, Zheng Tianyu, Zhang Yueqiao, Zhou Yaoqi for their valuable contributions.


Executive Summary The Great North China Initiative is a multifaceted research plan aimed at a holistic understanding of the Cenozoic evolution of the Earth systems in the Greater North China region (GNC). The GNC is chosen because (1) it hosts the most vital industrial, commercial, residential, and political centers of China; (2) it is one of the most geologically active continental regions in the world, with frequent and devastating earthquakes, floods, draughts, dust storms and other natural hazards; (3) it has a delicate ecosystem that is sensitive to the dynamic interplays among lithosphere, hydrosphere, atmosphere, and biosphere; (4) it is geologically the best studied regions in China where abundant geological and geophysical data provide a firm foundation for the proposed Earth system studies.

This research plan identifies some of the fundamental questions regarding the Cenozoic geodynamics of the Earth systems in the GNC, and outlines the required interdisciplinary approaches to address these questions. One focus area of this research plan is active crustal deformation and earthquakes – the GNC has the most active intracontinental seismicity in the world. Another focus area is asthenosphere-lithosphere interactions. In particular, effort is called to understand the mantle processes responsible for the thermal thinning of the GNC lithosphere since late Mesozoic, and the diffuse intraplate igneous activity throughout the Cenozoic. A third focus area is paleoclimate change in late Cenozoic and its relationship with tectonics, paleoecology, and hydrology. All these processes are integral parts of the system dynamics involving mantle flow, lithosphere deformation, air and water circulation, and bio-activity, hence requires cross-disciplinary approaches. In the past few years, the IPACES (International Professionals for the Advancement of Chinese Earth Sciences), with support from the National Science Foundation of China (NSFC), has conducted extensive discussions among its members and with scientists in China through a number of workshops. We are convinced that the Great North China Initiative (GNCI) will have a great impact on sustainable development of China in the next decade, while the cutting-edge integrated research in this plan will propel Chinese Earth Sciences to an international leadership position.


Table of Content 1. Introduction ……………………………………………………………………….5

1.1. Why North China? …………………………………………………………...5 1.2. Integrated Investigation of the GNC …………………………………………6

2. Lithospheric Deformation ………………………………………………………...8 2.1. Geographic Division of the GNC …………………………………………….8 2.2. Tectonic Development of GNC ………………………………………………8 2.3. Tectonic Boundary Conditions of the GNC ………………………………….11 2.4. Active Deformation …………………………………………………………..11 2.5. Major Questions……… …………..………………………………………….15 2.6. Recommendations …….……………..……………………………………….16

3. Mantle Processes ……………………………………………………………….....17 3.1. Seismic Velocity Structure …………………………………………………...17 3.2. Mesozoic to Cenozoic Modification of Lithospheric Mantle ………………..19 3.3. Cenozoic Volcanism in the Greater North China …………………………….21 3.4. Previous Studies on Cenozoic Volcanism in the GNC ………………………24 3.5. Major Questions ……………………………………………………………...28 3.6. Possible Research Directions ...………………………………………………29

4. Climate, Water, and Environment ………………………………………………...34 4.1. Introduction …………………………………………………………………..34 4.2. Paleoclimate ………………………………………………………………….35 4.3. Paleoecology……………………………………………………………...…..37 4.4. Hydrological Research ……………………………………………………….40

5. Integration ………………………………………………………………………...42 5.1. Climate Change, Surface Processes, and Tectonics ………………………….42 5.2. Crustal Stress, Earthquake Physics, and Hydrologic Processes …………...…43

6. Project Management ………………………………………………………...…….43


1. Introduction 1.1. Why North China?

The IPACES (International Professionals for the Advancement of Chinese Earth Sciences – www.ipaces.org) posted two questions when preparing this document: (1) What kind of research would have the greatest impact on sustainable development of China in the next decade? (2) What is the most effective research plan that would propel Chinese Earth Sciences to an international leadership position? After extensive discussion among the IPACES members and with geoscientists in China, we identify the Greater North China (GNC) region as the most promising target for addressing the above questions. Our decision is based on the following.

(1) The GNC covers about one-quarter of China’s territory (Fig. 1) and hosts the most vital industrial, commercial, residential, and political centers of the nation. This region is one of the most populated areas in China. Understanding its geologic setting is a prerequisite for making future strategic plans for the overall sustainable development of China.

(2) The GNC is one of the most geologically dynamic settings in the world, as indicated by the frequent occurrence of devastating earthquakes, floods, draughts, and dust storms. These natural hazards have enormous impacts on both the economy of China and the quality of life in the region. Remediation of these hazards requires a complete knowledge of geodynamic, climatic, and land-surface processes as well as the interactions among them.

(3) The GNC has a very delicate ecosystem and is long known for the lack of water resources. As the storitivity of any substantial aquifers depends critically on the climatic conditions, it is prudent to obtain paleoclimatic data that allow establishment of quantitative models to make specific predictions on the trend of climate variations at centennial to millennial scales. The models can provide a better guide for future planning of large urban centers whose survival will critically depend on the availability of water resources.

(4) GNC is a key area for testing many prominent geological hypotheses ranging from the nature of continental deformation to interactions between tectonics and climate changes. Because of this, GNC has become the focus of many international research groups in recent years. This provides a unique opportunity to the Chinese Earth Science community to showcase their scientific achievements in the international arena.

(5) A large amount of geological and geophysical data in the GNC region has accumulated over the past century. These data sets give the Chinese Earth scientists a unique advantage as the international geosciences community becomes increasingly more interested in understanding diffuse continental deformation in Asia and the coupled tectonics-climate processes.

(6) Both the IPACES and Chinese Earth Sciences community have extensive expertise and research experience for the studies of GNC. Many of them already have a strong track record in publications on regional climate variations, tectonics, geochemistry, and geophysics. With a coordinated integration of diverse research areas and


approaches, the Chinese Earth Science community is poised to take a prominent position in the regional studies of eastern Asian geology.

Fig. 1. Geological Map of Asia. Red area outlines the Greater North China region 1.2. Integrated Investigation of the GNC

In the past four decades, several major shifts of research focuses have occurred in Earth Sciences. The advent of the plate-tectonics theory in the 1960’s provides a simple kinematic description of lithospheric deformation that is most applicable to the oceanic regions [e.g., Morgan and McKenzie, 1965]. Research in the 1970’s and early 1980’s had focused on testing predictions of plate tectonics [e.g., Ernst, 1971; Dickinson, 1973] and applying the theory to reconstructing the history of ancient mountain belts [e.g., Dewey and Burke, 1973; Sengor, 1985]. Since the mid-1980’s, research on continental dynamics has flourished, which is mainly stimulated by the observation that active deformation over continental regions of Eurasia and western United States is distributed over broad zones (1000-2000 km wide) and located far away (> 1000 km) from the nearest plate boundaries [Molnar and Tapponnier, 1975; Proffett, 1977]. This new realization has led to the on-going debate on whether continental deformation can be described kinematically by the interaction of a few rigid blocks following the principles of plate tectonics [e.g., Tapponnier et al., 1986; Avouac and Tapponnier, 1993] or dynamically by continuum thin sheets [e.g., Bird and Piper, 1980; England and Houseman, 1986; Royden, 1996]. Although some compromise was reached by the later models that consider the role of faults or localized shear zones in the continuum approximation [e.g., Willett and Beaumont, 1994; Kong and Bird, 1996], these early workers


treated the asthenosphere as passive and frictionless media below deforming continental lithosphere. While some researchers were well aware of the important effect of continental deformation on regional and global climate changes [Kutzbach et al., 1989; Harrison et al., 1992; Molnar et al., 1993], the reverse role of climatic conditions in controlling the strain distribution and exhumation history of major orogenic systems and continental collision zones were not widely appreciated until very recently [e.g., Beaumont et al., 2001; Willett et al., 2001]. In addition, the role of asthenospheric flow in continental deformation has just began to be explored [e.g., Liu et al., 2004]. The studies of continental tectonics in the past two decades have resulted in a detailed understanding of particle paths (P-T-t) within zone of continental deformation [e.g., Harrison et al., 1998]. The nature of the paths was mostly attributed to the kinematic nature of deformation [e.g., thrusts vs. normal faults or extension vs. contraction] [e.g., Spear, 1993], with little or no concerns on surface processes that are responsible for exhuming the deep crustal rocks [e.g., England and Houseman, 1986]. In addition, most numerical models of continental tectonics consider only the boundary conditions on the sides of the plates, leaving the base and top undefined [cf. England and Houseman, 1989]. Recent advances in Quaternary geochronology, satellite images, and digital topography have revolutionized our views on the rates and physical mechanisms of surface processes that shape the Earth’s landscape via erosion and surface transport. Also, high-resolution seismic tomography and improved numerical models incorporating realistic features of the Earth have made the probing of deep-mantle dynamics possible.

The unprecedented capability available to Earth scientists to observe Earth’s surface and deep-mantle processes provides a new challenge and a unique opportunity in treating the asthenosphere-lithosphere-atmosphere as a unified and interactive system [e.g., Harrison et al., 1992; Molnar et al., 1993]. This is arguable the most exciting frontier in Earth Sciences as it lies at the interfaces of several seemingly unrelated disciplines in the past. We believe that the GNC is an ideal place to explore such an interactive system by investigating the Mesozoic and Cenozoic asthenospheric flow pattern, lithospheric deformation history, and surface processes via systematic and detailed geologic, geophysical, and geochemical studies. For example, the asthenospheric flow and its interaction with lithosphere may be established by the studies of seismic topography and igneous activity. Lithospheric deformation can be determined by integrated research incorporating structural geology, geochronology, quantitative metamorphic petrology, and sedimentology. Finally surface processes can be investigated by examining the variation of climatic conditions at different time scales and its relationships to tectonics and exhumation in shaping Earth’s surface.

Some of the unique geologic features in the GNC also make the proposed integrated system approach feasible. For example, the Loess Plateau is a direct result of Tibetan-plateau uplift created by Indo-Asian collision [An et al., 2001; Guo et al., 2002]. Its late Cenozoic deposits have provided one of the most complete records of climate change as a result of lithospheric deformation of the Indo-Asian collision zone. Widespread Cenozoic deformation in the GNC and their close association with igneous activity provide tractable clues in linking lithospheric deformation with asthenospheric processes. In the following, we propose a road map for deriving a quantitative and more holistic understanding of the interplay among Cenozoic lithospheric deformation, asthenospheric flow, and atmospheric circulation in the


context of the Greater North China. Reaching this goal requires closely integrated multidisciplinary studies including continental geodynamics, igneous activity, and climatic history that we discuss below. 2. Lithospheric Deformation 2.1. Geographic Division of the GNC

The geographically defined GNC is a large triangular region that has an east-west width 1500-2500 km and a north-south length of ~2000-2500 km. The area is bounded in the south by the east-trending late Triassic Qilian-Qinling-Dabie orogenic belt created by collision between North and South China Blocks and the Yangtze River delta (Fig. 1). The western boundary of the GNC follows the north-flowing Yellow River along the Yinchuan Valley in the south and the NNE-trending Greater Hinggan Range (i.e., Daxing’an Ling) that straddles between Inner Mongolia and Heilongjiang Province. The GNC is bounded in t he north by the east-flowing Heilongjing (i.e., Amor River) along the Sino-Russia and Sino-Mongolia border and in the east by the eastern Asian margin off the coast of eastern China and Korea north of the Yangtze River delta (Fig. 1). The major geographic provinces in the region include the Ordos Plateau with an average elevation of ~2000-2500 m in the west and the low-altitude Huabei and Songliao Basins with an average elevation of ~50-200 m in the east. The Huabei and Songliao basins are the largest contiguous flat areas of China and homes of some of the most vital political, agricultural and industrial centers of the nation. The topographic boundary between the 2-km high Ordos plateau and the low-altitude Huabei basin is abrupt along the eastern flank of the Taihang Shan (Fig. 1). This front is expressed by the largest gravity anomaly in east Asia [e.g., Ren et al., 1981; Ma, 1985]. The time and mechanism of this abrupt topographic division between the Orodos and Huabei has never been explored. It is possible that the boundary was developed during Eocene extension of the Huabei basin in the backarc region of the Japan trench [e.g., Yin and Chen, 2004]. It is also possible that the front was developed during the Jurassic or Cretaceous topographic collapse of a large plateau behind the Qinglin-Dabie orogenic belt [Yin and Nie, 1996].

2.2. Tectonic Development of GNC

Much of the basement rocks of the GNC belong to the North China Craton (NCC), part of the Sino-Korean craton with some of the oldest crust on Earth [Liu et al., 1992]. The NCC is believed to have been a coherent craton by at least ca. 1800 Ma as indicated by the metamorphic history of the orogenic belts joining the different Archaean blocks [e.g., Zhao, 2001; Kusky and Li, 2003; Zhai and Liu, 2003], the ca. 1770 Ma non-metamorphosed mafic dykes that cut the orogenic belts [Halls et al., 2000; Wang et al., 2004], and similar aged volcanic rocks in the cover successions [S. Zhang and Z.X. Li, unpublished SHRIMP results]. The interior of the NCC remained tectonically stable from ca. 1800 Ma until the end of the Paleozoic, as illustrated by the widespread, conformable or disconformable, shallow-marine clastic and carbonate sedimentary successions over much of the 1500 My [Wang et al., 1985]. Ordovician-age diamondiferous kimberlite pipes in eastern NCC indicate that the lithosphere there was no less than 180 km thick at that time [e.g., Griffin et al., 1992].


The lithosphere of NCC experienced significant thermal thinning during late Mesozoic to Cenozoic, associated with widespread rifting and volcanism. The abnormally thin lithosphere is clearly indicated by geological and geophysical data. Petrological and geochemical probing using xenolith from the upper mantle carried up by volcanic extrusions indicates that the lithosphere is no more than 80 km over much of eastern NCC, and is in places less than 60 km thick [e.g., Fan et al., 1993; Menzies et al., 1993; Griffin et al., 1998; Xu, 2001]. Geophysical analyses also point to a much thinner lithosphere in eastern NCC [e.g., Liu, 1987]. There has been much debate regarding the mechanism of the thinning. These can be summarised into three schools of thoughts: (1) orogenic related models [e.g., Fan and Menzies, 1996; Xu, 2001; Bryant et al., 2004] which suggest that Mesozoic orogenic root delamination/slab break-off, or erosion of an orogenically weakened lithosphere, caused the thinning, (2) Pacific rollback models [e.g., Ren et al., 2002; Northrup et al., 1995] involving ocean-ward migration of the western Pacific active plate margin, and (3) models involving the indentation of India with Eurasia [e.g., Menzies et al., 1993; Liu et al., 2004].

Cenozoic deformation in northern China south of latitude 40ºN is expressed by the development of the Paleogene Huabei basin and Neogene rift systems around the Ordos block [Ye et al., 1987; Zhang et al., 1998]. The Paleogene Huabei Basin formed by early Tertiary back-arc extension associated with Late Paleocene and Eocene basaltic eruptions [Ye et al., 1987]. Due to post-rifting thermal subsidence, the basin is largely covered by Neogene to Quaternary sediments [Ye et al., 1987].

There are three major graben systems around the Ordos block: the Yinchuan rift, the Hetao rift, and the Shan rift (Fig. 1). The Yinchuan rift has been assigned to initiate in the Oligocene because of the presence of Oligocene red beds in the rift basin [Ye et al, 1987]. However, a close examination of seismic reflection profiles across the rift [Ningxia BGMR, 1989] suggests that the syn-rift sediments are late Miocene and Pliocene in age. Similarly, the southernmost part of the Shanxi rift basin [i.e., the Weihe graben of Ye et al., 1987] contains Oligocene and possible Late Eocene strata. Because Paleogene strata are also widely distributed outside the southern segment of the Shanxi rift [Wang et al., 1996], it is possible that the inferred Paleogene initiation of rifting by Zhang et al. [1999] was due to assigning the pre-rift sequence to syn-rift sequence. The Yinchuan and Shanxi rifts terminate in the south at the left-slip Haiyuan and Qinling fault zones [Burchfiel et al., 1991; Zhang et al., 1998] that extends eastward to the Dabei Shan region [Ratschbacher et al., 2000]. The Hetao graben is the northern extension of the Yinchuan graben [Ye et al., 1987]. However, how the Hetao and Shanxi rifts terminate in the north is not clear. GPS studies in this region show a broad left-slip shear zone trending east-west separating the Hetao and Shanxi grabens in the south and the stable Amurian plate to the north [Shen et al., 2000]. The left-slip Qinling fault that terminates the Shanxi rift in the south may be linked with the east-trending left-slip Kunlun fault in central Tibet via a series of north-trending faults at the juncture of the westernmost Qinling and the Kunlun Mountains [Yin, 2000; Yin and Harrison, 2000].

The Tanlu fault zone bounds the eastern edge of the modern Huabei Basin and is a first order Cenozoic tectonic feature in East Asia (Fig. 1). Its Cenozoic development may have been related to opening of Bohai Bay during Paleogene back-arc extension [e.g., Allen et al., 1997; Ren et al., 2002a]. Zhang et al. [1999] showed that the southern segment of the Tanlu fault experienced three phases of deformation: (1) normal-slip, and (2) left-slip with a normal


component, and (3) right-slip with a minor normal component. GPS survey shows that the fault is extensional accommodating east-west extension [Shen et al., 2000].

To the east of the continental GNC are several large back-arc basins (Sea of Okhotsk, Japan Sea) and highly extended continental margins (Bohai Bay, East China Sea) (Fig. 1). Their tectonic development was closely related to that in the continental GNC, and the opening of the marginal seas off the east coast of the GNC and along other part of east Asian continent may have played important roles in change of ocean currents and consequently the climate.

The Japan Sea (also known as East Korea Sea) opened in the Late Paleocene and Eocene [Lallemand and Jolivet, 1986; Celaya and McCabe, 1987]. However, the oceanic crust in the basin was not created until Late Oligocene and lasted to the end of the Early Miocene (30-15 Ma). The Japan Sea is bounded in the northeast by the right-slip Sakhalin fault that may have accommodated about 400 km of displacement during the opening of the basin [Jolivet et al., 1994]. There are three models for the origin of the Japan Sea. Lallemand and Jolivet [1986] suggest that the Japan Sea was developed as a pull-apart basin between two right-slip faults. Yue and Liou [1999] proposed that the opening of the Japan Sea was related to the development of the left-slip Altyn Tagh fault in northern Tibet. Both models consider the development of the Japan Sea was associated with the Indo-Asian collision. Alternatively, the opening of the Japan Sea was attributed to back-arc extension [Jurdy, 1979; Celaya and McCabe, 1987] caused by slow convergence between Eurasia and Pacific plate [Northrup et al., 1995].

The Bohai Bay extensional domain is separated from Japan Sea by the Korea peninsular that appears to have experienced little Cenozoic extension (Fig. 1). This extensional system extends southward to the East China Sea and westward to the Huabei Basin [Zhao and Windley, 1990; Allen et al., 1997; Ren et al., 2002a]. Extension in the Bohai Bay region started in the Paleogene and was most active between the Eocene and latest Oligocene [Allen et al., 1997; Ren et al., 2002a]. Rift-related structures and sedimentary sequences are overprinted by Quaternary dextral transpressional deformation, causing inversion of some earlier normal faults [Allen et al., 1997].

The continental margin of East China Sea consists of three tectonic zones: (1) the East China Sea extended continental shelf, (2) the Taiwan-Sinzi folded zone, and (3) the Okinawa Trough (Fig. 1). The tectonic evolution of the East China Sea has been summarized by Zhou et al. [1989] and Kong et al. [2000]. Between the latest Cretaceous and earliest Paleocene, extension occurred in the East China Sea as expressed by the development of detachment faults. This extensional event is part of widely distributed extension in east Asia [Ren et al., 2002a]. Between the Late Paleocene and Early Oligocene, extension was focused in the East China Sea region. Significant crustal thinning during this period was manifested by the development of normal faults, development of a narrow basin, and rapid subsidence associated with the basin formation [Kong et al., 2000]. Contraction began in the central and northern Taiwan-Sinzi folded zone in the middle Oligocene [Kong et al., 2000] and was significantly intensified in the late Middle Miocene [Ren et al., 2002b]. This event may have been associated with the subduction of the Palau-Kyushu ridge on the Philippine plate [Kong et al., 2000]. Due to very thick sequence of syn- and post-rift sediments, the age of the southern Taiwan-Sinzi folded zone is poorly constrained. Kong et al. [2000] suggest that the


southern Taiwan-Sinzi folded zone initiated in the Late Miocene, possibly related to collision between the Luzon arc and Eurasia. In contrast, Sibuet et al. [2002] propose that the Taiwan-Sinzi folded belt terminated its development at ~15 Ma during a major plate reorganization at the junction of the Philippine Sea plate and South China Sea. The development of the Okinawa Trough is the youngest deformation event in the East China Sea. Its opening may have started in the Late Miocene associated with clockwise rotation of the Ryukyu arc [Sibuet et al., 1998]. 2.3. Tectonic Boundary Conditions of the GNC

Understanding the Cenozoic deformation history of the GNC requires the knowledge of the history of boundary conditions around the region. The area north of the GNC is an active zone of distributed deformation between the southern edge of the Siberia Craton as marked by the Baikal rift and the Sino-Mongolia border. This zone is dominated by extensional and strike-slip faults (Fig. 1). South of the GNC is the relatively stable South China Craton and the northern margin of the Tibetan plateau. The latter is marked by east-trending left-slip and thrust faults (Fig. 1). Because the diffuse nature of Cenozoic deformation north and south of the GNC, the exact kinematic history of the boundaries can only be established by systematic investigation of the deformation histories in the two regions.

The evolution of the margin seas along the eastern margin of Asia is also poorly understood. The key issue is the history of the Philippine plate motion over the Cenozoic. It remains uncertain about the past position of various plates over the western Pacific in the Cenozoic. This uncertainty presents a great challenge in using plate-tectonic boundary conditions to investigate the deformation history of the GNC region in a forward fashion, but at the same time it offers an opportunity to use land-based geology to inversely determine the possible plate boundary history such as strike-slip vs. subduction, Kula vs. Philippine plate reconstructions in the western Pacific [Yin and Chen, 2004].

2.4. Active Deformation Active tectonics of GNC is expressed by the frequent occurrence of large earthquakes. Seismicity in GNC tends to concentrate along the rims of the Huabei basin such as in the Shanxi rift system in the west, the Shanhaiguan uplift in the north, and the Tanlu fault zone in the east (Fig. 2). The GNC region has recorded some of the deadliest earthquakes in human history including the 1556 M8 Huaxian in Shanxi with a death toll of 830,000 and the 1976 M7.8 Tangshan in Hebei with a death toll of ~250,000. During the last major burst of seismicity in GNC, eight earthquakes with magnitude >M6.5 occurred between 1966 and 1976. Because GNC is the home of nation’s capital Beijing, which is the site of the 2008 Summer Olympic Games, it is urgently important to have a systematic assessment of earthquake potentials and their possible impacts in the major urban areas of northern China. Before discussing this issue in detail, we first outline the general progress in Chinese earthquake studies in recent decades.

Since 1960s, Chinese seismologists have done extensive work in GNC on lithosphere structure, earthquake processes, seismic stress field, as well as studies on earthquake precursors and earthquake prediction. During the past 20 years, Chinese geologists have compiled a “Map of Active Tectonics of China” at a scale of 1:4 million, in which more than


200 active tectonic zones have been identified [Deng et al., 2003]. This map delineates the active tectonic belts of China that bound relatively aseismic blocks by their slip rates that range from


is an order of magnitude lower than that along the block-bounding faults and that M=7 earthquakes generally occurred along the block boundaries.

Paleoseismic studies of the GNC region have been mostly concentrated on the rim of the Ordos plateau. Because the plateau is relatively aseismic, many Chinese geologists refer to it as the Ordos block. Along the northern edge of the Ordos block, Ran et al. [2003a] found that 62 paleo-earthquakes have occurred in the late Quaternary, of which 33 occurred in the Holocene. They also discovered that the recurrence intervals of major earthquakes differ for individual fault segments, individual faults, and composite fault zones. One of the major active faults along the northern edge of the Ordos block is the 220-km long Daqingshan normal fault zone. It bounds the NW-trending Hetao rift system along the northwestern edge of the Ordos plateau [Ran et al., 2003b]. This fault initiated in the Eocene with a total slip > 2.4 km since the Quaternary. A paleoseismic study shows that 7 major paleoseismic events occurred on the fault since 19 ka BP. They occurred at 18.75 ± 0.75 ka, 16.97 ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 6.83 ± 0.26 ka, and 4.50 ± 0.23 ka BP, with an average recurrence interval at 2.375 +/- 0.432 ka.

The NNE-trending Helan Shan-Yinchuan fault zone marks the western edge of the Ordos Plateau and GNC. Deng and Liao [1996] show that this is a normal-right-slip fault cutting late Pleistocene and Holocene alluvial fans and offseting the Great Wall of the Ming Dynasty built at ~400 years B.P. right-laterally for 1.45 m and vertically for 0.95 m. The offset event may result from the M = 8 Yinchuan-Pingluo earthquake of 1739. On the basis of terrace offset, scarp morphology, and paleoseismic trenching across the fault, Deng and Liao [1996] conclude that four large earthquakes with M = 8 occurred along the fault at 8400, 4600-6300 (or 5700), 2600, and 256 years B.P., with a recurrence interval of these earthquakes is 2300-3000 years. Paleoseimic studies were also conducted along the southern edge of the GNC region. Along the Luoshan Fault zone at the boundary between the GNC and NE margin of the Tibetan plateau [Wei et al., 2003]. The fault strikes N-S and is reverse right-slip and has a minimum slip rate of 2.15 ± 0.2 mm/yr. Four recent events have occurred on the fault zone: after 8200 +/- 600 years BP, between 3130 ± 240 years BP, at 4150 ± 120 year BP, and before 2230 ± 170 years BP.

Fig. 2. GPS site velocities relative to stable Eurasia. Data are from the CMONOC network [Wang et al., 2003]


GPS. The application of GPS technology has made it possible to observe continuous deformation of the crust, providing unprecedented constraints for understanding the deformation of continents and the genesis and rupture processes of earthquakes. The combination of these observations with physics-based dynamic models and the ever advancing computational technology now allows us to quantitatively interpret earthquake nucleation processes and earthquake rupture processes and predict propagation of seismic waves in complex media, simulation of strong ground motions, and earthquake hazards caused by strong ground motions. These breakthroughs not only represent major scientific achievements but also provide vital information for engineers, policy makers, and the general public. At the moment, we cannot predict earthquakes in a classical sense and we cannot predict major breakthroughs leading us to that goal. However, these new advances in seismology and crustal deformation have led us to a new stage of seismic hazard mitigation and will continue to provide us scientific bases for understanding earthquake phenomena and predictability.

Contemporary crustal deformation in North China has been measured since the 1950s. Early horizontal measurements were made mainly using triangulation, which however could not be used for precise surveying of tectonic deformation except for measuring coseismic displacements of large earthquakes [Huang, 1980]. Until the emergence of GPS technique in the early 1990s, for several decades leveling was the only effective means to detect tectonic deformation. The very first GPS network in North China was established in 1992, and expanded in 1995 to become the North China/Capital Circle Network [Li et al., 1995]. Using data from repeated surveys of this network, Shen et al. [2000] determined that the region moved about 3-8 mm/yr ESE with respect to the Eurasia plate. These results were confirmed by subsequent studies [Wang et al., 2001; Yang et al., 2002]. Monitoring of crustal deformation in North China was significantly improved after the founding of the Crustal Motion Observation Network of China (CMONOC) in 1998. About 300 CMONOC survey mode GPS stations are located in North China covering effectively all the known regional active faults, and have been surveyed in 1999, 2001, and 2004 respectively. Using GPS data from the North China/Capital Circle and the CMONOC networks many models have been proposed to quantify tectonic block motions in North China [Xu et al., 2002; Li et al., 2003; Yang et al., 2003; Huang et al., 2003]. Wang et al. [2003] analyzed the CMONOC data and found that the deformation field across the Zhangjiakou-Penglai seismic zone for the 1999-2001 time period was quite consistent with that of the 1992-1996 period. Deformation across the northern Shanxi Rift zone, however, showed insignificant slip for the 1999-2001 time period, different from the result obtained earlier for the 1992-1996 time period across the same segment of the fault. Such a discrepancy raises a question: is the rifting process across the Shanxi Rift varying with time [He et al., 2003], and if so, what is the cause of it? Interpolation of the CMONOC velocity field revealed dextral shear motion trending NNE in North China, at a rate of 2-4×10-8/yr for most part of the region except the area southwest of the Ordos plateau where the shear strain rate is up to 5-7×10-8/yr [Shen et al., 2003]. Liu and Yang [2005] derived the strain rates in north China using the updated Chinese GPS data. The high strains are found in the North China plain and around the Ordos block. Using the observed crustal kinematics as boundary conditions, they have shown in a 3D finite element model that the long-term distribution of high strain energy in the North


China plain and circum-Ordos rifts are consistent with seismic moment release in the past ~2000 years. Thus the intense seismicity in North China is not a transient phenomenon but reflect long-term strain energy accumulation and release resulting from crustal tectonics (Fig. 3).

Fig. 3. Left panel: GPS velocity (relative to stable Eurasia) and strain rates (background). Middle panel: predicted long-term strain energy (background) and stress states represented in sterographic lower-hemisphere projections. Right panel: seismic strain energy released in the past ~2000 years [Liu and Yang, 2005]. 2.5. Major Questions To better understand active tectonics and its control on earthquakes in the GNC region, multidisciplinary efforts are needed to address the following scientific issues. Some of the possible research directions and important questions are outlined below. (1) Structural and geological factors that give rise to intraplate earthquakes in north China.

• How are the intraplate earthquakes controlled by the far-field boundaries conditions such as India-Eurasia collision in the south Pacific subduction in the east? What dolithospheric structures control the spatial pattern of seismicity?

• Are the Cenozoic volcanism and active tectonics related and how? What is the role of igneous activity in creating lithospheric stress?

(2) Characterization of GNC earthquake sources, faults, and structure of the source region

• What are the characteristics of the NC earthquakes? How are the earthquakes distributed with spatially and with depth? Do the focal mechanisms change with depth? Are the earthquakes in NC fundamentally different from those in other regions, such as in the stress drops? What are the important features of the rupture processes of major NC earthquakes? What are the frictional features of the NC earthquake faulting?

Major tasks in this endeavor include (1) precise determination of earthquake locations, mechanisms, and rupture process; (2) mapping of active fault; (3) 3-D crustal structure around the active faults; and (4) basic understanding of the physics of faulting.


(3) Present-day crustal deformation and strain field • What is the present-day deformation (strain field) in NC? How is the deformation

related to geological structures? Why the present day deformation is mostly NNE dextral shear? How does it change from extension in the past to simple shear? Does deformation change with depth? How does the crustal deformation evolve with time? Why is the present-day deformation concentrated mainly in the area around the Ordos block and along the Zhang Jia Kou – Peng Lai seismic belt?

(4) Models of crustal deformation • How does NC continental lithosphere deform? Can the model of “Active Tectonic

Blocks” be applied to NC? What is the rheology profile of the lithosphere and how does it change laterally? How does the rheology depends on composition and thermal structure? What's the thermal profiles of the lithosphere of NC? How does it relate to reology of the lithosphere? What's the visco-elastic deformation style? Can we build a consistent dynamic model of deformation of the region?

• What is the role of mid crust and crust-mantle coupling in controlling the crustal deformation and earthquake genesis? What is the lithospheric strength profile? What's the nature of low velocity in midcrust? Does it relate to earthquake genesis?

• What is the nature and degree of coupling between crust and mantle? (5) Genesis of intraplate earthquakes

• What controls the seismic genesis of NC and Ordos? What controls the limit of the depth of the earthquakes? Is there systematic difference between earthquakes in the grabens and in the shear zones? How are earthquakes controlled by the properties of faults, such as strength, segmentation, curvature, and shape?

• How does stress evolve with time and space? How do faults interact with one another? What are the main factors triggering earthquakes (static or dynamic Coloumb stress change or other factors)? Why do present-day major earthquake concentrate on the Hetao–Zhangjiakou–Peng Lai seismic belt, and why is there no modern major earthquakes in the Fei-Wei seismic zone? Why in the Zhang Jia Kou – Peng Lai seismic belt, which is in the ESE direction, the earthquake faulting is usually along NNE direction?

(6) Earthquake hazard mitigation and earthquake prediction • A major application of this initiatives is the seismic hazard assessment and strong

ground motion simulation in NC. Given our understanding of the Earth structure in NC and earthquake sources and crustal deformation, can we achieve realistic strong ground motion simulation? How do the NC basins affect strong ground motions? Can the seismic zoning be made more accurate? Can we gain better understanding of long- and short- term earthquake predictions (one of the most fundamental goals of seismology)?

2.6. Recommendations (1) Establishing a National Quaternary Geochronological Center. An important

aspect of paleoseismic study is to determine the temporal evolution of major earthquakes and their evolution over space. For example, recent clustering of major earthquakes in eastern California of the United States, Turkey, the Kunlun Range of the central Qinghai-Xizang (Tibet) plateau, and Sumatra of Indonesia show


remarkable spatial migration of earthquakes over a few years. Determining the pattern of such clustering require more systematic studies of fault zones. However, as urban development has covered more and more land surfaces, trenching across some of the major faults may not be possible. This will require other means to determine the reoccurrence interval of major earthquakes such as determining the morphologic surfaces. This would require the application of a suite of analytical techniques such as the U-Th, thermal and optical luminescence, cosmogenic, and the traditional 14C dating methods.

(2) Blind structures in the urban areas of the GNC. A major change in the focus of paleoseismic studies occurred in the United States in the late 1980’s. Previously, research was mostly concentrated on faults that are capable of generating large earthquakes such as the San Andreas fault in southern California. However, the mid-80’s and early 90’s, two moderate earthquakes struck the Los Angeles region, the 1987 Whittier Narrow earthquake (Mw 5.9) and 1994 Northridge (Mw 6.7) earthquake. The latter caused a total economic damage of ~ US$40 billion. From our brief review above, we note that the paleoseimic studies in the GNC region have mostly focused on major block-bounding faults. However, some of the minor faults within the “stable” blocks may be capable of generating moderate size earthquakes. With a favorable condition, the possible moderate-sized earthquakes could produce more devastating effects on human lives and regional economy than the large faults located remotely from the major urban centers.

3. Mantle Processes 3.1. Seismic Velocity Structure Seismology is fundamentally an observational science. The Earth's major internal layers (the solid inner core, the fluid outer core, the lower and upper mantle, and the crust) were rapidly discovered in the first half of the 20th century following the development of modern seismometers in late 19th century. Systematic earthquake location and determination of earthquake mechanisms played a central role in formation of the plate tectonic theory in the late 1960s, following the deployment of the World-Wide Seismic Network in early 1960s. Accumulation of seismic data made it possible to conduct seismic tomography that started in 1970s and 1980s, which has formed a key to the understanding of the dynamics of Earth’s interior. The availability of modern high precision broadband digital seismometers is now making it possible to image delicate, yet vital features, such as, the plume structure, ultra low velocity structure in the lowermost mantle (D” region), and the layering of the solid inner core, and to detect temporal changes of earth’s structure or sources, such as the rotation of the inner core, the healing of the ruptured fault zones, and the plumbing of magma chambers. The availability of modern broadband portable digital seismic stations since the early 1990s has turned the nature into great field laboratories for seismologists and tectonicists. The observation and interpretation of these “field seismological laboratory” works have greatly increased seismic resolution of lithosphere structure and earthquake rupture processes and are playing increasingly important role in solving key geological and geodynamic problems.


Another major effort in the GNC region is deep seismic sounding (DSS) using active sources to image the crustal structure [e.g. Teng et al., 1979; Li and Mooney, 1998; Zhang et al., 1999; Li et al., 2001]. The coverage of the DSS profiles is particularly dense in North China [e.g., Duan et al., 2002]. These results show that the crust of North China thickens gradually from east to west, with its averaged thickness of about 35 km.

In recent year, a number of surface wave dispersion and inversion studies have been done, covering a large area of east Asia, using data from global stations of the Chinese Digital Seismic Network (CDSN) and other stations in adjacent regions [e.g., Wu et al., 1997; Ritzwoller and Levshin, 1998; Curtis et al., 1998; Xu et al., 2000; Zhu et al., 2002, Huang et al., 2003a; Lebedev and Nolet, 2003]. Body-wave studies on NC area from the western countries have mostly been limited, including body wave modeling [Beckers et al., 1994], propagation of Pn, Sn and Lg waves [Xie, 2002; Rapine and Ni, 2003], receiver functions [Mangino et al, 1999]. However, some high-resolution tomographic studies using regional and local travel-time data have been carried out for the whole country [e.g., Liu et al., 1990; Liu and Jin, 1993] and for a local area in the north and east China [Liu et al., 1986; Xu et al., 2001; Huang and Zhao, 2004], with most published in the Chinese literature.

Another significant effort in last few years, particularly in the last two years, is the use of the travel times of Pn waves to invert for the velocity and anisotropy distribution in the uppermost mantle and the crustal thickness of China [Song, 2004] (Fig. 4). Tomographic inversions of Pn waves haven been conducted in the whole country [Wang et al., 2002; Sun et al., 2004; Liang et al., 2004; Hearn et al., 2004] as well as in local regions of China, including the Tibetan plateau [Zhao and Xie, 1993; McNamara et al., 1997], Xinjiang and the western China [Pei et al., 2002], Southwest China (Sichuan-Yunnan region) [Huang et al., 2003b], the northeastern margin of Qinghai-Tibetan plateau [Xu et al., 2003], and the eastern and northern China Pn [Wang et al., 2003] and Sn [Pei et al., 2004]. These studies have revealed significant features of thin crustal thickness, lower upper mantle velocity, consistent with upper upwelling and lithospheric thinning, which may have controlled the genesis of the rich oil and gold deposits of the region [Song et al., 2004].

Pei et al. [2005] perform inversion of P-wave arrival times of both regional and

teleseismic earthquakes to obtain 3-dimensional P-wave seismic velocity variations within the upper mantle below the GNC region. The most important findings of their study are as follows. (1) No fast P-wave velocity anomalies can be related to subducted oceanic slabs

Fig. 4. Inversion results for Pn velocity (color) and anisotropy (bars) in China (from Liang et al., 2004). The bar indicates the fast Pn direction, and the length is proportional to the anisotropy amplitude, saturated at 4%.


beneath the 660-km discontinuity; instead the subducted oceanic slabs become flattened and stagnant along the transition zone. (2) The western end of the stagnant oceanic slabs lying along the 660-km transition zone can be correlated with the prominent surface topographic break in the GNC between the Ordos Plateau to the west and the Hubei plain to the east along the NNE-trending Taihang Shan Range (~105°E). The western end of the flat stagnant slabs is located ~ 1500 km west of the active trench in the western Pacific. (3) Slow P-wave velocity anomalies are present at depths of 100-250 km below the active volcanic arc and the stagnant slabs along the 660-km transition zone. A simple tectonic model is proposed to explain our observations and their potential correlation to the complex tectonic history of east Asia. In the model, vigorous convection is operating within this horizontally expanded “mantle wedge” above both the active subducting slab in the western Pacific and the ceased stagnant slabs beneath much of the North China plain. This horizontally expanded convection was probably resulted from both rapid eastward migration of the western Pacific trench system and the sinking of the Mesozoic and Cenozoic slabs now trapped at the 660-km transition zone. Both the widespread Cenozoic volcanism and associated extensional basins in east Asia may have been the manifestation of this vigorous upper mantle convection beneath the continental lithosphere. Finally the negative thermal anomaly associated with the stagnant slabs along the 660-km discontinuity has not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the above asthenosphere and continental lithosphere from any possible influence of mantle plumes originated from the lower mantle.

Fig. 5 Map view of the Vp variations at depths of 120, 300, and 500 km [Pei et al., 2005].

3.2. Mesozoic to Cenozoic Modification of Lithospheric Mantle

The lithosphere of the NCC is no longer as thick as 180 km or more. Petrological and geochemical probing using xenolith from the upper mantle carried up by volcanic extrusions indicates that the lithosphere is no more than 80 km over much of eastern NCC, and is in places less than 60 km thick [e.g., Fan et al., 1993; Menzies et al., 1993; Griffin et al., 1998; Xu, 2001]. Geophysical analyses also point to a much thinner lithosphere in eastern NCC [e.g., Liu, 1987]. Such a thin lithosphere is highly unusual for an old continental craton like the NCC, and would have significant bearings on modern seismicity.

Apart from the dramatic lithospheric thinning, the NCC also suffered two episodes of orogenic modifications during the Mesozoic. Major intro-cratonic structures produced by those events, like the Tan-Lu fault, could directly influence the distribution of epicentres. The first orogenic episode occurred during the Triassic to mid-Jurassic (Indosinian), corresponding to the collision of the NCC with the South China Block. The consumption of


over 200,000 km2 of continental crust had led to ultra-high-pressure (UHP) metamorphism. The collisional event also, for the first time since the formation of the NCC, caused major tectonism and relative elevations in the interior of the NCC. It modified the crustal and lithospheric architecture of the NCC in three ways. First, as shown in some tectonic models, major thick-skinned crustal thrusts of up to 15 km thick and hundreds of kilometres in dimensions may have developed in south-eastern NCC [Li, 1994, 1998]. This N to NNW-verging thrust system may extend to western Shandong to the west and southern Liaoning to the north. Minor, south-verging thick-skinned thrusting may also have occurred along northern NCC. Second, major crustal/lithospheric faults that cut cross the entire craton, e.g., the Tan-Lu Fault, were developed. Third, the continental collision event may have started the mantle erosion (lithospheric thinning) of the NCC [e.g., Menzies et al., 1993].

The second orogenic event was the so-called Yanshanian Orogeny developed along northern NCC. It was probably related to the closure of the Mongol-Okhotsk Ocean to the north. Although thin-skinned thrusting was widespread along northern NCC [e.g., Davis et al. 1998], the shallow nature of such structures may not have significant bearings on modern tectonics and seismicity.

Fig. 6. History of lithospheric thinning and regrowth in eastern NCC [Xu, 2001].

The lithospheric thinning occurred during late Mesozoic to Cenozoic. Presently active is occurring along the Shanxi rift system only whereas the thinned lithosphere is growing back in eastern NCC [for a review see Xu, 2001]. There has been much debate regarding the mechanism of the thinning. These can be summarised into three schools of thoughts: (1) orogenic related models [e.g., Fan and Menzies, 1996; Xu, 2001; Bryant et al., 2004] which suggest that Mesozoic orogenic root delamination/slab break-off, or erosion of an orogenically weakened lithosphere, caused the thinning, (2) Pacific rollback models [e.g., Ren et al., 2002; Northrup et al., 1995] involving ocean-ward migration of the western Pacific active plate margin, and (3) models involving the indentation of India with Eurasia [e.g., Menzies et al., 1993; Liu et al., 2004].


3.3. Cenozoic Volcanism in the Greater North China The plate tectonics theory has provided a solid framework for understanding the

distribution of volcanism along plate boundaries: (1) at seafloor spreading centers where two plates pull apart, the ocean crust is being continuously created by volcanism, and (2) at convergent boundaries where the oceanic plate returns into the Earth’s deep interior through subduction zones, volcanic arcs such as the “Pacific ring fires” are being built. However, plate tectonics theory, by its original definition, cannot explain earthquakes and volcanic activities occurring within plate interiors. Hotspots or deep-rooted mantle plumes have been widely invoked to be responsible for “intra-plate” volcanism. Intraplate volcanism is indeed widespread and is thus an important mode of mantle melting. Many of the intraplate volcanic activities are apparently associated with widely perceived mantle plumes/hotspots such as the Hawaii, Samoa, Tahiti volcanic islands, but such association is not clear in many other cases. For example, Cenozoic volcanic activities widespread in eastern Australia [e.g., Johnson ed., 1989], eastern China [e.g., Deng et al., 1998; Zhang et al., 1998; Liu, 1999], western and central Europe [e.g., Wilson & Patterson, 2001], the well-known Cameroon volcanic line straddling the Atlantic-African passive continental margin [e.g., Fitton and Dunlop, 1985; Halliday et al., 1988] and numerous seamounts scattered throughout much of the Earth’s ocean floor [e.g., Batiza, 1982] away from plate boundaries cannot be readily explained by either plate tectonics theory or mantle plume hypothesis. Mantle source “wet spots” [e.g., Green & Falloon, 1998] may explain some of the “intraplate” melting “anomalies”, but such mechanism alone cannot account for the aforementioned widespread and large scale volcanic activities.

It is possible that the melting anomalies of the kind may reflect a mode of mantle thermal anomalies that are yet to be established, or simply reflect mantle compositional anomalies. An understanding of the origin of these melting anomalies is fundamentally important because it will represent an advancement in our knowledge on how the Earth works within or outside the framework of the mature and widely accepted plate tectonics theory and mantle plume hypothesis. Furthermore, intraplate volcanic activities on land present a severe threat to human activities. Hence, such studies have both scientific significance and practical importance.

The GNC region is an ideal natural laboratory for examining intra-plate volcanism. For example, the documented lithosphere thinning [Menzies et al., 1993, Deng et al., 1996, 2004; Griffin et al., 1998; Zheng, 1999; Wu and Sun, 1999; Xu, 2001; Gao et al., 2002; Zhang et al., 2002; Yan et al., 2003] in the Mesozoic for an otherwise stable craton is inexplicable with existing theories. The widespread Mesozoic, and in particular, Cenozoic intraplate volcanism requires mechanisms that are beyond the scope of plate tectonics and mantle plumes hypothesis [Niu, 2005]. Only with combined multidisciplinary expertise, experienced field geologists, meticulous analysts, and creative thinking and modelling skills, is it possible to achieve the objectives with success and to lead to new advances towards understanding the working of the Earth within the interior of the plates and eastern China tectonics in particular.

While basaltic volcanism is ultimately caused by some sort of thermal or compositional (including volatiles) anomalies in the mantle, it is the volcanic products – the volcanic rocks that carry the messages about their source materials, histories and detailed processes. Hence,


detailed petrology and geochemistry of representative volcanic rocks is the first step for more comprehensive studies in the context of regional tectonic histories. Before we discuss this aspect of research, we briefly outline the igneous history of the GNC region below. Northeast China lies in Northeast mainland Asia and straddles two tectonic units: (i) North China-Korea Craton and [Powell and II] Mongol-Okhotsk Tectonic Belt. During late Jurassic and early Cretaceous, there was widespread volcanism of evolved magmas along Daxing’anling, mostly the calcalkaline series andesite-dacite-rhyolite, with some trachyandesite-trachyte. The widespread magmatism was part of the Yanshanian Orogeny in East Asia. The exact nature of the Yanshanian Orogeny is still being investigated, and one explanation is that it is due to subduction of both the Pacific plate and the Mongolo-Okhotsk plate during the early stage (160-130 Ma) and widespread intra-arc extension during the late stage (130-110 Ma) [e.g., Yin and Harrison, 1996]. More recent volcanism in Northeast China since about 80 Ma distinguishes from earlier volcanic activities in composition: Recent volcanic rocks are mostly basaltic (tholeiite, basanite and alkali olivine basalt) with minor evolved trachytes and rhyolite. In late Cretaceous, basaltic magmatism began to appear sporadically in Shandong and Liaoning. Most Cenozoic volcanos line up in linear belts on the sides of Northeast China Plain (Fig. 7). These belts will be referred to as “East”, “West” and “North” Volcanic Belts although they do not lie exactly on the east side, etc. Because many volcanoes along a belt are roughly synchronous (within the last 1.8 Ma), the belts do not seem to be hotspot tracks. Instead, they are more likely rifts or “hot lines”. At present, the nearest subduction zone (Japan Trench) is more than 1000 km away. Recent tomographic studies indicate that the subducting Pacific slab becomes stagnant in the mantle transition zone under Northeast Asia [e.g., Fukao et al., 1992; Kárason and van der Hilst, 2000; Zhao, 2001a, 2004; Chen et al., 2005; Pei et al., 2005].


Fig. 7. Known Cenozoic volcanoes in Northeast China, distinguished by their age. Overlapping symbols of different ages mean that the volcano was active over these periods. Small black dots mean volcanos of unknown age (but likely Cenozoic). Data for China are from Liu [1999], Ma et al. [2002], and references therein. For areas outside China, only Holocene volcanos are shown [Simkin and Siebert, 1994]. Names of volcanic fields (clockwise then center) are: PL: Penglai; QX: Qixia; LQ: Linqu; DT: Datong; HNB: Hannuoba; JN: Jining; ABG: Abaga; KSKT: Keshiketeng; HLH: Halaha; NMH: Nuominhe; KL: Keluo; WDLC: Wudalianchi; XK: Xunke; GS: Geshan; GDS: Gedashan; SYS: Shuangyashan; JD: Jidong; FZ: Fangzhen; SZ: Shangzhi; MDJ: Mudanjiang; ShuL: Shulan; JBH: Jingbohu; YT: Yitong; ZFS: Zengfengshan; TC: Tianchi; WTE: Wangtian’e; LG: Longgang; QY: Qingyuan; KD: Kuandian; SL: Shuangliao. For convenience, the volcanos from SYS to MDJ to JBH to KD to PL are referred to as the “East” Volcanic Belt. Those on the opposite side along Daxing’anling are referred to as the “West” Volcanic Belt. Those along Xiaoxing’anling are referred to as the “North” Volcanic Belt. Before 80 Ma, volcanic activities were calcalkaline and silicic. Basaltic volcanism occurred since about 80 Ma in the late Cretaceous, and it becomes more alkaline with time. There was sporadic basaltic volcanism in Shandong and Liaoning in the late Cretaceous. Paleogene volcanism mostly occurred in Xialiaohe, Bohai, as well as Huabei and Subei Plains. Often the basaltic flows have been covered by more recent sedimentation, but boreholes show that tholeiitic basaltic flows may be over 1000 m thick in places. Volcanic activities peaked in Neogene. In the west of Huabei Plain, Zhangjiakou-Weichang-Chifeng-Jining volcanic fields cover more than 20,000 km2, with both tholeiites and alkali basalts. In the east, along Tanlu Fault and Yilan-Yitong fault systems, there was widespread alkali basaltic eruption, especially Changbaishan volcano group (including Tianchi, Zengfengshan, Wangtian’e), which has been active for over 20 million years. Quaternary volcanism is distributed around Dongbei Plain. Individual volcanic centers are usually small and a volcanic field typically consists of several to tens of cinder cones. The most famous volcano is the Tianchi volcano, a typical stratovolcano with a caldera and also being the most recent. Its last major eruption in 1200 AD was explosive with a dense-rock equivalent volume of 32 km3 [Guo et al., 2002]. At 32km3, this is one of the largest historic eruptions, about twice the volume of the 1883 eruption of Krakatau that killed 36,000 people. Other volcanic fields consist of mostly cinder cones, such as Wudalianchi in Heilongjiang Province, Jinbohu and Longgang in Jilin Province, Kuandian in Lianing Province, and Datong in Shanxi Province. Neogene alkali basalts often contain mantle and crustal xenoliths. Both volcanic rocks and xenoliths can be used to study the interior of this region. Note that there are also volcanoes in Southeast China (and Southeast Asia), but there is a significant gap in between. Furthermore, some volcanos further south in SE China may be genetically associated with the India-Eurasia collision. These volcanoes are also important to investigate. In this Greater North China Initiative, the focus will be on volcanoes in NE China. Understanding the origin of the young volcanism in NE China is critical not only for understanding the tectonic evolution of the Greater North China, but also for the development of China. Volcanic eruptions are main geologic hazards. Tianchi volcano, for example, has the potential to produce large-scale explosive eruptions in the future. Tianchi is already a


tourist attraction (especially for Chinese and Koreans) and it will become more so as the economy of China develops. Understanding the past and monitoring the volcano will be the key in forecasting future eruptions and mitigation of life and property losses. 3.4. Previous Studies on Cenozoic Volcanism in the GNC

There have been numerous investigations on the volcanic rocks and mantle xenoliths in them in this vast area. Some are published in international journals [e.g., Zhou and Armstrong, 1982; Chen et al., 1984; Peng et al., 1986; Fan and Hooper, 1989, 1991; Song and Frey, 1989; Song et al., 1990; Zhi et al., 1990; Basu et al., 1991; Tatsumoto et al., 1992; Liu et al., 1994; Snyder et al., 1997; Griffin et al., 1998; Hsu et al., 1998; Xu et al., 1998a,b; Menzies and Xu, 1998; Chen et al., 2001; Liu et al., 2001; Ren et al., 2002; Xu, 2002; Wilde et al., 2003; Wu et al., 2003; Xu et al., 2003]. Most are understandably published in Chinese literature, especially in the 1980’s [e.g., Liu et al., 1979, 1981; Deng et al., 1980, 1987a,b; Lu et al., 1981, 1983; Wang et al., 1981, 1983, 1985; 1988; E et al., 1982, 1983; Feng et al., 1982; Liu and Wang, 1982; Du and Du, 1983; Hu et al., 1983; Sheng et al., 1983; Chen an Peng, 1985, 1986; Wu et al., 1985; Zhu et al., 1985, 1988; Chen et al., 1986, 1986a,b; Qiu et al., 1986a,b, 1988; E and Zhao, 1987; Liu, 1987, 1988, 1989; Xie et al., 1988, 1989a,b,c; Liu et al., 1989a,b; Wu, 1989; Zhi, 1989, 1990; Luo and Chen, 1990; Tang, 1990; Tang and Tian, 1990; Mu et al., 1992; Liu et al., 1993; Zhi et al., 1994; Liu and Xie, 1995; Deng et al., 1998; Fan et al., 1998, 1999a,b,c, 2000, 2001, 2002; Zhang et al., 2002]. There are some books in Chinese, one expounding a single theme [Deng et al., 1996], some timely collections of papers [e.g., Chi, 1988; Liu, 1992, 1995], and one single-authored book summarizing information on volcanoes in China [Liu, 1999]. These studies have provided a large amount of information, including petrographic and petrologic descriptions, geochemical studies, isotopic data and synthesis, inference of mantle compositions and conditions, and a basic picture of volcano distribution in space and time.

Fig. 8. North-south (a) and east-west (b) vertical cross sections of P-wave velocity images under the Changbai intraplate volcano in NE Asia [Zhao et al., 2004]. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown below the cross sections. Black triangles in (a) and (b) denote the intraplate volcanoes. White dots denote earthquakes that occurred within 100 km of the profiles. The two dashed lines denote the 410 and 660 km discontinuities. (c) Locations of the cross sections in (a) and (b). Black and red triangles denote seismic stations and volcanoes, respectively. The contour lines show the depths of the Wadati-Benioff deep seismic zone.

The numerous studies resulted in various proposed origins for volcanism in NE China, which includes almost all possibilities of intraplate volcanism (plume/hotspots, rifting,


back-arc extension, and lithosphere delamination and thinning), reflecting a lack of understanding of the region. (1) Deng et al. [1996, 1998] proposed that there are a number of mantle plumes responsible for diffuse volcanism over the large area. (2) Liu et al. [2001] suggested that back-arc extension related to the opening of Japan Sea contributed to the volcanism. Zhao et al. [2004], Lei and Zhao [2005], and Chen et al. [2005] based on imaged the presence of a flat slab at 400 to 600 km depth within the transition zone, also suggested a close relation between the stagnant slab and alkali basaltic volcanism (Fig. 8). (3) Ren et al. [2002] attributed the volcanism to a series of rift basins. (4) A fourth hypothesis that is gaining more support is that the volcanism is related to the delamination or thinning of the lithosphere beneath Northeast mainland Asia [e.g., Griffin et al., 1998; Menzies and Xu, 1998; Wilde et al., 2003]. (5) Niu [2005] suggested that because of a sudden change in the lithosphere thickness at the Great Gradient Line (at the boundary between East China and West China), eastward mantle flow crossing the Line experiences rapid decompression, leading to mantle partial melting and volcanism in North and NE China. Until now, the consequences of these various hypotheses have received only minimal discussion. Many Neogene and Quaternary alkali basalts contain mantle xenoliths, mainly spinel lherzolite and harzburgite. Xenoliths have been investigated extensively to understand the geotherm and mantle conditions [e.g., Deng et al., 1980; Liu et al., 1981; E et al., 1982; Feng et al., 1982; Du and Du, 1983; Lu et al., 1983; Sheng et al., 1983; E and Zhao, 1987; Xu et al., 1998b, 2003; Zhang et al., 1998, 2000; Zhi, 1989; Chen et al., 2001; Zheng, 2001; Xu, 2002; Zhou et al., 2002; Wilde et al., 2003], as well as He isotopes. Although there is some difficulty in obtaining pressure because the mantle xenoliths do not contain garnet, one major conclusion based on mantle xenolith and other studies is that the lithosphere has been thinned in the Mesozoic [e.g., Griffin et al., 1998; Xu et al., 1998a,b; Menzies and Xu, 1998]. With a large body of literature on NE China volcanoes, a large database on Northeast and North China volcanoes exists. Recently, Chen et al. [personal communication] have made an effort to compile the database. Examination of the compilation reveals that the reliability of the data is difficult to assess and there are inconsistencies. Preliminary comparison seems to show that Sr-Nd-Pb isotopic data are consistent among different groups and may be considered reliable [e.g., Zhou and Armstrong, 1982; Basu et al., 1991; Tatsumoto et al., 1992; Tu et al., 1992; Zhang et al., 1998; Chen et al., 2001; Wu et al., 2003]. On the other hand, the quality of trace element data, which has the potential to indicate whether subducted or subducting slabs may be involved in their petrogenesis, as well as other mantle characteristics, is not always high. For example, Figure 2 compares literature Ta versus Nb data in Quaternary NE China basalts [various authors, and names intentionally withheld] and recent data from the University of Michigan [Chen et al., personal communication]. There is a lot of scatter in the literature data, with Nb/Ta ratio varying by a factor of 10 (Fig. 9). More recent data show much less scatter with Nb/Ta ratio varying by only about 20%. It is almost certain that the large scatter in the literature data is due to poor analyses although one has to analyze exactly the same rock to make sure this is the case. Low-quality data do not allow unambiguous assessment of various models, such as the involvement of slab component in the derivation of primary magma.


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Fig. 9. Comparison of literature data and new and yet unpublished data by Chen et al. [personal communication]. There is large scatter in literature data.

Another example of questionable data quality is helium isotopic ratios. Helium isotopes can be diagnostic of a deep mantle reservoirs [e.g., Graham, 2002; Porcelli and Ballentine, 2002] although the exact depth (e.g., upper versus lower mantle) or source, or the uniqueness is still debated. For example, mid-ocean ridge basalts have a roughly uniform 3He/4He ratio (about 8 times the atmospheric ratio, or 8Ra; the atmospheric 3He/4He ratio is 1.4x10-6). Some ocean island basalts have high 3He/4He ratios (up to 35 times Ra), often interpreted to indicate a plume component. Continental crust usually has a low ratio of less than 0.1Ra. He isotope ratios in mantle xenoliths and megacrysts brought to the surface by basalts in NE China have only been measured by two groups and are highly variable [Xu et al., 1998; Li et al., 2002; He isotopic data in Xu and Liu, 2002 are the same as those in Xu et al., 1998]. The reported ratios vary widely. There are some intermediately high 3He/4He ratios, which, if verified, may suggest a mantle plume contribution for NE China volcanics. There are also some extremely high ratios, even higher than the ratio in the solar wind, as well as extremely low ratios. The very high ratios likely reflect cosmogenic 3He addition [Porcelli et al., 1987], especially the extremely high 3He/4He ratio (up to 700 Ra) reported by Li et al. [2002], but the authors did not carefully assess this possibility. Because He isotopes might provide key evidence for the involvement of plume component, it is critical to have reliable He isotope data. In a recent (and unpublished) study, Chen et al. [personal communication] obtained some He isotopic data, which showed helium isotopic ratio in mantle xenoliths in NE China volcanics is relatively uniform. Hence the high variability of He isotopic ratios in NE China volcanics and related rocks is almost certainly due to either sampling problems or analytical errors. By careful sampling, these problems can be avoided so that the true mantle component may be revealed by He isotopes. With regard to geochronology, although many K-Ar ages have been obtained before, the reliability of the ages is again difficult to assess and some ages have recently been found to be in error [Fan Qicheng, personal communications].


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Fig. 10. Comparison of literature data and unpublished six data points on three volcanic fields by Chen et al. [personal communication]. On the left-hand side of the data, typical 3He/4He ratios of various reservoirs are given. There is large scatter in literature data. Hence it is difficult to decide which data can be reliably used and which ones should not be used if one needs to use the data to test a hypothesis.

In recent years several geophysical studies were conducted to investigate the structure of the crust and upper mantle under the active volcanoes in NE China. Magnetotelluric soundings revealed low-resistivity anomalies in the crust under the Changbai volcano [Tang et al., 1997, 2001]. Seismic explosion experiments revealed low-velocity anomalies in the crust and upper mantle down to a depth of 40 km, suggesting the existence of magma chambers under the Changbai volcano [Zhang et al., 2002]. With the recent installation of 19 portable seismic stations in NE China [Wu and Hetland, 1999], a few studies have been made to determine the three-dimensional (3-D) structure of the crust and upper mantle beneath the Changbai volcano. For example, receiver function techniques were applied to the teleseismic waveforms recorded by the portable seismic network to map the geometry of the seismic discontinuities in the crust and upper mantle (the Moho, 410 and 670 km discontinuities) [Ai et al., 2003; Li and Yuan, 2003; Hetland et al., 2004]. These studies showed that the crust is thicker and contains low-velocity bodies beneath the Changbai volcano, and that the 670 km discontinuity is depressed under NE China, suggesting that the subducting Pacific slab is stagnant in the mantle transition zone. High-resolution seismic images of the mantle down to 800 km depth are determined beneath the Changbai volcano by applying a teleseismic tomography method to relative travel time residuals recorded by the portable seismic network [Zhao et al., 2004; Lei and Zhao, 2005]. The results show a columnar low-velocity anomaly extending to 400 km depth under the Changbai volcano. High-velocity anomalies are visible in the mantle transition zone, and deep earthquakes occur at depths of 500-600 km under the region, suggesting that the subducting Pacific slab is stagnant in the transition zone, as imaged clearly also by both global tomography [Zhao, 2004] and regional tomography [Pei et al., 2005; Chen et al., 2005]. These seismological results from tomographic and receiver function analyses suggest that the Changbai volcano is not a hotspot like Hawaii but a kind of back-arc intraplate volcano related to the upwelling of hot asthenospheric materials associated with the deep subduction, dehydration, and stagnancy of the Pacific slab under NE Asia [Zhao, 2004; Zhao et al., 2004; Lei and Zhao, 2005; Chen et al., 2005]. In summary, there has been extensive work on Cenozoic igneous activities and associated


mantle xenoliths in the GNC region. These works provide background information on which to build new studies. Nevertheless, many volcanic fields have not been investigated, or have not been investigated in any detail. Furthermore, for previous studies, the quality of the data is sometimes difficult to assess, and for some the quality is low, reflecting either analytical difficulties in the early years, or sloppiness in sampling and analyses. New, careful, systematic, and coordinated studies are necessary to improve our understanding of the regional volcanism in the GNC region. 3.5. Major Questions

Major questions remain regarding Cenozoic volcanism in NE China, including (the questions go from the more concrete to the more elusive):

(1) Structure of the deep mantle crust

• Exactly how thick is the NCC lithosphere? How did the thinning occur in late Mesozoic [e.g., Gao et al., 2004]?

• What’s the 3D velocity structure of the lithosphere and upper mantle in GNC and adjacent areas? What is the lithosphere thickness? What is the seismic anisotropy of the region? What is the structure in the deeper Earth beneath GNC? How is the structure in GNC related to regional structure and tectonics of the India-Eurasian collision and the subduction of the western Pacific?

• One of the most fundamental questions is how the intraplate earthquakes and tectonics in GNC are related to the deeper structure and dynamics. Is there correlation of earthquake locations with lithosphere velocity structure? How well can we map the velocity structure into temperature, composition, partial melt, and the rheology of the region? What is the origin of the seismic anisotropy and does it have any relation to stress field and deformation?

(2) Spatial and temporal variation of igneous activity • What is the spatial and temporal distribution of Cenozoic igneous activity GNC and

what control them? • What is the role of Pacific subduction vs. possible hot-spot activities in producing

igneous activities in GNC. • What are the spatial-temporal geochemical variations? Are there large-scale spatial

patterns, and if so, what is their significance? Are there general evolutionary trends for single, long-lived volcanoes, and for whole volcanic fields or regions? If so, what do they imply?

• How do mantle source composition and conditions of magma formation vary spatially and through time?

(3) Relation between seismic velocity structure and igneous activity • What is the distribution of the igneous activity in space and time? • What is the relation between deep structures and the distribution of igneous activities?

What is the lateral and depth extent of the low-velocity bodies under each of the active volcanoes such as the Changbai and Wudalianchi?

• Are the low-velocity bodies due to high temperature or high water content (or some other compositional differences)?


• Is the subducting Pacific slab really stagnant in the mantle transition zone under NE Asia? If so, how far does it penetrate westward to the interior of the Asian continent, and how does it affect the surface tectonics in addition to volcanism?

• Are there mantle plumes in the upper and/or lower mantle under NE Asia? Is there a mantle plume component characterized by high 3He/4He ratios? Is there contribution from subducted slabs to the composition of the igneous rocks in GNC (which would support the back-arc hypothesis)?

• Could there be interactions between a mantle plume and the subducting slab beneath GNC?

• What happened at about 80 Ma that led to the transition from calcalkaline silicic volcanism to basaltic volcanism? What led to the thinning of the lithosphere? What is the deep process and tectonic significance? What are the roles of the subducting Pacific plate, or back-arc extension, or India-Asia collision? Are there mantle plumes? What is the accurate timing of the transition? Does the transition age progress from west to east?

• What can we infer about mantle compositions, conditions and processes that contribute to the widespread volcanism?

3.6. Possible Research Directions Seismology. Permanent seismic stations in North China are very sparse. To determine the 3-D crust and mantle structure of this region, at least tens of portable seismic stations should be installed for a period of 6 months to 2 years. Waveforms from local, regional and teleseismic events can be recorded by the seismic network using state-of-the-art seismological methods. One method is seismic tomography for determining the 3-D P and S wave velocity variations in the crust and mantle. From the obtained P and S wave velocities (Vp, Vs), Vp/Vs ratio or Poisson’s ratio tomography can be estimated. Amplitudes of seismic waves can be used to determine seismic attenuation (seismic quality factor, Q) tomography. Hot magma chambers and mantle plumes would exhibit low-Vp, low-Vs, low-Q and high Poisson’s ratio, while cold subducting slabs exhibit high-Vp, high-Vs, high-Q and low Poisson’s ratio. From the seismic velocity and attenuation tomographic images, we can estimate the size and spatial extent of magma chambers, mantle plumes and subducting slabs [Zhao, 2001b; Zhao et al., 1997]. Poisson’s ratio (or Vp/Vs) is a key parameter in studying petrologic properties of crustal and mantle rocks [Christensen, 1996; Zhao et al., 1996] because it can provide tighter constraints on the composition than Vp or Vs alone. Its value in common rock types ranges from 0.20 to 0.35. Poisson’s ratio has proved to be very effective for detecting magma and fluids in the crust and mantle [Zhao et al., 1996, 2002]. From Vp, Vs, Q and Poisson’s ratio, temperature and content of melts and fluids can also be estimated. Another useful seismological method is teleseismic receiver functions which can be used to determine geometry and sharpness of seismic discontinuities such as the Moho, and the so-called “410” and “670” km discontinuities which represent the upper and lower boundaries of the mantle transition zone [e.g., Ai et al., 2003]. In and around the cold subducting slab, the 410-discontinuity is elevated, while the 670-discontinuity is depressed, thus the mantle transition zone would become thicker. In contrast, if a hot mantle plume exists, the 410-discontinuity will be depressed, while the 670-discontinuity will be elevated, and the


mantle transition zone will become thinner. Thus, receiver function analyses can also provide information on mantle plumes and subducting slabs, complementing tomographic imagings.

Field studies of Cenozoic igneous rocks. Careful and well-documented fieldwork is a prerequisite for all the subsequent laboratory studies. If detailed mapping has not been carried out yet for some major volcanic fields, mapping will be the first step. In the five years of the Greater North China Initiative, it is hoped that all major volcanic fields will be mapped. To collect samples for the purpose of laboratory analyses, for every sample, the longitude, latitude and elevation of the sample collection site will be recorded, plus other relevant information on the sample (such as quarry, road cut, valley, cliff, etc.) If the flow or eruption units have been mapped, such unit should be indicated for the sample. It is important to follow some sampling protocols, which may depend on the purposes of samples. For example, for He isotopic analysis, to avoid cosmogenic helium, it is important to sample fresh exposures, meaning rock interior many meters inside was recently (within the last tens of years) exposed. Recent quarries and road cuts are excellent sampling sites. If one simply picks up a megacryst on the ground, or if one hammers out a sample from a naturally exposed surface, the megacryst might have been exposed to cosmic ray bombardment for thousands or more years, and its helium signature may be largely cosmogenic. To avoid crustal contaminations, it is best to sample young fresh unaltered samples (such as Holocene eruptions), and samples away from any crustal xenoliths. To further coordinate the studies, it might help to choose a suite of samples from various volcanic fields as “reference” samples, for which all possible geochemical analyses and experimental work will be carried out on them (this was done in the “Basaltic Magmatism Project” in the 1970’s in the US). For geochemical modeling and for deeper understanding, very often it is critical to have all analyses done on a selected number of samples. However, the literature data often have major element and some trace element analyses for one rock, REE analyses for another, Sr-Nd isotopic analyses for another, Pb isotope analyses for another, etc.

Geochronology. A number of authors published dates on volcanic rocks in greater NE China [Chen and Peng, 1985; Liu, 1987; Wang et al., 1988; Luo and Chen, 1990; Liu et al., 1992; Chen et al., 1992a,b]. The data are extensive and cover many volcanic fields. However, there are important limitations. First, for some volcanic fields, there are no age data. Secondly, for most volcanic fields, the age data are insufficient to examine the full duration of volcanic activity, and therefore, how petrochemistry evolved with time. For example, there are about 20 eruption centers in Kuandian volcanic field [Liu et al., 1992], but only two samples have been dated to be 0.12 Ma for Liujia and 0.27 Ma for Huangyishan. Therefore, the age data are not enough to investigate the space-age relation of volcanic activities in the volcanic field and whether geochemical characteristics vary with time. Trace element characteristics in three eruption centers (Qingyishan, Huangyishan, and Liuja) of Kuandian volcanic field are quite different. Even in one eruption center (Huangyishan), there is significant variation in trace element geochemistry. Do these differences reflect time progression? Did the three eruption centers (only several km away from one another) erupt similar kind of magma at the same time? Or are the three centers fed by different magma chambers? More dating results will be necessary to address these questions.


To provide the spatio-temporal distribution of volcanic activities in the greater NE China, most or all volcanic centers need to be dated. For larger volcanos with prolonged eruption history, all major eruptions need to be dated. These studies will provide understanding of the spatio-temporal distribution of volcanic activity, as well as possible secular trend of primary magma composition in a single volcano and in the whole NE China volcanic province that might be related to change in mantle conditions. By the end of the Greater North China Initiative, it is hoped that there will be hundreds to thousands of new and accurate dates. With such data, a movie will be made at 1-Myr interval to show how the volcanic activity in the field area of Figure 1 evolved with time. That is, for every one million year interval, a movie frame will be created to show the distribution of the volcanic activities at that time period. For the 80-Ma interval, there will be 80 frames (roughly 3 seconds). This 3-second movie would show the spatial-temporal distribution of volcanic activities, the regional variation and pattern in eruptive activity, and would help to recognize possible hotspot tracks, rifts or hot lines, and other

greater north china initiative (gnci): cenozoic ...web.missouri.edu/~lium/pdfs/gnc-wp.pdflate...

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