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Showing papers on "Terrane published in 1987"


Journal ArticleDOI
TL;DR: In this article, the authors discuss early Mesozoic sinistral transcurrent faulting in the different regions of East Asia and discuss the characteristics of the Tancheng-Lujiang wrench fault system.

468 citations


Journal ArticleDOI
TL;DR: The LITHOPROBE seismic reflection project on Vancouver Island was designed to study the large-scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting Juan de Fuca plate as discussed by the authors.
Abstract: The LITHOPROBE seismic reflection project on Vancouver Island was designed to study the large-scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting Juan de Fuca plate. In this paper, we interpret two LITHOPROBE profiles from southernmost Vancouver Island that were shot across three important terrane-bounding faults—Leech River, San Juan, and Survey Mountain—to determine their subsurface geometry and relationship to deeper structures associated with modem subduction.The structure beneath the island can be divided into an upper crustal region, consisting of several accreted terranes, and a deeper region that represents a landward extension of the modern offshore subduction complex. In the upper region, the Survey Mountain and Leech River faults are imaged as northeast-dipping thrusts that separate Wrangellia, a large Mesozoic–Paleozoic terrane, from two smaller accreted terranes: the Leech River schist, Mesozoic rock...

226 citations


Journal ArticleDOI
Koshi Yamamoto1
TL;DR: In this article, the results of the fraction analyses for terrigenous, basaltic, hydrothermal, and biogenic end-members in the Franciscan rocks show that hydro-thermal emanations caused iron and manganese enrichment in the rock samples and played an important role in the formation of siliceous rocks.

216 citations


Journal ArticleDOI
TL;DR: The Coast Plutonic Complex between Prince Rupert and Terrace, British Columbia, developed in two stages between mid-Cretaceous and mid-Eocene time as discussed by the authors, and the earliest recognized events in the core of the orogen involve pervasive ductile deformation accompanied by emplacement of tonalite sills between about 85 and 50 Ma.
Abstract: The Coast Plutonic Complex between Prince Rupert and Terrace, British Columbia, developed in two stages between mid-Cretaceous and mid-Eocene time. The early stage (>100–70 Ma) involved crustal thickening as the amalgamated Alexander and Wrangellia terranes were emplaced against the Stikine terrane to the east. This thickening resulted from tectonic stacking of crustal slabs, lubricated by intrusion of melt generated at the base of the thickened crust. Emplacement of westward directed thrust slabs along the western margin of the orogen was accompanied by intrusion of a high pressure epidote-bearing tonalite pluton and associated sills. Thrusting resulted in inversion of the metamorphic sequence as supracrustal rocks buried early in the collision were tectonically exhumed. Pluton emplacement also accompanied eastward directed thrusting of high temperature gneisses over low-grade rocks on the eastern margin of the orogen as the tectonic welt was backthrust over Stikinia. The earliest recognized events in the core of the orogen involve pervasive ductile deformation accompanied by emplacement of tonalite sills between about 85 and 50 Ma. Uplift of the orogen core between 60 and 48 Ma coincided with and was facilitated by emplacement of large volumes of tonalitic magma, accompanied by anatexis and the development of ductile shear zones. One of these shear zones, the Work Channel lineament, presently separates the core of the orogen from the schists to the west. The main orogenic cycle ended by 48 Ma when the rocks cooled rapidly through the biotite and hornblende Ar blocking temperatures. The time and space associations of deformation, metamorphism, and plutonic intrusion imply that substantial differential movement occurred across zones which contained fluid during deformation of the Coast Plutonic Complex. In the region of westward directed thrusting and inverted metamorphism, melts generated in the tectonically thickened lower crust and fluids released during metamorphic recrystallization of supracrustal rocks, which were rapidly buried during tectonic thickening, acted to reduce the strength of the crust. In the core zone of the complex, injections of tonalite magma into the lower crust induced pervasive anatexis. The weakened core zone yielded along both low angle and steep shear zones that were further lubricated by melt as the relatively buoyant hot crust rose rapidly in response to continued compression and underthrusting.

201 citations


Journal ArticleDOI
TL;DR: The COCORP 40°N Transect of the Cordillera of the western United States has been studied in this paper, which provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion.
Abstract: The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust.

197 citations


Journal ArticleDOI
01 Dec 1987
TL;DR: In this article, it was shown that the deformation was not end-Silurian but of early Devonian age, climaxing in the Emsian and approximately synchronous with the Acadian orogeny of Canada.
Abstract: SUMMARY The late Caledonian structure of the Lower Palaeozoic slate belts which lie to the south of the Iapetus suture in Britain is not ‘Caledonoid’ (NE-SW) but characterised by arcuate trends. The significance of the major cleavage arc of northern England is the subject of this paper. Its exposed part, in the Lake District and adjacent inliers, is described and its regional extent inferred from the control exerted by Caledonian basement trends on early Carboniferous sedimentation patterns. The arc is shown to be a major feature of the orogen, marking a change from a NE-SW ‘Appalachian’ trend to the ESE-WNW ‘Tornquist’ trend of northern Germany and Poland. Evidence for the age of deformation in the British slate belts is reviewed. It is shown that the deformation was not ‘end-Silurian’ as previously supposed, but of early Devonian age, climaxing in the Emsian and approximately synchronous with the Acadian orogeny of Canada. The systematic variation in cleavage/fold transection angles around the arc is described and interpreted in terms of transpressive strains associated with the northward movement of a basement block, the Midlands Massif, which acted as a rigid indenter during accretion of the southern British terrane (Eastern Avalonia) onto the Laurentian margin. These new data on the timing and geometry of the Acadian accretion event in Britain go some way to resolving the current controversy concerning late Ordovician vs. Devonian closure of Iapetus.

197 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that many of the major strike faults in the British and Irish Caledonides were active as sinistral strike-slip zones in the end-Silurian to pre-mid-Devonian period.
Abstract: Evidence is presented that many of the major strike faults in the British and Irish Caledonides were active as sinistral strike-slip zones in the end-Silurian to pre-mid-Devonian period. Some, such as the Highland Boundary Fault, moved in this way at an earlier stage in the Ordovician. These data allow the Caledonian rocks lying between the Laurentian miogeocline (whose basement is represented by the Lewisian, Moine and possibly the Dalradian) and the Gondwanaland miogeocline (Midland Platform and Welsh Basin) to be re-analysed as a group of disorganized terranes which originated to the southwest in North America and southwest Europe/Africa prior to the Silurian. The Highland Border Terrane and Northern Belt Terrane are interpreted as duplicated pieces of a mid-Ordovician sequence which was a back are to northwest subduction. The Midland Valley Terrane is interpreted as a slice of Laurentian foreland onto which ophiolites were obducted in the lower Ordovician but which became the basement of a continental margin arc to northwest subduction in the mid-Ordovician. The Cockburnland Terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip. The Connemara Terrane is regarded as an allochthonous piece of the Dalradian miogeocline and the South Mayo Terrane as a remnant of an early Ordovician arc and fore arc which in mid-Ordovician times became a back arc/marginal basin to northwest subduction. The Lake District-Wexford Terrane is part of an arc to southeast subduction under Gondwanaland whose activity climaxed in the mid-Ordovician. The Central Terrane is interpreted as a Silurian overstep assemblage which blankets the junction between Laurentian- and Gondwanaland-derived oceanic terranes, and therefore Iapetus is regarded as an Ordovician ocean which closed prior to the Silurian. The model suggests that at the end of the Silurian, a clockwise-rotating Gondwanaland, having carried Laurentia into collision with Baltica, broke free and created a major sinistral strike-slip zone which disrupted the Ordovician palaeogeography in the British Isles/North American sector of Iapetus.

194 citations


Journal ArticleDOI
01 Jun 1987-Nature
TL;DR: In this paper, the authors suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust, and that granulite-facies metamorphism is a necessary complement of crustal melt ing.
Abstract: The discovery that granulites contain CO2-rich fluid inclusions while amphibolites in the same terrane contain H2O-rich or mixed CO2–H2O inclusions1 has led to a debate over the processes that produce granulite metamorphism. One theory2–4 maintains that granulites form as the result of a massive influx of CO2 from lower crustal or mantle sources. Such an influx could be responsible for the marked depletion many granulite terranes show in large-ion lithophile elements2,5. Others, however, maintain that granulite-facies metamorphism is a necessary complement of crustal melt ing6–9. Recently, Lamb and Valley10,11 have argued that granulite metamorphism may be a product of any one of three processes: CO2-streaming, partial melting, or recrystallization of originally dry rocks. Here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust.

166 citations


Journal ArticleDOI
TL;DR: A review of the present state of knowledge of the tectonic history of the southern and central Appalachian internides can be found in this article, with a focus on the development of the Appalachian orogen.
Abstract: The Appalachian orogen was constructed along the ancient Precambrian continental margin of eastern North America by a series of compressional events that began in the Ordovician and episodically spanned much of the Paleozoic era. The processes of accretion of suspect and exotic terranes, together with terrane collision and ultimately completion of the Wilson cycle by continent-continent collision, resulted in construction of the Appalachian orogen (Figure 1). These paleogeographic elements are rep­ resented by the various lithotectonic units visible today in the southern and central Appalachians (Figure 2). These subdivisions represent the late Precambrian rifted margin and early Paleozoic platform that were formed directly upon North American Grenvillian basement (Cumberland/ Allegheny Plateau, Valley and Ridge, and western Blue Ridge), the offshore North American slope-rise assemblage (Hamburg klippe, southern Appalachian eastern Blue Ridge and Inner Piedmont, and higher thrust sheets of the Maryland-northern Virginia Piedmont), and the exotic Carolina, or Avalon, and Wilmington terranes. The Carolina ter­ rane contains the Alleghanian high-grade metamorphic core. These elements form the basis for this analysis and review of the present state of knowledge of the tectonic history of the southern and central Appalachian internides. The Appalachian orogen has been the subject of study by geologists at least since the early nineteenth century. Early fundamental concepts, such as the geosynclinal theory of mountain building (Dana 1873, Hall 1883)

166 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used sequence mapping techniques to analyse the late Devonian to recent geologic evolution and hydrocarbon habitat of north Alaska and northwest Canada and identified eight depositional megasequences, each of which records a discrete, major phase of basin evolution.

147 citations


Journal ArticleDOI
TL;DR: In Ellesmere Island, the Canadian Shield and Arctic Platform are flanked on the northwest by the lower Paleozoic Franklinian mobile belt, which comprises an unstable shelf (miogeocline) and a deep-water basin, divisible into an inner sedimentary belt and an outer sedimentary-volcanic belt as discussed by the authors.
Abstract: In Ellesmere Island, the Canadian Shield and Arctic Platform are flanked on the northwest by the lower Paleozoic Franklinian mobile belt, which comprises an unstable shelf (miogeocline) and a deep-water basin, divisible into an inner sedimentary belt and an outer sedimentary–volcanic belt. Both are tied to the shelf by interlocking facies changes, but additional exotic units may be present in the outer belt.Pearya, bordering the deep-water basin on the northwest, is divisible into four successions. Succession I comprises sedimentary and(?) volcanic rocks, deformed, metamorphosed to amphibolite grade, and intruded by granitic plutons at 1.0–1.1 Ga. Succession II consists mainly of platformal sediments (carbonates, quartzite, mudrock), with smaller proportions of mafic and siliceous volcanics, diamictite, and chert ranging in age from Late Proterozoic (Hadrynian) to latest Cambrian or Early Ordovician. Its concealed contact with succession I is tentatively interpreted as an angular unconformity. Succession ...

Journal ArticleDOI
TL;DR: In this article, the authors used plate kinematics of the Pacific basin to model the movement of Baja BC along a transform margin with North America from 85 to 66 Ma.
Abstract: Paleomagnetic data suggest that a large part of British Columbia and southern Alaska was at the latitude of Baja California, Mexico, in early Late Cretaceous time. This allochthonous block (Baja BC) includes the Insular (Talkeetna in Alaska) and Intermontane composite terranes, the miniterranes between them, and possibly part of the Omineca metamorphic belt. Assuming that Baja BC was adjacent to North America by middle Cretaceous, an analysis using plate kinematics of the Pacific basin produces a model of northward movement of Baja BC which is compatible with the known geology of the reconstructed margin of western North America. Baja BC moved north 2400 km as part of the KuIa plate along a transform margin with North America from 85 to 66 Ma. This time interval is marked by the cessation of igneous activity in the Sierra Nevada, development of wrench faults and basins along coastal California, and initiation of Laramide-style tectonics to the east. Within Baja BC, diminished igneous activity to the west was coeval with possible wrench-tectonic basins along its eastern margin. By the end of the Cretaceous, Baja BC was positioned from Oregon to northern British Columbia. Between 66 and 56 Ma, Baja BC became attached to North America in a dextral-transpressive stage, during which the Kula--North America plate boundary expanded to encompass the whole block with a series of northwest trending strike-slip faults, a fold and thrust belt in the east, and large-scale uplifts and the formation of an incipient volcanic arc on the west. Southern Alaska was the site of convergence during the whole of the period of northward translation of Baja BC. A Late Cretaceous subduction zone in Alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of Baja BC collided with interior Alaska. The collision culminated in the Paleocene with formation of the McKinley granites and coeval dextral slip on the Denali fault system.


Journal ArticleDOI
TL;DR: In this paper, the authors show that the geologic record of the Alexander terrane is quite different from that in the Sierra-Klamath region and conclude that the two regions were not closely associated during Paleozoic time.
Abstract: The Alexander terrane consists of upper Proterozoic(?)-Cambrian through Middle(?) Jurassic rocks that underlie much of southeastern (SE) Alaska and parts of eastern Alaska, western British Columbia, and southwestern Yukon Territory. A variety of geologic, paleomagnetic, and paleontologic evidence indicates that these rocks have been displaced considerable distances from their sites of origin and were not accreted to western North America until Late Cretaceous-early Tertiary time. Our geologic and U-Pb geochronologic studies in southern SE Alaska and the work of others to the north indicate that the terrane evolved through three distinct tectonic phases. During the initial phase, from late Proterozoic(?)-Cambrian through Early Devonian time, the terrane probably evolved along a convergent plate margin. Arc-type(?) volcanism and plutonism occurred during late Proterozoic(?)-Cambrian and Ordovician-Early Silurian time, with orogenic events during the Middle Cambrian-Early Ordovician (Wales orogeny) and the middle Silurian-earliest Devonian (Klakas orogeny). The second phase is marked by Middle Devonian through Lower Permian strata which accumulated in tectonically stable marine environments. Devonian and Lower Permian volcanic rocks and upper Pennsylvanian-Lower Permian syenitic to dioritic intrusive bodies occur locally but do not appear to represent major magmatic systems. The third phase is marked by Triassic volcanic and sedimentary rocks which are interpreted to have formed in a rift environment. Previous syntheses of the displacement history of the terrane emphasized apparent similarities with rocks in the Sierra-Klamath region and suggested that the Alexander terrane evolved in proximity to the California continental margin during Paleozoic time. Our studies indicate, however, that the geologic record of the Alexander terrane is quite different from that in the Sierra-Klamath region, and we conclude that the two regions were not closely associated during Paleozoic time. The available geologic, paleomagnetic, and paleontologic data are more consistent with a scenario involving (1) early Paleozoic origin and evolution of the Alexander terrane along the paleo-Pacific margin of Gondwana, (2) rifting from this margin during Devonian time, (3) late Paleozoic migration across the paleo-Pacific basin in low southerly paleolatitudes, (4) residence in proximity to the paleo-Pacific margin of South America during latest Paleozoic(?)-Triassic time, and (5) Late Permian(?)-Triassic rifting followed by northward displacement along the eastern margin of the Pacific basin.

Journal ArticleDOI
TL;DR: In this paper, the results of Rb-Sr whole-rock and U-Pb zircon dating of gneisses and related basement units from the Wadi Feiran area in the Sinai peninsula have previously been suggested.
Abstract: Basement exposed on the perimeter of the Red Sea was created during the Pan-African event at the end of the Precambrian. Pre-Pan-African crust in the northern part of this region has not yet been identified. This paper reports the results of Rb–Sr whole-rock and U–Pb zircon dating of gneisses and related basement units from the Wadi Feiran area in the Sinai peninsula, where the existence of such older basement has previously been suggested. A post-tectonic extensional dyke gives a Rb–Sr age of 591 ± 9 Ma with an initial 87 Sr/ 86 Sr ratio of 0.7034 ± 0.0002. Rb–Sr whole-rock and thin slab dating of paragneisses gives ages of c . 610 Ma with an initial 87 Sr/ 86 Sr ratio of 0.7035. A U–Pb zircon age of 632 ± 3 Ma is interpreted as either the time of formation of these gneisses or the age of the crust sampled by protolith sediments. Granodiorite to the east gives a U–Pb zircon age of 782 ± 7 Ma and is interpreted as representing the westernmost extent of a 780 ± 50 Ma terrane that extends across Sinai into Jordan. Uplift and erosion of the 780 ± 50 Ma terrane supplied detritus to flanking terranes in N and SE Sinai. This region thus acted as a foreland to the younger accretionary and extensional units to the south and west that were active later in the Pan-African event. There is still no evidence for pre-Pan-African basement in the Precambrian units around the northern Red Sea east of the Nile.

Journal ArticleDOI
TL;DR: A compilation of U-Pb zircon dates for lower Proterozoic rocks in central Arizona shows that, although rocks tend to be older in the northwest (1800−1696 m.y.) than the southeast (1738−1630 my.), there is no single boundary separating distinct geochronologic provinces in Arizona as mentioned in this paper.
Abstract: A compilation of U-Pb zircon dates for lower Proterozoic rocks in central Arizona shows that, although rocks tend to be older in the northwest (1800−1696 m.y.) than the southeast (1738−1630 m.y.), there is no single boundary separating distinct geochronologic provinces in Arizona. Instead, the distribution of isotopic ages reflects the presence of two major tectonic provinces separated by a regionally subhorizontal boundary or boundaries. The northwestern part of central Arizona contains the Yavapai Series (1800−1755 m.y.) and calc-alkaline batholiths (1750−1696 m.y.), both believed to represent oceanic island-arc materials. The southeastern part of central Arizona is dominated by the Alder, Red Rock, and Mazatzal Groups and related hypabyssal intrusions (1710−1692 m.y.), with voluminous rhyolitic ash-flow tuffs and quartz arenite believed to record a relatively stable continental tectonic setting. Two working hypotheses emerge to explain the juxtaposition of representatives of these two tectonic provinces over a 100-km-wide zone in central Arizona. One interpretation (model 1) suggests that rocks of the southeast province were deposited with angular unconformity on newly accreted continental crust composed of northwest province rocks. A second interpretation (model 2) suggests that the two areas represent allochthonous terranes that evolved separately and were juxtaposed by large subhorizontal movements on thrusts and strike-slip faults. An important new constraint is that the 1699-m.y.-old strongly peraluminous Crazy Basin Quartz Monzonite was emplaced in the northwest province during ductile deformation at depths greater than 8 km at the same time that rhyolitic ash-flow tuffs and quartz arenite were being deposited in the southeast province. For model 1, this implies a rapid change of tectonic regimes about 1700 Ma, from convergence to uplift, erosion, sedimentation, and possibly extension. For model 2, the differences in crustal level, structural style, and petrologic affinity between ∼1700-m.y.-old rocks in both provinces are believed to result from juxtaposition of different crustal blocks after 1700 Ma.

Book ChapterDOI
01 Jan 1987
TL;DR: Petrological, geochronological and isotope geochemical studies over the past ten years are reviewed suggesting the following metamorphic and igneous evolution of the Cycladic crystalline complex as discussed by the authors.
Abstract: Petrological, geochronological and isotope geochemical studies over the past ten years are reviewed suggesting the following metamorphic and igneous evolution of the Cycladic crystalline complex: 1) Formation of high-pressure blueschist facies assemblages during the Eocene (40–45 Ma) as a result of continental collision processes. 2) During the Miocene (20–25 Ma) the Cycladic terrane became regionally overprinted by greenschist facies assemblages with local development of thermal domes. Isotopic studies show that infiltration of fluids is responsible for the propagation of the overprint. 3) Shortly after the culmination of the Miocene event (12–18 Ma) granitoids intruded on a number of islands, Ranging in composition from granodiorites and granites to monzonites the Cycladic granitoid province is interpreted to result from assimilation of crustal material by mantle-derived basaltic melts combined with fractional crystallization.


Journal ArticleDOI
TL;DR: In this paper, an extensive data set on the geochemistry of all types of rocks in the region culled from the literature is used to derive values for radiogenic heat production for each kind of crustal facies.
Abstract: The present geology of southern Scandinavia offers the unique opportunity to sample deep and intermediate levels from the same crustal section for both heat flow and heat production. In the central part of southern Norway, amphibolite facies terranes appear to lie on top of the same deeper crustal formations which crop out on their western and eastern margins. An extensive data set on the geochemistry of all types of rocks in the region culled from the literature is used to derive values for radiogenic heat production for each kind of crustal facies. Using constraints from heat flow data in the same area allows a reliable model of the distribution of crustal heat production. The average heat production of granulite facies terranes is 0.4 µW/m³, similar to values in other parts of the world. For amphibolite facies rocks, the value is 1.6 µW/m³. The present shield also includes heat producing element enriched granites formed in later events and the average heat production of presently exposed crust is 2.7 µ/m³. Using heat flow and radioactivity data from ten stations, the reduced heat flow in the area is 22 ± 2 mW/m². This corresponds to the heat flow at the top of 28 km of deep crustal facies, implying that the mantle heat flow is probably as low as 10 mW/m². Over the whole crustal thickness, the average amount of radiogenic heat is 31 mW/m².

Journal ArticleDOI
TL;DR: In this article, two separate two-dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved in the geologic record.
Abstract: Peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost U.S. Cordillera. Inverted metamorphic gradients are preserved in the Pelona-Orocopia Schist (southern California), the Catalina Schist (California borderland), the Central Metamorphic Belt (Klamath province), the South Fork Mountain Schist (northern California Coast Ranges), and possibly the Shuksan Suite (northern Washington). With the exception of the South Fork Mountain Schist, peak metamorphic temperatures increase structurally upward from ∼400°C to ∼650°C over 1–2 km, indicating metamorphic gradients in excess of −100°C km−1. Estimates of metamorphic gradients in some terranes, such as the Catalina Schist, depend critically on the effects of postmetamorphic faulting. The tectonic settings and high metamorphic pressures (∼800 MPa) suggest that the inverted metamorphic gradients formed in paleosubduction zones. Four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls, whereas the Pelona-Orocopia Schist formed beneath granitic crystalline rocks. The high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones. Two separate two-dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved. Rapid subduction (∼10 cm yr−1) beneath young oceanic lithosphere (<10 Ma) results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of −100°C km−1 in the top portion of the downgoing slab. In order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall. Accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and/or greenschist facies devolatilization reactions that weaken the descending slab. Alternatively, inverted metamorphic gradients may represent the continuous accretion of material at rates of 1–10 mm yr−1 under conditions of declining temperature. Continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 Ma. Continued subduction of ∼1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient. Synsubduction uplift and/or moderately fast postsubduction uplift rates of ∼1 mm yr−1 are required to preserve the inverted metamorphic gradient. The models predict that (1) the high-grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction, and (2) inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km.

Journal ArticleDOI
TL;DR: In this article, the ages of hornblende, muscovite, and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late Paleozoic tectonothermal evolution for t...
Abstract: 40Ar/39Ar incremental-release ages of hornblende, muscovite, and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late Paleozoic tectonothermal evolution for t...


Journal ArticleDOI
01 Feb 1987-Geology
TL;DR: In this article, large-scale over-thrusting and regional extension are both known to have been operative within the Shuswap metamorphic terrane of the southern Omineca belt.
Abstract: Large-scale over-thrusting and regional extension are both known to have been operative within the Shuswap metamorphic terrane of the southern Omineca belt. The Monashee core complex of the northern Shuswap terrane is interpreted to be a crustal-scale duplex that was developed during late Mesozoic shortening and thickening of the crust. Uplift of the Monashee complex, caused by basement duplexing, is estimated to have been in excess of 25 km. Late Cretaceous overthrusting of high-grade metamorphic and related plutonic rocks, during later stages of uplift and basement duplexing, led to weakening of the crust and to gravitationally driven extension of a regional crustal welt (Shuswap culmination). Denudation and spreading of this uplifted metamorphic terrane are attributed to displacements across listric-normal shear zones and faults localized along the outward-dipping flanks of the Shuswap culmination. Total extension across the Shuswap culmination at the northern end of the Monashee complex is estimated to be no greater than 30%. This model of denudation and gravity-induced spreading is extrapolated southward into the Valhalla and Okanagan regions, where extension may be far more significant.

Journal ArticleDOI
TL;DR: The Faeringehavn-Tre Brodre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event as discussed by the authors.
Abstract: The Faeringehavn–Tre Brodre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event. Contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes. The terrenes are: (1) Faeringehavn terrane, largely composed of early-Archaean Amitsoq gneiss cut by younger Archaean granitic gneiss defined here as the Satut gneiss; (2) Tre Brodre terrane comprising mid-Archaean Malene supracrustal rocks, anorthosite complex and polyphase Nuk gneisses; (3) Tasiusarsuaq terrane largely comprising mid-Archaean Nuk gneisses affected by c. 2800 Ma granulite facies metamorphism. The Tasiusarsuaq terrane is structurally above both the Faeringehavn and Tre Brodre terranes which at c. 2800 Ma experienced a lower grade of metamorphism. Juxtaposition of the terranes took place between 2800 and 2500 Ma and involved thrusting and crustal thickening. Subsequent re-equilibration involved folding, steeply inclined shear belts, intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages. This thrusting post-dates the 2800 Ma granulite facies metamorphism. It is younger and distinct from the thrusting postulated to explain the intercalation of the early-Archaean Amitsoq gneisses and the mid-Archaean Malene supracrustal rocks, associated with intrusion of the Nuk gneiss precursors described from Godthabsfjord. The tectonic breaks provide, for the first time, marker horizons which can be used to assess the amount and type of late Archaean deformation in the southern Godthabsfjord region.

Journal ArticleDOI
TL;DR: In this article, the D3 fabrics were superimposed on earlier structures which formed during initial amalgamation of the Meguma and Avalon Composite Terranes of the Maritime Appalachians and were interpreted to date diachronous formation of the S1 fabrics and concomitant low-grade regional metamorphism.
Abstract: The polygenetic Minas Geofracture separates the Meguma and Avalon Composite Terranes of the Maritime Appalachians. Dynamically recrystallized muscovite and biotite occur within D3 shear zones which developed during dextral movement along the Geofracture. These record 40Ar/39Ar plateau ages between circa 360 and 370 Ma. Comparable plateau ages are recorded by micas within undeformed portions of granitic plutons and their low-pressure, contact metamorphic aureoles. These are similar to U-Pb ages previously reported for igneous monazite, therefore indicating relatively rapid postmagmatic cooling at shallow crustal levels. The D3 fabrics were superimposed on earlier (S1) structures which formed during initial amalgamation of the Meguma and Avalon Composite Terranes. Whole-rock slate/phyllite samples collected outside high-temperature portions of the plutonic contact aureoles yield 40Ar/39Ar plateau ages which correspond to groupings of circa 400–395 Ma, 390–385 Ma, and 380–375 Ma. These are interpreted to date diachronous formation of the S1 fabrics and concomitant low-grade regional metamorphism. In the circa 25 Ma between initial amalgamation and granitic plutonism, it is inferred that there was a slow rise of isotherms through the crust as a result of overthrust tectonic thickening. This led to crustal melting and the production of widespread granitic magmas which were emplaced immediately prior to D3 dextral shearing.

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TL;DR: The Proterozoic Cheyenne belt, at the southern margin of the Archean Wyoming craton, consists of strongly deformed lithotectonic blocks bound by major mylonite zones.
Abstract: The Proterozoic Cheyenne belt, at the southern margin of the Archean Wyoming craton, consists of strongly deformed lithotectonic blocks bound by major mylonite zones. From north to south and structurally lowest to highest, these blocks include (1) Archean crystalline basement and associated early Proterozoic miogeoclinal rocks; (2) an amphibolite-grade orthogneiss terrane of unknown age containing probable rift-related mafic intrusive rocks; (3) a 1750–1790 Ma, marginal basin(?) succession consisting of upper amphibolite-grade pelitic and volcano-genie schist, associated peraluminous granite, and minor ultramafic rocks; and (4) a 1750–1790 Ma intermediate to mafic plutonic-metamorphic complex interpreted as the deep roots of an island-arc system. The earliest deformation, D1, produced a synmetamorphic, penetrative, horizontal transposition foliation and associated recumbent folds. Tectonic blocks were juxtaposed along major mylonite zones during D2. Macroscopic anil microscopic structures associated with D2 indicate northward thrusting along low-angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the Wyoming craton at about 1750 Ma. Thrusting occurred at minimum temperatures of 475 °C and produced an inverted metamorphic gradient in pelitic rocks north of the belt. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of dextral strike slip (D3) between 1750 and 1400 Ma. Cataclasis (D4) and brecciation associated with the Laramide orogeny (D5) locally overprint earlier structures. Oblique convergence between an island arc and the Archean continental nucleus may explain accretion, crustal shortening, and subsequent strike slip. The Phanerozoic accretionary events of eastern and western North America provide an analog for this model. Presence of similar lithologies and shear zones south of the Cheyenne belt suggest that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion.

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TL;DR: In this paper, the Southern Uplands were interpreted as an accretionary prism, and the model was reassessed in the light of advances made subsequently in understanding modern/Neogene prisms.
Abstract: Fore-arc regions, whether accretionary, erosional or composite, vary greatly both along strike and through time, in the geometries which they assume in response to subduction. There is no well-studied modern/Neogene margin which can be established as the 9type9 accretionary prism. All are individual in some respect or other. Nor is there an example of a well-studied modern/Neogene accretionary margin which can be compared closely to the Southern Uplands, though there are several good analogues for particular stages of its history. Accretion took place along the Southern Uplands margin for c. 50 million years. The range of geometries on modern/Neogene accretionary active margins, and the extent to which the behaviour of those margins can change during periods as long as 50 million years, is reviewed, emphasizing the accretionary prisms of Mexico, SW Japan and the Makran. The interpretation of the Southern Uplands as an accretionary prism, as presented in the late 1970s and early 1980s, is reassessed in the light of advances made subsequently in understanding modern/Neogene prisms. The recent alternative model of Stone et al. for the Southern Uplands envisages that the terrane evolved by the opening and closure (by arc-subduction) of a marginal basin. The new model draws heavily on arc-type detritus derived from the oceanward side of the ancient Southern Uplands trough. There are areas along the SW Japan fore-arc where oceanward-derived tubidites are accumulating in a trench (Nankai Trough), and where active submarine arc-type volcanism is occurring outboard of a currently growing accretionary prism. Such features in an ancient terrane like the Southern Uplands do not necessarily mean that an alternative model must be sought involving whole new arc systems. Furthermore, analysis of the life-spans and sedimentary fills of modern/Neogene marginal basins begs the conclusion that the Southern Uplands9 strata were not deposited in a marginal basin. It is most unlikely that the c. 50 Ma history of pelagic sedimentation preserved there can be manifesting anything other than the closure of a sizeable ocean basin, of a scale exceeding any known western Pacific-type marginal basin, and with a different sedimentation history from any such marginal basin.

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TL;DR: In this article, the Dupal isotopic anomaly is not restricted to the southern hemisphere, or to the Indian Ocean MORB and OIB, but is also present in the West Philippine arc of the Philippines.

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TL;DR: A suite of microconglomerates is recognized in Silurian rocks which occur on both sides of the proposed line of the Iapetus suture in Ireland.
Abstract: A suite of microconglomerates is recognized in Silurian rocks which occur on both sides of the proposed line of the Iapetus suture in Ireland. Clast composition and palaeocurrent data show that these conglomerates, which grade into the typical quartz-rich Silurian turbidites, were derived from two compositionally similar magmatic arc terranes which lay on either side of the present Silurian outcrop. In the Llandovery, derivation was from both the south and the north. In the Wenlock, derivation was from the north and sedimentation prograded southwards across the ‘suture’ and onto the southern margin. The source terrain in the south was probably the Ordovician Wexford–Lake District arc. We identify the northern source as another arc (Cockburnland) which has since been cut out by sinistral strike-slip against the Ordovician Northern Belt. These data imply that arc activity ceased synchronously on either side of Iapetus during the late Ordovician and this leads us to speculate that subduction of oceanic crust ended at that time. Closure was associated with deformation and uplift of the bounding Ordovician terrains. These rocks then contributed detritus to the Silurian infill of a successor basin. Regional sinistral transpression finally deformed and reorganized these units between the end Silurian and the early Devonian and led to the complete closure of the remaining Silurian seaway.

Journal ArticleDOI
Abstract: The contents of a computerized lexicon database are displayed in the form of a range chart that demonstrate the spartial and temporal relationships of lithtostratigraphic units to tectonostratigraphic terranes of New Brunsiwck. The chart provides a reference basis from which to derive the accretionary history of these terrance. The tectonostratigraphlc zonation of Hew Brunswick ia based upon the uniqueness of the pre-Taconlan stratigraphy within each fault-bounded terrane. From northwest to southeast, the following terranes and cover sequences are recognized: Matapedia Cover. Blmtree Terrane, Mlramichi Terrane, Frederlcton Cover, St. Croix Terrane, Hascarene Terrane, and Avalonian Terrane. Overstepping of the Matapedia Cover Sequence indicates that the Elmtree and Mlramichi terranes were docked with the North American craton by the Late Ordovician to Early Silurian. The presence of a similar early Paleozoic stratigraphy, tectonic style and major Silurian unconformity in the St. Croix Terrane suggests that it had become docked to the Mlramichi Terrane prior to this subduction-related Taconian event. Detritus and a similar fauna in the cover rocks of the St. Croix Terrane provide evidence that it was docked to the Hascarene Terrane by the Late Silurian. This docking coincided with the Salinic disturbance in northern Mew Brunswick where it is marked by an unconformity on the northwestern margin of the Mlramichi Terrane and by coarse clastic sedimentation and local deformation within the Matapedia Cover Sequence. Detritus links the Hascarene and Avalonian terranes by the Late Devonian. The Salinic disturbance and culminating Acadian orogeny are interpreted to be the reault of transcurrent convergence that accreted the Hascarene and Avalonian terranes to North America. RESUME On rend le contenu d'un lexique informatise sous forme d'un tableau qui demontre les relations spatiales et temporelles entre les unites lithostratlgraphiques et les lanieres tectonostratigraphlques presentes au Nouveau-Brunswick. Ce tableau sert de pivot pour deriver L’histoire de l'accretion de ces lanieres. On base le zonage tectonostratigraphique du Nouveau-Brunswick sur le caractere unique de la stratigraphie pretaconienne au aein de cheque laniere. Du nord-ouest au aud-est, on reconnalt les lanieres et les sequences de couverture suivantes: la Couverture de Matapedia, les lanieres d'Elmtree et de Mlramichi. la Couverture de Fredericton ainsi que les lanieres de St. Croix, Hascarene et d'Avalon. Leur empietement par la Sequence de Couverture de Matapedia demontre qu'a l'Ordovicien tardif. ou a l'Eosilurien, les lanieres d'Elmtree et de Mlramichi se sont deja juxtaposees au craton nord-americain. La similarite de leurs stratigraphies eopaleozoiquea et de leurs styles tectonigues, ainsi qu’une discordance silurienne importante dans la Laniere de St. Croix suggere l'accolement de cette dernlere sur la Laniere de Hlramichi avant cet evenement taconlen relie a une subduction. La preuve que la Laniere de St. Croix a'est deja accolee a la Laniere de Hascarene au Silurien tardif reside dans les fades detritiquea et une faune semblable dans les roches de couverture de cette premiere. Cette juxtaposition coincide avec le tumulte salinique au Nouveau-Brunswick septentrional ou elle se marque par une discordance sur la marge nord-ouest de la Laniere de Hlramichi ainsi que par un epandage detritique grossier et une deformation locale au aein de la Sequence de Couverture de Matapedia. Un lien detritique s'etablit entre les lanieres de Hascarene et d'Avalon avant le Devonien tardif. On interprete le tumulte salinique et le paroxysme de l’orogenie acadienne comme resultant d'un coulissage convergent qui a accretionne les lanieres de Hascarene et d'Avalon sur l'Aroerique du Nord. [Traduit par le journal]