Collaborative Research: Experimental and theoretical characterization of rapid Jurassic true polar wander

Harvard University, Cambridge MA

Investigators

Abstract

The motion of continents throughout the geologic history of the Earth strongly affects a vast number of processes on its surface. Among the effects of continental drift are the formation of mountain belts and volcanoes, the spread and extinction of biodiversity, and changes in climate at the local and global scales. Continents on Earth may move via one of two fundamentally different processes. The more familiar mechanism, known as plate tectonics, involves the differential motion of continents relative to each other. The second, less well-understood process is known as true polar wander (TPW), wherein the entire Earth rotates as a single unit such that the location of the present pole is transferred to a lower latitude. Theoretical studies show that TPW should have strong effects on the global environment, including regional sea-level changes of >100 m and drift of land surface across multiple, contrasting climate zones. However, the cause, rate, and even the existence of large-scale TPW events in Earth history have been controversial. Here we will measure the magnetism of rocks from the Late Jurassic (~165-150 million years before present) and pursue theoretical geodynamical computations to characterize the motion of the Earth during this most recent proposed episode of TPW. The results will have important implications for understanding the drivers of observed sea-level and climate changes throughout geologic time and for elucidating a fundamental process by which the global geography of the Earth evolves. This project is divided into two closely coupled parts that use complementary techniques to understand Jurassic TPW. First, the investigators will collect paleomagnetic rock samples from the La Negra Formation of Northern Chile and the Chon Aike province of Patagonian Argentina to quantify the position of continents during the 165-150 million year ago (Ma) interval, which has been identified by previous studies as the most recent time interval of potentially large amplitude (about 30 degrees) TPW. These rock units represent the most continuous deposits of igneous rocks from the candidate TPW interval, which implies the highest likelihood of recovering high precision paleogeographical information using paleomagnetic techniques. As part of this experimental component of this project, the team will collect samples for geochronological analysis using Ar-Ar and U-Pb in zircon techniques, which would provide more reliable rates of motion during this time span. Second, they will simulate the polar wander of the Earth using existing geodynamical code that accounts for the motion of mass anomalies in the mantle and lateral variations in the lithospheric strength of the Earth. A second, coupled geodynamical code will compute the expected changes in regional sea-level caused by the TPW motion. Combined with experimental data, these models will narrow the range of possible drivers of Late Jurassic TPW and evaluate its potential effect on records of climate and sea-level.