Lithospheric Electrical Characteristics of Eastern Jiangnan Orogen,

South China

Gengen Qiu

, Hui Fang, Qinyin Lv, Yan Pen, Dashuang He and Mexing He

Institute of Geophysical and Geochemical Exploration, Langfang 065000

Email: qiugengen@igge.cn

Keywords: Eastern Jiangnan orogeny, electrical characteristics, magnetotelluric sounding

Abstract: A large amount of mineral resources are distributed in the Southern China, which shows South China block

has experienced a complicated geological evolution process. Three dimensional (3D) lithospheric electrical

structure model of eastern Jiangnan orogen was established in this paper by 3D OCCAM inversion program.

The 3D lithospheric electrical structure model has a good corresponding relationship with the subsurface

geology and regional magnetic anomalies, and thus can provide more detailed information about

underground geological structure. From the perspective of 3D electrical structure model in combination

with regional geologic conditions and regional magnetic data, the eastern Jiangnan orogenic belt and

adjacent regions are divided into 4 first-order tectonic units and 16 secondary tectonic units. The 4 first-

order tectonic units include North China zone with low resistance, Dabie orogenic belt zone with high

resistance, Yangtze zone with low resistance and high-resistivity, and Cathaysia massif zone with high

resistance. The 3D model shows that layers and abnormal bodies with low resistivity in the crust and upper

mantle in the Southern China are widely developed. The characteristics of ‘U’ shape high resistance root in

the deep part of Dabie Mountains imply that the North China block and Yangtze block collided and

subducted beneath the Dabie Mountains on the north and south sides, suggesting strong compression uplift

tectonic settings. The overall performance of the Yangtze block is characterized by high resistance in the

middle and low resistance around. Huaiyu terrane located in the east of Yangtze block is of low resistance

characteristic, suggesting apparent obduction tectonic environment. Evidently the assaulted low resistance

anomalous in the lower section occurred along the collage tectonic belt, and these low resistance anomalous

could be related to the distribution and formation of a large amount of metal minerals above. The low

resistance anomalous whose tendency for south east along Jiangshao fault indicate the Cathaysia massif

obducted to Yangtze block. The establishment of 3D lithospheric electrical structure model of the eastern

Jiangnan orogen is to explore the relationships between different tectonic units and analyse the formation

and evolution of south China continent, which could have important reference value for forecasting and

prospecting of potential minerals.

1 INTRODUCTION

Southern China is composed of Yangtze block and

Cathaysia massif, which contains enrichment areas

of mineral resources in the mainland of China.

Stratigraphic sequence exposed in Southern China

spans from the Proterozoic to Cenozoic. Basic,

neutral and acidic igneous rocks are widely

distributed, especially Mesozoic granule volcanic-

intrusive rocks which are the most widely developed

(Dong and Ma, 2011). Southern China contains

abundant mineral resources, and various ores in

huge reserves. There are many metallogenic belts

distributed in the Southern China, such as the middle

and lower reaches of the Yangtze river metallogenic

belt and Qin-Hangzhou metallogenic belt. Its

distinctive geological pattern, unique crustal growth

and metallogenic regularity are rare in the

worldwide, suggesting a deep geodynamic

background and complicated tectonic evolution

history of the South China block. Nowadays, the

cause of abundant mineral deposits formation in this

metallogenic belt has been still a hot issue and have

attracted the attention of scholars of geologists.

Numerous geophysical prospecting works have

been carried out by predecessors in the Southern

China. After several generations of efforts, research

results are abundant, while there are still many basic

geological problems unsolved in the Southern China.

One of the simplest reasons is lack of knowledge

about deep geological information. Magnetotelluric

Qiu, G., Fang, H., Lv, Q., Pen, Y., He, D. and He, M.

Lithospheric Electrical Characteristics of Eastern Jiangnan Orogen, South China.

In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 415-420

ISBN: 978-989-758-342-1

415

(MT) sounding is one of the most important methods

for deep exploration (Chen and Liu, 1984), due to its

advantages in deep detecting, light equipment,

stationing flexibly, and low-cost. MT sounding is

widely used in the field of resource exploration and

deep structure study of crust and mantle. In order to

obtain deep structure information of the Southern

China, we carried out a large area of regional

magnetotelluric sounding work in the Southern

China during 2008-2016, and also completed a

measurement area of about 350 thousand square

kilometers. 525 wide-band magnetotelluric sounding

stations were acquired. At present, 1D and 2D

inversion methods of magnetotelluric data have been

developed, which have become the main means of

the actual data processing and interpretation (Dong

et al., 2012; Chen et al., 2011; Cai and Chen, 2010).

The nonlinear conjugate gradients algorithm

(NLCG) and the improved OCCAM method become

the mainstream inversion methods (Rodi and

Mackie, 2001; Rodi, 1976; Siripunvaraporn and

Egbert, 2000). With the development of 3D forward

calculation and the improvement of computer

performance, 3D magnetotelluric inversion method

is developing rapidly, and is gradually applied to

actual data processing (Newman and Alumbaugh,

2000; Patro and Egbert, 2008; TANG et al., 2014;

YANG et al., 2015; YANG et al., 2015; QIU et al.,

2014).

In this paper, we introduce the process of how to

construct 3D electrical structure model of eastern

Jiangnan orogen by WSINV3DMT inversion

program which is based on REBOCC 3D inversion

algorithm. Based on the analyses of 3D electrical

structure model, regional geology, regional gravity

and magnetic data, we have preliminarily divided

the tectonic units in the Southern China, and analyze

the electrical characteristics and contact relationship

of each structural unit. The 3D model provides

important information for the formation and

evolution of the mainland of Southern China and the

favorable direction of subsequent prospecting.

2 REBOCC INVERSION THEORY

Data-space method is proposed by professor

Weerachai Siripunvaraporn on the improvement of

Occam inversion algorithm (Siripunvaraporn et al.,

2004; Siripunvaraporn et al., 2005; Siripunvaraporn

and Egbert, 2009). It is based on the calculation and

storage format of the sensitivity function that

transform from original model space (M╳M) to

data space (N ╳ N) by derivation. The model

roughness is defined which can make the model

more flexible , and suppress the geoelectric structure

irrationality. In OCCAM inversion, by solving the

unqualified equation

(,)Um

can achieve the goal:

11 1 2

00 *

(,)( )( ){([])([])}

Um m m C m m d Fm C d Fm X

λλ

Τ− − Τ−

=− − + − − −

)

In order to get the required solution, the equation

is usually not solved directly , but by using the

penalty function as follows, because when fixed,

, and have the same value.

11 1

() ( ) ( ) {( []) ( [])}

W m m m C m m d Fm C d Fm

Τ− − Τ−

=− − + − −

)

refers to Lagrange multiplier, and is used to

control the smoothness of model fitting.

In general, the MT data is smooth and redundant,

so in the data space, we do not need to use all

sensitivity matrix as the base functions. The

minimization of equation (2) in number k iteration

can be expressed as a linear combination line of

smooth sensitivity matrix

10 1kmkk

mmCJ

−=

(3)

k+1

is unknown coefficient vector of basic

functions

[]

mk j

,(j=1,…,N). put (3) into (2), it can be

obtained:

11 1 1

{( ) ( )}

nnn

kkk k kk d k kk

WdCd

ββλ β β

Τ− Τ−

++ + +

=Γ + −Γ −Γ

(4)

kkmk

Γ=

is N╳N order "data subspace cross-

product" matrix. Differentiating Beta of equation

(4), make the result is zero, and rearranging,

unknown expansion coefficient can be written as:

(5)

By solving (5) over and over again, the optimal

inversion results can be obtained according to the

method of searching model space.

3 DATA ACQUISITION AND 3D

MODEL CONSTRUCTION

After MT curves editing and static shift correction,

the field observation data can be used for inversion

∂∂ ∂ ∂U/ m = W/ m

(,)Um

()Wm

1−

−

=+Γ()

kdkk

βλ

IWEG 2018 - International Workshop on Environment and Geoscience

416

calculation. 3D inversion is the first choice for

regional magnetotelluric data. A total of 525 wide-

band magnetotelluric sounding stations were

acquired in the Southern China. The distance

between each adjacent station is about 20-30

kilometers. Because a large amount of computer

memory is needed in 3D inversion calculation, the

whole work area is decomposed into 6 sub blocks to

carry out 3D inversion individually. A certain

overlap area is retained between the two adjacent

sub blocks so as to facilitate the post splicing work

of each sub inversion result. Figure 1 shows the

actual MT stations’ distribution in the Southern

China. Figure 2 shows the six sub blocks. 3D

inversion is carried out according to the following

steps and finally an accurate 3D electrical structure

model is achieved. 21 frequency data are uniformly

selected from the logarithmic range of 320Hz-

5000S. In the horizontal direction, the center of the

model within MT stations is divided by 10km

uniform grid. In the vertical direction, to start with

50m thickness as the first layer, and then step down

by 1.1 times gradually decline until reaching enough

depth. 100 Ω•m homogeneous half space are

selected as the initial model, and the mode of non

diagonal impedance tensor (Z

) is used for 3D

inversion calculation. After 3D inversion calculation

of each sub block is finished in turn, we can get the

fusion electrical structure model of the whole work

area using each sub block 3D result (Figure 3). The

RMS error of the model is 2.75, the data fitting is

relatively well, and the inversion results are credible.

Figure 1: Map of study area and location of MT stations

(The area of the work area is about 350 thousand square

kilometres. A total of 525 wide-band magnetotelluric

sounding stations were acquired.)

Figure 2: Actual MT stations and six sub blocks partition.

Figure 3: 3D lithosphere electrical structure model of

eastern Jiangnan orogen.

4 3D ELECTRICAL

CHARACTERISTICS OF

EASTERN JIANGNAN

OROGEN

4.1 Plane Electrical Characteristics and

Structural Units Division

The value of plane vertical conductance is calculated

by layer thickness and the corresponding resistance.

It is usually expressed in symbol S, S=H/ρ,

representing local rock conduction ability of a

certain thickness. The Southern China has

experienced a complex geological evolution history

that its geological structure is complicated. For the

theoretical of different geophysical exploration

methods are different, the methods of dividing

Lithospheric Electrical Characteristics of Eastern Jiangnan Orogen, South China

417

structural units may be different as well. Figure 4a

displays plane vertical conductance within 50 km

thickness of South China which is calculated using

3D electrical structure model. In accordance with

electrical characteristic differences, and additionally

combining with plane resistance characteristics at

the depth of 10 km (Figure4b), regional geology on

surface (Figure 4c) and satellite magnetic anomaly

(Figure 4d), the working area is divided into 4 first-

order tectonic units and 16 secondary tectonic units.

The 4 first-order tectonic units include low

resistance North China zone, high resistance Dabie

orogenic belt zone, low resistance and high

resistivity interval Yangtze zone and high resistance

Cathaysia massif zone. As can be seen from Figure

4, the electrical characteristics of each structural unit

have a good spatial correspondence with the regional

geology and the satellite magnetic anomaly.

Figure 4: Structural units division of eastern Jiangnan

orogen. (a) plane vertical conductance within 50 km

depth; (b) plane resistance characteristics within 10 km

depth; (c) regional geological map on earth surface; (d)

satellite magnetic anomaly. TLF:Tancheng-Lujiang fault,

XGF:Xiangfan-Guangji fault, YCF:Yangxing-Changzhou

fault, JSF:Jiangshan-Shaoxing fault.

4.2 Vertical Electrical Characteristics

and the Relationship between

Different Structural Units

As can be seen from the plane vertical conductance

map (Figure 4a), the North China block is of low

resistance characteristic, while the Dabie orogenic

belt and the Cathaysia massif are characterized by

high resistance. The Yangtze block is featured by

middle high resistance and low resistance around.

Different structural units in both horizontal and

vertical directions clearly show different electrical

characteristics.

Figure 5: Vertical electrical characteristics of two profiles.

(a) the position of two profiles AA’ and BB’. (b) electrical

characteristics of the West-East line BB’. (c) electrical

characteristics of the South-North line AA’.

In order to study the contact relationship between

various structural units, two electrical profiles are

selected in the work area (Figure 5). From profile C

(A-A') in Figure 5, the high resistance with a U-

shaped structure in the deep is performed in the

Dabie mountain, the deepest place can reach about

80 km depth, indicating obvious high resistance

"root" characteristic. There is a high conductivity

layer in the depth of about 30 km through deep high

resistance zone under the Dabie mountain. The

exploration results of the low resistivity layer are in

agreement with previous results. The fractures

located on both sides of Dabie mountain are steep,

and enclosed to the high conductivity layer in the

deeper. The Yangtze block is characterized by high

resistance in the middle and low resistivity around,

and there is a large low resistance anomaly under the

middle high impedance body. Low resistance belt

along the Yangtze River which is located in the

north of Yangtze massif shows apparent high

conductivity characteristic. The low resistivity zone

is linked to low resistance anomalous in deep

mantle. The Jiangshao fault located in the southern

margin of the Yangtze block is characterized by a

narrow range of low resistance zones under which

there is an local upward protruding structure. It can

be seen from the profile (B-B') in Figure 5 that the

middle part of the Yangtze block and Cathaysia

massif are characterized by a wide range of high

resistance. It is probably the base structure formed in

the Neoproterozoic Era, and its anomaly range and

depth of high resistivity are much larger than those

of the Dabie Mountains. HuaiYu terrane located in

the east of Yangtze block is of high conductivity

characteristic. The shallow low-resistance

anomalous connected with low resistance fluid in

deep mantle through a narrow passage. Tendency of

low resistance anomalies overall represents to

southeast. The above two profiles clearly

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demonstrate the electrical characteristics of each

structural unit and the contact relationship between

each other. The 3D electrical structure model is of

great value to study the formation and evolution of

the Southern China continent and the ore controlling

factors of deep deposit.

5 GEOLOGICAL SIGNIFICANCE

REVEALED BY THE 3D

ELECTRICAL MODEL

Generally the 3D electrical structure model contains

abundant underground information. The

characteristics of ‘U’ shape high resistance root in

the deep part of Dabie Mountains indicates that the

northside of North China block and the southside of

Yangtze block collided and subducted beneath the

high resistance bodies of the Dabie Mountains,

which made the Dabie mountains shows an obvious

compression uplift structure pattern. During the

Permian - Triassic period, the tectonic environment

in the Southern China area expanded, the low

resistivity layer was formed at the 30km depth

below the Dabie Mountain. Low resistance belt

along the Yangtze River located in front-end Dabie

orogenic belt shows significant high conductivity

characteristics. The low resistance abnormal body is

related with deep low resistance anomalous in upper

mantle, which could form the migration channel for

deep soft fluid. The low resistance channel may have

closed relationship with the distribution of abundant

of metal mineral resources above. Huaiyu terrane

located in the east of Yangtze block is of high

conductivity characteristic, suggesting obvious

collage of tectonic patterns, with which the two low

resistance layers in crust exist. The double low

resistivity layer may indicate the existence of a

double crustal structure. Double crust structure has

formed its unique form of mineral resources system.

Jiangshao fault shows an evident low resistivity

characteristic, and the low resistivity zone tends to

the Southeast. Its formation may be related to the

cracking events during Early Neoproterozoic-

Paleozoic era. At that same time, a lot of low

resistance layers were widely developed in crust

during the cracking period.

6 CONCLUSIONS

This paper introduces the process of how to

construct 3D electrical structure model of eastern

Jiangnan orogen by WSINV3DMT inversion

program which based on REBOCC 3D inversion

algorithm. Based on the 3D electrical structure

model and other related geophysical data, the 3D

electrical structure characteristics and geological

significance of Jiangnan orogenic belt are analyzed

and the following conclusions are obtained:

Using regional magnetotelluric data, the 3D

electrical structure model of eastern Jiangnan orogen

is established for the first time. Combined with

regional geology, regional gravity and magnetic

data, the eastern JiangNan orogen can be divided

into 4 first-order structural units and 16 secondary

structural units.

The characteristic of ‘U’ shape high resistance

root in the deep part of Dabie Mountains indicates

the North China block and Yangtze block collided

and subducted beneath the Dabie Mountains on the

north and south sides individually, suggesting strong

compression uplift tectonic settings.

Huaiyu terrane located in the eastern of Yangtze

block shows low resistivity, implying apparent

obduction tectonic environment. There are obvious

assaulting low resistance anomalous in the lower

section along the collage tectonic belt, and these low

resistance anomalous could be related to the

distribution and formation of a large number of

metal minerals above.

Jiangshao fault shows an obvious low

resistivitythat the low resistivity zone tends to the

Southeast, which means Cathaysia massif collided

over the Yangtze block.

ACKNOWLEDGMENTS

Thanks to all the staff involved in the field data

acquisition of this project, they have worked hard to

obtain valuable data. All the observation data in this

article are done with the support of multiple projects.

This study is supported by the National Key R&D

Program of China (No.2016YFC0600201), the

National Natural Science Fund (No.41574068),

Geological Investigation Project

(No.121201108000160913, No.12120114005801)

and CAGS basic research project (No.AS2016J12,

No.JYYWF20180902).

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