Deformation and motion along the southern margin of the Lhasa block (Tibet) prior to and during the India-Asia collision

J. Geodynamics Vol. 16, No. 1/2, pp, 21-54, 1992

0246-3707/92$5.00+0.00 Copyright © 1993 PergamonPress Ltd

Printed in Great Britain. All rights reserved

D E F O R M A T I O N A N D MOTION A L O N G THE S O U T H E R N M A R G I N OF THE L H A S A BLOCK (TIBET) P R I O R TO A N D D U R I N G THE I N D I A - A S I A COLLISION

LOTHAR RATSCHBACHER, 1 WOLFGANG and G U I T A N G P A N -2

FRISCH, 1 CHENGSHENG

C H E N "2

llnstitut ftir Geologie, Universitat, 7400 Ttibingen, Germany 2Chengdu Institute of Geology and Mineral Resources. Chengdu, Sichuan, People's Republic of China (Received 16 January 1992" aceepted 2 7 M~rch 1992)

Abstract--Field studies in the Lhasa block (central and eastern S-Tibet, long. 88 ° to 95°E and lat. 29.3 ° to 30°N) demonstrate N-S shortening with coeval and consecutive E - W extension and wrenching. Cretaceous, pre-collisional deformation was both coaxial and non-coaxial with top-to-S displacement in central S-Tibet and top-to-SE displacement in eastern S-Tibet. Post-Eocene, probably Miocene shortening as calculated from restored sections across the Qiuwu molasse and the Xigaze forearc basins caused 1>31% N-S contraction by folding and i>65% by folding, faulting and ductile deformation, with significant coeval E - W extension. Reduced stress tensors calculated from fault-striae data yield ,rl trending subhorizontally 010°(-+15 °) and ira trending subhorizontally 101 ° (___17°, azimuths of 22 sites in central S-Tibet). 0"2 is tensional. Post-Eocene structures related to N-S shortening are overprinted by structures indicating E - W compression in eastern S-Tibet (cq: 068°___11°, ,r3:155°+--16°, 6 sites) and by distributed deformation involving rifting and predominantly dextral wrenching (,r3:40°-+17°, 9 sites in central S-Tibet; 075°-+11 °, 6 sites in eastern S-Tibet). The eastern syntaxis forms an anticline (axis: 011°/61°), shows dextral-oblique normal faulting between Indian-plate and Lhasa-block rocks, and is cut in the NE by the active Karakoram-Jiali-Parlung fault zone. We discuss the geodynamics of the southern Lhasa block using all the data available: Pre-collisional deformation resulted from shortening during northward subduction and magmatic activity in an Andean-type continental margin. A major phase of exhumation in the early Miocene is correlated with major Tertiary folding and faulting. Associated strain accumulated pre-dominantly coaxilly, involved (major) subvertical and (minor) subhorizontal stretching, and reflects both thickening of Tibetan crust and bulk orogen-parallel (E-W) stretching. E - W extension is concentrated along distinct zones which coincide with margins of tectonostratigraphic units. The coaxiaUy deformed part of S-Tibet is probably bound to the S by the Miocene, dextral-oblique North Himalayan (normal) Fault Zone. This zone connects the western and eastern syntaxes and allowed the Miocene deformation of the Himalaya-Tibet system to be partitioned into a Himalayan zone thickened by simple-shear stacking and a Tibetan zone thickened by predominantly coaxial flattening-type shortening. At present strong eastward flow of the area S of the Karakoram-Jiali-Parlung fault zone is inhibited by pinning at the eastern syntaxis. The E--W compression in eastern S-Tibet results from the "dead corner" W of the syntaxis. Recent strike-slip faulting truncates the syntaxis and will probably relieve the locking of S-Tibet. 21

22

LOTHAR RA'ISCI-IBACHERet aL INTRODUCTION

Geological research in the Himalaya-Tibet system during the last decade focused on the Indian basement of the High Himalaya, the Indian (Tethyan) shelf sediments, and the (Indus)-Yarlung-Tsangpo (river) suture in Tibet. Comparatively few studies dealt with the Tibet plateau, the Asian hinterland of the Indian-Asian collision. In this paper we present the results of fieldwork in central and eastern S-Tibet along the southern margin of the Lhasa block, N of the Yarlung-Tsangpo suture (Fig. 1). The purpose is to augment structural and kinematic data on Cretaceous pre-collisional and Tertiary collisional shortening and on late Tertiary-Quaternary extension. We investigate deformation and displacement by structural and microfabric analysis, quantify the amount of post-Eocene shortening N of the Yarlung-Tsangpo suture by restoring sections across the Xigaze forearc basin and the Qiuwu molasse, and examine the paleostress field by quantitative analysis of small-scale fault-striae data. In addition, we present reconnaissance data on the structural evolution of the eastern (Namche Barwa) syntaxis. a

Tarim

basin

40°N --"-

sy

I n d i a 80"E

7°°E

/ b

-" 90"E

t Indian basement

I Indian shelf sediment=

NHFZ

Yarlung-TMngpo : suture :

wildflysch ~

I.haM block

GangdiM belt

100 km •

ophiolites

[ ] Xigaze flysch

[ ] Qiuwu molasee

Fi B. 1. (a) Sketch map of the Himalaya-Tibet system (light stipple: Indian basement of the High Himalaya bound to the S by the Main Central Thrust and to the N by the North Himalaya Fault Zone) showing Lhasa block (dense stipple) between Banggong-Nujiang and Yarlung-Tsangpo sutures. Rectangles show study areas and maps (Figs 3 and 9). (b) N~q cross section modified after Burg and Chen (1984).

Deformation and motion along margins of the Lhasa block

23

Finally, we discuss models for the Miocene to recent geodynamics of S-Tibet including the eastern syntaxis. GEOLOGIC

SETTING

The continental fragment of the Lhasa block (Fig. 1) separated from Gondwana in the Triassic and collided in the middle to late Jurassic with Eurasia, forming the Banggong-Nujiang suture (Chang et al., 1986; Seafle et al., 1987). A Carboniferous to Cretaceous sedimentary and volcanic sequence [youngest member Cenomanian red beds of Takena formation: e.g. Allbgre et al. (1984)] overlies crystalline basement. Wide areas of the southern Lhasa block are occupied by the Andean-type magmatic arc of the Gangdise (Transhimalaya) belt, which formed from late early Cretaceous through Eocene [110-40 Ma; e.g. Sch~irer et al. (1984); Maluski et al. (1982)]. The Linzizong formation encompasses the youngest volcanic rocks of this belt [60-48 Ma, e.g. Coulon et al. (1986)]. Immediately N of the Yaflung-Tsangpo suture lies the Xigaze forearc basin; biostratigraphic ages comprise Aptian-Albian (Cherchi and Schroeder 1980; Bassoullet et al., 1984), early Cretaceous (? Valanginian) to Turonian (Herin et al., 1983), and Cenomanian to Maastfichtian (Wan et al., 1982). It is partly floored by oceanic crust (Bassoullet et al., 1984) and received its sediments from the Lhasa block (S. Dtirr, personal communication). Molasse deposits attributed to Eocene-Oligocene age onlap both Gangdise and Xigaze rocks [Qiuwu formation, Wu et al. (1977): Eocene-Oligocene; Wang (1983), Chang (1984): late Eocene-Oligocene]; this sequence is correlated with the Indus molasse in Ladakh [Searle et al. (1990): mid-Eocene to late Miocene] and the Kailas molasse (Heim and Gansser, 1939). A problem with structural work in Tibet is the lack of detailed stratigraphic and radiometric data. In addition, several age data are not accompanied by fossil and/or collecting site records. For example, we were only able to confirm the mid Cretaceous age reported from the Xigaze basin and were unable to determine an age from the Qiuwu formation despite extensive sampling (H.P. Luterbacher, personal communication). We suggest that the Paleocene and early Eocene ages reported by Wan et al. (1982), Qian et al. [1982, cited in Burg et al. (1987)], Wang (1983) and Sun et al. (1984) for the Xigaze flysch should be used with caution until detailed data on collecting sites and fossils are provided. Folding in the southern Lhasa block (area around and NW of Lhasa) led to ca 40% shortening and occurred in the late Cretaceous, prior to the collision with India in the Eocene along the Yarlung-Tsangpo suture (Burg et al., 1983; Chang et aL, 1986). The India-Asia collision caused southward thrusting (Burg et al., 1983; Chang et al., 1986) and tight to generally open folding in the Linzizong formation [e.g. Tapponnier et al. (1981); Coward et al. (1988)]. Chang et al. (1986) suggested that post-Eocene shortening is responsible for the thickening of the Tibet plateau. Three deformation events (D1-D3) were involved in the structural evolution of the Lhasa block around Lhasa (Burg et al., 1983). D1, S facing, isoclinal

24

LOTHAR RA'ISCHBACHERet al.

mesoscopic folds are accompanied by a spaced or slaty cleavage (81) and a N trending stretching lineation. S directed tectonic transport was inferred from overturned folds and texture studies. Metamorphism [early Cretaceous according to Burg and Chen (1984)] and deformation increases southward. D2, a subvertical, often penetrative cleavage ($2) accompanies upright, open to tight, megascopic folds with subhorizontal E-trending axes and S1/$2 intersection lineations. D3, E trending kink bands and en 6chelon tension gashes characterize late-stage deformation. Chang etal. (1986) questioned D1 of Burg etal. (1983) by interpreting the structural difference between the strongly cleaved early Mesozoic rocks near Lhasa and the weakly cleaved late Cretaceous red-beds of the Takena formation around Maqu (NW of Lhasa) in terms of the same deformation event recorded at different structural levels. The Xigaze flysch sequence experienced ca 40% N-S shortening during a single deformation event and forms a synclinorium verging S [e.g. Burg et al. (1987)]. Cleavage is subvertical and rocks are very weakly to non-metamorphosed. The Oiuwu formation displays one phase of N verging folding and a subvertical, weak cleavage. The contact to the Gangdise rocks is generally a steeply S dipping (N directed) backthrust [e.g. Burg et al. (1987)]. Three events characterize the exhumation history of rocks along the southern margin of the Lhasa block. (1) Post-Cenomanian erosion and deformation (ca 90-60 Ma; after the deposition of the Takena formation). (2) Termination of sedimentation in the Xigaze basin (Cretaceous to early Eocene, depending on the reliability of the stratigraphic age of the youngest rocks), erosion of the Gangdise belt, and deposition of the Oiuwu molasse from the late Eocene on. (3) Rapid exhumation of the Gangdise belt (and possibly also the Xigaze and Oiuwu formations) in the early Miocene [20-17 Ma; Copeland et al. (1987), Richter et al. (1991)] and deposition of the Liuqu conglomerate [coarse conglomerates deposited in local basins and attributed to Oligocene-Miocene age; Zhang et al. (1980)] S of the Xigaze flysch along the Yarlung-Tsangpo suture. Cenozoic to recent E-W extension affected the southern Lhasa block (Armijo etal., 1986,1989). The Karakoram-Jiali dextral strike-slip fault zone and generally NNE-trending grabens form together with sinistral, NE-trending strike-slip faults N of the Lhasa block the tectonic escape system within which crustal wedges move eastward [e.g. Tapponnier et al. (1986)].

STRUCTURAL

ANALYSIS

Most of the data are summarized in Figs 2-10 and Tables 1-4. Observations are based mainly on mapping of meso- and microscale structures along cross sections. We draw the reader's attention to the work of Burg et al. (1983), Burg

Deformation and motion along margins of the Lhasa block

25

and Chen (1984), Chang et al. (1986) and Coward et al. (1988), reporting recent structural work in the Lhasa block. Central S- Tibet." Triassic metasedimentary an d metavolcanic rocks, Chagpori, Lhasa

The studied outcrops constitute a N-S profile across the former medicine-school hill (Chagpori; school destroyed, the site is presently occupied by a transmitter station) and encompasses shale, calcareous shale, marble, metagreywacke and metadacitic to metarhyolitic rocks. We confirmed the Triassic age (Bassoullet et al., 1984) by finding new fossils, i.e. Trachyceras sp. and Halobia sp. First deformation structures (D1, Fig. 2), clearly seen in iron--carbonate and calcitemobilisate layers interbedded with shale, encompass cm to m scale, tight to isoclinal similar folds (F1). An axial plane foliation ($1) dips steeply N-NNW and a stretching lineation (L1) parallels the ENE-trending F1 axes and is expessed by elongated minerals, preferred mineral growth, the long axes of pyrite concretions, oval-shaped reduction spots around pyrite, and deformed ammonites.

first deformation

WSW

..._ _ _

~ $ B

__._ENE

F.-"~

1

.....................

second deformation

WSW e e c l ~ . ~

eCeb~

___... ENE

......................................................................................

:

I I

~ . -

S2

c ~ ~

-

-

~

2 -

-

-

C

~

=

,J".

=

......

f

\-

\"

/

Fig. 2. Central S-Tibet: orientation data (lower hemisphere, equal area diagrams) and structural sketches of Triassic metasedimentary and metavolcanic rocks, Chagpori, Lhasa. (a) to (e) D1 structures: (a) stretching lineation (L1), foliation (S1), shear plane (C1) and fold axis (F1). (b) Calcite c [0001]- and (c) a(ll20)-axis textures (maxima stippled). Note asymmetry of texture skeletons and maxima to the L1-S1 coordinate system, indicating top-to-WSW vorticity. (d) Correlation function of texture asymmetry versus percentage of simple shear in bulk flow determined from texture asymmetry versus percentage of simple shear in bulk flow determined from texture simulations using the Taylor theory (from Wenk et al., 1987). Approximately 25% simple shear is indicated. (e) Sketch (XZ section; X~>Y~>Z, principal strain axes) of F 1 and SB (shear band structures). (f) to (h) D2 structures. (f) Multiple sets of ecc (extensional crenulation cleavage) and small-scale imbricate stack in dark marble. (g) Small-scale shear zone with Sz--C2 foliation-shear plane geometry and crenulation of S1 in black shale, and extensional faulting in competent voleanoclastic layer. (h) Orientation of structural elements.

26

LOqHAR RATSCHBACHERet a t

Pyrite concretions and competent tuff layers were stretched parallel L 1 by 33-85% [mean 40%, compare with the strain ratio of 1.48:1.12:0.55, k=0.33 reported by Burg et al. (1983) from a metarhyolite], giving a lower limit for D1 strain. Shear bands (White et al., 1980), developed in carbonate intercalations, offset both limbs of F1 in the same sense indicating formation late during D1, and cause foliation boudinage with variable boudin axes. Both shear bands and mm-scale pull-aparts filled with calcite fibers indicate sinistral-oblique, top-to-WSW displacement. Marble shows strong preferred orientation (texture) of calcite crystals [Fig. 2(b),(c)] and microstructure typical of crystal plastic deformaton (subgrains, multiple sets of deformation twins, recrystaUization). Texture, interpreted on the base of polycrystal-plasticity models and experimental data [e.g. Wenk et al. (1987), Ratschbacher et al. (1991)], indicates a deformation history with both pure and simple shear components (>125% simple shear). D 1 is contemporaneous with metamorphism characterized by sericite, chlorite and crystal plasticity in quartz. This assemblage, completed with the absence of epidote, defines the isograd band of the very low/low-grade metamorphic transition [300-350°C assuming low or medium pressures; Winkler (1979)]. In carbonatic shale a second cleavage ($2, Fig. 2) is developed in Srparallel shear zones; it shows a S-C fabric geometry [e.g. Berth6 etal. (1979)] and crenulates $1. $2 is a spaced fracture cleavage, indicating lower metamorphic temperatures than during D1. Outside D2 shear zones, $2 is observed only locally when the angle $1/$2 is greater than about 20°. In competent volcanoclastic rocks and marble, micro- to mesoscale fiber-filled tension gashes and multiple generations of brittle-ductile sets of extensional crenulation cleavage [ecc, Platt and Vissers (1980)] formed. Fiber orientation indicates ENE-WSW extension. In some cases, the compressional tip of eccs is formed by microscale imbricate stacks or $1 is crenulated by heterogeneous slip along $1. S-C fabrics, F2-vergence, and geometry of imbrication all indicate dextral-oblique displacement. D1 structures are truncated by the unconformity between the Takena and the Linzizong formations, and must therefore have formed between 90 and 60 Ma, prior to Eocene collision. We correlate D2 structures with those observed in the Linzizong formation and attribute them to collisional deformation. Dextral strike-slip accompanying N-S shortening may indicate a transpression setting. Strike-slip shear may also have rotated L1 from an initial N trend [see regional data below and in Burg et al. (1983)] to its present subhorizontal ENE trend. Central S-Tibet." southern edge o f the Gangdise belt

We analyzed granite, granodiorite, tonalite, gabbro, orthogneiss, amphibolite, paragneiss and marble along the road from Q~ixti to Xigaze (SW of Lhasa) along the Yarlung Tsangpo. Elongate amphibolite xenoliths in undeformed plutonites were aligned by magmatic flow [see also Burg et al. (1983)]. The Gangdise rocks show distributed, heterogeneous, generally non-penetrative ductile defor-

Deformation and motion along margins of the Lhasa block

27

mation [Fig. 3(a) for structural and kinematic data] and widespread fracture [Figs 3(b),(c) and 4 for structural data]. High-temperature metamorphism of metasedimentary rocks occurred in contact aureoles of plutons. Ductile deformation was syn- to late-intrusive or syn- to late-metamorphic - - determination of timing is therefore difficult. Most fractures and faults are coated with chloriteepidote-calcite and striae are composed of calcite-chlorite-epidote fibers behind fault steps. This indicates that brittle deformation was accompanied by hydrothermal activity, probably during final cooling of the plutons. Ductile deformation and metamorphism are constrained by the age of the Gangdise magmatic rocks (110-40 Ma) and are therefore pre-collisional. This is corroborated by the 76.9 Ma K/Ar age of syntectonic actinolite of sample TR221 [Fig. 3(a) for location and texture, Appendix 1 for isotopic data]. Qiuwu conglomerates, onlapping tonalite NE of Rinbung [89.9°E/29.15°N, E of Xigaze, Fig. 3(c)], exhibit fracture and hydrothermal mineralization identical to the underlying Gangdise rock; brittle deformation is therefore post-Eocene. The stretching lineation (L1) trends NNE and displacement is top-to-SSW [Fig. 3(a), S-C fabric geometry and texture analysis]. The regional consistency in orientation of L1 and sense of vorticity [see similar data in Burg et al. (1983)] indicates that deformation is not related to local pluton emplacement. A late fracture cleavage, observed in a granite SW of Lhasa, indicates local ENE-WSW compression. In five stations we measured mesoscale brittle failure structures to study the state(s) of paleostress associated with faulting in three dimensions. For assumptions involved in and critical analysis of methods of kinematic analysis of faults, the reader is referred to recent review papers [e.g. Angelier (1984, 1989)]. In Appendix 2 we describe the measurements and determinations made in the field and outline the techniques used for stress-tensor calculation from fault-striae data in this study. Overprinting criteria in the field and incompatible slip sense in the raw data (Fig. 4) indicate that superposition of homogeneous data sets occurred during successive deformations at nearly all stations. We differentiated two subsets, an older one comprising thrust, reverse, and strike-slip faults (set 1), and a younger including strike-slip and normal faults (set 3; we introduce a set 2, formed earlier than to coevally with set 3, later). Set 1 records NNE-trending subhorizontal compression, set 3 NE-trending extension. Stress orientation (trl>-cr2->cr3, principal stresses) is superposed on the subset plots of the fault-striae data (Fig. 4), and parameters of the deviatoric stress and exact location of stations are listed in Table 1. Central S-Tibet: Qiuwu molasse along the southern margin of the Gangdise belt The Qiuwu formation onlaps the Lhasa block rocks and comprises coarse conglomerate interbedded with red shale, sandstone and locally acid volcanic rocks. Combined with a sedimentologic and stratigraphic study (to be published

28

LOTHAR RATSCHBACHERet aL

e-

.Q

-~ •

C

~.

C

43

0

. ~

N c" ¢/) i

[]

~_~.~" ~o~/

Deformation and motion along margins of the Lhasa block

r, . . ,

29

k

r. n

ffl

2

tO

"o

o

'.~

t-"

~

,

~ oQ. q~

g,

q~

et-

* I

30

LOTHAR RA'ISCHBACHER et al.

"'! ~;!:iI :

~

..

~

.

~ .

c~ o

..

6~,~

~

~ . . . E o 0

o

~

o ~'~

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~

,-- i ~

,E~.~. u _ - ~

o

.

~

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c~

,.~

~

~,.~

~

.~'~ _'~!:~,-~

,,-~ v~ ["-I

~ ~

~.-~..j ~ , - ,

0"--

=9 I

~

<~ E ~ : ~ - ~

~_~

I

0

0 II

~--

~.-

~:

o

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,

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--:.

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,, .o

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× x

.~!

~ ~'~ = ~,~

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=~ ~. o ' - - , 8 - ~ ~ , ~ ~ . ~., ~

~

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_~_.

~

~

=

o

;

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~

.

=

0

I:

o ~

c~

Deformation and motion along margins of the Lhasa block

31

Table 1. Central S-Tibet, Gangdise belt: location of stations and parameters of the deviatoric stress tensor Sites

Method

E long

N lat

#

cr1

cr2

~r3

Dihedra P/T

89°50 ' 89050 '

29"18' 29"18'

14'13 05/05

021 03 327 13

114 48 066 34

288 42 218 53

Inversion P/T

89051 ' 89051 ,

29019 ' 29019 ,

12/12 09/08

356 07 135 19

252 63 330 70

089 25 227 05

Dihedra Iteration

90°03 ' 90°03 '

29=21 ' 29~21 '

10/10 34"25

029 13 304 30

143 61 160 54

293 26 044 17

Inversion P/T

90018 ' 90018 '

29"19' 29"19'

lull 21/19

183 54 126 09

088 47 297 80

276 42 031 01

Dihedra

90~23 '

29o18 '

09/09

115 24

289 63

020 12

Iteration

90052 ,

29%3'

14"12

020 00

110 77

290 15

R

F

0.22

15"

0.60

33*

0.23

10 °

0.7

21 °

(a) Tonalite 13-9 (1) 13-9 (3)

(b) Amphibolite 23-%1 (1) 23-%1 (3)

(c) Tonalite 23-%2 (1) 23-9-2 (3)

(d) Gabbro 23-9-3 (1) 23-%3 (3)

(e) Gneiss 23-%4 (3)

(f) Granite 23-9-5 (1)

# First number, number of measurements, second, number of measurements used for calculation. ~rl--cr3, azimuth (first number) and plunge (second number) of the principal stress axes. The stress ratio R = (or2-cr3)(~l-cr3) -1 (1, uniaxial extension; 0, uniaxial shortening). The fluctuation F gives the average angle between the measured striae and the orientation of the calculated theoretical shear stress. Letters (here: a-g) given prior to site numbers locate stations in Fig. 3. Numbers in parentheses after the site number give subsets separated from a single fault population (set 1, post-Eocene N-S compression; set 2, subrecent E-W compression W of the eastern syntaxis; set 3, recent E - W extension).

elsewhere), we mapped a 1.8 km long section with continuous outcrop W of Xigaze [Jiang Qin Zhe valley, Figs 3(c), 5 and Table 2 for location and structural data] and visited several localities S of the Yarlung Tsangpo (Fig. 6). Deformation led to open to tight, generally upright flexural-slip folds accompanied by an axial plane foliation in pelitic-psammitic layers. In conglomerate-dominated sequences, particularly where the molasse onlaps the Gangdise rocks, folding is poorly developed. The structural style is remarkably similar along strike. Minimum shortening determined by unfolding the section W of Xigaze is 31% [Fig. 5(a)]. Faulting is insignificant on macroscale. Given the 1.8 km of present N-S outcrop, the profile restores to 2.6 km depositional width; the minimum thickness is 500 m. We observed a maximum of 8.5 km of N-S outcrop width N of Sagia (N of the Yarlung Tsangpo along the road from Lhaz~ to Coq~n on the Tibet plateau). Applying the same shortening as calculated above, the minimum basin width in S-Tibet was 12.3 km, compared to 60 km in the Karakoram (Searle et al., 1990). We identified two subsets in the total fault pattern [Figs 5(b) and 6, Table 2, 9 stations]. Set 1 comprises pre-buckling layer-parallel shortening, flexural-gliding, and small-scale fold-propagation faults. These are cut by set 3, mostly strike-slip faults. Faulting and folding which developed simultaneously produced apparently complicated fault patterns. Appendix 3 provides a summary of the structural evolution of folded strata and the geometric analysis of set 1 faults on which we reconstructed the folding-faulting history and selected the fault-striae data for stress-tensor calculations. Several faults [station LRLATE, Fig. 5(b)] formed

32

Lo'n-IAR R

A q [ ~ A C I ~

et al.

Central S-Tibet: Gangdise belt

23-9-3~c ~

~

(~)

....'/

"~.

(3~ ~ ~ - ~

'~ unreliable (~a3 i very poor slip sense

Fig. 4. Central S-Tibet, southern edge of the Gangdise belt. Fault-striae data used to compute tr z to cr3 (1 to 3, Table 1 for parameters of deviatorie stress tensor and location of stations). Faults are drawn as great circles, striae as arrows with head pointing in direction oI displacement of the hanging wall. Because mistakes in slip-sense determination have severe effects on calculations, we assigned degrees of certainty to each observation; it is expressed in the head style of the striae arrows (see Figs 4--8 and 10); full, certain; open, reliable; half, unreliable; without head, very poor slip sense. Left hand row gives raw data (number refers to location, Table 1), following rows show subsets (sets and to set 3) determined using field evidence (see Appendix 2 and text).

Deformation and motion along margins of the Lhasa block

33

a .,,

e=.o.31

.......

N

* * o , * j

Jllng Qln Zhe

&

I

'v':'"'"

LRI41

. . . . . . . . . . . . . . . . . . .

s

LR130 LI~YN

ua~

i ............................................................................

.....

LR1 b

LRSUM •

_ _

"

.

.

ilie.

•

,

Fig. 5. Central S-Tibet, Qiuwu formation (late Eocene-Oligocene molasse). (a) Structural section established from field mapping along the southern rim of the Gangdise belt [vertical equals horizontal scale; Jian Qin Zhe valley, W of Xigaze, Fig. 3(c)]. Bold lines are marker beds used for restoration (between stippled arrows, 31% shortening). (b) Fault--striae data used to compute ~1 to cr3 (Table 2 for parameters of the deviatoric stress tensor and localities, Appendix 3 for discussion of relationship between faulting and folding). See Fig. 4 for explanation of symbols. LRSUM gives total data. LRLATE shows faults which formed late in the folding-faulting history, crosscut folds and/or older faults, and mainly comprise (top-to-N) backthrusts. LRJIA comprises poles to bedding (SS), foliation ($1) and calcite- or quartz-filled tension gashes (k). The latter indicate subvertical and subhorizontal E-W extension coeval with folding.

late in the folding-faulting history. These crosscut entire folds and/or reverse faults, and are nearly all (top-to-N) backthrusts. Dikes crosscutting the Qiuwu formation strike NNW, indicating a stress field similar to that portrayed by set 3 faults. The contact between the Qiuwu and the Xigaze formations is in general a steeply dipping backthrust.

34

LOTHAR RATSCHBACHER

et at

T a b l e 2. C e n t r a l S-Tibet, Q i u w u m o l a s s e : l o c a t i o n of s t a t i o n s a n d p a r a m e t e r s of the d e v i a t o r i c stress t e n s o r Sites

Method

E long

N lat

(g) 13-9(1)

Iteration Iteration Iteration Dihedral Iteration Iteration Iteration

89"50' 89050 , 89*48' 89048 ' 89*42' 89*42' 88046 '

29°18 ' 29o18 , 29017 ' 29017 ' 29019 ' 29019 ' 29019 '

13-9 (3) (h) 13-9-2 (1) 13-9-2 (3) (0 L R 5 5 (1) L R 5 5 (3) (]) L R 1 5 1 (1)

#

tr 1

~r2

~r3

R

F

09/07 11/11 20/19 04/04 17/15 20/17 10/09

220 10 338 30 170 00 105 23 040 00 339 20 00020

116 108 260 250 310 125 094

53 48 09 51 39 66 11

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35 26 81 18 51 12 67

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99199 12/12 30/26 34/21 12/10

188 012 186 010 170

098 282 276 102 080

00 03 10 09 06

007 161 088 232 260

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0.41 0.19 0.10 0.40

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262 13 285 47

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Jiang Q i n Z h e valley, t y p e p r o f i l e Q i u w u f o r m a t i o n L R S U M (1)

(k) L R 1 3 2 (1) (1) L R S Y N (1) (m) L R 1 4 1 (1) L R C R O S S (1)

Inversion Inversion Inversion Iteration Iteration

88039 , 88o39 ' 88°39 ' 88°39 ' 88°39 '

Xigaze flysch--Qiuwu formation contact (n) 14-9-2 (1) Inversion 89.51' (o) L R 1 3 0 (1)

Inversion

88039 ,

05 05 01 10 00

103 76 093 42

F o r l e g e n d see T a b l e 1.

Central S-Tibet: Xigaze flysch N of the Yarlung-Tsangpo suture The Xigaze basin comprises a sequence (>14.5 km) of flysch with minor conglomerate, marl and a basal layer of thickly bedded limestone. We mapped a section across the widest outcrop [Jiang Qin Zhe section, Figs 3(c), 7(a), S of the molasse, see above] and studied several localities along strike [Figs 7(c) and 8]. Deformation correlates with that affecting the Qiuwu formation (e.g. compare structural data of Figs 5-8). We verified the overall synclinorium structure proposed by earlier workers [e.g. Wan et al. (1982), Burg and Chen (1984)]. In our section, however, deformation intensity increases from S to N, with open to tight folds in the S where the Xigaze sequence thrust over Liuqu conglomerate, and tight folds and several thrust imbricates in the N [Fig. 7(a)]. Due to lack of marker horizons only the southern section can be restored accurately [>140% shortening, calculated by unfolding a massive conglomerate bed and a sequence comprising carbonate turbidites, Fig. 7(a)]. Total shortening, however, is larger due to small-scale folding, ductile deformation, and more intense contraction in the northern part of the profile. Strain, calculated from deformed concretions in strongly cleaved beds in the central part of the section yielded a ratio of 1.53:1.09:0.6 (40% N-S shortening) and a flattening geometry (k=0.5). We consider this to be a regionally representative value for the fine-grained rocks of the central and northern Xigaze basin. We estimate total shortening to be >165% (sum of shortening by buckling and ductile strain). Given the 23 km of present N-S outcrop this restores to a depositional width of >166 km, which we believe is a representative figure for the entire Xigaze basin in S-Tibet. This width is somewhat less than that of modern forearc basins [e.g. Karig et al. (1979)].

Deformation and motion along margins of the Lhasa block

35

Central S-Tibet: Qluwu formation (molasse)

.\ ---

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.,/) Fig. 6. Central S-Tibet, Qiuwu formation. Fault-striae data used to compute e 1 to ~3 (1-3, Table 2 for parameters of deviatorlc stress tensor and localities). See Fig. 4 for explanation of symbols. Left hand row gives raw data (number refers to location, Table 2), following rows show subsets (set 1 and 3) discriminated by field evidence (see text). Orientation diagrams comprise poles to bedding (SS), foliation (S1), tension gashes (k) and calcite or quartz fibers (L~) in tension gashes; k and L I mainly indicate subhorizontal E - W extension coeval with folding.

At one locality (western outskirts of Xigaze) we found the open folds [see above, F1 in Fig. 7(b) and (c)] affecting inverted beds [determined from graded bedding, sole marks, trace fossils and S0/S1 relationships, Fig. 7(b)]. This is evidence of older (pre-F1), isoclinal, mesoscale folding (F0 in Fig. 7(b)] which is widespread S of the suture. There, it is correlated with imbrication of Indian shelf rocks (Burg et al., 1987).

36

LOaHAR RA~CHBACHER et al.

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Deformation and motion along margins of the Lhasa block

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Fig. 8. Central S-Tibet: schematic sketch map of boundary between Oiuwu molasse, Xigaze llysch and ophiolitic melange of the suture E of Rinbung [89.9°E/29.15~1, Fig. 3(c)]. N--S shortening by backthrusting is connected with coaxial E - W extension along conjugate strike~lip faults. Diagrams give fault--striae data used to compute ~1 to ~3 (Table 3 for parameters of the deviatoric stress tensor) and orientation of structural elements. See Fig. 4 for explanation of symbols.

Table 3. Central S-Tibet, Xigaze flysch: location of stations and parameters of the deviatoric stress tensor Sites

Method

E long

N lat

#

(p) (q)

Iteration Inversion Inversion Inversion

88051 ' 88°23 ' 89052' 89052.5'

29°15 ' 29018 ' 29016 ' 29016'

10/09 05/05 08/08 10/10

LRXIG1 (1) L R J I D (1) (r) 14-9-1 (1) 14-9FOLD (1)

(r 1

(r2

(r3

169 197 173 326

20 00 01 24

262 107 081 092

08 05 16 53

014 288 286 223

68 85 ;2 27

011 021 317 202 192

03 01 00 02 01

280 291 226 112 282

21 09 85 05 04

109 119 047 312 084

69 81 05 84 86

R

F

0.70

12°

0.39

16 °

0.80

13°

Jialag Qin Zhe--Ka De valleys, type profile Xigaze formation

(s) X I G A A (t) X I G D D (1) X I G D D (3) (u) X I G E E (1) (v) LR145TO (1)

Dihedra Inversion P/I" P/I" Dihedra

88°37 ' 88037 ' 88037 ' 88031 ' 88030 '

29°19 ' 29°14 ' 29014 ' 29007 ' 29005 '

12/11 18/18 09/09 07/06 54/47

For legend see Table 1.

The total mesoscale fault pattern comprises two subsets [Figs 7(c) and 8; Table 3]. Set 1 indicates NNE-trending shortening. Similar to the molasse as small group of reverse faults crosscuts fold-related faults. Set 3 faults (see subset grouping above; NE-trending (r3) are scarce, but a well-developed dextral strike-slip zone occurs along a vertical fold limb in the S-central part of our section [station XIGDD(3), Fig. 7(a) and (c)]. We mapped the boundaries between Qiuwu molasse, Xigaze flysch, and ophiolitic melange of the suture NE of Rinbung

LO'IHAR RA'ISCHBACHER et at

38

(Fig. 8); N-S shortening by back-thrusting is associated with map-scale coaxial E-W extension along conjugate strike-slip faults. Eastern S-Tibet: southern edge of the Gangdise belt We sampled Permian metasedimentary rocks (quartzite, quartzphyllite, shale), acid volcanic rocks of the Linzizong formation, and plutonites of the Lhasa block, and Indian plate migmatites along the road from Lhasa to the eastern syntaxis [Tangmaj; Fig. 9(a)]. Deformation in the Permian rocks is penetrative and ductile with an S to ESE trending stretching lineation (L1, progressively more eastern trends are found eastwards) on $1, and tight to isoclinal folds with axes parallel to L1 [Figs 9(a) and 10]. Quartz textures [Fig. 9(b)] are monoclinic, typical of a strain path dominated by simple shear, and reveal a consistent topto-SE displacement. Deformation concurs with low grade metamorphism (biotite+garnet stable in pelite-rich layers) under falling temperatures (chloritization of garnet, biotite stable). The Gangdise plutonites [Cretaceous-Tertiary, after 'geological map of Qinghai-Xizang (Tibet) plateau and adjacent areas'] arc unaffected by the tectonometamorphic event in the Permian rocks. They are either undeformed or exhibit a NNW-striking, steeply dipping fracture cleavage with varying lineation (Fig. 10). Indian plate migrnatites and gneisses form a huge anticline (axis: 011°/61°) at the eastern syntaxis with a spectacular subvertical foliation and lineation (Fig. 10). The anticline is cut at its northeastern tip by the active dextral Jiali-Parlung fault (recent activity demonstrated by hot springs and landslides blocking rivers). The orientation of foliation and lineation and a top-to-NE vorticity [e.g. Fig. Table 4. Eastern S-Tibet, Gangdise belt: location of stations and parameters of the deviatoric stress tensor Sites

Method

E long

N lat

#

cr1

~r2

~3

Dihedra Iteration Inversion Iteration P/T Iteration

92°40" 92*40" 93.14' 93.14' 93.14' 94014 ,

29*53' 29*53' 29*52' 29*52' 29.52' 29*46'

18118 12/11 12/12 17114 26/21 06/06

22622 340 10 065 77 191 20 256 03 170 00

013 64 079 41 207 10 098 06 030 86 260 77

131 239 299 353 166 080

Iteration Iteration Iteration

93046 ' 93*46' 93*58'

29*50' 29*50" 29*47'

14/13 15/14 11/11

079 30 170 00 244 20

259 60 080 55 351 39

349 00 260 35 133 44

94"28' 94028 ' 94048 ' 94*48 ' 95003 '

29036 , 29*36' 29058 ' 29*58' 30006'

09/07 15/15 22/21 19117 31127

249 11 346 16 256 09 343 54 002 15

139 60 109 82 146 63 159 36 196 54

345 28 250 22 350 25 250 02 093 04

R

F

0.50 0.51 0.30

21 ° 10° 22 °

0.20

150

0.70 0.40 0.20

19° 15° 19.

0.62

12°

0.57

21"

Permian metasedimentary rocks 4-6 (2) 4-6 (3) 7-11D1 7-11 (1) 7-11 (2) 14 (3)

13 47 08 69 03 13

Plutonites 12 (2) 12 (3) 13 (2)

Migmatites, eastern syntaxis 15-16 15-16 20-23 20-23 24-25

(2) (3) (2) (3) (3)

P/T Iteration Dihedra P/T Inversion

For legend see Table 1.

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Deformation and motion along margins of the Lhasa block

43

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Deformation and motion along margins of the Lhasa block

45

9(b), monoclinic quartz texture of sample TR229, with foliation 026o/80° and lineation 062°/75 °] indicate a dextrally oblique normal fault geometry between Indian plate and Lhasa block rocks. Fault patterns are similar in the Permian, Gangdise and Indian-plate rocks (Fig. 10, Table 4). The Permian rocks locally have an early ductile-brittle normal fault pattern, interpreted as late-stage progressive stretching along L1. A fault pattern with a calculated NNE-trending 0.1 [Fig. 10, station 7-11(1)] also developed locally. Orientation and slip sense of these faults are similar to rare mesoscale faults accompanying open to tight folding around E-trending axes in the Linzizong formation. Two sets comprise most of the data [Figs 9(c), (d) and 10]. Fault development from brittle-ductile kinking indicates that set 2 is older than set 3.0"1, calculated from set 2, trends ENE, 0"3 SSE. Set 3 has 0"3 trending ENE, faulting is distributed and related to map-scale faults (e.g. Jiali-Parlung fault; fault along the Nyang river dextrally offsetting probably Cretaceous granite). We attribute set 2 faulting to E - W compression during syntaxis formation and set 3 faults to subrecent E - W extension (see below).

Partitioning of Tertiary deformation into frontal compression an d lateral extension To quantify the ratio between subvertical and subhorizontal extension during post-Eocene N-S shortening, we assume that the axial ratio of strain measured in the central portion of the Xigaze flysch profile [Fig. 7(a)] is representative of the ductile deformation along the southern edge of the Lhasa block. Strain geometry (k=0.5, see above) indicates that the subvertical extension was about five times the subhorizontal E - W extension. We also attempted to determine the relative importance of reverse, strike-slip, and normal faulting during post-collisional N-S compression (set 1 fault-striae stations). In addition to the orientation of each principal stress axis, the computation of the stress tensor enables one or determine the ratio R, which expresses the relationship between the magnitudes of the three principal stresses. Extreme values of R correspond to stress ellipsoids with 0"2=0" 3 (R=0) or 0"2=0"1 (R=I). As Tables 1 to 3 and Fig. 3(b) show, the ratios R for set 1 fault-striae stations are relatively consistent along the southern edge of the Lhasa block. The average ratio, calculated as the arithmetic mean of all numerical solutions (iteration and direct inversion methods, 16 sites) is R=0.3 (SD=0.2). Although such estimates are still questionable in their regional significance, because our observational data base is not dense enough, the calculated ratio indicates that the E - W oriented principal stress was tensional. In detail, most site analyses in the Qiuwu and Xigaze formations indicate that the 0"3 axes plunge close to vertically, showing that the reverse-slip tectonic regime dominated. However, the relative importance of strike-slip and reverse faulting changes spatially. Strike-slip (0"2 vertical, 0"3, subhorizontal and E - W ) plays a dominant role along several unit boundaries; e.g. at the southern margin of the Qiuwu molasse (station LR130, Fig. 5), along the zone of imbrication E

46

LOIHAR RATSCHBACHER et aL

of Rinbung (Fig. 8, stations 14-9) and, most prominently, along the southern margin of the Gangdise belt (Fig. 4). Contrarily, reverse faulting and folding dominates the interior of large coherent units (Figs 5 and 7). Both groups of stations show a similar orientation of 0.1, but 0.3 is generally subhorizontal along the margins of the major tectonostratigraphic units and subvertical within them; we infer that a spatial permutation in the orientation of 0.2/0.3 stresses occurred. A comparison of results of the quantitative fault-striae analysis and of strain gauges in S-Tibet points to a simple stress-strain analogy. The spatial partitioning in orientation of 0.2/0.3 stresses is accompanied by a spatial partitioning in orientation of the intermediate and maximum principal stretches (ez/e3). The orientation of tension gashes and related fibers indicates that e 3 is predominantly subvertical in stations where the calculated 03 axes plunge close to vertically (central portions of Qiuwu and Xigaze formations, Figs 5 and 7); both 0"3 and e 3 trend predominantly subhorizontally and E - W along unit boundaries [Figs 6, 7 (station XIGAA) and 8]. We conclude that post-Eocene (collisional) shortening across the southern margin of the Lhasa block involved a significant component of E-W extension; strain accumulated predominantly coaxially, intensity of E - W extension is distributed unevenly, and concentrated into E-W trending zones. Finally, because the R ratios of the post-Eocene shortening event [set 1 faultstriae stations, Fig. 3(b)] are generally low, the paleostress pattern is best defined by using 0"1 trends. On the other hand, R ratios are generally high (mean R =0.7; both 0"1 and 0"2 are compressive) for subrecent E-W extension (set 3 fault-striae stations), and we therefore describe the paleostress pattern using 0.3 trends [Figs 3(b) and 9(d)]. DISCUSSION

A collection of the available isotopic ages is given in Fig. 11 to constrain timing of deformation. Table 5 presents structural correlations. The intensity of pre-collision deformation in the southern Lhasa block shows that major crustal thickening along the Gangdise belt occurred prior to the India-Asia collision. Deformation may have been caused by folding during northward subduction and magmatic underplating as suggested by Burg et al. (1983) and Burg and Chen (1984). Based on similar structural style and paleostress orientation, and the age of the youngest rocks involved in deformation, we conclude that the Linzizong, Qiuwu and Xigaze formations were deformed together. Compression in the Qiuwu and Xigaze formations started post-Eocene (main folding and faulting) and continued with reverse faulting in the Miocene (late crosscutting backthrusts, age indicated by similar backthrusts affecting the Oligocene-Miocene Liuqu conglomerate). It is tempting to attribute the pulse of exhumation reported by Copeland et al. [(1987); they argue for surface uplift] and Richter et al. (1991) from the Qtixti pluton (SW of Lhasa, Fig. 11) and from the northwestern Himalaya

Deformation and motion along margins of the Lhasa block

47

Table 5. Metamorphism--deformation-time evolution of rocks along the southern margin of the Lhasa block according to literature and own data Deformation

Metamorphism

Age

Selected references

Late D3: coaxial E-W exNo metamorphism tension and dextral strikeslip in entire S-Tibet D3: local E--W compression (syntaxis formation)

Late Miocene (10-5 Ma) to recent

Mercier et aL (1987) Armijo et aL (1986) Armijo et al. (1989)

No metamorphism Late D2: 'backthrusting' along Yarlung-Tsangpo suture, in Liuqu conglomerates, Qiuwu molasse, Xigaze flysch, and along southern margin of Gangdise belt Partly anchimetamorphic D2: Gangdise belt: N--S compression, coaxial E-W extension, and local dextral transpression; Linzizong formation: N-S compression; Qiuwu and Xigaze formations: N--S compression and coaxial E-W extension

Post-Liuqu conglomerates (Oligocene-Miocene)

Mercier et al. (1987) Burg et aL (1987) Burg and Chen (1984)

Gangdise belt: postLinzizong (648 Ma), early Miocene: connection with rapid exhumation; Qiuwu and Xigaze formations: post-Eocene-Oligoce ne, post-early Miocene (correlation with Indus molasse)

Maluski et al. (1982) Chang et al. (1986) Coward et a/. (1986) Searle et aL (1990) Copeland et al. (1987) Richter et al. (1991)

Df Gangdise belt: N - - S (central S-Tibet) to ESEWNW (eastern S-Tibet) shortening; top-to-S to ESE displacement, crystal-plastic deformation dominant

Gangdise belt: 110--40Ma; Burg et al. (1983) Chang et al. (1986) generally pre- and synGangdise magmatism, pre- Sch~irer et al. (1984) Linzizong formation (<~60 Ma); radiometric ages for the Gangidse belt see Fig. 11

Gangdise belt: various P-T conditions (generally high T - low P) due to steep geothermal gradient caused by successive magma addition; Permian rocks: low grade (garnet+biotite); Triassic rocks: very low-low grade transition (sericite+ chlorite, no epidote)

by Zeitler (1985) to post-Eocene shortening. A top-to-S thrust, displacing amphibolite over granite, and top-to-N backthrusts, typical structures affecting the Linzizong, Xigaze and Qiuwu formations, are present in this area. Similar structures and similar orientation of stress in the Liuqu conglomerate suggest that its deformation is a progressive stage of the main deformation in the Xigaze and Qiuwu formations. Folding and faulting thickened the crust and may have induced tectonic uplift. Post-Eocene N-S shortening also involved stretching in an E - W direction concentrated along distinct, E-W trending zones; it may reflect bulk orogen-parallel extension facilitated by a weakly constrained lateral margin. Intensity of subrecent strike-slip deformation increases from central to eastern S-Tibet and is concentrated at the eastern syntaxis. Several fault zones [e.g. dextral cataclastic shear zones N and NW of Nyinchi; Fig. 9(c)] and the abrupt northeastern termination of the Indian plate at the Jiali-Parlung fault indicate that the syntaxis is currently fragmented. This may reflect southward side-stepping of deformation around dextral strike-slip faults locked by continuing S-N com-

48

LO'IHAR RA'ISCHBACHER et aL

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1 '~ ( /

+ 25"N

Fig. 12. Miocene and active tectonics in the Himalaya-Tibet system. YTS, Yarlung-Tsangpo suture; NHF'Z, North Himalaya Fault Zone, the dextral-oblique normal fault zone between the crystalline basement of the High Himalaya and the Indian shelf sediments to the N; MCT, main central thrust; MBT, main boundary thrust. (a) Model of the formation of the eastern syntaxis. The E - W compression (set 2 faults) in eastern S-Tibet is explained by the dead corner effect due to blocking of eastward material flow at the syntaxis. Inset (rectangle) shows recent fragmentation to overcome locking. (b) Model for partitioning of N-S shortening into subvertical and subhorizontal E - W stretching. (c) Recent and Miocene (NHFZ, MBT) structures, modified and completed from P~cher et al. (1991) and Armijo et aL (1986). (d) Miocene tectonic model. (e) Kinematics of active, tectonic escape of Tibet. Eastward flow of central S-Tibet is largely blocked at the eastern syntaxis, causing a dead corner effect with E - W shortening W of the syntaxis.

pression. Figure 12(a) gives a model for the formation of the eastern syntaxis based on the model proposed for the formation of the better studied western syntaxis (e.g. Treloar et al., 1991). Note that bending of the structures W of the syntaxis causes anticlockwise block rotation around a vertical axis. This rotation is reflected by the change in orientation of the stretching lineations in the Permian rocks and by the strike of the Indian plate rocks [Fig. 9(a)]. A jump from Cretaceous-early Tertiary to mostly Miocene biotite cooling ages coincides with the transition from the Lhasa block to the Indian plate [Fig. ll(b), the exact locations of the data of Zhang et al. (1981) are not known]. Synmetamorphic, dextral-oblique shear zones with a normal-fault geometry in the Indian plate rocks had a component of strike-slip deformation since the Miocene.

Deformation and motion along margins of the Lhasa block

,19

Syntaxis formation also explains the occurrence of the phase of E - W compression and N-S extension in eastern S-Tibet. This deformation appears to be restricted to eastern S-Tibet and occurred earlier to coeval with subrecent E-W extension. In our model [Fig. 12(a)], eastward material flow was blocked by the development of a 'dead corner' W of the syntaxis due to southward displacement along the central part of the Himalaya and pinning at the syntaxes. Pinning may have contributed to the radial displacement pattern observed along the Himalayan front (Brunel, 1986). Recent eastward lateral crustal extrusion N of the Himalaya amounts to ~1/3 of the present convergence between India and Siberia (Armijo et al., 1989). One of the most interesting questions on Tibetan tectonics is how much of the >~2000 km India-Asia convergence since collision was accomplished by lateral crustal extrusion. Our field work revealed no large-scale dextral strike-slip zones along the southern edge of the Lhasa block (this paper), along the Yarlung-Tsangpo suture, or within the Indian shelf sediments (unpublished results). Instead, our data indicate that together with N-S shortening and vertical lengthening, the crust was also stretched parallel to orogenic strike [Fig. 12(b)]. The incompatibilities in heterogeneous pure shear must have been accommodated by distributed shear. We suggest that some of this lateral shear is found in the meso-scale shear zones found throughout the southern Lhasa block, e.g. E of Rinbung (Fig. 8). We further speculate that the zone of coaxial deformation in S-Tibet is terminated to the S by the Miocene dextral-oblique normal fault zone between the Indian plate and the Lhasa block, observed at the eastern syntaxis. Its geometry and location is similar to the large-scale oblique-slip normal fault zone, the North Himalaya Fault Zone [NHFZ, Fig. 12(c)], mapped at several places along the boundary between the Indian basement and the Indian shelf sediments from the Karakoram to E of Mt Everest [see compilation in PScher et al. (1991); Fig. 12(c)]. This zone may have linked the western and eastern syntaxes and facilitated bulk Miocene E-W extension. Pinning at the syntaxes, but continuing motion along the central part of the Himalaya may have rotated this initially probably nearly vertical zone until it became a shallower, oblique-slip normal fault zone. This interpretation is supplementary or alternative to the interpretation citing gravitational forces [see Burg et al. (1984), Burchfiel and Royden (1985), but P~cher et al. (1991)]. The Miocene history of the Himalaya-Tibet system may thus have been that of simple shear stacking in the Himalaya and predominantly coaxial, flattening-type thickening in Tibet with the two zones separated by the dextral-oblique NHFZ [Fig. 12(d)]. At present the zone where Tibetan crust extrudes to the E is separated from the laterally confined zone in S-Tibet by the Karakoram-Jiali fault [see Mercier etal. (1987); Armijo etal. (1986,1989);Fig. 12(c,e)]. The NHFZ became inactivated by the formation of the arc along the Himalayan front. We speculate that the present fragmentation of the eastern syntaxis is to overcome the locking of S-Tibet.

50

LO'IHAR RATSCHBACHER et al.

Acknowledgments This study, financed by Deutsche Forschungsgemeinschaft, was part of the cooperative research project between the Institute of Geology and Mineral Resources, Chengdu, and the University of Ttibingen on the sedimentological evolution of the Yarlung-Tsangpo suture under the leadership of G. Einsele and B. Liu. Several people (in particular R. Caputo, K. Hardcastle, R. Ott, B. Sperner) contributed to the development of the stress-analysis laboratory at the University of Tiibingen. The manuscript benefitted from pre-submission reviews by S. Diirr, G. Einsele and U. Riller, from helpful comments from W. Jacoby, R. Lacassin and C. Talbot during editing and reviewing for the Journal, and from a final upgrading of the English text by P. O'Shea.

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Coulon C., Maluski H., Bollinger C. and Wang S. (1986) Mesozoic and Cenozoic volcanic rocks from central and southern Tibet: 39Ar-40Ar dating, petrological characteristics and geodynamical significance. Earth Planet. Sci. Letr 79, 281-302. Coward M. P., Kidd W. S. F., Pan Y., Shackleton R. M. and Zhang H. (1988) The structure of the 1985 Tibet geotraverse, Lhasa to Golmud. Phil, Trans. R. Soc. Lond. A 327, 307-336. Gephart J. W. (1990) FMSI: A Fortran program for inverting fault/slickenside and earthquake focal mechanism data to obtain the regional stress tensor. Comput. Geosci. 16, 953-989. Hardcastle K. C. and Hills L. S. (1990) BRUTE3 and SELECT: Quickbasic 4 programs for the determination of stress tensor configurations and separation of heterogeneous populations of fault-slip data. Comput. Geosci. 17, 23-43. Heim A. and Gansser A. (1939) Central Himalaya, geological observations of the Swiss expeditions 1936. M~m. Soc. helv. Sci. nat. LXXII, 1-245. Herm D., SchrOder R. and Binggao Z. (1983) Stratigraphic events during Cretaceous time in southern Xizang (Tibet). Terra cognita 3, 267. Karig D. E., Suparka S., Moore G. F. and Hehanussa P. E. (1979) Structure and Cenozoic evolution of the Sunda arc in the central Sumatra region. Am. Assoc. Petrol. Geol. Mere. 19, 223-237. Lisle R. J. (1979) The representation and calculation of the deviatoric component of the geological stress tensor. J. Struct. Geol. 1, 317-321. Maluski H., Proust F. and Xiao X. C. (1982) 39Ar/40Ar dating of the trans-Himalayan calc-alkaline magmatism of southern Tibet. Nature 298, 152-154. Mercier J. L., Armijo R., Tapponnier P., Carey-Gailhardis E. and Han T. (1987) Change from early Tertiary compression to Quaternary extension in southern Tibet during the India-Asia collision. Tectonics 6, 275-304. Platt J. P. and Vissers R. L. M. (1980) Extensional structures in anisotopic rocks. J. Struct. Geol. 2, 397-410. P6cher A., Bouchez J.-L. and Le Fort P. (1991) Miocene dextral shearing between Himalaya and Tibet. Geology 19, 683--685. Ratschbacher L., Wenk H.-R. and Sintubin M. (1991) Calcite textures: examples from nappes with strain-path partitioning. J. Struct. Geol. 13, 369-384. Ratschbacher L., Frisch W., Linzer H. G., Meschede M., Decker K. and Grygar R. (1993) The Pieniny Klippcn belt in the W-Carpathians of northeastern Slovakia: structural evidence for transpression. Tectonophysics (in press). Richter M. F., Lovera O. M., Harrison T. M. and Copeland P. (1991) Tibetan tectonics from 4°Ar/39Ar analysis of a single K-feldspar sample. Earth Planet. Sci. Lett. 105, 266-278. Sch~irer U., Xu R. H. and All~gre C. J. (1984) U-Pb geochronology of Gangdeze (Transhimalaya) plutonism in the Lhasa-Xigaze region, Tibet. Earth Planet. Sci. Len. 69, 311-320. Searle M. P., Windley B. F., Coward M. P., Cooper D. J~ W., Rex A. J., Rex D., Tingdong L., Xuchang X., Jan M. Q., Thakur V. C. and Kumar S. (1987) The closing of Tethys and the tectonics of the Himilaya. Geol. Soc. Am. Bull. 98, 678--701. Searle M. P., Pickering K. T. and Cooper D. J. W. (1990) Restoration and evolution of the intermontane Indus molasse basin, Himalaya, India. Tectonophysics 174, 301-314. Sun Y. Y., Chai Z. F., Mao X. Y. et al. (1984) The discovery of Iridium anomaly nearby the Cretaceous-Tertiary boundary, Zhongba country, Xizang (Tibet) and its significance. In Symposium on Himalayan Geology (abstracts) Chengdu, P.R. China. Tapponnier P., Mercier J. L., Proust F. et al. (1981) The Tibetan side of the India-Eurasia collision. Nature 294, 405-410. Tapponnier P., Peltzer G. and Armijo R. (1986) On the mechanics of the collision between India and Asia. In Collision Tectonics (Coward M. P. and Ries A. C., eds). Geol. Soc. London Spec. Publ. 19, 115157. Treloar P. J., Potts G. J., Wheeler J. and Rex D. C. (1991) Structural evolution and asymmetric uplift of the Nanga Parbat syntaxis, Pakistan Himalaya. Geol. Rdsctt 80, 411-428. Turner F. J. (1953) Nature and dynamic interpretation of deformation lamellae in calcite of three marbles. Am. J. Sci 251, 276-298. Wan Z., Li G., Cao Y., Gu Q., Zhou X., Zhang S., Wu Q. and Yuan X. (1982) Tectonics of Yarlung Zangbo suture Xizang (Tibet) - - Guide to geological excursion. Geol. Soc. Xizang, China 4, 1-49. Wang S. E. (1983) The age of the Qiuwu coal-bearing strata in Xizang (Tibet), China. In Colloque Franco-Chinois sur la g~ologie de l'Himalaya (abstracts) Montpellier. Wenk H.-R., Takeshita T., Bechler E., Erskine B. G. and Matthies S. (1987) Pure shear and simple shear calcite textures. Comparison of experimental, theoretical and natural data. J. Struct. Geol. 9, 731-745. White S. H., Burrows S. E., Carreras J., Shaw N. D. and Humphreys F. J. (1980) On mylonites in ductile shear zones. J. Strucr Geol. 2, 175-187. Winkler H. G. F. (1979) Petrogenesis o f Metamorphic Rocks. Springer, New York. Wu H. R., Wang D. G. and Wang L. C. 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Zhang R. Z. et aL (1980) A scienttfic guide book to South Xizang (Tibet), 2-14 June. Academica Sinica, Beijing. Zhang Y., Dai T. and Hong A. (1981) Isotopic geochronology of granitoid rocks in southern Xizang plateau. In Pro~ Syrup. Quinghai Xizamg (Tibet) plateau (Beijing, China. (Vol. 1: Geological History and Origin of Quinghai-Xizang pleateau), pp. 483--495. Science Press, Beijing. Zoback M. D., Zobaek M. L., Mount V. S. et al. (1987) New evidence on the state of stress of the San Andreas fault system. Science 238, 1105-1111.

APPENDIX

1

Additional K/Ar Data from S-Tibet Sample No.

Mineral

Grain size

%K

40Arrad scc/g.10-5

%40Arrad

Isotopic age (Ma)

TR221

actinolite*

>12.5 ttm

0.56

0.171

64.8

76.9--3.8

TR228

biotite1"

>0.5 cm

7.82

0.607

63.75

19.9_ + 1.0

*Syntectonic actinolite of Gangdise belt amphibolite: long. 89"58'E, lat. 29°19'N. tBiotite from anatectic pegmatite of Indian plate migmatite zone: long. 94040'. lat. 29*37'.

APPENDIX

2

Fault-Striae Analysis and Calculation of the Reduced Paleostress Tensor Fault size and attitude, striae orientation, sense of slip and polyphase slip and its chronology were measured and determined in the field. Fault size is classified qualitatively based on an estimation of the displacement and the lateral extent of the fault. The aim was to discriminate first order faults and to enable a comparison of faults measured in outcrops with those inferred from maps. Striae orientation and relative slip chronology enable the discrimination of superposed paleostress states. We used fault offset and superposition of fibers grown behind fault steps as chronology indicators. Riedel shears, steps on the fault surface, and fibers grown behind steps were used for sense of slip determination. We used four different methods to derive principal stress orientations and stress ratios from fault-striae data. The "direct inversion method" (Angelier and Goguel, 1979; Angelier, 1979) performs a least square minimization of the angular discordance between the calculated orientation of the maximum shear stress and the measured striae. The "grid search method" (Gephart, 1990; Hardcastle and Hills, 1990) identifies all possible tensors that fit all or portions of the data. The method tests thousands of tensors against the fault data. For each tensor position, the stress ratio is varied in increments. For each tensor configuration, it is determined whether the Mohr-Coulomb yield criterion ('r>~C+ ~o-,,) is satisfied for each fault. Parameters chosen in this study are C=0, because we assume pre-existing weaknesses (faults, fractures, bedding planes), and 1~=0.2-0.4 (depending on the rock type analyzed and the presence of fault gouge), which is close to natural observation [e.g. Zoback et al. (1987)] as opposed to 0.6 to 0.9 from laboratory experiments (Byerlee, 1978). The "right dihedra method" (Angelier and Mechler, 1977) calculates compression and extension right dihedra

Deformation and motion along margins of the Lhasa block

53

for each fault, superposes the dihedra, and derives areas of maximum compression and extension (containing 0.1 and 0.3, respectively). Conditioned least square fitting is used to derive orthogonalized loci of 0.1 to 0.3 (Caputo and Caputo, 1988). The "pressure-tension (P-T) axes method" (Turner, 1953) places 0-1 30 ° (as an empirical value) from the fault plane in the plane defined by the pole to the fault plane and the striae in such a way as to cause movement in the direction recorded for each fault. 0-3 is placed 90° from 0-1. Again a conditioned least square fit is used to locate mean 0-1 to 0-3 orientations. The quality and the quantity of field data determined selection of method used for calculations. The PT-axes and the dihedra methods were used with scarce data and where no time was available in the field for careful analysis of fault and striae characteristics. A comparison of methods is given by Ratschbacher et al. (1993). APPENDIX

3

Monophase Faulting and Folding in Qiuwu Molasse and Xigaze Flysch Relations between faulting and folding are illustrated using data from the Qiuwu molasse (Fig. 5). Prior to folding predominantly reverse faults developed (Fig. 13-1), with the S dipping set (bolt lines in Fig. 13) stronger developed than the N dipping one (dotted lines) as revealed by the raw data of station LRSYN which were collected along fiat-lying or shallowly dipping strata (Fig. 5). Concentric folding induced gliding of strata and striae developed on bedding surfaces (Fig. 13-3). The last stages of the folding-faulting process are reverse faults

N

$ dl

1: mixed reverse and normal fault systmms ~: beddlng pemllel gliding . ~ w I r e n t pure nonnmJfault sys-bmm

2: hinge co[kq:~e 4: elufy conjugate Nlveflm ~ u b letm, post-folding reverse faults

Fig. 13. Monophase compressional faulting and folding in Quiwu molasse and Xigaze flysch illustrated in fold profile and lower hemisphere stereoplots. Reconstruction of passive rotation of early conjugate reverse fault systems (4) during progressive folding (1, 5) and faults formed by bedding parallel gliding (3) and hinge collapse (2). Dotted faults play a minor role in the fault pattern of the Quiwu molasse [Fig. 5(b)].

54

LOTHAR RATSCttBACHER et aL

truncating entire folds [Fig. 5(b), station LRLATE]. The direction of crl remained constant relative to the fold axes during the progressive folding-faulting process. In some cases, reactivation of earlier conjugate fault systems occurred. Normal faults were produced in two ways: (1) tilting of early reverse fault systems; this resulted in mixed reverse and normal dip-slip faults and in apparent pure normal fault systems when bedding dip became larger than about 60° (Fig. 13-1, 13-5), (2) hinge collapse in overturned faults (Fig. 13-2). These fault-fold relations indicate that folding and set 1 faulting were widely coeval. Reduced stress tensors were calculated from data collected predominantly in hinge zones [e.g. Fig. 5(b), station LR132] or shallowly-dipping limbs [e.g. Fig. 5(b), station LRSYN]. In station LR141 [Fig. 5(b)] tensor calculation is based on the reverse faults collected in the broad hinge zone above Jiang Qin Zhe [Fig. 5(a)] and neglects the normal faults in the inverted limb to the S. The tensor calculated from the entire data collected along the profile [Fig. 5(b), LRSUM] closely correponds to those of the single stations along the profile. We attribute this to the following: (1) the generally symmetric fold shapes led to opposite and equal rotations of pre-existing faults in the limbs, (2) the predominantly dip-slip nature of the faults keep the poles to the faults and the striae on a great circle on the stereoplots during rotation of the limbs, and (3) the direct inversion method (see above) does not recognize sense of slip on the faults; slip sense on the apparent normal faults is reversed during tensor calculation.