Fault–Block Structure and State of Stress in the Earth’s Crust ...GEOTECTONICS Vol. 43 No. 1 2009 FAULT–BLOCK STRUCTURE AND STATE OF STRESS 69 It is assumed that the rift zone

67

ISSN 0016-8521, Geotectonics, 2009, Vol. 43, No. 1, pp. 67–84. © Pleiades Publishing, Inc., 2009.Original Russian Text © O.V. Lunina, A.S. Gladkov, 2009, published in Geotektonika, 2009, No. 1, pp. 78–96.

INTRODUCTION

The Gusinoozersky Basin is a reference one for the sys-tem of Late Mesozoic Transbaikal-type depressions [2].This basin extends in the northeastern direction from thelower reaches of the Temnik River to the Lower Ubukunsowneck, 65 km from Lake Baikal (Fig. 1). The basin is75 km long and 15 km wide, on average. The basin isframed by the Khambinsky Range in the northwest and bythe Monostoi Uplift in the southeast. Like the Baikal RiftZone, this structural assembly is asymmetric: the north-western wall of the basin is expressed more distinctly thanthe southeastern wall. The peaks of the KhambinskyRange reach 1420 masl and are often 200–300 m higherthan the opposite wall (Fig. 1). The bottom of the Gusi-noozersky Basin is located at 550 masl.

The Gusinoozersky Basin is much more well-stud-ied than other Transbaikal-type depressions [2], largelybecause of the active exploitation of coalfields in thisregion. Recently obtained data on sedimentary [16] andvolcanic rocks [4] provide insights into the Late Creta-ceous and Cenozoic evolution and geodynamics of thewestern Transbaikal region. However, the fault–blockstructure and state of stress in the Earth’s crust, as wellas the kinematics of faulting have not been studied indetail. Because the Gusinoozersky Basin is located nearthe tectonically active Baikal Rift Zone, information ofits structure is important in the light of the idea thatassumes westward migration of rifting. The develop-ment of this idea is outside the scope of this paper

because it requires the involvement of comprehensiveadditional data. Nevertheless, we suppose that theresults of the structural and tectonophysical studyreported in this publication will fill a gap in the knowl-edge of the structure and geodynamics of the Gusinooz-ersky Basin and the adjacent territory. These resultsmay be extended over the western Transbaikal region asa whole.

TECTONIC SETTING

According to current knowledge, the GusinoozerskyBasin and the adjacent uplifts are elements of the WestTransbaikal Rift Zone that extends in the northeasterndirection for 1000 km from the headwater of theSelenga River to the Vitim Highland. This zone wasfirst defined as the Selenga–Vitim region of Mesozoicdepressions formed as a result of linear warping of theEarth’s crust [15]. Recent investigations have providedevidence for the rift nature of the Transbaikal basins[4, 13, 19, 20]. Graben-like structure, normal faults,longitudinal dike swarms, and predominantly alkalibasaltic volcanism are typical attributes of rifts. Theoldest, Upper Jurassic–Lower Cretaceous sedimentsthat fill the basins and the Late Jurassic age of volcanicsknown in this region indicate that the West TransbaikalRift Zone started to evolve in the Late Mesozoic. Vol-canic activity has proceeded almost continuously overthe last 170 Ma, with varying intensity [19].

Fault–Block Structure and State of Stress in the Earth’s Crust of the Gusinoozersky Basin and the Adjacent Territory,

Western Transbaikal Region

O. V. Lunina and A. S. Gladkov

Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033 Russiae-mail: [email protected]

Received April 28, 2007

Abstract

—The geological structure and tectonophysics of the Gusinoozersky Basin—a tectonotype of Meso-zoic depressions in the western Transbaikal region—is discussed. New maps of the fault–block structure andstate of stress in the Earth’s crust of the studied territory are presented. It is established that the GusinoozerskyBasin was formed in a transtensional regime with the leading role of extension oriented in the NW–SE direc-tion. The transtensional conditions were caused by paths of regional tension stresses oriented obliquely to theaxial line of the basin, which created a relatively small right-lateral strike-slip component of separation (in com-parison with normal faulting) along the NE-trending master tectonic lines. The widespread shear stress tensorsof the second order with respect to extension are related to inhomogeneities in the Earth’s crust, including thosethat are arising during displacement of blocks along normal faults. Folding at the basin–range boundary wasbrought about by gravity effects of normal faulting. The faults and blocks in the Gusinoozersky Basin remainedactive in the Neogene and Quaternary; however, it is suggested that their reactivation was a response to tectonicprocesses that occurred in the adjacent Baikal Rift Zone rather than to the effect of a local mantle source.

DOI:

10.1134/S0016852109010051

68

GEOTECTONICS

Vol. 43

No. 1

2009

LUNINA, GLADKOV

Fig. 1.

3D topographic model of the Gusinoozersky Basin and the adjacent territory and location of observation points (OPs). White,gray, and black triangles are OPs documented in the Cenozoic, Mesozoic, and pre-Mesozoic rocks, respectively. The regional posi-tion of the study area is shown in the inset. The morphological section along line A–B is given below.

1400

0

Hei

ght,

m

Distance, km

600

1200

1000

800

A B

NW SE

5 10 15 20 25 30 35 40 45

106°00

′

106°15

′

106°30

′

106°45

′

107°00

′

E

18 km90

Selenduma

50°50

′

51°00

′

51°10

′

51°20

′

51°30

′

51°40

′ Ν

Siberian Platform

L. Ba

ikal

53°105°

51°51°

L. Gusinoe

Angara R.

Selenga R.

Dzhida

-

Vi t

i m

S ut u

r e

Chi

koi R

iver

val

ley

Selen

ga Ri

ver va

lley

Novoselenginsk

Mon

osto

i Ran

ge

Gusinoozersk

Gus

inoe

Lake

Kham

bins

ky

Rang

e

Ä

Å

sowneckLower-Ubukun

0507

0506

05050504

0307

05010502

0503

0306

0305 0304

0303 0302

0301 0604

04010402

0403

04040405Ó

0405k0206

0205

0204

02030202

02010602

06010406

0102

0603

06060605 0101

+108°+105°

Temnik River valley

GEOTECTONICS

Vol. 43

No. 1

2009

FAULT–BLOCK STRUCTURE AND STATE OF STRESS 69

It is assumed that the rift zone arose under intracon-tinental conditions above one of the hot mantle fields ofCentral Asia. The two mantle plumes correspond to thetwo long-lived magmatic centers [19]. The localizationof the Late Mesozoic basins was controlled by NE-trending deep faults. In particular, the GusinoozerskyBasin is confined to the ancient Dzhida–Vitim Suturethat separates the Baikalian and Caledonian fold sys-tems [1, 3].

The Gusinoozersky Basin is superimposed on gra-nitic basement broken into blocks [32]. The thicknessof the fresh-water continental sediments that fill thebasin gradually increases from the northwestern wall tothe southeast and reaches a maximum of 2500 m nearobservation point (OP) 0404 and ~9 km to the southeastof OP 0101 (Fig. 1). Near Cape Chana on the westerncoast of Lake Gusinoe opposite to OP 0504 and at itsnorthern end, the basement is uplifted and the thicknessof sediments is reduced to 1000 m. The entire sectionof the Lower Cretaceous conglomerate, gravelstone,sandstone, siltstone, and mudstone with coal seams [32]is overlapped in many places by Neogene and Quater-nary poorly cemented and loose sediments. The volca-nic field in the Khambinsky Range extends for morethan 40 km. The history of its evolution is divided intothree stages that cover an interval between 159 and117 Ma ago [4], i.e., from the Late Jurassic to the endof Early Cretaceous. Mesozoic granites crop out in bothuplifts that frame the Gusinoozersky Basin.

In structural terms, the Gusinoozersky Basin is ahomocline complicated by differential motions of base-ment blocks [2]. K.B. Bulnaev assigns the major role tothe Monostoi Normal Fault that controls subsidence,pointing out transverse and other faults of various agesbut disregarding the effect of the Khambinsky Fault onthe basin evolution. It is deemed that this fault did notexert any effect on the accumulation of Mesozoic sedi-ments [1]. However, reactivation of the KhambinskyRange in the Cenozoic is evident. A near-meridionalseismotectonic dislocation up to 2.5 km long is tracedin its southwestern portion as a scarp of normal fault

that cuts loose sediments of fans and is accompanied bylandslides and downfalls [9]. The trenching of this faultresulted in recognition of two seismic events: ayounger, less strong event happened after the formationof the buried humic horizon dated at 2680

±

60 yearsago, and an older and stronger event predated soil for-mation 5290

±

100 Ma ago [17]. Thus, the Late Meso-zoic and Cenozoic tectonics and geodynamics of theGusinoozersky Basin and the adjacent territory arerather complex and attract interest for investigationbecause this basin is located close to the Baikal RiftZone and is a classic example of coal-bearing basins.

FACTUAL DATA AND THEIR PROCESSING

The study of the fault–block structure and state ofstress in the Earth’s crust of the Gusinoozersky Basinwas performed using the technique applied previouslyto studying the Cenozoic Basins of the Baikal Rift Zone[10–12]. A network consisting of 35 observation pointshas been created (5 OPs in the Paleozoic rocks, 17 inthe Mesozoic, and 13 in the Cenozoic rocks) (Fig. 1). Inaddition to the standard description, the zones of crush-ing, foliation, fracturing, mylonitization, and/or cata-clasis were recorded, as well as the main fracture sys-tems, their relationships, ductile deformation, kine-matic indicators, and possible signs of Cenozoicfaulting.

The mass measurements of fracture orientationwere implemented at 33 OPs to prepare diagrams andthe subsequent reconstruction of the stress field usingthe techniques of Nikolaev [14] and Gzovsky [5]. Theformer technique was used to select the conjugate sys-tems by opposite dispersals at the maximums that lie onthe arc of a great circle, and the latter technique, for thedirect recreation of the position of the principal normalstresses. The kinematics of displacements along theconjugate fractures was determined from the recon-structed stress field. For two OPs where the conjugatesystems have not been established but the striation mea-surements are available, three kinds of kinematic meth-

Fig. 2.

View on the northwestern wall of the Gusinoozersky Basin near OP 0303.

Facets of normal fault Facets of normal faultKhambinsky Range

SW

70

GEOTECTONICS

Vol. 43

No. 1

2009

LUNINA, GLADKOV Fa

ctua

l dat

a an

d re

cons

truc

tion

of s

tres

s fi

elds

in th

e G

usin

ooze

rsky

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in a

nd it

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ount

ain

fram

ewor

k

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Lat

itude

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gi-

tude

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juga

te s

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gree

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3

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ess

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gree

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m.,

de-

gree

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m.,

deg

ree

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gree

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gree

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gree

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hed

GEOTECTONICS

Vol. 43

No. 1

2009


Tab

le.

(Con

td.)

OP

Lat

itude

Lon

gitu

deL

ithol

ogy

Age

of r

ocks

Con

juga

te s

yste

ms

I

An-

gle,

de

gree

σ

1

σ

1

σ

2

σ

2

σ

3

σ

3

Typ

e of

str

ess

fiel

dA

zim

., d

e-gr

ee

Dip

an

gle,

de

gree

Azi

m.,

de-

gree

Dip

an

gle,

de

gree

Azi

m.,

de-

gree

Dip

an

gle,

de

gree

Azi

m.,

de-

gree

Dip

an

gle,

de

gree

Azi

m.,

de-

gree

Dip

an

gle,

de

gree

0501

51.2

3610

6.26

3 T

rach

ybas

alt

Upp

er J

uras

-si

c–L

ower

C

reta

ceou

s

120

9035

080

1651

236

1230

7714

56

Shea

r

0502

51.2

5410

6.22

4G

rani

tePa

leoz

oic

130

7031

540

1370

299

7541

313

215

Ext

ensi

on

0503

51.2

6810

6.20

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rach

yand

esite

Upp

er J

uras

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ower

C

reta

ceou

s

6070

160

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8129

215

7070

199

13

Shea

r

0504

51.2

0710

6.30

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ndst

one

and

grav

elst

one

with

le

nses

of m

udst

one

and

coal

The

sam

eN

ot d

etec

ted

––

186

58–

–32

324

––

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161

224

231

528

––

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361

226

1232

226

––

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360

225

732

026

Tra

nste

nsio

n

0505

51.1

6210

6.22

7C

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ct o

f tra

chy-

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lt an

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nd-

ston

e

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er J

uras

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s

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est

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hed

140

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025

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320

6323

00

140

27

Ext

ensi

on

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51.1

3710

6.21

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rach

ybas

alt

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sam

e

120

8029

540

1260

309

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94

118

20

Ext

ensi

on

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51.0

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6.18

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rach

ybas

alt

The

sam

e

130

9032

030

961

300

5940

613

330

Tra

nste

nsio

n

0602

51.1

6910

6.53

6G

rani

tePa

leoz

oic

Not

det

ecte

d

––

210

73–

–31

03

––

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172

3917

307

5–

––

206

5821

3211

23

––

–20

668

3025

303

4

Ext

ensi

on

0603

51.1

0110

6.60

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ndy

loam

Hol

ocen

e

150

8031

545

1057

168

238

1214

418

Ext

ensi

on

7570

255

606

6025

580

165

075

10

Ext

ensi

on

0604

51.3

0710

6.59

3Sa

ndst

one

with

br

ecci

a fr

agm

ents

Upp

er J

uras

-si

c–L

ower

C

reta

ceou

s

Not

det

ecte

d

––

––

––

––

Not

est

ablis

hed

0605

51.4

0110

6.53

Sand

and

oth

er

clas

tic s

edim

ents

Hol

ocen

e

4060

110

8018

6816

017

3860

258

24

Shea

r

0606

51.4

310

6.71

0G

rani

tePa

leoz

oic

125

3030

080

1170

115

6521

03

302

25

Ext

ensi

on

0607

51.4

7810

6.74

7Sa

nd a

nd g

rave

lH

oloc

ene

6050

110

8016

5316

031

3146

268

27

Tra

nste

nsio

n

Not

e:32

sol

utio

ns a

t 33

OPs

wer

e ob

tain

ed, i

nclu

ding

13

solu

tions

(41%

) cor

resp

ondi

ng to

ext

ensi

on; 5

(15%

) to

tran

sten

sion

; 13

(41%

) to

shea

r; 1

to in

defin

ite ty

pe; a

nd 0

to tr

ansp

ress

ion

and

com

pres

sion

. Str

ess

field

was

not

est

ablis

hed

for

5 O

Ps w

ith m

ass

frac

ture

mea

sure

men

ts.

I

is th

e re

lativ

e in

tens

ity o

f th

e st

ress

fiel

d, w

hich

is th

e su

m o

f in

tens

ities

per

tain

ing

to m

axim

ums

of c

onju

gate

fra

ctur

e sy

stem

s. K

inem

atic

tech

niqu

es [

22, 2

3, 2

5] w

ere

used

at O

Ps 0

504

and

0602

; the

ave

rage

res

ult o

btai

ned

by th

e th

ree

tech

niqu

es is

sho

wn.

The

age

of r

ocks

are

giv

en a

fter

[2,

4, 1

9, 2

0].

72

GEOTECTONICS

Vol. 43

No. 1

2009

LUNINA, GLADKOV

Fig. 3.

Map of fault–block structure of the Gusinoozersky Basin and the adjacent territory. The rose diagram in the upper left cornerdemonstrates the strikes of the mapped faults (the total number of faults is 241; the step is 10

°

; the maximum percentage is 17%).(

1

) Regional faults: (

a

) mapped and (

b

) inferred; (2) local faults: (

a

) mapped and (

b

) inferred; (

3a

) normal and (

3b

) strike-slip faults;(

4

) dip azimuth and angle; (

5

) sedimentary rocks: (

a

) Quaternary, (

b

) Neogene, and (

c

) Lower Cretaceous; (

6

) crystalline basementand pre-Cenozoic volcanic rocks; (

7

) Cretaceous–Quaternary basalts, unspecified.

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106

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′

106

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30′ 106°45′ 107°00′ E50°50′

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51°30′

51°40′ Ν

60–65a b ca

a ab bb 1 2 3 4 5 6 7

0 9 18 km

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GEOTECTONICS Vol. 43 No. 1 2009


Fig. 4. Example of tectonic deformation in the zone affected by NE-trending fault. (A) Fracture zone that dips along an azimuth of 150° SEat an angle of 80–85° in the Quaternary loam with slightly rounded rock fragments, OP 0303. (B) Diagrams of mass measurements offractures and restored stress field at OP 0302 in the zone affected by the same fault, projection on the upper hemisphere. The window is 10.Contour lines of the density of fracturing maximums are spaced at 1.5, 2.5, 3.5, 4.5% and more; n is number of measurements. The dashedarrows inside the diagrams denote preferential directions of scattering in maximums of fracturing that indicate conjugation fracture sys-tems, after the Nikolaev method [14]. σ1 is the compression axis, σ2 is the intermediate axis, and σ3 is the tension axis. (C) Compressivedeformation at OP 0302 in the domain of local shear stresses at the boundary between the Gusinoozrsky Basin and the Khambinsky Range.The fold’s hinge strikes at an azimuth of 195° SSW and plunges at an angle of 25°.

A

B

C

NW

SE

1.5 m

00

σ3

σ2

σ1

OP 0302, n = 75

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LUNINA, GLADKOV

Fig. 5. Manifestation of the Khambinsky Fault in the southern Gusinoozersky Basin: (A) in satellite image (a fault segment rejuve-nated as a result of a paleoearthquake is indicated by arrows); (B) on the ground (seismotectonic escarpment is indicated by arrows);and (C) in zone of crushing and fracturing dipping along an azimuth of 125° SE and at an angle 70° at the contact between trachy-basalt and the Lower Cretaceous gray fine-grained sandstone, OP 0505. (D) Diagrams of mass measurements of fractures andrestored stress field at OPs shown in Fig. 5A. See Fig. 4B for explanation.

0506

0507

1 — 3680 ± 60 years2 — 5290 ± 100 years

Shaputy Valley

Khure Valley

Zun-Galtai

Murtoi Valley0505

Lake

Gus

inoe

A

B

C D N

OP 0505, n = 83

OP 0506, n = 100

OP 0507, n = 100

σ3

σ1

σ2

σ1

σ1

σ3

σ3

σ2

σ2

SW

NW

Murtoi Valley

TrachybasaltSandstone

Zone of crushing and fracturing,azimuth is 125°SE, dip angle is 70°

is OP

is the trench that stripped buried humic horizons [17];coordinates of the trench were presented by A.V. Chipizubov.



Fig. 6. Map of stress field in the Earth’s crust of the Gusinoozersky Basin and the adjacent territory. The rose diagram in the upper left cornerdemonstrates the strikes of tension axes inclined at angles of 0–30° (number of measurements is 31, step is 10°, maximum percentage is 19%).(1) Tension axis inclined at angles of (a) 0–30° and (b) 31–60° reconstructed at OPs; (2) compression axis inclined at (a) 0–30° and(b) 31–60° reconstructed at OPs; (3) interpreted paths of principal (a) tension and (b) compression vectors inclined at (a) 0–30° and(b) 31–60°; (4) sedimentary rocks of (a) Quaternary, (b) Neogene, and (c) Lower Cretaceous age; (5) crystalline basement and pre-Cenozoic volcanics; (6) Cretaceous–Quaternary volcanics, unspecified.

106°00′ 106°15′ 106°30′ 106°45′ 107°00′ Ε50°50′

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a b ca a ab bb

1 2 3 4 5 6

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0

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LUNINA, GLADKOV

ods [22, 23, 25] were applied, and the average solutionwas taken. Solutions of the stress field have beenobtained at 28 OPs of 33. The factual data and resultsof reconstruction are given in the Table. The method ofDanilovich girdles [7] was used in passing to determinethe vector of displacement at some OPs.

The new map of the fault–block structure (Fig. 3) isbased on several sources of information, including theresults of interpretation of 3D topography modelsaccessible at http://www.geomapapp.org and topo-graphic maps on scales of 1 : 100000 and 1 : 200000;faults and contours of sedimentary sequences depictedin the State geological maps on a scale 1 : 200000 pre-pared in 1960 and 1961; faults shown in the structuralscheme published in [2]; and our field observations andresults of their processing. After consideration of all theavailable data, only those faults are left on our mapwhich are clearly expressed in topography or revealed

by field observations, i.e., only the faults that wereactive in the Mesozoic and Cenozoic.

RESULTS

Examination of the map of the fault–block structureshows that NE-trending faults dominate in the studiedterritory; mostly, they extend between 40° and 50° NE(Fig. 3). The axis of the Gusinoozersky Basin is ori-ented at 35° NE parallel to the general trend of theKhambinsky Fault. The Monostoi Fault extends alongan azimuth of 36–38° NE. Thus, taking into account theformation of the fault pattern under different conditionsof loading [24], it may be suggested that the tensionforces responsible for the development of the basinwere oriented at an angle to its axis or were subse-quently turned clockwise by no less than 10°.

Fig. 7. NE-trending conjugate crush and fracture zones documented at OP 0502 and corresponding diagrams of mass measurementsof fractures and vectors of principal normal stresses. See Fig. 4B for explanations.

SE

140∠65° 340∠50°

0

σ3

σ2

σ1

OP 0502, n = 100



Tectonic deformation and stress field at thenorthwestern wall of the basin. The fault zones paral-lel to the Chambinsky Fault or approaching this fault atan acute angle (15–33°) are the best developed alongthe northwestern wall of the basin (Fig. 3). As followsfrom the character of fracturing, these faults havestrike-slip or normal–strike-slip kinematics. A fracturezone dipping at an azimuth of 150° SE and at angles80–85° and cutting the Quaternary loam with slightlyrounded rock fragments is an example (Fig. 4A). On theplace of the photograph (OP 0303), no measurementswere made; thus, the diagram of fracture orientationand the reconstructed stress field shown in Fig. 4B per-tain to OP 0302 located in the same fracture zone. Theobtained shear solution is consistent with the observedstructural situation. The hinge of the recumbent fold in

the limonitized Mesozoic sandstone has a strike at195° SSW and a dip angle of 25° practically coincidingwith the direction of extension of axis σ3 (20° NNE, dipangle is 13°). The thin beds within this fold are turnedup and displaced along a reverse fault (Fig. 4C). Thefold is located at the foot of a slope close to the bottomof the Gusinoozersky Basin. The observed structuralpattern indicates that thrusting of the gravity natureaccompanied normal faulting along the KhambinskyFault. The stress field was locally changed at the junc-tion of the slope and the basin bottom. Folds, as a resultof compression during synsedimentation subsidence ofthe basement in the Monostoi Fault Zone, are alsoknown on the southeastern coast of Lake Gusinoe [2].Both right- and left-lateral strike-slip offsets are notedalong the local NE-trending faults.

Fig. 8. Manifestations of fault zones of various directions in mudstone that hosts a thick coal seam, OP 0201: (A) general view ofsection; (B) fracture zone 0.4 m in apparent thickness; dip azimuth is 325–345° NW and dip angle is 50–65°; (C) downfaulting ofa coal seam along zone of mylonitization 0.2 m thick; dip azimuth is 10° NNE and dip angle is 75°; (D) diagrams of mass measure-ments of fractures and vectors of principal normal stresses at OP 0201. See Fig. 4B for explanations; (E) Danilovich girdle (high-lighted by gray) plotted on diagram of mass measurements of fractures and solution of displacement vector (right-lateral normal–strike-slip fault) along the NE-trending fracture zone at OP 0406.

NWσ3

σ2 σ1

OP 0201, n = 100

NE

NE

0D

E0

Direction of displacement,dip azimuth is 38° NE,

OP 0406, n = 100

CB

325–345∠50–65°

10∠75°

dip angle is 24°

A

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LUNINA, GLADKOV

The structural manifestation of the KhambinskyFault is most apparent in its southern part (Fig. 5) nearthe seismotectonic dislocation. At the same time, thefault is clearly expressed along its entire extent alongthe basin wall, and its normal-fault kinematics isemphasized in topography by triangular and trapezoidfacets (Fig. 2). The structure described by Lastochkin[9] continues northeastward, where it is traced for atleast 7 km from the Shaluty Valley with remnants oflandslides, escarpment, and trench. Near the MurtoiRiver, a seismotectonic fault is seen in the satelliteimage and observed in outcrops (Figs. 5A, 5B). ThreeOPs located in the fragment shown in Fig. 5A docu-ment crush and fracture zones more than 5 m thick,which dip to the ESE at angles of 50–70°. At OP 0505,trachybasalt and gray fine-grained sandstone come intocontact along the fault (Fig. 5C). It is noteworthy that insome places at the range–basin boundary (below thelevel of the aforementioned OPs), crush and foliationzones dipping at angles of 10–30° are observed in the

Lower Cretaceous sandstone and gravelstone that hostcoal seams (OP 0504).

Diagrams of mass measurements of fractures (Fig. 5D)allowed us to reconstruct the stress fields related to theKhambinsky Fault; these fields correspond to extension(OPs 0505 and 0506) and transtension (OP 0507) withthe NW–SE orientation of axis σ3. Note that solution atOP 0507 is close to pure extension (compression axisσ1 is inclined at an angle 59°). However, according tothe classification of stress fields proposed in [18],which we follow in this paper, this solution is referredto as transtension. A similar situation is characteristicof OP 0504, where the average solution is defined astranstension (angle of compression axis is 60°) (Table).Thus, in general, the Khambinsky Fault is classified asan almost pure normal fault. An insignificant right-lat-eral strike-slip component appears only locally. Thedeclination of striae on fracture planes that are orientedparallel to the Khambinsky Fault is not greater than 65°relative to the horizon. The shear stress tensors shownin Fig. 6 are related to local faults that approach

Fig. 9. Intersection of the NE- and NW-trending fracture zones in the Cretaceous basalt at OP 0403 and diagrams of mass measure-ments of fractures and vectors of principal normal stresses at OPs 0403 and 0405k. See Fig. 4B for explanations. Danilovich girdleis shown by gray.

NW

155∠70–85° 60∠50°

0

σ3

σ2σ1

OP 0403, n = 100

0 I II

II

III

I

OP 0405, n = 100

σ3 σ2

σ1

σ2

σ3

σ1



obliquely or perpendicular to the regional fault bound-ing the northwestern wall of the Gusinoozersky Basin.

The NE-trending crush and fracture zones traced6–7 km northwest of the Khambinsky Fault (Fig. 7) wereformed under effect of the same NW–SE extension, whichcaused the origin of the Gusinoe Lake Basin.

Tectonic deformation and stress field at thesoutheastern wall of the basin. In the southeasternframework of the Gusinoozersky Basin, as along itsopposite wall, the NE-trending fault zones are clearlytraced in outcrops, being parallel to the Monostoi Faultor approaching it at acute angles (

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LUNINA, GLADKOV

latitudinal fault and right-lateral–normal displacementalong the NE-trending fault. The same solution wasobtained at OP 0406 at another end of the same localfault approaching the Monostoi Fault (Figs. 1, 3). Azone of rough fracturing is documented here along a dipazimuth of 335° NW; dip angle is 50°; the Danilovichgirdle [7] is related to this zone (Fig. 8E). The direction

of displacement along an azimut 38° NE and at an angle 24°indicates right-lateral normal–strike-slip kinematics.

Two crosscutting fault zones are observed in the out-crop of basalt 100 Ma in age [19] at OP 0403 (Fig. 9).The first fracture zone, 2 m in apparent thickness, dipsalong an azimuth of 60° NE and an angle of 50°, whilethe second one is expressed in a series of crush and

Fig. 11. Seismites in the Lower Cretaceous and Neogene–Quaternary sedimentary rocks at OP 0203: (A) general view of the sec-tion; (B, C) large pockets and fragments of mudstone beds in the Neogene–Quaternary conglobreccia; (D) a mushroomlike bodythat replaces conglobreccia and contacting with Quaternary dark brown loam with rubble and gravel; (E) a sandstone lens from theunderlying bed in conglobreccia.

D E

B C

A

Talus

85°

Conglobreccia

Dump

C DB

E

Talus

Sandstone Mudstone

Mudstone

Dump

Sandstone

Conglobreccia

Sand and gravel

Conglobreccia

Sandstone

Mudstone

Mudstone

Sand and gravel

Sand and gravel

Sandstone

Conglobreccia

Mudstone

Conglobreccia

Sand and gravel

Sandstone



fracture zones of thickness up to 0.5 m dipping along anazimuth of 155° SE at angles of 70–85°. The trend ofthe basaltic field exactly coincides with orientation ofthe second fault zone, providing indirect evidence thatthis zone between the two parallel NW-trending faultsthat confine this field in the northeast and southwestwas a conduit for magma ascent (Figs. 1, 3). Becausebasaltic eruptions were controlled by these faults, theyoriginated before the eruptions and then were reacti-vated not earlier than in the Late Cretaceous. Extensionwith the northwestern orientation of axis σ3 recon-structed at OP 0403 provided normal displacementalong the zone of crushing and fracturing, which dipsalong an azimuth 155° SE° and at angles 70–85°; theDanilovich girdle (Fig. 9) supports this interpretation.At the same time, the fault zone that dips at an angle of50° along an azimuth of 60° NE provided a freer dis-placement of blocks along normal faults and had strike-slip kinematics. Similar structural pattern and interpre-tation of diagram of fracturing was obtained atOP 0405K (Fig. 9).

Fault zones are identified unambiguously in out-crops of bedrocks irrespective of their type and age.One more example of NE-trending fault zones is pre-sented in Fig. 10. This is a system of closely spacedshears dipping at angles 50–60° along an azimuth of320–335° NW and accompanied by a similarly orientedzone (0.2 m thick) of intense fracturing and grinding ofPaleozoic metasomatically altered granite (OP 0606).Limonitized slickenslides with striation declining to thenortheast at an angle 75° are observed on fault planes,indicating an insignificant right-lateral strike-slip off-set. According to the diagrams of fracture measure-ments, the fault zone was characterized by normal sep-aration (Fig. 10).

Obvious indications of Cenozoic reactivation offaults along the southeastern wall of the Gusinoozrsky

sBasin are recorded in a section ~150–200 m long and ~4 mhigh, which is exposed in the coal open pit at OP 0203(Fig. 11A). The lower portion of the section is com-posed of Lower Cretaceous white coarse-grained sand-stone pertaining to the Kholboldzha Formation. Neo-gene–Quaternary brown, poorly cemented conglobrec-cia overlaps the uneven surface of this formation. Somefragments in the conglobreccia are slightly rounded,but angular fragments are predominant. In some locali-ties, a bed of conglobreccia is replaced with large pock-ets or fragmented mudstone beds (Figs. 11B–11D).Over the entire extent, the bed is enriched in smallsandstone lenses derived from the underlying sequence(Fig. 11E). A light brown bed of sandy–gravely sedi-ments, which is an exposed upsection, is overlapped, inturn, by Quaternary (probably Holocene) dark brownloam with admixture of rubble and gravel. At the west-ern end of this section, the bed of conglobreccia is ter-minated. In terms of sedimentology, the unit with abun-dant deformed blocks from the adjacent beds is calledgravity or tectono-gravity mixtites formed owing tec-tonic movements along faults. A similar section of theMiocene–lower Pliocene Osinovsky Formation wasdescribed in the quarry near the town of Babushkinclose to the South Baikal Basin [16]. In terms of tecton-ics and seismology, the rocks similar to those atOP 0203 are named seismites [8, 21]. They are formedby dilution of the ground during strong earthquakes.Such rocks of the Holocene age were observed in theTunka Valley at the southwestern flank of the BaikalRift Zone [6, 10].

In general, quadrangular and less frequent triangularcrustal blocks are outlined in the Gusinoozersky Basinand the adjacent territories (Fig. 3). Stress tensors aredistributed by state of stress as follows: extension(41%, 13 solutions), transtension (15%, 5 solutions),shear (41%, 13 solutions), transpression (0%), com-

Fig. 12. Rose diagrams of strikes of conjugate fracture zones with different types of displacements. The general orientations of(1) Khambinsky and (2) Monostoi faults are shown by dashed lines.

0

90270

180

12

Right-lateral shear fractures Left-lateral shear fracturesstep is 10, number of fractures is 13,maximum percentage is 15%

step is 10, number of fractures is 13,maximum percentage is 23%

step is 10, number of fractures is 24;maximum percentage is 20%

Normal tension fractures,

0 0

90 90

180180

270 270

1 12 2

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LUNINA, GLADKOV

pression (0%), and indefinite type (3%, one solution)(Table; Fig. 6).

DISCUSSION

The specific features of faults and blocks, includingthe relationships between faults, the largely quadrago-nal shape of blocks, the kinematics of variously ori-ented blocks (Fig. 3), and the state of stress in theEarth’s crust of the Gusinoozersky Basin and the adja-cent territory (Fig. 6) are typical of rifting [4, 13, 19,20]. At the same time, the data obtained show that theleading role of extension in the studied region and WestTransbaikalia as a whole was combined with strike-slipfaulting (Table; Fig. 6). Both stress tensors are docu-mented in the Mesozoic and Cenozoic rocks, and there-fore, it can hardly be claimed that either of the deforma-tion regimes is more prevalent during different epochs.It is more likely that strike-slip faulting accompaniedextension of the Earth’s crust on the regional scale. Wemade an attempt to understand the cause of the ratherabundant shear component of deformation.

To estimate the significance of the obtained solu-tions of stress fields and their contribution to the gen-eral geodynamic setting, we constructed a series of rosediagrams of the strike of conjugate fracture systemsrelated to normal and right- and left-lateral strike-slipdisplacements (Fig. 12). Transitional solutions corre-sponding to transtension have been omitted. As followsfrom the rose diagrams, the strike-slip offsets are docu-mented largely along near-meridional and near-latitudi-nal fractures, which rarely reached the state of fault intheir evolution, whereas normal separations are typicalof the NE-trending fractures. The main peak on the rosediagram that illustrates normal displacements almostcoincides with the general trends of the Khambinskyand Monostoi faults (Fig. 12). Some fracture systemsoriented in the northeastern direction have right- or left-lateral strike-slip kinematics; however, the individualsolutions indicate that they are not related to the move-ment along the master faults that bound the Gusinooz-ersky Basin. The indefinite sense of offset (right or left)along faults of similar direction testifies to the instabil-ity of the source of shear stress. Most likely, this stresswas related to interblock slippage and variable orienta-tion of the principal tensile and compressive stresses inresponse to the heterogeneous geological medium orother factors. Our experience in structural studiesshows that bedding of sedimentary rocks is one of theheterogeneities that accommodates the realization ofstress in such a manner that the faults are formed at aright angle to bedding. We suppose that if stresses arerather high in magnitude, the variation of their vectorsis impossible; the orientation of weak stresses is morevariable, however. While principal normal stresses arereoriented, their magnitude can change, and oftenincreases, as follows from the relative intensity of stressfield (Table) determined by the degree of deformationalong conjugate fractures [12].

A strike-slip offset along the NE-trending faultscould have been caused by regional tensile stressbrought about by spreading of heated mantle materialrelative to the ancient Dzhida–Vitim Suture that con-trolled the Gusionoozersky Basin. The angle betweenthe paths of extension and the axis of the Early Creta-ceous rift basin varies from 90° to 55° (Fig. 6), in manyplaces creating conditions of oblique extension or tran-stension (a combination of extension and right-lateralshear). The general direction of tensile stress (310–330° NW) locally turns to the near-latitudinal direction,initiating a left-lateral strike-slip component along theNE-trending normal faults.

Thus, the strike-slip offsets in the GusinoozerskyBasin and near it were initiated by two causes. Theregional cause was related to the spreading of mantleplume and generation of horizontal tensile stress ori-ented in some places obliquely to the major Dzhida–Vitim Suture. This cause provoked primarily a lateraloffset along normal faults. The second cause was relatedto local variation in the regional state of stress induced byheterogeneities of the Earth’s crust, including bedding ofsedimentary rocks and local structural elements, e.g., theauxiliary NW-trending normal faults.

The activity of faulting in the Neogene and Quater-nary is noteworthy. These are the seismotectonic dislo-cations that cross the Murtoi Valley and the attendingcrush zones (Fig. 5); the fault zones in loam (Fig. 4A);and seismites, or tectono-gravity mixtites (Fig. 11).Morphostructural observations have shown that theGusinoozersky seismotectonic dislocations extend foralmost 10 km; previously, their extent was estimated at2.5 km [9]. Although the present-day seismic regime ofthe Gusinoozersky Basin and the adjacent territory iscomparable to that at the margin of the Siberian Plat-form, the aforementioned manifestations of neotecton-ics attract interest for the basin, which is located nearthe Baikal Rift and proceeded through active riftingover 120 Ma from the Early Cretaceous to Neogene. Asfollows from the regime of sedimentation and the rathersmoothed present-day topography, the tectonic activitymarkedly waned in the Quaternary. The seismotectonicreactivation of the Transbaikal region in the Pleistoceneand Holocene was a response to the processes that pro-ceeded in the adjacent Baikal Rift Zone. The active evo-lution of the Gusinoozersky Basin in the Late Creta-ceous, when the hot mantle plume operated beneath thelithosphere of West Transbaikalia, resulted in one-sidedsubsidence of its basement along the Monostoi Fault asthe southeastern boundary of the basin [2]. At present,this fault is poorly expressed in morphology in compar-ison with the Khambinsky Fault that controlled theregional seismic activity in the Pleistocene andHolocene [9]. Thus, in the Cenozoic or Late Creta-ceous, the tectonic activity shifted westward, havingprovided the same asymmetry of the GusinoozerskyBasin as in the Baikal Rift Zone.



CONCLUSIONS

The geological, structural, and tectonophysicalstudies and the data published previously have allowedus to prepare new maps of the fault–block structure andstate of stress in the Earth’s crust of the GusinoozerskyBasin and the adjacent territory and to draw some con-clusions about tectonic and geodynamic evolution ofthe studied area in the Late Mesozoic and Cenozoic.

(1) The Gusinoozersky Basin was formed in thetranstensional regime with predominance of the NW–SE extension initiated by hot mantle plume. The tran-stensional conditions were caused by oblique (relativeto basin axis) paths of regional tensile stresses. Theangle between the general trend of the GusinoozerskyDepression and the vector of regional tensile stress(interpreted paths) varied from 90° to 55°, providing aninsignificant (relative to normal separation) right-lat-eral offset along the master faults.

(2) Compressive deformation (folds, lenses) locallyobserved at the basin–range boundary is of the gravitynature and related to shearing of the second order withrespect to extension due to heterogeneity of the Earth’scrust. The shear stress field is often realized as offsetsalong the fractures that cut bedrocks and do not reachthe level of faults.

(3) As is indicated by the documented fault zonesand seismites in the Upper Cenozoic sediments, as wellas by the Holocene seismotectonic dislocation [17] upto 10 km in extent, the fault and block structure of theGusinoozersky Depression and the adjacent territoryremained active in the Neogene and Quaternary. Mostlikely, the tectonic reactivation of the GusinoozerskyBasin in the Late Cenozoic was a response to the tec-tonic processes that proceeded in the neighboringBaikal Rift Zone rather than to the activity of a localmantle source.

ACKNOWLEDGMENTS

We thank M.G. Leonov and V.V. Yarmolyuk fortheir helpful comments. This study was supported bythe Council for Grants of the President of the RussianFederation for Support of Leading Scientific Schools(grant no. MK-1323.2007.5), the Siberian Branch ofthe Russian Academy of Sciences (integration projectno. 6.13), International Association for the Promotionof Cooperation with Scientists from the Independent Statesof the Former Soviet Union (grant no. 05-109-4383), andthe Foundation for Support of Russian Science.

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25. A. Yamaji, “The Multiple Inverse Method: A New Tech-nique to Separate Stresses from Heterogeneous Fault-Slip Data,” J. Struct. Geol. 22, 441–452 (2000).

Reviewers: V.V. Yarmolyuk and M.G. Leonov

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Fault–Block Structure and State of Stress in the Earth’s Crust ...GEOTECTONICS Vol. 43 No. 1 2009 FAULT–BLOCK STRUCTURE AND STATE OF STRESS 69 It is assumed that the rift zone