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Chinese Science Bulletin

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Magnetostratigraphy of the Zanda basin in southwest

Tibet Plateau and its tectonic implications

WANG ShiFeng1, ZHANG WeiLin1, FANG XiaoMin1, DAI Shuang2 & Oliver KEMPF3

1

Institute of Tibetan Plateau Research, Chinese Academy Sciences, Beijing 100085, China;

2

Key Laboratory of Western China's Environmental Systems, Ministry of Education of China & College of Resources and Environ-

ment, Lanzhou University, Lanzhou730000, China;

3

Institut f r Umweltgeologie, Technische University Braunschweig, Braunschweig 38106, Germany

The Zanda basin is one of the very important basins at the north slope of the Himalaya Range. Thus the

study of the basin strata will provide critical information about the tectonic evolution of the Himalayan

Orogenic Belt. 268 oriented block samples were collected in the 750-m-thick sections of the Zanda ba-

sin. The characteristic remanent magnetization (ChRM) was isolated that decays linearly to the origin

between 500oC and 690oC for most studied samples. An age range of 9.5 2.6 Ma was estimated from

the correlation between our observed polarity column and the Geomagnetic Polarity Time Scale (GPTS).

The age of the Zanda basin does not support the models that the South Tibetan Detach system (STDS)

is one of the basin controlling faults. Given the sedimentological features in the basin and the tectonic

features at the north edge of the basin, the Zanda basin was a half graben that was possibly controlled

by the Karakorum fault on the northeast.

Zanda basin, magnetostratigraphy, basin controlling fault, Karakorum Fault, South Tibetan detachment system (STDS)

Continuing convergence between the Indian and the 7.0 6.7 Ma, the rapid uplift of the Himalayan Orogenic

Eurasian plates since 60 5 Ma ago led to the formation Belt occurred after 3.6 Ma.

of the Himalayan Orogenic Belt and the uplift of the Tectonically similar to the Gyirong Basin, the Zanda

Tibetan Plateau[1,2]. Opinions about the origin and evo-

GEOPHYSICS

basin is another very important basin at the north slope

lution of the Himalayan Orogenic Belt are widely dif- of the Himalaya Range. Due to the well preserved, con-

ferent and contradictory. For example, the study on the tinuous stratigraphic sequence and abundant fossils in it,

fault that controls the Thakkhola graben in the middle the Zanda basin is a prominent site for the study of

part of the Himalaya Range indicated the Belt attained sedimentology, climate change and south Tibetan tec-

its present elevation 14 Ma ago[3]. Based on the study of tonic evolution. The stratigraphy of the Zanda basin

C4 grasses in the diets of ancient mammals in the Gyi- consists of a suit of lacustrine and fluvial deposits in late

rong Basin at the north slope of the Himalaya Range, Cenozoic[9 11]. Eighty paleomagnetic block samples

Wang et al.[4] argued that the Belt was only 2900 3400 were collected in the 750-m-thick sediments, its pre-

m a.s.l. around 7 Ma. Shi et al.[5] and Xu et al.[6] held liminary magnetostratigraphic chronology shows that

that the Belt has risen at least 3000 m since Pleistocene sedimentation initiated from 7 Ma[12]. In the recent two

based on the study of plant fossil assemblage at the years, detailed sedimentary and magnetostratigraphic

Mount Xixabangma, and Huang et al.[7] further sup-

Received September 7, 2007; accepted December 26, 2007

ported this opinion with their first discovery of Hip- doi: 10.1007/s11434-008-0132-9

parion fauna in the Gyirong Basin. Magnetostratigraphic

Corresponding author (email: abpmrq@r.postjobfree.com)

data[8] in the Gyirong Basin shows that the elevation of Supported by the National Natural Science Foundation of China (Grant Nos.

40672142 and 40334038) and China National Key Project (Grant Nos.

south Tibet was almost the same as north China during 2005CB422000 and 2002CB412601)

Chinese Science Bulletin May 2008 vol. 53 no. 9 1393-1400

www.scichina.com csb.scichina.com www.springerlink.com

data were obtained in our Sino-Germany cooperation. posed of thick conglomerates with thin, grey-white beds

Part of the results will be presented in this article. of limestone and mudstone; they belong to interbedded

lacustrine and fluvial facies.

1 Geologic setting

2 Magnetostratigraphy of Zanda basin

The study area is located at the boundary between the

Himalayan orogenic belt and the Lhasa block. The 2.1 Sampling and analytical methods

basement in the west and southwest of the basin consists The section of sampling is from 20 km west of Zanda

of the Tethyan Himalayan arc, which forms the hanging Town to Bolin Village toward the southwest (Figure 1).

wall of the South Tibetan detachment system (STDS). Outcrop of the section is continuous and well preserved.

Zhu et al.[11] mentioned that there are two unconformity

Northeast of the basin is the Aylari Range where the

Zsangpo-Indus suture is situated, and the modern trace in the Zanda sediment sequence: the upper one is be-

of the Karakorum fault passes through the north and tween the Xiangzi Formation (Q1x) and the Zanda For-

south sides of the Range. The Zanda basin stretches in mation (N2z), and the lower one is between the Zanda

the NW-SE direction and is 150 km long and 20 50 km Formation (N2z) and the Tuolin Formation (N2t). But our

field work suggested that the sequence of Zanda Forma-

wide, trumpet-like in plane view. The almost horizontal

tion (N2z) and the Tuolin Formation (N2t) is continuous,

strata of the Zanda basin, superposed on the Jurassic and

and the lower unconformity identified by Zhu et al.[11]

Cretaceous shale and limestone, consist of weakly con-

represents a stream flow sedimentary structure. This

solidated clastic rocks with about 800 m maximum

view is according to the previous studies[12,13]. The upper

thickness. The sediment in the Zanda basin can be di-

vided into 3 parts[20]: The basal strata in the basin are unconformity interface is the upper boundary of our

sampled section. Considering the previous experience of

made up of partly imbricated conglomerate associated

sampling in northeast Tibet[14 16], samples were taken at

with coarse-grained, cross-bedded sandstone. The vari-

2 3 m stratigraphic intervals along 0.5 1-m-deep

ously stratified sandstones are interpreted as fluvial

channel deposits, while the fine-grained facies is the trenches through the entire length of the sections. Ex-

ceptions were made for conglomerates where sampling

products of overbank or swamp deposits. The middle

intervals were dependent on the availability of finer-

section in the basin is mainly siltstone associated with

grained lenses, so that some samples are about 5 m apart.

mudstone and marl which are mainly lacustrine and del-

All the samples are finer sandstones or mudstones to

taic facies. The strata on the top of the basin are com-

Location and tectonics map of the Zanda basin.

Figure 1

WANG ShiFeng et al. Chinese Science Bulletin May 2008 vol. 53 no. 9 1393-1400

1394

ARTICLES

of the samples show a clear decrease in magnetization

ensure reliability and accuracy. A total of 268 sites were

collected after two years of sampling and resampling. At around 350 400, accompanied by a clear change of

each site, one large oriented block was collected from remnant direction, indicating that the removal of a secon-

which two cubic sub-samples of 2 cm 2 cm 2 cm were dary remnant magnetization (SRM) stored by magnetiza-

taken respectively. A total of 387 sub-samples were tion. Further rock magnetism study should be necessary if

measured in the laboratories. the magnetic carriers are clearly known.

Systematic stepwise thermal demagnetizations (at

2.2 Magnetostratigraphy

least fourteen discrete steps between 50 and 690 at

Paleomagnetic directions were determined using

intervals of 50 below 550 and 10 20 above it)

Kirschvink principal component analysis of demagneti-

were done on the first set of specimens collected in 2005 zation patterns[17] in each sample, and the final ChRM

and second set of samples collected in 2006. Remnant direction at each site was obtained by Fisher averaging

intensities and directional measurements were done on a of the directions from the two suits of samples in 2005

2G Enterprises magnetometer in a magnetically shielded and one suit in 2006, totaling 384 specimens in 268 sites.

room, first in the Paleomagnetism Laboratory of the In-

Samples with maximum angular deviation (MAD) of the

stitute of Geology and Geophysics (Chinese Academy of ChRM greater than 15 and magnetizations with virtual

Sciences) and then in the Paleomagnetic and Rock geomagnetic pole (VGP) latitude values less than 20o

Magnetic Laboratory of Institut fur Geowissenschaften, are excluded. A total of 31 horizontal levels in the col-

Universitat Tubingen.

umn were excluded. The final remnannt directions are

Representative thermal demagnetization diagrams are averages for each level and then they are used to calcu-

shown in Figure 2. Most samples show simple demag- late VGPs which are plotted as a function of thickness

netization behavior. Many samples show a clear decrease after various paleomagnetic tests.

in magnetization around 200, accompanied by a clear A statistical bootstrap technique[18] has been used to

change of remnant direction, indicating that a viscous test whether the distributions of the ChRM vectors are

remnant magnetization (VRM) is readily removed. possibly non-Fisherian, and to characterize the associ-

Above 500 a characteristic magnetization (ChRM) is ated uncertainties for both normal and reversed ChRM

clearly isolated and decays nearly linearly to the origin. directions. The result of first reversal test shows that the

Most samples show an accelerated decay in remnant in- error of the average ChRM angle value is about 10 .

tensity just below 580 (Figure 2(d)) or 680 (Figure This means that there are still unsteady samples besides

the 31 sites. The retested result is good after excluding

2(a),(b),(c)), indicating that magnetite and hematite are

the major magnetic carriers in these rocks. A small part samples with remnant intensity that does not reach the

GEOPHYSICS

Figure 2 Thermal demagnetization diagrams of four representative samples with the sample number corresponding to the stratigraphic height in the

Zanda basin. Full (open) symbols represent projections onto the horizontal (vertical) plane.

WANG ShiFeng et al. Chinese Science Bulletin May 2008 vol. 53 no. 9-139*-****-****

Figure 3 (a) Equal area projections of all accepted ChRM directions and the normal and reversed polarity mean directions (with ovals of 95% confidence)

determined with the bootstrap method[18]. (b) Magnetostratigraphic jackknife analysis[19] for the Zanda section. The slope (J ) has a value of 0.2198, which

suggests that the section s record has recovered more than 95% of the number of polarity intervals. (c) Bootstrap reversal test diagram. Reversed polarity

directions have been inverted to their antipodes to test for a common mean shared by the normal and reversed magnetization directions. The confident

intervals for all components overlap, indicating a positive reversal test.

the samples belong to original remnant magnetization.

original beyond 690 . Figure 2(a) shows an equal-area

Four cheek teeth of Hipparion zandaense found at

projection of all 237 accepted ChRM directions of the

347 m of our Zanda section provide robust constraints

Zanda section. The histograms of the Cartesian coordi-

on our interpretation of the observed polarities. The

nates of bootstrapped means allow us to determine a

Hipparion zandaense was first postulated to be in late

95% level of confidence (ovals around the means in

Miocene period by Li Fenglin and Li Daliang in 1990.

Figure 2(a)) and to demonstrate that the bootstrap rever-

The holotype of Hipparion zandaense is a skull and jaw,

sal test is positive (Figure 2(a)). Furthermore, a jack-

knife technique[19] was used to quantify the reliability of first discovered in grey, grey-brown sandstone of Tuolin

Formation around Daba area 31 km south of Zanda

the magnetostratigraphy. The obtained jackknife pa-

Town. Similarly, Hipparion gyirongensis discovered in

rameter (J ) for the accepted sample-mean directions has

a value of 0.2198, which falls within the range of 0 to the Gyirong Basin records a late Miocene age, about 8

0.5 recommended by Tauxe and Gallet[19] for a robust 5.3 Ma as constrained by magnetostratigraphy in the

Gyirong Basin[8], and the first appearance of Hipparion

magnetostratigraphic data set, indicating that sampling

in north China was about 12 Ma. Moreover, there are

of the section has recovered more than 95% of the true

many other animal fossils discovered in the basin[22], for

number of polarity intervals (Figure 3(b)). Due to the

example, Ochotona sp. in the lower part of the section,

almost horizontal bedding in the section it is very diffi-

cult to perform McElhinny fold test[20], but the jackknife Adelinella regularis Y, Velutinopsis spiralis Y, Radix

Zandaensis sp. Nov, Hippeutis sp., and Dicerorhininae

and the bootstrap techniques assure that the ChRM of

WANG ShiFeng et al. Chinese Science Bulletin May 2008 vol. 53 no. 9 1393-1400

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in the upper part of the section. All the fossils are attrib- and long reversed intervals R2, R6 can be readily corre-

uted to Late Miocene to Early Pleistocene. So, the sedi- lated to the characteristic long normal and reversed

ment record in the Zanda basin should be younger than chrons C4n, C4An, C2Ar and 3r, respectively. Based on

12 Ma. these controlling polarity intervals, the N7-N12, R7-R12

Figure 4 shows a thickness vs. VGP plot of all the below the R6 can be correlated to C3An-C3Br of the

accepted and tested ChRM directions. It shows that there GPTS, the R13, N14, R14 between N13 and N15 corre-

are a total of 15 normal and 15 reversed polarity inter- lated to 4r. Then, R3-R5 are correlated to reversed

vals in the Zanda section, marked as N1-N15 and chrons in C3n, and N3-N6 to normal chrons in C3n. Fi-

R1-R15, respectively. The observed polarities can be nally, the N1, R1, N2 in the top of the section can be

correlated well with chrons 2An-4Br of the Geomag- correlated to C2An. Magnetic sample is not collected at

netic Polarity Time Scale (GPTS)[21] for most of the sec- the bottom and top of the section due to the coarse grain

of the sediment, but the age can be deduced by the

tion. First, the strikingly long normal intervals N13, N15

GEOPHYSICS

Correlation of the magnetostratigrahy of the Zanda section with the GPTS of Cande nad Kent[12].

Figure 4

WANG ShiFeng et al. Chinese Science Bulletin May 2008 vol. 53 no. 9-139*-****-****

sediment thickness and the sedimentation rate in the sec- thrust fault at the north edge of the basin and the strata

tion. Thus, the sedimentation in the basin was initiated at of this part should experience strong deformation, but

9.5 Ma, and the age of the upper unconformity is about this is not the case, for the depocenter of the basin is

located at south side of the basin[23,25] and the beds of the

2.6 Ma.

Figure 5 presents a thickness-vs.-age plot of the main basin are almost horizontal with weakly deformed beds.

chrons. It shows a linear relationship between the sam- Moreover, the last stage of STDS activity is before

11Ma[26 29], but the sediment in the basin is from 9.5 to

pling thicknesses and ages that agrees to a first order

(long-term change) with the lithologic change, the sedi- 2.6 Ma. This means that the STDS has no influence on

mentation rate also corresponds well to the lithological the basin evolution and it is not the basin-control fault.

Zhu et al.[11] suggested the basin model as a graben, due

change. For example, the sedimentation rate of the peb-

bled sandstones in the upper part and lower part of the to the STDS being not active since sedimentation of the

section (7 Ma) are 13.7 cm/ka and 11.9 basin started. The Zanda basin is strictly not a graben.

Shao et al.[24] argued that the basin was controlled by

cm/ka, respectively, but the sedimentation rate of the

fine-grained mudstone in the middle part of the section normal fault at south edge of the basin and by thrust

is only 10.1 cm/ka. The sedimentary features in Zanda fault tilted SW at north edge of the basin, so that the

basin lends further support to our interpretation. basin should be a seesaw type (extensional-compres-

sional basin). But this model also has several shortcom-

ings. First, the STDS (normal fault) is not active since

the basin started to accept sediment, and thus it is not the

basin-control fault. Second, if the north edge of the basin

controlled by the thrust fault is tilted SW, the position of

some bedding in the north side of the basin will change

from subsidence in the early stage to erosion in late

stage, but all the beddings in the north side of the basin

are continuous and almost at the same level. Third, the

basin is young in geological time and there should be

tectonic and geomorphic features of thrust fault in field

observation and satellite images. But the fault study at

the north edge of the basin suggests that[23] there are a

Figure 5 Thickness vs. age plots of the magnetic polarity chrons, sedi-

mentation rates are plotted also. series of E-W trending right lateral strike-slip faults with

normal fault component along the south side of the Ay-

3 Discussion and conclusions lari Range, which are branches of the Karakorum fault,

i.e., the basin is controlled by the Karakorum. Therefore,

3.1 Magnetostratigraphy constraining the active

the basin is not a seesaw type.

period of the Zanda basin-control faults

In summary, the magnetostratigraphy of the Zanda

Magnetostratigraphic data of the Zanda basin provide

basin shows that sedimentation in the basin started since

important evidence for the active faults controlling the

9.5Ma, and the basin is a half graben controlled by the

basin. Thus it is beneficial for exploring the issue of ba-

Karakorum Fault.

sin formation. There are four models about the Zanda

3.2 Magnetostratigraphy constraint on the fault

basin formation: compressional basin[9,10], graben[11,13],

activities

half graben[23] and extensional-compressional basin[24,25]

(Figure 6). Zhou et al.[9] argued that the south edge of Methods of study in fault chronology usually involve

thermal chronology (e.g. Ar39/Ar40, K-Ar, U-Pb, Sm/Nd)

the basin is controlled by the South Tibetan Detachment

by analysis of recrystallized minerals (e.g. k-feldspar,

System (STDS) and the north edge of the basin is con-

mica, muscovite, zircon) in the ductile shear zone. Due

trolled by the Aylari thrust fault which has a NE fault dip.

to the low temperature, there are little recrystallized

If this model is right, the sedimentary feature of the

minerals in brittle fault zones, in which low temperature

Zanda basin should have a character of foreland basin,

thermal chronology methods were usually applied, such

i.e. the depocenter of the basin should be close to the

WANG ShiFeng et al. Chinese Science Bulletin May 2008 vol. 53 no. 9 1393-1400

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Figure 6 The four basin type models of the Zanda basin. Model 1 modified after refs. [9, 10], model 2 modified after refs. [11,13], model 3 modified

after refs. [24, 25], model 4 modified after ref. [23].

as Fission track dating and U-Th-He dating using zircon beneficial for the question of brittle fault chronology.

and apatite. Their tectonic implications are related to There are a series of brittle strike slip fault developed in

uplift. But, till now, the age of brittle fault activity with late Cenozoic in Tibetan Plateau, such as the Xianshuihe

no relationship to uplift is very difficult to determine. Fault, the Red River Fault, the Haiyuan Fault, the East

The magnetostratigraphy in this article provides us an Kunlun Fault. The ages of these faults are still in debate

opportunity to discuss the time constraint of the fault due to the difficulties to find datable minerals. All these

activity based on the coupling between the sedimenta- strike slip faults transfer to thrust fault or pull-apart ba-

tion in the basin and the fault activity. sin at the end of the faults. Thus the magnetostratigraphy

The magnetostratigraphy in Zanda basin can be cor- study of the basins along the faults will provide an ap-

related to the thermal chronological date obtained from proach to an analysis of the fault age. For example, tens

the ductile shear zone. The 9.5Ma sediment age lags of kilometers offset along the Red River Fault trans-

behind the 12 10 Ma[30 33] U-Pb and Ar39/Ar40 age of ferred to the Dali Basin-range system at NW end of the

fault[38]. The sediment age in these basins will provide

the fault. If the sediment age represents the term of fault

an age of the fault. The same is the Xianshuihe Fault, in

activity, a reasonable interpretation of the delay is that

which the 60 km offset along the fault was transferred to

the formation of sediment basin will take thousands to

the pull-apart basins in Xiaojiang area[39]. The sedimen-

millions of years. For example, an 8.1 magnitude earth-

tation age in the basins will also provide information of

quake along the East Kunlun Fault can produce a 2 5

the fault chronology.

m-long sag-pot[34], but the development of a sediment

In summary, the accurate magnetostratigraphic date of

GEOPHYSICS

filled, pull-apart basin may take more than one million

the Zanda basin shows that the activity of the STDS is

years. This interpretation is attested by the fact that the

not coupled with the time of basin development. It is not

fault controlling the Thakkhola graben was initiated at

the basin-control fault. With sedimentary and tectonic

14 Ma[3], but the sediment in it was from 11 Ma[35]. An-

evidence, we show that the Zanda basin is a half graben

other interpretation of this lag is that the basin is pro-

controlled by the Karakorum Fault. The magnetostrati-

duced at a certain stage of fault activity, for example, the

graphic method provides a new insight in the fault

age of the Kunlun Pass basin along the East Kunlun

chronology based on the coupling of basin and fault de-

Fault is about 3.5 Ma[36], but the initiation time of the

velopments. The data of the magnetostratigraphy pro-

fault is about 12 10 Ma[37] as determined by the big-

vide an upper boundary of the fault activity.

gest offset along the fault and the Quaternary fault slip

rate, so that the sedimentary age in the basin provides an Thanks are given to Prof. Erwin Appel, Drs. Rachid El Bay and Ulrich

upper boundary of fault activity. If this is true, it will be Blaha for their help in the lab of Tubingen University.

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