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.
