Numerical terrain analysis of fluvial-marine watersheds of the hotspot island Santiago
Description
Numerical terrain analysis constitutes the missing link between the classical descriptive geomorphology and geomorphometry providing landform indices by means of which issues in applied and genetic geosciences can be solved. This method is fully satellite-based and supplemented by ground-truthing dealing with the interior of the positive landforms and the inter-landform sediments. This approach can successfully be taken prevalently for immature modern landscapes, exemplified by Cenozoic volcanic hotspot islands such as Santiago, the main island of the Cape Verde Archipelago. Corresponding to the scale of observation, a tripartite subdivision is achieved in numerical terrain analysis (1st order regional, 2nd order local, 3rd order outcrop scale). The compositional quantification encompasses mineralogy, sedimentology and isotope geochemistry (δ13C, δ18O, 14C-dating ). The first order indices allow for a tripartite compartmentalization of the volcanic island into the presumed paleosurface of all volcanic summits within a certain altitude (“Gipfelflur”), the volcanic pediment and the coastal zone. A binary subdivision is conducted in time into an Older Hotspot Island (OHI) with badlands characterized by strong vertical morphotectonic activity and a Younger Hotspot Island (YHI). The summits of the YHI are affected by strong lateral morphotectonic activity even though they are rising higher in the modern-day hinterland than the OHI. This activity resulted in a dissected shield area and a volcanic strike-slip wedge. The first order index called Vertical Sinuosity of Valleys (VeSival) index is crucial and reflects the intensity of relief of the volcanic landscape of the hotspot island. Among the second order indices, the VaSlAnalti index giving the variation of slope angle as function of altitude is an excellent environmental marker. Its correlation coefficients provide a measure for the homogeneity of volcanogenic, mass wasting, fluvial, and coastal landform series. Among the third order indices, the QuantGravsitu index (Quantification Gravel situmetry) lends much support to the afore-mentioned environmental markers. It is a meticulous measure of the modality, sharpness and fan width of the orientation of clasts with their data, illustrated in semicircle rose diagrams. It is used to fine-tune volcaniclastic deposits at outcrop scale. The current study is a combination of near-surface geoscientific disciplines where exogeneous processes shaping the volcanic landscape are used to translate into endogenous processes also applicable to economic geology. The metallogenic comparison of the Canary Islands and the Cape Verde Archipelago is based on terrain analysis and yields a contrasting picture of both alkali-magmatite-carbonatite hotspot provinces regarding their potential for REE-Nb-Ta-Be-Li metal deposits.