Tien Shan (China) - Institut de Physique du Globe de Paris

MARCH IO. I99I
JOURNAL OF GEOPHYSICALRESEARCH,VOL. 96, NO. 83, PAGES4065--1082.
PaleomagneticStudy of MesozoicContinentalSedimentsAlong the Northern
Strainin CentralAsia
Tien Shan(China)andHeterogeneous
AVOUAC,IPRUTTAPPO}.INER,1
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A paleomagnetic snrdy of rocks from the northern foot of the Tien Shan and the southern
border of the Dzungar Basin, east of Urumqi (44.2"N, 86.008)' sparuring ages from middle
Jurassic to early Tertiary was carried out to constrain the tectonic evolution in central Asia
since Mesozoic time. Five middle Jurassic sites reveal a remagnetized direction close to the
presenr Earth field in geographiccoordinates:D = 6.6", I = 72.6" (o95 = 7.4o), Thirteen out of
l7 upper Jurassic and lower Cretaceous sites yield a characteristic direction (stratigraphic
coordinates)of D = 12.7o,1= 48.6o (495 = 5'5o)' Nine of 16 upper Cretaceousand lower
Tertiary sites provide a characteristicdirection of D = 12.5",1= 51.3o (495 = 6.9o). The latter
two directions pass fold and reversal tests. The pole positions are close to each other and to the
Besse and Courtillor [1989, 1990] Eurasian apparent polar wander path, for ages ranging from
130 to 70 Ma. However, the diffcrence in paleolatitudes amounls to about 5.9" t 3.7o, which
could indicate significant continental shortening in the Altai Mountains and perhaps further
north, subsequenito India-Asia collision. The pole positions from the Dzu_ngarBasin are close
ro those found for rhe Tarim [Li et al., 1988a], leading to an insignificant paleolatitude
difference (3.0" t 6.9o), but showing a larger difference in declination (8.6' t 8.7'). These
paleomagnetic results are compatible with a model of heterogeneous deformation in the
western part of the collision zone between India and Siberia. A significant shortening in the
Altai, a slight counterclockwise rotation of the Dzungar block, the westward-increasing
shortening in rh" Ti"n Shan with attendant clockwise rotation of the Tarim block are all
consisteni with this model, in which Tibet, the Tien Shan anà the Altai undergo differential
suain along strike in a relay fashion, with the total India-Siberia convergence remaining
approximately constant.
units in the Permian, whereas t, [1980] finds no evidence for a
suture between the two.
A significant amount of paleomagnetic research has
accreted
which
blocks
of
Central Asia consiss of a mosaic
and eastern
since the early Paleozoic. Based mostly on paleontology and recently been published or is underway in central
et al',
Tibet
collected
from
have
been
Data
Asia.
a
[Achache
boundaries,
tectonic
of
stratigraphy, and on the recognition
19841, South China [e.g., Kent et al., 1986: Enkin et al.,
number of blocks with different geological histories have now
China le.g.Lin,
been identified: Siberia, Tarim, Kazakhstan, North China, 1991a, â), and farther north from North
et al., 1988a, à]' This leaves the
Li
the
Tarim
19841
urd
the
[e.g.,
However,
blocks.
the
Tibetan
and
Indochina,
China,
South
block relatively unexplored. Wiù a surface
timing of individual collisions is still debated, sometimes important Dzungar
this large basin has traditionally been
kmz,
130,000
area
of
the
North
propose
that
al.
et
Li
For
instance,
hotly so.
[1982]
part
the
Kazakhstan, wedged between the Tarim
a
of
considered
and South China blocks were accreted in the Triassic, whereas
(Figures 1 and 16). Its boundary with the
blocks
and
Siberia
propose'a
al.
et
and
Mattauer
U9851
Laveine et al. ll987l
former lies along the Tien Shan range and with the latter along
Paleozoic age.
the Altai range, where ophiolite suites and ophiolite mélanges
data
of
source
provides
an
independent
Paleomagnetism
have
been found [tr et aL,1982;Zhang andWu,1985).
bearing on this problem. Opdyl<eet aI. [19861 use such data !o
In
recent years, research carried out in the Gansu corridor
China
versus
South
of
North
age
post-Triassic
favor a
Tarim and North China, and in the Tarim and Dzungar
berween
collision. McElhinny et al. ll98ll and Li et a/. [1988b]
has
focused primarily on Pgrmian and Triassic
basins
as
independent
behaved
China
believe that Tarim and North
formations [Bai et al., 1987; Li, 1988; Li et al., 1989]' Yet
the configuration of the Eurasian mosaic has been profoundly
I hrtitut de Physiquedu Globede ParisandDépanernent
desSciences altered by the India-Asia collision fMolmr and Tappornier,
1975; Tapponnier and Molnar, 1979; Tapponnier et al',
de la ærrc,UniversiréParisVtr.
2 CeEss. UniversiréRennesI
19861. The first results from the French-Chinese
paleomagnetic work in Tibet suggested some 2000 km of
3 C"nt." National de la RechercheScientifique, Laboratoirede
shortening between Tibet and Siberia lAchache et al.,19841
Paléontologie
desVertébés, Univenité ParisVI.
and total shortening could be even larger, based on revisions
4 Xitr.li"ng EngineeringInstitute,People'sRepublicof China.
of the apparent polar wander path of Eurasia lBesse and
5 Burruo of seismologyof Xinjiang, People'sRepublicof China.
Courtillot, 1991] and on the age of the onset of collision
lJaeger et al.,1989).
Copyright l99l by the AmericanGeophysicalUnion.
It is therefore important to reconstruct the precollisional
of Asia and to determine in a quantitative
paleogeography
Papernumber90J802699.
fashion the amount of subsequent intracontinental
0 | 48-022'tt9 | t90JB-02699$05.00
IMTRoDUC,NON
4065
CHEN ET AL: MESOZOIC CONTINENTAL SEDIMENTS
81.
Fig. l. Topographic map of Cenrral Asia showing main basins (larim and Danngar) and ranges (fibet, Pamir, Tien Shan and
Altai); contotrrs every 1000 m. The box near Urumqi corrcsponds to the studied area (Figurc 2) [aher Tapporuier and Molnar,
1979; l.P. Avouac eI aJ., manuscript in prcparation, l99ll. The inset is a sketch map of the main tectonic unirs (see also
Figuæ 16).
deformation. This requires collection of Mesozoic and
Cenozoic formations, which have apparently been less
studie4 particularly in the Dzungar block. This paper reports
the first results of a paleomagnetic sampling in the region
west of Urumqi and north of the foot of the Tien Shan
Mountains (44.2'N, 86.0"E), along the southern edge of the
Dzungar basin (Figure 2).
Grolocynro
SewuNc
The well-exposed rock sequencesgenerally consist of soft,
coarse-grained continentâl sediments. Some harder strata were
found to be better suited for paleomagnetic sampling. Drill
cores (445) were taken from 38 sites in rocks dating from the
middle Jurassic to the early Tertiary. Dating the continental
*
r
qu*.,t*y
EII Lowereuatemary
g"66
E
E
siæ
El
;ulassiç
l-'l
Ct"t**us
@
Triassic and older
EEI
Roadarduill"g"
ffi
F"ult
E
r*-
Fig. 2. Schcmaric geological map of sampling arca, including site locations (solid circles). Cross sections AA' and BB' are
shown in Figure 3.
+067
CHEN ET AL: MESOZOIC CONTINENTAL SEDIMENTS
rocks of the Dzungar basin with some accuracy is difficult
our
because of the scarcity of fossils' Thus' we divided
paleomagnetic sampling into three age sequences: middle
iurassic.-late Jurassic-early Cretaceous and late Cretaceousearlv Tertiary. The sites are distributed along a 120-km-long
streich west of Urumqi (Table I and Figure 2)'
WanS U9851, this formation is Bathonian to Callovian in
age. Ostracodes reported from this formation, such as
T-imir iasevi a c aten ular i a M and elst atn and D arw inula imp udic a
Sharapova also suggest a Middle Jurassicage (J. F' Babinot'
personal communication, 1989). From this sequence we
coilected a total of 60 samples in five sites in deep-red suata
along the east side of the Totuenhe River (fable I and Figure
(J2)
Middle Jurassic Toluenhe Fornntion
medium-grained
green-grey
and
dark-red
of
This sequence
monoclinal with
,urràrrorr" i, .o." than 500 m thick' It is
3).and contains
n!* u"rti""t dips (even overturned, Figure
tu"it * Ginkgoites sibericus' B.aieria cf' gracilisi"*lf
-Àd pf*"
of Peu^oleum.Geology of
nrroptyllum
'personal sp. ftut"ou
éommunication' 1988)' According to
finjiu"g,
2).
Late Jurassic Qigou (Jjq)-Ecnly Cretaceous
H utubi ( K 1fi Forrnations
These iormations make up a total of more than 1000 m of
sediments. The rocks consist of beds of coarse to mediumgrained sandstones, some of them dark red, others grey-green'
TABLE 1. Site Data
Fossils
tl
t2
13
l4
rs
86
86
t2
88
t 1
78
12
a / 1
1'l
t2
(Jh-K&
Sandstone
Dark-RedCoarseMedium-Grained
01
02
03
04
19
20
l6
16
t6
l6
I
284
284
9
239
) 1
'))
24
25
28
29
30
31
a)
r2
r2
r2
9
10
l1
t2
13
r2
r3
JJ
05
06
07
08
09
l0
16
t7
l8
26
27
34
35
36
37
38
t2
t z
11
9
r3
r2
10
L2
tz
l4
t2
9
'l ')
r2
206
260
264
257
)R)
274
aÀa
50
50
58
43
75
t )
Ginkgoites sibericus
Baiera cf. gracilis
Pterophyllumsp.
(JtKU)
Rhinocypris
Cyp r idea unicost ata G al.
Cyp r idea t it a Lj ubimova
O rigoiliocypris cirrita Mad.
75
76
ll8
1 À
99
56
ll3
66
89
26
130
28
274
18
308
26
281
a ^
2-17
)1
281
(Kæ-El-22-(KuJD)
Sandstone
Fine-Grained
PaleRedColor
86
93
88
274
269
270
247
245
241
89
90
t2l
ll6
99
280
111
54
56
56
98
97
50
gastropod bivalve
79
75
64
68
30
JU
32
105
r00
= strike and dip (in degrees)of siæ beds(dips greaterthan
Abbreviarionsaren = numberof minicorescollectedfrom site; s, /
dip). Formationnamesarc 121= TotuenheFm. (middle
downward
from
90o denoteinvertedu"os; tuit" is counterclockwise
andE1=
Fm' (late Cretaceous,
Jurassic),J3o = eigou Fm. (late Jurassic),Klh = Hutubi Fm. (earlyCrelaceous),K2d Donggou
2z=ZniquaÀ Fm. (earlYTertiarY).
4068
CHEN ET AL: MESOZOICCONTINENTAL SEDIMENTS
?
,
/\
I
tl
l,
Fig. 3. Schemaric N-S cross-sections in eastem pan of sampling area (see Figure 2 for location).
The Qigou Formation has yielded ostracodes suggesring a
three-component cryogenic magnetometer. The remainder of
Jurassic age. The Hutubi Formation is reported to contain
measurements were done at the Paleomagnetic Laboratory of
Early Cretaceous ostracodes such as Cypridea unicostata
the Université de Rennes (UR), using a single axis cryogenic
G aleeva, C. trita Lj ubimova, Rhirccyp r is echinata Lj ubimova
magnetometer and a Schonstedt spinner flux g ate
al'd Origoiliocypris cirrita Mandelstam, The latter two species
magnetometer. A total of 282 specimens were measured. The
suggest a Barremian age (J. F. Babinot, personal - intensities of narural remanent magnetization (NRM) ranged
communication, 1989).
from 1 to 200 mA/m, averaging 2 mA/m. Most specimens
Stratigraphically, the Hutubi Formation is parr of rhe
were thermally demagnetized, in the laboratory-built fumace at
Tugulu Group, which according to Chen [983] spans the
the IPGP and in a Schonstedt furnace at the UR, and several
entfue early Cretaceous. The Hutubi Formation is overlain by
specimens were demagnetized by alternaring field (af).
the Shengjinkou Formation, itself overlain by the Lianmuqin
Thermal demagnetization was found, in general, to be more
efficient than af in sgparating magnetic components. Samples
Formation. In Wuerhe district, in the norrhwestern parr of the
were usually demagnerized over 10 to 16 steps, sample
Dzungar Basin, the Lianmuqin Formation has yielded a fairly
varied vertebrate fauna [Dong, 1973] containing, among
orientation in the furnace being inverted at each step to detect
others, the dinosaur Psittacosaurus. Psittacosaurus is known in
âny systematic magnerization resulting from an ambient
several other localities in Asia, from Siberia and Mongolia to
magnetic field in the furnaces. Demagnetization results were
plotted as orthogonal vector diagrams and as equal-area
northern China and Thailand fBuffetaw et al., 19891,in rocks
projections. After each temperature step, the bulk
which seem to correspond to the later part of the early
Cretaceous (Aptian/Albian). The older Hutubi Formation is
susceptibility of each specimen was measured. Curie
thus probably pre-Aptian in age.
temperature (CT) analyses in air and isothermal remanent
The strata outcrop on steeply dipping (>50o) limbs of
magnetization (RM) experiments were carried out on about 20
anticlines with roughly east-west fold axes (Figures. 2 and 3).
specimens.
Alogether, 211 cores were collected from 17 sites. Twelve of
Paleomagnetic directions and planes were determined using
the sites are from lhe northern limbs of the folds (Table I and principal
component analysis (PCA; Klrsclvint [1980]), and
Figure 2).
site means using McFadden's method [McFadden and
McElhinny, 19881. Twenty-eight out of 38 sites provided
Late Cretaceous Donggou (KZù - Early Tertiary
consistentresults, which are describedsequentially below.
(E
Ziniquan
f Z) Formatiotts
The Donggou Formation has yielded ostracodes;its lateral Middle Jurassic Totuenhe
Formation (J2s)
equivalent, the Ailikehu Formarion, has also yielded
Both thermal and af demagnetizarion methods were used,
hadrosaurid dinosaurs indicating a late Cretaceous age. These
but thermal demagnetization was much more effective for all
formations are referred to the Maastrichtian by Chen [19831
five sites (Figures 4a versus b). In general, vector diagrams
and to the Coniacian/Sanronian by Hao er a/. [1986]. In the
from both types of demagnetization demonsEate the presence
Ziniquan Formation, we have collected gastropods and
of two magnetic components with closely aligned directions.
bivalves of Tertiary aspect (A. Lauriat-Rage, personal
The low temperature component (LTC) was cleaned by about
communication, 1989). This formation has yielded few
20OoC. The Fisher average for this component, based on 20
fossils. It is referred to the PaleoceneÆarly Eocene by Li
specimens from the five sites, is Dg=t.Jo, Ig=66.9o (k=54.9
[ 1e84].
and g-95=4.2o) in geographic coordinates, i.e., close ro rhe
This strongly folded sequence crops our farrhesr North of
direction of the present Earth's field (PEF) (D=0", I=62').
the Tien Shan Mountains. The dips of the srrata are generally
In specimens from site 14, the high temperarure component
greater than 50o and are sometimes overturned. The fold axes
(HTC) is unbiocked by about 575oC for rhe specimens of sire
have orientations similar to those of folds to the south.
14 (Figure 5b); measurements of IRM and Tc on rhis site
Sampled rocks are fine-grained sandstones with a pale red
(Figures 4c ud 4d) suggest thar rtre remanence is carried by
color. A total of 174 cores were sampled from 16 sires, 8 of
magnetites. For the other sites, the HTC has an unblocking
which are on the northern limbs of the anticlines (Figure 3).
temperature of 620"C suggesting that hematite is a
contributing carrier (Figures 4a and 5a). Measurementsof lowP.rLrotrl,ccNgnc REST:LTS
field susceptibility do nor indicare major changes of magneric
mineralogy during thermal demagnetizarion (Figure 4e).
Minicores were collected with an electrically powered
Twenty samples from five sites yielded HTC directions rather
portable drill with standard 2.S-cmdiameter drill bits and were
close to the PEF in geographic coordinates, rhough with a
oriented with magnetic and Sun compasses. The average
siightly steeper inclination: Dg=6.6o, Ig=72.6o (t=107.9 and
magnetic declination in the region of study is 4oriy'. In the
rl95=7.4o). This direction becomes very shallow and mostly
laboraory the samples were cut into 2.2-cmlong specimens.
upward in stratigraphic coordinates: Ds= 353.9o, 1s=-10.1o
Most magnetic measurements were carried out in the
(k=50.7 and og5=10.8o; see Figures 6a ud 6b and Table 2).
magnetically shielded room of the Paleomagnetic Laboratory
at the Institut de Physique du Clobe de Paris (IPCP), wirh a
Because this formation is almost monoclinal. no
4069
CHEN gT AL: MESOZOICCONTINENTAL SEDIMENTS
1.0
Mmax= 13.5mA/m
Mmax= 16.2mA/m
1 n
x
o
X
(5
E
E
Altcrnating field (mT)
Tcmperelurc ('C)
b
60
=
ÈÂô
a
I
c
4
a
I
=
!
c
!zo
I
4
t
100
Mrgnctizing lield (mT)
200 300
400 500 600 700
T€mperrture (oq
d
c
Pigure 4
Tempcreturc('C)
Fig. 4. Results of magneric study for middle Jurassic (J2) samples. (a, à) Normaiized NRM intensity cuwes showing an
unblocking temp€raftre of about 650'C during thermal demagnetization in Figure 4a, and a rarher high coercivity during af
demagnetization in Figure 4à. Mmax is rhe maximum magnetization measured during demagnetization. (c) Acquisition of
IRM indicating predorninant magnerite for site 14. (d) Thermomagnelic curve in air showing a decrease of magneric
intensity ar about 580oC, also indicative of magnerite for the same site. The applied.magnetic field is about 0.5 T. (e)
Suscepribility curye demonstrating absence of magnetic mineral change during thermal demagnetization.
4070
CHEN ET AL: MESOZOICCONTINENTAL SEDIMËNTS
11 - 1 3 6 8
14-1738'
Geographic Coordinates
EUP
Scale = 2 mA/m
a o
Scale = 5 mA/m
Fig. 5. Orthogonal vector projecrion of representative thermal demagnetizarions from middle lurassic (J21) samples
demonstraring (l) different unblocking temperatures for differcnt siæs; (2) sæeper dirccrions rhan presenr Earth field
@EF)
in (a) geographic coordinates and (à) horizontal dircctions in stratigraphic coordinates. Solid symbols are in horizpntai
plane, and open symbols in NS vertical plane. Numben adjacent to data points indicate rempentur€s (degrces Celsius).
+
+
+
+
'ill.
*
.Tq
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Geognphic Coordlnrtes
Stntlgrephlc
Cocdlnrtes
Fig. 6. Equal-arca projection of high tcmpcrature cornponent (HTC) directions of all middlc Jurassic (J2f specimens (a)
beforc and (à) afær bcdding correction. Note that most dtections are steeper than present Eanh field @EF, shown by a star)
in gcographic coordinatcs and very shallow in stratigraphic coordinates. Closed (open) symbols are for directions in lower
(uppcr) hernispherc.
+
+
107I
CHEN ET AL: MESOZOICCON'NNENTALSEDIMENTS
Directioru for J6 Sites
TABLE 2. Paleomagnetic
Site
lI
"/ru
Dg
Ig
Ds
/r
414 16.8 69.3 3 5 9 . 0 -
k
c95
41.5
t4.4
28.4
10.6
36.3
r5.6
66.1
10.1
17.6
I L )
12
13
14
L5
Me"n
414 346. 75.4 348.4
5
414 23.6 70.3 4.0
4 1 4 33 8 .
4
68 . 1 3 4 4 . r
414 28.8 74.2 354.3 ?.8 3!.8
&
Is
Ds
Ig
nlN Dg
5
5
6.6 72.6
15.5
11.4
16.5
ç195
107.9 7.4
10.8
50.7
353.9 1 0 1.
The n/N is number of entries in the statistics/number of
demagnetized samples;
D, I, k,s95 is declination, inclination and Fisher [1953]
statistics of site-site data. Subscripts g and s stand for
geographic and stratigraphic coordinates, respectively.
significant fold test could be performed' although the best
esiimate (k) of the precision parameter decreasesby a factor of
2 upon tilt correction (this is not significant at the 5Vo leve|
for N=5 sites). Also, it is very unlikely that these beds have
acquired their magnetization before folding, since this should
imply a subequatorial paleolatitude of the area by the middle
Jurassic. On the other hand, if the characteristic component
corresponds to a remagnetization, it is clear from Figure 6 and
Table 2 that its direction is distinct from that of the PEF' but
with an inclination intermediate between PEF (1=62') and
bedding plane dip (85"N). Two possible causes could account
for this observation. Either (1) there is a discrete
remagnetization event before the end of folding of the units.
In this case, about l07o of partial unfolding would restore HTC
directions parallel to PEF; or (2) there is a deviating effect due
to a strong anisotropy of magnetic suscePtibility (AMS). A
few measurementsof the AMS of these samples were made on a
19831 at Rennes
Digico apparatus le.g., Collinson,
Universiry (e.g., see Cogné ll988al for the procedure)' They
show that the AMS ellipsoid is oblate with minimum
susceptibility normal to bedding and that the ratios of
maximum to minimum susceptibility are rather high,
averaging about I .15. A deviating effect of AMS on
magnetization aquisition is thus conceivable, as has already
been documented in other examples [Cogné, 1988à]. \flith the
data presently available, it is nor yet possible to conclude
which explanation is correct. However, a recent
remagnetization event leading to a PEF direction and deviated
by the anisotropy is probably a good hypothesis. As
mentioned above, the rocks making up this formation are
rather coarse and soft, so that the remagnetization was
possibly caused by chemical/fluid reactions.
Late Jurassic Qigou (Jjq) - Êaily Crctaceous
Hutubi (K1) Formations
It is clear that thermal demagnetization is much more
efficient than af for most specimens (Figures 7a and 7b).
Specimens from this formation exhibit two components of
magnetization. The low temperature component (LTC)
unblocks by 200" or 300"C. The mean direction of this
component is D8=2.8o, Ig=62.0o (k= 23.6' 6195=2.1o and
n=200) in geographic coordinates, and is not distinguishable
from the PEF direction. The t value is 10.7 times larger in
geographic than in stratigraphic coordinates, clearly
indicating a recent overprint. Because the Curie balance
experiments were carried out in air, the breakdown of the low
temperature part of the thermomagnetic curve in Figure 7d is
partly due to weight loss. However, the weight loss was
measured after the experiments and appeared to be
superimposed on a decay of magnetization due either to
reaching the Curie point or to the breakdown of a magnetic
phase. This suggests the presence of hydroxides such as
goethites, and/or sulfides such as pyrrhotites.
As far as the high temperature component (HTC) is
concerned, we can divide demagnetization chatacteristics into
three types of behavior. The first type shows magnetizations
which cannot be separated from the LTC, close to the PEF
(Figure 8a). The second type reveals two NRM components; a
LTC with unblocking temperature < 300oC and a HTC
characteristic component with both normal and reversed
(Figures 8b and 8c). In the third type,
polarities
demagnetization could not be completed, and the endpoints
show only a tendency toward reversed polarity (Figure 8d).
Comparison of nearby samples with Wpe 2 behavior suggests
that the IITC is the same in both tyPes 2 and 3. Hence there is
oniy one characteristic component in these samples. The
intersection of great circles, given by tJlreMcFadden and
McElhinny [988] combined line and plane data analysis
method, was used to determine the HTC of type 3 samples,
which could then be used in the computation of site mean
directions.
The FITCs have unblocking temPeraturesranging from 300o
to slightly below 600'C (Figure 7a), and IRM acquisjtion
curveJ shôw thatg}Vo saturation is reached in fîelds of 150 mT
for most specimens (Figure 7c). This evidence indicates that
the main magnetic carrier is magnetite' However, in some
samples, the high temPerature Part of thermomagnetic curves
reveals Curie poins above 600oC (Figure 7d). This suggests
that hematite is sometimes present (e.g., Figures 8â to 8d) but
does not carry a recoverable comPonent of magnetization. No
change of bulk susceptibility is found during heating (Figure
7e ) .
4072
CHEN ET AL: MESOZOICCONTINENTAL SEDIMENTS
M m a x= 2 2 . 2 m N m
Mmax= 8.60mÉr'm
1.0
x
(!
x
E
E
3
u
30
t
a
20
q
10
È
!
a
I
300
200
Mrgnctlzin3 fldd (n'I)
400
v)
I
0
êil s 0
a
tâ
400
200
600
Tcrnpcreturc ("C)
e
Fig. 7. Resultsof magneticstudyfor upperlurassicJowcrCretaceous
(J3o-K1) samples.(a, â) Normalizeddemagnerization
intensity cuwes (as in figurcs 4a and4b, but with unblocking temperâtureat about 580'C and lower coercivity); (c)
Acquisition of IRM showing that prominent magneticcarrier is magnetite.(d) Thermcnagnetic curve in air. Noæ that Curie
points of goethiteand magnetitearc ratherclearly discemedat about l00o and 580oC,togethcrwirh somehematitewith
Curie point above 600oC. The applicd magnericfield is about 0.6T. (e) Susceptibility curve for a rcpresentativespecimen
showingrhe lack of magneticmineral changeduring thermaldemagnerizarion.
From the 17 sites of this formation. four sites were McFadden and Lowes [1981] was used to check whetherthe
discarded (sites 3, 22, 23 and 24) because of total reversedand normal polaritiesbelong to one direction group.
remagnetizationin the PEF direction, or erratic behavior at The statisticp=(R1+R --R2l(R++R-))12(N-R+-R
-) (whereR, R*
high demagnetizationtemperafures(Figure 8a). Resultsfrom andR- are the lengthsof the vector sumsfor the total number
the remaining 13 sites are listed in Table 3. The method of of samples(N), normal-polarity samples only (N1) and
.r073
SEDIMENTS
CHEN ET AL: MESOZOICCONTINENTAL
Gcographic Coordlnater
Scalc - 2mA/n
b
NN
oo
EUP
o o
StrâtlgraphlcCoordlnstes
Scdc - 5 mA/m
Scale = I mA/rn
StratlgraphicCærdinates
of thermal demagnedzation
Fig. 8. Orthogonalveclor ProjectionsdiJolavins,differcnt types of magneticbehaviorfrom upper furassic.(J3o:k1) samplàs'(a) PEFonlv; (â' c) wèll-separaæd
ioi"i'Ci"ï""."s
and dual'polarities;(d) rendencytoward reversedpolarity-'
comDonents
direitions (not sirown)above575oC;(e) an example
;;;;;;';iil
rapidly
from reiectedsiæ 22 showingthat magneticintensitydecreases
and HTCIs difficuit to isolate,althougha revened"il."i1*p"-*rc
poi"f,y cômponent is discemable.Symbols and conventionsas in
Figure5.
4074
CHEN ET AL: MESOZOIC CONTINENTAL SEDIMENTS
TABLE 3. Paleomagnetic
Directionsfor JuKl sites
01
02
04
19
20
2r
25
28
29
30
J I
32
33
Mean
7lg
6ltl
7lr0
8/8
8/9
8/8
7ls
8t9
6ltl
8 / 1l
7t8
8/8
1 7 1 . 8 77.6
76 . 4
73.3
-42.0
-36.6
-34.4
2 0 .I
-22.6
79.3
6 6 .I
7 7. 7
6 6 .I
J.
68.9
r80.r
202.0
204.r
355.3
8.0
24.3
23.3
7.4
43.8
42.0
nlN
Ds
Ig
Ds
/"
55.8
69.2
iz.t
48.6
I 1.9
45.5
'-53.6
Normal 18
Reversed 5
33.7
23.2
310.1
305.7
313.2
t96.7
208.5
343.6
348.1
50.8
36 . 9
t4.3
2r.5
19.5
10.3
181.8
196.7
1.3
42.9
30.5
-57.r
-59.6
-59.3
-42.0
-47.9
 a 1
5r . 6
53.7
76 . r
àos.
r -:o.s
iso.t
l5.l
9.9
22.4
49.9
8.6
5r . 1
7.8
108.8 5.3
50.8
8.6
36.7
10.1
r8.2 l 3 . 3
9 7. 9
9.3
l 18.6 5.1
t< r
18.7
57.5
8.0
34.8
I
4.2
57.6
37.7
62.3
n a
61.9
cl95
kstkg f(Rs)
5.5
13.7
f(Rs)
7.30
f(Rc)
0.52
0.01
9.1
7.1
69.3
9.8
= 26.6ot 4.8o.Pole:latinrde= 72.3o,longitude
Paleolatitude
= 227.3o,A95= 4.B.The nlN, D, I, h a95 aresameas in Table
2' ThefiRg)
arecomputedasflR) = (Rs+Rru- R72\(R5+Rù)/2 (N-Rlu-Rs),whereRr, xs andrRlyare the lengrhof the
Td-l(nrl
vector suns of the site meandirectionsof all sites(R7), sites from the southemlimbs (R5) and from the
northern[r"U, 1ntr),
with l/ = NS sVN;,f(Rc) is the critical valueszt the95voprobabilitylevel; if
flR)>/(Rc), thî hypothesisof a commontrue mean
direction may be rejecæd[McFadden and Jorcs, l98ll.
reversed-polarirysamples(N_),with il=N++rV_, respectively)
gives 0.028 which is much smaller than rhe critical values ar
rhe 95Voprobabilirylevel, 0.28 (R_= 4.935,R+ = 6.917.R 11.82,N=13). This indicatesùar rhe rwo poladry groupsare
not significantly different from antipodal.
Only sites 25 and 28 are on the south limb of the fold.
However, the stratigraphiccorrectionproducesan increaseof
& from 4.2 to 57.6. The fold test is significanr ar the 99go
confiderrcelevel using both the McElhinny [1964] and the
{cFadden and. Jones [1981] fold rests(Figures 9à nd 9b,
Table 3). The mean paleomagneticdircction for this formarion
is thus Ds=12.7o, Is-48.6o (k= 57.6 and o,95=J.5o,i1
stratigraphiccoordinates,Table 3).
Late CretaceousDo.nggou(KZû - Eocene
Ziniquan (Et -Z) Forrnaions
About half of the samplesshowedonly the pEF or unstable
or random directions.The rest (45 out of 102 samples),
correspondingto 9 out of 16 sites, yielded two recognizable
components.Analysis of IRM and thermomagneticcurves,as
well as thermal demagnetizationsindicaæ that-LTCs and IITCs
correspondto distinct types of magneticcarriers. For LTCs,
the unblocking temperature ranges from l00o to 300oC
(Figures lla ro llc). The low temperarurepart of the
thermomagneticcurves (Figure l0e) suggests,as in the
previous.case, the presenceof goethites and/or pyrrhotites.
The statistics of this componeni shows a pEF direction as
above. For the HTCs, the unblocking temperaruresand
magnetic saturationsreveal two carriers. One, with a Curie
temperatureof about 650oC (Figure lOe), an unblocking
temp€ratureof 650oC(Figure lOa) and high coercivity (Figure
lOc) is probably hematite.The other, which reachei 90% of
magnetic saluration at 150 mT (Fig. lOd) and has an
unblockingtemperarweof about580.C (Figuresl0ô and lla)
is probablymagnerire.
The behavior of the HTCs is similar to that of the Late
J,urallilEarly Cretaceousformations.The sites showing only
the PEF direction were eliminatedfrom further analysis.Sitjmean directions were computedusing the combinedline and
gfqne a1t1 analysis method of McFadden and McElhinny
[1988]. No major changeof bulk susceptibilitywas founâ
during heating (Figure l0/). Sire mean and formation-mean
directionsare listed in Table 4.
Two of nine sites showed reversedpolarities. Comparing
!h9_tw_ogroups of polariries, rhe reversal test staristièp is
0.17 (R+= 6.900,N+=7, R-= 1.980,N_= 2, R=8.860,N= 9).
This is smaller than the crirical value at the 95Çoconfidence
level (0.534), which meens rhat rhe normal-polarity and
reversed-polaritygroups are not significantly different from
antipodal.
Figures l2a md 12à show siæ meanswittr o95 confidence
limits on equal-areaprojecrions before and after stratigraphic
corrections. The sites display a large range of structural
attitudes. After structural correction, t increasesltom 2.7 n
56.9. The fold test is positive at the 99goconfïdencelevel for
both the McElhirmy and rhe McFadden and Jones fold tests
(Table4). The formarionmeandirectionis D.rl2.5o. /.e51.3o
(ê.56.9 and 495=6.9o, in stratigraphiccoordinares,Table 4).
Dscusstox .cNDCoxeusrcx
A first sampling rrip !o the Northwestern foot of the Tien
Shan, West of Urumqi (44.2"N, 86.0.E), along the sourhern
edge of the Dzungar basin, has provided paleomagneticresults
from three age groups.Becauseage determinationson these
continental sediments are unfortunately rather coarse, the
1075
CHENET AL: MESOZOrcCONNNEIqIALSEDIMENTS
+
+
+
+
+
+
+
+
+
+
f
+
+
+
+
+
+
+
+
+
Gcogrephlc Coordlnrtcs
Str.tlgraphlc Cmrdlnrtes
(J:o-Ktt) sites'(a) before
from upperJurassic-lower.cretâceous
Fig. 9. Equal-areaprojecuonof HTC site meandirecdons
SyinUotsand convenlions
æst'
confiden"' à"*on'toting posidvefold
and (â) after bcddingcorrcction,wirh circlesof 957o
as in Figurc6.
been
than middle terms of paleolatitudes.This problem has.recently
three age Sroupscannorbe resolvedmore finely
1990a1'
atd
Levi'
sediments
natural
fArason
for
analvzed
Cretaceous,and laæ Cretaceouslite J^urassic-early
i**tii,
i"â"'pltirca sedimentslLevi and Banerjee, 19901'synthetic
TertiarY.
early
-lûiaOt"
-lArason and Kodama, 19901or by numerical
Jurassicsites appear to have been completely sediàents fDeamer
and Levi, l990bl' It appearsthat
in a direction "lot" to the PEF' after tectonic modeling
,"t*g""tir"a
clay-rich deep'seasediments
magnetite-bearing
àf
"à*f".ti6"
primary
deforiration. It seemsParticularly difficult to ottain
shallowing, as high as
inclination
a
significant
produce
may
formations
Jurassic
middle
directions from
oni"o-"stt"tic
'f#ôd;;
of initial inclination'
450-60o
tire
range
in
al'
"sp"cintly
iS;,
as has beenfound in SouthChina by Enkin et
which is na! systematicin deepphenomenon'
this
Howeuer,
(personal
communication'
ilS-trbf and M. Steiner et al'
that lower seasediments,is believedto be insignificançif not absent'in
ard Eesse[1986] als-osgss3.sfed
\*iej.'ciirillot
quoted by ouartz-rich sandstones lDeamei and Kodama' 1990:
China
South
i*r"ti" resule from both North and
et at., 19891.Becauseof the nature of the
3;;;;ï;;
-Lin et al. U9851might be laterremagnetizations'
our study (coarse-graincontinentalsandstones)'
in
Cretaceoussediments
late
the
Cretaceousand
fne- t"tà Jruassic--early
the
paleomagnetic we believe that paleomagneticinclinations parallel that of
early Tertiary age groups yield characteristicerror'
experimental
of
range
within
the
and
field
GAD
polarities
*fii"f .Ë likely primary with dual
à""i,r"*
ih" D"uttgar Basin was aPParently attached to the
gien in thlse formations'about 507o
l9i, potitit" fold tests.
Siberia and Tarim) by the
biocks (Kazakhstan,
random
'the
sunounAing
showed
in the PEF.or
of the sampleswereremagnetized
which we have ou oldest
for
time
i.e.,
Ju-Kl andKu-Tl poles,basedon 13 and9 siæs "ô"i i*Ëtic,
àl*ir*.'rrt"
with no major tectonic
constraints,
3
in
Tables
oaieomasnetic
listed
are
ôt,h t4 and 45 specimens)respécdvely,
âirof"."al*o mking place since.We nexl comPareour results
and4 andareshownin Figure 13'
with those availablefor other blocks'
symbols
It is clear that the t'"o-pol"t (triangleand diamond
-n;;;
their
of
jàint
intersection
ttt"
o'ltttitt
lie
i:l which
i"
Dzungaila versus Siberia
a 9 5 confidence intervals, ate not statistically --pi*-"gnedc
results from Siberia have been obtainedby
8'0o'
2'3o*
is
distance
angular
their
indeed,
diJtiirguishable;
beendiscussedby Wesplul .et
g;"u:t" the magnetizationsyielding thesetwo poles predate Soviet authàrs.However,as has
Courtillor
and
t19911,the datareliability
Besse
and
dual "i. tfSAOt
tectonic deformation, because they have recorded
the other hand, we cân follow the
On
evaluate.
to
aifnc"it
i,
most
are
they
minerals'
p"ioltl"t and are carried by distinct
periods of
ages urruÀption of Besse and Courtillot that for the
iit"iy a be primary' and thesetwo poles can b-e.assigned
We can
lo-Eurasia'
attached
from which they were i"a"t* here, Siberia was rigidly
from the
Siberia
of
ào"f ,o thË ages-of the formations
paleô-pôsitions
the
deduce
il;;;f"t;
obtained.
--en
p"f- wanderp"rft (epWp) of Europe'wheredata from
impficit assumptionin the following discussionis that "pp-""l
basedon
to the {di"" dai" and Nortir America have been included'
the recovleredpaleomagneticdirections-are parallel
1988'
Courtillat'
and
t;"onstructions lBesse
axiat aipote Geo) pa*neH directi,on' Indeed' we ;;;;;;
to
back
going
13'
;;;;"
Figure
in
shown
also
is
have isgil.iltit APWP
irust addressthe question of whether these directions
200 Ma at 10-Ma intervals.
-suffered from compaction-induced inclination shallowing
fii"-a"o of Figrue 13 are plotted in another way in Figure
in
interpreted
be
can
inclinations
before the paleomagnetic
4076
CHEN ÊT ÀL: MESOZOIC CONTINENTAL SEDIMENTS
Mmax = 1.75mA"/m
Mmax= 45.1mAlm
TarçanonfC)
E
.I
I
a
I
2
I
I
I
a
a
a
I
é
a
!
a
a
À
:
I
a
q
{
a
a
!
!
!
a
I
0
100 200 300 400 500 600 700
Tcrnpcnhæ
e
(oC)
0
400
200
600
Tctlpcnlùn(oC)(oC)
Tc|lpcnlùn
f
Fig. 10. Rcsults of magnetic study for upper Crcteceous-lowerTertiary (KZA-E1-2ù samples.Normalized denragnerization
intcnsity curves prcscnting clear unblocking tcmpcraures at about (a) 650" and (â) 550"C ; IRM acquisitior ctrves for rwo
typcr of sernples,with (c) high cocrcivity minerrls (gocthite and hcmatiæ) rnd (d) a low cocrcivity mineral (magneriæ);(a)
thcrmcnegnctic curvc in air, dcmonsrnring thc existcncc of gocrhiæ (about 100"C), hematitc (about 650"C) and a fraction
of magnetitc (580'C), the applicd mrgnaic ficld ic about 0.6T; (/) susceptibility curve showing stablc susceptibility during
thcrrnel dcmrgnctization.
4077
CHENET AL: MESOZOICCoNTINENTALSEDIMENTS
17-2A18
18-2198
Shrtigrrphlc Coordlnrter
Scalc = l0 mA.lrn
Scale= l0 mA/m
Scale=.5 mA/rn
Stratigrephic Coordlnrtec
c
Fig. ll. Orthogonal vcctor projcction of thcrmrl dcmagnctizationfrom uppcr Crctaccous-lowerTertiary (KU-E1-Zù
samplcr, dirplaying differcnt typcs of magnctic bchavior. (a,à) Well-scparateddircctiqrs with dual polarity; (c)
represcntetivi of some spccimenr ihowing otrly a tendencytoward r final dircction. Symbols and convcntions as in Figure
5.
14, where the paleolatitudes and declinations obtained for the
DanngarBasin are comparedwith thosededucedfrom the Besse
and Courtillot APWP for a referencesite at 44.20N,86.0oE.It
is clear from Figure 14â that at both periods the declinations
are virtually identical to thosepredicted for Eurasia.There is a
slightly larger, more systematic difference in terms of
paleolatitudes(Figure l4c). This difference is smallest,and
approximately constant, if the recorded ages of the
magnetizationsare older than 70 Ma and younger than 130
Ma. Although the agesbasedon paleontologyare allowed to
be some l0 Ma younger and older than these values,
respectively, this would imply increased paleolatitude
differences for which there is no geological evidence:
significant convergenceand a suturing event in the upper
Jurassic(?) on one hand, and divergenceand possibly basin
opening in the Eocene,i.e., at the time of onsetof the IndiaAsia collision. We assumethat sampleswith agesolder than
only a small part of
130 Ma and youngerthan 70 Ma rep,resent
our collection and have therefore little influence on the means.
The J/K and IVT poles are thereforeassignedagesof 130-ll0
Ma and 90-70 M4 respectively.
We seein Figure 13 that the IÇT pole of the Damgar block
is concordantwith 70-90 Ma polesof the Besseand Courtillot
APWP, with a large joint intersectionof confidenceintenrals
and an angulardistanceof 6.4ot6.6o.In the 90-70Ma window,
the paleolatitude and declination differences between Eurasia
and Dzungaria af,enot significantly different from zero (5.5ot
6.6o, and 3.4ot7.2", respectively,Figures14a and 14â). The
J/K pole of the Dzungarblock is also closeto the 130-110Ma
poles of the Besseand Cowtillot APWP (Figure 13), although
4078
CHEN ET AL: MESOZOIC CONTINENTAL SEDIMENTS
Directionsfor Ku/Tl Sites
TABLE 4. Paleomagnetic
Site
nlN
05
1.0
l6
st9
7t9
616
r7
7le
l8
26
34
35
36
8/8
,t<
Mean
nlN
)t1
4110
4t9
All
Dg
188.7
42.2
126.5
320.8
106.2
2.3
28.9
2r.9
l r.4
Ig
l .3
85.3
49.1
-29.1
46.2
-5.6
28.6
17.0
22.2
Dg
Ig
4t.4
41.9
Ds
Reversed2
Ds
og5
Is
51.3
43.2
36.6
-31.1
246.1
k
195.6 -5r.2 19.9
4.4
22.8
3 7. 3
4 9. 3 1 9 1. 0
8.4
t78.2 -64.7 5 1 . 3
2 8. 6
1 9 . 3 3 7. 0
5.2
58.3
2 7 . 2 5 8 . 5 1 7. 7
48.2
17.8
9.9
r2.9
54.6
r2.5
Normal 7
/s
49.3
22.5
26.9
kslk s
cl95
2.7
56.9
39.4
6.9
49.3
7.9
J.J
13.3
r 7. 6
r2.9
4.9
8.5
I0.5
s9.9
f(Rs)
ÂRs)
f(Rc)
0.19
0.94
3.91
2t.l
rs8.o -sà.2
=32.0o t 6.4o,Pole: latitude=74.3o,longin:de=223.1o, A95 = 6.4o,lægendas inTable 3.
Paleolatinrde
the intersection of confidence intervals is smaller and the
angular distance is statistically distinct from zero, at 6.2o t
5.1o. In the 130-110 Ma window, rather constant and
consistent paleolatitude and declination differences of
5.9"t5.2o and 2.5ot5.8o, respectively, are found (Figure l4).
Dzungaria versus Tarim
We next compare our Dzungar results from the northern
piedmont of the Tien Shan with those of Li et al. [988a]
which were obtained on the southem piedmonr of the range, at
the northem edge of the Tarim craton (Figures 1 and 16). We
note that the upper Cretaceous pole position listed by Li et al.
unfortunately contains a misprint, which is carried rhrough
their paper. Correct values are listed in Table 5 and displayed
in Figures 13 and 15. The two poles of Li et al. [1988a] are
within the joint intersection of their 957o confidence
intervals, which are rather large, and therefore not. statisrically
distinct at this probability level. If we compare rhe Dzungar
+
GcogrephicCærdlnrtes
+
+
+
+
+
+
+
+
+
+
+
+
+
Stratgrephic Coadlnates
Fig. 12. Equal-aree projection of HTC site mean dircctions from upper Cretaceous-lower Terriary (KZa-Et-Zù sites, (a)
bcfore and (à) afær bedding correction, with circles of 957o confidence demonstrating positive fold test. Symbols and
conventions as in Figurc. 6.
+
+
+
4079
CHEN ET AL: MESOZOIC CONTINENTAL SEDIMENTS
polesof Tarim, DzungarandEruasia
TABLE 5. Paleomagnetic
trP,oN
oP,"E
495, deg.
Tarim
(recalculated)
lLi et a1.,19881
64.6
208.4
9.6
66.3
,rt,, o*
8.7
Dzungar
(this paper)
72.3
111 1
4.8
74 . 3
223.1
6.4
Eurasia
IBesseand Courtillot, l99ll
75 . 5
t.9
76 . 4
1 9 9r.
1.8
201.6
op'"8
LP,oN
* The pole longitude, 0p, of the upper Cretaceous Kuche section is misprinted in the paper oî Li et a/. [1988] as 2I4" both in
their Table 2 and in the-text, but their global average is correct'
and Tarim poles for the JK and IÇT periods respectively, we
find that the Dzungar poles lie barely outside the large
uncetainry intervalsof the Tarim poles of conespondingage.
The angulardistancesbetweenthe pairs of poles are 10.3ot
10.7' fôr J/K and 8.0o 110.80 for K/I (Figure l3). This
translates into a Dzungaria versus Tarim paleolatitude
differenceof 1.9ot 10.?oand a declinationdifferenceof ll.5o
f ll.0o at J/K time, and correspondingvaluesof 6.7o+ 10'8o
and5.2o+ 11.0oat I?T time (Figure15).
I
h
g
h
I
H
h
'
fr
O
Tectonic I nterPr etation
The first-order result evident in Figures 13 to 15 is the
overall compatibility of the Siberian, Dzungar and Tarim
paleomagneticpoles at J/K and IVT times. This is consistent
with the proposalthat both Tarim and Dzungariawere attached
to Kazakhstanand Siberia(and the rest of Eurasiato the north
of Cenozoic ranges) prior to upper Jurassictime. The new
-50
-200
6t
90
70
çl
o
.tt
qô
ql
O
6l
30
È
-200
- I 50
-r00
-50
t0
0
Age (My)
Fig. 13. Apparcnt polar wander path of Eurasia efter Bessc ard
Civtillot tiistl, rna*ed wirh opcn squaresconnectedby a line (ime.
inærvel bctwcen succcrsivepoles is l0 Ma). Othcr poles are solid
squarc= J[/1(fiom TailmlLi ct al.,l988al, solidcircle =Ku from Tarim
(this study)'
lii ct a1.,1988a1.triangle = Ju/I(l from DzungarBasin
àiamond = Ku/Tl from Danngar Basin (this study) and asterisks=
Danngarand Tarirn Basin samplingarcas.
Fig. 14. (a,â) Prcdicted paleolatitudc and paleodeclination of sampling
area calculated frqn Eurasian AP1VP of Eassc ard Courtillot [1991]. (c)
Predicted paleolarirudc from /niag atd lrving [19821 APWP' Squares
are rcsults from this snrdy for Ju-Kl and Ku'Tl inærvals including enor
bars on ages and mean dircctions (stippled areas). Differcnces betwecn
results frqn this study and rcference cuwes are given.
4080
CHEN ET AL: MESOZOIC CONTINENTAL SEDIMENTS
o
g
(l
d
0
6t
q,
- r00
Age (My)
These values can be undersnod in terms of a simple model
of heterogeneous intracontinental deformation resulting from
5 0 the India-Asia collision (Figure 16 and J. P. Avouac et al.,
manuscript in preparation, 1991). It is of interest to examine
this deformation in relation to the NNE penetration of India
into Eurasia since the Eocene (Figure 16). Although
l 0 Dzungaria" Kazakhstan and Siberia appear to be parts of rhe
same Eurasian kernel. assembled before the end of the
Mesozoic, Dzungaria and Siberia are separatedby a major zone
of Cenozoic shortening, the Altai and Sayan Tuva ranges. This
r 0 zone is approximately 700 km wide and 2000 m high on the
averagei consistent with a S0-km-thick crust, based on
isostatic compensation. If this crust had an original thickness
of about 35 krn, the present thickness implies some 300 km of
shortening. This value is likely to be an underesrimare, since
we have not taker into account additional components of
shortening related to (l) the fact that the continental crust in
this part of Asia might have been initially thinner, (2) the
amount of material removed by erosion, or (3) displaced by
lateral extrusion. A total value of about 400 km is possible
and compatible with the paleomagnetic estimate of 650 t 410
2 0 km. The Dzungar Basin itself can be approximated by a
losange or domino-shaped block, bounded by the overlapping
Altai and Tien Shan ranges to the Norrh and South
respectively, and by NW to NNW striking right lareral faulrs
(Figure 1). NNE directed shortening should lead to CCW
Fig. 15. Detailof (a) Figuresl4a and14à, includingresultsfrom the
Tarim (circles; zfter Li et aJ. [988o]) and the Dzungar(squares;this
study) basins.
poles indirectly lend support to the hairpin loop in the
Eurasian APWP at Jurassic and Cretaceous times [Besse atd
Courtillot, 1988, 1991; see also Enkin et al., l99la, b).
These poles would have been interpreted to indicate large
latirudinal displacements, had the Irving and Iming Q9821
APWP been used. This is clear for instance in Figure 14c,
where the larger discrepancies and uncertainties in the
paleolatinrdes are readily apparent. Note that Li et al. [988a,
p. 219), when discussing Courtillot and. Besse's [1986]
speculations about differences between the East China and
Siberia Cretaceous poles, failed to quo[e one of their
suggestions, namely lhat the Cretaceous poles of Siberia
might be in error. This suggestion has been confirmed
subsequently by Besse ard Courtillot [988, l99l].
Ar a finer level of analysis, we note the following
observations. The small declination and paleolatitude
differences between Dzungaria and Siberia, and between
Dzungaria and the Tarim on the other hand, appear to be
systematic. These differences can be interpreted in terms of
deformation after ttre deposition age. If we average the IÇT and
J/I( values and correspondingly reduce the uncertainties, we
obtain a mean paleolatitude difference of 5.9o x 3.7o
(equivalent to 650 + 410 km of NS shortening) and a
declination difference (or CCW (counterclockwise) rotation) of
2.6" t 4.5o of Dzungaria with respect to Siberia. In the case of
Dzungaria with respect to the Tarim, we find a mean
paleolatitude difference of 3.0o t 6.9" (330 t 760 km) and a
declination difference (CW (clockwise) rotation of Tarim with
respect o Dzungaria) of 8.6o t 8.7o. The large uncertainties on
the Tarim poles of course adversely affect these
determinations. The values which are significantly different
from zero at the 95Vo confidence level are the shortening
between Dzungaria and Siberia, i.e., in the Altai and Sayan
Tuva ranges, and (at the edge of significance) the rotarion
between Dzungaria and the Tarim, i.e., in the Tien Shan.
Flg|,.
ta
Fig. 16. Schematictopographicmap showing major blocks and
mountain belts in central Asia, with cross sectionAB from Tarim to
Siberia. Thin great circles crudely ourline shapeof Tien Shan (J.P.
Avouac et al., manuscriprin preparation,l9l) and dashedgrcat circle
indicaæsCW rotation of northem edge of Tarim. Arrows show the
palcodeclinations of the different blocks. SIB = Siberia, KAZ =
Kazakhstan,DZUN = Dzungar,TAR = Tarim, TIB = Tibet, IND = India,
STS = southemfoot of Tien Shan,NTS = nonhem foot of Tien Shan,A
= southcm foot of Altai, S = nonhem foot of Sayan.Numben on croJJ
section AB are in kilometers (topography from Simkin et al., |9891
(Mercator projection)).
CHEN E'f AL: MESOZOIC CONTINEI'I-IAL SEDIMENTS
rotation of the basin le.g., Cobbold and Davy, 19881' Despite
its large uncertainty of 4.5o, the mean rotation which we find
(2.6') is in the conect sense and would conespond to a small
arnount of additional shortening (20 km).
The Dzungar Basin and the Tarim are separatedby the Tien
Shan range. This range is very narrow east of Hami, becomes
wider to the west and reaches a maximum width of about 400
km north of Kashgar (i.e., some 2000 km to the west),
suggesting that the amount of NS convergence increases
westward along the range. The wedge shape of the zone of high
relief of the range, as defined by its 2000 m altirude contour,
may be approximated by two great circles intersecting at a
pivot point P some 300 + 200 km to ttre East of Urumqi
(Figure 16). The mean elevation (about 3000 m) at the
longitude of Kashgar implies a crustal thickness of 60 km and
some 280 km of shortening with the same simple calculation
used above for the Altai. This corresponds to a CW rotation of
Tarim with respec! to Dzungaria of some 9o about P (the
distance between P and Kashgar is about 1800 km, hence 280
km/1800 km = 0.16 rad = 9o, with a combined uncertainry of
about 4o). The shortening between our sites in Dzungaria and
those of Li et al. [l988a] in the Tarim is inferred to be of the
order of 150-175 km (Figures 15 and 16). Despite large
uncertainties which we have stressed above, the orders of
magnitude are compatible with the paleomagnetic estimates
(9o t 4o to be compared with 8.6o t 8.7o, and 150-175 km t
507o with 330 f 760 km).
Paleomagnetic constraints now available indirectly for
Siberia fBesse and Courtillot, 19881 and directly from
Dzunguia (this paper) and the Tarim [Lr et al., 1988a1are thus
consistent with the simple model of heterogeneous
intracontinental deformation due to the India-Asia collision
summarized in Figure 16. The rather large shortening in the
Altai range, slight CCW rotation of Dzungaria with respect to
Siberia range, increasing shortening in the Tien Shan from
East to West and =10o CW rotation of is southern piedmont
with respect to its northern piedmont, observed or comparible
with the paleomagnetic data, are predicæd by this model. Note
that, as often seems to be the case in paleomagnetism, mean
values appear to make more sense than their uncertainties
might suggest, leading to the inference that these
uncertainties might be overestimated.
The tectonic model embodies rhe idea that the total amount
of shortening between India and Siberia, which is
approximately constant in the EW or ESE-WNW direction
between 70o and llOoE longitude, is spent in a nonuniform
"overlapping"
Tibetan Plateau,
way along the suike of the
Tien Shan and Altai ranges, with recent strain diminishing
toward the west in the Altai and Tibet and toward the east in the
Tien Shan. The Indian indenter is closest ro the Tarim at the
longitude of Kashgar. The width of Tibet is minimum there and
the northwest tip of India and the Pamirs directly abut the
deforming northem boundary of the Tarim, whereas moré
deformation is absorbed within Tibet to the east. This implies
Crff rotations of the Tarim and CCW rotalion of Dzungaria,
which are trapped between the overlapping convergence
zones. This means that the Tarim poles of Li et al. [l988al and
those given here for Dzungaria cannot be considered to be
rigidly fixed to either ttre Siberia block or the North China
block. Further work is required to reduce the uncertainties in
the Tarim data, to extend the geographical data base for the
Tarim and Dzungar blocks and to confirm the preliminary
results and interpretation outlined here. Direct Siberian data
are also badly needed. Field triPs to Siberia, northern
Dzungaria and southern Tarim are therefore plarured. Until
more data become available, the robust conclusion from this
sftdy and that of li et al. $988a1 is the first-order agreement
of Dzungar and Tarim paleomagnetic poles with the Besse atd
Courtillot [1988, 1991] APWP of Eurasia, and the rough
108I
agreement of second-order differences with an independently
in
derived tectonic model of heterogeneous deformation
overlapping zones of convergence.
Acknowledgment.r. R. Butler, N. Opdyke and a third anonymous
reviewer made valuable comments on the manuscript. We are also
thankful for the suggestions made by R. Enkin. This research has been
supported by the Institur National des Sciences de I'Univers and
Xinjiang Engineering lnstitute. This is IPGP contribution 1147.
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(ReceivedApril 24, 1990;
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accepedDecember13, 1990.)