For fifty years, the Tibetan Plateau has been recognized as the largest topographic feature that perturbs atmospheric circulation. It serves as an ideal field laboratory for understanding the geodynamic processes that build high terrain. Accordingly, the growth of the plateau should have altered atmospheric circulation and therefore written an evolving paleoclimatic signature not only on eastern Asian regional climates, but on global climate as well. Despite many recent studies, we still do not know precisely when the Tibetan Plateau reached its current dimensions and how it perturbs atmospheric circulation. This project brings together geodynamicists, atmospheric scientists, and paleoclimatologists in a multidisciplinary study of the when and the how.
One of the major goals of the project is to quantify the extent to which Tibet has grown by crustal thickening, by thrust faulting and folding, by flow within the crust that redistributes material there, or by replacement of cold mantle lithosphere with hotter material (all in a state of isostatic equilibrium). Such quantification will take big steps toward the understanding of how high plateaus are built and how continental lithosphere deforms, topics at the forefront of geodynamics.
Determining how Tibet has grown will require determining when crustal shortening and thickening occurred, using basic field methods and modern laboratory techniques, and quantifying paleoaltitudes with new isotopic tools. Applying such paleoaltimetric techniques, however, requires an understanding not only of how the atmosphere transports isotopes, but how the evolving high terrain affected surface temperatures at times in the past. Even if the project?s focus were solely on how Tibet has grown, a meteorological component of the study, focused particularly on eastern Asia?s hydrological cycle, would be necessary.
Most continental paleoclimatic indicators are thought to be more sensitive to precipitation than to temperature, and among the unknowns of future climate, the hydrological cycle stands out. Accordingly, a major focus will be on understanding how high terrain like Tibet affects the hydrological cycle of eastern Asia, and China in particular. These studies will focus on: (1) how the plateau, as both a topographic obstacle and a sink for solar radiation, affects atmospheric circulation; (2) how the atmosphere transports stable isotopes (ä18O and äD); (3) how it affects mid-latitude climate variability, including how, via lee cyclogenesis, it lofts and transports dust, and (4) how vegetation feeds back on atmospheric circulation and the hydrological cycle. As links from geologic processes occurring at multi-Myr time scales to those on human time scales, the Principal Investigators plan studies that specifically examine paleoprecipitation over the past few hundred thousand years, using both loess deposition and speleothems that quantify paleoclimate.
Regional to global scale interactions between climate and tectonics come from areas where the uplift of high topography occurs over thousands of kilometers. Despite more than 30 years of study of continental dynamics, the geodynamic processes responsible for broad areas of uplifted and deformed continental lithosphere remain controversial. The uplift of these wide, deformed regions depends on the material properties and deformation mechanisms of the deepest part of tectonic plates, which exert a primary control on how topography develops and how large of an area deforms by faults. Topographic change directly measures this deep-seated deformation and reflects the rheology of the lithosphere and driving forces of deformation. The Tibetan Plateau, the world’s highest landmass, continues to grow as a consequence of movement of the Earth’s tectonic plates. The continental "collision" of the Indian and Eurasian plate began 50 million years ago and continues today. During this time, the plateau became elevated to its modern height (5 km) due to the convergence of the plates and thickening of the continental crust to twice its normal thickness. The enormity and longevity of this active continental collision zone attracts many studies in the field of geology and geophysics. Our project focused on the mechanical growth of the northern Tibet and its consequences for tectonic-climate interactions. Many accept that the southern Tibetan Plateau was probably a high mountain range even prior to continental collision, much like the Andes Mountains today. However, much less is understood about when and how northern Tibet came to be. In this project, we have studied the age of several fault systems across the northern plateau. Folding and faulting of the crust in northern Tibet appears to coincide with the beginning of collision. This means that collision occurred across a wider region than previously understood, more than several thousand kilometers from the plate boundary. The distance over which deformation occurred at the time of plate collision and since helps us determine the strength of the rocks in the Earth’s crust and uppermost mantle. Interestingly, the rocks in northern Tibet record shortening and thickening of the Earth’s crust only during the early stages of continental collision, with little to no shortening in the last ~ 30 million years. Also, the thickening of the crust recorded by these faults is less than would be explained by the modern thickness of the crust beneath northern Tibet. This means that the elevation of the plateau was formed, at least in part, long ago and possibly much earlier than we understood previously. As well, some of the elevation of the plateau must be caused in part by forces acting in the deep crust or upper mantle because of the shortage of significant faults at the Earth’s surface.
