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Now showing items 1 - 16 of 19

  • Comparative K–Ar and Rb–Sr age determinations of retrograde processes on rocks from the KTB deep drilling project

    Wemmer, K.   Ahrendt, H.  

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  • K-Ar age determinations on the fine fractions of clay mineral “Crystallinity” Index Standards from the Palaeozoic mudrocks of SW England

    Schomberg, A. C.   Wemmer, K.   Warr, L. N.   Grathoff, G.  

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  • Spatio-temporal characterization of smectite-to-illite diagenesis in the Nankai Trough accretionary prism revealed by samples from 3 km below seafloor

    Hüpers, A.   Grathoff, G.   Warr, L. N.   Wemmer, K.   Spinelli, G.   Underwood, M. B.  

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  • The tectono-metamorphic evolution of the very low-grade hangingwall constrains two-stage gneiss dome formation in the Montagne Noire (Southern France)

    Doublier, M. P.   Potel, S.   Wemmer, K.  

    The Montagne Noire in the southernmost French Massif Central is made of an ENE-elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low-grade petrology, K-Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faugeres units, which constitute part of the southern hangingwall of the dome. It is shown that southward-directed Variscan nappe-thrusting (D-1) and a related medium-P metamorphism (M-1) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono-metamorphic event D-2/M-2 largely transposes D-1 structures, comprises a higher metamorphic thermal gradient than M-1 (transition low-P and medium-P metamorphic facies series) and affected the rocks between c. 309 and 300Ma, post-dating D-1/M-1 by more than 20Ma. D-2-related fabrics are refolded by D-3, which in its turn, is followed by dextral-normal shearing along the basal shear zone of both units at c.297Ma. In the western Mont Peyroux and Faugeres units, D-2/M-2 is largely synchronous with shearing along the southern dome margin between c. 311 and 303Ma, facilitating the emplacement of the gneiss dome into the upper crust. D-2/M-2 also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306Ma, and a related low-P/high-T metamorphism at c. 308Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D-2 fabrics and the M-2 metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone-epizone boundary to the diagenetic zone. The related zonation of the pre-D-3 metamorphic field gradients paralleled the dome. These observations indicate that D-2/M-2 is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D-2/M-2. Based on integration of these findings with regional geological constraints, a two-stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull-apart-like geometry. The second stage from 300Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by north-dipping detachments and hangingwall basin formation along the northeastern dome margin.
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  • Metamorphic evolution of the Tethyan Himalayan flysch in SE Tibet

    Dunkl, I.   Antolin, B.   Wemmer, K.   Rantitsch, G.   Kienast, M.   Montomoli, C.   Ding, L.   Carosi, R.   Appel, E.   El Bay, R.   Xu, Q.   Von Eynatten, H.  

    The metamorphic conditions and the age of thermal overprint were determined in metapelites, metaarenites and metabasites of the Tethyan Himalayan Sequence (THS) in SE Tibet using Kubler Index and vitrinite reflectance data and applying thermobarometrical (Thermocalc and PERPLEX) and geochronological methods (illite/muscovite K-Ar and zircon and apatite (U-Th)/ He chronology). The multiple folded thrust pile experienced a thermal overprint reaching locally peak conditions between the diagenetic stage (c. 170 degrees C) and the amphibolite facies (c. 600 degrees C at 10 kbar). Burial diagenesis and heating due to Early Cretaceous dyke emplacement triggered the growth of illite in the metapelites. Eocene collision-related peak metamorphic conditions have been reached at c. 44 Ma. During collision the different tectonic blocks of the THS were tectonically buried to different structural levels so that they experienced maximum greenschist to amphibolite facies metamorphism. Later, during Oligocene to Miocene times the entire THS underwent anchi- to epizonal metamorphic conditions, probably associated to continuous deformation in the flysch fold-thrust-system. This period terminated at c. 24-22 Ma. Adjacent to the north Himalayan metamorphic domes, the base of the THS was metamorphosed during Miocene times (c. 13 Ma). Post-metamorphic cooling below c. 180 degrees C lasted until Late Miocene and took place at different times.
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  • Major shear zones of southern Brazil and Uruguay: escape tectonics in the eastern border of Rio de La plata and Paranapanema cratons during the Western Gondwana amalgamation

    Passarelli, C. R.   Basei, M. A. S.   Wemmer, K.   Siga, O., Jr.   Oyhantcabal, P.  

    The Mantiqueira Province represents a series of supracrustal segments of the South-American counterpart formed during the Gondwana Supercontinent agglutination. In this crustal domain, the process of escape tectonics played a conspicuous role, generating important NE-N-S-trending lineaments. The oblique component of the motions of the colliding tectonic blocks defined the transpressional character of the main suture zones: Lancinha-Itariri, Cubato-Arcadia-Areal, Serrinha-Rio Palmital in the Ribeira Belt and Sierra Ballena-Major Gercino in the Dom Feliciano Belt. The process as a whole lasted for ca. 60 Ma, since the initial collision phase until the lateral escape phase predominantly marked by dextral and subordinate sinistral transpressional shear zones. In the Dom Feliciano Belt, southern Brazil and Uruguay, transpressional event at 630-600 Ma is recognized and in the Ribeira Belt, despite less coevally, the transpressional event occurred between 590 and 560 Ma in its northern-central portion and between ca. 625 and 595 Ma in its central-southern portion. The kinematics of several shear zones with simultaneous movement in opposite directions at their terminations is explained by the sinuosity of these lineaments in relation to a predominantly continuous westward compression.
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  • Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps

    Siegesmund, S.   Layer, P.   Dunkl, I.   Vollbrecht, A.   Steenken, A.   Wemmer, K.   Ahrendt, H.  

    The Ivrea Zone (southern Alps) is one of the key regions interpreted as exposing a section of the lower continental crust and was the subject of several review-type articles. The Ivrea-Verbano Zone was rotated into an upright position along the Insubric mylonite belt. In the southeast, this unit is in contact with the Strona Ceneri Zone, which is interpreted as upper continental crust crossing the Permian Cossato-Mergozzo-Brissagio Line (CMB Line). The CMB mylonites are locally overprinted by the mylonites and cataclasites of the Pogallo Line, which was active during the Jurassic. In addition, the sinistral, steeply inclined Rosarolo shear zone was active over a long time span from the ductile into the brittle field, i.e. from the Early Permian (high-temperature ultra-mylonites) to the Neo-Alpine basic dykes and pseudotachylites. The high-temperature mylonites accommodated crustal extension and may be related to normal faults generated by magmatic underplating. The reactivation at different crustal levels during exhumation and tilting is documented by strain increments at decreasing P/T conditions. Its present subvertical orientation was attained during the Neo-Alpine deformation. Constraints on its exhumation history are based on new (40)Ar/(39)Ar hornblende ages, K-Ar biotite ages and zircon fission-track data along the NE-SW trending Valstrona section. A re-interpretation of existing U-Pb monazite ages is included, based on a higher closure temperature for monazite. The oldest monazite ages are observed in proximity to the Pogallo Line (c. 292 Ma). Heat input by mafic intrusions was sufficient to reset the U-Pb monazite system, as is evidenced by the youngest ages in the vicinity of the Insubric Line. The re-interpretation favours the hypothesis that the oldest monazite ages are the result of complete resetting by a Permian thermal event. The 40Ar/39Ar homblende ages and K-Ar biotite ages document the cooling after Permian heating. Roughly parallel age progressions decrease from the Pogallo Line (homblende: 271 Ma vs. biotite: 227 Ma) towards the Insubric Line (homblende: 201 Ma vs. biotite: 156 Ma). Zircon fission-track ages run parallel to the biotite ages in the upper part of the profile, whereas towards the Insubric Line a significant deviation from the biotite age progression is attributed to tilting of the basement during the Oligocene. Zircon fission-track ages around 38 Ma are found close to the Insubric Line. No age offset, neither at the CMB nor at the Pogallo Line, is observed. This confirms the hypothesis that the Pogallo Line is an oblique normal fault, and that the CMB Line has accommodated only minor vertical displacement. The capture of the different cooling ages confirms the tilting of the Ivrea-Verbano Zone during the Neo-Alpine deformation and contradicts the tilting of the Ivrea-Verbano Zone during the Permian.
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  • Indication for clockwise rotation in the Siang window south of the eastern Himalayan syntaxis and new geochronological constraints for the area

    Liebke, Ursina   Antolin, B.   Appel, E.   Basavaiah, N.   Mikes, T.   Dunkl, I.   Wemmer, K.  

    Palaeomagnetic, rock magnetic and geochronological investigations were carried out on the Abor volcanics of Arunachal Pradesh, NE India. A Late Palaeozoic formation age for part of the Abor volcanics cannot be excluded based on K-Ar whole rock dating. Low-temperature thermochronometers - zircon (U-Th)/He and fission track analyses - yield a maximum burial temperature of c. 150-170 degrees C during Late Miocene. ZFT thermochronology of the Yinkiong and Miri Fms. indicates a post-Paleocene and post-Jurassic deposition age, respectively. This infers that the volcanic rocks intercalating or intruding them are not part of the Late Palaeozoic sequence but represent one or more, latest Cretaceous to Tertiary event(s). Therefore the Abor volcanics are connected to at least two separate events of volcanism. From palaeomagnetic sites, two characteristic magnetic remanence components were separated: a low-coercivity-component demagnetized below 20 mT and a high-coercivity-component demagnetized between 15 and 100 mT. Fold tests support a secondary origin of both components. Thermochronological and rock magnetic analyses indicate a low-grade overprint event between India-Asia collision and Miocene, which probably represents the time of remanence acquisition. The high-coercivity-component shows a trend of clockwise declinations, which is likely related to vertical-axis rotations of the eastern Himalayas due to eastward extrusion of the Tibetan Plateau.
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  • Spatiotemporal Characterization of Smectite-to-Illite Diagenesis in the Nankai Trough Accretionary Prism Revealed by Samples From 3km Below Seafloor

    Huepers, A.   Grathoff, G.   Warr, L. N.   Wemmer, K.   Spinelli, G.   Underwood, M. B.  

    The up-dip limit of seismogenesis in subduction zone forearcs depends on the lithological composition of the incoming sediment and its subsequent modification during compaction and diagenesis. Here we present results of a multimethodological approach to characterize the smectite-to-illite diagenesis in the accretionary prism of the Nankai Trough subduction zone offshore SW Japan. Our X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy clay mineralogical analysis reveal that advanced states of smectite-to-illlite diagenesis occur in samples recovered from down to 3km subseafloor at Integrated Ocean Drilling Program Site C0002, which sampled the inner accretionary prism. Our temperature- and time-dependent reaction kinetics models require elevated temperatures in the prism to explain the illitization, which is consistent with revised thermal models based on recent plate reconstructions. Biostratigraphic data suggest that the inner prism sediment was deposited during a period of slow or inactive subduction and buried in the accretionary prism after the Philippine Sea plate resumed subduction. Rapid burial in the past 6Ma led to the formation of two authigenic smectite-illite phases consistent with a broad 1-9.3Ma age determined by K-Ar dating. The low K+ smectite-illite reflects the long-term burial history, whereas the K+-rich smectite-illite was inherited from the younger accretion event. Our study predicts the illitization of smectite approaches completion at 5km below the seafloor in the hanging wall of the plate boundary and the megasplay fault zone, respectively, which coincides with the proposed seaward extent of coseismic slip in the 1944 M-w=3D8.1 Tonankai earthquake along those faults.
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  • P?diatrische radiologie. Lehrbuch in 2 b?nden: By F. Schmid (ed.), with the cooperation of W. Schuster, D. Beduhn, G. Fuchs, K. Gefferth, H. Giesen, H. Gutheil, H.-M. Heinisch, R. Kobel, H. Moll, M. Neuh?user, M. Schell-Schomberg, H. Schuster, U. Wemmer, W. Wenz and E. Zapp, Springer-Verlag, Berlin, 1973. Vol. 1 (Stützgewebe—Zentralnervensystem—Syndrome), xix + 504 pages, 491 illustrations, 86 tables, DM 248.00, US $ 101.20; subscription price (if 2 volumes are ordered) DM 198.40, US $ 81.00. Vol. 2 (Thoraxorgane—Verdauungstrakt—Urogenitaltrakt), xvi + 525 pages, 625 illustrations, 32 tables, DM 248.00, US $ 101.20; subscription price (if 2 volumes are ordered) DM 198.40, US $ 81.00

    G. du Boulay  

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  • New evidences for an early Birimian evolution in the West African Craton: An example from the Kedougou-Kenieba inlier, southeast Senegal RID F-5465-2010

    Gueye, M.   Siegesmund, S.   Wemmer, K.   Pawlig, S.   Drobe, A.   Nolte, N.   Layer, P.  

    The Paleoproterozoic rocks in the Kedougou Kenieba Inlier (KKI) consist of volcanic and volcanosedimentary rocks intruded by granitoids with a wide compositional spectrum. Geochronological data based on single-grain zircon Pb-Pb evaporation, U-Pb single grain, Ar-Ar and K-Ar dating of several plutons point to a long lasting emplacement history in the KKI. The zircon data presented here yielded Paleoproterozoic ages for the emplacement/crystallization of the Badon granodiorite (2213 +/- 3 to 2198 +/- 2 Ma), tonalitic gneiss from Sandikounda (2194 +/- 4 Ma), the Tinkoto pluton (2074 +/- 9 Ma) as well as for the felsic host rocks of the Mamakono pluton (2067 +/- 12Ma). The Badon granodiorite, formerly regarded to be relatively Young and post-tectonic, is substantially older than all other known KKI intrusions. As I consequence, this new age may constrain the ongoing discussion on the existence of an Early Birimian cycle in the West African Craton (WAC). Ar-Ar and K-Ar ages on hornblende (2112 +/- 12 and 2118 +/- 31 Ma, respectively, in the tonalitic gneiss from Sandikounda) and biotite (2098 +/- 26 and 2090 +/- 21 Ma, respectively, in the Badon granodiorite) define an age of ca 2090 Ma, which is interpreted to mark the major tectonomagmatic episode in this part of the WAC. This is confirmed by the Paleoproterozoic ages of the Saraya, Tinkoto and Mamakono granite. A hydrothermal event at 2020 Ma followed the emplacement of the granitoids and caused the alteration of the Saraya granite. The new zircon ages, reflecting the magmatic and tectonothermal events in the Study area correlate well with various magmatic and metamorphic events elsewhere in West Africa.
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  • The Itajai foreland basin: a tectono-sedimentary record of the Ediacaran period, Southern Brazil

    Basei, M. A. S.   Drukas, C. O.   Nutman, A. P.   Wemmer, K.   Dunyi, L.   Santos, P. R.   Passarelli, C. R.   Campos Neto, M. C.   Siga, O., Jr.   Osako, L.  

    The Itajai Basin located in the southern border of the Luis Alves Microplate is considered as a peripheral foreland basin related to the Dom Feliciano Belt. It presents an excellent record of the Ediacaran period, and its upper parts display the best Brazilian example of Precambrian turbiditic deposits. The basal succession of Itajai Group is represented by sandstones and conglomerates (BaA(0) Formation) deposited in alluvial and deltaic-fan systems. The marine upper sequences correspond to the Ribeiro Carvalho (channelized and non-channelized proximal silty-argillaceous rhythmic turbidites), Ribeiro Neisse (arkosic sandstones and siltites), and Ribeiro do Bode (distal silty turbidites) formations. The ApiA(0)na Formation felsic volcanic rocks crosscut the sedimentary succession. The Cambrian Subida leucosyenogranite represents the last felsic magmatic activity to affect the Itajai Basin. The Brusque Group and the Florianpolis Batholith are proposed as source areas for the sediments of the upper sequence. For the lower continental units the source areas are the Santa Catarina, So Miguel and CamboriA(0) complexes. The lack of any oceanic crust in the Itajai Basin suggests that the marine units were deposited in a restricted, internal sea. The sedimentation started around 600 Ma and ended before 560 Ma as indicated by the emplacement of rhyolitic domes. The Itajai Basin is temporally and tectonically correlated with the Camaqu Basin in Rio Grande do Sul and the Arroyo del Soldado/Piriapolis Basin in Uruguay. It also has several tectono-sedimentary characteristics in common with the African-equivalent Nama Basin.
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  • Reply to Oczlon's (2006) comment

    Huckriede, H.   Wemmer, K.  

    The comment of Martin Oczlon contains some significant contributions to the topics discussed in the article of Huckriede et al. (Int J Earth Sci 93:414-431, 2004). Contrary to Oczlon's comment, the central results of Huckriede et al. (Int J Earth Sci 93:414-431, 2004) are clearly different from the tectonic model of Oczlon (Geol Rundsch 83:20-31, 1994). Additionally, there is no reason for a new interpretation of the K-40/Ar-40 muscovite cooling-ages from allochthonous units.
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  • Schmid, F., Stadt. Kinderklinik Aschaffenburg Padiatrische Radiologie. Lehrbuch in 2 B?nden. Unter Mitarbeit von Schuster, W., Beduhn, D., Fuchs, G., Gefferth, K., Giesen, H., Gutheil, H., Heinisch, H.-M., Kobel, R., Moll, H., Neuh?user, G., Schell-Schomberg, M., Schuster, H., Wemmer, U., Wenz, W., Zapp, E. Band 2: Thoraxorgane. Verdauungstrakt. Urogenitaltrakt. 625 Abb. XVI, 525 Seiten. 1973. Gebunden DM 248,-: US $99·20. Subskriptionspreis gültig bei Abnahme Beider B?nde auch nach Erscheinen des Gesamtwerkes Gebunden DM 198·40; US $79·40. Berlin-Heidelberg-New York; Springer-Verlag. ISBM 3-540-06071-5.

    G.M. Steiner  

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  • Origin and K/Ar age of cryptomelane-bearing Sn placers on silcretes, SE Germany

    Dill, H. G.   Wemmer, K.  

    During the early Miocene (Aquitanian-Burdigalian) an alluvial-fluvial drainage system evolved along the boundary fault of the Munchberg Gneiss Complex in SE Germany. The resultant NE-SW trending channel system collected debris from the enclosing Paleozoic country rocks and from gossans of cassiterite-bearing veins exposed to denudation in the roof rocks of the adjacent Late Variscan Sparnberg-Pottiga Granite. Three different types of placer minerals occur: (1) isolated grains of cassiterite, (2) armored relics of cassiterite and barite, (3) remobilized and redeposited silica aggregates. Silica is present in a zone of silcretes, in mechanically redeposited pebbles within this depression and in cobbles of quartz gangue derived from the nearby vein-type deposits. These silcretes developed from alkaline fluids and were redeposited under semiarid conditions with dryer (period of solution) and wetter seasons (period of redeposition) at >= 23 Ma. Cryptomelane I formed under dry conditions around 23 Ma when the pore space of the channel lag deposits was incompletely filled with water, whereas goethite points to more humid climatic conditions between 23 and 17 Ma as the fluids were more acidic. Cryptomelane II filled the pore space of the clastic sediments to completeness on increasing humidity at <= 17 Ma. The mineral association under study pertains to the group of supergene Fe-Mn deposits (Hunsruck-type) whose ore-forming processes span a wide range from pedological/ground water-related Fe-Mn concentration in hydromorphic soils to alluvial-fluvial pebble or placer-type concentrations. The current investigations involved geological mapping and geoelectrical DC deep sounding in the field and optical microscopy, SEM-EDX, and K/Ar age dating of cryptomelane in the laboratory. (C) 2012 Elsevier B.V. All rights reserved.
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  • Pädiatrische radiologie. Lehrbuch in 2 bänden: By F. Schmid (ed.), with the cooperation of W. Schuster, D. Beduhn, G. Fuchs, K. Gefferth, H. Giesen, H. Gutheil, H.-M. Heinisch, R. Kobel, H. Moll, M. Neuhäuser, M. Schell-Schomberg, H. Schuster, U. Wemmer, W. Wenz and E. Zapp, Springer-Verlag, Berlin, 1973. Vol. 1 (Stützgewebe—Zentralnervensystem—Syndrome), xix + 504 pages, 491 illustrations, 86 tables, DM 248.00, US $ 101.20; subscription price (if 2 volumes are ordered) DM 198.40, US $ 81.00. Vol. 2 (Thoraxorgane—Verdauungstrakt—Urogenitaltrakt), xvi + 525 pages, 625 illustrations, 32 tables, DM 248.00, US $ 101.20; subscription price (if 2 volumes are ordered) DM 198.40, US $ 81.00

    G. du Boulay  

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