Drivers of eastern Andean Plateau incision from integrated thermochronology and thermo-kinematic modelling

Proposed drivers of eastern Andean Plateau river incision in the Pliocene include climate change, dynamically driven plateau uplift, and long-wavelength surface uplift above deep basement structures. However, the evaluation of each mechanism has been hampered in previous studies due to the lack of along-strike data on the timing and extent of canyon incision. In addition, the magnitude of exhumation, permissible structural geometries, and integration of the long-term deformation, erosion, exhumation, and sedimentation histories remain poorly understood.

This presentation focuses on two balanced geologic cross-sections and thermochronologic bedrock sample transects across the Andean Plateau, Eastern Cordillera, and Subandes in southern Peru. Based on (i) age-distance and age-elevation patterns of >80 new thermochronologic dates (apatite and zircon (U-Th)/He and fission-track) from plateau, interfluve, and canyon sample locations; (ii) inverse thermal history model results; and (iii) flexural and thermo-kinematic modeling, we highlight similarities and differences in thermochronometric age patterns, exhumation magnitude, structural geometries, and shortening rates between each section.

Results show that the first-order thermochronometric age pattern is a function of rocks' vertical and lateral movement over basement ramps and resulting exhumational erosion. This pattern is superimposed with a regional and synchronous incision-related exhumation signal since the Pliocene. While this incision occurred independent of structural deformation, the exhumation magnitude and difference in interfluve and canyon thermochronometric ages require the presence of a tectonic contribution to exhumation. We conclude that uplift over a basement ramp in the Eastern Cordillera and a decrease in shortening rates since ~10 Ma set the stage for climate-enhanced incision to occur in southern Peru.