Sounding of electrical structure of the crust and upper mantle along the eastern border of...

Vol 46 Supp SCIENCE IN CHINA (Series D) October 2003

Sounding of electrical structure of the crust and upper mantle along the eastern border of Qinghai-Tibet Plateau and its tectonic significance

SUN Jie (孙洁) JIN Guangwen (晋光文) BAI Denghai (白登海) amp Wang Lifeng (王立凤)

Institute of Geology China Seismological Bureau Beijing 100029 China Correspondence should be addressed to Sun Jie (email sunj66sohucom)

Received November 12 2002

Abstract Sounding and study on electrical structure of the crust and upper mantle within the eastern border region of Qinghai-Tibet Plateau by using the magnetotelluric sounding (simply MT) method permitted us to understand the characteristics of specific electrical structure in the region The sounding result clearly revealed that (1) The Xianshuihe fault zone represents a large-scale lithospheric fault and is an important boundary fault of the rhombic Sichuan-Yunnan block (2) The sounded region is a strong earthquake-prone zone The different crustal media of blocks on both sides of the fault became an important deep background for the strong seismo-active zone (3) A large-scale low-resistivity layer is found to exist at a depth more than ten kilometers beneath the northern part of the rhombic Sichuan-Yunnan block Its electrical resistivity is only several to tens Ωm The layer northeastward extends down at an angle of 45deg It is related to an obstacle to the lateral squeeze of Qinghai-Tibet Plateau and eastward flow of mass by the rigid block It is inferred from the characteristics of electrical property of deep media that the northern part of the recent rhombic Sichuan-Yunnan block is in a thermal state and is one of the recently fairly active blocks (4) The lithosphere in the sounded region is gradually thickened from the western segment (northern Sichuan-Yunnan block) to east (Yangtze block)

Keywords Qinghai-Tibet Plateau eastern border magnetotelluric sounding crust and upper mantle electrical structure

DOI 10136003dz0019

The study region is located in the eastern part of Qinghai-Tibet Plateau and is a conjunction zone between the northern part of rhombic Sichuan-Yunnan block[1] (99degmdash105degE and 29degmdash32degN) and Yangtze block It belongs to the conjunction-transition zone between Tethyan tectonic belt and circum-Pacific tectonic domain and is also a key section of the collision between Indian and Eura-sian plates Seismic data from 1970 to now provided by Chinese seismograph network indicate that the Sichuan-Yunnan region is one of the strongest seismo-active region in China continent Since the beginning of historical records hundreds of strong earthquakes of magnitudes Mge6 oc-curred in the region The specific tectonic environment and frequent seismic activity make the deep structure of the region be a research hotspot in the geoscientific circles and attract much at-tention of the scientists at home and aboard

A wealth of geological and geophysical data accumulated in successive implementation of a

244 SCIENCE IN CHINA (Series D) Vol 46

set of key research projects in the region during the eighth and ninth five-year plans provided us a certain scientific basis for the study of deep driving mechanism for recent tectonic movement and its controlling effect on strong earthquakes However no complete deep geophysical sounding profile was set up across the northern part of Sichuan-Yunnan region in the existing works Many basic problems such as the physical properties of subsurface media deep layer structure bound-ary outline of blocks and the relationship between them and the existence of anomalous body were not clarified Thus deepening research of the mechanisms for strong seismic activity in the region was obstructed For this reason by support of the 973 project the artificial seismic sound-ing and magnetotelluric sounding[2] were carried out in the West Sichuan-East Tibet region In this paper the result of magnetotelluric (MT) sounding is mainly presented

1 Field observation

Field magnetotelluric sounding was carried out in AugustmdashNovember of 2000 and the observation was performed at 76 sites According to the research purpose three MT profiles were laid out in the West Sichuan-East Tibet region The first profile was in the east-west direction and extended along the Sichuan-Tibet Highway from Ganlu Town (29deg52rsquoN 104deg46rsquoE) of Zizhong County Sichuan through Hongya Yarsquoan Kangding Yajiang Litang to Batang County (29deg59rsquoN 99deg05rsquoE) in West Sichuan for about 680 km in total length on which 48 observation sites were laid out The second profile was in SSW-NNE direction and extended from Sumdo Town of Daocheng County (29deg24rsquoN 100deg10rsquoE) Southwest Sichuan northeastward through Yajiang Dawu to Guanyinqiao of Jinchuan County (31deg41rsquoN 101deg39rsquoE) for about 350 km in length on which 18 observation sites were laid out In order to enhance control on the deep structure beneath the Xianshuihe fault zone another SSW-NNE-trending profile was added on the eastern side of the second profile It extended from Xinduqiao (30deg03rsquoN 101deg29rsquoE) northeastward through Qianning Danba to Anding Town of Jinchuan County (31deg17rsquoN 102deg01rsquoE) for about 170 km in length on which 10 observation sites were laid out The location of the sounding area and the dis-tribution of MT observation sites are shown in fig 1

The three sounding lines pass across the Xianshuihe fault and several large tectonic blocks in the study region (fig 1) The relief along the lines was complex and hence it was difficult to lay out the observation lines The spacing between observation sites was generally 15mdash20 km and densified on key structures and sections up to 5 km in individual areas

The instrument for field observation was two German sets of MMS-03e1) wide-band MT sounders produced by Metronix Company Germany The frequency band for recording was 256 Hzmdash4096 s For ensuring the picked information from the deep interior the record time was no less than 20 h at each site

1) Instruction and Userrsquos Guidebook of Software for MMS-03e Magnetotelluric Sounding System Metronix Company Germany 1995

Supp ELECTRICAL STRUCTURE SOUNDING OF EASTERN QINGHAI-TIBET PLATEAU 245

Fig 1 Tectonic background of the study region and locations of magnetotelluric sounding (MT) lines and observation sites in the region I Yangtze block II Songpan block III North Sichuan-Yunnan block IV Qamdo block

2 Analysis of the obtained data

The complex geological structures and large fluctuation of the relief in the sounded region make MT curves significantly distorted In order to extract real information of the electrical struc-ture in the region in addition to the conventional method for analysis of obtained data we used many analysis methods such as the distortion solution of impedance tensor Lillyrsquos Mohr cycle analysis and the tipple and inductance vector for grouping discriminating and correlating im-pedance and local distortion of structures in the region and determining the dimensional charac-teristics of structures in the region[3mdash6]

Analysis of data from all observation sites in the study region allowed us to obtain apparent electrical resistivity values and phase curves consistent with the characteristics of geological structures in the region These curves qualitatively reflect the following characteristics of the elec-trical structures in the region (1) The shallow structure (high-frequency band) is relatively simple and the deep structure (low-frequency band) is more complex (2) The major axes of electric structures at most observation sites are nearly consistent with the regional structures and are re-lated to the major fault zones The strike of regional structures in the whole region is mainly sub-longitudinal with the maximum deflection less than 30deg (3) Taking the Yarsquoan zone as a boundary the electrical resistivity of the crust-upper mantle in the eastern part is generally lower with a relatively weak electrical anisotropy while the electrical resistivity of the crust-upper man-tle in the western part is generally higher and sharply varies These characteristics indicate that the


crust and upper mantle in the western part are less stable and more complex in structure (4) The electricity is largely different on both sides of faults up to one to two orders of magnitudes lo-cally even much more The apparent resistivity and impedance phase curves are also different in their forms (5) Result of statistical analysis indicates that the small Mohr circle accounts for 64 or more It generally reflects that the region is dominated by 2D regional structures and the lateral anisotropy is not strong

3 Inversion and interpretation

During the last decade the methods for interpretating MT data have been rapidly developed One-dimensional (1D) inversion is fairly mature and two-dimensional (2D) inversion is also put in practice So the inversion and interpretation in the study were done mainly by using the 2D method and supplemented with the 1D inversion method

1D inversion can yield electrical parameters for the layering model corresponding to the ob-served curves In order to reduce the non-uniqueness of solution and improve the reliability of interpretation result we used many 1D-inversion methods for seeking out common characteristics of solution by different inversions in consideration of different constraints and for establishing an initial geoelectrical model for observation sites Then the thickness of strata is determined and the resistivity values of strata are more exactly determined from extreme value points on the resistiv-ity and impedance phase curves A 1D-inversion result for Batang-Zizhong (WE-trending) profile is given in fig 2

At present many methods exist for 2D inversion Of them the fast relaxation inversion and conjugate gradient inversion methods are more representative In this study we used the 2D fast relaxation auto-inversion method suggested by Smith and Booker [7] The method is more practical and yields better result With this method the horizontal gradient of electromagnetic field in the model is approximately calculated from the previous result of every iterations and only the con-ductivity disturbance amount below every observation site is calculated hence the problem of large partial derivative matrix for all model parameters in calculation of observation data is evaded An error information is brought in full play in the inversion and can control weight coefficient of data and automatic correlate distortion factor Thus the objectiveness and reliability of the interpretation result are largely improved

When the subsurface media are of 2D structure and the orientation of sounding lines is not strictly perpendicular to and does not largely depart from the strike of regional structure we can rotate MT impedance in a measurement coordinate system so as to obtain MT response parallel and vertical to the strike of regional structure (ie TE and TM modes) The concrete operation is that a straight-line perpendicular or approximately perpendicular to the strike of regional structure is drawn between the observation sites constituting the sounding line and its azimuth is deter-mined Then every point is vertically projected on the profile and simultaneously the impedance tensors are rotated so as E (TE) polarization is parallel to the regional structure strike and M (TM)


Fig

2

Ele

ctric

al st

ruct

ure

and

tect

onic

uni

ts a

long

the

Bat

ang-

Zizh

ong

(WE-

trend

ing)

pro

file

(com

pile

d fr

om 1

D in

vers

ion

resu

lt) G

eolo

gica

l cor

ridor

map

cite

d fr

om G

eolo

gica

lM

ap o

f Sic

huan

Pro

vinc

e 1

5000

00 E

29 o

bser

vatio

n si

te a

nd it

s num

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para

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yer (

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polarization is vertical to the regional structure strike According to the characteristics of regional structures in different zones the apparent resistivity and impedance phase data of different polari-zation modes are chosen for fitting inversion in order to make optimal fitting of multi-parameters on the whole cross-section Theoretical response of the 2D-inversion model and the fitting degree of the observed data are given in fig 3 The 2D-inversion results of three profiles are given in Plate I

Fig 3 Fitting degree of the 2D-inversion model (ρ ϕ) along an NE-trending profile (section from 0 to 340 km)

4 Analysis of results

The MT sounding result revealed that the electrical structure of the subsurface media in the study region is regular though the geological structure of the region is complex The results of 1D-inversion and 2D-inversion and interpretation along the Batang-Zizhong (WE-trending) pro-file (fig 2 and Plate I-1) and the 2D-inversion and interpretation results along NNE-trending Xiangcheng-Gora and Xinduqiao-Jinchuan profiles (Plate I-2 3) clearly show that the electrical structure of subsurface media in the study region is multi-layer in the longitudinal direction gen-erally 5mdash6 electrical layers can be delineated with exception of individual sites The lateral structure is of block character The region is separated by two electrical gradient zones corre-sponding to the Xianshuihe and Longmenshan fault zones into eastern middle and western elec-


trical blocks (or areas) of different electrical properties which are clearly controlled by the re-gional geological structures

Here the basic characteristics of electrical structure of blocks along the profiles are described as follows

Yangtze block (eastern block) is located east of Longmenshan fault zone and is a tectonic region from the western part of Yangtze Platform to Chengdu Basin Sichuan[8] Totally 13 obser-vation sites were arranged in the block The structure of electrical layer in the block is compara-tively simple electrically well stratified and laterally correlative The resistivity of the shallow part is lower and varies in a range from several to tens Ωmiddotm Its thickness varies and is the maximum in Hongya zone up to more than ten km But the layer tends to thin on its both sides and varies laterally It may be related to the sedimentary environment and subordinate block structures It is known from the regional geology that this set of thick low-resistivity layers mainly consists of a suit of lower metamorphosed basement rocks neritic debris rocks and continental sediments The result shows that this set of low-resistivity layers is similar to that of large thick-ness in many areas of Yangtze Platform detected by us in the 1990s

The lower crust along the profile has its resistivity from tens to hundreds Ωmiddotm and is a rela-tively high-resistivity and more homogeneous conductivity layer of relatively simple structure No high-conductivity layer was found The result of 1D-inversion and interpretation shows that an electrical discontinuity exists at a depth of about 32mdash35 km beneath nearly all the observation sites The resistivity of lower crustal media is some higher We suppose that it may be the Moho Discontinuity below the lower crust

The upper mantle high-conductivity layer occurs at a larger depth and consistently varies in a range of 200mdash300 km The above described characteristics reflect that the block is a stable block

Moreover it can be seen in profiles obtained by 1D- and 2D-inversions that the eastern boundary of the block is an east-dipping gradient zone of dense resistivity isolines which extends downward to the lower crust The electricity of both sides of the block is clearly different It well reflects the distribution condition of the Longmenshan fault zone in depth

Songpan block (middle block) is located between Yangtze block and the northern part of rhombic Sichuan-Yunnan block and tectonically belongs to the Songpan fold belt[12] The study region is significantly affected by Xianshuihe and Longmenshan fault zones The interpretation result of data from 12 observation sites indicate that the upper crust represents a high-resistivity block of large thickness except that a part of observation sites show a set of surface relatively low-resistivity thin layers of hundreds Ωmiddotm which is inferred to be related with the weathering

of rocks The block has its resistivity of several thousands Ωmiddotm on a large-scale and extends laterally westward to tens km west of Xianshuihe fault zone and longitudinally downward to several to tens km Its base plane largely fluctuates Analysis of surface outcrops indicates that the block in this area consists of large granite intrusions Precambrian metamorphic rocks and Kang-


ding complex rocks in the form of rock bodies or dykes Result of 1D-(and 2D-)interpretation shows that an electrical layer similar to the sedimentary cover was detected below the observation site j11 Its upper part has middle-low resistivity 250mdash400 Ωmiddotm and the middle part has

sub-high resistivity around 600 Ωmiddotm Thickness of the layer is more than ten km Analysis of regional geological data indicates that the layer is composed of Triassic-lower Paleozoic deposits metamorphic rocks or relict metamorphic basement rocks It is inferred that the block has under-gone multi-phase tectonic movement which has disturbed the mass in the mantle and hence made the magma to intrude along deep faults and cracks so that to form the recent tectonic framework of the area

The intracrustal high-conductivity layer is lacking within the block A gradient zone with a set of dense resistivity isolines dipping eastward at an angle of 45deg clearly exists beneath the Xianshuihe fault zone and extends down to the lower crust It can be seen in fig 2 that a sud-den-change zone of the conductivity layer structure in the upper mantle appears below the gradi-ent zone that is its occurrence depth changes from 140 km on the western side (beneath site j12) gradually down to 220 km on the eastern side (beneath site e09) with a fall up to 80 km The deep condition indicates the occurrence of Xianshuihe fault zone down to the sudden-change zone of the upper mantel structure Thus we can suppose that the fault zone is a large-scale lithospheric fault

Northern Sichuan-Yunnan block (western block) covers the whole western part of the sounded region from west of Xianshuihe fault zone to east of Jinshajiang fault and tectonically corresponds to the western part of rhombic Sichuan-Yunnan block

The block has its fairly distinct electrical structure In addition to a suit of sedimentary cover of middle electricity developed in the shallow part a large-scale low-resistivity body with its re-sistivity only of several to tens Ωmiddotm exists in the crust within the whole sounded region Its

thickness reaches 20mdash30 km in general and its occurrence depth varying significantly at 7mdash8 km

depth in minimum and at 40mdash50 km bottom depth in maximum This body extends eastward and

downward to the northeast at an angle of about 45deg We suggest that it may be a trace of lateral squeeze and eastward flow of the mass from the Qinghai-Tibet Plateau Many scientists have also proposed this suggestion[9mdash11] The MT sounding result provides a best evidence for the sugges-tion

It can be seen in the 2D profile that the high-conductivity body is not completely continuous in the block It reflects the different structural characteristics implying different tectonic mecha-nisms A clear discontinuity exists beneath Garzecirc-Litang fault and may be a reflection of the fault in depth A clear upward protrusion of the body occurs beneath the e27mdashj01 section thus the body thins and occurs at a smaller depth of 5 km just beneath the Yidun island-arc zone The available data indicate that a large number of island-arc magmatic rocks are distributed in this zone [12] and was inferred to be probably related with the upwelling of mantle-derived mass But


its mechanism remains to be further studied The lower crust media (the crust-upper mantle transition layer) has its resistivity of tens to

hundreds Ωmiddotm homogeneous electricity and relatively simple structure The occurrence depth

of the conductivity layer in the upper mantle in the region becomes clearly shallower about 80mdash120 km It implies that the western block is in a strongly active area of the crustal structure

In addition to the described above the Jinshajiang fault zone at western termination of the sounded region became a hotspot concerned by geologists The sounding line did not pass through the fault zone but three observation sites controlled it The 1D- and 2D-inversion results show that its electricity is fairly different from that of block east of it A set of middle-high-resistivity layers of 200mdash800 Ωmiddotm is developed in this zone The gradient zone of dense resistivity isolines is sharp and steep and extends downward The occurrence depth of high-conductivity layer in the upper mantle is around 80 km beneath the fault zone It is inferred from the existing MT data that the Jinshajiang fault zone is a large-scale deep-cutting lithospheric fault

5 New recognition and conclusions

The results of magnetotelluric (MT) sounding and study of the eastern border region of Qinghai-Tibet Plateau in combination with regional geological analysis permitted us to get the following recognition and to draw the following conclusions

(1) Deep structural blocks in the sounded region and their boundaries are fairly clearly ex-pressed in their electrical properties and well correspond to the regional geological structures

(2) Three large-scale blocks the Yangtze Songpan and northern rhombic Sichuan-Yunnan blocks are fairly different in the electricity of their subsurface media The Songpan block is a high-resistivity and relatively stale and rigid block The Yangtze block (Sichuan basin) has more homogeneous conductivity of its subsurface media and relatively simple structure No intracrustal high-conductivity layer was found in the block The upper mantle high-conductivity layer occurs in larger depth and is of a stable block character It can be inferred from the electricity of the me-dia that the recent northern rhombic Sichuan-Yunnan block is in a thermal state and is one of the recent active blocks The authors suggest that the different media between the blocks and on both sides of them may be an important deep background for the strong seismic activity

Moreover analysis of the MT sounding result allowed us to know that the characteristics of crustal media in the lower part of Songpan block are different from those of Yangtze and northern Sichuan-Yunnan blocks Thus the attitude and tectonic mechanism of the block remain to be fur-ther studies

(3) The Xianshuihe fault zone is a large-scale lithospheric fault It occurs steeply in subsur-face depth dips gradually to northeast at an angle of 45deg and extends downward beneath the Songpan and Yangtze blocks It is clear that it controls the structure of the blocks and represents a main boundary fault of the eastern part of active Sichuan-Yunnan block Meanwhile the result of MT sounding can also answer that the western boundary of Songpan block is the Xianshuihe tec-


tonic zone rather than the Garzecirc-Litang fault zone (4) The Longmenshan fault zone is clearly shown in the electrical structure The regional

geological data indicate that the fault zone has thrust from west to east in the shallow part The result of MT sounding indicates that the fault zone extends steeply downward into depth and gradually to southeast So it is a supracrustal fault next to the Xianshuihe fault zone in size The Jinshajiang fault zone is a steep downward and large-scale lithospheric fault The Garzecirc-Litang fault zone is a subordinate tectonic boundary fault

(5) It is clearly found that a trace of lateral flow of the mass exists at several to tens km depth beneath the eastern border of Qinghai-Tibet Plateau Why does a weak tectonic variation layer (or plastic layer) exist We consider that a best explanation is that the intense collision between Indian and Eurasian plates led the Qinghai-Tibet Plateau to rise and laterally squeeze and hence caused the mass in crust upper mantle and blocks in the periphery of the plateau to be disturbed mi-grated and laterally squeezed out and then to be obstructed by the Songpan rigid block so as to form a specific tectonic layer

(6) As regards the upper mantle asthenosphere it is also clearly shown in the profile obtained by 2D-inversion 1D-inversion and interpretation yielded 23 higher-quality data of electrical layer in the upper mantle Although the data are insufficient at all they can still reflect a basic frame-work and its variation characteristics of the upper mantle asthenosphere in the sounded region The MT sounding result shows a general occurrence depth of the conductivity layer in the upper mantle it gradually deepens from 80 km in the west (beneath northern rhombic Sichuan-Yunnan block) eastward to 240 km (beneath Yangtze block) But the occurrence of the upper mantle as-thenosphere beneath northern rhombic Sichuan-Yunnan block is shallower (the lithosphere thins) at 80mdash120 km depth reflecting a relatively active crust-mantle block The occurrence of the as-

thenosphere is deeper (the lithosphere thickens) beneath Yangtze block at 200mdash240 km depth reflecting a relatively stable crust-mantle block and is clearly at the sudden-change zone beneath the Xianshuihe tectonic zone This zone is an important tectonic transition zone and is worth deeply studying

The above-described characteristics well reflect a good correlative relation between the shal-low structures and deep processes

Acknowledgements The authors thank the MT Sounding Team of Zhejiang Oil Prospecting Division for its help in field data collection Associate Prof Tan Handong from China University of Geological Science for his help in data interpretation Thanks are also due to Zhong Dalai Prof Pan Yusheng from Institute of Geology the Chinese Academy of Sciences for their beneficial instruction in discussions of the work and to Prof Wang Yipeng for his direction in our work This work was supported by the National Key Basic Research and Development Project ldquo973rdquo (Grant No G9513-02-03)

References

1 Kan Rongju Zhang Sichang Yan Fengtong et al Discussion on characteristics of recent tectonic stress field and recent tectonic movement in southwestern China Acta Geophysica Sinica (in Chinese with English abstract) 1977 20(2) 96mdash108

2 Chen Leshou Wang Guangrsquoe Megnetotelluric Sounding Method (in Chinese) Beijing Geological Publishing House


1990 3 Bahr K Geological noise in magnetotelluric data a classification of distortion types Phys Earth Planet Science 1991

66(1) 24 mdash38 4 Bahr K Interpretation of the magnetotelluric impedance tensor regional induction and local telluric distortion Geophys-

ics 1998 62(1) 119mdash127 5 Lilley F E M Magnetotelluric tensor decomposition Part I Theory for a basic procedure Geophysics 1998 63(6)

1885mdash1897 6 Jin Guangwen Sun Jie Jiang Zhao Invariable of magnetotelluric impedance tensor and Mohr circle analysis of it Seis-

mology and Geology (in Chinese with English abstract) 1995 17(4) 439mdash445 7 Smith J T Booker J R Rapid inversion of two- and three-dimensional magnetotelluric data J Geophys Res 1991

96(B3) 3905mdash3922 8 Geological Bureau of Sichuan Province Regional Geological Records of Sihuang Province (in Chinese) Beijing

Geological Publishing House 1982 9 Xiong Jiong Relationship between eastward flow of mass and strength of strata in Qinghai-Tibet Plateau in Research on

Recent Crustal Movement and Geodynamics (3) Study on Recent Lithospheric Movement and Dynamics of Qing-hai-Tibet Plateau (in Chinese) Beijing Seismological Press 2001 177mdash184

10 Shen Jun Wang Yipeng Ren Jinwei Quaternary dextral strike-slip movement along Deqin-Zhongdian-Daju fault zone in Yunnan China in Research on Recent Crustal Movement and Geodynamics (3) Study on Recent Lithospheric Movement and Dynamics of Qinghai-Tibet Plateau (in Chinese) Beijing Seismological Press 2001 123mdash135

11 Teng Jiwen Physics and Dynamics of Lithosphere along Kang-Yunnan Tectonic Zone (in Chinese) Beijing Science Press 1994

12 Pan Yusheng Geological-Structural Zoning of Hengduan Mountains Region (in Chinse) Beijing Science Press 1994




1 Field observation





References


set of key research projects in the region during the eighth and ninth five-year plans provided us a certain scientific basis for the study of deep driving mechanism for recent tectonic movement and its controlling effect on strong earthquakes However no complete deep geophysical sounding profile was set up across the northern part of Sichuan-Yunnan region in the existing works Many basic problems such as the physical properties of subsurface media deep layer structure bound-ary outline of blocks and the relationship between them and the existence of anomalous body were not clarified Thus deepening research of the mechanisms for strong seismic activity in the region was obstructed For this reason by support of the 973 project the artificial seismic sound-ing and magnetotelluric sounding[2] were carried out in the West Sichuan-East Tibet region In this paper the result of magnetotelluric (MT) sounding is mainly presented

1 Field observation

Field magnetotelluric sounding was carried out in AugustmdashNovember of 2000 and the observation was performed at 76 sites According to the research purpose three MT profiles were laid out in the West Sichuan-East Tibet region The first profile was in the east-west direction and extended along the Sichuan-Tibet Highway from Ganlu Town (29deg52rsquoN 104deg46rsquoE) of Zizhong County Sichuan through Hongya Yarsquoan Kangding Yajiang Litang to Batang County (29deg59rsquoN 99deg05rsquoE) in West Sichuan for about 680 km in total length on which 48 observation sites were laid out The second profile was in SSW-NNE direction and extended from Sumdo Town of Daocheng County (29deg24rsquoN 100deg10rsquoE) Southwest Sichuan northeastward through Yajiang Dawu to Guanyinqiao of Jinchuan County (31deg41rsquoN 101deg39rsquoE) for about 350 km in length on which 18 observation sites were laid out In order to enhance control on the deep structure beneath the Xianshuihe fault zone another SSW-NNE-trending profile was added on the eastern side of the second profile It extended from Xinduqiao (30deg03rsquoN 101deg29rsquoE) northeastward through Qianning Danba to Anding Town of Jinchuan County (31deg17rsquoN 102deg01rsquoE) for about 170 km in length on which 10 observation sites were laid out The location of the sounding area and the dis-tribution of MT observation sites are shown in fig 1

The three sounding lines pass across the Xianshuihe fault and several large tectonic blocks in the study region (fig 1) The relief along the lines was complex and hence it was difficult to lay out the observation lines The spacing between observation sites was generally 15mdash20 km and densified on key structures and sections up to 5 km in individual areas

The instrument for field observation was two German sets of MMS-03e1) wide-band MT sounders produced by Metronix Company Germany The frequency band for recording was 256 Hzmdash4096 s For ensuring the picked information from the deep interior the record time was no less than 20 h at each site

1) Instruction and Userrsquos Guidebook of Software for MMS-03e Magnetotelluric Sounding System Metronix Company Germany 1995














Fig

2

Ele

ctric

al st

ruct

ure

and

tect

onic

uni

ts a

long

the

Bat

ang-

Zizh

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(WE-

trend

ing)

pro

file

(com

pile

d fr

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D in

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resu

lt) G

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d fr

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29 o

bser

vatio

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nd it

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para

met

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r ele

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in u

nit Ω

middotm

)












































References


















1 Field observation





References














Fig

2

Ele

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ruct

ure

and

tect

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uni

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long

the

Bat

ang-

Zizh

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(WE-

trend

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(com

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D in

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cite

d fr

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f Sic

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Pro

vinc

e 1

5000

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29 o

bser

vatio

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nd it

s num

ber

para

met

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r ele

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yer (

in u

nit Ω

middotm

)












































References


















1 Field observation





References












































References


















1 Field observation





References
















1 Field observation





References

Sounding of electrical structure of the crust and upper mantle along the eastern border of...

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