Showing papers on "Terrane published in 1977"

Wrangellia—A displaced terrane in northwestern North America

Abstract: A large terrane extending along the Pacific margin of North America, from Vancouver Island, British Columbia, to south-central Alaska, is characterized throughout by similar sequences of Triassic rocks These rocks, including a thick pile of tholeiitic flows and pillow lava (Nikolai Greenstone and Karmutsen Formation) capped with inner-platform carbonates (Chitistone Limestone, Whitestripe Marble, Kunga Formation, and Quatsino Limestone), overlie an upper Paleozoic andesitic arc sequence and Permian argillite and limestone This coherent terrane, herein named Wrangellia, is juxtaposed against unlike sequences of Triassic and older rocks throughout its extent and is interpreted to be allochthonous Paleomagnetic data obtained from the Nikolai Greenstone and published in a companion article by Hillhouse indicate that Middle and (or) Upper Triassic rocks in southern Alaska formed in low paleolatitudes, probably within 15" of the paleo-equator

Triassic blueschist from northern California and north-central Oregon

Abstract: Four samples of blueschist from the eastern Klamath Mountains near Yreka, northern California, and from north-central Oregon near Mitchell have radiometric ages of approximately 220 m.y. (Middle Triassic). In the Klamath Mountains, there are two types of blueschist occurrence: (1) tectonic blocks of mafic composition in a phyllitic quartzite and siliceous phyllite terrane, which has also undergone blueschist-facies meta-morphism and lies between a belt of serpentinite and an upper Paleozoic or lower Mesozoic greenstone-chert terrane, and (2) discontinuous layers and tectonic blocks in phyllitic rocks of the greenschist metamorphic facies, which occur beneath a thrust plate of lower Paleozoic sedimentary rocks. In north-central Oregon, blueschist blocks occur in strongly sheared upper Paleozoic metasedimentary and metavolcanic rocks overlain by Cretaceous sedimentary rocks. The blueschist blocks in the Klamath Mountains and north-central Oregon are inferred to have formed during subduction that accompanied widespread Middle Triassic tectonism in the western Cordillera.

Ancient borderland terranes of the North American Cordillera: Correlation and microplate tectonics

Abstract: Enigmatic Paleozoic and Precambrian sequences rich in volcanic and plutonic rocks form discrete terranes along the outer border of the North American Cordillera. These borderland terranes occur in California in the northern Sierra Nevada and Klamath Mountains, in central Oregon and eastern Oregon–westernmost Idaho, in Washington in the Northern Cascade Mountains and San Juan Islands, in British Columbia in Vancouver Island, and in Alaska in the Alexander Archipelago. The difficulty in relating the geology of the borderland terranes to that of the North American continent, the recognition of ophiolites and suture zones separating the terranes from the continent, plus the Asiatic affinity of certain of the borderland faunas indicate that the terranes are allochthonous relative to the North American continent. Furthermore, major differences in stratigraphy, magmatic activity, tectonic activity, metamorphism, and particularly the ages and types of basement between the terranes — when considered together with discordant paleomagnetic data — suggest that at least six lithospheric plates are represented. The terranes in the Klamath Mountains have an Ordovician ultramafic rock (ophiolite) basement. The Oregon terranes have ultramafic complexes (ophiolites) in close association with volcanic rocks (volcanic arcs) that form the basement. In the Northern Cascade Mountains and San Juan Islands, the terranes have, respectively, Precambrian and Ordovician crystalline metaplutonic (magmatic arc) basement. The terrane in the southern part of the Alexander Archipelago has a Precambrian crystalline meta-volcanic-metasedimentary (remnant arc) basement, but an Ordovician basaltic-andesitic basement (initial deposits of an upward-shoaling island arc) appears farther north. Transcurrent faults segment and truncate parts of the Cordillera, but since the borderlands are in themselves composed of several plates, models of a single allochthonous plate are difficult to apply. More likely, during Precambrian and Paleozoic time, multiple microcontinental plates and volcanic arcs moved outboard and inboard (away from and toward North America) to accommodate a succession of marginal ocean basins opening and closing behind migrating arcs. This was followed in Mesozoic and Cenozoic time by large-scale northwestward drift.

Blueschists of the Kodiak Islands, Alaska: An extension of the Seldovia schist terrane

Abstract: The Kodiak Islands are composed of a series of northeast-trending belts of schists and deep-sea rock types that are interpreted as having been accreted to the continental margin during several discrete phases of subduction since early Mesozoic time. The Kodiak Islands schist terrane is the oldest of these accretionary belts and crops out discontinuously along the northwest side of the islands. Metamorphic rocks in this belt include quartz-mica schist, marble, metachert, greenschist, blueschist, and epidote amphibolite; the rocks yield Early Jurassic K-Ar mineral ages. These ages apparently provide a measure for the age of emplacement of the Kodiak Islands schists and are consistent with independently determined age estimates based on (1) biostratigraphy of the associated forearc basin deposits and (2) K-Ar ages from the associated plutonic arc on the Alaska Peninsula. Similarities in rock types, mineral ages, and tectonic setting indicate that the Kodiak Islands schist terrane is the southwestern extension of the Seldovia blueschists of the Kenai Peninsula.

K-Ar ages of hornblendes from gabbroic rocks in Southwest Japan

Abstract: K-Ar age determinations have been carried out on 15 hornblendes from the Yakuno Basic Rocks and their affinities in the Inner Zone of Southwest Japan. Gabbroic rocks from the Yakuno Basic Rocks and those from the Sangun Metamorphic Terrane in eastern Chugoku gave ages ranging from 228 to 275 m.y.; more than half of them are concordant with the geologically estimated age of the rocks. Remaining older ages may indicate either the time of plutonism in Early Permian, which might have been contemporaneous and comagmatic with the basic volcanic activity in the same areas, or the existence of inherited argon in hornblendes. Ages of gabbroic rocks from the Sangun Metamorphic Terrane in northern Kyushu are 296-372m.y., all older than those from eastern Chugoku. Hornblendes from gabbroic rocks in the Nagasaki Metamorphic Terrane gave still older ages of 449-472m.y., suggesting that these rocks may be a part of old basement rocks in the Japanese Islands. Graphical presentation of argon and potassium data indicates that most of the hornblende samples contain no significant amount of excess argon.

Structural geology of the Nenana River segment of the Denali fault system, central Alaska Range

Abstract: The Denali fault system, one of the major tectonic elements of southern Alaska, forms an arc 2,100 km long across southern Alaska. In the central Alaska Range, the system consists of a northern Hines Creek strand and a southern McKinley strand, 30 km apart, which divide the area into northern, central, and southern terranes. There is evidence for at least two episodes of deformation in the northern terrane, four in the central, and two in the southern during Paleozoic and Mesozoic time. During each, the inferred axis of maximum compressive strain was subhorizontal and about north-south, but the direction shifted to north-northwest–south-southeast during a late Paleocene–Eocene folding episode. Tectonic stability during Oligocene–middle Miocene time was followed by differential uplift of crustal blocks during late Miocene–Pliocene time. The Hines Creek fault may preserve a record of the early history of the fault system. Strong contrasts between lower and middle Paleozoic rocks juxtaposed along the fault suggest large dextral strike-slip displacement, but major convergent movement cannot be ruled out. Movement throughout the Hines Creek fault ceased by middle Cretaceous time, but local dip-slip movements continued into the Cenozoic era. The McKinley fault is an active dextral strike-slip fault, with Cenozoic offset of probably at least 30 km and possibly much greater. Mean Holocene displacement rates are 1 to 2 cm/yr. These rates would produce a 30-km offset in 1.5 to 3.0 m.y., or a 400-km offset in 20 to 40 m.y. The Denali fault may be part of a transform fault system connecting the Juan de Fuca Ridge and the landward extension of the Aleutian subduction zone. However, it is more likely that the fault forms the northern boundary of a small lithospheric plate caught between the Pacific and American plates.

Significance of Mesozoic radiolarians from the pre-Nevadan rocks of the southern Klamath Mountains, California

Abstract: Ribbon cherts and siliceous tuffs of the North Fork and Rattlesnake Creek terranes of the Klamath Mountains yield Mesozoic radiolarians. Rocks of the North Fork terrane were previously considered to be of Paleozoic age and those of the Rattlesnake Creek to be of Paleozoic and Triassic age, on the basis of fossiliferous limestone bodies that are now considered to be exotic blocks. In both terranes, however, red cherts that are closely associated with ophiolitic rocks contain Late Triassic radiolarians; overlying cherts and siliceous tuffs contain Early or Middle Jurassic radiolarians. The Jurassic radiolarian fauna from the North Fork terrane is similar to a fauna contained in Franciscan chert near Santa Barbara in southern California. The change in age assignment of the dominant rocks of these terranes, based on this new radiolarian data, indicates that the suture between the North Fork terrane and the Devonian rocks of the central metamorphic belt on the east probably formed during Middle or Late Jurassic time.

Possible “Caledonian” subduction under the Domnonean domain, North Armorican area

Abstract: Geological and geophysical data recently gathered in the English Channel permit delineation of the main basement structures beneath the Mesozoic and Cenozoic cover. These are, from north to south, (1) a Paleozoic trough, (2) a mafic rock complex, probably ophiolitic, and (3) a Precambrian craton, with a thin Paleozoic cover restricted to some grabens. This disposition suggests the presence of a cryptic suture. Arguments based on stratigraphy (the terranes grow older to the north), structure (faults are parallel to the mafic rock complex, and horsts and grabens are found in the craton), geochronology (the pre-Hercynian intrusions show a distinct arrangement in time and space), petrography (calcalkalic intrusions and an arc of ignimbrites are found), and paleontology (two faunal realms existed during early Paleozoic time) suggest that subduction could have occurred between late Proterozoic and middle Paleozoic time under the Domnonean domain.

Fracture zone tectonics, continental margin fragmentation, and emplacement of the Kings-Kaweah ophiolite belt, Southwest Sierra Nevada, California

Abstract: The Sierra Nevada foothill met amorphic belt is a 450 km long assemblage of remnant continent-derived epiclastics, arc volcanics, pelagic-hemipelagic sediments, and ophiolite slices. The various lithologic units range in age from Ordovician to Jurassic. Litho logic units are lenticular at scales ranging up to 150 km and strike about N. 30°W. parallel to the trend of the metamorphic belt (Fig. 1). Many units are penetratively deformed with a variety of near vertical foliation surfaces. The lithologic units are generally bounded by steep dipping fault and melange zones, but locally depositional contacts can be recognized. From at least latitude 38°30'N southward, latest Paleozoic to possibly early Mesozoic disrupted ophiolite occurs as remnant oceanic basement beneath Triassic to Jurassic arc volcanics and interstratified continent-derived epiclastics. Along the northern part of this segment of the metamorphic belt the ophiolitic rocks occur as scattered basement exposures surrounded by the younger volcanic and epiclastic rocks (Morgan and Stern, 1977; Behrman, 1978; Saleeby, unpub. field data). Further south in the Kings-Kaweah terrane deeper structural levels of the foothill metamorphic belt are exposed. Here a nearly continuous 125 km long ophiolite belt occurs with scattered remnants of early Mesozoic arc volcanic and epiclastic rocks depositionally above It. The ophiolite belt is named informally the Kings-Kaweah ophiolite belt after the Kings and Kaweah Rivers which transect it. This ophiolite belt constitutes part of the same oceanic basement terrane that is locally exposed further north amidst the arc volcanics and epiclastics.

Late Paleozoic Orogeny and Sedimentation, Southern California, Nevada, Idaho, and Montana

Abstract: Upper Middle Pennsylvanian to Upper Permian conglomeratic, sandy, and silty rocks cover part of southern California, most of Nevada, and much of Idaho. Their distribution, facies trends, and current vectors indicate that detrital components were shed mainly from a tectonic belt of highlands that extended generally northward through southern California, central Nevada, and western Idaho. Their enormous volume suggests the provenance terrane was of major proportions. Sediments shed eastward from this belt were deposited in a shallow epicontinental sea, and they mingled with limestone, dolomite, evaporites, spicule chert, and phosphorite; but those shed westward were deposited in deeper water as debris flows and turbidites and they mingled with coarse volcanic sediments, tuffs, lava flows, and radiolarian cherts. The volcanic sediments, tuffs, and lavas emanated from an archipelago that lay parallel to, and a short distance west of, the tectonic belt. The tectonic highlands formed a barrier between the ocean to the west and the epicontinental sea to the east with profound effects on sedimentation in the latter. Along the California -Nevada border, the late Paleozoic tectonic belt transected older northeast-trending facies belts and structures of the Cordilleran geosyncline and replaced them with a northwest-trending pattern.

Paleotectonic and Paleogeographic Significance of the Calaveras Complex, Western Sierra Nevada, California

Abstract: The Calaveras Complex of the western Sierra Nevada, as defined here, consists of a 375 km long, 35 km wide belt of metasedimentary and metavolcanic rocks, bounded on the west by the Melones fault zone and Kings-Kaweah suture, and on the east by the Sierra Nevada batholith. The Calaveras Complex forms a continuous northwest-trending belt between the Placerville area and the Merced River area. South of the Merced River the belt extends in numerous roof pendants at least as far south as the Tule River. A sequence of four lithologic units is recognized, each of which is thousands of meters thick. Precise original stratigraphic thicknesses cannot be measured because of intense soft-sediment and post-consolidation deformation. The lowest unit consists of mafic pillow lava, breccia, tuff, and argilllte, and may represent layer 2 of oceanic crust. This basal unit is overlain by a predominantly chaotic unit of argillite with variable amounts of chert and siltstone often occurring as clasts in a diamictite. Olistoliths of shallow water limestone are locally an important component of this argillite unit. The overlying chert unit contains abundant large olistoliths of rhythmically bedded chert and locally important limestone olistoliths in a matrix of streaky argillite and diamictite. The highest unit included within the Calaveras Complex contains abundant, well-bedded quartzite with abundant interbedded olistostromes containing quartzite clasts and limes tone olistoliths. Fossils from limestone olistoliths reported here indicate a maximum Permo-Carboniferous age for the upper part of the argillite unit, and a maximum late Permian age for the over lying chert unit. Published fossil data indicate the upper parts of the quartzite unit are late Triassic to early Jurassic. The argillite and chert units apparently comprise numerous olistostromes that accumulated on oceanic crust in a marginal basin that was broad enough to have been relatively free of elastic detritus derived from the basin margins. Olistostromes apparently were shed from tectonically elevated areas within the marginal basin that were denuded of their pelagic and hemi pelagic cover. The quartzite unit may represent an early Mesozoic northwestward progradation of mature continent-derived sand across the western end of the late Paleozoic marginal basin. The marginal basin is considered to have been situated between the Cordilleran miogeocline to the southeast and a volcanic arc terrane to the northwest. The late Paleozoic Havallah sequence of north-central Nevada is believed to have accumulated in the same marginal basin. The Melones fault zone and Kings-Kaweah suture represent a zone of early Mesozoic tectonic truncation a long which the Calaveras Complex is juxtaposed against upper Paleozoic ophiolitic rocks and Jurassic volcanic and epiclastic rocks. Thus, we infer that the Calaveras Complex represents the westernmost exposure of the late Paleozoic marginal basin.

Subduction tectonics in Japan

Abstract: Surely, one of the real lessons to be learned from plate tectonics is that every major problem of historical geology must be treated in a global context. My personal horizons were markedly expanded by my experiences last summer, when I examined rock associations formed by subduction in Japan, a Neogene island arc. Cretaceous and Paleogene assemblages are similar to those in the California Coast Ranges. Lower Mesozoic and upper Paleozoic eugeosynclinal terranes in Japan and western North America are accreted oceanic facies that probably formed side by side within a vast Paleopacific Ocean.

Geology of the granitic terrane, south-west Stewart Island (note)

Abstract: Field, petrographic, and geochemical data are reported for the granite terrane at Port Pegasus, Stewart Island. Granodiorites predominate but some local variation to tonalite and granite may be recognised. Chemical data suggest emplacement and crystallisation from water-saturated melts at depths of 10–16 km. The granites are locally garnetiferous and an analysis of one such garnet is reported. A mylonite zone transects the western portion of the area and the Tin Range schist belt appears to continue SW as a train of amphibolitic xenoliths. These may be interpreted as a xenolith “screen” between deep level plutons rather than a coherent roof pendant as suggested by earlier workers.

A seismic wave velocity along the sanbagawa metamorphic belt in shikoku area, southwestern japan

Abstract: Explosion seismic observations were made in the Sanbagawa high-pressure metamorphic terrane, Shikoku area, Japan. The profile was nearly in the direction parallel to the trend of the metamorphic belt. In this direction a considerably high seismic wave velocity for the upper crust had been expected due to a large amount of basic rock on the surface and predominant schistosity. The observed velocity for the upper crust, however, was found to be around 6.1km/s, a value that is commonly observed in the continental area. The crossover-distance between a travel-time curve for the granitic layer and that for the basaltic layer or the upper mantle was greater than 150km. This fact may be interpreted, combined with the gravity data, as indicating that the thicknesses of the granitic and the basaltic layers under the terrane do not differ significantly from those of the normal continental structure.

Satellite image analysis in mineral exploration in complicated structural terranes, Great Northern Peninsula, Newfoundland

Abstract: In April 1975, the government of Newfoundland opened large areas of crown land in northern Newfoundland to staking. Philips Management Inc., in competition with other companies, sought to establish criteria for staking. The common objective was bedded sphalerite of the Daniel's Harbour type. The sphalerite in the Northern Peninsula is associated with algal dolostones of the Upper Ordovician St. George Group. These are mostly horizontal and overlain by repetitive unmineralized limestones of the Table Head Formation. The objective of the pre-exploration program was to locate the hitherto "unknown" contact of the two units and to stake the upper St. George Group. Landsat images at a scale of 1:1,000,000 were used in the initial rapid analysis and interpretation of a large incompletely mapped area. Four approaches were used in differentiating patterns: (1) structural where major "units" could be separated by outlining lineaments; (2) botanic where vegetation and snow cover were indicative of soil and rock differences; (3) geomorphic where erosional and depositional features related to relief, structure, and geology; and (4) a combination of 1, 2, and 3 where no single approach could explain a pattern or lack of pattern. The results show that the analysis of satellite images supplemented with aerial photography and existing geologic data provides for rapid and inexpensive exploration. A second more detailed image analysis using color composites (channels 4, 5, and 7) at 1:1,000,000 and 1:250,000 black and white images (channels 5 and 6) and conventional color photographs at 1:250,000 color infrared at 1:31,000 and black and white infrared at 1:15,500 was able to divide the Cambrian-Ordovician terrane into three structural regions and tentatively to reconstruct the events that had led to the formation of the Great Northern Peninsula.

Correlation of aeroradioactivity and geology in southeastern Connecticut and adjacent New York and Rhode Island

Abstract: Radioactivity patterns in southeastern Connecticut, adjacent Rhode Island, and New York closely reflect bedrock units. This relationship is not greatly affected by surficial deposits of Pleistocene age because most deposits are locally derived and bedrock outcrops are relatively abundant. Most bedrock units are medium- to high-grade metasedimentary, metavolcanic, and metaplutonic rocks that range in age from possible Precambrian to middle Paleozoic. The rest are postmetamorphic granite and pegmatite of late Paleozoic age. The area is on the west flank of an anticlinorium of granitic gneiss and subordinate metasedimentary and metavolcanic rocks at a place where the anticlinorium changes trend from north to east. The Honey Hill and Lake Char faults mark a zone of low-angle faulting between the granite-gneiss terrane and stratified rocks of the Merrimack synclinorium to the north and west. The east-trending Honey Hill fault marks the southern end of the Merrimack synclinorium. Four fields denoting different levels and patterns of radioactivity coincide with major geologic provinces. The granite-gneiss terrane, which contains abundant potassic, ledge-forming granitic rocks (the Sterling Plutonic Group), coincides with a field of relatively high radioactivity, in which subordinate lows reflect outcrop areas of quartzitic rocks of the Plainfield Formation. This terrane is flanked to the west and north by layered metavolcanic rocks of mafic to intermediate composition belonging mostly to the Quinebaug Formation. These rocks coincide with a field of relatively low to moderate levels of radioactivity. The lowest level within this field is produced by the Preston Gabbro, which is at the right-angle bend in the structure. A sequence of metasedimentary schist and gneiss of the Tatnic Hill Formation, Hebron Formation, Scotland Schist, and the sill-like Canterbury Gneiss overlies the Quinebaug Formation. These rocks produce a field of moderate levels of radioactivity but have nodes of high radioactivity where pegmatite is abundant. A field of moderate intensity south of the east-trending anticlinorium of granitic gneiss coincides with layered and massive granodioritic and quartz-dioritic gneiss and subordinate amphibolite equivalent stratigraphically to the layered metavolcanic rocks north of the anticlinorium. The late metamorphic to postmetamorphic, low-angle Honey Hill and Lake Char faults, which follow the bend in the structure at the base of and within the metavolcanic rocks on the north and west sides of the anticlinorium, respectively, can be recognized only by the radiometric pattern that reflects the tectonic pinch-out of the Quinebaug Formation westward along the Honey Hill fault. A pluton of Narragansett Pier Granite of late Paleozoic age, which has associated dikes of Westerly Granite, is reflected in a sub field of high radioactivity in southern Rhode Island. Material derived from these rocks is apparently responsible for a relatively high level of radioactivity in the Charlestown moraine immediately south of the out crop area.