Diagenetic controls on the structural evolution of siliceous sediments in the golconda allochthon, Nevada, U.S.A.

A model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper Paleozoic Golconda allochthon of Nevada, U.S.A. Numerous thrust faults slice the cherts into packets, each with a unique set of internal structures. Fold geometries, boudin profiles, pressure-solution features, etc. vary from packet-to-packet and layer-to-layer. Analogous variations in the diagenetically mixed siliceous sediments of the Miocene Monterey Formation. California, suggest the cherts of the Golconda allochthon were similarly mixed when they were deformed. Radiolarian sediments composed of biogenic silica (opal-A) developed ductile structures and pressure-solution features because of their high porosity, weak lithification and the high solubility of disordered silica. Lowporosity, strongly-lithified, relatively-insoluble quartz cherts, the final product of silica diagenesis, deformed in a brittle fashion and developed fewer pressure-solution features. Diagenetically intermediate CT-cherts and CT-porcelanites exhibited transitional behavior. Mixed diagenetic zones produced structures with layer-by-layer changes in structural style. The dehydration of opal-A and opal-CT helped create the excess pore fluid pressures responsible for thrust faults, hydraulic fractures, dilation breccias and elastic intrusions. The presence of ductile structures and numerous pressure solution features in the oldest cherts of the Havallah sequence suggests that these cherts were deformed while diagenetically immature, possibly within a long-lived upper Paleozoic accretionary prism.