Geomorphology 123 2010 305 319 Contents lists available at ScienceDirect Geomorphology j o u r n a l h o m e p a g e w w w e l s ev i e r c o m l o c a t e g e o m o r p h New physical evidence of the role of stream capture in active retreat of the Blue Ridge escarpment southern Appalachians Philip S Prince James A Spotila William S Henika Department of Geosciences Virginia Polytechnic Institute and State University Blacksburg VA 24061 USA a r t i c l e i n f o Article history Received 8 February 2010 Received in revised form 13 July 2010 Accepted 26 July 2010 Available online 11 August 2010 Keywords Appalachians Landscape evolution Escarpment retreat Passive margin escarpment Stream capture Divide migration a b s t r a c t The Blue Ridge escarpment of the southern Appalachian Mountains is a striking and rugged topographic feature of the ancient passive margin of eastern North America The crest of the escarpment generally coincides with an asymmetric regional drainage divide separating steep streams of the escarpment face from low gradient streams of the Blue Ridge Upland Recent exhumation and erosion rate studies suggest that the escarpment has evolved by inland erosional retreat but the mechanism timing and magnitude of retreat remain poorly understood Longitudinal stream pro les and slope drainage area relationships of several upland basins draining the divide have led to the identi cation of 14 previously unknown uvial terrace deposits preserved at the escarpment crest These relict terraces and the associated beheaded drainages indicate the role of large stream capture events in producing ongoing escarpment retreat through landward divide migration and subsequent topographic adjustment Terrace location and preservation suggest that rectilinear drainage patterns and divide asymmetry generate discrete high order captures and episodes of rapid localized retreat that collectively produce slower evolution of the escarpment at large While overall retreat magnitude and rate remain unknown roundness of terrace alluvium suggests that the most recent captures have locally produced tens of kilometers of retreat within the limited preservation lifetime of the deposits In contrast with recent numerical modeling and cosmogenic studies these data show the potential for stream capture and divide migration to sustain passive margin escarpment evolution long after the cessation of rifting The uvial record of divide retreat preserved atop the Blue Ridge escarpment suggests the potential for using eld methods to better constrain the histories of younger taller and potentially more dynamic passive margin escarpments 2010 Elsevier B V All rights reserved 1 Introduction Major seaward facing topographic escarpments are common features of rifted passive margins Such great escarpments are believed to initiate during rift ank uplift and subsequently evolve through erosional processes Ollier 1984 Kooi and Beaumont 1994 Gallagher and Brown 1997 Rapid erosion focused on the steep ank of a rift may produce inland retreat Ollier 1984 ten Brink and Stern 1992 Young and McDougall 1993 Tucker and Slingerland 1994 Seidl et al 1996 but the controls on the rate timing and mechanism of escarpment migration following rifting are poorly understood Some great escarpments such as southwestern Africa and southeastern Australia appear to have migrated little since the early stages of development Moore et al 1986 Gilchrist et al 1994 Bishop and Goldrick 2000 Cockburn et al 2000 Matmon et al 2002 Persano et al 2002 while other escarpments such as southeastern Brazil and the Blue Ridge escarpment of eastern North America may have continued to retreat into the late stages of escarpment development Bohannon et al 1989 Gallagher et Corresponding author Tel 1 864 320 3243 fax 1 540 231 3386 E mail address psprince vt edu P S Prince 0169 555X see front matter 2010 Elsevier B V All rights reserved doi 10 1016 j geomorph 2010 07 023 al 1994 Steckler and Omar 1994 Brown et al 2000 Spotila et al 2004 The existence of a universal paradigm applicable to great escarpment evolution is thus questionable Further constraining the chronology and mechanisms of great escarpment retreat particularly along mature passive margins is signi cant to improving our understanding of the evolution of passive margin landscapes After rifting escarpments may be rapidly excavated by downwearing seaward of a xed drainage divide van der Beek and Braun 1999 Matmon et al 2002 or experience slow but steady retreat in parallel with the divide King 1962 Fig 1 However no clear global relationship exists between escarpment age determined by age of rifting and distance from the coast the Sri Lanka escarpment 180 Ma 65 km from the present coastline Vanacker et al 2007 the Drakensberg escarpment 130 Ma 150 km from the present coastline Moore and Blenkinsop 2006 and the southeastern Australia escarpment 85 100 Ma 60 km from the coastline Weissel and Seidl 1998 imply different overall retreat distances and long term average rates The Blue Ridge escarpment BRE a passive margin escarpment often overlooked in great escarpment studies occurs along the oldest passive margin in the world initial rifting at 200 Ma Pique and Laville 1995 McHone 1996 yet it is located within 70 km of inland rift basins and maintains steep youthful topography Spotila et al 2004 The timing of 306 A P S Prince et al Geomorphology 123 2010 305 319 Preexisting inland divide Rift B Initial divide Rift C Initial divide Rift Fig 1 Comparison of three popular models of passive margin escarpment evolution Dotted lines trace the development of escarpment topography across equal time intervals All models ultimately produce escarpments of similar morphology despite differing mechanisms and rates of retreat A Rift related base level drop seaward of a preexisting drainage divide steepens large streams and rapidly excavates an escarpment at the divide Subsequent retreat is minimal B Rift ank uplift produces an asymmetric divide atop the rift shoulder Focused erosion on the steep ank of the divide produces steady parallel retreat of divide and escarpment C The parallel retreat mechanism of B decelerates because of the decrease in escarpment relief After van der Beek et al 2002 exhumation of the area seaward of the BRE is comparable with that observed seaward of the 100 Myr younger southeastern Australia escarpment each with low temperature U Th He apatite cooling ages of 85 110 Ma Persano et al 2002 Spotila