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Geology of the Grandfather Mountain Window and Vicinity, North Carolina and Tennessee

September 11, 2023

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 m.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. Layering and foliation in rocks of the Blue Ridge thrust sheet dip away from the Grandfather Mountain window on all sides, and broad flexures in these structures plunge away from its northwest and northeast corners. Minor folds in both the low- and medium-grade rocks are of two generations: (1) tight and isoclinal folds having axial planes parallel to foliation and layering and axes trending in various directions in the plane of the foliation, and (2) later open folds and crinkles having steep axial planes and northeast trending axes perpendicular to a well-developed northwest trending mineral lineation. The early folds, which are possibly 350 m.y. old, perhaps formed during an early stage of thrusting and were themselves deformed during continued thrust movement. The later folds formed in a late stage of the thrusting. In the northwest corner of the area, an intermediate sheet of partly metamorphosed Precambrian plutonic rocks occurs between the Blue Ridge thrust sheet and rocks of the Unaka belt in the Mountain City window. The Blue Ridge thrust sheet overrides both the intermediate sheet and the Mountain City window, in which rocks of the Chilhowee Group of Cambrian(?) and Early Cambrian age and the Shady Dolomite and Rome Formation of Early Cambrian age are exposed. The basement exposed in the Grandfather Mountain window is composed principally of nonlayered granitic gneiss (Wilson Creek Gneiss) and coarse-grained augen gneiss (Blowing Rock Gneiss), both 1,000 to 1,100 m.y. old. The plutonic basement rocks are stratigraphically overlain by the Grandfather Mountain Formation, a sequence at least 20,000 feet thick of arkose, siltstone, shale, and conglomerate of late Precambrian age. The formation also contains tuffaceous rocks; flows of basalt, quartz latite, and rhyolite; and sills of diabase. The sediments were derived mainly from adjacent plutonic rocks but partly from volcanic rocks similar to those found in the formation and were apparently deposited in a rapidly subsiding basin. Diabase and felsic porphyry intrusives in the basement rocks are probably related to the volcanic rocks in the Grandfather Mountain Formation. The main outcrop belt of the Grandfather Mountain Formation lies on the southeast limb of a large synclinorium overturned to the northwest. Medium- and small-scale folds are overturned to the northwest or west; they are isoclinal in the southeastern part of the belt and are more open in the northwestern part. In most of the outcrop belt, axes of minor folds are subhorizontal, and their axial planes strike northeast parallel to the trend of lithologic units. In the northern part of the outcrop belt, however, axes of minor folds plunge northeast and axial planes strike north or northwest at a large angle to the trends of the lithologic units. These folds are. evidently younger than, and superimposed on, the earlier major structure. Pervasive cleavage, parallel to the axial planes of the minor folds in the upper Precambrian rocks, is parallel to cataclastic foliation in the underlying basement rocks. Strongly developed cataclastic lineation plunges southeastward on the cleavage and foliation planes. It is generally normal to the axes of the minor folds and is evidently in the α direction. The rocks in the Grandfather Mountain window are of the same low metamorphic grade as the retrogressively metamorphosed rocks in adjacent parts of the Blue Ridge thrust sheet. Progressive metamorphism of the upper Precambrian rocks was concurrent with retrogressive metamorphism of the basement rocks. Typical metamorphic minerals in both groups of rocks are albite, microcline, epidote, actinolite, chlorite, and iron-rich muscovite. Basement rocks were converted to blastomylonitic and phyllonitic gneiss, phyllonite, and blastomylonite. Metamorphism of the rocks in the Grandfather Mountain window apparently reached a thermal climax about 350 m.y. ago. The large synclinorium in the upper Precambrian rocks formed prior to that time, but most of the minor folds and the cleavage, cataclastic foliation, and lineation formed during the metamorphic episode 350 m.y. ago. The Tablerock thrust sheet is a tectonic slice between the Blue Ridge thrust sheet and autochthonous rocks in the southwestern part of the Grandfather Mountain window It is composed of Shady Dolomite of Early Cambrian age and several thousand feet of quartizite, arkosic quartzite, and phyllite of the underlying Chilhowee Group of Early Cambrian(?) and Early Cambrian age. These rocks are metamorphosed to the same grade as the. rest of the window rocks, but their bedding and cleavage are approximately parallel with the cataclastic foliation in the Blue Ridge thrust sheet and are strongly discordant with structures in the underlying autochthonous rocks. Tight and isoclinal folds in rocks of the Tablerock thrust sheet have axial planes parallel with bedding and cleavage, and diversely oriented axes; superimposed on them are open folds having southeast-dipping axial planes and gently southwest-plunging axes which are approximately perpendicular to a well-developed mineral lineation. The geometry of these structures resembles that of the structures in the overlying Blue Ridge thrust sheet; both groups of structures probably formed and were rotated into their present orientation during thrusting. At the southwest end of the window the Tablerock thrust sheet is overturned, broken by faults, and overridden by sheets of basement rock. Layered gneiss, mica and sillimanite schist, amphibolite, and other associated metasedimentary and metavolcanic rocks in the Inner Piedmont belt are probably of late Precambrian or early Paleozoic age; they were metamorphosed in the early or middle Paleozoic. Where they have not been affected by later metamorphism related to the Brevard fault zone they contain the apparently stable mineral pairs sillimanite muscovite and epidote-diopside. Layering and foliation strike north and dip gently to moderately east. The axial planes of minor folds dip gently or moderately north or northeast, and axes plunge gently east, parallel to mineral lineation. Migmatite and granitic rocks in the Inner Piedmont were apparently formed by recrystallization and partial anatexis of the layered rocks of the Inner Piedmont during the climax of high-grade regional metamorphism. The Brevard fault zone is a narrow zone of strongly sheared and retrogressively metamorphosed rocks, including porphyroclastic blastomylonite gneiss, blastomylonite, and phyllonitic muscovite-paragonite schist. Most rocks in the zone were probably derived from the flanking rocks, but the paragonite-bearing schist is apparently an exotic tectonic slice. Foliation in the zone is steeply dipping or vertical. Rocks of both the Inner Piedmont and Blue Ridge belts near the fault zone show well-developed polymetamorphic textures characterized by porphyroclasts of potassic feldspar, plagioclase, or muscovite in a groundmass of recrystallized biotite, garnet, epidote, and oligoclase-andesine. Locally, adjacent to the fault zone, the rocks are retrogressively metamorphosed in the chlorite zone. A belt as much as 5 miles wide along the Brevard fault zone shows pervasive structural and metamorphic effects related to the faulting. The Piedmont rocks are overprinted by a subhorizontal northeast-trending cataclastic mineral lineation and a northeast-trending and southeast-dipping cleavage. Minor folds in the Piedmont rocks in this belt are subisoclinal, and their axes trend northeast, parallel to the mineral lineation. Folds become tighter and axial planes and cleavage become steeper as the Brevard fault zone is approached. Southeast-plunging mineral lineation in the Blue Ridge and Tablerock thrust sheets swings abruptly clockwise adjacent to the Brevard and becomes parallel to and indistinguishable from the subhorizontal northeast-trending cataclastic lineation along the southeast side of the fault zone. This observation suggests that strike-slip faulting along the Brevard was 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 close of the metamorphism 350 m.y. ago (Late Devonian) and before Late Triassic(?) time. Left-lateral strike-slip movement along the Brevard was concurrent with but may have lasted somewhat longer than, thrusting. Lateral displacement was greater than 135 miles.

Publication Year 2023
Title Geology of the Grandfather Mountain Window and Vicinity, North Carolina and Tennessee
DOI 10.5066/P9VIUB41
Authors Benjamin R Weinmann, Bruce Bryant, John C Reed
Product Type Data Release
Record Source USGS Digital Object Identifier Catalog
USGS Organization Florence Bascom Geoscience Center