ABSTRACT Miocene–Pliocene volcanism around Lake Tahoe, California/Nevada, USA, part of the Southern Ancestral Cascades arc, ceased at around 3 Ma as the southern edge of the subducting Juan de Fuca plate migrated north of the region. Post–3 Ma, arc volcanism continued north of Lake Tahoe, but modern subduction and arc volcanism now occur only north of the Lassen volcanic center. Miocene–Pliocene Tahoe arc lavas appear to include an older mantle source component that is not common in Quaternary Lassen arc rocks. The goal of this work was to investigate how magma sources and/or volcanic processes transitioned in the northern Sierra Nevada between Lake Tahoe and Lassen. The Sierra Nevada between Lake Tahoe and Lassen, or the North Sierra segment of the Ancestral Cascades, includes eroded remnants of Ancestral Cascades volcanic rocks, including lava flow complexes, intrusions, and landslide/debris-flow deposits. Lava samples from the North Sierra segment include calc-alkaline basalts to dacites, with rare rhyolites. All North Sierra segment lavas exhibit normalized incompatible-element patterns with negative Nb, Ta, and Ti anomalies and positive Pb, Sr, and Ba anomalies. The North Sierra segment is geochemically split into two parts: a northern group including lavas from the Susanville area, and a southern group consisting of arc rocks from the Portola, Sierraville, Henness Pass, and Sagehen areas. With the exception of the Sagehen area, the North Sierra segment shows little variation in radiogenic isotope ratios with SiO 2 , indicating that assimilation of crustal rocks was outweighed by liquid-crystal crystallization during magma evolution. Trace-element and isotopic ratios in mafic rocks of the northern group are more typical of Lassen area Quaternary volcanic rocks, whereas those of southern group mafic rocks are more typical of Miocene–Pliocene arc lavas of the Lake Tahoe area. The isotopic distinction between Lassen-like and Tahoe-like arc lavas is likely controlled by basement age and lithology, where Lassen-like magmas were derived largely by mantle wedge melting and Tahoe-like magmas were primarily partial melts of metasomatized Sierran lithospheric mantle. The Susanville area represents the “transition zone” between these two geochemically distinct primary magma sources.

We studied low prairie (Mima) mounds and ridges with sorted stone borders separated by broad rubbly soil intermounds in areas near Mount Shasta, northern California. An earlier study ascribed a purely physical origin for these soil features based on a four-stage conceptual model. Mounds were interpreted as periglacially produced clay domes formed in polygonal ground and stone perimeters as loose gravity accumulations in unexplained shallow trenches at dome peripheries. The model was widely cited to account for similar stone-bordered prairie mounds and rubbly soil intermounds in the Pacific Northwest. Our observations and measurements indicate, however, that these mounded landscapes are more complex, and that a polygenetic origin best explains them. We suggest that combined bioturbation, seasonal frost action, and erosion processes, with occasional eolian inputs, best account for the mounds, their well sorted stone borders, and the poorly sorted rubbly soil intermound pavements. We propose a transitional, eight-stage conceptual model to explain this complex landscape. The model may generally explain the origin of other similar strongly bioturbated, cold winter-impacted, erosion-prone mounded tracts in the Pacific Northwest.