Increased megathrust shear force drives topographic uplift in the Colombian coastal forearc

Abstract

The subsidence/uplift patterns of forearc basins are controlled by subduction dynamics and are strongly dependent on the nature of the margin (erosive vs. accretionary). Here, we explore the role that increased megathrust shear force plays in triggering and sustaining the short wavelength topography of coastal forearc regions at erosive margins, where subsidence is supposed to prevail. The northern Colombian coastal forearc contains a ~ 1000 m-high mountain range (Baudó Range) that abruptly stands-up for ~300 km along the northwestern Pacific margin of South America, where the buoyant Nazca plate is subducting at a shallow angle. Field observations and stratigraphic data of Cretaceous to late Neogene volcanic and sedimentary rocks, suggest that the Baudó Range has been uplifting since the middle-late Miocene. The uplift seems to continue nowadays as evidenced by the irregular cliffed and bluffed coastline, steep slopes and high topographic ruggedness in one of the wettest regions on Earth, where weathering and landscape smoothing is expected to be intense. Earthquake focal mechanisms, together with regional-scale N-S-trending sinistral reverse fault structures, west-verging folding, and NW- and NE-oriented topographic lineaments, seems to be related to a trench-perpendicular mostly compressional stress regime. The residual topography along the coastal range of the northern Colombian forearc indicates under-compensation, which may suggest that the relatively thin (~20 km-thick) crust cannot support isostatically the observed topography. We propose that increased shear force along a wide seismogenic zone linked to the shallow subduction of the Nazca plate and a poorly lubricated interface, due to limited sediment dispersal into the trench, drive the ongoing topographic uplift of the region. In contrast, the smoother relief and lower elevations in the southern Pacific coast of Colombia could be explained by a deficit of shear force available to support higher topography.