Showing papers on "Terrane published in 2007"

Tectonic model for the Proterozoic growth of North America

Abstract: This paper presents a plate-scale model for the Precambrian growth and evolution of the North American continent. The core of the North American continent (Canadian shield) came together in the Paleoproterozoic (2.0–1.8 Ga) by plate collisions of Archean continents (Slave with Rae-Hearne, then Rae-Hearne with Superior) as well as smaller Archean continental fragments (Wyoming, Medicine Hat, Sask, Marshfield, Nain cratons). The resulting Trans-Hudson orogen was a collisional belt similar in scale to the modern Himalayas. It contains mainly reworked Archean crust, but remnants of juvenile volcanic belts are preserved between Archean masses. The thick, buoyant, and compositionally depleted mantle lithosphere that now underlies North America, although dominantly of Archean age, took its present shape by processes of collisional orogenesis and likely has a scale of mantle heterogeneity similar to that exhibited in the overlying crust. In marked contrast, lithosphere of southern North America (much of the conti nental United States) was built by progressive addition of a series of dominantly juvenile vol canic arcs and oceanic terranes accreted along a long-lived southern (present coordinates) plate margin. Early juvenile additions (Pembine-Wausau, Elves Chasmarcs) formed at the same time (1.84–1.82 Ga) the core was assembling. Following final assembly of the Archean and Paleoproterozoic core of North America by 1.8 Ga, major accretionary provinces (defined mainly by isotopic model ages) were added by arc-continent accretion, analogous to present-day convergence between Australia and Indonesia. Also similar to Indonesia, some accreted terranes contain older continental crustal material [Archean(?) Mojavia], but the extent and geometry of older crust are not well known. Accretionary provinces are composed of numerous 10 to 100 km scale terranes or blocks, separated by shear zones, some of which had compound histories as terrane sutures and later crustal-assembly structures. Major northeast-trending provinces are the Yavapai province (1.80–1.70 Ga), welded to North America during the 1.71–1.68 Ga Yavapai orogeny; the Mazatzal province (1.70–1.65 Ga), added during the 1.65–1.60 Ga Mazatzal orogeny; the Granite-Rhyolite province (1.50–1.30 Ga), added during the 1.45–1.30 Ga tectonic event associated with A-type intracratonic magmatism; and the Llano-Grenville province (1.30–1.00 Ga), added during the 1.30–0.95 Ga broader Grenville orogeny. During each episode of addition of juvenile lithosphere, the transformation of juvenile crust into stable continental lithosphere was facilitated by voluminous granitoid plutonism that stitched new and existing orogenic boundaries. Slab roll back created transient extensional basins (1.70 and 1.65 Ga) in which Paleoproterozoic quartzite-rhyolite successions were deposited, then thrust imbricated as basins were inverted. The lithospheric collage that formed from dominantly juvenile terrane accretion and stabilization (1.8–1.0 Ga) makes up about half of the present-day North American continent. Throughout (and as a result of) this long-lived convergent cycle, mantle lithosphere below the accretionary provinces was more hydrous, fertile, and relatively weak compared to mantle lithosphere under the Archean core.

Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia

Abstract: The Archean Pilbara Craton contains five geologically distinct terranes – the East Pilbara, Karratha, Sholl, Regal and Kurrana Terranes – all of which are unconformably overlain by the 3.02- to 2.93-Ga De Grey Superbasin. The 3.53–3.17 Ga East Pilbara Terrane (EP) represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3.53–3.43, 3.35–3.29 and 3.27–3.24 Ga. Each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3.72 Ga, and melting of crust to generate first tonalite-trondhjemite-granodiorite (TTG), and then progressively more evolved granitic magmas. In each case, plume magmatism was accompanied by uplift and crustal extension. The combination of conductive heating from below, thermal blanketing from above, and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust. These mantle melting events caused severe depletion of the subcontinental lithospheric mantle, making the EP a stable, buoyant, unsubductable continent by c. 3.2 Ga. Extension accompanying the latest event led to rifting of the protocontinent margins at between 3.2 and 3.17 Ga. After 3.2 Ga, horizontal tectonic forces dominated over vertical forces, as revealed by the geology of the three terranes (Karratha, Sholl and Regal) of the West Pilbara Superterrane. The c. 3.12-Ga Whundo Group of the Sholl Terrane is a fault bounded, 10-km-thick volcanic succession with geochemical characteristics of modern oceanic arcs (including boninites and evidence for flux melting) that indicate steep Archean subduction. At 3.07 Ga, the 3.12-Ga Sholl Terrane, 3.27-Ga Karratha Terrane and c. 3.2-Ga Regal Terrane accreted together and onto the EP during the Prinsep Orogeny. This was followed by development of the De Grey Superbasin – an intracontinental sag basin and widespread plutonism (2.99–2.93 Ga) as a result of orogenic relaxation and slab break off. Craton-wide compressional deformation at 2.95–2.93 Ga culminated with 2.91-Ga accretion of the 3.18 Ga Kurrana Terrane with the EP. This compression caused amplification of the dome-and-keel structure in the EP. Final cratonization was effected by emplacement of 2.89–2.83 Ga post-tectonic granites.

Tectonic Evolution of the North China Block: From Orogen to Craton to Orogen

Abstract: The North China Craton contains one of the longest, most complex records of magmatism, sedimentation, and deformation on Earth, with deformation spanning the interval from the Early Archaean (3.8 Ga) to the present. The Early to Middle Archaean record preserves remnants of generally gneissic meta-igneous and metasedimentary rock terranes bounded by anastomosing shear zones. The Late Archaean record is marked by a collision between a passive margin sequence developed on an amalgamated Eastern Block, and an oceanic arc–ophiolitic assemblage preserved in the 1600 km long Central Orogenic Belt, an Archaean–Palaeoproterozoic orogen that preserves remnants of oceanic basin(s) that closed between the Eastern and Western Blocks. Foreland basin sediments related to this collision are overlain by 2.4 Ga flood basalts and shallow marine–continental sediments, all strongly deformed and metamorphosed in a 1.85 Ga Himalayan-style collision along the northern margin of the craton. The North China Craton saw relative quiescence until 700 Ma when subduction under the present southern margin formed the Qingling–Dabie Shan–Sulu orogen (700–250 Ma), the northern margin experienced orogenesis during closure of the Solonker Ocean (500–250 Ma), and subduction beneath the palaeo-Pacific margin affected easternmost China (200–100 Ma). Vast amounts of subduction beneath the North China Craton may have hydrated and weakened the subcontinental lithospheric mantle, which detached in the Mesozoic, probably triggered by collisions in the Dabie Shan and along the Solonker suture. This loss of the lithospheric mantle brought young asthenosphere close to the surface beneath the eastern half of the craton, which has been experiencing deformation and magmatism since, and is no longer a craton in the original sense of the word. Six of the 10 deadliest earthquakes in recorded history have occurred in the Eastern Block of the North China Craton, highlighting the importance of understanding decratonization and the orogen–craton–orogen cycle in Earth history. The Archaean North China (Sino-Korean) Craton (NCC) occupies about 1.7 10 km in northeastern China, Inner Mongolia, the Yellow Sea, and North Korea (Bai 1996; Bai & Dai 1996, 1998; Fig. 1). It is bounded by the Central China orogen (including the Qinling–Dabie Shan–Sulu belts) to the SW, and the Inner Monglia–Daxinganling orogenic belt (the Chinese part of the Central Asian Orogenic Belt) on the north (Figs 1 and 2). The western boundary is more complex, where the Qilian Shan and Western Ordos thrust belts obscure any original continuity between the NCC and the Tarim Block. The location of the southeastern margin of the craton is currently under dispute (e.g. Oh & Kusky 2007), with uncertain correlations between the North and South China Cratons and different parts of the Korean Peninsula. The Yanshan belt is an intracontinental orogen that strikes east–west through the northern part of the craton (Davis et al. 1996; Bai & Dai 1998). The NCC includes several micro-blocks and these micro-blocks amalgamated to form a craton or cratons at or before 2.5 Ga (Geng 1998; Zhang 1998; Kusky et al. 2001, 2004, 2006; Li, J. H. et al. 2002; Kusky & Li 2003; Zhai 2004; Polat et al. 2005a, b, 2006), although others have suggested that the main amalgamation of the blocks did not occur until 1.8 Ga (Wu & Zhang 1998; Zhao et al. 2001a, 2005, 2006; Liu et al. 2004, 2006; Guo et al. 2005; Kroner et al. 2005a, b, 2006; Wan et al. 2006a, b; Zhang et al. 2006). Exposed rock types and their distribution in these micro-blocks vary considerably from block to block. All rocks .2.5 Ga in the blocks, without exception, underwent the 2.5 Ga metamorphism, and were intruded by 2.5–2.45 Ga granitic sills and related bodies. Nd TDM models show that the main crustal formation ages in the NCC are between 2.9 and 2.7 Ga (Chen & Jahn 1998; Wu et al. 2003a, b). Emplacement of mafic dyke swarms at 2.5–2.45 Ga has also been From: ZHAI, M.-G., WINDLEY, B. F., KUSKY, T. M. & MENG, Q. R. (eds) Mesozoic Sub-Continental Lithospheric Thinning Under Eastern Asia. Geological Society, London, Special Publications, 280, 1–34. DOI: 10.1144/SP280.1 0305-8719/07/$15 # The Geological Society of London 2007.

Northern Cordilleran terranes and their interactions through time

Abstract: In the 25 years since the first application of the terrane concept to the North American Cordillera and the introduction of the term “suspect,” a pattern of interterrane stratigraphic and intrusive linkages and shared isotopic and faunal elements has emerged. Far from being restricted to late, postamalgamation overlaps, these linkages can be as old as the oldest rocks within the terranes. In the Canadian Cordillera, these linkages give a coherent sense to terranes that otherwise might appear to be a collection of isolated and unrelated fragments. Such observed linkages effectively eliminate some of the paleogeographic uncertainties that were previously inferred between adjacent terranes (although not necessarily with respect to the Laurentian continent) and highlight their common history. In light of these relationships, it is now possible to interpret terranes of the Canadian Cordillera in terms of shared geodynamic scenarios, such as repeated arc superposition on older arcs and/or basement and coexisting arc system components. A primary result of this analysis is that the Intermontane terranes represent one interrelated set of arcs, marginal seas, and continental fragments that once formed a Paleozoic to early Mesozoic fringe to North America, the peri-Laurentian realm. By contrast, the Insular terranes, along with the Farewell and Arctic-Alaska terranes, include crustal fragments that originated from separate sites within the Arctic realm in Paleozoic time.

Carboniferous granitic plutons from the northern margin of the North China block: implications for a late Palaeozoic active continental margin

Abstract: We report four late Palaeozoic zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages for granitic plutons from the Inner Mongolia Palaeo-uplift on the northern margin of the North China block. These cast a new light on the poorly understood tectonic history of the northern margin of the North China block and the Central Asian Orogenic Belt during the late Palaeozoic. The plutons have for a long time been considered to belong to the early Precambrian basement of the North China block. Our new SHRIMP U–Pb zircon dating of four plutons at Longhua, Daguangding, Boluonuo and Hushiha has yielded intrusive ages of 311 ± 2 Ma, 324 ± 6 Ma, 302 ± 4 Ma and 310 ± 5 Ma, respectively. Geochemical data suggest that these granitoids have a calc-alkaline, subduction-related I-type signature, indicating the existence of an Andean-style continental arc along the northern margin of the North China block during the late Palaeozoic. Our results also indicate that the Palaeo-Asian Ocean still existed during latest Carboniferous–earliest Permian time, and that the final collision between the southern Mongolia composite terranes and the North China block occurred later than c . 290 Ma. We suggest that the northern margin of the North China block was an active continental margin and the Inner Mongolia Palaeo-uplift is a deeply exhumed mid-crustal ‘root’ of a late Palaeozoic Andean-style continental arc.

Permo-Triassic Collision, Subduction-Zone Metamorphism, and Tectonic Exhumation Along the East Asian Continental Margin

Abstract: Convergent plate motion at ∼320–210 Ma generated the Tongbai-Dabie-Sulu (east-central China)-Imjingang-Gyeonggi (central Korea)-Renge-Suo (Southwestern Japan)-Sikhote-Alin orogen along the paleo-Pacific edge of cratonal Asia. This amalgamated belt reflects collision between the Sino-Korean and Yangtze cratons on the SW portion, and accretion of outboard oceanic arcs ± sialic fragments against the NE margin. Subducted Proterozoic-Paleozoic continental and oceanic crustal complexes underwent high- and ultrahigh-pressure metamorphism at low to moderate temperatures. Tectonic slices of sialic crust episodically disengaged from the downgoing plate and, driven by buoyancy, ascended rapidly to midcrustal levels from depths exceeding 90–200 km after continental collision in east-central China plus or minus Korea, and from ∼30–50 km after arrival of far-traveled oceanic terranes in SW Japan and the Russian Far East. On achieving neutral buoyancy and stalling out at 10–20 km depth, later doming, gravitation...

Crustal structure beneath the eastern margin of the Tibetan Plateau and its tectonic implications

Abstract: [1] Two crustal cross sections through the eastern margin of the Tibetan Plateau are jointly determined from deep seismic sounding. The E–W trending line AA’ passes through the western Sichuan plateau (including the Songpan-Garze terrane and the Longmenshan fault belt) and ends in the Sichuan basin (a part of the Yangtze craton). Line BB’ has a trend of NNE and crosses the Songpan-Garze terrane. Two-dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data. Our seismic velocity models indicate that the western Sichuan plateau and the Sichuan basin have crustal thicknesses of 62 and 43 km, average crustal P wave velocities of 6.27 and 6.45 km/s and lower crustal (Vp > 6.5 km/s) thicknesses of 27 and 15 km, respectively. Density models constructed from the seismic velocity models are consistent with observed Bouguer gravity anomalies. We infer that collision between the Tibetan Plateau and the Yangtze craton has caused thickening of the lower crust and uplift of the western Sichuan plateau. We detect a low-velocity layer in the upper crust of the western Sichuan plateau but observe no equivalence in the Sichuan basin; west dipping thrusts may detach into this low-velocity layer. The seismic phase PmP in the western Sichuan plateau has low amplitude, suggesting high attenuation in the lower crust (Qp of 100–300). We suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western Sichuan plateau.

Late Cretaceous to middle Tertiary basin evolution in the central Tibetan Plateau: Changing environments in response to tectonic partitioning, aridification, and regional elevation gain

Abstract: Located in the Bangong suture zone between the Lhasa and Qiangtang terranes of central Tibet, the Nima basin records Cretaceous–late Miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation. The Bangong suture zone originally formed during Late Jurassic–Early Cretaceous time as the Lhasa terrane collided with the Qiang-tang terrane. During Early to middle Cretaceous time, the region lay at northern near-equatorial paleolatitudes, near sea level. By Aptian time the Nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated Bangong suture zone. In the southern Nima basin, an ∼50 m.y. (Late Cretaceous–Eocene) depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the Lhasa terrane. In the northern Nima basin, deposition continued during latest Cretaceous time, recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits. By Oligocene time the Nima basin comprised two independent depocenters that accumulated coarse-grained alluvial, fluvial, lacustrine (evaporitic), and fan-delta deposits in close association with reactivated thrusts in the Bangong suture zone. Carbon and oxygen isotope data from Oligocene paleosol carbonate, reported elsewhere, indicate that regional paleoelevation during the late Oligocene was >4.6 km, as it is today. Overall, the depositional record of the Nima basin, combined with ongoing structural and geochronologic studies, demonstrates that the Bangong suture zone was reactivated during middle Cretaceous and middle Tertiary time, that the intervening ∼50 m.y. interval was a time of regional upper crustal shortening in the Lhasa terrane followed by regional ignimbrite eruptions, and that arid paleoclimate and high paleoelevation were established by the Late Cretaceous and late Oligocene, respectively. Within the context of other data sets from the Lhasa terrane, the record of deposition in the Nima basin is consistent with low-angle subduction of Neotethyan oceanic lithosphere and reactivation of the Bangong suture zone during the Early Cretaceous, followed by shortening within the Gangdese retroarc and northern Lhasa terrane thrust belts during the Late Cretaceous, lithospheric delamination-dripping and regional magmatic flare-up during latest Cretaceous through early Tertiary time, and underthrusting of Indian lower crust and lithosphere as far north as the Bangong suture zone during late Oligocene time.

Detrital zircon geochronology of Carboniferous-Cretaceous strata in the Lhasa terrane, Southern Tibet

Abstract: Sedimentary strata in the Lhasa terrane of southern Tibet record a long but poorly constrained history of basin formation and inversion. To investigate these events, we sampled Palaeozoic and Mesozoic sedimentary rocks in the Lhasa terrane for detrital zircon uranium-lead (U-Pb) analysis. The 4700 detrital zircon U-Pb ages reported in this paper provide the first significant detrital zircon data set from the Lhasa terrane and shed new light on the tectonic and depositional history of the region. Collectively, the dominant detrital zircon age populations within these rocks are 100-150, 500-600 and 1000-1400Ma. Sedimentary strata near NamCo in central Lhasa are mapped as Lower Cretaceous but detrital zircons with ages younger than 400Ma are conspicuously absent. The detrital zircon age distribution and other sedimentological evidence suggest that these strata are likely Carboniferous in age, which requires the existence of a previously unrecognized fault or unconformity. Lower Jurassic strata exposed within the Bangong suture between the Lhasa and Qiangtang terranes contain populations of detrital zircons with ages between 200 and 500Ma and 1700 and 2000Ma. These populations differ from the detrital zircon ages of samples collected in the Lhasa terrane and suggest a unique source area. The Upper Cretaceous Takena Formation contains zircon populations with ages between 100 and 160Ma, 500 and 600Ma and 1000 and 1400Ma. Detrital zircon ages from these strata suggest that several distinct fluvial systems occupied the southern portion of the Lhasa terrane during the Late Cretaceous and that deposition in the basin ceased before 70Ma. Carboniferous strata exposed within the Lhasa terrane likely served as source rocks for sediments deposited during Cretaceous time. Similarities between the lithologies and detrital zircon age-probability plots of Carboniferous rocks in the Lhasa and Qiangtang terranes and Tethyan strata in the Himalaya suggest that these areas were located proximal to one another within Gondwanaland. U-Pb ages of detrital zircons from our samples and differences between the geographic distribution of igneous rocks within the Tibetan plateau suggest that it is possible to discriminate a southern vs. northern provenance signature using detrital zircon age populations.

Geodynamic settings of mineral deposit systems

Abstract: Mineral deposits represent extraordinary metal concentrations that form by magmatic, magmatic–hydrothermal or hydrothermal processes in geodynamic environments typified by anomalously high thermal and/or mechanical energy near plate boundaries. As they require the conjunction of specific environmental conditions to form, particular mineral deposit types tend to occupy specific geodynamic niches. The temporal distributions of mineral deposit types reflect both formational and preservational processes. In the Archaean and Palaeoproterozoic, these were linked because of preservation in continental crust connected to thick buoyant subcontinental lithospheric mantle (SCLM), but were decoupled by the Neoproterozoic and Phanerozoic as a result of evolution to thinner, increasingly dense SCLM. The transition marks a change from mantle plume-influenced plate tectonics to modern-style plate tectonics, with broadly coincident environmental changes and a major impact on the nature and abundance of preserved mineral deposit types. As mineral deposits represent an integral part of tectonic process, they are essential indicators of that process and geodynamic settings, and should be incorporated into any holistic tectonic terrane analysis. Their distribution also provides a particularly critical test on ancient continental reconstructions derived from palaeomagnetic data. Conversely, such reconstructions provide a first-order targeting tool for the conceptual exploration required to discover new mineral provinces and deposits under cover.

Lithotectonic elements and geological events in the Hengshan-Wutai-Fuping belt: a synthesis and implications for the evolution of the Trans-North China Orogen

Abstract: The Hengshan–Wutai–Fuping belt is located in the middle segment of the Trans-North China Orogen, a Palaeoproterozoic continental collisional belt along which the Eastern and Western blocks amalgamated to form the North China Craton. The belt consists of the medium- to high-grade Hengshan and Fuping gneiss complexes and the intervening low- to medium-grade Wutai granite–greenstone terrane, and most igneous rocks in the belt are calc-alkaline and have affinities to magmatic arcs. Previous tectonic models assumed that the Hengshan and Fuping gneiss assemblages were an older basement to the Wutai supracrustal rocks, but recent studies indicate that the three complexes constitute a single, long-lived Neoarchaean to Palaeoproterozoic magmatic arc where the Wutai Complex represents an upper crustal domain, whereas the Hengshan and Fuping gneisses represent the lower crustal components forming the root of the arc. The earliest arc-related magmatism in the belt occurred at 2560–2520 Ma, marked by the emplacement of the Wutai granitoids, which was followed by arc volcanism at 2530–2515 Ma, forming the Wutai greenstones. Extension driven by widespread arc volcanism led to the development of a back-arc basin or a marginal sea, which divided the belt into the Hengshan–Wutai island arc (Japan-type) and the Fuping relict arc. At 2520–2480 Ma, subduction beneath the Hengshan–Wutai island arc caused partial melting of the lower crust to form the Hengshan tonalitic–trondhjemitic–granodioritic (TTG) suites, whereas eastward-directed subduction of the marginal sea led to the reactivation of the Fuping relict arc, where the Fuping tonalitic–trondhjemitic–granodioritic suite was emplaced. In the period 2360–2000 Ma, sporadic phases of isolated granitoid magmatism occurred in the Hengshan–Wutai–Fuping region, forming 2360 Ma, c. 2250 Ma and 2000–2100 Ma granitoids in the Hengshan Complex, the c. 2100 Ma Wangjiahui and Dawaliang granites in the Wutai Complex, and the 2100–2000 Ma Nanying granitoids in the Fuping Complex. At c. 1920 Ma, the Hengshan–Wutai island arc underwent an extensional event, possibly due to the subduction of an oceanic ridge, leading to the emplacement of pre-tectonic gabbroic dykes that were subsequently metamorphosed, together with their host rocks, to form medium- to high-pressure granulites. At 1880–1820 Ma, the Hengshan–Wutai–Fuping arc system was juxtaposed, intensely deformed and metamorphosed during a major and regionally extensive orogenic event, the Luliang Orogeny, which generated the Trans-North China Orogen through collision of the Eastern and Western blocks. The Hengshan–Wutai–Fuping belt was finally stabilized after emplacement of a mafic dyke swarm at 1780–1750 Ma.

Correlation of neoproterozoic terranes between the Ribeira Belt, SE Brazil and its African counterpart: comparative tectonic evolution and open questions

Abstract: Abstract Four main classes of tectonic entities may be considered for the Ribeira Belt and southwest African counterparts: (1) cratonic fragments older than 1.8 Ga and their passive margin successions, (2) reworked basement terranes with Mesoproterozoic and/or Neoproterozoic deformed cover, (3) magmatic arc associations, (4) terranes with Palaeoproterozoic basement and deformed Neoproterozoic back-arc successions. Based on comparative investigation, a tectonic model of polyphase amalgamation is proposed with c. 790 and 630–610 Ma major episodes of intra-oceanic and cordilleran arc magmatism along both sides of the Adamastor Ocean. Subsequent diachronous collision of the arc terranes and small plates followed at c. 630, 600, 580 and 530 Ma. The tectonic complexity reflects an accretionary evolution from Cryogenian to Cambrian times. The São Francisco–Congo and Angola palaeo-continents did probably not behave as one consolidated block, but rather may have accommodated considerable convergence during the Brasiliano/Pan-African episodes. The final docking of Cabo Frio and Kalahari in the Cambrian was coeval with the arrival of Amazonia on the opposite side, resulting in lateral reactivation and displacement between the previously amalgamated pieces. The transition between the Cambrian and the Ordovician is marked by the extensional collapse of the metamorphic core zones of the orogens.

Ages and origins of rocks of the Killingworth dome, south-central Connecticut: Implications for the tectonic evolution of southern New England

Abstract: The Killingworth dome of south-central Connecticut occurs at the southern end of the Bronson Hill belt. It is composed of tonalitic and trondhjemitic orthogneisses (Killingworth complex) and bimodal metavolcanic rocks (Middletown complex) that display calc-alkaline affinities. Orthogneisses of the Killingworth complex (Boulder Lake gneiss, 456 ± 6 Ma; Pond Meadow gneiss, ∼460 Ma) were emplaced at about the same time as eruption and deposition of volcanic-sedimentary rocks of the Middletown complex (Middletown Formation, 449 ± 4 Ma; Higganum gneiss, 459 ± 4 Ma). Hidden Lake gneiss (339 ± 3 Ma) occurs as a pluton in the core of the Killingworth dome, and, on the basis of geochemical and isotopic data, is included in the Killingworth complex. Pb and Nd isotopic data suggest that the Pond Meadow, Boulder Lake, and Hidden Lake gneisses (Killingworth complex) resulted from mixing of Neoproterozoic Gander terrane sources (high 207Pb/204Pb and intermediate eNd) and less radiogenic (low 207Pb/204Pb and low eNd) components, whereas Middletown Formation and Higganum gneiss (Middletown complex) were derived from mixtures of Gander basement and primitive (low 207Pb/204Pb and high eNd) sources. The less radiogenic component for the Killingworth complex is similar in isotopic composition to material from Laurentian (Grenville) crust. However, because published paleomagnetic and paleontologic data indicate that the Gander terrane is peri-Gondwanan in origin, the isotopic signature of Killingworth complex rocks probably was derived from Gander basement that contained detritus from non-Laurentian sources such as Amazonia, Baltica, or Oaxaquia. We suggest that the Killingworth complex formed above an east-dipping subduction zone on the west margin of the Gander terrane, whereas the Middletown complex formed to the east in a back-arc rift environment. Subsequent shortening, associated with the assembly of Pangea in the Carboniferous, resulted in Gander cover terranes over the Avalon terrane in the west; and in the Middletown complex over the Killingworth complex in the east. Despite similarities of emplacement age, structural setting, and geographic continuity of the Killingworth dome with Oliverian domes in central and northern New England, new and published isotopic data suggest that the Killingworth and Middletown complexes were derived from Gander crust, and are not part of the Bronson Hill arc that was derived from Laurentian crust. The trace of the Ordovician Iapetan suture (the Red Indian line) between rocks of Laurentian and Ganderian origin probably extends from Southwestern New Hampshire west of the Pelham dome of northcentral Massachusetts and is coverd by Mesozoic rocks of the Hartford basin.

Tectonic evolution of the active Hikurangi subduction margin, New Zealand, since the Oligocene

Abstract: [1] Deformation across the active Hikurangi subduction margin, New Zealand, including shortening, extension, vertical-axis rotations, and strike-slip faulting in the upper plate, has been estimated for the last ∼24 Myr using margin-normal seismic reflection lines and cross sections, strike-slip fault displacements, paleomagnetic declinations, bending of Mesozoic terranes, and seafloor spreading information. Post-Oligocene shortening in the upper plate increased southward, reaching a maximum rate of 3–8 mm/year in the southern North Island. Upper plate shortening is a small proportion of the rate of plate convergence, most of which (>80%) accrued on the subduction thrust. The uniformity of these shortening rates is consistent with the near-constant rate of displacement transfer (averaged over ≥5 Myr) from the subduction thrust into the upper plate. In contrast, the rates of clockwise vertical-axis rotations of the eastern Hikurangi Margin were temporally variable, with ∼3°/Myr since 10 Ma and ∼0°–1°/Myr prior to 10 Ma. Post 10 Ma, the rates of rotation decreased westward from the subduction thrust, which resulted in the bending of the North Island about an axis at the southern termination of subduction. With rotation of the margin and southward migration of the Pacific Plate Euler poles, the component of the margin-parallel relative plate motion increased to the present. Plate convergence dominated the Hikurangi Margin before ca. 15 Ma, with the rate of margin-parallel motion increasing markedly since 10 Ma. Vertical-axis rotations could accommodate all margin-parallel motion before 1–2 Ma, eliminating the requirement for large strike-slip displacements (for example, >50 km) in the upper plate since the Oligocene.

Signatures of mountain building: Detrital zircon U/Pb ages from northeastern Tibet

Abstract: Although detrital zircon has proven to be a powerful tool for determining provenance, past work has focused primarily on delimiting regional source terranes. Here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance, and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers. For such an analysis to succeed for a given mountain, discrete intrarange source terranes must have unique U/Pb zircon age signatures and sediments eroded from the range must have well-defi ned depositional ages. Here we use ~1400 single-grain U/Pb zircon ages from northeastern Tibet to identify and analyze an area that satisfi es these conditions. This analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete, punctuated changes in local source terranes. By tracking eroding rock units chronologically through the stratigraphic record, this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca. 8 Ma in the Laji Shan, a 10‐25-km-wide range in northeastern Tibet with a unique U/Pb age signature.

Accretionary orogens in space and time

Abstract: Accretionary orogens form along continental margins where oceanic lithosphere is subducted. They are primary sites of juvenile continental crust production and have been active on Earth since the earliest Archean. Orogen lifetimes expressed as accretion intervals range from 50 to over 300 m.y. The short duration of Late Archean accretionary orogens (<70 m.y.) may refl ect the short duration of a global mantle plume event at 2.7 and 2.5 Ga. Although there is no simple relationship between the onset or duration of accretionary orogens and the supercontinent cycle, many postArchean orogens terminate with continent-continent collisions during supercontinent assembly. Average terrane lifespan is typically 100‐200 m.y. in post‐1 Ga orogens, 50‐100 m.y. in pre‐1 Ga Proterozoic orogens, and 70‐700 m.y. in Archean orogens. Accretionary orogens can be grouped into two end members: simple orogens containing chiefl y juvenile terranes with lifespans of <100 m.y., and complex orogens with both juvenile accreted components and exotic microcratons, with terrane lifespans of ≥100 m.y. Terrane lifespan is controlled by (1) terrane tectonic setting, (2) complexity of precollisional terrane history, (3) availability of continental crust on Earth, and (4) plate history of ocean basins adjacent to accretionary orogens. Average accretion rates in accretionary orogens are 70 to 150 km 3 /km/m.y. in Phanerozoic orogens and 100 to 200 km 3 /km/m.y. in Precambrian orogens. Some orogens at 2.7 Ga have unusually high accretion rates greater than 300 km 3 /km/m.y., which may refl ect a global mantle plume event. Production rates of juvenile crust in accretionary orogens are typically 10%‐30% lower than total accretion rates, but can be up to 50% lower in Phanerozoic orogens.