Fluvial response to Andean-Amazonian Transition Dynamics: evidence from Huallaga River terraces in central Peru

The uplift of the Andes mountain range is widely recognized as a primary factor in shaping South America's climate patterns and transforming adjacent river landscapes. These changes have played a fundamental role in the dynamics of the rivers that drain the Amazon lowlands and shaping the physical landscapes and ecosystems over time. Here, we reconstruct the geomorphological and sedimentary evolution of the Huallaga River in central Peru. As part of the Amazon drainage system, the Huallaga River preserves a sedimentary record that allows us to disentangle the relative roles of tectonics and climate.This study uses geomorphological mapping, sedimentological characterization, and feldspar post-infrared infrared-stimulated luminescence (pIRIR) dating to investigate the geomorphological and sedimentary evolution of the upper Huallaga River . Feldspar pIRIR at 225oC and at 290oC was applied to determine sediment deposition ages from river terraces and the Juanjui Fm. in the Juanjuí region, which is located on the eastern edge of the Peruvian Andes. The Huallaga River deposits are generally characterized by thick sedimentary layers (0 - 75 m) composed of conglomerates supported by a fine sand matrix and framework. The Pliocene-Pleistocene Juanjui Fm. consists of polymictic conglomerates with a sandy matrix. The conglomerate framework consists of gneiss, volcanic rock, schist, and sandstone pebbles that were reworked and deposited in a fluvial-alluvial fan environment. Geomorphological mapping indicates eight distinct terrace levels, named T1 to T8 from lower to higher elevation ranging from 3 to 142 meters above the riverbed. Feldspar pIRIR ages range from 100 to 300 thousand years ago (ka), but some sediment layers have similar ages, indicating a fill-cut deposit. The evolution of this region can be divided into four phases. The first phase is represented by the deposition of the Pliocene Juanjuí Fm. over the Miocene Ipururu Fm., indicating a period of high-energy aggradation. The second phase is characterized by the beginning of uplift of a syncline, promoted by the Biabo fault. This uplift caused erosion of part of the Juanjuí Fm. due to incision by the ancient Huallaga River. This was followed by the deposition of alluvial fans in the axial portion of the river system. The third phase is characterized by continued uplift, which promoted the erosion of the Pleistocene alluvial deposits (now, exposed in terrace levels) and the onset of a new phase of river incision. The last phase records the current configuration of the fill-cut terraces. These minimum ages are older than previously reported ages for the top of the Juanjuí Fm. in a nearby anticline. An integrated analysis of mapping, sedimentology, and chronology allowed the interpretation of river terrace deposition and incision phases, supporting future links with regional tectonic deformation. These results improve our understanding of recent dynamics along the eastern Andean margin and its role in shaping the Amazon basin. Funding provided by FAPESP (23/16031-4 and 22/03007-5). Keywords: fluvial evolution, pIRIR dating, geomorphological mapping, Sub-Andean deposits