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Displacement and timing along the northern strand of the Altyn Tagh fault zone, Northern Tibet
EPSL ELSEVIER
Earth and Planetary
Science Letters 150
(1997)55-64
Displacement and timing along the northern strand of the Altyn Tagh fault zone, Northern Tibet Erchie Wang Department
*
of Earth, Atmospheric, and Planetary Sciences. Massachusetts Institute of Technology, Cambridge, MA V21.W USA Received
11 March 1997; revised 17 April 1997: accepted
17 April 1997
Abstract The northern strand of the Altyn Tagh fault has a late Cenozoic left-slip offset of 69-90 km near its northeastern end. Deformation began in this area as early as the Oligocene but left-slip displacement may be mainly of Middle Miocene to Recent age. Because the Altyn Tagh fault zone transfers strike-slip displacement into shortening in the Qilian Shan, the total offset changes along the fault, but the magnitude of displacement along this part of the fault zone suggests that only small-scale eastward movement of crustal rocks occurred in this part of the Tibetan plateau during the late Cenozoic. 0 1997 Elsevier Science B.V. Keww-ds: Qinghai-Xizang
Plateau; displacements;
fault zones; Cenozoic
1. Introduction The Altyn Tagh fault zone, which figures prominently in all interpretations of Cenozoic intracontinental deformation related to India/Eurasia convergence, strikes northeast for N 1600 km along the northern margin of the Tibetan plateau (Fig. 1). It has been interpreted as having a displacement of 150 km to 500-600 km (e.g. [l-4]) and, according to various workers, may have had its inception in the middle Miocene [3] or Pliocene [1,5]. However, data to support these interpretations are generally lacking. Despite the fact that the fault zone displays clear evidence for active left-lateral displacement [6-91 no
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evidence has yet been presented for total displacement or time of initiation for any part of the fault zone. Cenozoic rocks exposed in the Alkasay and Subei areas provide an opportunity to place bounds on total Cenozoic displacement and time of inception of faulting along this part of the Altyn Tagh fault zone. There may be a pre-Cenozoic history of the Altyn Tagh fault but it is not relevant to the present discussion.
2. Regional setting The Altyn Tagh fault zone forms a linear zone of faults striking about N 60” E and lying mainly within the northern part of the Tibetan plateau (Fig. 1). Along its eastern trace, the fault zone passes obliquely through the Altyn Tagh, an irregular group of mountains that separates the Tarim basin from the Qsi-
0012-821X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. P/I SOOl?-821X(97)00085-X
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and Planetary Science Letters 150 (1997) 55-64
damu basin and the Qilian Shan. To the southwest and northeast the Altyn Tagh fault zone merges with structures in the Kun Lun and Qilian Shan mountain belts respectively. In the area studied, the Altyn Tagh fault zone consists of three major faults, here referred to as the north, central and south Altyn Tagh faults (Figs. 1 and 2). The north Altyn Tagh fault comprises two linear segments east and west of Subei. The south Altyn Tagh fault is gently curved and partly forms the northern boundary of the Qsidamu basin southwest of Subei. The central Altyn Tagh fault contains an important right-stepping restraining bend south of Subei. Along the north and central faults, the main focus of this study, there is abundant evidence for active left slip [9]. Each of the three parts of the Altyn Tagh fault zone merges to the northeast with major thrust belts along the Qilian Shan.
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Rocks along the fault zone are dominated by lowto medium-grade late Proterozoic igneous and sedimentary rocks and early Paleozoic volcanic rocks, with less common outcrops of Mesozoic and Cenozoic rocks. The Precambrian metamorphic rocks present between the north and south Altyn Tagh faults range from the early Proterozoic Yemananshan Group, consisting of high-grade gneiss, schist and marble, to the late Proterozoic Danghe Group, consisting of low-grade slate, sandstone and stromatolitic carbonate rocks. Northeastward, these rocks extend into the Daxue Shan and South Danghe thrust belts. Precambrian rocks present north and south of the Altyn Tagh fault zone are assigned to the Dunhaung and Da-ken-da-ban groups of early Proterozoic age which form the basement rocks for the Qsidamu and Talimu basins [ 12,131. Tertiary rocks within the Altyn Tagh fault zone
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late Pleistocene and Holocen
Mesozoic and Paleozoic
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basement complexs of Qsidam
q and Tarim basins
Fig. 1. Simplified geological map of the northeastern part of the Tibetan plateau. Inset map shows the more regional setting of the part of the fault zone discussed in this paper and letters refer to locations mentioned in the text. Black areas in inset are locations of Tertiary rocks mentioned in the text. SD = South Danghe range; DX = Daxue Shan range; NBT = northern boundary thrust of the Qilian Shari;; NAT= North Ahyn Tagh fault; CAT= Central Altyn Tagh fault; SAT= South Ahyn Tagh fault.
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Science Letters I.50 (1997) 55-64
stream and Holocene late Pleistocene
: : ......
q early Pleistocene m middle Tertiary
q
pre-Cenozoic
Fig. 2. Generalized map along the northwestern part of the Altyn Tagh fault zone showing the location of the three areah of Tertiary rock interpreted as being offset across the fault zone. Upper limits for left-lateral displacement on the North and Central Altyn Tagh fnults are also shown.
are dated as late Oligocene to perhaps as young as early Middle Miocene (see below). They are terrestrial red beds and have experienced significant deformation. They are similar to Tertiary rocks present within the central Qilian Shan, but differ from correlative rocks within the Qsidamu and Talimu basins that form a continuous sequence of grey lacustrine and tluvial deposits from Paleogene to Neogene age.
3. Cenozoic history of the northern strand of the
Altyn Tagh fault zone Cenozoic rocks are present in the study area and can be used to evaluate the Cenozoic history of the Altyn Tagh fault zone. 3. I. Subei area
The most well studied Cenozoic rocks are exposed in the Subei area where they consist of three units. The oIdest unit rests unconformably on preCenozoic. mostly Precambrian, rocks and consists of
more than 2000 m of orange-weathering sandstone and mudstone interbedded with gypsum and conglomerate. The second unit paraconformably overlies the oldest unit and consists of about 200 m of greenish, coarse grained conglomerate, purple sandstone and siltstone. Bohlin [l 11 assigned these rocks an Oligocene to Miocene age based on abundant fossil assemblages in the area of Wugequan and Yandantu (see Fig. 3 and Appendix A). Unfortunately, he did not make clear the stratigraphic position of the fossil collections and later workers have assigned the rocks to either an Oligocene (unpublished data in Chinese) or Miocene [l 11age. Dr. Will Downs examined Bohlin’s fauna1 list and indicated the assemblage is from the Oligocene/Miocene transition and has no elements to indicate it is younger than 16 Ma or even 16 Ma (Downs. pers. commun., 1996). The younger of these two units correlates lithologically with the Miocene/Pliocene Shulehe Formation [ 121 present along the northern edge of the Tibetan plateau, with its type area in the Yumen area (see Fig. 1 inset), at the northeastern-most corner of the plateau. If this correlation is correct, it
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Earth and Planetary Science Letters 150 (1997) 55-64
94 :30” m
I:
stream late
Pleistocene
JJJearly
Pleistocene
5 Lm
m middle Tertiary npre-Cenozoic
El
Bohlin’s
locality
. i
fossil
::
ff:
1% 3f
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Fig. 3. Geological relations in the Subei area. Open boxes indicate the localities of Bohlin’s (1937) collections were not keyed to localities and attempts by many workers to recollect his material have been unsuccessful.
m \upper
Pleistocene m
early Pleistocene m
[l 11. Unfortunately
fossils
middle Tertiary (u55’1strike and dipping angle Paleozoic Proterozoic
Fig. 4. Geological relations in the Subei and Dabiegai areas discussed Altyn Tagh fault using different geological features.
in the text. Also shown are the magnitudes
of offset along the central
E. Wang/Earth
and Planeta~
Science Letters
would suggest none of Bohlin’s fossils came from the younger unit; however, attempts by many workers to collect new samples of Bohlin’s fauna have not been successful. Considering the uncertainties in the ages of these two units in the Subei area, these two older Tertiary units are combined into a single unit on the figures. The youngest unit consists of more than 150 m of grey and brown, poorly sorted, cobble and boulder conglomerate. Similar rocks are widespread throughout this part of the Tibetan plateau and the adjacent Tarim and Qsidamu basins where they contain Early Pleistocene fossils [ 121. In the Subei area these rocks are only in fault contact with the older two units (Figs. 2 and 4), but unconformably overlie them in the Liuchengzi area (Fig. 5). The oldest two rock units are strongly folded within a restraining bend of the Altyn Tagh fault zone (Fig. 4), and are bounded by two faults with Z-shaped arcuate traces that generally parallel fold axes within the Tertiary rocks. The southern fault
aHolocene
q upper
middle Pleistocene aearly
and late
f 1997)
Proterozoic(lower-grade
n
relations in the Liuchengzi
59
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dips south and thrusts Proterozoic rocks over Proterozoic and Tertiary rocks, and its footwall contains the depositional contact between the Tertiary rocks and Proterozoic basement. The northern fault is subvertical and juxtaposes folded Tertiary rocks against gently north-dipping, unfolded Lower Pleistocene rocks and is overlapped by the youngest alluvium. The northern fault may have been originally a south-dipping thrust fault later modified by younger deformation. The Z-shaped faults merge to the west and east with the north and central Altyn Tagh faults, respectively and the folded Tertiary rocks wedge out between them (Fig. 4). These relations are interpreted as being the result of a restraining bend between the north and central Altyn Tagh faults. The northeast projection of the north Altyn Tagh fault crosses Upper Pleistocene rocks for a distance of about 18-20 km north of Subei, and there is no evidence that this segment of the fault is active. Farther northeast, beyond outcrops of Upper Pleistocene rocks, a fault, parallel to the north Altyn Tagh
Proterozoic(high-grade
Hlower Pleistocene13aec(elevation in meters middle Tertiary (stream
Fig. 5. Geological
150
metamorphic
metamorphic rocks)
rocks)
N $ 5 km
area north of the North Altyn Tagh fault.
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fault west of Subei, juxtaposes Proterozoic and Cambrian rocks. This fault offsets rivers by l-4 km left-laterally (Fig. 4) and is considered here to be the eastern continuation of the north Altyn Tagh fault. Recent work by Meyer et al. 1141 has shown that the eastern part of the north Altyn Tagh fault is active and near its eastern exposures has a slip rate of 4 f 2 mm/a. Southwest of where the arcuate faults merge with the north Altyn Tagh fault, the Altyn Tagh fault is active and alluvial fans and rivers are offset by 50-300 m [lo]. Ge [lo] has shown that, in the A’kasay area, an alluvial fan dated at 35.4 k 0.26 Ka is offset 65 m left-laterally, yielding a slip rate of 1.8 mm/a. The oldest constrained age for initiation of deformation on this part of the Altyn Tagh fault is after the deposition of the two oldest Tertiary units. Unfortunately, the age of the youngest of these two units is not well constrained. It is certainly younger than 16 Ma but could be younger than Miocene/Pliocene, if the correlation with the rocks in the Yumen area is valid. The contrast in degree of deformation between the two Tertiary units and the Lower Pleistocene rocks suggests much of the deformation related to this part of the Altyn Tagh fault is older than Early Pleistocene.
Folds in the Tertiary rocks have WNW trending axes with bedding dips of 20-30”, except near the central Altyn Tagh fault where bedding dips are vertical and bedding strike becomes more nearly parallel to the fault. The central Altyn Tagh fault dips north and Proterozoic rocks are thrust south over the Tertiary rocks. The southern boundary of the Tertiary rocks is marked by the south-dipping south Altyn Tagh fault that thrusts late Proterozoic rocks over Tertiary rocks (Fig. 4). There are no unique piercing points from which to determine the offset of the Tertiary rocks across the central Altyn Tagh fault between the Dabiegai and Subei areas. A maximum offset of 18 km is indicated by correlating the NW trending unconformable contact between Upper Pleistocene and the Tertiary rocks in both areas (Figs. 2 and 4). A similar offset of 16 km is obtained by interpreting the deflection of the Danghe River as being caused by the left-lateral displacement on the central Altyn Tagh fault. Thus, a maximum of 16-18 km left-lateral offset is interpreted for the central Altyn Tagh fault. The Danghe river bends left laterally and follows the fault trace for 4 km before crossing the fault, suggesting some young displacement on the fault.
3.2. Dabiegai area
South and west of Liuchengzi, the Altyn Tagh fault zone shows evidence of shortening as well as left-lateral displacement (Fig. I). Between the north and south Altyn Tagh faults Proterozoic metamorphic rocks and early Paleozoic ultramafic rocks underlie the highest mountains in the region (up to 5798 m). Along the north Altyn Tagh fault these rocks are thrust northward onto Tertiary red beds in the Liuchengzi area. West of Liuchengzi the north and south Altyn Tagh faults merge and the wedgedshaped region of rocks between the faults to the east is interpreted to be in a zone of transpression. North of the north Altyn Tagh fault, in the Liuchengzi area, Proterozoic metamorphic rocks form the core of a NW trending dome-shaped mountain, reaching more than 4 km elevation. The Proterozoic rocks are unconformably overlain by orangeweathering sandstone and mudstone with some conglomerate exposed on the north and south flanks of
In the Dabiegai area, the central Altyn Tagh fault merges with the westernmost structures of the South Danghe range, part of the Qilian Shan (Figs. 1, 2 and Figs. 4). West of Dabiegai, rocks lithologically correlative to the older two Tertiary units in the Subei area rest unconformably on pre-Mesozoic rocks. No fossils are reported from these rocks but they lithologically correlate with the units at Subei, Yumen and with fossiliferous rocks of Oligocene age in the central Qilian Shan [ 111, and they are interpreted to be Oligocene/Miocene or perhaps as young as Miocene/Pliocene in age. The Tertiary rocks in the Dabiegai area are folded, but generally less strongly than near Subei. They are unconformably overlapped by gently dipping, unfolded Upper Pleistocene alluvial deposits which form the eastern boundary of the Tertiary rocks.
3.3. Liuchengzi
area
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the mountain (Fig. 5). No fossils have been reported from these rocks but they are lithologically similar to the oldest unit in the Subei area, thus they are tentatively assigned an Oligocene to early (?) Middle Miocene age. These rocks are unconformably overlain on the north side of the mountain by gently dipping, grey and brown, cobble and boulder conglomerate, assigned an Early Pleistocene age based on lithological correlation with dated rocks in the region. The Tertiary rocks in the Liuchengzi area are folded along WNW trending fold axes with bed dips of 20-25” (Fig. 5). Unconformably underlying Proterozoic metamorphic rocks are also folded along WNW trending axes, but with steep flank angles (60-80”) and local overturning. Locally, the Tertiary rocks are involved in Proterozoic-cored overturned folds or on the south side of the mountain are overthrust by Proterozoic rocks. This demonstrates that some structure within the Proterozoic rocks is also of Tertiary age. Near the north Altyn Tagh fault attitudes in both Proterozoic and Tertiary rocks curve into parallelism with the fault in a left-shear sense. Cobble and boulder conglomerate of Early Pleistocene age unconformably overlies the folded Tertiary rocks along the north ff ank of the mountain and is unfolded, but dips generally 10-30” N or NE. The rock types and structures in the Liuchengzi area are similar to those in the Subei and Dabiegai areas and lithological correlation with dated rocks in the Subei area indicate the timing of deformation between the areas is also similar. Caught within the north Altyn Tagh fault between the Liuchengzi and Subei areas are slivers of highly cataclastically deformed red and orange sedimentary Tertiary rocks. Locally, they are overlapped unconformably by coarse conglomerate of probable Early Pleistocene age. The conglomerate is locally cut by the fault, but not folded and dips gently to the north. It is either unconformably overlapped by younger Pleistocene rocks or truncated by an erosion surface that slopes north into the Tarim basin. These observations suggest that much of the displacement on the north Altyn Tagh fault is probably pre-Pleistocene. No piercing points can be determined between the Liuchengzi and Subei areas, so a unique magnitude of displacement cannot be determined. A possible maximum offset of 67 km is suggested by matching
the eastern limit of Tertiary rocks (Fig. 2). A possible minimum offset of 48 km is suggested by matching the western-most extent of the same rocks.
4. Cenozoic total offset, inception its implications
of faulting
and
The three areas of Tertiary rocks discussed above are interpreted as having formed a continuous belt of WNW trending folds prior to initiation of the Altyn Tagh fault, but probably during an early stage of left-lateral shear. Conglomerate in the Tertiary rocks contains cobbles dominated by Proterozoic lithologies, indicating some relief and perhaps some deformation began during their deposition. The nature of this deformation remains unknown but is suggested
ariy
middle Tertia
nd of middle Tertia
mProterozoic mcrystalline
in central Qilian basement of Tarim
Fig. 6. Possible structural evolution in the Subei, Dabiegai and Liuchengzi areas. Early middle Tertiary deformation expressed as NW trending positive and negative areas. possibly shortening structures related to early left-lateral shear. Initiation of Altyn Tagh fault zone and transfer of left-slip into NW trending structures in the Subei area in the Qilian Shan near end of the middle Tertiary; development of the restraining bend in the Subei area. Continued left shear along the Altyn Tagh fault and transfer into shortening within the Qilian Shan during the Quaternary.
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to be the beginning of folding in the area (Fig. 6). Following deposition of the Tertiary rocks, the main left-lateral displacement on the Altyn Tagh fault zone occurred, cutting through the folds and in the Subei and Dabiegai areas, and modifying the structures by the development of restraining bends (Fig. 61. Summing the range of total Tertiary offsets determined for the folded belt along the north and central Altyn Tagh faults yields a range of 64-85 km, with a value closer to the higher end of the range being more likely. Some uncertainty remains because piercing points cannot be established, but the general correlation of the belt of Tertiary rocks yields an offset within this range. Some additional displacement may occur in the restraining bend at Subei. Comparing the more open folds in the Dabiegai and Liuchengzi areas to the tighter folds at Subei suggests at least an additional shortening of about 5 km could be present, thus yielding a range of 69-90 km for the left-slip on the north and central Altyn Tagh faults. Only two other localities of Tertiary rocks present along the Altyn Tagh fault might be used to arrive at a different interpretation, but their deformational style and lithology suggest they are unlikely correlatives for rocks in the study area. One, about 60 km northeast of Subei (a, Fig. 1 inset), lies between the two branches of the north Altyn Tagh fault and the other, about 200 km southwest of Liuchengzi Cd, Fig. I), lies north of the fault zone. The Neogene rocks at point a are not as strongly folded as the other areas and do not appear to be lithologically correlative, and the rocks at point d have rock types unknown at Subei (such as limestone and gypsum) and are much thicker, by a factor of 2 or 3, than rocks at Subei. If the total offset of 69-90 km occurred at a uniform rate, only a range of long-term slip rates can be calculated based upon the time of inception of the faulting because the youngest age of the older two units in the Subei area remain uncertain. In all areas these two sedimentary rock units are without angular unconformity. All that can be said for certain is that folding and movement on the Altyn Tagh fault is younger than 16 Ma. If the younger unit is Miocene/Pliocene in age, based on correlation with the Yumen area, it would indicate folding of the
Tertiary rocks did not begin until after deposition of the Miocene/Pliocene conglomerate. The structural and stratigraphic evidence presented above suggests most of the offset took place in the interval between either 16 Ma or the Miocene/Pliocene and early Quaternary. This time interval is poorly determined and, depending upon what time is picked for the initiation of major left-lateral faulting, a long-term slip rate for this interval could range between 7-9 mm/a and 35-45 mm/a. The total offset on the north and central Altyn Tagh faults is very important because it begins to limit how much deformation can be associated with various models for the development of the Tibetan plateau. For example, Burchfiel et al. [15] and Tapponnier et al. [16] have suggested the Altyn Tagh fault zone functions as a transfer fault transferring left-slip displacement into shortening south of the Altyn Tagh fault zone. In general, transfer faults have variable magnitudes of displacement along them because, as they transfer motion from strike-slip to shortening or extension, the magnitude of strike-slip displacement must change. For the Altyn Tagh fault zone, if strike-slip is transferred into shortening south of the fault, the fault zone should loose displacement to the northeast and gain displacement to the southwest. Thus, the magnitude of displacement determined here is only valid for a small segment of the fault in the region studied. However, it does suggest that the amount of shortening in the Qilian Shan east of the Dabiegai area is about 69-90 km. No total offset for the south Altyn Tagh fault has been determined. It appears to merge and be transferred into shortening in the structures of the South Danghe Range in the Qilian Shan (Fig. 1) and thus does not effect structures farther to the northeast. However, it bounds the Qsidamu basin on the north and merges with the north Altyn Tagh fault to the southwest and would thus add to the total displacement on the fault zone farther to the southwest. Shortening in the Qsidamu basin is expressed in a series of WNW trending folds that expose mainly Quatemary rocks in their cores and appear to have less shortening than the structures in the Qilian Shan that contain rocks as old as Precambrian, suggesting the total offset southwest of where the north and south Altyn Tagh faults merge to the southwest is less than twice that measured in the study area. It
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and Planetary Science Letters I50 (1997) 55-64
further suggests that eastward movement of crustal rocks in this part of the Tibetan plateau has been of relatively small magnitude, perhaps less than 200 km.
Acknowledgements This work was supported by NSF grant 8904096 awarded to B.C. Burchfiel and L.H. Royden. The author thanks G.A. Davis, L. Ratschbacher, A.W. Bally. X. Fang and an unknown reviewer for helpful reviews. [RV]
A.2. Central Qilian Shan Oligocene fossils: Palaeoerinaceus kansuensis, Desmatolagus sp. (shargaltensis), Sinolagomys kansuensis, Tachyoryctoides sp., Paleoerinaceus cf. acridens, D. pamidens, D. large species, S. gracilis, Tuchyopctoides obrutschewi, T. intrrmedius. T. pachygnathus, Karakoromys cf. decessus. Leptotataromys gracilidens, Tsaganomys altncicus. Didymoeonus sp.. Indricotherium sp.
References [II B.C. Burchfiel.
Appendix A. Fauna1 list from Bohlin (1937), and his age assignments
A. 1. Subei area Oligocene fossils: Pulaeoerinaceus kansuensis, P. minimus, P. cf. rectus, Desmatolagus sp. (shargaltensis), Sinolagomys kansuensis, S. major, Parasminthus asiae-centralis, P. tangingoli, P. paroulus. Tachyovctoides sp., Tataromys grangeri, T. sigmodon, T. sigmodon, T. cf. plicidens. Yindirtemys woodi. Miocene fossils: Sayimys obliquidens. Schizotherium? sp., Kansupithecus sp., aff. Gomphotherium connexus, Testudo honanensis, T. tunhunensis, T. chienfuturgensis, Palaeoscaptor sp., Lnsecliuora sp., Palacoerinaceus cf. rectus, Matheweta granger, P. kansuensis, P. minimus, Soicidae sp.. Talpidec sp., Desriatolagus shargaltensis, D. paruidens. Sinolagomys kansucnsis, Tataromys phiwidens Matthew et Granger, Karakoromys cf. decescus. K. sp. Tachyovctoides ohrutschewi, T. interP. tangingoli, medius?, T. pachygnathus, Tsaganomys aitaicus Matthew et Granger, Sciurus sp. Parasminthusasiaecentralis, T. tangingoli, P. cf. Sicistinae sp.. Sicitinae parulus, Sp., cf. Cricetodon sp., aff. Eumys sp., Rhizomyidae sp., Tataromys grangeri Bohlin, Leptotaromys gracilidens Bohlin, Yindirtemys woodi, Schizotherium sp,, Eumerys sp.
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L.H. Royden. Tectonics of Asia SO years after the death of Emile Argand, Eclog. Geol. Helv. X4 ( 199 1) 599-629. [?I J.F. Dewey. S. Cande, W.C. Pitman III. Tectonic evolution of the India/Eurasia collision zone, Eclog. geol. Helv. X4 (1991) 717-734. [31 P. Tapponnier. G. Peltzer, R. Armijo. On the mechanics of the collision between India and Asia: in: M.P. Coward, A.C Ries (Eds.). Collision Tectonics. Geol. Sot. London Spec. Pub]. I9 (1986) 115-157. Formation and evolution of L41 G. Peltzer, P. Tapponnier. strike-slip faults. rifts, and basins during the India-Asia collision: an experimental approach. J. Geophya. Res. 93 (1988) 15085-151119. [51A.W. Bally, I.M. Chou, R. Clayton. H.P. Eugster. S. Kidwell, L.D. Meckel. R.T. Ryder, A.B. Watts. A.A. Wilson, Notes on sedimentary basins in China. Report of the American Sedimentary Basins Delegation to the People’s Republic of China. U.S. Geol. Surv. Open File Rep. X6-327, 1986. [61P. Molnar. P. Tapponnier. Cenozoic tectonics of Asia: Effects of a continental collision, Science I89 (1975) 419-426. [7] G. Peltzer. P. Tapponnier. R. Armijo. Magnitude of Late Quatemary left-lateral displacement along the northern edge of Tibet, Science 246 (1989) 1285-1289. [8] Y. Gaudemer. P. Tapponnier, D.L. Turcotte, River offsets across active strike-slip faults. Ann. Tecton. III (1989) 55-76. 191 P. Molnar. B.C. Burchfiel, L. K’uangyi. 2. Zhao, Geomorphic evidence for active faulting in the Altyn Tagh and northern Tibet and qualitative estimates of its contribution to the convergence of India and Eurasia. Geology 15 f 1987) 249-253. [IO] S. Gc. Altyn Tagh active fault zone. Seism. Press I I4 (1992). [I II B. Bohlin. OberoligozBne SIugetiere aus dem Shargalteintal western Kansu. China Region. Paleontol. Xinbingzhong 3 (1937). [I21 Anon, Regional Geology of Gansu Province, Beijing Publishing House, 1989. [I31 Anon, Regional Geology of Qinghai Province. Beijing Publi
64 [14] B. Meyer, P. Tapponnier,
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Y. Gaudemer, G. Peltzer, S. Guo, Z. Chen, Rate of left-lateral movement along the easternmost segment of the Altyn Tagh fault, east of 96” E (China), Geophys. J. Int. 124, 29-44. [15] B.C. Burchfiel, Q. Deng, P. Molnar, L.H. Royden, Y. Wang, P. Zhang, W. Zhang, Intracrustal detachment within zones of continental deformation, Geology 17 (1989) 448-452.
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[16] P. Tapponnier, B. Meyer, J.P. Avouac, G. Peltzer, Y. Gaudemer, G. Shunmin, X. Hongfa, Y. Kelun, C. Zhitai, C. Shuahua, D. Huagang, Active thrusting and folding in the Qilian Shan, and decoupling between upper cmst and mantle in northeastern Tibet, Barth Planet. Sci. Lett. 97 (1990) 382-403.
Earth and Planetary
Science Letters 150
(1997)55-64
Displacement and timing along the northern strand of the Altyn Tagh fault zone, Northern Tibet Erchie Wang Department
*
of Earth, Atmospheric, and Planetary Sciences. Massachusetts Institute of Technology, Cambridge, MA V21.W USA Received
11 March 1997; revised 17 April 1997: accepted
17 April 1997
Abstract The northern strand of the Altyn Tagh fault has a late Cenozoic left-slip offset of 69-90 km near its northeastern end. Deformation began in this area as early as the Oligocene but left-slip displacement may be mainly of Middle Miocene to Recent age. Because the Altyn Tagh fault zone transfers strike-slip displacement into shortening in the Qilian Shan, the total offset changes along the fault, but the magnitude of displacement along this part of the fault zone suggests that only small-scale eastward movement of crustal rocks occurred in this part of the Tibetan plateau during the late Cenozoic. 0 1997 Elsevier Science B.V. Keww-ds: Qinghai-Xizang
Plateau; displacements;
fault zones; Cenozoic
1. Introduction The Altyn Tagh fault zone, which figures prominently in all interpretations of Cenozoic intracontinental deformation related to India/Eurasia convergence, strikes northeast for N 1600 km along the northern margin of the Tibetan plateau (Fig. 1). It has been interpreted as having a displacement of 150 km to 500-600 km (e.g. [l-4]) and, according to various workers, may have had its inception in the middle Miocene [3] or Pliocene [1,5]. However, data to support these interpretations are generally lacking. Despite the fact that the fault zone displays clear evidence for active left-lateral displacement [6-91 no
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evidence has yet been presented for total displacement or time of initiation for any part of the fault zone. Cenozoic rocks exposed in the Alkasay and Subei areas provide an opportunity to place bounds on total Cenozoic displacement and time of inception of faulting along this part of the Altyn Tagh fault zone. There may be a pre-Cenozoic history of the Altyn Tagh fault but it is not relevant to the present discussion.
2. Regional setting The Altyn Tagh fault zone forms a linear zone of faults striking about N 60” E and lying mainly within the northern part of the Tibetan plateau (Fig. 1). Along its eastern trace, the fault zone passes obliquely through the Altyn Tagh, an irregular group of mountains that separates the Tarim basin from the Qsi-
0012-821X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. P/I SOOl?-821X(97)00085-X
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damu basin and the Qilian Shan. To the southwest and northeast the Altyn Tagh fault zone merges with structures in the Kun Lun and Qilian Shan mountain belts respectively. In the area studied, the Altyn Tagh fault zone consists of three major faults, here referred to as the north, central and south Altyn Tagh faults (Figs. 1 and 2). The north Altyn Tagh fault comprises two linear segments east and west of Subei. The south Altyn Tagh fault is gently curved and partly forms the northern boundary of the Qsidamu basin southwest of Subei. The central Altyn Tagh fault contains an important right-stepping restraining bend south of Subei. Along the north and central faults, the main focus of this study, there is abundant evidence for active left slip [9]. Each of the three parts of the Altyn Tagh fault zone merges to the northeast with major thrust belts along the Qilian Shan.
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Rocks along the fault zone are dominated by lowto medium-grade late Proterozoic igneous and sedimentary rocks and early Paleozoic volcanic rocks, with less common outcrops of Mesozoic and Cenozoic rocks. The Precambrian metamorphic rocks present between the north and south Altyn Tagh faults range from the early Proterozoic Yemananshan Group, consisting of high-grade gneiss, schist and marble, to the late Proterozoic Danghe Group, consisting of low-grade slate, sandstone and stromatolitic carbonate rocks. Northeastward, these rocks extend into the Daxue Shan and South Danghe thrust belts. Precambrian rocks present north and south of the Altyn Tagh fault zone are assigned to the Dunhaung and Da-ken-da-ban groups of early Proterozoic age which form the basement rocks for the Qsidamu and Talimu basins [ 12,131. Tertiary rocks within the Altyn Tagh fault zone
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late Pleistocene and Holocen
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q and Tarim basins
Fig. 1. Simplified geological map of the northeastern part of the Tibetan plateau. Inset map shows the more regional setting of the part of the fault zone discussed in this paper and letters refer to locations mentioned in the text. Black areas in inset are locations of Tertiary rocks mentioned in the text. SD = South Danghe range; DX = Daxue Shan range; NBT = northern boundary thrust of the Qilian Shari;; NAT= North Ahyn Tagh fault; CAT= Central Altyn Tagh fault; SAT= South Ahyn Tagh fault.
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stream and Holocene late Pleistocene
: : ......
q early Pleistocene m middle Tertiary
q
pre-Cenozoic
Fig. 2. Generalized map along the northwestern part of the Altyn Tagh fault zone showing the location of the three areah of Tertiary rock interpreted as being offset across the fault zone. Upper limits for left-lateral displacement on the North and Central Altyn Tagh fnults are also shown.
are dated as late Oligocene to perhaps as young as early Middle Miocene (see below). They are terrestrial red beds and have experienced significant deformation. They are similar to Tertiary rocks present within the central Qilian Shan, but differ from correlative rocks within the Qsidamu and Talimu basins that form a continuous sequence of grey lacustrine and tluvial deposits from Paleogene to Neogene age.
3. Cenozoic history of the northern strand of the
Altyn Tagh fault zone Cenozoic rocks are present in the study area and can be used to evaluate the Cenozoic history of the Altyn Tagh fault zone. 3. I. Subei area
The most well studied Cenozoic rocks are exposed in the Subei area where they consist of three units. The oIdest unit rests unconformably on preCenozoic. mostly Precambrian, rocks and consists of
more than 2000 m of orange-weathering sandstone and mudstone interbedded with gypsum and conglomerate. The second unit paraconformably overlies the oldest unit and consists of about 200 m of greenish, coarse grained conglomerate, purple sandstone and siltstone. Bohlin [l 11 assigned these rocks an Oligocene to Miocene age based on abundant fossil assemblages in the area of Wugequan and Yandantu (see Fig. 3 and Appendix A). Unfortunately, he did not make clear the stratigraphic position of the fossil collections and later workers have assigned the rocks to either an Oligocene (unpublished data in Chinese) or Miocene [l 11age. Dr. Will Downs examined Bohlin’s fauna1 list and indicated the assemblage is from the Oligocene/Miocene transition and has no elements to indicate it is younger than 16 Ma or even 16 Ma (Downs. pers. commun., 1996). The younger of these two units correlates lithologically with the Miocene/Pliocene Shulehe Formation [ 121 present along the northern edge of the Tibetan plateau, with its type area in the Yumen area (see Fig. 1 inset), at the northeastern-most corner of the plateau. If this correlation is correct, it
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94 :30” m
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stream late
Pleistocene
JJJearly
Pleistocene
5 Lm
m middle Tertiary npre-Cenozoic
El
Bohlin’s
locality
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fossil
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ff:
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Fig. 3. Geological relations in the Subei area. Open boxes indicate the localities of Bohlin’s (1937) collections were not keyed to localities and attempts by many workers to recollect his material have been unsuccessful.
m \upper
Pleistocene m
early Pleistocene m
[l 11. Unfortunately
fossils
middle Tertiary (u55’1strike and dipping angle Paleozoic Proterozoic
Fig. 4. Geological relations in the Subei and Dabiegai areas discussed Altyn Tagh fault using different geological features.
in the text. Also shown are the magnitudes
of offset along the central
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would suggest none of Bohlin’s fossils came from the younger unit; however, attempts by many workers to collect new samples of Bohlin’s fauna have not been successful. Considering the uncertainties in the ages of these two units in the Subei area, these two older Tertiary units are combined into a single unit on the figures. The youngest unit consists of more than 150 m of grey and brown, poorly sorted, cobble and boulder conglomerate. Similar rocks are widespread throughout this part of the Tibetan plateau and the adjacent Tarim and Qsidamu basins where they contain Early Pleistocene fossils [ 121. In the Subei area these rocks are only in fault contact with the older two units (Figs. 2 and 4), but unconformably overlie them in the Liuchengzi area (Fig. 5). The oldest two rock units are strongly folded within a restraining bend of the Altyn Tagh fault zone (Fig. 4), and are bounded by two faults with Z-shaped arcuate traces that generally parallel fold axes within the Tertiary rocks. The southern fault
aHolocene
q upper
middle Pleistocene aearly
and late
f 1997)
Proterozoic(lower-grade
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relations in the Liuchengzi
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dips south and thrusts Proterozoic rocks over Proterozoic and Tertiary rocks, and its footwall contains the depositional contact between the Tertiary rocks and Proterozoic basement. The northern fault is subvertical and juxtaposes folded Tertiary rocks against gently north-dipping, unfolded Lower Pleistocene rocks and is overlapped by the youngest alluvium. The northern fault may have been originally a south-dipping thrust fault later modified by younger deformation. The Z-shaped faults merge to the west and east with the north and central Altyn Tagh faults, respectively and the folded Tertiary rocks wedge out between them (Fig. 4). These relations are interpreted as being the result of a restraining bend between the north and central Altyn Tagh faults. The northeast projection of the north Altyn Tagh fault crosses Upper Pleistocene rocks for a distance of about 18-20 km north of Subei, and there is no evidence that this segment of the fault is active. Farther northeast, beyond outcrops of Upper Pleistocene rocks, a fault, parallel to the north Altyn Tagh
Proterozoic(high-grade
Hlower Pleistocene13aec(elevation in meters middle Tertiary (stream
Fig. 5. Geological
150
metamorphic
metamorphic rocks)
rocks)
N $ 5 km
area north of the North Altyn Tagh fault.
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fault west of Subei, juxtaposes Proterozoic and Cambrian rocks. This fault offsets rivers by l-4 km left-laterally (Fig. 4) and is considered here to be the eastern continuation of the north Altyn Tagh fault. Recent work by Meyer et al. 1141 has shown that the eastern part of the north Altyn Tagh fault is active and near its eastern exposures has a slip rate of 4 f 2 mm/a. Southwest of where the arcuate faults merge with the north Altyn Tagh fault, the Altyn Tagh fault is active and alluvial fans and rivers are offset by 50-300 m [lo]. Ge [lo] has shown that, in the A’kasay area, an alluvial fan dated at 35.4 k 0.26 Ka is offset 65 m left-laterally, yielding a slip rate of 1.8 mm/a. The oldest constrained age for initiation of deformation on this part of the Altyn Tagh fault is after the deposition of the two oldest Tertiary units. Unfortunately, the age of the youngest of these two units is not well constrained. It is certainly younger than 16 Ma but could be younger than Miocene/Pliocene, if the correlation with the rocks in the Yumen area is valid. The contrast in degree of deformation between the two Tertiary units and the Lower Pleistocene rocks suggests much of the deformation related to this part of the Altyn Tagh fault is older than Early Pleistocene.
Folds in the Tertiary rocks have WNW trending axes with bedding dips of 20-30”, except near the central Altyn Tagh fault where bedding dips are vertical and bedding strike becomes more nearly parallel to the fault. The central Altyn Tagh fault dips north and Proterozoic rocks are thrust south over the Tertiary rocks. The southern boundary of the Tertiary rocks is marked by the south-dipping south Altyn Tagh fault that thrusts late Proterozoic rocks over Tertiary rocks (Fig. 4). There are no unique piercing points from which to determine the offset of the Tertiary rocks across the central Altyn Tagh fault between the Dabiegai and Subei areas. A maximum offset of 18 km is indicated by correlating the NW trending unconformable contact between Upper Pleistocene and the Tertiary rocks in both areas (Figs. 2 and 4). A similar offset of 16 km is obtained by interpreting the deflection of the Danghe River as being caused by the left-lateral displacement on the central Altyn Tagh fault. Thus, a maximum of 16-18 km left-lateral offset is interpreted for the central Altyn Tagh fault. The Danghe river bends left laterally and follows the fault trace for 4 km before crossing the fault, suggesting some young displacement on the fault.
3.2. Dabiegai area
South and west of Liuchengzi, the Altyn Tagh fault zone shows evidence of shortening as well as left-lateral displacement (Fig. I). Between the north and south Altyn Tagh faults Proterozoic metamorphic rocks and early Paleozoic ultramafic rocks underlie the highest mountains in the region (up to 5798 m). Along the north Altyn Tagh fault these rocks are thrust northward onto Tertiary red beds in the Liuchengzi area. West of Liuchengzi the north and south Altyn Tagh faults merge and the wedgedshaped region of rocks between the faults to the east is interpreted to be in a zone of transpression. North of the north Altyn Tagh fault, in the Liuchengzi area, Proterozoic metamorphic rocks form the core of a NW trending dome-shaped mountain, reaching more than 4 km elevation. The Proterozoic rocks are unconformably overlain by orangeweathering sandstone and mudstone with some conglomerate exposed on the north and south flanks of
In the Dabiegai area, the central Altyn Tagh fault merges with the westernmost structures of the South Danghe range, part of the Qilian Shan (Figs. 1, 2 and Figs. 4). West of Dabiegai, rocks lithologically correlative to the older two Tertiary units in the Subei area rest unconformably on pre-Mesozoic rocks. No fossils are reported from these rocks but they lithologically correlate with the units at Subei, Yumen and with fossiliferous rocks of Oligocene age in the central Qilian Shan [ 111, and they are interpreted to be Oligocene/Miocene or perhaps as young as Miocene/Pliocene in age. The Tertiary rocks in the Dabiegai area are folded, but generally less strongly than near Subei. They are unconformably overlapped by gently dipping, unfolded Upper Pleistocene alluvial deposits which form the eastern boundary of the Tertiary rocks.
3.3. Liuchengzi
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the mountain (Fig. 5). No fossils have been reported from these rocks but they are lithologically similar to the oldest unit in the Subei area, thus they are tentatively assigned an Oligocene to early (?) Middle Miocene age. These rocks are unconformably overlain on the north side of the mountain by gently dipping, grey and brown, cobble and boulder conglomerate, assigned an Early Pleistocene age based on lithological correlation with dated rocks in the region. The Tertiary rocks in the Liuchengzi area are folded along WNW trending fold axes with bed dips of 20-25” (Fig. 5). Unconformably underlying Proterozoic metamorphic rocks are also folded along WNW trending axes, but with steep flank angles (60-80”) and local overturning. Locally, the Tertiary rocks are involved in Proterozoic-cored overturned folds or on the south side of the mountain are overthrust by Proterozoic rocks. This demonstrates that some structure within the Proterozoic rocks is also of Tertiary age. Near the north Altyn Tagh fault attitudes in both Proterozoic and Tertiary rocks curve into parallelism with the fault in a left-shear sense. Cobble and boulder conglomerate of Early Pleistocene age unconformably overlies the folded Tertiary rocks along the north ff ank of the mountain and is unfolded, but dips generally 10-30” N or NE. The rock types and structures in the Liuchengzi area are similar to those in the Subei and Dabiegai areas and lithological correlation with dated rocks in the Subei area indicate the timing of deformation between the areas is also similar. Caught within the north Altyn Tagh fault between the Liuchengzi and Subei areas are slivers of highly cataclastically deformed red and orange sedimentary Tertiary rocks. Locally, they are overlapped unconformably by coarse conglomerate of probable Early Pleistocene age. The conglomerate is locally cut by the fault, but not folded and dips gently to the north. It is either unconformably overlapped by younger Pleistocene rocks or truncated by an erosion surface that slopes north into the Tarim basin. These observations suggest that much of the displacement on the north Altyn Tagh fault is probably pre-Pleistocene. No piercing points can be determined between the Liuchengzi and Subei areas, so a unique magnitude of displacement cannot be determined. A possible maximum offset of 67 km is suggested by matching
the eastern limit of Tertiary rocks (Fig. 2). A possible minimum offset of 48 km is suggested by matching the western-most extent of the same rocks.
4. Cenozoic total offset, inception its implications
of faulting
and
The three areas of Tertiary rocks discussed above are interpreted as having formed a continuous belt of WNW trending folds prior to initiation of the Altyn Tagh fault, but probably during an early stage of left-lateral shear. Conglomerate in the Tertiary rocks contains cobbles dominated by Proterozoic lithologies, indicating some relief and perhaps some deformation began during their deposition. The nature of this deformation remains unknown but is suggested
ariy
middle Tertia
nd of middle Tertia
mProterozoic mcrystalline
in central Qilian basement of Tarim
Fig. 6. Possible structural evolution in the Subei, Dabiegai and Liuchengzi areas. Early middle Tertiary deformation expressed as NW trending positive and negative areas. possibly shortening structures related to early left-lateral shear. Initiation of Altyn Tagh fault zone and transfer of left-slip into NW trending structures in the Subei area in the Qilian Shan near end of the middle Tertiary; development of the restraining bend in the Subei area. Continued left shear along the Altyn Tagh fault and transfer into shortening within the Qilian Shan during the Quaternary.
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to be the beginning of folding in the area (Fig. 6). Following deposition of the Tertiary rocks, the main left-lateral displacement on the Altyn Tagh fault zone occurred, cutting through the folds and in the Subei and Dabiegai areas, and modifying the structures by the development of restraining bends (Fig. 61. Summing the range of total Tertiary offsets determined for the folded belt along the north and central Altyn Tagh faults yields a range of 64-85 km, with a value closer to the higher end of the range being more likely. Some uncertainty remains because piercing points cannot be established, but the general correlation of the belt of Tertiary rocks yields an offset within this range. Some additional displacement may occur in the restraining bend at Subei. Comparing the more open folds in the Dabiegai and Liuchengzi areas to the tighter folds at Subei suggests at least an additional shortening of about 5 km could be present, thus yielding a range of 69-90 km for the left-slip on the north and central Altyn Tagh faults. Only two other localities of Tertiary rocks present along the Altyn Tagh fault might be used to arrive at a different interpretation, but their deformational style and lithology suggest they are unlikely correlatives for rocks in the study area. One, about 60 km northeast of Subei (a, Fig. 1 inset), lies between the two branches of the north Altyn Tagh fault and the other, about 200 km southwest of Liuchengzi Cd, Fig. I), lies north of the fault zone. The Neogene rocks at point a are not as strongly folded as the other areas and do not appear to be lithologically correlative, and the rocks at point d have rock types unknown at Subei (such as limestone and gypsum) and are much thicker, by a factor of 2 or 3, than rocks at Subei. If the total offset of 69-90 km occurred at a uniform rate, only a range of long-term slip rates can be calculated based upon the time of inception of the faulting because the youngest age of the older two units in the Subei area remain uncertain. In all areas these two sedimentary rock units are without angular unconformity. All that can be said for certain is that folding and movement on the Altyn Tagh fault is younger than 16 Ma. If the younger unit is Miocene/Pliocene in age, based on correlation with the Yumen area, it would indicate folding of the
Tertiary rocks did not begin until after deposition of the Miocene/Pliocene conglomerate. The structural and stratigraphic evidence presented above suggests most of the offset took place in the interval between either 16 Ma or the Miocene/Pliocene and early Quaternary. This time interval is poorly determined and, depending upon what time is picked for the initiation of major left-lateral faulting, a long-term slip rate for this interval could range between 7-9 mm/a and 35-45 mm/a. The total offset on the north and central Altyn Tagh faults is very important because it begins to limit how much deformation can be associated with various models for the development of the Tibetan plateau. For example, Burchfiel et al. [15] and Tapponnier et al. [16] have suggested the Altyn Tagh fault zone functions as a transfer fault transferring left-slip displacement into shortening south of the Altyn Tagh fault zone. In general, transfer faults have variable magnitudes of displacement along them because, as they transfer motion from strike-slip to shortening or extension, the magnitude of strike-slip displacement must change. For the Altyn Tagh fault zone, if strike-slip is transferred into shortening south of the fault, the fault zone should loose displacement to the northeast and gain displacement to the southwest. Thus, the magnitude of displacement determined here is only valid for a small segment of the fault in the region studied. However, it does suggest that the amount of shortening in the Qilian Shan east of the Dabiegai area is about 69-90 km. No total offset for the south Altyn Tagh fault has been determined. It appears to merge and be transferred into shortening in the structures of the South Danghe Range in the Qilian Shan (Fig. 1) and thus does not effect structures farther to the northeast. However, it bounds the Qsidamu basin on the north and merges with the north Altyn Tagh fault to the southwest and would thus add to the total displacement on the fault zone farther to the southwest. Shortening in the Qsidamu basin is expressed in a series of WNW trending folds that expose mainly Quatemary rocks in their cores and appear to have less shortening than the structures in the Qilian Shan that contain rocks as old as Precambrian, suggesting the total offset southwest of where the north and south Altyn Tagh faults merge to the southwest is less than twice that measured in the study area. It
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further suggests that eastward movement of crustal rocks in this part of the Tibetan plateau has been of relatively small magnitude, perhaps less than 200 km.
Acknowledgements This work was supported by NSF grant 8904096 awarded to B.C. Burchfiel and L.H. Royden. The author thanks G.A. Davis, L. Ratschbacher, A.W. Bally. X. Fang and an unknown reviewer for helpful reviews. [RV]
A.2. Central Qilian Shan Oligocene fossils: Palaeoerinaceus kansuensis, Desmatolagus sp. (shargaltensis), Sinolagomys kansuensis, Tachyoryctoides sp., Paleoerinaceus cf. acridens, D. pamidens, D. large species, S. gracilis, Tuchyopctoides obrutschewi, T. intrrmedius. T. pachygnathus, Karakoromys cf. decessus. Leptotataromys gracilidens, Tsaganomys altncicus. Didymoeonus sp.. Indricotherium sp.
References [II B.C. Burchfiel.
Appendix A. Fauna1 list from Bohlin (1937), and his age assignments
A. 1. Subei area Oligocene fossils: Pulaeoerinaceus kansuensis, P. minimus, P. cf. rectus, Desmatolagus sp. (shargaltensis), Sinolagomys kansuensis, S. major, Parasminthus asiae-centralis, P. tangingoli, P. paroulus. Tachyovctoides sp., Tataromys grangeri, T. sigmodon, T. sigmodon, T. cf. plicidens. Yindirtemys woodi. Miocene fossils: Sayimys obliquidens. Schizotherium? sp., Kansupithecus sp., aff. Gomphotherium connexus, Testudo honanensis, T. tunhunensis, T. chienfuturgensis, Palaeoscaptor sp., Lnsecliuora sp., Palacoerinaceus cf. rectus, Matheweta granger, P. kansuensis, P. minimus, Soicidae sp.. Talpidec sp., Desriatolagus shargaltensis, D. paruidens. Sinolagomys kansucnsis, Tataromys phiwidens Matthew et Granger, Karakoromys cf. decescus. K. sp. Tachyovctoides ohrutschewi, T. interP. tangingoli, medius?, T. pachygnathus, Tsaganomys aitaicus Matthew et Granger, Sciurus sp. Parasminthusasiaecentralis, T. tangingoli, P. cf. Sicistinae sp.. Sicitinae parulus, Sp., cf. Cricetodon sp., aff. Eumys sp., Rhizomyidae sp., Tataromys grangeri Bohlin, Leptotaromys gracilidens Bohlin, Yindirtemys woodi, Schizotherium sp,, Eumerys sp.
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L.H. Royden. Tectonics of Asia SO years after the death of Emile Argand, Eclog. Geol. Helv. X4 ( 199 1) 599-629. [?I J.F. Dewey. S. Cande, W.C. Pitman III. Tectonic evolution of the India/Eurasia collision zone, Eclog. geol. Helv. X4 (1991) 717-734. [31 P. Tapponnier. G. Peltzer, R. Armijo. On the mechanics of the collision between India and Asia: in: M.P. Coward, A.C Ries (Eds.). Collision Tectonics. Geol. Sot. London Spec. Pub]. I9 (1986) 115-157. Formation and evolution of L41 G. Peltzer, P. Tapponnier. strike-slip faults. rifts, and basins during the India-Asia collision: an experimental approach. J. Geophya. Res. 93 (1988) 15085-151119. [51A.W. Bally, I.M. Chou, R. Clayton. H.P. Eugster. S. Kidwell, L.D. Meckel. R.T. Ryder, A.B. Watts. A.A. Wilson, Notes on sedimentary basins in China. Report of the American Sedimentary Basins Delegation to the People’s Republic of China. U.S. Geol. Surv. Open File Rep. X6-327, 1986. [61P. Molnar. P. Tapponnier. Cenozoic tectonics of Asia: Effects of a continental collision, Science I89 (1975) 419-426. [7] G. Peltzer. P. Tapponnier. R. Armijo. Magnitude of Late Quatemary left-lateral displacement along the northern edge of Tibet, Science 246 (1989) 1285-1289. [8] Y. Gaudemer. P. Tapponnier, D.L. Turcotte, River offsets across active strike-slip faults. Ann. Tecton. III (1989) 55-76. 191 P. Molnar. B.C. Burchfiel, L. K’uangyi. 2. Zhao, Geomorphic evidence for active faulting in the Altyn Tagh and northern Tibet and qualitative estimates of its contribution to the convergence of India and Eurasia. Geology 15 f 1987) 249-253. [IO] S. Gc. Altyn Tagh active fault zone. Seism. Press I I4 (1992). [I II B. Bohlin. OberoligozBne SIugetiere aus dem Shargalteintal western Kansu. China Region. Paleontol. Xinbingzhong 3 (1937). [I21 Anon, Regional Geology of Gansu Province, Beijing Publishing House, 1989. [I31 Anon, Regional Geology of Qinghai Province. Beijing Publi
64 [14] B. Meyer, P. Tapponnier,
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Y. Gaudemer, G. Peltzer, S. Guo, Z. Chen, Rate of left-lateral movement along the easternmost segment of the Altyn Tagh fault, east of 96” E (China), Geophys. J. Int. 124, 29-44. [15] B.C. Burchfiel, Q. Deng, P. Molnar, L.H. Royden, Y. Wang, P. Zhang, W. Zhang, Intracrustal detachment within zones of continental deformation, Geology 17 (1989) 448-452.
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[16] P. Tapponnier, B. Meyer, J.P. Avouac, G. Peltzer, Y. Gaudemer, G. Shunmin, X. Hongfa, Y. Kelun, C. Zhitai, C. Shuahua, D. Huagang, Active thrusting and folding in the Qilian Shan, and decoupling between upper cmst and mantle in northeastern Tibet, Barth Planet. Sci. Lett. 97 (1990) 382-403.