Quaternary deflection of the Macuma River as it crosses the actively growing Cutucú antiformal culmination in southeastern Ecuador

Models of fold growth involve simple mechanisms of tip propagation or an increase in amplitude and breadth by maintaining the tip fixed. Whatever the growth mechanism, the drainage network adapts to the newly created tectonic slopes. Lithological resistance to erosion, as well as climate and uplift rates, exerts a control on bedrock incision rates and, therefore, on whether a transverse river maintains its course or finally is deflected. In southeastern Ecuador, the northernmost end of the Cutucú Uplift corresponds to the Macuma anticline, a north-plunging periclinal fold that encroaches into the Amazonian plain. The east-flowing Macuma River exhibits a pronounced narrow curvature entrenched within the anticline structure. Morphometric analysis including river profiling, across-valley geometry, hypsometric-integral data, and small-scale saddle features such as wind gaps, evidence an antecedent east-flowing straight water gap in the initial stages of fold growth. Nevertheless, the gradual exposure of a rock-resistant core has deflected the Macuma River but only in its middle course while the general west-east trend is maintained. Consequently, the landscape resulted in a linked pair of elongated inner ridges bound by a horseshoe morphostructure. Through detailed geomorphic analysis, a complex history of lateral propagation has been established for the Macuma anticline. Thus, behind the frontal nose, an actively propagating core tip attained a point where it became fixed while the anticline began to grow in amplitude and breadth. Finally, a renewed phase of tip propagation encroached into the adjacent northern Amazonian plain and constructed a narrow anticlinal nose which is currently preserved as a pristine non-dissected surface.