CLIMATIC, LITHOLOGIC, AND TECTONIC CONTROLS ON TERRACE GENESIS IN ACTIVE OROGENS

In the Mediterranean region and elsewhere, the geodynamic processes building active orogens are constrained by the deformation of geomorphic and stratigraphic markers. Fluvial terraces are a common geomorphic and paleo-geodetic marker composed of both landforms and deposits that integrate tectonic, climatic, and geomorphic processes at the watershed scale. Terrace formation and preservation attests to a fundamental unsteadiness in the rate of vertical incision of a river as it carves its valley. The most common sources of that unsteadiness are vegetative, geomorphic, and hydrologic responses to climate, which for the Quaternary, are dominated by 100-k.y. glacial-interglacial cycles; however, the precise response depends much on watershed substrate and the climatic, tectonic, and base level setting. When coupled with numeric ages, the key terrace observable, the rate of incision, can be used to infer active tectonics. The Italian Apennines offer an excellent natural laboratory where terrace formation has been influenced by climatic, lithologic, and tectonic gradients. Northern Apennine rivers are dominated by bedrock channels and strath terraces, the formation of which is controlled by rapid rock uplift (~1 mm/yr) and watersheds underlain by siliciclastic rock-types. Here alluvium deposited during the last glacial maximum (LGM) climate is distinctly exposed as a terrace and provides a widespread, correlative geomorphic marker. Rivers in the central Apennines are dominated by thick fill terraces, the formation of which are controlled by modest rates of rock uplift (~0.25 – 0.5 mm/yr) and watersheds underlain by carbonates. Similarly, the LGM terrace is well exposed and regionally correlative. Terrace formation in both cases is in phase and paced by glacial-interglacial climate changes characteristic of the broader continental Europe. In contrast, the rivers of the southern Apennines are dominated by alluvial channels despite locally rapid rates of rock uplift (~1mm/yr), fill terraces with complex, composite stratigraphy, and diverse watershed geology. The drier, Mediterranean climate of southern Italy drives terrace genesis out of phase with respect to the northern and central Apennines. Most southern Italian streams have yet to incise and expose alluvium deposited during the LGM as a terrace, resulting in late-Pleistocene rates of incision that are slower than their more northern counterparts. These differences in terrace type and genesis color the tectonic interpretation of river incision, the correct understanding of which is rooted in a genetic rationalization of the local surficial processes driving terrace genesis.