The western border of the Indian plate: implications for Himalayan geology

The study of regions situated beyond the western margin of the present-day Indian plate (Afghanistan principally) point to the following facts:

1. (1) During the Late Precambrian—Early Paleozoic, stratigraphical continuity existed between western and central Iran, Central Afghanistan, Salt Range and western Pakistan.

2. (2) During the Paleozoic a similar epicontinental cover existed in central Afghanistan, Kashmir and Tibet, with Gondwana tillites and associated cold fauna, such as in India (Umaria); however, a so-called Hercynian zone exists also in northern Iran—Hindu Kush and northern Pamir: it exibits a Middle Paleozoic unconformity (Upper DevonianCarboniferous) on metamorphic Early Paleozoic.

3. (3) The end of the Paleozoic, is marked by: a fracturation of the basement of the Hercynian zone, with powerful volcanic eruptions at the northern part of Hindu Kush, Kashmir (Panjal trap) and also Nepal (Nar valley) the formation of a geosynclinal zone at the southern part of Hercynian zone (Turkman, Penjaw).

4. (4) During the Jurassic: the geosynclinal evolution of the Turkman—Penjaw furrow accelerated, with the accumulation of flysch, radiolarites, ophiolites, olistolites and incipient HP metamorphism. A general subduction took place followed by a Neocimmerian orogenic phase with overthrusting of the central Afghanistan ranges on the scar of the geosynclinal furrows.

5. (5) During the Cretaceous: the geosynclinal evolution ended: Lower Cretaceous lies unconformably on the folded Jurassic flysch. In eastern Afghanistan and northern Pakistan, during the Middle (?) or Upper Cretaceous, a new geosynclinal zone was created.

6. (6) During the Cenozoic, central Afghanistan was emerged; northwards, sedimentary basins were created along the Herat fault, with volcanic and magmatic activity. A southeastern geosynclinal furrow evolved with accumulation of flysch, ophiolites and finally molasse deposits (Katawas—Soleimans). Its western border began overthrusting, but this movement changed into a left lateral fault i.e., the presentday Chaman Arghandeh fault.

Conclusion: Two major phases of dislocation took place during the geological history of Gondwana: the first one began during the Permian and ended in the Jurassic; the second one began during the Cretaceous and is still active. The important Eocimmerian orogenic phenomena, existing in the Central Afghanistan and northern Pakistan, took place at the edge of a Gondwana continental fragment, which was larger than the presentday Indian plate. Coeval phenomena may exist in the Himalayan region and perhaps in one of the ophiolitic sutures of Tibet.     

Late Jurassic intraplate faulting in eastern Australia: A link to subduction in eastern Gondwana and plate tectonic reorganisation

Eastern Gondwana was subjected to subduction processes during the Middle-Late Jurassic, but how these processes affected intraplate deformation in eastern Australia is poorly understood. Here we present ⁴⁰Ar/³⁹Ar, K-Ar, and Rb-Sr geochronological data from illitic clay-bearing fault gouges associated with the northern part of the 200 km long, N-striking, dextral strike-slip, Demon Fault in eastern Australia. We show a major range of geochronological ages at 162.99 ± 0.74–152.1 ± 4.8 Ma, indicating that the Demon Fault was active during the Late Jurassic. This period partially coincides with the Middle-Late Jurassic deposition of widespread ash-fall tuffs in the Clarence-Moreton, Surat, and Eromanga basins. We propose that Middle-Late Jurassic intraplate tectonism in eastern Australia was influenced by subduction processes farther east, which produced extensive calc-alkaline magmatism in New Zealand from ~170 Ma. A global plate reorganisation event, related to the development of Early-Middle Jurassic sea-floor spreading of the Pacific Plate, possibly acted as the driving mechanism responsible for the intensification of magmatism and intraplate faulting in eastern Gondwana.     

The metamorphic sole of the western Tasmanian ophiolite: New insights into the Cambrian tectonic setting of the Gondwana Pacific margin

The Cambrian Ross–Delamerian Orogeny records the first phase of accretional tectonics along the eastern margin of Gondwana following breakup of the supercontinent Rodinia. Western Tasmania represents a key area for understanding the Cambrian tectonic setting of the eastern margin of Gondwana as it is one of the few places where a Tethyan-type ophiolite is preserved and contains the only known exposures of a sub-ophiolitic metamorphic sole associated with the Ross–Delamerian Orogen. This paper presents an integrated study of the field, petrographic, geochemical, and metamorphic characteristics of the metamorphic sole to the western Tasmanian ophiolite. The structurally highest levels of the metamorphic sole consist of granulite–upper amphibolite facies metacumulates and metagabbros. A transition to amphibolite and epidote–amphibolite facies conditions is recorded by metadolerites and metabasalts towards the base of the metamorphic sole. Kinematic indicators in mylonitic amphibolites suggest the metamorphic sole formed in an east-dipping subduction zone located to the east of the Proterozoic continental crust of Tasmania. Major and trace element whole rock and relict igneous spinel geochemistry indicates that the protoliths to the metamorphic sole formed at a back arc basin spreading centre. Our new data supports a model in which east-dipping subduction in Tasmania was driven by collapse of a back arc basin developed above an earlier west-dipping subduction zone outboard of the eastern margin of Gondwana. The proposed model may help to resolve a controversy related to apparent along-strike variations in subduction zone polarity during the Ross-Delamerian Orogeny and suggests a complex geodynamic setting had developed along the eastern margin of Gondwana by the Middle Cambrian. This study highlights the importance of considering the role of multiple subduction zones in generating metamorphic soles and emplacing ophiolites, which are key events associated with the construction of many orogenic belts worldwide.     

Aeromagnetic and gravity features of Gondwana and theirrelation to continental break-up: More pieces,less puzzle

Regional geophysical mapping techniques were initiated for economic exploration about 50 years ago and have now developed a completeness of coverage that can be exploited for geological research over large areas. The main strength of gravity and magnetic anomaly surveys lies in their ability to map ‘basement’ geology below cover. Suitably assembled and imaged at the continental-scale, the data give new insight into the mosaic of terranes that makes up the Precambrian continental crust, and into the margins of Precambrian continental fragments that have often been complicated by prolonged rifting before the onset of the drifting apart of continental fragments. Intrusions such as dykes, dyke swarms and plugs of small areal extent, that are often associated with continental disruption, can also be mapped with new totality. Examples using mainly aeromagnetic mapping are given to support a tight reassembly of the Precambrian crustal fragments of central Gondwana. In this, the outer margins of Precambrian blocks, known or interpreted from geophysical anomaly maps of the presently dispersed continents, are reassembled parallel and at a separation of only 50-80 km, typical of the width of present-day rift valleys. In the future, the wider availability of geophysical mapping data from both continents and oceans, with computer systems to process and interpret them, should contribute to a more fruitful co-operation of geologists and geophysicists in Gondwana research using more complete data coverage.     

藏东南始新世剑川盆地西缘沉积特征及构造意义

印度-欧亚大陆碰撞作用形成了以西藏-喜马拉雅造山带为主体的正向碰撞带和以"三江"造山带为主体的斜向碰撞带,二者之间的现代分界线为喜马拉雅东构造结.剑川盆地是目前已确定的、位于斜向碰撞带最南端的始新世盆地.通过对盆地西缘4条剖面的详细沉积学研究,发现盆地充填过程分为2个阶段.钾质-超钾质火山活动(37～34 Ma)之前的...     

Devonian basins of western norway - sedimentary response to tectonism and to varying tectonic context

The Devonian basins of western Norway are notable for their small size (<2000 km) and for the great stratigraphie thickness (up to 25 km) and the cyclic nature of their coarse-grained, alluvial infilling. These general features alone strongly suggest that tectonism was the dominant control on sedimentation but it is argued in particular that a coarsening-upwards motif, which permeates the three main basins usually on a scale of less than 200 m, was the direct sedimentary response to basin-floor subsidence. The coarsening-upwards is interpreted in terms of progradation of alluvial plain and alluvial fan facies in response to rapid lowering of the basin floor, while the overall cyclicity of the basin fills reflects repetition of such tectonic events. Detailed study within Hornelen Basin shows that the repeated coarsening-upwards theme overprints different sedimentary facies, and, in particular, the clastic wedges on opposite sides of the basin have contrasting thicknesses, radii, grain-size gradients and conglomerate types. These features suggest differing rates or degree of continuity of fault movement on opposite margins of the basin. Hornelen and Solund Basins, the two largest, are strongly contrasted in their differing mode of filling and sediment grade. It is suggested that this may reflect their development under strike-slip and dip-slip dominated regimes, respectively.     

Identifying Laurentian and SW Gondwana sources in the Neoproterozoic to Early Paleozoic metasedimentary rocks of the Sierras Pampeanas: Paleogeographic and tectonic implications

The provenance of Neoproterozoic to Early Paleozoic sedimentary rocks in the Sierras Pampeanas has been established using U–Pb SHRIMP age determination of detrital zircons in twelve metasedimentary samples, with supplementary Hf and O isotope analyses of selected samples. The detrital zircon age patterns show that the western and eastern sectors of the Sierras Pampeanas are derived from different sources, and were juxtaposed during the Early Cambrian ‘Pampean’ collision orogeny, thus defining initiation of the supercontinent stage of southwestern Gondwana. The Western Sierras Pampeanas (WSP), which extend northwards to the southern Puna (Antofalla) and the Arequipa Massif (Peru), constitute a single large continental basement of Paleoproterozoic age — the MARA block — that was reworked during the Grenvillian orogeny. The MARA block probably extends eastwards to include the Río Apa block (southern Brazil), but in this case without a Mesoproterozoic overprint. Detrital zircons from the WSP and Antofalla yield age peaks between 1330 and 1030 Ma, remarkably similar to the range of ages in the Grenville province of eastern Laurentia. The WSP Neoproterozoic sedimentary cover to this basement shows the same 1330–1030 component, but also includes important 1430–1380 Ma zircons whose juvenile Hf and O isotopic signatures strongly suggest derivation from the Grenville and the Southern Granite–Rhyolite provinces of eastern Laurentia. In contrast the Eastern Sierras Pampeanas metasedimentary rocks have a typically bimodal detrital zircon pattern with peaks at ca. 1000 and 600 Ma, which respectively indicate sources in the Natal–Namaqua belt and the East African orogen and/or the Dom Feliciano belt of SE Brazil and Uruguay. Sedimentary rocks in the Eastern Sierras Pampeanas and Patagonia deposited during the Late Early Cambrian–Early Ordovician interval, after the Pampean orogeny, have detrital patterns common to many sectors along the Terra Australis orogen, reflecting increasingly dominant input to the Paleozoic basins from the Neoproterozoic to Early Cambrian orogenic belts of the Gondwana margin.     

An overview of the Tertiary geology and hydrogeology of the northern part of the Arabian Gulf region with special reference to Kuwait

Tertiary sediments constitute the main source of usable groundwater in Kuwait. Tectonics, depositional environment, lithology and the hydrogeology of these sediments have been reviewed on a regional scale, with special reference to Kuwait. The review showed that Kuwait was situated at the boundary of the stable shelf towards southwest and the unstable shelf towards northeast throughout the Tertiary period. The Tertiary sedimentation started in this area with a marine transgression in the Palaeocene. Shallow marine to sabkha conditions prevailed in the area until the end of the Eocene; a carbonate-evaporite sequence (Umm Er-Radhuma, Rus and Dammam Formation) was deposited during this period. The sea regressed at the end of Eocene, and a widespread unconformity, causing the absence of Oligocene deposits over most of the area, marked the event. The karstification of the Dammam Limestone Formation, that provides localized easier pathways for ground water, took place during this period. The deposition of the clastic sediments of the Kuwait Group and its equivalents on the stable shelf, under mostly continental conditions, started in the early Miocene. The Tertiary aquifers of the area are recharged by occasional rainstorms in the outcrops of Saudi Arabia and Iraq. The water flows towards the north and east in the direction of the regional dip and discharges along the present-day coast of the Gulf. The water quality deteriorates in the same direction, being more mineralized as it flows through the aquifers. The aquifers are presently being exploited at a comparatively high rate. Since the aquifers extend beyond the political boundaries, cooperation among the countries of the region is recommended to ensure the optimum utilization of the scarce water resources of this desert environment.     

Mesozoic metallogeny in East China and corresponding geodynamic settings — An introduction to the special issue

The giant East China Mesozoic metallogenic province hosts some of the World’s largest resources of tungsten, tin, molybdenum, antimony and bismuth. Ores of gold, silver, mercury, lead, zinc, copper, uranium and iron are also of major importance. The province and its constituent metallogenic belts or regions (South China; Middle–Lower Yangtze River Valley; East Qinling–Dabie; Interior of North China Craton; Yan-Liao and North-east China) are the products of several pulses of igneous activity and mineralisation between ~240 and ~80 Ma. Each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the Mesozoic. This geodynamic evolution is linked to a complex series of tectonic events, involving far-field-subduction, plate collisions, crustal thickening, post-collision collapse and rifting.     

Source regions of granites and their links to tectonic environment: examples from the western United States

This review, in honor of Ilmari Haapala's retirement, reflects on lessons learned from studies of three granitic systems in western North America: (1) Mesoproterozoic samples from west Texas and east New Mexico; (2) Laramide granitic systems associated with porphyry-copper deposits in Arizona; and (3) granites of the Colorado Mineral Belt. The studies elucidate relationships amongst tectonic setting, source material, and magma chemistry.

Mesoproterozoic basement samples are from two different felsic suites with distinct elemental and isotopic compositions. The first suite, the “plutonic province”, is dominantly magnesian, calc-alkalic to alkali-calcic, and metaluminous. It has low K₂O/Na₂O and Rb/Sr, and Nd model ages of 1.56 to 1.40 Ga. The second suite, the “Panhandle igneous complex”, is magnesian, metaluminous, alkalic, and is part of the Mesoproterozoic belt of magmatism that extends from Finland to southwestern United States. Samples from the Panhandle igneous complex demonstrate three episodes of magmatism: the first pulse was intrusion of quartz monzonite at 1380 to 1370 Ma; the second was comagmatic epizonal granite and rhyolite at 1360 to 1350 Ma. Both of these rock types are high-K to slightly ultra-high-K. The third pulse at 1338 to 1330 Ma was intrusion of ultra-high-K quartz syenite. Nd model ages (1.94 to 1.52 Ga) are distinct from those of the “plutonic province” and systematically older than crystallization ages, implying a substantial crustal input to the magmas.

At the Sierrita porphyry-copper deposit in the Mazatzal Province of southeastern Arizona, trace element, Sr, and Nd isotopic compositions were determined for a suite of andesitic and rhyolitic rocks (67 Ma) intruded by granodiorite and granite. Isotopic composition and chemical evolution are well correlated throughout the suite. Andesite has the least negative initial ε_Nd (−4.3) and lowest ⁸⁷Sr/⁸⁶Sr_i (0.7069). It is also the oldest and chemically most primitive, having low concentrations of Rb, SiO₂, and high concentrations of transition elements. These parameters change through the system to the youngest unit (granite), which has the most negative ε_Nd (−8.5), the highest ⁸⁷Sr/⁸⁶Sr_i (0.7092), and is chemically most evolved. Correlation between chemical and Nd isotopic evolution probably resulted from a continuous process of progressive assimilation, in which mafic magmas invade and incorporate continental crust. Deposits in Arizona with ε_Nd values more negative than the −8.5 of Sierrita lie in the older Yavapai province in the northwestern part of the state. The difference in the most negative epsilon Nd implies that Nd isotopic signature is sensitive to the age of the Precambrian domain.

The granites from the Colorado Mineral Belt were emplaced during the transition from Laramide convergence to mid-Tertiary extension. Three different groups of granites are recognized. The first is Laramide and was formed during assimilation-fractional crystallization involving lower crustal mafic source materials; the second and third groups are mid-Tertiary and represent intracrustal melting of heterogeneous sources. This change in source regions and melt regimes in transition from convergence to extension is fundamental to the Mesozoic and Cenozoic evolution of western North America.