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Paleomagnetic Study Of Some Outcropping Rocks Northeastern Iraq And Its Tectonic Applications

19-09-2013


 Emad H. AL-Khersan

Department of Geology, College of Science, Basrah University
Email: emgeoph@yahoo.com 
 
Abstract
 
   A Paleocene-Lower Eocene Naoprdan limestone Formation at Chwarta and Zainal localities, Maastrichtian Aqra limestone Formation at Maukaba and Zardabe localities, Valanginian-Turonian Balambo limestone Formation at Azmar locality and igneous gabbros  intrusions at Kanaroe and Waraz localities were sampled for paleomagnetic investigations. 
(12) hand samples and (200) oriented, mostly limestones, drill cores were collected from these localities, which are situated at the northeast part of Iraq.
 
   The NRMs were measured using Spinner and Cryogenic Magnetometers that are available in Baghdad and Oklahoma Universities. Thermal and alternating field demagnetization were applied on (39) pilot specimens. Following stepwise thermal demagnetization procedures, two main magnetic components were determined; u a low-temperature-LT component at (20-200)ºC that is regarded as overprint secondary unstable magnetic component with magnetic direction around the present Earth’s field; and v medium - high temperature (MT and HT) component at 
(250-600)ºC which is carried by high coercivity magnetic grains. It shows stable magnetic component. Identification of the magnetic minerals which existed within the studied rocks was made by the assistance of the isothermal remanent magnetization technique for seven samples 
conducted in Oklahoma University. It reveals that the remanent magnetization in the Aqra limestone Formation is of a depositional origin and carried by a detrital magnetite grains. Rock magnetic analysis indicates that the primary magnetite is the dominant remanence carrying minerals observed in Maukaba and Zardabe specimens. In Chwarta, Zainal, Azmar, Kanaroe and Waraz localities, secondary haematite or maghemite is dominant. Chemical alterations could happened through the geologic time. We can say that the outcrop rocks of these localities are not good carrier for the paleomagnetic direction. They need more investigation and isolation of their magnetic components which could be related to some geological events in the past, and it can not be used for the predication of the paleo-position or the tectonic movements in the area. Therefore, only results from Maukaba and Zardabe rocks can be consider as reliable paleomagnetic results. Both of these rocks show stable and reverse paleomagnetic directions.
 
   The overall mean tilt-corrected paleomagnetic directions obtained in this study are Dec=140.7º, Inc=-26.5º, K=31.9, N=20, a95=4.5º and Dec=169.3º, Inc=-26.3º, K=28.5, N=25, a95=6.4º for Maukaba and Zardabe localities respectively. All computed VGPs for these two localities correspond to a reverse  polarity, and the overall mean VGP position of the Maukaba locality is Plat=44.4º S and Plong=279º, which is differ from that of the Zardabe locality (Plat=57.1º S and Plong=235º) with co-latitude (-14º) and (-13.9º) for these localities respectively. Accordingly, the paleo-positions of Aqra Limestones basin on Maastrichtian age are 14º Nlat and 13.9º Nlat. It seems that the Neotethys ocean to the north and northeast of studied area was still there during Maastrichtian age. The closure of this ocean apparently happened some time between the Maastrichtian and Lower Tertiary. The rotational movement of Afro-Arabian plate towards the northeast part of Iraq causing the closing of the ocean and then the collision with Iranian and Turkish micro plates. This movement caused the Arabian plate to move about 20 degrees in latitude. Then this will lead to the latitude positions of 34º N and 33.9º N for Maukaba and Zardabe locations. However, the present locations of these two localities are 35º 40¢ N and 
35º 50¢ N. It noticed that there is about 2 degrees difference between the present locations and the above calculated locations. This difference could be related to the model of the geocentric axial dipole and its related calculations or it could be related to other plate movements in Miocene ages, i.e. the opening of the Red Sea. The paleo-position of the Aqra Limestone depositional basin clearly suggests that the north part of Iraq was still in the warm environmental conditions near the equator during Maastrichtian times. This means that the oil accumulation can be found in rocks of ages for Maastrichtian and older than Maastrichtian.  The paleo-pole positions of Maukaba and Zardabe localities showed clear difference in their longitude values too, which reflect the difference in the declinations of the magnetic directions of these rocks. The difference in the declinations probably due to the existed transverse faults and local tectonic movements. These movements usually indicated by the divergence of fold axis of Azmar anticline. 
A reconstruction of this fold axis to its previous lineation resulted in a coincidence of the Maukaba and Zardabe magnetic declinations to form a single lineation represents the Azmar fold axis before divergence. This clearly indicates that the folding and the divergence in the fold axis happened after Aqra Limestones rocks acquired their primary magnetic directions.  In this study, Aqra Limestones rocks are considered as very good rocks for the purposes of paleomagnetic studies, because they recorded the paleomagnetic directions and kept them through the geologic times. Therefore, this formation can be considered as a key horizon used for paleomagnetic and local or even regional tectonic studies. This will help in reconstructing models for the tectonics and the structures of the north part of Iraq.
 
 
Phd. thesis presented By Emad H. AL-Khersan ,  Baghdad University, 2007
Supervised by; Dr. Basim R. Hijab /  Dr. Ezzadin N. Baban.

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