Alexandra Abrajevitch

Precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the Dazhuqu terrane, Yarlung-Tsangpo suture zone, Tibet

Well-preserved, abundant radiolarians provide high-precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the Yarlung–Tsangpo suture zone in southern Tibet. Dazhuqu terrane ophiolites were generated in an intra-oceanic supra-subduction zone setting within a relatively short (<10 Ma) interval from late Barremian to late Aptian. Accumulation of sediments upon the newly generated ophiolite initially occurred in a series of discrete rift-controlled sub-basins associated with various spreading centres. An increasing flux of arc-derived volcaniclastic sediment up-section indicates nearby volcanic arc activity. The Dazhuqu terrane developed in an intra-oceanic setting within Tethys where it was isolated from any continental influence.

Bainang Terrane, Yarlung–Tsangpo suture, southern Tibet (Xizang, China): a record of intra-Neotethyan subduction–accretion processes preserved on the roof of the world

The Bainang terrane, an intra-oceanic island arc subduction complex into which Tethyan oceanic rocks were accreted during the Cretaceous, is preserved within the Yarlung–Tsangpo suture zone of Tibet. The lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central Tethyan domain from Late Triassic to mid-Cretaceous time. These rocks are preserved within a south-verging imbricate thrust stack of thin (≪1 km thick) northward younging tectonic slices. Five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts. The northern tract, which accumulated on the north side of Neotethys, was probably separated from its southern counterpart by a mid-ocean ridge. Detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract. Oceanic plate stratigraphy of the northern tract records its northward travel and mid-Cretaceous (late Aptian) approach towards a south-facing intra-oceanic subduction zone. Rocks in the southern tract developed closer to the Indian subcontinent and experienced thermotectonic subsidence and Mid-Jurassic basic alkaline intraplate magmatism. They were probably accreted late in the Cretaceous. Variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion. The overall tectonostratigraphy of the terrane reflects its development in a remote intra-oceanic setting.

Paleomagnetism of mid-Paleozoic subduction-related volcanics from the Chingiz Range in NE Kazakhstan: The evolving paleogeography of the amalgamating Eurasian composite continent

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is key to the formation of the Eurasian composite continent during Paleozoic time, but the views on this complicated process remain disparate and sometimes controversial. A study of three volcanic formations of mid-Silurian, Lower to Middle Devonian, and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A single-polarity characteristic component in mid-Silurian andesites yields a positive intraformational conglomerate test, whereas dual-polarity prefolding components are isolated from the two Devonian collections. Post-folding, reversed-polarity overprint directions have also been isolated and are likely of Permo-Triassic age. These new data can be evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confi dence the polarity of each result, and hence to determine the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward, albeit with variable velocity, crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz Range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Middle Permian time. A comparison of the Chingiz paleolatitudes with those of Siberia shows, insofar as the sparse data allow, similarities between the northward motion of the Chingiz unit and that of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.

Stratigraphic and sedimentological constraints on the age and tectonic evolution of the Neotethyan ophiolites along the Yarlung Tsangpo suture zone, Tibet

Abstract Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models.

Diagenetic sensitivity of paleoenvironmental proxies: A rock magnetic study of Australian continental margin sediments

A rock magnetic study of the upper 40 m of the Ocean Drilling Program Site 133-820A recovered at the outer edge of the northeastern Australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level. We identified intervals enriched in single-domain (SD) magnetite in the studied section. Unlike previous studies that postulated a detrital source, we show that the bulk of the SD fraction is biologically produced and is likely to be authigenic. The abundance of SD magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms. The biogenic magnetite was preserved during the high sedimentation rate periods, likely due to a short residence time in the corrosive zone of active iron reduction. The presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution. More advanced dissolution during the periods of slow sedimentation, coincident with sea level highstands, resulted in significant changes in the composition of the detrital assemblage, particularly in relative abundances of different mineral phases. The overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite > magnetite > goethite. Due to the higher stability of hematite, reductive diagenesis in general will lead to the lowering of the goethite/hematite (G/H) ratio, which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy. The sensitivity of the G/H proxy to diagenesis should be taken into account in paleoenvironmental studies.

A new paleoprecipitation proxy based on soil magnetic properties: Implications for expanding paleoclimate reconstructions

Description of precipitation patterns and changes in the hydrological cycle during periods of past global change is crucial for providing an understanding of terrestrial climate systems and for predicting impacts of future climate change such as shifting water availability. While a number of proxies and climofunctions exist for reconstructing paleoprecipitation using paleosols, all of the available tools for reconstructing paleoprecipitation are either limited to certain precipitation ranges (effective only for low-precipitation regimes; e.g., depth to Bk, chemical index of alteration [CIA-K]), or are relevant only to a limited range of paleosols (single-pedotype relationships; e.g., calcium-magnesium index [CALMAG]). Here, we measure the acquisition of isothermal remanent magnetization in B horizons of modern soils to quantify the ratio of pedogenic magnetic minerals goethite and hematite, and we use the relationship between these soil magnetic properties and measured climatic variables at each soil site to derive a new quantitative proxy for precipitation. By compiling both literature-derived and measured goethite-hematite (G/H) ratios and mean annual precipitation estimates for a global suite of modern soils ( n = 70), we describe a strong linear relationship ( R 2 = 0.96) between the G/H ratios of soil B horizons and mean annual precipitation that can be used to estimate paleoprecipitation values for a wide range of climatic regimes (100–3300 mm yr –1 ) and soil types (Inceptisols, Alfisols, Ultisols, Oxisols, Mollisols, Aridisols, Spodosols). We tested the new climofunction using paleosols from the early Eocene of Wyoming, which show that estimates based on G/H ratios compare favorably to and expand upon previously published estimates based on paleosol data.

Rock magnetic record of the Triassic-Jurassic transition in pelagic bedded chert of the Inuyama section, Japan

The end-Triassic mass extinction event is regarded as one of the fi ve largest extinction events of the Phanerozoic. The emerging consensus points to volcanic activity at the Central Atlantic Magmatic Province (CAMP) as the ultimate cause of the extinction, yet the underlying mecha- nisms and the nature of global environmental changes that accompanied the biotic turnover remain elusive. We present a rock magnetic study of the extinction interval found within a continuous chert sequence that provides an uninterrupted record of pelagic sedimentation in the Panthalassa Ocean. The variations in the relative abundances and characteristics of authi- genic magnetic phases indicate that the Triassic-Jurassic transition progressed in two stages. The initial stage, characterized by a disappearance of the previously ubiquitous magnetofos- sils, started a few tens of thousands of years to 100 k.y. prior to the formal Triassic-Jurassic boundary as identifi ed by the diagnostic radiolarian species. The second stage, defi ned by signifi cant changes in optical and magnetic properties of hematite pigment, lasted a few tens of thousands of years. The stepwise change in magnetic properties is suggestive of the protracted environmental deterioration, likely prompted by the early episodes of the CAMP volcanism, which was followed by a sudden ocean acidifi cation event, perhaps triggered by a catastrophic release of gas hydrates.

Paleoenvironmental signature of the Deccan Phase-2 eruptions

The environmental impact of the Deccan trap volcanism is poorly understood as yet. The paucity of geological markers that can unambiguously be attributed to the Deccan volcanism and the temporal coincidence of the volcanism with an asteroid impact make evaluation of volcanic contribution to the end Cretaceous mass extinction difficult. Here we briefly review environmental proxy records of two reference Cretaceous-Tertiary boundary (KTB) sections, Bidart (France) and Gubbio (Italy). In both sections, a change in colour of sediments located just below the KTB is systematically associated with very low values of (low-field) magnetic susceptibility (MS). Rock magnetic characteristics suggest that the decrease in MS values results from the loss (dissolution) of ferrimagnetic mineral in this intervals. In addition to the characteristic change in magnetic assemblage, akaganeite (chlorine-bearing iron oxyhydroxide) is commonly observed under the scanning electron microscope in the low MS intervals at Bidart and Gubbio, but has never been detected in the remaining sedimentary successions. We suggest that the association of granular akaganeite and iron oxides dissolution features can be explained by an ocean acidification and aerosol deposition event linked to the Deccan Phase-2 volcanism.