Alexander L. Densmore

Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau

2007 American Geophysical Union. Densmore, A. L., M. A. Ellis, Y. Li, R. Zhou, G. S. Hancock, and N.Richardson, (2007), Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the TibetanPlateau, Tectonics, 26, TC4005, 10.1029/2006TC001987. To view the published open abstract, go tohttp://dx.doi.org and enter the DOIUse policy

Landsliding and the evolution of normal-fault-bounded mountains

Much of the tectonic and climatic history in high-relief regions, such as the mountains of the western U.S. Basin and Range province, is contained in the morphology of hillslopes, drainage networks, and other landforms that range in scale from 10−1 to 101km. To understand how these landforms evolve, we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules. Bedrock landsliding, long recognized as a significant geomorphic process in mountainous topography, is for the first time explicitly included in the rule set. In a series of numerical experiments, we generate synthetic landscapes that closely resemble mountainous topography observed in the Basin and Range. The production of realistic landscapes depends critically on the presence of bedrock landslides, and landsliding yields rates of long-term erosion that are comparable in magnitude to those of fluvial erosion. The erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate-limiting process in steady state experimental landscapes, Our experiments generate power law distributions of landslide sizes, somewhat similar to both field and laboratory observations. Thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal-fault-bounded ranges.

Extraordinary denudation in the Sichuan Basin: Insights from low- temperature thermochronology adjacent to the eastern margin of the Tibetan Plateau

The eastern margin of the Tibetan Plateau combines very high relief with almost no Tertiary foreland sedimentation and little evidence of Cenozoic tectonic shortening. While river incision and landscape development at the plateau margin have received significant attention over the last decade, little is known about the Cenozoic development of the adjacent Sichuan Basin. Here we assess the Cenozoic thermal history of this basin using detrital apatite fission track (AFT) and (U-Th)/He techniques and establish the presence of an exhumed AFT paleopartial annealing zone across much of the basin. This observation, combined with stratigraphic and borehole sections and inverse modeling of confined apatite fission tracks, indicates that the strata within the basin have undergone accelerated cooling after similar to 40 Ma, consistent with the widespread erosion of similar to 1 to 4 km of overlying sedimentary material. This regional-scale erosion is most likely a response to changes in the Yangtze River system draining and removing sediment from the basin. The base-level fall associated with this erosion contributed to a relative increase in relief across the Longmen Shan and may have helped drive Miocene-Recent incision and unloading of the plateau margin.

Topographic fingerprints of bedrock landslides

Bedrock landslides in mountainous regions may be triggered by either storms or earthquakes; the dominant mechanism in a region affects both landscape evolution and landslide hazard. We describe a simple observational test to distinguish between storm and earthquake triggers based on a probabilistic measure of hillslope morphology. In areas that are dominated by storm-triggered landslides, steep topographic slopes are concentrated on the lowermost parts of the hillslopes. Storm triggers act primarily on the hillslope toes, and landslides preferentially remove material from those locations, giving rise to inner gorges. Areas where most landslides are earthquake triggered have more uniform spatial distributions of steep topographic slopes, because coseismic shaking causes failures at both ridge crests and hillslope toes. Earthquake-triggered landslides lead to planar hillslopes and rare or absent inner gorges.

Seismic mountain building: Landslides associated with the 2008 Wenchuan earthquake in the context of a generalized model for earthquake volume balance

Here we assess earthquake volume balance and the growth of mountains in the context of a new landslide inventory for the Mw7.9 Wenchuan earthquake in central China. Coseismic landslides were mapped from high-resolution remote imagery using an automated algorithm and manual delineation, which allows us to distinguish clustered landslides that can bias landslide volume calculations. Employing a power-law landslide area-volume relation, we find that the volume of landslide-associated mass wasting (~2.8+0.9/-0.7 km3) is lower than previously estimated (~5.7-15.2 km3) and comparable to the volume of rock uplift (~2.6±1.2 km3) during the Wenchuan earthquake. If fluvial evacuation removes landslide debris within the earthquake cycle, then the volume addition from coseismic uplift will be effectively offset by landslide erosion. If all earthquakes in the region followed this volume budget pattern, the efficient counteraction of coseismic rock uplift raises a fundamental question about how earthquakes build mountainous topography. To provide a framework for addressing this question, we explore a group of scaling relations to assess earthquake volume balance. We predict coseismic uplift volumes for thrust-fault earthquakes based on geophysical models for coseismic surface deformation and relations between fault rupture parameters and moment magnitude, Mw. By coupling this scaling relation with landslide volume-Mw scaling, we obtain an earthquake volume balance relation in terms of moment magnitude Mw, which is consistent with the revised Wenchuan landslide volumes and observations from the 1999 Chi-Chi earthquake in Taiwan. Incorporating the Gutenburg-Richter frequency-Mw relation, we use this volume balance to derive an analytical expression for crustal thickening from coseismic deformation based on an index of seismic intensity over a defined area. This model yields reasonable rates of crustal thickening from coseismic deformation (e.g.~0.1-0.5 km Ma-1 in tectonically active convergent settings), and implies that moderate magnitude earthquakes (Mw≈6-8) are likely responsible for most of the coseismic contribution to rock uplift, because of their smaller landslide-associated volume reduction. Our first-order model does not consider a range of factors (e.g., lithology, climate conditions, epicentral depth and tectonic setting), nor does it account for viscoelastic or isostatic responses to erosion, and there remain important uncertainties on the scaling relationships used to quantify coseismic deformation. Nevertheless, our study provides a conceptual framework and invites more rigorous modeling of seismic mountain building.

Did incision of the Three Gorges begin in the Eocene

Like the other large river systems that drain the area of the India-Asia collision, the Yangtze River was assembled through a series of Cenozoic capture events. These events are important for orogenic erosion and sediment delivery, but their timing remains largely unknown. Here we identify enhanced cooling in the Three Gorges region in central China, a key capture site during basin development, beginning at 40–45 Ma. This event is not visible in regional thermochronological data, but is near-contemporaneous with the onset of widespread denudation in the Sichuan Basin, just upstream of the Three Gorges. While we cannot rule out alternative explanations, the simplest mechanism that links these events is progressive capture of the middle Yangtze River by the lower Yangtze and the onset of incision in the Three Gorges. This model agrees with independent mid-Cenozoic estimates for the timing of middle Yangtze River diversion and capture, and provides a plausible outlet for large volumes of erosional detritus from the Sichuan Basin.

Postglacial slip-rate increase on the Teton normal fault, northern Basin and Range Province, caused by melting of the Yellowstone ice cap and deglaciation of the Teton Range?

Along the eastern front of the Teton Range, Wyoming, prominent fault scarps offset Pinedale deposits by up to 30 m and document that multiple earthquakes ruptured the range-bounding Teton normal fault after the last glacial period. Paleoseismological data suggest that ~70% of the postglacial slip on the southern Teton fault accumulated during or shortly after deglaciation, before 8 ka. Here, we use a three-dimensional fi nite-element model to show that melting of the Yellowstone ice cap and the valley glaciers in the Teton Range may have caused the postglacial slip-rate increase on the Teton fault. During deglaciation, slip on our model fault accelerates by a factor of ~6 with respect to the long-term rate. Our model further shows that the impact of the melting Yellowstone ice cap on fault slip increases along-strike of the fault from south to north and is everywhere larger than the effect of the former valley glaciers in the Teton Range. The results demonstrate that postglacial slip on faults in glaciated regions may not have been uniform through time. Rather, a signifi cant fraction of slip may have accumulated within a few thousand years after the last glaciation. We hypothesize that the rebound caused by the Yellowstone ice cap has also triggered clusters of earthquakes on other normal faults in the surrounding Basin and Range Province.

Dynamic controls on erosion and deposition on debris-flow fans

Debris flows are among the most hazardous and unpredictable of surface processes in mountainous areas. This is partly because debris-flow erosion and deposition are poorly understood, resulting in major uncertainties in flow behavior, channel stability, and sequential effects of multiple flows. Here we apply terrestrial laser scanning and flow hydrograph analysis to quantify erosion and deposition in a series of debris flows at Illgraben, Switzerland. We identify flow depth as an important control on the pattern and magnitude of erosion, whereas deposition is governed more by the geometry of flow margins. The relationship between flow depth and erosion is visible both at the reach scale and at the scale of the entire fan. Maximum flow depth is a function of debris-flow front discharge and pre-flow channel cross-section geometry, and this dual control gives rise to complex interactions with implications for long-term channel stability, the use of fan stratigraphy for reconstruction of past debris-flow regimes, and the predictability of debris-flow hazards.

What sets topographic relief in extensional footwalls

We use three large normal fault arrays in the northeastern Basin and Range Province, western United States, to document catchment development and relief production during fault growth. Fault slip and slip rates increase systematically along strike from zero at the fault tips. Catchment relief and across-strike range width both increase as slip accumulates but reach maximum values at a distance of ∼15 km from the fault tips and remain uniform along strike over much of the footwalls. Catchment outlet spacing also increases away from the fault tips but does not reach a uniform value and may vary by a factor of 5–6 along strike. We infer that catchments first elongate in the across-strike direction as slip accumulates and the range half-width increases. Once the half-width reaches its maximum value, continued catchment growth is possible only by along-strike capture, which increases outlet spacing but not relief. The close correspondence between catchment relief and range half-width suggests that geomorphically limited hillslope and channel gradients are achieved within the 15 km tip zone. Thus, the limiting factor in footwall development is the width of the range, which is controlled by two external agents: the geometry and spacing of the major faults, and the elevations of base level on both flanks.

Controls on fluvial evacuation of sediment from earthquake-triggered landslides

Large earthquakes in active mountain belts can trigger landslides, which mobilize large volumes of clastic sediment. Delivery of this material to river channels may result in aggradation and flooding, while sediment residing on hillslopes may increase the likelihood of subsequent landslides and debris flows. Despite recognition of these processes, the controls on the residence time of coseismic landslide sediment in river catchments remain poorly understood. Here we assess the residence time of fine-grained ( 5 mm day–1). Together with previous observations from the C.E. 1999 Chi-Chi earthquake in Taiwan, our results demonstrate the importance of landslide density and runoff intensity in setting the duration of earthquake-triggered landslide impacts on river systems.