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Yang T.,Key Laboratory of Seismic Observation and Geophysical ImagingInstitute of Geophysics | Yang X.,State Key Laboratory of Earthquake DynamicsInstitute of Geology | Duan Q.,State Key Laboratory of Earthquake DynamicsInstitute of Geology | Chen J.,State Key Laboratory of Earthquake DynamicsInstitute of Geology | Dekkers M.J.,Paleomagnetic laboratory Fort Hoofddijk
Geochemistry, Geophysics, Geosystems

Fluid infiltration within fault zones is an important process in earthquake rupture. Magnetic properties of fault rocks convey essential clues pertaining to physicochemical processes in fault zones. In 2011, two shallow holes (134 and 54 m depth, respectively) were drilled into the Yingxiu-Beichuan fault (Longmen Shan thrust belt, China), which accommodated most of the displacement of the 2008 Mw 7.9 Wenchuan earthquake. Fifty-eight drill core samples, including granitic host rock and various fault rocks, were analyzed rock-magnetically, mineralogically, and geochemically. The magnetic behavior of fault rocks appears to be dominated by paramagnetic clay minerals. Magnetite in trace amounts is identified as the predominant ferrimagnetic fraction in all samples, decreasing from the host rock, via fault breccia to (proto-)cataclasite. Significant mass-losses (10.7-45.6%) are determined for the latter two with the "isocon" method. Volatile contents and alteration products (i.e., chlorite) are enriched toward the fault core relative to the host rocks. These observations suggest that magnetite depletion occurred in these fault rocks-exhumed from the shallow crust-plumbed by fluid-assisted processes. Chlorite, interpreted to result from hydrothermal activity, occurs throughout almost the entire fault core and shows high coefficients of determination (R2>0.6) with both low and high-field magnetic susceptibility. Close relationships, with R2>0.70, are also observed between both low and high-field magnetic susceptibility and the immobile elements (e.g., TiO2, P2O5, MnO), H2O+, and the calculated mass-losses of fault rocks. Hence, magnetic properties of fault rocks can serve as proxy indicators of fluid infiltration within shallow fault zones. © 2016. American Geophysical Union. All Rights Reserved. Source

Mourik A.A.,University Utrecht | Bijkerk J.F.,University Utrecht | Cascella A.,Italian National Institute of Geophysics and Volcanology | Husing S.K.,Paleomagnetic laboratory Fort Hoofddijk | And 3 more authors.
Earth and Planetary Science Letters

Continuous marine successions covering the Middle Miocene Climate Transition (MMCT; ~15-13.7Ma) are scarce and the lack of a high-resolution magnetobiostratigraphic framework hampers the construction of astronomically tuned age models for this time interval. The La Vedova High Cliff section, exposed along the coast of the Cònero Riviera near Ancona (Italy), is one of the few Mediterranean sections covering the critical time interval of the MMCT. Starting from an initial magnetobiostratigraphic age model, a robust astronomical tuning was constructed for the interval between 14.2 and 13.5Ma, using geochemical element data and time series analysis. A shift in δ18O of bulk sediment towards heavier values occurs between ~13.92 and 13.78Ma and could be related to the Mi3b oxygen isotope event, which reflects the rapid expansion of the East Antarctic Ice Sheet in the middle Miocene. The onset of the CM6 carbon excursion is reflected in the bulk record by a rapid increase in δ13C at 13.86Ma. Our results confirm the proposition that these events coincide with a 405-kyr minimum in eccentricity and a node in obliquity related to the ~1.2Myr cycle. From 13.8Ma onwards, distinct quadruplet cycles containing sapropelitic sediments were deposited. This may suggest a causal connection between the main middle Miocene cooling step and the onset of sapropel formation in the Mediterranean. © 2010 Elsevier B.V. Source

Ao H.,CAS Institute of Geology and Geophysics | Ao H.,CAS Institute of Earth Environment | Ao H.,University of Chinese Academy of Sciences | Deng C.,CAS Institute of Geology and Geophysics | And 2 more authors.
Earth and Planetary Science Letters

The Nihewan fluvio-lacustrine sequence (North China) has recorded late Pliocene-Pleistocene climatic and environmental changes, and contains valuable information on early human evolution in high-latitude East Asia. We carried out a combined mineral-magnetic and geochemical investigation on a sequence from the Xiantai section, eastern Nihewan Basin. Results suggest that large-amplitude magnetic-property variations between so-called 'high-magnetic' and 'low-magnetic' units mainly result from preservation/dissolution cycles of detrital magnetic minerals in alternating oxic and anoxic depositional environments. In our preservation/dissolution model, the high- and low-magnetic units represent glacial and interglacial deposits, respectively. This contributes to a better understanding of the link between magnetic properties and climate in the Xiantai fluvio-lacustrine sequence. Based upon this relationship, early humans may have occupied the Xiantai and Xiaochangliang sites during an interglacial period, during both interglacial and glacial periods at the Donggutuo site, and during a glacial period at the Maliang site. Our work provides a paleoenvironmental context for early human adaptation and occupation in mainland East Asia during the early Pleistocene. © 2010 Elsevier B.V. All rights reserved. Source

Vasiliev I.,Paleomagnetic laboratory Fort Hoofddijk | de Leeuw A.,Paleomagnetic laboratory Fort Hoofddijk | Filipescu S.,Babes - Bolyai University | Krijgsman W.,Paleomagnetic laboratory Fort Hoofddijk | And 3 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology

A marked paleoenvironmental change took place at the beginning of the late Miocene in the Central Paratethys, with dominantly marine Sarmatian successions grading rapidly into mainly brackish Pannonian deposits. A long and excellently exposed section comprising the Sarmatian-Pannonian transition has been investigated at Oarba de Mure? in the Transylvanian basin (Romania). In this paper, we focus on both radiometric and magnetostratigraphic dating to provide a chronology for the Sarmatian-Pannonian transition in Transylvania. Two volcaniclastic layers, located approximately 40m below the Sarmatian-Pannonian transition, yield excellent 40Ar/39Ar ages. The weighted mean plateau age for biotite and sanidine separates provided isotopic ages of 11.62±0.12Ma and 11.65±0.13Ma. This implies deposition during the magnetic chron C5r.2r, which is in agreement with the magnetostratigraphic results of the Oarba de Mure? composite section. Rock magnetic analyses indicate greigite as the main magnetic carrier, with characteristics very similar to the magnetosomal greigite found in the Carpathian foredeep. The newly obtained chronology at Oarba de Mure? constrains the age of the Sarmatian-Pannonian transition in the Transylvanian basin to 11.3±0.1Ma, slightly younger than the 11.61Ma postulated in the Styrian and Vienna Basins. © 2010 Elsevier B.V. Source

van den Berg B.C.J.,University of Salamanca | Sierro F.J.,University of Salamanca | Hilgen F.J.,University Utrecht | Flecker R.,University of Bristol | And 7 more authors.
Global and Planetary Change

We present a new high-resolution cyclostratigraphic age model for the Messinian sediments of the Montemayor-1 core. This core was drilled in the Guadalquivir Basin in southern Spain, which formed part of the marine corridor linking the Mediterranean with the Atlantic in the Late Miocene. Tuning of high-resolution geochemical records reveals a strong precessional cyclicity, with maximum clastic supply from river run off coinciding with maximum summer insolation. We recognize a gradual change in the nature of the typical cyclic fluctuations in elemental compositions of the sediments through the core, which is associated with a gradual change in depositional environment as the basin infilled. After applying the new age model, the upper Messinian glacial stages and deglaciation are clearly identified in the oxygen isotope records of the Montemayor-1 core. Reinterpretation of existing planktonic and benthic oxygen isotope records for the core and comparison with equivalent successions in the Rifian Corridor in northern Morocco allow the re-evaluation of the influence of the different water masses in the region: North Atlantic Central Water and Mediterranean Outflow Water. We observe no direct influence of MOW immediately before or during the Messinian Salinity Crisis. © 2015 Elsevier B.V. Source

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