Tectonically driven deposition and landscape evolution within upland incised valleys: Ambra Valley fill, Pliocene-Pleistocene, Tuscany, Italy

Sedimentation in the upstream reaches of incised valleys is predominantly of alluvial origin and, in most cases, independent from relative sea-level or lake-level oscillations. Preserved facies distributions record the depositional response to a combination of allogenic factors, including tectonics, climate and landscape evolution. Tectonics drive fluvial aggradation and degradation through local changes in gradient, both longitudinal and transverse to the valley slope. This article deals with a Pliocene–Pleistocene fluvial valley fill developed in the north-eastern shoulder of the Siena Basin (Northern Apennines, Italy). Evolution of the valley was not influenced by sea-level or lakelevel changes and morphological and depositional evolution of valley resulted from extensional tectonics that gave rise to normal and oblique-slip faults orthogonal and parallel to the valley axis. Data from both field observations and geophysical study are interpreted to develop a comprehensive tectono- sedimentary model of coeval longitudinal and lateral tilting of the developing alluvial plain. Longitudinal tilting was generated by a transverse, upstream-dipping normal fault that controlled the aggradation of finingupward strata sets. Upstream of the fault zone, valley back-filling generated an architecture similar to that of classic, sea-level-controlled, coastal incised valleys. Downstream of the fault zone, valley down-filling was related to an overwhelming sediment supply sourced and routed from the active fault zone itself. Lateral tilting was promoted by the activity of a fault oriented parallel to the valley axis, as well as by different offsets along near orthogonal faults. As a result, the valley trunk system experienced complex lateral shifts, which were governed by interacting fault-generated subsidence and by the topographic confinement of progradational, flank-sourced alluvial fans.