Structure of the Schoonover Sequence, Independence Mountains, Nevada: Emplacement Mechanisms for the Golconda Allochthon

Polyphase deformation and the interrelationship between chert diagenesis, rheology, and structural fabric in the Schoonover sequence, Independence Mountains, Nevada has contributed to resolving the controversy concerning the closure of the Havallah/Schoonover oceanic basin during the Late Permian to Early Triassic (?) Sonoma orogeny. The Schoonover sequence is an allochthonous upper Paleozoic assemblage of complexly deformed chert, argillite, siliciclastic, calcareous, and volcaniclastic turbidites, and mafic to intermediate volcanic rocks. The Schoonover sequence is lithologically similar to and has a deformational history paralIeIing that of the Havallah sequence, and both are considered pieces of the Late Devonian to Permian Golconda allochthon. The Late Permian to Early Triassic (?) emplacement of the Golconda allochthon onto North America is generally referred to as the Sonoma orogeny. Back-arc thrusting and prolonged stacking in an accretionary prism are alternative tectonic models proposed for closure of the Havallah/Schoonover ocean basin. The structural chronology of the Schoonover sequence suggests a deformational history with six possible events. These events include: (D0) original ceposttion and formation of mound features; (D1) bedding-paralleI pressure solution and boudin formation; (D2) NW - SE directed shortening and boudin formation; (D3) NE - SW directed shortening and boudin formation; (D4) SE - NW directed shortening and formation of the Golconda thrust; and (D4+) post-Sonoma deformation. A portion of Miller and others (1984) geologic map has been revised. The revisions include: addition of two map scale folds, reinterpretation of inverted lithotectonic units separated by a depositional contact as upright younger-over-older lithotectonic units separated by a thrust fault contact, reinterpretation of depositional contacts which are possibly thrust faults, and the relocation of a depositional contact based on biostratigraphy. New radiolarian age determinations have confirmed and strengthened the biostratigraphy. The existence of an undescribed piece of the Golconda allochthon in the Bull Run Mountains, in northern Nevada has been established. Eleven faunal groups were determined from 42 samples, and range in age from Late Devonian to Late Permian. Both older and younger chert units on map and microscopic scales display similar structural fabrics. Pressure solution found in the hinge regions of folds and in boudins, and fold hinge thickening suggests that deformation occurred while the siliceous sediment was still in an immature stage of opal-A or opal-CT. Conodont color indices and mineralogy support deformation occurring at low temperatures. Since older (Late Devonian and Mississippian) cherts units should show ductile fabrics if deformed prior to complete lithification in an accrectionary prism, and more brittle fabrics if deformed during back-arc thrusting, the data suggest the tectonic closure mechanism for the Havallah/Schoonover ocean basin was probably prolonged subduction with simultaneous stacking of diagenetically immature siliceous sediment in an accretionary prism. This accretionary assemblage was later thrust onto continental North America during the Late Permian to Early Triassic (?) Sonoma orogeny