An Integrated Investigation of Ultrahigh-Pressure (?) Metamorphism, North Qaidam, China

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

A recent discovery by geologists is that continental crust could be buried as deep as >90 km. This is a rather astonishing revelation because continental crust is typically about 30-35 km thick, and to have it buried below a depth of 90 km would require the continental crust to be surrounded by denser and fluid upper mantle rocks. This is rather difficult in physics because it requires the lighter continental crust to be dragged into the denser mantle materials. Exactly what forces have made the occurrence of this unusual process possible remains an active debate. A team of geologists from the University of California is currently investigating this problem. Because there are a variety of likely processes that may have induced continental rocks to decent to great depths, it is important to examine the rocks that bear the record of such deep burial in as many places on earth as possible. Another reason for geologists to examine as many cases of rock records of deep burial is that individual areas may not preserve a complete record of the burial process. Therefore, geologists as detectives, must use as many cases as possible to piece the complete history together. The study area that is currently under investigation is located in North Qaidam of the northern Tibetan plateau. This is a desert region exhibiting excellent exposure of rocks. Their research includes the following components. (1) Making a detailed geologic map of rocks that record deep-burial processes. The geologic map allows the naturally occurred geologic elements to be placed into a rigorous spatial context. As a result, the geometrical relationships among the different types of rocks can be systematically examined. (2) Sampling and petrologic analysis. Numerous samples collected from the field area are currently being analyzed for their mineralogical and chemical compositions using a variety of the state-of-the-art techniques and instruments. This analysis determines the exact depth that the rocks were buried and the temperature history associated with the rock burial. (3) Determination of age, duration, and rate of rock burial. This task is being carried out using the ion-microprobe and noble gas analyses. The main purpose of this investigation is to determine how fast the rock burial took place and how long it has taken to bring the rocks back to Earth's surface. The final completion of this research project will allow geologists to determine the forces that caused the down plunging of the light continental crust into the denser upper mantle, which in turn will reveal the fundamental aspects of physics and chemistry for the formation and evolution of Earth's crust that is central to Human's day-to-day livelihood.