Showing papers on "Terrane published in 1973"

Tibetan, Variscan, and Precambrian Basement Reactivation: Products of Continental Collision

Abstract: Extensive terranes of basement reactivation are interpreted as resulting from crustal thickening following continental collision. It is suggested that terranes, such as the Grenville Province and much of the Variscan orogenic belt in Europe, have their modern analog in the Tibetan Plateau. The Tibetan Plateau is underlain by a continental crust between 60 and 80 km thick and is characterized by extensive high-potash Neogene vulcanism. Following T. H. Green's arguments that partial melting of a dioritic lower crust may yield potassic granitic liquids and refractory anorthositic residues, we consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust. At high structural levels, silicic-potassic ignimbrites are extruded in intermontane basin-horst terranes, with subjacent granite plutons. At deeper levels, a dry refractory lower crust consisting of pyroxene granulites and anor-thosites is generated.

Interpretative Synthesis of Metamorphism in the Alps

Abstract: Compilation maps are presented showing major tectonic features, selected lithologies, and zones of progressive metamorphism in the eastern half of the western Alps (scale 1:400,000) and the eastern Alps (scale 1:800,000). Two principal complexes are distinguished on them: (1) the Caledonian and Hercynian metamorphosed terrane in the Southern Alps + Austroalpine sheets, overlain by deformed but largely unrecrystallized uppermost Paleozoic and younger platform-type sedimentary rocks; and (2), the tectonically lower Sesia-Lanzo + Lepontine-Pennine + Helvetic realms, a sequence of Hercynian and pre-Hercynian plutonic igneous + metamorphic rocks and a younger, chiefly Mesozoic cover sequence consisting of shelf, slope, and deep-sea sediments + ophiolites, incompletely to pervasively recrystallized during Alpine metamorphism. 1. The pre-Mesozoic metamorphism involved several cycles in both complexes, but it has not been possible to distinguish these on the maps. Judging from the mineral para-geneses, recrystallization events seem to have taken place under moderate to very high temperatures at low to moderately high pressures. 2. Three contrasting but intergradational, temporally overlapping episodes of Alpine metamorphism are recognized in the Sesia-Lanzo + Lepontine-Pennine + Helvetic terrane: (a) an early, high-pressure, low-temperature event syntectonic with nappe formation, which produced eclogites + albite amphibolites, glaucophane schists, and allied greenschists, with lower grade, more recently metamorphosed sections lying externally (that is, toward the European foreland) relative to the older, progressively higher grade, more internal, imbricated sections lying to the south and east; (b) a middle syntectonic to post-tectonic stage characterized by more “normal” physical conditions, resulting in the partial or complete conversion of the products of event (a) to greenschist (prasinite) and low-rank amphibolite facies metamorphic rocks; and (c), a late, and in most cases syntectonic to post-tectonic recrystallization involving moderately high temperatures and pressures, which locally obliterated the products of both (a) and (b). Of the recrystallization continuum, event (a) is best preserved in the Franco-Italian Alps, and in Switzerland in the cantons of Wallis and Graubunden, (b) is nearly ubiquitous in the Sesia-Lanzo + Pennine + Helvetic complex, and (c) is confined to the Lepontine gneiss area of the Italian and Swiss Alps, and to the central gneiss domes of the Tauern Fenster, central Austria. The timing of Alpine metamorphism evidently varied laterally along and across strike of the belt. For instance, in Austria, event (a) may have begun in Late Cretaceous (?) time, whereas it probably commenced during Paleocene-Eocene time in the western Alps. Moreover, “early” Alpine, low-grade zeolitization occurred in the external parts of the Helvetic realm probably during Oligocene time— nearly contemporaneously with the more internal “late” Alpine higher grade Lepontine recrystallization of event (c). The contact between (1) the Southern Alps + Austroalpine nappes on the one hand, and the structurally lower (2) Sesia-Lanzo + Lepontine-Pennine + Helvetic realms on the other, juxtaposes rocks of markedly contrasting petrologic and tectonic histories. This zone, here referred to as the Alpine Suture, is postulated to represent the crustal expression of a Late Mesozoic-early Tertiary convergent lithospheric plate junction. The early Alpine high-pressure paragenesis appears to reflect subduction and shuffling of the more northerly terrane beneath the stable lithospheric dab capped by the Southern Alps + Austroalpine sheet. If so, the observed blueschist-type metamorphic zoning probably was generated by progressively greater depths of underflow and a consequent depression of the isotherms. A variable rate or time at which the complex was exhumed locally could account for the later establishment of a more normal thermal regime, and thus, succeeding higher temperature mineral assemblages as displayed in the Lepontine gneiss area and the Tauern gneiss domes.

Archean Magmatism and Crustal Thickening

Abstract: Exposed stratigraphic thicknesses in Archean greenstone belts, high P-T mineral assemblages in Archean granulite-facies terranes, and K, Rb, and Sr geochemical polarity indices in Archean volcanic rocks suggest that large portions of the Archean crust were ≥25 km in thickness, and corresponding depths to subduction zone were ≥85 km prior to the emplacement of widespread Archean granitic rocks at 2.5 to 3.0 b.y. The polarity indices also suggest that the crust thickened to ≳30 km during emplacement of Archean granite. Compositional data suggest that at least some of the granitic rocks are not intrusive equivalents of greenstone volcanic rocks (at the same silica level). The thickness and seismic velocity distribution in the present crust do not appreciably change as a function of mean crustal age from terranes ≲225 m.y. to those >2,500 m.y. in age. A decrease in the range of crustal thickness with age is observed, however, probably reflecting an increasing degree of cratonization with age. Most data suggest that the Archean crust closely approached its present-day thickness before or during emplacement of Archean granitic rocks and that the crust has not significantly changed in thickness since that time.

Geochronology of Precambrian Gneisses in the Blue Ridge Province of Northwestern North Carolina and Adjacent Parts of Virginia and Tennessee

Abstract: Rb-Sr whole-rock radiometric age determinations were made on Precambrian basement rocks from the Blue Ridge province of northern North Carolina and adjacent areas of Tennessee and Virginia. Parts of the Cranberry Gneiss in North Carolina and the Grayson Gneiss in Virginia have ages of 1,252 ± 45 m.y. and 1,174 ± 14 m.y., respectively. Thus far, these are the oldest rocks found in the southern Appalachians, and their ages suggest that they are coeval with much of the basement of the stable U.S. interior. The Wilson Creek Gneiss of North Carolina yields model ages averaging 1,135 m.y. The Blowing Rock Gneiss of North Carolina and the Cranberry Gneiss in North Carolina and Tennessee have ages of 1,027 ± 36 m.y. and 1,063 ± 41 m.y., respectively. A third Cranberry Gneiss location, in Tennessee, may be only 871 ± 17 m.y. old. A radiogenic Sr87 growth curve for these gneisses is compatible with a model in which the Blue Ridge crust of the area studied has behaved as a closed Rb-Sr system for much of its Precambrian history. The 1,300- to 1,200-m.y.-old terrane was largely remobilized by a major orogenic episode about 1,050 m.y. ago, which suggests synchrony with the Grenville orogeny. The approximately 1,050-m.y.-old gneisses are products of anatexis. There is no evidence for the addition of new material to the Blue Ridge crust between 1,300 and 700 m.y. ago.

Tectonic significance of mineral ages of Blueschists near Seldovia, Alaska

Abstract: The 40K/40Ar ages of crossite and phengite from blueschists and actinolite and white mica-chlorite concentrates from intercalated greenschists in a metamorphic terrane near Seldovia, Alaska, indicate that metamorphism occurred in Late Triassic or Early Jurassic time. The Seldovia blueschist terrane is associated with cherts, mafic volcanics, and serpentinites, which are interpreted to be parts of a dismembered ophiolite. The terrane may be a segment of a fossil subduction zone.

Franciscan Rocks near Sur Fault Zone, Northern Santa Lucia Range, California

Abstract: The Sur fault zone is a complex series of high-angle and low-angle faults that juxtapose a northeast block of competent crystalline rocks against a southwest block of generally sheared Franciscan rocks. The Franciscan rocks are an assemblage of sandstone, siltstone, greenstone, and chert. This assemblage can be divided into a northwestern terrane characterized by unmetamorphosed sheared and unsheared units containing detrital K-feldspar and a southeastern terrane with a pervasive dip-slip tectonite fabric throughout, metamorphic mineral assemblages indicative of high-pressure, low-temperature metamorphism, and no K-feldspar. A lawsonite isograd in Franciscan sandstone crosses the southeastern terrane. K-Ar dates indicate a minimum age of Late Cretaceous for high-pressure metamorphism of the Franciscan. During mid- to Late Cretaceous time, accumulation of Franciscan rocks was closely followed by subduction and high-pressure, low-temperature metamorphism. Low-pressure, high-temperature metamorphism and plutonism occurred concurrently in the Salinian block. In Late Cenozoic time, the San Andreas fault probably offset the Sur-Nacimiento fault from the Great Valley subduction zone.

Early Paleozoic Volcanism and Metamorphism of the Moretons Harbour–Twillingate Area, Newfoundland

Abstract: In the Moretons Harbour area, at the eastern end of the Lushs Bight terrane of central Newfoundland, the volcanic rocks of the "Lushs Bight Supergroup" are divided into two new groups, viz, the Mor...

Interpretation of a High-Grade Precambrian Terrane in Northern Idaho

Abstract: A terrane of high-grade metamorphic rocks in northern Idaho and northeastern Washington is almost completely surrounded by low-grade rocks of the Precambrian Belt Supergroup. The high-grade terrane includes both Belt and pre-Belt rocks. Four events of folding and metamorphism occurred in the high-grade terrane. The first three events may have been associated with the Late Cretaceous emplacement of quartz monzonite of the Kaniksu batholith; the fourth may have been associated with a slightly later emplacement of granodiorite or with a Tertiary plutonic and volcanic episode. A much older event of plutonism in the high-grade terrane is recorded by zircon, which was dated by the Pb-U method at 1,500 m.y. from pre-Belt meta-igneous augen gneiss. Evidence of regional events intermediate in age between 1,500 and 100 m.y. has been found in the surrounding low-grade rocks but not in the high-grade terrane.

Tectonics and Oil Prospects of the Moluccas, Eastern Indonesia

Abstract: The greater Moluccas are bounded by th e Pacific and Indian oceanic plates and the greater Australian and Sunda continental crusta l plates or shelves. The effect of their opposing movement is a giant counterclockwise swor!' The island a rcs within this pattern are parts of subduction zones between the opposing plates. The Sorong transform left lateral fault resulted from these forces and offset remnants of the West Irian shelf terrane at least 700 kilometers westerly to the Sula Islands. Fossil subduction zones are present in the form of melanges including a persistent zone extending in an " S" pattern from T imor through Ce­ ram. Buru. and Sulawesi. T he age of emplacement of thi s zone in part is post-Lower Miocene pre-Middle Pliocene. Sedimentary rocks of pre-Pli ocene to Permi an age occur notably in Timor and th e Sula Islands. In most localities they are deformed and indurated to the extent of making them economic basement. Plio-P leistocene li near sed imentary basins follow the " S" trend and reach a maximum of 3.000 meters in thi ckness in northeast Ceram and Timor. Rocks include deep water claystones. shel f and lagoona l clays. bar and beach sands. and barrier coral reefs. Oil seeps occur on Ceram. Timor. a nd Buton (asphalt) mostly from Pli o­ Pleistocene rocks. Oil is produced at Bula. Ceram. from Plei stocene bar and shoreline sands in strat igraph ic traps. A recent development at Bula has established prolific production from an extremely permeable Pleistocene reef. The data suggest that oi l accumulation per unit volume of sediments is abnorm all y high in the Plio-Pleistocene. Reasonable extrapola­ tion indicates that major o il fields may be found in prospecting the "S" trend. Most likel y traps wi ll be stratigraphic with accumulations in beach. bar. a nd turbidite sands and reef limestones. Prospects a lso ex ist in other than Plio-Pleistocene in other portions of the Mo­ luccas. Prospecting for th ese subt le. elu s ive traps may be rewardin g if geologists exchange cer­ tain negative prejudices for a positive optimi sti c approach in combination with good ima­ gin at ive geology.

Structure and Stratigraphy of Eastern Alaska Range, Alaska: Regional Arctic Geology of Alaska

Abstract: The eastern Alaska Range, between 141°W (International Boundary) and 145°W long. in south-central Alaska, provides clues to the tectonic development of northwestern North America. The Denali fault system, a major structural feature extending in an arcuate path from the Bering Sea to the Gulf of Alaska, transects the eastern Alaska Range and separates extremely diverse geologic terranes. North of the Denali fault lies a widespread terrane of highly deformed, metamorphosed sedimentary and minor igneous rocks of Precambrian to Devonian age. South of the Denali fault system these rocks are absent, and the oldest rocks exposed are a heterogeneous series of Pennsylvanian(?) or Permian volcanic and volcaniclastic rocks derived from a late Paleozoic volcanic island arc probably built directly on oceanic crust. These rocks are overlain by a succession of Permian marine clastic beds and limestones; Triassic carbonaceous shales, subaerial tholeiitic basalt flows, and mari e limestones; and Jurassic-Cretaceous argillite, graywacke, and conglomerate. The cumulative thickness of the succession locally exceeds 10,000 ft (3,050 m). Sedimentation culminated in middle(?) Cretaceous time with a short-lived and restricted episode of andesitic volcanism. Relatively undeformed continental sedimentary rocks of Cretaceous age, or younger, and late Cenozoic terrestrial volcanic flows overlie the older rocks with marked angular unconformity. Linear bodies of serpentinized ultramafic rocks are present with the Permian rocks to the west in the central Alaska Range and to the east in Canada. In the eastern Alaska Range, ultramafic rocks have not been observed south of the Denali fault, but they do occur locally along the fault zone and in the older terrane just north of the fault. All pre-Late Cretaceous rocks south of the Denali fault system have been cut by high-angle normal faults and by numerous reverse and thrust faults that dip north toward the Denali fault. The Jurassic-Cretaceous marine sedimentary rocks also exhibit complex folding, locally isoclinal, and fold axes plunge at low angles generally toward the northwest. The geologic data suggest that the oceanic terrane south of the Denali fault collapsed against, and was added to, the continental American plate, probably in Early Triassic time. Since then, this terrane has undergone multiple deformation as later oceanic plates impinged against the continental margin. The Denali fault, which represents an ancient subduction zone, was reactivated as a ridge-arc dextral transform fault--probably during the early Pliocene--in response to a change in the direction of spreading in the North Pacific oceanic plate. The Totschunda fault system, which diverges from the Denali structure near 144°W long. and trends southeasterly toward the Fairweather fault in the Gulf of Alaska, is another major right-lateral strike-slip fault that may have developed as re ently as the middle Pleistocene. At present, the Denali fault system apparently is inactive southeast of the Denali-Totschunda junction.

The Perturbation of Alternating Geomagnetic Fields by an Island near a Coastline: Discussion

Abstract: tonic belts in eastern Newfoundland like those described by McCartney ( 1967 ) and reinterpreted by Hughes and Briickner ( 1971 ) ; (b) an undetected plutonic-metamorphic terrane to the northeast of the Avalon Peninsula now beneath the continental shelf, as supported by Papezik (1972) ; and (c) locally on the west, the plutonic-metamorphic rocks of the eastern crystalline belt of Jenness (1963), and Kennedy and McGonigal (1972). I suspect that the Cambrian sediments were derived mainly from Hadrynian rocks on the Avalon Peninsula and beneath the continental shelf. During the late Early Ordovician the undetected plutonicmetamorphic terrane became uplifted and sup plied quartz-rich sand and silt rich in detrital muscovite to a depressed platform upon which 5000 feet of strata accumulated (Rose 1952). The plutonic-metamorphic terrane, or more likely the older part of it in the eastern crystalline belt, may indeed extend eastward beneath the Avalon Peninsula and continental shelf as proposed by Papezik and thus be the source of the 'exotic' detrital minerals in the Hadrynian and lower Paleozoic strata. The age of this crystalline terrane supposedly beneath the continental shelf is unknown beyond being prelatest Precambrian; the 540-m.y. K-Ar date (Wanless et al. 1972) on detrital musco