The use of gravity and aeromagnetic data to define the structural configuration of the Western Desert, Iraq

Abstract

A gravity and magnetic analysis was performed over the Western Desert (WD) of Iraq to determine the structural makeup of the Phanerozoic sediments that overlie Precambrian lithologies. The WD contains a series of basins that were formed during a series of Precambrian to Paleozoic compressional tectonic events. Regional and residual gravity and magnetic anomaly maps created by a variety of methods, and a CET lineament analyzes of these maps indicate that the basins were formed mainly by strike-slip and transverse faults. These faults may have been the reactivation of faults formed during Precambrian compressional tectonic events. The prominent Nukhaib graben was found to be a series of smaller grabens and horsts whose origin is probably related to reactivation of Precambrian faults.

Introduction

The Iraqi Western Desert region which covers approximately one third of Iraq (Fig. 1), occupies a portion of the Arabian Plate (Fig. 1). The Western Desert is located in the northern portions of the Arabian Plate and lies within a portion of the Arabian Platform which has been divided into the Arabian Shield and Shelf provinces (Powers et al., 1966). While the Arabian Shield does not outcrop in Iraq it is known from scattered drill holes (Mousa et al., 2017). The Arabian plate is bounded by tectonically active margins (Fig. 1) that originated at approximately 25 Ma when the Gulf of Aden and Red Sea rifts developed (Stern and Johnson, 2010)

From the time when it separated from Africa, the Arabian plate has rotated anticlockwise and drifted toward the north (Bird, 2002). In the process of this northward drift, the Arabian plate closed the Tethys seaway as it subducted underneath Eurasia (Stern and Johnson, 2010). This subduction system is one of the active tectonic margins that currently bounds the entire Arabian plate (Stern and Johnson, 2010). Of importance to this study are the northern and eastern margins of the Arabian plate with the Bitlis suture zone to the north and the Zagros fold belt (Fig. 1) and the ongoing arc volcanism in the Urumieh Dokhtar arc of Iran (Fig. 1) to the east (Fox and Ahlbrandt, 2002, Alavi, 2004). Additionally, the oblique collision between the Arabian and Eurasian plate in the northwestern Arabian plate is evidenced by the slip on the East Anatolian fault and post collisional volcanism in eastern Anatolia.

While the Western Desert is part of the Arabian platform and shield, little is known about the structural makeup of the Phanerozoic sediments and underlying Precambrian basement due to the lack of outcrops, deep drill holes and deep seismic surveys (Al-Bassam et al., 1995, Jassim and Goff, 2006, Aqrawi et al., 2010). To date, the most useful studies to investigate the basins and the Precambrian basement have been based on the analysis of gravity and magnetic data (Al-Banna, 1992, Najar, 1999, Jassim and Goff, 2006, Mousa et al., 2017). These potential field studies used a variety of map transformations (e.g., wavelength filtering, derivatives, polynomial trend surfaces) to isolate gravity and magnetic minima to infer the location of Phanerozoic basins and structures within the Precambrian. In one of the most complete studies, Mousa et al. (2017) used spectral analysis methods to determine the depth of the Phanerozoic sediments.

The aim of this study is to analyze the available aeromagnetic and gravity data in combination with the available geological mapping, drill holes and seismic reflection data to determine the structure of the Phanerozoic sediments and more importantly, the structural makeup of these Phanerozoic basins. To accomplish this, a series of gravity and magnetic maps will be constructed using isostatic residual gravity, decompensative gravity (DAM) and differential reduction to the pole magnetic (DRTP) maps. These maps will be further enhanced using wavelength filtering, horizontal derivatives, vertical integration and CET (Center for Exploration Technology) lineament enhancement methods. The transformed gravity and magnetic anomaly maps will be broken into regional and residual anomaly maps that represent anomalies formed by basement and basin density/magnetic susceptibility contrasts, respectively. The gravity and magnetic analysis will be used to understand the tectonic history of the Western Desert, and will provide useful constraints in performing more detailed seismic reflection studies that are crucial in petroleum exploration.