Post-Mesozoic Transformation of Tectonic Domain in Southeastern China and Its Geodynamic Mechanism

Southeastern China, an important component of the eastern Asian continental margin, is a region where the Pacific Ocean plate and the Eurasian plate have interacted with each other since the Mesozoic. Southeastern China, studied by many Chinese and foreign geologists for a long time (Li, 1993; Ren, 1990; Li, 1973; Xie, 1965; Huang, 1954a, b), contains a number of important geological features including ancient Cathaysia, a tectono-magmatic belt of Mesozoic-Cenozoic and a polymetal circum-western Pacific mineral belt. There is no evidence that the southeastern China has experienced lengthy and complicated geological evolution. But further research is needed on its Mesozoic and Cenozoic transformation from the Tethyan-Himalayan tectonic domain to a circum-Pacific ocean tectonic domain. A systematic research on the depression of southwestern Fujian Province, one of the most significant tectonic and metallogenic unit of the southeastern China indicates that there exists a transformation from Tethyan-Himalayan tectonic domain into circum-Pacific tectonic domain in this area from Indosinian to Yanshanian since Mesozoic-Cenozoic. In this transformation, the geophysics, tectono-magmatic distribution, lithofacies and paleo-geography, tectonic system and regional metallogenisis experienced many distinctive changes in this area. The structural analysis shows that the formation of tectonic regime of southwestern Fujian Province resulted from the superimposition of the two tectonic domains. The geodynamic mechanism of the transformation is primarily expressed as the transverse and longitudinal heterogeneity of lithosphere, and the exchange between the crust and the mantle.

GEOPHYSICAL CHARACTERISTICS

Transformation of tectonic domains in the southeastern China is indicated by deep geophysical conditions of this area, most obviously in southwestern Fujian Province (Fig. 1). Although the Bouguer gravity anomaly of the southeastern China shows a north-northeast trending gradient, which is called the Daxing'anling-Taihangshan-Wuling large gravity gradient, southwestern Fujian Province and its adjacent region are located at the juncture between the east-west trending Nanling-Wuyishan lower gravity anomaly belt and northeast-north trending Fujian-Zhejiang gravity gradient (Ma, 1987). Accordingly, in this area, there probably exists superposed tectonic system between the east-west and the northeast trending structures. This superposed system reflects the transformation from Tethyan-Himalayan tectonic domain to circum-Pacific tectonic domain since the Mesozoic. Generally, seismic tomography may reflect the tectonic activity and deep material status on only the existing geophysical condition (Fig. 2). Seismic tomography in the southwestern Fujian Province also shows tectonic zones with east-west and northeast trends at the depth from the rheosphere to the upper crust (Liu et al., 1989). Evidently, the post-Mesozoic transformation from Tethyan-Himalayan tectonic domain to circum-Pacific tectonic domain still remains distinctive in this area. At the same time, it can be inferred from the geophysical characteristics that the lithosphere becomes much thinner from the western part to the eastern part, and that southwestern Fujian Province is situated along the transition between the lithospheric thicknesses. Therefore, the crust-mantle exchange with each other becomes a probable mechanism of the tectonic-domain transformation.

Figure  1.  Map of Bouguer gravity anomaly of the southeastern China

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Figure  2.  The characteristics of seismic tomography at 220 km beneath the earth in south China. 1. higher velocity zone; 2. lower velocity zone; 3. transitional velocity zone; 4. southwestern Fujian Province

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2. DISTRIBUTION OF TECTONO-MAGMATIC BELTS

Southwestern Fujian Province, a main tectonic domain in the southeastern China located within a major tectono-magmatic zone of the Mesozoic and the Cenozoic, has experienced many phases of magmatism since the formation of the continent's Archean nucleus. Among these phases, the magmatism, one of the Mesozoic ages, is the most violent. Before the Indosinian, this area had experienced formation of a paleo-continental nucleus, an increase in continental crust and formation of a stable platform or craton. During the Indosinian movement and the initiation of China's circum-Pacific tectonic domain, the craton and its inactive fold belts were reactivated. Because of much stronger lithospheric activity, the mantle was elevated, resulting in the extension and compression of the lithosphere. Consequently, a large-scale magmatic activity occurred at this time.

The study of the magmatism in the eastern China shows a slow and distinctively thinning of the lithosphere in the Mesozoic-Cenozoic, Yanshanian period resulted from diapric rise of the hot mantle. The lithospheric thinning was accompanied by the effect causing a transformation of tectonic domains.

The present magmatic belts have east-west and north-east trends, which accord with the main tectonic directions of the two tectonic domains. Furthermore, the distribution of Kn value (demarcating the K₂O content referred to as 60 % SiO₂ of every petrochemical sample) of Yanshanian plutons, identical with the main tectonic directions, reflects the activation of the circum-Pacific domain (Fig. 3).

Figure  3.  Distribution map showing the average Kn value of Yanshanian granite pluton in southwestern Fujian Province

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The distribution of the genetic types of intrusive rocks in southwestern Fujian Province indicates that different tectonic domains are characterized by their specific plutonic types. The remelting granites and the remelting-syntectic mixing granites, belonging to the Tethyan tectonic domain, spread in the western part of the mantle dpression. The syntectic granites are distributed in the transition zone from mantle depression to mantle uplift. And the syntectic differential granites, belonging to the circum-Pacific tectonic domain, are situated on the mantle uplift of the eastern part volcanic depression belt.

The magmatic activity prior to the Indosinian belonging to the Tethyan tectonic domain resulted from the folding orogenesis, while the magmatism of the circum-Pacific tectonic domain resulted from the oceanic subduction along the active continental margin.

In summary, the magmatism described above suggests that the transformation of the tectonic domain in southwestern Fujian Province since the Mesozoic also affected the distribution and genesis of the magma

LITHOFACIES AND PALEOGEOGRAPHY

The transformation of tectonic domains is certainly associated with changes of the lithofacies and paleogeography. The analysis of the lithofacies and paleogeography of southwestern Fujian Province indicates that the Middle Triassic was a major time when occurred the transformation from Tethyan-Himalayan tectonic domain to circum-Pacific tectonic domain. Fig. 4 and Fig. 5 illustrate the Late Devonian and the Early Triassic periods, respectively. Their characteristics are shown as follows: after the end of Caledonian movement, the sedimentation showed the east-west trends (Fig. 4), as did the contours of sedimentary thickness. In the Early Triassic, the contour of sedimentary thickness switched to northeast trend (Fig. 4). It is clear that the circum tectonic domain gradually affected the Tethys domain.

Figure  4.  Lithofacies and paleogeographical map of Late Devonian period in southwestern Fujian Province (modified from geological records of Fujian Province (1985)). 1. paleo-continent; 2. grit stone-silt stone-mud stone-shale assemblage; 3. grit stone assemblage; 4. contor of sedimentary thickness; 5. boundary of lithofacies; 6. littoral point-bar facies; 7. littoral basin facies

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Figure  5.  Lithofacies and paleogeographical map of Early Triassic in southwestern Fujian Province (modified from geological records of Fujian Province, 1985). 1. sand stone-silt stone assemblage; 2. shale-sand stone-silt stone assemblage; 3. contour of sedimentary thickness; 4. boundary of lithofacies; 5. strandflat facies; 6. intertidal basin facies

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EVOLUTION OF TECTONIC SYSTEM

The tectonic studies in southwestern Fujian Province indicate that its tectonic framework resulted from the transformation from Tethyan-Himalayan tectonic domain to circum-Pacific domain. Although the surface structures in southwestern Fujian Province are varied and complicated, they all are resulted from long-term tectonic evolution and violent tectonic activity. There exists an obvious transition in structural style from tectonic uplift and subsidence, and weak to the strong fault structures including tectonic slides, shear zones, thrusts, folds, fractures, deep and large faults. This kind of transformation became stronger just after the Indosinian. The new structures strongly transformed the old ones.

Tsinning-Caledonian epoch: This epoch was the main phase when the paleo-continent's area increased and experienced intense ductile deformation. The regional tectonic direction is mainly east and east-northeast and the stress field was compressed in one of the north-south trend. These kinds of structures mostly developed on the margin of paleo-continent nucleus (Fig. 6).

Figure  6.  Paleo-tectonic map during Caledonian in southwestern Fujian Province. 1. Caledonian uplift; 2. Caledonian faulting depression; 3. fault; 4. boundary of tectonic units

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Hercynian-Indosinian epoch: This epoch was the most important milestone of the formation of tectonic framework in southwestern Fujian Province during its tectonic evolution. Not only did the depression of southwestern Fujian Province occur at this time because of the crustal elevation elsewhere, but also the main direction of the tectonic trend shifted from the east-west trend to the northeast trend. The east-west trend structures in the western part of this area are much more reconstructed from west to east. Therefore, the main framework occurred in this area (Fig. 7).

Figure  7.  Paleo-tectonic map during Indosinian in southwestern Fujian Province. 1. Indosinian uplift; 2. Indosinian depression; 3. fault; 4. boundary of tectonic units; 5. compound anticline and compound syncline

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Yanshanian epoch: The Yanshanian framework in southwestern Fujian Province is controlled by block faulting, foldings and volcanic-emplacement (Fig. 8). The main tectonic direction is NE-NNE, and the structural types are NNE-trending basement uplifts and volcanic eruption zones, regional wide flexures, compound folds, fault belts. Most of the earlier structures, especially the folds and faults were reformed during this phase. It is clear that the Yanshanian structures belong to the circum-Pacific tectonic domain.

Figure  8.  Paleo-tectonic map during Yanshanian in southwestern Fujian Province. 1. Caledonian uplift; 2. depression of Late Paleozoic; 3. volcanic faulting belt of Mesozoic-Cenozoic; 4. fault; 5. compound anticline and compound syncline

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In his research on the intraplate tectonic stress field in eastern China during the Mesozoic-Cenozoic, Wan (1993) suggested that the tectonic styles, directions and strengths were obviously different between the Indosinian (258-208 Ma), Yanshanian (208-135 Ma) and the Sichuan epoch (135-52 Ma). The changes between these tectonic styles occurred rapidly. For example, the direction of maximum principal stress was north-south compression during Indosinian, WNW-ESE compression during the Yanshanian, and northeast-southwest compression during the Sichuan epoch. These changes correspond to the evolution of circum-Pacific tectonic domain in space and time.

GEODYNAMIC MECHANISM OF TRANSFORMATION OF TECTONIC DOMAIN IN MESOZOIC

This analysis of lithospheric deformation and geophysical data proves that the extension, compression and shearing within the crust of southwestern Fujian Province has greatly shifted and evolved since the Mesozoic, resulting from vertical and horizontal movements among different lithospheric layers within this area. Summing up the evidence of dynamic processes since the Mesozoic in the study area, we can postulate how the tectonic transformations occurred in the southeast China.

(1) From the Tsinning-Caledonian and Hercynian-Indosinian to Yanshanian, the evolution of the tectonic stress field was SN compression to NS compression with NS sinistral shearing, NNE sinistral shearing. Correspondingly, the tectonic framework evolved into E-W trends, NE+E-W trends and NNE trends. (2) At least since Caledonian, the mantle subsidence and uplift process and its corresponding extension and compression of the upper crust were instrumental in the formation of the tectonic framework of this area. Especially, due to the shift from mantle depression to mantle uplift (Morgan and Baker, 1983), the lithospheric thinning occurred abruptly. At the same time, the transformation of tectonic domain took place. (3) Because of the frequent transformations stress fields, the magmatic activity shows different strengths in this area. Obviously, the magmatic processes during the transitional Indosinian and Yanshanian were the strongest in the geological history. (4) The deep lithospheric zone with higher conductivity and lower density and velocity, discovered by many geophysical means, is a reasonable explanation for the existence of the concentration and mutation surface of stress and strain. According to the space-time distribution of the zones mentioned above, it is concluded that the stress release and transference from the deep to the shallow crust in the zone is a probable geodynamic mechanism of the transformation of tectonic domain since the Mesozoic.

ACKNOWLEDGMENTS

Thanks are due to Professor G. A. Davis for reviewing this paper and providing his helps in many aspects.