Indo-Asian Collision, the Qaidam Basin, and Paleomagnetism

by
Elizabeth Wilson
Spring 2002

ES767 Global Tectonics   *    Emporia State University


NOAA/NGDC


Abstract   *    Introduction   *    Qaidam Basin   *    Paleomagnetism   *    Conclusion    *    References


Abstract

The tectonic evolution of the India-Asia collision provides great insight to the sedentary position of the Qaidam Basin, a region north of the Tibetan Plateau. Ordinarily, the rapid, progressive movement of India into Asia would indicate considerable deformation and rotations to the localities that absorb the impact. Observations of red sedimentary rocks of the Qaidam Basin provide alternative conclusions. Contrary to what one might think, the Qaidam Basin does not suggest any significant deformation or rotation with respect to the Kunlun Fault, to the south, or the Tibetan Plateau. Paleomagnetic data obtained from the hematite-rich sedimentary rocks reveal lack of movement and may be the result of crustal thickening of the Tibetan Plateau.

Introduction

Plate tectonic research has revealed the rapid movement of the Indian plate. Since the break up of Pangaea, some 200 million years ago, India is documented to have been moving northward at an estimated rate of 10-20 centimeters per year (Molnar, 1986). India is attached to a large oceanic plate, which began subducting at the southern Asia margin approximately 70 million years ago. The lithospheric slab melted as it subducted into the mantle. Magma arose forming volcanoes in Tibet and thickened the crust beneath. As sea floor spreading continued in the Atlantic, oceanic crust continued to be consumed at the Asia continental margin. This conveyor belt motion allowed for the quickened subduction of the Indian plate beneath Asia.

India Collision into Asia
USGS.
In the Cenozoic, the landmasses of India and Asia converged. Keeping in mind that continental crust lighter and more buoyant than oceanic crust, India resisted the subduction process. Plate movement slowed its ongoing speed to its approximate rate of movement today, 6 centimeters per year. As India collided into Tibet, a fault transpired which pushed subducting crust further down and northward beneath Tibet. Part of the continent split off, thrust up, and eroded to form the Himalayan Mountains. The Tibetan Plateau is a result of the thickened crust arising from the Indo-Asian collision and the successive collision of India into Asia. Crust under the Tibetan Plateau is thus young and soft compared to that north of it. In fact, its thickness is approximately 60 km thick.

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Qaidam Basin

Altyn Tagh Fault
Arizona State University. (Click image for larger view).

The Qaidam Basin, a low-lying region formed from the Indo-Asian collision, resides in west-central China, north of the Tibetan Plateau. Although it is most known for petroleum and salt mining, it is underlain with old, Precambrian rock, which was pushed northward upon the collision (Zhu and Helmberger, 1998).
Space Image of Western Qaidam Basin
Geomorphology from Space. (Click image for larger view).

Compared to the Altyn Tagh Fault to the northeast and the Kunlun Fault to the south, the Qaidam basin exhibits little deformation since the Paleozoic (Zhu and Helmberger, 1998). Stable, hard, resistant crust is what prevents altering stress and acts an unfailing block to vertical axis rotations.
Kunlun Fault
USGS. (Click image for larger view).
The Qaidam Basin contains deposits of Tertiary, red sedimentary rocks. Specifically, hematite-rich sandstones. This mineral is the essential element for paleomagnetic observations in this region.
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Paleomagnetism

To briefly summarize the concept of paleomagnetism, when igneous rock cools, iron minerals align to the Earth’s magnetic field. Evidence of paleopoles has demonstrated that the Earth’s magnetic field has varied with time, containing periods of reversed and normal polarity. The term Apparent Polar Wandering Path (APWP) is indicative of how a continent has moved with respect to its rotation axis. Rocks obtain remnants of the magnetic field as the continent moves. Therefore, if one assumes the continent to be fixed, it appears as though the poles wander, hence the name apparent polar wander. Through examination of the declination of paleopoles compared to the current magnetic poles, one is able to reconstruct continental movement.


Apparent Polar Wander Path
Australian National University.

In order to determine movement of the Qaidam basin relative to the Altyn Tagh and Kunlun fault, paleomagnetic pole core samples were collected. Such cores were acquired from hematite rich sandstones in different localities within the Qaidam Basin. During rock formation, iron oxides in this sedimentary rock became remnants of magnetic declination. Paleomagnetism cannot provide reference to longitudal movement, however subsequent collision of India confirms northward movement of India, Tibet, and the Qaidam Basin. Paleomagnetic samples, on the other hand, conclude that there has been no latitude change of the Altyn Tagh Fault or the Qaidam Basin. "Mean paleomagnetic declinations from two locations separated by several hundred kilometers with the Qaidam Basin indicate no vertical-axis rotation during the past 30 m.y." (Dupont-Nivet, et al., 2002).

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Conclusion

During the past 30 million years, the strong crust of the Qaidam Basin has blocked continental stress of the Indo-Asian collision. It has driven rotation of the soft crust, Kunlun fault, clockwise adding to its deformation. Paleomagnetic data of this region reveals a rotation of 15-30 degrees (Dupont-Nivet, et al., 2002). The Altyn Tagh Fault, to the north of the Qaidam Basin is a left lateral slip fault, which absorbs stress of the collision, hence intense deformation (Bendick, et al., 2000). Paleomagnetic data for this region does not indicate any rotation as it forms the northern boundary of the Qaidam Basin. However, north of this fault, the Tarim Basin, experiences clockwise, vertical rotation while the Qaidam Basin and the Altyn Tagh Fault remain locked.

The Qaidam Basin essentially acts as a block that resists the crustal flow of the Tibetan Plateau. The high, uniform elevation of the Tibetan Plateau is, to a large extent, the result of crust thickening in response to the Indo-Asian collision (Owens and Zandt, 1997). The process is such that the lower crust of the Tibetan Plateaus grows vertically and restricts deformation of the plateau beneath to raise it uniformly. Crustal thickening and northward restraint, in turn, results in an eastward extension of the plateau and the driving mechanism for rotation of the Kunlun Fault.

Overall, the tectonic evolution of the Indo-Asian collision, the facts on the formation and features of the Qaidam Basin, and the aspects of paleomagnetism contribute to the understanding of the Qaidam Basins lack of rotation.

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References

Arizona State University
1999 Evidence for and Geometry of Repeated Large Magnitude Surface Ruptures Along the Central Altyn Tagh Fault.
http://www.asu.edu.
Permission granted from Ramón Arrowsmith for image reproduction.

Australian National University
2002 GEOL3017 Fundamentals of Geophysics. http://geology.anu.edu.au/.
Permission granted from Chris Klootwijk for image reproduction.

Bendick, R., R. Bilham, J. Freymueller, K. Larson, G. Yin
2000 Geodetic Evidence for a Low Slip Rate in the Altyn Tagh Fault System. Nature 404:69-72.

Dupont-Nivet, G., R.F. Butler, A. Yin, X. Chen
2002 Paleomagnetism Indicates No Neogene Rotation of the Qaidam Basin in Northern Tibet during Indo-Asian collision. Geology 30:263-266.

England, P., P. Molnar
1990 Right-Lateral Shear and Rotation as the Explanation for Strike-Slip Faulting in Eastern Tibet. Nature 344:140-142.

Flesch, L.
2000 Dynamics of Asia. http://geophysics.geo.sunysb.edu/~flesch/.

McNeely, L.
2001 Geomorphology from Space. http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_HOME_PAGE.html.

Molnar, P.
1986 The Structure of Mountain Ranges. Scientific American 9:125-138.

NOAA/NGDC
2002 NOAA Satellites and Data: National Geophysical Data Center. http://www.ngdc.noaa.gov/mgg/image/images.html.
Permission granted from Carla Moore for image reproduction.

Owens, T.J., G. Zandt
1997 Implications of Crustal Property Variations for Models of Tibetan Plateau Evolution. Nature 387:37-42.

Scotese, C.R.
2002 Paleomap Project. http://www.scotese.com/Default.htm.

Tapponnier, P.
1999 The Tectonics of Tibet: Where do we Stand? Journal of Conference Abstracts 4:1.

Wittlinger, G., P. Tapponnier, G. Poupinet, J. Mei, S. Danian, G. Herquel, F. Masson
1997 Tomographic Evidence for Localized Lithospheric Shear Along the Altyn Tagh Fault. Science 282:74-76.

Zhu, L., D.V. Helmberger
1998 Moho Offset Across the Northern Margin of the Tibetan Plateau. Science 281:1170-1172.

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This webpage was designed for ES767 Global Tectonics
Instructor: Dr. James S. Aber of Emporia State University
For questions or comments contact Elizabeth Wilson
Created on Apr 1, 2002.