Miocene Rejuvenation of Topographic Relief in the Southern Appalachians
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CELEBRATE GSA’S 125TH ANNIVERSARY FEBRUARY 2013 | VOL. 23, NO. 2 A PUBLICATION OF THE GEOLOGICAL SOCIETY OF AMERICA® Miocene rejuvenation of topographic relief in the southern Appalachians FEbruarY 2013 | VOLUME 23, NUMBER 2 SCIENCE ARTICLE GSA TODAY (ISSN 1052-5173 USPS 0456-530) prints news and information for more than 25,000 GSA member read- ers and subscribing libraries, with 11 monthly issues (April/ May is a combined issue). GSA TODAY is published by The Geological Society of America® Inc. (GSA) with offices at 3300 Penrose Place, Boulder, Colorado, USA, and a mail- ing address of P.O. Box 9140, Boulder, CO 80301-9140, USA. 4 Miocene rejuvenation of topographic relief GSA provides this and other forums for the presentation in the southern Appalachians of diverse opinions and positions by scientists worldwide, regardless of race, citizenship, gender, sexual orientation, Sean F. Gallen, Karl W. Wegmann, and religion, or political viewpoint. Opinions presented in this DelWayne R. Bohnenstiehl publication do not reflect official positions of the Society. © 2013 The Geological Society of America Inc. All rights Cover: Cullasaja Falls, North Carolina, USA. Photo reserved. Copyright not claimed on content prepared © Dave Allen Photography; used with permission. wholly by U.S. government employees within the scope of their employment. Individual scientists are hereby granted permission, without fees or request to GSA, to use a single figure, table, and/or brief paragraph of text in subsequent work and to make/print unlimited copies of items in GSA TODAY for noncommercial use in classrooms to further education and science. In addition, an author has the right to use his or her article or a portion of the article in a thesis or dissertation without requesting permission from GSA, provided the bibliographic citation and the GSA copyright credit line are given on the appropriate pages. 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North Carolina State University, 2800 Faucette Drive, Raleigh, Two hypotheses have been put forth to explain the occurrence North Carolina 27695, USA of locally high topographic relief, steep slopes, incised river gorges, and frequent mass-wasting events along the passive margin of the southern Appalachians (e.g., Gallen et al., 2011; Wooten et al., ABSTRACT 2008). One suggests that topography has persisted though time in Conventional wisdom holds that the southern Appalachian a dynamic equilibrium, with relief largely controlled by the Mountains have not experienced a significant phase of tectonic variable erodibility of rock units (Hack, 1960; Matmon et al., forcing for >200 myr; yet, they share many characteristics with 2003). The second posits that modern relief is a product of recent tectonically active settings, including locally high topographic rejuvenation (Hack, 1982); however, whether the process relief, steep slopes, incised river gorges, and frequent mass-wasting governing this resurgence is climate change (Molnar, 2004; events. Two competing hypotheses are commonly used to explain Hancock and Kirwan, 2007) or dynamic mantle processes forcing their modern topographic expression. One suggests that relief is epeirogenic uplift (Pazzaglia and Brandon, 1996) is debated. largely controlled by variable lithologic resistance to weathering Recent results obtained from the application of thermochronology and that their modern form has long persisted in a dynamic (Boettcher and Milliken, 1994) and terrestrial cosmogenic equilibrium. The second postulates that their relief is a product of radionuclides (CRNs; Matmon et al., 2003; Hancock and Kirwan, recent rejuvenation, driven either by climate change or the 2007) have not led to a consensus regarding the processes epeirogenic uplift of the land surface driven by mantle forcing. governing the evolution of relief within this landscape—a result Within portions of the Cullasaja River basin of the southern of contrasting interpretations drawn from different datasets. Appalachians, we show that relief has increased by >150% since We test the competing hypotheses of dynamic equilibrium and the Miocene. Evident within the basin are a set of retreating topographic rejuvenation with a study of the geomorphology of knickpoints that delineate a rugged, actively incising landscape the ~300 km2 Cullasaja River basin of the southern Appalachian from lower-relief relict topography. Constraints on the timing of Mountains in western North Carolina (Figs. 1A and 1B). The knickpoint entry into the basin suggest that the process of Cullasaja is a tributary to the Little Tennessee River, its waters landscape rejuvenation began well prior to the late Cenozoic traveling >1500 river kilometers before discharging into the Gulf (<4 myr) transition to a more oscillatory (glacial-interglacial) of Mexico (Fig. 1A). The timing and magnitude of changes in climate regime. Furthermore, the geomorphology of the Cullasaja relief within the basin are quantified through the analysis of a River basin is difficult to reconcile in the context of a transition to 6-m horizontal resolution LiDAR elevation dataset. Results a more erosive climatic regime but is consistent with an indicate that the Cullasaja basin landscape has undergone a period epeirogenically uplifted landscape. Consequently, these of rejuvenation, with relief increasing >150% since the Miocene. observations lend new support to the idea that the rugged The timing of this rejuvenation and the geomorphic expression of topography of the southern Appalachians has developed in the Cullasaja basin landscape, however, suggest that climate response to post-orogenic regional uplift in the Miocene. change is not the fundamental driving process (cf. Molnar, 2004). Rather, observational evidence favors a model where relief INTRODUCTION develops as the landscape is epeirogenically uplifted. Topographic relief exerts an essential control on the rates and STUDY AREA processes involved in landscape denudation (Ahnert, 1970; Montgomery and Brandon, 2002), influencing feedbacks between The Cullasaja River basin contains the geomorphic features atmospheric, earth-surface and rock exhumation processes, required to reconstruct its paleo-relief, including numerous active variations in sediment flux, and the magnitude and style of river knickpoints—sharp convexities in an otherwise concave-up gravity-driven natural hazards. A long-standing debate in the longitudinal river profile—and a preserved relict landscape geosciences is centered upon the nature of topographic decay in “surface.” The study area lacks evidence of late Pleistocene RY 2013 RY 2013 post-orogenic mountain ranges (Davis, 1889; Hack, 1960; Bishop, glaciation and because of its distance from the maximum extent A 2007). Central to this debate are the still-rugged terrains