The Blue Ridge belt in northwestern North Carolina and northeastern Tennessee is composed chiefly of 1,000-million to 1,100-million-year-old metamorphic and plutonic rocks that have been thrust many miles northwestward across unmetamorphosed Cambrian(?) and Cambrian sedimentary rocks of the Unaka belt. The Blue Ridge thrust sheet is rooted on the southeast along the Brevard zone, a zone of strike-slip faulting along which metamorphic and plutonic rocks of the Inner Piedmont belt are juxtaposed with rocks of the Blue Ridge.Near the southeastern edge of the Blue Ridge belt, the Blue Ridge thrust sheet is breached by erosion, and the rocks beneath are exposed in the Grandfather Mountain window, which is 45 miles long and as much as 20 miles wide; it is the only major window so far recognized in the Blue Ridge belt. The rocks exposed within it include 1,000-million-year to 1,100-million-year-old plutonic basement rocks, sedimentary and volcanic rocks of late Precambrian age, and an allochthonous tectonic slice of Lower Cambrian ( ? ) and Cambrian sedimentary rocks identified with the Chilhowee Group and Shady Dolomite in the Unaka belt 20 to 30 miles to the northwest.The Blue Ridge thrust sheet surrounding the Grandfather Mountain window consists largely of schist, gneiss, and amphibolite derived by •metamorphism of sedimentary and volcanic rocks 1,000 to 1,100 m.y. ago, and of Cranberry Gneiss, a complex of migmatite and granitic rocks which underlies the metasedimentary and metavolcanic rocks and which probably formed during the same metamorphic episode. The Cranberry Gneiss is intruded by the Beech Granite, by aegirine-augite granite, and by quartz monzonite, all of which were emplaced during a late stage of or after the plutonic metamorphism. Stocks and dikes of Bakersville Gabbro of late Precambrian(?) age and small bodies of ultramafic rock, granodiorite, and pegmatite of early or middle Paleozoic age intrude the earlier Precambrian rocks. Although all these rocks may have been metamorphosed about 450 nt.y. ago, the principal Paleozoic dynamothermal metamorphism occurred about 350 m.y. ago. At that time new medium-grade minerals, including staurolite, kyanite, monoclinic pyroxene, epidote, and calcic plagioclase, crystallized in the schist, gneiss, and amphibolite. During the late Paleozoic, most of the plutonic rocks were partly reconstituted to low-grade blastomylonitic and phyllonitic gneisses containing new biotite, albite, sericite, chlorite, actinolite, and epidote, whereas the overlying rocks were largely unaffected. The contact between low-and medium-grade rocks may be a fault. vard wa:s contemporaneous with and mechanically related to northwest movement of the Blue Ridge and Tablerock thrust sheets.A dike of unmetamorphosed Upper Triassic(?) diabase cuts the rocks of the Inner Piedmont, the Brevard fault zone, the Blue Ridge thrust sheet, and the Grandfather Mountain window.The rocks of the Blue Ridge thrust sheet moved northwestward at least 35 miles over the Grandfather Mountain window after the clos...

Apatite fission-track (AFT) data from Proterozoic and Paleozoic rocks in the mountains of north central Colorado (White RiverUplift, Gore Range, and western Front Range) record significant cooling that began with uplift and erosion related to the Laramide Orogeny and continued through the Tertiary to Pliocene time. The mountains immediately flanking the Blue River half graben (Williams Fork Mountains to the east and the Gore Range to the west) cooled significantly during the Neogene. The AFT ages along the flanks of the Blue River half graben are significantly younger than AFT ages farther to the east in the central and eastern Front Range and to the west in the White River uplift. In both of these areas, the apatite ages suggest Laramide cooling. The Williams Fork Mountains-Gore Range zone of young AFT ages extends southward adjacent to the axis of the Rio Grande rift through southern Colorado and New Mexico. These young ages result from a combination of elevated heat flow, uplift, and erosion along the axis of the Rio Grande rift during Neogene time.Zircons from Proterozoic rocks yield Proterozoic fission-track ages, indicating that this part of the Colorado basement has not been heated to temperatures Ͼ200 ЊC since Middle Proterozoic time.A sanidine 40 Ar/ 39 Ar age of 27 Ma from a rhyolite tuff just above a basal boulder conglomerate of the Troublesome Formation in a tilted fault block within the Blue River half graben shows that Tertiary deposition started there in middle Oligocene time. Xenocrystic sanidine from a basalt stratigraphically higher than the rhyolite tuff has an age of 24 Ma. Thus, the basalt is significantly younger than its postulated source, the 32 Ma laccolithic complex at Green Mountain.

No abstract

No abstract

40 Ar/ 39 Ar dating results of 133 samples from 84 late Cenozoic volcanic rocks provide emplacement ages that constrain the timing of evaporite collapse and the incision rates of the Colorado River. Our samples are from areas in west-central Colorado, both within and outside of the Carbondale and Eagle collapse centers. Significant pulses of volcanic activity occurred in the intervals from 24 to 22, 16 to 13, 11 to 9, and 8 to 7 Ma. In addition, small flows, widely spaced in time and space were emplaced during the last 4 m.y. Although individual basaltic flows appear to be chemically and isotopically homogeneous, there are significant geochemical and isotopic differences between flows, even between some flows that apparently have the same age within the limits of analytical precision.A low-relief early to middle Miocene erosional surface has been postulated in west-central Colorado. Our studies are consistent with the existence of a low-relief paleotopographic surface that is now at a minimum elevation range of ϳ2.9-3.4 km outside areas of collapse. Elevation departures from this range suggest that Ͼ1000 m of subsidence due to evaporite removal has locally occurred in the Carbondale and Eagle collapse centers. 40 Ar/ 39 Ar ages from downdropped and disrupted basaltic flows in the Carbondale center constrain initial collapse to Ͼ13 Ma, the timing of much of Downloaded from M.J. Kunk et al. 214 the evaporite-related collapse to the past 10-8 m.y., and an increase in the rate of collapse during the last 3 m.y.Ages and elevations of basaltic rocks above the Colorado River in Glenwood Canyon are used to calculate average apparent incision rates for the Colorado River in Glenwood Canyon of 24 mm/k.y. from 7.8 to 3.0 Ma. The average apparent incision rate increased by an order of magnitude to 242 mm/k.y. during the last 3 m.y. C o lo r a d o R iv e r E agle R iv e r F o r k Flank of Mt. Sopris E a gl e R iv e r W h it e R iv e r C o lo r a d o R iv er