Deep seismic probing of continental crust in the lower Yangtze region, eastern China

Tectonophysics,

291

173 (1990) 297-305

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Asia and Africa

Deep seismic probing of continental crust in the lower Yangtze region, eastern China WENG

SHIJIE

’ Institute of Petroleum

‘, CHEN

HUSHEN

2, ZHOU

3 and CUI

XUEQING

ZHICHEN

3

Geology, Ministry of Geology and Mineral Resources, 31 College Road, Haidian, Beijing IOOOOS3 (P.R.C.) ’ East China Bureau of Petroleum

-’ The Sixth Geophysical Exploration (Received

Geology, MGMR, Team, MGMR,

September

1,1988;

Pukou, Nanjing (P.R. C.)

21 Matai Street, Nanjing (P. R.C.)

accepted

March

7,1989)

Abstract Weng Shijie, Chen Hushen, lower Yangtae

region,

Zhou Xueqing

eastern

China.

and Cui Zhichen,

1990. Deep seismic probing

In: J.H. Leven, D.M. Finlayson,

(Editors),

Seismic Probing

of Continents

and their Margins.

In China,

seismic probing

of continental

crust started

seismic lines for petroleum extended

the seismic record

subsidence

histories

has been deformed foreland

length

During

have been transected

several

to obtain

oil prospects

deep crustal

by this deep seismic profile.

data.

with the recording

of regional

in the Lower Yangtze

Several

Beneath

faults which splay from a common

crust in the

and B.L.N. Kennett

173: 297-305.

years ago, associated

a survey for Palaeozoic

to 16 seconds

by a system of thrust

Tectonophysics,

of continental

J.C. Dooley

Cenozoic

these basins, detachment

basins

the Palaeozoic

surface

reg on, we

with d:fferent basement

in a style similar

to

basins.

A restite reflection structural

history

signature

below the Suzhou

of the Tangchen-Lujiang

Block was underthrust recently

exploration.

C. Wright,

beneath

and was influenced

the North

by the Cenozoic

granite

fault involved China

or Dabei

batholith thrust

suggests

it was formed

and subsequent

Block.

The structure

strike-slip

by crustal

remelting.

movement.

of the Moho

suggests

The

The ‘r’angtze it reformed

movements.

Introduction

structural framework of the Palaeozoic sequence. During this survey we extended the seismic record

In China, studies of the continental crust started in the 1950s mainly using deep seismic sounding

length to 16 seconds, and recorded reflection signals from the lower crust and Moho (Fig. 1). Due

methods. Today, refraction seismic techniques are still employed. Five long regional refraction profiles running northwest to southeast across China

to the complex shallow structure, a wide line technique using dynamite sources was employed, with 3-5 parallel lines of low-frequency recorders. The shot and receiver spacing were 80 m and 40 m respectively. In this paper some of the: prominent results from the five profiles shown in Fig. 1 will

have been recorded by the Chinese Ministry of Geology and Mineral Resources. However, these refraction data need to be supplemented by reflection profiling to resolve the finer details of crustal structure. The Lower Yangtze region is one of China’s productive petroleum regions and is a prospective target for Palaeozoic reservoirs. In the 198Os, a regional seismic survey was planned to study the 0040-1951/90/$03.50

0 1990 Elsevier Science Publishers

B.V.

be discussed. Formation of basins

the

Several study

Cenozoic sedimentary basins occur in region (Fig. 1). They are commonly

boundary

faults.

Events

in the section.

c.iown to

the Moho. show similar patterns. We suggest thih

structure was produced by horizontal i:ompression. These two examples serve to indicate that basins in the Lower Yangtze region are not simply analogous to the North Sea model. Neither do they match the pure shear model of McKenzie (1978). If we regard the fault as an extensional fault, the geometry can be explained by the simple shear model of Wernicke (1981). J

Fig. 1. Location

Hang=map showing

the seismic profiles

Thrust sheets

which corre-

spond to those of Figs. 3-7.

regarded to have resulted from rifting and stretching of the lithosphere. The typical model is an asymmetric half-graben in the lower half and a nearly symmetric downwarp in the upper half, somewhat like the Viking Basin of the North Sea (Chu Xia and Chen Huenjiang, 1987). Inversion of the gravimetric data suggests that the Moho has a structure which mirror images the base of the Cenozoic basins. However, reflection seismic data show this is not the case. No boundary fault is observed in the profile shown in Figs. 2 and 3. In Fig. 3, fault C is interpreted as a listric fault bordering the Kaoyou Basin. The line drawing indicates that the displacement of reflectors in the basement is small. Mesozoic events gradually converge to the basin margin, and have a different structure from the typical listric or growth fault sequence with its rotation of strata and truncation of events. The faulting gives the impression of strike-slip movement. The Kaoyou Basin overlies a slope on the Moho. East of the Kaoyou Basin, the Yangzhou uplift was high throughout the Mesozoic and Cenozoic, while related sequences thinned above it. Events between 3 and 4 s in Fig. 2 also bulge upwards and do not mirror image the Moho. Figure 4 is a section across Luzhi Basin (see Fig. 1) which is filled by late Tertiary and Eocene sediments. The base of the basin as shown in the seismic profile, curves with no indication of

Earlier shallow seismic surveys and geological field work have shown the existence of low angle thrust sheets in the study region, but their structure and spatial distribution are unknown. Figure 5 is a line drawing of the Son&n thrust sheet, which is located at the mouth of the Yangtze River. In this figure, A is interpreted as the base of Cenozoic sequence, B as the base of the Mesozoic sequence, and C as the main thrust fault which soles down to about 6 s. The antithetic normal fault accommodated Jurassic deposition. The initial thrust movement was pre-Jurassic and the fault was reactivated in the Tertiary. Below the thrust, there is a non-reflective zone, as observed in other thrust terranes (Cheadle et al., 1986). Some Palaeozoic exposures have been found to the northeast of the profile. Geological field surveys show that the lithofacies can be correlated with the Qiantang terrane, a elastic wedge near Hangzhou (Fig. l), but is quite distinct from exposures west of the thrust (JBGMR, 1986; SBGMR, 1989). When compared with other areas, there has been shortening of more than 30-40 km on this thrust. Many low angle thrusts (dips < 30’) have been mapped in the study region. Some are exposed, others buried to depths of I km or more. They are interpreted as thin-skin structures. However, the thrust C in Fig. 5 is interpreted as evidence for thick-skinned tectonics, since it extends below 10 km (6 s TWT) to the division between upper and lower crust. A magnetotelhuic survey suggests that there is a high-conductivity zone under the thrusts

DEE iP SEISMIC

1‘ROBING

OF CONTINENTAL

K,

CRUST

IN LOWER

YANGTZE

N

lM1

REGION

299

._

-1 ._... ---

-.._-

-__

_ _~___ ,sz ----_:

-_____

_.--I

_~ -

;:

--a:-. ._

-

-:

i-e---

_ s_._=L-_.c _--.

--. --“.<__ -=____._-__~_4-__^. --. ._

--

=.,.=; -

.-

:__;

_<

-I-;-_,

_.

_

__.

.-

---

.

.._r

---

,,-_:_;:--

2

.-

Fig. 3. Line drawing of Kaoyou Basin, taken from the tmmigrated time section of profile HQ-13 (2-2’ in Fig. 1). Reflections at A axe interpreted as the base of the Tertiary, those at B as the base of the Mesozoic sequence; C denotes thrusts, and M the Moho.

Fig. 4. Line drawing of Luzbi Basin, taken from the unmigrated time section of profile HQ-13 (3-3’ in Fig. 1). Note the downwarping of the events of the basinal sequence and the Moho.

DEEP

SEISMIC

PROBING

OF CONTINENTAL

CRUST

IN LOWER

YANGTZE

REGION

301 -0

-2

4

_.

-.

_--

-z

_&.-=

-_

_--

2-s

_ ___ -

Fig 5. Line drawing A are interpreted

of Songjian

thrust sheet, taken from the mm&rated

as the base of the Tertiary,

-I----

-~

-_=.__

--M _ --

time section of profile

those at B as the base of the Mesozoic

_.._ --

_

____;.__..

sequence;

-Y-z_ ---

_=

-

---7

HQ-13 (4-4’ in Fig. 1). Reflections C denotes

thrusts,

at

and M the Moho.

Note that the Moho is also displaced.

(Zhou Xueqing et al., 1989). Such high-conductivity zones are common along the other profiles (e.g.

interpreted as Palaeozoic country rocks upturned by the intrusive activity. The granite has a high Q

the thrusts

Granitic batholith

factor, so that high-quality reflections are obtained from beneath it. The batholith has a flat base near 4 s and hence the body i:; 9-11 km

The deep seismic profile cuts through Suzhou granitic batholith (Figs. 1 and 6). This is a com-

zontal reflections extends over 1 km OI more, and is relatively continuous. The spacing is dense with

in Fig. 7).

thick. Below the batholith

posite batholith

with several episodes

of emplace-

ment. Rocks are chiefly granite, silicic peralkaline granitoids and accompanying

granite, pegma-

tites. Petrographic work suggests this as a crustal remelting igneous body (Xue Keqing, 1984). This body is exposed at the surface 15 km south of the profile. whereas on the section it is covered by a thin veneer of Cenozoic sediments. In Fig. 6, the thin Cenozoic cover is shown as several distinct events. The main part of the batholith is seismically non-reflective, with no events coherent over more than 2-4 traces. On the northwest

side of Fig. 6, dipping

events

at D are

2-4

events

a sequence

of sub-hori-

per 100 ms. They have mcderate

am-

plitudes and positive polarity. This suggests that their density is relatively high, and they may be mafic in composition. However, this phenomenon is not observed elsewhere in the 2000 km of profiles of the Lower Yangtze region. Our data are similar to the reflection character of the granite batholith model proposed by Matthews (1987). This leads us to consider whether the events of the lower crust are reflections from the mafic restite. The spacing of the events in the lower crust and their configuration gives the impression that they may be grouped into 3-4 lenticular bodies. Can

Fig. 6. Line drawing of Suzhou batholith, taken from the unmigrated time section of profile HQ-13 (5-5’ in Fig. 1). A denotes the batholith. Reflections at C are interpreted as the restite layers of partial melting, those at D as the country rocks of the bath&th, and M as the Moho. The spacing of events beneath the batholith gives the impression that they may be contoured into several lens-like bodies reflecting the stages of remelting.

we correlate this structure with the three known emplacement stages of the batholith? The profile (Fig. 6) shows the batholith has a flat base, and is rootless as no supply pipes or feeders are observed. However, the presence of restite beneath the batholith is inferred from the seismic section. This supports the petrologists suggestion that this batholith was formed by crustal remelting. Scattered events below the Moho show similar character to those events above the Moho. Furthermore, the event density increases with depth. Thus the underplating process appears to have retained its signature.

The Tengchen-Lujiang deep fault in eastern China has a surface trace of more than 2000 km and separates major tectonic terranes. Its surface expression is four parallel faults. The eastern fault is commonly regarded as the boundary fault and defines the margin of Yangtze plate. West of the fault system is the North China plate, but within

the fault system some workers consider the rocks belong to the North China plate, whereas others assign them to the Dabei Block (ABGMB, 1983). Displacement on the fault system is dominantly left-lateral strike-slip. Estimates of displacement vary from a minimum of more than 100 km (Weng Shijie, 1984) to a maximum of more than 800 km (Xue Jiawei, 1982). In Fig. 7, A and B are respectively the west and east faults of the Tengchen-Lujiang Fault System. The west fault is nearly vertical and cuts through the whole crust. Structural features and events on both sides of this fault are quite distinct. A flower structure in the shallow seismic section (upper part of A) is not shown in the-line drawing. Using the criteria for identification of strike-slip fault (Zalan, 1987) it is interpreted as a strike-slip fault. The east fault (B) flattens at about 3-4 s, and is cross-cut by the east-dipping thrusts denoted E. This clearly demonstrates then eastern boundary fault of Tengchen-Lujiang- deep fault has a thrust component. The lower crust to the east of the fault A is continuous and shows similar

DEEP

SEISMIC

PROBING

OF CVNTINtN’IAL

CRUST

IN LOWER

YANGTZE

303

REGION

Fig. 7. Line drawing taken from the unmigratcd time section of profile HQ-13, which transects the Tengchen-Lujiang

Fault (6-6’ in Fig. 1). A = west boundary fault; B = east boundary fault; C = thrust within the metamorphosed rocks of the North China plate; D = thrust in the Yangtze plate which is not so evident at the surface but cuts through the whole crust; E = group of faults in the Tertiary basin which are now expressed as normal faults; F= base of ariother Tertiary basin and cut by three reverse faults which may he related with the thrusts in this section. Reflections at G are interpreted as stretched layers in the middle and lower crust of the North China plate: those at H as reflections from the Yangtze plate; M = Moho. Note that A cuts through the crust, while D and another fault also cut the Moho.

properties

(i.e.

they

are

the

basement

of

the

Yangtze plate). Xue Shutong et al. (1987) studied the relationship between the North China and

and wavelengths. Moreover, these events have considerable continuity; some extending for 5-8 km, and

the parallelism

Yangtze plates, and concluded that the North China plate over-thrust the Dabei plate, whereas

Smithson

(1986)

the

seismic profile

Dabei

plate

over-thrust

Deep seismic profiling still not widely accepted

the

supports

Yangtze

talline basement

plate.

this idea which is

by geologists.

is significant.

the framework

rocks in North

of crys-

America,

and our

shows many similar characteristics.

We have examined some properties of’ the lower crustal reflections, including length of events, intensity

Lower crust

of events

reviewed

of reflections,

their amplitude

and polari-

ties, and waveshape. The character of the North China Block is quite distinct from the Yangtze

The oldest rocks in the North China Block are Archaean granulites. Its lower crust is reflective

Block. yet

The lower crust of the Dabei Block has not explored by seismic reflection profiling.

been

and events show high intensity and large amplitudes (e.g. G, H in Fig. 7). The wavelets have a

When

compared

(1986)

and

long wavelength, suggesting that the reflectors arc thick. Boudinage-like events are common in the

America

original large-scale time section. The oldest rocks exposed on the surface in the Yangtze Block are

lower crust of the Yangtze Block only reached the lower metamorphic grade of amp~bo~ite facies.

middle to late Proterozoic. Seismic sections show the lower crust of the Yangtze region to be very

This

reflective.

The

spacing

of

reflectors

events per 100 ms. They have medium

reaches amplitudes

2

Block

with

Meissner

and Europe,

has a granulite

agrees

with

the

results

et al. (1987)

of

Smithson

from

North

we suggest the North

China

facies lower crust. while the

geological

fieid

observations.

Irrespective of whether Archaean strata existed at the depth within the Yangtze Block, we believe it has not reached

a high metamorphic

grade.

After migration,

the dipping

events towards

right hand of Fig. 7 dip at 30’ within a thickness

of 10 km. Because

these dipping

reflections

are unlikely

McGeary strated

United

Kingdom

Yangtze

and

have demonoc-

events and are cut by the

Can we interpret

in the Lower regard

et al. (1987)

lower crust. The reflections

cur as groups of dipping new Moho.

to be from a

States and Warner

(1987) in United subducted

a zone with

of this thickness.

single shear zone. The work of Nelson in the southeastern

the

similar

as subduction,

them as underthrusting

phenomena or at least

of the lower crust?

It is also interesting between

the

Cenozoic

basins

leum

used

gravimetric

the depth of the Moho. the

basins

surrounding

calculated mirror

surface

the Kaoyou

Basin

the

petro-

to deduce

anomalies

within

lower than

regions.

Thus

is always expressed

in the as a

basins.

to the Yangzhou

uplift.

the Moho rises gradually. Other seismic profiles in the Lower Yangtze, not shown in this paper, pro-

relatively low density mantle. The parallelism

gether they offer a combination of data that defines the depth of the Moho. In general, the Moho is

the Cenozoic

between

31 and

34 km depth.

It is

non (Meissner

plitude and intensity of the Moho event is strong, and there is good continuity. However, under the granitic batholith in Fig. 6, reflections occur from

Conclusions

the base of the igneous body throughout the lower crust. The lateral extent of events, the intensity and wavelength and density of events, and the total character do not change abruptly. Thus we can only identify the Moho with the aid of refracA preliminary

conclusion

is that

the

basins

material in the uppermost of the shape at the base of and

the Moho

reveals

they

have been deformed by the same geodynamic forces. As a whole, the Moho is a new phenome-

more clearly defined under Archaean rocks, where the upper mantle is nearly non-reflective, the arn-

data.

of

down below the Luzhi Basin.

In our surveys, refraction seismic profiling is combined with seismic reflection profiling. To-

tion

data

image of the base of the Cenozoic

The Moho is warped From

Bouguer

base

Formerly,

are commonly Palaeozoic

Moho

the relationships the

and the Moho.

geologists

sedimentary

of

vide similar results. These facts indicate that the shape of the granitic body is not only influenced by the low density basin fill, but also by the

Moho

undulating

to note

configuration

et al., 1987).

Results from the deep seismic reflection profile in Lower Yangtze have assisted the interpretation of other geophysical data and the understanding of the tectonic framework of the region. The Lower Yangtze region is composed of four large thrust sheets which moved from southeast to northwest, with the main detachment surface located around

ite.

lo-13 km depth. Movement of the Pacific plate to the east is the main cause of the deformation in the study region. The interior part of the Yang&e Block was thrust at shallow levels and underthrust in the lower crust. Deformation of Palaeozoic rocks occurred chiefly in Triassic time, but defor-

Moho events below the basins are clearer than those below the uplifts. In the former case, the Moho is strong and continuous, commonly defined as a single event or associated with one or two less distinct, less continuous ones. However, below the basement the Moho is often expressed as a transitional zone with a thickness from 2 to 3 km or more. Within the transition zone, reflectors show similar characteristics and it is difficult to define which is the Moho.

mation has continued to the present. The undulation of Moho and its transitional characteristics could reflect this. Based on deep seismic data, important reinterpretations of the Tangchen-Lujiang Fault are necessary. The west boundary fault is the largest and deepest, penetrating to the Moho. Its steeply dipping structure suggests that it is a strike-slip fault. Moreover, some flat events are interpreted as thrust faults which preceded the strike-slip move-

partial melting under the batholith was deep and the present Moho is a new interface which has not yet been isostaticalIy compensated. Underplating may not have yet homogenized the upper mantle and destroyed the reflective signature of the rest-

DEEP

SEISMIC

ment.

PROBING

OF CONTINENTAL

A two-stage

model

The characteristics the Yangtze

CRUST

IN LOWER

is therefore

YANGTZE

suggested.

of the lower crust suggest that

Block extends

fault. The dipping

to the west boundary

events near this fault are inter-

preted to indicate that the Yangtze Block is underthrust beneath the North China Block. The east boundary

fault

is an old thrust,

strike-slip

movement.

be extrapolated troversy

reactivated

Its surface

down

into

geology

is mainly

due to the uncertainty

Con-

on this fault

over the amount

in the Indosinian

amount of left-lateral Yanshanian movement.

cannot

the lower crust.

over the total displacement

of thrusting

by

movement

displacement

and the

during

the

305

RF.GION

1986. Memoir Geological tached

maps and English D., 1978. Some

sedimentary Meissner,

Nature

London, Meissner,

thank the participants in the meeting for discussions and exchange of ideas. We also thank L.D. Brown of Cornell University for his encouragement to present the poster paper. Sincere thanks are given to the editor and reviewers of this paper for their comments

and suggestions.

K.D.,

Geological tached Cheadle,

of Geology

on Regional

Publishing

House,

maps and an English M.J., Czuchra,

In:

Barazangi

Seismology: Geodyn.

The Continental

C.J., Byrne,

T., Brown,

S., 1986. Geometry Brown Crust.

of deep

Mojave

(Editors),

survey.

Reflection

Am. Geophys.

Union,

basins in China. (Jiangsu

Chinese

Bureau

Crust.

Geol.

Sot.

developmen

__Ann.

in

Geo-

S.. 1987. Results of recent

profiling

in the southeastern

(Shanghai

Bureau of Geology on Regional

Geological

3 attached Smithson,

Publishing

maps and English

America

based

Caswell et al. (Editors). Geol. Sot. London, Warner,

States. Geo-

and Mineral

Geology

Resources)%

of Shanghai

Muni-

House, Beijing. 280 pp. (with summary).

S.B.. 1986. Analytical

North

Unitec

Sot.. 89: 141-146.

model of the lower crust from

on

seismic

Nature

reflection.

In:

D.K.

of Lower Cont nental Crust,

Spec. Publ., 24: 22-34.

M. and McGeary,

cients from mantle

S., 1987. Seismic

fault zones. Geophys.

reflxtion

coeffi-

J. R. Astron.

Sot..

89: 223-230. Weng Shine. 1984. Discussion Tengchen-Lujiang

on the displacement

fault.

Tangcheng-Lujiang

Deep

In: Fault

Collected System.

of on

Geological

Puh-

Beijing, pp. 31-34.

B., 1981. Low angle normal Province:

nappe

tectonics.

faults in the Basin and Nature,

1982. On Tengchen-Lujiang

Inst. Geol. Miner. Resour., Xue Keying

vectors Papers

(Editor).

Bull., 3(l):

1984. Granitoids

291: 645-648.

deep

f.mlt. Nanjing

1- 11.

of South China.

Science

Press. Beijing, 230 pp. and

Zhou

Zhang

Evolution

China

Dong

1987.

of the Major

vince. China. Zalan,

Haiynan,

Wei~ng,

Structure

Ocean

P.V.. 1987. Identification

section.

Proc..

Am. Assoc.

Shuwen,

Chen Guanbao

Deformation

and

Elements

Press, Shanghai. of strike-slip Pet. Geol.

Tectonic

in Anhui

Pro-

310 pp. faults in seismic

Annu.

Meet..

198X.

pp. 116-118.

Ser., 14: 305-312.

Chu Xia and Chen Huenjiang, JBGMR

L.

3 at-

for crustal

McBride.

Xue Shutong,

from the COCORP

and

Province.

896 pp. (with

summary).

B.L., Ando,

faults, evidence

M.

Resources),

of Anhui

Beijing.

L.D., Oliver, J.E. and Kaufman crustal

and Mineral

Geology

Continental

J.A., Wille, f>.M.. Brown.

Xue Jiawei, Bureau

of

In: D.K. c_‘aswell et al.

J., Arnow,

Range

Memoir

reflection

L.D.. Oliver, J.E. and Kaufman.

Wernicke,

(Anhui

seismic

to our knowledge

phys.. 5B: 357-364. Nelson,

lishing House,

References

of deep

crust.

of Lower

implications

1989. Memoir

Continents and their Margins” symposium. We thank the organisers of the meeting and gratefully

years

a contribution

of

R., Wever, Th. and Fltih, E.R., 1987. The Moho

Europe:

SBGMR

in its original form as “‘Seismic Probing of

on the development Sci. Lett., 4C: 25-32.

Spec. Publ., 24: I-10.

phys. J.R. Astron.

This paper was presented a poster at the Canberra

remarks

of lower Variscan

(Editors),

Province.

566 pp. (with 3 at-

Earth Planet

in Germany:

the nature

of Jiangsu

Beijing,

summary).

R., 1986. Twenty

profiling

cipality.

1988.

basins.

Geology

House,

McKenzie,

COCORP

Acknowledgements

ABGMR

on Regional

Publishing

1987. Mesozoic

and Cenozoic

of Geology

and Mineral

Zhou Xueqing, comprehensive

Earth Sci., 1: 3-16. Resources),

Yangtze.

Ciu Zhichen geophysical

Acta Geophys.

and Chen Fenshen. prospecting (in press).

profile

1989. HQ-13: in the lower