Determination of the Paleostress Orientations and Magnitudes for Missan Structures, Southeastern Iraq

The use of Right dihedral method and Mohr daigram method allow determination of the paleostress orientation and its magnitude in Missan structures, Southeastern Iraq. The principal stress orientations and its magnitudes have been determined by measure of the striation on the faults planes. The measurement of orientation of the principal stress was deduced by different graphical methods, the horizontal maximum principal stresses (σ1) magnitudes were (3600, 4360,4650, 4750 and 5700) bars, the horizontal intermediate principal stresses (σ2) magnitudes were (1528, 1842,1962.5, 1998.5 and 2390.5) bars and the vertical minimum principal stresses (σ3) magnitudes were (544, 676,725, 753 and 919) bars. This study shows that area is located within unstable region, since the poles of measured faults lie in the area of reactivated faults in Mohr diagram. 1Introduction Most of the previous studies which have been carried out on the Folded Zone or other Zones of Iraq dealt with the orientation of the principal stresses [1], [2],[3] ,[4] and [5], but not with the shape of the stress tensor and the magnitudes of the fault planes paleostress. Missan Structures (Amara, Noor, Buzurgan and Halfaya Structures),southeastern part of Iraq are located between latitudes ( 31° 30° 32° 40° North) and longitudes (46° 50° 47° 40° East), covering about (2900) Km2.See Figure (1). Journal of University of Babylon for Pure and Applied Sciences (JUBAS) by University of Babylon is licened under a Creative Commons Attribution 4.0 International License. 2018.


2-Aims of the Study
Aims of this study are: 1-To determine model parameters (Orientation, Stress ratio (R) and Magnitudes).
2-To analyze any relationship between there parameters and other factors (e.g. overburden, rock types and age of the stresses). This attempt has been made to carry out a regional analysis of the stress in folded and faulted areas in south of Iraq.

3-Geological Setting
Tectonically, the study area belongs to the unstable shelf represented by two Zones ; The Foothills Zones, along the Iraqi-Iranian international boundaries and the Mesopotamian Zone, more precisely, the former is represented by Hemrin Sub zone ,whereas the latter by Tikrit-Amara Sub Zone [7] and [8].
Missan Structures (Amara, Noor, Buzurgan and Halfaya) locate at southeastern Iraq, Amara Structure is about (10)Km south western Amara city. Noor Structure is about (15) Subsurface geology could be known through studying these drilled wells which cover the these structure and penetrate the Tertiary period deposits and most of the Cretaceous period deposits. Most of Tertiary depositions consist of calcic rocks and there are some formations consist units of clastic deposits and have big thickness, were deposited in shallow marine environment [6] and [9]. The formations from the oldest to the youngest of Cretaceous period starting from Yamama, Ratawi, Zubair, Shuaiba, Nahr Umr, Mauddud, Ahmadi, Rumaila, Mishrif ,Khasib, Tanuma, Sadi, Hartha and Shiranish, while the Tertiary period is represented by Aaliji , Jaddala ,Krikuk , Jeribe and Fatha ( Oil Exploration Company) [10].

4-Methodology
Many authors [3], [15], [16] and [17] used the fault slip data to yield the orientation of the principal stress axes and stress ratio ( R) and then take into account the depth of overburden. [18] suggested that the lithostatic load gives value of the vertical stress. Experimental analyses shows and theoretical reasoning suggests that one of the principal stresses is generally vertical during the tectonic deformation.
In this study, Mohr daigram representation of the stresses in three dimensions from the basis of method used to check if the faults behaved as reactivated faults. When the occurrence of slip on discontinuities is controlled by the friction law [19], the plot points in the Mohr diagram should be found on or above the sliding line and beneath the failure envelope.
Several methods of fault plane striations analysis have been recently proposed for determining the axes of paleostress and deduce the directions and magnitude of the principal stress at the time faulting [20], [21] and [22]. Many authors [15], [16], [17], [18] and [23] have applied the reactivated fault model to estimate the magnitude of stresses.

1: Faults Analysis using Right dihedral method
Depending on the fault planes and slickensides which were measured in the study area, strike of these fault planes was (130), and its dip (40˚) in the east ( Cores measurements of Missan Structures).
Right dihedral method was used to determine directions of principal stresses axes (σ 1, σ 2, σ 3) of the study areas, as follows the maximum principal stress (σ 1) was horizontal and its strike (058), the intermediate principal stress (σ 2) was horizontal and its strike (328), and the minimum principal stress (σ 3) was sub vertical its strike (148), and dip of these faults in a SW trending. See Figure (6).
Where: e: is the pitch angle.
(I, m, n): are cosine of the angles (α , β, γ) respectively.    [26], the distribution of stress ratio ( R) values were calculated of all faults by Bott,1959 law, and calculated mean of the stress ratios ( R) value, which was ( R mean = 0.5). See Figure (

2: Determination of the paleostress magnitudes using Mohr diagram.
In the study area, can be determined some of the tectonic and structural factors are not changed as follows: a: Age of the studied formations refers to Cretaceous.
b: All rocks are sedimentary rocks.
c: All studied structures are subsurface.
There are other factors are change as density and thickness (overburden) of the rocks.
The lithostatic reference state is the simplest general stress model for the interior of the Earth. A rock volume with this condition cannot support differential stress over geologic time (σ1-σ3 = 0), which means that its state of stress is described as a point on the horizontal axis of the Mohr diagram. This means that stress is independent of the orientation: In this paper (σ3 = ρgz). The stress is, according to this model, completely controlled by the depth and density of the overlying rock column. For continental rocks, which have an vertical stress (σv) is close to the cap rock. Therefore, the vertical stress is given by: [21], [22] and [27].
Thrust faults measurements were obtained from cores of Missan Structures of the study area (The orientation is assumption), depth and lithology of the rocks were obtained from Missan Oil Company (M.O.C), table -2, therefore, can be estimated vertical stress (σ v = σ 3), the depth (z) from the table (2), (g) equal (9.8 m/ s²), density of the rock can be calculated of the maximum, minimum and average of the rock limestone and shale which are (2720, 2000 and 2360) kgm/m³ respectively.
Mohr diagram was drawn depending on the stress ratio (R) magnitude (R= 0.5) and the points (poles of the faults) were plotted depending on the angles (α, β, γ,) on the these circles [24], [25] and [34]. The plot points in Mohr diagram should be found above sliding line and beneath failure envelope. See Figure (9). Figure (10) shows the relationship between the principal stresses axes. It shows that increasing with the depth due to its extension resulted from overburden.

5-Results of paleostress magnitudes determination.
Orientation of stresses and the stress ratio ( R ), as discussed by [25], the maximum and minimum stresses (σ1 and σ3) are contained within right dihedral bounded by fault plane and imaginary plane which is perpendicular to the fault plane and to the direction of the maximum shear stress. Based on concepts similar to these geographical search procedure was illustrated by [20], [35] and [36] ,used individual faults to determine all possible orientations of (σ1 and σ3). Size of the fields containing (σ1 and σ3) are reduced by superimposing of the dihedral stresses from other faults.
The result obtained by application of this method are summarized in Table (3).

7-Conclusions
The fault structures of the studied area appear to have been produced by the NE-SW direction Alpine Orogenic compression. Paleostress analysis for fracture structures (Faults and Striations) indicated that the studied area was subjected throughout its geological history to a compression stresses which was perpendicular in orientation to the fold axis (NE -SW). Mohr diagram and Right dihedral methods suggest that area is affected by inhomogeneous deformation and shows a gradual change horizontal of these structures axes (This is a clear in in Buzurgan Structure axis is run a long a N50˚W , Halfaya Structure axis is run a long a N60˚W, Noor Structure axis is run a long a N65˚W and Amara Structure axis is run a long a N75˚W trending) and thrust orientation. Distribution of the stress ratio magnitudes on Lisle graph [26].) show the stress mean ( R) less than one, ( R= 0.5) that indicates shape of the stress was flattening (σ1 ≥ σ2 = σ 3). These changes may be related to where they have developed not to when they have formed. The stresses magnitudes increasing refer to overburden thickness. This study shows that area is located within unstable region, since the poles of measured faults lie in the area of reactivated faults in Mohr diagram.