AGU 2022 practice

Thomas Theunissen – Oblique continental rifting. Insights from 3-D forward geodynamic modelling coupled with surface processes and application to the Equatorial passive margins formation.

Sebastian Wolf – How high do mountains grow? quantifying growth and decay of topography in collision orogens

Björn Johan Emil Burr Nyberg – Global Scale Analysis on the Geomorphology of Rivers and Channel Belts

Thomas Theunissen – Oblique continental rifting. Insights from 3-D forward geodynamic modelling coupled with surface processes and application to the Equatorial passive margins formation.

Plate divergence (continental rifting) is often oblique to the rift axis or plate boundary, comprising many active rifts and mature rifted margins on Earth. Recent findings from field-based studies, plate reconstructions and numerical and analog models suggest that rift obliquity plays a critical role in rift evolution and imparts fundamental controls on the eventual architecture of mid-ocean ridges and mature rifted margins. These studies show the importance of vertical strike-slip and transform structures in the oblique extension process but also reveal that the initiation of syn-rift vertical strike-slip motion over long distances requires inherited weaknesses. As a typical example the Southern part of the Equatorial passive rifted conjugate margins exhibit segments with different tectonics and subsidence histories alternating with transform segment with different lengths and orientation. We aim in this study 1) to evaluate the role of the inheritance (obliquity, geometry of weaknesses) in controlling the pattern of faulting (type of fault, direction of propagation, and linkage), and 2) to evaluate the feedback between tectonics and surface processes in this context to understand consequences on landscape evolution. For this purpose, we use most recent advances in 3-D forward geodynamic modeling coupled with surface processes. Preliminary results confirm the importance of inheritance in shaping the continental rift by controlling fault propagation leading to oblique segments with variable tectonics and subsidence history and show some unexpected feedbacks between tectonics and surface processes. These results compare well to the Equatorial system and provide important insight on the causes that might explain timing and amplitude of vertical motions and related sediment flux.

Björn Nyberg – Global Scale Analysis on the Geomorphology of Rivers and Channel Belts

Rivers are fundamental geomorphic features on the Earth’s surface and have an important role in controlling human habitat, climate, ecosystems, and the hydrological cycle. However, only recently has the surface area of rivers globally been quantified, and there remains a significant knowledge gap in understanding the distribution of river types, the extent of channel belt deposits and their morphology. Yet it is these factors that ultimately control the behavior of river systems and the ecosystems and human livelihoods that depend on those rivers. Here we develop a novel machine learning method to recognize the patterns of different river channels and their deposits to classify their morphology. Our results quantify, for the first time, that the global observable extent of river channel belts covers an area of 30.5 x 10⁵ km², which equates to roughly the known size of lakes or 7 times larger than the extent of river channels. Of those channel belts, nearly 77% of the surface area are classified as meandering with the remainder defined by a more braided morphology. Furthermore, we quantify the hydrological, physio-climatic and tectonic controls governing the geomorphology of rivers globally. It is hoped that the new Global River Morphology (GRM) map and database may provide new insights into predicting the intensity and behavior of future floods, improve analysis of biogeochemical cycles and aid in water resource management

Sebastian Wolf – How high do mountains grow – quantifying growth and decay of topography in collisional orogens 

How does a collisional mountain belt grow and decay? It is widely recognized that crustal thickening creates topography, while river incision counteracts orogenic growth and leads to topographic decay, thus providing a link between climate, surface processes and tectonics. However, it remains uncertain whether surface processes or lithospheric strength control mountain belt height, width, and longevity, reconciling high erosion rates observed for instance in Taiwan and New Zealand, low erosion rates in the Tibetan and Andean plateaus, and long-term survival of mountain belts for several 100s of million years. Here we use a tight coupling between a landscape evolution model (FastScape) and a thermo-mechanically coupled mantle-scale tectonic model (Fantom) to investigate mountain belt growth and decay. Using several end-member models and introducing the new non-dimensional Beaumont number, Bm, we quantify how surface processes and tectonics control mountain growth and define three end-member types of growing orogens: Type 1, non-steady state, strength controlled (Bm > 0.5); Type 2, flux steady state, strength controlled (Bm ≈ 0.4−0.5); and Type 3, flux steady state, erosion controlled (Bm < 0.4). Bm can be assessed without complex measurements or assumptions, but simply by knowing a mountain belt’s convergence rate, height, width, first order shortening distribution, and widening rate. Applying our results to several orogens on Earth indicates that tectonics dominate in the Central Andes and Himalaya-Tibet (both Type 1), efficient surface processes balance high convergence rates in Taiwan (Type 2), and surface processes dominate in the Southern Alps of New Zealand (Type 3). Quantifying orogenic decay, we find that removal of topography is primarily determined by erosional efficiency and can be subdivided into two phases with variable isostatic rebound characteristics and associated timescales: First short-wavelength relief is removed within a few Myr, followed by removal of long-wavelength topography and effectively local isostatic rebound. Our results provide a simple unifying framework quantifying how surface processes and tectonics control the shape, height, and longevity of mountain belts on Earth.