A well-preserved set of partly eroded mid-Miocene scoria cones and tuff cones are exposed in the Jičín Volcanic Field, Czech Republic. Zebín Hill, a formerly quarried tuff cone with an exposed conduit and associated dikes, offers an opportunity to study magmatic processes within the high-level feeding system of a small monogenetic volcano. Two types of dikes and associated "blob-like" intrusions were observed. Pure basanite dikes predominate in the center of the cone, whereas its peripheries were intruded by mingled dikes. The mingled dikes consist of mm-scale basanitic domains enclosed in a network of trachyandesitic dikelets. Locally, hybrid domains with intermediate characteristics were observed. The basanite dikes and basanite enclaves within mingled dikes contain normally-zoned minerals (Fo 90 to Fo 70 olivine, Mg-rich to Mg-poor diopside, labradorite to andesine composition of plagioclase) indicative of basanite magma evolution during ascent. In contrast, the trachyandesite and hybrid enclaves in mingled dikes show reverse zoning of minerals (Mg-poor to Mg-rich diopside, sanidine to oligoclase feldspar composition) indicative of magma hybridization. This study suggests that the Zebín Volcano did not develop from a simple monogenetic system and through a single central axis feeder; rather, Zebín Hill evolved from a complex magma feeding/storage system and through a complex feeder network. We conclude that the simple external structure of a monogenetic volcano may sometimes hide rather complex magmatic plumbing systems encompassing compositionally contrasting rocks/magmas.

The mid‐Miocene Jack Springs Tuff (JST) outcrops from the Bodie Hills of eastern California into the western Mina Deflection, Nevada. Distal outcrops of the JST occur northwest of Mono Lake, CA, on a minimally rotated structural block. This provides a reference location for our paleomagnetic study to estimate relative vertical‐axis rotation. A new 40Ar/39Ar age places the eruption of the JST at 12.114 ± 0.006 Ma. Anisotropy of magnetic susceptibility data, once corrected for local tilt and vertical axis rotation, yields a mean imbrication that trends 178°. The probable source of the JST is located in the Huntoon Valley/Queens Valley region. Eighteen paleomagnetic sites constrain mid‐Miocene to recent vertical axis rotation within the region. All sites are discordant to the reference location with variable amounts of clockwise vertical axis rotation ranging from 25° to 104°. We hypothesize that a previously unrecognized early phase of deformation occurred that predates the deformation in the central Mina Deflection. These new data support the hypothesis that transtensional faulting associated with North American and Pacific plate interaction transferred deformation across the Mono Basin region that was partially accommodated by vertical axis rotation. By late Miocene, deformation had stepped east developing the Silver Peak‐Lone Mountain detachment system followed by the structures that presently define the central Mina deflection. This study further demonstrates that deformation occurred east of the Sierra Nevada Mountains at the latitude of the Mono Basin in western‐most Nevada, a region previously thought to be undeformed in terms of clockwise vertical axis rotation.

The Zebín Volcano, Czech Republic, is a quarried tuff cone that offers the opportunity to understand emplacement processes responsible for magma transport through a pyroclastic cone and magmatic processes involved in its development. A high‐resolution ground magnetometry survey, electrical resistivity tomography, and refraction seismic were conducted to supplement field mapping of the size, shape, and inner structure of the volcanic system. The Zebín Volcano yields normal polarity rocks, and the cone is underlain by a complex and branching magma feeder system with several diverging dikes. Samples were collected at 24 sites from feeder dikes and the main conduit for anisotropy of magnetic susceptibility and paleomagnetic analysis. Anisotropy of magnetic susceptibility results yield inferred magma flow directions indicating subhorizontal magma flow toward and away from the axial conduit as well as both upward and downward flow. Paleomagnetic data from all sites are normal polarity with statistically distinct remanence directions between some sites that indicate that significant time passed or deformation occurred during the growth of this volcanic system. 40Ar/39Ar isochron ages of groundmass provide emplacement ages of 18.38 ± 0.03 and 18.45 ± 0.03 Ma, whereas a weighted mean date of 18.52 ± 0.03 Ma for hornblende provides an eruption age. We argue that the Zebín Volcano evolved from a polyphase feeding system and through a complex feeder network. This detailed study shows that many monogenetic volcanoes deserve highly detailed study, as their subtleties can provide insight into broader crustal and magmatic environment during magmatism.

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