Thrusted Paleozoic Rocks in the Northern and Central HD Range, Northeastern Nevada
The northern and central HD Range in northeastern Nevada is formed of a succession of six major thrust plates that lie on Mississippian Chainman Shale in the central part and are juxtaposed or lie on Lower Permian quartzites and limestones at the northern end. Each plate has its own characteristic stratigraphy: the lowest plate consists of Ordovician chert unconformably overlain by Mississippian Diamond Peak quartzite; the succeeding three are characterized by thin, leptogeosynclinal, Ordovician-Silurian sections, or by intercalations of eu- and miogeosynclinal facies; the two upper plates have thin Ordovician-Silurian sections unconformably overlain by Pennsylvanian and Permian limestone and clastics. Three new formations have been differentiated in the Ordovician, one in the Silurian, and one in the Pennsylvanian. Directional structures and regional stratigraphic comparisons indicate that the last move of the plates, except the upper one, was in a south-southeasterly direction; more than one direction of movement is recorded within single plates, and rotation of large segments of one or more plates appears to have been common. Structures are the result of thrusting of thin, incompetent sections: imbrications, up- and overthrusting, and folding of the frontal ends of thrusts are common to many tectonic plates. The emplacement of the allochthon occurred in two stages: (a) the Ordovician and Silurian eugeosynclinal rocks were first affected in Late Devonian or Mississippian time by the Antler Orogeny and thrust eastward over the miogeosyncline from a source in southern Oregon or southwestern Idaho; subsequently, an overlap sequence of clastics and carbonates was laid down over them, (b) The allochthonous Ordovician-Silurian rocks and the overlap sequences were moved southeastward from south-central Idaho to their present position in early Mesozoic time prior to the intrusion of the mid-Jurassic Contact Pluton. This last phase of thrusting is possibly one of the earliest phases of deformation recorded in the eastern Great Basin east of the Antler Orogenic Belt and preceeded more extensive late Mesozoic thrusting in the Sevier and Laramide Belts of Utah and Wyoming. The occurrence of eugeosynclinal rocks in northeastern Nevada cannot be attributed directly to Antler thrusting: Mesozoic rethrusting of these rocks is to be considered in evaluating their tectonic history. The eastern limits of the Antler Belt do not extend to northeastern Nevada. The upper thrust plate in the north-central HD Range belongs to a separate tectonic succession in the southern part of the range and was emplaced from a more westerly source in late Mesozoic or Tertiary time.