Pre-Tertiary stratigraphy, magmatism, and structural history of the Central Jackson Mountains, Humboldt County, Nevada

Abstract

The Jackson Mountains (JM) are part of the early Mesozoic continental arc in northwest Nevada, which was constructed upon previously accreted Paleozoic basement. The stratigraphy of the Paleozoic basement exposed in the JM has been revised and correlations with nearby age-equivalent rocks in the Pine Forest Range and Bilk Creek Mountains are now more clearly recognized. Upper Triassic strata in the JM (the Carnian-Norian Boulder Creek Beds) herald the onset of Mesozoic arc activity in the region. The Boulder Creek Beds are both overlain and intruded by rocks of the Happy Creek Igneous Complex (HCC). Contact relations and internal features of the HCC indicate that mostly hypabyssal intrusive rocks are now exposed and that the bulk of the supracrustal volcanic succession was eroded prior to deposition of the King Lear Formation (KLF), which unconformably overlies the HCC. The HCC intrudes Norian strata and is cut by plutons that have yielded U-Pb zircon dates of 196-190 Ma and is probably entirely of Early Jurassic age. Igneous rocks associated with the KLF have yielded U-Pb zircon dates that indicate KLF deposition took place in the Early Cretaceous ($\sim$125 Ma). Two phases of Mesozoic deformation are recognized in the JM. The D$\sb1$ phase produced NW trending folds, an axial planar cleavage, and was associated with subgreenschist to amphibolite grade metamorphism. D$\sb1$ structures are found only in rocks older than the HCC and are truncated along intrusive contacts of the HCC. D$\sb2$ deformation produced NE trending folds, an axial planar cleavage, and was associated with very low grade metamorphism. D$\sb2$ affected the HCC and older rocks, but is absent in the KLF. Thus, D$\sb2$ shortening is constrained between late Early Jurassic to Early Cretaceous. D$\sb1$ correlates in style, orientation, and age with deformation in the adjacent Pine Forest Range, but the later D$\sb2$ event is apparently localized in the JM. In the JM, D$\sb2$ fabrics are better developed to the east, towards the back-arc region and may, therefore, have formed during juxtaposition of the arc terrane with the back-arc.