Geodynamic Analysis of the Zagros Foreland, Iraq

Schafer, Kirk ¹; DeCelles, Peter G.²; Kendall, Jerry ¹; Tai, Po ¹; Wyer, Paul ¹
(1) ExxonMobil Exploration Company, Houston, TX. (2) Department of Geosciences, University of Arizona, Tucson, AZ.

The evolution of the Zagros foreland basin system in Iraq is inferred based upon regional analysis of subsurface data coupled with both forward gravity modeling and 1-D subsidence analyses.

The proximal part of the Neogene foreland basin system consists of up to ~1 km of terrestrial sediments overlying the outer Zagros orogenic wedge. The wedge-top sediments transition southwestward into 2-3 km of foredeep sediments that taper onto the flank of the forebulge. In western Iraq topography rises to over ~300 m and averages ~700 m in Saudi Arabia. These high elevations mask the expression of the backbulge and are suggestive of an additional longer-wavelength geodynamic process (in addition to flexural isostasy).

A forward 2-D flexural-isostatic gravity model further constrains the flexural-isostatic signal associated with the Zagros Mountains. Modeling of the observed topographic and sediment loads predicts a flexural basin 200-300 km wide with a maximum deflection of 2-3 km and a 300-400 km wide forebulge centered along the southwestern border of Iraq. Despite a regional fit between the observed and predicted gravity, the model under-predicts the topography of western Arabia by as much as 1 km. The isostatically-balanced residual topography confirms a longer-wavelength process supporting this topography.

Paleogene counterparts to the Neogene foreland basin system are identified based upon borehole stratigraphic analysis. In northeastern Iraq, Early Miocene (Burdigalian) shallow-marine and terrestrial sediments unconformably overlie Late Eocene platform carbonates. The missing to highly-condensed interval (~34-19 Ma) is interpreted as an expression of a forebulge, which implies the onset of Arabia-Eurasia collision and the establishment of an orogenic wedge by Late Eocene.

The position of the present-day forebulge (western Iraq) relative to the position of the Oligocene forebulge (northeastern Iraq) implies ~350 km of flexural wave migration since Late Eocene providing an average migration rate of ~10-11 mm/yr. 1-D subsidence profiles for the Oligocene and Miocene time intervals show an abrupt increase in subsidence of the inferred backbulge nearly synchronous with both the onset of foredeep subsidence observed in northeastern Iraq (~19 Ma) and Red Sea rifting. The additional backbulge subsidence could be related to far-field geodynamic processes that modify the distal expression of a migrating flexural wave by wholesale eastward tilting.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.