TECTONIC GEOMORPHOLOGY AND EarthScope IN EASTERN NORTH AMERICA

Rivers, their long profiles, and terraces are well-accepted geomorphic markers of rock uplift and crustal deformation. The eastern U.S. landscape contains several well-documented anomalous geomorphic markers consistent with slow, but poorly understood processes of active crustal deformation in what is otherwise viewed as a passive margin. We focus on the longitudinal profiles and knickpoints of large Atlantic slope rivers including the Susquehanna and Potomac where they traverse the Fall Zone, an enigmatic geomorphic feature with 100 m of relief that may represent relatively recent rock uplift of the Appalachian Piedmont. We note that all mid-Atlantic rivers are deeply incised into the Fall Zone and Piedmont, and have several knickpoints with no apparent relation to rock type or structure. Mapping of these knickpoints in the Piedmont reveals that they cluster around three common elevations of ~45, ~75, and ~110 m, coincident with terraces or pediments suggestive of unsteady rock uplift. Even higher knickpoints of ~300 and 400 m preserved deep in the Appalachian landscape hint at long-term unsteadiness and long-lived transients working through the drainage network. Application of various stream-power based knickpoint retreat models estimate the time of rock uplift and support the notion that knickpoints can persist for several tens of millions of years. Recent geodynamic modeling of east coast dynamic topography predicts ~200 m of rock uplift in the past 30 m.y. in good general agreement with the elevation of the knickpoints. Integration of these kinds of geomorphic data sets with the seismology represented by the EarthScope transportable and flexible arrays presents an opportunity to investigate several active tectonic problems in the eastern U.S. including the origin of the Fall Zone, dynamic topography, and the underlying processes responsible for zones of seismicity and intervening seismic gaps.