Showing papers on "Terrane published in 1980"

Cordilleran suspect terranes

Abstract: Over 70% of the North American Cordillera is made up of ‘suspect terranes’. Many of these geological provinces are certainly allochthonous to the North American continent and seem to have been swept from far reaches of the Pacific Ocean before collision and accretion into the Cordilleran margin mostly in Mesozoic to early Cenozoic time.

Paleomagnetic record of plate‐margin tectonic processes along the western edge of North America

Abstract: The paleomagnetic record for the western edge of North America shows consistent, systematic discordance. Virtually none of roughly three dozen high-quality paleomagnetic studies of pre-Pliocene rock units located within several hundred kilometers of the continental margin are lying anywhere near their appropriate reference poles (constructed from high-quality paleomagnetic data from the craton). Nor do these discordant paleomagnetic studies show simple random scatter, as might be expected for rock bodies whose postmagnetization histories include reheating, complex tectonic deformation, or chemical change. Instead, their paleomagnetic poles are clearly displaced systematically away from the reference curve into the general area of the Atlantic Ocean. To accomplish this tectonically requires that the magnetized rock body move northward, rotate clockwise, or both, in relation to North America. The consistency of paleomagnetic record argues that most of the western edge of North America has undergone such block movement and therefore is allochthonous, at a scale at least as large as the area of a typical paleomagnetic study. It further argues that northward transport and clockwise rotation have been prime elements in shaping the Cordillera. Differences between paleomagnetic records for separate parts of the Cordillera probably reflect differences in specific platetectonic histories. For instance, although northward transport of allochthonous blocks is important south of Cape Mendocino and north of southern Vancouver Island, in the area between (the Pacific Northwest), only clockwise rotations are found. This is consistent with steady under-thrusting of the Pacific northwest coastline by the Farallon plate for most of the Tertiary, which is in contrast to the steady or intermittent transform activity to the north and south. Batholith belts, including the Peninsular Range batholith of Southern California and the Coast Plutonic Complex of British Columbia, also seem to have been involved in the general northward transport and clockwise rotation, although the possibility of undetected tectonic tilts clouds the record. However, consideration of the thermal behavior of large, tilted plutonic blocks suggests that post-magnetization tilts probably are small. If so, then most western Cordilleran Cretaceous batholiths (not including the Sierra Nevada) also are allochthonous. Northward transport and clockwise rotation could be accomplished by a variety of mechanisms, involving recognized plate tectonic processes. Some of these mechanisms are discussed. However, dextral shear between North America and plates to the west is central to each mechanism. It seems likely that shear also has disrupted and internally modified older accreted terranes, producing internal block rotations. Such older, disrupted allochthonous terranes appear to be drawn out into highly attenuated tectonic laminae plastered on the western edge of the continent.

Paleogene Tectonic Evolution of the Pacific Northwest

Abstract: Three consecutive tectonic regimes are recorded in the Paleogene of the North American Cordillera between 40° and 60° latitude. The Paleocene (65-53 Ma) regime produced crustal shortening along the eastern edge of the Cordillera, as well as a weak magmatic arc in British Columbia. The lower and middle Eocene regime (53-42 Ma) produced a robust volcanic arc superimposed on a network of strike-slip and normal faults and large north-northeast trending extensional metamorphic reset terranes. The Late Eocene and Oligocene regime (42-30 Ma) produced a north-south Cascade arc south of 50° latitude, and tectonic quiescence to the north and east. This sequence of events is consistent with the following tectonic model. In Paleocene time, continuous subduction took place from 40° to 60° latitude; low-angle subduction south of 49° created the Laramide magmatic gap. At about 53 Ma, amalgamation of the Pacific, Kula and northern Farallon plates created a Pacific-North America transform boundary north of 47° latitude. T...

The Great Lakes tectonic zone — A major crustal structure in central North America

Abstract: The Great Lakes tectonic zone is a major Precambrian crustal feature more than 1,200 km long extending eastward from Minnesota into Ontario, Canada. It is a zone of distinctive tectonism, affecting both Archean and early Proterozoic rocks, along the northern margin of the early Proterozoic Penokean fold belt adjacent to the Archean Superior province. The zone coincides with the boundary between two Archean crustal segments recognized in the region: a greenstone-granite terrane (∼2,700 m.y. old) to the north (Superior province) and an older (in part 3,500 m.y. old) gneiss terrane to the south. Tectonism along the zone began in the late Archean, during the joining together of the two terranes into a single continental mass, and culminated in the early Proterozoic, when steep or northward-facing overturned folds were formed in the supracrustal rocks, and intense cataclasis and a penetrative cleavage developed in subjacent basement rocks of the greenstone-granite terrane. The Proterozoic deformation took place under low to intermediate pressures. Movement occurred along the Great Lakes tectonic zone through much of the Precambrian time recorded in the region. In the early Proterozoic, crustal foundering, which was parallel to the zone and was diachronous, initiated the structural basins in which the early Proterozoic sequences of the Lake Superior and Lake Huron regions were deposited. Later, during the Penokean orogeny (∼1,850 to 1,900 m.y. ago), compression deformed the sequences in both regions. Still later, intermittent (∼1,850 to 1,100 m.y. ago) crustal extension provided sites for emplacement of abundant mafic igneous rocks. There is no definite evidence that any of the extensional events progressed to the stage of development of oceanic crust; probably the zone has been wholly intracratonal since its inception in late Archean time. During the Phanerozoic, minor differential movements occurred locally in the Great Lakes tectonic zone, as recorded by the thinning of Cretaceous strata and their subsequent tilting and by historic earthquakes in Minnesota.

Horizontal tectonic interaction of an Archean gneiss belt and greenstones, Pilbara block, Western Australia

Abstract: The Archean Pilbara block, Western Australia, is cited as a type example of a granite-greenstone terrane. New field evidence reveals that the internal structure of one of the major batholiths, the Shaw batholith, resembles an Archean gneiss belt. Greenstone intercalations within the gneiss belt can be traced into the surrounding greenstone sequence and were incorporated during subhorizontal thrusting and recumbent folding episodes. These early tectonic episodes preceded solid-state diapiric uprise of an overthickened crust. Granitic plutonism and partial melting of the sialic crust appear to have played an important role only in the later stages of the tectonic evolution of the Shaw batholith. Deformation and metamorphism are not a direct consequence of the emplacement of granitic magmas, contrary to the widely accepted magmatic model of batholith formation.

Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland

Abstract: Permian to middle Cretaceous strata in New Zealand can be genetically grouped into at least four tectonostratigraphic terranes. Each terrane is lithostratigraphically distinct, yet paleogeographic analyses indicate that each terrane is only part of a once larger geologic entity. The present distribution of terranes is inferred to represent accretionary processes along the margin of Gondwanaland. Accretion seems to have occurred intermittently until middle Cretaceous time, and accretion was immediately supplemented by rifting that broke New Zealand away from the Australia and Antarctica part of Gondwanaland. The terranes have been severed by Tertiary slip on the Alpine fault.

Paleomagnetic evidence for tectonic rotation of Jurassic plutons in Blue Mountains, eastern Oregon

Abstract: To test the hypothesis that pre-Tertiary rocks of eastern Oregon have undergone clockwise rotation, a paleomagnetic investigation was made of granitic and high-temperature metamorphic rocks of the upper Jurassic-Lower Cretaceous intrusive suite of the Blue Mountains province, northeastern Oregon. At nine sampling sites in three widely separated regions the paleomagnetic directions showed a high level of intrasite and intersite consistency, with a mean direction of I = 63°, D = 30°, a95 = 10°. The observed inclination is consistent with that expected on the basis of paleomagnetic results for stable North America, indicating that the intrusives have not undergone northward translation since cooling. The paleomagnetic declination, on the other hand, indicates that the region sampled has undergone a clockwise rotation of 60°±29° relative to the stable craton. Two of the bodies sampled are intrusive into the exotic Seven Devils terrane, and the remaining body is intrusive into the tectonic melange of a separate arc-trench terrane, which may well have been the North American margin. The consistency in magnetic directions between the three regions (k = 151) indicates that suturing of the exotic terrane took place prior to the emplacement of the intrusives. The early Tertiary Clarno Formation situated to the west of the region sampled has rotated substantially less than the Blue Mountains [Beck et al., 1978], indicating that much of the rotation of the Blue Mountains occurred prior to the Eocene. To account for the observed rotations, we discuss two alternative models. In the first, rotation accompanies back arc rifting associated with the Early Cretaceous westward rifting of the Klamath Mountains. In the second, rotation reflects Late Cretaceous regional dextral shearing in the Blue Mountain area, which is located several hundred kilometers south of major right lateral faults known to extend from Alaska down at least to northern Washington.

Paleozoic paleomagnetism and northward drift of the Alexander Terrane, southeastern Alaska

Abstract: Paleozoic limestone, graywacke, sandstone, milestone, red beds and volcanic rocks of the Alexander terrane, southeastern Alaska, have yielded six paleomagnetic pole positions after thermal and alternating-field demagnetization. These poles are from sample groups of late Middle Ordovician, Late Ordovician, Devonian, Late Devonian, and early and late Carboniferous age. To test various tectonic models for the structural development of this part of western North America, the paleomagnetic results are compared to those for the North American craton. It is found that the observed inclination and declination values deviate significantly from the values predicted for the present-day position of the Alexander terrane (55.5N, 133.5W). Better matching can be obtained for a paleoposition of the terrane at about 40N, 120W, in the present position of western Nevada and northeastern California. In addition, an in situ 25° clockwise rotation of the terrane is required to restore it to its original position.

Post-orogenic peralkaline and calc-alkaline granites and associated mineralization of the arabian shield, kingdom of saudi arabia

Abstract: Following a long period of island arc-like orogenic activity, post-orogenic granites of Pan-African age (670-550 Ma) were emplaced throughout the Arabian Shield. Most of these rocks are typical subsolvus calc-alkaline biotite monzogranite. The early Pan-African granites are associated with widespread small tholeiitic gabbro intrusions, many of which are layered. The calc-alkaline granites are not evenly distributed and their area of exposure relative to other rock types increases from S.W. to north and N.E. In the north-eastern part of the shield, they are associated spatially with rhyolites of similar composition and with molasse derived from granitic terrane.

Granitic ring complexes and Precambrian hot-spot activity in the St. Francois terrane, Midcontinent region, United States

Abstract: Surface and subsurface geologic data have led to the recognition of granitic ring complexes in the 1.5-b.y.-old St. Francois terrane of southeastern Missouri. The distinctive rock association of the ring complexes comprises alkaline silicic rocks with minor quartz monzonite, trachyte, and basalt. Three periods of granite emplacement are recognized: sub-volcanic granite massifs were emplaced below comagmatic rhyolites and ash-flow tuffs, quartz monzonite and associated hypabyssal rocks were intruded along ring fractures in cauldron-subsidence structures, and central plutons were emplaced in resurgent cauldrons. Similar magmatic complexes in the buried Precambrian basement of the Midcontinent indicate a period of widespread anorogenic magmatism that may have been induced by intraplate hot-spot activity. The post-Precambrian geologic record of the region suggests that recurrent, but localized and less intense, hot-spot activity was responsible for emplacements of alkaline ultramafic rocks, for tectonic adjustments, and for increased geothermal gradients that created favorable ore-concentrating environments in the sedimentary rocks.

Phanerozoic thrusting in Proterozoic belt rocks, northwestern United States

Abstract: Newly identified listric thrust faults show eastward translation north of the Lewis and Clark line across all 800 km of Belt terrane from Spokane, Washington, on the west to Glacier National Park on the east. Right-lateral slippage on the line was accompanied by clockwise rotation on the thrusts. These movements were probably in response to the complex plate interactions that began about 200 m.y. ago along the western continental margin. Gravity and magnetic data suggest that basement rock is involved in the thrusting. Reconcentration of strata-bound copper sulfides along bedding-plane shears may help form ore near one thrust. West of the Rocky Mountain Trench, Phanerozoic strata that elsewhere have oil and gas potential may have been stepped up by the thrusting, and eroded, rather than extending beneath most of the thrust belt.

Allochthonous Jurassic ophiolite in northwest Washington

Abstract: Fragments of Jurassic ophiolite having U-Pb zircon ages narrowly grouped at 160 to 170 m.y. are widespread over parts of northwest Washington. The Haystack thrust fault is inferred to mark the base of the ophiolite in the San Juan Islands and adjacent Cascade foothills; other bodies of mafic and ultramafic rock in the western Cascades may be klippen of the Haystack thrust plate. The Haystack thrust fault is probably the structurally highest and possibly most extensive thrust yet recognized within a family of Late Cretaceous thrust faults in northwest Washington. The ophiolite and its time of emplacement (bracketed between about 100 and 88 m.y.) suggest a similarity with the Coast Range thrust of California which thrust Upper Jurassic ophiolite and the Great Valley sedimentary sequence over the Franciscan assemblage. However, relations in the Cascades are complicated by the extraordinarily diverse character of lower plate rocks, of which very few resemble the Franciscan. We conclude that an original subduction system was modified by later tectonic activity so that a variety of terranes was juxtaposed as a family of rootless thrusts, with the ophiolite forming, at least in some areas, the uppermost structural unit. Perhaps the emplacement of Wrangellia, an allochthonous microcontinent west of the San Juan Islands, caused the thrusting.

Pre-Upper Devonian Quartzose Sandstones in the Shoo Fly Formation Northern California-Petrology, Provenance and Implications for Regional Tectonics

Abstract: The northern part of the Shoo Fly Formation, exposed in the northern Sierra Nevada, California, contains a thick and extensive deposit of pre-Upper Devonian age characterized by interbedded graded sandstone and phyllite. The sandstones have mature compositions with more than 90% quartz, rarely more than 10% feldspar, and 5% or less lithic grains. An analysis of quartz types in the sandstones indicates a source in a predominantly plutonic/metamorphic terrane. The quartzose sandstones and interbedded phyllites appear to have been deposited on oceanic crust and the sandstones probably were derived from a Precambrian basement in a nearby continental block. The depositional setting of the quartzose sandstones most likely was a passive continental margin. Lower Paleozoic quartzose sandstones in the eastern Klamath Mountains have essentially the same composition as those in the northern Sierra Nevada suggesting that the passive margin deposit extended northward at least as far as the Klamath Mountains. The lower...

Temperature profiles in the Barnes Ice Cap, Baffin Island, Canada, and heat flux from the subglacial terrane

Abstract: Temperature measurements were made in seven boreholes, ranging in depth from 50–276 m, in the Barnes Ice Cap. Holes B4, D4, and T0975 are approximately 1 km from the margin and an average of 8 km a...

Paleomagnetism of the Triassic Hound Island Volcanics, Alexander Terrane, southeastern Alaska

Abstract: The Hound Island Volcanics, a Late Triassic sequence of submarine basalts, has been investigated to determine possible large-scale movements of crust in southeastern Alaska. Data from oriented cores at 12 sites bordering Keku Strait yielded a mean paleomagnetic pole at latitude 23.0°N, longitude 188.8°E (α95 = 11.0°). Reliability of the pole is affirmed by a fold test and by the consistent antipolarity of normal and reversed magnetic directions. The anomalous position of this pole compared to that derived from coeval paleomagnetic data in central North America indicates that the region surrounding Keku Strait was rotated approximately 100° in a counterclockwise sense. The paleolatitude determined from the basalts is not significantly different from that predicted by data from the craton. Therefore the apparent northward drift of the Alexander terrane indicated by Paleozoic paleomagnetic results occurred before Late Triassic time.

Tertiary tectonic denudation of a Mesozoic–early Tertiary(?) gneiss complex, Rawhide Mountains, western Arizona

Abstract: The Rawhide fault in western Arizona is a major Miocene detachment surface that separates an autochthonous Mesozoic–early Tertiary(?) metamorphic complex from an allochthon of chaotically imbricated Precambrian(?) igneous and metamorphic, Paleozoic metasedimentary, Mesozoic(?) igneous and metamorphic, and Tertiary sedimentary and volcanic rocks. Lower-plate metamorphic rocks consist largely of mylonitic quartzofeldspathic gneisses with a strong mineral lineation trending 20° S, 48° W. These gneisses yield early Tertiary K-Ar ages. The allochthon is composed of two major sheets that are broken, independently, by a myriad of Miocene high- and low-angle normal faults. The Rawhide fault formed by the coalescing of listric normal faults along a favorable structural horizon, presumably near the top of the metamorphic complex. Tectonic denudation of the metamorphic complex along the Rawhide fault occurred between 16.2 and 9.6 m.y. ago. Structural analyses indicate movement of the allochthon toward the northeast. The nature, geometry, and style of deformation suggest that the highly disrupted upper-plate terrane was most likely produced by the detachment, probably by gravity, of the Precambrian(?) to Miocene cover rocks from the underlying Mesozoic–early Tertiary(?) metamorphic complex.

40Ar-39Ar ages of some pre-Tertiary plutonic and metamorphic rocks of eastern Oregon and their geologic relationships

Abstract: The “central melange” terrane of eastern Oregon, consisting mainly of an argillite-chert sequence and ophiolitic rocks, and the Seven Devils volcanic-arc terrane of northeastern Oregon and western Idaho probably formed in a volcanic island-arc and back-arc basin environment. New 40Ar-39Ar ages indicate that the ophiolitic rocks formed during the Permian and Late Triassic and that metamorphism synchronous with pervasive orogenic deformation occurred at the end of Triassic time. Accretion of the terrane to the North American continent in Late Jurassic time was accompanied by a second major orogenic event.

Structure and interpretation of the Caples terrane of the Thomson Mountains, northern Southland, New Zealand

Abstract: In the Thomson Mountains, vokaniclastic submarine fan and trench sediments of the Caples terrane have been deformed and metamorphosed and subsequently extensively faulted. The first of two phases of folding was dominated by large open to tight folds in bedding with subhorizontal axes and vertical axial planes. Few mesoscopic structures were developed in this phase. Metamorphism was coincident with, and may have outlasted, Phase 1 deformation; schistosity is developed in the east of the area and is axial plane to Phase 1 folds in bedding. Phase 2 deformation was limited to warping of metamorphic foliation. Phase 1 folding was accompanied by tectonic sliding. Tectonic slides are manifested as discrete fractures or as wide zones of deformation akin to melanges, intimately associated with major folds. Characteristic mesoscopic structures were developed within tectonic slide zones. Most of the area is within pumpellyite-actinolite metamorphic facies; prehnite occurs in veins and in one small area of p...

Tectonic implications of remagnetized upper Paleozoic strata of the northern Sierra Nevada

Abstract: A paleomagnetic study of a remagnetized Paleozoic terrane in the northern Sierra Nevada has provided new data on Cenozoic tectonics in the western United States. A combination of paleomagnetic and geologic evidence establishes that the remagnetization occurred during Late Jurassic time and that the rocks have experienced no major tectonic movement since. This conclusion establishes an eastern limit for the extent of large-scale tectonic rotations recently reported for Tertiary units on the western edge of the North American continent.

Zircon ages for the Pelly Gneiss and Klotassin granodiorite in western Yukon

Abstract: Two samples of Pelly Gneiss with different field relations and post-crystallization histories give different U/Pb ages that are Devonian and Permian (375 and 276 Ma). A sample of Klotassin granodiorite gave an age of 192 Ma confirming its Early Jurassic age. These Devonian, Permian, and Jurassic plutonic rocks in Yukon Crystalline Terrane lack equivalents in the adjacent Omineca Belt where sedimentary strata accumulated at these times. This suggests that the Yukon Crystalline Terrane is allochthonous with respect to the Omineca Belt.

Regional implications of Upper Triassic metavolcanic and metasedimentary rocks on the Chilkat Peninsula, southeastern Alaska

Abstract: A low-grade metamorphic sequence consisting of thick mafic volcanic rocks overlain by calcareous flysch with very minor limestone underlies much of the Chilkat Peninsula. Fossils collected from both units are of Triassic age, probably late Karnian. This sequence appears to be part of the Taku terrane, a linear tectono-stratigraphic belt that now can be traced for almost 700 km through southeastern Alaska to the Kelsall Lake area of British Columbia. The age and gross lithology of the Chilkat Peninsula sequence are comparable to Upper Triassic rocks that characterize the allochthonous tectono-stratigraphic terrane named Wrangellia. This suggests either that the two terranes are related in their history or that they are allochthonous with respect to one another and coincidentally evolved somewhat similar sequences in Late Triassic time.

Early Archean Sm-Nd model ages from a tonalitic gneiss, northern Michigan

Abstract: Sm-Nd model ages on a whole-rock sample of the gneiss at Watersmeet, Michigan, show that these rocks were added to the continental crust about 3,600 m.y. ago. These model ages are calculated by assuming that the rocks were derived from a mantle reservoir with the Sm/Nd ratio of CHUR, which corresponds approximately to the chondritic ratio. These data confirm the U-ThPb zircon results that indicated a minimum age of 3,410 m.y. and possibly an age as old as 3,800 m.y. This study further illustrates the utility of Sm-Nd model ages in deciphering the history of complex Precambrian terranes.

Precambrian(?) Crystallization and Permian(?) Metamorphism of Hypersolvus Granite in the Avalonian Terrane of Rhode Island

Abstract: The igneous and metamorphic rocks of southeastern Massachusetts and Rhode Island occupy one of the most poorly understood geologic terranes in the northern Appalachians Interest in this terrane has been renewed with the realization that it may contain importtant evidence concerning the opening and closing of the Atlantic Ocean and its predecessors (Wilson, 1966, 1973) and that some of the rocks in this region may be similar to the Avalon zone of Newfoundland (Williams, 1976; Robinson, 1976; Rast and others, 1976) In this paper, we describe the mineralogy and petrology of some Precambrian (?) granites in Rhode Island Despite superimposed deformation and metamorphism, it is possible to identify relict igneous textures and to show that these granites formed from iron-enriched, alkaline magmas at a high level in the crust High-grade meta-morphism and deformation has caused complete recrystallization of the feldspars in parts of the granite Some of this metamorphism occurred during Permian time, but the effects of earlier episodes cannot be evaluated Geologic Setting We have studied the Scituate Granite Gneiss (SGG), Hope Valley Alaskite Gneiss (HVA), and the Ten Rod Granite Gneiss (TRG) in southern and central Rhode Island (Quinn, 1971) The age of these granites is probably upper Precambrian or lower Paleozoic based on several whole-rock Rb-Sr isochrons Day (1968 and unpub data) has made preliminary analyses that suggest an age of about 600 my for the three units These preliminary data are supplemented by reports of granites with similar ages from nearby areas (Fairbairn and others, 1967; Hills and Dasch, 1972; Smith and Giletti, 1977; Kovach and others, 1977)

Gravity highs and crustal structure, Omineca crystalline belt, northeastern Washington and southeastern British Columbia

Abstract: The Omineca crystalline belt of northeastern Washington and southeastern British Columbia is characterized by a regional Bouguer gravity high, and individual gneiss domes within the terrane are marked by local gravity highs. Models of crustal structure which satisfy the limited available seismic refraction data and explain the gravity highs over the gneiss terrane permit the hypothesis that the metamorphic core complexes are the surface expression of a zone of dense infrastructure that makes up the top 20 km of the crust within the crystalline belt.

Petrology, structure, and regional tectonics of South Fork Mountain Schist, Pine Ridge Summit, northern California

Abstract: South Fork Mountain Schist of Pine Ridge Summit, northern California, is predominantly metasedimentary, of the glaucophanitic-greenschist facies, and has undergone at least two periods of folding. It was overridden by the Klamath Mountain block (Coast Range thrust fault) and later thrust over unmetamorphosed broken formation of the Franciscan Complex. The schist differs markedly from the subjacent Franciscan in being derived from a more distal submarine fan deposit. Chert breccia and volcanic boudins within the fault zone beneath the schist may have been derived from a melange terrane now situated beneath the Klamath block. The schist does not correlate lithologically or structurally with the nearby Redwood Mountain schist outlier, which may have been transported to its present position by strike-slip faulting.