Earthquakes Illuminate Mantle Under Tibet
San Francisco, Calif. -- A new way of looking at seismic waves recorded at monitoring stations in Tibet can shed light on the structure of the mantle beneath this immense plateau, according to a Penn State researcher.
"Tibet is interesting because of its high elevation and because there, the Indian and Asian continents meet and form the Himalayas," says Dr. Kevin P. Furlong, professor of geosciences. "This makes it a very important site to study the role of plate tectonics in the evolution of continents.
"We tend to know very little about the deformation beneath the crust. If we have some insight into how the crust and mantle deforms where continents collide, we can better model these collisions."
Previous research looked at averaged events and found a nearly uniform orientation of the mantle rock in an east-northeastern direction. However, the assumption has been that the mantle rock is horizontal, and the only orientation directions are considered two-dimensional, according to Furlong.
"When we look at the individual rather than averaged events, we find that each station shows a different orientation pattern, which implies that the mantle rock is not horizontal, but oriented in three dimensions," Furlong told attendees at the fall meeting of the American Geophysical Union Conference today (Dec. 16) in San Francisco. "The rock is not just oriented by cardinal direction, but is also tilted."
Furlong is trying to determine the orientation of olivine in the mantle beneath Tibet. Olivine, the principal mineral in the upper mantle, is a mineral composed of magnesium, iron and silica, and is anisotropic.
Anisotropic materials exhibit different physical properties depending on direction. For example, the direction in which seismic waves travel through the material. In this case, seismic p waves travel faster in one direction than in the other, and seismic s waves split into two polarized components which travel at different speeds.
The difference between the arrival times of the fast and slow s waves and the polarization directions are caused by the distance traveled and the effect of anisotropy.
"In simple areas of the Earth, the anisotropy found in olivine is unimportant because the minerals crystals are oriented at random and the s wave times average out," says Furlong. "However, in highly deformed areas, this deformation provides directionality and a fabric in the rocks becomes apparent. If we can read this fabric, we can use it as a proxy for how the rocks deformed."
Working with Thomas J. Owens of the University of South Carolina, Furlong looked at individual seismic events that occurred in the southwestern Pacific and western North America but were recorded at one of the 11 monitoring stations in Tibet.
"These seismic events are the proper distance from Tibet so that the seismic wave must go through the Earth's core before it arrives in the mantle beneath Tibet," says Furlong.
By going through the molten core, the original s wave is converted to a p wave and then changes back into an s wave when it re-enters solid mantle.
"By going through the core, we know we have a nice wave form to begin with," says Furlong. "Then we can evaluate the effects of the deformed mantle localized beneath Tibet."
These effects include identification of three distinctly different patterns in the seismic data, which theoretically imply three different directions of deformation of the olivine in the mantle beneath different parts of Tibet. The patterns are similar when comparing stations in the northern, middle or southern parts of the areas, but differ from area to area.
In the region of Tibet sampled by the seismic transect, there appears to be a systematic pattern, although it is not yet certain that this pattern of mantle deformation extends throughout Tibet. However, theoretically, the olivine fabric should relate to the forces that existed during collision and the actual deformation of the mantle rock.
"In theory, we should be able to look at the orientation of the fast and slow paths in the olivine and determine the direction and amount of inclination of the rock," says Furlong. "In practice, we are combining the information we have on the mantle fabric with models of rock deformation to determine the plate tectonic processes which have led to the development of the Himalayan and Tibetan Plateau."
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