Published: Wednesday, February 26, 2014
Carl Stearns, Pierre Arroucau, Gordana Vlahovic
Previous studies have shown that Digital Elevation Model (DEM) analysis could be used to quantify surface deformation in tectonically active regions, including slowly deforming areas such as intraplate continental interiors. Here, we investigate slope/area relationships and determine stream length index (SLI) spatial variations in 287 watersheds located in the Valley and Ridge physiographic province of the southern Appalachians, in a region known as the eastern Tennessee seismic zone (ETSZ). The goal is to identify possible spatial variations in drainage network characteristics that could reveal different deformation rates and styles within the study area. The ETSZ, although seismically active, does not show any evidence of recent surface deformation that could be related to tectonic activity. The earthquakes mostly occur between 5 and 25 km depth and their epicenters form a SSW-NNE trending, 300 km long by 100 km wide, band of diffuse seismicity that aligns along the New York Alabama (NYAL) magnetic lineament, a linear magnetic feature attributed to a fault affecting the Precambrian basement but without signature at the surface. DEMs with a resolution of 30 meters and watershed boundaries of 287 drainage basins were obtained from the United States Geological Survey (USGS) National Elevation Dataset (NED) and National Hydrography Dataset (NHD), respectively. After determining the local slope and drainage area for each 30 m x 30 m cell, reference concavity and steepness index values were calculated for the entire region. Then, the steepness index of each watershed was determined using the obtained reference concavity index. SLI values were also determined along extracted river profiles and average values calculated for each watershed. A good correlation is observed between steepness index and SLI, with low to mid-range values found in the Valley and Ridge province and higher values in two specific areas: at the transition between the Valley and Ridge province and the Cumberland Plateau, and at the transition between the Valley and Ridge and Blue Ridge provinces. High values of these indices are commonly interpreted as indicative of uplift; however they can simply reveal resistant lithologies. In order to address that question, we used the erosional resistance index (ERI) values determined in a previous study for rocks of eastern Tennessee and we assessed the rock erodibility at individual SLI measurement locations. The very poor correlation between ERI and SLI suggests that if high SLI and steepness index values correspond mostly to the expression of rock resistance to erosion, a contribution of tectonic uplift cannot be ruled out.