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