INTRODUCTION

The rock volume is like a tape recorder that is always running. The deformation path is recorded as features in the rock volume, including fractures, folds, and penetrative deformation. Paleostress analysis is the science of reading that tape and determining the deformation path. The deformation path is a powerful technique to provide additional constraint on structural interpretations [14]. Deformation structures that can be observed directly in individual outcrops or with hand specimen are commonly referred to as minor structures or mesostructures such as fractures, stylolites, striated fault planes, veins arrays and brittle shear zones. They nevertheless underpin many structural interpretations. They are the building blocks that allow to understanding of larger scale structures. In structural and tectonic studies these minor structures become widely known because they can be interpreted more accurately [18, 20]. The same approaches and methods cannot be used for studying both small and large structures, for example small faults, and sometimes those of an intermediate size, can be observed and studied at outcrops scale or hundreds of meters in scale. On the other hand, large structures are mainly studied using geological maps prepared from field studies and also from geophysical investigations or from interpretation of aerial photographs or satellite images [8]. Tension fracture is defined as a natural mode I rock fracture [10, 15, 17]. The plane of a propagating fracture is always perpendicular to the local least principal stress that prevailed during fracture propagation. The shear fractures form in undamaged rock initiate as conjugate sets that are oriented parallel to the intermediate stress axis and are generally ±2° to 30° from the maximum stress axis.

Fractures analysis carried out throughout traverses across Kalosh anticline. The anticline is located ~30 km south of Sulaimani city, Kurdistan region northeast Iraq (Fig. 1) [21]. It extends NW‒SE for ~17 km within the high folded zone of the western segment of the Zagros Foreland Fold Thrust Belt. It lies between longitudes (44°25′44″ E and 44°33′52″ E) and latitudes (35°512′09″ N and 36°16′13″ N). The area is extremely rugged mountainous with high amplitude anticlines.

Fig. 1.
figure 1

Location of the study area. The tectonic subdivisions of the Western Zagros Fold–Thrust Belt [12] overlaying digital elevation model of ALOS PALSAR [22].

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STRATIGRAPHIC SUCCESSIONS

The stratigraphic successions of the studied area are dominated by the exposures of Tertiary rocks [9] (Fig. 2). During this time, thick clastic unit was deposited in a flysch marginal, narrow NW‒SE trending trough Kolosh Formation [5]. Followed by shallow marine carbonate or mixed siliciclastic and carbonate Sinjar Formation of Early Eocene age. Then thick clastic unit was superimposed by red clastic molasses sequence of Gercus Formation of Middle Eocene age. This former molasse trough became a slightly subsiding, partly lagoonal, the basin filled in with the carbonates of the Pila Spi Formation of Middle-Late Eocene, [16]. They followed by thick evaporates, carbonate and marls of the Fatha Formation of Middle Miocene age [6], followed by the deltaic-pedimentary clastics which commence with red silts and marls of Injana Formation that belongs to Upper Miocene [4].

Fig. 2.
figure 2

Geological map and cross section (A–A−). A, B, C and D, are fracure stations along four traverses of Kalosh anticline.

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DATA AND METHODS

The fieldwork carried out through 24 stations distributed in the study area along the four traverses. The stations were selected far from plunging area to avoid the tectonic axes rotation and give the most realistic results (Fig. 2). All measurements and results are in Right Hand Rule (RHR). The measurements included the attitudes of bedding and fracture planes.

The fracture planes data analyzed stereographically using Dips software (Dips.v5) [21]. The results of fracture analysis were classified according to [13].

Win-Tensor program version 5-8-1 used for determines the paleostress directions (σ1, σ2, and σ3) from the average attitudes of the conjugate shear fracture planes. σ1 bisect acute angle between conjugate shear fracture planes, σ3 is perpendicular to σ1 and bisect obtuse angle, and σ2 represent the line of the intersection between the two fracture planes [7]. The tension fractures are always perpendicular to the least principal stress (σ3) and parallel to the maximum stress direction (σ1).

Finally, the output of the kinematic analyses of structural mode (fractures) unified to conclude the sequence of tectonic phases, which architecture the study area in the view of geotectonic setting of the studied area.

RESULTS

Geometry of the Fold

We studied the geometry of Kalosh anticline through four traverses perpendicular to its axis. These traverses show that the anticline follows the general Zagros fold trend NW‒SE, and the anticline is asymmetrical double plunge box fold. It consists of the crestal segment bounded by two limbs. The southwestern limb is slightly steeper while the northeastern limb is overturned. Figure 3 shows the synoptic stereographic pi-diagram of Kalosh anticline along cross section (A–A−). It shows that the anticline has two hinges. In the SW fold, the attitude of the axial plane is 042°/48°, the attitude of the fold axis is 126°/06°, the average attitude for the northeastern limbs is 202°/22°, the southwestern limb is 210°/63°, the interlimb angle is 138° and the anticline is gentle according to [20]. In the NE fold, the attitude of the axial plane is 208°/33°, the attitude of the fold axis is 124°/04°, the average attitude for the northeastern limbs is 203°/44°, the southwestern limb is 202°/22°, the Interlimb angle is 23° and the anticline is tight according to Fleuty classification [11].

Fig. 3.
figure 3

Synoptic pai diagram of the Kalosh anticline along cross section (A‒A−).

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Fractures Analysis

More than (450) readings of fractur planes were collected from 24 stations along four traverses (Fig. 3). Strike and dip were measured for the fracture planes as well as the attitude of the bedding plane, which contain the fracturs. Many of collected data were neglected due to the nonexistence of the two conjugate fractures of the system in the same station.

The stereographic projections of the fracture poles in the all 24 stations are shown in the (Figs. 4, 5). The paleostress analysis for all the 24 stations shown in the (Supplements 1, 2, 3 and 4). All data with paleostress classification are listed in Table 1.

Fig. 4.
figure 4

Pole projections (fractures) and classification for the stations on traverses A and B.

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Fig. 5.
figure 5

Pole projections (fractures) and classification for the stations on traverses C and D.

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Table 1.   Fracture types and paleostress analysis for all 24 stations in the study area
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Table 1.  (Contd.)

DISCUSSION

From the fracture analysis and classification, two orthogonal tension fracture sets ac and bc together with following shear systems (hk0) acute about (a), (hk0) acute about (b), h0l acute about (a), h0l acute about (c) and 0kl acute about (c) where distinguished in the study area. It is clear from table 1 that the most prevalent paleostress directions are NE‒SW and NW‒SE. The first NE‒SW compressive stress normal to the general trend of the major anticline is considerate as horizontal primary component of oblique collision between Arabian and Eurasian plates. This compressive phase led to initiation (ac) tension set, (hk0) acute about (a) and (h0l) acute about (a). (bc) tension set, and (hk0) acute about (b) indicate that they formed by other compressive stress in the direction NW‒SE parallel to sub parallel to the axes of the major fold. This stress considered as secondary stress developed during relaxation event after primary compressive stress. The (hk0) acute about (a) tectonic axis is one of the most prevalent shear fractures in the study area. The approximately (h0l) acute about (c) and (0kl) acute about (c) shear fractures indicate that these fractures may be developed by the extensional phase associated with NE‒SW and NW‒SE compressive stresses (Fig. 6).

Fig. 6.
figure 6

Relation between fractures set and systems with the stress directions in Kalosh anticline.

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The clockwise rotation of the NNE‒SSW paleostress direction to the NE‒SW which is perpendicular to sub-perpendicular to the general trend of the anticline with the following direction of maximum stress (σ1) (52°/184°) and (66°/250°) might be attributed to the oblique collision of the Arabian and Eurasian plates along their zigzag margins (Fig. 7).

Fig. 7.
figure 7

Different types of fractures in the study area.

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The combination of the above fractures, which analyzed to find the direction of maximum principal stress revealed that the anticline formed by the NE‒SW stress direction then developed by the uplifting process that make it to crestal extension in which the anticline changed from the symmetrical box fold to overturned NE vergence.

This two main compressional paleostress directions NE‒SW and NW‒SE, which are resulted from the fracture analysis for the study area, also mentioned by many authors who have investigated paleostress in different areas of north and northeast of Iraq such as; [1–3, 19].

CONCLUSIONS

(1) Paleostress analysis from fractures indicated that the study area was subjected to four stress states. Two of them are compression, and the rest two are extensions.

(2) The primary stress is the compressive stress with their maximum horizontal axis (σ1) in the direction of NE‒SW, seams as responsible to the initial folding and most of the brittle failures in the area.

(3) The NW‒SE compressive stress is parallel to sub-parallel of the fold axis, considered as secondary stress developed during the relaxation event succeeding the primary compressive pulse. This stress is responsible for the other brittle failures in the area.

(4) The NE‒SW extension stress considered as a releasing phase that associated with the final uplift of the main fold.

(5) The NW‒SE extension face considerate as an extension stress related to the primary NE‒SW compressive stress.

(6) The clockwise rotation of the NNE‒SSW paleostress direction to the NE‒SW it might be attributed to the oblique collision of the Arabian and Eurasian plates along their zigzag margins.