Crustal Thickness Variations across the Blue Ridge Mountains, Southern Appalachians: An Alternative Procedure for Migrating Wide-Angle Reflection Data

Abstract

Migration of wide-angle reflections generated by quarry blasts suggests that crustal thickness increases from 38 km beneath the Carolina Terrane to 47-51 km along the southeastern flank of the Blue Ridge. The migration algorithm, developed for generating single-fold images from explosions and earthquakes recorded with isolated, short-aperture arrays, uses the localized slant stack as an intermediate data set. In contrast with other methods, it includes an interpretive step that is based on the assumption that all coherent P-wave energy consists of reflections from planar interfaces. Each sample in the slant stack is mapped into a planar, dipping segment with a length that is determined by the recording aperture. Migrated sections from within the Blue Ridge show increases in reflectivity at depths of 20 and 40 km. High apparent reflectivity from 40 to 50-55 km suggests a layered zone in the lower crust that is similar to models proposed for the Cumberland Plateau in Tennessee and the Adirondacks. The migration results are consistent with regional gravity data and with the occurrence of crustal roots beneath the Urals, another Paleozoic orogen.

Isostatic compensation for a portion of the Southern Appalachians: Evidence from a reconnaissance study using wide-angle, three-component seismic soundings

“…For a 0.5 s delay (corresponding to energy near the end of a 0.5-s-duration source wavelet), the overestimate would range from 1.5 to 4 km (Hawman, 2008). In principle, depth errors for sections derived from migration of S-waves are smaller than those derived from migration of P-waves by a factor given by the Vp/Vs ratio, because of the smaller velocities.…”

“…Their models show a 50-kmthick crust and Moho transition zone ~5 km thick. More recent experiments using controlled blasts (Luetgert et al, 1994) and timed quarry blasts (Hawman, 1996(Hawman, , 2004(Hawman, , 2008 are discussed later herein.…”

“…7) were generated from wide-angle vertical-and transverse-component data using a simplifi ed procedure for migrating common-source gathers (Hawman, 2008). Shear-wave refl ections were migrated by converting P-wave velocity models to S-wave velocity using estimates of aver age crustal Vp/Vs.…”

“…Shear-wave refl ections were migrated by converting P-wave velocity models to S-wave velocity using estimates of aver age crustal Vp/Vs. To better evaluate and increase the reliability of events used for generating the migrated images, the data were fi rst coherency filtered using a setting that eliminated 99% of the samples in the noise window immediately preceding the arrival time for the direct P wave (Hawman, 2008). This minimized the inadvertent migration of random noise, while keeping the waveforms of coherent waveforms largely intact.…”

“…This is an update of an earlier map for the Blue Ridge Mountains (Hawman, 2008), extended to adjacent provinces and modifi ed to incorporate additional depth soundings and reinterpretations of some events within the Blue Ridge as refl ections from the uppermost mantle rather than the Moho.…”