Rates and mechanics of fold uplift and lateral bedrock planation in convergent foreland basins
Tectonic uplift and subsidence of rocks, together with their erosion, redistribution, and deposition by atmospheric, biologic, and gravitational processes shapes the topography of Earth’s surface. Studying the rates and mechanics of such processes and how they vary in space and time is critical to understanding the dynamic environment in which we live.
Topographic change is most dramatic along convergent plate boundaries in continental settings. Here, single faults and folds can uplift at rates of several millimeters per year and can present a significant seismic hazard. Understanding the rates of growth and propagation of contractional structures is integral to an assessment of such hazards, as well as to modeling the development of continental collision zones. Whereas point measurements of deformation rates are commonly possible, constraining the full temporal and spatial evolution of structures remains challenging.
Competing with rock-uplift that elevates Earth’s surface, landscape lowering is driven by the incision of rivers into bedrock. Rivers can also bevel laterally into bedrock, thereby planating topography and creating topographic markers, such as fluvial strath terraces, which are commonly used to infer climatic and tectonic changes. Significant uncertainties remain in understanding the mechanics of such lateral erosion by rivers and of strath-terrace formation in uplifting landscapes.
Three studies in this thesis address the competition between surface uplift and lateral erosion, as well as the temporal and spatial patterns of fold growth. Chapter 1 presents results from physical experiments on the interactions between alluvial rivers with a zone of uplift. From these experiments, a simple parameter emerges that predicts the width of active beveling as a function of the uplift rate and the mobility of channels. Chapter 2 describes a field-study of the extensive lateral planation of actively uplifting folds by rivers in the foreland of the Tian Shan, northwestern China. Here, geomorphic mapping and dating of Late Quaternary terraces reveals that, contrary to existing models of strath-terrace formation, changes in lateral erosion rates of 1-3 orders of magnitude strongly control formation of planation surfaces. The dated fluvial terraces presented in this chapter do not only constrain rates of bedrock erosion, but also add to the growing database of Quaternary shortening and rock-uplift rates of contractional structures along the deforming eastern Pamir-Tian Shan collision zone. In order to explore how these rapidly deforming structures evolve both in time- and space, new decadal uplift rates obtained from interferometric synthetic aperture radar time-series analyses are presented in Chapter 3. These data place constraints on the spatial patterns of surface- and rock-uplift rates. In combination with rock-uplift rates measured over geologic timescales, a probabilistic model shows that, where significant (mm/y) changes in peak rock-uplift rate occurred across the Quaternary, gradual variations in those rates are more likely than sudden, step-like changes.