Geosciences

2183 KiB

Open AccessArticle

Evaluating Complex Magma Mixing via Polytopic Vector Analysis (PVA) in the Papagayo Tuff, Northern Costa Rica: Processes that Form Continental Crust

by David W. Szymanski, Lina C. Patino, Thomas A. Vogel and Guillermo E. Alvarado

Geosciences 2013, 3(3), 585-615; https://doi.org/10.3390/geosciences3030585 - 29 Aug 2013

Cited by 9 | Viewed by 10401

Abstract

Over the last forty years, research has revealed the importance of magma mixing as a trigger for volcanic eruptions, as well as its role in creating the diversity of magma compositions in arcs. Sensitive isotopic and microchemical techniques can reveal subtle evidence of [...] Read more.

Over the last forty years, research has revealed the importance of magma mixing as a trigger for volcanic eruptions, as well as its role in creating the diversity of magma compositions in arcs. Sensitive isotopic and microchemical techniques can reveal subtle evidence of magma mixing in igneous rocks, but more robust statistical techniques for bulk chemical data can help evaluate complex mixing relationships. Polytopic vector analysis (PVA) is a multivariate technique that can be used to evaluate suites of samples that are produced by mixing of two or more magma batches. The Papagayo Tuff of the Miocene-Pleistocene Bagaces Formation in northern Costa Rica is associated with a segment of the Central American Volcanic Arc. While this segment of the arc is located on oceanic plateau, recent (<8 Ma) ignimbrites bear the chemical signatures of upper continental crust, marking the transition from oceanic to continental crust. The Papagayo Tuff contains banded pumice fragments consistent with one or more episodes of mixing/mingling to produce a single volcanic deposit. The PVA solution for the sample set is consistent with observations from bulk chemistry, microchemistry and petrographic data from the rocks. However, without PVA, the unequivocal identification of the three end-member solution would not have been possible. Full article

(This article belongs to the Special Issue Continental Accretion and Evolution)

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13056 KiB

Open AccessArticle

The Lost South Gobi Microcontinent: Protolith Studies of Metamorphic Tectonites and Implications for the Evolution of Continental Crust in Southeastern Mongolia

by Joshua P. Taylor, Laura E. Webb, Cari L. Johnson and Matthew J. Heumann

Geosciences 2013, 3(3), 543-584; https://doi.org/10.3390/geosciences3030543 - 21 Aug 2013

Cited by 17 | Viewed by 9412

Abstract

The Central Asian Orogenic Belt, or Altaids, is an amalgamation of volcanic arcs and microcontinent blocks that records a complex late Precambrian–Mesozoic accretionary history. Although microcontinents cored by Precambrian basement are proposed to play an integral role in the accretion process, a lack [...] Read more.

The Central Asian Orogenic Belt, or Altaids, is an amalgamation of volcanic arcs and microcontinent blocks that records a complex late Precambrian–Mesozoic accretionary history. Although microcontinents cored by Precambrian basement are proposed to play an integral role in the accretion process, a lack of isotopic data hampers volume estimates of newly produced arc-derived versus old-cratonic crust in southeastern Mongolia. This study investigates metamorphic tectonites in southern Mongolia that have been mapped as Precambrian in age, largely on the basis of their high metamorphic grade and high strain. Here we present results from microstructural analyses and U-Pb zircon geochronology on samples from Tavan Har (44.05° N, 109.55° E) and the Yagan-Onch Hayrhan metamorphic core complex (41.89° N, 104.24° E). Our results show no compelling evidence for Precambrian basement in southeastern Mongolia. Rather, the protoliths to all tectonites examined are Paleozoic–Mesozoic age rocks, formed during Devonian–Carboniferous arc magmatism and subsequent Permian–Triassic orogenesis during collision of the South Mongolia arc with the northern margin of China. These results yield important insights into the Paleozoic accretionary history of southern Mongolia, including the genesis of metamorphic and igneous basement during the Paleozoic, as well as implications for subsequent intracontinental reactivation. Full article

(This article belongs to the Special Issue Continental Accretion and Evolution)

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2207 KiB

Open AccessArticle

Detrital Zircon Fission-Track Thermochronology of the Present-Day Isère River Drainage System in the Western Alps: No Evidence for Increasing Erosion Rates at 5 Ma

by Matthias Bernet

Geosciences 2013, 3(3), 528-542; https://doi.org/10.3390/geosciences3030528 - 6 Aug 2013

Cited by 10 | Viewed by 7498

Abstract

The Isère River system drains parts of the Western Alps in south-eastern France. Zircon fission-track data of the Isère River and its tributaries show a range of apparent cooling ages from about 7 to 150 Ma. Zircons with Jurassic to early Tertiary cooling [...] Read more.

The Isère River system drains parts of the Western Alps in south-eastern France. Zircon fission-track data of the Isère River and its tributaries show a range of apparent cooling ages from about 7 to 150 Ma. Zircons with Jurassic to early Tertiary cooling ages are derived from partially reset or non-reset sedimentary cover units of the internal and external Alps, while grains belonging to the minimum age fraction are derived from areas of active river incision in the external crystalline massifs or from the Penninic front. With the absence of major normal faults, upper crustal exhumation in the Western Alps is driven by erosion. First-order long-term exhumation rate estimates based on minimum ages are about 0.5–0.6 km/Myr for the fastest exhuming areas, while drainage basin average rates based on central ages are about 0.2–0.4 km/Myr. These rates are slower than published short-term erosion rates determined from detrital quartz ¹⁰Be analyses in the Pelvoux massif. While present-day erosion is faster than the long-term average exhumation rates, the Isère River drainage zircon fission-track data do not show evidence for increasing erosion rates at 5 Ma. Exhumation has not been sufficient in this area to expose rocks with <5 Ma cooling ages today. The increase in erosion may have happened only in glaciated areas between 1 and 2 Ma. Full article

(This article belongs to the Special Issue Sedimentary Basins and Orogenic Belts)

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3374 KiB

Open AccessArticle

Provenance of Late Paleozoic-Mesozoic Sandstones, Taimyr Peninsula, the Arctic

by Xiaojing Zhang, Jenny Omma, Victoria Pease and Robert Scott

Geosciences 2013, 3(3), 502-527; https://doi.org/10.3390/geosciences3030502 - 29 Jul 2013

Cited by 32 | Viewed by 8956

Abstract

The sedimentary and provenance characteristics of seven Permo-Carboniferous and two early Cretaceous samples from the Taimyr Peninsula provide information about the latest evolution of Uralian orogeny and the opening of the Amerasian Basin. The Permo-Carboniferous samples have a mixed provenance of recycled and [...] Read more.

The sedimentary and provenance characteristics of seven Permo-Carboniferous and two early Cretaceous samples from the Taimyr Peninsula provide information about the latest evolution of Uralian orogeny and the opening of the Amerasian Basin. The Permo-Carboniferous samples have a mixed provenance of recycled and first cycle sediment, sourced from metamorphic and igneous terranes. U-Pb detrital zircon ages represent a mixture of Precambrian-Paleozoic grains with euhedral, penecontemporaneous late Carboniferous and Permian grains consistent with derivation from the Uralian Orogen, plus additional Timanian and Caledonian material presumably derived from Baltica. Differences between the late Permian sample and the other Carboniferous and early Permian samples are interpreted to reflect the final collisional stage of Uralian orogeny. Early Cretaceous sediments deposited at the time of the Amerasian Basin opening preserve a mixed provenance of mainly first cycle metamorphic and igneous source material, as well as an unstable heavy mineral assemblage dominated by staurolite, suggesting local derivation. Detrital zircon ages fall almost exclusively into one late Permian-early Triassic cluster, indicating a Siberia Trap-related magmatic source. The detrital zircon age spectra support a passive margin setting for Taimyr during the opening of the Amerasian Basin in the early Cretaceous. Full article

(This article belongs to the Special Issue Sedimentary Basins and Orogenic Belts)

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4004 KiB

Open AccessArticle

Lower Paleogene Tectonostratigraphy of Balochistan: Evidence for Time-Transgressive Late Paleocene-Early Eocene Uplift

by Intizar H. Khan and William C. Clyde

Geosciences 2013, 3(3), 466-501; https://doi.org/10.3390/geosciences3030466 - 29 Jul 2013

Cited by 32 | Viewed by 21650

Abstract

Analysis of lithofacies, paleoflow directions, and sandstone petrography of upper Paleocene-lower Eocene paralic and continental sediments exposed along the transpressional suture zone of the western margin of the Indian plate indicate that the process of deformation and uplift of the carbonate shelf in [...] Read more.

Analysis of lithofacies, paleoflow directions, and sandstone petrography of upper Paleocene-lower Eocene paralic and continental sediments exposed along the transpressional suture zone of the western margin of the Indian plate indicate that the process of deformation and uplift of the carbonate shelf in this area had started by late Paleocene time. This tectonic uplift and deformation is documented by: (1) an overall shallowing upward synorogenic sequence of sediments, (2) proximal conglomerate facies (consisting of lower Paleocene and Mesozoic clasts) dominating in the western part of the study area and distal facies of sandstone and shale dominating in the eastern part of the study area, (3) the existence of an unconformity of late Paleocene-early Eocene age in the Quetta and Kalat regions, (4) paleocurrent directions in deltaic and fluvial deposits indicating southeastward flowing sediment dispersal paths during late Paleocene-early Eocene time, which is opposite to that found in the late Cretaceous, suggesting a reversal in the depositional slope of the Cretaceous shelf, and (5) petrographic study of sandstones indicating a collision suture/fold thrust belt provenance. This episode of uplift and deformation could be the result of India-Arabian transpression with associated ophiolite obduction or, more likely, to represent the local response to initial India-Asia contact. The unroofing pattern and uplift geometry of the western Indian shelf suggests that this tectonism first started in the southern part of the study area (Kalat-Khuzdar area) during the late Paleocene-early Eocene and proceeded northward in a time-transgressive fashion. Full article

(This article belongs to the Special Issue Sedimentary Basins and Orogenic Belts)

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4069 KiB

Open AccessArticle

Nd Isotope Mapping of Crustal Terranes in the Parent-Clova Area, Quebec: Implications for the Evolution of the Laurentian Margin in the Central Grenville Province

by Mark Zelek and Alan Dickin

Geosciences 2013, 3(3), 448-465; https://doi.org/10.3390/geosciences3030448 - 18 Jul 2013

Cited by 9 | Viewed by 8065

Abstract

Over 100 new Nd isotope analyses for the central Grenville Province in the Parent-Clova region of Quebec help fill a major gap in understanding the crustal accretion history of the province. Nd model ages show that the Parent-Clova region consists of three crustal [...] Read more.

Over 100 new Nd isotope analyses for the central Grenville Province in the Parent-Clova region of Quebec help fill a major gap in understanding the crustal accretion history of the province. Nd model ages show that the Parent-Clova region consists of three crustal blocks: the Archean parautochthon in the north; a central block with mixed ages interpreted as an ensialic arc; and a southerly block forming an extension of the Mesoproterozoic Quebecia arc terrane. The Allochthon Boundary Thrust is believed to define the edge of the Archean parautochthon, which is bordered for a distance of 300 km by the ensialic arc block, within which model ages decrease consistently away from the craton. A similar negative correlation between Nd model age and distance from the craton is seen in published data for the Algonquin terrane in Ontario, but with a lower range of model ages. These comparisons show that in the Parent-Clova region, a Mesoproterozoic ensialic arc was established directly on the Archean margin, but further west, the Mesoproterozoic arc was built on a younger margin consisting of accreted Palaeoproterozoic arc crust. The use of large Nd data sets allows these distinct regional growth patterns to become clear and, hence, allows an understanding of Mesoproterozoic crustal evolution in the province as a whole. Full article

(This article belongs to the Special Issue Continental Accretion and Evolution)

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130 KiB

Open AccessEditorial

Extending the Avenues for Geosciences Research

by Carlos Alves

Geosciences 2013, 3(3), 446-447; https://doi.org/10.3390/geosciences3030446 - 9 Jul 2013

Viewed by 4749

Abstract

The special issue “Geoscience of the Built Environment” [1] has received diverse contributions that can be considered to be on something like the outer limits of geosciences, extending their methods and studies to what can be designated the extended anthroposphere, considering not only [...] Read more.

The special issue “Geoscience of the Built Environment” [1] has received diverse contributions that can be considered to be on something like the outer limits of geosciences, extending their methods and studies to what can be designated the extended anthroposphere, considering not only the human modified environment, but also the natural space that man enjoys and assumes the burden to manage. Perhaps because of the orientation of the work of the editor, most of the contributions concerned building materials in diverse perspectives: the characterization of building materials used either directly as built elements [2,3] or as elements of the preparation of manufactured building materials [4]; the implications of geologic materials to the performance of buildings—namely, regarding thermal stability [5], and the alteration [6] and preservation [7] of existing applications of natural stone. These papers reflect diverse epistemological concerns both at the level of basic conceptual research—namely, taxonomy; and applied research—durability of materials used in built works and the impacts of these natural materials on the comfort of built elements. [...] Full article

(This article belongs to the Special Issue Geoscience of the Built Environment)

26910 KiB

Open AccessReview

Volcanosedimentary Basins in the Arabian-Nubian Shield: Markers of Repeated Exhumation and Denudation in a Neoproterozoic Accretionary Orogen

by Peter R. Johnson, Galen P. Halverson, Timothy M. Kusky, Robert J. Stern and Victoria Pease

Geosciences 2013, 3(3), 389-445; https://doi.org/10.3390/geosciences3030389 - 9 Jul 2013

Cited by 69 | Viewed by 18186

Abstract

The Arabian-Nubian Shield (ANS) includes Middle Cryogenian-Ediacaran (790–560 Ma) sedimentary and volcanic terrestrial and shallow-marine successions unconformable on juvenile Cryogenian crust. The oldest were deposited after 780–760 Ma shearing and suturing in the central ANS. Middle Cryogenian basins are associated with ~700 Ma [...] Read more.

The Arabian-Nubian Shield (ANS) includes Middle Cryogenian-Ediacaran (790–560 Ma) sedimentary and volcanic terrestrial and shallow-marine successions unconformable on juvenile Cryogenian crust. The oldest were deposited after 780–760 Ma shearing and suturing in the central ANS. Middle Cryogenian basins are associated with ~700 Ma suturing in the northern ANS. Late Cryogenian basins overlapped with and followed 680–640 Ma Nabitah orogenesis in the eastern ANS. Ediacaran successions are found in pull-apart and other types of basins formed in a transpressive setting associated with E-W shortening, NW-trending shearing, and northerly extension during final amalgamation of the ANS. Erosion surfaces truncating metamorphosed arc rocks at the base of these successions are evidence of periodic exhumation and erosion of the evolving ANS crust. The basins are evidence of subsequent subsidence to the base level of alluvial systems or below sea level. Mountains were dissected by valley systems, yet relief was locally low enough to allow for seaways connected to the surrounding Mozambique Ocean. The volcanosedimentary basins of the ANS are excellently exposed and preserved, and form a world-class natural laboratory for testing concepts about crustal growth during the Neoproterozoic and for the acquisition of data to calibrate chemical and isotopic changes, at a time in geologic history that included some of the most important, rapid, and enigmatic changes to Earth’s environment and biota. Full article

(This article belongs to the Special Issue Sedimentary Basins and Orogenic Belts)

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3335 KiB

Open AccessReview

Continental Growth and Recycling in Convergent Orogens with Large Turbidite Fans on Oceanic Crust

by David A. Foster and Ben D. Goscombe

Geosciences 2013, 3(3), 354-388; https://doi.org/10.3390/geosciences3030354 - 5 Jul 2013

Cited by 30 | Viewed by 20535

Abstract

Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites) [...] Read more.

Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites) underlain by a layer of tectonically imbricated upper oceanic crust and/or thinned continental crust. When oceanic crust is converted to lower continental crust it represents a juvenile addition to the continental growth budget. This two-tiered accreted crust is often the same thickness as average continental crustal and is isostatically balanced near sea level. The Paleozoic Lachlan Orogen of eastern Australia is the archetypical example of a tubidite-dominated accretionary orogeny. The Neoproterozoic-Cambrian Damaran Orogen of SW Africa is similar to the Lachlan Orogen except that it was incorporated into Gondwana via a continent-continent collision. The Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas post-tectonic magmatism constrains the timing of chemical maturation and cratonization. Cratonization of continental crust is fostered because turbidites represent fertile sources for felsic magmatism. Recognition of similar orogens in the Proterozoic and Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of upper oceanic crust or mafic underplating does not readily result in the addition of voluminous zircon-bearing magmas at the time of accretion. This crust only produces significant zircon when and if it partially melts, which may occur long after accretion. Full article

(This article belongs to the Special Issue Continental Accretion and Evolution)

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Geosciences, Volume 3, Issue 3 (September 2013) – 9 articles , Pages 354-615

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