Layer parallel shortening; a mechanism for determining deformation timing at the junction of the Central and Southern Appalachians

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ID: 305347
1999
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Abstract
Near the transition from the southern Appalachians to the central Appalachians at the Roanoke recess, major southern Appalachian thrusts (D070°striking) with steep initial ramps (D25°to 30°) are associated with footwall synclines with nearly vertical to overturned southeast-limbs.The 170-km long, overturned, Glen Lyn footwall syncline (to the St. Clair thrust) forms the structural front of the southern Appalachian fold and thrust belt southwestward from the recess.Deformed mudcracks, o ¨olites, crinoid-stem plates, chert nodules, and bedding-normal burrows at 32 localities, mostly in the overturned limb, for 84 km from near Glen Lyn, Virginia (on the New River) to near Richlands, Virginia, primarily reflect presyncline layer parallel shortening (LPS) strain by intragranular mechanisms.The eastern part of the syncline averages 14 percent shortening attributable to LPS, assuming constant volume deformation, whereas in the western part, LPS averages 9 percent.Percent shortening for LPS strain is typically much lower (1-13 percent) in carbonates than it is in nearby clastic sedimentary rocks (3-21 percent), because strain is largely accommodated by outcrop-scale pressure solution along stylolites in the carbonates.Restoration of overturned bedding to horizontal, taking into account the trend and plunge of the segments of the fold axis, yields LPS directions of 264°to 342°with an average of 316°and a standard deviation of 19°.Although 25 percent of the LPS directions are in the range of southern Appalachian trends (330°-345°), most are in the ranges of central Appalachian (44 percent) and transitional (31 percent) trends.All are oblique to the local trends (067°-081°) of the segments of the axis of the Glen Lyn syncline used for restoration of bedding to horizontal.Hence, 28 LPS strain-ellipses were rotated about a vertical axis, so that the local segments were aligned with the trend (067°) of the axis at Glen Lyn.LPS directions then range from 262°to 331°, with an average of 308°and a standard deviation of 17°.If anomalous shortening directions obtained for carbonates with very low percent (1 percent) LPS are excluded, then the average LPS direction is 311°.This principal LPS direction lies 26°from the perpendicular to the Glen Lyn syncline (067°) but is more consistent with shortening directions of D300°in the central Appalachians reported by other workers.Earlier papers concluded that development of central Appalachian structures postdated development of southern Appalachian structures.However, preservation of central/transitional LPS strain in the overturned limb of this major southern Appalachian fold indicates that central/transitional Appalachian deformation initiated prior to formation of the Glen Lyn syncline, a southern Appalachian fold.
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openalex_W1993079322 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors A. E. Whitaker
Journal american journal of science
Year 1999
DOI
10.2475/ajs.299.3.238
URL
https://ajsonline.org/article/60813.pdf
Keywords Keywords not found

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