Bridging timescales of tectonically driven landscape evolution of the Chilean forearc

Abstract

This thesis investigates the interplay between tectonic processes, structural inheritance, and landscape evolution along the Chilean forearc across contrasting climatic zones. By integrating geomorphic mapping and morphometric analysis of remotely sensed datasets, structural field data, and seismostratigraphic interpretation for kinematic modelling, this research shows how transients of crustal deformation and upper-plate fault reactivation shape the forearc landscape over multiple timescales (from decades to several millions of years).

In Northern Chile (19-20°S), the long-standing hyperarid climate led to a sediment-starved trench that abuts a margin impacted by subduction erosion and aseismic ridge subduction. Here, the focus lies first on identifying and characterizing previously unmapped faults within the Coastal Range and Central Depression transition, which subtly deform Quaternary drainages. Results reveal the presence of slow-deforming, east-vergent compressive blind faults with two main tectonic phases: Late Miocene–Pliocene reverse faulting that inverted the Andean piedmont, followed by Pliocene–Quaternary transpression extending southward. These faults actively modulate topography, force Quaternary aluvial-alluvial drainages to reorganize, and pose unrecognized seismic hazards. Seismostratigraphic interpretation and kinematic modelling based on reprocessed seismic data and lithologic well reveal that the east-vergent faults are steep crustal structures inherited from Early Cretaceous extensional tectonics and have been episodically reactivated since the Late Cretaceous. Their reactivation reflects changes in plate convergence and the arrival of subducting ridges to the trench, causing intermittent uplift of the Coastal Range block. Available microseismicity and focal mechanisms show the reactivation and switching kinematics of these persistent upper-plate faults within the ongoing seismic cycle of subduction earthquakes.

In Southern Chile (37-39°S), the forearc has experienced intense glaciation since the Pliocene. The trench sediment infill is thick, and the margin is impacted by accretionary processes. Here, the focus lies in exploring the history, patterns, and drivers of the rapid Late Neogene uplift of the coastal Nahuelbuta Range and upper-plate fault reactivation along its boundary with the Central Depression. Geomorphic mapping and river profile analysis indicate an onset of uplift between 3 and 2.5 Ma, with subsequent discrete pulses of rapid uplift lasting ~1 Ma. Paleoclimate proxies show that these pulses correlate with Southern Hemisphere glacial cycles and consequent sudden increases in sediment influx to the trench, meaning larger volumes of subducted material. Further analytical models show that the duration and wavelength of transient uplift pulses are compatible with underplating nappes of sizes and depth matching thick and sediment-rich portions of the subduction channel.

This thesis emphasizes the oscillating nature of tectonically driven forearc landscape evolution across diverse geological timescales and climate settings. It highlights the importance of integrating geomorphologic analysis and field data with subsurface imaging to understand the interplay between upper-plate faults, the seismic cycle, and subduction zone processes, which contribute to crustal deformation and seismic hazard in active convergent margins.