Tectonic-fluvial interactions in the Sierra la Laguna, Mexico: insights from geomorphology, numerical modelling and cosmogenic radionuclides

Abstract

Tectonics work to build topography. Earth surface processes respond by eroding. In this thesis I am interested in the interactions between tectonics and erosion. I begin by considering the components of tectonic displacement. Tectonic displacement consists of vertical uplift or subsidence, and horizontal advection. I focus on the effects of tectonic advection on mountain range topography, surface drainage patterns and drainage divide dynamics. By numerically modelling a normal fault I find that advection promotes the elongation of catchments. I also demonstrate that at steady-state a mountain range experiencing advection displays a spatial disequilibrium between uplift and erosion. This disequilibrium induces a migration of the main drainage divide of the mountain range towards the fault. Comparisons of topographic observations in the Sierra la Laguna (Mexico), are consistent with topographic analysis of my modelling results when advection is included.

Building on this transfer of drainage area from the proximal (fault adjacent) to the distal (far side) flank of the mountain range, I explore the effects of drainage reorganisation on erosion rates. Divide migration and drainage capture contribute to drainage reorganisation. I numerically model divide migration and drainage capture, and monitor the effects of each on catchment-averaged erosion rates. In the growing catchment, erosion rates increase, whereas in the shrinking catchment erosion rates decrease. Drainage capture initiates knickpoints that migrate upstream. This means the drainage capture signal is preserved longest in the headwaters of the captured area. I compare numerical modelling erosion rate signals to ¹⁰Be-derived catchment-averaged erosion rates in the Sierra la Laguna, downstream of a capture point. ¹⁰Be-derived catchment-averaged erosion rates are found to be twice as fast in the suspected growing catchment headwaters (0.17 mm yr⁻¹) relative to the shrinking catchment headwaters (0.09 mm yr⁻¹). The wider catchment-averaged erosion rate signal across the mountain range shares similarities with numerical modelling derived catchment-averaged erosion rates for drainage capture.

Finally, I consider the role of boulders. Are the boulders in the Sierra la Laguna predominantly immobile, or are boulders mobilised during large magnitude flood events? By analytically modelling boulders in a channel, I consider the influence of a boulder’s presence on channel flow dynamics. I find at low flow, boulders can act to decrease channel shear stress, whereas at high flow, boulders can act to increase channel shear stress. By measuring channel width and slope in the Sierra la Laguna for two trunk channels I calculate channel shear stress. In addition, mapping boulders in the Sierra la Laguna allows me to calculate the critical shear stress of boulders in these trunk channels. A comparison of channel shear stress and boulder critical shear stress values along trunk channels suggests the boulders are not mobilised during high flow events. However, shear stress and boulder critical shear stress values reflect a similar pattern with distance downstream. This leads me to consider the role of sand as a tool that lowers the critical shear stress of boulders in these channels.