THE Himalayas get their height from India, and we aren't talking about genes. About 50 million years ago, the Indian subcontinent collided with Asia, and the two continents continue to converge at a rate of about 5 centimeters every year. The ongoing collision has been violent enough to push up the Himalayas, shove Southeast Asia further and further southeast, and perhaps most impressively, raise the Tibetan Plateau-a landmass as large as two-thirds of the lower 48 states-to an average elevation of 5,000 meters. Uplift of the Tibetan Plateau has been linked to intensification of the Asian monsoon and, by virtue of its erosion products, to gradual changes in seawater chemistry over long time periods. The Indo-Asian collision thus provides not only a natural laboratory for studying the mechanical response of Earth to plate tectonic forces but also an opportunity to explore the links between tectonics, climate, and ocean history.|
Livermore geophysicists Rick Ryerson, Jerome van der Woerd, Bob Finkel, and Marc Caffee, along with collaborators from the University of California at Los Angeles and from Paris and Beijing, have been studying this terrestrial wrestling match for several years, making the first-ever measurements of long-term movement along large faults in northern Tibet. The Kunlun, Altyn Tagh, and Haiyuan faults are strike-slip faults that allow blocks of Earth's crust to slide past one another, often with disastrous consequences. All of these faults have experienced large earthquakes ranging in intensity from 7.5 to 8.7.
The function of the faults is a subject of considerable geophysical controversy. Faults may define major discontinuities in Earth's lithosphere (the outer 100 kilometers of the crust that define the plates in plate tectonics) and thus absorb a significant portion of the convergence between India and Asia. Or they may be shallow features that play a secondary role in a more fluid lithosphere. Some research indicates that the Kunlun and Altyn Tagh faults extend to the base of Earth's lithosphere, suggesting that they indeed define continental plates. A first step in deciding the faults' extent and function is to obtain accurate, long-term slip rates at enough sites along the faults to characterize their large-scale behavior.
AMS and the Dating Game|
To derive rates of motion along faults, scientists first identify a site where lateral offset has occurred and then measure the offset and determine its age. Commercially available satellite imagery, with resolution to 10 meters, allowed the team to select regions where tectonic offsets are best preserved, such as abandoned stream beds and surfaces formed by glacial action. As shown in the figure below, the team took measurements at several sites where the faults cross alluvial fans formed by melting glaciers. As a glacier shrinks, the stream running from it becomes narrower, leaving behind an older, wider streambed in a series of terraces. The boundaries between different terrace levels represent lateral offset markers.
To determine the age of these surfaces and thus a slip rate, the team relied on experts from Livermore's Center for Accelerator Mass Spectrometry. Conventional mass spectrometry measures the concentrations of different isotopes of the same element. Accelerator mass spectrometry (AMS) does the same job but is much more sensitive than conventional mass spectrometry. The most common dating method is to measure carbon-14 relative to other carbon isotopes. AMS, for instance, can find one atom of carbon-14 in a quadrillion other carbon atoms, which means that extremely small samples can be studied.
However, in high, arid mountain ranges, fossil organic remains are often hard to find. Moreover, the ages of some surfaces may be too old to measure by carbon-14 methods. Therefore, for its first study, on the Kunlun Fault, the team compared slip rates obtained through radiocarbon dating with those obtained by measuring the cosmogenic nuclides beryllium-10 and aluminum-26 in quartz rock. Cosmogenic nuclides are produced through the interaction of surface samples with cosmic rays. Whereas carbon-14 levels fall over time, the levels of cosmogenic nuclides rise the longer a sample resides at Earth's surface. But the amounts they contain are still small. It has only been in the last 10 years, with advancements in such techniques as AMS, that measuring cosmogenic isotopes has been possible at all.
Under optimal conditions, samples taken from the surface will yield the true age of the surface. But a surface may also contain samples that were previously exposed to cosmic rays. Or rocks may simply roll downhill and contaminate a previously abandoned surface. Scientists must, therefore, collect many samples, both buried and from the surface, to account for all sources of contamination and derive a site's true age.
For the Kunlun studies, slip rates derived from beryllium-10 and aluminum-26 ages compared extremely well with those from radiocarbon dating.
What the Data Say|
The figure below, showing one alluvial fan along the Karakax Valley segment of the Altyn Tagh Fault, is an example of the many sites studied on both faults. The fault is clearly visible in part (a) of the figure as is lateral offset of terrace levels along the fault. Parts (b) and (c) show two interpretations of the evolution of this site.
Measurements at 10 sites along the Altyn Tagh Fault yielded slip rates as high as 3 centimeters per year in the west, decreasing to rates below 1 centimeter per year at the fault's eastern end. The rate decreases as lateral movement in the west is transformed into the vertical uplift that has created young mountains in northeastern Tibet. In contrast, using on measurements at six sites along a 600-kilometer length of the Kunlun Fault, the team found a uniform slip rate of about 12 millimeters per year over a time span of 40,000 years.
Comparison of the two faults suggests that the Kunlun Fault may be a more mature version of the Altyn Tagh. While the Altyn Tagh Fault is still in the process of propagating eastward, piling up new mountains along its bow, the Kunlun's movement appears to be fully transferred to other faults to the east. But Ryerson is quick to add, "We don't really understand yet what is happening at the eastern end of the Kunlun Fault."|
These data indicate that the birth and growth of strike-slip faults has been moving north with time, suggesting that the northern portion of the Tibetan Plateau has been uplifted by successive episodes of eastward fault propagation coupled with the uplift of young mountain ranges. Sediments from the young mountain ranges accumulate in closed basins that are in turn uplifted by fault movement. Ironically, much of one of the greatest mountain ranges on Earth, the Tibetan Plateau, may have been built not of mountains but of basins.
The relatively high slip rates observed along the Altyn Tagh and Kunlun faults are consistent with the models showing that these faults play an important role in accommodating Indo-Asian convergence. Livermore's data indicate that the models representing the lithosphere as a fluid may be flawed.
The first stage of this work, assessing the slip rates on active faults in northern Tibet, is nearing completion. Barely begun, however, is the next stage-extrapolating these observations of active faulting back to the early history of the collision. Meanwhile, continents continue to collide in Tibet.
Key Words: accelerator mass spectrometry, cosmogenic isotopes, dating techniques, faults, plate tectonics, Tibet.
For more information contact Rick Ryerson (925) 422-6170 (firstname.lastname@example.org).