Arthur W. Snoke

Confirmation of the Carolina Slate Belt as an Exotic Terrane

An assemblage of Middle Cambrian Atlantic faunal province trilobites has been found in the rocks of the Carolina slate belt near Batesburg, South Carolina. Geologic and paleomagnetic data suggest that the Carolina slate belt and the adjacent Charlotte belt constitute an exotic terrane that was accreted to North America in early to middle Paleozoic time.

Significance of mid-Mesozoic peridotitic to dioritic intrusive complexes, Klamath Mountains–western Sierra Nevada, California

A mid-Mesozoic plutonic suite ranging from ultramafic-gabbroic rocks to dioritic rocks and commonly intruded by younger granitoids is widespread in the Klamath Mountains–western Sierra Nevada, California. The ultramafic-gabbroic rocks are clinopyroxene-rich and commonly vary from wehrlite to olivine-hornblende clinopyroxenite and melagabbro. Associated dioritic rocks include biotite–two-pyroxene diorite–monzodiorite, and the granitic rocks typically range from tonalite to granodiorite. These plutonic complexes are coextensive and broadly coeval with a suite of weakly metamorphosed volcanic rocks ranging in composition from basalt to basaltic andesite to andesite. The volcanic rocks are chiefly volcaniclastic and are characterized by several distinct phenocryst assemblages: clinopyroxene + plagioclase (± olivine or hornblende). The plutonic complexes and the associated volcanic rocks are restricted to terranes that lack continental (sialic) crust and that, at least locally, contain juvenile ophiolitic crust generated adjacent to the locus of magmatism. Thus, their petrogenesis may be related to recurrent fracturing and magmatism within a rifted ensimatic arc.

A thrust plate of ophiolitic rocks in the Preston Peak area, Klamath Mountains, California

In the Preston Peak area, Klamath Mountains, California, a regional thrust fault separates metasedimentary rocks of the Late Jurassic Galice Formation from an overlying plate of older ophiolitic rocks. The ophiolite consists of a basal sheet of ultramafic tectonite overlain and intruded by a heterogeneous mafic complex that in turn is overlain by metabasaltic and metasedimentary rocks. Field relations indicate that the ophiolite is polygenetic, with a major temporal hiatus separating the tectonitic ultramafic rocks and the associated mafic rocks. Mineral assemblages and textures in the ultramafic rocks suggest high-temperature recrystallization and penetrative deformation. In contrast, diabase and diabase breccia, the most abundant constituents of the mafic complex, are nonschistose rocks metamorphosed to lower greenschist facies. Contacts between ultramafic rocks and rocks of the mafic complex are fault contacts, intrusive contacts, or both. Mafic rocks occur in the ultramafic rocks as diabase dikes with chilled margins and as tectonic inclusions. Piecemeal growth of the ophiolite is also indicated by minor features: scarce jackstraw-textured talc-olivine rocks in tectonitic peridotite, cognate xenoliths of gabbro and olivine clinopyroxenite in diabase, and scattered dikes of intermediate composition in both ultramafic and mafic rocks. Field aspects of the ophiolite appear more compatible with a primitive island-arc setting than with a spreading oceanic ridge or marginal-basin model. The temporal relations between the ultramafic and mafic rocks, the presence of pyroclastic breccias, and the character of associated epiclastic rocks support this hypothesis. On the basis of this interpretation, the tectonic history of this segment of the Klamath Mountains during late Paleozoic to Jurassic time was dominated by island-arc genesis and westward extensional rifting. The ultimate collapse of this system occurred during the Late Jurassic (Nevadan orogeny) when the Galice Formation (Jurassic island arc and associated sedimentary basin) was thrust beneath the Preston Peak ophiolite, a Permian-Triassic remnant arc.

Jackstraw-textured talc-olivine rocks, Preston Peak area, Klamath Mountains, California

An unusual ultramafic rock characterized by elongate olivine crystals forms several small masses in alpine-type, tectonitic peridotite of the Preston Peak ophiolite, Klamath Mountains, California. The largest mass of this distinctive ultramafic rock, which has been mapped in detail, has contacts with the surrounding peridotite tectonite that vary from indistinct to sharp. In several areas, the country-rock tectonitic peridotite is impregnated by a network of talc-olivine veins locally containing elongate olivine crystals. Within the mass, the elongate olivine crystals, many of them pseudomorphosed by serpentine, are typically arranged in a crisscrossed pattern (jackstraw texture), but radial and parallel patterns also occur. The olivine crystals are bladelike but lack the skeletal or dendritic textures characteristic of elongate igneous olivines. Microprobe analyses of the olivine blades indicate that they are magnesium-rich (Fo 89.4 ) and unzoned. The coexisting mineral assemblage includes talc, tremolite, magnesite, chlorite, and pentlandite, minerals that are not compatible with a magmatic origin. Furthermore, at several localities within the mass, relict structures and minerals of the peridotite tectonite are preserved in the Jackstraw-textured rocks, indicating a replacement origin. Major- and minor-element chemical data support a replacement origin, in that the Jackstraw-textured talc-olivine rocks are similar in chemical composition to the surrounding tectonitic peridotite of the ophiolite. The most obvious difference is that the jackstraw-textured rocks contain more sulfur. Experimental data on the coexisting mineral assemblages indicate a low-temperature (500 to 600 °C) origin, and field relations suggest the importance of a volatile-rich phase.

E-5 Cumberland Plateau to Blake Plateau

DNAG Transect E-5. Part of GSAs 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.