The Main Lineament Trends of Central Iraq, Depending on the THD of Residual Gravity and Magnetic Maps Obtained from Four Upward Continuation Levels

Abstract


Introduction
The Geological and geophysical data are essential in providing information about the surface structural features (folds and faults).The gravity method involves the measurement of variations in the gravity field of the earth which is produced by local differences in the density of the subsurface rocks (McDowell et al., 2002).The magnetic method is one of the oldest methods used to explore the subsurface and is considered one of the potential methods exploited for detecting the magnetic susceptibility of subsurface rocks, potential methods used to measure the magnetic susceptibility of materials (Dobrin and Savit, 1988).Generally, most of long continuous natural geological features, such as folds or faults, can be reflected on the geophysical 2D surfaces or maps as lineaments, so these methods are usually used to detect the faults and their trends in the basement (Hung et al., 2005).
The Arabian plate (including Iraq) suffered many events from the pre-Cambrian era to the present, which passed through many movements such as Kibaran Orogeny, Hijaz Tectonic Cycles, Nejd Orogeny, Caledonian, Acadian Orogeny, Hercynian, Early Kimmerin, Late Kimmerin, Austrian Orogeny, Subhercynien, Laramide, Late Turonian Orogeny, and Alpine Orogeny (Numan, 1997;Jassim and Goff, 2006).The process of collision of the Arabian Plate with the Eurasian Plate began during the Early Tertiary and their action continued and featured within the modern tectonic state, particularly in Zagros Thrust faulting belt.The tectonic action reflected on the surface as mountainous features in the eastern and northeastern parts of Iraq in modern tectonics (Zagors Thrust Fault), represented at the surface by mountain features in the eastern and northeastern parts of Iraq.This tectonic action was also effective in developing the sedimentary basins in central Iraq (Buday and Jassim, 1987).The tectonic processes control the formation and diversity of structural characteristics within the earth's crust and its evolution through geological times, which include the movement of continents, the building of mountains, the faults, and other geological processes within the earth's crust (Sharland et al., 2001).Gravity Method is sensitive to density contractions within the sub-surface geology, so it is ideal for exploring major sedimentary basins with a significant density contrast between the lighter sediments and the denser underlying rocks (Blacky, 1995;Reynolds, 1997).
This research aims to investigate the lineament trends and boundaries of geological structures in central Iraq using aeromagnetic and Bouguer gravity data and applying the Total Horizontal Derivative (THD) technique.

Location of the study area
The study area is located in the Middle of Iraq, within latitudes of 30.7° to 34.5° N and longitudes of 38.8° to 45.7° E. It represents a strip-like crossing of Iraq transversely in the NE-SW direction.It extends from the Rutba tectonic subzone of Western Iraq (near the Iraq-Jordan and Saudi Arabia borders) passes through the Mesopotamian zone towards the east, and ends in Khanaqin (near the Iraq-Iran border) (Fig. 1).

Geology and Tectonic of the Study Area
The geology of the study area is characterized by Mesozoic formation on the west, and Quaternary sediments on the eastern side (Jassim and Goff, 2006).The regional geology of the study area is characterized by the dipping strata toward the northeast, which coincide with the surface land slope.At the same time, the sedimentary layers from Rutba town dipped toward the west (Buday, 1980;Konert et al., 2001;Jassim and Goff, 2006;Aqrawi et al., 2010;Sissakian and Fouad, 2015).Al-Ma'aniyah Depression area is located above Nukhaib Graben on the stable shelf in the southwestern part of Iraq within Al-Najaf province.
The interpretation of gravity and magnetic data in Western Iraq indicates that the basement rocks consist of acidic or ultra-basic regional groups (Al-Banna, 1999).The basement rocks of central Iraq are composed of Granite, Granodiorite, Schist, and phyllite rocks (Jassim and Goff, 2006).A group of many formations from Late Permian (Ga'ara Formation) on the west side to Quaternary sediment on the eastern part, were outcropped at the surface of the study area (Jassim and Goff, 2006;Sissakian and Mohammed, 2007) (Fig. 2).
Fig. 2. The geological map of Iraq shows the study area in the red polygon (modified after Jassim and Goff, 2006).
Tectonically, the study area is divided into two units, the stable shelf in the west and the unstable shelf in the east.The stable shelf is divided into two zones, the Rutba-Jezira Zone to the west and the Salman Zone to the east (Buday andJassim, 1984 and1987), at the same time it referred to this part as the inner platform (Fouad, 2015;Al-Banna and Ali, 2018).The eastern part of the study area represents the unstable shelf or outer platform Mesopotamia Basin from the tectonics point of view (Fouad, 2015;Abdulrahim et al., 2022).
The derived Faults from various sources including satellite images, gravity and magnetic gradients, and, to a lesser extent seismic data (Jassim and Goff, 2006).The Total Horizontal Derivative (THD) of gravity was found to be the most useful parameter to detect the fault.The main tectonic fault that crosses the study area is the N-S Nabitah (Idsas) System.The NW-SE Najd System is represented in the study area by the Tar Al Jil Fault, Euphrates Boundary Fault, Ramadi-Musaiyib Fault, Tikrit-Amara Fault, and Makhul-Hemrin Fault.The NE-SW or E-W Transversal System is resented by the Amij-Samarra, Sirwn, and Kut-Dezful faults (Fig. 3).These fault systems formed during the Late Precambrian Nabitah Orogeny.They were reactivated repeatedly during the Phanerozoic (Jassim and Goff, 2006).

Data Acquisition
The gravity data of Iraq was reprocessed by the British Getech Group (Getech, 2010).Oil Exploration Company (O.E.C.) (2005) concluded that the Iraqi regional gravity map of the sedimentary basin dipped towards the east.The Bouguer gravity anomaly map of the study area of grid 1X1 kilometers shows gravity values ranging between 9 to and -97 mgal, (Fig. 4).The French company carried out the aeromagnetic surveys in Iraq in 1974 at 140 meters above the topographic surface, and the data is processed and interpreted to construct the basement depth map of Iraq (Getech, 2010).The aeromagnetic data is reprocessed and reinterpreted by Getech for the oil exploration company and the geological survey of Iraq (Getech, 2010).The Total Magnetic Intensity (T.M.I.) values in the study area ranged between 4766 and 5230 nT (Fig. 5).The Oil Exploration Company (2005) supplied the gravity and magnetic data.The effect of a magnetic dipole source in an aeromagnetic anomaly map converted to a one-pole anomaly source using a mathematical process called the Reduction to Pole (RTP) technique by using the Oasis Montaj Program.The inclination with value (50.1) and Declination angles (4.47) were calculated through the application of the International Geomagnetic Reference Field (IGRF) Program (Baranov, 1957).The RTP map, which is derived from the T.M.I. magnetic map of the study, shows magnetic values range 4836 to 5219 nT (Fig. 6).

Total Horizontal Derivatives (THD) Technique
Total horizontal derivative (THD) is used to identify the subsurface of the fault structure body.After applying the upward continuation filter to calculate regional anomalies and subtract from the Bouguer and RTP Magnetic maps to obtain the residual anomalies.The maximum THD values delineate the lineaments related to the body boundaries or faults (Cordell and Gauch, 1985;Al-Rahim and Abdulrahim, 2021;Al-Dulaimy and Al-Banna, 2022).
The total horizontal derivative is calculated either by dT/dx and dT/dy, or by the second horizontal derivative d 2 T/dx 2 and d 2 T/dy 2 .The THD is usually used to delineate magnetic and gravity contact.The Total Horizontal Derivative (THD) is determined as in the equation below (Blakely, 1995): THD=√(/) 2 + (/) 2 (1) Where: T is the anomaly, dx: are horizontal derivatives in the x direction, dy: is horizontal derivatives in the y direction.
The Total Horizontal derivatives were obtained from the study area's residual gravity and magnetic data according to the following techniques, Geosoft Oasis Montaj, Version 8.4, 2015 (Gravity and

Results
The upward continuation filter was applied to obtain the regional and residual anomalies maps of the gravity and RTP magnetic data for four elevations: 2, 12, 16, and 22 km.The residual gravity maps show positive and negative anomalies (Oasis Montaj, 2015;Arc Map 10.7, 2018;and Rockworks 16, 2023).
The locations of the prominent positive and negative anomalies in gravity and magnetic residual maps are not change, indicating that these anomalies many be related to the deep source (Figs. 7 and 8).The THD maps were used to delineate the lineaments of gravity and magnetic data for four upward elevation residual maps (2 km, 12 km, 16 km, and 22 km) (Figs. 9 and 10).The upward continuation technique is used to obtain the regional anomaly maps in the first stage and then determine the residual anomaly maps in the second stage, using four levels to separate the causative sources at various depth levels.A rose diagram was made for the lineaments to obtain the main trends (Figs. 11 and 12).The results of the gravity and magnetic lineaments trends were summarized in Tables (1 and 2).According to the application of the THD technique on the residual anomalies of gravity and magnetic data at various levels, the faults are traced and related to different depths.The maximum values of THD increase within deeper levels, which indicate the increase of lateral variation in density of deeper lithology (mostly within the deeper rocks (basement).

Discussion
The upward continuation method processed the gravity and RTP magnetic data to separate the residual anomalies from regional anomalies.The residual gravity and RTP magnetic with 2km, 12km, 16km, and 22km upward levels were obtained (Figs. 7 and 8).These maps were processed by the THD technique to delineate the maximum values, which may be related to the boundary of sources or faults, from gravity and magnetic data at certain levels, respectively (Figs. 9 and 10).The maximum values of THD were traced as lineaments in Figs.(9 and 10).These lineaments were plotted as a rose diagram.The main trends appear from gravity data in descending order as N55W, N45W, N35W, N-S, and N65W, with a general trend of NW-SE in deep sources related to the Najd fault system (Table 1 and Fig. 11).
The number of lineaments and their trends from magnetic data reflect the main trend in descending order as N-S, N55W, N35W, N45W, N45E, N35E, N65W, and N55E (Fig. 12).The lineaments trend from magnetic data at 2 km is NW-SE, while at other levels, concern with deeper sources shows that the general trend is N-S (Table 2).The N-S trends of lineaments of the deeper sources related to the Nabitah fault system.Finally, the main lineament trends obtained for shallow sources within the upper 2 km from gravity and magnetic data analysis indicate that the main trend is NW-SE.The deeper levels show more fault numbers in gravity and magnetic processed maps, this may be due to the relatively high lateral variation within the density or susceptibility in the deeper level (basement and deeper sedimentary rocks).

Conclusions
The total horizontal derivative of gravity and RTP magnetic residual anomaly maps were calculated.The lineament trends were determined using 2, 12, 16, and 22 Km upward continuation elevation levels in the central part of Iraq.Many conclusions were obtained, including: The lineament from gravity data has been classified into five orders.These are N55W, N45W, N35W, N-S and N65W, respectively The dominant lineament trend of gravity at shallow and deep sources from all upward elevation levels is N55W, with a general trend of NW-SE in the basement and all sedimentary columns which is related to the Najd fault system.The deeper levels show more fault numbers in gravity and magnetic processed maps, this observation may be due to the relatively high lateral variation in density in the deeper level (basement and deeper sedimentary rocks).
The lineaments of RTP magnetic data are classified into eight orders: N-S, N55W, N35W, N45W, N45E, N35E, N65W, and N55E.The dominant trend at shallow sources (2 km) is N55W, while the dominant trend was NS at 12 km, 16 km, and 22 km upward levels (related to deeper sources).
The N-S lineaments, from the magnetic data analysis, appear in deep structures and may related to related to the Nabitah system (680 Ma).This trend is found in basement rocks and all the sedimentary cover, which belongs to the mega-sequences 1-11.

Fig. 1 .
Fig.1.Location map of the study area.

Fig. 3 .
Fig. 3.The Tectonic map of Iraq shows the study area in the red polygon (modified after Jassim and Goff, 2006).

Table 1 .
The main lineaments trends of the study area, those obtained from THD of residual gravity deduced from upward continuation elevations 2, 12, 16, and 22 km.

Table 2 .
The main lineaments trends of the study area, those obtained from THD of residual RTP Magnetic deduced from upward continuation elevations 2, 12, 16 and 22 km.