STRUCTURAL ANALYSIS OF INWARD-VERGENT WALL ROCK ANTICLINES FLANKING THE SILVER ZONE PASS PLUTON, NEVADA

Inward-vergent wall rock anticlines are common at pluton margins and have been inferred to be genetically related to the plutons. Proposed mechanisms to form the anticlines include emplacement of the plutons as lopoliths or saddle reefs; diapiric return flow; and post-crystallization sinking of plutons. Glazner and Miller (1997, Geology) proposed that, after it crystallized, the Silver Zone Pass pluton (SZP) in northeastern Nevada was denser than its wall rocks and sank through them. However, new results indicate that the anticlines adjacent to the SZP formed before its emplacement and that the SZP is not denser than its wall rocks. The SZP is a Jurassic granodiorite that intrudes Cambrian silty limestones. Geologic mapping defines anticlines that plunge gently toward about 130 and 115 at the northern and southern margins of the SZP, respectively. A NE-striking (left?-)reverse fault cuts and offsets the northern anticline and is in turn truncated by the pluton contact. Microstructures of contact-metamorphic rocks adjacent to the pluton indicate post-kinematic metamorphic mineral growth. Both map relations and petrography thus indicate that the anticlines predate emplacement of the SZP. Both the folds and the reverse fault record roughly N-S contraction that we attribute to regional tectonics rather than local plutonic processes. Other observations reinforce the lack of post-emplacement sinking but emplacement mechanism(s) remain uncertain. Measured rock densities indicate minimal contrast between the SZP (~2700 kg/m^3) and wall rocks (2690-2740 kg/m^3) with the SZP generally slightly less dense than wall rocks. A lack of syn- or post-plutonic ductile strain in the contact aureole, including undeformed meter-thick granodiorite dikes that cut wall rocks adjacent to the pluton, is inconsistent with diapiric emplacement as well as post-emplacement sinking. The partially preserved roof does not appear domed, arguing against laccolithic emplacement. The data permit but do not demand floor subsidence to emplace the SZP as a lopolith. The results illustrate the importance of case-by-case study of plutons with flanking anticlines. If there is a genetic relationship between the SZP and the adjacent folds, it can only be that folding somehow prepared a favorable site for later pluton emplacement.