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Sample records for adjacent continental crust

  1. Freshly brewed continental crust

    NASA Astrophysics Data System (ADS)

    Gazel, E.; Hayes, J. L.; Caddick, M. J.; Madrigal, P.

    2015-12-01

    Earth's crust is the life-sustaining interface between our planet's deep interior and surface. Basaltic crusts similar to Earth's oceanic crust characterize terrestrial planets in the solar system while the continental masses, areas of buoyant, thick silicic crust, are a unique characteristic of Earth. Therefore, understanding the processes responsible for the formation of continents is fundamental to reconstructing the evolution of our planet. We use geochemical and geophysical data to reconstruct the evolution of the Central American Land Bridge (Costa Rica and Panama) over the last 70 Ma. We also include new preliminary data from a key turning point (~12-6 Ma) from the evolution from an oceanic arc depleted in incompatible elements to a juvenile continental mass in order to evaluate current models of continental crust formation. We also discovered that seismic P-waves (body waves) travel through the crust at velocities closer to the ones observed in continental crust worldwide. Based on global statistical analyses of all magmas produced today in oceanic arcs compared to the global average composition of continental crust we developed a continental index. Our goal was to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust. We suggest that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone, a process probably more common in the Achaean where most continental landmasses formed, can produce the starting material necessary for juvenile continental crust formation.

  2. The Continental Crust.

    ERIC Educational Resources Information Center

    Burchfiel, B. Clark

    1983-01-01

    Continental crust underlies the continents, their margins, and also small shallow regions in oceans. The nature of the crust (much older than oceanic crust) and its dynamics are discussed. Research related to and effects of tectonics, volcanism, erosion, and sedimentation on the crust are considered. (JN)

  3. Raising the continental crust

    NASA Astrophysics Data System (ADS)

    Campbell, Ian H.; Davies, D. Rhodri

    2017-02-01

    The changes that occur at the boundary between the Archean and Proterozoic eons are arguably the most fundamental to affect the evolution of Earth's continental crust. The principal component of Archean continental crust is Granite-Greenstone Terranes (GGTs), with granites always dominant. The greenstones consist of a lower sequence of submarine komatiites and basalts, which erupted onto a pre-existing Tonalite-Trondhjemite-Granodiorite (TTG) crust. These basaltic rocks pass upwards initially into evolved volcanic rocks, such as andesites and dacites and, subsequently, into reworked felsic pyroclastic material and immature sediments. This transition coincides with widespread emplacement of granitoids, which stabilised (cratonised) the continental crust. Proterozoic supra-crustal rocks, on the other hand, are dominated by extensive flat-lying platform sequences of mature sediments, which were deposited on stable cratonic basements, with basaltic rocks appreciably less abundant. The siliceous TTGs cannot be produced by direct melting of the mantle, with most hypotheses for their origin requiring them to be underlain by a complimentary dense amphibole-garnet-pyroxenite root, which we suggest acted as ballast to the early continents. Ubiquitous continental pillow basalts in Archean lower greenstone sequences require the early continental crust to have been sub-marine, whereas the appearance of abundant clastic sediments, at higher stratigraphic levels, shows that it had emerged above sea level by the time of sedimentation. We hypothesise that the production of komatiites and associated basalts, the rise of the continental crust, widespread melting of the continental crust, the onset of sedimentation and subsequent cratonisation form a continuum that is the direct result of removal of the continent's dense amphibole-garnet-pyroxenite roots, triggered at a regional scale by the arrival of a mantle plume at the base of the lithosphere. Our idealised calculations suggest

  4. Continental crust: a geophysical approach

    SciTech Connect

    Meissner, R.

    1986-01-01

    This book develops an integrated and balanced picture of present knowledge of the continental crust. Crust and lithosphere are first defined, and the formation of crusts as a general planetary phenomenon is described. The background and methods of geophysical studies of the earth's crust and the collection of related geophysical parameters are examined. Creep and friction experiments and the various methods of radiometric age dating are addressed, and geophysical and geological investigations of the crustal structure in various age provinces of the continents are studied. Specific tectonic structures such as rifts, continental margins, and geothermal areas are discussed. Finally, an attempt is made to give a comprehensive view of the evolution of the continental crust and to collect and develop arguments for crustal accretion and recycling. 647 references.

  5. Nd isotopic variation of Paleozoic-Mesozoic granitoids from the Da Hinggan Mountains and adjacent areas, NE Asia: Implications for the architecture and growth of continental crust

    NASA Astrophysics Data System (ADS)

    Yang, Qidi; Wang, Tao; Guo, Lei; Tong, Ying; Zhang, Lei; Zhang, Jianjun; Hou, Zengqian

    2017-02-01

    There is a long-standing controversy regarding the tectonic division, composition and structure of the continental crust in the Da Hinggan Mountains and adjacent areas, which are mainly part of the southeastern Central Asian Orogenic Belt (CAOB). This paper approaches these issues via neodymium isotopic mapping of Paleozoic-Mesozoic (480 to 100 Ma) granitoids. On the basis of 943 published and 8 new whole-rock Nd isotopic data, the study area can be divided into four Nd isotopic provinces (I, II, III and IV). Province I (the youngest crust, Nd model ages (TDM) = 0.8-0.2 Ga) is a remarkable region of Phanerozoic crustal growth, which may reflect a major zone for closures of the Paleo-Asian Ocean. Province II (slightly juvenile crust, TDM = 1.0-0.8 Ga), the largest Nd isotopic province in the southeastern CAOB, is considered to reflect the recycling of the initial crustal material produced during the early stage (Early Neoproterozoic) evolution of the Paleo-Asian Ocean. Province III (slightly old crust, TDM = 1.6-1.1 Ga) is characterized by ancient crustal blocks, such as the central Mongolian, Erguna, Dariganga and Hutag Uul-Xilinhot blocks, which represent micro-continents and Precambrian basements in the southeastern CAOB. Several parts of Province III are located along the northern margin of the North China Craton (NCC), which is interpreted as a destroyed cratonic margin during the Paleozoic and Mesozoic. Province IV (the oldest crust, TDM = 2.9-1.6 Ga) mainly occurs within the NCC and reflects its typical Precambrian nature. These mapping results indicate that the boundary between Provinces II and III (the northern margin of the NCC) along the Solonker-Xar Moron Fault can be regarded as the lithospheric boundary between the CAOB and NCC. Provinces I and II account for 20% and 44% of the area of the southeastern CAOB, respectively, and therefore the ratio of continental growth is 64% from the Neoproterozoic to the Mesozoic, which is typical for this part of the

  6. Palaeomagnetism and the continental crust

    SciTech Connect

    Piper, J.D.A.

    1987-01-01

    This book is an introduction to palaeomagnetism offering treatment of theory and practice. It analyzes the palaeomagnetic record over the whole of geological time, from the Archaean to the Cenozoic, and goes on to examine the impact of past geometries and movements of the continental crust at each geological stage. Topics covered include theory of rock and mineral magnetism, field and laboratory methods, growth and consolidation of the continental crust in Archaean and Proterozoic times, Palaeozoic palaeomagnetism and the formation of Pangaea, the geomagnetic fields, continental movements, configurations and mantle convection.

  7. Composition of the Continental Crust

    NASA Astrophysics Data System (ADS)

    Rudnick, R. L.; Gao, S.

    2003-12-01

    The Earth is an unusual planet in our solar system in having a bimodal topography that reflects the two distinct types of crust found on our planet. The low-lying oceanic crust is thin (˜7 km on average), composed of relatively dense rock types such as basalt and is young (≤200 Ma old) (see Chapter 3.13). In contrast, the high-standing continental crust is thick (˜40 km on average), is composed of highly diverse lithologies (virtually every rock type known on Earth) that yield an average intermediate or "andesitic" bulk composition (Taylor and McLennan (1985) and references therein), and contains the oldest rocks and minerals yet observed on Earth (currently the 4.0 Ga Acasta gneisses (Bowring and Williams, 1999) and 4.4 Ga detrital zircons from the Yilgarn Block, Western Australia (Wilde et al., 2001)), respectively. Thus, the continents preserve a rich geological history of our planet's evolution and understanding their origin is critical for understanding the origin and differentiation of the Earth.The origin of the continents has received wide attention within the geological community, with hundreds of papers and several books devoted to the topic (the reader is referred to the following general references for further reading: Taylor and McLennan (1985), Windley (1995), and Condie (1997). Knowledge of the age and composition of the continental crust is essential for understanding its origin. Patchett and Samson (Chapter 3.10) review the present-day age distribution of the continental crust and Kemp and Hawkesworth (Chapter 3.11) review secular evolution of crust composition. Moreover, to understand fully the origin and evolution of continents requires an understanding of not only the crust, but also the mantle lithosphere that formed more-or-less contemporaneously with the crust and translates with it as the continents move across the Earth's surface. The latter topic is reviewed in Chapter 2.05.This chapter reviews the present-day composition of the

  8. Continental crust beneath southeast Iceland

    PubMed Central

    Torsvik, Trond H.; Amundsen, Hans E. F.; Trønnes, Reidar G.; Doubrovine, Pavel V.; Gaina, Carmen; Kusznir, Nick J.; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D.; Griffin, William L.; Werner, Stephanie C.; Jamtveit, Bjørn

    2015-01-01

    The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland—and especially the Öræfajökull volcano—is characterized by a unique enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and 207Pb/204Pb. Here, we demonstrate through modeling of Sr–Nd–Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2–6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume. PMID:25825769

  9. Continental crust beneath southeast Iceland.

    PubMed

    Torsvik, Trond H; Amundsen, Hans E F; Trønnes, Reidar G; Doubrovine, Pavel V; Gaina, Carmen; Kusznir, Nick J; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D; Griffin, William L; Werner, Stephanie C; Jamtveit, Bjørn

    2015-04-14

    The magmatic activity (0-16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland--and especially the Öræfajökull volcano--is characterized by a unique enriched-mantle component (EM2-like) with elevated (87)Sr/(86)Sr and (207)Pb/(204)Pb. Here, we demonstrate through modeling of Sr-Nd-Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2-6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume.

  10. Earthquakes in stable continental crust

    SciTech Connect

    Johnson, A.C.; Kanter, L.R. )

    1990-03-01

    Earthquakes can strike even in stable crust, well away from the familiar earthquake zones at the edges of tectonic plates, but their mere occurrence is both a source of concern in planning critical facilities such as nuclear power plants. The authors sought answers to two major questions: Just how much seismic activity does take place within the stable parts of continents And are there specific geologic features that make some areas of stable crust particularly susceptible to earthquakes They began by studying North America alone, but it soon became clear that the fairly short record of these rare events on a single continent would not provide enough data for reliable analysis. Hence, they decided to substitute space for time--to survey earthquake frequency and distribution in stable continental areas worldwide. This paper discusses their findings.

  11. Continental crust generated in oceanic arcs

    NASA Astrophysics Data System (ADS)

    Gazel, Esteban; Hayes, Jorden L.; Hoernle, Kaj; Kelemen, Peter; Everson, Erik; Holbrook, W. Steven; Hauff, Folkmar; van den Bogaard, Paul; Vance, Eric A.; Chu, Shuyu; Calvert, Andrew J.; Carr, Michael J.; Yogodzinski, Gene M.

    2015-04-01

    Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10 Myr ago--coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone--a process probably more common in the Archaean--can produce juvenile continental crust.

  12. Earthquakes in Stable Continental Crust.

    ERIC Educational Resources Information Center

    Johnston, Arch C.; Kanter, Lisa R.

    1990-01-01

    Discussed are some of the reasons for earthquakes which occur in stable crust away from familiar zones at the ends of tectonic plates. Crust stability and the reactivation of old faults are described using examples from India and Australia. (CW)

  13. Workshop on the Growth of Continental Crust

    NASA Technical Reports Server (NTRS)

    Ashwal, Lewis D. (Editor)

    1988-01-01

    Constraints and observations were discussed on a fundamental unsolved problem of global scale relating to the growth of planetary crusts. All of the terrestrial planets were considered, but emphasis was placed on the Earth's continental crust. The title of each session is: (1) Extraterrestrial crustal growth and destruction; (2) Constraints for observations and measurements of terrestrial rocks; (3) Models of crustal growth and destruction; and (4) Process of crustal growth and destruction.

  14. Continental Lower Crust: Wavespeeds, Composition, and Relamination

    NASA Astrophysics Data System (ADS)

    Hacker, B. R.; Kelemen, P. B.; Behn, M. D.

    2015-12-01

    The composition of much of Earth's lower continental crust is enigmatic. The available heat-flow and wavespeed constraints can be satisfied if lower continental crust elsewhere contains anywhere from 49 to 62 wt% SiO2 (similar to andesite and dacite), with high to moderate concentrations of K, Th and U. Beneath shields and platforms, Vp suggests that 20-30% of lower crust is mafic. A large fraction of this material could be denser than peridotite. In these settings the underlying upper mantle is too cold to permit development of a convective instability. High Vp lithologies in these settings may be the result of mafic underplating, or slow metamorphic growth of large proportions of garnet. Vp from lower crust of Paleozoic-Mesozoic orogens indicates a smaller amount of mafic rock and little or no material that is denser than peridotite. Beneath rifts, arcs, and volcanic plateaux and beneath continent-collision zones, ~10-20% of lower crust is mafic, and about half that is denser than peridotite. The inferred gravitational instability and high Moho temperatures suggest that the mafic lower crust in these regions may be temporary. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogite and may continue to descend into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than continental upper crust. These more-felsic rocks may rise buoyantly, undergo decompression melting and melt extraction, and may be relaminated to the base of the crust. As a result of this refining/differentiation process, such relatively felsic rocks could form much of lower crust.

  15. Electrical conductivity of the continental crust

    SciTech Connect

    Glover, P.W.J.; Vine, F.J. |

    1994-11-01

    Geophysical measurements indicate that the Earth`s continental lower crust has a high electrical conductivity for which no simple cause has been found. Explanation usually relies on either saline fluids saturating the pores, or interconnected highly conducting minerals such as graphite, Fe/Ti oxides and sulfides, providing conducting pathways. Attempts in the laboratory to clarify the problem have, hitherto, been unable to recreate conditions likely to be present at depth by controlling the confining pressure and pore fluid pressure applied to a rock saturated with saline fluids at temperatures between 270 C and 1000 C. Here we report conductivity data obtained using a cell designed to make such measurements on rocks saturated with saline fluids. Our results show that the conductivity of saturated samples of acidic rocks is explicable entirely in terms of conduction through the pore fluid whereas the conductivity of saturated basic rocks requires the presence of an additional conduction mechanism(s). We have used the experimental data to construct electrical conductivity/depth profiles for the continental crust, which, when compared with profiles obtained from magnetotelluric observations, demonstrate that a mid to lower crust composed of amphibolite saturated with 0.5 M NaCl shows electrical conductivities sufficient to explain conductivity/depth profiles for the continental crust inferred from geophysical measurements.

  16. A relatively reduced Hadean continental crust

    NASA Astrophysics Data System (ADS)

    Yang, Xiaozhi; Gaillard, Fabrice; Scaillet, Bruno

    2014-05-01

    Among the physical and chemical parameters used to characterize the Earth, oxidation state, as reflected by its prevailing oxygen fugacity (fO2), is a particularly important one. It controls many physicochemical properties and geological processes of the Earth's different reservoirs, and affects the partitioning of elements between coexisting phases and the speciation of degassed volatiles in melts. In the past decades, numerous studies have been conducted to document the evolution of mantle and atmospheric oxidation state with time and in particular the possible transition from an early reduced state to the present oxidized conditions. So far, it has been established that the oxidation state of the uppermost mantle is within ±2 log units of the quartz-fayalite-magnetite (QFM) buffer, probably back to ~4.4 billion years ago (Ga) based on trace-elements studies of mantle-derived komatiites, kimberlites, basalts, volcanics and zircons, and that the O2 levels of atmosphere were initially low and rose markedly ~2.3 Ga known as the Great Oxidation Event (GOE), progressively reaching its present oxidation state of ~10 log units above QFM. In contrast, the secular evolution of oxidation state of the continental crust, an important boundary separating the underlying upper mantle from the surrounding atmosphere and buffering the exchanges and interactions between the Earth's interior and exterior, has rarely been addressed, although the presence of evolved crustal materials on the Earth can be traced back to ~4.4 Ga, e.g. by detrital zircons. Zircon is a common accessory mineral in nature, occurring in a wide variety of igneous, sedimentary and metamorphic rocks, and is almost ubiquitous in crustal rocks. The physical and chemical durability of zircons makes them widely used in geochemical studies in terms of trace-elements, isotopes, ages and melt/mineral inclusions; in particular, zircons are persistent under most crustal conditions and can survive many secondary

  17. Cyclic growth in Atlantic region continental crust

    NASA Technical Reports Server (NTRS)

    Goodwin, A. M.

    1986-01-01

    Atlantic region continental crust evolved in successive stages under the influence of regular, approximately 400 Ma-long tectonic cycles. Data point to a variety of operative tectonic processes ranging from widespread ocean floor consumption (Wilson cycle) to entirely ensialic (Ampferer-style subduction or simple crustal attenuation-compression). Different processes may have operated concurrently in some or different belts. Resolving this remains the major challenge.

  18. Growth of the lower continental crust

    NASA Technical Reports Server (NTRS)

    Rudnick, Roberta L.

    1988-01-01

    One of the largest uncertainties in crustal composition and growth models is the nature of the lower continental crust. Specifically, by what processes is it formed and modified, and when is it formed, particularly in reference to the upper crust? The main reason for this lack of information is the scarcity of lower crustal rock samples. These are restricted to two types: rocks which outcrop in granulite facies terrains and granulite facies xenoliths which are transported to the earth's surface by young volcanics. The important conclusions arising from the xenolith studies are: the majority of mafic lower crustal xenoliths formed through cumulate process, resitic xenoliths are rare; and formation and metamorphism of the deep crust is intimately linked to igneous activity and/or orogeny which are manifest in one form or another at the earth's surface. Therefore, estimates of crustal growth based on surface exposures is representative, although the proportion of remobilized pre-existing crust may be significantly greater at the surface than in the deep crust.

  19. Density Sorting During the Evolution of Continental Crust

    NASA Astrophysics Data System (ADS)

    Kelemen, P. B.; Behn, M. D.; Hacker, B. R.

    2015-12-01

    We consider two settings - in addition to "delamination" of arc lower crust - in which dense, mafic eclogites founder into the convecting mantle while buoyant, felsic lithologies accumulate at the base of evolving continental crust. Arc processes play a central role in generating continental crust, but it remains uncertain how basaltic arc crust is transformed to andesitic continental crust. Dense, SiO2-poor products of fractionation may founder from the base of arc crust by "delamination", but lower arc crust after delamination has significantly different trace elements compared to lower continental crust (LCC). In an alternative model, buoyant magmatic rocks generated at arcs are first subducted, mainly via subduction erosion. Upon heating, these buoyant lithologies ascend through the mantle wedge or along a subduction channel, and are "relaminated" at
the base of overlying crust (e.g., Hacker et al EPSL 11, AREPS 15). Average buoyant lavas and plutons
for the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs fall within the range of estimated LCC major and trace elements. Relamination is more efficient in generating continental crust than delamination. Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle. There is a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across the region where Tibetan crust overlies Indian crust, into thickened Tibetan crust. About half the subducted Indian crust is present, whereas the other half is missing. Data (Vp/Vs; Miocene lavas formed by interaction of continental crust with mantle; xenolith thermometry) indicate 1000°C or more from ca 50 km depth to the Moho since the Miocene. We build on earlier studies (LePichon et al Tectonics 92, T'phys 97; Schulte-Pelkum et al Nature 05; Monsalve et al JGR 08) to advance the hypothesis that rapid growth of garnet occurs at 70-80 km and 1000°C within subducting Indian crust. Dense eclogites founder

  20. Large-scale subduction of continental crust implied by India-Asia mass-balance calculation

    NASA Astrophysics Data System (ADS)

    Ingalls, Miquela; Rowley, David B.; Currie, Brian; Colman, Albert S.

    2016-11-01

    Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth's surface today--represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry.

  1. Emergence of modern continental crust about 3 billion years ago

    NASA Astrophysics Data System (ADS)

    Dhuime, Bruno; Wuestefeld, Andreas; Hawkesworth, Chris J.

    2015-07-01

    The continental crust is the principal record of conditions on the Earth during the past 4.4 billion years. However, how the continental crust formed and evolved through time remains highly controversial. In particular, the composition and thickness of juvenile continental crust are unknown. Here we show that Rb/Sr ratios can be used as a proxy for both the silica content and the thickness of the continental crust. We calculate Rb/Sr ratios of the juvenile crust for over 13,000 samples, with Nd model ages ranging from the Hadean to Phanerozoic. The ratios were calculated based on the evolution of Sr isotopes in the period between the TDM Nd model age and the crystallization of the samples analysed. We find that the juvenile crust had a low silica content and was largely mafic in composition during the first 1.5 billion years of Earth’s evolution, consistent with magmatism on a pre-plate tectonics planet. About 3 billion years ago, the Rb/Sr ratios of the juvenile continental crust increased, indicating that the newly formed crust became more silica-rich and probably thicker. This transition is in turn linked to the onset of plate tectonics and an increase of continental detritus into the oceans.

  2. Generation of continental crust in intra-oceanic arcs

    NASA Astrophysics Data System (ADS)

    Gazel, E.; Hayes, J. L.; Kelemen, P. B.; Everson, E. D.; Holbrook, W. S.; Vance, E.

    2014-12-01

    The origin of continental crust is still an unsolved mystery in the evolution of our planet. Although the best candidates to produce juvenile continental crust are intra-oceanic arcs these systems are dominated by basaltic lavas, and when silicic magmas are produced, the incompatible-element compositions are generally too depleted to be a good match for continental crust estimates. Others, such as the W. Aleutians, are dominated by andesitic melts with trace element compositions similar to average continental crust. In order to evaluate which intra-oceanic arcs produced modern continental crust, we developed a geochemical continental index (CI) through a statistical analysis that compared all available data from modern intra-oceanic arcs with global estimates of continental crust. Our results suggest that magmas from Costa Rica (<10 Ma) have a CI <50, closer to the CI (~20) computed from available average continental crust estimates. Transitional CI values of 50-100 were found in the Aleutians, the Iwo-Jima segment of Izu-Bonin, the L. Antilles, Panama, Nicaragua, and Vanuatu. The geochemical signature of the Costa Rican lavas is controlled by melts from the subducting Galapagos tracks. Iwo-Jima and Vanuatu are in a similar tectonic scenario with subducting intraplate seamounts. Melts from the subducting oceanic crust are thought to significantly control the geochemical signature in the W. Aleutians and Panama. In the L. Antilles and E. Aleutians the continental signature may reflect recycling of a component derived from subducting continental sediments. Most of Izu-Bonin, Marianas, S. Scotia and Tonga arcs with a CI >100 have the least continent-like geochemical signatures. In these arcs the subducting plate is old (>100 Ma), not overprinted by enriched intraplate volcanism and the geochemistry may be dominated by slab-derived, aqueous fluids. We also found a strong correlation between the CI and average crustal P-wave velocity, validating the geochemical index

  3. Granitic Perspectives on the Generation and Secular Evolution of the Continental Crust

    NASA Astrophysics Data System (ADS)

    Kemp, A. I. S.; Hawkesworth, C. J.

    2003-12-01

    Every geologist is acquainted with the principle of "uniformitarianism," which holds that present-day processes are the key to those that operated in the past. But the extent this applies to the processes driving the growth and differentiation of the Earth's continental crust remains a matter of debate. Unlike its dense oceanic counterpart, which is recycled back into the mantle by subduction within 200 Ma (see Chapter 3.13), the continental crust comprises buoyant quartzofeldspathic materials and is difficult to destroy by subduction. The continental crust is, therefore, the principal record of how conditions on the Earth have changed, and how processes of crust generation have evolved through geological time. It preserves evidence of secular variation in crustal compositions, and thus the way in which the crust has formed throughout Earth's history. Exploring the nature and origin of these variations is the focus of this chapter.Continental rocks are highly differentiated, and so the crust is enriched in incompatible components compared to the primeval chondritic composition (see Chapter 3.01). Of these, water is perhaps the most relevant, both for the origin and evolution of life, and also for many models of crust generation and differentiation. Similarly, the mass of continental crust is just 0.57% of the silicate Earth, and yet it contains ˜35% of the potassium (using the crustal composition estimates in Table 1). Continental rocks comprise the buoyant shell that was once thought to float on a basaltic substratum, inferred from the wide distribution of chemically similar continental flood basalts (von Cotta, 1858). The links with the adjacent oceans were perhaps unclear, "the greatest mountains confront the widest oceans" ( Dana, 1873). Yet, it has long been argued that the rock that has the most similar composition to the average continental crust, andesite, may be generated by fractional crystallization of basalt ( Daly (1914) and Bowen (1928); but see the

  4. Water depth-composition trends in ferromanganese crusts adjacent to the California margin compared to those in equatorial Pacific crusts

    NASA Astrophysics Data System (ADS)

    Conrad, T. A.; Hein, J. R.

    2013-12-01

    Ferromanganese (Fe-Mn) crusts have been used as proxies for paleo-seawater chemistry; however, element concentrations and growth rates in crusts can vary depending on the region, latitude, and water depth. Here we will look at 130 Fe-Mn crusts from seven seamounts adjacent to the California (CA) margin to explore trends in composition with water depth and latitude. Crusts were collected by ROV, resulting in a dataset with exact water depth and location coordinates. Water depth ranges from 570 to 3,934 m along a 700-km transect running roughly parallel to the CA margin. Crust samples used for comparison were collected by dredging along transects following the Gilbert Ridge and Tokelau Seamounts in the western equatorial Pacific, with water depths ranging from about 1,500 to 4,800 m. In addition to variations with latitude and water depth, element concentrations in CA margin crusts are influenced by high primary productivity in surface waters, terrestrial input, and upwelling along the continental margin. Elements associated with terrestrial input, including Na, Si, Al, K, Pb, and particularly Th, are enriched in CA margin crusts relative to crusts from the equatorial Pacific transects. Si is also associated with the biogenic phase, as are P, Ba, and Cu but these elements are lower in CA margin crusts. Ba is a proxy for primary productivity. CA margin crusts show Ba increasing with increasing water depth, while equatorial Pacific crusts show the inverse trend. In equatorial Pacific crusts, Ba correlates with decreasing latitude, which reflects increasing proximity to the high productivity zone of equatorial upwelling; additionally, local obstructional upwelling associated with primary productivity around seamounts and islands enhances the productivity signal. Cu, which is associated with the manganese oxide phase, in addition to the biogenic phase, also increases with water depth along the CA margin; this is consistent with the seawater profile for dissolved Cu. In

  5. Seismic anisotropy in the continental crust of northwestern Canada

    NASA Astrophysics Data System (ADS)

    Dalton, Colleen A.; Gaherty, James B.

    2013-04-01

    Most studies of the seismic structure of continental crust assume that the wave speeds are isotropic at seismic wavelengths. The ability to measure surface wave propagation speed from the cross-correlation of ambient seismic noise provides new opportunities to image the crust and uppermost mantle. We investigate radial anisotropy in the continental crust of northwestern Canada from group-velocity curves of Love and Rayleigh waves obtained from ambient-noise cross-correlation. We test the null hypothesis that the Love and Rayleigh group-speed curves can be simultaneously fit by an earth model containing isotropic seismic velocities throughout the crust. Group velocity is predicted for 200 000 one-dimensional earth models, which are generated by randomly varying the crustal shear velocity and radial anisotropy within a prescribed range. The goodness-of-fit of the predictions is assessed by comparison with two sets of observed dispersion curves that correspond to two tectonically distinct terranes: the Archean/early Proterozoic craton and the transition from craton to Cordillera. The majority of best-fitting models contain VSH > VSV (4-5 per cent) in the middle crust. The finding that the middle/lower crust is seismically anisotropic across a large swath of northwestern Canada, combined with recent observations of anisotropic crust in much of the western United States, suggests that anisotropy may be ubiquitous in the continental crust.

  6. Does subduction zone magmatism produce average continental crust

    NASA Technical Reports Server (NTRS)

    Ellam, R. M.; Hawkesworth, C. J.

    1988-01-01

    The question of whether present day subduction zone magmatism produces material of average continental crust composition, which perhaps most would agree is andesitic, is addressed. It was argued that modern andesitic to dacitic rocks in Andean-type settings are produced by plagioclase fractionation of mantle derived basalts, leaving a complementary residue with low Rb/Sr and a positive Eu anomaly. This residue must be removed, for example by delamination, if the average crust produced in these settings is andesitic. The author argued against this, pointing out the absence of evidence for such a signature in the mantle. Either the average crust is not andesitic, a conclusion the author was not entirely comfortable with, or other crust forming processes must be sought. One possibility is that during the Archean, direct slab melting of basaltic or eclogitic oceanic crust produced felsic melts, which together with about 65 percent mafic material, yielded an average crust of andesitic composition.

  7. Imaging proto-oceanic crust off the Brazilian Continental Margin

    NASA Astrophysics Data System (ADS)

    Klingelhoefer, F.; Evain, M.; Afilhado, A.; Rigoti, C.; Loureiro, A.; Alves, D.; Leprêtre, A.; Moulin, M.; Schnurle, P.; Benabdellouahed, M.; Baltzer, A.; Rabineau, M.; Feld, A.; Viana, A.; Aslanian, D.

    2015-01-01

    During the Sanba (Santos basin seismic transect) experiment in 2010-2011, a 380-km-long combined wide-angle and reflection seismic profile has been acquired using 30 ocean-bottom seismometers, a 4.5 km seismic streamer and a 8900 in.3 airgun array. The Sanba 3 profile crosses the southern flank of the Sao Paulo Plateau, the Sao Paulo Ridge and the easternmost Santos Basin in an east-west direction. Its eastern end is located on undisturbed oceanic crust. Tomographic and forward modelling of the wide-angle seismic data reveals that the sedimentary thickness is variable with only 1-2 km on top of the ridge and thickening to 4-5 km in the basin. Crustal thickness at the ridge is about 18 km and the relative layer thickness and velocity gradients indicate a continental origin of this ridge. The eastern Santos Basin is underlain by crust of only 5 km thickness, characterized by high seismic velocities between 6.20 km s-1 in the upper crust and 7.40 km s-1 in the lower crust. Three hypotheses for the nature of the crust in this region are tested here: (i) thinned continental crust, (ii) serpentinized upper mantle material, (iii) thin oceanic crust. As seismic velocity gradients seem to rule out a continental origin of this region, and clear Moho reflections argue against serpentinized upper mantle, we propose that the crust underlying the easternmost Santos Basin is of oceanic origin. Deviations from normal oceanic crustal velocities in the lower crust (6.70-7.00 km s-1) can be explained by accretion at slow spreading rates leading to the inclusion of serpentinite into the lower crust at the onset of organized seafloor spreading.

  8. Continental crust composition constrained by measurements of crustal Poisson's ratio

    NASA Astrophysics Data System (ADS)

    Zandt, George; Ammon, Charles J.

    1995-03-01

    DECIPHERING the geological evolution of the Earth's continental crust requires knowledge of its bulk composition and global variability. The main uncertainties are associated with the composition of the lower crust. Seismic measurements probe the elastic properties of the crust at depth, from which composition can be inferred. Of particular note is Poisson's ratio,Σ ; this elastic parameter can be determined uniquely from the ratio of P- to S-wave seismic velocity, and provides a better diagnostic of crustal composition than either P- or S-wave velocity alone1. Previous attempts to measure Σ have been limited by difficulties in obtaining coincident P- and S-wave data sampling the entire crust2. Here we report 76 new estimates of crustal Σ spanning all of the continents except Antarctica. We find that, on average, Σ increases with the age of the crust. Our results strongly support the presence of a mafic lower crust beneath cratons, and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions, followed by magmatic underplating, or a model in which crust formation processes have changed since the Precambrian era.

  9. The origin of continental crust: Outlines of a general theory

    NASA Technical Reports Server (NTRS)

    Lowman, P. D., Jr.

    1985-01-01

    The lower continental crust, formerly very poorly understood, has recently been investigated by various geological and geophysical techniques that are beginning to yield a generally agreed on though still vague model (Lowman, 1984). As typified by at least some exposed high grade terranes, such as the Scottish Scourian complex, the lower crust in areas not affected by Phanerozoic orogeny or crustal extension appears to consist of gently dipping granulite gneisses of intermediate bulk composition, formed from partly or largely supracrustal precursors. This model, to the degree that it is correct, has important implications for early crustal genesis and the origin of continental crust in general. Most important, it implies that except for areas of major overthrusting (which may of course be considerable) normal superposition relations prevail, and that since even the oldest exposed rocks are underlain by tens of kilometers of sial, true primordial crust may still survive in the lower crustal levels (of. Phinney, 1981).

  10. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction.

    PubMed

    Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

    2013-12-04

    Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins.

  11. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction

    PubMed Central

    Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

    2013-01-01

    Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins. PMID:24301173

  12. Deep observation and sampling of the earth's continental crust (DOSECC): Continental scientific drilling workshop

    SciTech Connect

    Not Available

    1985-01-01

    Research summaries are presented of ongoing or proposed deep drilling programs to explore hydrothermal systems, buried astroblemes, continental crust, magma systems, mountain belt tectonics, subduction zones, and volcanoes. Separate abstracts have been prepared for individual papers. (ACR)

  13. Radiogenic heat production in the continental crust

    NASA Astrophysics Data System (ADS)

    Jaupart, Claude; Mareschal, Jean-Claude; Iarotsky, Lidia

    2016-10-01

    The thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust. Determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density. Enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust. Many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events, implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels. For the same total heat production, concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust, thereby allowing stabilization of the crust. The present-day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself. This review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces. Analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness, showing that the average crustal heat production is not constant. Differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition. Collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age. This trend can be accounted for by radioactive decay, indicating that thermal conditions at the time of crustal stabilization have not changed significantly. For the average crustal thickness of 40 km, Moho temperatures are near solidus values at the time of stabilization

  14. Reflection seismology: The continental crust (1986). Volume 14

    SciTech Connect

    Barazangi, M.; Brown, L.

    1986-01-01

    This volume explores the major problems of deep geology and continental tectonics, the issues of deep crustal fluids, and the geological significance of deep reflections. It reports on seismic results from Precambrian crust to Paleozoic structures, including seismic results from the Appalachian-Hercynian system and new results from the Cordillera of western North America, the young subduction zone of western Canada, and Antarctica.

  15. Permeability of continental crust influenced by internal and external forcing

    USGS Publications Warehouse

    Rojstaczer, S.A.; Ingebritsen, S.E.; Hayba, D.O.

    2008-01-01

    The permeability of continental crust is so highly variable that it is often considered to defy systematic characterization. However, despite this variability, some order has been gleaned from globally compiled data. What accounts for the apparent coherence of mean permeability in the continental crust (and permeability-depth relations) on a very large scale? Here we argue that large-scale crustal permeability adjusts to accommodate rates of internal and external forcing. In the deeper crust, internal forcing - fluxes induced by metamorphism, magmatism, and mantle degassing - is dominant, whereas in the shallow crust, external forcing - the vigor of the hydrologic cycle - is a primary control. Crustal petrologists have long recognized the likelihood of a causal relation between fluid flux and permeability in the deep, ductile crust, where fluid pressures are typically near-lithostatic. It is less obvious that such a relation should pertain in the relatively cool, brittle upper crust, where near-hydrostatic fluid pressures are the norm. We use first-order calculations and numerical modeling to explore the hypothesis that upper-crustal permeability is influenced by the magnitude of external fluid sources, much as lower-crustal permeability is influenced by the magnitude of internal fluid sources. We compare model-generated permeability structures with various observations of crustal permeability. ?? 2008 The Authors Journal compilation ?? 2008 Blackwell Publishing Ltd.

  16. Tectonic escape in the evolution of the continental crust

    NASA Technical Reports Server (NTRS)

    Burke, K.; Sengor, C.

    1986-01-01

    The continental crust originated by processes similar to those operating today and continents consist of material most of which originated long ago in arc-systems that have later been modified, especially at Andean margins and in continental collisions where crustal thickening is common. Collision-related strike-slip motion is a general process in continental evolution. Because buoyant continental (or arc) material generally moves during collision toward a nearby oceanic margin where less buoyant lithosphere crops out, the process of major strike-slip dominated motion toward a 'free-face' is called 'tectonic escape'. Tectonic escape is and has been an element in continental evolution throughout recorded earth-history. It promotes: (1) rifting and the formation of rift-basins with thinning of thickened crust; (2) pervasive strike-slip faulting late in orogenic history which breaks up mountain belts across strike and may juxtapose unrelated sectors in cross-section; (3) localized compressional mountains and related foreland-trough basins.

  17. Continental crust under the southern Porcupine Seabight west of Ireland

    NASA Astrophysics Data System (ADS)

    Makris, J.; Egloff, R.; Jacob, A. W. B.; Mohr, P.; Murphy, T.; Ryan, P.

    1988-08-01

    Two new seismic refraction/wide-angle reflection profiles demonstrate that the crust beneath the southern Porcupine Seabight, out to water depths in excess of 4000 m, is of continental type. They also reveal the rifted margin of the Porcupine basin on its eastern side. Crustal thickness under the Seabight, inclusive of sediments which are up to 6 km thick, decreases from 23 km in the east to about 10 km at a sharp continent-ocean transition in the west.

  18. Precipitous Continental Slopes and Considerations on the Transitional Crust.

    PubMed

    Emiliani, C

    1965-01-08

    The continental slope bordering the Bahamian Platform to the east descends precipitously from a shallow edge to oceanic depths. Sustained slopes of at least 40 degrees have been found east of San Salvador. Considerations based on isostatic equilibrium suggest that in this area the oceanic crust may approach the edge of the shelf within a distance as short as 8.5 km. If so, it may be possible to drill and sample a substantial portion of the transitional crust by means of directional drilling techniques from a rig located at the edge of the shelf. It may even be possible, at some future time, to cross the entire crust, pierce the Mohorovicić discontinuity, and penetrate the mantle.

  19. Recycling lower continental crust in the North China craton.

    PubMed

    Gao, Shan; Rudnick, Roberta L; Yuan, Hong-Ling; Liu, Xiao-Ming; Liu, Yong-Sheng; Xu, Wen-Liang; Ling, Wen-Li; Ayers, John; Wang, Xuan-Che; Wang, Qing-Hai

    2004-12-16

    Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

  20. Sulfur and metal fertilization of the lower continental crust

    NASA Astrophysics Data System (ADS)

    Locmelis, Marek; Fiorentini, Marco L.; Rushmer, Tracy; Arevalo, Ricardo; Adam, John; Denyszyn, Steven W.

    2016-02-01

    Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (< 300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe-Ni-Cu-PGE sulfide mineralization. Stratigraphic relationships, mineral chemistry, geochemical modeling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite. Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt. New high-precision chemical abrasion TIMS U-Pb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1 ± 0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (~ 420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (< 300 Ma). Unlike previous models, outcomes from this study suggest a

  1. Experimental constraints on phase relations in subducted continental crust

    NASA Astrophysics Data System (ADS)

    Hermann, Jörg

    2002-01-01

    Synthesis piston cylinder experiments were carried out in the range 2.0-4.5 GPa and 680-1,050 °C to investigate phase relations in subducted continental crust. A model composition (KCMASH) has been used because all major ultrahigh-pressure (UHP) minerals of the whole range of rock types typical for continental crust can be reproduced within this system. The combination of experimental results with phase petrologic constraints permits construction of a UHP petrogenetic grid. The phase relations demonstrate that the most important UHP paragenesis consists of coesite, kyanite, phengite, clinopyroxene, and garnet in subducted continental crust. Below 700 °C talc is stable instead of garnet. As most of these minerals are also stable at much lower pressure and temperature conditions it is thus not easy to recognize UHP metamorphism in subducted crust. A general feature, however, is the absence of feldspars at H2O-saturated conditions. Plagioclase is never stable at UHP conditions, but K-feldspar can occur in H2O-undersaturated rocks. Mineral compositions in the experiments are fully buffered by coexisting phases. The Si content of phengite and biotite increase with increasing pressure. At 4.0 GPa, 780 °C, biotite contains 3.28 Si per formula unit, which is most probably caused by solid solution of biotite with talc. Above 800 °C, the CaAl2SiO6 component in clinopyroxene buffered with kyanite, coesite and a Mg-phase increases with increasing temperature, providing a tool to distinguish between 'cold' and 'hot' eclogites. Up to 10% Ca-eskolaite (Ca0.5[]0.5AlSi2O6) in clinopyroxene has been found at the highest temperature and pressure investigated (>900 °C, 4.5 GPa). Garnet buffered with coesite, kyanite and clinopyroxene displays an increase of grossular component with increasing pressure for a given temperature. Although the investigated system represents a simplification with respect to natural rocks, it helps to constrain general features of subducted continental

  2. The effect of thicker oceanic crust in the Archaean on the growth of continental crust through time

    NASA Technical Reports Server (NTRS)

    Wilks, M. E.

    1988-01-01

    Present crustal evolution models fail to account for the generation of the large volume of continental crust in the required time intervals. All Archaean plate tectonic models, whether invoking faster spreading rates, similar to today's spreading rates, or longer ridge lengths, essentially propose that continental crust has grown by island arc accretion due to the subduction of oceanic crust. The petrological differences that characterize the Archaean from later terrains result from the subduction of hotter oceanic crust into a hotter mantle. If the oceanic crust was appreciably thicker in the Archaean, as geothermal models would indicate, this thicker crust is surely going to have an effect on tectonic processes. A more valid approach is to compare the possible styles of convergence of thick oceanic crust with modern convergence zones. The best modern analog occurs where thick continental crust is colliding with thick continental crust. Oceanic crustal collision on the scale of the present-day Himalayan continental collision zone may have been a frequent occurrence in the Archaean, resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts, leaving a depleted stabilized basic residuum. Present-day island arc accretion may not have been the dominant mechanism for the growth of the early Archaean crust.

  3. Crustal redistribution, crust-mantle recycling and Phanerozoic evolution of the continental crust

    NASA Astrophysics Data System (ADS)

    Clift, Peter D.; Vannucchi, Paola; Morgan, Jason Phipps

    2009-12-01

    We here attempt a global scale mass balance of the continental crust during the Phanerozoic and especially the Cenozoic (65 Ma). Continental crust is mostly recycled back into the mantle as a result of the subduction of sediment in trenches (1.65 km 3/a), by the subduction of eroded forearc basement (1.3 km 3/a) and by the delamination of lower crustal material from orogenic plateaus (ca. 1.1 km 3/a). Subduction of rifted crust in continent-continent collision zones (0.4 km 3/a), and dissolved materials fixed into the oceanic crust (ca. 0.4 km 3/a) are less important crustal sinks. At these rates the entire continental crust could be reworked in around 1.8 Ga. Nd isotope data indicate that ca. 80% of the subducted continental crust is not recycled by melting at shallow levels back into arcs, but is subducted to depth into the upper mantle. Continent-continent collision zones do not generally form new crust, but rather cause crustal loss by subduction and as a result of their physical erosion, which exports crust from the orogen to ocean basins where it may be subducted. Regional sedimentation rates suggest that most orogens have their topography eliminated within 100-200 million years. We estimate that during the Cenozoic the global rivers exported an average of 1.8 km 3/a to the oceans, approximately balancing the subducted loss. Accretion of sediment to active continental margins is a small contribution to crustal construction (ca. 0.3 km 3/a). Similarly, continental large igneous provinces (flood basalts) represent construction of only around 0.12 km 3/a, even after accounting for their intrusive roots. If oceanic plateaus are accreted to continental margins then they would average construction rates of 1.1 km 3/a, meaning that to keep constant crustal volumes, arc magmatism would have to maintain production of around 3.8 km 3/a (or 94 km 3/Ma/km of trench). This slightly exceeds the rates derived from sparse seismic experiments in oceanic arc systems. Although

  4. Evolution of the continental crust as recorded in accessory minerals

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi

    2013-04-01

    Recent developments in precise in situ isotopic analysis by LA-ICPMS and SIMS allow correlating multiple isotopic systems within single grains of accessory minerals such as zircon and monazite. The combined isotope systematics have provided valuable insights into the evolution of the continental crust. Zircon, a common accessory phase in granitoids, can be precisely dated by the U-Pb system. Zircon Lu-Hf isotopic composition is a function of crustal residence time of the magmatic protolith, whereas the O isotopic composition is a sensitive record of reworking of mature sediments such as pelite. An integration of U-Pb, Lu-Hf and O isotopic data for detrital zircons from modern large rivers indicates that: (1) the preserved continental crust dominantly formed between 3.6 and 1.0 Ga, (2) the major mode of crustal development would change during the supercontinent cycle, i.e., the generation of juvenile crust during supercontinent fragmentation versus the stabilization of the generated crust via crustal remelting during supercontinent fragmentation, and (3) reworking of mature sediments increased abruptly at ca. 2.1 Ga. No granitoids are known to have survived since 4.03 Ga. Yet evidence of an even older evolved crust is provided by detrital zircons with ages up to 4.4 Ga from Mt. Narryer and Jack Hills metasedimentary rocks in the Yilgarn Craton, Western Australia. Recently, such Hadean zircons have been found from outside the Yilgarn Craton, indicating that the young Earth had widespread granitoid crust. In addition, another accessory phase, monazite, in the Mt. Narryer and Jack Hills metasedimentary rocks offers an unique opportunity to advance our knowledge of early crustal evolution. Monazite, a light rare earth element phosphate mineral, occurs as an igneous accessory phase particularly in low-Ca granitoids, in contrast to the occurrence of igneous zircon in a wide range of granitoids. U-Pb and Sm-Nd isotope systematic of monazite are analogous to U-Pb and Lu

  5. Influence of Earth crust composition on continental collision style in Precambrian conditions: Results of supercomputer modelling

    NASA Astrophysics Data System (ADS)

    Zavyalov, Sergey; Zakharov, Vladimir

    2016-04-01

    A number of issues concerning Precambrian geodynamics still remain unsolved because of uncertainity of many physical (thermal regime, lithosphere thickness, crust thickness, etc.) and chemical (mantle composition, crust composition) parameters, which differed considerably comparing to the present day values. In this work, we show results of numerical supercomputations based on petrological and thermomechanical 2D model, which simulates the process of collision between two continental plates, each 80-160 km thick, with various convergence rates ranging from 5 to 15 cm/year. In the model, the upper mantle temperature is 150-200 ⁰C higher than the modern value, while the continental crust radiogenic heat production is higher than the present value by the factor of 1.5. These settings correspond to Archean conditions. The present study investigates the dependence of collision style on various continental crust parameters, especially on crust composition. The 3 following archetypal settings of continental crust composition are examined: 1) completely felsic continental crust; 2) basic lower crust and felsic upper crust; 3) basic upper crust and felsic lower crust (hereinafter referred to as inverted crust). Modeling results show that collision with completely felsic crust is unlikely. In the case of basic lower crust, a continental subduction and subsequent continental rocks exhumation can take place. Therefore, formation of ultra-high pressure metamorphic rocks is possible. Continental subduction also occurs in the case of inverted continental crust. However, in the latter case, the exhumation of felsic rocks is blocked by upper basic layer and their subsequent interaction depends on their volume ratio. Thus, if the total inverted crust thickness is about 15 km and the thicknesses of the two layers are equal, felsic rocks cannot be exhumed. If the total thickness is 30 to 40 km and that of the felsic layer is 20 to 25 km, it breaks through the basic layer leading to

  6. The global Moho depth map for continental crust

    NASA Astrophysics Data System (ADS)

    Baranov, Alexey; Morelli, Andrea

    2014-05-01

    Different tectonic units cover the continents: platform, orogens and depression structures. This structural variability is reflected both in thickness and physical properties of the crust. We present a new global Moho map for the continental crust, derived from geophysical data selected from the literature and regional crustal models. The Moho depth is represented with a resolution of 1x1 on a Cartesian grid. A large volume of new data has been analyzed: mostly active seismic experiments, as well as receiver functions and geological studies. We have used the following regional studies: for Europe and Greenland, models EPcrust [Molinari and Morelli, 2011]and EUNAseis [Artemieva and Thybo, 2103]; for North Asia, Moho models from [Cherepanova et al., 2013; Iwasaki et al., 2013; Pavlenkova, 1996]; for Central and Southern Asia, model AsCrust [Baranov, 2010] with updates for India [Reddy and Rao, 2013]; China [Teng et al., 2013];Arabian [Mechie et al., 2013]; for Africa, the model by [Pasyanos and Nyblade, 2007] as a framework and added many others regional studies; for South America, models by [Assumpção et al.,2013; Chulick et al.,2013; Lloyd et al., 2010]; for North America, the model by [Keller, 2013]; for Australia, the model by [Salmon et al., 2013]; for Antarctica, model ANTMoho [Baranov and Morelli, 2013] with update for West Antarctica (POLENET project, [Chaput et al., 2013]). For two orogens we have found the maximum depth at - 75 km (Tibet and Andes). In our model the average thickness of the continental crust is about 34 km (st. deviation 9 km) whereas in CRUST 2.0 model the average Moho for continental areas is about 38 km. The new Moho model for continents exhibits some remarkable disagreement at places with respect to global model CRUST 2.0. The difference in crustal thickness between these two models may amount up to 30 km, mainly due to improved resolution of our model's Moho boundary. There are significant changes in several regions: among them

  7. Ultrahigh-pressure metamorphism: tracing continental crust into the mantle

    NASA Astrophysics Data System (ADS)

    Chopin, Christian

    2003-07-01

    More and more evidence is being discovered in Phanerozoic collision belts of the burial of crustal rocks to previously unsuspected (and ever increasing) depths, presently on the order of 150-200 km, and of exhumation from such depths. This extends by almost one order of magnitude the depth classically ascribed to the metamorphic cycling of continental crust, and demonstrates its possible subduction. The pieces of evidence for this new, ultrahigh-pressure (UHP) metamorphism exclusively occur in the form of relics of high-pressure minerals that escaped back-transformation during decompression. The main UHP mineral indicators are the high-pressure polymorphs of silica and carbon, coesite and microdiamond, respectively; the latter often demonstrably precipitated from a metamorphic fluid and is completely unrelated to kimberlitic diamond or any shock event. Recent discoveries of pyroxene exsolutions in garnet and of coesite exsolutions in titanite suggest a precursor garnet or titanite containing six-fold coordinated silicon, therefore still higher pressures than implied by diamond stability, on the order of 6 GPa. The UHP rocks raise a formidable geological problem: that of the mechanisms responsible for their burial and, more pressingly, for their exhumation from the relevant depths. The petrological record indicates that large tracts of UHP rocks were buried to conditions of low T/ P ratio, consistent with a subduction-zone context. Decompression occurred in most instances under continuous cooling, implying continuous heat loss to the footwall and hangingwall of the rising body. This rise along the subduction channel - an obvious mechanical discontinuity and weak zone - may be driven by buoyancy up to mid-crustal levels as a result of the lesser density of the acidic crustal rocks (even if completely re-equilibrated at depth) after delamination from the lower crust, in a convergent setting. Chronological studies suggest that the rates involved are typical plate

  8. New Tracers of Gas Migration in the Continental Crust

    SciTech Connect

    Kurz, Mark D.

    2015-11-01

    Noble gases are exceptional tracers in continental settings due to the remarkable isotopic variability between the mantle, crust, and atmosphere, and because they are inert. Due to systematic variability in physical properties, such as diffusion, solubility, and production rates, the combination of helium, neon, and argon provides unique but under-utilized indices of gas migration. Existing noble gas data sets are dominated by measurements of gas and fluid phases from gas wells, ground waters and hot springs. There are very few noble gas measurements from the solid continental crust itself, which means that this important reservoir is poorly characterized. The central goal of this project was to enhance understanding of gas distribution and migration in the continental crust using new measurements of noble gases in whole rocks and minerals from existing continental drill cores, with an emphasis on helium, neon, argon. We carried out whole-rock and mineral-separate noble gas measurements on Precambrian basement samples from the Texas Panhandle. The Texas Panhandle gas field is the southern limb of the giant Hugoton-Panhandle oil and gas field; it has high helium contents (up to ~ 2 %) and 3He/4He of 0.21 (± 0.03) Ra. Because the total amount of helium in the Panhandle gas field is relatively well known, crustal isotopic data and mass balance calculations can be used to constrain the ultimate source rocks, and hence the helium migration paths. The new 3He/4He data range from 0.03 to 0.11 Ra (total), all of which are lower than the gas field values. There is internal isotopic heterogeneity in helium, neon, and argon, within all the samples; crushing extractions yield less radiogenic values than melting, demonstrating that fluid inclusions preserve less radiogenic gases. The new data suggest that the Precambrian basement has lost significant amounts of helium, and shows the importance of measuring helium with neon and argon. The 4He/40Ar values are particularly useful

  9. The silicon isotope composition of the upper continental crust

    NASA Astrophysics Data System (ADS)

    Savage, Paul S.; Georg, R. Bastian; Williams, Helen M.; Halliday, Alex N.

    2013-05-01

    The upper continental crust (UCC) is the major source of silicon (Si) to the oceans and yet its isotopic composition is not well constrained. In an effort to investigate the degree of heterogeneity and provide a robust estimate for the average Si isotopic composition of the UCC, a representative selection of well-characterised, continentally-derived clastic sediments have been analysed using high-precision MC-ICPMS. Analyses of loess samples define a narrow range of Si isotopic compositions (δ30Si = -0.28‰ to -0.15‰). This is thought to reflect the primary igneous mineralogy and predominance of mechanical weathering in the formation of such samples. The average loess δ30Si is -0.22 ± 0.07‰ (2 s.d.), identical to average granite and felsic igneous compositions. Therefore, minor chemical weathering does not resolvably affect bulk rock δ30Si, and loess is a good proxy for the Si isotopic composition of unweathered, crystalline, continental crust. The Si isotopic compositions of shales display much more variability (δ30Si = -0.82‰ to 0.00‰). Shale Si isotope compositions do not correlate well with canonical proxies for chemical weathering, such as CIA values, but do correlate negatively with insoluble element concentrations and Al/Si ratios. This implies that more intensive or prolonged chemical weathering of a sedimentary source, with attendant desilicification, is required before resolvable negative Si isotopic fractionation occurs. Shale δ30Si values that are more positive than those of felsic igneous rocks most likely indicate the presence of marine-derived silica in such samples. Using the data gathered in this study, combined with already published granite Si isotope analyses, a weighted average composition of δ30Si = -0.25 ± 0.16‰ (2 s.d.) for the UCC has been calculated.

  10. Influence of water on rheology and strain localization in the lower continental crust

    NASA Astrophysics Data System (ADS)

    Getsinger, A. J.; Hirth, G.; Stünitz, H.; Goergen, E. T.

    2013-07-01

    We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnøya, Norway. Over a distance of ˜1 m, the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice-preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (aH2O) during deformation must have been high (˜1) in the sheared gabbro compared with the nonhydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promote strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.

  11. Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons

    NASA Technical Reports Server (NTRS)

    Stevenson, Ross K.; Patchett, P. Jonathan

    1990-01-01

    Results from the fractionation of zircon by sedimentary processes into continental margin sandstone yield information on the preservation of preexisting continental crust in the form of zircon, making it possible to distinguish between the contrasting theories of gradual growth versus constant volume of continental crust over geologic time. In this work, Hf-176/Hf-177 ratios were determined for detrital zircon fractions from 2.0-2.5, 2.6-3.0, and pre-3.0 Gyr old sandstones from the Canadian-Shield, the North-Atlantic, the Wyoming, and the Kaapvaal Cratons. Results pointed to small amounts of continental crust prior to 3.0 Gyr ago and a rapid addition of continental crust between 2.5 and 3.0 Gyr ago, consistent with the gradual growth of continental crust, and giving evidence against no-growth histories.

  12. Physical processes in the growth of the continental crust

    NASA Technical Reports Server (NTRS)

    Schubert, G.

    1988-01-01

    Major mechanisms of crustal addition are volcanism and plutonism at plate boundaries and within plate interiors. One approach to deciding if island arc magmatism dominated ancient crustal growth is to assess the rate at which the process has operated in the recent past. The localized addition rates were found to be comparable to present day global rates. One physical observable that was used to constrain models of crustal growth is sea level. A simple physical model was developed to explore the consequences of constant freeboard (the height of the continents above sea level). Global geoid and sea floor topography data were used to identify and study oceanic plateaus and swells that have either continental crustal roots or anomalously thick ocean crusts.

  13. Growth of early continental crust by partial melting of eclogite.

    PubMed

    Rapp, Robert P; Shimizu, Nobumichi; Norman, Marc D

    2003-10-09

    The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.

  14. The Crust Has Changed: Evidence for and implications of age dependent Sm/Nd ratios in juvenile continental crust

    NASA Astrophysics Data System (ADS)

    Brown, S. T.

    2011-12-01

    The Nd isotope systematics of crustal granites and their presumed source materials are used to reconstruct the Sm/Nd ratios of the continental crust with distinct mantle extraction ages (based on DePaolo, 1988 EPSL and Bennett and DePaolo, 1987 GSA Bull). A more extensive, literature-derived data set for the western USA supports the isotopic mapping of Bennett and DePaolo. Calculated Sm/Nd ratios of the crust show a secular increase from the Archean to approximately 1.5 Ga. Modern island arc lavas, analogs for juvenile continental crust have the lowest Sm/Nd, consistent with the inferred trend from the granites. Possible reasons for the increasing Sm/Nd include changes in the Sm/Nd ratio of the mantle reservoir and/or changes in the mineralogy of the reservoir from which the crust was extracted. It is postulated that the primary control of Sm/Nd in the continental crust is the residual mineralogy in the reservoirs from which the crust was extracted. This is possible because changes in the geothermal gradient through Earth's history affect the residual mineralogy during the extraction of continental crust. Recent phase equilibria studies relevant to modern island arcs suggest that accessory minerals such as allanite buffer the LREE budget from the subducting sediment and basaltic slab, the primary source of REE in island arc lavas. Crust production early in Earth's history likely occurred at temperatures too high for allanite stability, meaning Sm/Nd ratios were likely controlled by garnet and/or amphibole in either subducted crust or the mantle reservoir. Modeling results will show how changing residual mineralogy during crust extractions can produce the observed changes in Sm/Nd ratios. More broadly the inferred time dependence for Sm/Nd may relate to the silica content of juvenile crust throughout Earth's history.

  15. Geometries of hyperextended continental crust in northeastern continental brazilian margin: insights from potential field and seismic interpretation

    NASA Astrophysics Data System (ADS)

    Magalhães, José; Barbosa, José; Ribeiro, Vanessa; Oliveira, Jefferson; Filho, Osvaldo; Buarque, Bruno

    2016-04-01

    The study region encompasses a set of three basins located at Northeast Brazilian continental margin: Pernambuco (south sector), Paraíba and Natal platform (north sector). These basins were formed during the last stage of separation between South America and African plates during Cretaceous. The continental breakup in these regions occurred probably during the Middle-Upper Albian (~102 m.y). The adjacent basement rocks belong to Borborema Province (BP), which was formed due a complex superposition between Pre-Cambrian orogenic cycles. The structural framework of BP is dominated by large shear zones that divided this province in three main tectonic domains: South, Central and North. The Pernambuco Basin is located in the South Domain and the Paraíba and Natal platform basins are related to the Central Domain. The tectonic and magmatic evolution of the Pernambuco Basin was influenced by oblique rifting (~ 35° to rift axis) and a thermal anomaly probably caused by the Santa Helena hotspot. The north sector represents a continental shelf characterized by basement high with a narrow platform and an abrupt shelf break on transition to the abyssal plain. The continental platform break of this sector was parallel to the rift axis. In this way, we present a regional structural interpretation of these sectors of Brazilian rifted margin based on interpretation and 2D forward modeling of potential field and 2D seismic data. The magnetic maps (Reduction to magnetic pole and Analytic signal) revealed the influence of an alternating pattern of large narrow magnetic and non-magnetic lineaments, oriented NE-SW, E-W and NW-SE. In the Pernambuco Basin these lineaments (NE-SW and E-W) are related to shear zones in the hyperextended basement which is interpreted as a continuation of the granitic-gneissic and metasedimentary rocks of the South Domain of BP. The Paraíba and Natal platform basins show a slight change in the orientation of structures trending E-W (shear zones in

  16. Excavation and melting of the Hadean continental crust by Late Heavy Bombardment

    NASA Astrophysics Data System (ADS)

    Shibaike, Yuhito; Sasaki, Takanori; Ida, Shigeru

    2016-03-01

    No Hadean rocks have ever been found on Earth's surface except for zircons-evidence of continental crust, suggesting that Hadean continental crust existed but later disappeared. One hypothesis for the disappearance of the continental crust is excavation/melting by the Late Heavy Bombardment (LHB), a concentration of impacts in the last phase of the Hadean eon. In this paper, we calculate the effects of LHB on Hadean continental crust in order to investigate this hypothesis. Approximating the size-frequency distribution of the impacts by a power-law scaling with an exponent α as a parameter, we have derived semi-analytical expressions for the effects of LHB impacts. We calculated the total excavation/melting volume and area affected by the LHB from two constraints of LHB on the Moon, the size of the largest basin during LHB, and the density of craters larger than 20 km. We also investigated the effects of the value of α. Our results show that LHB does not excavate/melt all of Hadean continental crust directly, but over 70% of the Earth's surface area can be covered by subsequent melts in a broad range of α. If there have been no overturns of the continental crust until today, LHB could be responsible for the absence of Hadean rocks because most of Hadean continental crust is not be exposed on the Earth's surface in this case.

  17. Ages and Growth of the Continental Crust from Radiogenic Isotopes

    NASA Astrophysics Data System (ADS)

    Patchett, P. J.; Samson, S. D.

    2003-12-01

    The development and application of radiogenic isotopes to dating of geologic events, and to questions of growth, evolution, and recycling processes in the continental crust are mature areas of scientific inquiry. By this we understand that many of the approaches used to date rocks and constrain the evolution of the continents are well established, even routine, and that the scope of data available on age and evolution of continents is very large. This is not to say that new approaches have not been developed in recent years, or that new approaches and/or insights cannot be developed in the future. However, the science of continental crustal evolution is definitely a domain where many of the problems are well defined, the power of the techniques used to solve them are well known, and the limitations of field and laboratory databases, as well as the preserved geologic record, are understood.From the very early days of crustal evolution studies, it was innovations and improvements in laboratory techniques that drove the pace of discovery (e.g., Holmes, 1911; Nier, 1939). This remained true through all the increments in capability reviewed in this chapter, up to the present day. Thus, continental crustal evolution is an area of Earth science where a species of very laboratory-oriented investigator, the "radiogenic isotope geologist" or "geochronologist," has made major advances, even breakthroughs, in understanding. This is true in spite of the fact that many of the individuals of the species may have lacked field expertise, or even more than a primitive level of geologic background. Because design and building of instruments like radiation detectors or mass spectrometers requires a knowledge of physics, many of the early practitioners of rock dating were physicists, like Alfred Nier (cited above). Since the 1970s, essentially all mass spectrometers have been constructed by specialized commercial firms, and the level of physics expertise among isotope geologists has

  18. Density constraints on the formation of the continental Moho and crust

    NASA Technical Reports Server (NTRS)

    Herzberg, C. T.; Carr, M. J.; Fyfe, W. S.

    1983-01-01

    As part of a general review of recent high-resolution seismological data, constraints are established for the distribution of mass in the continental crust and upper mantle of the earth. It is shown that the densities of mantle magmas such as MORB-like tholeiites, oicrites, and komatites at 10 kilobars are greater than densities in the continental crust. In order to explain the deviation, a mechanism is proposed for the control of mass transfer during igneous and metamorphic processes throughout geological time. Within the context of a description of the mechanism, the continental crust is characterized as a density filter through which only highly evolved magmas with H2O may pass. It is shown that the removal of less evolved magmas can lead to an over accretion (penetration) or an under accretion (underplating) of magmas in the continental Moho. Some implications of the proposed mechanism for current theoretical models of crust formation are discussed in detail.

  19. The growth of the continental crust: Constraints from radiogenic isotope geochemistry

    NASA Technical Reports Server (NTRS)

    Taylor, Paul N.

    1988-01-01

    Most models for evolution of continental crust are expressed in the form of a diagram illustrating the cumulative crustal mass (normalized relative to the present crustal mass) as a function of time. Thus, geochronological data inevitably play a major role in either constructing or testing crustal growth models. For all models, determining the start-time for effective crustal accretion is of vital importance. To this end, the continuing search for, and reliable characterization of, the most ancient crustal rock-units remains a worthy enterprise. Another important role for geochronology and radiogenic isotope geochemistry is to assess the status of major geological events as period either of new crust generation or of reworking of earlier formed continental crust. For age characterization of major geological provinces, using the critieria outined, the mass (or volume) of crust surviving to the present day should be determinable as a function of crust formation age. More recent developments, however, appear to set severe limitations on recycling of crust, at least by the process of sediment subduction. In modeling crustal growth without recycling, valuable constaints on growth rate variations through time can be provided if variations in the average age of the continental crust can be monitored through geological history. The question of the average age of the exposed continental crust was addressed by determining Sm-Nd crustal residence model ages (T-CR) for fine-grained sediment loads of many of the world's major rivers.

  20. 40K-(40)Ar constraints on recycling continental crust into the mantle

    PubMed

    Coltice; Albarede; Gillet

    2000-05-05

    Extraction of potassium into magmas and outgassing of argon during melting constrain the relative amounts of potassium in the crust with respect to those of argon in the atmosphere. No more than 30% of the modern mass of the continents was subducted back into the mantle during Earth's history. It is estimated that 50 to 70% of the subducted sediments are reincorporated into the deep continental crust. A consequence of the limited exchange between the continental crust and the upper mantle is that the chemistry of the upper mantle is driven by exchange of material with the deep mantle.

  1. Crustal radiogenic heat production and the selective survival of ancient continental crust

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1985-01-01

    It is pointed out that the oldest terrestrial rocks have so far revealed no evidence of the impact phase of Earth evolution. This observation suggests that processes other than impact were dominant at the time of stabilization of these units. However, a use of the oldest terrestrial rocks as a sample of the early terrestrial crust makes it necessary to consider the possibility that these rocks may represent a biased sample. In the present study, the global continental heat flow data set is used to provide further evidence that potassium, uranium, and thorium abundances are, on the average, low in surviving Archean crust relative to younger continental crust. An investigation is conducted of the implications of relatively low crustal radiogenic heat production to the stabilization of early continental crust, and possible Archean crustal stabilization models are discussed.

  2. Crustal radiogenic heat production and the selective survival of ancient continental crust

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1985-01-01

    It is pointed out that the oldest terrestrial rocks have so far revealed no evidence of the impact phase of earth evolution. This observation suggests that processes other than impact were dominant at the time of stabilization of these units. However, a use of the oldest terrestrial rocks as a sample of the early terrestrial crust makes it necessary to consider the possibility that these rocks may represent a biased sample. In the present study, the global continental heat flow data set is used to provide further evidence that potassium, uranium, and thorium abundances are, on the average, low in surviving Archean crust relative to younger continental crust. An investigation is conducted of the implications of relatively low crustal radiogenic heat production to the stabilization of early continental crust, and possible Archean crustal stabilization models are discussed.

  3. On the relationship between sequential faulting, margin asymmetry and highly thinned continental crust

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Heine, Christian; Pérez-Gussinyé, Marta; Sobolev, Stephan

    2014-05-01

    The architecture of magma-poor continental margins is remarkably variable. The width of highly thinned continental crust (with a thickness < 10 km) varies from 70 km off Iberia, and 200 km offshore Angola, to over 300 km in the Antarctic Enderby Basin. The respective conjugate margin, however, is restricted to few tens of kilometres resulting in large scale crustal asymmetry. Growing evidence from rifted continental margins in the North and South Atlantic, as well as from the East Australia/Lord Howe Rise margin pair supports the idea that rifts with a very wide margin and a narrow conjugate are rather the rule than the exception. In this study, we use numerical thermo-mechanical models to investigate the dynamics of rifting. Our simulations apply an elasto-visco-plastic rheology formulation that relies on laboratory-derived flow laws for crustal and mantle rock. The models are constrained by geophysical and geological observations like limited melt generation, cold initial geotherms, and mafic lower crustal rheology. We show that small-scale lateral rift migration simultaneously explains the observed margin asymmetry and the presence of highly thinned continental crust. Rift migration results from two fundamental processes: (1) Strain hardening in the rift centre due to cooling of upwelling mantle material; (2) Formation of a low viscosity exhumation channel adjacent to the rift centre that is generated by heat transfer from the upwelling mantle and enhanced by viscous strain softening. Rift migration takes place in a steady-state manner and is accomplished by oceanward-younging sequential faults within the upper crust and balanced through lower crustal flow. We demonstrate that the rate of extension has paramount control on margin width. Since higher velocities lead to elevated heat flow within the rift and hence to hot and weak lower crust, a larger low-viscosity exhumation channel is generated that facilitates rift migration leading to wider margins. The South

  4. Flow of material under compression in weak lower continental crust can cause post-rift uplift of passive continental margins

    NASA Astrophysics Data System (ADS)

    Chalmers, James

    2014-05-01

    There are mountain ranges up to more than 2 km high along many passive continental margins (e.g. Norway, eastern Australia, eastern Brazil, SE and SW Africa, east and west Greenland etc.), dubbed Elevated Passive Continental Margins (EPCMs). EPCMs contain several features in common and observations indicate that uplift of these margins took place after continental break-up. There are many explanations for their formation but none that satisfy all the observations. Lack of a geodynamical mechanism has meant that there has been difficulty in getting the community to accept the observational evidence. Formation of a passive continental margin must take place under conditions of tension. After rifting ceases, however, the margin can come under compression from forces originating elsewhere on or below its plate, e.g. orogeny elsewhere in the plate or sub-lithospheric drag. The World Stress Map (www.world-stress-mp.org) shows that, where data exists, all EPCMs are currently under compression. Under sufficient compression, crust and/or lithosphere can fold, and Cloetingh & Burov (2010) showed that many continental areas may have folded in this way. The wavelengths of folding observed by Cloetingh & Burov (2010) imply that the lower crust is likely to be of intermediate composition; granitic lower crust would fold with a shorter wavelength and basic lower crust would mean that the whole lithosphere would have to fold as a unit resulting in a much longer wavelength. Continental crust more than 20 km thick would be separated from the mantle by a weak layer. However, crust less thick than that would contain no weak layers would become effectively annealed to the underlying strong mantle. Under sufficient horizontal compression stress, material can flow in the lower weak layer towards a continental margin from the continental side. The annealed extended crust and mantle under the rift means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material

  5. Beyond KTB - electrical conductivity of the deep continental crust

    NASA Astrophysics Data System (ADS)

    Glover, Paul W. J.; Vine, F. J.

    1995-01-01

    Great strides have been made in understanding the upper part of the crust by in-situ logging in, and laboratory experiments on core recovered from super-deep bore-holes such as the KTB. These boreholes do not extend into the lower crust, and can contribute little to the elucidation of mechanisms that produce the high electrical conductivities that are commonly observed therein by magneto-telluric (MT) methods. Laboratory studies at simulated lower crustal conditions of temperature, pressure and saturation, on electrolyte saturated rocks thought to have been derived from the lower crust, have not been possible up until now due to their experimental difficulty. It is necessary to subject electrolyte-saturated rock samples to independently controlled confining and pore-fluid pressure, which implies that the rock be sleeved in some impermeable but deformable material, that can withstand the very high temperatures required. Metals are the only materials capable of being used, but these cause great difficulties for cell sealing and conductivity measurement. In this paper we describe recent breakthroughs in experimental work, specifically the development of two new types of sophisticated metal/ceramic seal, and a conductivity measurement technique that enables the measurement of saturated rock conductivity in the presence of a highly conducting metallic sleeve. The advances in experimental technique have enabled us to obtain data on the electrical conductivity of brine saturated basic, acidic and graphite-bearing rocks at lower crustal temperatures and raised pressures. These data have facilitated the comparison of MT derived crustal electrical conductivity profiles with profiles obtained from laboratory experiments for the first time. Initial modelling shows a good agreement between laboratory derived and MT derived profiles only if the mid-crust is composed of amphibolite pervaded by aqueous fluids, and the lower crust is composed of granulite that is saturated with

  6. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 ga.

    PubMed

    Harrison, T M; Blichert-Toft, J; Müller, W; Albarede, F; Holden, P; Mojzsis, S J

    2005-12-23

    The long-favored paradigm for the development of continental crust is one of progressive growth beginning at approximately 4 billion years ago (Ga). To test this hypothesis, we measured initial 176Hf/177Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. epsilonHf (deviations of 176Hf/177Hf from bulk Earth in parts per 10(4)) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/Hf approximately 0.01), perhaps as early as 4.5 Ga. Positive epsilon(Hf) deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle.

  7. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius

    NASA Astrophysics Data System (ADS)

    Ashwal, Lewis D.; Wiedenbeck, Michael; Torsvik, Trond H.

    2017-01-01

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U-Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U-Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ~700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.

  8. Exploring the Geological Structure of the Continental Crust.

    ERIC Educational Resources Information Center

    Oliver, Jack

    1983-01-01

    Discusses exploration and mapping of the continental basement using the seismic reflection profiling technique as well as drilling methods. Also discusses computer analysis of gravity and magnetic fields. Points out the need for data that can be correlated to surface information. (JM)

  9. No evidence for Hadean continental crust within Earth's oldest evolved rock unit

    NASA Astrophysics Data System (ADS)

    Reimink, J. R.; Davies, J. H. F. L.; Chacko, T.; Stern, R. A.; Heaman, L. M.; Sarkar, C.; Schaltegger, U.; Creaser, R. A.; Pearson, D. G.

    2016-10-01

    Due to the acute scarcity of very ancient rocks, the composition of Earth's embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today's oceanic crust, or included significant amounts of continental crust remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth--a 4,019.6 +/- 1.8 Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.

  10. Implications of Continental Crust Extension for Plate Reconstruction: AN Example from the Gulf of Mexico

    NASA Astrophysics Data System (ADS)

    Dunbar, John A.; Sawyer, Dale S.

    1987-12-01

    The prerift reconstruction of continental plates bounded by rifted margins requires the closure of relative motion accomplished both by seafloor spreading and by the extension of continental crust during the rift phase of continental breakup. Continental extension is not expected to be pervasive throughout the plate, but to be confined to a zone of up to several hundred kilometers in width. The direction of particle motion in this zone is expected to be parallel to flow lines followed by the rigid portions of the plates. The "best fit" between rifted continental margins is then described by the rotation angle about an Euler pole which best closes both the oceanic crust and the extension within the continental crust, along small circles about the Euler pole. As an example application of these concepts, the pattern of Late Mesozoic crust extension within the Gulf of Mexico basin is used to constrain the location of the Euler pole and angle of rotation of the Yucatan block with respect to North America. Both crust type and the degree of extension within the transitional crust surrounding the basin are estimated on a point-by-point basis from bathymetry and basement depth. The root-mean-square (rms) misfit in the apparent total closure of both oceanic crust and extension within the continental crust is computed for a range of possible poles of opening. A contour map of misfit versus pole location reveals a global minimum rms misfit of ±47 km for a 45° counterclockwise opening of the Gulf of Mexico basin about an Euler pole at 25°N, 79°W. An elongate trend of poles, for which the misfit is less than 100 km, extends from the Bahama Islands eastward across the Atlantic and through North Africa. The reconstruction of the Gulf of Mexico for the best fit pole places the palinspastically restored Yucatan block in contact with North America along the Texas and Louisiana shelf. The resulting restored southern margin of North America fits against the the northern margin of

  11. Misho mafic complex - A part of paleotethyan oceanic crust or a magmatism in continental rift?

    NASA Astrophysics Data System (ADS)

    Azimzadeh, Zohreh; Jahangiri, Ahmad; Saccani, Emilio; Dilek, Yildirim

    2013-04-01

    Misho Mafic Complex (NW Iran) represents a significant component of the West Cimmerian domain in Paleo-Tethys. The Misho Mafic Complex (MMC) consists of gabbro (mainly) and norıte,olivine gabbro, anorthosite and diorite with the east- west sereight. MMC has ıntrussıved ın Kahar sedımrtery Infta- Cambrıan rocks, crosscut by abundant basaltic dykes and the overlying basaltic sheeted dyke complex. Kahar sedimentary rocks are representing the northern margin of Gondwana. Misho mafic complex are covered by Permian sedimentary rocks. The gabbros and basaltic dykes have MORB affinities. MMC formed as a product of interactions between a depleted MORB-type asthenosphere and plume-type material. Mafic rocks represent an early Carboniferous magmatic event developed during the continental break-up of the northern edge of Gondwanaland that led to the opening of Paleotethys. Alternatively, these magmas may have been emplaced into the continental crust at the continental margin soon after the oceanic crust was formed (that is the oceanic crust was still narrow). There is no data for discriminating between these two hypotheses. In first hypothesis MMC is a part of ophiolites related to paleotethyan oceanic crust and the rocks that were above this crustal level should have necessarily been eroded. In another hypothesis Misho complex represents an aborted rift in a triple junction. Above a mantle plume, the continental crust breaks along three directions at 120 degrees. But, soon after, the extension proceeds along two of these three direction. Between them is formed the oceanic crust. The continental extension along the third direction is aborted. Here no oceanic crust if formed and there is only rifted, thinned continental crust. But, also in the aborted branch MORB magmatism can occur for short time. In this hypothesis, the Misho complex was never associated with oceanic crust, but was anyway associated with the opening of the Paleotethys. This magmatism was originally

  12. Growth of early continental crust controlled by melting of amphibolite in subduction zones.

    PubMed

    Foley, Stephen; Tiepolo, Massimo; Vannucci, Riccardo

    2002-06-20

    It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process.

  13. Continental crust in deep-water basins of East Arctic region

    NASA Astrophysics Data System (ADS)

    Artyushkov, Eugene; Belyaev, Igor; Chekhovich, Peter; Poselov, Victor

    2013-04-01

    The nature of the crust in deep-water basins (1.5-4 km) in East Arctic is a matter of debates. The occurrence of continental crust has been demonstrated by deep-sea drilling only for the central part of the Lomonosov Ridge. Many authors suggest that the Mendeleev High and the Makarov and Podvodnikov basins in the Amerasian Basin are underlain by oceanic crust. In these regions the mean P-wave velocities in the consolidated crust are higher than in most continental areas. However, the thickness of this layer is several times larger (15-30 km) than that of the oceanic crust (7 km) and it includes a thin granitic layer (2-5 km). To explain this anomalous structure and thickness of the crust it is commonly supposed that in the Late Jurassic and Cretaceous the oceanic crust was formed in the above regions by sea-floor spreading accompanies by melting out of large masses of crustal material on a hot spot like on the present Iceland hot spot. Other investigators consider the crust in the above regions as a continental one. An important argument is the evolution of the subsidence in time which is quite different from a square root of time that typical of oceanic crust. Thus, according to the dredging data, the Mendeleev High remained near to sea level for 170 Myr since the Late Silurian and until the Early Permian. This would be absolutely impossible for a cooling hot spot on the oceanic crust. Furthermore, the structure of consolidated crust in these areas is similar to that in some ultradeep basins within the continents and on their passive margins, e.g., in the East Barents, North Caspian and North Chukchi basins which were originally formed on continental crust. To produce the water loaded subsidence by 1.5-4 km by lithospheric stretching, the lithosphere should be stretched by 1.5-4 times. However, in most of the seismic reflection profiles, no large tensile deformations can be observed. Under such circumstances the transformation of gabbro in the lower crust into

  14. Active Dehydration, Delamination and Deformation of Transitional Continental Crust in an Arc-Continent Collision, Taiwan

    NASA Astrophysics Data System (ADS)

    Byrne, T. B.; Rau, R. J.; Chen, K. H.; Huang, H. H.; Wang, Y. J.; Ouimet, W. B.

    2014-12-01

    A new study of the 3-D velocity structure of Taiwan, using a new tomographic model (Vp and Vs; Huang et al., 2014), suggests that subducted continental crust is delaminated from the subducting mantle of the Eurasia plate and progressively deformed by the subducting Philippine Sea plate. In southern Taiwan, vertical sections show an east-dipping, asymmetric lobe of low velocity that projects down dip to a band of seismicity interpreted as the Wadati-Benioff zone of the subducting Eurasian plate. Seismic tremors in the mid-crust also suggest dehydration (Chuang et al., 2014), consistent with prograde metamorphism of crustal materials. In central Taiwan, however, the seismicity of the W-B zone progressively disappears and the low velocity lobe shallows and broadens. The velocity structure of the lower and middle crust (represented by the 7.5 and 6.5 km/sec isovelocity surfaces, respectively) also appear distinctly out-of-phase, with the lower crust forming a broad, smooth synformal structure that contrasts with the higher amplitude undulations of the middle crust. These mid-crust structures appear as smaller irregular lobes separated by patches of higher velocity. In northern Taiwan, the velocity structure of the lower and middle crust again appear "in phase" and form a symmetrical crustal root centered beneath the Central Range. Seismicity patterns and 3-D analysis of the velocity structure also show the western edge of the PSP subducting beneath the eastern Central Range. We interpret these south-to-north changes to reflect the partial subduction (southern Taiwan), delamination (central Taiwan) and deformation (northern Taiwan) of continental-like crust. Support for these interpretation comes from: 1) unusually high rates of surface uplift (up to 15 mm/yr; Ching et al., 2011); 2) Vp and Vs attenuation studies that suggest anomalously high temperatures; 3) evidence for NE-SW extension; and 4) anomalous areas of low topographic relief.

  15. On the origin of hot metasedimentary quartzites in the lower crust of continental arcs

    NASA Astrophysics Data System (ADS)

    Chin, Emily J.; Lee, Cin-Ty A.; Tollstrup, Darren L.; Xie, Liewen; Wimpenny, Josh B.; Yin, Qing-Zhu

    2013-01-01

    Volcanic arcs associated with subduction zones are thought to be the primary building blocks of continents. The composition of the magmas, particularly in continental arcs, is the product of mixing between differentiation of juvenile magmas and pre-existing crustal wallrock, the former being typically mafic and the latter more silicic. Because the upper continental crust is on average thought to be more silicic than the mafic lower crust, mixing with silicic endmembers should occur primarily in the upper crust. However, we show here that the lower crust of continental arcs contains silicic metasediments. We examine garnet-bearing, granulite-facies sedimentary quartzite xenoliths from the Sierra Nevada batholith in California, a Cretaceous continental arc. The quartzites have equigranular textures and contain quartz (>50%), plagioclase (<30%), garnet (10%), and small amounts (<1%) of rutile, aluminosilicate, biotite, monazite, zircon, graphite and trace orthopyroxene. Cathodoluminescent images show zircons with rounded detrital cores mantled by metamorphic overgrowths. Hf isotopic model ages and U-Pb upper intercept ages, for a given zircon, are similar, but the zircon population shows variable protolith ages ranging from Proterozoic to Archean. In contrast, all zircons share similar lower intercept U-Pb ages (103±10 Ma), which coincide with the peak of arc magmatism in the Sierra Nevada. The Precambrian protolith ages are similar to North American cratonal basement, and together with the abundance of quartz and detrital zircons, suggest that these quartzites represent ancient, passive margin sediments instead of juvenile active margin sediments in the oceanic trench and accretionary prism. Importantly, these quartzites record peak metamorphic temperatures and pressures of 700-800 °C using Ti-in-quartz thermometry and 0.7-1.1 GPa using garnet-aluminosilicate-plagioclase thermobarometry, indicating that these xenoliths experienced significant heating and possible

  16. A relatively reduced Hadean continental crust and implications for the early atmosphere and crustal rheology

    NASA Astrophysics Data System (ADS)

    Yang, Xiaozhi; Gaillard, Fabrice; Scaillet, Bruno

    2014-05-01

    It is widely believed that the Earth was strongly reduced during its early accretion, however, the transition from the reduced state that prevailed during Earth's early period to the modern oxidized crust and mantle has never been captured by geochemical surveys on Earth materials as old as ∼4.0 billion years ago. By combining available trace-elements data of igneous zircons of crustal origin, we show that the Hadean continental crust was significantly more reduced than its modern counterpart and experienced progressive oxidation till ∼3.6 billions years ago. We suggest that the increase in the oxidation state of the Hadean continental crust is related to the progressive decline in the intensity of chondritic addition during the late veneer. Inputs of carbon- and hydrogen-rich chondritic materials during the formation of Hadean granitic crust must have favored strongly reduced magmatism. The conjunction of cold, wet and reduced granitic magmatism during the Hadean implies the production of methane-rich fluids, in addition to the CO- and H2-rich volcanic species produced by degassing of hot reduced basaltic melts as modified by delivered materials during the late veneer. When the late veneer events ended, magma produced by normal decompression melting of the mantle imparted more oxidizing conditions to erupted lavas and the related crust, emitting CO2- and H2O-rich gases. Our model suggests that the Hadean continental crust was possibly much weaker than present-day, facilitating intrusion of underplating magma and thus allowing faster crustal growth in the early Earth.

  17. From a collage of microplates to stable continental crust - an example from Precambrian Europe

    NASA Astrophysics Data System (ADS)

    Korja, Annakaisa

    2013-04-01

    Svecofennian orogen (2.0-1.7 Ga) comprises the oldest undispersed orogenic belt on Baltica and Eurasian plate. Svecofennian orogenic belt evolved from a series of short-lived terrane accretions around Baltica's Archean nucleus during the formation of the Precambrian Nuna supercontinent. Geological and geophysical datasets indicate W-SW growth of Baltica with NE-ward dipping subduction zones. The data suggest a long-lived retreating subduction system in the southwestern parts whereas in the northern and central parts the northeasterly transport of continental fragments or microplates towards the continental nucleus is also documented. The geotectonic environment resembles that of the early stages of the Alpine-Himalayan or Indonesian orogenic system, in which dispersed continental fragments, arcs and microplates have been attached to the Eurasian plate margin. Thus the Svecofennian orogeny can be viewed as proxy for the initial stages of an internal orogenic system. Svecofennian orogeny is a Paleoproterozoic analogue of an evolved orogenic system where terrane accretion is followed by lateral spreading or collapse induced by change in the plate architecture. The exposed parts are composed of granitoid intrusions as well as highly deformed supracrustal units. Supracrustal rocks have been metamorphosed in LP-HT conditions in either paleo-lower-upper crust or paleo-upper-middle crust. Large scale seismic reflection profiles (BABEL and FIRE) across Baltica image the crust as a collage of terranes suggesting that the bedrock has been formed and thickened in sequential accretions. The profiles also image three fold layering of the thickened crust (>55 km) to transect old terrane boundaries, suggesting that the over-thickened bedrock structures have been rearranged in post-collisional spreading and/or collapse processes. The middle crust displays typical large scale flow structures: herringbone and anticlinal ramps, rooted onto large scale listric surfaces also suggestive

  18. The formation of deep basins in High Arctic from metamorphism in continental crust

    NASA Astrophysics Data System (ADS)

    Artyushkov, Eugene; Belyaev, Igor; Chekhovich, Peter; Petrov, Eugene; Poselov, Viktor

    2014-05-01

    In the East Barents and North Chukchi basins, 16-20 km deep, the crystalline crust is attenuated to 12-18 km (reference profiles 2-AR, 4-AR and 5-AR). P-wave velocities and densities in this layer are characteristic of the oceanic crust. However, the subsidence history in the basins is quite different from that typical of the oceanic crust. In both basins the subsidence continued for several hundred million years and one half of the deposits or more was formed long after the start of the subsidence when cooling of the oceanic plate would be already over. Moreover, the basins are 4-5 km deeper than it could be expected according to the thickness of the crystalline crust above the Moho boundary. In the absence of large free-air gravity anomalies, joint analysis of the gravity and seismic data indicates the existence under the Moho of thick layers of high-density and high-velocity eclogites. As can be seen in high resolution seismic profiles, the intensity of crustal stretching did not exceed 10% in the basins, and their formation can be predominantly attributed to a high-grade metamorphism in the mafic lower part of continental crust. At some episodes, strong increase in the rate of subsidence occurred in the basins. This indicates acceleration of metamorphism catalyzed by infiltration of mantle fluids. A set of the above features, abnormally large depth, long subsidence history with its acceleration at the late stages, and episodes of pronounced acceleration of the subsidence represent characteristic features of some other large hydrocarbon basins, e.g., of the North and South Caspian basins. These features can be used for prospecting new prolific provinces on the Arctic shelf. The Lomonosov ridge, Mendeleev high and the Makarov basin pertain to the same structural type. In the Oligocene they underwent erosion near to sea level with the formation of pronounced unconformity. Then at the end of Oligocene deep-water basins were formed in these regions. Rapid crustal

  19. Causes of long-term landscape evolution of "passive" margins and adjacent continental segments at the South Atlantic Ocean.

    NASA Astrophysics Data System (ADS)

    Glasmacher, Ulrich Anton; Hackspacher, Peter C.

    2013-04-01

    During the last 10 years research efforts have been devoted to understand the coupling between tectonic and surface processes in the formation of recent topography. Quantification of the rate at which landforms adapt to a changing tectonic, heat flow, and climate environment in the long term has become an important research object and uses intensively data revealed by low-temperature thermochronology, terrigenous cosmogenic nuclides, and geomorphological analyses. The influence of endogenic forces such as mantle processes as one of the causes for "Dynamic Topography Evolution" have been explored in a few studies, recently. In addition, the increased understanding how change in surface topography, and change in the amount of downward moving cold surface water caused by climate change affects warping isotherms in the uppermost crust allows further interpretation of low-temperature thermochronological data. "Passive" continental margins and adjacent continental segments especially at the South Atlantic ocean are perfect locations to quantify exhumation and uplift rates, model the long-term landscape evolution, and provide information on the influence of mantle processes on a longer time scale. This climate-continental margin-mantle process-response system is caused by the interaction between endogenic and exogenic forces that are related to the mantle-process driven rift - drift - "passive" continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Furthermore, the influence of major transform faults (also called: transfer zones, Fracture Zones (FZ)) on the long-term evolution of "passive" continental margins is still very much in debate. The presentation will provide insight in possible causes for the differentiated long-term landscape evolution along the South Atlantic Ocean.

  20. The speciation of marine particulate iron adjacent to active and passive continental margins

    NASA Astrophysics Data System (ADS)

    Lam, Phoebe J.; Ohnemus, Daniel C.; Marcus, Matthew A.

    2012-03-01

    We use synchrotron-based chemical-species mapping techniques to compare the speciation of suspended (1-51 μm) marine particulate iron collected in two open ocean environments adjacent to active and passive continental margins. Chemical-species mapping provides speciation information for heterogeneous environmental samples, and is especially good for detecting spectroscopically distinct trace minerals and species that could not be detectable by other methods. The average oxidation state of marine particulate iron determined by chemical-species mapping is comparable to that determined by standard bulk X-ray Absorption Near Edge Structure spectroscopy. Using chemical-species mapping, we find that up to 43% of particulate Fe in the Northwest Pacific at the depth of the adjacent active continental margin is in the Fe(II) state, with the balance Fe(III). In contrast, particulate iron in the eastern tropical North Atlantic, which receives the highest dust deposition on Earth and is adjacent to a passive margin, is dominated by weathered and oxidized Fe compounds, with Fe(III) contributing 90% of total iron. The balance is composed primarily of Fe(II)-containing species, but we detected individual pyrite particles in some samples within an oxygen minimum zone in the upper thermocline. Several lines of evidence point to the adjacent Mauritanian continental shelf as the source of pyrite to the water column. The speciation of suspended marine particulate iron reflects the mineralogy of iron from the adjacent continental margins. Since the solubility of particulate iron has been shown to be a function of its speciation, this may have implications for the bioavailability of particulate iron adjacent to passive compared to active continental margins.

  1. Do the trace element compositions of detrital zircons require Hadean continental crust?

    NASA Astrophysics Data System (ADS)

    Coogan, Laurence A.; Hinton, Richard W.

    2006-08-01

    The trace element compositions of Hadean zircons have been used in two ways to argue for the existence of Hadean continental crust. One argument is based on low crystallization temperatures of Hadean zircons that have been determined using a novel geothermometer based on the Ti content of zircons in equilibrium with rutile. The second argument is based on using the trace element abundances in zircons to calculate their parental melt compositions, especially the rare earth elements. Here we demonstrate that zircons that grow from a melt formed by basalt differentiation at modern mid-ocean ridges cannot be unambiguously distinguished from Hadean zircons on either of these grounds. Thus, we conclude that the trace element compositions of Hadean zircons are permissive of models that do not include the generation of continental crust in the Hadean.

  2. Decoupled Rb-Sr and Sm-Nd isotopic evolution of the continental crust

    NASA Technical Reports Server (NTRS)

    Goldstein, S. L.

    1988-01-01

    Evidence was presented that the Rb-Sr and Sm-Nd isotopic systems are decoupled in crust-mantle evolution. Rare earth element (including Sm and Nd) residue principally in silicates, and are resistant to mobilization by weathering and metamorphism. In contrast, Rb and Sr are easily fractionated by crustal processes and residue in carbonates as well as in silicates. As a result, continental Sr, but not Nd, can be recycled into the mantle by exchange of seawater with basalt at spreading ridges and by subduction of carbonates associated with ridge processes. These effects result in mean Rb-Sr ages of the continental crust and of the upper mantle that are too young. Crustal growth curves based largely on Rb-Sr data, such that of Hurley and Rand, are therefore incorrect.

  3. Li Isotopic Composition and Concentration of the Upper Continental Crust: New Insights from Desert Loess

    NASA Astrophysics Data System (ADS)

    Sauzeat, L.; Rudnick, R. L.; Chauvel, C.

    2014-12-01

    The isotopic composition of lithium (δ7Li) is recognized to be an excellent proxy of near-surface fluid-rock reactions during weathering. Using Li as a tracer of these processes however requires constraints on the average Li composition of terrestrial reservoirs, in particular that of the upper continental crust. To date, only one value for the average δ7Li value of the upper continental crust, derived from periglacial loess, shales, and granites is available in the literature (7δLi = 0 ± 4 (2σ), Teng et al., 2004). Several values exist for the average [Li] of the upper crust, but they differ by more than 30%. We measured the Li isotopic composition of about 30 desert and periglacial loess (unweathered windblown sediments) from several parts of the world (Europe, Argentina, China and Tajikistan). We demonstrate that desert loess, which is more homogeneous and representative of larger portions of the Earth's surface, provides a better proxy for the average composition of the upper continental crust compared to periglacial loess. The Li isotopic compositions and concentrations of desert loess are controlled by eolian sorting, which can be quantified as a binary isotopic mixing between a weathered fine-grained end-member and an unweathered coarse-grained end-member. Using correlations between Li isotopic compositions, Li concentrations and trace element concentrations in desert loess, we estimate new average values for the upper continental crust: 1 ± 2 (2σ); [Li] = 35.3 ± 4.6 (2σ) ppm. This δ7Li value is slightly higher than that previously published in Teng et al. (2004), but overlaps within uncertainty, whereas the [Li] is identical to that of Teng et al. (2004: 35 ± 11, 2σ); both new estimates have lower uncertainty. Our new estimate of [Li], along with that of Teng et al. (2004), are higher than all previous estimates for the upper continental crust, raising the question as to whether the average concentrations of other mobile alkali metals such as

  4. Numerical Simulations of the Incremental Intrusion of Granitic Magma into Continental Crust

    NASA Astrophysics Data System (ADS)

    Cao, W.; Kaus, B. J.; Paterson, S. R.

    2012-12-01

    We have employed the visco-elasto-plastic Finite-Element & Marker-in-cell code, MILAMIN_VEP, to carry out a 2D modeling study of the incremental intrusion of granitic magma into continental crust. Algorithms of multiple pulses of magma and pseudo-diking are implemented into the code. New magma of an initial circular shape is regularly replenished at "magma source" regions at sub-crustal depths. Pseudo-dikes of rectangular shapes are added at location where the maximum differential stress along the melt-solid interface is greater than an assigned tensile strength of the surrounding solid host rock. Preliminary results show that when diking and multiple pulses of magma are included, later pulses of magma rise higher and faster and even reach the Earth's surface in some cases by taking advantage of the pre-heated low-viscosity pathways created by earlier dikes and pulses of magma. Host rocks display bedding rotation, and downward flow at two sides of a growing magma chamber but show discordantly truncation when magma ascend through the weak channels made by dikes. The effect of the thermal structure of the crust was tested as well. In a cold crust, "diking" is critical in breaking the high-viscosity crust, guiding the direction of magma rising, and facilitating later magma pulses to form chambers. In a warmer crust, magma rises in the form of diapirs, after which dikes take over in transporting later pulses of magma to the surface. The simulations also suggest that a magma chamber incrementally constructed by multiple magma bathes is a very dynamic environment featuring intra-chamber convection and recycling previous batches of magma. In simulations without diking and multiple pulses, magma is unable to reach the shallow crust. Instead, it is stuck in the middle crust, as the viscosity of the upper crust is too large to permit rapid motion, and at the same time magma-induced stresses are insufficient to deform the upper crust in a plastic manner. Intra

  5. Do the trace element compositions of Hadean zircons require Hadean continental crust?

    NASA Astrophysics Data System (ADS)

    Coogan, L. A.; Hinton, R. W.

    2006-12-01

    The trace element composition of Hadean zircons have been used to argue for the existence of Hadean "continental crust". One argument is based on the low Ti content of Hadean zircons that, based a novel geothermometer that assumes equilibrium with rutile (Watson and Harrison, 2005), suggests a low crystallization temperature. The second argument is based on using the trace element abundances in zircons to calculate their parental melt compositions, especially the rare earth elements. We have undertaken an empirical study to determine if these approaches can unambiguously distinguish Hadean zircons from zircons grown from the late stage differentiates produced by basalt fractionation. We analysed the compositions of zircons in samples from the plutonic section of the oceanic crust collected from both fast- and slow-spreading ridges. This tectonic setting ensures that there was no contamination by continental material and thus these zircons should be broadly compositionally similar to zircons grown from the differentiation products of basaltic melt in the Hadean. The Ti-derived temperatures, and REE patterns, are similar to those of Hadean zircons. Thus we conclude that the trace element composition of Hadean zircons are permissive of models that do not include the generation of continental crust in the Hadean. Watson EB, Harrison TM (2005) Zircon thermometer reveals minimum melting conditions on Earliest Earth. Science 308:841-844

  6. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius

    PubMed Central

    Ashwal, Lewis D.; Wiedenbeck, Michael; Torsvik, Trond H.

    2017-01-01

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U–Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U–Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ∼700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India. PMID:28140395

  7. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius.

    PubMed

    Ashwal, Lewis D; Wiedenbeck, Michael; Torsvik, Trond H

    2017-01-31

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U-Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U-Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ∼700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.

  8. Constraints on the development of the early continental crust from isotopic data

    NASA Technical Reports Server (NTRS)

    Jacobson, S. B.

    1985-01-01

    Aspects of the origin and development of the early (AE) continential crust are addressed by radiogenic isotope and trace element studies. The most important ones are: (1) at what time did the earliest continental crust form; (2) what was its composition; (3) by what processes did it grow and by what processes was it destroyed; (4) what were the rates of production and destruction as a function of time during this time period? Nd is isotopic data on the oldest terrestrial rocks indicate that the mantle at this time had already suffered substantial depletion in incompatible elements due to earlier continent forming events. Isotopic data on young volcanic rocks derived from the depleted mantle show no evidence of this early history. The observed isotopic patterns of Nd, Sr, Hf and Pb through time together with the presently observed age spectrum of crustal rocks are considered. These patterns can be modelled by a transport model in which the continental growth and destruction rates are allowed to vary as a function of time. It is suggest that the mass of the continents at 3.8 AE ago was about 25% of the current continental mass. However, due to the very high recycling rates obtained in the early Archean only a few percent of this crust has been preserved up to the present.

  9. Has 7% of Continental Crust been Lost since Pangea Broke Up?

    NASA Astrophysics Data System (ADS)

    Scholl, D. W.; Stern, R. J.

    2012-12-01

    After modern plate tectonics began, the net growth or loss of continental crust predominantly involved the mass balance at subduction zones (SZs) between the yin of adding mantle-sourced arc igneous rocks and the subtracting yang of recycling existing crust back into the mantle. Field observations suggest that during Mesozoic and Cenozoic time, a rough long-term balance existed at ocean-margin SZs (e.g., W. N. America, Andes). But a different picture, one of net loss, emerges when additions and losses at collisional or crust-suturing SZs (e.g., India-Tibet) are considered. GAINS AND LOSSES SINCE ~200 Ma Because Mesozoic and Cenozoic convergent margins can be field inspected, the net growth of continental crust after the breakup of Pangea at ~200 Ma can be estimated. Pangea breakup also marked the beginning of the present supercontinent cycle. Newly established (Eocene) ocean-margin SZs (e.g., IBM, Tonga-Kermadec) added juvenile arc crust for at least 10-15 Myr at rates ~10-15 times higher than later and elsewhere at long-established SZs (~30 km3/Myr/km). During the Cenozoic, at colliding SZs (e.g., Alps, India-Tibet, Arabia-Eurasia) tomographic and geological data document losses of subducted continental crust sustained for 15-50 Myr at rates ~15 times that typical from the upper plate at ocean-margin SZs (~70 km3/Myr/km). For additions, we considered that as the Atlantic opened in early Jurassic time, new, prodigiously productive SZs were initiated along the western margin of North and Middle America but not along western South America and the eastern margin of Eurasia. In the Cretaceous, new SZs formed along much of the northern margin of the Tethys, along western Sumatra and southern Java, and at the great arc of the Caribbean. In the early Eocene, in the offshore, a lengthy (~20,000 km) curtain of new, voluminously productive intra-oceanic SZs formed from the Aleutian Islands southward to the Kermadec Islands. For subtractions, we applied subduction losses (~70

  10. Constraints from loess on the Hf-Nd isotopic composition of the upper continental crust

    NASA Astrophysics Data System (ADS)

    Chauvel, Catherine; Garçon, Marion; Bureau, Sarah; Besnault, Adeline; Jahn, Bor-ming; Ding, Zhongli

    2014-02-01

    Knowledge of the average composition of the upper continental crust is crucial to establish not only how it formed but also when. While well constrained averages have been suggested for its major and trace element composition, no values exist for its Nd and Hf isotopic compositions even though radiogenic isotopic systems provide valuable information on its average model age. Here we present Nd and Hf isotopic data determined on a large number of loess deposits from several continents. We demonstrate that these deposits have very uniform Nd and Hf isotopic compositions. We obtain an average Nd isotopic composition that is similar to previous estimates for the upper continental crust (εNd=-10.3±1.2(1σ)) and we establish a new Hf average value at εHf=-13.2±2(1σ). This average falls on the “Terrestrial Array”, demonstrating that the two parent-daughter ratios are not decoupled during crust formation. Trace element data acquired on the same set of samples allow us to calculate an average 147Sm/144Nd ratio for the upper continental crust: 0.1193±0.0026, a value slightly higher than previous estimates. Based on the relationship between Sm/Nd and Nd isotopes, we estimate the average Nd extraction age of upper continental crust from the depleted mantle at TDM(Nd)=1.82±0.07 Ga. This model age is entirely consistent with previous suggestions made for example by Goldstein et al. (1984). Assuming that for each individual sample, the Hf model age cannot be younger than the Nd model age, our new Nd-Hf isotopic data provide a value for the very poorly known 176Lu/177Hf ratio of the upper crust. Our estimate is 176Lu/177Hf = 0.0125 ± 0.0018, a value significantly lower than commonly used values (0.0150-0.0159; Griffin et al., 2002; Goodge and Vervoort, 2006; Hawkesworth et al., 2010) but higher than Rudnick and Gao's (2003) estimate of 0.0083. The impact of our new 176Lu/177Hf ratio on crustal model ages of zircon populations is not simple to evaluate but the Hf model

  11. Grain size reduction, fluid infiltration, and extreme weakening in the continental lower crust

    NASA Astrophysics Data System (ADS)

    Menegon, Luca; Stünitz, Holger; Nasipuri, Pritam

    2013-04-01

    Knowledge of the rheology of the continental lower crust is essential when trying to model major geodynamic processes and to understand the mechanics and distribution of earthquakes at different depths in the lithosphere. However, whether the lower crust is mechanically strong or weak is still a debated issue, and convincing evidence for both, a weak and a strong lower crust has been shown. A key aspect in this controversy is the role of aqueous fluids, with anhydrous conditions typically invoked as the main reason for high strength of large parts of the granulite facies lower crust. The Lofoten islands in northern Norway represent an outstanding natural laboratory to investigate the progressive microstructural and rheological evolution of localized shear zones exhumed from the continental lower crust. In the anhydrous mangerite-charnockite rocks from Lofoten, deformation is strongly localized along discrete ductile shear zones developed under upper amphibolite to granulite facies conditions (T=730° C, P=0.65 GPa). Shear zone formation was associated with an extreme grain size reduction of the main minerals of the magmatic protolith (perthite + clinopyroxene). Grain size reduction occurred by fracturing and neocrystallization in perthites, and by fracturing and hydration reactions in pyroxene, which was replaced by amphibole + quartz + calcite. Recrystallized perthites and reaction products show fine grain size (20-30 μm), equant shape, dispersed phase distribution, and the lack of a crystallographic preferred orientation. These observations are consistent with diffusion creep as the dominant deformation mechanism in the shear zones. The occurrence of new phases (amphibole, quartz) in triple and quadruple junctions, as well as the concentration of fine quartz grains along bands with a C' orientation strongly suggest the concomitant operation of grain boundary sliding and cavitation during grain-size sensitive creep. Bulk-rock chemical analysis indicates an

  12. Structure of thinned continental crust across the Orphan Basin from a dense wide-angle seismic profile and gravity data

    NASA Astrophysics Data System (ADS)

    Lau, K. W. Helen; Watremez, Louise; Louden, Keith E.; Nedimovíć, Mladen R.

    2015-09-01

    We present a 500-km long, 2-D P-wave velocity model across the Orphan Basin, offshore NE Newfoundland, Canada, from Flemish Cap to the Bonavista Platform, formed using refraction and wide-angle reflection data from 89 ocean-bottom seismometers. This layered model builds on a recent traveltime tomography result using additional constraints from coincident multichannel seismic reflection and gravity data plus borehole logs from three wells. The model shows (i) post-rift Tertiary (velocities ˜1.7-3.5 km s-1) and (ii) both post-rift and syn-rift, Cretaceous and Jurassic sediments (˜4-5 km s-1), deposited within an eastern and a western sub-basin that are separated by a major basement block. The existence of Jurassic sediments indicates a pre-Cretaceous rifting phase in the eastern sub-basin, and possibly in the western sub-basin. However, there is no evidence that Triassic sediments are widespread across the Orphan Basin. Two upper crustal sublayers and one lower crustal layer are defined by differences in velocities (5.4-6.1, 6.1-6.5 and 6.3-7.1 km s-1, respectively) and vertical velocity gradients (mean = 0.14, 0.10 and 0.05 s-1, respectively). Crustal thinning is asymmetric across the Orphan Basin. Within the eastern sub-basin, continental crust beneath Flemish Cap (˜32 km thick; β ˜ 1.1) thins westward into a 35-km-wide zone of hyperextended crust (<10 km thick; β > 3.4) beneath an 11-km-deep sedimentary basin. Within the western sub-basin, the Bonavista Platform crust (˜32 km thick) thins eastward into a 116-km-wide zone of hyperextended crust. Two zones of thicker crust (β = 2-3.5) exist within the central section, with muted topography within the eastern part and large basement highs in the western part, separated by the eastward dipping White Sail Fault (WSF). The zone to the east of the WSF displays higher velocities in the lower crust than to the west. This can only be explained by a lateral ductile flow across the zone boundary. By combining the two

  13. Steep Satellite Altimetry Gradients as a Proxy to the Edge of the Continental Crust

    NASA Astrophysics Data System (ADS)

    Lawver, L. A.; Gahagan, L. M.

    2005-05-01

    Tight-fit plate reconstructions are produced using a global database constrained by marine magnetic anomalies tied to a consistent timescale, paleomagnetic poles, seafloor age dates based on drilling results, and fracture zone and transform fault lineations picked from ship-track and satellite altimetry data. Where a prominent steep gradient in the satellite altimetry data is present near the continental-ocean transition, it is used as a proxy to the continental shelf break [CSB]. Continental block outlines are based on digitization of the steep gradient. In some places, notably off Namibia, there is a very close correlation between that gradient and the ocean-continent boundary deduced from seismic refraction and reflection data. In other regions, there may be some stretched continental crust oceanward of the steep gradient but for reconstruction purposes we assume the crust to be predominantly continental landward of the boundary and oceanic, seaward of the line. Good matchs for conjugate CSBs are found in many places world-wide along passive margins and these will be highlighted. Particularly good matches are observed between the cratonic edges of East Antarctica as determined by sub-ice topographic highs seen along the margins of East Antarctica with respect to Madagascar, Sri Lanka, the southern half of the eastern margin of India, and the region of Australia between 124° E and 133° E along the Great Australian Bight (GAB). There are overlaps of the reconstructed conjugate CSBs, with one overlap between India and East Antarctica (70° E to 85° E) and one between East Antarctica and the western section of the GAB (105° E to 120° E). These two overlaps are coincident with the outer margins of the Lambert Graben - Prydz Bay Basin and the Aurora Subglacial Basin, respectively. It is known that there are substantial glacially-derived sediments prograded off the continental margin onto oceanic crust at Prydz Bay where there may be as much as 200 km in width of

  14. No significant production of continental crust prior to 3.8 Ga

    NASA Astrophysics Data System (ADS)

    Vervoort, J. D.; Kemp, T.; Fisher, C. M.

    2012-12-01

    In his final paper before his death in 1991 (The Persistent Myth of Crustal Growth), Dick Armstrong argued that continental crustal recycling has kept pace with crustal growth and that the volume of continental crust has been in an approximate steady state (no net growth) for at least the past 4 Ga. Although this has been a minority viewpoint (most scientists favor some manner of progressive crustal growth) Armstrong's no-growth model has had some vehement supporters over the years. Since this paper was published, most of the discussion in the literature has focused on the isotopic record—particularly that of the oldest preserved igneous rocks. Recently, attention has shifted to detrital zircons and, in particular, the Hf isotopic record in the oldest known zircons. Two main features from these data have been used to argue for widespread crust in the early Earth: 1) heterogeneous Hf isotopic compositions of the oldest zircons; and 2) Hf model ages in Archean detrital zircons that are much older than their crystallization ages. The dataset on which these observations have been based, however, is not robust. Many data are compromised by complexities in age and isotopic composition. Hf zircon model ages are even more problematic as they are loaded with assumptions and based on a depleted mantle evolution reference that is not well known, is most likely wrong prior to 3.8 Ga, and has uncertain relevance for understanding crustal growth. In order to provide a less ambiguous isotopic record, a better approach is needed: the integration of age (U-Pb zircon) and isotopic information (Hf-Nd whole rock, Hf-O in zircons) from the oldest, but also least complicated and altered, magmatic samples. Several lines of evidence suggest that formation of continental crust did not begin in earnest until ca. 3.8-3.5 Ga: 1) lack of older crust or inherited pre-4 Ga zircon cores in the geological record in shield areas; 2) conspicuous lack of pre-4 Ga zircons in nearly all Archean

  15. Insights into chemical weathering of the upper continental crust from the geochemistry of ancient glacial diamictites

    NASA Astrophysics Data System (ADS)

    Li, Su; Gaschnig, Richard M.; Rudnick, Roberta L.

    2016-03-01

    Glacial diamictites, with ages ranging from ∼2900 to 0.01 Ma, record the changing composition of the upper continental crust through time (Gaschnig et al., 2014). Li concentrations and isotopic compositions, combined with Pb isotopic compositions, chemical index of alteration (CIA) values and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these deposits and the origin of this signature. The δ7Li values of most of the diamictites (ranging from -3.9 to +3.5) are lower than those of mantle-derived basalts (+3.7 ± 2, 2σ), and the low δ7Li values are generally accompanied by high CIA and low Sr/Sr∗ values (or Sr depletion factor, Sr/Sr∗ = Sr/(Ce∗Nd)0.5), reflecting a weathering signature that may have derived from pre-depositional, syn-depositional, and/or post-depositional weathering processes. Profiles through three glacial diamictites with relatively high CIA (a fresh road cut of the Neoproterozoic Nantuo Formation (CIA = 62-69), and drill cores through the Paleoproterozoic Timeball Hill (CIA = 66-75) and Duitschland Formations (CIA = 84-91)) do not show evidence of significant post-depositional weathering. High Th/U, reflecting loss of uranium during oxidative weathering, is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits. Pb isotopic systematics suggest that this signature was largely inherited from preexisting crust, although a subset of samples (the Neoproterozoic Konnarock, Paleozoic Dwyka, and several of the Paleoproterozoic Duitschland samples) appears to have experienced post-depositional U loss. Modern glaciomarine sediments record little weathering (CIA = 47, Sr/Sr∗ = 0.7, δ7Li = +1.8), consistent with the cold temperatures accompanying glacial periods, and suggesting that limited syn-depositional weathering has occurred. Thus, the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper

  16. Is the Continental Crust Stronger or Weaker than the Mantle Lithosphere ?

    NASA Astrophysics Data System (ADS)

    Houseman, G. A.; Billen, M. I.

    2005-12-01

    Standard models of the stress that can be supported by the continental lithosphere (encapsulated in the so-called Christmas-tree diagram) suggest that the deep continental crust is relatively weak compared to the mantle lithosphere, primarily because of the effect of thermal activation. Such models, however, are based on laboratory measurements that must be extrapolated over roughly seven orders of magnitude, in both spatial and temporal scales, to say nothing of the effects of variable crustal composition and, in particular, varying concentration of water. Direct measures of crustal strength, however, may be obtained from geodetic measurements of strain rate and stress estimates that are calibrated against the effect of gravity on an inferred density structure. Crustal strength in this case is defined by apparent viscosity: the ratio of stress difference to strain-rate. Such measurements, though less precise than laboratory measurements, are directly applicable on the time and length scales on which lithosphere deforms. When lithospheric deformation is driven by internal buoyancy forces in tectonically active areas the implied viscosity of the mantle lithosphere is on the order of 1021 Pa s or less. In such systems, deformation of the crust follows that of the mantle lithosphere, and the resulting spatial variations in crustal thickness and surface topography provide direct constraints on the relative crustal viscosity. The general stability of continental lithosphere suggests that it is relatively strong compared to typical intra-plate deviatoric stress, but may be weakened by stress, temperature, or fluids. Analysis of tectonic systems from a number of regions also shows that there are very large regional differences in the apparent strength of the crust. In the Transverse Ranges of California, for example, steep topographic gradients imply that the crust is relatively strong compared to the mantle. In contrast the relatively uniform elevation of the Tibetan

  17. Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi; Komiya, Tsuyoshi; Rino, Shuji; Maruyama, Shigenori; Hirata, Takafumi

    2010-04-01

    We have determined U-Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U-Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7-2.5, 2.2-1.9, 1.7-1.6, 1.2-1.0, 0.9-0.4, and <0.3 Ga in the accumulated age distribution. These peaks correspond well with the timing of supercontinent assembly. The Hf isotopic data indicate that many zircons, even those having Archean U-Pb ages, crystallized from magmas involving an older crustal component, suggesting that granitoid magmatism has been the primary agent of differentiation of the continental crust since the Archean era. We calculated Hf isotopic model ages for the zircons to estimate the mean mantle-extraction ages of their source materials. The oldest zircon Hf model ages of about 3.7 Ga for the river sands suggest that some crust generation had taken place by 3.7 Ga, and that it was subsequently reworked into <3.3 Ga granitoid continental crust. The accumulated model age distribution shows peaks at 3.3-3.0, 2.9-2.4, and 2.0-0.9 Ga. The striking attribute of our new data set is the non-uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3-2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than

  18. Felsic Magmatism through Intracrustal Melting of Previously Formed Volcanic-Arc Crust: Implications for Differentiation and Secular Evolution of the Continental Crust

    NASA Astrophysics Data System (ADS)

    G R, R. K.; C, S.

    2015-12-01

    The fundamental challenge in understanding the origin and evolution of the continental crust is to recognize how primary mantle source, and oceanic crust, which are essentially mafic to ultramafic in composition, could differentiate into a more or less felsic compositions. It is possible to understand growth and differentiation of the continental crust by constraining the interplay of magmatism, deformation, and high-grade metamorphism in the lower crust. Here, we apply this knowledge on the lower crustal granitoids of southern India and speculate on the variations in geochemistry as a consequence of differentiation and secular evolution of the continental crust.The major groups of granitoids of southern India are classified as metatonalites, comparable to typical Archaean TTGs with pronounced calc-alkaline affinity, and metagranites which are magmatic fractionation produced by reworking of early crust. Metatonalites are sodic-trondhjemites with slightly magnesian, moderate LREE (average LaN = 103) and low HREE (average YbN = 2) characerestics, where as metagranites are calc-alkaline ferroan types with enriched LREE (average LaN = 427) and HREE (average YbN = 23). Petrogenetic characteristics of granitoids illustrate continuous evolution of a primary crust into diverse magmatic units by multiple stages of intracrustal differentiation processes attributed to following tectonic scenarios: (1) formation of tonalitic magma by low- to moderate-degree partial melting of hydrated basaltic crust at pressures high enough to stabilize garnet-amphibole residue and (2) genesis of granite in a continental arc-accretion setting by an episode of crustal remelting of the tonalitic crust, within plagioclase stability field. The first-stage formed in a flat-subduction setting of an volcanic-arc, leading to the formation of tonalites. The heat budget required is ascribed to the upwelling of the mantle and/or basaltic underplating. Progressive decline in mantle potential temperature

  19. Geological cycles and a two-stage history of the Continental Crust (Robert Wilhelm Bunsen Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Hawkesworth, Chris J.; Cawood, Peter A.; Dhuime, Bruno

    2014-05-01

    The continental crust is the archive of Earth history, and the apparently cyclical nature of geological evolution is a feature of the geological record. The advent of radiometric ages has highlighted that the spatial and temporal distribution of the Earth's record of rock units and events is heterogeneous with distinctive peaks and troughs in the distribution of ages of igneous crystallization, metamorphism, continental margins and mineralization. It is argued that the temporal distribution largely reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. In contrast there are other signals, such as the Sr isotope ratios of seawater, mantle temperatures, and redox conditions on the Earth, where the records are regarded as primary because they are not sensitive to the numbers of samples of different ages that have been analysed. Models based on the U-Pb, Hf and O isotope ratios of detrital zircons suggest that at least ~60-70% of the present volume of the continental crust had been generated by 3 Ga. The sedimentary record is biased by preferential sampling of relatively young material in their source terrains. The implication is that there were greater volumes of continental crust in the Archaean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but the rates of continental growth were significantly higher before 3 Ga than subsequently. The time-integrated Rb/Sr ratios, and the average SiO2 contents, indicate that new continental crust was largely mafic over the first 1.5 Ga of Earth's evolution, and that significant volumes of pre-3 Ga crust may have been associated with intraplate magmatism. Since ~3 Ga there has been an increase in Rb/Sr, SiO2, and the inferred thickness of new crust, consistent

  20. Thallium isotope composition of the upper continental crust and rivers - An investigation of the continental sources of dissolved marine thallium

    USGS Publications Warehouse

    Nielsen, S.G.; Rehkamper, M.; Porcelli, D.; Andersson, P.; Halliday, A.N.; Swarzenski, P.W.; Latkoczy, C.; Gunther, D.

    2005-01-01

    The thallium (Tl) concentrations and isotope compositions of various river and estuarine waters, suspended riverine particulates and loess have been determined. These data are used to evaluate whether weathering reactions are associated with significant Tl isotope fractionation and to estimate the average Tl isotope composition of the upper continental crust as well as the mean Tl concentration and isotope composition of river water. Such parameters provide key constraints on the dissolved Tl fluxes to the oceans from rivers and mineral aerosols. The Tl isotope data for loess and suspended riverine detritus are relatively uniform with a mean of ??205Tl = -2.0 ?? 0.3 (??205Tl represents the deviation of the 205Tl/203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 104). For waters from four major and eight smaller rivers, the majority were found to have Tl concentrations between 1 and 7 ng/kg. Most have Tl isotope compositions very similar (within ??1.5 ??205Tl) to that deduced for the upper continental crust, which indicates that no significant Tl isotope fractionation occurs during weathering. Based on these results, it is estimated that rivers have a mean natural Tl concentration and isotope composition of 6 ?? 4 ng/kg and ??205Tl = -2.5 ?? 1.0, respectively. In the Amazon estuary, both additions and losses of Tl were observed, and these correlate with variations in Fe and Mn contents. The changes in Tl concentrations have much lower amplitudes, however, and are not associated with significant Tl isotope effects. In the Kalix estuary, the Tl concentrations and isotope compositions can be explained by two-component mixing between river water and a high-salinity end member that is enriched in Tl relative to seawater. These results indicate that Tl can display variable behavior in estuarine systems but large additions and losses of Tl were not observed in the present study. Copyright ?? 2005 Elsevier Ltd.

  1. Native iron in the continental lower crust - petrological and geophysical implications

    SciTech Connect

    Haggerty, S.E.; Toft, P.B.

    1985-08-01

    Lower crustal granulite xenoliths recovered from a kimberlite pipe in western Africa contain native iron (Fe) as a decomposition product of garnet and ilmenite. Magnetic measurements show that less than 0.1 percent (by volume) of iron metal is present. Data from geothermometry and oxygen geobarometry indicate that the oxide and metal phases equilibrated between iron-wuestite and magnetite-wuestite buffers, which may represent the oxidation state of the continental lower crust, and the depleted lithospheric upper mantle. Ferromagnetic native iron could be stable to a depth of about 95 kilometers and should be considered in the interpretation of long-wavelength static magnetic anomalies. 32 references.

  2. Native iron in the continental lower crust: petrological and geophysical implications.

    PubMed

    Haggerty, S E; Toft, P B

    1985-08-16

    Lower crustal granulite xenoliths recovered from a kimberlite pipe in western Africa contain native iron (Fe(0)) as a decomposition product of garnet and ilmenite. Magnetic measurements show that less than 0.1 percent (by volume) of iron metal is present. Data from geothermometry and oxygen geobarometry indicate that the oxide and metal phases equilibrated between iron-wüstite and magnetite-wüstite buffers, which may represent the oxidation state of the continental lower crust, and the depleted lithospheric upper mantle. Ferromagnetic native iron could be stable to a depth of approximately 95 kilometers and should be considered in the interpretation of long-wavelength static magnetic anomalies.

  3. Native iron in the continental lower crust - Petrological and geophysical implications

    NASA Technical Reports Server (NTRS)

    Haggerty, S. E.; Toft, P. B.

    1985-01-01

    Lower crustal granulite xenoliths recovered from a kimberlite pipe in western Africa contain native iron (Fe) as a decomposition product of garnet and ilmenite. Magnetic measurements show that less than 0.1 percent (by volume) of iron metal is present. Data from geothermometry and oxygen geobarometry indicate that the oxide and metal phases equilibrated between iron-wuestite and magnetite-wuestite buffers, which may represent the oxidation state of the continental lower crust, and the depleted lithospheric upper mantle. Ferromagnetic native iron could be stable to a depth of about 95 kilometers and should be considered in the interpretation of long-wavelength static magnetic anomalies.

  4. Volcanic-plutonic parity and the differentiation of the continental crust.

    PubMed

    Keller, C Brenhin; Schoene, Blair; Barboni, Melanie; Samperton, Kyle M; Husson, Jon M

    2015-07-16

    The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks-volcanic and plutonic rocks-and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation.

  5. Processes of lithosphere evolution: New evidence on the structure of the continental crust and uppermost mantle

    USGS Publications Warehouse

    Artemieva, I.M.; Mooney, W.D.; Perchuc, E.; Thybo, H.

    2002-01-01

    We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomography studies of Eurasia and from global and regional data on seismic anisotropy. These data as documented in the papers of this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passive and active), subduction, and lithospheric basal erosion due to a relative motion of cratonic keels and the convective mantle. ?? 2002 Elsevier Science B.V. All rights reserved.

  6. Recycling of lower continental crust through foundering of cumulates from contaminated mafic intrusions

    NASA Technical Reports Server (NTRS)

    Arndt, Nicholas T.; Goldstein, Steven L.

    1988-01-01

    A mechanism is presented for recycling of lower continental material back into the mantle. Picritic magmas, possible parental to volumious continental volcanics such as the Karoo and Deccan, became trapped at the Moho, where they interacted with and become contaminated by lower crustal materials. Upon crystallization, the magmas differentiated into lower ultramafic cumulate zones and upper gabbroic-anorthositic zones. The ultramafic cumulates are denser than underlying mantle and sink, carrying lower crustal components as trapped liquid, as xenoliths or rafts, and as constituents of cumulate minerals. This model provides a potentially significant crust-mantle differentiation mechanism, and may also represent a contributing factor in crustal recycling, possibly important in producing some OIB reservoirs.

  7. Tectonic controls on hydrothermal mineralisation in hot continental crust: Thermal modelling and spatial analysis

    NASA Astrophysics Data System (ADS)

    Gessner, K.; Porwal, A.

    2009-04-01

    Hydrothermal ore deposits provide a record of excess energy flux and mass transfer in the Earth's lithosphere. The heterogeneous distribution of ore deposits in space and time provides a challenge to uniformitarian geodynamic and tectonic concepts, but unusual thermal and structural events often coincide with high mineral endowment. In the Australian Proterozoic continental backarcs and intracratonic rifts host large resources of base metals, gold, and uranium. We present thermal models and spatial analyses of mineral occurrences within the Mount Isa Inlier, an inverted Mesoproterozoic rift in northwest Queensland, Australia, to demonstrate how thermal structure, tectonic style and crustal scale fluid flow are related. In the Mount Isa Inlier, radiogenic heat production contributes significantly to present day surface heat flow, and Mesoproterozoic geotherms of 40°C km-1 in the upper crust can be inferred from lithosphere-scale conductive models. The combination of thick continental crust and high temperatures implies that localization of deformation was limited to a thin upper crustal layer. During rifting mid-crustal rocks intruded by syn-extensional granites were exhumed as metamorphic core complexes in strike-parallel linear basement belts. The resulting horizontal strength contrast between sedimentary basins and shallow basement domains became a focus for deformation during subsequent crustal shortening. Our spatial analysis of mineral occurrences demonstrates that epigenetic copper mineralization at Mount Isa correlates positively with steep fault zones bounding linear basement domains, and granites within these domains. Mineralization potential is greatly increased, because high permeability along steep fault zones enables hydrothermal fluid flow between magmatic, metamorphic and sedimentary reservoirs. We argue that the deformation behavior of hot continental lithosphere generates a favorable environment for hydrothermal mineralization by linking shallow

  8. Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

    USGS Publications Warehouse

    Jonathan, Patchett P.; Kouvo, O.; Hedge, C.E.; Tatsumoto, M.

    1982-01-01

    We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from ??Hf=0 to +14, or about 60% of the variability of the present mantle. The approximate ??Hf=2??Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=??? 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive ??Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle. ?? 1981 Springer-Verlag.

  9. Direct observation of adakite melts generated in the lower continental crust, Fiordland, New Zealand.

    NASA Astrophysics Data System (ADS)

    Stevenson, J.; Daczko, N.; Clarke, G.; Pearson, N.; Klepeis, K.

    2003-04-01

    Adakite igneous rocks have a distinctive chemistry that links them to melting of a mafic source at high pressure (P > 1.2 - 1.5 GPa; e.g. Peacock et al. 1994). They have been attributed to melting of subducted oceanic crust (Kay, 1978; Defant and Drummond, 1990) or melting of the crustal roots of thick continental arcs (Atherton and Petford, 1993). We report the first direct evidence for the generation of adakite melts in mafic lower continental crust. The Pembroke Granulite represents the deepest crust (P = 12 - 14 kbar , T = 750 - 800^oC; Daczko et al. 2001) in an exhumed Cretaceous arc in the South Island of New Zealand (Clarke et al., 2000). The Pembroke Granulite has the bulk chemistry, assemblage, and partial melting textures involving peritectic garnet, to be the source region for an adakite melt. The partial melting textures form the source for numerous trondhjemitic vein-filled fractures, which we suggest were the initial conduit for the adakite melt as it migrated away from its source. LA-ICMPS point analyses of minerals in the dioritic gneiss host rock, partial melting textures, and trondhjemitic veins of the Pembroke Granulite are consistent with this interpretation. The originally overlying Separation Point Batholith contains rocks of adakitic composition thought to have been formed through melting in a continental arc (Muir et al., 1995; 1998); the melts formed in the Pembroke Granulite are texturally, compositionally, geochemically, and structurally consistent with being the source of the Separation Point adakites. References: Atherton, M. P. and Petford, N.: Generation of sodium-rich magmas from newly underplated basaltic crust. Nature, 362, 144--146, 1993. Clarke, G. L., Klepeis, K. A. and Daczko, N. R.: Cretaceous high-P granulites at Milford Sound, New Zealand: their metamorphic history and emplacement in a convergent margin setting. J. Metamorphic Geol., 18, 359--374, 2000. Daczko, N. R., Klepeis, K. A. and Clarke, G. L.: Evidence of Early

  10. The fate of mafic and ultramafic intrusions in the continental crust

    NASA Astrophysics Data System (ADS)

    Roman, Alberto; Jaupart, Claude

    2016-11-01

    Geochemical and petrological data indicate that the bulk continental crust results from the fractionation of basaltic magmas followed by the foundering of residual mafic cumulates. Structural and geological evidence for foundering has been elusive and it is argued that it lies in the shapes of mafic intrusions that have been preserved in the crust. Numerical calculations of visco-elasto-plastic deformation induced by a dense intrusive body in continental crust have been carried out for a wide range of physical conditions. Three regimes are defined on the basis of the amount of dense material that remains at the original emplacement level as well as on the shape of the residual body. With strong encasing rocks, the intrusion deforms weakly in a sagging regime characterized by downwarping of the floor. At the other extreme, the intrusion sinks through weak surroundings, leaving behind a very small volume of material. In an intermediate regime, the intrusion does not sink wholesale and undergoes a dramatic change of shape. A residual body is preserved with a shape that depends on the aspect ratio of the initial intrusion. For aspect ratios of order one, the residual body is funnel-shaped above a thin and deep vertical extension. For the small aspect ratios that typify large igneous complexes such as the Bushveld, South Africa, the residual body is characterized by thick peripheral lobes with inward-dipping igneous layers and a thinner central area that has lost some of the basal cumulates. The transitions between these regimes depend on the rheology and temperature of encasing rocks.

  11. Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia

    NASA Astrophysics Data System (ADS)

    Rusmore, Margaret E.; Bogue, Scott W.; Dodson, Karen; Farley, Kenneth A.; Woodsworth, Glenn J.

    2010-08-01

    New structural, paleomagnetic, and apatite (U-Th)/He results from the continental margin inboard of the Queen Charlotte fault (˜54°N) delineate patterns of brittle faulting linked to transform development since ˜50 Ma. In the core of the orogen, ˜250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ˜50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until <9 Ma. Local tilting in the core of the orogen is associated with glacially driven, post-4 Ma exhumation. Integration with previous results shows that post-50 Ma dextral and normal faulting affected a region ˜250 km inboard of the transform and ˜300 km along strike. Initially widespread, the zone of active extension narrowed and migrated toward the transform ˜25 Ma after initiation of the transform, while dextral faulting continued throughout the region. Differential amounts of post-50 Ma extension created oroclines at the southern and northern boundaries of the deformed region. This region approximately corresponds to continental crust that was highly extended just prior to transform initiation. Variation in Neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid-Cretaceous plutons. Similarities to the Gulf of California suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops.

  12. Brittle-viscous deformation cycles in the dry lower continental crust

    NASA Astrophysics Data System (ADS)

    Menegon, Luca; Pennacchioni, Giorgio

    2015-04-01

    the main deformation mechanisms in the ultramylonites. Nucleation of hornblende indicates synkinematic fluid infiltration. Ongoing measurements of intracrystalline water content along gradients from the pristine anorthosite to the ultramylonite will shed light on the effect of water infiltration on the deformation mechanisms of plagioclase and clinopyroxene. In summary, this study indicates that brittle (coseismic) fracturing was essential to induce grain size reduction and fluid infiltration in the dry and strong lower crust. These processes promoted weakening by activating grain size sensitive creep in the fine-grained hydrated material and resulted in the ductile shear zones localized to the brittle precursors. In the absence of intense fracturing dry granulites would not undergo deformation and metamorphism, and would survive metastably in the course of Wilson cycles. This has obvious implications for long-term continental dynamics and for strain localization at plate boundaries, and will need to be included in future geodynamic models.

  13. Evaluating the influence of chemical weathering on the composition of the continental crust using lithium and its isotopes

    NASA Astrophysics Data System (ADS)

    Rudnick, R. L.; Liu, X.

    2011-12-01

    The continental crust has an "intermediate" bulk composition that is distinct from primary melts of peridotitic mantle (basalt or picrite). This mismatch between the "building blocks" and the "edifice" of the continental crust points to the operation of processes that preferentially remove mafic to ultramafic material from the continents. Such processes include lower crustal recycling (via density foundering or lower crustal subduction - e.g., relamination, Hacker et al., 2011, EPSL), generation of evolved melts via slab melting, and/or chemical weathering. Stable isotope systems document the influence of chemical weathering on the bulk crust composition: the oxygen isotope composition of the bulk crust is distinctly heavier than that of primary, mantle-derived melts (Simon and Lecuyer, 2005, G-cubed) and the Li isotopic composition of the bulk crust is distinctly lighter than that of mantle-derive melts (Teng et al., 2004, GCA; 2008, Chem. Geol.). Both signatures mark the imprint of chemical weathering on the bulk crust composition. Here, we use a simple mass balance model for lithium inputs and outputs from the continental crust to quantify the mass lost due to chemical weathering. We find that a minimum of 15%, a maximum of 60%, and a best estimate of ~40% of the original juvenile rock mass may have been lost via chemical weathering. The accumulated percentage of mass loss due to chemical weathering leads to an average global chemical weathering rate (CWR) of ~ 8×10^9 to 2×10^10 t/yr since 3.5 Ga, which is about an order of magnitude higher than the minimum estimates based on modern rivers (Gaillardet et al., 1999, Chem. Geol.). While we cannot constrain the exact portion of crustal mass loss via chemical weathering, given the uncertainties of the calculation, we can demonstrate that the weathering flux is non-zero. Therefore, chemical weathering must play a role in the evolution of the composition and mass of the continental crust.

  14. Transition from rifted continental to oceanic crust at the southeastern Korean margin in the East Sea (Japan Sea)

    NASA Astrophysics Data System (ADS)

    Cho, H.; Kim, H.; Jou, H.; Hong, J.; Baag, C.

    2004-12-01

    The southeastern Korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the East Sea (Japan Sea). In this study, we present the transitional structure of the southeastern Korean margin and its formation process from rifted continental to 10 km thick normal oceanic crust. The two-dimensional P velocity model of the southeastern Korean margin was computed from ocean bottom seismometer data by tomographic inversion and distilled by iterative forward modeling. The crustal structure shows the emplacement of high-velocity (>7 km/s) lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust. The high-velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting. Magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the southeastern Korean Peninsula that is assumed to represent volcanic extrusives and intrusives. The continental margin featuring a rapid transition from continental to oceanic crust exhibits a remarkable decrease in crustal thickness accompanied by shallowing of the Moho over a distance of about 50 km. It thus appears that the Korean margin experienced intense tectonism comprising crustal deformation and volcanism associated with the opening of the East Sea and consequently registered the early history of continental rifting and subsequent sea floor spreading. We suggest that the rifting and subsequent seafloor spreading at the Korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature.

  15. Archean Arctic continental crust fingerprints revealing by zircons from Alpha Ridge bottom rocks

    NASA Astrophysics Data System (ADS)

    Sergeev, Sergey; Petrov, Oleg; Morozov, Andrey; Shevchenko, Sergey; Presnyakov, Sergey; Antonov, Anton; Belyatsky, Boris

    2015-04-01

    nowadays iceberg formation. Moreover, no Grenvillian-age zircons were revealed in studied sample. The nearest areas of Paleo- and Mesoarchean rocks appearance are deeply inland territories like Acasta Gneiss Complex, Uivak Gneisses, the Superior Province, Anabar and Aldan Shields, Northern China craton (Anshan complex), thus, a only possibility to bring some fragments of basement rocks from these areas to oceanic coast is transcontinental river transportation. The second possibility - this gneissic clast has a local provenance and has undergone a submarine weathering, shallow marine avalanche, proximal transportation by alongside ocean flows, tidal waves abrasion, and as so, reflects local bedrock geology, i.e. adjacent Alpha Ridge rock composition. Additional confirmation of this can be seen in a wide distribution of Qu-sandstones with Paleo-Mezoarchean zircons, and finds of similar allochtonous zircons in dolerites along Alpha-Mendeleev Ridge profile. The studied fragment is a unique evidence for the possible existence of Paleoarchean continental crust within the submarine Alpha-Mendeleev Ridge in Arctic Ocean.

  16. Sources of Mesoproterozoic igneous rocks and formation time of the continental crust of the Kokchetav Massif (Northern Kazakhstan)

    NASA Astrophysics Data System (ADS)

    Tretyakov, A. A.; Kovach, V. P.; Degtyarev, K. E.; Shatagin, K. N.

    2016-12-01

    Within the Kokchetav massif (Northern Kazakhstan), Mesoproterozoic granites and acid volcanics are widespread: these are the youngest Precambrian igneous rocks forming basement of the region. The Nd isotopic characteristics (ɛNd( t)-4.4 ÷-9.6, t Nd(DM) 2.1-2.6 Ga) obtained for these rocks indicate that the source of their melts was the Early Precambrian continental crust. Thus, the continental crust of the Kokchetav Massif had basically been formed by the beginning of the Mesoproterozoic and during the Late Precambrian: later it became a source for the granitoid melts.

  17. Hyper-extended continental crust deformation in the light of Coulomb critical wedge theory

    NASA Astrophysics Data System (ADS)

    Nirrengarten, Michael; Manatschal, Gianreto; Yuan, Xiaoping; Kusznir, Nick; Maillot, Bertrand

    2016-04-01

    The rocks forming the wedge shape termination of hyper-extended continental crust are deformed in the frictional field during the last stage of continental rifting due to cooling and hydration. Seismic interpretation and field evidence show that the basal boundary of the wedge is a low frictional décollement level. The wedge shape, the frictional deformation and the basal décollement correspond to the requirements of the critical Coulomb wedge (CCW) theory which describes the stability limit of a frictional wedge over a décollement. In a simple shear separation model the upper-plate margin (in the hangingwall of the detachment fault) corresponds to a tectonic extensional wedge whereas the lower plate (in the footwall of the detachment fault) is a gravitational wedge. This major difference causes the asymmetry of conjugate hyper-extended rifted margins. We measure a dataset of upper and lower hyper-extended wedge and compare it to the stability envelope of the CCW theory for serpentine and clay friction. We find a good fit by adjusting fluid pressure. The main results of our analysis are that the crustal wedges of lower plate margins are close to the critical shape, which explains their low variability whereas upper plate wedges can be critical, sub- or sup- critical due to the detachment evolution during rifting. On the upper plate side, according to the Coulomb tectonic extensional wedge, faults should be oriented toward the continent. Observations showed some continentward faults in the termination of the continental crust but there are also oceanward faults. This can be explained by two processes, first continentward faults are created only over the detachment, therefore if part of the hyper-extended upper plate crust is not directly over the detachment it will not be part of the wedge. Secondly the tip block of the wedge can be detached creating an extensional allochthon induced by the flattening of the detachment near the surface, therefore continentward

  18. Historic surface faulting in continental United States and adjacent parts of Mexico

    USGS Publications Warehouse

    Bonilla, M.G.

    1967-01-01

    This report summarizes geometric aspects of approximately 35 instances of historic faulting of the ground surface in the continental United States and adjacent parts of Mexico. This information is of immediate importance in the selection and evaluation of sites for vital structures such as nuclear power plants. The data are presented in a table and graphs which show the quantitative relations between various aspects of the faulting. Certain items in the table that are uncertain, poorly known, or not in the published literature are briefly described in the text.

  19. Natural constraints on the rheology of the lower continental crust (Musgrave Ranges, Central Australia)

    NASA Astrophysics Data System (ADS)

    Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Camacho, Alfredo; Pennacchioni, Giorgio

    2015-04-01

    Current models and extrapolated laboratory data generally predict viscous flow in the lower continental crust and any localized brittle deformation at these depths has been proposed to reflect downward propagation of the frictional-viscous transition zone during short-term seismic events and related high strain rates. Better natural constraints on this proposed rheological behaviour can be obtained directly from currently exposed lower crust that has not been strongly overprinted during its exhumation. One of the largest and best preserved lower crustal sections is located in the Musgrave Ranges, Central Australia. The Petermann Orogeny (550 Ma) in this area is characterized by the development of localized shear zones on a wide range of scales, overprinting water-deficient granulites of Musgravian age (1.2 Ga) as well as younger granites and gabbros. Shearing is rarely localized on lithological inhomogeneities, but rather on precursor fractures and on commonly associated pseudotachylytes. The only exception is that older dolerite dykes are often exploited, possibly because they are planar layers of markedly smaller grain size. Sheared pseudotachylyte often appears caramel-coloured in the field and has a fine grained assemblage of Grt+Cpx+Fsp. Multiple generations of pseudotachylyte formed broadly coeval with shearing are indicated by clasts of sheared pseudotachylyte within pseudotachylyte veins that then themselves subsequently sheared. The ductile shear zones formed under sub-eclogitic conditions of ca. 650°C and 1.2 GPa, generally typical of the lower continental crust. However, the P-T conditions during pseudotachylyte formation cannot be readily determined using classical geothermobarometry, because of the fine grain sizes and possible disequilibrium. The software "Xmaptools" (by Pierre Lanari) allows the quantification of X-ray maps produced by EDS or WDS. It provides both very precise definition of local mineral compositions for exchange geothermobarometry

  20. The Mohorovičić discontinuity beneath the continental crust: An overview of seismic constraints

    NASA Astrophysics Data System (ADS)

    Carbonell, Ramon; Levander, Alan; Kind, Rainer

    2013-12-01

    relatively simple, well understood process, the continental Moho can be subject to an extensive variety of tectonic processes, making overarching conclusions about the continental Moho difficult. Speaking very broadly: 1) In orogenic belts still undergoing compression and active continental volcanic arcs, the Moho evolves with the mountain belt, 2) In collapsed Phanerozoic orogenic belts the Moho under the collapse structure was formed during the collapse, often by a combination of processes. 3) In regions having experienced widespread basaltic volcanism, the Moho can result from underplated basalt and basaltic residuum. In Precambrian terranes the Moho may be as ancient as the formation of the crust, in others Precambrian tectonic and magmatic processes have reset it. We note that seismic reflection data in Phanerosoic orogens as well as from Precambrian cratonic terranes often show thrust type structures extending as deep as the Moho, and suggest that even where crust and mantle xenoliths provide similar age of formation dates, the crust may be semi-allochothonous.

  1. Giant magmatic water reservoirs at mid-crustal depth inferred from electrical conductivity and the growth of the continental crust

    NASA Astrophysics Data System (ADS)

    Laumonier, Mickael; Gaillard, Fabrice; Muir, Duncan; Blundy, Jon; Unsworth, Martyn

    2017-01-01

    The formation of the continental crust at subduction zones involves the differentiation of hydrous mantle-derived magmas through a combination of crystallization and crustal melting. However, understanding the mechanisms by which differentiation occurs at depth is hampered by the inaccessibility of the deep crust in active continental arcs. Here we report new high-pressure electrical conductivity and petrological experiments on hydrated andesitic melt from Uturuncu volcano on the Bolivian Altiplano. By applying our results to regional magnetotelluric data, we show that giant conductive anomalies at mid-crustal levels in several arcs are characterized by relatively low amounts of intergranular andesitic partial melts with unusually high dissolved water contents (≥8 wt.% H2O). Below Uturuncu, the Altiplano-Puna Magma Body (APMB) displays an electrical conductivity that requires high water content (up to 10 wt.%) dissolved in the melt based on crystal-liquid equilibria and melt H2O solubility experiments. Such a super-hydrous andesitic melt must constitute about 10% of the APMB, the remaining 90% being a combination of magmatic cumulates and older crustal rocks. The crustal ponding level of these andesites at around 6 kbar pressure implies that on ascent through the crust hydrous magmas reach their water saturation pressure in the mid-crust, resulting in decompression-induced crystallization that increases magma viscosity and in turn leads to preferential stalling and differentiation. Similar high conductivity features are observed beneath the Cascades volcanic arc and Taupo Volcanic Zone. This suggests that large amounts of water in super-hydrous andesitic magmas could be a common feature of active continental arcs and may illustrate a key step in the structure and growth of the continental crust. One Sentence Summary: Geophysical, laboratory conductivity and petrological experiments reveal that deep electrical conductivity anomalies beneath the Central Andes

  2. Geological implications of a permeability-depth curve for the continental crust

    USGS Publications Warehouse

    Ingebritsen, S.E.; Manning, C.E.

    1999-01-01

    The decrease in permeability (k) of the continental crust with depth (z), as constrained by geothermal data and calculated fluid flux during metamorphism, is given by log k = -14 - 3.2 log z, where A is in meters squared and z is in kilometers. At moderate to great crustal depths (>???5 km), this curve is defined mainly by data from prograde metamorphic systems, and is thus applicable to orogenic belts where the crust is being thickened and/or heated; lower permeabilities may occur in stable cratonic regions. This k-z relation implies that typical metamorphic fluid flux values of ???10-11 m/s are consistent with fluid pressures significantly above hydrostatic values. The k-z curve also predicts that metamorphic CO2 flux from large orogens may be sufficient to cause significant climatic effects, if retrograde carbonation reactions are minimal, and suggests a significant capacity for diffuse degassing of Earth (1015-1016 g/yr) in tectonically active regions.

  3. Missing Lead and High 3He/4He in Ancient Sulfides Associated with Continental Crust Formation

    PubMed Central

    Huang, Shichun; Lee, Cin-Ty A.; Yin, Qing-Zhu

    2014-01-01

    Major terrestrial reservoirs have Pb isotopes more radiogenic than the bulk silicate Earth. This requires a missing unradiogenic Pb reservoir, which has been argued to reside in the lower continental crust or dissolved in the core. Chalcophile element studies indicate that continent formation requires the formation of sulfide-bearing mafic cumulates in arcs. Because Pb, but not U, partitions into sulfides, we show that continent formation must have simultaneously generated time-integrated unradiogenic Pb reservoirs composed of sulfide-bearing cumulates, now recycled back into the mantle or stored deep in the continental lithosphere. The generation of such cumulates could also lead to coupled He-Pb isotopic systematics because 4He is also produced during U-Th-Pb decay. Here, we show that He may be soluble in sulfide melts, such that sulfide-bearing cumulates would be enriched in both Pb and He relative to U and Th, “freezing” in He and Pb isotopes of the ambient mantle at the time of sulfide formation. This implies that ancient sulfide-bearing cumulates would be characterized by unradiogenic Pb and He isotopes (high-3He/4He). These primitive signatures are usually attributed to primordial, undifferentiated mantle, but in this case, they are the very imprint of mantle differentiation via continent formation. PMID:24937103

  4. The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

    PubMed Central

    Shen, Bing; Jacobsen, Benjamin; Lee, Cin-Ty A.; Yin, Qing-Zhu; Morton, Douglas M.

    2009-01-01

    Continental crust is too Si-rich and Mg-poor to derive directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires a second step involving Mg loss, which is thought to be dominated by internal igneous differentiation. However, igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here, we show that granites from the eastern Peninsular Ranges Batholith (PRB) in southern California are isotopically heavy in Mg compared with PRB granodiorites and canonical mantle. Specifically, Mg isotopes correlate positively with Si content and O, Sr, and Pb isotopes and negatively with Mg content. The elevated Sr and Pb isotopes require that a component in the source of the granitic magmas to be ancient preexisting crust making up the prebatholithic crustal basement, but the accompanying O and Mg isotope fractionations suggest that this prebatholithic crust preserved a signature of low-temperature alteration. The protolith of this basement rock may have been the residue of chemical weathering, which progressively leached Mg from the residue, leaving the remaining Mg highly fractionated in terms of its isotopic signature. Our observations indicate that ancient continental crust preserves the isotopic signature of compositional modification by chemical weathering. PMID:19920171

  5. The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation.

    PubMed

    Shen, Bing; Jacobsen, Benjamin; Lee, Cin-Ty A; Yin, Qing-Zhu; Morton, Douglas M

    2009-12-08

    Continental crust is too Si-rich and Mg-poor to derive directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires a second step involving Mg loss, which is thought to be dominated by internal igneous differentiation. However, igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here, we show that granites from the eastern Peninsular Ranges Batholith (PRB) in southern California are isotopically heavy in Mg compared with PRB granodiorites and canonical mantle. Specifically, Mg isotopes correlate positively with Si content and O, Sr, and Pb isotopes and negatively with Mg content. The elevated Sr and Pb isotopes require that a component in the source of the granitic magmas to be ancient preexisting crust making up the prebatholithic crustal basement, but the accompanying O and Mg isotope fractionations suggest that this prebatholithic crust preserved a signature of low-temperature alteration. The protolith of this basement rock may have been the residue of chemical weathering, which progressively leached Mg from the residue, leaving the remaining Mg highly fractionated in terms of its isotopic signature. Our observations indicate that ancient continental crust preserves the isotopic signature of compositional modification by chemical weathering.

  6. Cenozoic analogues support a plate tectonic origin for the Earth’s earliest continental crust

    NASA Astrophysics Data System (ADS)

    Hastie, A. R.; Kerr, A. C.; Mitchell, S. F.; McDonald, I.; Pearce, J. A.; Millar, I. L.; Wolstencroft, M.

    2009-12-01

    crust. The Newcastle magmas ascended and erupted without coming into contact with a mantle wedge thus forming the low MgO, Ni and Cr contents. Most Cenozoic adakites have compositions similar to the middle-late Archaean TTG suite of igneous rocks. In contrast, early (>3.5 Ga) Archaean TTG crustal rocks have lower Sr, MgO, Ni and Cr concentrations and prior to this study had no modern adakite analogue. However, the Newcastle adakites have similar compositions to the, early Archaean TTG. The discovery of these rocks has important implications for our understanding of the formation of the Earth’s earliest continental crust and so it is proposed that the Newcastle lavas be classified as a unique sub-group of adakites: Jamaican-type adakite.

  7. Adjoint tomography of crust and upper-mantle structure beneath Continental China

    NASA Astrophysics Data System (ADS)

    Chen, M.; Niu, F.; Liu, Q.; Tromp, J.

    2013-12-01

    Four years of regional earthquake recordings from 1,869 seismic stations are used for high-resolution and high-fidelity seismic imaging of the crust and upper-mantle structure beneath Continental China. This unprecedented high-density dataset is comprised of seismograms recorded by the China Earthquake Administration Array (CEArray), NorthEast China Extended SeiSmic Array (NECESSArray), INDEPTH-IV Array, F-net and other global and regional seismic networks, and involves 1,326,384 frequency-dependent phase measurements. Adjoint tomography is applied to this unprecedented dataset, aiming to resolve detailed 3D maps of compressional and shear wavespeeds, and radial anisotropy. Contrary to traditional ray-theory based tomography, adjoint tomography takes into account full 3D wave propagation effects and off-ray-path sensitivity. In our implementation, it utilizes a spectral-element method for precise wave propagation simulations. The tomographic method starts with a 3D initial model that combines smooth radially anisotropic mantle model S362ANI and 3D crustal model Crust2.0. Traveltime and amplitude misfits are minimized iteratively based on a conjugate gradient method, harnessing 3D finite-frequency kernels computed for each updated 3D model. After 17 iterations, our inversion reveals strong correlations of 3D wavespeed heterogeneities in the crust and upper mantle with surface tectonic units, such as the Himalaya Block, the Tibetan Plateau, the Tarim Basin, the Ordos Block, and the South China Block. Narrow slab features emerge from the smooth initial model above the transition zone beneath the Japan, Ryukyu, Philippine, Izu-Bonin, Mariana and Andaman arcs. 3D wavespeed variations appear comparable to or much sharper than in high-frequency P-and S-wave models from previous studies. Moreover our results include new information, such as 3D variations of radial anisotropy and the Vp/Vs ratio, which are expected to shed new light to the composition, thermal state, flow

  8. iSIMM (Integrated Seismic Imaging and Modelling of Margins): Seismic Acquisition on the Faroes Shelf, Hatton Bank and adjacent Continental Margins

    NASA Astrophysics Data System (ADS)

    White, B.; Kusznir, N.; Christie, P.; Roberts, A. M.; Lunnon, Z.; Roberts, A. W.; Hurst, N.; Smith, L.; Parkin, C.; Surendra, A.; Davies, A.

    2002-12-01

    The iSIMM project is using state-of-the art seismic techniques with long-offset and wide-angle data, to image the crust formed on volcanic continental margins in parallel with developing and testing new quantitative models of rifted margin formation, incorporating heterogeneous stretching, the effects of melt generation and emplacement and varying thermal anomalies in the mantle. During June-July 2002, we used RRS Discovery to acquire wide angle and normal incidence seismic data on the Faroes Shelf and adjacent continental margin, Hatton-Rockall Basin, Hatton Bank and the adjacent oceanic crust using OBS and MCS. In August 2002, WesternGeco's Topaz used three single-sensor, Q-Marine streamers, 12km plus two 4km, to overshoot the wide-angle profiles on the Faroes Shelf and adjacent continental margin. In the Faroes region we deployed 85 4-component ocean bottom seismometers (OBS) and 5 vertical arrays along a 350km-long profile extending from the Faroes-Shetland Channel across the Faroes Shelf and continental margin into the oceanic crust of the Norwegian Sea. The entire profile was shot twice. First with a 6,300 cubic inch airgun array towed at 20 m depth and tuned to enhance the initial peak output pressure pulse. Second, with the airguns reconfigured to enhance the low-frequency bubble waveform, producing a source rich in low frequency energy (centred on 10-12 Hz), and with a broad bandwidth. Shots were spaced at either 75 m or 100 m, giving shot repetition rates in excess of 60 secs, thus avoiding contaminating with wrap-round energy from the previous shot. The Q-Marine acquisition used a 48-gun, 10,170 cu. in. airgun array, also tuned to enhance the low-frequency bubble signature, shooting at 50m/20s intervals and recorded on individual sensors for optimal grouping. The streamer configuration provides swath coverage at shorter offsets, while the long offsets record diving waves and wide-angle reflections. Shot-by-shot source signature recording will facilitate

  9. Continental crust under compression: A seismic refraction study of South Island Geophysical Transect I, South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    van Avendonk, Harm J. A.; Holbrook, W. Steven; Okaya, David; Austin, Jeffrey K.; Davey, Fred; Stern, Tim

    2004-06-01

    The 1996 South Island Geophysical Transect (SIGHT) active source seismic survey was designed to show the style of lithospheric thickening due to late Cenozoic oblique convergence across the Australian-Pacific plate boundary in New Zealand. As part of this study, two seismic refraction lines were shot across central South Island and offshore extensions of the continental crust in the Tasman Sea and Pacific Ocean. We present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects. A tomographic inversion of 62,563 travel times from crustal and upper mantle refractions and wide-angle reflections resulted in a model with a two-layer crust. Upper crustal velocities were between 5.9 and 6.3 km/s, and lower crustal velocities were between 6.5 and 7.0 km/s. Continental compression has locally reduced the seismic velocities in the Pacific plate crust by 0.2-0.3 km/s, a possible effect of high strain and fluids in the crust. The thickening of the crust from 28 km at the east coast of South Island to 37 km beneath the Southern Alps can account for about 25% of the 80-110 km shortening of Pacific plate crust, while the rest must be accounted for by rapid erosion of Mesozoic sedimentary rocks on the west side of the orogen. In our model the lower crust forms a continuous 2-6 km thick layer beneath central South Island. The asymmetric topography of the Southern Alps is reflected in the crustal root which has a steeper flank at the west coast. This observation is consistent with westward underthrusting of Pacific lower lithosphere beneath South Island that has been suggested in earlier studies.

  10. Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project

    NASA Astrophysics Data System (ADS)

    Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Harding, Alistair J.; Rymer, Michael J.; González-Fernández, Antonio; Lázaro-Mancilla, Octavio

    2016-10-01

    A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched preexisting crust or higher-grade metamorphosed sediment. The lower crust below 12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 km depth as it does to the south, and a weak reflection suggests Moho at 28 km depth. Structure in adjacent Mexico has slower midcrustal velocity, and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

  11. The influence of partial melting and melt migration on the rheology of the continental crust

    NASA Astrophysics Data System (ADS)

    Cavalcante, Geane Carolina G.; Viegas, Gustavo; Archanjo, Carlos José; da Silva, Marcos Egydio

    2016-11-01

    The presence of melt during deformation produces a drastic change in the rheological behavior of the continental crust; rock strength is decreased even for melt fractions as low as ∼7%. At pressure/temperature conditions typical of the middle to lower crust, melt-bearing systems may play a critical role in the process of strain localization and in the overall strength of the continental lithosphere. In this contribution we focus on the role and dynamics of melt flow in two different mid-crustal settings formed during the Brasiliano orogeny: (i) a large-scale anatectic layer in an orthogonal collision belt, represented by the Carlos Chagas anatexite in southeastern Brazil, and (ii) a strike-slip setting, in which the Espinho Branco anatexite in the Patos shear zone (northeast Brazil) serves as an analogue. Both settings, located in eastern Brazil, are part of the Neoproterozoic tectonics that resulted in widespread partial melting, shear zone development and the exhumation of middle to lower crustal layers. These layers consist of compositionally heterogeneous anatexites, with variable former melt fractions and leucosome structures. The leucosomes usually form thick interconnected networks of magma that reflect a high melt content (>30%) during deformation. From a comparison of previous work based on detailed petrostructural and AMS studies of the anatexites exposed in these areas, we discuss the rheological implications caused by the accumulation of a large volume of melt "trapped" in mid-crustal levels, and by the efficient melt extraction along steep shear zones. Our analyses suggest that rocks undergoing partial melting along shear settings exhibit layers with contrasting competence, implying successive periods of weakening and strengthening. In contrast, regions where a large amount of magma accumulates lack clear evidence of competence contrast between layers, indicating that they experienced only one major stage of dramatic strength drop. This comparative

  12. Evolution of Continental Lower Crust Recorded By an Exhumed Deep Crustal Intracontinental Shear Zone

    NASA Astrophysics Data System (ADS)

    Dumond, G.; Mahan, K. H.; Regan, S. P.; Williams, M. L.; Goncalves, P.; Wood, V. R.

    2014-12-01

    Exposures of deep crustal shear zones are fundamental records of strain localization and the temporal evolution of ductile to brittle behavior as these tectonites were exhumed to the surface. We present results from a decade of field-based research on a deeply exhumed (~35 km-paleodepths) strike-slip shear zone in the western Churchill province of the Canadian Shield. The Grease River shear zone is a >400 km-long and 7 km-thick structure that cuts the Athabasca granulite terrane, North America's largest exposure of continental lower crust (>20,000 km2). The shear zone is dominated by granulite- to amphibolite-grade L-S and L>S tectonites characterized by penetrative NE-striking steeply-dipping foliations with gently-plunging to sub-horizontal stretching and intersection lineations. These fabrics are locally overprinted by pseudotachylyte and narrow (<500 m-thick) greenschist-grade zones of cataclasite. Dextral kinematics are defined by deflected foliation trajectories, C' shear bands, and well-developed σ- and δ-type porphyroclasts of Kfs + Pl + Opx + Grt + Hb in felsic to intermediate granulite paragneisses and orthogneisses. Data collected along a well-exposed, nearly 150 km-long segment of the shear zone documents a >100 m.y. episodic record of transpressive to strike-slip intracontinental strain accumulation that coincided with two oppositely convergent orogenies: the east-vergent arc-continent collision of the 1.94-1.90 Ga Taltson orogen and the west-vergent continent-continent collision of the 1.9-1.8 Ga Trans-Hudson orogen. Deformation mechanisms evolved from distributed ductile dynamic recrystallization and grain-size reduction to localized pseudotachylyte development, cataclastic flow, and brittle faulting. Lower crustal behavior during strain localization was dynamic. Melt-weakened mono-cyclic crust was juxtaposed against strong isobarically-cooled poly-cyclic crust along the shear zone at 1.92-1.90 Ga. Brittle-ductile reactivation of the structure

  13. Noble gases preserve history of retentive continental crust in the Bravo Dome natural CO2 field, New Mexico

    NASA Astrophysics Data System (ADS)

    Sathaye, Kiran J.; Smye, Andrew J.; Jordan, Jacob S.; Hesse, Marc A.

    2016-06-01

    Budgets of 4He and 40Ar provide constraints on the chemical evolution of the solid Earth and atmosphere. Although continental crust accounts for the majority of 4He and 40Ar degassed from the Earth, degassing mechanisms are subject to scholarly debate. Here we provide a constraint on crustal degassing by comparing the noble gases accumulated in the Bravo Dome natural CO2 reservoir, New Mexico USA, with the radiogenic production in the underlying crust. A detailed geological model of the reservoir is used to provide absolute abundances and geostatistical uncertainty of 4He, 40Ar, 21Ne, 20Ne, 36Ar, and 84Kr. The present-day production rate of crustal radiogenic 4He and 40Ar, henceforth referred to as 4He* and 40Ar*, is estimated using the basement composition, surface and mantle heat flow, and seismic estimates of crustal density. After subtracting mantle and atmospheric contributions, the reservoir contains less than 0.02% of the radiogenic production in the underlying crust. This shows unequivocally that radiogenic noble gases are effectively retained in cratonic continental crust over millennial timescales. This also requires that approximately 1.5 Gt of mantle derived CO2 migrated through the crust without mobilizing the crustally accumulated gases. This observation suggests transport along a localized fracture network. Therefore, the retention of noble gases in stable crystalline continental crust allows shallow accumulations of radiogenic gases to record tectonic history. At Bravo Dome, the crustal 4He*/40Ar* ratio is one fifth of the expected crustal production ratio, recording the preferential release of 4He during the Ancestral Rocky Mountain orogeny, 300 Ma.

  14. What governs the enrichment of Pb in the continental crust? An answer from the Mexican Volcanic Belt

    NASA Astrophysics Data System (ADS)

    Goldstein, S. L.; Lagatta, A.; Langmuir, C. H.; Straub, S. M.; Martin-Del-Pozzo, A.

    2009-12-01

    One of Al Hofmann’s many important contributions to our understanding of geochemical cycling in the Earth is the observation that Pb behaves like the light rare earth elements Ce and Nd during melting to form oceanic basalts, but is enriched in the continental crust compared to the LREE by nearly an order of magnitude (Hofmann et al. 1986). This is unusual behavior, and has been called one of the Pb paradoxes, since in most cases, the ratios of elements are effectively the same in the continental crust and oceanic basalts if they show similar mantle melting behavior. One of several mechanisms suggested to mediate this special enrichment is hydrothermal circulation at ocean ridges, which preferentially transports Pb compared to the REE from the interior of the ocean crust to the surface. We confirm the importance of hydrothermal processes at the East Pacific to mediate Pb enrichment at the Trans-Mexican Volcanic Belt (TMVB, through comparison of Pb isotope and Ce/Pb ratios of TMVB lavas with sediments from DSDP Site 487 near the Middle America trench. The lavas of the Trans-Mexican Volcanic Belt include “high Nb” alkali basalts (HNAB), whose trace element patterns lack subduction signatures. The HNAB basalts and hydrothermally affected sediments from DSDP 487, form end-members that bound calcalkaline lavas from volcanoes Colima, Toluca, Popocatépetl, and Malinche in Ce/Pb versus Pb isotope space. The HNAB represent the high Ce/Pb and high Pb-isotope end-member. The hydrothermal sediments have Pb isotopes like Pacific MORB but Ce/Pb ratios typical of the arcs and the continental crust, and an order of magnitude lower than MORB. No analyzed calcalkaline lavas are have compositions outside of the bounds formed by the HNAB and the hydrothermal sediments. The Ce/Pb and Pb isotope ratios show that the calcalkaline lava compositions are inconsistent with contributions from HNAB and EPR MORB, rather the contributions are from HNAB upper mantle and subducted

  15. Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic

    NASA Astrophysics Data System (ADS)

    Gall, L.; Williams, H. M.; Siebert, C.; Halliday, A. N.; Herrington, R. J.; Hein, J. R.

    2013-08-01

    The global variability in nickel (Ni) isotope compositions in ferromanganese crusts is investigated by analysing surface samples of 24 crusts from various ocean basins by MC-ICPMS, using a double-spike for mass bias correction. Ferromanganese crusts have δ60Ni isotopic compositions that are significantly heavier than any other samples thus far reported (-0.1‰ to 0.3 ‰), with surface scrapings ranging between 0.9 ‰ and 2.5 ‰ (relative to NIST SRM986). There is no well resolved difference between ocean basins, although the data indicate somewhat lighter values in the Atlantic than in the Pacific, nor is there any evidence that the variations are related to biological fractionation, presence of different water masses, or bottom water redox conditions. Preliminary data for laterite samples demonstrate that weathering is accompanied by isotopic fractionation of Ni, which should lead to rivers and seawater being isotopically heavy. This is consistent with the slightly heavier than average isotopic compositions recorded in crusts that are sampled close to continental regions. Furthermore, the isotopic compositions of crusts growing close to a hydrothermal source are clustered around ∼ 1.5 ‰, suggesting that hydrothermal fluids entering the ocean may have a Ni isotopic composition similar to this value. Based on these data, the heavy Ni isotopic compositions of ferromanganese crusts are likely due to input of isotopically heavy Ni to the ocean from continental weathering and possibly also from hydrothermal fluids. A depth profile through one crust, CD29-2, from the north central Pacific Ocean displays large variations in Ni isotope composition (1.1 - 2.3 ‰) through the last 76 Myr. Although there may have been some redistribution of Ni associated with phosphatisation, there is no systematic difference in Ni isotopic composition between deeper, older parts and shallower, younger parts of the crust, which may suggest that oceanic sources and sinks of Ni have

  16. Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites

    NASA Astrophysics Data System (ADS)

    Gaschnig, Richard M.; Rudnick, Roberta L.; McDonough, William F.; Kaufman, Alan J.; Valley, John W.; Hu, Zhaochu; Gao, Shan; Beck, Michelle L.

    2016-08-01

    The composition of the fine-grained matrix of glacial diamictites from the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic, collected from four modern continents, reflects the secular evolution of the average composition of the upper continental crust (UCC). The effects of localized provenance are present in some cases, but distinctive geochemical signatures exist in diamictites of the same age from different localities, suggesting that these are global signatures. Archean UCC, dominated by greenstone basalts and to a lesser extent komatiites, was more mafic, based on major elements and transition metal trace elements. Temporal changes in oxygen isotope ratios, rare earth elements, and high field strength elements indicate that the UCC became more differentiated and that tonalite-trondhjemite-granodiorite suites became less important with time, findings consistent with previous studies. We also document the concentrations of siderophile and chalcophile elements (Ga, Ge, Cd, In, Sn, Sb, W, Tl, Bi) and lithophile Be in the UCC through time, and use the data for the younger diamictites to construct a new estimate of average UCC along with associated uncertainties.

  17. The structure of continental crust: comparison of body wave apparent incidence angle and receiver function results

    NASA Astrophysics Data System (ADS)

    Cunningham, E. E.; Lekic, V.

    2015-12-01

    The dense coverage of the EarthScope USArray presents an unprecedented opportunity to systematically investigate the structure of the continental crust across different tectonic regimes. Receiver function analysis of body wave arrivals can isolate converted phases (Ps or Sp) produced across velocity discontinuities beneath a seismometer and constrain relative vertical density and seismic velocity variations. Analysis of receiver functions computed for stations across the footprint of the USArray can be used to constrain both the strength and topography of crustal interfaces. However, complications in receiver function analysis arise from trade-offs among compressional (Vp) and shear (Vs) wave velocity variations, as well as reverberations caused by sediment-dominated regions within the United States. We show that by measuring the apparent incidence angle of P waves - their relative amplitude on the vertical and radial components of the seismogram - computed for different time-windows, we can obtain an estimate of crustal Vs layering at each station. The calculated Vs estimate is in terms of absolute as opposed to relative velocity variations and is independent of Vp. Using synthetic waveforms, we quantify to what extent absolute velocity inferences obtained using the apparent-incidence angle method are affected by multiple reverberations in shallow layers. We then use both synthetics and data to evaluate the potential of the apparent incidence method for constraining anisotropy, and compare it to receiver functions. Finally, we compare and contrast results from receiver function and apparent incidence angle analyses in different tectonic settings across North America.

  18. LITHOPROBE: a scientific update-new images of the continental crust

    NASA Astrophysics Data System (ADS)

    Clowes, Ron M.

    1990-02-01

    Seismic line run across the continental margin northeast of Newfoundland help determine the nature of extensional tectonics and associated sedimentary basin evolution on the rifted margin. The GLIMPCE Transect has enabled a glimpse of the mid-continent crust by recording marine reflection data in the Great Lakes. The sections from Lake Superior reveal — 10 km of sediments overlying a 20 km thick sequence of strongly reflective volcanics and inter-flow sediments, perhaps representing the greatest vertical extent of intra-continental rift deposits on Earth.The original Archean/Early Proterozoic crust is greatly thinned beneath the rift basin. In the Lake Huron/Georgian Bay profile, the Grenville front is imaged as the westernmost reflection of a spectacular 70 km wide band of southeast-dipping reflections that clearly truncates prominent sub-horizontal crustal reflections of the Superior granite-rhyolite terrane. The Kapuskasing Structure Zone is a sequence of lower crustal rocks that have been thrust upward along the Ivanhoe Lake cataclastic zone and are now exposed at the surface. As part of the Kapuskasing Transect, 340 km of regional deep reflection data and 20 km of " high resolution" data (20 m station spacing, 20 m source interval and a higher sweep range) over the cataclastic zone were recorded in 1987-1988. The Abitibi sub-province of Ontario and Quebec contains mineralized belts for which it is famous. As a preliminary program for the Abitibi-Grenville Transect, 130 km of regional and 65 km of high-resolution Seismic profiles also were recorded in 1987-1988. These were centred over major fault zones with which mineralization is related. Data quality is excellent and the data are in the process of being interpreted.

  19. Construction of mass-age curves for the continental crust: An empirical model and an example from the western US

    NASA Technical Reports Server (NTRS)

    Depaolo, D. J.; Schubert, G. S.; Linn, Ann

    1988-01-01

    The methodology of determining crustal mass-age curves is discussed. Problems in doing this include determination of model ages and accounting for mixing of materials of different ages during crust forming processes. These difficulties can be overcome with some reasonable assumptions and estimates of rock volumes based on areal proportions. This technique was used to construct a reasonably well constrained mass-age curve for the southwestern U.S. based on isotopic measurements on over 100 samples. The results imply that the crust in this area grew episodically at 2.8, 1.8, and 0.1 Ga. It is estimated that it would take on the order of 10000 to 100000 individual Sm-Nd isotopic measurements to carry out a similar exercise for continental crust world wide.

  20. ON THE WIND-INDUCED EXCHANGE BETWEEN INDIAN RIVER BAY, DELAWARE AND THE ADJACENT CONTINENTAL SHELF. (R826945)

    EPA Science Inventory

    The structure of the wind-induced exchange between Indian River Bay, Delaware and the adjacent continental shelf is examined based on current measurements made at the Indian River Inlet which represents the only conduit of exchange between the bay and the coastal ocean. Local ...

  1. Intrusion of granitic magma into the continental crust facilitated by magma pulsing and dike-diapir interactions: Numerical simulations

    NASA Astrophysics Data System (ADS)

    Cao, Wenrong; Kaus, Boris J. P.; Paterson, Scott

    2016-06-01

    We conducted a 2-D thermomechanical modeling study of intrusion of granitic magma into the continental crust to explore the roles of multiple pulsing and dike-diapir interactions in the presence of visco-elasto-plastic rheology. Multiple pulsing is simulated by replenishing source regions with new pulses of magma at a certain temporal frequency. Parameterized "pseudo-dike zones" above magma pulses are included. Simulation results show that both diking and pulsing are crucial factors facilitating the magma ascent and emplacement. Multiple pulses keep the magmatic system from freezing and facilitate the initiation of pseudo-dike zones, which in turn heat the host rock roof, lower its viscosity, and create pathways for later ascending pulses of magma. Without diking, magma cannot penetrate the highly viscous upper crust. Without multiple pulsing, a single magma body solidifies quickly and it cannot ascent over a long distance. Our results shed light on the incremental growth of magma chambers, recycling of continental crust, and evolution of a continental arc such as the Sierra Nevada arc in California.

  2. Continental like crust beneath the Andaman Island through joint inversion of receiver function and surface wave from ambient seismic noise

    NASA Astrophysics Data System (ADS)

    Gupta, Sandeep; Borah, Kajaljyoti; Saha, Gokul

    2016-09-01

    We study shear wave velocity structure of the crust beneath the Andaman Island through joint inversion of the teleseismic receiver function and Rayleigh wave group velocity measurements from 10 broadband seismographs over the Island. The group velocities in the periods from 5 to 21 s are obtained using cross-correlation of six month's ambient seismic noise data recorded by these seismic stations. Joint inversion results show 2 to 6 km thick subsurface low shear velocity (Vs 1.3-2.5 km/s) layer followed by a 12-14 km thick layer of silicic material (average Vs 3.5 km/s). The lower crust is mapped as an 8-12 km thick mafic layer with Vs 4.0 km/s. Uppermost mantle shear wave velocity is 4.55 km/s. The near-surface low-velocity layer is interpreted as the Andaman flysch sediments. The crustal thickness beneath the Andaman Island varies from 24 km in the north to 32 km in the south. The shear wave velocity-depth results show that the crustal structure beneath the Andaman Island is akin to continental crust, possibly the Burma continental crust. The subducting Indian plate may lie down below this overriding plate.

  3. 2.6-2.7 Ga continental crust growth in Yangtze craton, South China

    NASA Astrophysics Data System (ADS)

    Chen, K.; Gao, S.; Wu, Y.

    2013-12-01

    A combined study of zircon U-Pb and Lu-Hf isotopes and whole rock major and trace elements and Sr-Nd isotopes has been conducted for 10 granitic and tonalitic-trondhjemitic-granodioritic (TTG) gneisses from the Kongling terrain, the only known Archean microcontinent in the Yangtze craton, South China. The results reveal a significant magmatic event at ~2.6-2.7 Ga (Fig. 1), in addition to the previously reported ~2.9 Ga and ~3.2-3.3 Ga magmatism (Zhang et al., 2006; Jiao et al., 2009; Gao et al., 2011). The ~2.6-2.7 Ga rocks show relatively high REE (530-1074 ppm), apparently negative Eu anomaly (Eu/Eu* = 0.22-0.35), low #Mg (19.51-22.63) and low LaN/YbN (10.3-24.2). Besides, they have high K-feldspar proportion and relatively evaluated (K2O+Na2O)/CaO, TFeO/MgO, Zr, Nb, Ce and Y contents. Their 10000 × Ga/Al ratios range between 3.00 and 3.54. All these features suggest that the protoliths of these gneisses are A-type granites. Most of the ~2.6-2.7 Ga zircon grains have ɛHf(t) values >0 (up to 7.93, close to the depleted mantle value). This clearly indicates a considerably higher proportion of new crustal components in the ~2.6-2.7 Ga granitoids compared to the ~3.2-3.3 Ga and ~2.9 Ga TTGs. Our results support the conclusion of worldwide studies of igneous and detrital zircons that age peaks at 2.65-2.76 Ga represent increases in the volume of juvenile continental crust. The present study also confirms the existence of the two older magmatic events in the Kongling terrain. Both whole rock ɛNd(t) values (-3.74 to 1.59) and the zircon ɛHf(t) values (-11.18 to 3.55) for the ~2.9 Ga TTG and the Hf isotopes of ~3.2-3.3 Ga igneous zircons (-7.37 to 3.12) are chondritic or subchondritic, suggesting that they were mainly generated by reworking of older rocks with a small amount of new crustal additions. References Gao, S., Yang, J., Zhou, L., Li, M., Hu, Z.C., Guo, J.L., Yuan, H.L., Gong, H.J., Xiao, G.Q., Wei, J.Q., 2011. Age and growth of the Archean Kongling terrain

  4. New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature

    NASA Astrophysics Data System (ADS)

    Sauzéat, Lucie; Rudnick, Roberta L.; Chauvel, Catherine; Garçon, Marion; Tang, Ming

    2015-10-01

    Lithium isotopes are increasingly used to trace both present-day and past weathering processes at the surface of the Earth, and could potentially be used to evaluate the average degree of past weathering recorded by the upper continental crust (UCC). Yet the previous estimate of average δ7Li of the UCC has a rather large uncertainty, hindering the use of Li isotopes for this purpose. New δ7Li for desert and periglacial loess deposits (windblown dust) from several parts of the world (Europe, Argentina, China and Tajikistan) demonstrate that the former are more homogeneous than the latter, and may therefore serve as excellent proxies of the average composition of large tracts of the UCC. The Li isotopic compositions and concentrations of desert loess samples are controlled by eolian sorting that can be quantified by a binary mixing between a weathered, fine-grained end-member, dominated by phyllosilicates and having low δ7Li, and an unweathered, coarse-grained end-member, that is a mixture of quartz and plagioclase having higher δ7Li. We use correlations between insoluble elements (REE, Nd/Hf and Fe2O3/SiO2), Li concentrations (henceforth referred as [Li]), and δ7Li to estimate a new, more precise, average Li isotopic composition and concentration for the UCC: [ Li ] = 30.5 ± 3.6 (2 σ) ppm, and δ7Li = + 0.6 ± 0.6 (2 σ). The δ7Li for desert loess deposits is anti-correlated with the chemical index of alteration (CIA). Using this relationship, along with our average δ7Li, we infer that (1) the present-day CIA of the average UCC is 61-2+4 (2 σ), higher than the common reference value of 53, and (2) the average proportion of chemically weathered components is as high as 37-10+17 (2 σ)% at the surface of the Earth.

  5. The Siberian Flood Basalts: Connecting the Mantle, the Continental Crust, and the Atmosphere

    NASA Astrophysics Data System (ADS)

    Elkins-Tanton, L. T.; Arndt, N.; Black, B. A.; Fristad, K. E.; Kiehl, J. T.; Lamarque, J.; Meyer, K. M.; Payne, J.; Planke, S.; Shields, C. A.; Svensen, H.

    2012-12-01

    Around 252 million years ago the Siberian flood basalts intruded into and erupted onto the Siberian craton. At that time the craton was at about its present latitude, and was a part of Pangaea, with inward-dipping subduction on several sides. The flood basalt event is among several possible causes for the end-Permian extinction, the largest extinction in Earth history. Our team hypothesizes that the magmas caused the injection of sufficient volatiles into the atmosphere to produce global climate change. These volatiles were in part sweated out of the crustal rocks by the chambered magmas, and in part assimilated from the crust by the magmas and released upon eruption. The magmas intruded a 12-km-deep evaporate basin containing hydrocarbon reservoirs. The complex interactions of heat and rock with silicate, hydrous, and hydrocarbon fluids produced rich ore bodies, a variety of magmatic rocks including carbonatites, and significant volumes of carbon, sulfur, chlorine, and fluorine-bearing volatiles. We will present an overview of our results to date, including paleomagnetic data on rates of eruption, dynamic models of plume-lithosphere interaction, isotopic evidence of the changes in ocean chemistry, field and laboratory data on volatile release in intrusive aureoles, melt inclusion measurements of magmatic volatile loads, and early climate model results. Because of its volume and the specific crustal region it passed through, the magmatic event produced significant chemical and heat transfer between Earth's interior and its surface. Though other continental flood basalts are similarly sized, the Siberian event interacted with particularly toxic crustal rocks and is a likely candidate for triggering the end-Permian global climate change and mass extinction.

  6. Interdisciplinary approach to exploit the tectonic memory in the continental crust of collisional belts.

    NASA Astrophysics Data System (ADS)

    Gosso, G.; Marotta, A. M.; Rebay, G.; Regorda, A.; Roda, M.; Spalla, M. I.; Zanoni, D.; Zucali, M.

    2015-12-01

    Collisional belts result by thoroughly competing thermo-mechanical disaggregation and coupling within both continental and oceanic lithospheric slices, during construction of tectono-metamorphic architectures. In multiply reworked metamorphics, tectonic units may be contoured nowadays on the base of coherent thermo-baric and structural time-sequences rather than simply relying on lithologic affinities. Sequences of equilibrium assemblages and related fabric imprints are an approach that appears as a more reliable procedure, that enables to define tectonic units as the volume of crustal slices that underwent corresponding variations during the dynamics of an active margin and takes into account a history of physical imprints. The dimensions of these tectonic units may have varied over time and must be reconstructed combining the tracers of structural and metamorphic changes of basement rocks, since such kind of tectono-metamorphic units (TMUs) is a realistic configuration of the discrete portions of orogenic crust that experienced a coherent sequence of metamorphic and textural variations. Their translational trajectories, and bulk shape changes during deformation, cannot simply be derived from the analysis of the geometries and kinematics of tectonic units, but are to be obtained by adding the reconstruction of quantitative P-T-d-t paths making full use of fossil mineral equilibria. The joint TMU field-and-laboratory definition is an investigation procedure that bears a distinct thermo-tectonic connotation, that, through modelling, offers the opportunity to test the physical compatibilities of plate-scale interconnected variables, such as density, viscosity, and heat transfer, with respect to what current interpretative geologic histories may imply. Comparison between predictions from numerical modelling and natural data obtained by this analytical approach can help to solve ambiguities on geodynamic significance of structural and thermal signatures, also as a

  7. Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

    NASA Astrophysics Data System (ADS)

    Zhao, Fang; Alves, Tiago M.; Wu, Shiguo; Li, Wei; Huuse, Mads; Mi, Lijun; Sun, Qiliang; Ma, Benjun

    2016-07-01

    Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K-Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean-continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup.

  8. Sea-floor drainage features of Cascadia Basin and the adjacent continental slope, northeast Pacific Ocean

    USGS Publications Warehouse

    Hampton, M.A.; Karl, Herman A.; Kenyon, Neil H.

    1989-01-01

    Sea-floor drainage features of Cascadia Basin and the adjacent continental slope include canyons, primary fan valleys, deep-sea valleys, and remnant valley segments. Long-range sidescan sonographs and associated seismic-reflection profiles indicate that the canyons may originate along a mid-slope escarpment and grow upslope by mass wasting and downslope by valley erosion or aggradation. Most canyons are partly filled with sediment, and Quillayute Canyon is almost completely filled. Under normal growth conditions, the larger canyons connect with primary fan valleys or deep-sea valleys in Cascadia Basin, but development of accretionary ridges blocks or re-routes most canyons, forcing abandonment of the associated valleys in the basin. Astoria Fan has a primary fan valley that connects with Astoria Canyon at the fan apex. The fan valley is bordered by parallel levees on the upper fan but becomes obscure on the lower fan, where a few valley segments appear on the sonographs. Apparently, Nitinat Fan does not presently have a primary fan valley; none of the numerous valleys on the fan connect with a canyon. The Willapa-Cascadia-Vancouver-Juan de Fuca deep-sea valley system bypasses the submarine fans and includes deeply incised valleys to broad shallow swales, as well as within-valley terraces and hanging-valley confluences. ?? 1989.

  9. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

    USGS Publications Warehouse

    Von Huene, R.; Scholl, D. W.

    1991-01-01

    At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (???19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-mediumsized prisms have formed (???16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (???8,200 km), roughly 70% of the incoming trench floor section is

  10. Continental crust subducted deeply into lithospheric mantle: the driving force of Early Carboniferous magmatism in the Variscan collisional orogen (Bohemian Massif)

    NASA Astrophysics Data System (ADS)

    Janoušek, Vojtěch; Schulmann, Karel; Lexa, Ondrej; Holub, František; Franěk, Jan; Vrána, Stanislav

    2014-05-01

    The vigorous Late Devonian-Early Carboniferous plutonic activity in the core of the Bohemian Massif was marked by a transition from normal-K calc-alkaline, arc-related (~375-355 Ma), through high-K calc-alkaline (~346 Ma) to (ultra-)potassic (343-335 Ma) suites, the latter associated with mainly felsic HP granulites enclosing Grt/Spl mantle peridotite bodies. The changing chemistry, especially an increase in K2O/Na2O and 87Sr/86Sri with decrease in 143Nd/144Ndi in the basic end-members, cannot be reconciled by contamination during ascent. Instead it has to reflect the character of the mantle sources, changing over time. The tectonic model invokes an oceanic subduction passing to subduction of the attenuated Saxothuringian crust under the rifted Gondwana margin (Teplá-Barrandian and Moldanubian domains). The deep burial of this mostly refractory felsic metaigneous material is evidenced by the presence of coesite/diamond (Massonne 2001; Kotková et al. 2011) in the detached UHP slices exhumed through the subduction channel and thrusted over the Saxothuringian basement, and by the abundance of felsic HP granulites (> 2.3 GPa), some bearing evidence for small-scale HP melt separation, in the orogen's core (Vrána et al. 2013). The subduction channel was most likely formed by 'dirty' serpentinites contaminated by the melts/fluids derived from the underlying continental-crust slab (Zheng 2012). Upon the passage through the orogenic mantle, the continental crust-slab derived material not only contaminated the adjacent mantle forming small bodies/veins of pyroxenites (Becker 1996), glimmerites (Becker et al. 1999) or even phlogopite- and apatite-bearing peridotites (Naemura et al. 2009) but the felsic HP-HT granulites also sampled the individual peridotite types at various levels. Eventually the subducted felsic material would form an (U)HP continental wedge under the forearc/arc region, to be later redistributed under the Moldanubian crust by channel flow and crustal

  11. Creep of mafic dykes infiltrated by melt in the lower continental crust (Seiland Igneous Province, Norway)

    NASA Astrophysics Data System (ADS)

    Degli Alessandrini, G.; Menegon, L.; Malaspina, N.; Dijkstra, A. H.; Anderson, M. W.

    2017-03-01

    A dry mafic dyke from a continental lower-crustal shear zone in the Seiland Igneous Province (northern Norway) experienced syn-kinematic melt-rock interaction. Viscous shearing occurred at T ≈ 800 °C, P ≈ 0.75-0.95 GPa and was coeval with infiltration of felsic melt from adjacent migmatitic metapelites. The dyke has a mylonitic microstructure where porphyroclasts of orthopyroxene, clinopyroxene and plagioclase are wrapped by a fine-grained (4-7 μm) polyphase mixture of clinopyroxene + orthopyroxene + plagioclase + quartz + ilmenite ± K-feldspar ± apatite. Microstructural observations and electron backscatter diffraction analysis indicate that the porphyroclasts deformed by a combination of dislocation glide and fracturing, with only a limited record of dislocation creep, recovery and dynamic recrystallization. We identified diffusion creep as the dominant deformation mechanism in the mixture based on the small grain size, phase mixing and weak crystallographic preferred orientation of all phases (interpreted as the result of oriented grain growth during viscous flow). The polyphase mixture did not form by dynamic recrystallization or by mechanical fragmentation of the porphyroclasts, but rather by melt-rock interaction. Thermodynamic models indicate that the syn-kinematic mineral assemblage results from the chemical interaction between a pristine mafic dyke and ca. 10 vol.% of felsic melt infiltrating from the adjacent partially molten metapelites. Extrapolation of laboratory-derived flow laws to natural conditions indicates that the formation of interconnected layers of fine-grained reaction products deforming by diffusion creep induces a dramatic weakening in the mafic granulites, with strain rates increasing up to 2-3 orders of magnitude. The reaction weakening effect is more efficient than the weakening associated with melt-assisted diffusion creep in the presence of up to 10 vol.% of infiltrated melt without formation of fine-grained reaction products

  12. The interplay and effects of deformation and crystallized melt on the rheology of the lower continental crust, Fiordland, New Zealand

    NASA Astrophysics Data System (ADS)

    Miranda, Elena A.; Klepeis, Keith A.

    2016-12-01

    Microstructural, electron backscatter diffraction (EBSD), and misorientation analyses of a migmatitic granulite-facies orthogneiss from the exhumed lower crust of a Cretaceous continental arc in Fiordland, New Zealand show how deformation was accommodated during and after episodes of melt infiltration and high-grade metamorphism. Microstructures in garnet, omphacite, plagioclase, and K-feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase, with subsequent deformation becoming partitioned into plagioclase. Continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite, where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding, most likely accommodated by diffusion creep, in fine recrystallized plagioclase grains. Our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc, where initial weakening was primarily achieved by deformation of both omphacite and plagioclase. Subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite, and that changes in deformation mechanisms within plagioclase, and an evolution of its strength, primarily control the rheology of the lower crust during and after episodes of melting and magma addition.

  13. The deep crust beneath island arcs: Inherited zircons reveal a Gondwana continental fragment beneath East Java, Indonesia

    NASA Astrophysics Data System (ADS)

    Smyth, H. R.; Hamilton, P. J.; Hall, R.; Kinny, P. D.

    2007-06-01

    Inherited zircons in Cenozoic sedimentary and igneous rocks of East Java range in age from Archean to Cenozoic. The distribution of zircons reveals two different basement types at depth. The igneous rocks of the Early Cenozoic arc, found along the southeast coast, contain only Archean to Cambrian zircons. In contrast, clastic rocks of north and west of East Java contain Cretaceous zircons, which are not found in the arc rocks to the south. The presence of Cretaceous zircons supports previous interpretations that much of East Java is underlain by arc and ophiolitic rocks, accreted to the Southeast Asian margin during Cretaceous subduction. However, such accreted material cannot account for the older zircons. The age populations of Archean to Cambrian zircons in the arc rocks are similar to Gondwana crust. We interpret the East Java Early Cenozoic arc to be underlain by a continental fragment of Gondwana origin and not Cretaceous material as previously suggested. Melts rising through the crust, feeding the Early Cenozoic arc, picked up the ancient zircons through assimilation or partial melting. We suggest a Western Australian origin for the fragment, which rifted from Australia during the Mesozoic and collided with Southeast Asia, resulting in the termination of Cretaceous subduction. Continental crust was therefore present at depth beneath the arc in south Java when Cenozoic subduction began in the Eocene.

  14. Granitoid pluton formation by spreading of continental crust: the Wiley Glacier complex, northwest Palmer Land, Antarctica

    NASA Astrophysics Data System (ADS)

    Vaughan, Alan P. M.; Wareham, Christopher D.; Millar, Ian L.

    1997-12-01

    The emplacement mechanism, geometry, and isotope geochemistry of plutons of the Wiley Glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the Antarctic Peninsula magmatic arc in Early Cretaceous times. The suggested mechanism is analogous to basalt dyke injection during sea-floor spreading. During normal-dextral shear, the Burns Bluff pluton, a sheeted, moderately east-dipping, syn-magmatically sheared tonalite-granodiorite intruded syn-magmatically sheared quartz diorite of the Creswick Gap pluton and 140 ± 5 Ma hornblende gabbro. UPb dating of zircon and ArAr dating of hornblende and biotite suggest that both granite s.l. plutons were emplaced between 145 and 140 Ma, but that extensional shearing was active from the time of emplacement until ca. 127 Ma. The Burns Bluff pluton is chilled at its margin, and grades through mylonitised, porphyritic tonalite-granodiorite sheets and tonalite-granodiorite sheets with minor chilling, to a kilometre-scale body of coarse-grained, hypidiomorphic tonalite-granodiorite. Co-magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves. These dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal-dextral shear. Pluton-marginal porphyritic and hypidiomorphic tonalite-granodiorite forms large, fault-hosted sheets emplaced progressively under extension with minor dextral shear. Kinematic indicators from pluton-marginal granite s.l. dykes suggest that early in pluton accretion, intrusive sheets cooled rapidly, with simple shear prior to full crystallisation changing to ductile simple shear during cooling. Kinematic indicators towards the pluton core suggest that as the pluton grew, and cooled more slowly, emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma. Cross

  15. Making Earth's earliest continental crust - an analogue from voluminous Neogene silicic volcanism in NE-Iceland

    NASA Astrophysics Data System (ADS)

    Berg, Sylvia E.; Troll, Valentin R.; Burchardt, Steffi; Riishuus, Morten S.; Deegan, Frances M.; Harris, Chris; Whitehouse, Martin J.; Gústafsson, Ludvik E.

    2014-05-01

    Borgarfjörður Eystri in NE-Iceland represents the second-most voluminous exposure of silicic eruptive rocks in Iceland and is a superb example of bimodal volcanism (Bunsen-Daly gap), which represents a long-standing controversy that touches on the problem of crustal growth in early Earth. The silicic rocks in NE-Iceland approach 25 % of the exposed rock mass in the region (Gústafsson et al., 1989), thus they significantly exceed the usual ≤ 12 % in Iceland as a whole (e.g. Walker, 1966; Jonasson, 2007). The origin, significance, and duration of the voluminous (> 300 km3) and dominantly explosive silicic activity in Borgarfjörður Eystri is not yet constrained (c.f. Gústafsson, 1992), leaving us unclear as to what causes silicic volcanism in otherwise basaltic provinces. Here we report SIMS zircon U-Pb ages and δ18O values from the region, which record the commencement of silicic igneous activity with rhyolite lavas at 13.5 to 12.8 Ma, closely followed by large caldera-forming ignimbrite eruptions from the Breiðavik and Dyrfjöll central volcanoes (12.4 Ma). Silicic activity ended abruptly with dacite lava at 12.1 Ma, defining a ≤ 1 Myr long window of silicic volcanism. Magma δ18O values estimated from zircon range from 3.1 to 5.5 (± 0.3; n = 170) and indicate up to 45 % assimilation of a low-δ18O component (e.g. typically δ18O = 0 ‰, Bindeman et al., 2012). A Neogene rift relocation (Martin et al., 2011) or the birth of an off-rift zone to the east of the mature rift associated with a thermal/chemical pulse in the Iceland plume (Óskarsson & Riishuus, 2013), likely brought mantle-derived magma into contact with fertile hydrothermally-altered basaltic crust. The resulting interaction triggered large-scale crustal melting and generated mixed-origin silicic melts. Such rapid formation of silicic magmas from sustained basaltic volcanism may serve as an analogue for generating continental crust in a subduction-free early Earth (e.g. ≥ 3 Ga, Kamber et

  16. Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

    USGS Publications Warehouse

    Akinin, V.V.; Miller, E.L.; Wooden, J.L.

    2009-01-01

    Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep

  17. RESEARCH NOTE: Physical description of Lg waves in inhomogeneous continental crust

    NASA Astrophysics Data System (ADS)

    Vaccari, Franco; Gregersen, Søren

    1998-11-01

    their neighbouring modes. A characteristic of the regularity is that there is no mode conversion for very long-period modes. In contrast, a pattern cannot be recognized for waves of shorter period, and one may use the term ill-behaved. The Lg waves in the continental crust are in the short-period range because their wavelengths are short compared to thickness changes as well as to the thicknesses of crustal structures. This leads to the view that Lg waves are superpositions of many modes, changing into even more modes at any discontinuity. Since Rayleigh and Love waves are coupled at non-perpendicular incidence and the Earth has many discontinuities, the superposition of modes is constantly changing in relative proportions, Rayleigh and Love modes being intermingled.

  18. Experimental study of continental crust partial melting: rheological and microstructure effects.

    NASA Astrophysics Data System (ADS)

    Fauconnier, Julien; Stünitz, Holger; Rosenberg, Claudio; Labrousse, Loïc; Jolivet, Laurent

    2014-05-01

    Deformation on the orogen-scales involves localization due to weakening, and partial melting of the continental crust is one of the most important weakening agent. In order to understand the causes of melt-induced weakening and the way melt moves through an anisotropic medium (generally consisting of a foliation and/or layering) we conducted a series of high-pressure experiments to deform partially molten, crustal-like materials and compare the experimental microstructures with those of natural migmatites. Deformation experiments were performed in simple shear mode in a Griggs-type apparatus. The starting material consists of 90% quartz powder (crushed Fontainebleau sandstone; 10 to 20 µm grain size) and 10% biotite powder (50 to 100 µm grain size). In order to control the amount of melt, 5% to 10% of haplogranitic glass (HPG) were added to the starting material. Experiments were performed in the biotite stability field, but above the HPG transition temperature (1 GPa confining pressure, temperature between 700°C and 900°C). Deformation took place at a strain rate of 10-5 s-1,to a finite shear strain of up to γ ≡ 4. Our first results show the following features: (1) The presence of melt weakens the sample only during the first increments of deformation (below γ ≡ 2) compared to experiments without glass. (2) Small amounts of weak phase (5% HPG or 10% biotite) result in the nucleation of shear bands which localize deformation. (3) Larger amounts of melt (10% HPG) are associated with weakening by a factor of 3 compared to 5% melt samples, accompanied by a more homogeneously distributed deformation and absence of shear bands. The increase from 5 to 10% HPG changes the microstructures from dominant crystal plasticity of quartz to microstructures indicating grain boundary sliding. (4) Surprisingly, the weakening caused by 5% HPG in a pure quartz sample at 800°C (i.e. 5% melt) is very similar to the weakening effect of 10% biotite (after γ ≡ 2) without melt

  19. Modification of an oceanic plateau, Aruba, Dutch Caribbean: Implications for the generation of continental crust

    NASA Astrophysics Data System (ADS)

    White, R. V.; Tarney, J.; Kerr, A. C.; Saunders, A. D.; Kempton, P. D.; Pringle, M. S.; Klaver, G. T.

    1999-01-01

    The generation of the continental crust may be connected to mantle plume activity. However, the nature of this link, and the processes involved, are not well constrained. An obstacle to understanding relationships between plume-related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships, particularly in ancient greenstone belts. Studies of younger analogous regions may help to clarify these relationships. On the island of Aruba in the southern Caribbean, a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed. The mafic lavas show geochemical and isotopic affinities with other basaltic, picritic and komatiitic rocks that crop out elsewhere in the Caribbean—these are well documented as belonging to an 88-91 Ma plume-related oceanic plateau, which is allochthonous with respect to the Americas, and is thought to have been formed in the Pacific region. The ˜85 to ˜82 Ma tonalitic rocks share some geochemical characteristics (high Sr and Ba, low Nb and Y) with Archaean tonalite-trondhjemite-granodiorite (TTG) suites. Field relationships suggest that deformation of the plateau sequence, possibly related to collision with a subduction zone, was synchronous with intrusion of the Aruba batholith. New incremental heating 40Ar/ 39Ar dates, combined with existing palaeontological evidence, show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence. Sr-Nd isotopic data for both rock suites are uniform ( 87Sr/ 86Sr i≈0.7035 , ɛNd i≈+7), whereas Pb isotopes are more variable (Plateau sequence: 206Pb/ 204Pb =18.6-19.1 , 207Pb/ 204Pb =15.54-15.60 , 208Pb/ 204Pb =38.3-38.75 ; Aruba batholith: 206Pb/ 204Pb =18.4-18.9 , 207Pb/ 204Pb =15.51-15.56 , 208Pb/ 204Pb =38.0-38.5 ). This suggests that there has been a minor sedimentary input into the source region of the batholith. However, the limited time interval

  20. The Origin of Voluminous Dacite (vs. Andesite) at Mature, Thick Continental Arcs: A Reflection of Processes in the Deep Crust

    NASA Astrophysics Data System (ADS)

    Lange, R. A.

    2013-12-01

    An outstanding question is why some continental arc segments are characterized by voluminous eruptions of dacite (65-70 wt% SiO2), whereas others erupt more andesite (58-64 wt% SiO2) than any other magma type. An example of the former is the Altiplano-Puna region of the central Andean arc, which has erupted a predominance of dacite over all magma types 10-1 Ma (de Silva, 1989). In contrast, a 200-km arc segment of the Mexican volcanic arc (Michoacán-Guanajuato arc segment) has erupted ~75% andesite, ~26% basaltic andesite and < 10% basalt, with no dacite, over the last 1-2 Myrs (e.g., Ownby et al., 2011). Possible plutonic equivalents of these two contrasting arc segments are: (1) the eastern Peninsular/Sierra Nevada batholiths, where plutons of dacitic (65-70 wt% SiO2) composition predominate, and (2) the western Peninsular/Sierra Nevada batholiths, where plutons with 58-65 wt% SiO2 are more abundant than those with 65-70 wt% SiO2 (e.g., Gromet and Silver, 1987; Oliver et al., 1993). A consistent feature is the predominance of dacitic magma where subduction occurs along mature, thick (50-75 km) continental crust, whereas andesite is more abundant where crustal thickness is average (~35-45 km). For the Michoacán-Guanajuato arc segment, on the basis of phenocryst modes, major- and trace-element data, as well as phase-equilibrium experiments from the literature, it is proposed that the andesites were derived by partial melting (>20%) of hornblende-rich (~40%) gabbronorite in the deep crust, driven by mantle-derived basalt intrusions at depths of 30-40 km. The absence of any dacite or rhyolite along this arc segment indicates that interstitial liquid from crystal-rich andesites never segregated to form eruptible magma. Thus, little upper-crust differentiation occurred along this arc segment. On the basis of phase-equilibrium experiments in the literature (e.g., Sisson et al., 2005), it is proposed that rhyolite and dacite did form during partial melting of the lower

  1. Copper systematics during mantle melting and crustal differentiation in arcs: implications for S and Pb budgets of the continental crust

    NASA Astrophysics Data System (ADS)

    Lee, C.; Chin, E. J.; Dasgupta, R.; Luffi, P. I.; Le Roux, V.

    2010-12-01

    During mid-ocean ridge melting, Cu behaves like Sc and is therefore moderately incompatible, as evidenced by the twofold increase in Cu content in MORBs compared to the mantle. However, Cu content in the continental crust is comparable to that of the mantle, implying that during continental crust formation, Cu becomes effectively compatible. Cu is one of the only elements that exhibits Jekyll and Hyde behavior. This switch to being compatible is consistent with the observation that for the majority of arc magmas (as well as MORBs), Cu decreases with increasing SiO2 and decreasing MgO. Using natural samples, we infer new partition coefficients that indicate Cu is incompatible in olivine, pyroxenes, amphiboles, and biotite. The only mineral to exert significant control on Cu partitioning is sulfide. Cu behaves incompatibly during mantle melting because the modal abundance of sulfides relative to silicate minerals is extremely low. The monotonic decrease in Cu in most differentiating arc magmas requires sulfide saturation. In addition, the similar abundances of Cu in many primitive arc magmas compared to MORBs suggests that mantle melting in both environments occurs not only at sulfide saturation but without the need for excess Cu (or S). In a few cases, however, primitive arc magmas begin with high Cu or show increases in Cu with differentiation, which most likely requires unusually high oxygen fugacities in the source or magmatic evolution towards high oxygen fugacity. Such cases may be important for the origin of Cu porphyry deposits, but are generally rare. Because of the close link between Cu and sulfide during magma differentiation, Cu can be used as a proxy for the pre-degassed S content of arc magmas. The S content of continental crust, like many volatile elements, is basically unconstrained, but it can be inferred from Cu, which is much better constrained. Finally, the fact that the continental crust is highly depleted in Cu implies that there is a missing

  2. Inherent gravitational instability of thickened continental crust with regionally developed low- to medium-pressure granulite facies metamorphism

    NASA Astrophysics Data System (ADS)

    Gerya, Taras V.; Maresch, Walter V.; Willner, Arne P.; Van Reenen, Dirk D.; Smit, C. Andre

    2001-08-01

    Petrological arguments show that regionally developed low- to medium-pressure, high-temperature granulite facies metamorphism may critically enhance the lowering of crustal density with depth. This leads to gravitational instability of homogeneously thickened continental crust, mainly due to changes in mineral assemblages and the thermal expansion of minerals in conjunction with the exponential lowering of the effective viscosity of rocks with increasing temperature. It is argued that crustal processes of gravitational redistribution (crustal diapirism) contributing to the exhumation of granulite facies rocks may be activated in this way.

  3. Growth of early archaean crust in the ancient Gneiss complex of Swaziland and adjacent Barberton Greenstone Belt, Southern Africa

    NASA Technical Reports Server (NTRS)

    Kroener, A.; Compston, W.; Tegtmeyer, A.; Milisenda, C.; Liew, T. C.

    1988-01-01

    The relationship between early Archean greenstones and high grade gneisses in the Ancient Gneiss Complex of Swaziland and the neighboring Barberton greenstone belt in Southern Africa is discussed. New high precision zircon analyses reveal a complex history in individual zircons from tonalitic orthogneisses, with ages as old as 3644 + 4 Ma. This suggests the presence of continental crust prior to the formation of the supracrustal rocks of the Barberton greenstone belt, which have been previously considered the earliest rocks in the area. The author suggested that these data are incompatible with the intraoceanic settings that have been widely accepted for this terrane, and favors either a marginal basin or rift environment. By using the detailed age information obtained from zircons in combination with Ar-40 and Ar-39 and paleomagnetic measurements, the author estimated that plate velocities for this part of Africa craton were about 10 to 70 mm/yr, during the period 3.4 to 2.5 Ga. This is not incompatible with the idea that Archean plate velocities may have been similiar to those of today.

  4. Origin of granulite terranes and the formation of the lowermost continental crust.

    PubMed

    Bohlen, S R; Mezger, K

    1989-04-21

    Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystallization of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.

  5. Origin of granulite terranes and the formation of the lowermost continental crust

    USGS Publications Warehouse

    Bohlen, S.R.; Mezger, K.

    1989-01-01

    Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystalliztion of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.

  6. The Rock Record Has It About Right—No Significant Continental Crust Formation Prior to 3.8 Ga

    NASA Astrophysics Data System (ADS)

    Vervoort, J. D.; Kemp, A. I.; Fisher, C.; Bauer, A.; Bowring, S. A.

    2015-12-01

    Although limited in its exposed extent and by its quality of preservation, the geologic record through the first two billion years of Earth's history provides surprisingly tight constraints on the growth and evolution of continental crust. The magmatic zircon record for this period is dominated by Neoarchean U-Pb ages, with greatly diminishing abundance of older rocks and no known zircon bearing rocks much older than 4.0 Ga. A similar pattern exists for the ages of detrital zircons but with important addition of zircons as old as 4.4 Ga, mostly from the Jack Hills metaconglomerate. It has been suggested that this represents an artefact of preservation rather than the actual production rate of older crust, with the implication that large volumes of older crust have been destroyed by various recycling processes. This undoubtedly has happened to some extent, but there is considerable uncertainty as to how much has been destroyed and the nature of the early-formed crust. Here is where the long-lived radiogenic isotopic record, particularly Lu-Hf, can provide important information on the sources of the zircons by integrating age and tracer isotopic information in not only the same sample or zircon but even in the same domain of zircon. Using the most robust data from zircon and whole rock samples, excluding those with unconstrained ages and mixed-domain analyses, the most radiogenic Hf isotope compositions are characterized by ~ chondritic Hf isotopic compositions from 4.4 to ~ 3.8 Ga and a nearly linear evolution trend from epsilon Hf of 0 at 3.8 Ga to ~ epsilon Hf of +16 at present. There remains no evidence from the Hf isotope record for widespread mantle depletion prior to 3.8 Ga. Excluding the Jack Hills zircons, there is a conspicuous lack of pre 3.9 Ga zircons in even the oldest sediments1. This indicates that crust prior to 3.8 Ga was likely small in volume and/or effectively recycled back into the mantle on short time scales and did not result in significant

  7. Anatexis of accretionary wedge, Pacific-type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Jiang, Y. D.; Schulmann, K.; Sun, M.; Å típská, P.; Guy, A.; Janoušek, V.; Lexa, O.; Yuan, C.

    2016-12-01

    Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide.

  8. Growth of continental crust: Clues from Nd isotopes and Nb-Th relationships in mantle-derived magmas

    NASA Technical Reports Server (NTRS)

    Arndt, N. T.; Chauvel, C.; Jochum, K.-P.; Gruau, G.; Hofmann, A. W.

    1988-01-01

    Isotope and trace element geochemistry of Precambrian mantle derived rocks and implications for the formation of the continental crust is discussed. Epsilon Nd values of Archean komatiites are variable, but range up to at least +5, suggesting that the Archean mantle was heterogeneous and, in part, very depleted as far back as 3.4 to 3.5 Ga. This may be taken as evidence for separation of continental crust very early in Earth history. If these komatiite sources were allowed to evolve in a closed system, they would produce modern day reservoirs with much higher epsilon Nd values than is observed. This implies recycling of some sort of enriched material, perhaps subducted sediments, although other possibilities exist. Archean volcanics show lower Nb/Th than modern volcanics, suggesting a more primitive mantle source than that observed nowadays. However, Cretaceous komatiites from Gorgona island have similar Nb/Th to Archean volcanics, indicating either the Archean mantle source was indeed more primitive, or Archean magmas were derived from a deep ocean island source like that proposed for Gorgona.

  9. Temporal Evolution of the Upper Continental Crust: Implications for the Mode of Crustal Growth and the Evolution of the Hydrosphere

    NASA Astrophysics Data System (ADS)

    Rudnick, R. L.; Gaschnig, R. M.; Li, S.; Tang, M.; Qiu, L.; Valley, J. W.; Zurkowski, C.; McDonough, W. F.

    2014-12-01

    The upper continental crust (UCC), the interface between the atmosphere and solid Earth, is the site of weathering that produces sedimentary rocks, influences ocean chemistry through runoff of soluble elements, and affects climate through CO2 draw-down. The UCC also contains more than 50% of the crust's highly incompatible element budget (including K, Th, and U). Therefore, understanding its composition and evolution provides insight into how continents have formed, evolved, and interacted with the hydrosphere. New major and trace element compositions of >100 glacial diamictites and >100 Archean shales, plus δ7Li and δ18O for a subset of these samples, combined with data from the literature, show that the average composition of the UCC has changed through time, reflecting both the rise of atmospheric oxygen and its attendant effects on weathering, as well as the mode of crust formation and differentiation. Some changes that occur as a step function near the Archean/Proterozoic boundary (increased Th/U, decreased Mo/Pr, V/Lu) reflect the rise of oxygen at the great oxidation event (GOE) and its influence on chemical weathering signatures in the UCC. Other changes are more gradual with time (e.g., higher Th/Sc and δ18O, lower Ni/Co, La/Nb, Eu/Eu* and transition metal abundances) and reflect an UCC that has transitioned from a more mafic to a more felsic bulk composition, and which experienced increased interaction with the hydrosphere with time. The gradual nature of these compositional changes likely reflects the waning heat production of the Earth, rather than an abrupt change in tectonics or style of crust formation. These more gradual changes in crust composition, which contrast with the abrupt changes associated with the GOE, suggest that a fundamental change in the nature of crust differentiation is unlikely to be responsible for the rise of atmospheric oxygen (cf. Keller and Schoene, 2012). Indeed, it appears that the opposite may be true: that the rise of

  10. Formation of metamorphic core complexes in non-over-thickened continental crust: A case study of Liaodong Peninsula (East Asia)

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Burov, Evgueni; Gumiaux, Charles; Chen, Yan; Lu, Gang; Mezri, Leila; Zhao, Liang

    2015-12-01

    Pre-thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes (MCCs). However, the discovery of MCCs in the Liaodong Peninsula, where the crust has a normal thickness (~ 35 km), challenges the universality of this scenario. Therefore, we implement a series of 2-D numerical thermo-mechanical modeling experiments in which we investigate the conditions of MCC formation in normal crusts, as well as the relationships between the underlying mechanisms and the syn-rift basin evolution. In these experiments, we explore the impact of the lithostratigraphic and thermo-rheological structure of the crust. We also examine the lithosphere thickness, strain softening, extension rate, and surface erosion/ sedimentation processes. The experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome, which is geometrically incompatible with the observations of the MCCs in the Liaodong Peninsula. According to our further findings, the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift, while synchronous surface erosion controls the polarity of the syn-rift basin. The synthetic model data are compatible with the geological observations and cooling history based on the thermo-chronology for the eastern part of the East Asia during the late Mesozoic to the early Cenozoic. The model-predicted P-T-t paths are essentially different from those inferred for the other known MCCs, confirming the exceptional character of the MCC formation in the wide rift system of the East Asia.

  11. Mineralogical Evidence for the Bulk Transformation of Continental Crust to Ultrahigh-Pressure Conditions in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Peterman, E. M.; Hacker, B. R.; Kylander-Clark, A. R.

    2005-12-01

    Evidence for (ultra)high-pressure --(U)HP-- metamorphism in modern orogenic belts and the preservation of exhumed (U)HP terranes around the world suggest that subduction and exhumation of continental crust plays an important role in Phanerozoic plate tectonics. The Western Gneiss region (WGR) of Norway, a major (U)HP province extending over 60,000 km2, provides an excellent opportunity to study how subduction to depths >100 km affects continental crust. By studying a ~60 km wide transect bounded to the north by Vartdalsfjorden and Rovdefjorden and the south by the Möre og Romsdal county boundary, we are able to examine mineralogical changes that occurred during subduction and exhumation within a rock composed predominantly of orthogneiss and variably transformed mafic bodies, which indicate the depths to which these rocks were subducted. Previous studies (e.g. Hacker et al., 2005) have suggested that Caledonian deformation in WGR host gneisses is primarily limited to brittle-ductile fabrics characterized by greenschist to lower-amphibolite facies metamorphism; the majority of the deformation in the rocks, including the pervasive foliation and foliation-parallel isoclinal folds, occurred between 1200 and 900 Ma. On the northern half of our study area, however, locally occurring neoblastic garnet crosscuts the foliation in the gneiss. The boundary of this garnet zone coincides with the local HP-UHP boundary, as determined by the presence of coesite in eclogite. Because garnet can retain information about changes in pressure and temperature, as well as the availability of water within the crust to catalyze chemical reactions, our findings suggest that 1) portions of the orthogneiss did transform at high pressures, 2) the presence of garnet within the orthogneiss may indicate conditions that approximate UHP and can therefore be useful in defining the boundaries between UHP and HP conditions, and 3) the growth of garnet during (U)HP metamorphism may be controlled by

  12. Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America

    NASA Technical Reports Server (NTRS)

    Patino Douce, Alberto E.; Humphreys, Eugene D.; Johnston, A. Dana

    1990-01-01

    This paper presents a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening exemplified by the Sevier hinterland in western North America, and uses the model to examine the geological and physical processes leading to crustally derived magmatism. The results of numerical experiments show that anatexis was an inevitable end-product of Barrovian metamorphism in the thickened crust of the late Mesozoic Sevier orogenic belt and that the advection of heat across the lithosphere, in the form of mantle-derived mafic magmas, was not required for melting of metasedimentary rocks. It is suggested that, in the Sevier belt, as in other intracontinental orogenic belts, anatexis occurred in the midcrust and not at the base of the crust.

  13. Application program of CRUST-1 10km continental scientific drilling rig in SK-2 scientific drilling well

    NASA Astrophysics Data System (ADS)

    Sun, Youhong; Gao, Ke; Yu, Ping; Liu, Baochang; Guo, Wei; Ma, Yinlong; Yang, Yang

    2014-05-01

    SK-2 Well is located in DaQing city,where is site of the largest oil field in China,Heilongjiang province, north-east of China.The objective of SK-2 well is to obtain full cores of cretaceous formation in Song Liao basin,and to build the time tunnel of Cretaceous greenhouse climate change,and to clarify the causes,processes and results of the formations of DaQing oil field. This will ensure to achieve our ultimate goals,to test the CRUST-1 drilling rig and improve China's deep scientific drilling technology,to form the scientific drilling technology,method and system with independent intellectual property rights,and to provide technical knowledge and information for China's ten kilometers super-deep scientific drilling technical resources.SK-2 Well is at 6400 meter depth, where the drilling inclination is 90 degree and the continuous coring length is 3535 meter that from 2865 to 6400 meter,the recovery rate of the core is greater or equal to 95 percent with 100 millimeters core diameter and 3.9 degree per 100 meter geothermal gradient.The CRUST-1 rig is designated with special drilling equipment for continental scientific drilling combined to the oil drilling equipment ability with advanced geological drilling technology which is highly automatic and intelligent. CRUST-1 drilling ability is 10000 meter with the maximum hook load 700 tons, the total power is 4610 Kilowatt.CRUST-1 will be integrated with a complete set of automation equipment,including big torque hydraulic top drive,high accuracy automatic drilling rod feeding system, suspended automatic drill string discharge device,hydraulic intelligent iron roughneck,and hydraulic automatic catwalk to fully meet the drilling process requirements of SK-2.Designed with advanced drilling technique for 260 degree in the bottom of SK-2 well and hard rock,including the drilling tools of high temperature hydraulic hammer,high temperature resistance and high strength aluminum drill pipe,high temperature preparation of mud

  14. Gas-particle partitioning of polycyclic aromatic hydrocarbons in urban, adjacent coastal, and continental background sites of western Greece.

    PubMed

    Terzi, Eleni; Samara, Constantini

    2004-10-01

    Particle- and gas-phase polycyclic aromatic hydrocarbons (PAHs) were collected from an urban, an adjacent coastal, and a continental background site located in Eordea basin, western Greece, to investigate their gas/ particle distributions. Thirteen two- to six-ring PAHs, included in the U.S. EPA priority pollutant list, were determined in 24-h integrated glass fiber filters and polyurethane foam samples. At the prevailing ambient temperature levels, the three-ringed species (phenanthrene, anthracene) and the four-ringed fluoranthene and pyrene were primarily found in the gas phase. Conversely, the five- and six-ring PAHs were mainly associated with the particle phase. Gas/particle partitioning coefficients, Kp, were calculated, and their relationship with the subcooled liquid vapor pressure p degrees L of individual PAHs was investigated. Despite the large variability among samples, a good linear relationship between log Kp and log p degrees L was obtained for all sampling sites following the equation log Kp = m(r) log p degrees L + b(r). In the majority of sampling events, particularly in the adjacent coastal and the continental background sites, slopes (m(r)) were found to be shallower than the value of -1, which has been suggested as reflecting equilibrium partitioning. The deviations from predicted aerosol behavior observed in the present study may be attributed to several reasons, such as the presence of nonexchangeable PAH fraction, nonequilibrium as well as different particle characteristics.

  15. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.

    PubMed

    Wilde, S A; Valley, J W; Peck, W H; Graham, C M

    2001-01-11

    No crustal rocks are known to have survived since the time of the intense meteor bombardment that affected Earth between its formation about 4,550 Myr ago and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss of northwestern Canada. But evidence of an even older crust is provided by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where grains as old as approximately 4,276 Myr have been found. Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404+/-8 Myr--about 130 million years older than any previously identified on Earth. We found that the zircon is zoned with respect to rare earth elements and oxygen isotope ratios (delta18O values from 7.4 to 5.0%), indicating that it formed from an evolving magmatic source. The evolved chemistry, high delta18O value and micro-inclusions of SiO2 are consistent with growth from a granitic melt with a delta18O value from 8.5 to 9.5%. Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone low-temperature interaction with a liquid hydrosphere. This zircon thus represents the earliest evidence for continental crust and oceans on the Earth.

  16. Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton: Implications for magnetism in the continental lower crust

    NASA Astrophysics Data System (ADS)

    Li, Zhiyong; Zheng, Jianping; Zeng, Qingli; Liu, Qingsheng; Griffin, W. L.

    2014-02-01

    Studies of the petrology, mineral chemistry, and rock magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility (χ) and saturation isothermal remanent magnetization (Mrs) of 39.59 × 10-8 m3 kg-1 and 12.05 × 10-3 Am2 kg-1, respectively. The magnetic minerals are mainly magnetite, pyrrhotite, and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial microcrystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths' ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has ~ 50% mole fraction of Fe3O4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these pyroxenite xenoliths would be nonmagnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~ 32-42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~ 32 km) in Hannuoba, northern North China Craton.

  17. Granodiorite and alkaline suite at Gale crater: continental crust on early Mars

    NASA Astrophysics Data System (ADS)

    Sautter, V.; Toplis, M. J.; Cousin, A.; Fabre, C.; Wiens, R. C.; Mangold, N.; Forni, O.; Gasnault, O.; Pinet, P.; Rapin, W.; Fisk, M.; Le Deit, L.; Meslin, P.-Y.; Maurice, S.; Lasue, J.; Stolper, E.; Beck, P.; Wray, J.; Bridges, J. C.; Le Mouelic, S.

    2015-10-01

    The Curiosity rover landed at Gale, an early Hesperian age crater formed within Noachian terrains on Mars. The rover encountered a great variety of felsic igneous float rocks ranging from granodiorite to trachy andesite and trachyte during the first part of the traverse up to sol 550. They are the first in-situ evidence of low density early Noachian crust on Mars, sampled by Peace Vallis river cross-cutting the crater wall over a 2-3km thick vertical section, below the basaltic regolite.

  18. Trend and dynamic cause of sediment particle size on the adjacent continental shelf of the Yangtze Estuary

    NASA Astrophysics Data System (ADS)

    Yang, Yun-ping; Zhang, Ming-jin; Li, Yi-tian; Fan, Yong-yang

    2016-12-01

    Based on the measured data in recent 20 years, the variation trends of the median grain size of the surface sediment, the sand-silt boundary and the mud area on the adjacent continental shelf of the Yangtze Estuary were analyzed in depth, and the effects of natural mechanism and human activities were discussed. The results show that: (1) In recent years (2006-2013), the median grain size of sediment and the distribution pattern of grouped sediments in the adjacent continental shelf area to the Yangtze Estuary have presented no obvious change compared with those before 2006; (2) The median diameter of the surface sediment in the continental shelf area displayed a coarsening trend with the decrease of sediment discharge from the basin and the drop in suspended sediment concentration in the shore area; (3) In 2004-2007, the sand-silt boundary in the north part (31°30'N) of the continental shelf area presented no significant changes, while that in the south part (31°30'S) moved inwards; In 2008-2013, both the sand-silt boundaries in the north and south parts of the continental shelf area moved inwards, mainly due to the fact that in the dry season, a relatively enhanced hydrodynamic force of the tides was generated in the Yangtze River, as well as a decreased suspended sediment concentration and a flow along the banks in North Jiangsu; (4) The mud area where the maximum deposition rate is found in the Yangtze Estuary, tends to shrink due to the drop in sediment discharge from the basin, and the decrease in suspended sediment concentration in the shore area and erosion in the delta. Moreover, it tended to shift to the south at the same time because the implement of the training works on the deep-water channel of the North Passage changed the split ratio between the North and South Passages with an increase in the power of the discharged runoff in the South Passage.

  19. Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe, western Alps

    NASA Astrophysics Data System (ADS)

    Lapen, Thomas J.; Johnson, Clark M.; Baumgartner, Lukas P.; Piaz, Giorgio V. Dal; Skora, Susanne; Beard, Brian L.

    2007-02-01

    High precision U Pb geochronology of rutile from quartz carbonate white mica rutile veins that are hosted within eclogite and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz carbonate white mica rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480 570°C, >1.3 1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261 281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore, the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally overlying Zermatt-Saas ophiolite [˜50 40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425 438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57 72]. The nearly identical P T t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role in exhumation and preservation of the

  20. Evidence of low density sub-crustal underplating beneath western continental region of India and adjacent Arabian Sea: Geodynamical considerations

    NASA Astrophysics Data System (ADS)

    Pandey, O. P.; Agrawal, P. K.; Negi, J. G.

    1996-07-01

    The known high mobility of the Indian subcontinent during the period from 80 to 53 Ma has evoked considerable interest in recent times. It appears to have played an important role in shaping the subcontinental structures of western India and the adjoining Arabian Sea. During this period, a major catastrophic event took place in the form of Deccan volcanism, which coincides with the biological mass extinction at the K-T boundary, including the death of dinosaurs. The origin of Deccan volcanism is still being debated. Geophysically, western India and its offshore regions exhibit numerous prominent anomalies which testify to the abnormal nature of the underlying crust-lithosphere. In this work, we develop a two-dimensional structural model of these areas along two long profiles extending from the eastern basin of the Arabian Sea to about 1000 km inland. The model, derived from the available gravity data in the oceanic and continental regions, is constrained by seismic and other relevant information in the area, and suggests, for the first time, the presence of an extensive low-density (2.95-3.05 g/cm 3) sub-crustal underplating. Such a layer is found to occur between depths of 11 and 20 km in the eastern basin of the Arabian Sea, and betweeen 45 and 60 km in the continental region where it is sandwiched in the lower lithosphere. The low density may have been caused as a result of serpentinization or fractionation of magma by a process related in some way to the Deccan volcanic event. Substantial depletion of both oceanic and continental lithosphere is indicated. We hypothesize that the present anatomy of the deformed lithosphere of the region at the K-T boundary is the result of substantial melt generated owing to frictional heat possibly giving rise to a hot cell like condition at the base of the lithosphere, resulting from the rapid movement of the Indian subcontinent between 80 and 53 Ma.

  1. The Ivrea zone as a model for the distribution of magnetization in the continental crust

    NASA Technical Reports Server (NTRS)

    Wasilewski, P.; Fountain, D. M.

    1982-01-01

    Units are identified within the Ivrea zone of northern Italy exhibiting magnetic susceptibilities greater than 0.0005 cgs, saturation magnetization values above 0.009 emu/cu cm, and Curie points as high as 570-580 C. Amphibolites from the granulite-amphibolite facies transition, and the mafic-ultramafic granulite facies lithologies exhibit high values of initial susceptibility and saturation remanence, are laterally continuous, and may be considered as a deep crustal source for long-wavelength anomalies in low-geothermal gradient areas. Evidence is presented which suggests that such mafic-ultramafic bodies as those exposed in the Toce valley were synmetamorphic additions to the base of the crust.

  2. Identification of Geostructures of the Continental Crust Particularly as They Relate to Mineral Resource Evaluation. [Alaska

    NASA Technical Reports Server (NTRS)

    Lathram, E. H. (Principal Investigator)

    1974-01-01

    The author has identified the following significant results. A pattern of very old geostructures was recognized, reflecting structures in the crust. This pattern is not peculiar to Alaska, but can be recognized throughout the northern cordillera. A new metallogenic hypothesis for Alaska was developed, based on the relationship of space image linears to known mineral deposits. Using image linear analysis, regional geologic features were also recognized; these features may be used to guide in the location of undiscovered oil and/or gas accumulations in northern Alaska. The effectiveness of ERTS data in enhancing medium and small scale mapping was demonstrated. ERTS data were also used to recognize and monitor the state of large scale vehicular scars on Arctic tundra.

  3. Permeability of the continental crust: Implications of geothermal data and metamorphic systems

    USGS Publications Warehouse

    Manning, C.E.; Ingebritsen, S.E.

    1999-01-01

    In the upper crust, where hydraulic gradients are typically 10 MPa km-1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ~10-16 m2 to ~10-18 m2 between 5- and 12-km depth. Below ~12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ~10(-18.5??1) m2. Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k ~ -3.2 log z - 14, where k is in meters squared and z is in kilometers. At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat. Such conditions may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling.

  4. Synchronous oceanic spreading and continental rifting in West Antarctica

    NASA Astrophysics Data System (ADS)

    Davey, F. J.; Granot, R.; Cande, S. C.; Stock, J. M.; Selvans, M.; Ferraccioli, F.

    2016-06-01

    Magnetic anomalies associated with new ocean crust formation in the Adare Basin off north-western Ross Sea (43-26 Ma) can be traced directly into the Northern Basin that underlies the adjacent morphological continental shelf, implying a continuity in the emplacement of oceanic crust. Steep gravity gradients along the margins of the Northern Basin, particularly in the east, suggest that little extension and thinning of continental crust occurred before it ruptured and the new oceanic crust formed, unlike most other continental rifts and the Victoria Land Basin further south. A preexisting weak crust and localization of strain by strike-slip faulting are proposed as the factors allowing the rapid rupture of continental crust.

  5. Late Quaternary Paleoenvironmental History of the Peru-Chile Current System and Adjacent Continental Chile

    NASA Astrophysics Data System (ADS)

    Lamy, F.; Hebbeln, D.; Kaiser, J.; Mohtadi, M.; Ninnemann, U.

    2004-12-01

    A combined analysis of terrigenous and biogenic compounds in marine sediments from the Chilean continental margin allows detailed reconstructions of the paleoclimatic and paleoceanographic history of this region during the last ca. 120,000 years. Based on several sediment cores recovered during two German cruises and ODP Leg 202 (Site 1233), we found evidence for changes both in continental rainfall, most likely induced by latitudinal shifts of the Southern Westerlies, and marine productivity as well as sea surface temperature and salinity changes within the Peru-Chile Current system on time scales ranging from Milankovitch to centennial-scale. On Milankovitch time-scales, we found strong evidence for precession-controlled shifts of the Southern Westerlies implying for example generally more humid conditions during the LGM and a trend towards more arid climates during the deglaciation culminating in the early Holocene. These shifts are paralleled by paleoceanographic changes indicating generally higher productivity during the LGM mainly caused by increased advection of nutrients from the south through an enhanced Peru-Chile current. North of 33°S, these general productivity patterns are complicated by additional impacts from the tropics resulting in maximum paleoproductivity during the deglaciation and prior to the LGM. On shorter time-scales, extremely high resolution sediment cores from the southern Chilean margin provide evidence of significant short-term Holocene climate variability with bands of variability centred at ca. 900 and 1500 years, periodicities also well known from Northern Hemisphere records. Recently drilled ODP Site 1233 allowed to prolong these records into the last glacial. The available data show millennial-scale SST changes that closely follow the temperature pattern known from Antarctic ice-cores. Including other records from the Southern Hemisphere mid-latitudes, our data suggest a quasi-hemisphere-wide response that is consistent with the

  6. Subsurface geology of upper Tertiary and Quaternary deposits, coastal Louisiana and adjacent Continental Shelf

    SciTech Connect

    McFarlan, E. Jr.; Leroy, D.O.

    1988-09-01

    Upper Tertiary and Quaternary deposits thicken seaward from a feather edge on the outcrop in the uplands of southern Louisiana to more than 7000 ft (2134 m) beneath the middle continental shelf. Through a study of cores and cuttings from 100 control wells and electric-log pattern correlations from 350 water and petroleum industry wells with seismic corroboration in the offshore area, these deposits have been divided into six major time-stratigraphic units, four of which correlate to outcropping terraces. This investigation presents a regional stratigraphic framework of the major upper Tertiary and Quaternary units from their updip pinch-outs in and beneath the terraced uplands, into the subsurface, across the coastal plain to the Louisiana offshore area.

  7. Heavy metal distribution in surface sediments from the continental shelf adjacent to Nazaré canyon

    NASA Astrophysics Data System (ADS)

    Oliveira, A.; Palma, C.; Valença, M.

    2011-12-01

    The continental shelf surface sediments around the Nazaré canyon have been analyzed for heavy metal contamination. Organic carbon content and grain size were also determined. The shelf area around the Nazaré canyon shows mean concentrations of Cr, Cu, Hg, Ni, Pb and Zn within the limits recommended by OSPAR (2008). Some enrichment and human impact was found for Cr and especially Pb. Local rivers, mainly the Mondego and southern creeks seem to be the major sources of heavy metals for the Nazaré area, showing similar contamination levels. A set of reference values for non-contaminated fine surface sediments (<63 μm) in the North-Eastern Atlantic are proposed for As, Al, Cd, Cr, Cu, Fe, Hg, Li, Mn, Ni, Pb and Zn.

  8. The crust and upper mantle of central East Greenland - implications for continental accretion and rift evolution

    NASA Astrophysics Data System (ADS)

    Schiffer, Christian; Balling, Niels; Ebbing, Jörg; Holm Jacobsen, Bo; Bom Nielsen, Søren

    2016-04-01

    The geological evolution of the North Atlantic Realm during the past 450 Myr, which has shaped the present-day topographic, crustal and upper mantle features, was dominated by the Caledonian orogeny and the formation of the North Atlantic and associated igneous activity. The distinct high altitude-low relief landscapes that accompany the North Atlantic rifted passive margins are the focus of a discussion of whether they are remnant and modified Caledonian features or, alternatively, recently uplifted peneplains. Teleseismic receiver function analysis of 11 broadband seismometers in the Central Fjord Region in East Greenland indicates the presence of a fossil subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of hydrated mantle peridotite. This model is generally consistent with gravity and topography. It is shown that the entire structure including crustal thickness variations and sub-Moho heterogeneity gives a superior gravity and isostatic topographic fit compared to a model with a homogeneous lithospheric layer (1). The high topography of >1000 m in the western part of the area is supported by the c. 40 km thick crust. The eastern part requires buoyancy from the low velocity/low density mantle wedge. The geometry, velocities and densities are consistent with structures associated with a fossil subduction zone. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere and that significant present-day dynamic topography seems not to be required. Further, this structure is suggested to be geophysically very similar to the Flannan reflector imaged north of Scotland, and that these are the remnants of the same fossil subduction zone, broken apart and separated during the formation of the North Atlantic in the early Cenozoic (2). 1) Schiffer, C., Jacobsen, B.H., Balling, N., Ebbing, J. and Nielsen, S

  9. Metal elements in the bottom sediments of the Changjiang Estuary and its adjacent continental shelf of the East China Sea.

    PubMed

    Cao, Lu; Hong, Gi Hoon; Liu, Sumei

    2015-06-15

    The metal elements (Al, Fe, Mn, Cr, Co, Ni, Cu, Zn, As, Pb and Ca) in the bottom sediment of the Changjiang Estuary and its adjacent continental shelf of the East China Sea were studied to map their spatial distribution and to assess their potential risk to the marine biota. These metal concentrations except Ca were generally higher in the inner shelf and northeastern part, and were found to decrease from the coast to the offshore of the Changjiang Estuary. Sedimentary Ca was most abundant in the outer shelf sediments and decreased in inner shelf. Arsenic (As) appeared to be contaminated due to economic development from 1980s in the inner shelf overall, but the potential ecological risk from the selected metals was low in the coastal sea off the Changjiang.

  10. Tectonic evolution of Ovda Regio: An example of highly deformed continental crust on Venus?

    NASA Astrophysics Data System (ADS)

    Romeo, I.; Capote, R.

    2011-10-01

    A detailed structural analysis of several selected areas of Ovda Regio provides evidence of a complex tectonic evolution. We have reported thrusting in the marginal fold belts indicating together with the presence of short-wavelength folds a significant amount of shortening. Extensional tectonics postdate at least in some locations contraction, while the contrary was not observed. Both contraction and extension occur on a complex layered crust yielding contemporary structures of different wavelengths. The thrust and fold belts of the plateau margins are characterized by concentric contraction followed by concentric contraction with perpendicular extension and finally radial extension. Deformation in the thrust and fold belts of Ovda margins is gradually transmitted to the external plains. A complex tectonic history has been revealed in the internal area of Ovda, basically characterized by contraction in different directions generating basin and dome interference at different wavelengths. Small amounts of a non-coaxial component of deformation have been observed both in the margins and in the central area of the plateau. All the reported observations can be explained if Ovda Regio is a continent that survived a global subduction event.

  11. Bayesian Monte Carlo inversion of multiple data sets to image the crust and uppermost mantle beneath the continental United States

    NASA Astrophysics Data System (ADS)

    Shen, Weisen

    The recent deployment of the USArray/Transportable Array (USArray/TA) has stimulated new methods (e.g., ambient noise eikonal tomography; teleseismic Helmholtz tomography) to produce high resolution surface wave dispersion maps. These dispersion maps, combined with other geophysical data sets derived from the array such as receiver functions, present the opportunity to image the crust and uppermost mantle for the continental US at unprecedented resolution. However, new methods are needed to overcome the limitations of traditional methods that may generate unstable models and do not estimate model uncertainties. In this thesis, I present a new approach that jointly interprets new surface wave observations with other geophysical observables using a Bayesian Monte Carlo framework. In this approach, prior constraints and assumptions are explicitly expressed as prior distributions, and data uncertainties are rigorously interpreted in the resulting models by the Monte Carlo sampling of the posterior distributions. Thus, a 3D model with attendant uncertainties at all depths and for all discontinuities is estimated. I show that with this approach it is feasible to interpret both surface wave data and other geophysical data observed at most stations from the USArray/TA with simple models. In addition, the vertical resolution of the model is enhanced by improvements to estimates of Moho depth and upper crustal structures using both receiver functions and Rayleigh wave H/V ratio. By applying the new method to multiple data sets, a set of 3-D models is constructed for the crust and uppermost mantle beneath the contiguous US. These models reveal many geological features.

  12. Depth- and Pressure dependent Permeability in the Upper Continental Crust - data from the Urach 3 geothermal well -

    NASA Astrophysics Data System (ADS)

    Stober, I.

    2010-12-01

    Since the late seventies the 4500 m deep research borehole at Urach (South Germany) has been extensively used for hydraulic testing of the crystalline basement. The data permit a general interpretation of the hydraulic properties of the crystalline continental upper crust at different depth intervals. The typical gneissic basement contains an interconnected fluid-filled fracture system. Low-pressure hydraulic tests show that the basement on a larger scale can be described as a homogeneous, isotropic aquifer and this characteristic hydraulic behavior persists at least several hundred meters away from the borehole. This demonstrated homogeneity of the aquifer, together with the highly saline water in an interconnected system of copious fractures is characteristic of the continental upper crust in general. On a smaller scale however the fractures, crossing the uncased sections in the borehole, will define the flow-behavior in detail. So, at the beginning of a hydraulic test the pressure-data show the influence of wellbore-storage and skin, followed by a linear and a bilinear flow-period, and later on by a pseudo-radial flow-period. The transmissivity of the bulk rock will be given by evaluating the data of the pseudo-radial flow-period. Within this presentation on the one hand a compilation of existing, a description and interpretation of new hydraulic test data in the Urach 3 well are given together with a global interpretation of all test campaigns at Urach 3. Within the crystalline basement of the Urach 3 well permeability is decreasing with depth. On the other hand the pressure-dependant hydraulic phenomena is studied and an interpretation of this particular data in terms of pressure-dependant-permeability is given. In the Urach borehole a lot of high-pressure tests with well-head pressures of more than 600 bar, corresponding to an overpressure of more than 700 bar, were carried out as well. The results of these tests will be presented. The tests clearly show a

  13. Roles of magmatic oxygen fugacity and water content in generating signatures of continental crust in the Alaska-Aleutian arc

    NASA Astrophysics Data System (ADS)

    Kelley, K. A.; Cottrell, E.; Brounce, M. N.; Gentes, Z.

    2014-12-01

    Early depletion of Fe during magmatic differentiation is a characteristic of many arc magmas, and this may drive them towards the bulk composition of continental crust. In the Alaska-Aleutian arc, magmas are strongly Fe-depleted both in the east, where the arc sits atop pre-existing continental crust, and in the west, where the system is oceanic but convergence is highly oblique. Primary basaltic arc magmas may achieve early Fe depletion through a combination of high magmatic H2O, which delays silicate saturation, and high oxygen fugacity (fO2), which promotes early onset of Fe-oxide crystallization. Alternatively, low-Fe, high Mg# magmas may emerge directly from the arc mantle, possibly due to slab melting, driving mixing with Fe-rich basaltic magmas. Yet, the relative importance of H2O, fO2, and magmatic bulk composition in generating Fe-depletion is not clearly resolved. Here, we present new measurements of the oxidation state of Fe (Fe3+/∑Fe ratio; a proxy for magmatic fO2), in combination with major element and volatile data, of olivine-hosted melt inclusions from four Alaska-Aleutian arc volcanoes (Okmok, Seguam, Korovin, Augustine), acquired using XANES spectroscopy. We use the Tholeiitic Index (THI) of Zimmer et al., 2010 to quantify the behavior of Fe in each volcano magma series (<1 is Fe-depleted, >1 is Fe-enriched). These volcanoes span a range of THI, from 0.9-0.65. The Fe3+/∑Fe ratios of Aleutian basalts, corrected for fractional crystallization to 6 wt.% MgO (i.e., Fe3+/∑Fe6.0) range from 0.22-0.31 and correlate strongly with THI (r2>0.99), such that more Fe-depleted magmas contain a greater proportion of oxidized Fe. The maximum dissolved H2O contents of basaltic melt inclusions from these volcanoes also strongly correlate with THI (r2>0.96), and with measured Fe3+/∑Fe ratios (although H2O is not the direct cause of oxidation). These links point to a slab-derived origin of both H2O and oxidation and thus relate slab fluxes to the Fe

  14. Distribution of Pasiphaea japonica larvae in submarine canyons and adjacent continental slope areas in Toyama Bay, Sea of Japan

    NASA Astrophysics Data System (ADS)

    Nanjo, Nobuaki; Katayama, Satoshi

    2014-09-01

    The horizontal and vertical distribution of Pasiphaea japonica larvae, which included larval stages and postlarval or later stages, were investigated in Toyama Bay located in central Japan. The horizontal distributions in the inner part of the bay were investigated by oblique hauls from 10 m above the sea-bottom to the surface using a Remodeled NORPAC net (LNP net) in May, August, November 2005, January, March, April, July, September, December 2006, March-September, November-December 2007, and January-March 2008. The vertical distributions were investigated by concurrent horizontal hauls at the depths of 0, 50, 100, 150, 200, and 250 m using a Motoda net (MTD net) in January, March, April, July, September, and December 2006. Mean density of larvae was higher in submarine canyons which dissect the continental shelf and run to the mouth of river, than adjacent continental slope areas. Larvae densely aggregated in the canyon head. Vertical distribution of the larval stages concentrated in the depth range of 100-150 m in both daytime and nighttime, and larvae in the postlarval or later stages showed diel vertical distribution over a wider depth range than larval stages. Our results indicate the possibility of a larval aggregation in energy-rich habitats, and indicated two important roles of submarine canyons, which were larval retention and high food supply.

  15. High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomargin

    NASA Astrophysics Data System (ADS)

    Clerc, C.; Lahfid, A.; Monié, P.; Lagabrielle, Y.; Chopin, C.; Poujol, M.; Boulvais, P.; Ringenbach, J.-C.; Masini, E.; de St Blanquat, M.

    2015-06-01

    An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40Ar-39Ar (amphibole, micas) and U-Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have

  16. The Alegre Lineament and its role over the tectonic evolution of the Campos Basin and adjacent continental margin, Southeastern Brazil

    NASA Astrophysics Data System (ADS)

    Calegari, Salomão Silva; Neves, Mirna Aparecida; Guadagnin, Felipe; França, George Sand; Vincentelli, Maria Gabriela Castillo

    2016-08-01

    The structural framework and tectonic evolution of the sedimentary basins along the eastern margin of the South American continent are closely associated with the tectonic framework and crustal heterogeneities inherited from the Precambrian basement. However, the role of NW-SE and NNW-SSE structures observed at the outcropping basement in Southeastern Brazil and its impact over the development of those basins have not been closely investigated. In the continental region adjacent to the Campos Basin, we described a geological feature with NNW-SSE orientation, named in this paper as the Alegre Fracture Zone (AFZ), which is observed in the onshore basement and can be projected to the offshore basin. The main goal of this work was to study this structural lineament and its influence on the tectonic evolution of the central portion of the Campos Basin and adjacent mainland. The onshore area was investigated through remote sensing data joint with field observations, and the offshore area was studied through the interpretation of 2-D seismic data calibrated by geophysical well logs. We concluded that the AFZ occurs in both onshore and offshore as a brittle deformation zone formed by multiple sets of fractures that originated in the Cambrian and were reactivated mainly as normal faults during the rift phase and in the Cenozoic. In the Campos Basin, the AFZ delimitates the western side of the Corvina-Parati Low, composing a complex fault system with the NE-SW faults and the NW-SE transfer faults.

  17. Brittle versus ductile deformation as the main control of the deep fluid circulation in continental crust

    NASA Astrophysics Data System (ADS)

    Violay, Marie; Madonna, Claudio; Burg, Jean-Pierre

    2016-04-01

    The Japan Beyond-Brittle Project (JBBP) and the Taupo Volcanic Zone-Deep geothermal drilling project in New Zealand (TVZ-DGDP) proposed a new concept of engineered geothermal development where reservoirs are created in ductile rocks. This system has several advantages including (1) a simpler design and control of the reservoir due to homogeneous rock properties and stress states in the ductile domain ,(2) possible extraction of supercritical fluids (3) less probability for induced earthquakes. However, it is at present unknwon what and how porosity and permeability can be engineered in such environments. It has been proposed that the magmatic chamber is surrounded by a hot and ductile carapace through which heat transfer is conductive because the plastic behaviour of the rock will close possible fluid pathways. Further outward, as temperature declines, the rock will encounter the brittle-ductile transition with a concomitant increase in porosity and permeability. The thickness of the conduction-dominated, ductile boundary zone between the magmatic chamber and the convecting geothermal fluid directly determines the rate of heat transfer. To examine the brittle to ductile transition in the context of the Japanese crust, we conducted deformation experiments on very-fine-grain granite in conventional servocontrolled, gas-medium triaxial apparatus (from Paterson instrument). Temperature ranged from 600° C to 1100° C and effective confining pressure from 100 to 150 MPa. Dilatancy was measured during deformation. The method consisted in monitoring the volume of pore fluid that flows into or out of the sample at constant pore pressure. Permeability was measured under static conditions by transient pressure pulse method. Mechanical and micro-structural observations at experimental constant strain rate of 10-5 s-1 indicated that the granite was brittle and dilatant up to 900 ° C. At higher temperatures the deformation mode becomes macroscopically ductile, i

  18. Heat flow and near-surface radioactivity in the Australian continental crust

    USGS Publications Warehouse

    Sass, J.H.; Jaeger, J.C.; Munroe, Robert J.

    1976-01-01

    Heat-flow data have been obtained at 44 sites in various parts of Australia. These include seven sites from the old (~ 2500 m.y.) Precambrian shield of Western Australia, seventeen from the younger (~ 600- 2000 m.y.) Precambrian rocks of South Australia, the Northern Territory, and Queensland, and twenty within the eastern Paleozoic and younger rocks. Thirty of the sites are located where no previous heat-flow data existed, and the remainder provide significant extensions or refinements of areas previously studied. Where the holes studied penetrated the crystalline basement rocks, or where the latter rocks were exposed within a few kilometers of the holes, the upper crustal radiogenic heat production has been estimated based on gamma-ray spectrometric determinations of U, Th, and K abundances. Three heat-flow provinces are recognized in Australia based on the linear relation (q = q* + DA0 ) between heat flow q and surface radioactivity A0. New data from the Western Australian shield support earlier studies showing that heat flow is low to normal with values ranging from 0.7 to 1.2 hfu and with the majority of values less than 1.0 hfu, and the parameters q* = 0.63 hfu and 0 = 4.5 km determined previously were confirmed. Heat flow in the Proterozoic shield of central Australia is quite variable, with values ranging between about l and 3 hfu. This variability is attributed mainly to variations in near-surface crustal radioactivity. The parameters of the heat-flow line are q* = 0.64 hfu and 0 = 11.1 km and moderately high temperatures are predicted for the lower crust and upper mantle. Previous suggestions of a band of l ow- to - normal heat flow near the coast in eastern Australia were confirmed in some areas, but the zone is interrupted in at least one region (the Sydney Basin), where heat flow is about 2.0 hfu over a large area. The reduced heat flow, q*, in the Paleozoic intrusive rocks of eastern Australia varies from about 0.8 to 2.0 hfu . This variability might

  19. Fractional Crystallisation of Archaean Trondhjemite Magma at 12-7 Kbar: Constraints on Rheology of Archaean Continental Crust

    NASA Astrophysics Data System (ADS)

    Sarkar, Saheli; Saha, Lopamudra; Satyanarayan, Manavalan; Pati, Jayanta

    2015-04-01

    Fractional Crystallisation of Archaean Trondhjemite Magma at 12-7 Kbar: Constraints on Rheology of Archaean Continental Crust Sarkar, S.1, Saha, L.1, Satyanarayan, M2. and Pati, J.K.3 1. Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee-247667, Haridwar, India, 2. HR-ICPMS Lab, Geochemistry Group, CSIR-National Geophysical Research Institute, Hyderabad-50007, India. 3. Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad-211002, India. Tonalite-Trondhjemite-Granodiorite (TTGs) group of rocks, that mostly constitute the Archaean continental crusts, evolved through a time period of ~3.8 Ga-2.7 Ga with major episodes of juvenile magma generations at ~3.6 Ga and ~2.7 Ga. Geochemical signatures, especially HREE depletions of most TTGs conform to formation of this type of magma by partial melting of amphibolites or eclogites at 15-20 kbar pressure. While TTGs (mostly sodic in compositions) dominates the Eoarchaean (~3.8-3.6 Ga) to Mesoarchaean (~3.2-3.0 Ga) domains, granitic rocks (with significantly high potassium contents) became more dominant in the Neoarchaean period. The most commonly accepted model proposed for the formation of the potassic granite in the Neoarchaean time is by partial melting of TTGs along subduction zones. However Archaean granite intrusive into the gabbro-ultramafic complex from Scourie, NW Scotland has been interpreted to have formed by fractional crystallization of hornblende and plagioclase from co-existing trondhjemitic gneiss. In this study we have studied fractional crystallization paths from a Mesoarchaean trondhjemite from the central Bundelkhand craton, India using MELTS algorithm. Fractional crystallization modeling has been performed at pressure ranges of 20 kbar to 7 kbar. Calculations have shown crystallization of garnet-clinopyroxene bearing assemblages with progressive cooling of the magma at 20 kbar. At pressure ranges 19-16 kbar, solid phases

  20. Geochemical and Sr-Nd-Pb isotopic evidence for ancient lower continental crust beneath the Xi Ujimqin area of NE China

    NASA Astrophysics Data System (ADS)

    Gao, Xiaofeng; Guo, Feng; Xiao, Peixi; Kang, Lei; Xi, Rengang

    2016-05-01

    The Central Asian Orogenic Belt (CAOB) is the largest Phanerozoic accretionary orogen on Earth. The role that Precambrian continental microblocks played in its formation, however, remains a highly controversial topic. New zircon U-Pb age data and whole-rock geochemical and Sr-Nd-Pb isotopic studies on Permian (253-251 Ma) andesites from the Xi Ujimqin area provide the first evidence for the existence of a continental lower mafic crust in the eastern segment of the CAOB. These Permian lavas generally have chemical compositions similar to experimental melts of garnet pyroxenites. Based on Sr-Nd-Pb isotopic compositional differences, they can be further subdivided into two groups. Group 1 has moderately radiogenic Sr (87Sr/86Sr(i) = 0.7060-0.7062) and nonradiogenic Nd (εNd(t) = - 9.0-8.3) and Pb (e.g., 206Pb/204Pb = 17.18-17.23) isotopic compositions similar to the ancient lower mafic crust beneath the North China Craton (NCC). Compared with Group 1, Group 2 has less radiogenic Sr (87Sr/86Sr(i) = 0.7051-0.7055), and more radiogenic Nd (εNd(t) = - 0.2-+1.4) and Pb (e.g., 206Pb/204Pb = 18.04-18.20) isotopic compositions as observed in the Phanerozoic granitoids and felsic lavas of the CAOB. The combined geochemical and isotopic data indicate that Group 1 was derived from ancient lower mafic crust of the NCC affinity, with a residual assemblage of pyroxene + plagioclase + amphibole. The source for Group 2 was a mixture of ancient lower mafic crust and a juvenile crustal component, and melting left a residue of orthopyroxene + clinopyroxene + plagioclase + garnet + amphibole. Generation of these two types of late Permian andesites favors a model whereby breakoff of a subducted slab and subsequent lithospheric extension triggered extensive asthenospheric upwelling and melting of the continental mafic lower crust of the eastern CAOB. The discovery of ancient lower continental crust of the NCC affinity in the CAOB implies that the NCC experienced continental breakup during

  1. Mid to late proterozoic magmatism within Northeastern North America and its iplications for the growth of the continental crust

    NASA Technical Reports Server (NTRS)

    Mclelland, J. M.

    1988-01-01

    Recent studies of the mangerite-charnockite-alaskite suite exposed in the Adirondack Highlands strongly suggest that these rocks were emplaced under anorogenic, or mildly extensional, conditions. The characteristic signatures of the rocks are high (FeO/FeO+MgO) and (FeO/Fe2O3); mildly alkaline to subalkaline and metaluminous trends; high Ga/Al2O3; and within plate concentrations of Nb vs Y, Ta vs Yb, Rb vs (Y+Nb), and Rb vs (Yb+Ta). Evolved members of the series are low in CaO and MgO and high in alkalies and halogens. All of these properties are consistent with anorogenic magmatism comprising acidic crustal melts and mantle derived mafic additions to the crust. Major and minor element trends, as well as field evidence, strongly suggest that the anorogenic acidic suite is coeval, but not comagmatic, with closely associated anorthositic massifs. Present outcrop configurations are consistent with the evolution of the acidic and anorthositic rocks in zoned, bimodal magma chambers cored by the mafic constituents and overlain by explosive, caldera-type volcanism. Age determinations indicate that the emplacement of the anorthosite-charnockite suite extended over approximately 500 Ma in the Grenville and adjacent Nain, Provinces.

  2. Arc-continent collision and the formation of continental crust: A new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

    USGS Publications Warehouse

    Draut, Amy E.; Clift, Peter D.; Amato, Jeffrey M.; Blusztajn, Jerzy; Schouten, Hans

    2009-01-01

    Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U-Pb zircon dating yields ages of 493 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc-continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian-Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc-continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust. ?? 2009 Geological Society of London.

  3. Measured and calculated elastic wave speeds in partially equilibrated mafic granulite xenoliths: Implications for the properties of an underplated lower continental crust

    NASA Astrophysics Data System (ADS)

    Rudnick, Roberta L.; Jackson, Ian

    1995-06-01

    Ultrasonic compressional wave velocities measured at 1.0 GPa and room temperature are compared with calculated velocities (based on single-crystal data and modal mineralogy) for a suite of mafic granulite xenoliths from the Chudleigh volcanic province, north Queensland, Australia. The xenoliths have nearly constant major element compositions but widely variable modal mineralogy, reflecting recrystallization under variable pressure-temperature conditions at depth in the continental crust (20-45 km). They thus provide an excellent opportunity to investigate velocity variation with depth in a mafic lower crust. Measured P wave velocities, corrected for the decompression-induced breakdown of garnet, range from 6.9 to 7.6 km/sec and correlate with derivation depth. These velocities are 5-12% lower than the calculated velocities (7.5-8.0 km/sec), apparently as a result of grain boundary alteration as well as irreversible changes that occurred in the xenoliths during rapid decompression. Calculated P wave velocities are similar to those estimated by Furlong and Fountain (1986) and Sobolev and Babeyko (1989) for mafic granulites formed through basaltic underplating of the continental crust. Depending upon in situ temperature, P wave velocities in the deepest samples may be interpreted as crustal (e.g., 7.3-7.6 km/sec, if heat flow is high) or mantle (7.7-7.8 km/sec, in areas of low heat flow). The range of velocities in the xenolith suite is larger than predicted for a fully equilibrated underplated basaltic layer, highlighting the importance of kinetic effects in determining the ultimate velocity profile of magmatically underplated crust. Comparison of our results with seismic profiles illustrates that the lower crust rarely reaches such high velocities, suggesting quartz-bearing rocks (country rocks?) are present within magmatically underplated layers of the deep crust.

  4. Evolution of the depleted mantle and growth of the continental crust: An early beginning or a slow start?

    NASA Astrophysics Data System (ADS)

    Vervoort, Jeff

    2014-05-01

    A fundamental principle of the geochemical evolution of the Earth holds that continental crust formed by extraction of melts from the mantle leaving part of the mantle depleted in incompatible elements. Nd and Hf isotopes have long been used to show that this process has been an essential feature of the Earth throughout its history, but the details of the record—and its implications for for addressing questions of the mechanisms, timing, and volumes of crustal production—remain hotly debated, particularly for the early Earth. One reason for the uncertainty in the isotopic record is a paucity of Archean rocks > 3.5 Ga and the ones that have survived often have had complex geologic histories, resulting in either compromised isotopic systematics and/or complicated mixtures of components with different ages and isotopic compositions. To address these potential complexities two approaches have been used to constrain the isotopic record: 1) Nd and Hf isotopic compositions of least altered rocks with well-constrained crystallization ages; and 2) Hf isotope composition of zircons in both magmatic rocks and in clastic sediments. The latter approach has found great utility by virtue of coupling U-Pb age constraints with Lu-Hf tracer isotopic information. A further advantage of the integrated U-Pb and Hf approach has been to examine zircons bereft of their corresponding whole rock. This has allowed the extension of the Hf isotopic record to the Hadean by examining detrital zircons from places like the Jack Hills. One unintended consequence of two different (Nd, Hf) approaches has been the creation of isotopic records not in agreement with each other, resulting in different interpretations. The Nd isotopic record, based solely on whole-rock analyses, shows evidence for the development of a depleted mantle signature in the oldest mantle-derived rocks. These data appear to correlate with positive 142 Nd anomalies, consistent with very early development of depleted and

  5. Brittle/Ductile deformation at depth during continental crust eclogitization (Mont-Emilius klippe, Western Internal Alps).

    NASA Astrophysics Data System (ADS)

    Hertgen, Solenn; Yamato, Philippe; Morales, Luiz; Angiboust, Samuel

    2016-04-01

    Eclogitic rocks are important for understanding tectonics at large scale as they provide key constraints on both the evolution (P-T-t-ɛ paths) and the deformation modes of the crust along the subduction interface. We herein focus our study on eclogitized mafic dykes remnants exposed within granulites from the continental basement silver of the Mt. Emilius klippe (Western Internal Alps, Italy). These eclogites exhibit highly deformed garnetite and clinopyroxenite levels. In some places, these rocks with a ± mylonitic aspect can be found as clasts within meter-thick brecciated fault rocks formed close to metamorphic peak conditions in eclogite facies. Especially, the garnet-rich levels tend to behave in a brittle fashion while deformation within clinopyroxene-rich levels is mostly accommodated by creep. This is evidenced by the presence of elongated grains, subgrain boundaries and intense grain size reduction close to rigid garnets. Crystallographic preferred orientation (CPO) measurements in garnets indicate a quasi-random distribution. In most of the clinopyroxenes levels nevertheless, the CPO is relatively strong, with multiples of uniform distribution varying from 4 to 5.5 (value of 1 is random texture). This CPO is characterized by a strong alignment of poles (001) parallel to the lineation and (100) and [010] distributed along girdles cross-cutting the foliation plane. Our study thus attests that the materials along the subduction interface at P~2.0-2.5 GPa and T~500-550°C can locally be brittle where deformation is classically envisioned as ductile. In addition to this deformation analysis, we present a petrological study of these eclogites, from the outcrop to the microscopic scale, tracking the chemical evolution associated to the observed deformation. Based on all these data, we finally propose a tectono-metamorphic history for these rocks allowing to explain the co-existence of ductile and brittle features developed in the same metamorphic facies, and

  6. Nitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust

    NASA Astrophysics Data System (ADS)

    Inguaggiato, S.; Taran, Y.; Grassa, F.; Capasso, G.; Favara, R.; Varley, N.; Faber, E.

    2004-12-01

    The Jalisco Block (JB) is a geologically and tectonically complex part of northwestern Mexico characterized by active subduction-type volcanism, rifting, and old stable structures. Thermal springs and groups of springs are widely distributed over JB. Bubbling gas from seven thermal springs located within different tectonic environments of the JB was analyzed for He, 20Ne, and N2 concentrations and δ15N ratios. All gases are N2-dominant (>84%) with the exception of one sample (Rió Purificación), which has a significant CH4 content (about 50%). All collected gas samples are relatively high in He, up to 1500 ppm vol and with 3He/4He values ranging from 0.6 to 4.5 Ra. All measured nitrogen isotope ratios are heavier than air with δ15N values ranging from 0.5 to 5.0‰. The relative N2 excess with respect to air-saturated water computed on the basis of N2 and 20Ne contents indicates the contribution of a nonatmospheric N2 source. All the samples show a good correlation between δ15N and the relative excess of N2 with δ15N ˜ +5.3‰ for the maximum N2 excess of 100%. Due to a presumed lack of seafloor sediment involved in the subduction process, such a δ15N positive value seems to reflect the addition to the fluids of a heavy nitrogen originating from metamorphism processes of rocks occurring within the overlying continental crust.

  7. The Cool Early Earth: Oxygen Isotope Evidence for Continental Crust and Oceans on Earth at 4.4 Ga

    NASA Astrophysics Data System (ADS)

    Valley, J. W.; King, E. M.; Peck, W. H.; Graham, C. M.; Wilde, S. A.

    2001-05-01

    Zircons preserve the best record of U-Pb crystallization age and oxygen isotope ratios of igneous rocks. The d18-O of non-metamict zircon is unaffected even by hydrothermal alteration and high-grade metamorphism. Ion microprobe analysis of detrital zircons from the \\sim3 Ga Jack Hills metaconglomerate (Narryer Gneiss Terrane, Yilgarn Craton, Western Australia) yield U-Pb ages from 3.1 to 4.4 Ga (SHRIMP II, Wilde et al. 2001 Nature) and d18-O from 5 to 8 permil (Cameca 4f, Peck et al. 2001 GCA). The d18-O of these zircons averages 6.3, and is 1 permil higher than that in equilibrium with the mantle and that of normal Archean granitic zircons (5.3+-0.3, 5.5+-0.4, respectively; King et al. 1998 Pre-C Res, Peck et al. 2000 Geology). The distribution of mantle-like vs. mildly elevated d18-O values for magmas is constant from 2.7 to 4.4 Ga, and on 4 continents. The age of 4.404+-0.008 Ga from one 200 micron zircon is >99% concordant and represents the oldest recognized terrestrial material. This crystal is zoned in d18-O (5.0+-0.7 vs. 7.4+-0.7) and REEs (La=0.3 to 13.6 ppm), and contains inclusions of SiO2. REE patterns are HREE enriched with positive Ce and negative Eu anomalies; calculated melts are LREE enriched. Taken together, these results suggest crystallization from a quartz-saturated granitic magma and thus the existence of continental crust, possibly in a setting like Iceland. The high d18-O portion of the crystal would be in equilibrium with a magma at d18-O(WR)= 8.5-9.5. There is no known mantle reservoir with such high values. d18-O(WR) values above 8.5 are typical of "S-type" granites that have melted or assimilated material that was altered by low temperature interaction with water at the surface of the Earth (i.e., weathering, diagenesis, low T hydrothermal alteration). Thus the high d18-O value of the 4.4 Ga zircon suggests that surface temperatures were cool enough for liquid water suggesting that the early steam-rich atmosphere condensed to form

  8. Magma chamber evolution: implication for the generation of continental crust: A case study in Kekeli batholith, North Qilian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Huang, H.; Niu, Y.

    2013-12-01

    plus minor other felsic minerals (Kfs, Qtz). The charnockite sample QL10-26 has the highest TiO2, pyroxene with Amp reaction rims and the most radiogenic WR-Hf (ɛHf(t) = 26) and therefore represent the most primitive mafic endmember. The leucogranite samples have the lowest ɛHf(t) and ɛNd(t) and thus represent the crustal endmember. Other samples with intermediate composition between these two endmembers also have the intermediate isotopic compositions in between. The various zircon types with complex internal structures present in most samples reflect complex source materials. All these observations are consistent with the scenario that the mantle-derived basaltic magmas interacted with granitic magmas derived from the lower crust. Considering the petrotectonic associations, it is probable that the Kekeli Batholith may represent a fossil magma chamber that crystallized at the base of a continental arc crust associated with a subduction zone. Schmitz, M.D., Vervoort, J.D., Bowring, S.A., Patchett, P.J., 2004. Decoupling of the Lu-Hf and Sm-Nd isotope systems during the evolution of granulitic lower crust beneath southern Africa. Geology 32, 405.

  9. Linking the tectonic evolution with fluid history in magma-poor rifted margins: tracking mantle- and continental crust-related fluids

    NASA Astrophysics Data System (ADS)

    Pinto, V. H. G.; Manatschal, G.; Karpoff, A. M.

    2014-12-01

    The thinning of the crust and the exhumation of subcontinental mantle is accompanied by a series of extensional detachment faults. Exhumation of mantle and crustal rocks is intimately related to percolation of fluids along detachment faults leading to changes in mineralogy and chemistry of the mantle, crustal and sedimentary rocks. Field observation, analytical methods, refraction/reflection and well-core data, allowed us to investigate the role of fluids in the Iberian margin and former Alpine Tethys distal margins and the Pyrenees rifted system. In the continental crust, fluid-rock interaction leads to saussuritization that produces Si and Ca enriched fluids found in forms of veins along the fault zone. In the zone of exhumed mantle, large amounts of water are absorbed in the first 5-6 km of serpentinized mantle, which has the counter-effect of depleting the mantle of elements (e.g., Si, Ca, Mg, Fe, Mn, Ni and Cr) forming mantle-related fluids. Using Cr-Ni-V and Fe-Mn as tracers, we show that in the distal margin, mantle-related fluids used detachment faults as pathways and interacted with the overlying crust, the sedimentary basin and the seawater, while further inward parts of the margin, continental crust-related fluids enriched in Si and Ca impregnated the fault zone and may have affected the sedimentary basin. The overall observations and results enable us to show when, where and how these interactions occurred during the formation of the rifted margin. In a first stage, continental crust-related fluids dominated the rifted systems. During the second stage, mantle-related fluids affected the overlying syn-tectonic sediments through direct migration along detachment faults at the future distal margin. In a third stage, these fluids reached the seafloor, "polluted" the seawater and were absorbed by post-tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the thinned continental crust, that the mantle-related fluids

  10. The stoichiometric ratio during biological removal of inorganic carbon and nutrient in the Mississippi River plume and adjacent continental shelf

    NASA Astrophysics Data System (ADS)

    Huang, W.-J.; Cai, W.-J.; Powell, R. T.; Lohrenz, S. E.; Wang, Y.; Jiang, L.-Q.; Hopkinson, C. S.

    2012-02-01

    The stoichiometric ratios of dissolved inorganic carbon (DIC) and nutrients during biological removal have been widely assumed to follow the Redfield ratios (especially the C/N ratio) in large river plume ecosystems. However, this assumption has not been systematically examined and documented because DIC and nutrients are rarely studied simultaneously in a river plume area, a region in which they can be affected by strong river-ocean mixing as well as intense biological activity. We examined stoichiometric ratios of DIC, total alkalinity (TA), and nutrients (NO3, PO43- and Si(OH)4) data during biological removal in the Mississippi River plume and adjacent continental shelf in June 2003 and August 2004 with biological removals defined as the difference between measured values and values predicted on the basis of conservative mixing determined using a multi-endmember mixing model. Despite complex physical and biogeochemical influences, relationships between DIC and nutrients were strongly dependent on salinity range and geographic location, and influenced by biological removal. Lower C/Si and N/Si ratios in one nearshore area were attributed to a potential silicate source induced by water exchange with coastal salt marshes. When net biological uptake was separated from river-ocean mixing and the impact of marshes and bays excluded, stoichiometric ratios of C/N/Si were similar to the Redfield ratios, thus supporting the applicability of the Redfield-type C/N/Si ratios as a principle in river-plume biogeochemical models.

  11. Geothermal modelling of faulted metamorphic crystalline crust: a new model of the Continental Deep Drilling Site KTB (Germany)

    NASA Astrophysics Data System (ADS)

    Szalaiová, Eva; Rabbel, Wolfgang; Marquart, Gabriele; Vogt, Christian

    2015-11-01

    The area of the 9.1-km-deep Continental Deep Drillhole (KTB) in Germany is used as a case study for a geothermal reservoir situated in folded and faulted metamorphic crystalline crust. The presented approach is based on the analysis of 3-D seismic reflection data combined with borehole data and hydrothermal numerical modelling. The KTB location exemplarily contains all elements that make seismic prospecting in crystalline environment often more difficult than in sedimentary units, basically complicated tectonics and fracturing and low-coherent strata. In a first step major rock units including two known nearly parallel fault zones are identified down to a depth of 12 km. These units form the basis of a gridded 3-D numerical model for investigating temperature and fluid flow. Conductive and advective heat transport takes place mainly in a metamorphic block composed of gneisses and metabasites that show considerable differences in thermal conductivity and heat production. Therefore, in a second step, the structure of this unit is investigated by seismic waveform modelling. The third step of interpretation consists of applying wavenumber filtering and log-Gabor-filtering for locating fractures. Since fracture networks are the major fluid pathways in the crystalline, we associate the fracture density distribution with distributions of relative porosity and permeability that can be calibrated by logging data and forward modelling of the temperature field. The resulting permeability distribution shows values between 10-16 and 10-19 m2 and does not correlate with particular rock units. Once thermohydraulic rock properties are attributed to the numerical model, the differential equations for heat and fluid transport in porous media are solved numerically based on a finite difference approach. The hydraulic potential caused by topography and a heat flux of 54 mW m-2 were applied as boundary conditions at the top and bottom of the model. Fluid flow is generally slow and

  12. A new estimate for the composition of weathered young upper continental crust from alluvial sediments, Queensland, Australia

    NASA Astrophysics Data System (ADS)

    Kamber, Balz S.; Greig, Alan; Collerson, Kenneth D.

    2005-02-01

    We present new major element, trace element and Nd-isotope data for 30 alluvial sediments collected from 25 rivers in Queensland, E Australia. Samples were chosen to represent drainage from the region's most important lithologies, including Tertiary intraplate volcanic rocks, a Cretaceous igneous province (and sedimentary rocks derived thereof) as well as Proterozoic blocks. In most chemical and isotopic aspects, the alluvial sediments represent binary or ternary mixing relationships, with absolute abundances implied to reflect the proportion of lithologies in the catchments. When averaged, the studied sediments differ from other proxies of upper continental crust (UCC) mainly in their relative middle rare earth element enrichment (including an elevated Sm/Nd ratio), higher relative Eu abundance and higher Nb/Ta ratio. These features are inherited from eroded Tertiary intraplate basalts, which commonly form topographic highs in the studied region. Despite the high degree of weathering strong to excellent coherence between similarly incompatible elements is found for all samples. From this coherence, we suggest revisions of the following upper crustal element ratios: Y/Ho = 26.2, Yb/Tm = 6.37, Th/W = 7.14, Th/Tl = 24 and Zr/Hf = 36.9. Lithium, Rb, Cs and Be contents do not seem depleted relative to UCC, which may reflect paucity of K-feldspar in the eroded catchments. Nickel, Cr, Pb, Cu and Zn concentrations are elevated in polluted rivers surrounding the state capital. River sediments in the Proterozoic Georgetown Inlier are elevated in Pb, Cu and Zn but this could be a natural phenomenon reflecting abundant sulphide mineralisation in the area. Except for relative Sr concentrations, which broadly anticorrelate with mean annual rainfall in catchments, there is no obvious relationship between the extent of weathering and climate types, which range from arid to tropical. The most likely explanation for this observation is that the weathering profiles in many

  13. Iron, zinc, magnesium and uranium isotopic fractionation during continental crust differentiation: The tale from migmatites, granitoids, and pegmatites

    NASA Astrophysics Data System (ADS)

    Telus, Myriam; Dauphas, Nicolas; Moynier, Frédéric; Tissot, François L. H.; Teng, Fang-Zhen; Nabelek, Peter I.; Craddock, Paul R.; Groat, Lee A.

    2012-11-01

    The causes of some stable isotopic variations in felsic rocks are not well understood. In particular, the origin of the heavy Fe isotopic compositions (i.e., high δ56Fe values, deviation in ‰ of the 56Fe/54Fe ratio relative to IRMM-014) of granites with SiO2 > 70 wt.% compared with less silicic rocks is still debated. It has been interpreted to reflect isotopic fractionation during late stage aqueous fluid exsolution, magma differentiation, partial melting, or Soret (thermal) diffusion. The present study addresses this issue by comparing the Fe isotopic compositions of a large range of differentiated crustal rocks (whole rocks of migmatites, granitoids, and pegmatites; mineral separates) with the isotopic compositions of Zn, Mg and U. The samples include granites, migmatites and pegmatites from the Black Hills, South Dakota (USA), as well as I-, S-, and A-type granitoids from Lachlan Fold Belt (Australia). The nature of the protolith (i.e., I- or S-type) does not influence the Fe isotopic composition of granitoids. Leucosomes (partial melts in migmatites) tend to have higher δ56Fe values than melanosomes (melt residues) indicating that partial melting of continental crust material can possibly fractionate Fe isotopes. No clear positive correlation is found between the isotopic compositions of Mg, U and Fe, which rules out the process of Soret diffusion in the systems studied here. Zinc isotopes were measured to trace fluid exsolution because Zn can easily be mobilized by aqueous fluids as chloride complexes. Pegmatites and some granitic rocks with high δ56Fe values also have high δ66Zn values. In addition, high-SiO2 granites show a large dispersion in the Zn/Fe ratio that cannot easily be explained by magma differentiation alone. These results suggest that fluid exsolution is responsible for some of the Fe isotopic fractionation documented in felsic rocks and in particular in pegmatites. However, some granites with high δ56Fe values have unfractionated δ66

  14. Formation of continental crust in a temporally linked arc magma system from 5 to 30 km depth: ~ 90 Ma plutonism in the Cascades Crystalline Core composite arc section

    NASA Astrophysics Data System (ADS)

    Ratschbacher, B. C.; Miller, J. S.; Kent, A. J.; Miller, R. B.; Anderson, J. L.; Paterson, S. R.

    2015-12-01

    Continental crust has an andesitic bulk composition with a mafic lower crust and a granodioritic upper crust. The formation of stratified continental crust in general and the vertical extent of processes active in arc crustal columns leading to the differentiation of primitive, mantle-derived melts entering the lower crust are highly debated. To investigate where in the crustal column magma mixing, fractionation, assimilation and crystal growth occur and to what extent, we study the ~ 90 Ma magmatic flare-up event of the Cascades arc, a magma plumbing system from ~ 5 to 30 km depth. We focus on three intrusive complexes, emplaced at different depths during major regional shortening in an exceptionally thick crust (≥ 55 km1) but which are temporally related: the upper crustal Black Peak intrusion (1-3 kbar at 3.7 to 11 km; ~ 86.8 to 91.7 Ma2), the mid-crustal Mt. Stuart intrusion (3.5-4.0 kbar at 13 to 15 km; 90.8 and 96.3 Ma3) and the deep crustal Tenpeak intrusion (7 to 10 kbar at 25 to 37 km; 89.7 to 92.3 Ma4). These intrusive complexes are well characterized by geochronology showing that they have been constructed incrementally by multiple magma batches over their lifespans and thus allow the monitoring and comparison of geochemical parameters over time at different depths. We use a combination of whole rock major and trace element data and isotopes combined with detailed investigation of amphibole, which has been recognized to be important in the generation of calc-alkaline rocks in arcs to test the following hypotheses: (a) compositional bimodality is produced in the lower crust, whereas upper crustal levels are dominated by mixing to form intermediate compositions, or (b) differentiation occurs throughout the crustal column with different crystallizing phases and their compositions controlling the bulk chemistry. 1. Miller et al. 2009: GSA Special Paper 456, p. 125-149 2. Shea 2014: PhD thesis, Massachusetts Institute of Technology 3. Anderson et al. 2012

  15. The syncollisional granitoid magmatism and continental crust growth in the West Kunlun Orogen, China - Evidence from geochronology and geochemistry of the Arkarz pluton

    NASA Astrophysics Data System (ADS)

    Zhang, Yu; Niu, Yaoling; Hu, Yan; Liu, Jinju; Ye, Lei; Kong, Juanjuan; Duan, Meng

    2016-02-01

    The West Kunlun orogenic belt (WKOB) at the northwest margin of the Greater Tibetan Plateau records seafloor subduction, ocean basin closing and continental collision with abundant syncollisional granitoids in response to the evolution of the Proto- and Paleo-Tethys Oceans from the early-Paleozoic to the Triassic. Here we present a combined study of detailed zircon U-Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf isotopic geochemistry on the syncollisional Arkarz (AKAZ) pluton with mafic magmatic enclaves (MMEs) exposed north of the Mazha-Kangxiwa suture (MKS) zone. The granitoid host rocks and MMEs of the AKAZ pluton give the same late Triassic age of ~ 225 Ma. The granitoid host rocks are metaluminous granodiorite and monzogranite. They have initial 87Sr/86Sr of 0.70818 to 0.70930, εNd(225 Ma) = - 4.61 to - 3.91 and εHf(225 Ma) = - 3.01 to 0.74. The MMEs are more mafic than the host with varying SiO2 (51.00-63.24 wt.%) and relatively low K2O (1.24-3.02 wt.%), but have similar Sr-Nd-Hf isotope compositions to the host ((87Sr/86Sr)i = 0.70830-0.70955, εNd(225 Ma) = - 4.88 to - 4.29, εHf(225 Ma) = - 2.57 to 0.25). Both the host and MMEs have rare earth element (REE) and trace element patterns resembling those of bulk continental crust (BCC). The MMEs most likely represent cumulate formed from common magmas parental to the granitoid host. The granitoid magmatism is best explained as resulting from melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional similarity to the BCC and the inherited mantle-like isotopic compositions. Simple isotopic mixing calculations suggest that ~ 80% ocean crust and ~ 20% continental materials contribute to the source of the AKAZ pluton. Thus, the hypothesis "continental collision zones as primary sites for net continental crust growth" is applicable in the WKOB as shown by studies in southern Tibet, East Kunlun and Qilian orogens. In

  16. Continental hyperextension, mantle exhumation and thin oceanic crust at the continent-ocean transition, West Iberia: new insights from wide-angle seismic

    NASA Astrophysics Data System (ADS)

    Davy, Richard; Minshull, Tim; Bayrakci, Gaye; Bull, Jon; Klaeschen, Dirk; Papenberg, Cord; Reston, Timothy; Sawyer, Dale; Zelt, Colin

    2016-04-01

    Anomalously thin oceanic crust and expanses of exhumed and serpentinised mantle material at magma-poor rift margins are now a globally observed phenomena that characterizes the seaward limit of the continent-ocean transition. Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S-reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been suggested to delimit the seaward extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160-km-long wide-angle seismic profile (WE-1). Forward modelling and travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks overlying the S-reflector, which correlates with the 6.0 - 7.0 kms-1 velocity contours, corresponding to peridotite serpentinization of 60 - 30 %, respectively. West of the Peridotite Ridge we observe a basement layer which is 2.8 - 3.5 km thick in which velocities increase smoothly and rapidly from ˜4.6 kms-1 to 7.3 - 7.6 kms-1,with an average velocity gradient of 1.00 s-1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle material with depth. However, sparse Moho reflections indicate the onset of an anomalously thin oceanic crust, which increases in thickness from ˜0.5 km to ˜1.5 km over a distance of 35 km, seaward.

  17. High temperature pseudotachylytes and ductile shear zones in dry rocks from the continental lower crust (Lofoten, Norway)

    NASA Astrophysics Data System (ADS)

    Menegon, Luca; Pennacchioni, Giorgio; Harris, Katherine; Wood, Elliot

    2014-05-01

    Understanding the mechanisms of initiation and growth of shear zones under lower crustal conditions is of fundamental importance when assessing lithosphere rheology and strength. In this study we investigate brittle-ductile shear zones developed under lower crustal conditions in anorthosites from Nusfjord, Lofoten (northern Norway). Steep ductile shear zones trend E-W to ESE-WSW and have a stretching lineation plunging steeply to the SSW or SSE. The shear sense is normal (south block down to the south) as indicated by SC and SC' fabrics and sigmoidal foliations. The shear zone show a mylonitic to ultramylonitic fabric, sharp boundaries to the host anorthosites, and abundant anastomosing dark fine-grained layers along the main foliation. The fine-grained layers localized much of the strain. Relatively lower strain domains within or adjacent to shear zones indicate that the fine dark bands of mylonites represent transposed pseudotachylyte which still locally preserve the pristine structures such as chilled margins, breccia textures with angular clasts of the host rock and injection veins; intersecting veins of pseudotachylyte record multiple stages of seismic slip. The orientation of injection veins and marker offset along the most preserved pseudotachylyte fault veins indicate approximately a sinistral strike slip kinematic during faulting event responsible for the friction-induced melting. These observations indicate that ductile shear zones exploited pre-existing brittle fault zones including a network of pseudotachylytes, and that the fine-grained "ultramylonites" derive from former fine-grained pseudotachylytes. The pseudotachylyte microstructure is dominated by plagioclase microlites dispersed in a groundmass of fine-grained clinopyroxene. Clinopyroxene recrystallizes in the damage zone flanking the pseudotachylytes, indicating high metamorphic grade during pseudotachylyte formation. Small idioblastic or cauliflower garnet are scattered through the matrix and

  18. Degrading permafrost and gas hydrate under the Beaufort Shelf and marine gas hydrate on the adjacent continental slope

    NASA Astrophysics Data System (ADS)

    Paull, C. K.; Dallimore, S. R.; Hughes Clarke, J. E.; Blasco, S.; Melling, H.; Lundsten, E.; Vagle, S.; Collett, T. S.

    2011-12-01

    The sub-seafloor under the Arctic Shelf is arguably the part of the Earth that is undergoing the most dramatic warming. In the southern Beaufort Sea, the shelf area was terrestrially exposed during much of the Quaternary period when sea level was ~120m lower than present. As a consequence, many areas are underlain by >600m of ice-bonded permafrost that conditions the geothermal regime such that the base of the methane hydrate stability can be >1000m deep. Marine transgression has imposed a change in mean annual surface temperature from -15°C or lower during periods of terrestrial exposure, to mean annual sea bottom temperatures near 0°C. The thermal disturbance caused by transgression is still influencing the upper km of subsurface sediments. Decomposition of gas hydrate is inferred to be occurring at the base and the top of the gas hydrate stability zone. As gas hydrate and permafrost intervals degrade, a range of processes occur that are somewhat unique to this setting. Decomposition of gas hydrate at depth can cause sediment weakening, generate excess pore water pressure, and form free gas. Similarly, thawing permafrost can cause thaw consolidation, liberate trapped gas bubbles in ice bonded permafrost. Understanding the connection between deep subsurface processes generated by transgression, surficial sediment processes near the seafloor, and gas flux into the ocean and atmosphere is important to assessing geohazard and environmental conditions in this setting. In contrast, conditions for marine gas hydrate formation occur on the adjacent continental slope below ~270m water depths. In this paper, we present field observations of gas venting from three geologically distinct environments in the Canadian Beaufort Sea, two on the shelf and one on the slope. A complimentary paper by Dallimore et al reviews the geothermal changes conditioning this environment. Vigorous methane venting is occurring over Pingo-Like-Features (PLF) on the mid-shelf. Diffuse venting of

  19. Crustal structure of the Altiplano from broadband regional waveform modeling: Implications for the composition of thick continental crust

    NASA Astrophysics Data System (ADS)

    Swenson, Jennifer L.; Beck, Susan L.; Zandt, George

    2000-01-01

    We have modeled the full waveforms from six intermediate-depth and two shallow earthquakes recorded at regional distances by the BANJO Broadband Andean Joint Experiment (BANJO) and Seismic Exploration of the Deep Altiplano (SEDA) portable seismic networks in the central Andes. In this study we utilize data from those BANJO and SEDA stations located within the Altiplano and Eastern Cordillera. We used reflectivity synthetic seismograms and a grid search to constrain four parameters of the Altiplano-Eastern Cordillera lithosphere: crustal thickness, average crustal velocity (Vp), and crustal and upper mantle Poisson's ratios (σcrust and σmantle). Using our grid search, we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths. Robust models for the Altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average Vp of 5.75-6.25 km/s, crustal thicknesses of 60-65 km, σcrust = 0.25, and σmantle = 0.27-0.29. We find a north-south variation in the structure of the Altiplano, with the crust south of the BANJO transect characterized by either lower than average crustal P wave velocities or a slightly higher σcrust relative to crust north of the BANJO transect. These results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust, rather than by underplating or magmatic intrusion from the mantle.

  20. Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean

    NASA Astrophysics Data System (ADS)

    Bouyahiaoui, B.; Sage, F.; Abtout, A.; Klingelhoefer, F.; Yelles-Chaouche, K.; Schnürle, P.; Marok, A.; Déverchère, J.; Arab, M.; Galve, A.; Collot, J. Y.

    2015-06-01

    We determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009, which included a wide-angle, 240-km-long, onshore-offshore seismic profile, multichannel seismic reflection lines and gravity and magnetic data, complemented by the available geophysical data for the study area. The analysis and modelling of the wide-angle seismic data including refracted and reflected arrival travel times, and integrated with the multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ˜5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P-wave velocities between 4.8 and 6.0 km s-1, from the top to the bottom. The lower crust is defined by a lower velocity gradient and P-wave velocity between 6.0 and 7.1 km s-1. The Poisson ratio in the lower crust deduced from S-wave modelling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P-wave velocities between 5.2 and 7.0 km s-1, from the top to the bottom, shows a gradual seaward thinning of ˜15 km over an ˜35-km distance. This thinning is regularly distributed between the upper and lower crusts, and it characterizes a rifted margin, which has resulted from backarc extension at the rear of the Kabylian block, here represented by the Edough Massif at the shoreline. Above the continental basement, an ˜2-km-thick, pre-Messinian sediment layer with a complex internal structure is interpreted as allochthonous nappes of flysch backthrusted on the margin during the collision of Kabylia with the African margin. The crustal structure, moreover, provides evidence for Miocene

  1. First findings of Paleo- and Mesoarchean zircons in the rocks from the Central Arctic province of oceanic rises as an evidence of the ancient continental crust

    NASA Astrophysics Data System (ADS)

    Sergeev, S. A.; Presnyakov, S. L.; Antonov, A. V.; Belyatsky, B. V.; Rodionov, N. V.; Shevchenko, S. S.

    2015-07-01

    This report presents the results of local U-Pb zircon dating (SIMS SHRIMP II) for a sample of migmatite gneiss dredged on the western slope of Alpha Ridge in the Arctic Ocean in the course of the "Arktika-2012" Russian polar expedition. The distribution of U-Pb ages of the examined zircon points to the Early Precambrian origin of this gneiss, for the bulk of the zircon was crystallized at least 3450 Ma ago from a magmatic melt under acidic volcanism at the primary crust formation. Zircon of the second generation was crystallized 3300 Ma ago under the remelting of acid volcanics and appearance of migmatite gneisses under the amphibolite facies of metamorphism. Most likely, a partial recrystallization of zircon and formation of microfolded structures and foliation took place 3000 Ma ago at the stage of rocks deformation. The latest zircon was formed 1900 Ma ago from the crust fluid or melt under the low-gradient metamorphism. In view of the possibility of the appearance of the treated clastogenic gneiss fragment under current oceanic erosion, the obtained results allow one to affirm that the occurrence of a fragment of the most ancient sialic continental crust formed at least 3450 Ma ago is possible at the submarine rises of the Arctic Ocean (Alpha Ridge and the Mendeleev Rise).

  2. Influence of chemical weathering on the composition of the continental crust: Insights from Li and Nd isotopes in bauxite profiles developed on Columbia River Basalts

    NASA Astrophysics Data System (ADS)

    Liu, Xiao-Ming; Rudnick, Roberta L.; McDonough, William F.; Cummings, Michael L.

    2013-08-01

    Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8-9 m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including "mobile" and some "immobile" elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and ɛNd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20-60 wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and ɛNd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust.

  3. Ocean-continent transition and tectonic framework of the oceanic crust at the continental margin off NE Brazil: Results of LEPLAC project

    NASA Astrophysics Data System (ADS)

    Gomes, Paulo Otávio; Gomes, Benedito S.; Palma, Jorge J. C.; Jinno, Koji; de Souza, Jairo M.

    In 1992, Brazilian Navy and PETROBRAS carried out a geophysical survey along the continental margin off northeastern Brazil, as part of a governmental plan to delineate the "Legal Continental Shelf" according to the international Law of the Sea. This data set is leading to a better understanding of the crustal transition processes and on the evolution of the oceanic crust over that part of the Brazilian continental margin. On our seismic transects, we show a rifted marginal plateau (Pernambuco Plateau) where crustal extension was controlled by detachment faulting, possibly in a non-volcanic margin setting. Farther north, dealing with the ocean-continent transition nearby a major transform margin, we found a normal passive margin-style transition zone instead of transform-related structures. With the support of multichannel seismic profiles and gravity data derived from GEOSAT altimetry, several well-known oceanic fracture zones and structural lineaments were properly located and correlated. The relationship of these structures with volcanic ridges and extensional, compressive and strike-slip tectonic reactivations suggests that fracture zones at this area behaved either as zones of weakness or as locked transform fault scars. Striking lithospheric flexural deformation is also related to FZs in this region. In the surroundings of the Fernando de Noronha Ridge, lithospheric flexure represents an isostatic response to volcanic loading, while bending across Ascension FZ is likely to have been caused by differential subsidence in crustal segments of contrasting ages. We also correlate some other deformation of the oceanic crust with changes in spreading directions that possibly took place at the Upper Cretaceous.

  4. Growth of continental crust and its episodic reworking over >800 Ma: evidence from Hf-Nd isotope data on the Pietersburg block (South Africa)

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Zeh, Armin; Moyen, Jean-François; Doucelance, Régis; Martin, Hervé

    2014-05-01

    The formation and evolution of the continental crust during the Precambrian, and in particular during the Archaean eon (4.0-2.5 Ga), is still a matter of debate. In particular, it is not yet clear in which tectonic environment the genesis of crust took place and how the large volume of granitoid rocks that form ~70% of the Archaean crust were extracted from the mantle. Many studies highlighted that radiogenic isotope systems, especially Lu-Hf and Sm-Nd, are powerful tools to unravel the respective extent of crustal growth and recycling in Archaean terranes. This work presents coupled Hf and Nd isotope data (analyzed both in situ in accessory minerals and in whole rock samples) of Meso- to Neoarchaean granitoids, applied to unravel the processes of crust formation and evolution of the Pietersburg crustal block in South Africa. This crustal segment, the northermost one of the Archaean Kaapvaal Craton, is separated from older crust (3.65-3.10 Ga) by a large-scale suture zone, and the processes related to amalgamation of both blocks and their subsequent evolution are still unclear. The Pietersburg block is made up of a wide range of Archaean granitoid rocks, including tonalite-trondhjemite-granodiorite (TTG) series, high-K monzogranites as well as (grano)diorites belonging to the so-called "sanukitoid" group [1], all intruded by late Paleoproterozoic alkaline complexes. Age determinations highlighted two stages of granitoid formation: (1) TTG magmatism took place episodically over >400 Ma between 3.34 and 2.89 Ga, with a major pulse at 2.97-2.90 Ga; while (2) all the other (high-K) granitoid types emplaced subsequently between 2.84 and 2.69 Ga before a long magmatic shutdown until the intrusion of alkaline complexes at ~2.00 Ga [2-3]. Isotope systematics reveal that these two stages are related to juvenile crust formation and crust reworking, respectively. Indeed, all Hf-Nd isotope data from TTG gneisses are suprachondritic, pointing to a juvenile origin and precluding

  5. Seismicity and S-wave velocity structure of the crust and the upper mantle in the Baikal rift and adjacent regions

    NASA Astrophysics Data System (ADS)

    Seredkina, Alena; Kozhevnikov, Vladimir; Melnikova, Valentina; Solovey, Oksana

    2016-12-01

    Correlations between seismicity, seismotectonic deformation (STD) field and velocity structure of the crust and the upper mantle in the Baikal rift and the adjacent areas of the Siberian platform and the Mongol-Okhotsk fold belt have been investigated. The 3D S-wave velocity structure up to the depths of 500 km has been modeled using a representative sample of Rayleigh wave group velocity dispersion curves (about 3200 paths) at periods from 10 to 250 s. The STD pattern has been reconstructed from mechanisms of large earthquakes, and is in good agreement with GPS and structural data. Analysis of the results has shown that most of large shallow earthquakes fall in regions of low S-wave velocities in the uppermost mantle (western Mongolia and areas of recent mountain building in southern Siberia) and in zones of their relatively high lateral variations (northeastern flank of the Baikal rift). In the first case the dominant STD regime is compression manifested in a mixture of thrust and strike-slip deformations. In the second case we observe a general predominance of extension.

  6. Brittle extension of the continental crust along a rooted system of low-angle normal faults: Colorado River extensional corridor

    NASA Technical Reports Server (NTRS)

    John, B. E.; Howard, K. A.

    1985-01-01

    A transect across the 100 km wide Colorado River extensional corridor of mid-Tertiary age shows that the upper 10 to 15 km of crystalline crust extended along an imbricate system of brittle low-angle normal faults. The faults cut gently down a section in the NE-direction of tectonic transport from a headwall breakaway in the Old Woman Mountains, California. Successively higher allochthons above a basal detachment fault are futher displaced from the headwall, some as much as tens of kilometers. Allochthonous blocks are tilted toward the headwall as evidenced by the dip of the cappoing Tertiary strata and originally horizontal Proterozoic diabase sheets. On the down-dip side of the corridor in Arizona, the faults root under the unbroken Hualapai Mountains and the Colorado Plateau. Slip on faults at all exposed levels of the crust was unidirectional. Brittle thinning above these faults affected the entire upper crust, and wholly removed it locally along the central corridor or core complex region. Isostatic uplift exposed metamorphic core complexes in the domed footwall. These data support a model that the crust in California moved out from under Arizona along an asymmetric, rooted normal-slip shear system. Ductile deformation must have accompanied mid-Tertiary crustal extension at deeper structural levels in Arizona.

  7. Constraints on the viscosity of the continental crust and mantle from GPS measurements and postseismic deformation models in western Mongolia

    USGS Publications Warehouse

    Vergnolle, M.; Pollitz, F.; Calais, E.

    2003-01-01

    We use GPS measurements and models of postseismic deformation caused by seven M6.8 to 8.4 earthquakes that occurred in the past 100 years in Mongolia to assess the viscosity of the lower crust and upper mantle. We find an upper mantle viscosity between 1 ?? 1018 and 4 ?? 1018 Pa s. The presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath Mongolia. The viscosity of the lower crust is less well constrained, but a weak lower crust (3 ?? 1016 to 2 ?? 1017 Pa s) is preferred by the data. Using our best fit upper mantle and lower crust viscosities, we find that the postseismic effects of viscoelastic relaxation on present-day horizontal GPS velocities are small (<2 mm yr-1) but still persist 100 years after the 1905, M8.4, Bolnay earthquake. This study shows that the GPS velocity field in the Baikal-Mongolia area can be modeled as the sum of (1) a rigid translation and rotation of the whole network, (2) a 3-5 mm yr-1 simple shear velocity gradient between the Siberian platform to the north and northern China to the south, and (3) the contribution of postseismic deformation, mostly caused by the 1905 Bolnay-Tsetserleg sequence and by the smaller, but more recent, 1957 Bogd earthquake. Copyright 2003 by the American Geophysical Union.

  8. Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth

    NASA Technical Reports Server (NTRS)

    Rapp, R. P.

    1994-01-01

    Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and

  9. Two-stage exhumation of subducted Saxothuringian continental crust records underplating in the subduction channel and collisional forced folding (Krkonoše-Jizera Mts., Bohemian Massif)

    NASA Astrophysics Data System (ADS)

    Jeřábek, Petr; Konopásek, Jiří; Žáčková, Eliška

    2016-08-01

    The Krkonoše-Jizera Massif in the northern part of the Variscan Bohemian Massif provides insight into the exhumation mechanisms for subducted continental crust. The studied region exposes a relatively large portion of a flat-lying subduction-related complex that extends approximately 50 km away from the paleosuture. wide extent of HP-LT metamorphism has been confirmed by new P-T estimates indicating temperatures of 400-450 °C at 14-16 kbar and 450-520 °C at 14-18 kbar for the easternmost and westernmost parts of the studied area, respectively. A detailed study of metamorphic assemblages associated with individual deformation fabrics together with analysis of quartz deformation microstructures and textures allowed characterisation of the observed deformation structures in terms of their subduction-exhumation memory. An integration of the lithostratigraphic, metamorphic and structural data documents a subduction of distal and proximal parts of the Saxothuringian passive margin to high-pressure conditions and their subsequent exhumation during two distinct stages. The initial stage of exhumation has an adiabatic character interpreted as the buoyancy driven return of continental material from the subduction channel resulting in underplating and progressive nappe stacking at the base of the Teplá-Barrandian upper plate. With the transition from continental subduction to continental collision during later stages of the convergence, the underplated high-pressure rocks were further exhumed due to shortening in the accretionary wedge. This shortening is associated with the formation of large-scale recumbent forced folds extending across the entire studied area.

  10. Comment on "Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga".

    PubMed

    Valley, John W; Cavosie, Aaron J; Fu, Bin; Peck, William H; Wilde, Simon A

    2006-05-26

    Harrison et al. (Reports, 23 December 2005, p. 1947) proposed that plate tectonics and granites existed 4.5 billion years ago (Ga), within 70 million years of Earth's formation, based on geochemistry of >4.0 Ga detrital zircons from Australia. We highlight the large uncertainties of this claim and make the more moderate proposal that some crust formed by 4.4 Ga and oceans formed by 4.2 Ga.

  11. Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block.

    PubMed

    Zuo, Xuran; Chan, Lung Sang; Gao, Jian-Feng

    2017-01-01

    The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab.

  12. Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block

    PubMed Central

    Chan, Lung Sang; Gao, Jian-Feng

    2017-01-01

    The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab. PMID:28182640

  13. Continental hyperextension, mantle exhumation, and thin oceanic crust at the continent-ocean transition, West Iberia: New insights from wide-angle seismic

    NASA Astrophysics Data System (ADS)

    Davy, R. G.; Minshull, T. A.; Bayrakci, G.; Bull, J. M.; Klaeschen, D.; Papenberg, C.; Reston, T. J.; Sawyer, D. S.; Zelt, C. A.

    2016-05-01

    Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0-7.0 km s-1 velocity contours, corresponding to peridotite serpentinization of 60-30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8-3.5 km, with an average velocity gradient of 1.0 s-1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

  14. Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy

    NASA Astrophysics Data System (ADS)

    Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

    2006-12-01

    Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural

  15. 87Sr/86Sr Ratios in Carbonate From the Red Lake and Steep Rock Groups in Canada Suggest Rb-enriched Continental Crust was Influencing Seawater Chemistry Prior to 3.0 Ga

    NASA Astrophysics Data System (ADS)

    Satkoski, A.; Fralick, P. W.; Beard, B. L.; Johnson, C.

    2015-12-01

    Previous work has suggested that prior to 2.5 Ga, Sr isotope compositions of seawater were essentially mantle buffered and the effects of continental weathering on seawater chemistry were negligible. To test this, we collected Sr isotope data from 2.93 and 2.80 Ga carbonates that are part of the Red Lake and Steep Rock groups (Canada), respectively. To better understand carbonate formation and any post-depositional alteration, Sr isotopes are considered with O isotopes and REEs, as well as Rb and Sr contents, including correction for decay of 87Rb. All samples have Y/Ho ratios higher than chondrite and have positive La anomalies, which, combined with low Rb contents suggests that clastic contamination is negligible. Samples we consider near pristine have δ18O (VSMOW) values >20‰. Samples with the highest Y/Ho ratios and largest La anomalies from Red Lake and Steep Rock have initial 87Sr/86Sr ratios of 0.7018-0.7020. This Sr isotope composition is significantly more radiogenic than contemporaneous mantle (0.7011-0.7012), especially at a time when the isotopic difference between the crust and mantle was much less than today. This implies that radiogenic continental crust was emergent and shed detritus into the world's oceans prior to 3.0 Ga, in contrast with proposals for submerged continental crust, but in line with new estimates that continental crust at 3.0 Ga was 60-70% of current volume. We contend that this large amount of crust combined with enhanced Archean weathering could account for the radiogenic Sr isotope compositions we report here, and suggests a significant impact from continental weathering on ocean chemistry during the Archean.

  16. Continental growth through time by underplating of subducted oceanic crust: Evidence from kimberlites in South Africa and SW Pacific

    NASA Technical Reports Server (NTRS)

    Taylor, Lawrence A.; Neal, Clive R.

    1988-01-01

    In the dynamic model of plate tectonics, it is evident that crustal components are returned to the mantle by subduction. Chemical signatures of these subducted components were identified in ocean island volcanics and in island arc volcanics. Indeed, an origin involving a subducted protolith was postulated for certain types of xenoliths in kimberlite, including diamonds. Recent studies of eclogite xenoliths in kimberlite from southern Africa and megacrysts form the Malaitan alnoite, Solomon islands, indicate that lithospheric underplating by subducted oceanic crust has occurred in these two contrasting areas. The results of new eclogite studies from the Bellsbank kimberlite, South Africa, and isotopic data from the Malaitan alnoite megacryst suite. This forms the basis for discerning the role of lithospheric underplating in the growth of cratons and in the evolution of mantle-derived magma.

  17. Anisotropy in the subducted oceanic crust and the overlying continental crust explain the existence of a double tectonic tremor zone in the flat portion of the Mexican subduction zone.

    NASA Astrophysics Data System (ADS)

    Husker, A. L.; Castillo, J. A.; Perez-Campos, X.; Frank, W.; Kostoglodov, V.

    2015-12-01

    Tectonic tremor (TT) in Mexico has a complicated behavior due to the shape of the subducted plate. In the flat section the slab dives from the trench to a depth of 40 km at 150 km from the trench where it turns to be flat. It remains at 40 km depth till about 290 - 300 km from the trench where it continues to steeply dive into the mantle. All TT activity is within the flat slab section. An LFE catalog and the vertically averaged shear wave anisotropy observed from receiver functions at the slab interface are used to divide the region into 4 zones. (1) The Transient Zone located at the corner of the slab when it first arrives at 40 km depth (~130 km - 165 km from the trench) where the majority of LFE's are seen in small bursts that produce TT. (2) The Buffer Zone has almost no LFE and is located ~165 km - 190 km from the trench. (3) The sub-Sweet Spot is located ~190 - 204 km from the trench and seems to share many characteristics of the Sweet Spot, but has less than half the LFE activity observed in the Sweet Spot in addition to different anisotropy. (4) The Sweet Spot has the overwhelming majority of LFE and is located ~204 km - 245 km from the trench. No LFE is found from 245 km to 300 km from the trench despite the plate still being at 40 km depth. The anisotropy percentage in the continental crust drops significantly above the Transient Zone and Sweet Spot suggesting the crust acts as a seal in those two zones permitting trapped fluids to generate TT/LFE activity there as has been observed in other zones. The Buffer Zone coincides with a region of high fluid flow in the crust (Jodicke et al., 2005) suggesting that there is no seal in this zone allowing fluids to escape thereby limiting TT/LFE generation. The convergence of the zone would imply that the anisotropy preferred orientation at the plate interface should be perpendicular to the trench as much of it is. However, the fast azimuth direction rotates to be trench parallel in the region of the large SSE

  18. An archean suture zone in the Tobacco Root Mountains? (1984) Evolution of Archean Continental Crust, SW Montana (1985)

    NASA Technical Reports Server (NTRS)

    Mogk, D. W.; Kain, L.

    1985-01-01

    The Lake Plateau area of the Beartooth Mountains, Montana were mapped and geochemically sampled. The allochthonous nature of the Stillwater Complex was interpreted as a Cordilleran-style continental margin. The metamorphic and tectonic history of the Beartooth Mountains was addressed. The Archean geology of the Spanish Peaks area, northern Madison Range was addressed. A voluminous granulite terrain of supracrustal origin was identified, as well as a heretofore unknown Archean batholithic complex. Mapping, petrologic, and geochemical investigations of the Blacktail Mountains, on the western margin of the Wyoming Province, are completed. Mapping at a scale of 1:24000 in the Archean rocks of the Gravelly Range is near completion. This sequence is dominantly of stable-platform origin. Samples were collected for geothermometric/barometric analysis and for U-Pb zircon age dating. The analyses provide the basis for additional geochemical and geochronologic studies. A model for the tectonic and geochemical evolution of the Archean basement of SW Montana is presented.

  19. Accessory Mineral Depth-Profiling Applied to the Corsican Lower Crust: A Continuous Thermal History of Mesozoic Continental Rifting

    NASA Astrophysics Data System (ADS)

    Seymour, N. M.; Stockli, D. F.; Beltrando, M.; Smye, A.

    2015-12-01

    Despite advances in understanding the structural development of hyperextended magma-poor rift margins, the temporal and thermal evolution of lithospheric hyperextension during rifting remains only poorly understood. In contrast to classic pure-shear models, multi-stage rift models that include depth-dependent thinning predict significant lower-crustal reheating during the necking phase due to buoyant rise of the asthenosphere. The Santa Lucia nappe of NE Corsica is an ideal laboratory to test for lower-crustal reheating as it preserves Permian lower crust exhumed from granulitic conditions during Mesozoic Tethyan rifting. This study presents the first use of apatite U-Pb depth-profile thermochronology in conjunction with novel rutile U-Pb and zircon U-Pb thermo- and geochronology to reconstruct a continuous t-T path to constrain the syn-rift thermal evolution of this exposed lower-crustal section. LASS-ICP-MS depth-profile analyses of zircon reveal thin (<10 μm) ~210-180 Ma overgrowths on 300-270 Ma cores in lower-crustal lithologies, indicative of renewed thermal activity during Mesozoic rifting. Cooling due to rapid rift margin exhumation is recorded by the topology of rutile and apatite depth profiles caused by thermally-activated volume diffusion at T >400°C. Lower-crustal rutile reveal a rounded progression from core plateaus at ~170 Ma to 150-145 Ma at the outer 8-10 μm of grains while middle-crustal apatite records 170 Ma cores grading to 140-135 Ma rims. Inverse modeling of rutile profiles suggests the lower crust cooled from 700°C at 200 Ma to 425°C at 140 Ma. Middle-crustal apatite yield a two-stage history, with rapid cooling from 500°C at 200 Ma to 420°C at ~180 Ma followed by slow cooling to 400°C by 160 Ma. Combined with zircon overgrowth ages, these data indicate the Santa Lucia nappe underwent a thermal pulse in the late Triassic-early Jurassic associated with depth-dependent thinning and hyperextension of the Corsican margin.

  20. Limited Flow of Continental Crust at UHP Depths: Coupled Age and Trace-Element Analyses of Titanite in the Western Gneiss Region, Norway

    NASA Astrophysics Data System (ADS)

    Garber, J. M.; Hacker, B. R.; Kylander-Clark, A. R.

    2015-12-01

    Coupled age and trace-element data from titanites in the Western Gneiss Region (WGR) of Norway suggest that continental crust underwent limited recrystallization and ductile flow through ~40 My of deep subduction and subsequent exhumation. Precambrian igneous titanites in granitic to tonalitic orthogneisses from the WGR were metastably preserved though Caledonian ultrahigh-pressure (UHP) metamorphism and variably recrystallized through subsequent amphibolite-facies metamorphism from ~420-385 Ma. The inherited Precambrian titanites are not present everywhere but rather cluster primarily in a cooler "southern domain" (peak T ~650oC) and a hotter "northern domain" (peak T ~750-800oC).Titanite data were collected using LASS (laser-ablation split stream inductively-coupled plasma mass spectrometry) at UCSB, and a principal component analysis (PCA) was used to define age and trace-element populations. These data indicate that inherited titanites are LREE-enriched, HFSE-enriched, and have higher Th/U, consistent with Precambrian neocrystallization from a granitic melt. In contrast, the recrystallized titanites have generally lower Th/U and flat, LREE-depleted, or hump-shaped trace-element patterns. These data suggest that (1) Caledonian titanite recrystallization occurred in the presence of LREE-depleted melts or fluids, or that (2) recrystallization was accompanied by a "typical" granitic melt, but that titanite/bulk-rock distribution coefficients are different for neo- and recrystallization; on-going whole-rock analyses will clarify these hypotheses. Critically, the geochemical signature of recrystallized titanite in felsic orthogneisses is comparable across the entire WGR - emphasizing that the petrologic process of titanite recrystallization was similar orogen-wide, but was less extensive in the domains where inherited titanite was preserved. In this case, large volumes of crust outside of the "old domains" may also have retained metastable titanite during subduction

  1. Destabilization of a 650 km chemical boundary layer and its bearing on the evolution of the continental crust

    NASA Technical Reports Server (NTRS)

    Herzberg, C. T.; Forsythe, R. D.

    1983-01-01

    A mechanism for the production of a chemical change in the mantle, from primordial silicate compositions above the 650 km discontinuity to differentiated compositions below, is reviewed. Some consequences of this are the stabilization of two layer convection with a temperature contrast between the anhydrous mantle solidus and the geotherm which, at 650 km depth, is lower than any other location in the mantle. With thermal contributions from the concentration of the heat producing elements U, Th and K below the 650 km mantle boundary layer and the higher geotherms in the past, widespread or catastrophic melting may have taken place at this location. An episodic breach of this boundary layer by extensive heat and mass transport may have periodically destroyed any simple two-layer convection geometry in the mantle. Such episodic injections of mass and energy into the upper mantle from below may have been the mechanism responsible for episodes of enhanced surface tectonism and thermal activity which appear to be recorded in apparent polar wandering paths and radiometric ages of continental rocks.

  2. High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the north Pyrenean paleo-passive margin

    NASA Astrophysics Data System (ADS)

    Clerc, C.; Lahfid, A.; Monié, P.; Lagabrielle, Y.; Chopin, C.; Poujol, M.; Boulvais, P.; Ringenbach, J.-C.; Masini, E.; de St Blanquat, M.

    2015-02-01

    An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a dataset of more than one hundred peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This dataset is completed by previous PT estimates based on mineral assemblages, and new Ar-Ar (amphibole, micas) and U-Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key-localities of the Mauléon Basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyse the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps, and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.

  3. Ancient plate kinematics derived from the deformation pattern of continental crust: Paleo- and Neo-Tethys opening coeval with prolonged Gondwana-Laurussia convergence

    NASA Astrophysics Data System (ADS)

    Kroner, Uwe; Roscher, Marco; Romer, Rolf L.

    2016-06-01

    The formation and destruction of supercontinents requires prolonged convergent tectonics between particular plates, followed by intra-continental extension during subsequent breakup stages. A specific feature of the Late Paleozoic supercontinent Pangea is the prolonged and diachronous formation of the collisional belts of the Rheic suture zone coeval with recurrent continental breakup and subsequent formation of the mid-ocean ridge systems of the Paleo- and Neo-Tethys oceans at the Devonian and Permian margins of the Gondwana plate, respectively. To decide whether these processes are causally related or not, it is necessary to accurately reconstruct the plate motion of Gondwana relative to Laurussia. Here we propose that the strain pattern preserved in the continental crust can be used for the reconstruction of ancient plate kinematics. We present Euler pole locations for the three fundamental stages of the Late Paleozoic assembly of Pangea and closure of the Rheic Ocean: (I) Early Devonian (ca. 400 Ma) collisional tectonics affected Gondwana at the Armorican Spur north of western Africa and at the promontory of the South China block/Australia of eastern Gondwana, resulting in the Variscan and the Qinling orogenies, respectively. The Euler pole of the rotational axis between Gondwana and Laurussia is positioned east of Gondwana close to Australia. (II) Continued subduction of the western Rheic Ocean initiates the clockwise rotation of Gondwana that is responsible for the separation of the South China block from Gondwana and the opening of Paleo-Tethys during the Late Devonian. The position of the rotational axis north of Africa reveals a shift of the Euler pole to the west. (III) The terminal closure of the Rheic Ocean resulted in the final tectonics of the Alleghanides, the Mauritanides and the Ouachita-Sonora-Marathon belt, occurred after the cessation of the Variscan orogeny in Central Europe, and is coeval with the formation of the Central European Extensional

  4. Reply to comment by J.D. Clemens and G. Stevens on 'Water-fluxed melting of the continental crust: A review'

    NASA Astrophysics Data System (ADS)

    Weinberg, Roberto F.; Hasalová, Pavlína

    2015-10-01

    Airport halls and kitchen benches everywhere attest to the importance of water-fluxed melting. They commonly show migmatites lacking anhydrous peritectic minerals, as is expected from the reactions listed in Table 2 in Weinberg and Hasalová (2015), W&H henceforth. The question we should ask is not whether it is a significant process but how and when it happens. We start this response by clarifying that in W&H, we never meant to imply that water-fluxed melting is the dominant melting process, but that it is widespread and relevant to understanding the evolution of the continental crust. The aim of our paper was to review the many reported cases in which water-fluxed melting has been inferred, to draw attention to its common occurrence, to suggest how this may take place and discuss its possible role in crustal evolution. In order to avoid confusion, in this reply we refer consistently to their Fig. 1, referring to Fig. 1 in Clemens and Stevens (2015), or to our Fig. 1 shown here.

  5. Fluid pressure and flow at great depth in the continental crust. A discussion in relation to topography, temperature and salinity distribution using as an example the KTB Fault Zones in connection with the Eger Rift Hot Spot.

    NASA Astrophysics Data System (ADS)

    Kessels, W.; Kuhlmann, S.; Li, X.

    2006-12-01

    Hydraulic investigations in and between the two KTB boreholes have shown that groundwater flow is possible at great depth in the crystalline crust. Remarkable permeability was found particularly in the SE1 and SE2 fault zones. The results from a long term pump and injection test, and the related three-dimensional groundwater modelling (Graesle et al., 2006), document the existence of a large-scale (more than 10 km) hydraulic reservoir in the crystalline crust. According to this calculation, an overpressure of 0.4 MPa can be still be expected in KTB-HB in 2009, 4 years after the end of the injection. The good match with the measurement data confirms groundwater pathways at a scale of more than 10 km. The isotopic water composition recovered from the KTB pilot hole indicates a downward water flow along the SE2 fault zone, which is in contact with the Franconian Line. Moreover, there is a deep upward groundwater flow 60 km away in the western Eger Rift Valley as indicated e.g. by the temperature signature and gas flow observations. Therefore, the demand for fluid mass continuity means that water is being supplied by a downstream groundwater flow, probably from the Franconian Line. The question of potential driving processes must be answered to understand and quantify the flow in the deeper crust at a scale of 10 km to 100 km. The processes must result in a sufficient horizontal pressure gradient to allow groundwater flow at great depth. The density variations of groundwater with depth are highly relevant for the calculation of horizontal pressure differences. The two independent potential fields of gravity and pressure have to be considered. Differentiation into 4 relevant driving processes is required: \\bullet The groundwater surface topography related to the groundwater recharge and mean regional distance between neighbouring valleys \\bullet Geothermal gradient and water density depending on temperature and pressure \\bullet Different salt contents in adjacent

  6. Crust structure of the Northern Margin of North China Craton and adjacent region from Sinoprobe-02 North China seismic WAR/R experiment

    NASA Astrophysics Data System (ADS)

    Li, W.; Gao, R.; Keller, G. R.; Li, Q.; Cox, C. M.; Hou, H.; Guan, Y.

    2011-12-01

    The Central Asian Orogen Belt (CAOB) or Altaids, situated between the Siberian craton(SC) to the north and north China craton (NCC) with tarim to the south, is one of the world's largest accretionary orogens formed by subduction and accretion of juvenile material from the Neoproterozoic through the Paleozoic. The NCC is the oldest craton in China, which suffered Yanshan intercontinental orogenic process and lithosphere thinning in Mesozoic. In the past 20 years, remarkable studies about this region have been carried out and different tectonic models were proposed, however, some crucial geologic problems remain controversial. In order to obtain better knowledge of deep structure and properties of crust on the northern margin of north China craton, a 450 km long WAR/R section was completed jointly by Institute of Geology, CAGS and University of Oklahoma. Our 450 km long NW-SE WAR/R line extends from west end of the Yanshan orogen, across the Bainaimiao arc, Ondor sum subduction accretion complex to the Solonker suture zone. The recording of seismic waves from 8 explorations was conducted in 4 deployments of 300 reftek-125A records and single-channel 4.5Hz geophones with station spacing of 1km. The shooting procedure was employ 500 or 1500kg explosives in 4-5 or 15-23 boreholes at 40-45m depth. The sampling rate was 100 HZ, and recording time window was 1200s. The P wave field on the sections got high quality data for most part of the profile, but have low signal-to-noise for the south end, where closed to Beijing with a lot of ambient noise from traffic, industry and human activity. Arrivals from of refracted and reflected waves from sediments and basement (Pg), intracrust (Pcp, Plp) and Moho (Pmp) were typically observed, but Pn phase through the upper most mantle was only observed for 2 shots. Identification and correlation of seismic phases was done manually on computer screen Zplot software. Each trace has been bandpass filtered (1-20Hz) and normalized with AGC

  7. Habitat use and preferences of cetaceans along the continental slope and the adjacent pelagic waters in the western Ligurian Sea

    NASA Astrophysics Data System (ADS)

    Azzellino, A.; Gaspari, S.; Airoldi, S.; Nani, B.

    2008-03-01

    The physical habitat of cetaceans occurring along the continental slope in the western Ligurian Sea was investigated. Data were collected from two different sighting platforms, one of the two being a whale-watching boat. Surveys, conducted from May to October and from 1996 to 2000, covered an area of approximately 3000 km 2 with a mean effort of about 10,000 km year -1. A total of 814 sightings was reported, including all the species occurring in the area: Stenella coeruleoalba, Balaenoptera physalus, Physeter macrocephalus, Globicephala melas, Grampus griseus, Ziphius cavirostris, Tursiops truncatus, Delphinus delphis. A Geographic Information System was used to integrate sighting data to a set of environmental characteristics, which included bottom gradient, area between different isobaths, and length and linearity of the isobaths within a cell unit. Habitat use was analysed by means of a multi-dimensional scaling, MDS, analysis. Significant differences were found in the habitat preference of most of the species regularly occurring in the area. Bottlenose dolphin, Risso's dolphin, sperm whale and Cuvier's beaked whale were found strongly associated to well-defined depth and slope gradient characteristics of the shelf-edge and the upper and lower slope. The hypothesis of habitat segregation was considered for Risso's dolphin, sperm whale and Cuvier's beaked whale. Canonical discriminant functions using depth and slope as predictors outlined clear and not overlapping habitat preferences for Risso's dolphin and Cuvier's beaked whale, whereas a partial overlapping of the habitat of the other two species was observed for sperm whale. Such a partitioning of the upper and lower slope area may be the result of the common feeding habits and suggests a possible competition of these three species. A temporal segregation in the use of the slope area was also observed for sperm whales and Risso's dolphins. Fin whales, and the occasionally encountered common dolphin and long

  8. Generation of continental crust in the northern part of the Borborema Province, northeastern Brazil, from Archaean to Neoproterozoic

    NASA Astrophysics Data System (ADS)

    de Souza, Zorano Sérgio; Kalsbeek, Feiko; Deng, Xiao-Dong; Frei, Robert; Kokfelt, Thomas Find; Dantas, Elton Luiz; Li, Jian-Wei; Pimentel, Márcio Martins; Galindo, Antonio Carlos

    2016-07-01

    This work deals with the origin and evolution of the magmatic rocks in the area north of the Patos Lineament in the Borborema Province (BP). This northeastern segment of NE Brazil is composed of at least six different tectonic blocks with ages varying from late-Archaean to late-Palaeoproterozoic. Archaean rocks cover ca. 5% of the region. They were emplaced over a period of 700 Ma, with at least seven events of magma generation, at 3.41, 3.36, 3.25, 3.18, 3.12, 3.03, and 2.69 Ga. The rocks are subalkaline to slightly alkaline, with affinity to I- and M-type magmas; they follow trondhjemitic or potassium calc-alkaline differentiation trends. They have epsilon Nd(t) of +1.4 to -4.2 and negative anomalies for Ta-Nb, P and Ti, consistent with a convergent tectonic setting. Both subducted oceanic crust and upper mantle (depleted or metasomatised) served as sources of the magmas. After a time lapse of about 350 m y., large-scale emplacement of Paleoproterozoic units took place. These rocks cover about 50% of the region. Their geochemistry indicates juvenile magmatism with a minor contribution from crustal sources. These rocks also exhibit potassic calc-alkaline differentiation trends, again akin to I- and M-type magmas, and show negative anomalies for Ta-Nb, Ti and P. Depleted and metasomatised mantle, resulting from interaction with adakitic or trondhjemitic melts in a subduction zone setting, is interpreted to be the main source of the magmas, predominanting over crustal recycling. U-Pb ages indicate generation of plutonic rocks at 2.24-2.22 Ga (in some places at about 2.4-2.3 Ga) and 2.13-2.11 Ga, and andesitic volcanism at 2.15 Ga. Isotopic evidence indicates juvenile magmatism (epsilon Nd(t) of +2.9 to -2.9). After a time lapse of about 200 m y. a period of within-plate magmatic activity followed, with acidic volcanism (1.79 Ga) in Orós, granitic plutonism (1.74 Ga) in the Seridó region, anorthosites (1.70 Ga) and A-type granites (1.6 Ga) in the Transverse Zone

  9. Devonian granitoids and their hosted mafic enclaves in the Gorny Altai terrane, northwestern Central Asian Orogenic Belt: crust-mantle interaction in a continental arc setting

    NASA Astrophysics Data System (ADS)

    Chen, Ming; Sun, Min

    2016-04-01

    Granitoids are a major component in the upper continental crust and hold key information on how did the continental crust grow and differentiate. This study focuses on the Yaloman intrusive complex from the Gorny Altai terrane, northwestern Central Asian Orogenic Belt (CAOB). The association of granitoids and mafic enclaves can provide important clues on the source nature, petrogenetic processes and geodynamic setting of the Yaloman intrusive complex, which in turn will shed light on the crustal evolution in the northwestern CAOB. Zircon U-Pb dating shows that the granitoids, including quartz diorites and granodiorites, were emplaced in ca. 389-387 Ma. The moderate Na2O + K2O contents and low A/CNK values indicate that these rocks belong to the sub-alkaline series with metaluminous to weakly peraluminous compositions. The granitoids yield two-stage zircon Hf model ages of ca. 0.79-1.07 Ga and whole-rock Nd model ages of ca. 0.90-0.99 Ga, respectively, implying that they were mainly sourced from Neoproterozoic juvenile crustal materials. The mafic enclaves show an almost identical crystallization age of ca. 389 Ma. The identification of coarse-grained xenocrysts and acicular apatites, together with the fine-grained texture, makes us infer that these enclaves are likely to represent magmatic globules commingled with the host magmas. The low SiO2 and high MgO contents of the mafic enclaves further suggest that substantial mantle-derived mafic melts were probably involved in their formation. Importantly, the SiO2 contents of the granitoids and mafic enclaves are well correlated with other major elements and most of the trace elements. Also a broadly negative correlation exists between the SiO2 contents and whole-rock epsilon Nd (390 Ma) values of the granitoids. Given the observation of reversely zoned plagioclases within the granitoids and the common occurrence of igneous mafic enclaves, we propose that magma mixing probably played an important role in the formation

  10. Timing and mechanisms of mafic magma ascent/emplacement in the continental middle crust: an example from the Permian Sondalo gabbroic complex (Alps, N-Italy)

    NASA Astrophysics Data System (ADS)

    Petri, Benoît; Mohn, Geoffroy; Skrzypek, Etienne; Mateeva, Tsvetomila; Galster, Federico; Robion, Philippe; Schulmann, Karel; Manatschal, Gianreto; Müntener, Othmar

    2015-04-01

    We explore the mechanisms of mafic magma ascent and emplacement in the continental crust by studying the mid-crustal Permian Sondalo gabbroic complex (Campo unit, Eastern Central Alps, N-Italy). We characterized the structure and anisotropy of magnetic susceptibility (AMS) fabric of the concentric gabbroic to dioritic intrusions. We used Laser Ablation ICP-MS U-Pb zircon dating on magmatic and metamorphic rock samples, zircon trace element geochemistry and existing P-T estimates to constrain the timing and depth of magma emplacement. Petrological and geochemical observations provide insights on the crystallization sequence in the magmatic rocks and facilitate the interpretation of the AMS record. The magmatic and magnetic fabrics (foliations and lineations) of the pluton reflect their original orientations and are essentially vertical, indicating vertical magma transfer through the crust. The intrusion was emplaced in two phases. (1) The concordant orientation between the main magmatic foliation and the host-rock xenoliths elongation and foliation in the centre of the pluton suggest that the first magma ascent phase occurred through fracture opening parallel to the vertical fabric of the host metasedimentary rocks. Trace element analyses point to late-magmatic zircon crystallization, which enable to interpret the associated U-Pb results of 289-288 Ma as the age of this initial emplacement stage. (2) The second magma ascent phase is marked by a rheological change in the host-rock. The temperature increase in the contact aureole induced partial melting and decreasing mechanical strength in the metasediments. This resulted in the formation of a vertical foliation in the metamorphic aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase occurred at 288-285 Ma and accounts for the contrasted P-T evolution of metasedimentary rocks in the contact aureole. Thermal models of the intrusion indicate that the contact aureole

  11. Continental magnetic anomaly constraints on continental reconstruction

    NASA Technical Reports Server (NTRS)

    Vonfrese, R. R. B.; Hinze, W. J.; Olivier, R.; Bentley, C. R.

    1985-01-01

    Crustal magnetic anomalies mapped by the MAGSAT satellite for North and South America, Europe, Africa, India, Australia and Antarctica and adjacent marine areas were adjusted to a common elevation of 400 km and differentially reduced to the radial pole of intensity 60,000 nT. These radially polarized anomalies are normalized for differential inclination, declination and intensity effects of the geomagnetic field, so that in principle they directly reflected the geometric and magnetic polarization attributes of sources which include regional petrologic variations of the crust and upper mantle, and crustal thickness and thermal perturbations. Continental anomalies demonstrate remarkably detailed correlation of regional magnetic sources across rifted margins when plotted on a reconstruction of Pangea. Accordingly, they suggest further fundamental constraints on the geologic evolution of the continents and their reconstructions.

  12. Platinum-group element abundances and Re-Os isotopic systematics of the upper continental crust through time: Evidence from glacial diamictites

    NASA Astrophysics Data System (ADS)

    Chen, Kang; Walker, Richard J.; Rudnick, Roberta L.; Gao, Shan; Gaschnig, Richard M.; Puchtel, Igor S.; Tang, Ming; Hu, Zhao-Chu

    2016-10-01

    The fine-grained matrix of glacial diamictites, deposited periodically by continental ice sheets over much of Earth history, provides insights into the average composition and chemical evolution of the upper continental crust (UCC) (Gaschnig et al., 2016, and references therein). The concentrations of platinum-group elements (PGEs, including Os, Ir, Ru, Pt and Pd) and the geochemically related Re, as well as 187Re/188Os and 187Os/188Os ratios, are reported here for globally-distributed glacial diamictites that were deposited during the Mesoarchean, Paleoproterozoic, Neoproterozoic and Paleozoic eras. The medians and averages of PGE concentrations of these diamictite composites decrease from the Mesoarchean to the Neoproterozoic, mimicking decreases in the concentrations of first-row transition elements (Sc, V, Cr, Co and Ni). By contrast, Re concentrations are highly variable with no discernable trend, owing to its high solubility. Assuming these diamictites are representative of average UCC through time, the new data are fully consistent with the previous inference that the Archean UCC contained a greater proportion of mafic-ultramafic rocks relative to younger UCC. Linear regressions of PGEs versus Cr and Ni concentrations in all the diamictite composites from the four time periods are used to estimate the following concentrations of the PGEs in the present-day UCC: 0.059 ± 0.016 ng/g Os, 0.036 ± 0.008 ng/g Ir, 0.079 ± 0.026 ng/g Ru, 0.80 ± 0.22 ng/g Pt and 0.80 ± 0.26 ng/g Pd (2σ of 10,000 bootstrapping regression results). These PGE estimates are slightly higher than the estimates obtained from loess samples. We suggest this probably results from loess preferentially sampling younger UCC rocks that have lower PGE concentrations, or PGEs being fractionated during loess formation. A Re concentration of 0.25 ± 0.12 ng/g (2σ) is obtained from a regression of Re versus Mo. From this, time-integrated 187Re/188Os and 187Os/188Os ratios for the UCC are

  13. Sudbuction-related tectonic mixing between serpentinized mantle and continental crust in the internal Western Alps: exhumed portion of a marble cake?

    NASA Astrophysics Data System (ADS)

    Spalla, M.; Zucali, M.; Cantù, M.; Roda, M.; Marotta, A.

    2011-12-01

    The pre-Alpine continental crust of the Western Alps is widely affected by subduction-related high-pressure low-temperature (HP-LT) metamorphism and the Sesia-Lanzo Zone (SLZ) represents the widest crustal complex re-equilibrated under HP conditions during Late-Cretaceous (early-Alpine) times. Its Alpine tectonic evolution is compatible with an uplift during active oceanic lithosphere subduction (e.g. Spalla et al., 1996; Meda et al., 2010; Zucali & Spalla, 2011). In the SLZ three main lithologic complexes have been recognized (e.g. Compagnoni et al., 1977): i) Gneiss Minuti complex (GMC); ii) Eclogitic Micaschists complex (EMC); iii) II Dioritic-Kinzigitic Zone (IIDK). IIDK consists of kilometer-size lenses of pelitic and mafic granulites that escaped eclogite facies re-equilibration. EMC and GMC, both pervasively eclogitised, strongly differ in the volume percentage of greenschist facies re-equilibration (Stuenitz, 1989; Spalla et al., 1991). In the southern SLZ the metamorphic complex of Rocca Canavese thrust sheets (RCT; Pognante, 1989 a; Pognante, 1989b) has been recognised, on the ground of its strongly contrasted Alpine metamorphic evolution, characterized by a P-climax recorded under very LT conditions. A new multi-scale structural analysis, performed to acquire new detailed petrologic data and to refine the tectonic trajectory of such a peculiar metamorphic complex, pointed out that RCT is a tectonic mixing of serpentinized mantle and continental rocks deformed together under high pressure conditions. Field analysis reports a superposed grid of metamorphic foliations allowing regional scale correlation of structural and metamorphic stages, after micro-structural identification of mineral assemblages marking successive fabrics in each rock type. Chemical data on variations of mineral compositions in different micro-structural sites, led to the reconstruction of a PTdt (Pressure-Temperature-relative deformation time) path and opened therefore the possibility

  14. Geophysical study of the crust and upper mantle beneath the central Rio Grande rift and adjacent Great Plains and Colorado Plateau

    SciTech Connect

    Ander, M.E.

    1981-03-01

    As part of the national hot dry rock (HDR) geothermal program conducted by Los Alamos Scientific Laboratory, a regional deep magnetotelluric (MT) survey of Arizona and New Mexico was performed. The main objective of the MT project was to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau, Basin and Range Province, and Rio Grande rift. Three MT profiles cross the Jemez lineament. Preliminary one-dimensional analysis of the data suggest the lineament is associated with anomalously high electrical conductivity very shallow in the crust. An MT/audiomagnetotelluric (AMT) study of a 161 km/sup 2/ HDR prospect was performed on the Zuni Indian Reservation, New Mexico. Two-dimensional gravity modeling of a 700-km gravity profile at 34/sup 0/30'N latitude was used to study the crust and upper mantle beneath the Rio Grande rift. Several models of each of three consecutive layers were produced using all available geologic and geophysical constraints. Two short-wavelength anomalies along the gravity profile were analyzed using linear optimization techniques.

  15. Boron isotopes in tourmaline from the ca. 3.7-3.8 Ga Isua supracrustal belt, Greenland: Sources for boron in Eoarchean continental crust and seawater

    NASA Astrophysics Data System (ADS)

    Grew, Edward S.; Dymek, Robert F.; De Hoog, Jan C. M.; Harley, Simon L.; Boak, Jeremy; Hazen, Robert M.; Yates, Martin G.

    2015-08-01

    Boron is highly concentrated in Earth's crust relative to primitive mantle. However, when present-day crustal concentrations were achieved remains debatable. It has been proposed that seawater boron δ11B was lower than at present, consistent with a model relating increase in sea-water δ11B to the proportion of B extracted from Earth's mantle into the oceans and crust. Our in situ ion microprobe analyses of tourmaline in 17 samples from the Eoarchean Isua supracrustal belt, Southwest Greenland, gave the following average δ11B with uncertainties ranging from ±0.4 to ±1.9‰: δ11B = -7.1 to -11.5‰ in amphibolite; δ11B = -10.5 to -25.3‰ in mica schist; δ11B = -19.2‰ in metachert (one sample), and δ11B = -21.9‰ in metaconglomerate (one sample). Tourmaline is largely schorl-dravite, rarely uvite-feruvite, and shows color and compositional zoning. δ11B varies from grain to grain in most samples; grains in a kyanite-staurolite schist are isotopically zoned, possibly because the rims incorporated B released by muscovite breakdown. The patterns in color-zoned tourmaline grains in our samples are not consistent with detrital origin of the cores, which rules out the possibility of there being tourmaline detritus from pre-existing continental crust in the studied samples. The tourmaline-bearing rocks are found in both the ca. 3700 Ma northern and ca. 3800 Ma southern terranes in the Isua supracrustal belt. Following an approach suggested by Chaussidon and Appel, we estimated Eoarchean seawater δ11B by calculating back from δ11B of tourmaline in metasedimentary rocks using fractionation of boron isotopes between clays and muscovite, tourmaline and aqueous fluid. This calculation gave an estimated δ11B ≈ +14 ± 15‰ for Eoarchean seawater, 25‰ lower than present-day seawater (δ11B = +39.5‰). For comparison, an estimate obtained simply by direct comparison of δ11B for Eoarchean and Phanerozoic tourmaline presumed to have crystallized in similar

  16. Mg-O isotopes trace the origin of Mg-rich fluids in the deeply subducted continental crust of Western Alps

    NASA Astrophysics Data System (ADS)

    Chen, Yi-Xiang; Schertl, Hans-Peter; Zheng, Yong-Fei; Huang, Fang; Zhou, Kun; Gong, Ying-Zeng

    2016-12-01

    Fluids are important for mass transfer at the slab-mantle interface in subduction zones. However, it is usually difficult to trace fluids from specific sources in a subducting slab, especially those derived from dehydration of serpentinite. Coesite-bearing whiteschist at Dora-Maira in the Western Alps is characterized by strong Mg enrichment relative to the country rocks, which requires infiltration of Mg-rich fluids into the supracrustal rock. In order to constrain the origin of such Mg-rich fluids, we have performed an integrated study of whole-rock Mg and O isotopes, zircon U-Pb ages and O isotopes for the whiteschist and related rocks. Zircons in the whiteschist show two groups of U-Pb ages at ∼262 Ma and ∼34 Ma, respectively, for relict and newly grown domains. The Permian U-Pb ages of relict magmatic domains are consistent with the protolith age of host metagranite, suggesting that their common protolith is the Permian granite. The Tertiary U-Pb ages occur in coesite-bearing metamorphic domains, consistent with the known age for ultrahigh-pressure metamorphism. The metamorphic domains have δ18O values of 5.8- 6.8 ‰, whereas the relict magmatic domains have high δ18O values of ∼ 10 ‰. Such high δ18O values are also characteristic of the metagranite, indicating that the whiteschist protolith underwent metasomatism by metamorphic fluids with low δ18O value of ∼ 2- 4 ‰. The whiteschist mostly has whole-rock δ26Mg values of -0.07 to 0.72‰, considerably higher than country-rock δ26Mg values of -0.54 to - 0.11 ‰. Thus, the metamorphic fluids are not only rich in Mg but also heavy in Mg isotopes. They were probably derived from the breakdown of Mg-rich hydrous minerals such as talc and antigorite in serpentinite at the slab-mantle interface in the subduction channel. Therefore, the dehydration of mantle wedge serpentinite during the subduction and exhumation of continental crust can provide the Mg-rich fluids responsible for the metasomatism

  17. Lu-Hf systematics of the ultra-high temperature Napier Complex, East Antarctica: evidence for the early Archean formation of continental crust

    NASA Astrophysics Data System (ADS)

    Choi, S.; Mukasa, S. B.; Andronikov, A. V.; Osanai, Y.; Harley, S. L.; Kelly, N. M.

    2009-12-01

    The Napier Complex in East Antarctica comprises some of the oldest rocks on earth (~3.8 billion years old), overprinted by an ultra-high temperature (UHT) metamorphic event near the Archean-Proterozoic boundary. Garnet, orthopyroxene, sapphirine, osumilite, rutile and a whole rock representing an equilibrated assemblage from this belt yield a Lu-Hf isochron age of 2,403 ± 43 Ma. Preservation of the UHT mineral assemblage in the rock analyzed suggests rapid cooling with closure likely to have occurred for the Lu-Hf system at post-peak UHT conditions near a temperature of ~800C. Individual zircon grains from Gage Ridge within the Napier Complex yielded a remarkably uniform range of 176Hf/177Hf values between 0.280433 ± 7 and 0.280505 ± 10, corresponding to ɛHf > +5.6 at 3.85 Ga relative to the chondritic uniform reservoir (CHUR). Because of their exceedingly low Lu/Hf values (<0.001), the grains are effectively recording the initial Hf isotope composition of the magmatic systems from which the gneiss protoliths crystallized. These results indicate that (1) the source of the crustal materials that formed the Napier Complex at 3.85 Ga were depleted relative to the CHUR. The extent of depletion involved is higher than has been predicted by extrapolation from the Lu-Hf isotopic evolution inferred for the source of Proterozoic and Phanerozoic basalts, judging from an fLu/Hf value of 0.51, (2) the depleted mantle reservoir has been in existence since very early in Earth’s history, in agreement with the early differentiation of the Earth that the latest core formation models require, and (3) an extremely depleted source also mean that the bulk of continental crust was extracted from the mantle by ~3.8 Ga. Moreover, the results demonstrate that even the oldest silicic rocks in the complex are not likely to have formed from remobilized older crustal materials, but were instead juvenile products of mantle melting. In addition, zircons with metamorphic rims have a similar

  18. Ultrasonic velocity drops and anisotropy reduction in mica-schist analogues due to melting with implications for seismic imaging of continental crust

    NASA Astrophysics Data System (ADS)

    Almqvist, Bjarne S. G.; Misra, Santanu; Klonowska, Iwona; Mainprice, David; Majka, Jarosław

    2015-09-01

    Melt generation in the continental crust is thought to significantly influence seismic bulk velocities and anisotropy, although existing laboratory data provide limited constraints on such seismic attributes. In this study we measured ultrasonic compressional wave speeds (Vp) and anisotropy during sample compaction, mica-breakdown and melt-generation in synthetic, foliated quartz-muscovite aggregates. Measurements were performed at peak conditions of 300 MPa hydrostatic confining pressure and 750 °C, over a six hour time period, for three separate samples where wave propagation directions were aligned at 0°, 45° and 90° with respect to the foliation plane. The experiments are initially marked by sample compaction and rapid reduction in porosity (from 25% to <2 vol%), with a corresponding increase in Vp. During initial stages of the experiment Vp ranges from 4.65 to 5.45 km/s, depending on the direction of the P wave propagation, resulting in up to ∼16% anisotropy, which originates mainly from the preferred orientation of muscovite and possible incipient melt lenses oriented parallel to foliation. As the experiment progresses the bulk Vp decreases gradually and Vp anisotropy drops from ∼16% to ∼2% after five hours into the experiment. The drop in anisotropy is caused by muscovite-breakdown and increasing amount of partial melting of the aggregate. The bulk velocities are considerably lower than predicted for a mixture of muscovite and β-quartz (stable at 300 MPa and 750 °C). Thermodynamic modeling indicates that mica breakdown and production of melt with ∼2 wt% dissolved water in the aggregate can explain the low Vp, even though β-quartz is stable; the modeled Vp is ∼4.75 km/s at 300 MPa and 750 °C. These ultrasonic attributes signify movement of the melt to fill vacant pores and coating grain boundaries, effectively connecting melt films and reducing seismic anisotropy. The results may apply to regions where crustal melting is on-going, in

  19. Fluid induced metamorphism and strength of the middle to lower continental crust - field and textural examples from Bergen Arcs, Western Norway

    NASA Astrophysics Data System (ADS)

    Austrheim, H.; Putnis, A.; Putnis, C. V.

    2011-12-01

    Fluids may change the rheology of the lithosphere both by changing the deformation mechanism of minerals and by inducing metamorphic reactions. In the present account the influence of fluid-induced metamorphic reactions on the mineralogical and structural evolution of a thickened continental crust is described from anorthositic granulites in the Lindås Nappe, Bergen Arcs, Norway, where the Grenvillian age (~930 My) granulites (T 800°C, P≤10kbar) are transformed to Caledonian age (~420My) eclogite (~650°C and ≤ 20kbar) and amphibolite facies assemblages. The anorthosite complex ranges in composition from pure anorthosite via gabbroic anorthosite to gabbro with lenses of peridotite and pyroxenites which allow us to study the mineral reactions and assess relative rock strength in a wide range of compositions. The complex is locally banded with up to meter thick garnet-pyroxene rich layers alternating with plagioclase rich layers. In other localities the granulite facies structure is defined by oriented disc-shaped corona textures in a plagioclase rich matrix. The eclogites (garnet, omphacite, amphibole, kyanite, white micas ± plagioclase) and amphibolites (plagioclase, hornblende, kyanite, and white micas) are formed along fluid pathways such as fractures and shear zones. Breccias, where rotated blocks of granulites are surrounded by anastamosing eclogite- and amphibolite facies shear zones, outcrop over areas of km2. Pseudotachylytes are developed in the granulites while the hydrated rocks in the shear zone respond by ductile deformation. A hierachy of rock strength can be inferred from these field observations. Notably the relict granulites form rotated angular blocks within the shear zones suggesting that granulites, independent of composition, are stronger than hydrous eclogitites and amphibolites. The garnet pyroxenite layer forms rigid blocks in eclogites suggesting that the mafic parts of the granulite complex must have been stronger than the

  20. Argon, oxygen, and boron isotopic evidence documenting 40ArE accumulation in phengite during water-rich high-pressure subduction metasomatism of continental crust

    NASA Astrophysics Data System (ADS)

    Menold, Carrie A.; Grove, Marty; Sievers, Natalie E.; Manning, Craig E.; Yin, An; Young, Edward D.; Ziegler, Karen

    2016-07-01

    were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40Ar/39Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40ArE concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low phengite/fluid partition coefficients for argon (Dphg/fluid Ar =10-3to 10-5) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40ArE partitions are able to accumulate within phengite. We alternatively propose that phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10-2 to allow phengites to accumulate large amounts of 40ArE from aqueous fluids under HP to UHP conditions.

  1. Mineral Dilution and Shallow Groundwater Dynamics as Motor to Drive Fluid Migration in the Deep Crystalline Crust - Interpretation of Hydraulic Investigations From the 9,101 m Super Deep German Continental Drillhole -

    NASA Astrophysics Data System (ADS)

    Kessels, W.; Graesle, W.

    2002-12-01

    . 633-635. KESSELS, W. and KšCK, J (1995): Hydraulic Communication in the Crystalline Rock Between the two Boreholes of the Continental Deep Drilling Programme in Germany, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 32, S. 37-47. M™LLER, P., WEISE, S., ALTHAUS, E., BACH, W., BEHR, H. J., BORCHARDT, R, BRŽUER, K., DRESCHER, J., ERZINGER, J., FABER, E., HORN, E., HUENGES, E., KŽMPF, W., KESSELS, W., KIRSTEN, T., LANDWEHR, D., LODEMANN, M., MACHON, L., PEKDEGER, A., PIELOW, H.-U., REUTEL, C., SIMON, K., WALTER, J., WEINLICH, F. H., ZIMMER, M. (1997): Paleo- and Recent Fluids in the Upper Continental Crust - Results from the German Continental Deep Drilling Projekt (KTB), Journal of Geophysical Resarch, 102, B8, S 18223 - 18254. HUENGES, E., ENGESER, B., ERZINGER, J., KESSELS, W., KšCK, J. (1997): The Permeable Crust: Geohydraulic Properties Down to 9000 m Depth - Results from the German Continental Deep Drilling Project (KTB), Journal of Geophysical Resarch, 102, B8, S 18 255 -18 265.

  2. Continental-Margin Processes Recorded in Shelf and Canyon Sediments. Sediment Deposition, Erosion and Accumulation on a Tidal Flat Adjacent to a River Mouth

    DTIC Science & Technology

    2007-01-01

    Sequence Stratigraphy , IAS Spec. Pub. 37, Blackwell Publishing, Oxford, 549 pp. (2007). C.A. Nittrouer, J.A. Austin, M.E. Field, J.H. Kravitz, J.P.M...Continental Margin Sedimentation: From Sediment Transport to Sequence Stratigraphy , IAS Spec. Pub. 37, Blackwell Publishing, Oxford, 1-48 (2007... Stratigraphy , IAS Spec. Pub. 37, Blackwell Publishing, Oxford, 157-212 (2007). L.F. Pratson, C.A. Nittrouer, P.L.Wiberg, M.S. Steckler, J.B. Swenson

  3. Drift of continental rafts with asymmetric heating.

    PubMed

    Knopoff, L; Poehls, K A; Smith, R C

    1972-06-02

    A laboratory model of a lithospheric raft is propelled through a viscous asthenospheric layer with constant velocity of scaled magnitude appropriate to continental drift. The propulsion is due to differential heat concentration in the model oceanic and continental crusts.

  4. A geodynamic model of the evolution of the Arctic basin and adjacent territories in the Mesozoic and Cenozoic and the outer limit of the Russian Continental Shelf

    NASA Astrophysics Data System (ADS)

    Laverov, N. P.; Lobkovsky, L. I.; Kononov, M. V.; Dobretsov, N. L.; Vernikovsky, V. A.; Sokolov, S. D.; Shipilov, E. V.

    2013-01-01

    The tectonic evolution of the Arctic Region in the Mesozoic and Cenozoic is considered with allowance for the Paleozoic stage of evolution of the ancient Arctida continent. A new geodynamic model of the evolution of the Arctic is based on the idea of the development of upper mantle convection beneath the continent caused by subduction of the Pacific lithosphere under the Eurasian and North American lithospheric plates. The structure of the Amerasia and Eurasia basins of the Arctic is shown to have formed progressively due to destruction of the ancient Arctida continent, a retained fragment of which comprises the structural units of the central segment of the Arctic Ocean, including the Lomonosov Ridge, the Alpha-Mendeleev Rise, and the Podvodnikov and Makarov basins. The proposed model is considered to be a scientific substantiation of the updated Russian territorial claim to the UN Commission on the determination of the Limits of the Continental Shelf in the Arctic Region.

  5. Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons: Ion microprobe evidence for high δ 18O continental crust and oceans in the Early Archean

    NASA Astrophysics Data System (ADS)

    Peck, William H.; Valley, John W.; Wilde, Simon A.; Graham, Colin M.

    2001-11-01

    Ion microprobe analyses of oxygen isotope ratios in Early Archean (Hadean) zircons (4.0- to 4.4-Ga) reveal variable magmatic δ 18O values, including some that are high relative to the mantle, suggesting interaction between magmas and already-formed continental crust during the first 500 million yr of Earth's history. The high average δ 18O value of these zircons is confirmed by conventional analysis. A metaconglomerate from the Jack Hills in the Yilgarn Craton (Western Australia) contains detrital zircons with ages > 4.0 Ga (Compston and Pidgeon, 1986) and one crystal that is 4.40-Ga old (Wilde et al., 2001). The newly discovered 4.40-Ga grain is the oldest recognized terrestrial mineral. The Jack Hills metaconglomerate also contains a large 3.3- to 3.6-Ga-old zircon population with an average δ 18O value of 6.3 ± 0.1‰ (1 s.e.,; n = 32 spot analyses). Two 4.15-Ga zircons have an average δ 18O of 5.7 ± 0.2‰ ( n = 13). In addition, a 4.13-Ga zircon has an average δ 18O of 7.2 ± 0.3‰ ( n = 8) and another 4.01-Ga zircon has an average δ 18O of 6.8 ± 0.4‰ ( n = 10). The oldest grain (4.40 Ga) is zoned with respect trace element composition (especially LREE), and intensity of cathodoluminescence, all of which correlate with oxygen isotope ratios (7.4‰ vs. 5.0‰). High LREE and high-δ 18O values from the 4.01- to 4.40-Ga grains are consistent with growth in evolved granitic magmas (δ 18O(WR) = 8.5 to 9.5‰) that had interacted with supracrustal materials. High δ 18O values show that low-temperature surficial processes (i.e., diagenesis, weathering, or low-temperature alteration) occurred before 4.0 Ga, and even before 4.40 Ga, shortly following the hypothesized date of core differentiation and impact of a Mars-sized body to form the Moon at ˜4.45 Ga. This is the first evidence of continental crust as early as 4.40 Ga and suggests differentiation during the period of intense meteorite bombardment of the early Earth. The magnitude of water and rock

  6. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

    Levander, Alan; Bezada, Maximiliano; Niu, Fenglin; Palomeras, Imma; Humphreys, Eugene; Carbonell, Ramon; Gallart, Josep; Schmitz, Michael; Miller, Meghan

    2016-04-01

    Subduction recycling of oceanic lithosphere, a central theme of plate tectonics, is relatively well understood. Recycling continental lithosphere is more difficult to recognize, can take a number of different forms, and appears to require an external trigger for initiation. Delamination and localized convective downwelling are two processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. We describe a related process that can lead to the loss of continental lithosphere adjacent to a subduction zone: Subducting oceanic plates can entrain and recycle lithospheric mantle from an adjacent continent and disrupt the continental lithosphere far inland from the subduction zone. Body wave tomograms from dense broadband seismograph arrays in northeastern South America (SA) and the western Mediterranean show larger than expected volumes of positive velocity anomalies which we identify as the subducted Atlantic slab under northeastern SA, and the Alboran slab beneath the Gibraltar arc (GA). The positive anomalies lie under and are aligned with the continental margins at sublithospheric depths. The continental margins along which the subduction zones have traversed, i.e. the northeastern SA plate boundary and east of GA, have significantly thinner lithosphere than expected. The thinner than expected lithosphere extends inland as far as the edges of nearby cratons as determined from receiver function images and surface wave tomography. These observations suggest that subducting oceanic plates viscously entrain and remove continental mantle lithosphere from beneath adjacent continental margins, modulating the surface tectonics and pre-conditioning the margins for further deformation. The latter can include delamination of the entire lithospheric mantle and include the lower crust, as around GA, inferred by results from active and passive seismic experiments. Viscous removal of continental margin lithosphere creates LAB topography leading

  7. 3.30 Ga high-silica intraplate volcanic-plutonic system of the Gavião Block, São Francisco Craton, Brazil: Evidence of an intracontinental rift following the creation of insulating continental crust

    NASA Astrophysics Data System (ADS)

    Zincone, Stefano A.; Oliveira, Elson P.; Laurent, Oscar; Zhang, Hong; Zhai, Mingguo

    2016-12-01

    High-silica rhyolites having U-Pb zircon ages of 3303 ± 11 Ma occur along the eastern border of the Gavião Block (Brazil) associated with the Contendas-Mirante and Mundo Novo supracrustal belts. Unlike many Archean greenstone sequences, they are not interlayered with mafic to intermediate units. Instead, they belong to an inter-related plutonic-volcanic system, together with granitic massifs having similar zircon crystallization ages of ca. 3293 ± 3 Ma and 3328 ± 3 Ma and plotting along the same geochemical trends as the rhyolites. The rhyolites show well-preserved primary volcanic features such as magma flow textures and euhedral phenocrysts. High emplacement temperatures are indicated by petrographic evidence (β-quartz phenocrysts), zircon saturation temperatures (915-820 °C) and geochemical data, especially high SiO2 (74-79 wt.%) together with elevated Fe2O3(T) ( 3 wt.%), MgO (0.5-1.5 wt.%) and low Al2O3 (< 11 wt.%). The rhyolites show homogeneous trace element ratios (La/YbN 4.8 ± 1.8; EuN/Eu* 0.55; Sr/Y 0.7) and negative ԐHf(3.3 Ga) from 0 to - 7, indicating derivation from a single crustal source for both occurrences. Specifically, the rhyolites would have derived from extraction and eruption of highly silicic residual liquid formed by crystallization of granitic magma in a relatively shallow (< 10 km) reservoir, now represented by the granite massifs. The granite magma was formed by melting or differentiation of material similar to the diorite gneiss that occurs regionally. The 3.30 Ga volcanic-plutonic systems formed after a period of crustal growth and stabilization of a thick continental lithosphere, represented by massive 3.40-3.33 Ga TTG and medium to high-K calk-alkaline magmatism in the Gavião Block. The 3.30 Ga-old rhyolites and granites would therefore have formed in an intracontinental tectonic setting after the formation and stabilization of new continental crust, and accordingly would represent the first stages of rifting and continental

  8. Continental crustal composition and lower crustal models

    NASA Technical Reports Server (NTRS)

    Taylor, S. R.

    1983-01-01

    The composition of the upper crust is well established as being close to that of granodiorite. The upper crustal composition is reflected in the uniform REE abundances in shales which represent an homogenization of the various REE patterns. This composition can only persist to depths of 10-15 km, for heat flow and geochemical balance reasons. The composition of the total crust is model dependent. One constraint is that it should be capable of generating the upper granodioritic (S.L.) crust by partial melting within the crust. This composition is based on the andesite model, which assumes that the total crust has grown by accretion of island arc material. A representation of the growth rate of the continental crust is shown. The composition of the lower crust, which comprises 60-80% of the continental crust, remains a major unknown factor for models of terrestrial crustal evolution. Two approaches are used to model the lower crust.

  9. Prograde infiltration of Cl-rich fluid into the granulitic continental crust from a collision zone in East Antarctica (Perlebandet, Sør Rondane Mountains)

    NASA Astrophysics Data System (ADS)

    Kawakami, Tetsuo; Higashino, Fumiko; Skrzypek, Etienne; Satish-Kumar, M.; Grantham, Geoffrey; Tsuchiya, Noriyoshi; Ishikawa, Masahiro; Sakata, Shuhei; Hirata, Takafumi

    2017-03-01

    input of Cl-rich fluids through tectonic boundaries during continental collision.

  10. Layered granitoids: Interaction between continental crust recycling processes and mantle-derived magmatism: Examples from the Évora Massif (Ossa-Morena Zone, southwest Iberia, Portugal)

    NASA Astrophysics Data System (ADS)

    Moita, Patrícia; Santos, José F.; Pereira, M. Francisco

    2009-08-01

    of the leucogranite magma. The Almansor outcrop is then interpreted as the remnants of a shear zone that operated as a pathway for melts that moved upward through the crust providing the locus for differentiation and mingling/mixing of magmas, whilst Alto de São Bento would correspond to the zone, at a higher crustal level, where magmas were trapped and forced to spread horizontally. At Valverde (the third area) foliated and non-foliated granitoids are spatially related and field criteria links these rocks to metamorphic protolith and anatectic melt, respectively. However, petrographic, geochemical and isotopic information shows that they all are compositionally identical trondhjemites with no evidence of metamorphic fabric. In the foliated rocks, mesoscopic features are interpreted as resulting from melt segregation structures formed in a crystallizing mush. In contrast to the previous areas, the Valverde trondhjemites probably do not belong to the main calc-alkaline plutonic suite of the Évora Massif, since they have a distinct Sr and Nd isotope signature.

  11. Active seismic monitoring of changes of the reflection response of a crystalline shear zone due to fluid injection in the crust at the Continental Deep Drilling Site, Germany

    NASA Astrophysics Data System (ADS)

    Beilecke, T.; Kurt, B.; Stefan, B.

    2005-12-01

    In theory and in the laboratory variations of the hydraulic pressure can be detected with seismic methods: A lowering of the hydraulic pressure leads to the closure of micro-cracks within the rock (increase of the differential or effective pressure). Subsequently, the seismic velocities increase. An increase of the hydraulic pressure leads to reverse seismic effects. Consequently, seismic impedance contrasts and associated reflection amplitudes vary in the case of a propagating fluid pressure front in a rock matrix with inhomogeneous permeability - as is the case at shear zones. The largest amplitude changes can be expected with vertical ray inclination on the impedance contrast. Generally, the expected effects are small however (Kaselow, 2004). The practical utilization of active seismics for the detection of pressure changes at large scale in hard rock is currently being studied at the Continental Deep Drilling Site (KTB). The injection of water (200 l/min) in a depth of about 4000 m into the so-called SE2 shear zone in the KTB pilot hole was monitored with active seismics between May 2004 and April 2005. The core of the experiment layout is a fixed 5-arm geophone array consisting of 24 3-component geophones, buried at about 70 cm depth. The source signal is a vertical vibrator sweep of 30 s length with the spectrum 30-120 Hz. The signal is sent into the ground 32 times during each cycle, detected with the array and recorded separately for each geophone channel, without prior correlation with the source signal. This allows maximum post-processing with seismic processing and analysis tools and especially permits the use of array properties to increase the signal-to-noise ratio. Critical parameters of the experiment are the repeatability of the source signal as well as the stability of the receiver properties. Another pivot is the hydraulic pressure and its distribution built up within the rock matrix. Estimations based on model calculations show that a change of

  12. He and N isotopes in thermal springs of the Mexican Pacific coast: subducting slab, continental crust and mantle contributions to fluids of a forearc zone.

    NASA Astrophysics Data System (ADS)

    Taran, Yuri; Inguaggiato, Salvatore; Varley, Nicholas; Ramirez Guzman, Alejandro

    2010-05-01

    Two oceanic plates are subducting beneath the continent along the Mexican Pacific coast: Cocos Plate south of Colima graben (~19°N) and a young Rivera Plate to the north of Colima graben. The trench is situated ~ 70 km from the shore line which is very close comparing with other continental margins. There are 26 groups of thermal springs between 16°N and 21°N, in a 30 km-wide zone along the coast. The temperature and salinity ranges are 40-90°C and 100-20,000 ppm, respectively. The springs are mainly of a low salinity (< 1000 ppm), high pH (8-10) and temperatures of 37-50°C. Almost all springs discharge bubbling gas with N2 as a predominant component and have He content between 400 and 1500 ppmV. Two groups of springs are methane-rich (70 and 10 vol%). The CO2-rich springs and high-HCO3 waters are absent. All springs to the south of the Michoacan-Guerrero boarder (~ 18°N) are characterized by N2/Ar ≈ 100, δ15N ≈0 and 3He/4He ratios lower than 0.2Ra (where Ra=1.4x10-6, the air ratio) except the Paso Real springs (0.9Ra) located within a Coyuca seismogenic fracture zone. Springs along the Michoacan coast, the northern part of the Cocos Plate subduction, discharge gases with 1.5Ra < 3He/4He <2.5Ra but still low, close to the atmospheric N2/Ar and δ15N ≈0. All springs located within the Colima graben have high 3He/4He (up to 4.5Ra) and elevated N2/Ar and δ15N. The El Salitre (La Tuna) springs located within the southern board of the Colima graben discharge saline Na-Ca-Cl water (46°C, Cl=15,000 ppm) with N2/Ar > 400, δ15N = +4.6‰, almost no CH4 (<0.1 %) and 3He/4He = 2.3Ra. The only group of hot springs within the Jalisco Block and close to the shore line, Rio Purificacion, discharge hot, saline Na-Cl water (80°C, 12,000 ppm of Cl), with N2/Ar > 300, δ15N = +5‰ and 3He/4He = 0.4Ra. A number of hot and warm springs associated with Puerto Vallarta graben are characterized by high 3He/4He up to 4.5Ra, elevated N2/Ar and δ15N. The last group, Punta

  13. H.R. 73: A Bill to protect the ecologically fragile coastal resources of south Florida by prohibiting offshore oil and gas activities and by cancelling Federal leases in the area of the Outer Continental Shelf adjacent to the south Florida coast. Introduced in the House of Representatives, One Hundred Fourth Congress, First session

    SciTech Connect

    1995-12-31

    This document contains H.R. 73, A Bill to protect the ecologically fragile coastal resources of south Florida by prohibiting offshore oil and gas activities and by cancelling Federal leases in the area of the Outer Continental Shelf adjacent to south Florida. This Bill was introduced in the House of Representatives, 104th Congress, First Session, January 4, 1995.

  14. A Geo-traverse at a Passive Continental Margin: the Tagus Abyssal Plain, West Iberia

    NASA Astrophysics Data System (ADS)

    Afilhado, A.; Matias, L.; Mendes-Victor, L.

    2006-12-01

    transitional domain suggests a layered structure for the lithospheric mantle. The location of the main features and trend changes seen on the magnetic anomaly and the Bouguer anomaly maps are compatible to the seismically defined transition to the oceanic domain. Both magnetic and free air profiles modeling are also consistent to a major rock property contrast at this location. Minor features within the oceanic domain indicate some variability of the mass and magnetic dipoles distribution that might be related to different cross cutting of the segment axis and/or non stationary thermo-mechanical conditions of the sea floor spreading process. From 9.4W to the foot of the continental slope, at 10.2W, in a 65km distance, the Bouguer anomaly strongly increases due both to continental crust thinning and Moho shallowing. Magnetic anomalies having peak to peak values of 30 to 90nT and wave length of at least 30km plus the high density blocks modeled at shallow levels, indicate that dense and magnetic rocks should be present at the continental margin. A large body of exhumed continental mantle at the OCT is hardly supported by the data. Even so, the eastern 35km segment of the abyssal plain adjacent to the continental margin is a mass excess segment, bordered by significant magnetic anomalies, in good agreement to an intruded and partly ultramafic lower transitional crust, as suggested by the seismic data.

  15. Off-shore to near-shore transects of ferromanganese crusts adjacent to the California margin Tracey A. Conrad1, James R. Hein2, Adina Paytan1 1University of California Santa Cruz, CA, 95064 (tconrad@ucsc.edu) 2USGS, Menlo Park, CA, 94025, USA (jhein@usgs.gov)

    NASA Astrophysics Data System (ADS)

    Conrad, T. A.; Hein, J. R.; Paytan, A.

    2012-12-01

    Marine ferromanganese (Fe-Mn) crusts growing on seamounts along the California Margin (CM) are influenced by terrestrial and biogenic input. These continental margin crusts have higher concentrations of Si, K, Fe, Na, Ag, Cr, B, and Ba than Fe-Mn crusts from the global open-ocean. Al is also higher but only relative to Pacific open-ocean crusts. These relative enrichments may reflect the high primary productivity near the CM caused by seasonal upwelling and high sediment transport to the region from river/eolian input and cliff erosion. Two transects with samples from five seamounts each are used to compare seaward changes. Transect A includes analyses of 66 bulk samples from Flint, Ben, and Little Joe seamounts, Patton Escarpment, and Northeast Bank. It spans ~400 km of seafloor heading ~58N and coming within ~220 km of the shoreline with samples collected at water depths ranging from 570-2925 m. Transect B includes analyses of 136 bulk samples from Adam, Hoss, San Marcos, San Juan, and Rodriguez seamounts at water depths ranging from 692-3880 m. This transect spans ~240 km heading ~10N and comes within ~75 km of the shoreline near the base of the continental slope. For both transects, mean water depth increases with mean longitude, and latitude is fairly constant varying by approximately 2 degree latitude for transect A and 1degree for B. Both transects show statistically significant trends at the 99% confidence level for element concentrations versus water depth. Concentrations of Fe, Ca, P, Co, and Pb increase as water depth decreases. For transect (A), Mn and Mg also follow this trend, as do Mo and Al for transect (B); Mn also shows this trend for transect (B) but at the 95% confidence level. For both transects, Cu and Zn show the opposite trend, with concentrations increasing in crusts with increasing water depth. For Transect (B), Ni and Al also show this trend. Si and K show no statistically significant trends for either transect. In open-ocean samples

  16. Crusts: biological

    USGS Publications Warehouse

    Belnap, Jayne; Elias, Scott A.

    2013-01-01

    Biological soil crusts, a community of cyanobacteria, lichens, mosses, and fungi, are an essential part of dryland ecosystems. They are critical in the stabilization of soils, protecting them from wind and water erosion. Similarly, these soil surface communities also stabilized soils on early Earth, allowing vascular plants to establish. They contribute nitrogen and carbon to otherwise relatively infertile dryland soils, and have a strong influence on hydrologic cycles. Their presence can also influence vascular plant establishment and nutrition.

  17. Gravity and structure of the continental margins of southwestern Mexico and northwestern Guatemala

    NASA Astrophysics Data System (ADS)

    Couch, Richard; Woodcock, Stephen

    1981-03-01

    Geophysical measurements over the eastern end of the Tehuantepec Ridge and adjacent continental margins of southern Mexico and northern Guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces. A positive free-air gravity anomaly which extends northwestward along the outer continental shelf of Guatemala curves abruptly landward in the Gulf of Tehuantepec. The positive shelf anomaly is on trend with the positive anomaly of the Nicoya Peninsula, Costa Rica, and suggests that rocks genetically related to Cretaceous rocks of the Nicoya Complex extend northwestward along the continental shelf to the Gulf of Tehuantepec. A crustal and subcrustal cross section of the continental margin of Guatemala, constrained by gravity, magnetic, and seismic refraction data, indicates that the rock strata causing the outer shelf gravity high dip landward, consistent with imbricate thrusting of the oceanic crust beneath and into the continental margin. A model crustal cross section of the continental margin of southern Mexico, north of the Tehuantepec Ridge, shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis.

  18. Heat flow and continental breakup: The Gulf of Elat (Aqaba)

    NASA Technical Reports Server (NTRS)

    Ben-Avraham, Z.; Vonherzen, R. P.

    1985-01-01

    Heat flow measurements were made in the major basins of the Gulf of Elat (Aqaba), northern Red Sea. The gulf is located at the southern portion of the Dead Sea rift which is a transform plate boundary. Gradient measurements at each site were made with a probe which allows multiple penetration of the bottom during a single deployment of the instrument. Thermal conductivity was determined by needle probe measurements on sedimentary cores. The mean heat flux, about 80 mWm(-2), is significantly above the continental mean, and probably also above that from the adjacent Sinai and Arabian continental blocks. The heat flow appears to increase from north to south. Such an increase may be related to the more advanced rifting stage of the Red Sea immediately to the south, which presently includes creation of an oceanic crust. This trend also corresponds to the general trend of the deep crustal structure in the gulf. Evidence from various geophysical fields suggest a gradual thinning of the crust towards the direction of the Red Sea where a normal oceanic crust exists. The heat flow data, together with other geophysical data, indicate a propagation of mature rifting activity from the Red Sea into the Gulf of Elat. This process is acting simultaneously with the transform motion along the Dead Sea rift.

  19. Continental collisions and seismic signature

    NASA Astrophysics Data System (ADS)

    Meissner, R.; Wever, Th.; Sadowiak, P.

    1991-04-01

    Reflection seismics in compressional belts has revealed the structure of crustal shortening and thickening processes, showing complex patterns of indentation and interfingering of colliding crusts and subcrustal lithospheres. Generally, in the upper crust large zones of detachments develop, often showing duplexes and 'crocodile' structures. The lower crust from zones of active collision (e.g. Alps, Pyrenees) is characterized by strongly dipping reflections. The base of the crust with the Moho must be continuously equilibrating after orogenic collapse as areas of former continental collision exhibit flat Mohos and subhorizontal reflections. The depth to the Moho increases during collision and decreases after the onset of post-orogenic extension, until finally the crustal root disappears completely together with the erosion of the mountains. Processes, active during continental collisions and orogenic collapse, create distinct structures which are imaged by reflection seismic profiling. Examples are shown and discussed.

  20. Continental Growth and the Sedimentary Record

    NASA Astrophysics Data System (ADS)

    Dhuime, B.; Hawkesworth, C. J.; Robinson, R. A. J.; Cawood, P. A.

    2014-12-01

    Detrital sedimentary rocks provide average samples of the continental crust formed at different times and in different places. Some materials are more susceptible to erosion and/or to preservation bias than others, and one issue is to understand how the compositions of a range of source rocks are then recorded in the sediments. Here we considered two different approaches to model the growth of the continental crust: (i) The variation of Nd isotopes in continental shales with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter usually referred to as 'K'. The determination of K, and the extent to which it varies in different erosion systems, thus have fundamental implications for the models of continental growth based on radiogenic isotopes in continental sediments. (ii) The variations in U-Pb, Hf and O isotopes in detrital zircons, from 'modern' sediments sampled worldwide. In this approach, O isotopes are used to screen 'hybrid' Hf model ages (i.e. ages resulting from mixing processes of crustal material from different ages) from Hf model ages that represent actual crust formation ages. These two approaches independently suggest that the continental crust has been generated continuously, but with a marked decrease in the continental growth rate at ~3 Ga. The >4 Ga to ~3 Ga period is characterised by relatively high net rates of continental growth (~3.0 km3.a-1), which are similar to the rates at which new crust is generated, and destroyed, at the present time. Net growth rates are much lower since 3 Ga (~0.8 km3.a-1), which may be attributed to higher rates of destruction of continental crust. The inflexion in the continental growth curve at ~3 Ga indicates a change in the way the crust was generated and preserved. This change may be linked to onset of subduction-driven plate tectonics and discrete subduction zones.

  1. Reaction zone between pre-UHP titanite and host rock: insights into fluid-rock interaction and deformation mechanisms during exhumation of deeply subducted continental crust (Dabie Shan UHP unit, China)

    NASA Astrophysics Data System (ADS)

    Wawrzenitz, N.; Romer, R. L.; Grasemann, B.; Morales, L. F. G.

    2012-04-01

    Exhumed crustal UHP rocks may occur as relict blocks in strongly metasomatized matrix rocks. Due to variations in competence between the mm to km sized blocks and their ductile matrix, the largely undeformed blocks may preserve the pre-subduction and the prograde history, whereas the matrix rocks have been ductilely deformed to high magnitudes and record successive stages of deformation. The reaction zones between blocks and matrix, however, provide insights into the fluid-rock interaction, deformation and the deformation mechanisms active during the exhumation of deeply subducted continental crust in the subduction channel. We investigate a titanite megacryst (3 cm in diameter) in a calc-silicate marble from the UHP unit in the Dabie Shan, China. The core of the titanite megacryst grew prograde during subduction. Its U-Pb system remained closed and yields a maximum age for UHP metamorphism. Sr and Nd isotope compositions in the core demonstrate that the titanite megacryst precipitated from a homogeneous fluid source. During metamorphism in the subduction zone, infiltration of external fluids resulted first in Sr-loss from the marbles and then introduction of Sr with unusually low 87Sr/86S values (Romer et al., 2003), leading to the contrasting 87Sr/86Sr values in the titanite megacryst and the hosting UHP marbles (Wawrzenitz et al., 2006). Related to deformation in the calc-silicate marble matrix, the rim of the titanite megacryst has been replaced during the following dissolution-precipitation reactions: (i) Pseudomorphic replacement of the old titanite megacryst by coupled dissolution-reprecipitation. Fluid migrated into the old grain producing a sharp boundary of the replacement front. (ii) New small titanite grains grew with their long axes parallel to the foliation of the marble matrix, reflecting the activation of dissolution precipitation creep. In the matrix, the foliation is defined by the orientation of the basal planes of phengitic white mica. The new

  2. The Role of Plumes in Breakup Processes - Traces Found in the Deep Crustal Structure at the Intersection of Walvis Ridge with the African Continental Margin

    NASA Astrophysics Data System (ADS)

    Fromm, T.; Jokat, W.; Behrmann, J. H.; Ryberg, T.; Weber, M. H.

    2014-12-01

    Large igneous provinces (LIP) are often found in close temporal and spatial proximity with continental breakups, supporting the model, that an arriving mantle plume produces large amounts of melt and has a massive influence on the breakup process. The South Atlantic is a classical example, with flood basalts on both adjacent continents and a paired age progressing ridge system connecting them with the current hotspot location at Tristan da Cunha. To estimate the influence of the plume on the preexisting continental crust, a large-scale geophysical experiment was conducted in 2011 at the intersection of Walvis Ridge with the African continent. We present four P-wave velocity models derived from seismic refraction data. One extends 430 km along the ridge crest and continues onshore to a total length of 730 km, while the other three crossing the ridge perpendicular: one (480 km long) far offshore in the oceanic regime, one (600 km) close to shelf break and the last one (400 km) onshore. Crustal velocities beneath Walvis Ridge range between 5.5 km/s and 7.0 km/s, which are typical velocities for oceanic crust. The crustal thickness, however, is approximately three times larger than of normal oceanic crust: 17 km in the western part increasing to 22 km towards the continent. The continent ocean transition is characterized by 30 km thick crust with a high velocity body (HVB) in the lower crust and seismic velocities up to 7.5 km/s. The western extend of the HVB is to a similar distance from shore as for HVBs observed south of Walvis Ridge. In contrast, the eastern boundary lies well within the continental domain, at the 40 km thick crust of the Kaoko fold belt. Here, the variation of seismic velocities indicates that hot material intruded the continental crust during the initial rifting stage. However, beyond this relatively sharp boundary (40 km wide), the remaining continental crust seems unaffected by intrusions and the root of the Kaoko belt is no eroded. The cross

  3. Modelling Sea Floor Spreading Initiation and Depth Dependent Stretching at Rifted Continental Margins

    NASA Astrophysics Data System (ADS)

    Kusznir, N. J.; Tymms, V.

    2003-12-01

    subsidence, water depths and top basement heat-flow. The ocean-ridge fluid-flow models predict advection of continental lithosphere material and depth dependent stretching which is highly sensitive to the ratio of Vz/Vx. Convection modelling of sea-floor spreading initiation at volcanic margins (Nielsen & Hopper 2002) suggests that Vz/Vx > 5 for early sea-floor spreading, reducing to Vz/Vx ~ 1.3 after a few Ma. Application of this Vz/Vx history predicts that lower continental crust and lithospheric mantle adjacent to the site of sea-floor spreading initiation is advected continentward producing a distribution of depth dependent stretching consistent with that observed at volcanic margins. Mantle exhumation is not predicted for Vz/Vx >>1. For non-volcanic margins, Vz/Vx at sea-floor spreading initiation is expected to be ~ 1. Models of sea-floor spreading initiation with Vz/Vx ~ 1 predict that lower continental crust and continental lithospheric mantle adjacent to the site of sea-floor spreading initiation are advected oceanward and result in rifted margin depth dependent stretching and continental mantle exhumation consistent with that observed at non-volcanic margins.

  4. Thermal models pertaining to continental growth

    NASA Technical Reports Server (NTRS)

    Morgan, Paul; Ashwal, Lew

    1988-01-01

    Thermal models are important to understanding continental growth as the genesis, stabilization, and possible recycling of continental crust are closely related to the tectonic processes of the earth which are driven primarily by heat. The thermal energy budget of the earth was slowly decreasing since core formation, and thus the energy driving the terrestrial tectonic engine was decreasing. This fundamental observation was used to develop a logic tree defining the options for continental growth throughout earth history.

  5. Did Flow of Weak Middle or Lower Crust Accommodate Extension in the Gulf of California and Salton Trough?

    NASA Astrophysics Data System (ADS)

    Black, N.; Axen, G. J.; Lizarralde, D.; Kent, G. M.; Harding, A. J.; Holbrook, W. S.; Umhoeffer, P. J.; Fletcher, J. M.; Gonález-Fernández, A.

    2003-12-01

    The strength of middle and/or lower crust (MLC) is key to understanding continental deformation and controls whether or not brittle upper crust and mantle lithosphere are coupled: weak MLC may flow and decouple the adjacent layers. We analyze the extent of MLC flow during rifting in the northern Gulf of California and Salton Trough (NGC/ST; in the NSF MARGINS Rupturing of Continental Lithosphere focus site) using area-balance techniques to compare upper crustal extension and MLC extension. Mid-Tertiary extension in SE CA, W AZ and W mainland Mexico thinned the crust before ˜E-W extension localized in the GC/ST at ˜12 Ma. By ˜6 Ma, NW-SE Pacific-N. American plate separation was concentrated in the NGC/ST, which began to subside below sea level. By ˜3.6 Ma, transform faults connected pull apart basins forming ocean crust (S gulf) and proto-oceanic crust in the NGC/ST. The Peninsular Ranges west of the NGC/ST were not extended at the surface. MLC flow into the NGC/ST is suggested by (1) the fact that the Moho dips W under the unextended Peninsular Ranges (W of the rift) to a maximum depth well W of the range crest (Lewis et. al., 2000, 2001), (2) by an apparent surplus of MLC relative to upper crust within the rift (e.g., Couch et al., 1991), and (3) by the magnitude of extension ( ˜255 km) between initially adjacent areas on either side of the NGC/ST (Oskin et al., 2001). We constrain the extent of MLC flow two ways: by comparing known upper crustal extension (e.g., Lewis and Stock, 1998; Axen and Fletcher, 1998) to (1) whole-crustal and (2) lower-crustal extension. Whole and lower crustal extension are obtained from cross-sectional areas constrained by depth to Moho and/or to the base of the upper crust (references above), by the extent of new proto-oceanic crust (e.g., Fuis et.al., 1984), and by assuming end-member initial crustal thicknesses (e.g., ˜37-40 km maximum thickness of the Peninsular Ranges or ˜25 km minimum from previously extended crust east of

  6. Rheological Stratification of the Continental Lithosphere: Constraints from Space Geodesy

    NASA Astrophysics Data System (ADS)

    Thatcher, W.

    2003-12-01

    Postseismic transient deformation, isostatic rebound from removal of pluvial lake loads, and lithospheric deflection due to reservoir impoundment are each converging on consistent rheological models for the crust and upper mantle of actively deforming continental regions. These results imply a strong elastic crust 25-40 km thick overlain by a viscoelastic substrate with an effective viscosity of ~10**18 to 10**19 Pa-s. The most surprising result of these studies is that the upper mantle is weaker than the lower crust. However, the lower crust in these regions may deform by ductile flow on longer time scales, and the data provide a lower bound of ~10**20 Pa-s for its effective viscosity. This bound on lower crustal viscosity is consistent with spectral admittance studies of the gravity field and its relation to topography in the western U. S. (Lowry et al., 2000). These results indicate effective elastic lithospheric thickness is 5-15 km in the same regions where the post-loading results indicate the entire crust is strong over about 10 to 10,000 years. Recent (and not so recent) relevant results include: (1) Deformation imaged by InSAR and GPS following the 1992 Landers and 1999 Hector Mine, California earthquakes; (2) Leveling surveys following the 1959 M=7.3 Hegben Lake, Montana earthquake; (3) Isostatic rebound of Lake Bonneville, Utah; (4) Leveling surveys following filling of Lake Mead, Arizona in 1935. Postseismic transient deformation observed following several other recent large earthquakes provides potential constraints on bulk rheology of the lithosphere. However, deformation following events at major plate boundaries, including the 1993 Hokkaido-oki (M=7.8), 1999 Taiwan (M=7.6) and 1999 Izmet (M=7.5) earthquakes is dominated by the effects of buried aseismic afterslip, making it difficult to extract any signals that may be due to bulk relaxation of the lower crust and upper mantle. This suggests that large intraplate earthquakes on faults adjacent to

  7. Composition of the continental plates

    USGS Publications Warehouse

    Gilluly, J.

    1954-01-01

    The structures of continental plates and of oceanic basins suggested by several seismologists are utilized to estimate the relative volumes of sial and sima in the earth's crust. It seems that sial of the composition of the average igneous rock constitutes fully 26% and perhaps as much as 43% of the total crust. This ratio is far higher than seems likely if the sial had been entirely derived through fractional crystallization of a basaltic magma. The relative paucity of intermediate rocks as compared with granite and gabbro in the crust points in the same direction. The tentative conclusion is reached that the sial owes a large part of its volume to some process other than fractional crystallization of basalt-possibly to the emanation of low-melting constituents such as water, silica, potassa, soda, and alumina directly from the mantle to the crust. ?? 1954 Springer-Verlag.

  8. La marge européenne de la Téthys jurassique en Corse : datation de trondhjémites de Balagne et indices de croûte continentale sous le domaine Balano-LigureThe European margin of the Jurassic Tethys in Corsica: dating of Balagne trondhjemites and evidence to support a continental crust beneath the Balagne-Ligurian domain

    NASA Astrophysics Data System (ADS)

    Rossi, Philippe; Cocherie, Alain; Lahondère, Didier; Fanning, C. Mark

    Vein trondhjemite in gabbro of the Carnispola Bridge has been dated to 169±3 Ma (UPb on zircon). This date indicates that E-MORB-type ophiolites were emplaced in the marginal Balagne part of the Ligurian Jurassic basin about some 10 Ma before the emplacement of N-MORB ophiolites in the most central part of the ocean. In addition, the presence of inherited zircons with Ordovician (431±8 Ma) and Archean (2693±12 Ma) ages reveals that the Balagne ophiolites were emplaced on a thinned continental crust. Finally, the 298±4 Ma age of zircons from eclogitised meta-arkose in the eclogitic Morteda-Farinole unit ('Schistes lustrés' zone) confirms the attribution of these rocks to a palaeogeographic area that laid between continent and ocean, along the edge of the Hercynian granite batholith in Corsica. To cite this article: P. Rossi et al., C. R. Geoscience 334 (2002) 313-322.

  9. 1900-Ma ocean crust in Canada

    NASA Astrophysics Data System (ADS)

    Maggs, William Ward

    The oldest known occurrence in North America of an ophiolite, considered to be a piece of ancient ocean crust, has been reported in the Cape Smith Belt in northern Quebec, Canada.The recognition last summer of a key structural component of the characteristic ophiolite suite has buttressed confidence in the theory that the 1900-Ma fragments of an ocean basin were accreted to an early Proterozoic Canadian continent. The tectonic mixing of oceanic and continental crust is strong evidence for the operation of plate tectonics early in Earth's history.

  10. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics.

    PubMed

    Tang, Ming; Chen, Kang; Rudnick, Roberta L

    2016-01-22

    The Archean Eon witnessed the production of early continental crust, the emergence of life, and fundamental changes to the atmosphere. The nature of the first continental crust, which was the interface between the surface and deep Earth, has been obscured by the weathering, erosion, and tectonism that followed its formation. We used Ni/Co and Cr/Zn ratios in Archean terrigenous sedimentary rocks and Archean igneous/metaigneous rocks to track the bulk MgO composition of the Archean upper continental crust. This crust evolved from a highly mafic bulk composition before 3.0 billion years ago to a felsic bulk composition by 2.5 billion years ago. This compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3.0 billion years ago.

  11. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics

    NASA Astrophysics Data System (ADS)

    Tang, Ming; Chen, Kang; Rudnick, Roberta L.

    2016-01-01

    The Archean Eon witnessed the production of early continental crust, the emergence of life, and fundamental changes to the atmosphere. The nature of the first continental crust, which was the interface between the surface and deep Earth, has been obscured by the weathering, erosion, and tectonism that followed its formation. We used Ni/Co and Cr/Zn ratios in Archean terrigenous sedimentary rocks and Archean igneous/metaigneous rocks to track the bulk MgO composition of the Archean upper continental crust. This crust evolved from a highly mafic bulk composition before 3.0 billion years ago to a felsic bulk composition by 2.5 billion years ago. This compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3.0 billion years ago.

  12. Relamination of mafic subducting crust throughout Earth's history

    NASA Astrophysics Data System (ADS)

    Maunder, Ben; van Hunen, Jeroen; Magni, Valentina; Bouilhol, Pierre

    2016-09-01

    Earth has likely cooled by several hundred degrees over its history, which has probably affected subduction dynamics and associated magmatism. Today, the process of compositional buoyancy driven upwelling, and subsequent underplating, of subducted materials (commonly referred to as ;relamination;) is thought to play a role in the formation of continental crust. Given that Archean continental crust formation is best explained by the involvement of mafic material, we investigate the feasibility of mafic crust relamination under a wide range of conditions applicable to modern and early Earth subduction zones, to assess if such a process might have been viable in an early Earth setting. Our numerical parametric study illustrates that the hotter, thicker-crust conditions of the early Earth favour the upward relamination of mafic subducting crust. The amount of relaminating subducting crust is observed to vary significantly, with subduction convergence rate having the strongest control on the volume of relaminated material. Indeed, removal of the entire mafic crust from the subducting slab is possible for slow subduction (∼2 cm/yr) under Archean conditions. We also observe great variability in the depth at which this separation occurs (80-120 km), with events corresponding to shallower detachment being more voluminous, and that relaminating material has to remain metastably buoyant until this separation depth, which is supported by geological, geophysical and geodynamical observations. Furthermore, this relamination behaviour is commonly episodic with a typical repeat time of approximately 10 Myrs, similar to timescales of episodicity observed in the Archean rock record. We demonstrate that this relamination process can result in the heating of considerable quantities of mafic material (to temperatures in excess of 900 °C), which is then emplaced below the over-riding lithosphere. As such, our results have implications for Archean subduction zone magmatism, for

  13. Early formation of evolved asteroidal crust.

    PubMed

    Day, James M D; Ash, Richard D; Liu, Yang; Bellucci, Jeremy J; Rumble, Douglas; McDonough, William F; Walker, Richard J; Taylor, Lawrence A

    2009-01-08

    Mechanisms for the formation of crust on planetary bodies remain poorly understood. It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics, whereas the Moon acquired its feldspar-rich crust by way of plagioclase flotation in a magma ocean. Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large-scale differentiation processes. Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak (GRA) 06128 and GRA 06129. These meteorites are feldspar-rich, with andesite bulk compositions. Their age of 4.52 +/- 0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide-rich nature of the meteorites are most consistent with an origin as partial melts from a volatile-rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early-formed materials in the Solar System.

  14. Temperature distribution in the crust and mantle

    NASA Technical Reports Server (NTRS)

    Jeanloz, R.; Morris, S.

    1986-01-01

    In an attempt to understand the temperature distribution in the earth, experimental constraints on the geotherm in the crust and mantle are considered. The basic form of the geotherm is interpreted on the basis of two dominant mechanisms by which heat is transported in the earth: (1) conduction through the rock, and (2) advection by thermal flow. Data reveal that: (1) the temperature distributions through continental lithosphere and through oceanic lithosphere more than 60 million years old are practically indistinguishable, (2) crustal uplift is instrumental in modifying continental geotherms, and (3) the average temperature through the Archean crust and mantle was similar to that at present. It is noted that current limitations in understanding the constitution of the lower mantle can lead to significant uncertainties in the thermal response time of the planetary interior.

  15. Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

    PubMed Central

    Liu, Xiao-Ming; Rudnick, Roberta L.

    2011-01-01

    Chemical weathering, as well as physical erosion, changes the composition and shapes the surface of the continental crust. However, the amount of continental material that has been lost over Earth’s history due to chemical weathering is poorly constrained. Using a mass balance model for lithium inputs and outputs from the continental crust, we find that the mass of continental crust that has been lost due to chemical weathering is at least 15% of the original mass of the juvenile continental crust, and may be as high as 60%, with a best estimate of approximately 45%. Our results suggest that chemical weathering and subsequent subduction of soluble elements have major impacts on both the mass and the compositional evolution of the continental crust. PMID:22184221

  16. Coseismic and postseismic Coulomb stress changes on intra-continental dip-slip faults and the role of viscoelastic relaxation in the lower crust: insights from 3D finite-element models

    NASA Astrophysics Data System (ADS)

    Bagge, Meike; Hampel, Andrea

    2016-04-01

    Investigating the stress interaction of faults plays a crucial role for assessing seismic hazard of a region. The calculation of Coulomb stress changes allows quantifying stress changes on so-called receiver faults in the surrounding of a source fault that was ruptured during an earthquake. Positive Coulomb stress changes bring receiver faults closer to failure, while a negative value indicates a delay of the next earthquake. Besides the coseismic ('static') stress changes, postseismic ('transient') stress changes induced by postseismic viscoelastic relaxation occur. Here we use 3D finite-element models with arrays of normal or thrust faults to study the coseismic stress changes and the stress changes arising from postseismic relaxation in the lower crust. The lithosphere is divided into an elastic upper crust, a viscoelastic lower crust and a viscoelastic lithospheric mantle. Gravity is included in the models. Driven by extension or shortening of the model, slip on the fault planes develops in a self-consistent way. We modelled an earthquake on a 40-km-long source fault with a coseismic slip of 2 m and calculated the displacement fields and Coulomb stress changes during the coseismic and postseismic phases. The results for the coseismic phase (Bagge and Hampel, Tectonophysics in press) show that synthetic receiver faults in the hanging wall and footwall of the source fault exhibit a symmetric distribution of the coseismic Coulomb stress changes on each fault, with large areas of negative stress changes but also some smaller areas of positive values. In contrast, faults positioned in along-strike prolongation of the source fault and outside of its hanging wall and footwall undergo mostly positive stress changes. Postseismic stress changes caused by viscous flow modify the static stress changes in a way that the net Coulomb stress changes on the receiver faults change significantly through space and time. Our models allow deciphering the combined effect of stress

  17. Reconstruction of the East Africa and Antarctica continental margins

    NASA Astrophysics Data System (ADS)

    Nguyen, L. C.; Hall, S. A.; Ball, P.; Bird, D. E.

    2015-12-01

    The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana. Previously proposed reconstruction models often contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate margin of Antarctica near the Riiser Larsen Sea. New satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones in the study area. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Information on crustal thickness along with the identification of fracture zones reveal the COBs that are located significantly closer to the coasts of Africa and Antarctica than previously recognized. Correlation of both fracture zone azimuths and the identified COBs over the conjugate margins suggest Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. Of several scenarios examined, the Beira High is most likely oceanic and may be a conjugate feature of the southern Astrid Ridge. An areal-balancing method that involves restoring the crust to a uniform pre-rift thickness has been used to perform the non-rigid reconstruction for both non-volcanic and volcanic margin with magmatic underplating. Based on the results, Africa underwent extension of 65-105 km while Antarctic crust was stretched by 90-190 km. Both margins reveal a trend of increasing extension from east to west. Various models tested to determine the direction of extension during rifting suggest that Antarctica underwent a counter-clockwise rotation with respect to Africa between 186-171 Ma prior to the onset of seafloor spreading.

  18. Reconstruction of the East Africa and Antarctica continental margins

    NASA Astrophysics Data System (ADS)

    Nguyen, Luan C.; Hall, Stuart A.; Bird, Dale E.; Ball, Philip J.

    2016-06-01

    The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana and Pangea. Previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean-boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate Antarctic margin near the Riiser-Larsen Sea. Satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Crustal thicknesses together with fracture zone terminations reveal COBs that are significantly closer to the African and Antarctic coasts than previously recognized. Correlation of fracture zone azimuths and identified COBs suggests Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. An areal-balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins. Both margins reveal a trend of increasing extension from east to west. Our results suggest Africa underwent extension of 60-120 km, while Antarctic crust was stretched by 105-180 km. Various models tested to determine the direction of extension during rifting suggest that Antarctica moved away from Africa in a WNW-ESE direction during the period between 184 and 171 Ma prior to the onset of seafloor spreading.

  19. Mantle plumes and continental tectonics.

    PubMed

    Hill, R I; Campbell, I H; Davies, G F; Griffiths, R W

    1992-04-10

    Mantle plumes and plate tectonics, the result of two distinct modes of convection within the Earth, operate largely independently. Although plumes are secondary in terms of heat transport, they have probably played an important role in continental geology. A new plume starts with a large spherical head that can cause uplift and flood basalt volcanism, and may be responsible for regional-scale metamorphism or crustal melting and varying amounts of crustal extension. Plume heads are followed by narrow tails that give rise to the familiar hot-spot tracks. The cumulative effect of processes associated with tail volcanism may also significantly affect continental crust.

  20. Imaging crust and mantle discontinuities across tectonic boundaries in North America with Sp receiver functions

    NASA Astrophysics Data System (ADS)

    Fischer, Karen M.; Hopper, Emily

    2015-04-01

    When broadband stations are spaced at ~70 km or less, as with the EarthScope Transportable Array in North America, common conversion point stacking of Sp receiver functions is capable of continuous three-dimensional imaging of velocity gradients at shallow mantle depths, provided that the gradients are localized over ~30 km or less. In the tectonically active western United States, Sp common conversion points stacks reveal a strong and coherent negative velocity gradient (velocity drop with increasing depth) that falls within the transition from high velocity lithosphere to low velocity asthenosphere seen in surface wave tomography. This negative velocity gradient is interpretable as the seismological lithosphere-asthenosphere boundary. Its depth varies significantly across certain tectonic boundaries at horizontal length scales of less than ~75 km, consistent with a rheologically strong mantle lithosphere in which strain can localize. When station spacing is sufficiently dense (~5 km) coherent imaging of discontinuities in the upper and lower crust is possible, even for Sp phases with dominant periods close to 10 s. With data from the 85 broadband stations of the SESAME array in the southeastern United States (an EarthScope Flexible Array experiment) and adjacent Transportable Array and permanent stations, common conversion point stacking of Sp phases resolves strong velocity gradients in the upper and lower crust that are continuous over hundreds of horizontal kilometers. Across the Suwannee suture (the northern edge of the Gondwanan or peri-Gondwanan Suwannee lithosphere that accreted to Laurentia in the last stages of the Appalachian orogeny) a strong positive velocity discontinuity dips southward from the surface expression of the suture to depths of 25-30 km. Modeling with common conversion point stacks of synthetic Sp phases demonstrates that Sp data can resolve the dipping discontinuity, despite the presence of sediment-filled Mesozoic rift basins and

  1. Stored mafic/ultramafic crust and early Archean mantle depletion

    NASA Technical Reports Server (NTRS)

    Chase, Clement G.; Patchett, P. J.

    1990-01-01

    Both early and late Archean rocks from greenstone belts and felsic gneiss complexes exhibit positive epsilon(Nd) values of +1 to +5 by 3.5 Ga, demonstrating that a depleted mantle reservoir existed very early. The amount of preserved pre-3.0 Ga continental crust cannot explain such high epsilon values in the depleted residue unless the volume of residual mantle was very small: a layer less than 70 km thick by 3.0 Ga. Repeated and exclusive sampling of such a thin layer, especially in forming the felsic gneiss complexes, is implausible. Extraction of enough continental crust to deplete the early mantle and its destructive recycling before 3.0 Ga ago requires another implausibility, that the sites of crustal generation of recycling were substantially distinct. In contrast, formation of mafic or ultramafic crust analogous to present-day oceanic crust was continuous from very early times. Recycled subducted oceanic lithosphere is a likely contributor to present-day hotspot magmas, and forms a reservoir at least comparable in volume to continental crust. Subduction of an early mafic/ultramafic oceanic crust and temporary storage rather than immediate mixing back into undifferentiated mantle may be responsible for the depletion and high epsilon(Nd) values of the Archean upper mantle.

  2. Modes of continental decretion in western Canada

    NASA Astrophysics Data System (ADS)

    Cook, F. A.

    2010-12-01

    Between the Archean Slave Province and the Pacific Ocean, the North American continent in Canada appears to have ‘grown’ westward during Paleoproterozoic (Wopmay) and Paleozoic-Mesozoic (Cordilleran) orogenesis. However, results from geological observations combined with regional geophysical surveys lead to interpretations in which pre-orogenic basement in both the Wopmay and the Cordillera projects far beneath proposed ‘accreted’ terranes. In the Wopmay orogen (1.89-1.84 Ga), deep geophysical images indicate that Archean Slave basement projects in the subsurface up to 50 km west of the Medial zone (formerly Wopmay fault), a boundary that was previously interpreted as the transition from Slave basement to accreted terrane basement. The Slave basement appears as a west-tapering tectonic wedge into accreted(?) rocks, consistent with an interpretation by Hildebrand and Bowring (Geology, 1999) that the Archean lithosphere that was formerly west of the wedge broke off and was recycled. In the Cordillera, cross sections of the lithosphere illustrate that lower crustal and upper mantle rocks as much as 2/3 of the distance across the orogen can be stratigraphically, geologically and seismically correlated to the ancient cratonic margin. Retrodeformation of rocks that were deposited on or adjacent to the craton leads to the conclusion that, prior to the onset of terrane accretion, the North American margin and associated rocks projected even farther west (today’s coordinates) - at least as far as the modern margin. Apparently, as flakes of terranes were added to the surface in the western regions of the Cordillera, the North American lithosphere, which may have been foreshortened during contraction, was tectonically and/or thermally eroded from below. As a result, terrane accretion in the Cordillera apparently resulted in a net decretion of continental lithosphere. Together, these observations lead to the interpretation that the processes responsible for

  3. Magnesium isotope fractionation during continental weathering

    NASA Astrophysics Data System (ADS)

    Teng, F. Z.; Huang, K. J.; Li, W.; Liu, X. M.; Ma, L.

    2014-12-01

    Continental weathering links the atmosphere, hydrosphere and continents as it regulates the CO2 content of the atmosphere, shifts the composition of the continental crust from basaltic to andesitic, and ultimately controls the chemical composition of river waters and seawater. Magnesium is a water-soluble major element in the hydrosphere, continental crust and the mantle, and has three stable isotopes (24Mg, 25Mg and 26Mg). Studies of Mg isotopes during continental weathering may help to document the interactions between hydrosphere, crust and mantle. Previous studies have shown that the continental crust has a heterogeneous but on average heavier Mg isotopic composition than the mantle, whereas the hydrosphere is isotopically light. The complementary characteristics of Mg isotopic compositions between continental and hydrosphere have been attributed to continental weathering, with light Mg isotopes partitioned into water, leaving heavy Mg isotopes behind in the crustal residue. Here we summarize our studies of Mg isotope fractionation in four weathering profiles under various climate conditions. We show that large Mg isotope fractionation can occur during continental weathering. Although the weathered residue is usually enriched in heavier Mg isotopes than unaltered parent rocks, some heavily weathered products can be quite light in Mg isotopic composition. The complicated behaviors of Mg isotopes reflect different control factors during weathering such as parent rock lithology, primary mineral dissolution, secondary mineral formation, ion exchange, vegetation uptake etc. Though studies of natural samples can provide direct evidence on isotope fractionation, more well-controlled laboratory experiments on Mg isotope fractionation between fluids and minerals are needed in order to fully understand the behaviors of Mg isotopes during weathering, which ultimately lays the foundation for making Mg isotope geochemistry an important tool for studying different geological

  4. Geochemistry, geochronology and isotope geology of Nakfa intrusive rocks, northern Eritrea: products of a tectonically thickened Neoproterozoic arc crust

    NASA Astrophysics Data System (ADS)

    Teklay, M.; Kröner, A.; Mezger, K.

    2001-08-01

    The north-south-trending Neoproterozoic volcano-sedimentary plutonic associations in northern Eritrea are part of the Nubian Shield. The Nakfa intrusive rocks range in composition from gabbro to syeno-diorite to granite and alkaline syenite and intrude supracrustal rocks of volcanic and sedimentary origin. All granitoid rocks are metaluminous or slightly peraluminous and have typical I-type chemical signatures. The calc-alkaline intrusive rocks and the alkaline syenites have geochemical characteristics (e.g. low Nb values) typical of arc intrusives and plot as volcanic arc granites on various discriminant diagrams. Single zircon evaporation Pb-Pb ages and conventional multigrain U-Pb ages on zircons and titanites yielded emplacement ages of ˜620-640 Ma. These are comparable to those of adjacent juvenile terranes in the Nubian Shield. No pre-Pan-African rocks have so far been found in northern Eritrea. Isotopic data show a limited range, with initial ɛ Nd values ranging from 3.5 to 5.6 and initial Sr ratios from 0.7018 to 0.7037. The high positive initial ɛ Nd values and low initial Sr ratios indicate that the granitoid rocks were derived from a mantle and/or juvenile crustal source with no, or only insignificant, contribution from an older continental component. This is further supported by the absence of inherited zircons and the lack of rocks of continental affinity. Leached K-feldspars from Nakfa intrusive rocks have Pb isotope ratios ( 206Pb/ 204Pb = 17.60-17.88, 207Pb/ 204Pb = 15.49-15.53 and 208Pb/ 204Pb = 37.12-37.37) similar to those for 'oceanic leads' from Saudi Arabia, which are interpreted as manifesting a mantle source. Hence, the Pb isotope ratios, in agreement with the Sr and Nd isotopic data, indicate an insignificant involvement of older crustal components in the generation of Pan-African crust in northern Eritrea. The ages and isotopic characteristics of the Nakfa intrusive rocks are comparable to those of adjacent juvenile terranes in the

  5. Crust and mantle discontinuities beneath the southern Appalachians

    NASA Astrophysics Data System (ADS)

    Hopper, E.; Fischer, K. M.; Wagner, L. S.; Hawman, R. B.; Parker, E. H.; Howell, D. K.

    2013-12-01

    In this study we image crust and mantle discontinuities to better understand processes of continental accretion and rifting in the southeastern U.S. Targets of particular interest are the Laurentia-Gondwana suture proposed in southern Georgia and adjacent regions of Mesozoic extension and magmatism, including the South Georgia Rift basin. We are analyzing Sp and Ps phases recorded by SESAME (the Southeastern Suture of the Appalachian Margin Experiment) which includes 85 broadband EarthScope Flexible Array stations, as well as adjacent Transportable Array and permanent stations. We have obtained Sp and Ps receiver functions for individual waveforms using an extended time multi-taper deconvolution method. Sp receiver functions were migrated into a 3D volume using common conversion point stacking and a spline function representation of phase Fresnel zones. Data collection is still underway, and at this stage image interpretation is more robust beneath western Georgia where stations have been installed the longest (since 2010 and 2011). The Sp stack reveals gently south-dipping phases in the crust. The most prominent is a negative arrival (indicative of a decrease in velocity with depth) that is continuous over more than 500 km of N-S distance beneath western Georgia. This phase appears in the upper crust beneath the mountains of northwestern Georgia and descends to depths just above the Moho in Florida. The top of this phase coincides with the depth of the Appalachian decollement inferred from COCORP reflection data in central Georgia (McBride et al., 2005). The phase is too deep to be associated with the base of the South Georgia Rift Basin, given constraints on sediment thickness from COCORP data and on Vp/Vs ratios from H-k stacking of Ps receiver functions. A similar phase is beginning to emerge from data recorded in eastern Georgia, suggesting that this south-dipping feature exists over ~170 km along the strike of the suture zone. We tentatively interpret this

  6. Continental underthrusting and obduction during the Cretaceous closure of the Rocas Verdes rift basin, Cordillera Darwin, Patagonian Andes

    NASA Astrophysics Data System (ADS)

    Klepeis, Keith; Betka, Paul; Clarke, Geoffrey; Fanning, Mark; Hervé, Francisco; Rojas, Lisandro; Mpodozis, Constantino; Thomson, Stuart

    2010-06-01

    The Patagonian Andes record a period of Cretaceous-Neogene orogenesis that began with the compressional inversion of a Late Jurassic rift called the Rocas Verdes basin. Detrital zircon ages from sediment that filled the southern part of the basin provide a maximum depositional age of ˜148 Ma, suggesting that the basin opened approximately simultaneously along its length during the Late Jurassic. Structural data and U-Pb isotopic ages on zircon from granite plutons near the Beagle Channel (55°S) show that basin inversion involved two stages of shortening separated by tens of millions of years. An initial stage created a small (˜60 km wide) thrust wedge that placed the basaltic floor of the Rocas Verdes basin on top of adjacent continental crust prior to ˜86 Ma. Structures and metamorphic mineral assemblages preserved in an exhumed middle to lower crustal shear zone in Cordillera Darwin suggest that this obduction was accompanied by south directed subduction of the basaltic crust and underthrusting of continental crust to depths of ˜35 km beneath a coeval volcanic arc. A subsequent stage of out-of-sequence thrusting, culminating in the Paleogene, shortened basement and Upper Jurassic igneous rock in the internal part of the belt by at least ˜50 km, forming a bivergent thrust wedge. This latter period coincided with the exhumation of rocks in Cordillera Darwin and expansion of the fold-thrust belt into the Magallanes foreland basin. This orogen provides an important example of how orogenesis initiated and led to continental underthrusting and obduction of basaltic crust during closure of a quasi-oceanic rift basin.

  7. The Oceanic Crust.

    ERIC Educational Resources Information Center

    Francheteau, Jean

    1983-01-01

    The earth's oceanic crust is created and destroyed in a flow outward from midocean ridges to subduction zones, where it plunges back into the mantle. The nature and dynamics of the crust, instrumentation used in investigations of this earth feature, and research efforts/findings are discussed. (JN)

  8. Temperature Control of Continental Lithosphere Elastic Thickness: Effective Elastic Thickness Te vs Upper Mantle Velocity Vs

    NASA Astrophysics Data System (ADS)

    Hyndman, R. D.; Currie, C. A.; Mazzotti, S.; Frederiksen, A.

    2006-12-01

    The elastic thickness of continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes. Recently it has been questioned whether the elastic thickness and strength are dependent on crust and upper mantle temperatures and compositions in the way predicted by laboratory data. We test this dependence in western North America by a regional comparison of the effective elastic thickness (Te) from topography-gravity coherence, and upper mantle temperatures mapped by tomography shear wave velocities (Vs). We find a good correlation between Te and Vs of the form expected based on the thermal and laboratory data. The Te distribution is strongly bimodal as previously found globally, less than 20 km for the high temperature Cordillera and over 100 km for the adjacent cold stable Canadian Shield. Only intermediate thermal regimes have intermediate Te that suggests a weak layer in the lower crust over a stronger upper mantle. Strength envelopes based on laboratory data correspond to the observed Te for thermal regimes with temperatures at the Moho of 800-900C for the Cordillera and 400-500C for the Shield, in agreement with temperatures from Vs and other estimators. Our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature and that laboratory- based rheology provides a good first order estimate of the deformation behaviour of the crust and upper mantle. The Cordillera and other continental backarcs are weak enough to be deformed by plate boundary forces, whereas cratons are generally much too strong. In the Cordillera, the upper mantle is too hot for brittle failure and earthquakes occur only in the upper 10-15 km of the crust. In the cool craton, earthquakes occur rarely in the upper mantle because the total lithosphere strength is too great for significant deformation by plate tectonic forces.

  9. Evolution of the Archaean crust by delamination and shallow subduction.

    PubMed

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E

    2003-01-16

    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time.

  10. Seismic-reflection signature of cretaceous continental breakup on the Wilkes Land margin, Antarctica

    USGS Publications Warehouse

    Eittreim, S.L.; Hampton, M.A.; Childs, J. R.

    1985-01-01

    The passive (rifted) continental margin of Wilkes Land, Antarctica, is characterized on seismic reflection records by (i) in the south, a block-faulted sequence of highly stratified continental beds overlain by two distinct unconformities; (ii) a transitional, greatly thinned continental crust overlain by material interpreted to be flood basalt; and (iii) in the north, oceanic crust with a boundary ridge at its edge. The Mohorovic??ic?? discontinuity can be followed across the continent-ocean boundary and shows a progressive thinning of continental crust to a minimum of 2.5 kilometers at its northern edge.

  11. Seismic-reflection signature of cretaceous continental breakup on the wilkes land margin, antarctica.

    PubMed

    Eittreim, S L; Hampton, M A; Childs, J R

    1985-09-13

    The passive (rifted) continental margin of Wilkes Land, Antarctica, is characterized on seismic reflection records by (i) in the south, a block-faulted sequence of highly stratified continental beds overlain by two distinct unconformities; (ii) a transitional, greatly thinned continental crust overlain by material interpreted to be flood basalt; and (iii) in the north, oceanic crust with a boundary ridge at its edge. The Mohorovicić discontinuity can be followed across the continent-ocean boundary and shows a progressive thinning of continental crust to a minimum of 2.5 kilometers at its northern edge.

  12. Influence of the mechanical coupling and inherited strength variations on the geometry of continental rifts.

    NASA Astrophysics Data System (ADS)

    Philippon, Melody; van Delft, Pim; van Winden, Matthijs; Zamuroviç, Dejan; Sokoutis, Dimitrios; Willingshofer, Ernst; Cloetingh, Sierd

    2013-04-01

    The geometry of continental rifts is strongly controlled by the rheology of the lithosphere at the onset of rifting. This initial geometry will further control the development of ocean spreading centers and the structure of adjacent passive margins. Therefore, understanding the influence of coupling between the different layers of the lithosphere with and without laterally variable strength in the crust is key when investigating continental rifts. In this study we infer the influence of coupling in the crust on the rift geometry by means of crustal scale analogue experiments, where we characterize the response of the crust to deformation in terms of the strength ratio between brittle and ductile crust. The degree of coupling has been varied for setups containing or not a pre-existing weak zone. To allow a better description of the geometry obtained in our models, some key observations such as: a) the degree of tilting of the blocks, b) the total width of the graben, c) the displacement along the main fault and d) the distribution of thinning in the lower crust are monitored. Models containing a weak zone are compared to natural examples of the inherited Mozambique Ocean suture zones (MOSZ) in the Red Sea rift. The modelling results suggest that deformation is not a-priori localized within pre-existing weak zones unless the coupling between the brittle and the ductile crust is high. With respect to the MOSZ, we infer that: (1) Jurassic NW-SE trending grabens developed parallel to but not within the MOSZ and hence reflect a low degree of coupling whereas (2) Eocene rifting in the Red Sea occurred under coupled conditions as deformation strongly focused within the MOSZ. Models without weak zone shows that large-scale detachment faults can also form within a highly coupled crust, which is at variance to the common perception that detachment faulting demands strong decoupling. Our findings shed light on natural rift systems, which show a wide range of geometries that

  13. Spreading and slope instability at the continental margin offshore Mt Etna, imaged by high-resolution 2D seismic data

    NASA Astrophysics Data System (ADS)

    Gross, Felix; Krastel, Sebastian; Behrmann, Jan-Hinrich; Papenberg, Cord; Geersen, Jacob; Ridente, Domenico; Latino Chiocci, Francesco; Urlaub, Morelia; Bialas, Jörg; Micallef, Aaron

    2015-04-01

    Mount Etna is the largest active volcano in Europe. Its volcano edifice is located on top of continental crust close to the Ionian shore in east Sicily. Instability of the eastern flank of the volcano edifice is well documented onshore. The continental margin is supposed to deform as well. Little, however, is known about the offshore extension of the eastern volcano flank and its adjacent continental margin, which is a serious shortcoming in stability models. In order to better constrain the active tectonics of the continental margin offshore the eastern flank of the volcano, we acquired and processed a new marine high-resolution seismic and hydro-acoustic dataset. The data provide new detailed insights into the heterogeneous geology and tectonics of shallow continental margin structures offshore Mt Etna. In a similiar manner as observed onshore, the submarine realm is characterized by different blocks, which are controlled by local- and regional tectonics. We image a compressional regime at the toe of the continental margin, which is bound to an asymmetric basin system confining the eastward movement of the flank. In addition, we constrain the proposed southern boundary of the moving flank, which is identified as a right lateral oblique fault movement north of Catania Canyon. From our findings, we consider a major coupled volcano edifice instability and continental margin gravitational collapse and spreading to be present at Mt Etna, as we see a clear link between on- and offshore tectonic structures across the entire eastern flank. The new findings will help to evaluate hazards and risks accompanied by Mt Etna's slope- and continental margin instability and will be used as a base for future investigations in this region.

  14. Continuous Mantle Exhumation at the Outer Continental Margin of the Santos, Campos and Espírito Santo Basins, Brazil

    NASA Astrophysics Data System (ADS)

    Zalan, P. V.; Severino, M. G.; Rigoti, C. A.; Magnavita, L. P.; Oliveira, J. B.; Viana, A. R.

    2011-12-01

    continental crust pinches out invariably on the flanks of exhumed mantle. This gives rise to a remarkable long (900 km along a N-S direction and 600 km in E-W direction), relatively narrow (15 to 70 km wide) and continuous belt of exhumed mantle that marks the passage from continental crust to oceanic crust in all three basins. The Santos, Campos and Espírito Santo Basins thus form a typical magma-poor passive margin. These are in sharp contrast with the adjacent basin to the south, the Pelotas Basin, that in turn is a typical volcanic passive margin displaying a long (1000 km in a N-S direction) and wide (100 to 220 km) belt of seaward-dipping reflectors at its outer margin and no exhumation of the mantle at the continent-ocean boundary.

  15. Continental volume and freeboard through geological time

    NASA Technical Reports Server (NTRS)

    Schubert, G.; Reymer, A. P. S.

    1985-01-01

    The consequences of approximately constant freeboard for continental growth are explored using a model that relates the volumes of isostatically compensated continents and oceans to the secular decline in terrestrial heat flow. It is found that a post-Archean increase in freeboard by 200 m requires continental growth of only 10 percent, while a decrease in freeboard by 200 m during this same period necessitates a crustal growth of 40 percent. Shrinkage of the continental crust since the end of the Archean can be ruled out. Changes of more than 10 percent in post-Archean crustal thickness are highly unlikely.

  16. Moroccan crustal response to continental drift.

    PubMed

    Kanes, W H; Saadi, M; Ehrlich, E; Alem, A

    1973-06-01

    The formation and development of a zone of spreading beneath the continental crust resulted in the breakup of Pangea and formation of the Atlantic Ocean. The crust of Morocco bears an extremely complete record of the crustal response to this episode of mantle dynamics. Structural and related depositional patterns indicate that the African margin had stabilized by the Middle Jurassic as a marine carbonate environment; that it was dominated by tensile stresses in the early Mesozoic, resulting in two fault systems paralleling the Atlantic and Mediterranean margins and a basin and range structural-depositional style; and that it was affected by late Paleozoic metamorphism and intrusion. Mesozoic events record the latter portion of African involvement in the spreading episode; late Paleozoic thermal orogenesis might reflect the earlier events in the initiation of the spreading center and its development beneath significant continental crust. In that case, more than 100 million years were required for mantle dynamics to break up Pangea.

  17. Continental shelves

    SciTech Connect

    Postma, H.; Zijlstra

    1987-01-01

    Continental shelves form a relatively narrow fringe, of varying width, around the continents. Altogether they take up only about 7% of the ocean's surface and less than 0.2% if its volume. Nevertheless, their specific biological characteristics and economical importance justify a separate discussion in this series. Ecosystems of the World. The specific biological characteristics are due to the position of continental shelves between the land masses on one side and the oceans on the other, to their relative shallowness and variable sea-floor texture and to the fact that, besides residual currents, tidal streams exert a great influence on the movements of water bodies.

  18. Evolution of the earth's crust: Evidence from comparative planetology

    NASA Technical Reports Server (NTRS)

    Lowman, P. D., Jr.

    1973-01-01

    Geochemical data and orbital photography from Apollo, Mariner, and Venera missions were combined with terrestrial geologic evidence to study the problem of why the earth has two contrasting types of crust (oceanic and continental). The following outline of terrestrial crustal evolution is proposed. A global crust of intermediate to acidic composition, high in aluminum, was formed by igneous processes early in the earth's history; portions survive in some shield areas as granitic and anorthositic gneisses. This crust was fractured by major impacts and tectonic processes, followed by basaltic eruptions analogous to the lunar maria and the smooth plains of the north hemisphere of Mars. Seafloor spreading and subduction ensued, during which portions of the early continental crust and sediments derived therefrom were thrust under the remaining continental crust. The process is exemplified today in regions such as the Andes/Peru-Chile trench system. Underplating may have been roughly concentric, and the higher radioactive element content of the underplated sialic material could thus eventually cause concentric zones of regional metamorphism and magmatism.

  19. What Hf isotopes in zircon tell us about crust-mantle evolution

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi; Yamaguchi, Takao; Itano, Keita; Hibiya, Yuki; Suzuki, Kazue

    2017-03-01

    The 176Lu-176Hf radioactive decay system has been widely used to study planetary crust-mantle differentiation. Of considerable utility in this regard is zircon, a resistant mineral that can be precisely dated by the U-Pb chronometer and record its initial Hf isotope composition due to having low Lu/Hf. Here we review zircon U-Pb age and Hf isotopic data mainly obtained over the last two decades and discuss their contributions to our current understanding of crust-mantle evolution, with emphasis on the Lu-Hf isotope composition of the bulk silicate Earth (BSE), early differentiation of the silicate Earth, and the evolution of the continental crust over geologic history. Meteorite zircon encapsulates the most primitive Hf isotope composition of our solar system, which was used to identify chondritic meteorites best representative of the BSE (176Hf/177Hf = 0.282793 ± 0.000011; 176Lu/177Hf = 0.0338 ± 0.0001). Hadean-Eoarchean detrital zircons yield highly unradiogenic Hf isotope compositions relative to the BSE, providing evidence for the development of a geochemically enriched silicate reservoir as early as 4.5 Ga. By combining the Hf and O isotope systematics, we propose that the early enriched silicate reservoir has resided at depth within the Earth rather than near the surface and may represent a fractionated residuum of a magma ocean underlying the proto-crust, like urKREEP beneath the anorthositic crust on the Moon. Detrital zircons from world major rivers potentially provide the most robust Hf isotope record of the preserved granitoid crust on a continental scale, whereas mafic rocks with various emplacement ages offer an opportunity to trace the Hf isotope evolution of juvenile continental crust (from εHf[4.5 Ga] = 0 to εHf[present] = + 13). The river zircon data as compared to the juvenile crust composition highlight that the supercontinent cycle has controlled the evolution of the continental crust by regulating the rates of crustal generation and intra

  20. Icelandic-type crust

    USGS Publications Warehouse

    Foulger, G.R.; Du, Z.; Julian, B.R.

    2003-01-01

    Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ?? 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15-30 ?? 5 km thick and begins where the velocity gradient decreases radically. This generally occurs at the V p ??? 6.5 km s-1 level. A low-velocity zone ??? 10 000 km2 in area and up to ??? 15 km thick occupies the lower crust beneath central Iceland, and may represent a submerged, trapped oceanic microplate. The crust-mantle boundary is a transition zone ???5 ?? 3 km thick throughout which V p increases progressively from ???7.2 to ???8.0 km s-1. It may be gradational or a zone of alternating high- and low-velocity layers. There is no seismic evidence for melt or exceptionally high temperatures in or near this zone. Isostasy indicates that the density contrast between the lower crust and the mantle is only ???90 kg m-3 compared with ???300 kg m-3 for normal oceanic crust, indicating compositional anomalies that are as yet not understood. The seismological crust is ???30 km thick beneath the Greenland-Iceland and Iceland-Faeroe ridges, and eastern Iceland, ???20 km beneath western Iceland, and ???40 km thick beneath central Iceland. This pattern is not what is predicted for an eastward-migrating plume. Low attenuation and normal V p/V s ratios in the lower crust beneath central and southwestern Iceland, and normal uppermost mantle velocities in general, suggest that the crust and uppermost mantle are subsolidus and cooler than at equivalent depths beneath the East Pacific Rise. Seismic data from Iceland have historically been interpreted both in terms of thin-hot and thick-cold crust models, both of which have been cited as supporting the plume

  1. Dynamics of continental accretion.

    PubMed

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

  2. Dynamics of continental accretion

    NASA Astrophysics Data System (ADS)

    Moresi, L.; Betts, P. G.; Miller, M. S.; Cayley, R. A.

    2014-04-01

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

  3. Dynamics of Continental Accretion

    NASA Astrophysics Data System (ADS)

    Moresi, L. N.; Betts, P. G.; Miller, M. S.

    2013-12-01

    Subduction zones become congested when they try to swallow buoyant exotic crust. Accretionary mountain belts (orogens) that form at these convergent plate margins are the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North and South American Cordilleras and southwest Pacific. The geologic record is riddled with accretionary orogens, such as the Tasmanides along the eastern margin of the supercontinent Gondwana and the Altaides that formed on the southern margin of Laurasia. Both the modern and ancient examples are characterised by episodic switches between extension and shortening associated with transitions from collision of exotic crust and subduction related rollback. We present three-dimensional dynamic models that show for the first time how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back arc region. The complexity of the morphology and evolution of the system are driven by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonal to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, but infer that this is a global phenomena throughout Earth's evolution.

  4. Secular changes recorded in mineralization of African crust

    NASA Astrophysics Data System (ADS)

    Mabidi, T.; Thiart, C.; de Wit, M. J.

    2007-02-01

    Variations in enrichment of mineral deposits in continental crust over time may be one way to test for secular changes in crustal genesis. We present spatial and chemical information about African mineral deposits with which to 'fingerprint' the metal endowment of African crust of different age. We then compare three regions of juvenile African crust, all with similar geology, tectonic history, and mineral deposits, but each of a different age. Each region was formed during rapid accretion of similar tectonic units derived from the mantle over ˜500 million years, and is apparently devoid of older recycled continental crust. Together, the three areas span 2500 million years of Earth history, from 0.5 Ga to 3.0 Ga, (e.g. the Zimbabwe Craton (2.5-3.0 Ga), the Birimian Shield (1.8-2.3 Ga), and the Arabian-Nubian Shield (0.5-1.0 Ga)). The three areas have a studied total of 2671 mineral deposits that are divided into six groups according to their geochemical affinities. Using these known deposits, a measure of spatial association (spatial coefficient) is derived. We show that each region has a unique metal endowment and that, per unit area, there is a greater concentration of mineral deposits in the crust of the Archean Zimbabwe Craton relative to the younger crust of the Birimian Shield and in turn the Arabian-Nubian Shield. This study quantitatively corroborates past studies that suggest older crust is more mineral diverse and enriched in mineral deposits than younger crust. Thus, a secular change in mineralization is implicated, and the mantle derived metal endowment of the African crust has undergone major evolutionary changes from Archean to Neoproterozoic time.

  5. The nature of orogenic crust in the central Andes

    NASA Astrophysics Data System (ADS)

    Beck, Susan L.; Zandt, George

    2002-10-01

    The central Andes (16°-22°S) are part of an active continental margin mountain belt and the result of shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. We have combined receiver function and surface wave dispersion results from the BANJO-SEDA project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure. The major results are as follows: (1) The crust supporting the high elevations is thick and has a felsic to intermediate bulk composition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west (65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but does not underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminating piecemeal under the Altiplano-EC boundary but is not completely removed beneath the central Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation, leading to lower crustal flow and flat topography. In contrast, in the high-relief, inland-sloping regions of the EC and sub-Andean zone, the upper crust is still strongly coupled across the basal thrust of the fold-thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustal shortening between the Altiplano and Brazilian lithosphere appears to be accommodated by delamination near the Altiplano-EC boundary. Our study suggests that orogenic reworking may be an important part of the "felsification" of continental crust.

  6. The Mafic Lower Crust of Neoproterozoic age beneath Western Arabia: Implications for Understanding African Lower Crust

    NASA Astrophysics Data System (ADS)

    Stern, R. J.; Mooney, W. D.

    2011-12-01

    We review evidence that the lower crust of Arabia - and by implication, that beneath much of Africa was formed at the same time as the upper crust, rather than being a product of Cenozoic magmatic underplating. Arabia is a recent orphan of Africa, separated by opening of the Red Sea ~20 Ma, so our understanding of its lower crust provides insights into that of Africa. Arabian Shield (exposed in W. Arabia) is mostly Neoproterozoic (880-540 Ma) reflecting a 300-million year process of continental crustal growth due to amalgamated juvenile magmatic arcs welded together by granitoid intrusions that make up as much as 50% of the Shield's surface. Seismic refraction studies of SW Arabia (Mooney et al., 1985) reveal two layers, each ~20 km thick, separated by a well-defined Conrad discontinuity. The upper crust has average Vp ~6.3 km/sec whereas the lower crust has average Vp ~7.0 km/sec, corresponding to a granitic upper crust and gabbroic lower crust. Neogene (<30 ma) lava fields in Arabia (harrats) extend over 2500 km, from Yemen to Syria. Many of these lavas contain xenoliths, providing a remarkable glimpse of the lower-crustal and upper-mantle lithosphere beneath W. Arabia. Lower crustal xenoliths brought up in 8 harrats in Saudi Arabia, Jordan, and Syria are mostly 2-pyroxene granulites of igneous (gabbroic, anorthositic, and dioritic) origin. They contain plagioclase, orthopyroxene, and clinopyroxene, and a few contain garnet and rare amphibole and yield mineral-equilibrium temperatures of 700-900°C. Pyroxene-rich and plagioclase-rich suites have mean Al2O3 contents of 13% and 19%, respectively: otherwise the two groups have similar elemental compositions, with ~50% SiO2 and ~1% TiO2, with low K2O (<0.5%) and Na2O (1-3%). Both groups show tholeiitic affinities, unrelated to their alkali basalt hosts. Mean pyroxene-rich and plagioclase-rich suites show distinct mean MgO contents (11% vs. 7%), Mg# (67 vs. 55), and contents of compatible elements Ni (169 vs. 66 ppm

  7. A large-scale detachment fault system in deep water area of South China Sea under the background of continental passive rifted margin: A case study of Heshan Sag

    NASA Astrophysics Data System (ADS)

    Lin, Zi; Ren, Jianye; Han, Xiaoying; Chen, Lin

    2015-04-01

    Heshan sag locates in the deep-water area of continental passive rifted margin of northern South China Sea, where preserved strong thinning crust of the ocean-continent transition zone. This area owns particular and unique tectonic settings with the geological characteristics of lithospheric ductile deformation and high temperature gradient, which highly differs from the continental shelf area. In this research, we try to determine the detachment faults within Heshan sag and build its tectonic-stratigraphic framework and geological evolution history as well. The research achievements are as followed: (1) According to the interpretation of 2D seismic profiles covering the whole study area, we basically recognized that Heshan sag was a detachment basin which is floored by a large-scale seaward detachment fault with a very low angle of 10 degrees to 20 degrees and overlain by tilted and hyper extended hanging wall block. And the heave of detachment fault has been extended to approximately 20 km. (2) Based on the latest geophysical and geological data, in the northern part of South China Sea, the strike of Moho surface is from NE to SW. The overall Moho depth is between 10 and 29 km from north to south, from shelf to continental slope and abyssal plain, mirroring the topography of the sediment basement, which means the crustal thickness decreases from land to ocean. Our study area exactly exists in the zone that the crust is hyper-extended and has the characters of continental crust and transitional crust. (3) Continental passive rifted margin closely relates to the ocean-continent transition zone (OCT). Guided by the basin dynamic analysis and lithosphere extension and breakup theory, combined with overseas and domestic research status on OCT and the latest information of adjacent areas, we tried to elucidate the geological nature of OCT and construct continental lithosphere configuration in different tectonic units on the northern continental margin in South China

  8. Ductile extensional shear zones in the lower crust of a passive margin

    NASA Astrophysics Data System (ADS)

    Clerc, Camille; Jolivet, Laurent; Ringenbach, Jean-Claude

    2015-12-01

    We describe and interpret an unpublished set of ION Geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the Uruguayan volcanic rifted margin. We distinguish two main groups of reflectors in the lowermost continental crust. A first group, at depths ranging from 32 km below the continent to 16 km in the continent-ocean transition, comprises reflectors continuous over tens of kilometers, peculiarly visible near the mantle-crust boundary. A second group of reflectors dipping toward the ESE (oceanward) is widely distributed in the lower crust. These reflectors are slightly curved and tend to merge and become sub-parallel with the first group of reflectors. Together they draw the pattern of thick shallow-dipping top-to-the continent shear zones affecting the lower continental crust. Such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn-tectonic volcanic formations (SDR). A major portion of the continental crust behaved in a ductile manner and recorded a component of top-to-the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle.

  9. Melt evolution and residence in extending crust: Thermal modeling of the crust and crustal magmas

    NASA Astrophysics Data System (ADS)

    Karakas, Ozge; Dufek, Josef

    2015-09-01

    Tectonic extension and magmatism often act in concert to modify the thermal, mechanical, and chemical structure of the crust. Quantifying the effects of extension and magma flux on melting relationships in the crust is fundamental to determining the rate of crustal melting versus fractionation, magma residence time, and the growth of continental crust in rift environments. In order to understand the coupled control of tectonic extension and magma emplacement on crustal thermal evolution, we develop a numerical model that accounts for extension and thermal-petrographic processes in diverse extensional settings. We show that magma flux exerts the primary control on melt generation and tectonic extension amplifies the volume of melt residing in the crustal column. Diking into an extending crust produces hybrid magmas composed of 1) residual melt remaining after partial crystallization of basalt (mantle-derived melt) and 2) melt from partial melting of the crust (crustal melt). In an extending crust, mantle-derived melts are more prevalent than crustal melts across a range of magma fluxes, tectonic extension rates, and magmatic water contents. In most of the conditions, crustal temperatures do not reach their solidus temperatures to initiate partial melting of these igneous lithologies. Energy balance calculations show that the total enthalpy transported by dikes is primarily used for increasing the sensible heat of the cold surrounding crust with little energy contributing to latent heat of melting the crust (maximum crustal melting efficiency is 6%). In the lower crust, an extensive mush region develops for most of the conditions. Upper crustal crystalline mush is produced by continuous emplacement of magma with geologically reasonable flux and extension rates on timescales of 106 yr. Addition of tectonic effects and non-linear melt fraction relationships demonstrates that the magma flux required to sustain partially molten regions in the upper crust is within the

  10. Dynamics of Pre-3 Ga Crust-Mantle Evolution

    NASA Astrophysics Data System (ADS)

    Patchett, P. J.; Chase, C. G.; Vervoort, J. D.

    2004-05-01

    During 3.0 to 2.7 Ga, the Earth's crust underwent a non-uniformitarian change from a pre-3.0 Ga environment where long-term preservation of cratons was rare and difficult, to post-2.7 Ga conditions where cratons were established and new continental crust generation took place largely at craton margins. Many models view the Earth's surface during pre-3 Ga time as broadly equivalent to the post 2.7 Ga regime. Any such uniformitarian or gradual evolution cannot explain the conundrum that only a tiny amount of pre-3 Ga crust is preserved today coupled with the fact that very little pre-3 Ga crust was incorporated into the large amount of new craton that came into existence during 3.0-2.7 Ga. If large volumes of pre-3 Ga continental crust existed, it disappeared either just prior to 3 Ga, or during 3.0-2.7 Ga. To explain sudden appearance of surviving but dominantly juvenile continental crust in a model where continents were large prior to 3 Ga, it would be necessary either that pre-3 Ga continent was recycled into the mantle at sites systematically different from those where new 3.0-2.7 Ga crust was made, or that widespread continent destruction preceded the 3.0-2.7 Ga crustal genesis. From expected mantle overturn in response to the heat budget, it is likely that most pre-3 Ga crust was both more mafic and shorter-lived than after 3 Ga. Although Nd and Hf ratios for pre-3 Ga rocks are uncertain due to polymetamorphism, it appears that depleted upper mantle was widespread by 2.7 Ga, even pre-3 Ga. Depletion may have been largely achieved by formation, subduction and storage of mafic crust for periods of 200-500 m.y. The rapid change to large surviving continents during 3.0-2.7 Ga was due to declining mantle overturn, and particularly to development of the ability to maintain subduction in one zone of the earth's surface for the time needed to allow evolution to felsic igneous rock compositions. In as much as storage of subducted slabs is probably occurring today, and

  11. Physical constraints on dolomite crust formation, Ambergris Cay Belize

    SciTech Connect

    Birdwell, B.A.; Bischoff, W.D.; Mazzullo, S.J. )

    1990-05-01

    Dolomitic crusts forming on a peritidal flat on Ambergris Cay, Belize, occur beneath surface sediment adjacent to, but not within, small saline (60-90 ppt) ponds. Upper crusts, 2-12 cm thick forming at or slightly below the water table (approximately equivalent to lagoon water level) are areally restricted by (1) ponds where sediment lies below 20-50 cm of water, (2) high and relatively dry areas where sediment accumulation of more than 15 cm above water level supports diverse vegetation, and (3) low areas affected by mangrove encroachment where preexisting crusts are perforated by roots and displaced. The lower crusts occur immediately above the Pleistocene in lows beneath the Holocene sediment and on exposed Pleistocene surfaces. Estimates from x-ray diffraction analysis indicate 80-100% dolomite content within the upper crusts and 50-60% dolomite content in the lower crusts. Unlithified sediment above and below the upper crust contain up to 80% dolomite. Compositions range from Ca{sub 56}, Mg{sub 44} in the upper crusts to Ca{sub 60} Mg{sub 40} in the lower crusts. There is no correlation between stoichiometry and ordering in the dolomites; all are poorly ordered as indicated by very weak (015) and (021) superstructure peaks. Where crusts are not 100% dolomite, the dolomite is evident as euhedral cements within pores, especially within foraminiferal tests, and as micrite along algal laminations and walls of burrows. However, preliminary examinations with scanning electron microscopy and energy dispersive x-ray mapping show that magnesium enrichment is pervasive within these crusts and may represent Mg-enrichment of calcite as an intermediate stage in dolomite formation.

  12. Ophiolites and oceanic crust

    USGS Publications Warehouse

    Moores, E.M.; Jackson, E.D.

    1974-01-01

    OPHIOLITES consist of a pseudostratiform sequence, of harzburgite, tectonite, ultramafic and mafic cumulates sometimes including gabbro and quartz diorite (plagiogranite) intrusions, dolerite dyke swarms, pillow lava 1, and deep-sea sediments2-4. This assemblage occurs in all Phanerozoic mountain systems and is interpreted as fossil oceanic crust and uppermost mantle5-10. Outstanding problems include differences between the chemical properties of Ophiolites and rocks thought to represent present-day oceanic crust11,12, the lack in some complexes of recognised dyke swarms or cumulates, and the relative thinness of ophiolite mafic rocks compared with standard oceanic crustal sections5,8,13. ?? 1974 Nature Publishing Group.

  13. A study of models and controls for basin formation during continental collision: (1) Australian lithosphere along Banda orogen (Indonesia) and (2) Alboran Sea basin (western Mediterranean)

    NASA Astrophysics Data System (ADS)

    Tandon, Kush

    Mechanisms for the formation of a foreland basin at the beginning of continental collision (Pliocene-Recent Australian continental foreland along the Banda orogen) and a post-orogenic, continental, Miocene extensional basin (Alboran Sea) are studied. Such a study investigates the controls on the basin formation during the start of the Wilson cycle and later during the break up of a thickened continent in a collisional environment. Effective Elastic Thickness (EET) of the Australian continental lithosphere from Roti to the Kai Plateau ({˜}121sp°{-}137sp°E longitude) are estimated using an elastic-half beam model to match the sea floor bathymetry and the Bouguer gravity anomalies. Range of constant EET values from 27-75 km across the shelf of Australian lithosphere shows a variation of 64% with the highest value in the vicinity of central Timor where the collision is most advanced. Downdip on the Australian continental lithosphere from shelf to beneath the Banda orogen, the reduction in EET is from ˜90 km-˜30 km (66%). Variations in EET can be explained by inelastic yielding (brittle and plastic failure, crust-mantle decoupling in the lower crust and brittle-ductile decoupling in the upper-middle crust) in the Australian lithosphere. Change in EET occurred at the start of continental subduction due to change in curvature, both in map and cross-sectional view. Oroclinal bending of the continental Australian lithosphere increased the inelastic failure in the eastern end. Different mechanisms of basin formation at a site of post-orogenic collapse are studied by constraining the timing of rifting in the western, eastern, and northern parts of the Alboran Sea basin on seismic reflectors via synthetic seismograms using ODP Leg 161 and Andalucia A-1 data. Regions of adjacent coeval compression and extension are found in the Alboran Sea basin. Normal faulting continues in parts of the eastern Alboran Sea basin later than in the western Alboran Sea basin. The development

  14. Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin

    NASA Astrophysics Data System (ADS)

    Yegorova, Tamara; Bakhmutov, Vladimir; Janik, Tomasz; Grad, Marek

    2011-01-01

    The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

  15. Continental rifts and mineral resources

    SciTech Connect

    Burke, K. . Geosciences Dept.)

    1992-01-01

    Continental rifts are widespread and range in age from the present to 3 b.y. Individual rifts may form parts of complex systems as in E. Africa and the Basin and Range. Rifts have originated in diverse environments such as arc-crests, sites of continental collision, collapsing mountain belts and on continents at rest over the mantle circulation pattern. Continental rift resources can be classified by depth of origin: For example, in the Great Dike, Norilsk and Mwadui magma from the mantle is the host. At shallower depths continental crust partly melted above mafic magma hosts ore (Climax, Henderson). Rift volcanics are linked to local hydrothermal systems and to extensive zeolite deposits (Basin and Range, East Africa). Copper (Zambia, Belt), zinc (Red Dog) and lead ores (Benue) are related to hydrothermal systems which involve hot rock and water flow through both pre-rift basement and sedimentary and volcanic rift fill. Economically significant sediments in rifts include coals (the Gondwana of Inida), marine evaporites (Lou Ann of the Gulf of Mexico) and non-marine evaporites (East Africa). Oil and gas in rifts relate to a variety of source, reservoir and trap relations (North Sea, Libya), but rift-lake sediment sources are important (Sung Liao, Bo Hai, Mina, Cabinda). Some ancient iron ores (Hammersley) may have formed in rift lakes but Algoman ores and greenstone belt mineral deposits in general are linked to oceanic and island arc environments. To the extent that continental environments are represented in such areas as the Archean of the Superior and Slave they are Andean Arc environments which today have locally rifted crests (Ecuador, N. Peru). The Pongola, on Kaapvaal craton may, on the other hand represent the world's oldest preserved, little deformed, continental rift.

  16. Workshop on the Deep Continental Crust of South India

    NASA Technical Reports Server (NTRS)

    Ashwal, Lewis D. (Editor)

    1988-01-01

    The various theories in the field on the Southern Indian granulite formation are discussed in this workshop proceeding. That some widely accepted ideas about the origin of granulites, particularly relating to the role of metamorphic fluids, will have to be modified, is one outcome of the workshop. Extended abstracts of the papers presented, summaries of the attendant discussions, up-to-date accounts of the geology of the South Indian Precambrian Shield, and detailed field trip guides to all areas visited are presented. It should serve as a convenient source of information and reference to those interested in the classic Precambrian high grade terrains on Earth.

  17. Chemical transport through continental crust: (Annual) progress report, 1988

    SciTech Connect

    Not Available

    1989-03-20

    The main objective of these studies is to understand the extent and mechanisms of chemical migration over a range of temperatures and in diverse geologic media. During 1988--1989 we continued to attack these problems through studies in the granite-pegmatite systems of the Black Hills, South Dakota. Mineral chemistry, major element chemistry and trace element modeling of the Harney Peak Granite (Black Hills, South Dakota) suggest that 75% to 80% fractional crystallization was the dominant mechanism in producing evolved tourmaline-bearing granite (high B, Li, Rb, Cs, Be, Nb) from a biotite-muscovite granite. To evaluate the petrogenetic-evolutionary relations between the granite and the surrounding rare-element pegmatite field, over 500 K-feldspars (Kf) were analyzed from 60 unzoned to complexly zoned pegmatites. Pegmatites with Kf relatively high in Ba (>140 ppM) and relatively low in Rb (<1000 ppM) and Cs (<30 ppM) are distributed in regions of high pegmatite density (>200 pegmatites/sq. mile), whereas highly evolved pegmatites with Kf enriched in Rb (>4000 ppM) and Cs (>500 ppM) are distributed in regions of low pegmatite density (<100 pegmatites/sq. mile). The extent of pegmatite evolution as reflected in the Kf documents the relation between the degree of fractionation and internal zoning characteristics. Modeling of these data is a major task for the next grant year to provide new insights into chemical and thermal transport in the midcrust.

  18. Chemical constraints on the evolution of Archean continental crust

    NASA Technical Reports Server (NTRS)

    Weaver, B. L.; Tarney, J.

    1985-01-01

    One of the challenges of Archean geochronology is to find isotopic systems that preserve an indication of a rock's primary age in spite of the effects of later metamorphism. Zircon dating has been used widely with considerable success but not without difficulty, especially in polymetamorphic terrains. Zircons in such cases commonly are found to have lost radiogenic Pb, and despite fractionizing the zircons or abrading them to remove disturbed portions; it often is not possible to define a pattern of Pb loss from which the original age can confidently be inferred. The refinement of techniques to enable extremely small samples, or even single crystals, to be analyzed has contributed greatly to solving the problem but even those techniques cannot resolve the micron scale isotopic heterogeneities within single zircons in which much of their history is recorded. That can only be done by ion microprobe. Progress reports on studies of four Archean rocks, each of which illustrates the power and potential of ion microprobe analysis in solving problems of Archean geochronology are discussed.

  19. Flexural extension of the upper continental crust in collisional foredeeps

    USGS Publications Warehouse

    Bradley, D.C.; Kidd, W.S.F.

    1991-01-01

    Normal faults on the outer slopes of trenches and collisional foredeeps reveal that high-amplitude lithospheric flexure can result in inelastic extensional deformation of the convex side of a flexed plate. This process, which we call "flexural extension', differs fundamentally from rifting in that the lower lithosphere contracts while the upper lithosphere extends. Structural evidence does not support common illustrations of flexural normal faults as planar-irrotational structures which simply die out at shallow crustal depths. Instead, the surface geology shows that flexural normal faulting must be rotational with respect to the enveloping surface of the flexed plate. This toppled domino geometry implies the presence at depth of a detachment or zone of distributed ductile simple shear where fault displacement and block rotation are accommodated. -from Authors

  20. Mineralization through geologic time: Evolution of continental crust

    NASA Astrophysics Data System (ADS)

    Veizer, Jan; Laznicka, Peter; Jansen, S. L.

    In analogy to living systems, geologic entities (e.g., rocks, mineral deposits, tectonic realms and domains) are involved in the process of perpetual generation and destruction (birth/death cycles). This results in time distribution patterns akin to age structures in living populations and the systematics is amenable to treatment by the concepts of population dynamics. Utilizing this theoretical approach, the survivorship patterns for major realms of the plate tectonic system, for consitutent rocks, and for the entombed mineral resources are predicted. The present inventory encompasses global economic accumulations of metals by geologic age. The deposits of these metals were assigned to nine broad genetic categories, with an attempt to relate each category to tectonic setting within the framework of global plate tectonics.

  1. Mineralization through geologic time: Evolution of continental crust

    NASA Technical Reports Server (NTRS)

    Veizer, Jan; Laznicka, Peter; Jansen, S. L.

    1988-01-01

    In analogy to living systems, geologic entities (e.g., rocks, mineral deposits, tectonic realms and domains) are involved in the process of perpetual generation and destruction (birth/death cycles). This results in time distribution patterns akin to age structures in living populations and the systematics is amenable to treatment by the concepts of population dynamics. Utilizing this theoretical approach, the survivorship patterns for major realms of the plate tectonic system, for consitutent rocks, and for the entombed mineral resources are predicted. The present inventory encompasses global economic accumulations of metals by geologic age. The deposits of these metals were assigned to nine broad genetic categories, with an attempt to relate each category to tectonic setting within the framework of global plate tectonics.

  2. Stress pattern in Portugal mainland and the adjacent Atlantic region, West Iberia

    NASA Astrophysics Data System (ADS)

    Ribeiro, A.; Cabral, J.; Baptista, R.; Matias, L.

    1996-06-01

    The Portuguese mainland territory is located close to the Azores-Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [Cabral, 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (SHmax). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean SHmax trend. These differences in stress trend suggest a regional progressive rotation of the SHmax direction from NNW-SSE to WNW-ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW-SE SHmax trend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continental crust.

  3. FAST TRACK PAPER: Older crust underlies Iceland

    NASA Astrophysics Data System (ADS)

    Foulger, G. R.

    2006-05-01

    The oldest rocks outcropping in northwest Iceland are ~16 Myr old and in east Iceland ~13 Myr. The full plate spreading rate in this region during the Cenozoic has been ~2 cm a-1, and thus these rocks are expected to be separated by ~290 km. They are, however, ~500 km apart. The conclusion is inescapable that an expanse of older crust ~210 km wide underlies Iceland, submerged beneath younger lavas. This conclusion is independent of any considerations regarding spreading ridge migrations, jumps, the simultaneous existence of multiple active ridges, three-dimensionality, or subsidence of the lava pile. Such complexities bear on the distribution and age of the older crust, but not on its existence or its width. If it is entirely oceanic its maximum age is most likely 26-37 Ma. It is at least 150 km in north-south extent, but may taper and extend beneath south Iceland. Part of it might be continental-a southerly extension of the Jan Mayen microcontinent. This older crust contributes significantly to crustal thickness beneath Iceland and the ~40 km local thickness measured seismically is thus probably an overestimate of present-day steady-state crustal production at Iceland.

  4. Phytoplankton Communities in Louisiana coastal waters and the continental shelf

    EPA Science Inventory

    Louisiana coastal waters and the adjacent continental shelf receive large freshwater and nutrient inputs from the Mississippi and Atchafalaya Rivers, creating favorable conditions for increased phytoplankton productivity. To examine inshore-offshore patterns in phytoplankton comm...

  5. Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

    USGS Publications Warehouse

    Grimes, Craig B.; John, Barbara E.; Kelemen, P.B.; Mazdab, F.K.; Wooden, J.L.; Cheadle, Michael J.; Hanghoj, K.; Schwartz, J.J.

    2007-01-01

    We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB. ?? 2007 The Geological Society of America.

  6. Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

    NASA Astrophysics Data System (ADS)

    Grimes, C. B.; John, B. E.; Kelemen, P. B.; Mazdab, F. K.; Wooden, J. L.; Cheadle, M. J.; Hanghøj, K.; Schwartz, J. J.

    2007-07-01

    We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

  7. Psoriasis or crusted scabies.

    PubMed

    Goyal, N N; Wong, G A

    2008-03-01

    We describe a case of a 67-year-old woman with a 1-year history of nail thickening and a non-itchy erythematous scaly eruption on the fingertips. She was diagnosed with psoriasis and started on methotrexate after having had no response to topical calcipotriol. The diagnosis was reviewed after it was revealed by another consultant that the patient's husband had been attending dermatology clinics for several years with chronic pruritus, which had been repeatedly thought to be due to scabies. Our patient was found to have crusted scabies after a positive skin scraping showed numerous mites. She was treated with topical permethrin, keratolytics and oral ivermectin. We also review the literature on crusted scabies and its management, with recommendations.

  8. Continental and oceanic crustal magnetization modelling

    NASA Technical Reports Server (NTRS)

    Harrison, C. G. A.; Hayling, K. L.

    1984-01-01

    Inversion of magnetic data from the MAGSAT satellite, to arrive at intensities of magnetization of the Earth's crust, was performed by two different methods. The first method uses a spherical harmonic model of the magnetic field. The coefficients believed to represent sources in the Earth's crust can then be inverted to arrive at vertical dipole moments per unit area at the Earth's surface. The spherical harmonic models contain coefficients of degrees of harmonics up to 23. The dipole moment per unit area for a surface element can then be determined by summing the contribution for each individual degree of harmonic. The magnetic moments were calculated for continental and oceanic areas separately as well as over certain latitudinal segments. Of primary concern was to determine whether there are any differences between continental and oceanic areas. The second analysis with magnetization intensities was made using narrower ranges of degrees of harmonics, assuming that higher degrees are present in the core field signal.

  9. Oceanic Crust in the Canada Basin of the Arctic Ocean

    NASA Astrophysics Data System (ADS)

    Hutchinson, Deborah; Chian, Deping; Jackson, Ruth; Lebedeva-Ivanova, Nina; Shimeld, John; Li, Qingmou; Mosher, David; Saltus, Richard; Oakey, Gordon

    2015-04-01

    Crustal velocities from 85 expendable sonobuoys in the Canada Basin of the Arctic Ocean acquired between 2007 and 2011 distinguish oceanic, transitional, and extended continental crust. Crustal type was based on objective assignments of diagnostic velocities - oceanic from the presence of layer 3 velocities (6.7-7.2 km/s); transitional from the presence of a lower-most, high velocity layer (7.2-7.7 km/s), and continental for velocities typical of continental crust (≤6.6 km/s). Combined interpretations of sonobuoys, coincident multichannel seismic reflection profiles and existing maps of potential field (gravity and magnetic) are used to refine the distribution of oceanic crust. Oceanic crust forms a polygon approximately 320-350 km wide (east-west) by ~500 km (north-south). The northern segment of the Canada Basin Gravity Low (CBGL) bisects this zone of oceanic crust, as would be expected from the axis of the spreading center. The multichannel profiles also image a prominent bathymetric valley along this segment of the CBGL, similar to axial valleys found on slow and ultra-slow spreading ridges. Paired magnetic anomalies are associated only with crust that has typical oceanic velocities and are interpreted to represent possibly Mesozoic marine magnetic anomalies M0r - M4 (?), for a duration of opening of 8 million years, and a half spreading rate of ~10 mm/a. The southern segment of the CBGL, where it trends toward the Mackenzie Delta/fan, is associated with transitional velocities that are interpreted to represent serpentinized peridotite (mantle). As a result of being close to the inferred pole of rotation, this southern area may have had a spreading rate too low to support magmatism, producing amagmatic transitional crust. Further north, near Alpha Ridge and along Northwind Ridge, transitional crust is interpreted to be underplated or intruded material related to the emplacement of the High Arctic Large Igneous Province. Seismic reflection profiles across the

  10. Formation of Continental Fragments: The Tamayo Bank, Gulf of California

    NASA Astrophysics Data System (ADS)

    van Wijk, J.; Abera, R.; Axen, G. J.

    2015-12-01

    Potential field data are used to construct a two-dimensional crustal model along a profile through the Tamayo Trough and Bank in the Gulf of California. The model is constrained by seismic reflection and refraction data, and field observations. The potential field data do not fit a model where the crust of the Tamayo trough is continental, but they fit well with a model where the Tamayo trough crust is oceanic. This implies that the Tamayo Bank is entirely bounded by oceanic crust and is a microcontinent. The oceanic crust of the Tamayo trough that separates the Tamayo Bank from the mainland of Mexico is thin (~4 km), so oceanic spreading was probably magma-starved before it ceased. This led us to come up with a model that explains the formation of microcontinents that are smaller in size and are not found in the proximity of hotspots. At first, seafloor spreading commences following continental breakup. When the magma supply to the ridge slows down, the plate boundary strengthens. Hence, the ridge may be abandoned while tectonic extension begins elsewhere, or slow spreading may continue while a new ridge starts to develop. The old spreading ridge becomes extinct. An asymmetric ocean basin forms if the ridge jumps within oceanic lithosphere; a microcontinent forms if the ridge jumps into a continental margin. This model for formation of continental fragments is applicable to other regions as well, eliminating the need of mantle plume impingement to facilitate rifting of a young continental margin and microcontinent formation.

  11. The dynamics of continental extension and divergent margin formation

    SciTech Connect

    Sawyer, D.S. )

    1990-05-01

    Continental breakup is a highly variable process. Differences occur in the relative timing and extent of volcanism, uplift, and graben formation as well as in the mode and amount of continental extension before breakup. The authors propose a model that reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends. Continental lithosphere is viewed as a composite material composed of two strong layers, one in the upper mantle and one in the middle crust. Finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust. This leads to early melt segregation and volcanism, margin uplift during the late stages of the extension process, and relatively narrow symmetrical extended margins. In contrast, failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle. This produces initial graben formation, cooling in the lower crust and upper mantle, and broad asymmetrical extended margins. Volcanism only occurs late in the process. Failure at laterally offset weaknesses within both strong layers, perhaps the most common case, leads to a deformation pattern dominated by simple shear. Thus, differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup. They find that certain features of the evolution of the US Atlantic margin, particularly the formation of the hinge zone and the distribution and timing of extension may be better explained using these models.

  12. Using garnet peridotites as tools to reconstruct paleo-geodynamic settings of fossil continental collision zones

    NASA Astrophysics Data System (ADS)

    Zhang, Cong; van Roermund, Herman; Zhang, Lifei

    2010-05-01

    Orogenic garnet peridotites (metamorphic rocks containing the characteristic HP garnet-olivine mineral assemblage) form volumetrically minor, but important components of (ultra)high pressure (UHP) metamorphic terranes. Such terranes form along convergent plate margins where two adjacent plates collide and one of the plates is subducted below the other. After eduction back to the surface such fossil continental subduction/collision zones form the basic components of exposed (U)HP metamorphic terranes. In the absence of significant amounts of tectonic overpressure (Vrijmoed et al., 2010), the discovery of diamond and majoritic garnet in (U)HP metamorphic terranes provide evidence that subduction of continental crust into the mantle was deep enough to reach the garnet stability field in the overlying mantle wedge above the subduction zone. Brueckner (1998) was the first author who noticed that garnet peridotite bodies, present in such mantle wedges, could be transferred during collision from the mantle wedge into the subducted continental crust. Subsequent buoyancy, most likely generated by slab break off of previously subducted oceanic crust, is the most likely candidate to enable the subducted continental crust and its garnet peridotite "cargo" to return back to (sub)crustal levels. During the latter process mantle wedge garnet peridotite may recrystallize (partly or completely) into what will be called here subduction zone garnet peridotite. Alternatively subduction zone garnet peridotite may be formed by prograde subduction of ultramafic protoliths (serpentinites, Fe-Ti peridotite) that may be present in subducting continental crust prior to subduction. Subdivision between these two basic types of orogenic garnet peridotites (mantle wedge - versus subduction zone peridotite) allows however that in mantle wedge garnet peridotite the subcontinental lithospheric mantle (SCLM) - versus crustal-incorporation processes can be identified which a.o. has lead to the recent

  13. Scales and effects of fluid flow in the upper crust.

    PubMed

    Cathles, L M

    1990-04-20

    Two of the most important agents of geological change, solar energy and internal heat from the mantle, meet and battle for dominance in propelling aqueous and related fluids in the earth's upper crust. Which prevails and how they interact are subjects of active research. Recent work has demonstrated that both agents can propel fluids over nearly continental-scale distances in a fashion that influences a host of important geological processes and leaves a record in chemical alteration, mineral deposits, and hydrocarbon resources.

  14. Continental tectonics in the aftermath of plate tectonics

    NASA Technical Reports Server (NTRS)

    Molnar, Peter

    1988-01-01

    It is shown that the basic tenet of plate tectonics, rigid-body movements of large plates of lithosphere, fails to apply to continental interiors. There, buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity. The role of crustal blocks and of the detachment of crustal fragments in this process is discussed. Future areas of investigation are addressed.

  15. Anatexis in Himalayan Crust: Evidences and Implications

    NASA Astrophysics Data System (ADS)

    Yang, X.; Ma, J.

    2004-12-01

    Here we present results of chemical features of migmatites in Higher Himalayan Crystallines (HHC) and K-Ar and Ar-Ar ages of the leucosomes (type-I and type-II) in the migmatites. The mass leucosomes distributed in HHC were generated by partial melting and consequently they are ideal specimens to determine the timing of anatexis, to study the relationship between migmatization and leucogranite formation, and to understand the role of partial melting in crustal evolution in continental collision orogenic belt. Mass balance relationship among mesosome, type-I leucosome and melanosome implies that they are likely generated in the same process. The compositions of Type-I leucosome are identical with those of melt produced by dehydration melting of biotite-plagioclase gneiss but different from those of type-II leucosome which compositions are similar to HHL. These compositional characteristics of leucosomes reflect that Type-I leucosome is the product of crystallization from melt generated by partial melting of mesosome in the source region, but Type-II leucosome and HHL probably underwent crystallization differentiation of plagioclase during melt aggregation and migration. Prime partial melting occurred at 22.7-24.7 Ma based on the date of type-I leucosome. The ages of type-II leucosomes (ranging from 14.82 to 18.37 Ma) are consistent with that of HHL. Very young age of 6.2-8.3 Ma of type-II leucosome provides a new time constraint on magma activity in the central segment of Higher Himalayas. These ages reveal some dynamic correlation summarized as follows. (1) The age of about 24 Ma (the beginning of anatexis), which is a little younger than that of MCT movement, does not support the view that anatexis was derived from MCT movement; on the contrary, but rather, the anatexis may have played an important role in the formation of MCT and Southern Tibetan Detachment System. (2) Decompression melting associated with STDS movement and crust uplifting contributed greatly to

  16. Structure of the Crust beneath Cameroon, West Africa, from the Joint Inversion of Rayleigh Wave Group Velocities and Receiver Functions

    SciTech Connect

    Tokam, A K; Tabod, C T; Nyblade, A A; Julia, J; Wiens, D A; Pasyanos, M E

    2010-02-18

    The Cameroon Volcanic Line (CVL) is a major geologic feature that cuts across Cameroon from the south west to the north east. It is a unique volcanic lineament which has both an oceanic and a continental sector and consists of a chain of Tertiary to Recent, generally alkaline volcanoes stretching from the Atlantic island of Pagalu to the interior of the African continent. The oceanic sector includes the islands of Bioko (formerly Fernando Po) and Sao Tome and Principe while the continental sector includes the Etinde, Cameroon, Manengouba, Bamboutos, Oku and Mandara mountains, as well as the Adamawa and Biu Plateaus. In addition to the CVL, three other major tectonic features characterize the region: the Benue Trough located northwest of the CVL, the Central African Shear Zone (CASZ), trending N70 degrees E, roughly parallel to the CVL, and the Congo Craton in southern Cameroon. The origin of the CVL is still the subject of considerable debate, with both plume and non-plume models invoked by many authors (e.g., Deruelle et al., 2007; Ngako et al, 2006; Ritsema and Allen, 2003; Burke, 2001; Ebinger and Sleep, 1998; Lee et al, 1994; Dorbath et al., 1986; Fairhead and Binks, 1991; King and Ritsema, 2000; Reusch et al., 2010). Crustal structure beneath Cameroon has been investigated previously using active (Stuart et al, 1985) and passive (Dorbath et al., 1986; Tabod, 1991; Tabod et al, 1992; Plomerova et al, 1993) source seismic data, revealing a crust about 33 km thick at the south-western end of the continental portion of the CVL (Tabod, 1991) and the Adamawa Plateau, and thinner crust (23 km thick) beneath the Garoua Rift in the north (Stuart et al, 1985) (Figure 1). Estimates of crustal thickness obtained using gravity data show similar variations between the Garoua rift, Adamawa Plateau, and southern part of the CVL (Poudjom et al., 1995; Nnange et al., 2000). In this study, we investigate further crustal structure beneath the CVL and the adjacent regions in

  17. Implication of Flow in the Lower Crust on Strain Localization

    NASA Astrophysics Data System (ADS)

    Le Pourhiet, Laetitia

    2016-04-01

    A major difference between oceanic and continental crust is the capacity of the lower crust to flow. This has been the moto of the research group centered around Genia Burov over the last 15 years and I will try to summarize the results of number of numerical models run in different geodynamic setting to tackle the question of the rheology of the lithosphere and crust at the scale of plate tectonics. I will insist on how apriori very complex numerical models have helped the community to build our intuition on geodynamics processes and change the way of thinking the interactions between mantle process and crustal processes which are the core of plate tectonic and beyond. I will finally discuss what have we learn about the rheology of the lithosphere so far and how we intend to pursues evgeni's fundamental contribution to the field.

  18. Modelling Sea-floor Spreading Initiation and Rifted Continental Margin Formation: Does Depth Dependent Stretching Occur Pre- or Syn-breakup?

    NASA Astrophysics Data System (ADS)

    Kusznir, N. J.; Karner, G. D.

    2004-05-01

    Depth dependent stretching, in which upper continental crustal extension is much less than that of the lower crust and lithospheric mantle, is a characteristic of both non-volcanic and volcanic rifted margins. A key question is whether depth dependent stretching of rifted margin lithosphere occurs during the initiation of sea-floor spreading or during pre-breakup rifting. A kinematic fluid-flow model of sea-floor spreading initiation has been developed to determine the concomitant distribution and amplitude of rifted continental margin lithosphere thinning and thermal evolution. The ocean-ridge initiation model uses an iso-viscous corner-flow stream-function solution (Batchelor 1967) to predict the divergent lithospheric and asthenospheric fluid-flow field of the extending continental margin lithosphere. The thinning of the continental margin lithosphere is calculated by material advection in the newly initiated ocean ridge fluid-flow field. The ocean-ridge initiation model predicts depth dependent stretching of continental lithosphere. Rifted continental margin lithosphere thinning and thermal evolution are dependent on ocean-ridge spreading rate (Vx), the mantle upwelling velocity beneath the ridge axis (Vz), and the pre-breakup lithosphere beta stretching factor. The sea-floor spreading initiation model predicts that the distribution and magnitude of depth dependent stretching of continental margin lithosphere is highly sensitive to the ratio of Vz/Vx. For volcanic margins Vz/Vx may be > 5 during sea-floor spreading initiation, reducing to Vz/Vx ~ 1 after a few Myr (Nielsen & Hopper 2002), while for non-volcanic margins, lower values of Vz/Vx (~ 1) are expected during sea-floor spreading initiation. For Vz/Vx ~ 1 the sea-floor spreading initiation model predicts that lower continental crust and continental lithospheric mantle adjacent to the margin are advected oceanward and result in rifted margin depth dependent stretching and continental mantle exhumation

  19. Analog Modeling of Continental Lithosphere Subduction

    NASA Astrophysics Data System (ADS)

    Willingshofer, E.; Sokoutis, D.; Luth, S.; Beekman, F.; Cloetingh, S.

    2012-12-01

    Lithospheric-scale analog modeling sheds light on the consequences of decoupling within the continental lithosphere and along plate interfaces during continental collision. The model results provide valuable information in terms of strain localization, deformation of the subducting slab and the evolution and architecture of the overlying mountain belt and its topography. A weak layer has been implemented in three-layer models to simulate decoupling along the plate interface and at different levels of the lithosphere (brittle-ductile transition, entire lower crust, crust-mantle boundary). Additionally, varying the strength of the mantle lithosphere of both the upper as well as the lower plate regulated the degree of plate coupling. Plate boundaries were orthogonal to the convergence direction. All models emphasize that strong decoupling at the plate interface is a pre-requisite for the subduction of continental lithosphere. In addition, deformation of the subducting slab was found to be sensitive to the strength contrast between the subduction zone and the mantle lithosphere of the downgoing as well as the upper plate. As such, a low strength contrast between the plate interface and the lower plate leads to deformation of the subducting slab by thickening and the development of a shallow slab. Conversely, when the strength contrast is high, deep slabs evolve which undergo relatively less deformation. Furthermore, the level of decoupling in the downgoing plate governs how much continental crust is subducted together with the mantle lithosphere. Shallow decoupling, at the brittle-ductile transition, results in subduction of the lower crust whereas small amounts of lower crust are subducted when decoupling occurs at the level of the Moho. Weak plate coupling and a weak lower crust of the lower plate steer the evolution of mountain belts such that deformation propagates outward, in the direction of the incoming plate, by successive imbrication of upper crustal thrust

  20. Aleutian basin oceanic crust

    USGS Publications Warehouse

    Christeson, Gail L.; Barth, Ginger A.

    2015-01-01

    We present two-dimensional P-wave velocity structure along two wide-angle ocean bottom seismometer profiles from the Aleutian basin in the Bering Sea. The basement here is commonly considered to be trapped oceanic crust, yet there is a change in orientation of magnetic lineations and gravity features within the basin that might reflect later processes. Line 1 extends ∼225 km from southwest to northeast, while Line 2 extends ∼225 km from northwest to southeast and crosses the observed change in magnetic lineation orientation. Velocities of the sediment layer increase from 2.0 km/s at the seafloor to 3.0–3.4 km/s just above basement, crustal velocities increase from 5.1–5.6 km/s at the top of basement to 7.0–7.1 km/s at the base of the crust, and upper mantle velocities are 8.1–8.2 km/s. Average sediment thickness is 3.8–3.9 km for both profiles. Crustal thickness varies from 6.2 to 9.6 km, with average thickness of 7.2 km on Line 1 and 8.8 km on Line 2. There is no clear change in crustal structure associated with a change in orientation of magnetic lineations and gravity features. The velocity structure is consistent with that of normal or thickened oceanic crust. The observed increase in crustal thickness from west to east is interpreted as reflecting an increase in melt supply during crustal formation.

  1. Early Continental Rifting of the South China Sea

    NASA Astrophysics Data System (ADS)

    Lee, C.; Chiu, M.; Chan, C.

    2010-12-01

    Combined two years (2007 and 2008) of OBS and MCS studies in the northern slope of the South China Sea, we suggest that the early rifting, probably during 60 - 30 mabp, is an asymmetrical Atlantic-type continental rifting. The crust thin out from 35-40 km of possible continental crust to about 10-15 km of typical oceanic crust. Along the continent-ocean boundary, we observe an intrusion of the high P-wave velocity (about 7.5-8.0 km/sec). This is possible of mantle exhumation as comparable to other Atlantic-type continental margins. The OBS result is revealed by the gravity data. Along the upper layers of the continental crust as well as the oceanic crust, the MCS and multi-beam bathymetry data show that they are covered by numerous submarine seamounts. This probably relate to a volcanic origin of the Cenozoic sea-floor spreading during 30-15 mabp as mapped by previous magnetic anomalies in this region. The sea-floor spreading spread apart in the central, NW and SW sub-basins with several different episodes. Lack of the deep crustal data in the southern slope of the South China Sea, particularly around the Sprately area, the interpretation is speculative. However, several very large-size atolls (150 - 200 km in diameter), such as the Chen-Ho, Shun-Zu, Chung-Yeh and Chiu-Cheng fringing reefs, are sub-parallel located along the south margins. We interpret that these are the upper portions of the continental rifting. Combined the two tectonic stories in the northern and southern slope of the South China Sea, we believe that it is in consistent with the complicate nature of the South China Sea crust.

  2. Microtopography of manganese crusts

    NASA Astrophysics Data System (ADS)

    Morgan, Charles L.

    Quantitative examination of the seafloor surface roughness will be necessary for any design of equipment intended for use in collecting surface deposits such as cobalt-rich manganese crusts or nodules. Furthermore, it is an essential prerequisite to the confident interpretation of returns from high frequency side-scan and other acoustic systems. The objectives of the project were to develop the capability at the University of Hawaii of generating high resolution (less than 1 cm horizontal and vertical) topographic models of the seafloor from 35 mm stereo photographs; to produce such models from existing photographs of cobalt-rich manganese crust deposits; and to optimize the configuration of the existing Hawaii Undersea Research Laboratory (HURL) camera system for stereo photograph collection and correlation of acoustic data with the photographic ground-truth. These tasks were accomplished and have also led to the development of a follow-on project (MMTC/OBD Project 1512) dedicated to the simultaneous acquisition of both optical and side-scan acoustic data for future accurate determination of seabed microtopography.

  3. Earth's continental crustal gold endowment

    NASA Astrophysics Data System (ADS)

    Frimmel, H. E.

    2008-03-01

    The analysis of the temporal distribution of gold deposits, combined with gold production data as well as reserve and resource estimates for different genetic types of gold deposit, revealed that the bulk of the gold known to be concentrated in ore bodies was added to the continental crust during a giant Mesoarchaean gold event at a time (3 Ga) when the mantle temperature reached a maximum and the dominant style of tectonic movement changed from vertical, plume-related to subhorizontal plate tectonic. A magmatic derivation of the first generation of crustal gold from a relatively hot mantle that was characterized by a high degree of partial melting is inferred from the gold chemistry, specifically high Os contents. While a large proportion of that gold is still present in only marginally modified palaeoplacer deposits of the Mesoarchaean Witwatersrand Basin in South Africa, accounting for about 40% of all known gold, the remainder has been recycled repeatedly on a lithospheric scale, predominantly by plate-tectonically induced magmatic and hydrothermal fluid circulation, to produce the current variety of gold deposit types. Post-Archaean juvenile gold addition to the continental crust has been limited, but a mantle contribution to some of the largest orogenic or intrusion-related gold deposits is indicated, notably for the Late Palaeozoic Tien Shan gold province. Magmatic fluids in active plate margins seem to be the most effective transport medium for gold mobilization, giving rise to a large proportion of volcanic-arc related gold deposits. Due to their generally shallow crustal level of formation, they have a low preservation potential. In contrast, those gold deposits that form at greater depth are more widespread also in older rocks. This explains the high proportion of orogenic (including intrusion-related) gold (32%) amongst all known gold deposits. The overall proportion of gold concentrated in known ore bodies is only 7 × 10- 7 of the estimated total

  4. Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?

    USGS Publications Warehouse

    Churkin, M.; McKee, E.H.

    1974-01-01

    The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton. ?? 1974.

  5. Numerical Geodynamic Experiments of Continental Collision: Past and Present

    NASA Astrophysics Data System (ADS)

    Gray, Robert

    Research explores deep continental lithosphere (i.e., the continental lower crust and mantle lithosphere) deformation during continental collision. I found that depending on the composition/rheology of the crust and the amount of radiogenic heat production in the crust, three dominant modes of mantle lithosphere deformation evolve under Neoarchean-like conditions: (1) a pure-shear thickening style; (2) an imbrication style; (3) and a "flat-subduction" style. The imbrication and the flat-subduction styles result in the emplacement of "plate-like" mantle lithosphere at depths between 200 km and 325 km. The imbrication style behavior shifts to the "flat-subduction" style behavior after a crustal inversion event. I investigated mature Phanerozoic-style collision and found that it is sensitive to mantle lithosphere density, mantle lithosphere yield stress, lower-crustal strength and to the presence of phase change-related density changes in the lower crust. The early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate. Next, the subducting lower crust and mantle lithosphere retreat from the collision zone, permitting the sub-lithospheric mantle to upwell and intrude the overriding plate. Next, the lower crust and mantle lithosphere of the overriding plate delaminate from the overlying crust. This process produces plateau-like uplift. These modeling results are interpreted in the context of available geological and geophysical observables for the Himalayan-Tibetan orogen. I quantitatively investigated the effects that sediment deposition may have on continental lithosphere deformation during collision. In the absence of sedimentation, the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere beneath the overriding plate. Next, the subducting lower crust and mantle lithosphere retreat from the collision zone. This permits the sub

  6. Fractal behavior in continental crustal heat production

    NASA Astrophysics Data System (ADS)

    Vedanti, N.; Srivastava, R. P.; Pandey, O. P.; Dimri, V. P.

    2011-02-01

    The distribution of crustal heat production, which is the most important component in the elucidation of continental thermal structure, still remains a theoretical assumption. In general the heat production values must decrease with depth, but the form of decrease of heat production in the crust is not well understood. The commonly used heat production models are: "block model", in which heat production is constant from the surface to a given depth and the "exponential model", in which heat production diminishes as an exponential function of depth. The exponential model is more widely used wherein sources of the errors are heterogeneity of rock and long wavelength changes due to changes in lithology and tectonic elements, and as such exponential distribution does not work satisfactorily for the entire crust. In the present study, we analyze for the first time, deep crustal heat production data of six global areas namely Dharwar craton (India), Kaapvaal craton (South Africa), Baltic shield (Kola, Russia), Hidaka metamorphic belt (Japan), Nissho pluton (Japan) and Continental Deep Drilling site (KTB, Germany). The power spectrum of all the studied data sets exhibits power law behaviour. This would mean slower decay of heat production with depth, which conforms to the known geologic composition of the crust. Minimum value of the scaling exponent has been found for the KTB borehole, which is apparently related to higher heat production of gneisses, however for other study areas, scaling exponent is almost similar. We also found that the lower values of scaling exponents are related to higher heat production in the crust as is the case in KTB. Present finding has a direct relevance in computation of temperature-depth profiles in continental regions.

  7. Subduction-driven recycling of continental margin lithosphere.

    PubMed

    Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S

    2014-11-13

    Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones.

  8. Plume-lithosphere interactions near a passive continental margin: a thermo-mechanical modelling study

    NASA Astrophysics Data System (ADS)

    François, Thomas; Cloetingh, Sierd; Burov, Evgueni; Matenco, Liviu

    2015-04-01

    Plume head-lithosphere (PLI) interactions have important consequences both for tectonic and mineralogical evolution of the lithosphere and are often considered to be an important factor of continental break-up. Nevertheless, the interaction between plume and post break-up tectonics (i.e. evolution of passive margins) remain unclear. The passive margins represent important geometrical, thermal and rheological barriers that interact with the plume head material during its emplacement below the lithosphere. For example on the Scandinavia's North Atlantic passive margin the large Cenozoic uplift comprised uplift of basin margins as well as accelerated subsidence of basin centres adjacent to the uplifted landmasses while the compressional reactivation coincides with the postulated Iceland-plume events associated with massive magma emplacement. The goal of this study is to understand the role of the Iceland plume in the Cenozoic evolution of the Scandinavia's North Atlantic passive margin. To investigate the interactions between the plume and passive margin we use fully coupled thermo-mechanical 2D numerical code (Flamar v12). The model area is 700 km deep and 1500 km wide comprising rheologically realistic lithosphere and the entire upper mantle Our models have free upper surface boundary, surface erosion, account for the rheological stratification (upper crust, lower crust, lithospheric mantle and asthenosphere), brittle-elastic-ductile rheology, metamorphic phase changes (density and physical properties) and for the specific crustal and thermal structure of the Scandinavia's North Atlantic passive margin. We have tested several parameters including the lateral position of the plume, the rate of extension and the thermo-rheological profile of the continental lithosphere.

  9. KTB and the electrical conductivity of the crust

    NASA Astrophysics Data System (ADS)

    Haak, V.; Simpson, F.; Bahr, Karsten; Bigalke, J.; Eisel, M.; Harms, U.; Hirschmann, G.; Huenges, E.; Jödicke, H.; Kontny, A.; Kück, J.; Nover, G.; Rauen, A.; Stoll, J.; Walther, J.; Winter, H.; Zulauf, G.; Wolfgang, J.

    1997-08-01

    The German Continental Deep Drilling Program (KTB) drilled two holes through crystalline rocks which are rich in both high-salinity fluids and graphite accumulated along shear zones. Analyses of a large number of borehole measurements yield models for the electrical resistivity of the upper and middle crust in the vicinity of the KTB holes. High observed resistivity, of more than 105Ωm in the lowermost part of the 9000 m deep main hole, in a rather ``wet'' crust, indicates that effective mechanisms exist to cut down connections between fluid accumulations and therefore that fluids are not the likely cause of high-conductivity anomalies. On the other hand, graphite accumulations appear to be connected along shear lineaments over hundreds of meters or more. Structural, mineralogical, and geochemical studies suggest a tectonic model which explains the deposition of graphite as the relic and witness of a shearing process that occurred during the late Variscan (Upper Carboniferous) thrusting. This process took place while this part of the crust resided at temperatures between 240° and 380°C. Subsequent independent reverse faulting lifted this part to the Earth's surface. Our conclusion is that the KTB case indicates how high electrical conductivities in the upper crust, which originated from the middle to lower crust, are caused by graphite accumulations, rather than by fluids, and that these anomalies are related to shearing processes. Such graphite accumulations may exist elsewhere and may be of relevance in the context of present-day midcrustal conductors.

  10. The spin zone: Transient mid-crust permeability caused by coseismic brecciation

    NASA Astrophysics Data System (ADS)

    Melosh, Benjamin L.; Rowe, Christie D.; Gerbi, Christopher; Bate, Charlotte E.; Shulman, Deborah

    2016-06-01

    Pore fluids migrating through the deep section of continental strike-slip fault zones have been invoked to explain such phenomena as tectonic tremor, stress transfer across the brittle-ductile transition, and short timescales of co-seismic healing. In this contribution, we describe a coseismic mechanism for forming transient vertical fluid conduits within dilational jogs in strike-slip faults. We present field observations of breccias that formed coseismically at dilational stepovers in the dextral Pofadder Shear Zone, a ∼ 1 Ga exhumed continental strike-slip fault in South Africa and Namibia. These breccias are interpreted to have formed when tensile fractures emanating from rupture tips intersected mylonitic foliation parallel to the rupture surface, which then failed, disaggregating the rock. We used quartz textures in the mylonites determined by electron backscatter diffraction to uniquely compare the orientation of each clast to the neighboring wall rock and constrain finite clast rotation within breccia bodies. Comparison of two- and three-dimensional rotation patterns show that clast trajectories are highly scattered when decoupled from wall rock, suggesting that Pofadder breccias were not formed by gradual plucking of clasts during slip. The dilational breccia bodies have sub-vertical geometries and high porosities relative to the host mylonites. We infer that the opening of these breccias may have created instantaneous, temporary vertical pathways for fluid draining through the brittle-plastic transition. These pathways healed post-seismically by cementation or ductile creep along the fault. The connection of many adjacent and overprinting breccia bodies through time provides a mechanism for fluid transport on a 10 s of km scale though the middle crust.

  11. Shallow subduction, ridge subduction, and the evolution of continental lithosphere

    SciTech Connect

    Helmstaedt, H.; Dixon, J.M.; Farrar, E.; Carmichael, D.M.

    1985-01-01

    Subduction of oceanic lithosphere beneath continental crust at a shallow angle has occurred throughout the Phanerozoic Eon. Ridge subduction often follows shallow subduction and causes bimodal volcanism and crustal rifting, forming back-arc basins. Recent models for Archean plate tectonics propose very fast rates of spreading (400-800 km/Ma) and convergence, and sinking rates comparable to or slower (<10 km/Ma) than those of today. As faster convergence and slower sinking correspond to subduction at shallower angles, shallow subduction and ridge subduction must have been ubiquitous during the Archean permobile regime. This is compatible with a back-arc-basin origin for Archean greenstone belts. The common coexistence of tholeiitic and calc-alkaline igneous rocks in Archean greenstone belts, also implies ridge subduction. The authors envisage a transition, between 2.4 and 1.8 Ga., from a regime dominated by shallow subduction and repeated ridge subduction to one of normal plate tectonics with steeper subduction. Spreading rates decreased; continental plates became larger and stable shelves could develop at trailing margins. Shallow subduction became the exception, restricted to episodes of abnormally fast convergence; nevertheless, the long span of post-Archean time makes it unlikely that any part of the continental crust has escaped shallow subduction and ridge subduction. These processes recycle much volatile-rich oceanic crust into the sub-continental upper mantle, thereby underplating the crust, effecting upper-mantle metasomatism and affecting intraplate magmatism.

  12. Peridotite-pyroxenite intraplating near the continental Moho (Ivrea-Verbano Zone, Western Alps)

    NASA Astrophysics Data System (ADS)

    Berno, Davide; Tribuzio, Riccardo; Zanetti, Alberto

    2016-04-01

    The Sesia magmatic system from Ivrea-Verbano Zone (hereafter IVZ) comprises intrusive and volcanics rocks of Lower Carboniferous to Upper Permian age (Sinigoi et al., 2010), which are exposed in a virtually complete section across the pre-Mesozoic Adriatic continental crust. Intrusions are present at different stratigraphic levels. Gabbroic to dioritic rocks of the Sesia Mafic Complex intruded the granulite- to amphibolite-facies metasedimentary and metavolcanic rocks of the lower crust. Granitoid plutons are dominant in the adjacent intermediate to upper crustal sequence of the Serie dei Laghi, whose emplacement was accompanied by intermediate to acidic volcanism. Recent investigations have shown that the geodynamic evolution of the northernmost tip of the IVZ (Finero area) involved Triassic-Lower Jurassic magmatic episodes (Zanetti et al., 2013; Schaltegger et al., 2015), thereby suggesting the presence of important geological heterogeneities within IVZ. This presentation deals with the petrology and the geochemistry of the lower crust intrusions that are geographically intermediate between Sesia and Finero areas. We focus on the so-called Monte Capio sill, which is found near the deepest level of the exposed continental crust, close to the Insubric line. The sill is about 4 km long, spreading from Strona to Sesia valley, and has maximum thickness of 600 m. Peculiar characteristics of the Monte Capio sill are: (i) dominance of ultramafic rocks, and (ii) close association with garnet-bearing gabbros and granulite-facies SiO2-rich rocks ("stronalites"). U-Pb zircon geochronology returned ages spanning from 314.0 ± 5.2 to 283.2 ± 6.0 Ma, which were tentatively interpreted as crystallization and recrystallization ages, respectively (Klotzli et al., 2014). The Monte Capio sill mostly consists of amphibole-bearing peridotites to pyroxenites. The pyroxenites typically contain Fe-Ni-Cu mineralizations that were exploited in the past century through mine activity. In

  13. Dependency of continental crustal rupture, decompression melt initiation and OCT architecture on lithosphere deformation modes during continental breakup: Numerical experiments

    NASA Astrophysics Data System (ADS)

    Jeanniot, L.; Kusznir, N. J.; Manatschal, G.

    2012-12-01

    During the continental breakup process, the initiation of sea-floor spreading requires both the rupture of the continental crust and the initiation of decompression melting. Using numerical experiments, we investigate how the deformation mode of continental lithosphere thinning and stretching controls the rupture of continental crust and lithospheric mantle, the onset of decompression melting and their relative timing. We use a two dimensional finite element viscous flow model to describe lithosphere and asthenosphere deformation. This flow field is used to advect lithosphere and asthenosphere material and temperature. Decompression melting is predicted using the parameterization scheme of Katz et al. (2003). Consistent with the observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996), we assume that the topmost continental and oceanic lithosphere, corresponding to the cooler brittle seismogenic layer, deforms by extensional faulting (which we approximate to pure-shear deformation) and magmatic intrusion. Beneath this topmost lithosphere layer approximately 15-20 km thick, we assume that deformation occurs in response to passive upwelling and thermal and melt buoyancy driven small-scale convection. The relative contribution of these deformation components is parameterised by the ratio Vz/Vx, where Vx is the half spreading rate applied to the topmost lithosphere deformation and Vz is the upwelling velocity associated with the small scale convection. We use a series of numerical experiments to investigate the dependency of continental crust and lithosphere rupture, decompression melt initiation, rifted margin ocean-continent transition architecture and subsidence history on the half-spreading rate Vx, buoyancy driven upwelling rate Vz, the ratio Vz/Vx and upper lithosphere pure-shear width W. Based on the numerical experiment results we explore a polyphase evolution of deformation modes leading to continental breakup, sea

  14. Evidence for mechanical coupling and strong Indian lower crust beneath southern Tibet.

    PubMed

    Copley, Alex; Avouac, Jean-Philippe; Wernicke, Brian P

    2011-04-07

    How surface deformation within mountain ranges relates to tectonic processes at depth is not well understood. The upper crust of the Tibetan Plateau is generally thought to be poorly coupled to the underthrusting Indian crust because of an intervening low-viscosity channel. Here, however, we show that the contrast in tectonic regime between primarily strike-slip faulting in northern Tibet and dominantly normal faulting in southern Tibet requires mechanical coupling between the upper crust of southern Tibet and the underthrusting Indian crust. Such coupling is inconsistent with the presence of active 'channel flow' beneath southern Tibet, and suggests that the Indian crust retains its strength as it underthrusts the plateau. These results shed new light on the debates regarding the mechanical properties of the continental lithosphere, and the deformation of Tibet.

  15. Generation of continental adakitic rocks: Crystallization modeling with variable bulk partition coefficients

    NASA Astrophysics Data System (ADS)

    Dai, Hong-Kun; Zheng, Jianping; Zhou, Xiang; Griffin, W. L.

    2017-02-01

    The geochemical signatures (i.e., high Sr/Y and La/Yb ratios) of adakitic rocks in continental settings, which are derived from the continental lower crust rather than from subducted slabs, may reflect high-pressure melting in the lower crust or may be inherited from their sources. The North China Craton (NCC) is an ideal place for investigation of this type of adakites due to its ubiquitous distribution. As an example, we explore the petrogenesis of the Jurassic ( 163 Ma) adakitic rocks in western Liaoning, in the NE part of the NCC, using elemental and Sr-Nd isotopic analysis and crystallization modeling based on Rhyolite-MELTS. The modeling demonstrates that adakitic signatures can be generated by fractional crystallization of magmas within crust of normal thickness (i.e., 33 km). Partial-melting modeling based on the composition of the lower continental crust shows that only the adakitic rocks from orogenic belts require a thickened crust (i.e., 45 km). We suggest that continental adakitic rocks are not necessarily linked to high-pressure processes and their use as an indicator of thickened/delaminated continental crust should be regarded with caution.

  16. Seismic imaging of extended crust with emphasis on the western United States

    USGS Publications Warehouse

    McCarthy, J.; Thompson, G.A.

    1988-01-01

    Understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust. This is particularly true in areas where the last tectonic event was extensional, such as the Basin and Range province of the western United States and much of western Europe. In these regions, a characteristic reflective pattern has emerged, whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective. In the metamorphic-core-complex belt in the western United States, where extension can be as much as an order of magnitude greater than in the more classic continental rift zones, the lower crustal reflectivity thickens and rises, yielding a picture of a crust that is reflective throughout. If metamorphic core complexes are representative of extended continental crust world-wide, then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world. -from Authors

  17. Rifted continental margins: The case for depth-dependent extension

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2014-12-01

    Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Type I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

  18. Rifted Continental Margins: The Case for Depth-Dependent Extension

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2016-04-01

    Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at; 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Types I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented

  19. Rifted Continental Margins: The Case for Depth-Dependent Extension

    NASA Astrophysics Data System (ADS)

    Huismans, R. S.; Beaumont, C.

    2015-12-01

    Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at; 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Types I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

  20. On the differences in continental rifting at the Earth, Mars and Venus

    NASA Technical Reports Server (NTRS)

    Nikishin, A. M.; Milanovsky, E. E.

    1985-01-01

    During the process of continental rifting on Earth, the lower ductile crust stretches, forming a neck, while the upper brittle crust is broken in blocks by faults, and the blocks sink down the thinned lower crust; if the stretching continues, the neck may break and a newly originated oceanic crust is formed at this place. The rift system structure depends on the depth of the boundary surface between the brittle crust and the ductile crust, the litospheric thickness, the tension value, etc.. The rigid brittle rifting when narrow necks form in the lower crust is characteristic of the contemporary Earth; on Mars the brittle rifting with large subsidence was characteristic of the Tharsis upland formation epoch. The ductile rifting is typical of the Venus. The differences in rheologic features of the lithospheres of different planets causes the variation in types of rifting.

  1. Lomonosov Ridge as a Natural Component of Continental Margin

    NASA Astrophysics Data System (ADS)

    Poselov, V.; Kaminsky, V. D.; Butsenko, V. V.; Grikurov, G. E.

    2010-12-01

    In geodynamic context, Lomonosov Ridge is interpreted as a rifted passive margin framing the Eurasian oceanic basin. At the same time its near-Siberian segment is intimately associated with the Russian Arctic shelf, as evidenced by morphological data and the results of “Trans-Arctic 1992” and “Arctic-2007” geotransect studies. Coring and ACEX data demonstrated the presence in the uppermost geological section of the ridge of Late Cretaceous through Cenozoic sediments and Jurassic-Cretaceous sedimentary rocks; the latter may belong to deeper levels of sedimentary cover, or may represent the Mesozoic folded basement. Coarse bottom debris contains also the fragments of Riphean-Paleozoic rocks probably derived from the local bedrock source. Structure of sedimentary cover is imaged by continuous seismic observations from the shelf of East Siberian Sea along the length of Lomonosov Ridge to 85 N. In the upper part of the section there are two sedimentary sequences separated by a regional unconformity; their seismic velocities are 2.4-3.1 km/s in the upper sequence and 3.4-4.0 km/s in the lower one, and the total thickness reaches ~ 8 km in the deepest part of New Siberian Basin. Both these sequences and the unconformity are traced from Lomonosov Ridge into Amundsen Basin on seismic reflection sections obtained by drifting ice stations North Pole 2479 and 2480. The low-velocity sediments are underlain by a metasedimetary sequence with velocities decreasing from 4.7-4.9 km/s on the shelf to 4.4-4.9 km/s beneath continental slope and 4.2-4.8 km/s on Lomonosov Ridge. The thickness of metasedimentary sequence is about 7 km on the shelf, up to 3.5 km under continental slope, and strongly variable (1-5 km) on Lomonosov Ridge. The upper layer of consolidated crust is 8-9 km thick on the shelf with velocities 6.1-6.2 km/s; on Lomonosov Ridge both its thickness and velocities increase to 10 km and 6.0-6.4 km/s, respectively. In the lower crust the velocities do not exceed 6

  2. Habitability Of Europa's Crust

    NASA Astrophysics Data System (ADS)

    Greenberg, R.; Tufts, B. R.; Geissler, P.; Hoppa, G.

    Physical characterization of Europa's crust shows it to be rich in potentially habitable niches, with several timescales for change that would allow stability for organisms to prosper and still require and drive evolution and adaptation. Studies of tectonics on Europa indicate that tidal stress causes much of the surface cracking, that cracks pen- etrate through to liquid water (so the ice must be thin), and that cracks continue to be worked by tidal stress. Thus a global ocean is (or was until recently) well linked to the surface. Daily tidal flow (period~days) transports substances up and down through the active cracks, mixing surface oxidants and fuels (cometary material) with the oceanic reservoir of endogenic and exogenic substances. Organisms moving with the flow or anchored to the walls could exploit the disequilibrium chemistry, and those within a few meters of the surface could photosynthesize. Cracks remain active for at least ~10,000 yr, but deactivate as nonsynchronous rotation moves them to different stress regimes in less than a million yr. Thus, to survive, organisms squeezed into the ocean must migrate to new cracks, and those frozen in place must hibernate. Most sites remelt and would release captive organisms within about a million yr based on the prevalence of chaotic terrain, which covers nearly half of Europa. Linkage of the ocean to the surface also could help sustain life in the ocean by delivering oxidants and fuels. Suboceanic volcanism (if any) could provide additional sites and support for life, but is not necessary. Recent results support this model. We further constrain the non-synchronous rotation rate, demonstrate the plausibility of episodic melt-through, show that characteristics of pits and uplift features do not imply thick ice, and demonstrate polar wander, i.e. that the ice crust is detached from the solid interior and has slipped as a unit relative to the spin axis. Thus Europa's biosphere (habitable if not inhabited) likely

  3. Conditions for lower lithosphere exhumation from continental collision: South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Cunje, A.; Pysklywec, R. N.

    2013-12-01

    The South Island of New Zealand provides a unique opportunity for the investigation of active deep crustal dynamics and the effects of surface processes during collision, as a fairly young and relatively well-constrained convergent plate boundary. One of the uncertainties of the orogenesis is the fate of the lower crust during the continental collision: portions of the crust are exhumed along the Alpine Fault, but the lowermost crust does not seem to follow. This work focuses on the fate of the mid- and lower-crust during the collision, investigating several of the primary controls - rheology, boundary conditions, temperature - that regulate the behaviour of the crust during an idealized continental collision event. We use forward thermo-mechanical numerical modelling of the mantle and lithosphere, with variable surface boundary conditions of erosion and deposition, to explore the deformation of the crust and mantle lithosphere via the collision; the 2D models are configured for a South Island-type system using available observational constraints. The models show several end member modes of behaviour of the lower crust from complete exhumation, to 'ponding'/ accumulation at the base of the orogen, to subduction and deep entrainment. The rheology of the lower crust is the dominant factor controlling these behaviours, although there is also modification of the dynamics depending on the rates of continental convergence, the presence of active and varying degrees of erosion, and the effects of differing ratios of deposition.

  4. Continental accretion: contrasting Mesozoic and Early Proterozoic tectonic regimes in North America

    NASA Astrophysics Data System (ADS)

    Condie, Kent C.; Chomiak, Beverly

    1996-11-01

    Juvenile continental crust was accreted to southern and western North America during the Early Proterozoic and Mesozoic, respectively. Graywacke, granite, granodiorite, and basalt comprise most of the accreted Early Proterozoic crust, whereas graywacke, andesite, basalt, and granodiorite comprise most of the Mesozoic crust. In addition, carbonates, ultramafics, pelagic sediments, and tonalite/diorite are minor but important components in the juvenile Mesozoic crust, whereas rhyolites are important in the Early Proterozoic crust. Mesozoic supracrustal rocks vary significantly in chemical composition, while Early Proterozoic supracrustals have a limited compositional range and exhibit a linear relation between many element concentrations suggesting a genetic linkage between accreted terranes. Although SiO 2, Al 2O 3, FeO, and incompatible elements are more enriched in Early Proterozoic than in Mesozoic supracrustal rocks, negative Eu anomalies are typical of rocks of both ages. Early Proterozoic granitoids are enriched in LILE (large ion lithophile elements) compared to Mesozoic granitoids, and granitoids of both ages of are enriched in LILE and have larger Eu anomalies than associated supracrustal rocks. Accreted Mesozoic upper crust is similar to andesite in chemical composition, and the bulk crust is similar to basaltic andesite. In contrast, accreted Early Proterozoic upper crust and bulk crust are similar to granodiorite and andesite, respectively. Incompatible elements are depleted in the Mesozoic compared to the Early Proterozoic crust, but both crustal types have negative NbTa anomalies. Depending on the composition assumed for the lower crust, both ages of crust have either very small or negligible Eu anomalies. Lifespans of the Early Proterozoic terranes (time interval between oldest rocks in a terrane and its collision with North America) are 20-80 My, whereas lifespans of Mesozoic terranes are 50-500 My, with most falling between 50 and 200 My. Within

  5. Surface coating for prevention of crust formation

    DOEpatents

    Kronberg, James W.

    1994-01-01

    A flexible surface coating which promotes the removal of deposits as they reach the surface by preventing adhesion and crust formation. Flexible layers are attached to each side of a flexible mesh substrate comprising of a plurality of zones composed of one or more neighboring cells, each zone having a different compressibility than its adjacent zones. The substrate is composed of a mesh made of strands and open cells. The cells may be filled with foam. Studs or bearings may also be positioned in the cells to increase the variation in compressibility and thus the degree of flexing of the coating. Surface loading produces varying amounts of compression from point to point causing the coating to flex as deposits reach it, breaking up any hardening deposits before a continuous crust forms. Preferably one or more additional layers are also used, such as an outer layer of a non-stick material such as TEFLON, which may be pigmented, and an inner, adhesive layer to facilitate applying the coating to a surface.

  6. Tectonic structure and evolution of the Atlantic continental margin

    SciTech Connect

    Klitgord, K.D.; Schouten, H.; Hutchinson, D.R.

    1985-01-01

    The Atlantic continental margin developed across the boundary between continental and oceanic crust as rifting and then sea-floor spreading broke apart and separated the North American and African plates, forming the Atlantic Ocean Basin. Continental rifting began in Late Triassic with reactivation of Paleozoic thrust faults as normal faults and with extension across a broad zone of subparallel rift basins. Extension became localized in Early to Middle Jurassic along the zone that now underlies the large marginal basins, and other rift zones, such as the Newark, Hartford, and Fundy basins, were abandoned. Rifting and crustal stretching between the two continents gave way to sea-floor spreading Middle Jurassic and the formation of oceanic crust. This tectonic evolution resulted in formation of distinctive structural features. The marginal basins are underlain by a thinner crust and contain a variety of fault-controlled structures, including half-grabens, seaward- and landward-tilted blocks, faults that die out within the crust, and faults that penetrate the entire crust. This variable structure probably resulted from the late Triassic-Early Jurassic pattern of normal, listric, and antithetic faults that evolved from the Paleozoic thrust fault geometry. The boundary between marginal basins and oceanic crust is marked approximately by the East Coast Magnetic Anomaly (ECMA). A major basement fault is located in the Baltimore Canyon trough at the landward edge of the ECMA and a zone of seaward dipping reflectors is found just seaward of the ECMA off Georges Bank. The fracture zone pattern in Mesozoic oceanic crust can be traced landward to the ECMA.

  7. Linking biological soil crust diversity to ecological functions

    NASA Astrophysics Data System (ADS)

    Glaser, Karin; Borchhardt, Nadine; Schulz, Karoline; Mikhailyuk, Tatiana; Baumann, Karen; Leinweber, Peter; Ulf, Karsten

    2016-04-01

    Biological soil crusts (BSCs) are an association of different microorganisms and soil particles in the top millimeters of the soil. They are formed by algae, cyanobacteria, microfungi, bacteria, bryophytes and lichens in various compositions. Our aim was to determine and compare the biodiversity of all occurring organisms in biogeographically different habitats, ranging from polar (both Arctic and Antarctic), subpolar (Scandinavia), temperate (Germany) to dry regions (Chile). The combination of microscopy and molecular techniques (next-generation sequencing) revealed highly diverse crust communities, whose composition clustered by region and correlates with habitat characteristics such as water content. The BSC biodiversity was then linked to the ecological function of the crusts. The functional role of the BSCs in the biogeochemical cycles of carbon, nitrogen and phosphorous is evaluated using an array of state of the art soil chemistry methods including Py-FIMS (pyrolysis field ionization mass spectrometry) and XANES (x-ray absorbance near edge structure). Total P as well as P fractions were quantified in all BSCs, adjacent soil underneath and comparable nearby soil of BSC-free areas revealing a remarkable accumulation of total phosphorous and a distinct pattern of P fractions in the crust. Further, we observed an indication of a different P-speciation composition in the crust compared with BSC-free soil. The data allow answering the question whether BSCs act as sink or source for these compounds, and how biodiversity controls the biogeochemical function of BSCs.

  8. Potentially exploitable supercritical geothermal resources in the ductile crust

    USGS Publications Warehouse

    Watanabe, Noriaki; Numakura, Tatsuya; Sakaguchi, Kiyotoshi; Saishu, Hanae; Okamoto, Atsushi; Ingebritsen, Steven E.; Tsuchiya, Noriyoshi

    2017-01-01

    The hypothesis that the brittle–ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10−16 m2) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability–depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350–500 °C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic–plastic transition) and, importantly, causes no ‘jump’ in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450 °C may form at depths of 2–6 km even in the nominally ductile crust.

  9. Potentially exploitable supercritical geothermal resources in the ductile crust

    NASA Astrophysics Data System (ADS)

    Watanabe, Noriaki; Numakura, Tatsuya; Sakaguchi, Kiyotoshi; Saishu, Hanae; Okamoto, Atsushi; Ingebritsen, Steven E.; Tsuchiya, Noriyoshi

    2017-01-01

    The hypothesis that the brittle-ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10-16 m2) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability-depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350-500 °C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic-plastic transition) and, importantly, causes no `jump' in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450 °C may form at depths of 2-6 km even in the nominally ductile crust.

  10. Deformation and seismicity associated with continental rift zones propagating toward continental margins

    NASA Astrophysics Data System (ADS)

    Lyakhovsky, V.; Segev, A.; Schattner, U.; Weinberger, R.

    2012-01-01

    We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as thetectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for thetectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.

  11. Tertiary evolution and petroleum potential of Oregon-Washington continental margin

    SciTech Connect

    Snavely, P.D. Jr.

    1986-07-01

    The Oregon-Washington continental margin was the site of a deep marginal basin in which more than 7000 m of Tertiary sedimentary and volcanic rocks accumulated. Oceanic basalts of Paleocene to early Eocene age form the basin floor and are interpreted to represent eruptions in an elongate trough formed by rifting of the continental margin. Middle Eocene turbidite sandstone overlapped both the oceanic crust and the pre-Tertiary rocks of the Klamath Mountains, thus indicating that suturing of the Coast Range-Olympic terrane to North America was about 50 Ma. Oblique convergence between the Farallon and North American plates occurred during most of the middle Eocene to middle Miocene. Sedimentation, punctuated by episodes of volcanism, was essentially continuous in a forearc basin whose axis lay along the present inner continental shelf. The oblique interaction between the plates was interrupted by two periods of more head-on convergence during the middle late Eocene and late middle Miocene. Thick accretionary melange wedges of Eocene and of late Oligocene to late middle Miocene ages were formed during these strongly compressive episodes. Geochemical analyses indicate that the accretionary melange wedges, which crop out along the west side of the Olympic Peninsula and beneath the adjacent shelf, have the highest potential for oil and gas generation. They are the source rocks for numerous gas seeps and oil and gas shows in exploratory wells, and for the 12,000 bbl of 38.9 /sup 0/ API paraffin-based oil produced from a well drilled on the southwest Washington coast. Potential exploration targets exist where the Eocene and Oligocene-Miocene melanges are underplated to a position beneath the lower Eocene oceanic basalt. Hydrocarbons generated in the melanges could migrate upward into structures in strata that overlie the basalt in the upper plate.

  12. Correlations between the Lomonosov Ridge, Marvin Spur and adjacent basins of the Arctic Ocean based on seismic data

    NASA Astrophysics Data System (ADS)

    Langinen, A. E.; Lebedeva-Ivanova, N. N.; Gee, D. G.; Zamansky, Yu. Ya.

    2009-07-01

    Seismic profiles across the Lomonosov Ridge, Marvin Spur and adjacent basins, acquired near the North Pole by the drifting ice-station NP-28, provide a reflection image of the upper parts of the Ridge that is readily correlatable with those acquired by the Alfred Wegner Institute closer to the Siberian margin. A prominent flat-lying composite reflection package is seen in most parts of the Ridge at a few hundred meters below the sea bottom. Underlying reflections are variable in intensity and also in dip. The base of this reflection package is often accompanied by a sharp increase in P-velocity and defines a major angular discontinuity, referred to here as the Lomonosov Unconformity. The Arctic Coring Expedition (ACEX) cored the first c. 430 m section on the Lomonosov Ridge near the North Pole, in 2004 defining the deeper water character of the Neogene and the shallower water Paleogene sediments. These boreholes penetrated the composite reflection package towards the base of the hole and identified sediments (our Unit III) of late Paleocene and early Eocene age. Campanian beds at the very base of the hole were thought to be representative of the units below the Lomonosov Unconformity, but the P-velocity data suggest that this is unlikely. Correlation of the lithologies along the top of the Lomonosov Ridge and to the Marvin Spur indicates that the Marvin Spur is a sliver of continental crust closely related to, and rifted off the Ridge. This narrow (50 km wide) linear basement high can be followed into, beneath and across the Makarov Basin, supporting the interpretation that this Basin is partly resting on thinned continental crust. In the Makarov Basin, the Paleogene succession is much thicker than on the Ridge. Thus, the condensed, shallow water succession (with hiati) was deposited on the Ridge during rapid Eocene to Miocene subsidence of the Basin. In the Amundsen Basin, adjacent to the Lomonosov Ridge, the sedimentary successions thicken towards the Canadian

  13. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    SciTech Connect

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.

  14. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    NASA Astrophysics Data System (ADS)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

  15. Layered basic complex in oceanic crust, romanche fracture, equatorial atlantic ocean.

    PubMed

    Melson, W G; Thompson, G

    1970-05-15

    A layered, basic igneous intrusion, analogous in mineralogy and texture to certain large, continental layered complexes, is exposed in the Romanche Fracture, equatorial Atlantic Ocean. Crustal intrusion of large masses of basic magmas with their subsequent gravity differentiation is probably one of a number of major processes involved in the formation of new oceanic crust during sea-floor spreading.

  16. Coupled onshore erosion and offshore sediment loading as causes of lower crust flow on the margins of South China Sea

    NASA Astrophysics Data System (ADS)

    Clift, Peter D.

    2015-12-01

    Hot, thick continental crust is susceptible to ductile flow within the middle and lower crust where quartz controls mechanical behavior. Reconstruction of subsidence in several sedimentary basins around the South China Sea, most notably the Baiyun Sag, suggests that accelerated phases of basement subsidence are associated with phases of fast erosion onshore and deposition of thick sediments offshore. Working together these two processes induce pressure gradients that drive flow of the ductile crust from offshore towards the continental interior after the end of active extension, partly reversing the flow that occurs during continental breakup. This has the effect of thinning the continental crust under super-deep basins along these continental margins after active extension has finished. This is a newly recognized form of climate-tectonic coupling, similar to that recognized in orogenic belts, especially the Himalaya. Climatically modulated surface processes, especially involving the monsoon in Southeast Asia, affects the crustal structure offshore passive margins, resulting in these "load-flow basins". This further suggests that reorganization of continental drainage systems may also have a role in governing margin structure. If some crustal thinning occurs after the end of active extension this has implications for the thermal history of hydrocarbon-bearing basins throughout the area where application of classical models results in over predictions of heatflow based on observed accommodation space.

  17. Copper systematics in arc magmas and implications for crust-mantle differentiation.

    PubMed

    Lee, Cin-Ty A; Luffi, Peter; Chin, Emily J; Bouchet, Romain; Dasgupta, Rajdeep; Morton, Douglas M; Le Roux, Veronique; Yin, Qing-zhu; Jin, Daphne

    2012-04-06

    Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper's (Cu's) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

  18. Cobalt in ferromanganese crusts as a monitor of hydrothermal discharge on the Pacific sea floor

    USGS Publications Warehouse

    Manheim, F. T.; Lane-Bostwick, C. M.

    1988-01-01

    Ferromanganese oxide crusts, which accumulate on unsedimented surfaces in the open ocean1-6, derive most of their metal content from dissolved and particulate matter in ambient bottom water7,8, in proportions modified by the variable scavenging efficiency of the oxide phase for susceptible ions9. They differ in this respect from abyssal nodules, much of whose metals are remobilized from host sediments. Here we present maps of cobalt concentration and inferred accumulation rate of ferromanganese crusts from the Pacific Ocean. We propose that depletion of cobalt in Pacific crusts measures the location and intensity of submarine hydrothermal discharge. Use of the 'cobalt chronometer', an algorithm inversely relating cobalt content and crust growth rate, permits mapping of the accumulation rate of ferromanganese crusts with only indirect recourse to radioactivity-based dating methods. These maps show that crusts in hydrothermal areas grow from two to more than four orders of magnitude faster than in the Central Pacific Ocean. Cobalt-enriched crusts are found where water masses are most isolated from continental-coastal and hydrothermal sources of metals, now and in the past. This relationship can resolve the problem of cobalt enrichment in crusts without recourse to hypotheses invoking special cobalt sources or enrichment mechanisms. ?? 1988 Nature Publishing Group.

  19. Osmium isotope stratigraphy of a marine ferromanganese crust

    USGS Publications Warehouse

    Klemm, V.; Levasseur, S.; Frank, M.; Hein, J.R.; Halliday, A.N.

    2005-01-01

    Ferromanganese crusts provide records of long term change in ocean circulation and continental weathering. However, calibrating their age prior to 10 Ma has been entirely based on empirical growth rate models using Co concentrations, which have inherently large uncertainties and fail to detect hiatuses and erosional events. We present a new method for dating these crusts by measuring their osmium (Os) isotope record and matching it to the well-known marine Os isotope evolution of the past 80 Ma. The well-characterised crust CD29-2 from the central Pacific, was believed to define a record of paleooceanographic change from 50 Ma. Previous growth rate estimates based on the Co method are consistent with the new Os isotope stratigraphy but the dating was grossly inaccurate due to long hiatuses that are now detectable. The new chronology shows that it in fact started growing prior to 70 Ma in the late Cretaceous and stopped growing or was eroded between 13.5 and 47 Ma. With this new technique it is now possible to exploit the full potential of the oceanographic and climatic records stored in Fe-Mn crusts. ?? 2005 Elsevier B.V. All rights reserved.

  20. A Comparison of Microbial Communities from Deep Igneous Crust

    NASA Astrophysics Data System (ADS)

    Smith, A. R.; Flores, G. E.; Fisk, M. R.; Colwell, F. S.; Thurber, A. R.; Mason, O. U.; Popa, R.

    2013-12-01

    Recent investigations of life in Earth's crust have revealed common themes in organism function, taxonomy, and diversity. Capacities for hydrogen oxidation, carbon fixation, methanogenesis and methanotrophy, iron and sulfur metabolisms, and hydrocarbon degradation often predominate in deep life communities, and crustal mineralogy has been hypothesized as a driving force for determining deep life community assemblages. Recently, we found that minerals characteristic of the igneous crust harbored unique communities when incubated in the Juan de Fuca Ridge flank borehole IODP 1301A. Here we present attached mineral biofilm morphologies and a comparison of our mineral communities to those from a variety of locations, contamination states, and igneous crustal or mineralogical types. We found that differences in borehole mineral communities were reflected in biofilm morphologies. Olivine biofilms were thick, carbon-rich films with embedded cells of uniform size and shape and often contained secondary minerals. Encrusted cells, spherical and rod-shaped cells, and tubes were indicative of glass surfaces. We also found that the attached communities from incubated borehole minerals were taxonomically more similar to native, attached communities from marine and continental crust than to communities from the aquifer water that seeded it. Our findings further support the hypothesis that mineralogy selects for microbial communities that have distinct phylogenetic, morphological, and potentially functional, signatures. This has important implications for resolving ecosystem function and microbial distributions in igneous crust, the largest deep habitat on Earth.

  1. Magmatic consequences of cold plumes: A possible off-crust generation of batholiths

    NASA Astrophysics Data System (ADS)

    Castro, A.; Gerya, T.

    2006-12-01

    Numerical and laboratory experiments strongly support that large volumes of silicic melts are generated in the mantle wedge overlying subducting ilthospheric plates. Subducted mélanges composed of oceanic crust, sediments and hydrated mantle form cold plumes producing hydrous, silicic melts that may survive reaction with the mantle peridotite by the formation of pyroxenite bands and, hence, they can be transported through the mantle. These composite magamtic bodies may intrude into the continental crust forming calc-alkaline batholiths. Melt compositions, fertility and reaction between silicic melts an the peritotite mantle (both hydrous and dry) were tested by means of piston-cylinder experiments at condiotions of 1000 °C and pressures of 2.0 and 2.5 GPa. The results indicate that great amounts of silcic melts of trondhjemite and granodiorite compositions are procuced in these ascending megastructures. It is suggested that these silicic melts may intrude into the crust forming batholiths. This off-crust generation of granitoids may account for the growing of the continental crust by met additions of silicic magmas, being the recycling of subducted oceanic crust one of the main contributors for crustal growing. The observed jump in the MgO/MgO+FeO ratio of granodiorites since the last 1.8 Ga, if due to limited interaction between mantle peridotite and silicic melts, may imply that this process of diapiric upwelling and batholith generation was not active before this age.

  2. Seismic structure of the U.S. Mid-Atlantic continental margin

    NASA Astrophysics Data System (ADS)

    Holbrook, W. Steven; Purdy, G. M.; Sheridan, R. E.; Glover, L., III; Talwani, M.; Ewing, J.; Hutchinson, D.

    1994-09-01

    Multichannel and wide-angle seismic data collected off Virginia during the 1990 EDGE Mid-Atlantic seismic experiment provide the most detailed image to date of the continent-ocean transition on the U.S. Atlantic margin. Multichannel data were acquired using a 10,800 cu inch (177 L) airgun array and 6-km-long streamer, and coincident wide-angle data were recorded by ten ocean bottom seismic instruments. A velocity model constructed by inversion of wide-angle and vertical-incidence travel times shows strong lateral changes in deep-crustal structure across the margin. Lower-crustal velocities are 6.8 km/s in rifted continental crust, increase to 7.5 km/s beneath the outer continental shelf, and decrease to 7.0 km/s in oceanic crust. Prominent seaward- dipping reflections comprise a 100-km-wide, 25-km-thick ocean- continent transition zone that consists almost entirely of mafic igneous material accreted to the margin during continental breakup. The boundary between rifted continental crust and this thick igneous crust is abrupt, occupying only about 20 km of the margin. Appalachian intracrustal reflectivity largely disappears across this boundary as velocity increases from 5.9 km/s to greater than 7.0 km/s, implying that the reflectivity is disrupted by massive intrusion and that very little continental crust persists seaward of the reflective crust persists seaward of the reflective crust. The thick igneous crust is spatially correlated with the East Coast magnetic anomaly, implying that the basalts and underlying intrusives cause the anomaly. The details of the seismic structure and lack of independent evidence for an appropriately located hotspot in the central Atlantic imply that nonplume processes are responsible for the igneous material.

  3. Nature of crust in the central Red Sea

    NASA Astrophysics Data System (ADS)

    Mitchell, Neil C.; Park, Yongcheol

    2014-07-01

    A transition between continental crust in the northern Red Sea and oceanic crust in the southern Red Sea coincides broadly with a southward increase in plate tectonic separation rate and with a decrease in upper mantle seismic velocity. We re-evaluate here the nature of crust in the intervening central Red Sea with the results of legacy seismic refraction experiments and recently released marine gravity anomalies derived from satellite altimeter measurements. In the refraction data, collected east of Thetis Deep, velocities of 6.6-6.9 km s- 1 of a deep refracting layer, which are similar to measured velocities of unaltered gabbro samples, extend outside the deep to 65 km from the axis. The new version of the marine gravity field reveals trends crossing the central Red Sea. Whereas some of them connect with major lineaments in the surrounding African-Arabian shield, those around Thetis Deep die out towards the coastlines. They can be paired across the ridge and lie slightly oblique to plate motions, as is typical of oceanic fracture zones or non-transform discontinuities migrating away from hotspots. Taken together these observations support the view that an oceanic rather than extended continental crust underlies this part of the central Red Sea. The crestal mountains around the median valleys of slow-spreading ridges are typically 500-1000 m lower at spreading discontinuities. Around Thetis Deep, the similar pattern in the gravity field to those of slow-spreading ridges suggests that the crestal mountains may variably block or impede flowage of evaporites towards the spreading centre, whereas the discontinuities may mark areas where flowage is unobstructed. Limited multibeam data collected in transits outside Thetis Deep show oblique fabrics as expected from these predicted movements.

  4. Mapping of lithologic and structural units using multispectral imagery. [Afar-Triangle/Ethiopia and adjacent areas (Ethiopian Plateau, Somali Plateau, and parts of Yemen and Saudi Arabia)

    NASA Technical Reports Server (NTRS)

    Kronberg, P. (Principal Investigator)

    1974-01-01

    The author has identified the following significant results. ERTS-1 MSS imagery covering the Afar-Triangle/Ethiopia and adjacent regions (Ethiopian Plateau, Somali Plateau, and parts of Yemen and Saudi Arabi) was applied to the mapping of lithologic and structural units of the test area at a scale 1:1,000,000. Results of the geological evaluation of the ERTS-1 imagery of the Afar have proven the usefullness of this type of satellite data for regional geological mapping. Evaluation of the ERTS images also resulted in new aspects of the structural setting and tectonic development of the Afar-Triangle, where three large rift systems, the oceanic rifts of the Red Sea and Gulf of Aden and the continental East African rift system, seem to meet each other. Surface structures mapped by ERTS do not indicate that the oceanic rift of the Gulf of Aden (Sheba Ridge) continues into the area of continental crust west of the Gulf of Tadjura. ERTS data show that the Wonji fault belt of the African rift system does not enter or cut through the central Afar. The Aysha-Horst is not a Horst but an autochthonous spur of the Somali Plateau.

  5. Mantle exhumation and OCT architecture dependency on lithosphere deformation modes during continental breakup: Numerical experiments

    NASA Astrophysics Data System (ADS)

    Jeanniot, Ludovic; Kusznir, Nick; Manatschal, Gianreto; Cowie, Leanne

    2013-04-01

    The initiation of sea-floor spreading, during the continental breakup process, requires both the rupture of the continental crust and the initiation of decompression melting. This process results in mantle upwelling and at some point decompressional melting which creates new oceanic crust. Using numerical experiments, we investigate how the deformation mode of continental lithosphere thinning and stretching controls the rupture of continental crust and lithospheric mantle, the onset of decompression melting, their relative timing, and the circumstances under which mantle exhumation may occur. We assume that the topmost continental and ocean lithosphere, corresponding to the cooler brittle seismogenic layer, deforms by extensional faulting (pure-shear deformation) and magmatic intrusion, consistent with the observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). We assume that deformation beneath this topmost lithosphere layer (approximately 15-20 km thick) occurs in response to passive upwelling and thermal and melt buoyancy driven small-scale convection. We use a 2D finite element viscous flow model (FeMargin) to describe lithosphere and asthenosphere deformation. This flow field is used to advect lithosphere and asthenosphere temperature and material. The finite element model is kinematically driven by Vx for the topmost upper crust inducing passive upwelling beneath that layer. A vertical velocity Vz is defined for buoyancy enhanced upwelling as predicted by Braun et al. (2000). Melt generation is predicted by decompression melting using the parameterization and methodology of Katz et al. (2003). Numerical experiments have been used to investigate the dependency of continental crust and lithosphere rupture, decompression melt initiation, rifted margin ocean-continent transition architecture and subsidence history on the half-spreading rate Vx, buoyancy driven upwelling rate Vz, the relative contribution of these deformation

  6. The gravity field of the U.S. Atlantic continental margin

    USGS Publications Warehouse

    Grow, A.J.; Bowin, C.O.; Hutchinson, D.O.

    1979-01-01

    Approximately 39,000 km of marine gravity data collected during 1975 and 1976 have been integrated with U.S. Navy and other available data over the U.S. Atlantic continental margin between Florida and Maine to obtain a 10 mgal contour free-air gravity anomaly map. A maximum typically ranging from 0 to +70 mgal occurs along the edge of the shelf and Blake Plateau, while a minimum typically ranging from -20 to -80 mgal occurs along the base of the continental slope, except for a -140 mgal minimum at the base of the Blake Escarpment. Although the maximum and minimum free-air gravity values are strongly influenced by continental slope topography and by the abrupt change in crustal thickness across the margin, the peaks and troughs in the anomalies terminate abruptly at discrete transverse zones along the margin. These zones appear to mark major NW-SE fractures in the subsided continental margin and adjacent deep ocean basin, which separate the margin into a series of segmented basins and platforms. Rapid differential subsidence of crustal blocks on either side of these fractures during the early stages after separation of North America and Africa (Jurassic and Early Cretaceous) is inferred to be the cause of most of the gravity transitions along the length of margin. The major transverse zones are southeast of Charleston, east of Cape Hatteras, near Norfolk Canyon, off Delaware Bay, just south of Hudson Canyon and south of Cape Cod. Local Airy isostatic anomaly profiles (two-dimensional, without sediment corrections) were computed along eight multichannel seismic profiles. The isostatic anomaly values over major basins beneath the shelf and rise are generally between -10 and -30 mgal while those over the platform areas are typically 0 to +20 mgal. While a few isostatic anomaly profiles show local 10-20 mgal increases seaward of the East Coast Magnetic Anomaly (ECMA: inferred to mark the ocean-continent boundary), the lack of a consistent correlation indicates that the

  7. Collision zone magmatism aids continental crustal growth

    NASA Astrophysics Data System (ADS)

    Savov, Ivan; Meliksetian, Khachatur; Ralf, Halama; Gevorg, Navasardian; Chuck, Connor; Massimo, D'Antonio; Samuele, Agostini; Osamu, Ishizuka; Sergei, Karapetian; Arkadi, Karakhanian

    2014-05-01

    The continental crust has a broadly andesitic bulk composition and is predominantly generated at convergent margins. However, estimates of the bulk composition of oceanic arcs indicate a bulk composition closer to basalt than to andesite. Hence, reworking processes that transform basaltic island arc crust into andesitic continental crust are essential[1] and explaining growth of andesitic continental crust via accretion of arc crustal fragments remains problematic. Recent studies of magmatism in the Great Tibetan Plateau[2], as site of multiple and still active continent-continent collisions, have proposed that andesitic CC is generated via amalgamation of large volumes of collision-related felsic magmas generated by melting of hydrated oceanic crust with mantle geochemical signatures. We aim to test this hypothesis by evaluating geochemical data from the volcanically and tectonically active Lesser Caucasus region (Armenia, Azerbaijan, Georgia and E. Turkey), as the only other region where active continent-continent collision takes place. We will benefit from the newly compiled volcano-tectonic database of collision-related volcanic and plutonic rocks of Armenia that is comparable in quality and detail to the one available on Tibet. Our dataset combines several detailed studies from the large Aragats shield volcano[3] and associated monogenetic volcanic fields (near the capital city of Yerevan), as well as > 500 Quaternary to Holocene volcanoes from Gegham, Vardenis and Syunik volcanic highlands (toward Armenia-Nagorno-Karabakh-Azerbaijan-Iran border). The Armenian collision-related magmatism is diverse in volume, composition, eruption style and volatile contents. Interestingly, the majority of exposed volcanics are andesitic in composition. Nearly all collision-related volcanic rocks, even the highly differentiated dacite and rhyolite ignimbrites, have elevated Sr concentrations and 87Sr/86Sr and 143Nd/144Nd ratios varying only little (average ~ 0.7043 and ~ 0

  8. CHIC - Coupling Habitability, Interior and Crust

    NASA Astrophysics Data System (ADS)

    Noack, Lena; Labbe, Francois; Boiveau, Thomas; Rivoldini, Attilio; Van Hoolst, Tim

    2014-05-01

    We present a new code developed for simulating convection in terrestrial planets and icy moons. The code CHIC is written in Fortran and employs the finite volume method and finite difference method for solving energy, mass and momentum equations in either silicate or icy mantles. The code uses either Cartesian (2D and 3D box) or spherical coordinates (2D cylinder or annulus). It furthermore contains a 1D parametrised model to obtain temperature profiles in specific regions, for example in the iron core or in the silicate mantle (solving only the energy equation). The 2D/3D convection model uses the same input parameters as the 1D model, which allows for comparison of the different models and adaptation of the 1D model, if needed. The code has already been benchmarked for the following aspects: - viscosity-dependent rheology (Blankenbach et al., 1989) - pseudo-plastic deformation (Tosi et al., in preparation phase) - subduction mechanism and plastic deformation (Quinquis et al., in preparation phase) New features that are currently developed and benchmarked include: - compressibility (following King et al., 2009 and Leng and Zhong, 2008) - different melt modules (Plesa et al., in preparation phase) - freezing of an inner core (comparison with GAIA code, Huettig and Stemmer, 2008) - build-up of oceanic and continental crust (Noack et al., in preparation phase) The code represents a useful tool to couple the interior with the surface of a planet (e.g. via build-up and erosion of crust) and it's atmosphere (via outgassing on the one hand and subduction of hydrated crust and carbonates back into the mantle). It will be applied to investigate several factors that might influence the habitability of a terrestrial planet, and will also be used to simulate icy bodies with high-pressure ice phases. References: Blankenbach et al. (1989). A benchmark comparison for mantle convection codes. GJI 98, 23-38. Huettig and Stemmer (2008). Finite volume discretization for dynamic

  9. Continental emergence in the Late Archean reconciles early and late continental growth models

    NASA Astrophysics Data System (ADS)

    Flament, Nicolas; Coltice, Nicolas; Rey, Patrice

    2014-05-01

    The analysis of ancient sediments (Rare Earth Element composition of black shales, isotopic strontium composition of marine carbonates, isotopic oxygen composition of zircons) suggests that continental growth culminated around the Archean-Proterozoic transition. In stark contrast, the geochemical analysis of ancient basalts suggests that depletion of the mantle occurred in the Hadean and Eoarchean. This paradox may be solved if continents were extracted from the mantle early in Earth's history, but remained mostly below sea level throughout the Archean. We present a model to estimate the area of emerged land and associated isotopic strontium composition of the mantle and oceans as a function of the coupled evolution of mantle temperature, continental growth and distribution of surface elevations (hypsometry). For constant continental hypsometry and four distinct continental growth models, we show that sea level was between 500 and 2000 m higher in the Archean than at present, resulting in < 12% of emerged land, compared to ~ 28% at present. If in addition the hot Archean lithosphere could not sustain high relief, as little as 2-3% of Earth's surface would have been emerged in the Archean. Using a geochemical box model for the strontium isotopic composition of the mantle and oceans, we show that a reduced area of emerged continental crust can explain why the geochemical fingerprint of continents extracted early in Earth's history was not recorded at the surface of the Earth until the late Archean.

  10. Helium isotopes in ferromanganese crusts from the central Pacific Ocean

    USGS Publications Warehouse

    Basu, S.; Stuart, F.M.; Klemm, V.; Korschinek, G.; Knie, K.; Hein, J.R.

    2006-01-01

    Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/4He of VA13/2 samples varies from 18.5 to 1852 Ra due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

  11. Partitioning of crustal shortening during continental collision: 2-D thermomechanical modeling

    NASA Astrophysics Data System (ADS)

    Liao, Jie; Gerya, Taras

    2017-01-01

    Partitioning of crustal shortening between the colliding continental plates is highly variable in nature. Physical controls of such variability remain largely enigmatic and require quantitative understanding. In this study, we employ 2-D thermomechanical numerical modeling to investigate the influence of the rheological properties of the continental crust on the dynamics and distribution of crustal shortening during continental collision. Three major physical parameters, (i) the mechanical strength of the upper crust, (ii) the Moho temperature, and (iii) the convergence rate, are investigated, and their influences on crustal shortening partitioning between the lower and upper plates are systematically documented. Numerical modeling results suggest that a strong upper crust of the lower plate, high Moho temperature, and slow convergence rate favor migration of crustal shortening from the lower to the upper plate. Our numerical modeling results compare well with natural observations from the Alpine orogenic system where variable partitioning of crustal deformation between the plates is documented.

  12. Thermal perturbation, mineral assemblages, and rheology variations induced by dyke emplacement in the crust

    NASA Astrophysics Data System (ADS)

    Lavecchia, Alessio; Clark, Stuart R.; Beekman, Fred; Cloetingh, Sierd A. P. L.; Burov, Evgueni

    2016-05-01

    We constructed a thermomechanical model to examine the changes in rheology caused by the periodic intrusion of basaltic dykes in a two-layered continental crust. Dyke intrusion can locally change the mineralogical composition of the crust in space and time as a result of temperature-induced metamorphism. In our models we paid particular attention to determine how different mineral assemblages and reaction kinetics during metamorphism impact on the thermomechanical behavior of the crust, in terms of differential stress values. We investigated several lithologies characteristic for intracontinental crust: (1) a quartz-feldspathic crust (QF), (2) a crust with a mineralogical assemblage resembling the average chemical composition occurring in literature (CC), and (3) a micaschist crust (MS). Our model shows that temperature profiles are weakly influenced by metamorphism, with negligible variations in the T-t paths. The results indicate that intrusion-induced changes in the crustal rheology are strongly dependent on mineralogical assemblage variation. The strength of a dyke aureole in the upper crust increases during dyke emplacement, which may cause migration of later dykes and influence the dyke spacing. In contrast, in the lower crust the strength of a dyke aureole decreases during dyke emplacement. Fast kinetics results in a ductile lower crust in proximity of the dykes, whereas slower kinetics leads to the formation of partial melts and subsequent switch from ductile to brittle behavior. Lithology exerts a dominant role on the quantity of melt produced, with higher volume percentages occurring in the MS case study. Produced melts may migrate and support acidic volcanic activity.

  13. Paleostress adjacent to the Alpine Fault of New Zealand - Fault, vein, and styolite data from the Doctors Dome area

    NASA Astrophysics Data System (ADS)

    Nicol, Andrew; Wise, Donald U.

    1992-11-01

    Doctors Dome, 75 km north of Christchurch, New Zealand, is an early Pleistocene to Recent structure being deformed along the southeast edge of the Pacific-Australian plate boundary. Paleostress in the area has been determined in basement rocks of the Mesozoic meta-graywacke Torlesse Supergroup which lies unconformably beneath Cretaceous and younger cover rocks. Inversion of basement fault data for the area indicates a general northwest compression with two peaks, one WNW-ESE parallel to the shortening suggested by the older vein system and the other parallel to southeast-northwest stylolite columns in the cover rocks. This direction is approximately parallel to regional indicators of contemporary deformation in and adjacent to the Alpine Fault Zone and suggests that the stress field affecting these rocks has not changed significantly since the late Pliocene-early Pleistocene. Like the San Andreas system, this compression is at a high angle to the strike of the zone as a whole, but is compatible with the direction of plate convergence and motion of the major faults. Between the overlappig ends of the Alpine Fault and the Hikurangi Subduction Zone the Alpine Fault may become subhorizontal at middle-lower crustal levels, partially decoupling the crust from underlying structures, and thus allowing oblique motion to be transferred directly onto the fault from the subduction complex, while aiding the change from subduction to continental collision.

  14. Heat flow in eastern Egypt - The thermal signature of a continental breakup

    NASA Technical Reports Server (NTRS)

    Morgan, P.; Boulos, F. K.; Hennin, S. F.; El-Sherif, A. A.; El-Sayed, A. A.

    1985-01-01

    It is noted that the Red Sea is a modern example of continental fragmentation and incipient ocean formation. A consistent pattern of high heat flow in the Red Sea margins and coastal zone, including Precambrian terrane up to at least 30 km from the Red Sea, has emerged from the existing data. It is noted that this pattern has important implications for the mode and mechanism of Red Sea opening. High heat flow in the Red Sea shelf requires either a high extension of the crust in this zone (probably with major basic magmatic activity) or young oceanic crust beneath this zone. High heat flow in the coastal thermal anomaly zone may be caused by lateral conduction from the offshore lithosphere and/or from high mantle heat flow. It is suggested that new oceanic crust and highly extended continental crust would be essentially indistinguishable with the available data in the Red Sea margins, and are for many purposes essentially identical.

  15. Evolution of Oxidative Continental Weathering

    NASA Astrophysics Data System (ADS)

    Konhauser, Kurt; Lalonde, Stefan

    2014-05-01

    The Great Oxidation Event (GOE) is currently viewed as a protracted process during which atmospheric oxygen levels increased above 10-5 times the present atmospheric level. This value is based on the loss of sulphur isotope mass independent fractionation (S-MIF) from the rock record, beginning at 2.45 Ga and disappearing by 2.32 Ga. However, a number of recent papers have pushed back the timing for oxidative continental weathering, and by extension, the onset of atmospheric oxygenation several hundreds of million years earlier despite the presence of S-MIF (e.g., Crowe et al., 2013). This apparent discrepancy can, in part, be resolved by the suggestion that recycling of older sedimentary sulphur bearing S-MIF might have led to this signal's persistence in the rock record for some time after atmospheric oxygenation (Reinhard et al., 2013). Here we suggest another possibility, that the earliest oxidative weathering reactions occurred in environments at profound redox disequilibrium with the atmosphere, such as biological soil crusts, riverbed and estuarine sediments, and lacustrine microbial mats. We calculate that the rate of O2 production via oxygenic photosynthesis in these terrestrial microbial ecosystems provides largely sufficient oxidizing potential to mobilise sulphate and a number of redox-sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant S-MIF signature. These findings reconcile geochemical signatures in the rock record for the earliest oxidative continental weathering with the history of atmospheric sulphur chemistry, and demonstrate the plausible antiquity of a terrestrial biosphere populated by cyanobacteria. Crowe, S.A., Dossing, L.N., Beukes, N.J., Bau, M., Kruger, S.J., Frei, R. & Canfield, D.E. Atmospheric oxygenation three billion years ago. Nature 501, 535-539 (2013). Reinhard, C.T., Planavsky, N.J. & Lyons, T.W. Long-term sedimentary recycling of rare sulphur isotope anomalies. Nature 497

  16. Pre-breakup continental extension in East Gondwanaland and the early opening of the eastern Indian Ocean

    NASA Astrophysics Data System (ADS)

    Powell, C. McA.; Roots, S. R.; Veevers, J. J.

    1988-12-01

    Extension of continental crust by up to 360 km on a north-northeast azimuth in the Great Australian Bight occurred prior to the Middle Cretaceous (96 Ma) onset of seafloor spreading between Australia and Antarctica. This large amount of continental extension is constrained to the Late Jurassic (≈ 160 Ma)-Middle Cretaceous interval, about a pole estimated to lie about 40° southeastward of southeastern Australia. Using bathymetric data combined with seismic and magnetic determinations of the continent-ocean boundaries off Australia, India and Antarctica, we determine a revised fit of East Gondwanaland prior to continental extension, and at various stages thereafter. In the first stage from 160 to 132.5 Ma (≈ M11), India-Australia rotated from Antarctica with continental crustal extension between Australia and Antarctica and largely transform motion between the Coromandel Coast margin of India and the Kron Prins Olav Kyst margin of Antarctica. In the second stage, from 132.5 to 96 Ma, India rotated northwestward away from Australia-Antarctica, producing M10 and younger magnetic anomalies along the western margin of Australia. Continental extension was greatest between Australia and Antarctica during this stage. The newly determined 132.5-96 Ma rotation between India and Australia provides an improved fit between the orientation of observed M10-M0 magnetic anomalies and the southward decrease in spreading rate off the western Australian margin. Together with bathymetry, the new rotation parameters lead to identification of the abandoned 96 Ma spreading ridge along the Lost Dutchmen and Dirck Hartog ridges. The Naturaliste Fracture Zone is found to lie nearly parallel to the Early Cretaceous transform direction between India and Antarctica suggesting that slow seafloor spreading, required between Australia and Antarctica to accommodate the continental extension further east, occurred between Naturaliste Fracture Zone and the M4-M0 anomalies adjacent to the

  17. Crust and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data (Postprint). Annual Report 1

    DTIC Science & Technology

    2012-05-10

    upper mantle velocity structure of the region expressing the continental collision between the Arabian and Eurasian plates is being investigated using a...Eurasian plates using a joint inversion of body wave arrival times and satellite gravity. The body wave data set is derived from previous and on-going work...velocity structure in the crust and upper mantle of the region expressing the continental collision between the Arabian and Eurasian plates . Our strategy

  18. Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

    NASA Astrophysics Data System (ADS)

    Rudnick, R. L.; Liu, X. M.

    2012-04-01

    The continental crust has an "intermediate" bulk composition that is distinct from primary melts of peridotitic mantle (basalt or picrite). This mismatch between the "building blocks" and the "edifice" that is the continental crust points to the operation of processes that preferentially remove mafic to ultramafic material from the continents. Such processes include lower crustal recycling (via density foundering or lower crustal subduction - e.g., relamination, Hacker et al., 2011, EPSL), generation of evolved melts via slab melting, and/or chemical weathering. Stable isotope systems point to the influence of chemical weathering on the bulk crust composition: the oxygen isotope composition of the bulk crust is distinctly heavier than that of primary, mantle-derived melts (Simon and Lecuyer, 2005, G-cubed) and the Li isotopic composition of the bulk crust is distinctly lighter than that of mantle-derive melts (Teng et al., 2004, GCA; 2008, Chem. Geol.). Both signatures mark the imprint of chemical weathering on the bulk crust composition. Here, we use a simple mass balance model for lithium inputs and outputs from the continental crust to quantify the mass lost due to chemical weathering. We find that a minimum of 15%, a maximum of 60%, and a best estimate of ~40% of the original juvenile rock mass may have been lost via chemical weathering. The accumulated percentage of mass loss due to chemical weathering leads to an average global chemical weathering rate (CWR) of ~ 1×10^10 to 2×10^10 t/yr since 3.5 Ga, which is about an order of magnitude higher than the minimum estimates based on modern rivers (Gaillardet et al., 1999, Chem. Geol.). While we cannot constrain the exact portion of crustal mass loss via chemical weathering, given the uncertainties of the calculation, we can demonstrate that the weathering flux is non-zero. Therefore, chemical weathering must play a role in the evolution of the composition and mass of the continental crust.

  19. Crustal and basin evolution of the southwestern Barents Sea: From Caledonian orogeny to continental breakup

    NASA Astrophysics Data System (ADS)

    Gernigon, L.; Brönner, M.; Roberts, D.; Olesen, O.; Nasuti, A.; Yamasaki, T.

    2014-04-01

    A new generation of aeromagnetic data documents the post-Caledonide rift evolution of the southwestern Barents Sea (SWBS) from the Norwegian mainland up to the continent-ocean transition. We propose a geological and tectonic scenario of the SWBS in which the Caledonian nappes and thrust sheets, well-constrained onshore, swing from a NE-SW trend onshore Norway to NW-SE/NNW-SSE across the SWBS platform area. On the Finnmark and Bjarmeland platforms, the dominant inherited magnetic basement pattern may also reflect the regional and post-Caledonian development of the late Paleozoic basins. Farther west, the pre-breakup rift system is characterized by the Loppa and Stappen Highs, which are interpreted as a series of rigid continental blocks (ribbons) poorly thinned as compared to the adjacent grabens and sag basins. As part of the complex western rift system, the Bjørnøya Basin is interpreted as a propagating system of highly thinned crust, which aborted in late Mesozoic time. This thick Cretaceous sag basin is underlain by a deep-seated high-density body, interpreted as exhumed high-grade metamorphic lower crust. The abortion of this propagating basin coincides with a migration and complete reorganization of the crustal extension toward a second necking zone defined at the level of the western volcanic sheared margin and proto-breakup axis. The abortion of the Bjørnøya Basin may be partly explained by its trend oblique to the regional, inherited, structural grain, revealed by the new aeromagnetic compilation, and by the onset of further weakening later sustained by the onset of magmatism to the west.

  20. Identification of hyper-extended crust east of Davie Ridge in the Mozambique Channel

    NASA Astrophysics Data System (ADS)

    Klimke, Jennifer; Franke, Dieter

    2015-04-01

    Davie Ridge is a ~1200 km wide, N-S trending bathymetrical high in the Mozambique Channel. Today, it is widely accepted that Davie Ridge is located along a fossil transform fault that was active during the Middle Jurassic and Early Cretaceous (~165-120 Ma). This transform fault results from the breakup of Gondwana, when Madagascar (together with India and Antarctica) drifted from its northerly position in the Gondwana Supercontinent (adjacent to the coasts of Tanzania, Somalia and Kenya) to its present position (e.g. Coffin and Rabinowitz, 1987; Rabinowitz et al., 1983; Segoufin and Patriat, 1980). The southward motion of Madagascar relative to Africa is constrained by the interpretation of magnetic anomalies in the Western Somali Basin, located north of Madagascar (e.g. Rabinowitz et al., 1983). According to Bird (2001), sheared margins share typical characteristics and a common evolution: 1. The transition from continental to oceanic crust is relatively abrupt (~ 50-80 km). 2. Along the continental side of the margin, complex rift basins form that display a wide range of faults. 3. Prominent marginal ridges form along the sheared margin that probably originate from the propagation of the oceanic spreading center along the plate boundary (Bird, 2001). In February and March 2014, a dense geophysical dataset (multichannel seismic, magnetics, gravimetry and bathymetry) with a total of 4300 profile km along the sheared margin was acquired with the R/V Sonne by the Federal Institute for Geosciences and Natural Resources (BGR). A special objective of the project, amongst others, is the characterization and interpretation of the continent-ocean transition seaward of Davie Ridge in the Mozambique Channel. Seismic profiles located east of Davie Ridge in the Western Somali Basin reveal a wide sequence of half-grabens bounded by listric normal faults. We tentatively suggest that this crust is of continental origin and results from rifting between Africa and Madagascar during