Bedrock river channel steepness index can be used to infer relative differences in rock uplift rates across a landscape, given uniform lithology and climatic forcing. River incision, E, is
A sedimentologic, petrologic, and geochemical evaluation of lacustrine carbonates from the Eocene-Oligocene Fenghuoshan Group of north-central Tibet provides insight into the paleoenvironmental and paleolimnological setting of the Hoh Xil basin during the mid-Tertiary. Fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones. These carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial/alluvial plain deposits. Individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward. Fenghuoshan carbonates also contain abundant ostracode, bivalve, and charaphyte fossils. Sedimentologic evidence suggests that carbonate deposition occurred in shallow, relatively short-lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems. Stable C and O isotopic analysis of Fenghuoshan carbonates show d 18 O values ranging from Ϫ11.7‰ to Ϫ10.3‰ (Vienna Pee Dee Belemnite [VPDB]) and d 13 C values between Ϫ7.1‰ and Ϫ2.2‰ (VPDB). The lack of correlation between the two isotopic systems supports the sedimentological interpretation of a shallow, hydrologically open lacustrine setting during the time of carbonate deposition. This interpretation is reinforced by the mineralogy and Mg/Ca molar ratios from Fenghuoshan lacustrine carbonates that indicate fresh water conditions. In addition, petrographic analysis and the d 18 O composition of samples suggest that Fenghuoshan lacustrine rocks have not undergone significant diagenetic alteration. Collectively, these data indicate that the isotopic composition of Fenghuoshan Group carbonates may be used to infer the oxygen isotopic composition of meteoric water feeding these lacustrine basins at the time of deposition, from which we derive estimates of the paleoaltimetry of north-central Tibet during Middle Eocene time. Model results using the isotopic data from Fenghuoshan carbonates indicate that the hypsometric mean elevation of the drainage basins feeding Hoh Xil lakes was Շ2 km. Together with estimates of the Eocene paleoaltimetry from the Lunpola basin to the south, these results provide the first direct evidence for the differential uplift of the northern margin of the Tibetan Plateau.
[1] The Sila Massif in the Calabrian Arc (southern Italy) is a key site to study the response of a landscape to rock uplift. Here an uplift rate of $1 mm/yr has imparted a deep imprint on the Sila landscape recorded by a high-standing low-relief surface on top of the massif, deeply incised fluvial valleys along its flanks, and flights of marine terraces in the coastal belt. In this framework, we combined river longitudinal profile analysis with hillslope erosion rates calculated by 10 Be content in modern fluvial sediments to reconstruct the long-term uplift history of the massif. Cosmogenic data show a large variation in erosion rates, marking two main domains. The samples collected in the high-standing low-relief surface atop Sila provide low erosion rates (from 0.09 AE 0.01 to 0.13 AE 0.01 mm/yr). Conversely, high values of erosion rate (up to 0.92 AE 0.08 mm/yr) characterize the incised fluvial valleys on the massif flanks. The analyzed river profiles exhibit a wide range of shapes diverging from the commonly accepted equilibrium concave-up form. Generally, the studied river profiles show two or, more frequently, three concave-up segments bounded by knickpoints and characterized by different values of concavity and steepness indices. The wide variation in cosmogenic erosion rates and the non-equilibrated river profiles indicate that the Sila landscape is in a transient state of disequilibrium in response to a strong and unsteady uplift not yet counterbalanced by erosion. Be erosion rates and river longitudinal profile analysis, Tectonics, 31, TC3007,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.