Two Lithotectonic Boundaries In Western North Carolina: Geologic Interpretation of A Region Surrounding Sylva, Jackson County

Understanding the geology of any region begins with the construction of high-quality geologic maps. As maps for a specific region are revised in greater detail, geologic understanding is improved. This study involved detailed geologic mapping (at the scale of 1 : 24,000) of a portion of the southern Appalachian Blue Ridge that was previously only mapped by reconnaissance. It is hoped that the research herein will add to the present understanding of the Blue Ridge province and of the entire Appalachian orogenic belt. During the course of field mapping, three lithotectonic units were divided, each separated from the others by a major fault. Unit 1, located in the northwesternmost portion of the study area, is part of the Great Smoky Group and is comprised of feldspathic metaquartzite and metagraywacke interlayered with subordinate aluminous schist. Unit 1 is bounded on the south by the Hayesville fault. Unit 2 occurs immediately south of the Hayesville fault and is composed of feldspathic metagraywacke, biotite gneiss, and interlayered amphibolite. This sequence is correlated with the Tallulah Falls Formation. In addition, Unit 2 contains large masses of amphibolite and ultramafic rock that are not considered part of the Tallulah Falls stratigraphic sequence. Unit 3 is the southeastemmost unit and consists of an assemblage of aluminous metagraywacke, feldspathic metaquartzite, aluminous schist, and interlayered amphibolite. These rocks are correlated with the Otto Formation which has been mapped directly along strike to the southwest. A small ultramafic body is also present in Unit 3. Unit 3 is separated from Unit 2 by the Soque River fault. Unit 2 is the hanging wall for both the Hayesville and Soque River faults. The study area is structurally complex. Structural and metamorphic evidence indicates that all lithotectonic units were juxtaposed prior to the onset of metamorphism and early-stage deformation. Six separate fold generations have been recognized. Early-stage folding (F1 and F2) are isoclinal flowage folds, each of which involved transposition of a preexisting layering and development of a regionally penetrative foliation. The dominant foliation now present in the study area was designated S2 because F2 folds fold a preexisting foliation. Intermediate-stage folds are tight, passive flow to flexural flow folds. Intermediate-stage fold geometries indicate that rheologic contrasts developed and affected fold mechanics as the rocks cooled. Late-stage folds are open flexural flow and flesural slip buckle folds indicating that the rocks behave more rigidly and had cooled still further. Late stage deformation is responsible for the development of the Webster-Addie dome , a prominent doubly-plunging antiform in the study area. A window through Unit 2, exposing the underlying rocks of Unit 3, has developed in the core of the Webster-Addie dome. One major regional metamorphism affected the rocks in the study area is associated with the Taconic orogeny. Rocks in this study area underwent medium-pressure facies series metamorphism, ultimately attaining kyanite-sillimanite grade in the amphibolite facies. The sillimanite isograd has been domed in the study area exposing higher pressure (kyanite grade) rocks in the center. Textural relationships indicate that garnet growth continued well after the regional metamorphic peak. Thermobarometric calculations based on mineral equilibria indicate that perhaps closure temperatures varied between pelites, gneisses, and amphibolites. Maximum conditions for metamorphism (recorded by rocks with the highest closure temperature) were approximately 725°C and 9 kb. Metamorphic conditions recorded by rocks with the lowest closure temperature were 550°C and 5.5 kb. Retrograde metamorphism (chlorite-biotite zone) is evident in some samples and may be associated with Acadian regional metamorphism. The geochemistry of amphibolites occurring in Units 2 and 3 indicates that they were mafic igneous rocks prior to metamorphic recrystallization. Trace element abundances indicate that amphibolites originated as ocean floor basalts. This suggests that Units 2 and 3 were deposited in a tectonic regime in which lavas from a depleted mantle source could be tapped and emplaced without significant crustal contamination