Makran-Geology

Geology of Sistan and Baluchestan Province

Sistan and Baluchestan Province, located in southeastern Iran, is one of the most important parts of the Neotethys that has undergone stages of development from oceanic to continental crust. Based on the geological and metallogenic characteristics of the region, it can be divided into the Makran belts, the Lut and Hirmand blocks, and the Iran Shahr-Birjand shear fault zone. The Makran ophiolites and their colorful assemblages can be considered as the process of subduction of the Neotethys oceanic crust.

Sistan and Baluchestan Province includes parts of Central Iran (Lut Block), the Zabol Zone, the Sistan Flysch Zone, and the Makran Zone, and covers the B, Sistan, and Makran plains.
 

There is a wide range of igneous, sedimentary, and metamorphic rocks with different origins. These rocks include ophiolite belts, granitoid belts, volcanic belts, and extensive sedimentary metamorphic sequences, including Eocene flysch. The Sistan and Baluchestan province encompasses three main geological-structural zones:

1.Flysch Zone or Nehbandan-Khash Zone

2.Lut Block

3.Makran Zone and Jazmurian Basin

 

Due to the presence of two main sedimentary-structural plains, the East Iran Flysch Basin and the Makran Basin, older rocks than Cretaceous are less common in Sistan and Baluchestan Province. The oldest geological units in the province can be considered the ophiolites that outcrop along the main faults of the province. Most of the province is covered by old sedimentary units and Quaternary unconsolidated sediments, followed by ophiolitic and igneous units. Metamorphic units are very rare in this province.

Tectonics

Based on the morphology of Sistan and Baluchestan Province, its elevations and depressions follow three general trends:

a- North-south trends are more common in the northern parts of the province. b- Northwest-southeast trends in the southern parts of Sistan, between Saravan and Iranshahr to the Khash area. c- Approximate east-west trends, which are the dominant trend of the inner and outer Makran Mountains in the south of the Jazmurian Basin and south of the Mashkil River to the Oman Sea coast.

 

The two Jazmurian and Moshkel Basins, located between the volcanic arc and the wedge, are known as forearc basins.

Ophiolitic mélanges (colored complexes) separate the Jazmurian basin from the Makran accretionary wedge.

Seismic and earthquake data (marine section) indicate the subduction of the oceanic lithosphere with a slope of less than 3 degrees to the north.

The depth of this plate reaches 30 kilometers below the Jazmurian basin.

Seismic data show a thickness of about 3 kilometers of sediments in the foreland of the wedge, which has led to the formation of a narrow (low-angle) wedge with seaward growth due to shortening to the north.

Accretion and continuous underthrusting of sediments occur along a north-dipping detachment surface.

The present Makran wedge has developed since the Early-Middle Miocene.

Makran comprises four structural-stratigraphic domains separated by major thrusts (from north to south: North Makran, Internal, Outer, Coastal).

North Makran: Includes ophiolites and overlying sediments mainly of Late Cretaceous to Eocene age. This domain is separated from the Internal Makran by the Beshagard thrust.

Internal Makran: Mainly consists of Eocene to Late Oligocene turbidites and minor shallow marine sediments of Early to Middle Miocene. This domain is between the Beshagard and Qasr-e-Qand thrusts to the north and south, respectively.

Outer Makran: Located between the Qasr-e-Qand and Chah Khan faults to the north and south, respectively, and consists mainly of shallow marine sediments of Early to Middle Miocene. There are no outcrops of older turbidites, and the style of folding is similar to that of Miocene sediments in the Internal Makran structural domain.

Coastal Makran: It is located south of the Chah Khan thrust and extends to the seashore. The shallow marine sediments of the Late Miocene have the most outcrops and are structurally characterized by very gentle folding without the occurrence of reverse faults, and normal faults are instead exposed in this area.

Geomorphology of the study area

The rocks in this area are generally composed of shale, marl, sandstone, and conglomerate. Due to the higher erodibility of shale and marl compared to conglomerate and sandstone, areas with shale and marl tend to have lower-altitude topography and a more subdued, rolling morphology. The mountain ranges in the area have also been formed as a result of this difference in the composition of the formations. In areas where the Flysch block of Dehar outcrops, due to the difference in composition between the blocks (hard igneous and sedimentary rocks and the surrounding shale and marl) and the difference in their erosion rates, the blocks have taken the form of prominent cones in low-lying and hilly areas. The course of the main rivers and many of their tributaries is controlled by folded structures, faults, bedding, and the composition of the rocks that make them up.
 

The specific shape of the watercourses has created a badland topography on the more marly units of the region.

As a result of wind and rain erosion in soft marly terrain covered by relatively hard conglomerate, hoodoo-like features have been formed.

In this unit, where the dust and silt particles between the pebbles have been eroded by wind and only the coarse pebbles remain, a geomorphological phenomenon is visible.

Since lithology and strong erosion play a role in the morphology of the region, anticlines are usually the prominent and high points and synclines create the low-lying and sunken areas compared to the surrounding terrain.

The course of the main rivers and many of their tributaries is controlled by folded structures, faults, bedding, and the composition of the rocks that make them up.

 

Seismicity Of Makran Region

In most areas of the Earth where the subduction phenomenon occurs, the seismic potential is very high. However, in Makran, as an active subduction zone, relatively low seismicity has been recorded. One of the reasons for its relatively low seismicity compared to other subduction zones in the world can be the low slope of the subduction zone.

b) Two-dimensional cross section in the Makran subduction zone at the longitudes of Bazman (red elements) and Sultan (blue elements) volcanos. The small circles are the epicenters of the thick intersecting lines of location uncertainty in the X and Z directions. The revised center yellow star for the 2013 Khash earthquake is the global average depth of the plate under the volcanic arc based on Wada and Wang, 2009.
 

Surface observations (tectonics, magnetism and geodesy) indicate a compressive discontinuity consisting of several faults that strike obliquely to the convergent plane movement. While deeper (seismological) observations of smooth transition across the fault system no lithospheric transform faults have been established.

This syncline is located in the western part of Makran and due to the lack of device data related to previous earthquakes and also due to low population density, there is no data related to historical earthquakes in this area.

But the subduction setting with low slope of Makran may make it prone to big earthquakes.











 

Map of Makran faults in Iran and Pakistan and related areas according to topography. The faults of Iran are from the active faults map of Iran and the faults of Afghanistan and Pakistan. Faults and fracture zones in the Indian Ocean and Oman Sea belong to. The brown arrows pointing south indicate the continental microplate tectonic depressions of the Lut and Hirmand blocks (Eurasia) on the Arabian plate. The blue vectors in the north also show the relative values of internal deformation of the Zagros, Lut and Hirmand blocks. The easternmost blue vector shows the tectonic depressions of the Indian to Eurasian plate.

Considering the role of segmentation in deformation control in subduction zones, it is necessary to determine the main segments of a subduction zone based on transform faults in order to know as much as possible the possible risks of earthquakes and tsunamis caused by it.

 

Structural map of Makran fault and faults affecting its geometry. The Makran fault ends at the triple junction in the eastern part and at the Minab transform margin (ZMPF) in the western part.

Investigations show that the Makran fault consists of 6 main segments with a step arrangement. In various reflection seismic studies that have been carried out on the hanging walls of these six segments. The folds and thrusts of the surface effects of the faults in the eastern part of the Oman Sea.

On the topography images of the sea floor, the Sonne fault and other transform faults that cut the Makran fault can be traced. So that their continuation can also be seen on land. Along these fault lineaments, displacement of accretionary prisms can be seen.

b) Faults along with a number of mechanisms of earthquakes occurred in the region

Faults parallel to the Makran fault can be seen in the sea.