Study of Joints in Baba Dome-NE Iraq

The present work involves detailed study for joints in Baba dome within the foreland fold belt of north Iraq. It aims to identification and classification of various joint sets and systems, extraction predominant directions of joint sets and systems and estimation stress direction in the study area. The study includes only Injana (Miocene) and Mukdadiya (Pliocene) formations. The field work has been executed by means of nine traverses, each of them includes mainly two stations, as a result, 719 measurements of joint planes were extracted. The study comprises several categories: carefully scanning the joint plane for any shear movement, measuring the attitude, classification of joints according to tectonic axes a, b and c, determining the predominant direction, calculation of causative stress, estimation the structural history of Baba Dome .The types of joint which distinguished are (ab, bc, ac, hko system acute about a, hko set acute about b, hol system acute about a and hkl set).All measured joints have no visible shear displacement on their planes. The hko system and hol system appeared as single set and as conjugate system. The predominant direction of the joints is determined to be nearly along the strike of exposed beds. The mean causative stress direction is calculated to be: 046° from hko system and 315° from hol system. Trend of measured joints ( ab = 315° , ac = 045° , bc = 315° , hko1>a = 058° , hko2>a = 033° , hko3>b = 336° , hol1>a = 316° , hol2>a = 312° and hkl = 257° ). An attempt to put the measured joints in order according to timing sequence and from older to younger: hko system acute about a &ac set, hol system acute about a and hko system acute


INTRODUCTION
The NW and SW ends of Baba Dome (Study area) lie on the Lesser Zab River and Tarjil Village respectively (Figure 1.A). It covers 250km 2 . The length and width of the study area attain 52km and 4-6km respectively.
The study area appears on the surface, i.e on the geological map as a trace of reverse fault, passes roughly through hinge line of the subsurface anticline (Figure 1.B). The mentioned fault causes the NE limb overrides the SW limb causing the latter obscure.
The present study comprises classification and extraction of stress from the measured joints. The classification depends mainly on [1 and 2].Numerous studies concerning joints in Iraq for an instant [3and 4] from NE Iraq. Other studies outside Iraq treat joints from different view like [5, 6, 7, 8 and 9].
The aim of the present study is to study joints from different view: classifying joints, estimating the stress orientation and deduce the tectonic history of the study area.
Tectonically, the study area lies within low folded zone of Zagros Fold-Thrust Belt ,that consists of a series of widely spaced, low amplitude gentle folds trending NW -SE but change gradually to E -W as they extend northwestwards [4] . This zone is characterized by the presence of regional detachment consists of Middle Miocene salt layers of Fat'ha Formation, that have caused decoupling of the surface structures from their subsurface counterparts [ 4 ] .
Structurally, the study area is a part of Kirkuk Structure, that is an anticline trending NW-SE, divided by two prominent saddles into three major structural domes, Amsha saddle  Fig. (2) . Kirkuk Structure is overthrust from the NE in the exposed beds ,due to sliding on the Fat'ha salt, but the fold at medium depth , below the salt zone of the Fat'ha , is simple and almost symmetrical [10]. The study area, on the surface, represented by NE limb of the anticline with inclined bed (31°-60°) toward NE , which resulted from the intersection of the thrust fault plane with the surface.
The lithology of rock units exposed in the study area is built up of alternating beds of

Methodology
Nine traverses have been chosen to cover the study area. The method of work depends upon [2 and 11] .Joint planes were measured by compass in the pre-located and accessible stations oriented along nearly straight line (traverses) perpendicular to strike of bed. The field data are plotted on Schmidt net using Stereonet program. Then the joints were classified depending on tectonic axes a, b and c. The mentioned program also calculates the orientation of bisector of the acute angle between planes of joint system representing the orientation of stress.

Classification and Description of Joints in the study area 1-Bedding joints (ab)
The planes of this set are parallel to tectonic axes a and b, and perpendicular to c Fig. (3).
Therefore, they are parallel to the bedding plane. Their strikes are NW-SE direction with bearing ranges from 306º to 324 º. The mean strike is 315º and the mean dip angle is 044º toward NE. It appears in all stations. It has 13.7% of the measured joints Fig. (4) (Plate 1.A).

2-Strike Joints (bc)
The planes of this set is parallel to tectonic axes b and c and perpendicular to axis a, so it is perpendicular to the bedding plane along strike of beds Fig. (3). It has NW-SE direction with bearing range from 306° to 318°.Its mean strike is 315º and mean dip angle is 46º toward SW.
It appears in all stations, having 15.4% of the total measured joints Fig. (4) (Plate 1.C).

3-Dip Joints (ac)
The planes of this set are parallel to tectonic axes a and c and perpendicular to tectonic axis b Fig. (3), so they are nearly perpendicular to the bedding plane. It has NE-SW direction with bearing ranges from 36º to 54 º. The mean strike is 045º and perpendicular to the bedding.
This set appears in all stations. It has, 16.1% of the total measured joints Fig. (4) (Plate 1.B).

4-1 hko joints
The hko joints, in the study area, appear as set and system. They are of two types represented by hko set acute about b and hko system acute about a. The hko system acute about a intersects the tectonic axes a and b and parallel to c tectonic axis. This system occurs in the form of two sets : hko1 and hko2, Fig. (3) .The hko1 has a mean strike and dip angle attaining 058°and 64° toward NW respectively while the hko2 has a mean strike and dip angle attaining 033°and 81° toward NE respectively. This system appears as conjugate planes in

4-2 hol System acute about a
This system appears as single set and conjugate system in the study area.
The mention joints are of hol > a type. The planes of this system intersects tectonic axes a &c and parallel to b Fig. (3 Fig. (1.B).

Discussion
Kinematic analysis shows the relationship between the joints (sets and systems) and the stages of the study area formation . It has direction NW-SE.
The hol system acute about a axis is formed under stress regime in which greatest principal stress (σ1) parallel to tectonic axis a and the intermediate principal stress (σ2) is parallel to b axis (both σ1 and σ2 lie within the bedding plane) while the least principal stress (σ3) parallel to the tectonic axis c (perpendicular to the bedding plane) .This system appears asymmetrical about tectonic axis a. The (σ1) bisect the acute angle between conjugate joints while σ3 bisects the obtuse angle. The mean acute angle between this type attain (82º), this means that the effective stress (σ1-σ3) is relatively low during the formation of this type of joints.
The relatively constant relation between the angles of the planes of this type and bedding plane (10º -15º) indicates that σ1 varies with the dip of beds and being parallel to the bedding plane during the formation of this type. The stress direction of this type is calculated to be 315º.
During the end of Late Upper Miocene and mainly during the Pliocene the Folded Zone was progressively involved in to Orogenic uplift [14]. This phase include the collision of Stress mentioned started to grow to commence shortening in the sedimentary pile. This shortening made by folding and later on thrusting on the folded beds. This scenario (tectonic model) is best put forward by [15].
In the study area, incompetent salty beds of Fat'ha Formation ruptured along shallow décollement (Imbricate fan in a thrust system with a basal décollement). Moreover, the earlier buckled beds are now serving as an obstacle or ramp structure which forces the propagated thrust sheet to rise and become shallower as it is furtherly pushed along the lubricant surface until finely crops out at the surface. This condition provided an ideal condition to develop a fault-fold geometry .The latter geometry has a typical asymmetrical foreland-verging characteristic of the folded zone associated with major Zagros thrust fault.
The timing mentioned and mechanism can be translated or linked to the joint and stress directions that are detected in this study and as follows: -The NW-SE stress orientation: As the stress direction mentioned has continued and more Generally the orientation of joint planes has occurred parallel, perpendicular and oblique to the strike of the beds.

5-
The Sulfurous bed of Fat'ha Formation, perhaps, has prevented or isolated the joints in the upper part of the lower part of the mentioned bed .Therefore, the detected joints probably have no relation to joints that developed beneath the salfurous bed .On the contrary, if the mentioned possibility isn't true, the joints beneath the salfurous bed could coincide with the joints in the upper part.