Geomorphic characteristics of the Minjiang drainage basin ... · To the west of Minjiang Fault, HF and ... River valley. The Minshan Mountain is approxi- ... in our target area

Island Arc

(2006)

15,

239–250

© 2006 The AuthorsJournal compilation © 2006 Blackwell Publishing Asia Pty Ltd

doi:10.1111/j.1440-1738.2006.00524.x

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1038-48712006 Blackwell Publishing Asia Pty LtdJune 2006152239250Research Article

Geomorphology of the Minjiang drainage basinH.-P. Zhang

et al.

*Correspondence.

Received 20 December 2004; accepted for publication 5 January 2006.

Research Article

Geomorphic characteristics of the Minjiang drainage basin (eastern Tibetan Plateau) and its tectonic implications: New insights from a digital

elevation model study

H

UI

-P

ING

Z

HANG

,

1,2,3

* S

HAO

-F

ENG

L

IU

,

1,2,3

N

ONG

Y

ANG

,

4

Y

UE

-Q

IAO

Z

HANG

4

AND

G

UO

-W

EI

Z

HANG

5

1

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (CUGB), Beijing 100083, China (email: [email protected]),

2

School of Earth Sciences and Resources, China University of Geosciences (CUGB), Beijing,

3

Key Laboratory of Lithosphere Tectonics, Deep-level Process and Lithoprobing Technology of Ministry of Education, China University of Geosciences (CUGB), Beijing,

4

Institute of Geomechanics, Chinese Academy of Geological Sciences (CAGS), Beijing and

5

Department of Geology, North-west University,

Xi’an, China

Abstract

The Minshan Mountain and adjacent region are the major continental escarp-ments along the eastern Tibetan Plateau. The Minjiang drainage basin is located withinthe plateau margin adjacent to the Sichuan Basin. Based on the analysis of the digitalelevation model (DEM) acquired by the Shuttle Radar Topography Mission (SRTM), weknow that the Minjiang drainage basin has distinct geomorphic characteristics. The reg-ular increasing of local topographic relief from north to south is a result of the Quaternarysediment deposition within the plateau and the holistic uplift of the eastern margin of theTibetan Plateau versus the Sichuan Basin. Results from DEM-determined Minjiang drain-age sub-basins and channel profiles show that the tributaries on the opposite sides areasymmetric. Lower perimeter and area of drainage sub-basins, total channel length andbifurcation ratio within eastern flank along the Minjiang mainstream are the result of theQuaternary differential uplift of the Minshan Mountain region. Shorter stream lengthsand lower bifurcation ratio might be the indications of the undergrowth and newbornfeatures of these eastern streams, which are also representative for the eastern uplift ofthe Minshan Mountain.

Key words:

geomorphology, Minjiang drainage basin, Shuttle Radar Topography Mission-digital elevation model, Tibetan Plateau.

INTRODUCTION

Although the central Tibetan Plateau is remark-ably flat (Fielding

et al

. 1994; Fielding 2000), mostof the marginal mountain belts bordering the pla-teau are characterized by steep topographic gra-dients (Clark & Royden 2000). In contrast to thenorthern and southern margins, most of the east-ern margin is more irregular and diffuse (Kirby

et al

. 2000, 2002). One notable exception is the

Longmenshan and Minshan Mountains (Deng

et al

. 1994; Zhao

et al

. 1994a,b; Searle 2001), whichform one of the most remarkable continentalescarpments (Kirby

et al

. 2002). The topographicelevation decreases gradually from 5000 to 6000 min the eastern Tibetan Plateau to 600 m in theSichuan Basin within a horizontal distance of only50–60 km (Zhao

et al

. 1994b; Searle 2001). Thedrainage basin of the Minjiang River is locatedalong this plateau margin. In this region manyprevious researchers have focused on the tectonicevolution and geomorphic characteristics of thedifferent geological units, such as the Longmen-shan Tectonic Belt (LTB) (Chen

et al

. 1994a,b;Deng

et al

. 1994; Zhao

et al

. 1994a; Chen & Wilson

240

H.-P. Zhang

et al.


1996), the Minjiang Fault zone and Minshan uplift(Tang

et al

. 1991; Zhao

et al

. 1994b, Zhou

et al

.2000), and presented many conceptual models ofthe Mesozoic and Quaternary geological evolution(Chen

et al

. 1994a; Deng

et al

. 1994; Chen & Wil-son 1996; Wallis

et al

. 2003). However, few of thesestudies have emphasized the relationship betweendrainage and the neotectonics in this region. Kirby

et al

. (2000, 2002) and Yang

et al

. (2003) investi-gated geomorphic features of this area with thepurpose of relating this morphology to chronolog-ical studies. The detailed geomorphic features ofthe Minjiang drainage networks and drainage sub-basins, the stream morphology and the morpho-metric relationship between the slope and eleva-tion of the Minjiang drainage basin are stillunknown. Recent topographic analyses of moun-tain belts perpendicular to the main structures ofdifferent orogens (Burbank 1992; Fielding

et al

.1994; Gilchrist

et al

. 1994; Fielding 2000; Zhang &Liu 2004) indicate that the digital elevation model(DEM)-based topographic analysis now makes itpossible to rapidly quantify the topographic char-acteristics of areas ranging from local drainagebasins to entire continents (Burbank 1992).

Based on numerical analysis of the morphome-try from the Shuttle Radar Topography Mission(SRTM) DEM, together with a geological field sur-vey, the present study aims to analyze the geomor-phic characteristics of the Minjiang drainage basinand to discuss its implications for the tectonicactivities along the eastern margin of the TibetanPlateau.

GEOLOGICAL AND TOPOGRAPHIC SETTING

QUATERNARY FAULTS

The Minshan region and Minjiang drainage basinare located along the eastern margin of theTibetan Plateau, and separate the Songpan-Garzêterrane (SGT) from the Qinling Orogenic Belt(Fig. 1). There are three major Quaternary faultsidentified in these areas: the westernmost north-east–trending Minjiang Fault zone, the westtrending Xueshan Fault (XF) and the easternmostnorth-west–trending Huya Fault (HF) (Fig. 1).

The Minjiang Fault zone consists of severalstrands of high-angle (60

°−

80

°

) western dippingreverse faults (Zhao

et al

. 1994b; Qian

et al

. 1995,

Fig. 1

Simplified geological map of the Minjiangdrainage basin and adjacent regions. Index mapshows the main tectonic elements of China, and lightgray box shows approximate location of the studyarea. CR, Changjiang River (Yangtze River); HF, HuyaFault; HR, Huanghe River (Yellow River); LT, Lhasaterrane; LTB, Longmenshan Tectonic Belt; MF, Min-jiang Fault; NCB, North China block; QB, QaidamBasin; QT, Qiangtang terrane; SCB, South Chinablock; SGT, Songpan-Garzê terrane; XF, XueshanFault; ZB, Zhangla Basin.

F

1

,

F

2

and

F

3

represent thethree major thrust faults within LTB.

250 50 km

100 km

Pre-Mesozoic

Pre-Mesozoic granite

Thrust fault zone

Thrust fault

Strike slip fault

General fault

Drainage watershed

Mesozoic

Mesozoic granite

Cenozoic

MFZB

XF

HF

LTB

LTB

QinlingOrogen

TibetanPlateau

SichuanBasin

Minjiang

Longmensh

an Mt.

Minshan M

t.

River

.

.

SGT

31°

32°

33°

102° 45´ 103° 30´

Dujiangyan

Maoxian

ZhanglaHongyuan

Songpan

Heishui

F1

F2 F3

LT

QT

QBATFNCB

SCB

SGT

Sichuan

Basin

HR

CR

Geomorphology of the Minjiang drainage basin

241


1999; Kirby

et al

. 2000; Zhou

et al

. 2000) withnortheastern strikes parallel to the west side of theMinshan Mountain. These fault strands extendalong the Minjiang River south-westwards toSongpan for nearly 100 km. The easternmost Min-jiang Fault is a Quaternary active fault and itsrecent activities result in displacements of theCenozoic deposits (Zhao

et al

. 1994b; Zhou

et al

.2000) and some earthquakes with high seismicintensity (Chen

et al

. 1994b; Qian

et al

. 1995, 1999).The Quaternary Zhangla Basin and the dis-tribution of Quaternary sediments are closelycontrolled by this active fault (Fig. 1). The east–west-trending Xueshan Fault terminates againstthe Minjiang Fault to the southwest of Zhangla.Kirby

et al

. (2000) interpret the Xueshan Fault asa Mesozoic active structure that played a minorrole in Cenozoic deformation within the MinshanMountain. The HF is located at the eastern marginof the Minshan Mountain (Deng

et al

. 1994; Kirby

et al

. 2000; Zhou

et al

. 2000), and extends in theNNW–SSE direction and dips westwards atapproximately 70

°−

90

°

(Kirby

et al

. 2000). Itsnorthern end is covered by the XF, but can extendnorthwards as a blind structure (Zhao

et al

. 1994b;Kirby

et al

. 2000; Zhou

et al

. 2000). Many historicearthquakes have occurred along this fault (Chen

et al

. 1994b; Zhou

et al

. 2000). The HF played animportant role in tectonic deformation along theMinshan Mountain (Kirby

et al

. 2000). The distri-bution of hypocenters along the HF suggests itremains steep throughout much of the upper crust(Kirby

et al

. 2000).Besides the above Quaternary faults, three

major thrust faults (Fig. 1;

F

1

,

F

2

and

F

3

), whichbelong to the LTB, control the southeastern flankof the Minjiang drainage basin. These faults areMesozoic structures that were reactivated duringthe Cenozoic (Burchfiel

et al

. 1995).

STRATIGRAPHIC DISTRIBUTION WITHIN THE MINJIANG DRAINAGE BASIN

The whole Minjiang drainage basin can be dividedinto three tectonic units: the Qinling orogen, theSongpan Garzê Mesozoic basin and the Longmen-shan orogen (Fig. 1) each with distinct strati-graphic characteristics. Outcrops east of theMinjiang Fault, HF and north of the XF, parts ofQinling orogen, are mainly pre-Mesozoic shallow-water siliciclastic rocks and reef-bearing lime-stones. To the west of Minjiang Fault, HF andsouth of the XF, almost all the outcrops are Triassicchanging from shallow-water limestones to sev-

eral-kilometers-thick flysch deposits in the Song-pan Garzê basin (Fig. 1; Ministry of Geology andMineral Resources, PRC 1991; Burchfiel

et al

.1995). A suite of Mesozoic plutons (generally gra-nodiorite to monzonite) emplaced into the Triassicflysch southwest in the Minjiang drainage basin(Fig. 1; Ministry of Geology and MineralResources and PRC 1991) and a few are developedto the northwest of the drainage watershed(Zhang

et al

. 2004). The deposits within the Ceno-zoic Zhangla Basin are a semicontinuous sequenceof Pliocene conglomerates, Middle Pleistocene-upper Pleistocene alluvial conglomerates andupper Pleistocene-Holocene alluvium related withthe current Minjiang drainage. The Longmenshanfold and thrust belt consists mainly of pre-Mesozoic metamorphic and plutonic rocks.Metamorphic grade ranges from low to loweramphibolite (Burchfiel

et al

. 1995). More detailedinformation is given in Huang

et al

. (2003) andWallis

et al

. (2003).

GENERAL TOPOGRAPHIC SETTING

The Minjiang drainage basin is located along theplateau margin. The headwater region of the Min-jiang River is located to the north of Zhangla(Figs 1,2a). The Minjiang River runs southwardinto the Sichuan Basin and the elevation dropbetween source and Dujiangyan is approximately3000 m (Fig. 2a,b).

To the northwest adjacent to the Minjiangdrainage basin, the altitude is approximately 3500–3800 m (Fig. 2a). Eastwards, the elevation variesfrom 4000 m to 4500 m and reaches the MinjiangRiver valley. The Minshan Mountain is approxi-mately 40–50 km wide along the eastern side of theMinjiang River and contains many peaks above4500 m (Fig. 2a,c), and the highest peak reaches5588 m southeast of Zhangla. To the east, the ele-vation decreases gradually to approximately 1000–3000 m into the Qinling Orogen adjacent to theMinjiang drainage basin. There is a very largedifference in altitude between the Minjiang drain-age basin and the Sichuan Basin associated withsteep gradients (Fig. 2c–e).

DATA AND PROCESSING METHODS

SRTM-DEM DATA

Our analysis is based on the dataset that containsa concatenation of 3-arcsecond DEM acquired by

242

H.-P. Zhang

et al.


the United States Geological Survey SRTMproject (Rabus

et al

. 2003). A 0.5

×

2.0 degreesquare is used (Fig. 2a). The data represent spotheights on a 3

×

3 arcsecond rectangular grid.Each grid cell is roughly 71 m wide by 92 m longin our target area. The spatial accuracy of theseDEM data is approximately

±

16 m absolute and

±

6 m relative vertical accuracy. Data for process-ing and analysis were clipped from the originSRTM-DEM database by the Minjiang drainagebasin boundary, which was extracted by means ofgeographic information system software.

PROCESSING METHODS

Local topographic relief

Local topographic relief is defined by the differ-ence in height between the peaks with highest ele-vation and the valley bottoms in a region, which isan expression of incision by rivers or glaciers. It

can be obtained by subtracting the base level ele-vations from the summit level elevations (Deffon-taines

et al

. 1994, 1997; Kühni & Pfiffner 2001).The base level and summit level elevation mapsare computed by dividing the DEM into squaresof equal size (filter window) 1 km

×

1 km(Fig. 3a,b). For each of these squares, the coordi-nates of the points of lowest or highest elevationsare determined and saved. A new surface is calcu-lated that contains all these defined points. Toobtain the same resolution as the initial SRTM-DEM, a standard Kriging method linear vario-gram model is applied to the areas between thestored points of minimum or maximum elevations.It is known that the DEM is the numerical expres-sion of the earth surface, and the grid DEM isusually presented as a regular matrix. By meansof a raster calculator imbedded in the GeographicInformation System (GIS) software, the localtopographic relief of our target area can beobtained by subtracting base level elevation

Fig. 2

Morphology of the Minjiang drainage basin and adjacent regions. (a) Elevation-shaded digital elevation model overlain by major tectonic contactsshows the location and geometrical relationships with these contacts. (b) Longitudinal profile of the Minjiang River. Note the sudden changes of channelgradient caused by Quaternary faults activities. (c–e) Topographic profiles along the eastern margin of the Tibetan Plateau. Note the deeply dissected,high relief escarpment along the margin.

C´B´

A´

a

F1

F2

F3

25 50 km0

31°

32°

33°

102° 45´ 103° 30´

Maoxian

Wenchuan

Zhangla

Hongyuan

Dujiangyan

Songpan

Heishui

C

B

A

1000

2000

3000

4000

5000 m4000 m3000 m2000 m1000 m

00030 200100

S

F2 F3

1000

2000

3000

4000

50 100 150

A A´

B B´

C C´

SB

Minshan MountSE

Longmenshan Mount

F1 F2

F3

1000

2000

3000

4000

5000

20 40 60 80 100 120

Ele

vati

on (

m)

Distance (km)

Ele

vati

on (

m)

Ele

vati

on (

m)

Ele

vati

on (

m)

Miniang River

SB

Longmenshan MountSE

F1 F2F3

1000

2000

3000

4000

50 100 150

Miniang RiverSB

Longmenshan MountSE

F1F2

F3

b

c

e

d

a

Geomorphology of the Minjiang drainage basin

243


matrix from summit level elevation matrix(Fig. 3c).

Extraction of stream networks and drainage basins from DEM

Conventional quantitative parameters (channellength, number of streams, bifurcation ratio, ele-vation difference, slope, perimeter and area ofdrainage basins) were extracted for 67 drainagesub-basins from the DEM of the Minjiang drain-age basin area. Table 1 shows the attributesdefined for drainage networks and basins analysis.

Because only the SRTM-DEM of the whole Min-jiang drainage basin is available in this 0.5

×

2.0degree square area, surface stream networks ofthe Minjiang were automatically extracted byArcGIS (Geographic Information Systems soft-

ware developed by ESRI; Redlands, Ca, USA) andRivertools software (Research Systems; Boulder,Colorado, USA). The orders of the extractedstreams were further defined according toStrahler’s system (Strahler 1952), which is thesimplest, and certainly the most widely acceptedmethod today. In this classification approach initialtributaries, i.e. streams with no tributaries areallotted rank of one and called first-order streams.A second-order stream is formed below the junc-tion of two first-order streams (Fig. 4). In general,two streams of the same order increased by one.Note that the order number of a stream isunchanged irrespective of the number of tributar-ies of a lesser order that flow into it. Figure 5ashows the extracted streams of more than thethird-order in the whole Minjiang drainage basin.

Fig. 3

(a) Elevation-shaded base level map and (b) summit level map of the Minjiang drainage basin (sampling grid width: 1 km × 1 km). Localtopographic relief calculated by subtracting (a) from (b) of the Minjiang drainage basin is shown in (c). Relief profiles along northern section (NS), centralsection (CS), and southern section (SS) are shown in Fig. 8.

Table 1 Attributes defined for drainage sub-basins

No Attribute Definitions Methods

1 Perimeter Perimeter of drainage sub-basins (km) ArcGIS polygon feature2 Area Area of drainage sub-basins (km2) ArcGIS polygon feature3 Max. Elev. Maximum elevation of special size filter window (m) ArcGIS Grid4 Min. Elev. Minimum elevation of special size filter window (m) ArcGIS Grid5 Ave. Slp. Average topographic slope (°) ArcGIS Grid6 Length Length of all streams in drainage basin (km) ArcGIS polyline feature7 Number (n) Number of stream segments (order n) ArcGIS polyline feature8 Rb Bifurcation ratio of stream channels ArcGIS polyline feature

244 H.-P. Zhang et al.


A basin is defined by the location of its outlet,and by definition, one basin has one single outlet.The outlet is the relatively narrow section alongthe boundary (drainage divide) of a basin acrosswhich water and sediment are collected by thechannels in the basin. Generally, the outlet islocated within the main channel and its elevationbecomes the local base level of a single sub-basin.The drainage basin as a whole is normally allottedthe number of the highest order stream that flowsin it. Figure 3, for example, represents a fourth-order drainage sub-basin.

In the current study, Dujiangyan within theSichuan Basin was defined as the outlet of theMinjiang drainage basin (Figs 1,2a), and 67 fourthand more than fourth-order drainage sub-basinsare extracted, in which 31 sub-basins (labeled as1–31 in Fig. 4b) and 36 sub-basins (labeled as 32–67 in Fig. 4b) are located to the western and east-ern sides of the north–south-trending MinjiangRiver (Fig. 5b), respectively.

MORPHOMETRIC EXTRACTION

Perimeter and area of drainage sub-basins

Based on obtained distribution of the Minjiangdrainage basin and sub-basins, the attributes suchas perimeter and area of the basins can beextracted by ArcGIS feature topology (Fig. 6a,b).A native Gauss-Kruger projected coordinate sys-tem (PCS) with Beijing 1954 geography coordi-nate system datum embedded in ArcGIS softwarewas chosen for morphometric calculation.

Total channel length and bifurcation ratio

According to distribution of drainage networksextracted from the DEM in different drainagesub-basins, the total channel length was easilydetermined (Fig. 6c) by adding the channel length

Fig. 5 Digital elevation model-extracted MinjiangRiver drainage networks and sub-basins. (a) Streamsof third-orders and above (for clarity, streams of lowerorders are not shown), and (b) 67 sub-basins of fourth-orders or above extracted for morphometric analysis.The western sub-basins are designated from number 1to number 31 and the eastern ones – from number 32to number 67, upstream of the Minjiang main channel.

Fig. 4 Example of drainage basin and stream networks indicating thedefinition of Strahler order (Strahler 1952) for streams and basins.

1

23

1

1

11

1 1

1

11

1

11

1

1

11

1

11

1

11

11

1

1

2 2 2 2

22

2

2

4

3

3

4

23 4

Geomorphology of the Minjiang drainage basin 245


of all different order streams within the corre-sponding drainage basin. This morphometricparameter is statistically analyzed by using theattribute tables of different ArcGIS features, suchas polylines and polygons.

The bifurcation ratio is derived by calculatingthe mean of the ratios between the total numberof streams of each order and the total for the nextorder above it (McCullagh 1978). For example, thebifurcation ratio for Fig. 4, in which there are27 streams of the first-order, 10 of the second-order, four of the third-order and one of the fourth-order, is:

All of the fifth-order drainage basins within the67 sub-basins were chosen for calculating the indi-vidual bifurcation ratio according to the aboveequation (Fig. 6d).

Channel profiles

The river channel profiles of the fifth-order andgreater order basins were extracted from SRTM-DEM. For comparison between the vertical com-ponents of 13 west tributaries and 10 east ones,the relative altitudes of fifth-order streams abovethe Minjiang main channel were plotted againstthe river running distance (Fig. 7a,b).

PROCESSING RESULTS

LOCAL TOPOGRAPHIC RELIEF OF THE MINJIANG DRAINAGE BASIN

A local topographic relief map of the Minjiangdrainage basin is obtained by subtracting base

Rb

NN

NN

NN=

1

2

2

3

3

4

+ +=

+ +=

3

2710

104

41

33 07.

Fig. 7 Longitudinal profiles of the longest river for comparison the vertical components of the drainage systems. (a) Streams flowing east into theMinjiang River, (b) streams flowing west into the Minjiang River. Note the differences of running distances and elevation drops between two flanks.

Alt

itu

db

ov

inj i

anm

)

15

9

24

31

26

25

19

6

14

1110

12

a

324042

5839

57

33

46

5966

b

Fig. 6 Plots illustrating the variations in geomorphic parametersbetween the western sub-basins and the eastern ones. (a) Area of sub-basins, (b) perimeter of sub-basins, (c) total channel length within sub-basins, (d) bifurcation ratio. Class breaks were determined statisticallyby finding adjacent feature pairs that have relatively large gaps betweenthem.



level elevations from summit level elevations(Fig. 3). Using a filter window size of 1 km × 1 kmscale, the results indicate that with increasing dis-tance downstream the Minjiang River is associ-ated with higher local topographic relief. Therelief increases from 0 m to near 800 m betweenthe northernmost headstream and the local outletof the Minjiang drainage basin at Dujiangyan.The minimum relief within the whole drainagebasin occurs at near Zhangla and southeast ofHongyuan (Fig. 3c; see Fig. 2a for the locations).In contrast to the relatively high relief of thesouthern region, two very low relief singularitiescan be detected along the mainstream channelnear Dujiangyan and Maoxian (Fig. 3c; see Fig. 2afor the locations).

CHARACTERISTICS OF THE MINJIANG DRAINAGE SUB-BASINS

On the eastern side of the Minjiang main channel,the areas and perimeters of nearly all drainagesub-basins are less than 100 km2 and 50 km(Fig. 6a,b). The exceptions are Dujiangyan (num-ber 32), Maoxian (number 46) and Songpan (num-ber 58). However, on the opposite side of theMinjiang channel entirely different characteristicsare observed. Areas of four sub-basins (number 3,9, 15 and 24) and the perimeters of nine sub-basins(number 1, 3, 6, 9, 15, 19, 24, 25 and 27) are morethan 1000 km2 and 100 km, respectively (Fig. 6a,b).The area of number 15 even reaches 7202.2 km2.

The total channel lengths in the each sub-basinalong the eastern side of the Minjiang are less than200 km except the channels within number 32, 46and 58 sub-basins. The total lengths of five westerntributary streams exceed 1000 km (number 3, 9,15, 24 and 27) and six streams are between 200 kmand 1000 km (Fig. 6c). The bifurcation ratios of thefifth-order streams are calculated and plotted(Fig. 6d). Within the 10 eastern sub-basins, onlytwo tributaries show higher bifurcation ratiosbetween 4.1 and 5.0 (number 32 and 46). However,eight of the 13 western ones are presented withhigher bifurcation ratios.

TRIBUTARY CHANNEL MORPHOMETRY

Within all of the fifth-order sub-basins along theMinjiang main channel, 23 longest stream channelprofiles are plotted to compare the vertical compo-nents of the drainage systems (Fig. 7). Althoughthe western tributary profiles indicate that eightstreams run less than 50 km, the stream lengths

of number 9, 15 and 24 are more than 150 km(Fig. 7a). But along the eastern side, every channelis definitely no longer than 50 km, and is mainlyconfined within 30 km except number 32 and 58(Fig. 7b). Most tributary channel drops (differencebetween stream source and outlet) are less than3000 m within the fifth-order sub-basins.

INTERPRETATION OF THE RESULTS AND DISCUSSION

LOCAL TOPOGRAPHIC RELIEF AND ITS TECTONIC IMPLICATIONS

Local relief is often used to describe the morpho-logical characteristics of a mountain belt (Burbank1992; Fielding et al. 1994; Gilchrist et al. 1994;Fielding 2000). It is the result of erosion processesacting to destroy relief that is built up or hasalready been built by tectonic processes. Althoughinterpreting the significance of local topographicrelief is difficult because local relief depends onnumerous interdependent processes, it is still aparameter to describe the maximum dissection ofa landscape as a result of valley incision (Kühni &Pfiffner 2001).

Our processing results of local topographicrelief indicate that the relief increases graduallyfrom north to south, and usually it is lower alongthe stream channels but becomes very high justaway from streams (Fig. 8a–c). As shown inFig. 8a, the closer to the Minshan uplift region, thehigher relief appears. The incision processes anddifferential uplift construct approximately 450 mrelief. The maximum valley incision (800 m or so)occurs nearly above the regional base level (theoutlet at Dujiangyan within the Sichuan Basin)along the southernmost LTB and southern seg-ment of the Minshan uplift zone (Figs 3c,8c). Astudy on the Minjiang terraces and their ages alsoproves downstream increase of Quaternary valleyincision (Li et al. 2005).

The longitudinal gradient ratio of the Minjiangdownstream channel is even higher because of thelarge amount of surface uplift above the level ofthe Sichuan Basin (Fig. 2). Streams continuouslyincise the basement to maintain their course. Theresult is maximum relief developed along the tec-tonically active Longmenshan Front. Minimumrelief within the whole drainage basin is nearZhangla and in the southeast region of Hongyuan.Two very low relief singularities can be detectedalong the mainstream channel near Dujiangyanand Maoxian (Figs 3c,8b). Our field reconnais-



sance shows that Zhangla, Hongyuan and Maoxianregions are Quaternary sedimentary basins(Fig. 9a,b). Quaternary sediments into basinsaccelerate lowering stream gradient and power,and then low incision and continuous sedimentsfinally result in the lower relief within these areas.Dujiangyan low relief singularity can be inter-preted by its geographic and geological setting andit is just located within the low relief SichuanBasin.

MORPHOMETRY OF DRAINAGE SUB-BASINS AND ITS TECTONIC IMPLICATION

Almost all the conventional quantitative parame-ters (perimeter and area of drainage sub-basins,total stream channel length and bifurcation ratio)of the Minjiang drainage basin show the obviousdifferences between western drainage sub-basinsand eastern ones. Areas and perimeters of 33 east-ern drainage sub-basins are less than 100 km2 and50 km (Fig. 5a,b). However, areas of four sub-basins and perimeters of nine sub-basins are morethan 1000 km2 and 100 km, respectively (Fig. 5a,b).The total channel lengths in the each sub-basinalong the eastern side of the Minjiang are less than200 km, except the channels within number 32, 46and 58 sub-basins. In contrast, the total lengths offive western tributary streams exceed 1000 kmand six streams are between 200 km and 1000 km(Fig. 5c). Within the 10 eastern sub-basins, onlytwo tributaries show higher bifurcation ratiosbetween 4.1 and 5.0. However, eight of the 13 west-ern sub-basins have streams associated with highbifurcation ratios.

Some exceptions to the above-mentionedparameters exist within numbers 32, 46 and 58sub-basins (Figs 6,7; see Fig. 3 for sub-basin num-ber). The three sub-basins are just located nearthe transition regions. Number 58 sub-basin isnear the south end of the Minjiang Fault, number46 is at the intersection of the Minshan and theLongmenshan Mountains and number 32 isbetween the boundary of the Longmenshan Moun-tain and the Sichuan Basin. All the mainstreamswithin these three sub-basins are almost parallelto the Minjiang mainstream channel. Modern glo-bal positioning satellites researches show that thestudied area is still under the compressive stressfield of near east–west direction, which also resultsin the east–west shortening (Zhang et al. 2002).Therefore, the following development of north–south striking thrust faults and their associatednorth–south striking bedding and foliation pro-vides more evolution spaces for these above excep-tional streams.

Considering the regional geological setting,these calculated parameters indicate that thesedifferences result from the Cenozoic tectonicactivities along the eastern margin of the TibetanPlateau. The Minshan Mountain is a rapid Qua-ternary uplift zone clamped by western MinjiangFault and eastern HF (Fig. 1). Compared withrapid uplift and tilt of the Minshan Mountainarea (Kirby et al. 2000), the western side of the

Fig. 8 Relief profiles within the Minjiang drainage basin. (a) Northernsection, (b) Central section, and (c) Southern section. Locations areshown in Fig. 3. Note the increase in relief towards the east and south,and low relief within the Quaternary basins.

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Fig. 9 Photographs of low relief Quaternary basins. (a) Zhangla Basinand (b) Maoxian Basin. Note the low relief within these basins and theterraces of lower elevations above river level. Quaternary sedimentsdeposited within these basins result in lower local relief. Locations of thebasins are shown in Fig. 2a.

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Minjiang Fault was relatively stable, althoughuplift also occurs within this region. Previousseismic studies on the Minjiang and HF alsoshow that they are still very active nowadays.Quaternary differential uplift of the Minshanfinally results in lower perimeter and area ofdrainage sub-basins, total channel length andbifurcation ratio within eastern flank of the Min-jiang mainstream. At the same time, the shorterchannel lengths and higher drops of easternstreams possibly testify to the existence of differ-ential tectonic activities (Fig. 7b). All the streamskeep on incising the bedrocks to reach the localbase level and to balance the stream power;therefore, streams should probably become moredeveloped if there are no external influences. Thelonger the streams run and the more tributariesare generated, the more one drainage basin isdeveloped. Therefore, the shorter paths of east-ern networks and lower bifurcation ratios mightindicate the undergrowth and newborn feature ofstreams, which also results from the eastern dif-ferential uplift.

In summary, geomorphic characteristics of theMinjiang drainage basin and sub-basins resultfrom the Quaternary tectonic activities of the east-ern margin of the Tibetan Plateau. The evolutionof the Minjiang drainage basin shows one of theclearest cases for differential tectonic upliftbetween east and west of the Minjiang streamchannel (Fig. 10).

CONCLUSIONS

Based on the results of analysis of the DEM in theMinjiang drainage basin, the major topographiccharacteristics can be summarized as follows:1. The regular increase in the local topographic

relief from north to south results from thedeposition of the Quaternary sediments withinthose basins along the Minjiang River and theoverall uplift of the eastern margin of theTibetan Plateau versus the Sichuan Basin.

2. Lower perimeter and area of drainage sub-basins, total channel length and bifurcationratio within the eastern flank of the basin, alongthe Minjiang mainstream, result from the Qua-ternary differential uplift of the Minshan Moun-tain region.

3. Shorter lengths and lower bifurcation ratio ofthe eastern streams indicate the undergrowthand newborn features, which also are theresults of the eastern active uplift of the Mins-han Mountain.

ACKNOWLEDGEMENTS

The current study was funded jointly by NationalScience Foundation of China (Grant no. 40234041,40272055) and Department of Land and Resourcesof China (Grant no. 20010202). We would like tothank Professor Paul H. Heller (Wyoming Univer-sity) for providing the SRTM-DEM data, Mr Qun-wei Xue (Institute of Geological EnvironmentMeasurement, Beijing) for access to the ArcGISsoftware environment, and Professor Yu Wang forvaluable suggestions and earnest instructions. Wegratefully acknowledge comments and sugges-tions for modifications of the manuscript by MrDaniel Costich (CUGB, Beijing), Dr Bo Lu (Uni-versity of Texas at Austin), Dr Margaret E.McMillan (University of Arkansas at Little Rock)and Professor Simon R. Wallis (Nagoya Univer-sity). Constructive reviews of Professor RunshengWang, Dr P. Gautam and Professor V. Dangolhelped to improve this paper.

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Geomorphic characteristics of the Minjiang drainage basin ... · To the west of Minjiang Fault, HF and ... River valley. The Minshan Mountain is approxi- ... in our target area