A numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes. Crustal deformation is calculated by a two-dimensional finite element model of deformation in response to subduction and accretion of continental crust. Erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power. The relative importance of the tectonic and erosion processes is given by a dimensionless “erosion number” relating convergence velocity, rock erodibility, and precipitation rate. This number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt. Fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction. These characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt. The two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity. Models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior. Models of the latter case predict a focused zone of exhumation at the margin of the orogen and, at high erosion number, a reversal in the topographic asymmetry. Natural examples of these two cases are presented. The Southern Alps of New Zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate. The asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry. The contrasting example of dominant wind in the direction of subduction motion is provided by the Olympic Mountains of Washington State. In this case, exhumation of deep levels of the Cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900248

Orogeny and orography: The effects of erosion on the structure of mountain belts

Erosion of orogenic mountain ranges exhumes deeply buried rocks and controls weathering, climate, and sediment production and transport at a variety of scales. Erosion also affects the topographic form and kinematics of orogens, and it may provide dynamic feedbacks between climate and tectonics by spatially focused erosion and rock uplift. Thermochronology measures the timing and rates at which rocks approach the surface and cool as a result of exhumation. Relatively well-understood noble gas and fission-track thermochronometric systems have closure temperatures ranging from ∼60 to ∼550 ◦ C, making them sensitive to exhumation through crustal depths of about one to tens of kilometers. Thus, thermochronology can constrain erosion rates and their spatial-temporal variations on timescales of ∼10 5 –10 7 years, commensurate with orogenic growth and decay cycles and possible climate-tectonic feedback response times. Useful methods for estimating erosion rates include inverting ages for erosion rates using crustal thermal models, vertical transects, and detrital approaches. Spatial-temporal patterns of thermochronometrically determined erosion rates help constrain flow of material through orogenic wedges, orogenic growth and decay cycles, paleorelief, and relationships with structural, geomorphic, or climatic features.

/pdf/using-thermochronology-to-understand-orogenic-erosion-3sbe4s46h9.pdf

Using thermochronology to understand orogenic erosion

The dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state. This concept is often invoked, but the nature of the steady state is commonly not specified. We identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical-model results and natural examples. These types are (1) flux steady state, (2) topographic steady state, (3) thermal steady state, and (4) exhumational steady state: they refer to the erosional flux, the topography, the subsurface temperature field, and the spatial pattern of cooling ages, respectively. Models suggest that the topography will reach a steady mean form at the scale of an orogenic belt, but perfect topographic steady state is unlikely to be achieved at shorter length scales. Thermal steady state is a precondition for exhumational steady state and in the case of temperature-dependent deformation, topographic steady state. Exhumational steady state is characterized by reset age zones spatially nested according to closure temperature, as illustrated in natural systems from New Zealand, the Cascadia accretionary margin, and Taiwan.

On steady states in mountain belts

[1] Many important insights regarding the coupling among climate, erosion, and tectonics have come from numerical simulations using coupled tectonic and surface process models. However, analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered. We present an approximate analytical solution for two-sided orogenic wedges obeying a frictional rheology, and in a condition of flux steady state, that makes explicit the nature and sensitivity of the coupling between climate and deformation. We make the simplifying assumption that the wedge grows in a self-similar fashion consistent with Airy isostasy such that topographic taper is invariant with orogen width, tectonic influx rate, and climate. We illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen-scale erosion rule. We show that steady state orogen width, crest elevation, and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process. Remarkably, we show that for most combinations of parameters in the erosion law, rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux. Further, assuming frontal accretion with no underplating, the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro-wedge and retro-wedge and the time-averaged trajectories of rocks through the orogen. The restriction to invariant frictional properties is conservative in these respects; systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate, erosion, and deformation than shown here.

/pdf/controls-on-the-strength-of-coupling-among-climate-erosion-154nug31gr.pdf

Controls on the strength of coupling among climate, erosion, and deformation in two-sided, frictional orogenic wedges at steady state

Reconciling the Intertropical Convergence Zone, Himalayan/Tibetan tectonics, and the onset of the Asian monsoon system.

Orogeny and metamorphism; a model for deformation and P-T-t paths with applications to the Central and Southern Appalachians

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Orogeny and metamorphism; a model for deformation and P-T-t paths with applications to the Central and Southern Appalachians