Subsurface Structure of the Eastern Edge of the Zagros Basin as Inferred from Gravity and Satellite Data

Abstract

A data set of 10,505 points of land gravity measurements from southeast Iran obtained from the Bureau Gravimetrique International, combined with Landsat imagery, was used to investigate crustal and Cenozoic lithospheric structure. Interpretation of the Bouguer anomalies reveals three primary structural features. The Zagros Mountain belt is characterized by a progressive decrease in gravity values from -70 mGal near the Persian Gulf to -160 mGal over the suture zone between the Arabian margin and central Iran crustal blocks. The second feature is marked by a backward-L-shaped pair of anomalies that extends from the eastern peripheries of the Zagros basin and wraps around southern Iranian shores. These 15- to 20-km-deep source anomalies, with amplitudes of as much as 10 mGa , are interpreted as intrabasement intrusions demarcating an ancient rift axis. The shallow (6-8 km) east-west-trending anomalies are perhaps interbasement uplifts bordered by reverse faults. The third structure, observed on both gravity and Landsat displays, a north-striking eastward-facing topographic escarpment, has a gravity gradient of 0.85 mGal/km, and is right laterally offset approximately 100 km by the Zagros main recent fault. On Landsat, this escarpment is manifest by a system of north-trending faults that seem to diverge into a horsetail pattern toward the east, and by a set of north-trending dike swarms that cut Jurassic volcano-sedimentary rocks. It separates two distinct structural and sedimentary styles, the Zagros foreland basin anticlines of shallow marine Precambrian to Pliocene rocks on the west and the Makran deep-water Eocene flysch belt on the east. A comparison of gravity features with surface structures on Thematic Mapper and Landsat Multispectral Scanner imagery indicates that a northeast-trending fault system is the result of post-Miocene pervasive transpressive stress coupled with clockwise rotation of underlying basement blocks following the collision of Arabia and Iran. Accommodation structures such as forced folds and rabbit-ear anticlines may develop over and on the flanks of the basement blocks, providing remigration and trapping mechanisms for new oil and gas plays.