Showing papers on "Terrane published in 1981"

Crustal Evolution of Southern Africa: 3.8 Billion Years of Earth History

Abstract: 1 Tectonic Framework.- 1.1. Cratons, Mobile Belts, and Structural Provinces.- 1.2. Gravity Field and Crustal Structure.- 1.3. Evolutionary Stages in the Southern African Crust.- 1.4. Stage 1: Archean Crustal Development.- 1.5. Stage 2: Early Proterozoic Supracrustal Development.- 1.6. Stage 3: Proterozoic Orogenic Activity.- 1.7. Stage 4: The Gondwana Era.- 1.8. Stage 5: After Gondwana.- Stage 1: Archean Crustal Evolution.- 2 Granite-Greenstone Terrane: Kaapvaal Province.- 2.1. The Early Gneiss Terranes.- 2.2. Swaziland Supergroup: A Uniquely Preserved Early Archean Supracrustal Pile.- 2.3. Other Kaapvaal Greenstone Belts.- 2.4. Archean Cratonization: Granitoid Emplacement in the Eastern Kaapvaal Province.- 2.5. Pongola Supergroup: The Oldest Cratonic Cover.- 2.6. Post-Pongola Magmatism.- 2.7. Broad Implications of Archean Crustal Development in the Kaapvaal and Zimbabwe Provinces.- 3 Granulite-Gneiss Terrane: Limpopo Province.- 3.1. Extent of Limpopo Province.- 3.2. Northern Marginal Zone.- 3.3. Central Zone-Limpopo Valley.- 3.4. Central Zone-Botswana.- 3.5. The Southern Marginal Zone.- Stage 2: Early Proterozoic Supracrustal Development.- 4 The Golden Proterozoic.- 4.1. Dominion Group: The Witwatersrand Protobasin.- 4.2. West Rand Group: The Witwatersrand Sea.- 4.3. Central Rand Group: Alluvial-Fan Environments.- 4.4. Ventersdorp Supergroup: Crustal Fracturing.- 5 The Transvaal Epeiric Sea.- 5.1. Protobasinal Phase.- 5.2. Inundation of the Kaapvaal Province.- 5.3. Sedimentation in a Clear-water Epeiric Sea.- 5.4. Renewed Terrigenous Influx and Progradation.- 5.5. Depositional History of the Epeiric Sea.- 6 The Bushveld Complex: A Unique Layered Intrusion The Vredefort Dome: Astrobleme or Gravity-Driven Diapir?.- 6.1. Framework of the Complex.- 6.2. Magmatic and Volcanic Stratigraphy.- 6.3. Age of the Bushveld Event.- 6.4. Geochemistry.- 6.5. Petrogenesis: Origin of Parent Magmas and Igneous Layering.- 6.6. Contact Metamorphism.- 6.7. Sulfide Mineralization.- 6.8. Vredefort Dome.- 6.9. Structural Setting and Mechanics of Intrusion.- 7 The Earliest Red Beds.- 7.1 The Intracratonic Waterberg Group.- 7.2. Soutpansberg Trough.- 7.3. The Miogeoclinal Umkondo Group.- 7.4. The Craton-Edge Matsap Group.- 7.5. Synthesis.- Stage 3: Proterozoic Orogenic Activity.- 8 Namaqua-Natal Granulite-Gneiss Terranes.- 8.1. The Natal Province.- 8.2. The Namaqua Province.- 8.3. Eastern Marginal Zone of the Namaqua Province.- 8.4. Western Zone of the Namaqua Province.- 8.5. Central Zone of the Namaqua Province.- 9 The Pan African Geosynclines.- 9.1. The Gariep Geosyncline.- 9.2. The Intracratonic Nama Platform Succession.- 9.3. The Malmesbury Geosyncline in the Western Saldanian Province.- 9.4. Pre-Cape Basins in the Eastern Saldanian Province.- 9.5. The Damara Province: Keystone of the Pan African Framework.- Stage 4: The Gondwana Era.- 10 The Cape Trough: An Aborted Rift.- 10.1. Table Mountain Group: The Quartz Arenite Problem.- 10.2. The Natal Embayment.- 10.3. Paleogeographic Synthesis of the Table Mountain and Natal Groups.- 10.4. Bokkeveld Group: Allocyclic Control Over Delta Progradation and Reworking.- 10.5. Witteberg Group: The Cape-Karoo Transition.- 11 The Intracratonic Karoo Basin.- 11.1. Glaciogene Dwyka Sedimentation.- 11.2. Postglacial Epicontinental Ecca Basin.- 11.3. The Beaufort Group: Fluvial Aggradation in a Foreland Basin.- 11.4. Upper Karoo Sedimentation.- 11.5. Cape Orogeny.- 11.6. Karoo Volcanism.- Stage 5: After Gondwana.- 12 Fragmentation and Mesozoic Paleogeography.- 12.1. The Proto-Atlantic Margin.- 12.2. Evolution of the Southern Continental Margin.- 12.3. The Transkei Swell and the Zululand Basin.- 12.4. Synthesis.- 13 Kimberlites and Associated Alkaline Magmatism.- 13.1. Carbonatites.- 13.2. Alkaline Complexes.- 13.3. Kimberlites.- 13.4. Petrogenesis of Alkaline Rocks.- 14 Changing Climates and Sea Levels: The Cenozoic Record.- 14.1. Tertiary Coastal Environments.- 14.2. Tertiary Shelf Sedimentation.- 14.3. Quaternary Transgressions and Regressions.- 14.4. The Interior Basin.- 14.5. Cenozoic Biogeography and Climatic Evolution.- References.

Continental Accretion: From Oceanic Plateaus to Allochthonous Terranes

Abstract: Some of the regions of the anomalously high sea-floor topography in today's oceans may be modern allochthonous terranes moving with their oceanic plates. Fated to collide with and be accreted to adjacent continents, they may create complex volcanism, cut off and trap oceanic crust, and cause orogenic deformaton. The accretion of plateaus during subduction of oceanic plates may be responsible for mountain building comparable to that produced by the collision of continents.

Chapter 11 Proterozoic Chronology and Evolution of the Midcontinent Region, North America

Abstract: Early Proterozoic rocks of the midcontinent region consist of mafic to felsic volcanic rocks, thick sequences of metasedimentary rocks, and calc-alkaline plutons, all of varying age In the Penokean Fold Belt the principal orogenic activity occurred about 1820—1860Ma ago; about 80Ma later a terrane of rhyolite and epizonal granite formed to the south of the Penokean rocks The early Proterozoic basement of the western United States represents two distinct periods of orogenic development, 1690–1780 and 1610—1680 Ma ago, but units of Penokean age are apparently absent In the central midcontinent area, the basement is composed of gneissoid granitic rocks and small volumes of metasedimentary and metavolcanic rocks In the northern part of this region some of the rocks may be coeval, and possibly correlative, with either the 1690–1780 Ma old terrane of the Rocky Mountains or the 1820–1900 Ma old Penokean terrane (or both), but precise age confirmation is lacking At least some of the granitic rocks in northern Kansas and Missouri formed about 1625 Ma ago and are thus coeval with plutonic and volcanic rocks in Arizona and New Mexico Metamorphic effects of that age occur in the Penokean terrane, suggesting that the 1610–1680 Ma old belt extends as far east as the southern Great Lakes region A striking feature of the southern and eastern midcontinent region is a great terrane of rhyolite and epizonal granite that stretches from northern Ohio across Indiana, Illinois, Missouri, southern Kansas, and Oklahoma at least into the Texas panhandle These rocks were formed in middle Proterozoic time, mostly in the interval 1380–1480 Ma ago The most important characteristics of the Proterozoic rocks of the midcontinent and their distribution with regard to the possible operation of plate-tectonic mechanisms in the Proterozoic in this region are: (1) the steady progression of younger and younger rocks southward from the Archaean craton of the Canadian Shield; and (2) the absence of typical island-arc rock assemblages in the northern midcontinent and the great abundance of granite and rhyolite in the southern midcontinent We believe that these terranes were probably formed by convergent processes on the margin of the continent despite their lack of similarity with modern circum-Pacific rock assemblages

Rb-Sr and U-Pb geochronology and distribution of rock types in the Precambrian basement of Missouri and Kansas

Abstract: Most of what we know about basement rocks in Kansas and Missouri is derived from cores and cuttings from deep drilling. These rocks may be divided into a northern terrane, underlain by rocks consisting of abundant granite commonly showing cataclastic textures and by metavolcanic to metasedimentary rocks; and a southern terrane, underlain almost exclusively by rhyolitic flows and ash-flow tuffs and epizonal granite plutons. The northern terrane is interrupted in central Kansas by mafic igneous rocks and flanking arkosic sedimentary rocks of the Central North American Rift System. Data from 63 Rb-Sr whole-rock analyses yield ages ranging from 1,153 to 1,748 m.y., but these are considered to represent only the minimum ages of the rocks. Ages derived from U-Pb analyses of suites of cogenetic zircons from 22 rock samples indicate that some rocks of the northern terrane were formed 1,610 to 1,650 m.y. ago. These are apparently intruded by younger granite plutons formed 1,450–1,470 m.y. and 1,340–1,380 m.y. ago. Rocks of the southern terrane were formed 1,460–1,480 m.y. ago in the St. Francois Mountains terrane of southeastern Missouri and its buried equivalents, but these are about 1,380 m.y. old in southwestern Missouri and southern Kansas. Rocks of the Central North American Rift System in Kansas are assumed to be about 1,100 m.y. old by geophysical and drill-hole extension to their outcrop in the Lake Superior region, where they have been dated. Both the northern and southern terranes are notable for the great abundance of granitic rocks and the scarcity of intermediate to mafic igneous rocks. Quartzite is the most abundant metamorphic rock. Although the decrease in ages from north to south in the mid-continent region suggests their sequential accretion at the edges of a pre-existing continent, the rock assemblages are not consistent with convergent plate-margin suites of the Andean type. The great volumes of rhyolite and epizonal granite of the southern terrane may represent melting of thickened, somewhat older crust following accretion at the continental margin. Major-element chemical data are presented for basement rocks from Kansas, Oklahoma and Texas, and these are compared with the compositions of similar rocks from the St. Francois Mountains. Names and locations are given for wells from which samples were obtained.

Ocean floor accretion and volcanoplutonic arc evolution of the Mesozoic Sierra Nevada

Abstract: Recent studies on the history of the Pacific Ocean floor coupled with structural, paleomagnetic, and paleobiogeographic studies of the North American Cordillera indicate that the Pacific floor moved northward and beneath the California continental margin throughout much of Mesozoic time. Northward movement began by Early Jurassic time and possibly as early as Late Triassic time. The northward movement brought equatorial ocean floor into the western Sierran region as an allochthonous ophiolitic basement terrane. Accretion of the oceanic basement was complex, involving both transcurrent faulting and subduction processes. Native basement consisting of Paleozoic borderland sequences and the North American continental shelf sat inboard of the exotic oceanic basement. The contact between the native and exotic terranes was an active break of long duration, herein named the Sierran Foothills suture. The signature of plate convergence in the Sierran region is a composite volcano-plutonic arc. Voluminous magmatism proceeded semicontinuously from Early Jurassic through Late Cretaceous time. Arc rocks of all ages show compositional variations that reflect the nature of the preexisting basement rocks. Gabbroic-basaltic associations occur primarily within the limits of the oceanic basement, whereas voluminous granitic-rhyolitic assemblages are associated with continental basement. The depositional environments of the volcanic arc rocks varied greatly due to tectonic instability within the arc terrane. These environments consisted of an inherited continental shelf, uplifted and eroded continental and oceanic basement, sedimentary basins receiving epiclastic as well as volcaniclastic turbidites, and newly formed intraarc basins floored by juvenile crust. The Mesozoic plate juncture history along the California margin was complex, being dominated by oblique subduction with probable interludes of transform faulting. The Sierra Nevada coincided with the axis of volcano plutonic arc activity from Early Jurassic through Cretaceous time. No consuming plate juncture of Jurassic or younger age can be identified within the Sierran region. Late Triassic to possibly earliest Jurassic consumption occurred in the western foothills during the accretion of the ophiolitic basement terrane. Jurassic consumption occurred west of the Sierran region, but any subduction complex that may have existed was removed by Jurassic and Cretaceous rifting and transcurrent faulting. Cretaceous consumption in the Coast Ranges is marked by the Franciscan Complex. The Sierra Nevada evolved as one composite arc terrane containing numerous internal structural breaks. Deformation of the arc consisted of large-scale dextral wrench faulting with a family of wrench-related structures, oblique rifting, and the development of fold-thrust welts. The result of these processes is a complex igneous and metamorphic terrane consisting of interleaved volcanic arc and older basement fragments that are cut by highly to only slightly deformed plutons of a wide range of ages. Similar structural complexities are evolving today in modern arc terranes with analogous plate-tectonic settings. The processes that operate along a convergent plate juncture that also has a significant strike-slip component are more complex than junctures with normal or near-normal convergence. The unique tectonic style resulting from prolonged oblique convergence is termed transpression after Harland (1971).

Geology of the crystalline basement complex, Eastern Transverse Ranges, Southern California: Constraints on regional tectonic interpretation

Abstract: About 3000 km2 within the crystalline basement complex of the Eastern Transverse Ranges in the Chuckwalla, Orocopia, Eagle, Cottonwood, Hexie, Little San Bernardino, and Pinto Mountains of Riverside County, California were mapped at scales of 1:36,000 and 1:62,500 and compiled at 1:125,000 (Plate I). Pre-Jurassic(?) (i.e., older than the Mesozoic batholiths) rocks of the crystalline complex comprise two lithologically distinct terranes. These terranes are called the Joshua Tree and San Gabriel terranes for regions of southern California in which their lithologies were initially characterized. The two terranes are superposed along a previously unrecognized low-angle fault system of regional extent, the Red Cloud thrust. During the course of this study, the structurally lower Joshua Tree terrane has been defined as a stratigraphically coherent group of crystalline rocks that consists of Precambrian granite capped by a paleo-weathered zone and overlain nonconformably by orthoquartzite that interfingers westward with pelitic and feldspathic granofelses. The quartzite contains near-basal quartz/quartzite clast conglomerates, and has well-preserved cross-bedding that appears upright wherever it has been observed. Pelitic and feldspathic granofelses crop out to the west of the quartzite exposures in four lithologically different belts that trend northnorthwest throughout the area mapped. These lithologic belts are interpreted to have been derived from stratigraphically interfingering sedimentary protoliths deposited in a basin offshore from a quartzose beach-sand protolith. In proximity to the early Red Cloud thrust, this whole stratigraphic package was pervasively deformed to granite gneiss, stretched pebble conglomerate, lineated quartzite, and schist. A northeast-trending pattern of metamorphic isograds was orthogonally superimposed on the northnorthwest-trending protoliths of the Pinto gneiss. A central andalusite zone, located in the southern Little San Bernardino and Hexie, and northern Eagle Mountains, is flanked to the northwest and southeast by sillimanite zones. Coincident with this symmetrical distribution of aluminosilicates is an asymmetrical distribution of other pelitic mineral zones, with prograde cordierite-aluminosilicate-biotite- and K-feldspar-aluminosilicate-bearing assemblages to the northwest in the northern Little San Bernardino and Pinto Mountains, staurolite-bearing assemblages in a narrow zone in the southern Little San Bernardino-Hexie and northern Eagle Mountains, and retrograde chlorite-muscovite-bearing assemblages in the southernmost Little San Bernardino, Cottonwood, southern Eagle, Orocopia, and Chuckwalla Mountains. One occurrence of chloritoid-sillimanite in the central Eagle Mountains is apparently also retrograde. The crossing isograds are interpreted to result from a temporal increase in PH2O relative to PT from south to north through the field area. Comparison of the pelitic assemblages with experimental studies suggests peak conditions of PT ≈ 3.5 to 4 kb, T ≈ 525 to 625°C. The early prograde metamorphism pre-dated the thrusting event; the retrograde stage may have overlapped in time with the emplacement of the San Gabriel terrane allochthon. Cordierite-orthoamphibole-bearing assemblages are present in one stratigraphic zone of the Pinto gneiss. In this study, the Precambrian lithologies of the San Gabriel terrane are viewed as a three-part deep crustal section, with uppermost amphibolite grade pelitic (Hexie) gneiss intruded by granodioritic (Soledad) augen gneiss at the highest level, retrograded granulite (Augustine) gneiss at an intermediate level, and syenite-mangerite-jotunite at the lowest level exposed in the Eastern Transverse Ranges. The Hexie gneiss, characterized by sillimanite-garnet-biotite-bearing assemblages, is thrust over andalusite-bearing granofels of the Pinto gneiss. The Red Cloud thrust system is inferred to have developed in three or four sequential structural events: 1) early thrusting that probably moved parallel to the ENE mineral lineations recorded in both plates; 2) regional folding of the initial thrust surface around NNE-trending axes; 3) later thrusting that broke with some component of westward movement across a fold in the older thrust surface to produce a stacking of crystalline thrust plates of the two terranes; 4) continued or renewed folding of both thrust faults with eventual overturning toward the SW. It is consistent with all observations to date to link these structural events into a single regional tectonic episode that resulted in westward-vergent allochthonous emplacement of the San Gabriel terrane over Joshua Tree terrane. The thrust timing can only be loosely bracketed in time between 1195 m.y. and 165 m.y. ago. The pre-batholithic terranes and the westward-vergent Red Cloud thrust are considered to be exotic with respect to the pre-batholithic rocks and structures exposed to the north and east of the field area. The bounding discontinuity has been obliterated by intrusion of both suites of Mesozoic batholithic rocks. The Mesozoic plutonic rocks comprise two batholithic suites, both of which intrude the Joshua Tree and San Gabriel terranes and the Red Cloud thrust system. NW-SE trending belts of plutonic lithologies have been mapped within each suite: the oldest lithology of the younger suite intrudes the youngest lithology of the older suite. The older suite, Jurassic(?), lying to the NE, appears to have an alkalic character; the younger suite, Cretaceous(?), appears calc-alkaline. The older suite consists of biotite- and K-feldspar-bearing gabbro-diorites intruded by low-quartz monzogranites. The younger suite includes hornblende-biotite-sphene granodiorite intruded by porphyritic monzogranites, intruded in turn by nonporphyritic monzogranite. The Eastern Transverse Ranges south of the Pinto Mountain fault are defined by several Cenozoic E-W left-lateral strike-slip faults that have a cumulative westward displacement from S to N of about 50 km. The left-lateral faults are interpreted to form part of a conjugate fault set with complementary right-lateral faults in the Mojave and Colorado Deserts. Along the western boundary of the Eastern Transverse Ranges in the Little San Bernardino Mountains, the crystalline rocks have been pervasively cataclasized by an event that post-dates intrusion of the Cretaceous(?) plutonic rocks. The cataclasis is attributed to the Vincent-Orocopia-Chocolate Mountain thrust that is thought to superpose the diverse pre-batholithic and batholithic rocks of the Eastern Transverse Ranges above Pelona-type schist. The cataclastic foliation is folded along the length of the Little San Bernardino Mountains in an antiform that is inferred to be cored with Pelona-type schist. This fold may have formed a single antiformal feature comprising all the crystalline-rock antiforms now recognized along the San Andreas fault that are cored by Pelona-type schist. Displacements of the piercing points formed by the antiformal axis apparently indicate 220 km of right-lateral offset on the present San Andreas strand and about 80 km of right-lateral offset along a fragmented older San Andreas strand that consisted of the San Francisquito, Fenner, and Clemens Well faults and a buried extension of this fault beneath the alluvial fill of the valley between the Chocolate and Chuckwalla Mountains.

Research in the Geysers-Clear Lake Geothermal Area, Northern California

Abstract: From abstract: The Geysers-Clear Lake geothermal area lies within the central belt of the Franciscan assemblage in northern California. The structure of this terrane is characterized by northeast-dipping imbricate thrust slices that have been warped and cut by steeply dipping strike-slip and normal faults. Introduction of magma into the crust beneath the Geysers-Clear Lake area can be related to eastsoutheast extension accompanying northward propagation of the San Andreas transform system between the Clear Lake region and Cape Mendocino within the last 3 million years. The initiation of strike-slip faulting during this time terminated subduction of elements of the Farallon plate beneath North America as strike-slip motion was taken up along the Pacific-North American plate boundary. The mechanism for magma generation appears to require a heat source in the mantle that mixed mantle-derived melts with various crustal rocks. These crustal rocks may have included the Franciscan central and coastal belts, ophiolite, Great Valley sequence, and possibly middle and late Tertiary rocks subducted before initiation of strike-slip faulting.

Tectonic history of the Queen Charlotte Islands and adjacent areas—a model

Abstract: The region including Queen Charlotte Islands, Hecate Strait, and Queen Charlotte Sound is underlain by two allochthonous terranes, Wrangellia and the Alexander terrane. The suture between them occurs in central Graham Island and central Hecate Strait and is coincident with the traces of the Sandspit and Rennell Sound fault zones, each of which developed in response to crustal rifting in Queen Charlotte Sound during mid-Tertiary time.The stratigraphic succession comprises four tectonic assemblages. (1) The allochthonous assemblages comprise Paleozoic rocks of the Alexander terrane and Upper Triassic and Jurassic rocks of Wrangellia, which on the basis of paleomagnetic and biogeographical data are clearly exotic. The distribution of these terranes beneath Queen Charlotte Sound and Hecate Strait is supported by geophysical information and subsurface data obtained from offshore wells. (2) The suture assemblage is represented by extremely coarse conglomerates, massive graywackes, and turbidites of Early Cretac...

Accretion of continental crust: thermal and geochemical consequences

Abstract: The irreversible chemical differentiation of the Earth’s mantle to produce sialic crust over the past 3900 Ma has most probably occurred during widely separated, but short-lived, accretion episodes. These episodes involved the massive addition of juvenile sialic magma to the Earth’s surface, thickening pre-existing crust. Simple numerical simulations, based on tectonic, petrological and geochemical observations on Archaean high-grade orthogneiss terranes, have been used to explore the metamorphic and geochemical consequences of massive thickening of sialic crust during short-lived accretion episodes. The location of the main sites of magmatic addition within the crust exert a profound influence on the thermal regimes. Geochemical differentiation of the continental crust by partial-melt and vapour-phase-controlled processes, and the development of granulite facies mineral assemblages can be integrated with the simple numerical models. Finally, the survival of thick Archaean continental crust imples the contemporaneous stabilization of thick lithospheric substructures to the newly formed continental masses.

Late Cenozoic deformation in the Matanuska Valley, Alaska: Three-dimensional strain in a forearc region

Abstract: The Matanuska Valley is located in the forearc terrane of the Alaska-Aleutian volcanic arc. A period of extensive faulting occurred in the western Matanuska Valley during the Neogene. Faulting was a response to intraplate strain caused by subduction of the Pacific plate beneath the convergent margin. The Neogene fault system consisted of three sets of faults: two sets of north-trending strike-slip faults and a set of east-northeast–trending reverse faults. Three fault sets were required to accommodate the general three-dimensional deformation. Crustal shortening in a north-northwest direction was accompanied by lateral and vertical extension. The direction of maximum horizontal shortening strain has apparently been constant since the Neogene, although there is no evidence for extensive faulting of Quaternary deposits in the area we have studied. The western Matanuska Valley is located only 50 km above the Benioff zone, and the predicted maximum principal shortening direction is north-northwest, on the basis of the slip-vector azimuth between the North American and Pacific plates and the nature of surficial ground displacements during the 1964 (M = 8.4) subduction zone earthquake. Focal mechanisms for earthquakes in the vicinity of the Benioff zone and in the lower part of the North American plate indicate west-northwest extension and north-northeast shortening. This means that the strain field at depths below 20 km may differ significantly from the near-surface strain field.

Late Palaeozoic palaeomagnetic data for Wrangellia: resolution of the polarity ambiguity

Abstract: Wrangellia and the Alaska Peninsula terrane are two of the many allocthonous terranes making up southern Alaska today. Palaeomagnetic data from these areas clearly indicate low palaeolatitudes in the early Mesozoic. Because of local and regional rotations, the mean declination of the palaeomagnetic vectors is uncertain. This, in addition to incomplete temporal coverage, has led to an ambiguity in the polarity of the vector, and hence an ambiguity in whether the palaeolatitudes determined are in the Northern or Southern Hemisphere. The data considered here have resolved the ambiguity for Wrangellia, and by implication for the Alaska Pennisula terrane, and also allow the construction of a highly speculative apparent polar wander curve.

A lithologic- tectonic framework for the metallogenic provinces of California.

Abstract: The lithologic-tectonic framework of California developed principally during Mesozoic time when various terranes of oceanic crust and island-arc crust were accreted to older sialic crust, resulting in westward growth of the continent. Emplacement of great batholithic masses of granitoid rocks cutting all these crustal types also took place during the Mesozoic period. The discrete tectonostratigraphic terranes that resulted from these events and subsequent Tertiary and Quaternary volcanic events are characterized by specific types of metallic mineral deposits or, in some terranes, by the virtual absence of deposits.Lead-silver-zinc replacement-type deposits are common in the Paleozoic carbonate terrane in the eastern part of the state and occur sporadically elsewhere in the miogeoclinal and cratonal terranes but are absent from the oceanic and island-arc terranes. The vast majority of contact metasomatic tungsten deposits, including all the large ones, are in pendants of miogeoclinal rocks in the Sierra Nevada batholith, but the important Atolia deposits reside in granitoid rocks that invade oceanic terrane. Molybdenum distribution closely follows that of tungsten. All the large contact metasomatic iron deposits in California are in craton and miogeoclinal terranes, but sparse small deposits of this type also occur in island-arc terranes of the northern Sierra Nevada and eastern Klamath Mountains.Lode gold deposits, although widely scattered, show a marked preference for oceanic and island-arc terranes that have been invaded by granitoid plutons. All the major deposits, including late Tertiary bonanza deposits such as Bodie, are in such terranes. It appears that magmatic processes were responsible for mobilizing and transporting the gold, but the metal was perhaps derived from the eugeosynclinal rocks, notably the mafic volcanics.Most mercury deposits are found in the Coast Ranges, where they commonly occur in silica-carbonate rock, an alteration product of serpentinite. The deposits appear to be spatially related to the Coast Range thrust, and the source of the mercury may have been sedimentary rocks of the underlying Franciscan assemblage.Epigenetic mineralization occurred at several different times during the Mesozoic, and again during Miocene and Pliocene time. The timing of mineralization events and the distribution of various deposit types indicate that no broad-scale zoning of epigenetic deposits exists around the Sierra Nevada batholith.Syngenetic deposits are represented mainly by massive sulfides, chert-associated manganese, and chromite. The massive sulfide deposits, with one exception, are restricted to island-arc terranes, and nearly all of these deposits are in silicic volcanic rocks. They are interpreted to be syngenetic with the enclosing rocks, although some redistribution of metals may have occurred after the original deposition. The deposits occur in volcanic sequences of at least five different ages ranging from Early Devonian to Late Jurassic or Early Cretaceous and, along with their enclosing rocks, were probably formed at some distance from their present sites.Chert-associated manganese deposits occur mainly in exotic blocks of oceanic crust in melange and probably formed in fairly deep ocean environments. Chromite is confined to ultramafic rock, much of which occupies suture zones separating various accreted terranes.

Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite

Abstract: The Precambrian Coal Creek serpentinite, Llano Uplift, Texas, occurs within the upper part of the Packsaddle Schist, which appears to represent a thick sequence of shelf-edge volcaniclastic arc-flank metasediments. The serpentinite has a foliation defined by lizardite pseudomorphs after original foliated harzburgite tectonite, which deviates as much as 15° from the regional country-rock foliation. A few samples contain the relict assemblage olivine + orthopyroxene + anthophyllite reflecting re-equilibration to regional metamorphic conditions near 710 °C at ∼3.5 kb. Oxygen and deuterium isotopic data indicate the lizardite could have formed in equilibrium with magmatic water at about 300 to 400 °C during regional uplift. A likely petrogenetic model involves the olistostromal emplacement of a fragment of ophiolitic material, the Coal Creek ultramafic body, into the volcaniclastic arc-flank sediments. The island-arc model suggested for the Llano Uplift and the existence of ophiolitic material imply that brittle plate collisions were locally important 1,200 m.y. ago and that the orogenic event that affected the Llano terrane (that is, the Grenvillian orogeny) involved island-arc-continent interactions. The Llano Uplift contains the only exposed ophiolitic material along the entire “Grenvillian” orogenic belt.

Chapter 6 Geotectonic Evolution of the Archaean Successions in the Barberton Mountain Land, South Africa

Abstract: The debate on the geotectonic evolution of Archaean granite-greenstone terranes is briefly reviewed to highlight some of the principal avenues of contention that surround this formative period in the earth's history. It is concluded that the majority of current opinion concedes that the Archaean record reflects broad-scale similarities with modern arc-trench systems but that the number of reservations expressed preclude direct comparisons with contemporary plate-tectonic mechanisms. The controversy regarding the nature of the early crust is discussed and evidence from the Barberton Mountain Land relating to the problem is considered. It is concluded that the oldest recognizable rocks in this area are ensimatic in character (peridotitic and basaltic komatiites dated at 3.54 Ga) and are compositionally compatible with the probable source material from which large volumes of intrusive tonalites and trondhjemites were generated following partial melting at depth. These early sialic additions, some of which yield ages approximating that of the Barberton greenstone belt, both arose from and interacted with the supracrustal assemblages at deep and shallow crustal levels, respectively, to form complex migmatites and the protocontinental crust of the developing cratons. A model of crustal evolution is outlined which draws heavily on the available evidence accumulated in the Barberton Mountain Land and adjoining territory of Swaziland. It is maintained that there is no evidence, either in the Barberton greenstone belt or in the surrounding granite-greenstone terrane, to suggest that plate tectonics, in the modern sense of the term, may have been responsible for the development of the region. Rather, it is claimed, the ancient crust evolved as a response to vertical tectonics involving the sinking of simatic lithospheric slabs and the diapiric upwelling of a succession of granitoids that commenced with early Na-rich phases but which later changed to K-rich magma types during the final stages of Archaean crustal consolidation 3.2–3.0 Ga ago.

Structure and stratigraphy of the Luning allochthon and the kinematics of allochthon emplacement, Pilot Mountains, west-central Nevada

Abstract: The northern Pilot Mountains of west-central Nevada consist of a complexly deformed terrane of imbricate thrust sheets composed of rocks of Mesozoic and possible late Paleozoic ages. Thrust nappes within the allochthonous terrane, named the Luning allochthon, are principally constituted by the Upper Triassic Luning Formation. Locally, the nappes contain numerous imbricate thrust slices, some of which are composed of undifferentiated Sunrise-Gabbs Formation (Triassic and Jurassic) and unnamed volcanic-carbonate assemblages of probable Permian or Triassic age. Three episodes of folding and thrusting are recognized on the basis of folded thrusts and thrusted folds, and deformation and emplacement of the allochthon is constrained as late Mesozoic. Folds apparently formed coevally with thrust faults and fold geometry is used to determine approximate directions of thrust transport. Thrust displacements are responsible for 35 to 40 km of an estimated 70 km of intra-allochthon contraction which is inferred from lithofacies analysis of rocks juxtaposed by thrust nappes and from the structural overlap of imbricate thrusts. The history of thrust displacement is complex and involves three directions of motion on a regionally extensive detachment surface, the Luning thrust. First motion, from northwest to southeast, resulted in the major component of stratal contraction and is the probable result of northwest-southeast regional compression. The final two episodes of motion are northeast to southwest followed by east to west; they resulted in small displacements and are possibly the product of gravity sliding of the thrust sheets into a depression formed beneath part of the allochthon within the autochthon. The site of downwarp in the autochthon may have formed by either load-induced subsidence or regional compression.

Geology of the Victorville Region, California

Abstract: Late Precambrian and Paleozoic strata of the Cordilleran continental margin (miogeocline) form a westward-thickening wedge of sedimentary rocks in the southern Great Basin region (Stewart, 1970). Facies boundaries and isopachs of these rocks can be traced across this region into the eastern Mojave Desert, southeastern California, where these older trends are crosscut by the younger, Mesozoic-age magmatic arc terrane (Fig. 1). Miogeoclinal rocks involved in the Cordilleran fold-and-thrust belt in the eastern Mojave Desert become more highly deformed and metamorphosed southwestward as the magnatic arc is approached. The geologic complexity of the central and western Mojave Desert, however, has posed a major stumbling block in attempts to reconstruct the paleogeography and tectonic history of the southern part of the Cordilleran orogenic belt. In the central and western Mojave Desert, bed-rock exposures consist chiefly to Tertiary volcanic rocks and Mesozoic granitic rocks. Because of the lack of exposure of older rocks in this terrrane, the southwestward continuation of miogeoclinal facies and isopach trends and the timing and nature of structural events in this region are very poorly known in comparison to what is known about the eastern Mojave Desert region.

Submarine fan facies in the Torlesse terrane, New Zealand

Abstract: The Torlesse terrane is an assemblage of late Palaeozoic and Mesozoic greywackes allochthonous to other coeval rocks in New Zealand. The sedimentology of these greywackes indicates submarine fan depositional processes; two exceptions are Middle Triassic (Kaihikuan) non-marine to shelf deposits and one occurrence of Jurassic non-marine beds. The source terrane is inferred to be an ensialic arc, and deposition occurred in a variety of basinal settings in a region of subduction. The internal tectonic complexity of the greywacke strata and the rather simple map pattern of the biostratigraphic zones reflect tectonic kneading and a systematic outbuilding of an accretionary prism.

Allochthonous terranes in Alaska: Implications for the structure and evolution of the Bering Sea shelf

Abstract: Abundant evidence has been gathered in support of the concept that much of the Pacific margin of North America consists of allochthonous terranes that were accreted during Mesozoic and early Tertiary time. In particular, southern Alaska is almost completely composed of separate terranes whose stratigraphy and paleomagnetism indicate distant origins. The Bering Sea continental shelf is among the largest shelves in the world, almost half as large as Alaska itself. Some of the allochthonous terranes in Alaska probably continue beneath the continental shelf, but their distribution beyond the shoreline has not been determined. We believe that much of the continental shelf is probably composed of allochthonous terranes, in much the same way as southern Alaska. Geophysical data from the Bering continental shelf have been used to test this hypothesis. Although inconclusive without extensive drilling, multichannel seismic profiles and magnetic data indicate the composite character of the shelf basement. The magnetic anomalies can be separated into specific domains of limited extent. These magnetic variations suggest a corresponding variation of rock terranes beneath the shelf. In addition, multichannel seismic profiles show possible thrust faults within the shelf basement that could be sutures between separate terranes.

Geology and Uranium Deposits Along the Northeastern Margin, McDermitt Caldera Complex, Oregon

Abstract: The adjoining Aurora and Bretz uranium prospects are located along the northeastern ring-fracture system of the Miocene McDermitt caldera. A series of block faults, constituting the ring-fracture system, divide the area into two contrasting terranes. The northern terrane includes a series of mafic to silicic lavas and rhyolite ash-flow tuff (Bretz series). Rocks of the southern terrane (Aurora series) represent filling in of the caldera after collapse. Uranium concentrations occur along several horizons, including: geologic contacts, unconformities, and redox boundaries in the Bretz series; a widespread horizon within the tuffaceous lake sediments; and potentially commercial deposits along flow boundaries and interflow breccia in the Aurora lavas.

Seismic history of Minnesota and its geological significance

Abstract: Twelve earthquakes have been documented in Minnesota in the last 120 yr. The first nine were felt, whereas the last three (all in 1979) were detected instrumentally by a six-element seismic array which has recently been put into operation. Estimated magnitudes range from 0.1 (instrumental only) to 4.8, with four of magnitude 4.3 or greater. The highest intensity values were VI to VII. Depths where obtainable are estimated at 5 to 20 km. The best documented event occurred on 9 July 1975 near Morris, Minnesota, with a magnitude of 4.6, a maximum intensity of VI, and a felt area of 82,000 km 2 covering parts of four states. The event was recorded to epicentral distances of at least 38°. The epicenters show a clear relationship to tectonic features of the state. Four epicenters lie along the newly defined Great Lakes Tectonic Zone, an east-northeast-trending belt extending across several states and into Canada. The zone separates 3,000 to 3,600 m.y. rocks of a gneissic terrane to the south from 2,700 m.y. rocks of a greenstone-granite terrane to the north. Four other events lie on known major northwest-trending faults in the greenstone-granite terrane. Two and possibly three events are associated with the western margin of the Midcontinent Rift System.

Lower Palaeozoic strata on the Pacific Plate of North America

Abstract: The portion of California (USA) and Baja California (Mexico) lying west of the San Andreas–Gulf of California plate boundary is part of the Pacific Plate, and consists of at least seven distinct lithological terranes (Fig. 1). The only pre-Carboniferous rocks previously identified in any of these terranes are the Pre-cambrian gneiss and plutonic rocks of the San Gabriel Mountains in California (Fig. 1, terrane III). Previous reports of Palaeozoic strata west of the San Andreas Fault in southern California are either erroneous1 or unconfirmed (R. V. Sharp, personal communication). Fossils near El Volcan, Baja California, originally reported as possibly Palaeozoic2, are now recognized to be early Triassic3. Unidentified cup corals and crinoid-like specimens were reported in metamorphosed limestone of the Gabilan Range, central California (Fig. 1, terrane I)4. Unidentified brachiopods and crinoidal debris in the Sierra Pinta of Baja California were assigned to the Carboniferous (Fig. 1, terrane VII; ref. 5). Fossils of Pennsylvania age were found in a block of limestone and quartzite in a melange on Isla Cedros.6 We describe here the first Lower Palaeozoic rocks from the Pacific Plate; this carbonate–quartzite sequence contains Lower Ordovician conodonts and is exposed at San Marcos, 50-km south of the International Border between Tecate and Ensenada (Fig. 1).

Guidebook to the geology and ore deposits of the St. Francois Mountains, Missouri

Abstract: The St. Francois Mountains constitute the exposed portion of an extensive Precambrian terrane of anorgenic, granitic ring complexes identified on the basis of drillhole data and aeromagnetic maps. The terrane is characterized by the predominance of alkaline-silicic over mafic rocks, and trachytic intermediate rocks. The distinctive ore deposits are (1) magmatic and hydrothermal iron - apatite deposits (Kiruna type); (2) hypo-exothermal vein deposits of tungsten, silver and lead; and (3) vein and replacement deposits of manganese. This guidebook presents routes featuring the principal rock types and provides an opportunity to study the only extensive outcrops of Precambrian rocks in the continental interior of the US. (ACR)

Geology of an alpine-type peridotite in the Mount Sorenson area, east-central Alaska

Abstract: An alpine-type peridotite of the harzburgite subtype, about 47 km in area, crops out in the vicinity of Mount Sorenson in the Eagle and Charley River quadrangles of east-central Alaska. The peridotite consists mainly of harzburgite, some dunite, and minor clinopyroxenite. All of the peridotite has been serpentinized to some extent, most extensively in the eastern part. Lizardite is the dominant serpentine mineral. Large tectonic inclusions of diabase have been altered, probably by metasomatism during serpentinization. Four small quartz-carbonate veins are also included. The peridotite crops out as a synform mass trending nearly east-west. Aeromagnetic contours closely follow the configuration of the outcrop. The rocks surrounding the peridotite include greenstone, metamorphosed basalt and gabbro, quartzite, mica-schist, greenschist, chert, phyllite, and other metamorphosed rocks of probable Paleozoic age. The ultramafic rocks are believed to have been tectonically emplaced and may be part of a dismembered ophiolite. The nearby Tintina fault, a major fault system with significant, right-lateral displacement, may have been a deep suture along which mantle material was derived and obducted onto a slice of northwestward-moving continental crust. INTRODUCTION A large serpentinized peridotite body, about 47 km2 , crops out in the northeastern Eagle and the southwestern Charley River 1:250,000 quadrangles in the Yukon-Tanana Upland of east-central Alaska (fig. 1). Because it includes part of Mount Sorenson, the most prominent landmark of the area, the rocks of this peridotite body are referred to as the peridotite of Mount Sorenson. The Yukon-Tanana Upland (called the Yukon crystalline terrane by Tempelman-Kluit, 1976, p. 1343 where it extends into Canada) is an area of greenschistto amphibolite-facies metamorphic rocks that have been intruded by Mesozoic and Tertiary granitic rocks (Foster, 1976). The peridotite body crops out as a synform mass trending nearly east-west and having two limbs, a northern limb about 30 km long and a southern limb 14 Cache Creek Exploration Company, Reno, Nevada. Skyline Labs., Inc., Wheat Ridge, Colo. 'Hawley and Hawley Assayers and Chemists (Division Skyline Labs., Inc.), Tucson, Ariz. Fault—Dotted where uncertain

Aeromagnetic survey of Kansas

Abstract: An aeromagnetic survey of Kansas, conducted by the Kansas Geological Survey during the past 5 years, has been completed. The total intensity magnetic field contour map, along with spectrally filtered versions, reveals largescale magnetic patterns, which we correlate with basement composition and paleotectonics. A second vertical derivative map clearly demonstrates that the southern part of the Proterozoic Central North American Rift System (CNARS) does not terminate in central Kansas but continues along a southwestern trend to at least the Oklahoma border. Some of the current seismicity within the state appears to be correlated with reactivated faults within the CNARS. Apparent in northeastern Kansas is the presence of numerous highly magnetic shallow granitic plutons, known from drilling to be approximately 1350 m.y. old, embedded within the older, 1600—1700-m.y.-old mesozonal granitic crust. The total intensity magnetic field map reveals a series of magnetic lows trending approximately east-west across the state. These may correspond to a paleoplate boundary between the older granitic terrane to the north and the younger, 1400-m.y. rhyolitic and epizonal granitic terrane to the south.