Showing papers on "Terrane published in 1995"

The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: A geodynamic model

Abstract: Paleontologic and stratigraphic evidence points to the early Paleozoic Precordilleran terrane of western Argentina as being the conjugate rift pair of the Appalachians. Stratigraphic similarities of the Cambrian and early Arenig carbonate series and very strong affinities among trilobite, conodont, and brachiopod faunas show their close relationship. The most probable provenance areas are the Appalachian-Ouachita rifted margin and the Newfoundland Appalachians, although the former fits better with geometric and drifting paths suggested by faunal affinities. Increasing Celtic and Baltic brachiopod genera and divergent stratigraphy since the Arenig indicate the drifting of the Precordilleran terrane. Collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed. The collision of the Precordillera with western Gondwana occurred during the mid-Llanvirn to Llandeilo. A magmatic arc related to eastward subduction (present coordinates) was active in the Famatina Range east of the Precordillera. This region of Celtic affinity shows faunal exchange with the Precordilleran terrane since the late Arenig and may represent accreted intra-Iapetus volcanic island-arc complexes. The rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the Precordilleran terrane from periequatorial to peripolar latitudes between the Cambrian and latest Ordovician. The deep ocean to the west of the Precordillera started to close by the Late Ordovician with the eastward drift of the Chilenian terrane. Absence of volcanic or pyroclastic arc-derived rocks in the Precordillera indicate west-dipping subduction. As Chilenia approached the continental margin, a new forebulge was established on the former collided Precordilleran terrane, developing an erosional unconformity in central Precordillera (Talacasto-Tambolar arch). A Gondwanic signature was fully developed by the Middle Silurian when the Malvinokaffric Clarkeia Fauna flourished. Before then, the Late Ordovician glacial record and associated Hirnantia Fauna were the first clear tie to Gondwana. During the Silurian the marginal basin behaved as a foreland, with lithosphere rheology and eustasy governing the sequence stratigraphy. Wrench faulting along its eastern boundary displaced the Precordillera toward the south. Continued shortening during closure with the Chilenian terrane in the mid-Devonian produced thrust loading of the basin and generated a thick graywacke succession. Final accretion of Chilenia (Late Devonian) generated a regional angular unconformity between the lower and upper Paleozoic. New eastward subduction was initiated west of the accreted Chilenian terrane during the Late Permian–Triassic as indicated by the Choiyoi volcanic complex, which presently outcrops in the Frontal Cordillera.

Contrasting plate-tectonic styles of the Qinling-Dabie-Sulu and Franciscan metamorphic belts

Abstract: The Dabie Mountains are part of the >2000-km-long Qinling-Dabie-Sulu suture zone juxtaposing the Sino-Korean and Yangtze cratons. An eastern extension apparently crosses Korea and lies along the Japan Sea side of Honshu as the Imjingang and Sangun terranes, respectively; a northeastern segment may be present in Sikhote-Alin, Russian Far East. This orogenic belt records late Paleozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs in east-central China. Coesite and microdiamond inclusions in strong, refractory minerals of eclogite facies ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu area attest to profound subduction of a leading salient of the old, cold Yangtze craton, now recovered through tectonic exhumation and erosion. Northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high-pressure (HP) and UHP metamorphism documented in the Western Alps. In both regions, subduction of narrow prongs of continental material, UHP metamorphism, and return toward midcrustal levels of relatively lower density, buoyant microcontinental blocks resulted from delamination of these rocks from the descending, higher density, oceanic-crust-capped lithospheric plate. Such salients of continental crust represent an integral structural part of the downgoing slab, remain intact, and may be dragged to great depths before disengaging and rising differentially as coherent blocks. UHP parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700–900 ° C and 28–35 kbar or more. In contrast, Pacific-type HP metamorphic belts, as represented by the Franciscan Complex of western California, recrystallized under physical conditions up to 200–500 ° C, 10 ± 3 kbar. In this setting, voluminous quartzo-feldspathic and graywacke debris was carried downward on oceanic-crust-capped lithosphere, choking the subduction zone with incompetent material. Sited between both plates, and strongly adhering to neither, this buoyant, largely sedimentary complex decoupled at 25–30 km depth, and ascended toward the surface. In both Alpine-type intracontinental collision and Pacific-type underflow, light sialic material displaced dense mantle; thus, the return to midcrustal levels was propelled dominantly by body forces.

Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution

Abstract: New Caledonia preserves evidence that constrains models for the tectonic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subduction-related terranes and their accretion to the Gondwana margin; (2) Cretaceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene arrival of thinned Gondwana margin crust at a southwest-facing subduction zone, resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast; and (4) detachment faulting during extensional collapse, resulting in unroofing of metamorphic core complexes. The last phase explains supposedly anomalous metamorphic gradients in the northeast of the island.

Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence

Abstract: The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW–SE decreasing granitoid intrusion age trend (∼1.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW–SSE trending Cretaceous-Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (∼7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40–25 Ma, with a velocity of 6.5–4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological observations and paleomagnetic data do not give strong support for large-scale right-lateral displacements of crustal blocks like the Baja California. Given the isotopic data presented, the continental margin truncation in SW Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment. On the other hand, the lateral removal of material associated with the displacement of Chortis is more important in the southeastern segment.

Detrital zircon reference for Cambrian to Triassic miogeoclinal strata of western North America

Abstract: U-Pb analyses of 656 single zircon grains from Cambrian to Triassic miogeoclinal strata provide a latitudinal and temporal reference for the ages of grains that accumulated along the western margin of North America. Comparisons between this detrital zircon reference and the ages of grains in potentially displaced terranes outboard (west) of the miogeocline should help establish when the terranes first arrived in sedimentary proximity to western North America. North-south variations in the ages of grains in Cambrian and Devonian to Triassic strata, which reflect the north-south changes in the age of cratonal rocks near the margin, should also help place constraints on a terrane's paleolatitude during these time periods. The technique cannot be used to determine paleolatitude during Ordovician time, however, because miogeoclinal strata from northern Canada to northern Mexico are dominated by grains shed from the Peace River arch (northwestern Canada).

Thermal evolution of the southeastern Canadian Cordillera

Abstract: The eastern metamorphic culmination of the southern Canadian Cordillera is a composite core complex, which at low structural levels exposes the Monashee decollement, a major contractional fault with large Late Cretaceous to Paleocene east-directed displacement. The hanging wall of this fault, the Selkirk allochthon, is a sheared thrust sheet, recording metamorphic and deformational events spanning the period from ca. 170 to 60 Ma, with younger kinematic and thermal events recorded at progressively deeper levels. The Monashee complex, the footwall terrane of the Monashee decollement, consists of an Early Proterozoic crystalline basement complex overlain by Late Proterozoic and perhaps Phanerozoic metasedimentary rocks. The Monashee complex was significantly metamorphosed and deformed in Paleogene time (60–55 Ma), on the basis of U–Pb data presented in this paper. Analysis of U–Pb titanite data show that the duration of this metamorphic event was but a few million years at most, and it provides a strong arg...

Trace elements in shale as indicators of crustal provenance and terrane accretion in the southern Canadian Cordillera

Abstract: In this study, the trace-element geochemistry of shale is used to infer the nature of adjacent crustal blocks during terrane amalgamation and accretion in southern British Columbia. The geochemistry of about fifty shale samples from one of the best-dated stratigraphic sections in the North America Cordillera is used to (1) infer proximity to source regions characterized by juvenile ocean arcs, continental arcs, and ophiolites and (2) address a controversy concerning the timing of terrane amalgamation and accretion. This study includes analyses of shale from the entire Mesozoic stratigraphic section of the Cadwallader terrane and overlying Tyaughton basin, as well as argillites from the structurally adjacent Bridge River Complex. The mid-Cretaceous (Albian) strata of the upper Taylor Creek Group, show light rare earth (LREE) enrichment (La N /Sm N of ∼3.0) and anomalously high Cr and Ni. Older units (Upper Triassic–Lower Cretaceous) that stratigraphically underlie the upper Taylor Creek Group have less LREE enrichment, high Ba N /La N ratios, and no Cr or Ni anomaly. The mid-Cretaceous strata were probably derived from a continental arc during a period of uplift and erosion of ultramafic rocks (probably ophiolites) and the co-extensive nature of these sediments suggests accretion of many basement terranes to the edge of a continent. The trace-element geochemistry of the older strata is similar to that of sediment derived from an oceanic arc; however, Lower Cretaceous strata of the Relay Mountain Group may record the incipient collision and progressive closure of this ocean basin. The Bridge River Complex, which is dominated by oceanic rocks and is interpreted to be an accretionary complex, contains two suites of argillites, one of which shows significant LREE enrichment, and isotopic data suggest a Jurassic-Cretaceous age for these argillites that compose the matrix in this complex (Leitch et al., 1991). We suggest that the Bridge River Complex was a Jurassic-Cretaceous accretionary complex that formed outboard of the Methow section of rocks and was not juxtaposed with the Cadwallader terrane and Tyaughton basin until the Albian when these terranes were accreted to North America. Paleomagnetic data from these rocks (Maxson et al., 1993) indicate that accretion took place at the latitude of central Mexico and that subsequent strike-slip faulting brought these terranes to their present position by Eocene time.

A Pan‐African granulite facies metamorphic episode in Prydz Bay, Antarctica: Evidence from Sm‐Nd garnet dating

Abstract: The granulite facies gneisses along the Prydz Bay coastline, Antarctica, have long been regarded as part of an extensive Proterozoic (ca 1000 Ma) terrane, based on correlations with other parts of eastern Antarctica. New Sm‐Nd garnet‐whole rock dates for various rocks indicate that the main metamorphic episode that has affected the rocks in Prydz Bay is of Pan‐African age (515–490 Ma). Only in one locality, in the western part of the area, have older garnet ages been found. On Sostrene Island, where petrographic and analytical work suggests an earlier higher pressure metamorphic event, an age of 990 Ma correlates with some well‐established metamorphic ages from the Northern Prince Charles Mountains to the south, the Rayner Complex, Enderby Land to the southwest and part of the Rauer Group to the east. Evidence for reworking of the rocks on Sostrene is provided by the textures and by the 500 Ma ages which have been determined for interlayered pelitic and felsic gneisses. The notion of a continuous Late Pro...

Structural and Tectonic Controls of Basin Evolution in Southwestern Gondwana During the Phanerozoic

Abstract: The continental lithosphere of southwestern Gondwana, comprising the southern part of South America and southern Africa, was largely assembled before the end of the Proterozoic. Geologic studies indicate that the basement anisotropy that controlled the development of Phanerozoic basins was established by Neoproterozoic-Early Cambrian tectonism. This tectonism reactivated the older terrane boundaries or cut across them. The backbone linking the system of Pan-African and Brasiliano basins was a system of northeast-trending structures. There were four areas of pronounced Neoproterozoic-Early Cambrian basin subsidence in the study area: the Chiquitanas trough in Bolivia, the Puncoviscana basin in Argentina, the Dom Feliciano-Ribeira basins in Brazil, and the Damara-Nama basin complex of southern Africa.

Basin Development in an Accretionary, Oceanic-Floored Fore-Arc Setting: Southern Coastal Ecuador During Late Cretaceous-Late Eocene Time

Abstract: Southern coastal Ecuador is an accreted terrane underlain by an oceanic crust formed during the AptianAlbian. To the southeast, the oceanic crust is overlain by Cenomanian-Coniacian fine-grained pelagic deposits, coarse-grained volcaniclastic turbidites of Santonian-Campanian age, and Maastrichtian-middle Paleocene tuffaceous shales. Toward the northwest, late Campanian-Paleocene volcaniclastic beds and lava flows of island arc composition rest on the oceanic crust. This results from the opening of a marginal basin between an early Late Cretaceous island arc (Cayo arc) and a latest Cretaceous-Paleocene island arc (San Lorenzo arc). In the late Paleocene, the accretion of the Cayo remnant arc to the Andean continental margin caused a major deformation phase that affected only the southern part of coastal Ecuador. There, deformation was sealed by thick, coarse-grained, quartz-rich turbidites that constitute the infilling of an early fore-arc or slope basin. A subsequent tectonic event in the early Eocene is believed to have resulted in emergence of the entire area. At the early-middle Eocene boundary, new fore-arc basins were created that filled with mud and clastic shelf deposits. A marked disconformity is overlain by coastal to continental coarse-grained deposits of late middle-early late Eocene age. These express a major tectonic phase attributed to definitive collision of coastal Ecuador with the Andean margin. The entire area then emerged, until the formation of new fore-arc basins in the latest Oligocene-Miocene. The late Paleocene, earliest Eocene, and early late Eocene tectonic events are the most important deformation phases to affect southern coastal Ecuador and represent its progressive accretion to the margin. The creation of repeated fore-arc basins can be attributed to subsidence from crustal erosion of the upper plate because each subsidence event succeeded an important compressive phase that must have favored coupling and tectonic erosion. This complex geologic history has implications for burial and maturation of organic matter and must be taken into account in guiding oil exploration in coastal Ecuador.

Facies, Paleogeography, and Sedimentary History of the Southern Permian Basin in Europe

Abstract: The structural position of the Southern Permian Basin (SPB) is controlled by the Caledonian and Variscan tectonic framework (Fig. 1). The basin is located between the mid-North Sea High and Ringkobing-Fyn High in the north, the East European Platform in the east and the Variscan tectogene in the south (Fig. 2). In the area of maximum subsidence (i.e. in the Variscan Foreland), the substrate is composed of consolidated Caledonian massifs. This consists of (1) the Netherlands-North German Massif (possibly a terrane), considered now (Hoffmann 1990) as a separate structural unit of Precambrian age which was established finally in Caledonian time, and (2) a part of the Holstein-Rugen-Pomorze Terrane (Caledonian fold zone; Franke 1990) limited by strike-slip faults from the North German-Polish Variscan orogenic belt. The Southern Permian Basin has several narrow connections with adjacent basins (for references, see Sorensen and Martinsen 1987) and possible temporary connections with the Tethys domain via the Polish-Dobrugea trough, along a rift zone (Ziegler 1987) and with small basins on the Inner Variscan domain. These connections and the SPB tectonic framework were induced by pre-Permian tectonics generally described as a subequatorial structural plan and submeridional deep fractures (Franke 1990).

Tertiary calc-alkaline magmatism associated with lithospheric extension in the Pacific Northwest

Abstract: The long-standing assumption that calc-alkaline magmatism requires contemporaneous subduction is critically reviewed. Two of many widely spread fields of Tertiary calc-alkaline magmatism in the Pacific Northwest, one Eocene and one Miocene, provide clear examples of the association between calc-alkaline magmatism and lithospheric extension. We suggest that the calc-alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle, enriched by previous subduction events, than by contemporaneous Eocene subduction. This model proposes that extension will cause calc-alkaline-dominated magmatism in areas of continental crust with a history of previous subduction, as along the western margin of North America. Currently popular models for the tectonic/magmatic evolution of the Basin and Range province, in which subduction-driven calc-alkaline magmatism is followed by extension-related bimodal magmatism, may need modification; much more of this magmatism may result from extension. In the Pacific Northwest, we suggest that subduction beneath the Idaho batholith ceased at the end of the Cretaceous, blocked by the accreted terranes, to be reactivated in its present position beneath the Cascades during the Eocene. The various isolated Eocene volcanic fields spread across the Pacific Northwest were due to lithospheric extension, not to the presence of a subducting plate immediately below.

Cadomian terranes, wrench faulting and thrusting in the central Europe Variscides: geophysical and geological evidence

Abstract: Sixty five per cent of the Paleozoic basement of western and central Europe is hidden by a sedimen- tary cover and/or sea. This work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the Variscan or- ogeny. It is based on the interpretation of various forms of data: (a) published gravity maps corrected for the ef- fects of the crust-mantle boundary topography and light sedimentary basins; (b) aeromagnetic maps; (c) measurements of densities; and (d) induced and rema- nent magnetizations on rocks from Paleozoic outcrops of the upper Rhenish area. From the northern B ohem- ian Massif to the eastern Paris Basin, the Saxothurin- gian is characterized by a 500 km long belt of gravity highs, the most important being the Kraichgau high. Most of the corresponding heavy bodies are buried un- der a post-early Visdan cover. They are interpreted as relics of Late Proterozoic terranes overlain by an Early to Middle Paleozoic sequence, equivalent to the Bo- hemian t errane in the Bohemian Massif. The most probable continuation of these dense Bohemian ter- ranes toward the west is the Southern Channel-North- ern Brittany Cadomian terrane. The gravity lows are correlated with Variscan granites and pre- and early Variscan metagranites. Gravity and magnetic maps demonstrate large-scale displacement in Devonian-Early Carboniferous times along the parallel and equidistant, NW-SE striking, Vistula, Elbe, Bavarian, Bray and South Armorican dextral wrench faults. In the Vosges-Schwarzwald and Central Massif the faults continue with the east-west striking Lalaye-Lubine-Ba den-Baden and Marche faults and with south vergent thrusts. The Bavarian faults shift the Kraichgau terrane by 150 km relative to the Bohemian terrane, whereas the offset of the North- ern Brittany Cadomian relative to the Northern Vosges-Kraichgau terranes is estimated at 400km along the Bray fault. Sinistral wrench faults are the NE-SW striking Sillon Houiller, Rheingraben, Rodl, Vitis and Diendorf faults. The southern Vosges- Schwarzwald Devonian-Dinanti an basin is interpreted as a pull-apart basin at the south-easterly extremity of the Bray fault. The Bohemian and Kraichgau body form allochthonous terranes which were thrust over the Saxothuringian crust. Thrusting to the north-west was accompanied by back-thrusting and led to the forma- tion of pop-up structures. Contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision. The zonal structure of the Varis- cides in the sense of Kossmat (1927) is relevant only to the Rhenohercynian Foreland Belt. Kossmat (1927) al- ready spoke of a Moldanubian Region because it dis- plays no real zonal structure. The Saxothuringian Zone was formed by terrane accretion. Their apparent zonal structure is not a pre-collisional feature, but only the result of accretion and collision.

Tectonic role of ophiolite‐bearing terranes in the development of the Southern Apennines orogenic belt

Abstract: New data on ophiolite-bearing terranes of the Liguride Complex, together with some information on the terranes of the Sicilide Complex, result in a better understanding of the role and tectonic significance of these units in the construction of the Southern Apennines orogenic belt. The Liguride Complex is composed of two main tectonic units overlain by a thick turbiditic sequence of Late Oligocene-Middle Miocene age. The uppermost one (Frido Unit) is a polydeformed and polymetamorphosed sequence, composed of two tectonic subunits of shales and calc-schists, respectively, containing blocks of ophiolite, garnet gneiss, amphibolites and granitoids. This unit is thrust over the un-metamorphosed terranes (Calabro–Lucano Flysch Unit) consisting of a broken formation with blocks of Late Jurassic ophiolite and their sedimentary cover, Cretaceous-Eocene pelagic sediments and Late Oligocene volcaniclastic deposits. The Frido Unit underwent HP/LT metamorphism (P= 8–10 Kb; T= 400–500 °C) resulting in glaucophane and lawsonite assemblages in the ophiolitic rocks and aragonite in the meta-limestones and calc-schists, followed by greenschist fades metamorphism (P= 4 Kb; T= 300–350 °C). From a structural point of view units of the Liguride Complex comprise structures developed at different structural levels, indicating progressive non-coaxial deformation in response to tectonic transport towards the N-NE. The ophiolite-bearing terranes of the Liguride Complex can be considered as a remnant of an accretionary complex in which the Calabro Lucano Flysch Unit represents the toe of the wedge where frontal accretion processes occur and the Frido Unit is a deeper portion. Emplacement of the Frido Unit is explained as being due to formation of a deep duplex structure during the early stage of continental collision processes. The polarity of tectonic transport provides new evidence that the Liguride Complex represents a suture zone between the Apulian and the Calabrian blocks. The age of collision appears to be not older than late Oligocene. The allochtonous terranes of the Liguride and Sicilide Complexes, therefore, represent a complete accretionary wedge which records, first, subduction of the Neotethys ocean beneath the Calabrian (Europe) continental margin and, later, continental collision with the African block.

Structural history of the Chugach metamorphic complex in the Tana River region, eastern Alaska: A record of Eocene ridge subduction

Abstract: The Chugach metamorphic complex of southern Alaska is an Eocene high-temperature ( T ), low-pressure ( P ) fore-arc metamorphic belt related to subduction of the Kula-Farallon spreading center beneath western North America. The Chugach metamorphic complex has a three-phase ductile deformational history that records major changes in kinematic axes during a short interval of geologic time (∼8 m.y.). The earliest deformation (D 1 ) is a regional event recognized throughout the flysch subterrane of the Chugach terrane. D 1 is a regional layer-parallel slaty/phyllitic cleavage developed during accretion and subsequent shortening. In the Chugach metamorphic complex, D 1 predates high-temperature metamorphism. During prograde metamorphism, there were two major structural events. D 2 records orogen-parallel extensional accompanied by vertical shortening with components of pure shear and top-to-the-east simple shear. D 2 is synchronous with melt injections (Ti 2 ) in the gneissic core of the complex and large plutons throughout the complex. D 3 records a return to subhorizontal contraction perpendicular to the margin and is interpreted as a dextral transpressional event. D 3 contraction produced a dramatic thickening of the complex in a regional-scale D 3 anticlinorium. In the gneissic core, the presence of melt (Ti 3 ) strongly influenced D 3 . Finite strain data and field observations indicate that both F 2 and F 3 have axes that are parallel to the stretching direction, yet these are not sheath folds because strains are too low. Instead the structures are examples of folds that developed with their axes parallel to the elongation axis. Together these observations provide further evidence for our previous interpretations that the Chugach metamorphic complex is a manifestation of an Eocene plate reorganization at ca. 56–52 Ma. Plate models predict that before 56 Ma the Kula–Farallon–North American triple junction migrated southward and is associated with a time-transgressive fore-arc plutonic belt. After plate reorganization, the triple junction either backtracked northward (Kula Plate model) or continued southward with intermittent northward motion (Pacific-Farallon model). We interpret the D 2 -to-D 3 progression as either a result of highly oblique subduction of the Kula plate followed by more orthogonal—but still dextral-oblique—convergence of the Farallon plate (Kula Plate model), or a special case of Pacific–Farallon–North American interaction.

Ultradeep crustal metamorphism: A prospective view

Abstract: Ultradeep, or ultrahigh-pressure (UHP), metamorphism of crustal rocks at depths greater than those of the quartz-coesite and even graphite-diamond transitions has been firmly established from petrological studies in at least three collisional belts. The typically disrupted structural settings of the UHP rocks raise important questions about the scale and extents of UHP metamorphism and whether any present example can be interpreted as a regional terrane across which metamorphic P-T gradients can be defined. Improvements in the precision with which UHP metamorphism can be characterized will rely on extending experimental studies to a greater range of chemical systems and solid solutions and on developments in the application of microbeam techniques to experimental products. The role of fluids in continental crust undergoing subduction, important to any understanding of melt generation in convergent zones, needs to be considered further using the available UHP areas as natural laboratories and through high-pressure experimental studies of the solubilities of minerals in aqueous and saline fluids. Pressure-temperature paths provide an essential constraint on the choice of possible tectonic models to explain the exhumation of UHP metamorphic rocks and therefore must be determined in detail using all the petrological tools available and integrated with precise geochronology focused on the timing of mineral growth in relation to structural markers and P-T evolution. Extension of thickened crust or a crustal wedge experiencing continued underplating is a favored mechanism for the uplift and exhumation of UHP areas which will continue to be evaluated as new P-T-t data constraining the rates of exhumation and cooling of these remarkable rocks become available.

Late Paleozoic collision, delamination, short-lived magmatism, and rapid denudation in the Meguma Terrane (Nova Scotia, Canada): constraints from 40Ar/39Ar isotopic data

Abstract: The Meguma Terrane consists of 8–15 km of early Paleozoic stratified rocks, deposited on a continental basement, that were deformed and metamorphosed during the late Paleozoic as a result of lithospheric plate collision. The oldest cleavage (previously published, whole-rock, 40Ar/39Ar plateau ages of 415–395 Ma) dates the onset of crustal thickening, which was followed by voluminous, but short-lived, Late Devonian granitic and minor mafic magmatism (380–370 Ma). This magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere, which effected melting above the new Moho and resulted in intrusion at depths of 5–12 km. 40Ar/39Ar plateau ages of hornblende, muscovite, and biotite indicate that, at the present erosion level, most of the Meguma Terrane cooled through ~300 °C by 368–360 Ma, slightly earlier than for the southwestern Meguma Terrane (ca. 345 Ma). The present erosion level was exhumed by the latest Devonian–Early Carboniferous (Visean): the age of the ...

Age profile of ophiolitic rocks across the Late Palaeozoic New England Orogen, New South Wales: Implications for tectonic models

Abstract: Zircon U‐Pb ages have been determined from various ophiolitic and associated rocks across the southern part of the New England Orogen in northern New South Wales. Ophiolitic rocks of the Weraerai terrane originated during the earliest Cambrian and are the oldest rocks known from this region. The Weraerai terrane is tectonically juxtaposed against Siluro‐Devonian Gamilaroi and Djungati terranes but no unambiguous pre‐Permian links exist between these three terranes. Age and geochemical characteristics of the Weraerai terrane are consistent with its interpretation as a low Ti‐tholeiitic supra‐subduction‐zone ophiolite, analogous to those within the Lachlan Fold Belt located to the west of the New England Orogen. A felsic volcanic rock which occurs as a tectonic block in melange along the Peel Fault zone is geochemically identical to intra‐oceanic island arc rocks of the Gamilaroi terrane, the only likely nearby source. U‐Pb ages for this rock indicate arc development in the Silurian. Zircon ages from a plag...

Sources of continental magmatism adjacent to the late Archean Kolar Suture Zone, South India: distinct isotopic and elemental signatures of two late Archean magmatic series

Abstract: Conspicuous Nd, Sr and Pb isotopic differences exist between the Archean gneiss terranes adjoining the suture at the Kolar Schist Belt, south India. These gneisses, which are the deformed equivalents of plutonic and volcanic rocks, have known or inferred igneous ages of 2630 to 2530 Ma. Initial isotopic ratios of Nd, Sr and Pb suggest that metaplutonic gneisses west of the Kolar Schist Belt were emplaced into, and variably contaminated by, an evolved continental crust that formed prior to 3200 Ma. Felsic metaigneous gneisses that occur as slivers on the western margin of the schist belt have an isotopic character similar to that of the metaplutonic rocks on the same side of the Kolar Schist Belt. On the east side of the Kolar Schist Belt the isotopic evidence suggests that the 2530 Ma granitic gneisses were not derived from or contaminated by an older continental crust. Their source probably evolved with a Nd isotopic composition similar to that of typical Archean mantle, but became light rare earth element enriched after 2900 to 2700 Ma. The inferred tectonic setting for the west side of the Kolar Schist Belt is an Andean continental magmatic arc. For the east side of the Kolar Schist Belt, a possible Phanerozoic analog is an evolved island arc, such as Japan.

The Lithoprobe corridor across the Vancouver Island continental margin: the structural and tectonic consequences of subduction

Abstract: The deep structure and tectonic history of the continental margin of southwestern Canada have been determined by phase I of the Lithoprobe program across south-central Vancouver Island and associated marine studies across the continental shelf and slope. This article reviews results from the marine portion of the corridor but also presents continuous onshore–offshore data and interpretation. The geophysical data include multichannel seismic reflection, seismic refraction, magnetics, gravity, bathymetry, sea-floor acoustic imagery, heat flow, and seismicity. There has been Ocean Drilling Program (ODP) coring and downhole measurements on the continental slope. The margin structure and Cenozoic tectonic history are dominated by the consequences of subduction. Two narrow terranes, the Eocene volcanic Crescent and the Mesozoic mainly sedimentary Pacific Rim, were emplaced along the coast at the time of north Pacific plate reorganization at 43 Ma. They provide the landward-dipping backstop to a large accretiona...

Nd isotopic evidence for juvenile crust in the Carolina terrane, southern Appalachians

Abstract: Nd isotopic analyses of whole-rock samples from the older portion of the Carolina terrane, one of the largest terranes in the Appalachian orogen, demonstrate that part of this terrane is composed of juvenile, mantle-derived crust. These data suggest that the terrane may not have originally been built upon old, evolved basement material but rather may have been built upon oceanic crust. A recent study by other workers demonstrates a more crustally evolved Nd isotopic signature for younger components of the Carolina terrane. These data may indicate that the terrane interacted with evolved crust at a later time, possibly by amalgamation with a more evolved crustal fragment before final accretion to Laurentia, rather than indicating a primary old basement. A juvenile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal fragment that formed in a continental margin setting — a scenario proposed to explain the high proportion of felsic volcanic rocks within the terrane. It is herein suggested that Carolina is a chemically evolved but isotopically juvenile crustal fragment, because it remained in an oceanic setting for an unusually long time. In this regard the Carolina terrane is similar to some of the large accreted terranes in the Canadian Cordillera, such as Wrangellia and Alexander. The presence of juvenile crust in the Carolina terrane documents that at least part of the southern Appalachian orogen is not composed solely of reactivated pre-existing continental crust. The importance of this part of the orogen in terms of the volume of juvenile Phanerozoic crustal material in North America may be larger than previously thought. However, until additional major Appalachian terranes have been isotopically characterized the volume of juvenile crust in the whole orogen remains unknown. The isotopic make-up of a terrane can be an important aspect of terrane analysis as different terranes may have significantly different isotopic compositions, while even widespread pieces of a single terrane should have very similar isotopic characteristics. The Nd isotopic data for the Carolina terrane form the beginning of an isotope database for terranes in the southern Appalachians.

Circum-Pacific mid-Cretaceous deformation and uplift: A superplume-related event?

Abstract: Evidence of short-lived episodes of mid-Cretaceous deformation, metamorphism, uplift, and hiatus in sedimentation is widespread in the Lower Cretaceous rocks that bordered the Cretaceous Pacific basin. I present a model in which these coeval but widely spaced events are linked to increased ridge-push force at subduction zones triggered by large-scale upwelling of mantle associated with the mid-Cretaceous superplume. This model can account for mid-Cretaceous terrane accretion and ophiolite obduction events and can explain brief compressional phases in predominantly extensional orogenies as recognized in Alaska, British Columbia, California, western South America, and West Antarctica.

Geochemical and Nd/Pb Isotopic Evidence for the Provenance of the Early Proterozoic Virginia Formation, Minnesota. Implications for the Tectonic Setting of the Animikie Basin

Abstract: The Early Proterozoic Animikie Basin is located at a major crustal boundary between Archean crust of the Superior craton and penecontemporaneous Early Proterozoic crust of the Wisconsin magmatic terranes. The provenance of sedimentary rocks within the Animikie Basin constrains the tectonic evolution of the Early Proterozoic margin of the Superior craton. Published geological, petrographic, and Nd-Pb isotopic data indicate that the Pokegama Quartzite (lower passive margin unit of the Animikie Group) has a Late Archean provenance. In contrast, the Virginia Formation (upper deep water turbidite-shale unit of the Animikie Group) was derived from dominantly Early Proterozoic sources, with isotopic compositions similar to igneous rocks of the Wisconsin magmatic terranes. Shales of the Virginia Formation have Nd depleted mantle model ages 2.14 to 2.35 Ga, and $\mu_{1}$ of 7.81 to 8.09. Ashes within the Virginia are more evolved geochemically than shales and have younger model ages of 1.86 and 1.99 Ga, and simila...