Accretion Of Island Arcs Research Articles

U–Pb dating of the Verkniy Ufaley intrusion, middle Urals, Russia: a minimum age for subduction and amphibolite facies overprint of the East European continental margin

The Palaeozoic Uralian orogen formed by accretion of island arcs and microcontinents along the eastern margin of the East European platform. In the middle Urals, a promontory of the East European platform, the Ufaley Complex experienced eclogite facies conditions followed by an amphibolite facies overprint. The Verkniy Ufaley intrusion crosscuts the amphibolite facies foliation in the East Ufaley Complex and thus postdates the two metamorphic events. A concordant U–Pb age of 316±1 Ma obtained on magmatic titanite dates the intrusion of the pluton and provides a minimum age for subduction and subsequent amphibolite facies metamorphism of the East European continental margin. Zircon yields discordant age data that reflect the involvement of Proterozoic crustal sources.

New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laurentia

A question regarding the 1900–1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40 Ar/ 39 Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes (F 1 ) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest–trending upright F 2 folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40 Ar/ 39 Ar hornblende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is ∼50–60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810–1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780–1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

Geochemistry of neoproterozoic granitoids from the Axum area, northern Ethiopia

The granitoids of Chila, Rama, Medebay and Mereb cover ca 470 km 2 and represent 22% of the exposed Precambrian basement rocks of the Axum area. They are circular, sub-circular and elliptical in shape and appear to have been emplaced syn/late to post tectonically, generally show calc-alkaline affinities and are similar to plutonic rocks generated in subduction-related environments. The Chila Granitoid (800 Ma), which is composed of granodiorite, diorite and tonalite with subordinate gabbro and granite, appears to represent the earliest granitoid magmatism in the area and is characterised by a wide range of SiO 2 (54–71 wt%), high Al 2O 3 (15–19 wt%), slight to moderate LREE enriched patterns [(La/Yb) N = 10 to 28], slight Eu anomalies and a low Rb/Sr ratio (average 0.07). At comparable silica values, the Chila samples are similar to the associated island-arc volcanics, except that they have higher Al 2O 3, Na 2O, Sr and Rb contents. The Chila Granitoid was superseded (around 750 Ma) by the Rama and Medebay Granitoids, which are predominantly composed of granodiorite and monzogranite, though with a broad compositional spectrum extending from gabbro to leuco syenogranite. These granitoids are relatively enriched in incompatible element contents (K, Th, U, Rb), slightly depleted in REE, and highly depleted in Nb and Ti. These rocks have higher SiO 2 values, are more potassic and are chemically similar to a continental margin batholith. They may have originated either as final products of a mature arc environment, or during a massive fusion event following accretion of island-arcs. The granitoid magmatism in the Axum area ceased at around 600 Ma with the emplacement of the post-tectonic Mereb Granite, which shows a strong enrichment in Ba, Rb, Th and LREE and, by contrast, a depletion in Nb and Ti.

The Sholl Shear Zone, West Pilbara: evidence for a domain boundary structure from integrated tectonostratigraphic analyses, SHRIMP UPb dating and isotopic and geochemical data of granitoids

The Pilbara Block provides a record of Archaean continental growth involving the tectonic accretion of outboard island-arcs and collisions with other continental-scale fragments. This record of continental growth is balanced by breakup and strike-slip dismemberment of the continent. New SHRIMP UPb in zircon ages and SmNd data provide evidence in the West Pilbara which demonstrates that subduction-related and tectonic-accretion processes at the western margin of that ancestral continent between 3.15-2.78 Ga were coeval with, and genetically related to, crustal-scale tectonics and basin formation inboard of that margin. The tectonic division of the West Pilbara is defined by integrated tectonic analyses, geochronology, geochemistry and isotopic analyses. Geochronological studies clearly indicate that the western Pilbara comparises two domains with different recorded geohistories, whereas geochemistry and isotopic systematics reflect the changing tectonic regimes through time. In combination, these studies allow the development of a reconstruction of the relative positions of the domains through time on the western margin of the Pilbara Block. The supracrustal rocks of the northern Roebourne Lithotectonic Complex (Domain 6 in a Pilbarawide scheme) were formed in an island arc setting, facing an ocean to the north-west, prior to 3260 Ma, the time of emplacement of voluminous granitoids into the complex. In contrast, the supracrustal rocks of the southern Sholl Belt (Pilbara Domain 5) were formed in a back-arc setting behind a north-west-facing arc between 3125 and 3112 Ma, with more-or-less synchronous granite emplacement at about 3115 Ma. The two domains were tectonically juxtaposed, between 2991 and 2925 Ma, by the Sholl Shear Zone, a largely sinistral shear zone, with subsequent volcanism in both domains to about 2925 Ma. The Roebourne Lithotectonic Complex (Domain 6) is interpreted to be an allochthonous terrane, which formed north-east relative to its present position, but indigenous to the Pilbara Block rather than an exotic terrane. The East Pilbara is interpreted to have acted as a cratonic hinterland during the convergent margin tectonics that affected the two West Pilbara domains.

Landsat MSS imagery of a Lower Cretaceous regional dyke swarm, Damaraland, Namibia: a precursor to the splitting of western Gondwana : Lord J., Oliver G.J.H. & Soulsby J.A., International Journal of Remote Sensing, 1996, 17/15 (2945–2954)

The rocks of the Wadi Umm Gheig/El-Shush area in the central Eastern Desert of Egypt form part of the Nubian Shield, a component of the Neoproterozoic Pan-African Orogeny. The rocks have been divided into three units: (i) low-grade metamorphosed rocks, which consist of metavolcanic rocks interleaved with slices of ophiolitic melange; (ii) high-grade metamorphic rocks, which consist of syn-tectonic granitoids; and (iii) post-tectonic granites, which intrude into both the low- and high-grade rocks.Three distinct tectonic and two magmatic events have been deduced from the structural analysis of the area. These are listed in chronological order: (i) the formation of the major D1 sinistral strike-slip El-Shush Shear Zone, which occurs within the granitoid rocks (six individual granitoid bodies, all now intensely sheared, are thought to have been intruded into the active El-Shush Shear Zone); (ii) the emplacement of the metavolcanic rocks over the granitoid rocks by major D2 thrusting along a low angle dip-slip shear zone; (iii) upright open folding of the rocks during D3; and (iv) the intrusion of late stage granites, which are virtually undeformed.A model for the tectonic evolution of the study area is proposed. It is argued that the major strike-slip shear zone of the region is possibly related to island-arc accretion and that the various granitoid rocks were intruded along this active strike-slip zone. During the collision associated with island-arc accretion, ophiolite sheets were interleaved with the volcanic rocks and together were thrust over the granitoid basement. Field observations show that an important episode of folding occurred after thrusting and that the folds possess an axial planar fracture cleavage. They are, therefore, the result of a sub-horizontal tectonic compression rather than of diapirism as previously suggested. Thus, the domal structures of the Eastern Desert may be the result of folding rather than diapirism.

Mechanism and sequence of assembly and dispersal of supercontinents

The term continent implies a major land mass, with adjacent shelves and possessing one or more internal stable regions known as cratons. In geological time very large land masses had originated as supercontinents and superoceans formed outboard of these. Motions of former cratons and continents have been investigated paleomagnetically and by tectonic tracing. Thus, supercontinents were formed and fragmented in the process of continental drift and ocean spreading. The growth of supercontinents is the consequence of collision with other continents, smaller continental fragments, as well as island arcs, accretionary prisms, and a variety of oceanic plateaux. Dispersal of supercontinents occurred as fragmentation of larger continental masses into smaller blocks took place. There is some evidence that growth and dispersal of supercontinents developed periodically every few hundred million years. The best known relatively young dispersion of a supercontinent is that of Pangea. However, the amalgamation of this supercontinent was more complex and some of its aspects are disputed among those who invoke normal plate tectonic processes and those who suggest the operation of superplumes. The amalgamation of continental fragments occurred by: (i) the development of cold subsiding regions in the mantle; (ii) the adherence of island arcs: or, (iii) the movement of diverse terranes and their collision and accretion to continental masses and those to each other. Several similar scenarios have been suggested in Archean and Proterozoic times. At present Archean plate tectonics are still controversial, but abundant evidence suggests that collisional and accretionary events occurred even then. In the Lower-Middle Proterozoic times amalgamation and accretion became significant, while in the Upper Proterozoic other new cratons originated, possibly by island arc accretion. It is suggested that the break up of continental plates as a consequence of distentional events was prompted by convective changes in the mantle.

Formation of Neoproterozoic metamorphic complex during oblique convergence (Eastern Desert, Egypt)

Major portions of the Pan-African Orogen in the Eastern Desert of Egypt were formed by island-arc accretion in the Neoproterozoic. These areas are characterized by their lack of major crustal thickening. Metamorphic core complexes occur parallel to the strike of the Eastern Desert Orogen. These domes exhibit polyphase metamorphism and deformation in contrast to the structurally overlying nappes which include ophiolitic melanges and island-arc volcanic rocks. These nappes show northwest directed, orogen-parallel thrusting in the internal parts and west to southwest directed imbrication in the external parts of the orogen. Structures related to exhumation of the metamorphic core complexes partition into different displacement paths localized within a crustal-scale wrench corridor of the Najd fault system. Northwest trending orogen-parallel, sinistral strike-slip faults define the western and eastern margins of the domes. North and south dipping low-angle normal faults developed along the northern and southern margins of the domes and form extensional bridges between them. 40 Ar 39 Ar ages obtained from syntectonic muscovites within the shear zones gave Neoproterozoic ages of 595.9±0.5 and 588.2±0.3 Ma. The synchronous activity of strike-slip and normal faults suggests a regional east-west shortening which was accomodated by deep-level basal decollement beneath the metamorphic core complexes and a coeval northwest-southeast, orogen-parallel extension. This extension was accompanied by intramontane molasse sedimentation and emplacement of calc-alkaline plutons. Since the rapid exhumation of gneisses in the core complexes cannot be explained by thickening of the crust, the authors favour a model which calls for enhanced heat flow along the Najd fault system which would have enabled the formation of syn-extensional plutonism and triggered the exhumation of the metamorphic core complexes. Lateral buoyancy forces were concentrated within the Najd wrench corridor and enabled orogen-parallel extension.

Early Paleozoic Island arc accretion to the North China craton and the Shang Dan fault zone: A major paleoplate boundary in eastern Asia

The early Paleozoic tectonic evolution of part of the Qinling‐Dabie‐Shandong orogenic belt is reconstructed on the basis of the results of field mapping, geochemical, and geochronological studies. The Qinling orogenic belt separates the Archean‐Proterozoic North China craton (NCC) from the South China craton (SCC, also Yangtze craton). The study area is located about 90 km southwest of the city of Xi'an. Three lithologically distinctive zones, separated from each other by fault zones, can be distinguished. Passive continental margin deposits of the NCC (zone 1) have εNd(t) values between −6.16 and −12.54 and TDM ages between 1.29 and 2.20 Ga. They are separated from an early Paleozoic island arc sequence (zone 2) by the sinistral Chenche fault. Mafic volcanic lithologies in the southern part of zone 1 are associated with the development of a passive continental margin in late Proterozoic time. Similiar lithologies of zone 2 have geochemical characteristics of an oceanic island arc. Accretion was accompanied by a strong deformation event between early Ordovician and late Silurian time. Magmatic intrusions in zone 2 and the southern part of zone 1 are associated with a short‐lived magmatic arc that developed above a north dipping subduction zone on the modified continental margin of the NCC in late Silurian to early Devonian time. The εNd(t) values of felsic intrusions from the northern part of zone 2 decrease in a northern direction from 1.78 to −1.05. Mafic to intermediate intrusions in the southern part of zone 2 have positive εNd(t) values from 1.28 to 2.04 as well as low 87Sr/86Sr(i) values between 0.705 and 0.706 indicating an increasing mantle component toward the south. Low‐grade metamorphic pelites are separated from the island arc sequence of zone 2 by a ≤2 km wide Shang Dan shear zone. Isotopic studies show strong similarities between a granite sample from zone 3 and samples from the SCC. 87Sr/86Sr(i) values increase from 0.709 to 0.713, whereas εNd(t) values decrease from −1.90 to −5.34 in a southern direction. The shear zone that separates zone 2 from 3 is of fundamental importance and marks the boundary between the NCC and the SCC. The results presented in this paper document a previously unknown early Paleozoic tectonic evolution of the Qinling orogenic belt and delineate one of the fundamental paleoplate boundaries in eastern Asia.

Seismic refraction measurements within the Peninsular terrane, south central Alaska

We present an interpretation of crustal seismic refraction data from the Peninsular terrane, one of the many exotic terranes that have been accreted to the continental margin of southern Alaska in the past 200 m.y. A seismic refraction line was collected along the Glenn Highway in the Copper River Basin of south central Alaska in 1984 and 1985, as part of the U.S. Geological Survey Trans‐Alaska Crustal Transect (TACT) program. P wave velocities of 2.7–3.5 km/s and thicknesses of 1–2 km characterize post‐Lower Jurassic sedimentary rocks that underlie most of the seismic refraction line. An average crustal velocity structure includes the following five velocity divisions. Beneath the sedimentary rocks lie 1–2 km of 4.0–4.6 km/s materials, correlating with andesitic volcaniclastic sedimentary rocks and lava flows of the Lower Jurassic Talkeetna Formation. Below these rocks, seismic velocity increases rapidly, from 5.0 to 6.1 km/s, in 2–3 km. At 7–8 km depth, velocity jumps to 6.3 km/s and increments to 6.6 km/s by 10–12 km depth. Velocities increase from 6.8 to 7.0 km/s between 12 to 20 km depth. At about 22 km depth, a jump in velocity from 7.0 to 7.4 km/s is inferred but is poorly resolved. Depth to the Moho discontinuity could not be determined from our data. The absence of clear PmP reflections may indicate that Moho is deeper than 40 km. Data from two offset shotpoints northeast of the line and within the Wrangellia terrane constrain the deep structure transition between Peninsular and Wrangellia terranes. The 6.3–6.6 km/s material thickens to the northeast, toward the suture between Peninsular and Wrangellia terranes, but southwest of its mapped trace at the West Fork fault. Peninsular terrane crustal structure appears dissimilar to that of continental interiors. It is similar to velocity structures determined for accreted island arc fragments in California, such as the basement of the Great Valley and the Klamath Mountains.

Kinematics of arc‐continent collision in the eastern Central Range of Taiwan inferred from geomorphic analysis

Active arc‐continent collision in Taiwan is an example of a typical collisional orogen, with a deformed continental margin (Foothills Belt, Hsueshan Range, and Central Range) separated from an accreted island arc (Coastal Range) by a collapsed forearc basin (Longitudinal Valley). The geomorphology of the eastern Central Range reflects spatial and temporal variations in uplift rate and pattern along the orogen during late Quaternary time. Digitized drainage basin perimeters, stream channels, and the mountain‐piedmont junction of a 200‐km‐long segment of the eastern Central Range provide data for calculating quantitative geomorphic parameters. Statistical analyses of these parameters define four domains of distinct landscape character. The northern domain extends northward along the coast from the northern end of the Longitudinal Valley (LV) and has landscape characteristics suggesting uplift near the mountain front. The central two domains are adjacent to the LV and show evidence of uplift concentrated in the interior of the range, away from the front. The boundary between the two central domains occurs near the middle of the LV. The southern domain extends southward along the coast from the southern end of the LV and has characteristics suggesting a transient response to increased uplift rate. Analysis of fluvial terrace profiles in the Central Range adjacent to the southern half of the LV indicates a change in uplift pattern through time, with migration of maximum uplift rate from the mountain front to the interior of the range. This change probably occurred just prior to late Pleistocene time.

Geochemistry and evolution of late Archean plutonism and its significance to the tectonic development of the Slave craton

The intrusion of voluminous Late Archean plutonic rocks is the final event leading to the stabilization of the Slave craton. Three groups of plutonic rocks are recognized in the central part of the province: Group 1 (∼2689–2650 Ma) includes high-Al 2O 3 (Olga Suite) and low-Al 2O 3 (Gondor and Wishbone Suites) trondhjemites and diorite (Central Belt Suite) all of which are temporally or spatially associated with volcanic rocks; Group 2 (∼2610–600 Ma) includes syn- to late deformation hornblende-biotite monzodiorite through granodiorite (Concession Suite) and trondhjemites (Siege Suite); and Group 3 (∼2599–2580 Ma) consists of post-deformational muscovite-biotite granites (Contwoyto Suite) and biotite granites (Yamba Suite). Volcanic and associated mafic plutonic rocks are calc-alkaline and have trace element characteristics diagnostic of rocks formed in modern subduction zone settings. The high-A1 20 3 and low-A1 20 3 trondhjemite suites may be derived by high and low pressure partial melting of mafic crust, respectively. Rocks of the Concession Suite are calc-alkaline, metaluminous and have high contents of Sr, Ba and LREE, high (Ce/Yb) Nratios and negative Nb anomalies. Parental magmas are derived from melting enriched mantle sources and related rocks were produced through assimilation-fractional crystallization and mixing processes in the crust. The younger granites are derived from mixing and homogenization of mantle- and crust-derived materials (Yamba Suite) and the partial melting of metasedimentary protoliths (Contwoyto Suite). Volcanic and Group 1 rocks are interpreted to be remnants of an allochthonous island arc terrane which was accreted to a continental block. The secular evolution in the composition of Group 2 and 3 plutonic rocks reflects a change from dominantly mantle- to crustal-derived magmatism during and following collision. Thickening of the lithosphere during collision, followed by lithospheric detachment caused melting of previously subduction-modified mantle beneath the accreted island arc terranes and generation of magmas parental to the Concession Suite. Intrusion of mantle-derived magmas, in combination with crustal thickening resulted in high T, low P metamorphism, crustal melting and the generation of the two Group 3 plutonic suites.

Accretion of Japanese island arcs and implications for the origin of Archean greenstone belts

The present‐day region of arc‐arc collision between the Izu‐Bonin Arc and mainland Japan (Honshu Arc) in the western Pacific (called the Izu Collision Zone or ICZ) provides a useful kinematic model for the development of many Archean greenstone belts. The compositional assemblage, thermal structure and structural style of the crust in the ICZ are very similar to those inferred for many Archean greenstone belts. In both cases, thick, young and flaky arc crust with a high heat flux is in collision with another crustal block. The ICZ has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms. Many Archean greenstone belts show similar geological features, and we suggest that the incremental imbrication of the crust was a common tectonic style in some Archean convergent margins.

Archean tectonic and metallogenic evolution of the superior province of the canadian shield

Abstract The Superior Province of the Canadian Shield has mainly experienced an Archean tectonic history, comprising several cycles of volcanism, plutonism and tectonism. Although the rocks of the Superior Province span more than 500 million years of Archean time, most mineral deposits formed in an interval of 100 million years or less, corresponding to a period of accretion of oceanic and pre-existing continental fragments in the late Archean at ∼2.7 Ga. In accord with recent tectonic syntheses which argue for the accretionary origin of the Superior Province, the metallogenic history of the Superior Province can also be interpreted in the same context provided secular anomalies in the early history of the earth are acknowledged for relatively unique deposit types such as komatiitic nickel deposits and voluminous iron formations. Volcanogenic massive sulphide, Iode gold and granitoid-associated deposits, as well as NiCu, PGE, Cr and Ti deposits associated with mafic/ultramafic intrusions, are readily interpreted in terms of the construction and accretion of late Archean island arcs.

Late Proterozoic evolution of the northern part of the Hamisana zone, northeast Sudan: constraints on Pan-African accretionary tectonics

New structural data from the Red Sea Hills, northeast Sudan, add constraints to accretionary tectonic processes that acted in the Arabian-Nubian shield during the late Proterozoic. Geometric constraints provided by field data are used together with Landsat Thematic Mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages. In contrast to interpretations reported elsewhere, these studies indicate that the Hamisana zone, a prominent structural feature in the northern Red Sea Hills, post-dated terrane accretion. It is characterized by east-west crustal shortening and by steep folds and thrust faults. Calc-alkaline magmatism was contemporaneous with and outlasted main phase deformation, thus deformation occurred in an intra-arc setting. Thrust faults were reactivated after east-west contraction, compromising their usefulness in delineating 'sutures' or as piercing points across older faults. Pan-African accretionary processes may have been analogous to Phanerozoic ophiolite and island arc accretion in the western North American Cordillera, where penetrative deformation occurred in response to periodic intra-plate shortening events rather than an ultimate collision of unrelated crustal fragments. Such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long-lived convergent margin setting, consistent with the long-standing observation that Pan-African magmatism became more evolved with time. This view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening, in contrast to models where crustal shortening results directly from collision of a volcanic arc, microcontinent, or other buoyant features.

Common Pb systematics of Precambrian granitic rocks of the Nubian Shield (Egypt) and tectonic implications

We report the Pb isotopic compositions for alkali feldspars separated from 34 granitic rocks from the late Proterozoic assemblages of the Eastern Desert and the Sinai Peninsula (Egypt) in the Nubian segment of the Arabian-Nubian Shield. The Eastern Desert shows a range of initial Pb isotopic compositions ( 206 Pb/ 204 Pb = 17.375-19.176; 207 Pb/ 204 Pb = 15.462-15.629; 208 Pb/ 204 Pb = 37.023-38.349) that extend from model mantle toward upper-crustal values. Initial Pb isotopic compositions do not define geographic provinces, except at Aswan and surrounding areas, where the most radiogenic values were obtained (Aswan, 207 Pb/ 204 Pb = 15.611; Gebel El Hudi, 207 Pb/ 204 Pb = 15.629; and Wadi Mariya, 207 Pb/ 204 Pb = 15.615). Samples from these areas are separated from the less radiogenic samples to the east by trails of serpentinites that we interpret from Landsat thematic mapper images as the extension of the Allaqi-Heiani ophiolitic belt. Our data support previous interpretations of the belt as a suture location and suggest that its western margin might coincide with the boundary between the Nubian Shield and the old African continent. We interpret the Pb isotopic compositions from the Eastern Desert and the previously identified Group I and II Pb (Stacey and others, 1980) from the Arabian-Nubian Shield to indicate various degrees of mixing between mantle-derived, juvenile oceanic component(s) and pre-Pan-African crustal material. On 207 Pb/ 204 Pb- 208 Pb/ 204 Pb plots, the feldspar and galena data display variations characteristic of modern island-arc magmas, consistent with models that relate Shield assembly to accretion of island arcs. The crustal Pb isotopic signature of sample from Aswan and surrounding areas is best explained by interaction of arc magmas with an overriding crustal plate, whereas the less radiogenic nature of the Eastern Desert Pb9s is probably related to subduction of sediments derived from adjacent continent or deposition of these sediments in near-arc basins. Accounting for recycled pre-Pan-African crustal material in the Shield results in normal growth rates (0.44 km 3 /yr), using reasonable assumptions for initial Pb isotopic compositions and concentrations.

Paleomagnetism of the Moreton's Harbour Group, northeastern Newfoundland Appalachians: Evidence for an Early Ordovician Island Arc near the Laurentian Margin of Iapetus

Paleomagnetic results have been obtained from mafic volcanic units in the upper part of the Moreton's Harbour Group, which is part of an accreted island arc terrane that is preserved in the Notre Dame Bay subzone of the Central Mobile Belt of the Newfoundland Appalachians. Detailed thermal and alternating field demagnetization reveals a stable characteristic component of magnetization, carried by magnetite, at a large number of sites in pillow basalts and coeval basaltic dikes. The intrusives contain both polarities, and a primary age for the characteristic magnetization is indicated by a positive contact test for one of the dikes, and by a positive structural test involving a correction for block rotations (strike‐correction). The overall mean direction for the characteristic component after tectonic (tilt and strike) correction (flows and intrusives: D = 171°, I = +22°, k = 22.6, α95 = 6.5°, pole 29°N, 135°E; flows: D = 166°, I = +22°, k = 33.2, α95 = 6.5°) corresponds to an Early Ordovician paleolatitude for the arc of 11°S, which is indistinguishable from the expected paleolatitude of the North American margin. This implies that the arc formed at or near the margin of the craton. In contrast, the Avalon block, which formed the other margin of Iapetus, was widely separated from the arc and the craton at this time. The Early Ordovician paleolatitude of the arc terrane supports a tectonic model in which coeval ophiolitic sequences inboard of the arc were formed in a narrow ocean basin between the arc and the craton. Subsequent convergence between the arc and Laurentia in Middle Ordovician time resulted in closure of this narrow ocean basin and obduction of the back‐arc basin oceanic crust onto the ancient margin of the craton, thus giving rise to the Taconic orogenic pulse in the Newfoundland Appalachians.

Deep structure of an arc‐continent collision: Earthquake relocation and inversion for upper mantle P and S wave velocities beneath Papua New Guinea

We examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction, by analyzing the seismicity and upper‐mantle velocities in the New Guinea arc‐continent collision. Earthquakes in the Papua New Guinea (PNG) region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities, using P and S wave arrival times recorded by seismographs in PNG. A total of 957 well‐recorded earthquakes were chosen for use in three‐dimensional velocity inversions from an initial catalog of 12,960 events, based on the stability of initial hypocenter relocations. Constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters, and to exploit a priori knowledge of the geometry of velocity anomalies. Spherical geometry was used to accurately calculate long ray paths (up to 1500 km in length). Most of the ∼700 best located hypocenters from the PNG data set are in the upper mantle beneath the Finisterre‐Huon (FH) ranges (the newly accreted island arc terrane), beneath the Papuan Peninsula, and in the New Britain seismic zone (an oceanic subduction system). These hypocenters show a well‐defined seismic zone dipping vertically or steeply to the north beneath the northern FH ranges from 125‐ to 250‐km depth, continuous along strike with the New Britain seismic zone to the east (which shows earthquakes to 600 km depth). The steeply dipping intermediate‐depth seismicity flattens near 100 km depth beneath the FH ranges and forms a subhorizontal seismic zone beneath the FH ranges. By contrast, seismicity continues upward to the surface beneath New Britain where the arc has not yet collided with New Guinea. The 100‐km‐deep flat zone does not continue south of the FH ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane, and no clear evidence for a south‐dipping seismic zone could be found. The relationship of the flat part of the slab 100 km beneath the FH ranges and surface faulting is difficult to discern; the apparent surface suture bounding the south side of the FH ranges is 100 km directly above the southern termination of intermediate‐depth seismicity. Nowhere in PNG could clear evidence for arc polarity reversal be found from seismicity. All of the well‐located subcrustal earthquakes beneath the Papuan peninsula lie in a narrow horizontal line between 125‐ and 175‐km depth that follows the center of the peninsula, and are distinctly separated from the FH seismic zone to the north. These events do not require a southward subducting plate beneath the Papuan peninsula. An alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere. Information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals. Allowing for only crustal heterogeneity reduces travel time residual variances by 40% from a simple one‐dimensional structure, while allowing for additional mantle heterogeneity results in ≤ 10% further reduction for the various parameterization s attempted here. Inversions with several different block geometries show low velocities at 35‐ to 171‐km km depth beneath the Bismarck Sea back‐arc basin and high velocities just south of the intermediate depth FH seismic zone. The pattern of lateral heterogeneity supports the inference from seismicity of north‐dipping slabs beneath both northeast New Guinea and New Britain.

Changing source regions of magmas and crustal growth in the Trans-Himalayas: evidence from the Chalt volcanics and Kohistan batholith, Kohistan, northern Pakistan

The Kohistan batholith and Chalt volcanics form a major part of the Kohistan island arc terrane in northern Pakistan. They record some 70–80 Ma of magmatism within the northwestern Himalayan orogen; the Chalt volcanics are Albian-Aptian (mid-Cretaceous), and currently available Rb Sr age data for the batholith indicate an intrusive age span of 102 to 30 Ma. The volcanics are composed of medium-K, calc-alkaline andesites and low-K, high-Mg, tholeiites, some of which have boninitic and basaltic komatiitic chemistry. Three intrusive stages are recognised in the batholith: (1) (110–90 Ma) comprises low-K trondhjemites and medium-high-K calc-alkaline gabbro-diorites with associated hornblendite cumulates; (2) (85–40 Ma) comprises low-high-K, calc-alkaline gabbro-diorites (with hornblendite cumulates, granodiorites and granites; and (3) (circa 30 Ma) comprises biotite ± muscovite ± garnet leucogranites. 87Sr/ 86Sr 0 for the batholith range between 0.7039 and 0.7052. Four magmatic source regions (Sce 1–Sce 4) have been identified. Sce 1 is a variably metasomatised mantle wedge situated above an active subduction zone during stages 1 and 2. Sce 2 is a harzburgite depleted with respect to major elements and incompatibles, whilst retaining “subduction-related” trace element ratios. Sce 2 melted during stage 1 within a fore-arc position during the subduction of young, hot oceanic crust to provide the necessary extra thermal input for harzburgite anatexis. Melts from Sce 2 produced the high-Mg tholeiitic volcanics. Sce 3 melted during stage 1 to produce the low-K trondhjemites, and it is envisaged as incompatible element-depleted primitive arc crust with a similar composition to the depleted Chalt volcanics. Sce 4 melted at 30 Ma during stage 3 as India underthrust Eurasia: frictional heating and dehydration of the Indian plate caused crustal anatexis within the deep Kohistan arc and these melts formed the stage 3 leucogranites. The low 87Sr/ 86Sr initial ratios for the great bulk ( > 95%) of the batholith indicate that it represents a major juvenile addition to the continental crust. This conclusion has important implications for crustal growth theories of island arc accretion and batholith underplating.

Tectonic evolution in the Archaean and Proterozoic

Evidence for continental motion since at least 3.3 Ga and at minimum average velocities comparable to those of today leaves little doubt that Precambrian tectonic styles are essentially comparable to those of the Phanerozoic plate-tectonic regime and were governed by deformation along and within continental plates. The detailed mechanisms for these motions, however, remain a matter of considerable dispute. For the earliest Archaean, from > 4.2 Ga to perhaps 3.9 Ga, plate tectonics appear unlikely because of thin and “soft” lithosphere: high heat flow probably favoured global hotspot activity with crustal growth of Iceland type through vertical magmatic accretion. The tectonic environment for the generation of about 3.9-2.5 Ga granite-gneiss-greenstone terrains with voluminous production of tonalite-trondhjemite-granodiorite magmas is still uncertain, and both plate margin and intraplate scenarios are possible. Deformation styles in greenstones and adjacent high-grade orthogneisses with intercalated shallow-water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking. This probably resulted from collisional and/or rotational motion and produced significant intracrustal melting as early as 3.9 Ga. By the end of the Archaean, and locally much earlier, crustal thicknesses reached 35–50 km or more, and the extraordinary high global crust-production rate between about 2.8 and 2.5 Ga may be ascribed to both subduction-related lateral accretion and to extensive magmatic underplating, which finally led to large, stable cratons. The apparent worldwide gap in major tectonic and magmatic activity during the early Proterozoic between about 2.5 and 2.2 Ga is an artifact due to lack of precise age data; both intracontinental as well as arc-type magmatic rocks were produced, though only locally. The voluminuous and worldwide crust-formation event at about 2.1-1.8 Ga provides the first evidence of modern-style plate tectonics, many magmatic rocks, such as in the Canadian Trans-Hudson orogen and in the Svecofennian of southwest Finland, have arc-type geochemical signatures, and both obducted ophiolites and blueschist assemblages have recently been discovered. In addition, the Proterozoic is characterized by long, linear mobile belts whose origin and evolution are still under debate. Many of these belts have tectonic features comparable to Phanerozoic collisional orogens, but in Australia and parts of southern Africa some Proterozoic belts appear to have formed through crustal stretching without lithospheric separation, and subsequent shortening leading to orogenic deformation may have been due to delamination and crustal subduction. Towards the end of the Proterozoic, evidence for modern-style plate tectonics becomes abundant through the recognition of island arc accretion, ophiolite obduction, foreland thrust and fold belts and exotic terranes. One of the best documented example for this tectonic environment is the Arabian-Nubian shield. Although it is doubtful whether modern plate tectonic settings can serve as analogues for all Precambrian tectonic styles, it is clear that global crustal evolution since the early Archaean was governed by the motion of plates which repeatedly came together to form supercontinents and then dispersed again. We shall not understand the detailed mechanism of Precambrian crustal tectonics before we learn how to interpret recent large-scale crustal deformation and its causes.

Ophiolites in Northeast and East Africa: implications for Proterozoic crustal growth

The presence of ophiolite complexes in NE and E Africa has been documented using Landsat, field and geochemical studies. The present work identifies five major ophiolitic sutures in NE Africa, while plate reconstruction of Africa and Madagascar suggests a possible sixth ophiolite belt to the east. The ophiolites are considered to be remnants of supra-subduction zones and back-arc basins. The ophiolites are dismembered, and their mode of occurrence varies widely resulting in different structural relationships. In Western Ethiopia, the Yubdo complex is formed of harzburgite which grades into a cumulate sequence of ultramafic and gabbroic rocks and metabasalts. In Kenya, the Baragoi complex is formed of tectonized harzburgite with dunite and chromite pods, a cumulate sequence of ultramafic and gabbroic units and a dyke unit. Trace element data from the Baragoi complex show a transitional mid-ocean ridge basaltic to island arc tholeiitic affinity, and the presence of boninites suggests a supra-subduction setting, while data from the Adola-Moyale belt (S. Ethiopia-NE Kenya) indicate an island-arc and MORB geochemistry, which developed in a back-arc tectonic setting. In Sudan, the Ingessana complex which has an island arc tholeiitic tectonic affinity indicates development in a supra-subduction setting, while the trace element suggest that the Sol Hamed complex developed in a back-arc basin. These ophiolite belts represent sutures marking the position of island arcs extending to those in Saudi Arabia on a pre-Red Sea drift reconstruction and further south to Mozambique. The terrane delimited by a proposed Ingessana-Port Sudan suture and by the Adola-Moyale ophiolite belts as defined in this study encompasses an area in which the crust was derived from oceanic and island arc material. Relative age reconstruction across the Arabian-Nubian Shield in East Africa broadly indicates eastward younging of the ophiolite belts. Regional geological, tectonic and geochemical studies suggest rifting at c. 1200 Ma and subsequent convergence led to the development of intra-oceanic arcs and associated marginal basins in the north and narrow basins within the sialic basement gneisses further south in Kenya and Tanzania. This was followed by continent-continent collision which led to accretion of island arcs by gentle collision from the northeast in Saudi Arabia and severe crustal shortening in S Sudan, Kenya and SE Ethiopia as compared to Saudi Arabia, NE Sudan and N and W Ethiopia owing to oblique collision from the southeast.