On the geotectonic nature of the Fen-Wei rift system

Tecionophysics, 143 (1987) 217-223 Elsevier Science Publishers

217

B.V., Amsterdam

- Printed

On the geotectonic

in The Netherlands

nature of the Fen-Wei rift system CHEN GUODA

Changsha Institute of Geotectonics, Academia Sinica, Changsha Hunan (P.R. of China) (Received

February

2,1985;

accepted

June 7,1986)

Abstract Chen Guoda,

1987. On the geotectonic

Qvale (Editors), The Fen-Wei following depths

Continental rift system,

a zigzag path of 5-6

displacement

marginal activation formed

a maximum

uplift zones. Possibly

Originally

km wide,

sediments

to be rather

a mantle

the North

as a result of contraction

high as marked

arched

belt exists below.

China

diwa

ancient

and

continental

abroad.

However,

geodepression,

is derived

from Chinese

words

the

meaning

being a special kind of intermountain-struct-

ural basins occurring

mostly

on the ancient

platform

region,

filled chiefly

with

mollasoid

sediments.

A diwa

marked

high

relief

and

by basins,

diwa

with ranges

as positive

units called

with

units intercalated domes”.

It is a post-platform

tectonic

element

of the continental

nal and platform (Chen,

1959,

“activated it is formed

mobile

regions,

1960b,

region”

proposed

1965,

proposed

by the activation

1980,

region,

by the author

“geo-

the third

by the author Its

is

as negative

crust besides etc).

region

geo-

geosyncliin 1959

synonym

is

in 1956, because

of the platform

(Chen

1956,

1960a).

0040-1951/87/$03.50

0 1987 Elsevier Science Publishers

part

km). Accompanying of alkaline

In the upper platform.

Bouguer

anomalies

are

than that of the

VP = 7.95 km/s.

In the Early Jurassic,

region *. In the Cenozoic, with creeping

part).

took place 24 times

2-4 km thinner

mantle,

vertical

in the southern

basalts

Negative

2 km) reaching

the mainly

up to 7 km (greatest mainly

of East China,

altitude

the latter underwent

the Fen-Wei

of asthenospheric

rift zones in E.E. Milanovsky’s

rift system

material.

It may

classification.

Beginning at the end of the 19th century, both German and American geologists made geological surveys in this region during their investigation in China. The geology of the region was also described in reports of the Geological Survey of

problem of the geotectonic nature of this region has remained unsolved until relatively recently. * The term “diwa”

(average

by 17 hot springs.

of the crust in this region combined

to the crevice type of post-platform

at home

China

(geodepression)

The Fen-Wei Graben is a world famous graben which has long attracted the attention of geoloboth

3-4

and G.

143: 217-223.

in the northern

Plateau

laterally

M = 8. Eruptions

Introduction

gists

is situated

(thickness

extends

E.E. Milanovsky

Tectonophysics,

of - 200 mGal. The crust is 38 km thick on average,

the rift system was part of the broad

and became

be referred

Cenozoic

km deep are frequent;

Heat flow appears

reaching

lo-100

In: LB. Ramberg,

Characteristics.

It cuts the Shanxi-Shaanxi

fault zones, the rift system

with foci lo-30

rift system.

and Regional

NNE-SSW.

km, and is filled with

in the Cenozoic.

of the Fen-Wei

1200 km long and

trending

of the marginal

Earthquakes dominant,

nature

Rifts-Principal

B.V.

China and in the geological map of Asia published by the Association of Earth Sciences of Tokyo. Beginning in the 1920’s, the tectonics, volcanoes and geomorphology of the region has been described by a number of authors including Wang (1925) and Yun (1977). In the 1930’s, the American geologist Barbour (1931) made an analysis of the Taigu deposits in Shanxi Province and the climatic environment for their deposition. Since then a number of geological and geomorphological investigations, such as the study of the Fen River by Guo et al. (1956), and the Lishan area by Zhang et al. (1957) have served to lay the foundation for a discussion of the tectonics of the region.

218

Studies of the Fen-Wei Graben as an example of rifting began in the 1970’s. Xi et al. (1977) called its northern segment the “Shanxi Rift”, while the term “Fen-Wei graben rift-system” used by Liu et al. (1979) embraces the whole region. Research on the relations~p between aeromagnetic anomalies and deep structure carried out by local geologists and a discussion of the relationship between crustal structures and seismicity by Wu et al. (1981) have greatly contributed to an understanding of the rift nature of the region. based on geophysical and crustal structure evidence. The present author began to study the geology of the region in the 1930’s (Chen, 1936) investigating the tectonics, geomorphology and Cenozoic sediments of the Fen-Wei rift system successively in its northern, central and southern segments. This paper is based on data collected during field investigations combined with geological, seismological and geophysical data provided by other authors and organizations. It attempts to discuss the geotectonic nature of the region from the viewpoint of crustal evolution. Geographical and geomorphological

position

Geographically the Fen-Wei rift system is situated in the northern part of East China (Fig. 1) and includes the Fen and Wei River valleys (Fig. 2). It extends from north of Datong through Taiyuan and Taigu in Shanxi Province to west of Xi’an, and follows a zigzag path trending NNE-SSW, then turning ENE-WSW. The rift system forms an elongate belt 1200 km long and lo-100 km wide. Tectono-geomorphologically it lies in the east part of the ShantiShaanxi Plateau (Fig. 3) and consists of minor grabens and intervening or concealed horsts having an en echelon arrangement trending NE or NNE and locally ENE (Fig. 2). It is bounded by uplift zones, the Taihang Mt. uplift zone in the east and the Luliang Mt. uplift zone in the west (Figs. 2 and 3). Both of them form today moderately high mountains with an average altitude of 2,000 m above sea level, on which relicts of multistep ancient planation surfaces and numerous erosion and accumulation terraces are still preserved.

Fig. 1. Sketch map of China showing the regional setting of the Fen-Wei rift system.

The latter are developed largely in minor grabenbasins, as for example the terraces composed of the Early Quaternary Taigu deposits in the Taigu Basin on the Taihang Mt. uplift zone. The Fen-Wei rift system is separated from the two uplift zones by a series of NNE-, NE- or ENE-trending normal faults with varying degrees of strike-slip component. The fault planes dip towards the interior of the rift system, with angles 50”~60 o or more. Vertical displacement on each fault is estimated, according to stratigraphic evidence, to be 500-1000 m or more. These fault planes are expressed geomorphologically as clear fault scarps or fault-line scarps (Figs. 3, 4 and 5). Triangular facets on them are well developed and can be seen, for example, in the Lintong fault of Shaanxi Province (Fig. 4). There are numerous springs arranged in sets at the bases of these scarps arranged in an en echelon manner. Examples are the Jiuquansi (Figs. 3 and 5) fault spring on the eastern margin, and the Jinci (Fig. 3) fault spring on the western margin of the rift system. Deep structure and crustal construction

The Fen-Wei rift system cuts deeply into the crust from the Shanx-Shaanxi Plateau which is on average about 2000 m above sea level. The graben floor attains a depth of 5-6 km, filled with

219

Cenozoic

diwa sediments

Cambro-Ordivician

m D

,-I

Fracture

,

/ Epicenter of M=8 _j earthquake

rzxI

Epicenter of M=7 earthquake

L--_-l rx

Quaternary

crater

Area of Fig. 3

Fig. 2. Tectonic map of the Fen-Wei rift system (based on Tectonic

Cenozoic sediments on average 3-4 km thick, but up to 6 km near Taiyuan. Accompanying the mainly vertical displacements of the marginal fault zones, the rift system extended laterally. According to the dip angles of all the fault planes and displacement of the faults, it can be calculated that the total amount of the lateral displacement may in places be up to 7 km or more (e.g. in the southern segment of the rift system). The crustal thickness in the rift system averages 38 km, and locally even less, e.g., 27 km near Xi’an (Chen and Chen, 1974). In the adjacent uplift zones it is generally 40 km and up to 45 km in some places (e.g., west of Taiyuan) (Wu et al., 1981). An average reduction in thickness of 2-4

Map of China, 1: 4,000,ooO; Chen Guoda et al., 1977a).

km in the Fen-Wei area indicates that a mantle arched belt possibly exists beneath the rift system (Chen and Chen, 1974); see Fig. 6. Gravity Bouguer anomalies in the rift system are dominantly negative, usually -200 mGa1. Aeromagnetic anomalies show a NNE-tren~ng negative axis. In the upper mantle VP= 7.95 km/s. Earthquakes in the Fen and Wei region occur frequently with foci lo-30 km deep, and have a mag~tude up to 8. From Oligocene to late Pleistocene, 24 volcanic eruptions, mainly of alkaline basalts, occurred at Datong in the northern segment of the rift system while some volcanic cones are still recognizable today (Figs. 2 and 3). Hot springs are common on

220 SHANXISHAANXI LULIANC

MT. UPLIFT ZONE:,

TAIYUAN

-

PLATEAU

TAIH.4NG MI‘. UPLIFT ZONE

-

Cenozoic sediments Permian sandstones Cambrian shales Cambro-Ordivician Iimrstones

Fig. 3. Simplified structural and geomorphological block-diagram of the northern segment of the Fen-Wei rift system showing the uplift zones on its east and west sides and fault zones between them. FENGSHANSI ZHOUQUANSI

- --; -;

/

i--IAI(;U

Fig. 4. Scarp at the Lintong fault, Shaanxi Province. Fig. 5. The Taigu fault scarp, Shanxi Province, with fault springs at its foot.

Fig. 6. Ideal cross section of the earth’s crust of the Fen-Wei rifts system at Taiyuan. (Data according to Wu Lie et al., 1981).

the two sides of the rift system, totalling up to 17, indicative of high terrestrial temperature. History of crustal evolution The history of crustal evolution in the Fen-Wei region and its adjacent areas can be traced back to the Archean. According to drill hole data, Archean rnet~o~~c rocks exist in the central part of the folded basement within the Datong fault basin in the northern segment. On the Luliang Mt. uplift zone, gneiss and schist of late Archean age are extensively exposed. These ancient metamorphic sequences consist of pre-geosynclinal and geosynclinal sediments. Ordovician limestone and Carboniferous coal measures are known in the marginal parts of the Datong fault basin. These Paleozoic rocks outcrop more widely on the Taihang Mt. uplift zone. They belong to platform type sediments. In the central and southern segments of the Fen-Wei area the Paleozoic platform sediments are also seen, with a varying extent, on the adjacent uplift zones. The above-mentioned strata indicate that prior to Mesozoic times the history of crustal evolution of the region passed through pre-g~s~c~nal, geosynclinal and platform stages. Evidently this region was part of the ancient North China platform region of post-Luliang age. In the Jurassic the ancient North China platform region began to undergo activation and was transformed into a diwa (geodepression) region (Chen, 1960a; Chen et al., 1977b). Consequently the Fen-Wei region entered a new stage, becoming part of the North China diwa region (Chen et al., 1977a, Chen, 1981). The development of the diwa region may be subdivided into three periods. In the early and middle periods, heat flow is increasing while in the last period it decreases (Chen, 1965). In Cenozoic time, the development of the North China diwa region entered the last period (Chen et al., 1977b), and the earth’s crust was contracting due to the decrease of heat flow. The result was predominantly tensile stresses. Beginning in the Paleocene, and especially marked during the Miocene (e.g. at Datong), various extensional faults were generated and developed in the marginal fault zones of the

Fen-Wei River valley to accompany the crustal contraction. Their vertical displacement gradually increased, giving rise to fault basins in the northem, central and southern segments of the region, and the grabens became morphologically distinct. In the Pliocene, development of the fault basins reached a climax and the basins spread over the whole region, resulting in a large single rift system, composed of a series of minor grabens and extending for more than 1000 km. In the northern segment of the rift system there were several volcanic eruptions mainly of alkaline basal&, indicating that the faults had cut through the lower crust. Rifting still occurs in Quatemary time, forming the present-day geomorphological features. It can be seen from the Taigu fault scarp on the eastern side of the Fen-Wei rift system that the early Quatemary Taigu deposits are cut by the fault (Fig. 5). Moreover, the faults on each side of the Fen-Wei rift system are famous throughout history as seismogenic structures, where earthquakes of great magnitude have been located. One must therefore assure that they are still active today. Conclusions The Fen-Wei rift system has long been interpreted as a platform region, which belongs to the North China platform. However, from the tectonic history of the rift system, crustal construction, geological and geophysical characteristics, it can be concluded that the region has now become a diwa region (Chen et al., 1977a, b). It is a negative unit of the North China diwa region-a rift-type diwa system. Next, in the classification of rifts in terms of their geotectonic nature, the Fen-Wei area is a post-platform rift system. It was very active in the Tertiary, cutting down through the lower crust, and has also been operative in the Quatemary. It can be compared with such famous grabens as the Rhine Graben in Europe and the Baikal Graben in Asia, investigated by the author himself in 1980 and 1984 respectively. They all belong to diwa regions. The Fen-Wei area can be referred to the crevice type of post-platform continental rift zones in the classification of the world’s rifts proposed by Milanovsky in 1972 and revised in 1976.

222

Geologists’ opinions differ on the problem of the stress field in the study region and adjacent areas during the formation of the Fen-Wei rift system. It is known that all Cenozoic diwa basins appearing in the diwa regions of East China (especially those trending in a NNE direction) are extensional in nature. This indicates that various parts of this broad domain, including the Fen-Wei rift system, have been governed by the same stress field. Stress analysis shows that horizontal tension in a NNW-SSE direction was a dominant factor in the whole eastern part of China at that time. It is probably related to movement of subcrustal material in East Asia since the Cenozoic. According to satellite gravity data (Anonymous, 1978) there are four regions of high gravity anomalies, one in Japan, one in the Philippines, and the other two in the Qin~ai-Xi~g Plateau. East China shows a gravity low. These data reflect a difference in density of mantle material at depths of 100400 km. It can be inferred that the difference in density of asthenospheric material in the East Asia-West Pacific domain resulted during the Cenozoic in a general tendency for creep from northwest to southeast. Dragged by such a creep the crust in East China underwent tension in a correspon~ng direction, which gave rise to a series of NNE-, NE- or ENE-trending rift-type and other extensional diwa basins. The Fen-Wei rift-type diwa system is just one of such basins. The formation of the Fen-Wei rift-type diwa system is therefore related not only to the internal cause crustal contraction and prevailing tension during the last period of development of the diwa region, but also to the important external cause described above. It is only in this way that the present rift system with an axis trending generally in a NNE direction can have come into being.

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I am grateful to Xi Guojin and Wu Gongjian for providing geological and geophysical data. Huang Su and Tan Keren read the manuscript and offered constructive comments. I thank them and am especially grateful to Zhou Yufan for his help in the preparation of the paper.

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