Abstract The tectonomagmatic evolution of eastern Chukotka, NE Russia, is important for refining the onset of Pacific plate subduction, understanding the development of the Amerasia Basin, and constraining Arctic tectonic reconstructions. Field mapping and strategic sample collection provide relative age constraints on subduction-related continental arc magmatism in eastern Chukotka. Ion microprobe U–Pb zircon ages provide absolute constraints and identify five magmatic episodes ( c. 134, 122, 105, 94 and 85 Ma) separated by three periods of uplift and erosion ( c. 122–105, 94–85 and post-85 Ma). Volcanic rocks in the region are less contaminated than their plutonic equivalents which record greater crustal assimilation. These data, combined with xenocrystic zircons, reflect the self-assimilation of a continental arc during its evolution. Proto-Pacific subduction initiated by c. 121 Ma and arc development occurred over c. 35–50 myr. Crustal growth was simultaneous with regional exhumation and crustal thinning across the Bering Strait region. Ocean–continent subduction in eastern Chukotka ended at c. 85 Ma. The timing of events in the region is roughly synchronous with the inferred opening of the Amerasia Basin. Simultaneous arc magmatism, extension and development of the Amerasia Basin within a back-arc basin setting best explain these coeval tectonic events. Supplementary material: Includes SIMS U–Pb and geochemistry data tables, detailed geological map and geochemical figures which are available at https://doi.org/10.6084/m9.figshare.c.3784565

We examined numerous coeval mid-Cretaceous intrusions emplaced at different depths into Triassic slate and phyllite to evaluate wall-rock features and contact relations associated with emplacement mechanisms. The pre-emplacement regional deformation of the wall rocks is well characterized, which facilitated clear and unambiguous identification of emplacement-related structures. The depth of intrusion in the different study areas ranged from ~5 to 12 km, which allowed us to examine emplacement mechanisms from the ductile to the brittle regime. Intrusions range in size from dikes and small pods to stocks and plutons up to 90 km 2 in area. Our analyses indicate several important findings: (1) At all depth levels, the pre-emplacement (Jurassic) structural grain of the wall rocks (pervasive foliation, folds at various scales, reverse faults) had significant influence on the geometry and distribution of intrusions. (2) Evaluation of emplacement mechanisms in this study was facilitated by observing several intrusions that offered “snapshots” of different emplacement depths and different levels of the intrusive system (from the roofs to sides). (3) Emplacement mechanisms and intrusion geometry vary with depth, time, and location; as a result, emplacement via multiple mechanisms is pervasive. At the shallowest levels, magma was intruded primarily by diking and merging of dikes via stoping into small stocks that typically had highly irregular wall-rock contacts. Faulting associated with roof uplift and/or cauldron subsidence is locally evident where larger intrusive bodies reside at depth. At intermediate levels, intrusions were emplaced by a combination of stoping and rigid host-rock displacement (roof uplift and lateral flexure), with minor radial expansion. Discordant pluton–wall-rock relations are prevalent, and markedly irregular contacts are still seen, but dikes are much less common. Stoping has removed portions of the intrusion contact aureoles, and has likely obliterated much evidence for processes associated with initial intrusion. At the deepest levels, intrusions are nearly circular in shape and have more common concordant contacts and ductile wall-rock features (synemplacement folding and foliation). These deeper-level stocks were emplaced as diapirs, which then experienced late-stage radial expansion and rigid host-rock displacement (lateral flexure and roof uplift?).

The Franciscan Complex, Great Valley Group, and Sierra Nevada batholith have long been considered to represent a Cretaceous convergent margin assemblage. This subduction complex, forearc basin, and magmatic arc triad has also been considered to have formed essentially in place with little or no Cretaceous-age translation between any of the three parts. Below we explore the possibility that the Great Valley Group accumulated in a basin that was translated parallel to the convergent margin as a forearc sliver during the Late Jurassic–Early Cretaceous. There are three different scales of evidence that lead to this hypothesis. The first comes from the processes operating at modern convergent plate boundaries. The second line of evidence is based on analysis of the geologic relations where the Coast Ranges meet the Klamath Mountains province in northern California. Thirdly, we explore published and some new detrital zircon age data in the context of a translational model for the Great Valley forearc basin. We conclude that the Great Valley forearc basin is bounded on its eastern and northern sides by a strike-slip fault that accommodated several hundreds of kilometers of dextral offset in the Late Jurassic–Early Cretaceous. This boundary is now a highly modified fault separating the Klamath Mountains province and the Coast Ranges, across which are juxtaposed two fundamentally different parts of the Great Valley Group. The boundary continues to the south between the Sierra Nevada and the Coast Ranges, where it is buried beneath younger sediments of the Sacramento Valley and/or perhaps includes structures in the Sierran foothills such as the Melones fault. Detrital zircon data suggest to us that the most likely original location of the Coast Ranges Great Valley Group, prior to strike-slip offset, was offshore of the continental arc in the southwest Cordillera (southeast California to northwest Mexico). In addition, we discuss evidence that the boundary between the Franciscan subduction complex and Great Valley forearc basin experienced significant dextral displacement. Finally, we suggest that these plate-boundary-parallel faults are part of an even larger system of Early Cretaceous dextral strike-slip faults in the U.S. Cordillera, including the Mojave–Snow Lake fault, western Nevada shear zone, and Idaho shear zone.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

New geologic mapping and fossil data from the Pine Forest Range, Black Rock Desert, northwest Nevada, indicates that the range contains a structurally intact sequence of variably metamorphosed middle (and early?) Paleozoic through latest Triassic strata. The oldest rocks in the range include metamorphosed quartzo-feldspathic sedimentary rocks and mafic volcanic and volcaniclastic rocks of Mississippian and/or older age. Overlying fan facies chert/argillite/quartz-rich clastic rocks are of post–Late Devonian(?) and pre–Late Mississippian age, and are succeeded by shallower marine Upper Mississippian to Lower Pennsylvanian(?) volcanic rocks, volcanic-lithic–rich clastic rocks, and limestone. The remainder of Paleozoic time is characterized mostly by shallow marine conditions and the development of several unconformities. A thin sequence of shallow marine carbonates and clastic sediments, yielding early Late Permian fossils at the top, overlies Pennsylvanian(?) strata across an unconformity that may span early Pennsylvanian through Early Permian time. Upper Permian(?) chert and shale unconformably overlie older rocks and reflect some subsidence in Late Permian(?) time. A third unconformity separates Paleozoic and Triassic rocks and spans latest Permian(?) through Middle or Late Triassic time. Triassic strata in the Pine Forest Range record two distinct periods of deposition: (1) fan facies sedimentary-lithic–rich sediments and basinal carbonates were deposited from Ladinian or Carnian (late Middle or early Late Triassic) through early Norian (late late Triassic) time, and (2) mafic to intermediate composition lavas and associated volcanic-lithic– and crystal-rich fan facies sediments were deposited during most of the remainder of Norian time. Lavas exhibit the trace-element characteristics of volcanic arc magmas. Relatively deep marine conditions of deposition occurred throughout Middle(?) to Late Triassic time. The Paleozoic stratigraphic record in the Pine Forest Range shows important similarities to that of other Paleozoic arc sequences in the western U.S. Cordillera, including those in the northern Sierra Nevada and eastern Klamath Mountains (California), Blue Mountain province (Oregon), and Chilliwack terrane (Washington). These similarities support an interpretation of paleogeographic and tectonic ties between the Black Rock Desert and these other arc sequences in Mississippian (and early Paleozoic?) through Permian time. In addition, the presence of a Permo-Triassic unconformity in the Pine Forest Range represents new evidence that these Paleozoic arc sequences were characterized by uplift and erosion during the time of the Sonoma orogeny. Early Mesozoic strata in the Pine Forest Range provide a record of volcanism and sedimentation that is similar to that in other early Mesozoic volcanic arc sequences from the southwestern United States through northern California. These similarities support an interpretation that early Mesozoic arc sequences in northwest Nevada, as well as northern California, form the northern continuation of the west-facing early Mesozoic arc documented in the southwestern United States. In addition, the Triassic record in the Pine Forest Range suggests that extension-related intra-arc subsidence, inferred to have characterized the southwestern United States during early Mesozoic time, may also have affected early Mesozoic rocks of the Black Rock Desert.