T33D-01 13:40h
Timescale of Orogen Response to Tectonic and Climatic Forcing
We have recently published an analytical solution that quantifies the strength of coupling among climate, topography, erosion, and deformation in frictional orogenic wedges at mass flux steady state. Here we extend this work to address the transient evolution of frictional orogenic wedges in response to changes in climatically controlled erosional efficiency and tectonically driven accretionary flux. Interestingly, the coupling between climate and tectonics is shown to be stronger during transients than at steady state. Significant, potentially diagnostic, differences are seen in orogen response to climatic versus tectonic perturbations. By assuming that wedge geometry, during growth and decay, is self-similar, and that response time is controlled by mass balance not drainage basin rearrangement, we derive an approximate analytical solution for orogen response time. Following a step-function change in either erosional efficiency or accretionary flux, wedge cross-sectional area asymptotically approaches a new steady state condition. The transient evolution of wedge cross-sectional area is approximately exponential, such that orogen response time can be characterized by a half-life. Remarkably, the transient response time is determined primarily by the erosional efficiency associated with the final climate state, and only weakly influenced by the initial conditions, the magnitude of the climatic or tectonic perturbation, and the accretionary flux. A robust implication is that, for a given set of initial conditions, orogen response to an increase in the erosional efficiency will be faster than that to a change in accretionary flux, which, in turn, will be faster than that to a decrease in erosional efficiency. The transient response time is determined to have a sub-linear dependence on the details of the erosion rule and the rheology of the orogenic wedge, varying by less than a factor of two for a wide range of model parameters.
T33D-02 13:55h
Co-evolution of spatial patterns of precipitation and topography
A necessary step towards understanding the coupled climate-tectonics-erosion system is learning how, on geologic timescales, patterns of climatic forcing create patterns of erosion. Precipitation patterns in mountainous landscapes are strongly related to topography, while at the same time, precipitation fundamentally affects the ability of rivers and glaciers to erode, thus directly influencing topographic development. We present observations that reflect significant and robust connections between topography and precipitation and numerical models that show significant landscape modifications resulting from these feedbacks. Mesoscale numerical weather prediction via MM5 and field measurements in the Olympic mountains of Washington state reveal a sustained 2-4 fold enhancement of precipitation on ridges relative to valleys separated by horizontal distances of ~10 km and elevation differences of ~500 m. These strong gradients in precipitation are observed on annual, seasonal, and individual event timescales despite variability in wind speed, direction and temperature. In the Matheny ridge area, field measurements of rain and snow support numerical predictions of the location and amplitude of spatial gradients in precipitation. Incorporation of a linear precipitation model that reproduces, for the Olympics, the observed locations and magnitudes of spatial gradients in precipitation in the CASCADE landscape evolution model allows for exploration of the influence of spatial patterns of precipitation on topographic development. We present simulations of the co-evolution of precipitation and topography with a distribution of atmospheric conditions meant to simulate climatic variability. Additionally, we have modeled this co-evolution with several different imposed uplift patterns. Topography that co-evolves with precipitation is distinct from topography created under uniform precipitation conditions in several ways. First, on the windward side, precipitation is enhanced on ridges relative to valleys and results in smaller ridge-valley elevation differences and higher channel concavities than predicted by simple erosion laws. In addition, the topographic divide is shifted down wind and highest topography is shifted even farther down wind than the divide. On the lee side, rivers are deeply entrenched between high ridges and the ridge-valley elevation difference is more extreme while channel concavities are lower than predicted by erosion laws.
T33D-03 INVITED 14:10h
Testing Glacial Limits to Mountain Building: The Buzz Saw in the Chugach/St. Elias Range, Alaska
As effective agents of erosion, glaciers act as an "ultimate" erosional control on orogenic development. Widespread alpine glaciation occurs when a significant fraction of a mountain range lies above a local elevation threshold for glaciers, an occurrence which, by itself, implies fluvial systems are locally incapable of limiting peak elevation and relief in mountain belts. The glacial buzz saw hypothesis states that glaciers impose a limit to orogenic mean elevation, via feedback loops between topography, erosion rate, and glacial equilibrium line altitude (ELA). If rock uplift exceeds fluvial exhumation, mean elevation can rise to the threshold at which glaciers form. In a buzz saw world, glacial erosion and associated mass wasting and sediment transport are hyper effective, thus limiting further surface uplift. This implies that ELA, which varies in space and time depending on latitude, mean annual temperature, and precipitation, fundamentally limits topography and thereby influences orogenesis. Empirical tests of the glacial buzz saw hypothesis have mainly focused on topography, including comparisons of mean elevation, ELA, slope distribution, and relief. Although applicable to some ranges, not all active, glaciated mountain belts meet the predictions of the hypothesis. An alternate approach is to compare orogenic flux rates. If glacial erosion can keep pace with tectonic rock uplift, topography should be steady state, exhumation should equal rock uplift, and the rate of tectonic influx should be equal to the rate of erosional efflux at the orogen scale. Testing for a steady-state flux, however, is difficult, requiring absolute comparisons between disparate data sets. In the Chugach/St. Elias Range of southern Alaska, erosional efflux varies over the timescale of measurement. Long-term exhumation averages ~1-2 mm/yr, but this estimate is plagued by thermochronometric errors and uncertainties in geothermal conditions along crustal trajectories. The tectonic influx is more poorly constrained. Estimates of total volumetric influx can be made based on approximate dimensions of the colliding Yakutat microplate and rate of convergence. However, there are large uncertainties in crustal thickness, crustal composition, degree to which convergence is accommodated by accretion, and distribution of plate motion on individual structures. Based on existing data, inferring steady-state flux is permissible for this range, consistent with predictions of the buzz saw hypothesis. Yet, the uncertainties of this comparison demonstrate the difficulty of quantifying the coupling of erosion and tectonics in field settings. More data are required to further calibrate the applicability of the glacial buzz saw hypothesis in southern Alaska.
T33D-04 INVITED 14:25h
Contribution of Orography, Structure, and Geography to Deformation, Exhumation, and Topography of an Active Glaciated Collisional Orogen
Accretion of the Yakutat terrane (YT) to North America (NA) across the Chugach- St. Elias thrust fault (CSE) is forming the Chugach/St. Elias Range in southern Alaska. Glacial erosion has accompanied development of the orogenic belt over the last $\sim$6 My. A windward southern and leeward northern flank defines the orographic configuration across the range. Glacier redistribution on the windward side of the range is large (10's of km up- and down-valley ) relative to the leeward side of the range during Holocene glacial cycles (10$^{3}$ yr ). Glacier distribution in the landscape in space and time affects the proportion of landscape covered by glaciers, which in turn modulates base level for hillslopes, rivers in unglaciated portions of valleys, and sediment production, routing, storage, and delivery out of the orogen. Within the windward flank, low-T cooling ages imply a progressive increase in exhumation (from $<$1 mm/yr to $\sim$2 mm/yr) northward across strike from the deformation front on the south to the CSE on the windward flank. The magnitude of exhumation is smaller on the leeward side of the range relative to the windward flank. Orography thus controls the magnitude and frequency of glacier coverage within the orogen, which appears linked to larger degree of exhumation of the windward relative to the leeward flank of the range. Whereas YT-NA convergence velocity is $\sim$40 mm/yr, restoration of an area-balanced cross-section indicates $\sim$55 km of internal shortening has been accommodated within the YT on the windward flank near the eastern edge of the collision. Assuming this internal deformation accumulated after 6 Ma, $\sim$25% of the YT tectonic influx has accreted at $\sim$10 mm/yr onto the NA upper plate. Progressive exposure of deeper structural levels constrains erosional denudation from $<$ 5 km to $>$ 7 km from south-to-north, respectively. Structural variables such as plate interface dip, detachment stratigraphic locations, and convergence obliquity control the relatively small tectonic influx, shortening distribution, and exhumation and rock uplift rates. Mean elevation corresponds closely with the modern equilibrium line altitude of glaciers (ELA). The relatively low mean elevation of the range (2500 to 1100 m) reflects the 60$^{\circ}$ latitude and maritime setting of active deformation. Geography apparently dictates the dominant erosional process and topographic amplitude. Together, orography controls long-term exhumation magnitude because of the contrast in glacier distribution in space and time between the windward and leeward flanks, structure paces long-term exhumation rates because they are linked to rock uplift patterns, and geography determines erosion process and topographic form of this active orogenic belt.
T33D-05 14:40h
The results of coupled climate/tectonic numerical and analytical models of margin development during episodes of climate change
Current theory suggests that rock uplift in active orogens is coupled to surface processes, which are in turn coupled to climate. This idea is both analytically and numerically tested. A critical wedge based analytical model is compared with a geodynamic model that couples a thermo-mechanical model of crustal shortening with a landscape evolution model that includes glacial evolution. The analytical model predicts that at an active orogen and in a cooling climate, the "glacial buzzsaw hypothesis" produces an instantaneous increase in the rate of erosion, and that this increase can lead to a doubling in both rates of erosion and uplift. The numerical results reproduce the large increase in rock uplift and erosion, but also indicate that there is a significant delay in the coupled system. This delay is of the order of hundreds of thousands of years. The numerical model also suggests that the "glacial buzzsaw hypothesis" is inconsistent with current theories of glacial erosion in coupled orogenic systems.
T33D-06 14:55h
Chinese loess as a paleoenvironmental indicator of tectonics or climate: the role of the Arctic, cold air outbreaks, and lee cyclogenesis?
The evolution of the Tibetan Plateau is perhaps the archetypal example of using tectonic, erosional, and paleoenvironmental indicators to reconstruct the best possible history of tectonic and topographic development. One key piece of paleoenvironmental evidence is the very significant increase in the deposition of windblown dust in the region around Lanzhou, China, at around 7-8 million years ago. The resulting loess plateau is one of the most important repositories of paleoclimate information in Asia. Previous geological interpretations have suggested that increased loess deposition implies a strengthening of the wintertime atmospheric circulation regime, caused in part by enhanced deflection of the prevailing winds around the topography developing at that time. However, analyses of the controlling factors in the current climate show that dust storms are almost exclusively a springtime phenomenon, and occur during the passage of cold fronts. Moreover, enhanced wintertime circulations actively suppress dust storms, and therefore inhibit loess deposition. Instead, the important environmental controls on dust storms in the region are lee cyclogensis (storm development due to flow over topography), and cold air outbreaks (intrusions of cold air originating far to the north). In the current climate interannual variability of sea-ice extent in the Arctic Ocean is significantly correlated with the occurrence of these cold air outbreaks. Thus the increase in loess deposition resulted from a combination of the development of a springtime reservoir of cold air to the North, consistent with the onset of glaciation in Greenland at around the same time, and topographic development, which generated the weather disturbances necessary for dust storms.
T33D-07 15:10h
Rocks, Rivers, and Rain: Controls on Exhumation in Orogenic Belts?
Coupling between tectonics, erosion and climate is evident in many aspects of orogen evolution, including the pattern of exhumation observed at the Earth's surface. Indeed, there is strong evidence from numerical modeling studies that the distribution of precipitation is a primary control of the pattern of tectonic deformation. Further, because the exhumation pattern integrates information about the internal mechanisms of orogenesis, it is a useful tool for examining the manner and magnitude of the linkages between tectonics, erosion and climate. Currently lacking from analyses of natural orogens, however, is a sound theoretical understanding of how the components of the coupled system are expected to control the pattern of exhumation. We investigate this question with analytical and numerical models in which critical wedge theory describes the behavior of the tectonic system (e.g., fold-and-thrust belts and small accretionary orogens such as the Olympic mountains of Washington state). Critical wedge theory is arguably the simplest framework in which to consider this question; it predicts that the mean topographic slope in the direction of convergence maintains a critical taper angle and that perturbations are compensated by tectonic deformation. The analytical and numerical models contain different but self-consistent descriptions of surface erosion and tectonic deformation. In the analytical model, critical wedge theory constrains the mean topographic slope, and the stream-power law is used to simulate fluvial erosion. In the numerical model, a planform landscape evolution model, incorporating fluvial erosion and landsliding, is coupled with a model that simulates deformation of a Coulomb-plastic material. The most important difference between the two models is that ridge-valley relief is not allowed to develop in the former, whereas it arises naturally in the latter. With each model, we explore two scenarios: uniform precipitation and non-uniform precipitation with a boxcar increase above a background value. In the first scenario, we find that, 1) the erosion law determines the first-order form of uplift pattern, 2) non-uniform uplift is a natural response to uniform precipitation, and 3) uplift rate correlates well with ridge-valley relief in the numerical experiments. In the case of non-uniform precipitation, the analytical model predicts that the uplift rate increases both within and downstream of the precipitation increase, whereas the numerical model predicts that the uplift response matches the precipitation increase in both form and extent. From these results, we conclude that the uplift pattern in this tectonic framework is controlled primarily by erosion and the distribution of precipitation and that the development of ridge-valley relief has important implications for the form of the uplift and exhumation patterns.
T33D-08 15:25h
Exhumation and Plateau Growth in Northeastern Tibet
The lateral expansion of high topography in northern and eastern Tibet is important to both our understanding of the mechanics of plateau growth and tectonic/climate interactions related to the aridification of central Asia and the intensification of the east Asian and Indian monsoons. In particular, the initiation of basin sedimentation in northern Tibet has been used as a proxy for the development of high mean elevations in northern Tibet. However, poorly constrained thermal histories in adjacent fault bounded ranges and lack of provenance data from basin deposits make linkages between orogenesis, exhumation/sedimentation and climate tentative. Preliminary apatite (U-Th)/He ages from northeastern Tibet are Middle Miocene (7-12 Ma) at low structural positions in the West Qinling and Lajieshan ranges and early Cenozoic to Mesozoic (40-200 Ma) at progressively higher structural positions and within the Yellow River gorge. These data suggest initiation of fault activity by middle Miocene time, but that overall exhumation is limited to a few kilometers and occurs proximal to principle structures. Geomorphic surfaces and Oligocene helium ages (30 Ma) at low structural positions on the Eastern Kunlun Range (Qaidam Fault) suggest minimal amounts of shortening on steep faults along the Qaidam Basin plateau margin despite having a steep topographic escarpment and a major step in the Moho at depth. The Nd isotopic record from Linxia Basin sediments suggests that early mudstone deposition (since 29 Ma) was dominated by loess and that significant unroofing of plutonic and metasedimentary source rocks in the adjacent West Qinling range did not begin until $\sim$14 Ma, coeval with \delta$^{18}$O data suggesting increased aridity at this time (Garzione et al., this session). These data suggest that loess deposition and flexural loading began 15 Ma prior to the major erosional episode that exhumed the W. Qinling range. Differences in ages between initial basin sedimentation and major exhumation of adjacent fault bounded ranges suggested by Garzione et al., and limited erosional exhumation of these structures indicated by our helium data, suggests that the timing of initial basin formation in northeastern Tibet need not be a primary indicator of high topography. Low surface strain and a relatively arid climate could effectively limit any feedback mechanisms between erosion and orogenic growth- a principle difference between northern and southern Tibet despite similarities in their extreme mean elevation and regional topographic gradients.