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ORIGINAL PAPER
Paleostress regime reconstruction based on brittle structure analysis
in the Shekarab Mountain, Eastern Iran
Maryam Ezati
1
&Ebrahim Gholami
1
&Seyed Morteza Mousavi
1
Received: 1 April 2020 /Accepted: 1 November 2020
#Saudi Society for Geosciences 2020
Abstract
Eastern Iran, including the Sistan suture zone, comprises the boundary between Lut block and Afghan block. This research aims
to reconstruct the stress regime evolution from the upper Cretaceous to Quaternary based on the brittle tectonic analysis. In this
study, three episodic changes in stress regimes were recognized in the Shekarab Mountain using data inversion. In places where
conglomerate outcrops are present, the Quaternary stress state is obtained using the youngestslickensides. The Quaternary stress
state indicates that the direction of σ
Hmax
is close to N026°, which is compatible with the present-day Arabia-Eurasia convergence
direction. Reconstruction of stress fields using age and sense of motion of faults shows that the stress regime during the
Cretaceous was compressional, which caused the uplift of peridotites and ophiolites in the eastern part of the study area. The
state of stress in the upper Eocene and Oligocene was transpressional; in the eastern part of the study area, there isa change from
transpression to transtension. The exhumation of igneous rocks in the eastern part of the Shekarab Mountains is due to the local
change of the stress regime. According to the results of this study, the first stage of stress state in the Shekarab Mountains was
compressive and the average direction of maximum stress axis (σ1) was toward N337°. In Eocene, the tectonic regime was
transpressional and the average direction of maximum stress axis (σ1) was toward N003°. In Quaternary, the tectonic regime is
strike-slip and the average direction of maximum stress axis (σ1) is toward N026°. This implies that at least 49° clockwise
rotation of σ1 happened in the Shekarab Mountain.
Keywords Brittle structures .Paleostress .Shekarab Mountain .Eastern Iran
Introduction
The present tectonics of Iran is due to the north-south conver-
gence between the Afroarabian and the Eurasia plates
(Jackson and Mc Kenzie 1984). This convergence is accom-
modated across the Iranian Plateau and adjacent deformed
zones; the deformation is not uniformly distributed. Much of
the deformation is concentrated in the Zagros fold and thrust
belt in the southwest, Alborz Thrust Belt is bordering the
oceanic crust of the South Caspian depression, Kopeh-Dagh
active thin-skinned Fold Belt in the northeast, and the east
Iranian ranges (Hollingsworth 2007). Results from a regional
GPS network indicate that the total convergence across Iran is
25 mm/year in eastern Iran (Vernant et al. 2004). The late
Cretaceous to Tertiary rocks of the Sistan Suture Zone is char-
acterized by different structural elements and rock associations
and now separates two structurally coherent continental blocks:
the Lut block to the west and the Afghan block to the east
(Berberian and Yeats 1999). Continental collision includes brit-
tle and coeval compressional, extensional, and strike-slip
faulting in the upper crust (Molnar and Tapponnier 1975).
Paleostress analysis a branch of Structural Geology whose
aim is describing stress systems doing in the past from their
record in deformation structures, singularly from fault-slip
data (Simon 2018). Determining stresses allows a better un-
derstanding of the mechanical manner of geological materials
and decipher tectonic mechanisms from those associated with
plate motion at a large scale to those creating jointing and
faulting (Parlangeau et al. 2017). The orientation and shape
of the stress ellipsoid with respect to the earth’s surface show
the type, direction, and slip sense of faults developed in an
Responsible editor: François Roure
*Maryam Ezati
M.Ezati@Birjand.ac.ir; Geology1200@yahoo.com
1
Department of Geology, Faculty of Science, University of Birjand,
P.O. Box: 97175-615, University Blvd, Birjand,Southern Khorasan,
Iran
Arabian Journal of Geosciences (2020) 13:1232
https://doi.org/10.1007/s12517-020-06235-4
area (Angelier 1994). The inversion results include the azi-
muth and plunge of the main stress axes (Angelier and
Goguel 1979). Stress inversions from fault slip measurements
have now become a standard tool in tectonics; stress inver-
sions are used for characterizing ancient stress fields and also
inactive tectonics (e.g., Angelier 1994). The chronology of
different stress states can be determined from the age of the
rock formations affected by corresponding tectonic regimes
(Shabanian et al. 2010). Brittle tectonic analysis, including
stress tensor inversion, is useful to decipher the sequence of
deformational events that resulted in the present-day structures.
The standard step in brittle tectonic analysis involves data col-
lection in the field, data separation based on age recognition,
computation of stress fields, and finally, characterization and
classification of different events (Navabpour et al. 2007). In this
paper, we attempt to reconstruct the stress regime history of the
study area based on new constraints obtained through the brittle
tectonic analysis of faults and paleostress determinations. The
stress pattern of Shekarab Mountain based on brittle structures
like faults has been presented in this paper. This work intends to
reconstruct the stress regimes that are related to the evolution of
the Shekarab Mountain structures.
This study focuses on the evolution of the upper
Cretaceous to Quaternary tectonic stress regime using brittle
structures; this kind of methodology has been found to be
useful in various researches such as stress field rotation or
block rotation in the Lake Mead fault system (Ron et al.
1993), variation of relative paleostress magnitudes and orien-
tations (Kaymakci 2006), paleostress regimes of brittle struc-
tures (Heuberger et al. 2010), paleostress field reconstruction
(Sippel et al. 2010), paleostress reconstructions of Jabal Hafit
Structures United Arab Emirates (UAE) (Zaineldeen 2011),
paleostress reconstruction and multiphase weak deformation
in cratonic interior (Tripathy and Saha 2015), brittle structural
data evaluation and paleostress calculation (Sasvari and
Baharev 2014), late Cartaceous and Cenozoic paleostress his-
tory (Coubal et al. 2015), kinematic development and
paleostress pattern of the Edremit Basin, western Turkry
(Gürer et al. 2016), paleostress of Huangshan Basin of south-
eastern China (Xu et al. 2016), brittle deformation and state of
paleostress constrained in the central German platform
(Navabpour et al. 2017), stress regime changes in the Main
Boundary Thrust zone, Eastern Himalaya (Patra and Saha
2019), and exhumation of granitoid plutons in the Eastern
Iran (Samimi et al. 2020).
Geological setting
Three phases of Cenozoic deformation have been responsible
for the configuration of the suture zone at present. The defor-
mation of Iran is due to the Arabia-Eurasia collision (Vincent
et al. 2005). The initial Arabia-Eurasia collision probably
dates from as early as ~ 35 Ma (e.g., Agard et al. 2005;
Vincent et al. 2005). The active tectonics of Iran is controlled
by the northward motion of Arabia, at a velocity of ~ 25 mm/
year at longitude 60°E, with respect to the interior of Eurasia
(Vernant et al. 2004). Iran consists of three major orogenic
belts aligned parallel to the border of the country, i.e., the
Alborz-Kopeh Dagh ranges to the north, the Zagros fold and
thrust belt to the southwest, and the Sistan Suture Zone to the
east (Shafiei et al. 2009). The Sistan Suture Zone represents a
deformed accretionary prism that was emplaced during the
destruction of a small Neotethyan ocean basin, referred to as
the Sistan Ocean, which once separated the Afghan and Lut
continental blocks (Sengor et al. 1988). The Sistan Suture
Zone represents a narrow oceanic lithosphere. Since the early
Cretaceous, the Sistan Suture Zone has undergone a rather
complicated history marked by changes in the tectonic envi-
ronment and corresponding changes in the stress regime.
Rifting, subductions, ophiolite emplacement, continental
trench collision, uplift, and at least three phases of Cenozoic
deformation have been responsible for the present configura-
tion of the Sistan suture zone; several magmatic episodes have
been recorded throughout its dynamic history (Camp and
Griffis 1982). The thickness in the Sistan Suture Zone is
thought to be ~ 35–40 km (Dehghani and Makris 1983).
The northward motion of central parts of Iran relative to
western Afghanistan results in north-south right-lateral
shear of ~ 16 mm/year at the present day in eastern Iran
between the longitudes 56°E and 62°E (Vernant et al.
2004). The N-S right-lateral shear component between
central Iran and Afghanistan is accommodated by several
right-lateral strike-slip fault systems bordering the Dasht-e-
Lut desert (Walker and Khatib 2006; Meyer and Le Dortz
2007). This results in a right-lateral shear in the east of Iran
as central Iran moves N-NNE relative to stable Afghanistan
(Walker et al. 2003). The Iran-Afghan border in Sistan is
also a tectonic boundary, separating the distributed defor-
mation in Iran from the virtually aseismic region of western
Afghanistan (Berberian et al. 2000). As Iran is shortened by
the Eurasia-Arabia collision, North-South right-lateral
shear is taken up partly along the west boundary of the
Lut block in central Iran. To the south, the Sistan ranges
merge with the E-W coastal ranges of the Makran (Jackson
and Mc Kenzie 1984; Jackson 1992). The activity on the N-
S Sistan faults decreases to the south because central Iran is
rotating clockwise relative to Afghanistan about an axis
close to the junction of the Sistan ranges with the Makran
(Jackson and Mc Kenzie 1984). The right-lateral strike-slip
faults of the Sistan Suture Zone die away in the thrust faults;
this transition from N-S to the E-W faults involves a change
from nonrotational to rotational deformations in Sistan
Suture Zone (Walker and Khatib 2006). The Nehbandan
fault system with a strike-slip mechanism by N-S general
trend has sub-branches in the Northern and Southern
1232 Page 2 of 18 Arab J Geosci (2020) 13:1232
terminals. The northern terminal of the Nehbandan fault has
rotated toward the west and its southern terminal toward the
east (Khatib 1998). The Sistan suture zone is dominated by
major N-S or NNW-SSE right-lateral strike-slip faulting
with some NW-SE reverse faults and some E-W left-lateral
strike-slip faults (Camp and Griffis 1982; Berberian and
Yeats 1999). Shekarab Mountain is located in the Sistan
suture zone, eastern Iran (Fig. 1a, b). Shekarab Mountain
was created by a terminal splay of Nehbandan fault (Tirrul
et al. 1983). The Geology map shows that Shekarab
Mountain is composed of peridotite (upper Cretaceous),
phyllite (upper Cretaceous), flysch (upper Cretaceous-
Paleocene), tuff (middle Eocene), andesite (upper Eocene-
Oligocene), and dacite (Neogene) (Fig. 1c).
Fig. 1 aTectonic map of the Iranian and Arab plate as well as theeastern position of Iran on the map. bStructural map of east Iran (Baniadam et al. 2019)
and the study areas location. cGeological map of the study area
Arab J Geosci (2020) 13:1232 Page 3 of 18 1232
Material and methods
In this study, the direct stress tensor inversion method
(Angelier 2002) was adopted for computation of the state of
stress. The stress tensor inversion method is based on maxi-
mizing the sum of slip shear stress in the direction of actual
slip for the entire data sets. This sum is calculated as a function
of four independent variables of the reduced stress tensor, i.e.,
three angular parameters for orientation of the principal stress
axes σ1≥σ2≥σ3, as well as the ratio of the principal stress
magnitude differences calculated as (Φ) given the direction
and shape of stress ellipsoid (Angelier 2002). In this study,
to obtain the stress tensor, one type of paleostress inversion
method, right dihedron (Angelier and Mechler 1977), was
used.
Physical basis of the methods
The general stress tensor (first matrix term (T)) is associated
with the stress tensor in the principal stress coordinate system
as follows:
adf
dbe
fec
0
@1
A¼
x1 x2 x3
y1 y2 y3
z1 z2 z3
0
@1
A:
σ10 0
0σ20
00σ3
0
@1
A:
x1 y1 z1
x2 y2 z2
x3 y3 z3
0
@1
A
ð1Þ
This is the matrix expression of a tensor rotation: the trans-
fer from the system of principal axes into a general system of
Cartesian coordinates. The expressions
x1
y1
z1
0
@1
A;
x2
y2
z2
0
@1
A;and
x3
y3
z3
0
@1
Aare the unit vectors along
σ1, σ2, and σ3. The stress vector σacting on a fault plane
characterized by its normal unit vector nis given (Fig. 2), in
vector and matrix notations successively, by the following
equations:
σ¼Tn ð2Þ
σx
σy
σz
0
@1
A;
adf
dbe
fec
0
@1
A:
x
y
z
0
@1
Að3Þ
The modulus of the normal stress (ν) is given by the scalar
result of the stress vector using the normal unit vector by the
following equations:
ν
jj
¼σnð4Þ
Order ν
jj
¼xσxþyσyþzσzð5Þ
The normal stress vector (ν)isthen:
v¼ν
jj
nð6Þ
νx
νy
νz
0
@1
A¼ν
jj
x
y
z
0
@1
Að7Þ
Recognizing the stress vector (σ), the normal stress vector
(ν), and the shear stress vector (τ)is
σ=ν+τthat is:
τx
τy
τz
0
@1
A¼
σx
σy
σz
0
@1
A−
νx
νy
νz
0
@1
Að8Þ
Reduced stress tensor: by adding to the stress tensor isotro-
pic stress defined by │=−σ
3
, then multiplying the tensor by
the positive constant k=1/(σ1−σ3), one obtains
ab c
Fig. 2 aStress states σ1, σ2, and σ3 are the principal stress axes. bFault
plane, unite vector perpendicular to fault plane (n), fault plane (F), stress
vector actingon fault plane (σ), normal stress perpendicular to F(ν), shear
stress parallel to F(τ). cDemonstrations of misfit angle fault plane, actual
slip (s) and theoretical shear stress (τ)onafaultplane,(α)misfitangle
(Angelier 1994)
1232 Page 4 of 18 Arab J Geosci (2020) 13:1232
σ10 0
0σ20
00σ3
0
@1
A→
100
0Φ0
000
0
@1
A, both tensors are
equivalent in terms of directions and senses of shear stresses
(remember Φ=(σ2-σ3) / (σ1-σ3)). The resulting “reduced
stress tensor”contains four independent variables and simply
depends on the orientation of the principal stress axes and the
ratio Φ
x1 x2 x3
y1 y2 y3
z1 z2 z3
0
@1
A:
100
0Φ0
000
0
@1
A:
x1 y1 z1
x2 y2 z2
x3 y3 z3
0
@1
Að9Þ
The variables kand │cannot be determined from fault-slip
orientations and senses. Whereas the reduced stress tensor has
four autonomous variables, therefore, at least four distinct
fault-slip data are needed to calculate it (Angelier 1994).
Inversion methods
The inverse problem includes determining the mean stress
tensor (T) and recognizing the senses of slip-on numerous
fault planes. The tensor Thas four passives, but the number
of equations (i.e., the number of fault slips) is much larger;
most “direct inversion”approaches use a least-square criterion
in the minimization process. Assuming the least-square crite-
rion, the average reduced stress tensor, T, which best fits the K
fault slip data of the set considered, is obtained by minimizing
asumS, written as follows:
Sm¼∑K¼k
K¼1WkFmk
ðÞ
2ð10Þ
In this total expression, W
k
is the weight of the datum
number k(a fault-slip set), and F
k
is a function which ex-
presses the deviation of this datum (F
k
= 0 corresponds to a
perfect fit with the average stress tensor, and increasing values
of F
k
represent increasing misfits). The term mrefers to the
particular form adopted for F
k
and consequently, S(Angelier
1994).
Misfit criteria
Misfit criteria are the angle between the shear stress (τ)and
the actual unit-slip vector (s) calculated as a function of the
stress tensor (Fig. 2c) defined as follows:
F1k ¼s;τðÞ ð11Þ
Angelier (1975)used
F2k ¼sin s;τðÞ
2ð12Þ
F
1k
changes in the range 0° to 180°, F
2k
changes from 0 to
1, and due to the half-angle (0–90°), this function allows to
take the slip sense into calculate. Functions like F
1k
and F
2k
(and similar others) solely depend on the angle between ob-
served slip and computed shear. The form of the reduced
stress tensor is chosen according to the mathematical necessity
of the methods, which depend on the form of the misfit crite-
rion:
x1 x2 x3
y1 y2 y3
z1 z2 z3
0
@1
A:
100
0Φ0
000
0
@1
A:
x1 y1 z1
x2 y2 z2
x3 y3 z3
0
@1
Að13Þ
COS−φαγ
αCOS φþ2π
3
β
γβCOS φþ4π
3
0
B
B
B
B
@
1
C
C
C
C
A
ð14Þ
where φ(psi) is an expression for Φthat depends on the
orientations of stress axes (Angelier 1994).
Date separation
Inversion results include the azimuth and plunge of the prin-
cipal stress axes (σ1, σ2, and σ3) as well as a stress ellipsoid
shape parameter Φratio (Φ=(σ2-σ3) / (σ1-σ3) ≤1). The
inversion method is based on the reduced misfit angle (α),
the misfit angle calculated as the minimization of the angle
between the real striae (s) and the calculated relative shear
stress (τ); acceptable for τ< 25° (Angelier 1984,1990). In
this research, the fault kinematics data, including spatial ori-
entation of fault planes and associated striae, have been mea-
sured from 38 sites (Fig. 3). We carried out stress calculations
from fault data within Cretaceous to Quaternary age in order
to constrain the orientation of the paleostress fields during the
evolution of Shekarab Mountain. The state of stress of
Quaternary deduced from the analysis of the youngest fault
slips observed at sites where there an outcropping of
Quaternary conglomerates. For obtaining stress tensor inver-
sion, brittle structures, including the orientation of fault sur-
faces, slickenside lineation, and sense of motion indicators,
were collected. For determining stress tensor orientation and
stress field in different geological age, we separated the data
sets into homogeneous subsets in order to reconstruct various
stress fields in the study area. We separate brittle structures
related to different geological age, the state of stress, and stress
tensor orientation computed using Wintensor software. In this
study, brittle tectonic analyzed, based on stress tensor inver-
sion of fault slip data, led us to determine the different stress
regimes of the Shekarab Mountain during the upper
Cretaceous to Quaternary age. Inversion of the separated data
sets allows us to distinguish that there are episodic changes in
stress regimes in the Shekarab Mountain. To determine the
stress field rotations, structural evidence were used; calculated
stress fields were defined by direction and plunge of the
Arab J Geosci (2020) 13:1232 Page 5 of 18 1232
principal stress axes (σ1, σ2, and σ3), the ratio of stress mag-
nitude differences (Φ), number of data (N), and the reduced
misfit angle (α) were attached to each determination, because
local results of the stress calculations are not equal.
Results
Paleostress analysis
In this paper, brittle structures, including spatial orien-
tation of fault planes and associated striae, were collect-
ed from 38 sites. The faults and sites of collected data
indicated in the Geological map and Structural map; in
order to reconstruct the stress history of the Shekarab
Mountain, we first attempted to analyze the brittle tec-
tonic data that were collected from different sites. This
paper aims to investigate the variations of stress tensor’s
direction from Cretaceous time to younger times. Here,
we attempt to make a relationship between stress regime
changes and the exhumation of igneous rocks using
stress tensor variation, so this allows us to analyze re-
gional tectonic regimes and corresponding stress re-
gimes deduced from inversions of fault kinematics. In
this paper, the direction of three principal stress axes
such as major stress axis (σ1), medium stress axis
(σ2), and minor stress axis (σ3) in 38 sites from the
different geological time are determined and presented
in Table 1. Stress determination based on the direct
inversion method (DI) revealed various stress regimes
in the Shekarab Mountain.
Paleostress reconstruction in the Upper Cretaceous to
Eocene age
The paleostress state of upper Cretaceous is the oldest stress
state distinguished since 83 Ma in the Shekarab Mountain; the
Pre-Cretaceous kinematics history and paleostress states re-
main unclear. To determine the stress state in the study area,
the inversion analysis is performed for the fault populations,
including the upper Cretaceous fault kinematics. The
Cretaceous rocks are distributed throughout the study area;
eastern sites are 13, 17, and 19, and western sites are 33 and
34. There are two outcrops of Cretaceous time in the peridotite
and ophiolites rocks (sites 13 and 17), faults observed in these
two sites are reverse, and stress regimes are compressional
(Fig. 4a). Operations of compressional stress regime in the
eastern partof the study area caused the uplifting of peridotites
and ophiolites in the east of the study area, and major stress
axis (σ1) in sites 13 and 17 had an NW-SE trend. Analysis of
data collected from sites 33 and 34 located in the F8 and F9
reverse faults indicates a compressional stress regime. In the
sites 13, 17, 19, and 33, major stress axis (σ1) had an NW-SE
trend; there is also one outcrop (site 34) showing a different
direction of σ1(Table1,Fig.4a). Additionally, the data from
Cretaceous locations have shown compression in the NW-SE
direction. On the other hand, five stress configurations indi-
cate compression which, consistent with the compressional
regime in the region. The NE-SW direction σ
hmin
(σ
3
)
paleostress field represents 74% of the calculated stress ten-
sors. It has been identified at five sites. The estimated mean
tensor has principal stress axes oriented: σ1 = 337/26, σ2=
070/06, and σ3 = 172/64 (Table 2). The ratio of stress
Fig. 3 Structural map of the studied structures and sites of collected data on the shaded relief map
1232 Page 6 of 18 Arab J Geosci (2020) 13:1232
Table 1 Geological position of brittle structures and reconstructed stress regimes. N, number of faults slips data. Φ, the ratio of stress magnitude
differences (Φ=(σ2-σ3)/(σ1-σ3)). α, the average angle between observed slip and computed shear in degrees
Site NStratigraphic age σ1Dir,Plung σ2Dir,Plung σ3Dir,Plung ΦαLocation Latitude (N°)
Longitude (E°)
1 9 Miocene 311.24 043.05 144.65 0.5 22 Eastern part 33°05́48.05″N
59°15′54.74″E
2 11 Eocene 069.21 162.09 273.67 0.45 10 Eastern part 33°08′32.12″N
59°12′12.56″E
3 10 Eocene 067.21 161.09 272.67 0.45 3 Eastern part 33°04′44.82″N
59°14′13.90″E
4 12 Eocene 275.31 014.15 126.55 0.79 12 Eastern part 33°07′10.09″N
59°07′57.91″E
5 13 Eocene 296.06 029.27 194.62 0.5 8 Eastern part 33°59′46.58″N
59°11′54.61″E
6 12 Eocene 340.06 248.18 089.71 0.5 19 Eastern part 32°59′00.43″N
59°12′40.76″E
7 13 Eocene 024.12 289.22 141.64 0.5 11 Eastern part 32°58′10.87″N
59°14′26.72″E
8 13 Eocene 266.34 005.14 115.52 0.77 7 Eastern part 32°56′57.38″N
59°13′21.78″E
9 10 Oligocene 059.65 164.07 257.24 0.5 13 Eastern part 33°03
́52.02 N
59° 4
́40. 56 E
10 12 Quaternary 220.07 314.35 120.55 0.83 15 Eastern part 33°55
́08.00″N
59°11′39.23″E
11 12 Quaternary 221.06 313.16 112.73 0.5 11 Eastern part 32°55′19.96″N
59°05′42.03″E
12 10 Oligocene 282.11 187.22 037.65 0.5 14 Eastern part 32°57′36.69″N
59°01′46.18″E
13 12 Upper Cretaceous 280.08 173.64 014.25 0.5 6 Eastern part 32°59′26.07″N
59°04′23.42″E
14 12 Paleocene 286.07 187.52 021.37 0.54 4 Eastern part 33°01′39.38″N
59°02
́16.94″E
15 13 Oligocene 349.13 256.13 123.71 0.5 11 Eastern part 32°59′24.36″N
59°06′14.51″E
16 13 Oligocene 008.60 257.11 162.27 0.65 20 Eastern part 32°58
́45.66 N
58°59
́24.30 E
17 13 Upper Cretaceous 342.04 251.20 084.69 0.5 12 Eastern part 33°00′19.05″N
59°09′02.00″E
18 12 Eocene 241.01 151.01 018.88 0.71 22 Eastern part 33°03
́52.02 N
59°4́40. 56 E
19 12 Upper Cretaceous 040.70 139.03 230.20 0.5 12 Eastern part 33° 04́30.36 N
59° 4
́40. 56 E
20 11 Paleocene 307.11 042.24 193.64 0.95 17 Middle part 32°56
́37.20 N
58°51
́34.74 E
21 12 Eocene 027.31 295.03 199.58 0.5 24 Middle part 32°58
́17.28 N
58°51
́08. 46 E
22 12 Oligocene 056.29 318.14 204.57 1 8 Middle part 32° 56
́17.28 N
58° 51
́34.74 E
23 10 Eocene 068.26 328.19 206.57 0.95 7 Middle part 32°56′30.04″N
58°54′38.91″E
24 11 Quaternary 072.28 334.14 219.58 1 5 Middle part 32°55′38.76″N
58°52′35.85″E
25 12 Oligocene 081.03 348.38 175.52 0.21 3 Middle part 32°57′05.93″N
58°53′18.58″E
26 13 Oligocene 302.12 039.31 194.56 0.5 14 Middle part 32°59
́15. 24 N
58°46
́46.98 E
Arab J Geosci (2020) 13:1232 Page 7 of 18 1232
magnitude difference (Φ) value is 0.1, confirming that this
tectonic regime was the coexistence of thrust faulting and
compressional stress regimes. There are two outcrops of
Paleocene rocks in the Shekarab Mountain; we collected data
of Paleocene time from sites 14, and 20 inversions of the fault
slip vector indicate that the direction of the major stress axis
(σ1) was NW-SE (Fig. 4b). The Eocene rocks are distributed
throughout the entire region of Shekarab Mountain. There
are 17 outcrops of Eocene in sandstone, limestone, con-
glomerate, and andesite rocks (Fig. 4c); faults observed
are mainly reverse with a strike-slip component. Based
on the direction of the major stress axis (σ1), two subsets
were separated: one with σ1 around N35°E (sites 2, 3, 4, 7,
8, 18, 21, 31, 35, 36, 37, and 38) and one σ1 around
N45°W (sites 5, 6, 23, and 32); there is also one outcrop
(site 28) showing a different direction of σ1(Table1). The
Eocene age mean major stress axis (σ1) had an approxi-
mately North-South trend (Table 2). The N-S σ
Hmax
(σ
1
)
paleostress field in the second stage of the stress regime
represented that 71% of the calculated stress tensors is
identified at 17 sites. The estimated mean tensor has prin-
cipal stress axes oriented: σ1 = 003/31, σ2=110/26,σ3=
232/47 (Table 2). The ratio of stress magnitude differences
(Φ) value is 0.29 (Table 2), also confirming that this tec-
tonic regime was the coexistence of transpressive regimes.
Change in the stress regime
The fault plane data of Oligocene time from sites 9, 12, 15, 16,
22, 25, and 26 were collected; in these sites, the Φratio was
from 0.21 to 1. Two distinct stress regimes have been distin-
guished throughout the region in Oligocene time: (1)
transpressional stress regimes presented by the sites compres-
sional along major reverse faults and (2) transtensional stress
regime controlled by normal faults. Transtensional stress re-
gime was observed in normal faults such as the F19, F20, and
F26. The two transtensional tensors along with the F20 and
F26 normal faults are controlled by local changes in the fault
geometries (Fig. 5a). Geometric and kinematic analysis of
brittle structures indicate that most faults have reverse compo-
nents which reveal existing of compressional stress in
Shekarab Mountain (Fig. 6), in the eastern part of Shekarab
Mountain as locally there are normal faults; the data of sites 9
and 16 give a transtensional stress regime in the eastern part of
the study area. There is evidence for both transpressional and
transtensional stress regimes in Shekarab Mountain. In the
east part of Shekarab Mountain regionally, there are indicators
of transtensional stress regime such as sigmoidal opening and
extrusion of veins in small shear zones (Fig. 7). Most of the
transtensional regimes are observed along the normal faults,
while the compressional stress regimes are distributed
Table 1 (continued)
Site NStratigraphic age σ1Dir,Plung σ2Dir,Plung σ3Dir,Plung ΦαLocation Latitude (N°)
Longitude (E°)
27 12 Quaternary 260.04 354.39 165.51 0.54 17 Middle part 32°59′43.16″N
58°49′32.98″E
28 11 Eocene 083.24 350.06 246.65 0.5 20 Western part 33°00
́44.22 N
58°42
́46.98 E
29 10 Quaternary 068.11 338.00 246.79 0.5 11 Western part 32°55′59.27″N
58°44′47.57″E
30 10 Quaternary 064.04 334.01 237.86 0.75 13 Western part 32°57′05.93″N
58°44′08.26″E
31 13 Eocene 038.00 308.20 128.70 0.5 19 Western part 33°01
́19.32 N
58°39
́21.06 E
32 13 Eocene 337.03 233.65 070.24 0.5 14 Western part 33°01
́38.34 N
58°41
́19.44 E
33 12 Upper Cretaceous 100.18 007.07 257.71 1 23 Western part 33°00′07.94″N
58°39′01.48″E
34 12 Upper Cretaceous 209.03 300.18 109.72 0.58 9 Western part 33°01́16.56 N
58°41
́07.92 E
35 9 Eocene 217.07 113.63 310.26 0.67 6 Western part 33°01′53.91″N
58°41′52.38″E
36 10 Eocene 224.00 133.70 314.20 0.3 8 Western part 33°01′57.33″N
58°43′19.55″E
37 9 Eocene 036.00 127.69 306.21 0.5 5 Western part 33°01′55.62″N
58°45′27.73″E
38 10 Eocene 037.02 132.63 306.27 0.5 9 Middle part 33°02′05.87″N
58°49′28.71″E
1232 Page 8 of 18 Arab J Geosci (2020) 13:1232
Fig. 4 aCompression recorded in the upper Cretaceous rocks. bCompression recorded in the Paleocene rocks. cCompression recorded in the eocene; Φ
ratio is indicated in the bracket
Arab J Geosci (2020) 13:1232 Page 9 of 18 1232
throughout the region. The main question of this paper is
why igneous rocks such as andesite and dacite are more
outcrops in the eastern part of the study area? There are
only a few sites where igneous rocks have been ob-
served; however, they are distributed throughout the
eastern part of the study area (Fig. 1c). To reveal the
reason for more outcropping andesitic and dacitic rocks
in eastern Shekarab Mountain, faults data adjacent to
andesitic and dacitic rocks separated from other
Eocene and Oligocene data, these kinds of faults appear
as a normal fault with a strike-slip component example
of such normal faults indicated in Fig. 5. In the sites,
we observe the exhumation of igneous rocks, the Φ
ratios are close to 0.5, and the stress regime is
transtensional due to a local change in stress regime
(Figs. 5a and 8). We observed the age of Eocene and
Oligocene igneous rocks in the eastern part of Shekarab
Mountain. Consequently, stress regime determination for
these regions needs careful data separation, data analy-
sis, and stress regime computation. The stress tensors
from sites 9 and 16 are different from the regional
Oligocene stress state pattern; those two sites are situat-
ed at the adjacent igneous rocks (Fig. 5a). Our stress
analysis reveals that the outcrop of the igneous rocks in
the eastern part of the study area was due to local
changes of stress regime from transpressive to
transtensive; thus, we determine in these regions as lo-
cally σ1 was vertical and the Φratio was close to 0.5;
therefore, stress regime was transtensive (Table 1). The
transtensive stress regime has caused the exhumation
and outcrop of igneous rocks in the eastern part of
Shekarab Mountain. Stress analysis of Miocene rocks
reveals that in the Miocene time, stress regime was
strike-slip (Fig. 5b). Stages of applied stress based on
brittle structures were analyzed and provided valuable
information about the evolutionary process of the
Shekarab Mountain. As indicated by reconstructed stress
axes variations, our brittle tectonic analysis revealed dif-
ferent directions of compression. Based on the direct
inversion method (DI), the stress determination for σ1
data indicates a variety of σ1 trends within the studied
area ranging (Table 1). Direct inversion methods have
been applied to analyze the kinematic data.
Quaternary stress state
The present-day mean direction of σ
Hmax
(σ
1
)measuredby
GPS motion relative to Arabia-Eurasia convergence is around
N10°E; the active tectonics of Iran is controlled by the north-
ward movement of Arabia with respect to Eurasia (Vernant
et al. 2004). The computed deviatoric stress tensors, which
belong to the modern stress state of Sistan suture zone defor-
mation domains, reveal a homogenous stress field. The focal
mechanism analysis of the earthquake that occurred in the
Sistan suture zone suggests that strike-slip faulting associated
with the NE-SW compression direction was generated
(Dziewonski et al. 1981). The present-day state of stress was
analyzed by Jentzer et al. (2017) based on the inversion of
earthquake focal mechanisms; they determined that the direc-
tion of σ1 for the present day is N25°E, and it agrees with the
shortening axis given by GPS measurements (Masson et al.
2005;Tavakoli2007). We collected the data of Quaternary
time in the six sites 10, 11, 24, 27, 29, and 30 (Fig. 5c);
analysis of the data of site 27 shows that the stress regime in
this site is strike-slip and the direction of major stress axis (σ1)
is NE-SW. The NE-SW direction σ
Hmax
(σ
1
) paleostress field
represents 64% of the calculated stress tensors, and it has been
identified at six sites. The estimated mean tensor has principal
stress axes oriented: σ1 = 026/10, σ2 = 193/72, and σ3 = 119/
08 (Table 2). The ratio of stress magnitude difference (Φ)
valueis0.5,confirmingthatthistectonicregimewasacoex-
istence of strike-slip regimes. The modern state of stress was
deduced from the analysis of the youngest fault slip observed
at individual inspected sites where the majority of fault slips
data was measured in Plio-Quaternary conglomerates. The
modern stress field is characterized by N26°E trending hori-
zontal maximum stress axes (σ1) for almost all the studied
sites. However, five of six sites (10, 11, 27, 29, and 30) show
direction of σ1 axes with respect to the regional pattern (Fig.
8). The last one indicates the perturbed and different direction
from the regional modern stress state pattern. This site is sit-
uated at the junction of the two NE-SW trending fault systems.
The average values of σ
1
axes (NE-SW in the Quaternary)
generated the strike-slip regime resulting from the kinematics
of the faults; this result is consistent with the GPS vectors in
the region. The direction of maximum horizontal stress
axes (σ
Hmax
) of modern stress solutions is presented in
Table 2. Even though the region is under the influence
of both the Arabia-Eurasia collision and Makran sub-
duction, geometric and kinematic data of structural fea-
tures evidenced the effect of the Arabia-Eurasia current
Shekarab Mountain strike-slip regime. The mean direc-
tion of σ1 computed for the Quaternary times is NE-
SW and stress regime is strike-slip (Table 2). The ratio
Table 2 The direction of main stresses of the axes and ratio of stress
magnitude differences for different geological times
Geological time σ1Dir,Plung σ2Dir,Plung σ3Dir,Plung Φ
Upper Cretaceous 337/26 070/06 172/64 0.1
Paleocene 342/16 199/70 075/11 0.75
Eocene 003/31 110/26 232/47 0.29
Oligocene 006/38 116/52 238/05 0.52
Miocene 020/01 164/28 341/62 0.5
Quaternary 026/10 193/77 119/08 0.5
1232 Page 10 of 18 Arab J Geosci (2020) 13:1232
of stress magnitude differences (Φ)obtainedinthe38
sites; our analysis indicates that in most sites, shape of
stress ellipsoid is between prelate and oblate and Φ=
0.5 (Fig. 9). Our results demonstrate three dominant
minimum horizontal stress (σ
hmin
) axes: NE-SW, N-S,
and NW-SE (Table 2).
Fig. 5 aCompression recorded in the Oligocene rocks. bCompression recorded in the Miocene rocks. cCompression recorded in the Quaternary; Φ
ratio is indicated in the bracket
Arab J Geosci (2020) 13:1232 Page 11 of 18 1232
Discussion
Stress regime direction
There have been four distinct stress regimes in the study area:
(1) compression, (2) transpression, (3) transtension, and (4)
strike-slip. There are three distinct major stress axis (σ1) di-
rections in the Shekarab Mountain, which represents succes-
sive but discontinuous tectonic regimes. Cretaceous time is
the oldest state of stress in Shekarab Mountain. The analysis
of stress stages in the study area shows that the first stage in
Shekarab Mountains was compressional tectonic regime
Fig. 6 Field view of study area’s faults, aF17 reverse with a dextral strike-slip component, bF6 sinistral strike-slip with a reverse component, cF9
reverse with a dextral strike-slip component, dF26 normal with a dextral strike-slip component
Fig. 7 Field views of shear zone
indicators in the eastern part of the
Shekarab Mountain, a
photograph of the sigmoidal
opening in a small shear zone
related to shale rocks, b
photograph of calcite veins being
extruded by shear zone in
ultramafic rocks, cveins created
by a small shear zone in
calcareous rocks
1232 Page 12 of 18 Arab J Geosci (2020) 13:1232
along with the σ1 = 337/26, σ2 = 070/06, σ3 = 172/64 stress
directions and stress ratio was 0.1, which has caused the uplift
of peridotites and ophiolites in the eastern part of the study
area. The Eocene state of stress was deduced from the analysis
of the fault kinematics at the study area; fault slip data collect-
ed at the adjacent andesite and dacite rocks are normal with a
strike-slip component. The second stage of stress has been
transpressive with the direction of the main stress axes σ1=
003/31, σ2 = 110/26, σ3 = 232/47, and the ratio of stress
magnitude differences was 0.29. The third stage of stress re-
gime in Shekarab Mountain is strike-slip with the direction of
the main stress axes σ1 = 026/10, σ2 = 193/72, σ3 = 119/08,
and ratio of stress magnitude differences 0.5 (Table 2). Three
different tectonic regimes were identified following a structur-
al and kinematic study of faults formed during the upper
Cretaceous to the Quaternary periods in the Shekarab
Mountain. In the Oligocene time, stress regime was
transpressional, and in the eastern part, there is a different
stress regime, regionally change of stress regime in the
eastern study area keeping in the mind that this different
stress regime is responsible for the emplacement of an-
desite and dacite rocks. We used the fault data sets to
determine the paleostress history of the study area, and
stress field rotations inferred from regional structural
and tectonic features. Any cluster of fault orientations
is associated with a corresponding direction of the stress
field. Most of the complexities observed in the fault
patterns of the study area are due to distinct stress re-
gimes; therefore, multiple fault sets are formed by dif-
ferent stress regimes.
Fig. 8 Synthesis of upper Cretaceous brittle deformation to Quaternary in the study area, three successive directions of compression (σ1) identified
Arab J Geosci (2020) 13:1232 Page 13 of 18 1232
Stress field rotation and structural model
It is probable that in many cases, stress field rotations
take place over geological time; these rotations are
probably slow and gradual (Ron et al. 1993). The sep-
aration of fault kinematics data in the Shekarab
Mountain reveals the existence of three successive and
continuous stress regimes; as shown in Fig. 10,thereis
about 49° clockwise stress field rotation since upper
Cretaceous time in the Shekarab Mountain. There are
other regions that the tectonic history involves the stress
field rotation, such as there is 60° clockwise stress field
rotation in the Basin and Range (Ron et al. 1993),
northern Greece (Pavlides and Kilias 1987), and south-
ern California (Nicholson et al. 1986). The structural
paleostress indicators from the study area indicate that
the sense of this paleostress rotation is consistent with
the sense of brittle structure orientation (Fig. 11). The
current maximum horizontal stress, as inferred from
earthquake fault plane solutions, is oriented N25°E
(Jantzer et al. 2017). This direction is in good agree-
ment with stress orientation derived from Quaternary
deduced from fault kinematics. Inversions of the sepa-
rated data sets allow us to distinguish major changes in
σ1 direction such as the NW-SE trend in Cretaceous
time, approximately N-S trend in Eocene time, and the
NE-SW trend in Quaternary time. Accordingly, the ma-
jor stress axis (σ1) in Shekarab Mountain had clockwise
rotation (Figs. 10 and 11). Similar interpretations of
stress field rotations were suggested for California
(Terres and Luyendyk 1985), Central America (Menton
1987), and Alaska (Stamatakos et al. 1988).
The stress regime rotation in the Shekarab Mountain
is due to the variation of the minor stress fields and
changes in the principal stress field of Arabia plate.
Nehbandan fault system is a shear zone on the edge
of the collision zone between Arabia and Eurasia plates.
In the shear zones, there are minor stress fields, and
direction of these minor stress fields has rotations. The
Arabian plate movement towards Eurasia is a general
motion, and because of this, a number of minor stress
fields have been created and the direction of these mi-
nor stress fields has constantly changed. These minor
stress fields show a number of rotations in the direction
of the principal stresses. Nehbandan fault system is lo-
cated in the eastern part of Iran. It is created by the
relative movement of the Arabian plate to the north
(relative to the Eurasian plate). The principal stress field
of Iran is affected by the Arabia plate motions; the
motion direction of the Arabia plate had changed from
N-S to NE-SW during geological times.
Our results also have implications for understanding
the paleostress orientation during the upper Cretaceous
to Quaternary age and emphasize using the brittle struc-
tures to infer a schematic model for the study area
Fig. 9 The ratio of stress magnitude differences (Φ) histograms for the
paleostress fields obtained in the study area
Fig. 10 Density stereoplots for σ1 orientations for the three paleostress fields obtained,aupper Cretaceous time, bEocene time, cQuaternary time. One
percent contour intervals represented in lower-hemisphere, equal-area projection
1232 Page 14 of 18 Arab J Geosci (2020) 13:1232
using stress regime changes. Reconstruction of the
structural model from an initial compressional stage dur-
ing the upper Cretaceous stage to the final strike-slip
stage during Quaternary reveals the existence of three
successive and continuous stress regimes in the
Shekarab Mountain (Fig. 12). In Cretaceous, major
stress axis (σ1) had NW-SE trend, and stress regime
was compressional, so the compressional stress regime
activities in the NW-SE trend have uplifted the perido-
tites and ophiolites in the eastern part of the study area
(Fig. 12a). The second stage of stress in the upper
Eocene to the Oligocene time was transpressional, and
the major stress axis (σ1) had an N-S trend. The upper
Eocene to the Oligocene magmatic rocks (andesite and
dacite) emplace over the eastern part of Shekarab
Mountain (Fig. 12b). The local change of stress regime
from transpressive to transtensive was responsible for
the exhumation and exposure of andesite and dacite
rocks in the eastern part of the study area. Analyzing
the tectonic pattern of the study area and its evolution
during geological times indicates that Shekarab
Mountain is now a sinistral shear zone. Figure 12 c
presents our proposed model for the third stage of stress
in the study area; in this model, a schematic sinistral
shear zone and related structures based on stress chang-
es and structural analysis along the mountain range
were drawn. In this model, the NW-SE striking thrust
faults and chains are perpendicular to the direction of
current maximum horizontal stress (NE-SW) as inferred
from earthquake fault plane solutions. These results pro-
vide a neat and simple explanation for the puzzling
discrepancy between apparent change in stress orienta-
tion and the exhumation of andesite and dacite rocks in
the eastern part of the study area; therefore, the avail-
able data conclude that the extension in the Shekarab
Mountain is with respect to the regional deformation
event in this sinistral shear zone. Based on the results,
the geometrical relationship between volcanic rocks and
structural patterns of the study area is compatible with
the structural evolution of a sinistral shear zone.
Conclusion
In the north Sistan suture zone, the direction of compressional
stress is close to N25°E. This direction has been obtained from
Fig. 11 Distribution of the σ1 axes obtained from brittle structures throughout the studied area from upper Cretaceous to Quaternary age
Arab J Geosci (2020) 13:1232 Page 15 of 18 1232
the inversion of focal mechanism analysis of earthquake
which occurred between 1976 and 2015 (Jentzer et al.
2017). The Quaternary stress states, deduced from fault kine-
matics analysis, indicate that direction of compression (σ1) in
the Shekarab Mountain is close to N26°E, while the
Quaternary σ1 direction is relatively coincident with the
present-day Arabia-Eurasia convergence direction. Our stress
analysis of brittle structures in the Shekarab Mountain reveals
three drastic changes in stress tensors from the upper
Cretaceous to Quaternary.
The results show that the direction of compression (σ1)
was N337°, N003°, and N026° during Cretaceous-
Paleocene, Eocene, and Oligocene to Quaternary, respective-
ly. Therefore, the direction of compression (σ1) was rotated in
a clockwise by at least 49° over the last 83 Myr.
Evolutions of the stress regime in the Shekarab Mountain
correspond to various tectonic events. The first stress stage
was compressional, which caused the uplifting of peridotites
andophiolitesintheeasternpartofthestudyarea.The
Eocene-Oligocene second stage shows two distinct
transpressional and transtensional tectonic regimes. The
changes from transpression to transtension are due to local
variation of stress regime that is responsible for the andesite
and dacite exposures in the eastern part of Shekarab
Mountain. Ultimately, based on the results of this study, the
third stress stage of the study is strike-slip.
Nomenclature Dir, direction; DI, direct inversion method; F, fault; F
k
,
function which expresses the deviation of datum; GPS, Global
Positioning System; K, fault slip data; Ma, million years before the pres-
ent; Myr, million years ago; N, number of faults; n, normal unit vector of
faults; Plung, plunge; s, real striae; T, the stress tensor; σ
Hmax
,maximum
horizontal stress; σ
hmin
,minimum horizontal stress; σ1, major stress axis;
σ2, medium stress axis; σ3, minor stress axis; Φ,ratio of stress magnitude
differences; α,misfit angle; τ,calculated relative shear stress; ν,normal
stress vector; W
k
,the weight of the datum number k
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