Neogene variations in slab geometry drive topographic change and drainage reorganization in the Northern Andes of Colombia

Highlights

• Geomorphic analysis indicates a strong, recent increase in uplift in the northern Central Cordillera.

• Strong increase in surface uplift is most likely driven by flattening of the subducting slab <8 Ma ago.

• Large-scale drainage reorganization can be related to slab flattening and volcanism.

• Changes in topography and drainage network likely affected biodiversity and biogeography in the region

Abstract

The tropical Northern Andes of Colombia are one the world's most biodiverse places, offering an ideal location for unraveling the linkages between the geodynamic forces that build topography and the evolution of the biota that inhabit it. In this study, we utilize geomorphic analysis to characterize the topography of the Western and Central Cordilleras of the Northern Andes to identify what drives landscape evolution in the region. We supplement our topographic analysis with erosion rate estimates based on gauged suspended sediment loads and river incision rates from volcanic sequences. In the northern Central Cordillera, an elevated low-relief surface (2500 m in elevation, ~40 × 110 km in size) with quasi-uniform lithology and surrounded by knickpoints, indicates a recent increase in rock and surface uplift rate. Whereas the southern segment of the Central Cordillera shows substantially higher local relief and mostly well graded river profiles consistent with longer term uplift-rate stability. We also identify several areas of major drainage reorganization, including captures and divide migrations. These changes in the topography coincide with the proposed location of a slab tear and flat slab subduction under the northern Central Cordillera, as well as with a major transition in the channel slope of the Cauca River. We identify slab flattening as the most likely cause of strong and recent uplift in the Northern Andes leading to ~2 km of surface uplift since 8–4 Ma. Large scale drainage reorganization of major rivers is likely driven by changes in upper plate deformation in relation to development of the flat slab subduction geometry; however, south of the slab tear other factors, such as emplacement of volcanic rocks, also play an important role. Several biologic observations above the area of slab flattening suggest that surface uplift isolated former lowland species on the high elevation plateaus, and drainage reorganization may have influenced the distribution of aquatic species.

Introduction

Since Alexander von Humboldt's work on the Chimborazo volcano, the Northern Andes of South America have been noted as one of Earth's most biodiverse regions (Rahbek et al., 2019; von Humboldt and Bonpland, 2013). Many studies have shown that topography and its evolution through time are important predictors of modern-day biodiversity globally, and especially within the Northern Andes (Antonelli et al., 2018; Antonelli and Sanmartín, 2011; Badgley et al., 2017). Therefore, a clear understanding of the timing and spatial patterns of topographic growth is necessary to discern the generation of the observed modern biodiversity patterns in the Andes (Baker et al., 2014; Hoorn et al., 2010; Luebert and Weigend, 2014), but is also critical to identifying the tectonic, geodynamic and climatic processes that generate topography (Garzione et al., 2017; Horton, 2018; Schildgen and Hoke, 2018). The Northern Andes of Colombia are a region of complex topography above the Nazca subduction zone, with three roughly north-south striking parallel mountain chains separated by intermontane basins. The regional topography is overall controlled by subduction processes, yet we know little about the topographic growth especially of the Western and Central Cordillera. A change in subduction geometry from steep to shallow beginning around 6–8 Ma has been proposed, which is expected to have a significant effect on topography (e.g., Eakin et al., 2014) and by extension, an imprint in modern biodiversity. However, the topographic evolution of the Western and Central Cordillera remains elusive, as do the contributions of different drivers of topographic change such as subduction geometry and drainage reorganization.

Extensive geochronology and geochemistry of rocks in the Central and Western Cordillera have been used to decipher the Mesozoic and Cenozoic evolution of the magmatism and terrane accretion events (Kerr et al., 1998; Kerr et al., 1997; Villagómez et al., 2011). Specifically, thermochronology, a method that records the cooling of rocks as they are advected towards the surface via the removal of overlying rocks, termed exhumation (Malusà and Fitzgerald, 2019; Reiners and Brandon, 2006), has been applied to identify periods of mountain building. Thermochronology data from the Central Cordillera generally point towards high rates of exhumation in the Late Cretaceous to Paleogene between ~50–70 Ma, related to the accretion of oceanic terranes (Villagómez et al., 2011; Zapata et al., 2020). The Western Cordillera shows a pulse of exhumation at ~40 Ma followed by a decrease in rates (Villagómez and Spikings, 2013). Yet, few data exist to constrain the Neogene topographic evolution of the Western and Central Cordilleras of the Northern Andes (e.g., Mora et al., 2019).

The main Neogene tectonic events are the collision of the Panama Block during the middle Miocene (ca. 12–15 Ma) with South America (Farris et al., 2011; Montes et al., 2015; Montes et al., 2012) followed by tearing of the Nazca slab at ca. 6–8 Ma and subsequent initiation of flat slab subduction north of ~5°N (Fig. 1A; Chiarabba et al., 2016; Vargas and Mann, 2013; Wagner et al., 2017). Thermal history models from apatite fission track data in the Western and Central Cordilleras show higher rates of exhumation south of the slab tear over the past 40 Ma (Villagómez and Spikings, 2013). Lower temperature apatite (UTh)/He (AHe) ages of the Central Cordillera, which record exhumation from ~2–3 km in the crust, are younger south of the slab tear (Fig. 1B), indicating higher exhumation compared to the north.

The cause of the differences in thermochronology ages, and the general effects of the transition from normal to flat slab on the Western and Central Cordilleras' topography remain elusive. Flat slab subduction is generally associated with changes in the rates of patterns of strain in the upper plate while also inducing dynamic vertical motions (Dávila and Lithgow-Bertelloni, 2013; Espurt et al., 2008; Gutscher et al., 2000; Horton, 2018; Martinod et al., 2020). Several studies, advocate for increased crustal shortening and rock uplift above the zone of flat slab subduction due to increased coupling between the upper and lower plates (Espurt et al., 2008; Gutscher et al., 2000) or isostatic adjustment (Eakin et al., 2014), yet thermochronological data record more and faster exhumation in the southern steeper slab segment (Villagómez and Spikings, 2013). The rate dependent integration time of the employed thermochronometry may be too long to capture a recent increase in uplift rates in the north in response to slab flattening. Such changes in subduction dynamics and tectonic uplift rates may also induce drainage reorganization. Yet, there is no data on past and present rates of modern drainage reorganization within the Central and Western Cordillera.

In this paper, we use geomorphic tools to characterize the topography of the Western and Central Cordilleras of the Northern Andes (Colombia), identify areas and mechanisms of drainage reorganization, and discuss the roles of tectonic events such as slab flattening, Panama Block collision and volcanism in the topographic evolution of the region. Our analysis combines simple topographic observations through swath profiles and detailed analyses of the river network. We employ the analysis of river long profiles to map knickpoints (kinks in river profiles) that can be related to temporal changes in tectonic uplift rates (e.g., Wobus et al., 2006), and river steepness to elucidate spatial patterns of uplift and erosion rates. We also investigate metrics that indicate drainage reorganization, e.g., the χ-index to map the stability of drainage basins (Forte and Whipple, 2018; Scherler and Schwanghart, 2020; Willett et al., 2014). We integrate our topographic observations with geological data and erosion rate estimates based on gauged suspended sediment loads. The topographic features we identify indicate a dynamic landscape that is responding to spatial and temporal changes in rock uplift and drainage reorganization. Our observations help identify the potential drivers of topographic change and highlight linkages between landscape evolution and the modern distribution of species.