[ghsc-seminars] GHSC Seminar THIS MORNING, April 20th @ 10am: Sean Gallen, Colorado State University

[Oliver Boyd]

"Lithologic controls on landscape dynamics, aquatic species evolution and intraplate seismic zones in a decay phase orogen."

Professor Sean Gallen

Colorado State University

Contrary to the paradigm of slow, steady topographic decay after orogenesis ceases, nearly all ancient mountain belts exhibit evidence of unsteady landscape evolution at large spatial scales, and many of these setting host anomalous zones of intraplate seismicity. External forcing from dynamic uplift due to mantle processes, climate change or some other exotic process is commonly invoked to explain the unexpected dynamics of dead orogens, yet direct evidence supporting such inferences is generally lacking. Here we use quantitative analysis of fluvial topography in the southern Appalachian Mountains to show that the exhumation of rocks of variable erosional resistance exerts a fundamental, autogenic control on the evolution of post-orogenic landscapes that continually reshapes river networks. We demonstrate that the transient, cascading impact of landscape change imposed on regions with variable lithology can act to focus erosion and perturb stress in the upper-to-mid crust, which might explain some intraplate seismic zones. We characterize the spatial pattern of erodibility associated with individual rock-types in the Upper Tennessee drainage basin. Inverse modeling of river profiles is used to argue for a ~150 m base level fall event at 9 ± 3 Ma in the Upper Tennessee basin, which lies directly above the Eastern Tennessee Seismic Zone (ETSZ), second most seismically active region east of the Rocky Mountains. This analysis, combined with existing geological and biological data, indicates that base level fall was triggered by capture of the Upper Tennessee River basin by the Lower Tennessee River basin in the Late Miocene. Modeling results indicate that rock-type triggered changes in river network topology gave rise to the modern Tennessee River system and enhanced erosion rates, changed sediment flux and dispersal patterns, and altered bio-evolutionary pathways in the southeastern U.S.A., a biodiversity hotspot. Paleo-topographic reconstructions indicate that base level fall in the Upper Tennessee basin preferentially eroded ~3,550 ± 800 km3 (mean ± 1𝜎) of rock from a ~70 km wide by ~350 km long corridor of highly erodible units in the ancient fold-thrust belt of the Late Paleozoic Alleghanian orogeny, directly above the ETSZ. Stress modeling indicates that spatially focused erosion reduced fault clamping stress at 15 km depth on average by ~2.5 MPa, with average annual unclamping rates of ~0.3 Pa yr-1. Under the assumption that the crust is critically stressed, it is argued that preferential erosion of less competent overburden created a zone of reduced clamping stress, allowing for slip on pre-existing basement structures in the ambient stress field. Collectively, these findings suggest that rock-type imparts a fundamental control on the dynamics of ancient mountain settings, driving landscape disequilibrium, augmenting the evolution of aquatic species and perturbing crustal stress fields long after tectonic activity ends.

Friday, April 20th, 2018, 10-11am (Mountain Time)


USGS, 1711 Illinois Street, Golden, CO
Entry Level Seminar Room

*Note: Please arrive ~5 minutes early and bring photo ID for airport-style security measures now in place at the USGS building.

Thank you,
GHSC Seminar Committee 

Mirus, Ben - bbmirus at usgs.gov
Josh Rigler - erigler at usgs.gov
Oliver Boyd - oboyd at usgs.gov

 

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