Earth-Science Reviews, 2023,
242, p. 104452 [32 p.] ISSN 0012-8252
The tropical cratonic geosystem encompasses non-orogenic continental domains of the tropical belt that developed and preserved Cenozoic lateritic regolith mantles (i.e., the products of intense rock weathering). These domains represent a significant part of the continental landsurface contributing to regulate global biogeochemical cycles. Here I address the surface evolution mechanisms of the tropical cratonic geosystem based on a comprehensive review and an original map of the landform-regolith associations (combinations of reliefs and regolith mantles of specific nature and age) of Northwestern Africa, an archetype of the tropical cratonic geosystem. The landform-regolith pattern is a composite record of long-term climate variability modulated by the composition of the bedrock. Relictual, pan-tropical associations attest to (i) very intense, greenhouse-driven weathering starting in the Late Cretaceous and culminating at the Early Eocene thermal maximum, (ii) intense Late Oligocene weathering driven by seasonally humid climate, (iii) dry erosional climate during the Early and Late Miocene, with a return to moderate weathering-prone, seasonally dry climate around the Miocene climatic optimum (18-11 Ma) and at the End-Miocene (7-6 Ma). Post Miocene functional landform-regolith associations are latitudinally zoned, reflecting installation of the modern climatic zonation in the tropics. Modern regional susceptibility to erosion has to be partitioned among the functional landform-regolith associations, which tend towards pedoclimatic equilibrium, and relict associations, which are mostly fossil. Moreover, results show that (i) past etchplains and pediplains ("paleo-surfaces") are unreliable topographic gauges of mantle-driven deformation, (ii) staircase patterns of successive etchplains or pediplains attributed to positive epeirogenic pulses are invalid and (iii) low-temperature thermochronology fails to document final rock exhumation paths in cratonic contexts.
