Pampean Orogen: An Intraplate Component of Early Paleozoic Deformation?

After accretion of the Pampean continental fragment to the western Rio de la Plata craton margin (530 Ma), subsequent deformation, crustal anatexis and plutonism may have been intraplate responses to Brasiliano-PanAfrican collisional tectonism on the eastern margin during the amalgamation of Gondwana. Investigations of intraplate orogens such as the Tien Shan and the Ancestral Rocky Mountains, as well as of analogue and numerical models, permit discrimination of two early Paleozoic tectonomagmatic phases in the Sierras Pampeanas. The first involved marginal trough subduction and calc-alkaline magmatism, culminating in accretion of the Pampean terrane to the western craton edge; the second was characterized by crustal anatexis and peraluminous plutonism, penetrative deformation and high-angle reverse faulting resulting from continental collision on the eastern craton margin.Field observations from modern (Tien Shan) and ancient (Ancestral Rocky Mountains) intraplate chains, deep seismic and borehole data, radiometric and fission-track data constitute control for analogue and numerical models of intraplate deformation resulting from continental collision. Near-simultaneous continent-wide deformation, regularly spaced ranges/buckles, reverse-fault initiation at fold hinges of buckles, and doubling of crustal thickness are replicated in structural arrays formed in four-layer analogue models of lithospheric buckling. These data have significant implications for the ductile deformation, crustral thickening and post-subduction plutonism that spanned central South America in Late Cambrian time.     

Caledonide development offshore–onshore Svalbard based on ocean bottom seismometer, conventional seismic, and potential field data

The western Barents Sea and the Svalbard archipelago share a common history of Caledonian basement formation and subsequent sedimentary deposition. Rock formations from the period are accessible to field study on Svalbard, but studies of the near offshore areas rely on seismic data and shallowdrilling. Offshore mapping is reliable down to the Permian sequence, but multichannel reflection seismic data do not give a coherent picture of older stratigraphy. A survey of 10 Ocean Bottom Seismometer profiles was collected around Svalbard in 1998. Results show a highly variable thickness of pre-Permian sedimentary strata, and a heterogeneous crystalline crust tied to candidates for continental sutures or major thrust zones. The data shown in this paper establish that the observed gravity in some parts of the platform can be directly related to velocity variations in the crystalline crust, but not necessarily to basement or Moho depth. The results from three new models are incorporated with a previously published profile, to produce depth-to-basement and -Moho maps south of Svalbard. There is a 14 km deep basement located approximately below the gently structured Upper Paleozoic Sørkapp Basin, bordered by a 7 km deep basement high to the west, and 7–9 km depths to the north. Continental Moho-depth range from 28 to 35 km, the thickest crust is found near the island of Hopen, and in a NNW trending narrow crustal root located between 19°E and 20°E, the latter is interpreted as a relic of westward dipping Caledonian continental collision or major thrusting. There is also a basement high on this trend. Across this zone, there is an eastward increase in the V_P, V_P/V_S ratio, and density, indicating a change towards a more mafic average crustal composition. The northward basement/Moho trend projects onto the Billefjorden Fault Zone (BFZ) on Spitsbergen. The eastern side of the BFZ correlates closely with coincident linear positive gravity and magnetic anomalies on western Ny Friesland, apparently originating from an antiform with high-grade metamorphic Caledonian terrane. A double linear magnetic anomaly appears on the BFZ trend south of Spitsbergen, sub-parallel to and located 10–50 km west of the crustal root. Based on this correlation, it is proposed that the suture or major thrust zone seen south of Svalbard correlates to the BFZ. The preservation of the relationship between the crustal suture, the crustal root, and upper mantle reflectivity, challenges the large-offset, post-collision sinistral transcurrent movement on the BFZ and other trends proposed in the literature. In particular, neither the wide-angle seismic data, nor conventional deep seismic reflection data south of Svalbard show clear signs of major lateral offsets, as seen in similar data around the British Isles.     

Trace element fractionation during high-grade metamorphism and crustal melting—constraints from ion microprobe data of metapelitic, migmatitic and igneous garnets and implications for Sm–Nd garnet chronology

Rare earth element (REE) and other trace element (Y, Sr, Ti, Cr, V, Na) abundances in garnet from a garnet-bearing metapelite, a pelitic migmatite, a syn-tectonic granite and a post-tectonic leucogranite were measured by secondary ion mass spectrometry (SIMS) in order to identify the effective variables on the trace element distribution between garnet and the host rock. Garnet from the garnet-bearing metapelite, the pelitic migmatite and the syn-tectonic granite is zoned with respect to REE. The cores are enriched by a factor of 2–3 relative to the rims. For the garnets from the garnet-bearing metapelite equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for Ti, V, Cr and REE from core to rim. These features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open-system behaviour in these rocks at a time when the garnet, originally nucleated in the metamorphic environment was incorporated into the melt. The garnet from the syn-tectonic granite shows stepwise decreasing concentrations in REE from core to rim: a REE-rich core can be distinguished from a broad REE-depleted rim. Notably, from core to rim an inflection of the Yb / Er and Yb / Dy ratios is visible. Whereas the decrease of HREE abundance in the core region of the garnet from the syn-tectonic granite may arise from equilibrium partitioning during garnet growth, the inflection can be interpreted as a result of partial melting. Garnet cores with high Yb / Er and Yb / Dy >  1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower Yb / Er and Yb / Dy <  1 crystallized in the presence of a melt. Garnet from the leucogranite has lower REE abundances and is considered to be of igneous origin. In contrast to garnet from the other samples, its core has low trace element abundances, whereas its rim is significantly enriched in REE but depleted in Ti. These features suggest that only the outermost rim was in equilibrium with the melt. For this garnet, liquid diffusion controlled partitioning is more likely to explain the extreme trace element variation. An evaluation of Sm and Nd concentrations in garnet and a comparison of Sm–Nd and U–Pb garnet ages and U–Pb monazite ages form the terrane indicate that the observed LREE systematics in the different garnet species are a primary feature and are not homogenized by volume diffusion during high grade amphibolite facies conditions.     

Temperature–time paths from phosphate accessory phase paragenesis in the Honey Brook Upland and associated cover sequence, SE Pennsylvania, USA

Analysis of monazite-bearing lithologies from the Precambrian Honey Brook Upland (HBU) and overlying metasedimentary Paleozoic Chester Valley Sequence (CVS) (SE PA, USA) reveals overprinting of primary major and accessory phase parageneses by texturally and compositionally disparate secondary accessory phase parageneses. Two-pyroxene temperatures of 915–945 °C for reconstituted pyroxene reflect emplacement temperatures of felsic plutonic rocks (opdalite, charnockite) prior to Mesoproterozoic metamorphism. Monazite in metavolcanic felsic gneiss yields three age domains at 1009 ± 4 Ma (2 s.e.), 965 ± 6, and 876 ± 10 Ma. The first two domains record metamorphism of the HBU after anorthosite intrusion; peak monazite–xenotime temperatures for the monazite core domain are 700 °C, and high Th/U values in the second (overgrowth) age domain likely reflect a second high-T monazite growth episode. Formation of cummingtonite coronas on orthopyroxene in opdalite constrains maximum 1010 Ma metamorphic temperatures in the “granulite-facies” terrane to 730–740 °C. Evidence of increased Cl fluid activity in the 965 Ma metamorphism includes higher Cl content of matrix apatite relative to garnet-included apatite (metavolcanics), and Cl-bearing K-hornblende succeeding cummingtonite in coronal overgrowths (opdalite). Extreme monazite Th/U values (75–250) in the rim domain suggest growth during low-T hydrothermal alteration. In the opdalite, secondary singe-grain monazite and monazite + xenotime metasomites in apatite yield ages of 714 ± 24 and 586 ± 88 Ma, temperatures of 325–425 °C, and are interpreted to reflect thermal disturbances associated with late Proterozoic plutonic and volcanic activity in the Upland. This thermal disturbance may be recorded by Rb–Sr age of 567 Ma for biotite from a HBU gneiss. Monazite age domains in metaquartzite (378 ± 28, 272 ± 44 Ma) suggest that low-grade metamorphism (260–320 °C, Mnz–Xno thermometry) of the CVS is not a result of Taconian orogenesis.     

The Ross orogeny of the transantarctic mountains: a northern Victoria Land perspective

Northern Victoria Land (Antarctica) is made up of three terranes of Cambrian–Ordovician rocks: the Wilson (WT), Bowers (BT) and Robertson Bay terranes (RBT). The WT comprises a low- to high-grade metasedimentary sequence intruded by calc-alkaline plutons with magmatic arc affinity; the BT is composed of low-grade metavolcanic and metasedimentary rocks usually interpreted as an intra-oceanic arc; the RBT is a very low-grade flysch-like sequence. Terrane juxtaposition has traditionally been attributed to accretion during the Cambro-ordovician Ross orogeny. We propose a new model in which the WT, BT and RBT are interpreted as an arc/back-arc/trench system, developed in the context of a SW-dipping subduction zone. The subducting plate carried a continent originally located outboard of the turbidite fan of the RBT. Collision between this continent and the East Antarctic craton caused partial subduction of the intervening back-arc basin and, ultimately, the end of Ross-orogenic subduction. The turbidite fan of the RBT originally sedimented above the trench and on the subducting oceanic plate; due to collision it was thrusted on the continent, that constitutes, at least in part, the present basement of the RBT turbidite. The eastern portions of this continental mass were later dissected by the tensile tectonics related to the opening of the of the Southern Ocean.     

Correlation of the Late Cenozoic endogenic events and climatic variations in Central Asia

Trend of climatic changes in geological history of the Earth was determined by gradual decrease in the global surface temperature. Substantial deviations from this trend depended on the prevalent type of volcanism: predominantly explosive volcanism at convergent boundaries between lithospheric plates led to cooling and onset of glacial epochs, while intense intraplate volcanism strengthened greenhouse effect and resulted in global warming. During cold epochs, orogenic processes played an important role in climatic variations. The most frequent and regular climatic variations are controlled by the Earth position in solar orbit (Milankovitch cycles). The Late Cenozoic variations of cold climate were interrelated with orogenic processes caused by collision between the Indian and North Asian lithospheric plates. The first event of considerable cooling in the Northern Hemisphere (2.8–2.5 Ma ago) coincided with a rapid growth of mountains throughout the collision belt. The Tibetan Plateau formed in South Asia. In Central Asia, the large (> 1.5 × 10⁶ km²) Khangai-Altai-Sayan mountain system appeared 3 Ma ago. Total area subjected to orogenic processes in Central and South Asia exceeded 9 × 10⁶ km². The intense intraplate volcanism suggests that sublithospheric mantle was involved into orogenic processes. Alternation of glacial and interglacial climatic epochs during the last 1.8 m.y. is recorded in Central Asia. These climatic variations are compatible with the Milankovitch cycles. As is established, climatic events recognizable in the Baikal sedimentary record are correlative with interglacial and glacial epochs detectable in volcanic lavas of the East Sayan Mountains. There are indications of lava eruptions into ice during the cold periods. It is assumed therefore that all the cooling epochs detectable in the Baikal sedimentary record after 1.8 Ma were associated with development of mountain glaciation that formed glacial sheet up to 3 km thick and 100 000 km² in size. During the Brunhes Chron, there were eight glaciations at least. The endogenic (volcanism and orogeny) and exogenic (glaciation) processes during the last 3 m.y. are shown to be correlative. The intermittent development and degradation of thick ice sheets was responsible for oscillation of lithospheric load on the asthenosphere, and this caused periodical magma generation in marginal parts of volcanic provinces.     

中国东部中生代软流层上涌造山作用

中国东部中生代造山带不同于陆缘俯冲作用和陆间大陆碰撞造山带,也不是陆缘和陆间碰撞造山带发展演化的某一个特定阶段的产物。它是一种由深部软流层上涌造山作用形成的一个新类型的造山带,又称东亚型造山作用。它的造山作用过程是:(1)早中生代(230～180Ma)的前和初始造山幕,深部软流层物质上涌和底侵作用导致冷、强的大陆岩石圈地幔线状破裂与局部拆沉;(2)中、晚侏罗世(180～140Ma)主造山幕,软流层大规模上涌并沿着岩石圈底部壳-幔边界横向侵入和伸展,使垂向差异运动转变为水平挤压作用,结果地壳表层发生大规模的褶皱构造变形和推覆构造,使陆壳加厚形成山根,岩石圈根发生部分拆沉;(3)白垩纪(140～65Ma)的晚期造山幕,加厚的陆壳山根与岩石圈根的大规模拆沉,岩石圈进入全新的从挤压向伸展转变和巨大减薄阶段,软流层大规模上涌成山。     

一种新的地质地貌景观类型——西藏札达盆地古格地貌的特征与成因初探

札达盆地位于西藏西南边陲,构造上处于喜马拉雅断块挠起带头部内侧,呈NW-SE走向,长约1000km,宽约70km,总面积约40000km2,海拔约4500m.盆地中地层呈明显的二元结构特征.基底岩石为三叠纪-侏罗纪灰岩、砂岩和板岩,盖层为上新世-早更新世早期沉积的固结和半固结岩石,因受喜马拉雅断块挠起运动的影响,形成NW、NE和近EW、SN向等多组节理裂隙.晚新生代以来,盆地上升,在构造节理的主导下,穿越盆地的象泉河及其支流对岩石地层进行侵蚀、切割,在雨水的淋蚀、寒冻风化的剥蚀作用下,逐渐形成现今大小不同和形态奇特的半固结砂泥岩塔林地貌景观-古格地貌,这是中国地质地貌景观的一种新类型.     

Hornblende Ar/Ar geochronology across terrane boundaries in the Sveconorwegian Province of S. Norway

Hornblende incremental heating ⁴⁰Ar/³⁹Ar data were obtained from augen gneiss and amphibolite of the Sveconorwegian Province of S. Norway. In the Rogaland-Vest Agder and Telemark terranes, four pyroxene-rich samples, located close (≤ 10 km) to the anorthosite-charnockite Rogaland Igneous Complex, define an age group at 916 + 12/ − 14 Ma and six samples distributed in the two terranes yield another group at 871 + 8/ − 10 Ma. The first age group is close to the reported zircon U---Pb intrusion age of the igneous complex (931 ± 2 Ma) and the regional titanite U---Pb age (918 ± 2 Ma), whereas the second group overlaps reported regional mineral Rb---Sr ages (895-853 Ma) as well as biotite K---Ar ages (878-853 Ma). In the first group, the comparatively dry parageneses of low-P thermal metamorphism (M2) associated with the intrusion of the igneous complex are well developed, and hornblende ⁴⁰Ar/³⁹Ar ages probably record a drop in temperature shortly after this phase. In other hornblende + biotite-rich samples, with presumably a higher fluid content, the hornblende ages are probably a response to hornblende-fluid interaction during a late Sveconorwegian metamorphic or hydrothermal event. A ca 220 m.y. diachronism in hornblende ⁴⁰Ar/³⁹Ar ages is documented between S. Telemark (ca 870 Ma) and Bamble (ca 1090 Ma). Differential uplift between these terranes was mostly accommodated by shearing along the Kristiansand-Porsgrunn shear zone. The final stage of extension along this zone occurred after intrusion of the Herefoss post-kinematic granite at 926 ± 8 Ma. On the contrary, the southern part of the Rogaland-Vest Agder and Telemark terranes share a common cooling evolution as mineral ages are similar on both sides of the Mandal-Ustaoset Line the tectonic zone between them. The succession within 20 m.y. of a voluminous pulse of post-tectonic magmatism at 0.93 Ga, a phase of high-T-low-P metamorphism at 0.93-0.92 Ga, and fast cooling at a regional scale ca 0.92 Ga, suggests that the southern parts of Rogaland-Vest Agder and Telemark were affected by an event of post-thickening extension collapse at that time. This event is not recorded in Bamble.     

Meso-Cenozioc Tectonic Attribute of Southern China and Folding of the Nanhua (South Cathay) Orogenic Belt

The Meso-Cenozoic tectonic attribute of southern China is a continental tridirectional orogenic belt formed by subsynchronous interaction among the Tethys, Northwest Pacific and Kunlun-Qinling tectonic domains. It was created by superimposition of repeated orogenies since the Late Permian. The Indosinian folds therein are gentle and localized.