Abstract Cleavage refraction angles are used to estimate effective viscosity contrasts between interlayered quartzites and phyllites within the Baraboo Syncline, Wisconsin, USA. Both types of layers contain two major phases, quartz and pyrophyllite, with minor amounts of hematite. Quartz (with minor hematite) behaves as the strong phase and pyrophyllite acts as the weak phase. Cleavage refraction directly relates to mineralogy with a linear relationship between bedding/cleavage angle and strong-phase concentration. Mineralogy exerts first-order control over effective viscosity contrasts, which are generally small, in most cases <10. Effective viscosity contrasts are consistent across the fold, so are likely not to be highly strain dependent and indicate approximate linear viscous rheology. Microstructures suggest deformation was dominated by dislocation creep in layers with high quartz concentrations and diffusive mass transfer in layers with lower quartz concentrations, and that the transition of the deformation mechanism is gradual. Thus, the rheological flow laws at the small scale may not reflect the bulk flow law at the large scale over the span of the deformation. Effective viscosity contrasts allow an evaluation of samples compared to theoretical two-phase mixtures. The analysed samples most closely resemble the Reuss bound of two-phase mixtures, regardless of the mineralogy.

Abstract Mid-Palaeozoic assembly models for the Arctic Alaska–Chukotka microplate predict the presence of cryptic crustal sutures, the exact locations and deformational histories of which have not been identified in the field. This study presents data on the provenance of polydeformed and metamorphosed strata in the southern Brooks Range Schist Belt and Central Belt of presumed Proterozoic–Devonian depositional age, as well as for the structurally overlying strata, to help elucidate terrane boundaries within the Arctic Alaska–Chukotka microplate and to add new constraints to the palaeogeographical evolution of its constituent parts. The protoliths identified support correlations with metasedimentary strata in the Ruby terrane and Seward Peninsula and suggest a (peri-) Baltican origin in late Neoproterozoic–early Palaeozoic time. Proximity to Laurentia is only evident in what are inferred to be post-early Devonian age strata. By contrast, the North Slope and Apoon terranes originated proximal to Laurentia. The mid-Palaeozoic boundary between these (peri-) Baltican and (peri-) Laurentian terranes once lay between rocks of the Schist/Central belts and those of the Apoon terrane, but is obscured by severe Mesozoic–Cenozoic deformation. Whether this boundary represents a convergent or transform suture, when exactly it formed and how it relates to broader Caledonian convergence in the North Atlantic are still unresolved questions. Supplementary material: Details of the analytical methods together with zircon U-Pb and Lu-Hf isotopic data tables are available at https://doi.org/10.6084/m9.figshare.c.3805696

This paper is an in-depth review of the architecture and evolution of the Western Sierra Nevada metamorphic province. Firsthand field observations in a number of key areas provide new information about the province and the nature and timing of the Nevadan orogeny. Major units include the Northern Sierra terrane, Calaveras Complex, Feather River ultramafic belt, phyllite-greenschist belt, mélanges, and Foothills terrane. Important changes occur in all belts across the Placerville–Highway 50 corridor, which may separate a major culmination to the south from a structural depression to the north. North of the corridor, the Northern Sierra terrane consists of the Shoo Fly Complex and overlying Devonian to Jurassic–Cretaceous cover, and it represents a Jurassic continental margin arc. The western and lowest part of the Shoo Fly Complex contains numerous tectonic slivers, which, along with the Downieville fault, comprise a zone of west-vergent thrust imbrication. No structural evidence exists in this region for Permian–Triassic continental truncation, but the presence of slices from the Klamath Mountains province requires Triassic sinistral faulting prior to Jurassic thrusting. The Feather River ultramafic belt is an imbricate zone of slices of ultra-mafic rocks, Paleozoic amphibolite, and Triassic–Jurassic blueschist, with blueschist interleaved structurally between east-dipping serpentinite units. The Downieville fault and Feather River ultramafic belt are viewed as elements of a Triassic–Jurassic subduction complex, within which elements of the eastern Klamath subprovince were accreted to the western edge of the Northern Sierra terrane. Pre–Late Jurassic ties between the continental margin and the Foothills island arc are lacking. A Late Jurassic suture is marked by the faults between the Feather River ultramafic belt and the phyllite-greenschist belt. The phyllite-greenschist belt, an important tectonic unit along the length of the Western Sierra Nevada metamorphic province, mélanges, and the Foothills island arc terrane to the west were subducted beneath the Feather River ultramafic belt during the Late Jurassic Nevadan orogeny. South of the Placerville–Highway 50 corridor, the Northern Sierra terrane consists of the Shoo Fly Complex, which possibly contains structures related to Permian–Triassic continental truncation. The Shoo Fly was underthrust by the Calaveras Complex, a Triassic–Jurassic subduction complex. The Late Jurassic suture is marked by the Sonora fault between the Calaveras and the phyllite-greenschist belt (Don Pedro terrane). As to the north, the phyllite-greenschist belt and Foothills island arc terrane were imbricated within a subduction zone during the terminal Nevadan collision. The Don Pedro and Foothills terranes constitute a large-magnitude, west-vergent fold-and-thrust belt in which an entire primitive island-arc system was stacked, imbricated, folded, and underthrust beneath the continental margin during the Nevadan orogeny. The best age constraint on timing of Nevadan deformation is set by the 151–153 Ma Guadelupe pluton, which postdates and intruded a large-scale megafold and cleavage within the Mariposa Formation. Detailed structure throughout the Western Sierra Nevada metamorphic province shows that all Late Jurassic deformation relates to east-dipping, west-vergent thrusts and rules out Jurassic transpressive, strike-slip deformation. Early Cretaceous brittle faulting and development of gold-bearing quartz vein systems are viewed as a transpressive response to northward displacement of the entire Western Sierra Nevada metamorphic province along the Mojave–Snow Lake fault. The preferred model for Jurassic tectonic evolution presented herein is a new, detailed version of the long-debated arc-arc collision model (Molucca Sea–type) that accounts for previously enigmatic relations of various mélanges and fossiliferous blocks in the Western Sierra Nevada metamorphic province. The kinematics of west-vergent, east-dipping Jurassic thrusts, and the overwhelming structural evidence for Jurassic thrusting and shortening in the Western Sierra Nevada metamorphic province allow the depiction of key elements of Jurassic evolution via a series of two-dimensional cross sections.

At 100 Ma, about midway through the history of the emplacement of the northern part of Peninsular Ranges batholith, a regional-scale deformation zone 1 developed that juxtaposed two major prebatholithic units. Deformation occurred as subduction transitioned from beneath oceanic crust to beneath continental crust. The deformation zone is well exposed at Searl Ridge, where the intensity of deformation attendant to the juxtaposition of the two prebatholithic units progressively increased from west to east over a distance of 5.5 km. In this 5.5-km-wide zone, a series of three progressive structural transpositions is recorded in the metasedimentary rocks. Many outcrops have two intersecting planar fabrics and a linear fabric produced by the intersecting planar fabrics. West to east within the deformation zone, the rock fabric changes from phyllitic to schistose, and the grade of metamorphism increases from greenschist to lower granulite. Plutonic rocks predating the deformation zone at the west edge of Searl Ridge were likewise deformed locally into rocks having a pronounced gneissic fabric. The transposition displacement produced a 300 °C gradient over a 4-km-wide zone of low-pressure, high-temperature, Buchan-type metamorphic mineral assemblages. The 300 °C gradient is based on biotite forming at a temperature of ~425 °C, and K-feldspar at ~750 °C. A 3–4 kbar pressure gradient across the deformation zone is inferred from a crystallization pressure of 2 kbar for the 120 Ma Domenigoni Valley pluton, located at the western edge of the deformation zone, and a 5–6.3 kbar pressure for the 100 Ma Lakeview Mountains pluton, emplaced in the eastern part, the latter being coeval with the formation of the deformation zone. If temperature increase is attributable entirely to difference in depth, and assuming an idealized 0.3 kbar increase in pressure per 1 km increase in crustal depth, the vertical displacement component of deformation within the deformation zone was 10–13 km.