No abstract

Numerous empirical and model-based studies argue that, in general, hillslopes and river channels increase their gradients to accommodate high rates of base-level fall. To date, however, few data sets show the dynamic range of both these relationships needed to test theoretical models of hillslope evolution and river incision. Here, we utilize concentrations of 10 Be in quartz extracted from river sand on the eastern margin of the Tibetan Plateau to explore relationships among short-term (10 2 -10 5 a) erosion rate, hillslope gradient, and channel steepness. Our data illustrate nonlinear behavior and a threshold in the relationship between erosion rate and mean hillslope gradient, confi rming the generalization that hillslopes around the world are limited by slope stability and cease to provide a metric for erosion at high rates (>~0.2 mm/a). The relationship between channel steepness index and erosion rate is also nonlinear, but channels continue to steepen beyond the point where threshold hillslopes emerge up to at least 0.6 mm/a, demonstrating that channel steepness is a more reliable topographic metric than mean hillslope gradient for erosion rate and that channels ultimately drive landscape adjustment to increasing rates of base-level fall in tectonically active settings.

New (U-Th)/He analyses from three elevation transects collected within river gorges that dissect the eastern margin of the Tibetan Plateau provide constraints on the rates and timing of accelerated river incision into the high-elevation, low-relief topography of the region. Apatite He data from the easternmost transect (Dadu River), ~120 km from the plateau margin adjacent to the Sichuan Basin, indicate that rapid river incision of ~0.33 ± 0.04 km/m.y. began at ca. 10 Ma and has continued to the present. Apatite He and zircon He data from the middle transect (Yalong River), collected ~225 km SSW of the Dadu data, indicate that rapid river incision of ~0.34 ± 0.02 km/m.y. began prior to ca. 14 Ma and continued until the early Quaternary, when it increased, likely in response to local uplift and erosion associated with active faults nearby. Apatite He and zircon He data from the westernmost transect (Yangtze River), ~210 km W of the Yalong data, indicate that rapid river incision of ~0.38 ± 0.04 km/m.y. began at or prior to ca. 10 Ma, though likely not prior to ca. 15 Ma, and has continued to the present. The regional consistency of these data indicates that the entire eastern margin of the Tibetan Plateau was being dissected by 10 Ma and that incision has been relatively constant and uniform in the region since that time; this pattern is consistent with an erosional response to broad regional uplift at or before 10 Ma. The larger amount, and earlier onset, of exhumation observed in the Yalong River gorge shows that certain areas on the eastern margin deviate from a simple regional pattern of epeirogenic uplift and subsequent river incision and probably refl ect the superposition of local upper-crustal deformation and uplift on the broad regional pattern.

Efforts to extract information about climate and tectonics from topography commonly assume that river networks are static. Drainage divides can migrate through time, however, and recent work has shown that divide mobility can potentially induce changes in river profiles comparable to changes caused by variation in rock uplift, climate, or rock properties. We use 1‐D river profile and 2‐D landscape evolution simulations to evaluate how mobile divides influence the interpretation of river profiles in tectonically active settings. We define a nondimensional divide migration number, NDm, as the ratio of the timescale of channel profile response to a change in drainage area (TdA) to the timescale of divide migration (TDm). In simulations of headward divide migration, NDm is much less than unity with no measurable perturbation of channel profiles. Only in simulations configured to induce rapid lateral divide migration are there occasional large stream capture events and zones where localized drainage area loss is fast enough to support NDm values near unity. The rapid response of channel profiles to changes in drainage area ensures that under most conditions profiles maintain quasi‐equilibrium forms and thus generally reflect spatiotemporal variation in rock uplift, climate, or rock properties even during active divide migration. This implies that channel profile form may not reliably record divide mobility, so we evaluate alternate metrics of divide mobility. In our simulations and an example in Taiwan, we find that simple measures of cross‐divide contrasts in topography are more robust metrics of divide mobility than measures of drainage network topology.

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that signifi cant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6-10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500-1000 m may account for ~25%-50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.