Sedimentary basins in the interior of the Tibetan Plateau ... · Sedimentary basins in the interior of the Tibetan Plateau: recording environmental changes and tectonic processes

Sedimentary basins in the interior of the Tibetan

Plateau: recording environmental changes and

tectonic processes

Sara Keil

Institute for Geology, Bernhard-v.-Cotta Str. 2, 09599 Freiberg, Germany

Abstract. The Nima basin area in central Tibet and the Nangqian-Yushu region in

east-central Tibet exhibit plateau-interior basins that provide information about

paleogeography, paleoelevation and tectonical processes. Divided into independ-

ent sub-basins they are controlled by thrust-fold deformation. Especially the sedi-

mentary record in the Nima basin area shows that continued convergence long af-

ter intitial collision of the Lhasa and Qiangtang terranes induces crustal

shortening, thrust-fold deformation and consequently basin development. Fur-

thermore sedimentary record is evidence for small climate influence on deposi-

tion. Reconstruction of paleoelevation in these areas supports the model of oblique

stepwise rise and growth of the Tibetan plateau (Tapponnier et al. 2001).

Introduction

Sedimentary basins along the margins of the Tibetan plateau have been studied in

a variety of investigations (Metivier et al. 1998; DeCelles et al. 1998; Zhang 2000;

Liu and Wang 2001). However, the record of basin evolution in the remote inte-

rior of the Tibetan plateau has received considerably less attention (Liu et al.

2001; Schneider et al. 2003; Zhang et al. 2004). These interior basins provide in-

formation on not only the erosional and depositional processes associated with re-

gional denudation and relief production, but also the tectonic processes that con-

trol crustal thickening and surface uplift.

Recent publications of DeCelles et al. (2007) and Horton et al. (2002) present re-

sults of investigations undertaken in interior-plateau, sedimentary basins of the

Nima area in central Tibet and the Nangqian-Yushu region of east-central Tibet,

respectively. By the combination of sedimentological, modal petrographical, geo-

2 Sara Keil

chronological, and structural data, they conclude on environmental changes, pa-

leoelevation and tectonic processes for these regions.

The purpose of this paper is to introduce to the evolution of sedimentary basins

of the Nima area and the Nangqian-Yushu region and to make the attempt to (1)

explain why these basins developed long after the collision of relevant terranes;

(2) estimate the influence of climate on erosion compared with tectonics; (3) find

evidence for stepwise rise and growth of the Tibetan plateau suggested by Tap-

ponnier et al. (2001).

Geological setting

The Tibetan Plateau occupies an area of around 2.5 million km² and has an aver-

age elevation of more than 4,500 m. It is composed of three major terranes, from

the south to the north the Lhasa, Qiangtang, and Songpan-Ganzi terrane; they are

separated by suture zones that formed as each terrane in succession collided with

the southern margin of Eurasia (Dewey et al. 1988; Yin and Harrison 2000). The

southernmost Lhasa terrane is located between the Indus-Yarlung suture in the

south and the Bangong suture in the north. The latter recorded Late Jurassic-Early

Cretaceous collision between the Lhasa block and the Qiangtang terrane (Dewey

et al. 1988). The Qiangtang terrane is separated from the Songpan-Ganzi terrane

by the Jinsha suture, representing latest Triassic collision. Reactivation of these

suture zones during the India-Asia collision resulted in substantial Tertiary short-

ening and strike slip faulting (Yin and Harrison 2000). The Nima basin area is lo-

cated in the Bangong suture zone between the Lhasa and Qiangtang terranes of

central Tibet within an ~50 km radius of the town of Nima (Fig. 1). The basins of

the Nangqian-Yushu region occur in the northern part of the Qiangtang terrane be-

tween the cities of Yushu in the north and Nangqian in the south (Fig. 3).

Nima basin area

The Nima basin records Cretaceous to late Miocene nonmarine sediment accumu-

lation during a period of drastically changing paleogeography and paleoelevation

(DeCelles et al. 2007). The study area lies near the geographic center of the Ti-

betan plateau, ~450 km NW of the Tibetan capital of Lhasa. It covers the north-

central part of the Lhasa terrane and the adjacent southernmost fringe of the

Qiangtang terrane, extending on the trace of the Bangong suture zone (Fig 1). The

greater Nima basin developed as a single, wide basin during middle Cretaceous

time and was subsequently shortened and tectonically partitioned into the northern

and the southern Nima sub-basins during Late Cretaceous to middle Tertiary time

(Fig.2; DeCelles et al. 2007). The Nima basin fill rests unconformably on top of

penetratively deformed, low-grade metasedimentary rocks assigned to the Upper

Jurassic- Lower Cretaceous (Fig. 2; Kapp et al. 2007).

Sedimentary basins in the interior of the Tibetan Plateau: recording environmental

changes and tectonic processes 3

Fig.1. Tectonic and topographic map of the Tibetan Plateau, after DeCelles et al. (2007).

Southern polygon outlines the study area (Fig. 2).

The following information is given because of its importance for provenance ana-

lysis: The southern boundary of the Nima basin is marked by the southward-

dipping Gaize-Siling Tso thrust with the northernmost exposures of the Aptian-

Albian limestones of the Langshan Formation in its hanging wall (Kapp et al.

2005). The northward-dipping Muggar thrust, which carries Jurassic (?) metape-

litic and metavolcanic rocks and Cretaceous (ca. 118 Ma; Kapp et al. 2007) gran-

ites in its hanging wall, forms the northern boundary of the Nima basin. Other im-

portant thrust faults within the Nima basin are the Nima, Puzuo, Zanggenong and

Queri-Malai thrust systems (Fig. 2; Kapp et al. 2007).

Stratigraphy, Sedimentology and Provenance

Southern Nima basin

Cretaceous and Tertiary intervals are separated by a poorly defined chronostrati-

graphic gap of 50 m.y. duration (DeCelles et al. 2007). The southern Nima Creta-

ceous deposits contain the Basal Volcaniclastic, Lower Redbed, Middle Volcanic,

and Upper Conglomerate units, being exposed in the northward-overturned syn-

cline in the proximal footwall of the Gaize-Siling Tso thrust (DeCelles et al.

2007). It shows an overall transition from vent-proximal, lava and pyroclastic flow

4 Sara Keil

Fig. 2. Geological map of the Nima basin area, after DeCelles et al. (2007). ZTS—

Zanggenong thrust system (in northeastern part of map area).

deposits, to volcanogenic alluvial fan deposits, to sediment-clast-rich alluvial fan

deposits that were closely associated with thrust faulted highlands to the south in

the hanging wall of the Gaize-Siling Tso thrust (DeCelles et al. 2007). Deposits

were mainly alluvial fans and shallow braided streams, without evidence for ma-

rine or deep lacustrine environments (DeCelles et al. 2007). Paleoflow directions

reversed, from southward during deposition of the volcanogenic deposits to

northward during deposition of the alluvial fan deposits of the Upper Conglomer-

ate unit, which were derived from the uplifted highlands south of the trace of the

Gaize-Siling Tso thrust (DeCelles et al. 2007).



The southern Nima Tertiary deposits contain the older Nima Redbed unit and the

younger Gray Conglomerate unit, which are best exposed in a belt of outcrops

along the south side of the Dagze Lake and along the Mochang River south and

west of the town of Nima (Fig. 2; DeCelles et al. 2007). In this part of the Nima

basin, depositional environments during the middle Tertiary were laterally vari-

able (DeCelles et al. 2007). The commingling of massive disorganized conglom-

erate with cross-stratified conglomerate indicates that the sediment gravity flows

were occasionally able to reach the gravelly shoreline. That implies that the relief

was relatively steep and that the source terrane was nearby (DeCelles et al. 2007).

During deposition of lacustrine fan deltas, paleogeography must have been similar

to the present geography of the Nima basin region, characterized by alternating

mountain ranges and basins that are occupied by large lakes (both fresh and sa-

line), braided fluvial systems, fan deltas and alluvial fans, with a basin floor eleva-

tion of typically 4.5 to 4.7 km (DeCelles et al. 2007). The Gray Conglomerate unit

was deposited by a large gravelly braided river flowing toward the east, implying

that the paleodrainage pattern in the Nima basin region was radically altered

sometime in the late Miocene, from internally drained to open toward the east

(DeCelles et al. 2007).

Cretaceous-Tertiary sandstones exhibit a change through time from magmatic-

arc-dominated to thrust-belt-dominated provenance coeval with a change in pa-

leocurrent directions from southward to northward and the onset of erosion of the

Aptian-Albian limestone in the hanging wall of the Gaize-Siling Tso thrust. (De-

Celles et al. 2007).

Northern Nima basin

The stratigraphic succession of the northern Nima basin is well exposed in the

river valley that flows southward from the glaciated 6000 m peaks in the Muggar

Gangri. As in the southern Nima basin, the lower part of the succession is domi-

nated by volcaniclastic-rich interval, and the upper part of the succession is com-

posed of clastic red beds, sandstone, conglomerate and lacustrine marl. Radiomet-

ric and palynological ages, however, suggest that the northern Nima basin

stratigraphy is not directly correlatable with that in the southern Nima basin (De-

Celles et al. 2007).

The Lower Cretaceous Volcaniclastic unit shows an assemblage of lithofacies

that suggests deposition in shallow braided fluvial to alluvial fan systems that pro-

graded into the area from local topographic highlands. Abundant paleosols count

for landscape stability and slow sediment accumulation during deposition of the

lower 200 m of the succession (DeCelles et al. 2007).

The Upper Cretaceous-lower Paleocene Lower Muggar unit rests in angular

unconformity on the top of the Volcaniclastic unit. The assemblage of lithofacies

leads to the interpretation that deposits of gravelly fluvial channels and distal allu-

vial fan systems with southward paleoflow directions, are laterally adjacent to

eolian and sand-flat environments. Paleowind directions in eolian and sand-flat

6 Sara Keil

environments are variable but generally northeastward or southeastward, associ-

ated fluvial channels flowed southeastward (DeCelles et al. 2007).

The middle Tertiary Upper Muggar unit displays an assemblage of lithofacies

suggesting deposition in shallow, restricted evaporitic lakes and fluvial systems on

a lacutrine coastal plain. Massive red gypsiferous siltstones represent the most

abundant lithofacies. In contrast to large Tertiary paleolakes documented in the

southern Nima basin, paleolakes in the northern Nima basin were shallow and

ephemeral. Paleocurrent data imply southward and eastward paleoflow of fluvial

channels. (DeCelles et al. 2007).

Modal petrographic data show that an overall trend of increasing erosion depth

is evident, from volcanic edifice, to plutonic roots and associated metasedimentary

rocks.

Major differences between the composition of sandstones from the southern

Nima basin and those of the northern Nima basin suggest distinct and separate

source terranes (DeCelles et al. 2007).

Results

Provenance and paleocurrent data (DeCelles et al. 2007) and structural reconstruc-

tions (Kapp et al. 2007) demonstrate that the southern and northern Nima basin ar-

eas received sediment derived from southern and northern terranes, respectively,

and were separated by a topographic barrier in the hanging wall of the Puzuo

thrust during Late Cretaceous through Tertiary time. Thus the depositional sys-

tems in both regions developed independently (DeCelles et al. 2007).

By Early to middle Cretaceous the region lay at northern near-equatorial pa-

leoaltitudes, near sea level. By Aptian time the Nima basin was located above sea

level and strongly influenced by nearby volcanic activity and crustal shortening

along the reactivated Bangong suture zone, about 20 Ma after the initial collision

of the Lhasa and Qiangtang terranes. Continued convergence during Middle to

Late Cretaceous time between these terranes was accomodated along the Bangong

suture by northward underthrusting of the Lhasa terrane and the establishment of

the Gaize-Siling Tso, Nima, Puzuo thrust systems in the Nima basin region (Kapp

et al. 2007). Coarse-grained alluvial fans flanked growing topographic ranges and

fed more distal fluvial, sand-flat and small lacustrine deposystems (DeCelles et al.

2007).

During Late Cretaceous to middle Tertiary time, the Nima basin was topog-

raphically partitioned into separate depocenters in the north and south. In the

southern Nima basin an ~50 m.y. (Late Cretaceous-Eocene) depositional hiatus

correlates with major crustal shortening and ensuing voluminous ignimbrite erup-

tions in the Lhasa terrane (DeCelles et al. 2007; Kapp et al. 2007). In the northern

Nima basin deposition continued during latest Cretaceous time, recording arid pa-

leoclimate in evaporitic lacustrine and eolian dune-field deposits (DeCelles et al.

2007).

By Oligocene time the independent depocenters of northern and southern Nima

basin accumulated coarse-grained alluvial, fluvial, lacustrin (evaporitic), and fan-



delta deposits in close association with reactivated thrusts in the Bangong suture

zone (DeCelles et al. 2007).

An important implication of this reconstruction is that paleoclimate in central

Tibet was already arid by Late Cretaceous time. Another evidence is provided by

stable C and O isotopic data from paleosol carbonate and lacustrine marl in the

Nima Redbed unit, suggesting low soil respiration rates and highly evaporated

lake waters (DeCelles et al. 2007b). Stable isotopic data from the Nima basin

(DeCelles et al. 2007b) and the Lunpola basin ~250 km to the east (Rowley and

Currie 2006) suggest that regional paleoelevation was ≥4600 m (equal to modern

base elevations in the region) by late Oligocene time (DeCelles et al. 2007).

Nangqian-Yushu region

The Nangqian-Yushu region of east-central Tibet is located in the northern part of

the Qiangtang terrane (near the headwaters of the Mekong and Yangtze Rivers;

Fig. 3).

Fig. 3. Simplified tectonic map of the Tibetan plateau, including sedimentary basins, faults,

suture zones, and rivers (modified from Horton et al. 2002). Shaded areas represent Ceno-

zoic nonmarine deposits, including a belt of basins (denoted by arrows) that stretches from

north-central Tibet (HXB—Hoh Xil basin; FSB—Fenghuo Shan basins) to east-central Ti-

bet (NYB—Nangqian-Yushu basins) to southeastern Tibet and Yunnan province, south

China (GB—Gongjue basin; LB—Lijian basin; JB—Jinggu basin). Also shown are sutures

(JS—Jinsha suture; BNS—Bangong-Nujiang suture; IYS—Indus-Yalu suture) and major

Ganzi-Yushu fault; XXF—Xiangshuihe-Xiaojiang fault system; LSTB—Longmen Shan

thrust belt; RRF—Red River fault; GF—Gaoligong fault; JF—Jiali fault). Rectangular out-

line shows location of Nangqian-Yushu study region (Fig. 2 refers to Fig. 4 here).

8 Sara Keil

Four elongate basins in this region indicate nonmarine, lacustrine-dominated

sedimentation synchronous with Paleocene-Eocene northeast-southwest shorten-

ing (Fig. 4; Horton et al. 2002; Spurlin et al. 2005).

Sedimentological, stratigraphic, compositional and structural data lead to the fol-

lowing results: Differences in local lithofacies, paleocurrents and conglomerate

composition in addition to the lack of single stratigraphic unit that can be corre-

lated regionally among the basins implies that each basin developed independently

(Horton et al. 2002), comparable with the Nima basin evolution. Alluvial –fan and

fan-delta deposition was narrowly limited to basin margins (Horton et al. 2002;

Fig. 4, 5).

Results

Early Tertiary basin development was controlled by thin-skinned fold-thrust de-

formation (Spurlin et al. 2005; Fig. 5). Growth strata along the margins of the out-

crop belts provide evidence for sedimentation contemporaneous with motion on

thrust-fold structures (Horton et al. 2002).

Sedimentologic and provenance evidence for internal drainage, limited unroofing

and relatively low average sediment-accumulation rates in these syncontractional,

plateau-interior basins indicate generally small drainage systems, short main-stem

rivers, shallow regional slopes and limited denudation in the east-central Tibet

during early Tertiary time (Horton et al. 2002). Evidence for internal drainage are

centrally directed paleocurrents, dominantly lacustrine depositional conditions,

and the overall distribution of facies (coarse-grained facies limited to the basin

margins and lack of regionally correlative strata; Horton et al. 2002).

The reconstructed geomorphic conditions, which are similar to the modern low-

relief interior of the Tibetan plateau, suggest that the deeply incised Mekong and

Yangtze Rivers of eastern Tibet were not established until after the termination of

Paleogene basin development in the region (Horton et al. 2002). The predomi-

nance of lacustrine deposits – the sources of which were small drainage networks

with short length-rivers, rather than large river systems or fluvial megafans de-

rived from extensive drainage networks – requires that the large rivers were not

present in the Nangqian-Yushu region during early Tertiary basin development

(Horton et al. 2002). Thus, they must have been established during Neogene, con-

sistent with predominantly increases in mass-accumulation rates documented at

the mouths of major Asian rivers (Métivier et al. 1999). That implies that strongest

uplift in east-central Tibet started in Neogene.



10 Sara Keil

Fig. 4. Generalized geologic map and cross section of the Nangqian-Yushu region (location

shown in Fig. 3). (A) Map depicting structures and rock types of four Cretaceous–

Paleogene outcrop belts and underlying Carboniferous–Triassic rocks, after Qinghai

BGMR (1983a, 1983b) and Spurlin et al. (2000). Locations of 25 measured stratigraphic

sections (sections A–Y) are shown. Basin fill includes primarily fine-grained deposits

(shaded patterns). Coarse-grained deposits (random-dot pattern) are confined to margins of

outcrop belts. (B) Cross section illustrating surface and inferred subsurface structural ge-

ometries. Basin dimensions are controlled by spacing of thin-skinned thrusts above an ~5-

km-deep décollement. Numerals 1–4 refer to four Triassic units, and "V" pattern denotes

Paleogene volcanic rocks. After Horton et al. (2002).

Fig. 5. Schematic block diagram of basin development in the Nangqian-Yushu region. Ba-

sin subsidence occurs in footwall of active thrust fault. Small drainage networks provide

sediment derived from Carboniferous–Triassic rocks (no pattern). The basin is dominated

volumetrically by fine-grained lacustrine deposits (shaded); proximal conglomeratic facies

(random-dot pattern) are confined to the faulted margin of the basins. Most sediment is de-

rived from thrust-sheet hanging wall. Bedding orientations (short line segments) show

growth-strata geometries adjacent to thrust and on flanks of small anticline (small arrows).

After Horton et al. (2002).

Conclusion

The results of investigations in the Nima area and Nangqian-Yushu region demon-

strate that basins in the remote interior of the Tibetan plateau recorded similar

processes of basin evolution. The basin development was controlled by thrust-

fold-deformation, induced by continuing convergence of the Lhasa and Qiangtang

terranes. Distinct sub-basins developed as a result of crustal shortening and repre-

sent independent depocenters. However, they show similar facies distribution with

alluvial-fan-deposits along the margins and lacustrine deposits in the interior.

Continued convergence of the Lhasa and Qiangtang terranes may explain that

the Bangong suture zone was reactivated two times, during middle Cretaceous and

middle Tertiary time, inducing the development of thrust-fold systems and associ-

ated basins about 20 m.y. after the initial collision of the Lhasa and Qiangtang-

terranes.



During Late Cretaceous to middle Tertiary time there is a 50 m.y. depositional

hiatus in the southern Nima basin, whereas deposition in the adjacent northern

Nima basin continued. According to the small distance between both depocenters

it is improbable that differences in climate conditions induced differences in sedi-

mentary record. More likely, crustal shortening was responsible for the deposi-

tional hiatus in the southern Nima basin. Thus the tectonic flux seems to be much

more important for erosion and deposition than the climate flux.

Investigations in the Nima basin area (DeCelles et al. 2007b) and the Lunpola

basin ~250 km to the east (Rowley and Currie 2006) demonstrate that regional pa-

leoelevation in central Tibet was ≥4600 m (equal to modern base elevations in the

region) by late Oligocene time. By investigations of sedimentary basins in the

Nangqian-Yushu region, Horton et al. (2002) suggests that the deeply incised Me-

kong and Yangtze Rivers of eastern Tibet were not established until after the ter-

mination of Paleogene basin development in the region, implying that east-central

Tibet underwent strongest uplift starting in Neogene time. These results support

the model of northeast-directed oblique and stepwise rise and growth of the Tibet

Plateau (Tapponnier et al. 2001).

References

DeCelles, P.G., and Hertel, F., 1989, Petrology of fluvial sands from the Amazonian fore-

land basin, Peru and Bolivia: Geological Society of America Bulletin, v. 101, p. 1552–

1562.

DeCelles, P.G., Kapp, P., Ding, L., Gehrels, G.E., 2007, Late Cretaceous to middle Tertiary

basin evolution in the central Tibetan Plateau: Changing environments in response to

tectonic partitioning, aridification, and regional elevation gain: GSA Bulletin;

March/April 2007; v. 119; no. 3/4; p. 654–680.

DeCelles, P.G., Quade, J., Kapp, P., Fan, M., Dettman, D.L., and Ding, L., 2007b, High and

dry in central Tibet during the late Oligocene: Earth and Planetary Science Letters, v.

253, no. 3/4, p. 389–401.

Dewey, J.F., Shackleton, R.M., Chang, C., and Sun, Y., 1988, The tectonic evolution of the

Tibetan plateau: Philosophical Transactions of the Royal Society of London, v. A327,

p. 379–413.

Horton, B.K., Yin, A., Spurlin, M.S., Zhou, J., and Wang, J., 2002, Paleocene–Eocene syn-

contractional sedimentation in narrow, lacustrine-dominated basins of east-central Ti-

bet: Geological Society of America Bulletin, v. 114, p. 771–786.

Kapp, P., Yin, A., Harrison, T.M., and Ding, L., 2005, Cretaceous–Tertiary shortening, ba-

sin development, and volcanism in central Tibet: Geological Society of America Bulle-

tin, v. 117, p. 865–878.

Kapp, P., DeCelles, P.G., Gehrels, G.E., Heizler, M., Ding, L., 2007, Geologic records of

the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet: Geo-

logical Society of America Bulletin; July/August 2007; v. 119; no. 7/8; p. 917–932.

Liu, Z., and Wang, C., 2001, Facies analysis and depositional systems of Cenozoic sedi-

ments in the Hoh Xil basin, northern Tibet: Sedimentary Geology, v. 140, p. 251–270.

12 Sara Keil

Liu, Z, Wang, C., and Yi, H., 2001, Evolution and mass accumulation of the Cenozoic Hoh

Xil basin, northern Tibet: Journal of Sedimentary Research, v. 71, p. 971–984.

Metivier, F., Gaudemer, Y., Tapponnier, P., and Meyer, B., 1998, Northeastward growth of

the Tibet plateau deduced from balanced reconstruction of two depositional areas: The

Qiadam and Hexi Corridor basins, China: Tectonics, v. 17, p. 823–842.

Me´tivier, F., Gaudemer, Y., Tapponnier, P., and Klein, M., 1999, Mass accumulation rates

in Asia during the Cenozoic: Geophysical Journal International, v. 137, p. 280–318.

Rowley, D.B., and Currie, B.S., 2006, Palaeo-altimetry of the late Eocene to Miocene Lun-

pola basin, central Tibet: Nature, v. 439, p. 677–681.

Schneider, W., Mattern, F., Wang, P., and Li, C., 2003, Tectonic and sedimentary basin

evolution of the eastern Bangong zone (Tibet): A Reading cycle: Geologische Rund-

schau, v. 92, p. 228–254.

Spurlin, M.S., Yin, A., Horton, B.K., Zhou, J., and Wang, J., 2005, Structural evolution of

the Yushu-Nangqian region and its relationship to syncollisional igneous activity, east

central Tibet: Geological Society of America Bulletin, v. 117, p. 1293–1317.

Tapponnier, P., Zhiqin, X., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., and Jingsui,

Y., 2001, Oblique stepwise rise and growth of the Tibet Plateau: Science, v. 294, p.

1671–1677.

Yin, A., and Harrison, T.M., 2000, Geologic evolution of the Himalayan-Tibetan orogen:

Annual Review of Earth and Planetary Sciences, v. 28, p. 211–280.

Zhang, K.J., 2000, Cretaceous palaeogeography of Tibet and adjacent areas (China): Tec-

tonic implications: Cretaceous Research, v. 21, p. 23–33.

Zhang, K.-J., Xia, B.-D., Wang, G.-M., Li, Y.-T., and Ye, H.-F., 2004, Early Cretaceous

stratigraphy, depositional environments, sandstone provenance, and tectonic setting of

central Tibet, western China: Geological Society of America Bulletin, v. 116, p. 1202–

1222.

Documents

Sedimentary basins in the interior of the Tibetan Plateau ... · Sedimentary basins in the interior of the Tibetan Plateau: recording environmental changes and tectonic processes