Hess’ ideas related to the tectonic significance of ultramafic rocks, ultramafic magmas, evolution of oceanic crust, and H 2 O outgassing, all emanating from his abiding interest in serpentinite. Hess recognized the prime importance of ultramafic rocks in understanding the tectonic development of collisional (Alpine-type) mountain belts. They are not magmatic intrusions, as he proposed, but tectonic remnants of fossil plate boundaries. Hess was the formulator of the concept of sea-floor spreading. His ideas on the nature of oceanic crust changed through the years, but he repeatedly argued that oceanic crust was partly or predominantly serpentinite. Despite the lack of subsequent popularity of this view, he was at least partly right. Serpentinite is a part of oceanic crust, notably in technically thinned regions along rifted ridges, near transform faults, and along obliquely rifted continental margins, such as the Mesozoic western Alpine continental margin.

The Jurassic Smartville arc sequence in the northern Sierra Nevada foothills is bounded both in the north and east by ophiolitic and marine rocks of a disrupted oceanic terrane. The Jarbo Gap ophiolite in the north consists of serpentinized harzburgite and dunite intruded by gabbro and diorite and overlain by volcanic and sedimentary rocks, all of which are metamorphosed to upper greenschist-amphibolite facies. Contacts between the ophiolitic subunits, which commonly have lenticular shapes, are generally tectonic and discontinuous along-strike. Ophiolitic rocks display a penetrative fabric of a west-northwest–striking and steeply to moderately north-northeast–dipping foliation. Locally, mafic to intermediate volcanic rocks within the ophiolite are cut by fault-bounded and/or diapiric intrusions of serpentinized peridotites. Contact relations and the rock types indicate that the Jarbo Gap ophiolite is part of a multiply deformed, heterogeneous oceanic crust. Marine rocks exposed south of and structurally beneath the Jarbo Gap ophiolite form a chert-argillite unit that is composed mainly of blocks of sedimentary, volcanic, plutonic, and ultramafic rocks in an argillite matrix. The matrix displays a northwest-trending, northeast-dipping penetrative foliation, whereas some of the blocks in the matrix show little or no deformation. Olistostromal origin of these undeformed blocks suggests that gravity debris sliding in addition to tectonic mixing was a significant agent for development of the generally chaotic internal structure of the chert-argillite unit. Ophiolitic blocks and detritus in the matrix indicate an older ophiolitic provenance, whereas blocks and clasts of relatively undeformed and fresh volcanic rocks, which occur as olistoliths in a mudstone-sandstone sequence, suggest volcanic activity adjacent to the depocenter(s) of the chert-argillite unit. Age relations, reconstructions of stratigraphic, structural, and contact relations, and regional correlations suggest that the Jarbo Gap ophiolite may have been originally overlain by the chert-argillite unit. Thus the Jarbo Gap ophiolite and the overlying chert-argillite unit are interpreted to have constituted a late Paleozoic–early Mesozoic oceanic basement. The existence of similar structural and stratigraphic relations between stratigraphically and chronologically correlative ophiolitic and marine rocks exposed in the Central Belt east of the Smartville complex suggests that much of the Central Belt is composed of this late Paleozoic–early Mesozoic oceanic basement. Both the Jarbo Gap ophiolite and the chert-argillite unit are intruded by dikes of a basaltic andesite composition that display low FeO and high MgO contents and enrichment in compatible as well as some LIL elements. These dikes, which are Lower Jurassic in age and chemically unrelated to the ophiolitic sequence, show petrographic and petrologic characteristics typical of boninitic affinity and are interpreted as products of primary melts generated by high degrees of melting of a metasomatically enriched source during early stages of subduction. We infer that this subduction during Early Jurassic time may have under-plated the preexisting oceanic basement and produced a submarine volcanic arc terrane. Part of the Smartville complex and the entire Slate Creek complex are interpreted as fragments of this arc terrane, which was built on and across the preexisting oceanic basement. This model suggests that the Western Belt and much of the Central Belt in the northern Sierra Nevada are part of a single ensimatic and polygenetic arc terrane that evolved in early Mesozoic time. This interpretation precludes tectonic scenarios suggesting the existence in the northern Sierra Nevada metamorphic belt of a number of discrete, exotic island-arc systems that were presumably accreted into the North American continental margin during early Mesozoic time. The current structural grain defined by northwest-striking and steeply northeast-dipping faults and associated structures is a relatively late-stage feature that modified the primary contact relations during a period of regional contraction in Late Jurassic to Early Cretaceous time.

Abstract The North American Continent-Ocean Transects Program is a study of the tectonics and Phanerozoic evolution of the transitional region of the North American hemisphere between its bordering ocean basins and the craton or longstable continental interior. Corridor CI transects the North American Cordillera from the active Mendocino triple junction off the northern California coast, to the North American craton in Wyoming. The North American Cordillera in this transect displays examples of terranes presently being displaced, ancient displaced terranes and deformed North America. The primary aims of this explanatory pamphlet are: 1) to guide and assist the reader in the use of the graphic display; 2) to provide documentation on the sources of data and manner of construction for the graphic display; and 3) to supplement the graphic display with supporting discussions on crustal structure, tectonostratigraphic units, major structures and tectonic evolution. An in-depth synthesis of the CI display will be integrated with that of the C2 display (central California offshore to Colorado Plateau) in the North America Transects Synthesis Volume to be published as part of the DNAG (Decade of North American Geology) series. In order to give the reader a reasonable geological context from which to explore the graphic display and explanatory pamphlet, the first section of the pamphlet will provide an overview of the major tectonic features displayed. From west to east Corridor C-l (Fig. 1) crosses a number of major tectonic features that record the growth of the North American continental margin throughout the Phanerozoic, as well

Abstract DNAG Transect C-1. Part of GSA's DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.