Abstract The overwhelming documentation of coarse-grained alluvial fans dominated by mass flow contrasts with the scarce accounts of finer grained, traction-dominated alluvial fans. To fill this gap, we present sedimentological and architectural data from a set of sand-rich, streamflow-dominated Pleistocene fans flanking the eastern Upper Valdarno Basin, Italy. The routing of sand-rich sediment resulted from the fast, intense weathering of the feldspar-rich, carbonate-deprived sandstone bedrock underlying the fan catchments. Although capable of entraining large boulders, high water discharge sustained tractional reworking along the proximal facies belts, hindering mass flow. The medial facies belts have a channelized, braided planform and are dominated by processes hardly distinguishable from those characterizing fluvial environments. Along the distal facies belts, extensive overbank tracts are composed of terminal splays and crevasse lobes, the spatial arrangement of which controlled the evolution of through-going tributary channels connected to the axial basin drainage. This study focused on the sedimentary processes, stratal architecture and morphodynamics of the alluvial fans and considered the effect of bedrock inheritance on their development. The results underline how granulometry and mass flow dominance are not distinctive of alluvial fan sedimentation per se and indicate how the critical detection of piedmont, radial palaeomorphology is crucial in the identification of ancient alluvial fans.

The Messinian mélange of the Tertiary Piedmont Basin is the product of different but interrelated processes (tectonic, gravitational, and diapiric) that operated sequentially over a short time span (intra-Messinian time) and in a geodynamic environment (episutural basin) for which mélanges have so far been poorly described. It is composed of different mappable bodies of (non-metamorphic) mixed rocks characterized by a strong facies convergence. Their geometric and stratigraphic position, the internal organization, and the nature of the bounding surfaces allow the defining of some criteria to distinguish different units of mixed rocks (tectonically disrupted unit, gravity-driven sedimentary unit, and diapiric disrupted unit) , in each of which the role of a different prevailing mélange-forming process can be inferred. None of these processes operated in isolation. They were linked by complex and intimate mutual interactions and triggered by intra-Messinian tectonics. The latter produced self- generating processes of mélange formation in which gravitational and diapiric processes triggered and affected each other. Different pulses of overpressured fluids (often rich in methane) strongly governed sediment deformation and also played a crucial role in influencing the time relationships and causative links between the different mélange-forming processes. Faulting may have triggered gas hydrate dissociation, promoting the upward rise of overpressured fluids. These fluids reduced the shear strength of the overlying sediments, promoting large-scale gravity-driven phenomena. Loading provided by rapid emplacement of the gravity-driven sedimentary bodies could have, in turn, developed new overpressured conditions necessary to promote the upward rise of poorly consolidated sediments and shale diapirism.

Deformed Tertiary sedimentary rocks and geomorphic features within the northern foothills of the Alaska Range illustrate the development of a fold-and-thrust belt during the past ca. 3 m.y. The northern foothills form a northward-convex salient at the apex of the Alaska Range and the Denali fault. The neotectonic framework of this region has not previously been established despite the proximity of the northern foothills to the Denali fault, several historic large-magnitude earthquakes, and clear topographic evidence of Quaternary deformation. A distinct east-trending topographic grain corresponds with the orientation of folds and faults defined both by bedrock and geomorphic features. To characterize the active structures of the region, we interpreted previous geologic mapping, developed structural cross sections, analyzed topographic and stream profiles, mapped a sequence of Quaternary fluvial terraces, and surveyed several terrace treads. A northward topographic slope across the northern foothills coupled with the pattern of faulting and folding suggests the presence of an orogenic wedge overlying a south-dipping basal detachment. Mapping and surveying of the terraces document continued uplift, folding, and faulting during terrace formation. Geomorphic analyses demonstrate deformation and differential uplift over the entire foothills belt. Together, these data and interpretations indicate the northern foothills are an active fold-and-thrust belt that is propagating northward into the Tanana basin. The identification of this fold-and-thrust belt documents a significant contractional component to the late Cenozoic evolution of the Alaska Range in addition to the obvious strike-slip motion on the Denali fault. Also, the tectonic activity of these structures indicates that this region represents a potential seismic hazard for nearby military facilities and important transportation corridors.

Evidence of former equilibrium line elevations on Mount Olympus, Greece, coupled with estimates of uplift rate, point to more extensive Pleistocene glaciation and far colder climates than previous studies have indicated. These findings are supported by the record of glacial deposition both on the mountain and across the adjacent piedmont. The data not only provide evidence of significant equilibrium line altitude depression from a present-day elevation as much as 600 m above the mountain's summit (2917 m), but also show that Mount Olympus was glaciated on several occasions, and that the first episode of glaciation significantly predated the late Pleistocene. Piedmont sediments east and west of Mount Olympus record three discrete stages of deposition, each of which can be related to glacial activity on the mountain. Soils that separate these sedimentary units correspond to nonglacial intervals and can be correlated to a dated soil succession south of Olympus. This correlation suggests that the oldest soils correspond to the isotope stage 7 (Mindel/Riss) interglacial event (ca. 210,000 yr before present; U/Th disequilibrium) and that the oldest Pleistocene sediments record isotope stage 8 (Mindel) glaciation in the Olympus region. Subsequent stages of deposition are interpreted to record glaciation on the mountain during the isotope stage 6 (Riss) and isotope stages 4–2 (Würm) glacial events. Sedimentary units defined on the piedmont are also recognized on the Olympus upland and within valley-head cirques, where they correspond to three stages of cirque development. The distribution of these materials, as well as the occurrence of glacial erosional and depositional landforms, indicates that Mount Olympus supported upland ice during the first and second episodes of glaciation and that the first glaciation was sufficiently extensive to produce piedmont ice lobes that covered parts of the eastern, northern, and western piedmont of the mountain. Uplift-corrected cirque floor elevations, coupled with the distribution of glacial sediments, indicate that the Pleistocene equilibrium line altitude during each episode of glaciation was depressed at least 1400–1500 m, to elevations of 2000–2100 m above present sea level (asl). Assuming that lowering of the equilibrium line was solely a function of temperature, this would correspond to a mean annual temperature decrease of 8–9 °C. This contrasts with previous studies of the Mount Olympus region, which suggest that glaciation was restricted to small valley glaciers in upland and valley-head positions, that the regional Pleistocene equilibrium line was lowered only to elevations of 2200–2400 m asl, and that active glaciation was restricted to the latest Pleistocene (Würm). Study of the neotectonic history of Mount Olympus suggests that uplift persisted throughout the mid-Pleistocene–Holocene at a rate of ∼1.6 m/k.y.

Mountain ranges in the southern Rocky Mountains, first uplifted during the early Cenozoic Laramide orogeny, have followed separate landscape evolutionary pathways in the late Cenozoic. We present a model that reconstructs the post-Laramide tectonic and geomorphic history of Sierra Nacimiento and the Taos Range, two nearly adjacent rift-flank ranges in north-central New Mexico that serve to illustrate the various processes shaping landscapes across the southern Rocky Mountains. The Sierra Nacimiento landscape reflects the exhumation of hard Precambrian rocks from beneath a softer Phanerozoic sedimentary cover. The exhumation is continuous, but not steady, being driven by distal base-level fall. Downstream diverging river terraces in the Jemez River valley on the eastern flank of Sierra Nacimiento and late Pliocene to Holocene fluvial deposits on the western Sierra Nacimiento piedmont document the base-level fall. The timing and contemporary rates of incision from these river systems suggest that exhumation is being propagated from south to north as knickzones work their way headward from the Rio Grande. In contrast, the Taos Range landscape reflects alternating active stream incision and aggradation astride, and throttled by, an active range-front normal fault. The distinction between the exhumation-dominated and tectonic-dominated mountain front is best quantified by analyses of first-order stream gradients and a watershed metric we call the drainage basin volume to drainage basin area ratio ( R va ). Gradients of first-order streams in the exhumation-dominated Sierra Nacimiento have a mode of 6.8 degrees, significantly less than the 17.7 degrees obtained from a comparable data set of Taos Range first-order streams. The distinct stream gradient and R va populations hint at an important change in the processes shaping hillslopes and low-order channels, which is supported by the lack of slope-clearing landslides in the Sierra Nacimiento landscape and the presence of such landslides in the Taos Range. Analogue and numeric models find that steep, rugged, faceted topography associated with tectonically active mountain fronts like the Taos Range can only be produced and maintained by creep and landslides where the sediment flux scales as a power law with respect to average hillslope or low-order channel gradient. Here, the fingerprint of active tectonics is recorded by both high R va values and steep modal channel gradients. By comparison, the Sierra Nacimiento landscape is shaped primarily by creep where the sediment flux has a linear relationship to average hillslope and low-order channel gradient. In this situation, the signatures of distal base-level fall are low R va values and relatively gentle modal channel gradients.