Showing papers on "Terrane published in 1972"

Cenozoic Volcanism and Plate-Tectonic Evolution of the Western United States. I. Early and Middle Cenozoic

Abstract: Variations in Cenozoic volcanism in the Western United States correlate rather closely with changes in tectonic setting: intermediate-composition rocks and their associated differentiates were erupted through orogenic or fairly stable crust, whereas basaltic or bimodal basalt-rhyolite suites were erupted later-concurrently with crustal extension and normal faulting. Lower and middle Cenozoic continental lavas, erupted onto postorogenic terranes, are predominantly intermediate types (andesite to rhyodacite), commonly with closely associated more silicic ash-flow sheets. Compositional zonations in individual ash-flow sheets, from rhyolite upward into quartz latite, record magmatic differentiation in underlying batholithic source chambers. The intermediate lavas probably represent the greater part of these batholiths and the ash-flow tuffs their differentiated tops. Continental volcanic activity of this type was most voluminous in the northwestern United States in Eocene time, but shifted southward in the Oligocene; contemporaneous sea-floor basalts occur in the Oregon-Washington coast ranges. Largely intermediate-composition calc-alkalic igneous suites, that become more alkalic toward the continental interior, are characteristic of most of the North and South American cordilleran belt. Similar volcanic associations are forming now around most of the Pacific margin where continental plates override oceanic crust along active subduction systems, marked by Benioff seismic zones and oceanic trenches. A similar subduction mechanism probably operated in the Western United States until late Cenozoic time. Analogy with chemical variations across active island arcs suggest that early and middle Cenozoic subduction occurred along two subparallel imbricate zones that dipped about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate, partly coupled to the western zone below the low-velocity layer. Predominantly intermediate-composition volcanism persisted throughout the Western United States until the initial intersection of North America with the East Pacific rise started the progressive destruction of the subduction system.

Precambrian Belt Basin of Northwestern United States: Its Geometry, Sedimentation, and Copper Occurrences

Abstract: Geometry and provenances of the Belt basin at various stages of its 600-m.y. time span (between about 1,450 to 850 m.y. ago) are decipherable for the middle and upper parts of the Ravalli Group, the Helena-Wallace Formations, and the lower part of the Missoula Group. The Revett and overlying St. Regis Formations (middle and upper parts of the Ravalli Group) had a cratonic source in the south and southwest and were deposited in troughs that reflect the west-northwest trend of the Osburn fault zone (or the Lewis and Clark line whose west-central part includes the Osburn fault zone). The Spokane Formation, correlative of St. Regis, had a source in the Canadian Shield and was deposited in a north-to northwest-trending trough in northwestern Montana, west of, but approximately parallel to, the north-northwest trend of the much younger Montana disturbed belt. The Helena and Wallace Formations were deposited in a broad simple northwest-trending basin between the previous troughs, which received clastic sediments from both the east and southwest, and in which extensive carbonate deposits formed on the eastern shelf. Tectonic adjustment in early Missoula time resulted in a long low dome in the Idaho Panhandle area and in the rejuvenation of the north-northwest-trending trough in the east. Clastic sediments containing abundant hematite were deposited on Helena-Wallace carbonate rocks, and Purcell Lava was poured out in the northeast. Anomalously high amounts of copper (100 or more ppm) are scattered throughout thousands of square miles of Belt terrane. The copper occurs in almost all formations, but it is most common in green strata. This mode of occurrence suggests a syngenetic or diagenetic origin. Stratabound copper ores, however, are known only as disseminations, discrete blebs, and veinlets in white quartzites and siltites of the Revett Formation in a “copper sulfide belt” along the northwestern Montana border. This belt is perpendicular to the Revett trough but parallel to the dome formed in early Missoula time, suggesting post-Revett epigenetic reconcentration of copper. Geochronologic data as interpreted by several authors are in apparent conflict with geologic history of the basin as read from the sedimentation record. Many of the conflicts can be resolved by reinterpretation of existing geochronologic and geologic data. It is obvious, however, that the dating of many events, including those that may have caused migration of copper, is in need of refinement.

Terranes of the western paleozoic and triassic belt in the southern klamath mountains, california

Abstract: The western Paleozoic and Triassic belt of the Klamath Mountains is herein subdivided from west to east into the Rattlesnake Creek, Hayfork, and North Fork terranes, further emphasizing the previously known composite nature of the province. The Rattlesnake Creek terrane is dominantly ophiolitic, includes appreciable soda granite, is structurally incompetent, and contains many deposits of chromite and manganiferous chert. The Hayfork terrane is in large part andesitic, includes the syenodioritic Ironside Mountain batholith, is relatively competent, and contains lode-gold deposits. The North Fork terrane is also ophiolitic, but metalliferous deposits similar to those of the Rattlesnake Creek terrane seem nearly absent. All three terranes include lenses of Permian limestone, but the Rattlesnake Creek terrane also includes Triassic limestone. The lithologies of the terranes suggest that the Rattlesnake Creek and North Fork terranes are tectonic slices of oceanic crust and that the Hayfork terrane originated as an island arc.

Pre-Permian Global Tectonics: A Tectonic Test

Abstract: Post-Permian sea-floor spreading coupled with fragmentation and widespread drift (> 1,000 km) of large segments (>107 km2) of continental crust appear established. However, studies of the nature and patterns of orogenic belts formed from the early Archean to the Recent suggest the post-Permian, large-scale dispersive movements of major continental fragments are uncommon, possibly unique in the geological record. Widespread drift and agglomeration of arcs, inter-arc basins, seas, and protocontinental segments also are indicated in conjunction with numerous subparallel spreading centers and subduction zones in the relict Archean rock complexes formed between 2.5 and 3.5 b.y. (b.y. = 109 yrs). But the Archean-type global tectonics were characterized by both spreading and accretive movements of evolving arcs, intervening basins, and protocontinental segments. Accretive motions dominated throughout the final 500 m.y. of Archean time, culminating in perhaps one, or, at most, three major protocontinents at about 2.5 aeons ago. The ensialic nature of most Proterozoic orogenic belts and their lithic components, as well as their interrelations to Archean terranes, indicate that they evolved in large part on and between the closely spaced Archean protocontinental clusters. These underwent deformation, refractionation, and thickening as the Proterozoic orogens evolved, but without the major fragmentation, widespread dispersion, and recollision of continents typical of the post-Permian. Late Proterozoic and early Paleozoic orogenic patterns and features, including ophi-olitic series within some orogens, indicate rifting, rotation, subordinate drift, and collision of large fragments of the evolving megacontinental masses. But the dominating continental component in associated orogenic belts indicates the scale of drift was limited to “second-order” movements commonly involving much less than 1,000-km drift, and no more than 20° rotation of these fragments. The rifting and collisions indicated in the late Proterozoic and early Paleozoic appear to be the incipient phase of the global dispersal of major continents in the post-Permian. Structural evidence for these conclusions appears in the accordant tectonic patterns that result when the Americas, Africa, India, and Australia are reclustered into the pre-Triassic Gondwana and western Laurasia defined by many independent lines of evidence. In this megacontinent, especially in Gondwana, fold belts of Paleozoic, Proterozoic, and Archean ages are juxtaposed into subparallel, coherent entities. In general the fold belts decrease in age from north to south. These first-order features seem to require a consistent interrelation and orientation of southern Gondwana with remarkably uniform regional force fields in the mantle, under, and seaward beyond Gondwana throughout at least 3.5 b.y. of geologic time. The further accordance, along great circles or parallels, of the Archean fold belts of Permian North America with those in Permian Gondwana suggests either a single mega-Archean orogenic system, or two complementary subparallel systems which have undergone little differential reorientation throughout the entire Proterozoic and Paleozoic. Because the motions necessary to generate the subparallel mega-Archean fold systems of the world did not deviate widely from the contemporary meridians of latitude, they seem largely of internal (radiogenic?) origin or only indirectly related to obvious extraterrestrial forces, such as changes in rates of rotation of the earth.

The Precambrian Gneisses and Supracrestal Rocks of the Western Shore of Kaipokok Bay, Labrador, Newfoundland

Abstract: In the Kaipokok Bay area the amphibolite facies Hopedale Complex is overlain by the English River Greenstones. The Hopedale Complex is formed of gneisses, amphibolites, and associated granites and shows indications of being polycyclic, possibly derived from an older (Aphebian?) granulite facies terrane. The English River Greenstones are a supracrustal sequence of dominantly metavolcanic rocks and are the equivalents of the Aillik series. Hudsonian orogenic effects were superimposed on both rock units and caused locally intense refoliation of the gneisses and the development of a strong foliation in the greenstones. During this orogenic cycle an early period of intense zonal flattening was followed by a more regionally developed folding and weak tectonite fabric development. All of these rocks were cut by later acidic and basic intrusions. The geological history of the area shows a general similarity to that of southwestern Greenland.

New Fossil Localities in the Lush's Bight Terrane of Central Newfoundland

Abstract: Three new fossil localities have been discovered in the Cutwell Group on Long Island in western Notre Dame Bay, Central Newfoundland. Conularids, crinoids, ostracodes, and cephalopods occur in smal...

Guidebook 5. Field Trip Guide Book for Precambrian Migmatitic Terrane of the Minnesota River Valley

Abstract: Prepared for the 85th Annual Meeting of THE GEOLOGICAL SOCIETY OF AMERICA and ASSOCIATED SOCIETIES Minneapolis, Minnesota, 1972. SPECIAL PAPERS: Precambrian geology of the Minnesota River Valley between Morton and Montevideo: Grant, J.A., Pt. 1— Geology and structure; and Pt. 2—Geochronology and geochemistry.