120 to 0 Ma tectonic evolution of the southwest Pacific and analogous geological evolution of the 600 to 220 Ma Tasman Fold Belt System

We review the tectonic evolution of the SW Pacific east of Australia from ca 120 Ma until the present. A key factor that developed early in this interval and played a major role in the subsequent geodynamic history of this region was the calving off from eastern Australia of several elongate microcontinental ribbons, including the Lord Howe Rise and Norfolk-New Caledonia Ridge. These microcontinental ribbons were isolated from Australia and from each other during a protracted extension episode from ca 120 to 52 Ma, with oceanic crust accretion occurring from 85 to 52 Ma and producing the Tasman Sea and the South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins was assisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the Lord Howe Rise, which in turn contributed to rapid and efficient eastward trench rollback. A major change in Pacific plate motion at ca 55 Ma initiated east-directed subduction along the recently extinct spreading centre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000 km of plate boundary in this region, and growth of the Loyalty-Entrecasteaux arc. Continued subduction of South Loyalty Basin crust led to the arrival at about 38 Ma of the 70-60 million years old western volcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of the New Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Paleocene rift tholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a poorly exposed boninitic lava slice, which itself was overridden by the massive ultramafic sheets that cover large parts of New Caledonia and are derived from the colliding forearc of the Loyalty-Entrecasteaux arc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which have blueschist and eclogite facies metamorphic assemblages. Following locking of this subduction zone at 38-34 Ma, subduction jumped eastward, to form a new west-dipping subduction zone above which formed the Vitiaz arc, that contained elements which today are located in the Tongan, Fijian, Vanuatu and Solomons arcs. Several episodes of arc splitting fragmented the Vitiaz arc and produced first the South Fiji Basin (31-25 Ma) and later (10 Ma to present) the North Fiji Basin. Collision of the Ontong Java Plateau, a large igneous province, with the Solomons section of the Vitiaz arc resulted in a reversal of subduction polarity, and growth of the Vanuatu arc on clockwise-rotating, older Vitiaz arc and South Fiji Basin crust. Continued rollback of the trench fronting the Tongan arc since 6 Ma has split this arc and produced the Lau Basin-Havre Trough. This southwest Pacific style of crustal growth above a rolling-back slab is applied to the 600-220 Ma tectonic development of the Tasman Fold Belt System in southeastern Australia, and explains key aspects of the geological evolution of eastern Australia. In particular, collision between a plume-triggered 600 Ma volcanic passive margin and a 510-515 Ma boninitic forearc of an intra-oceanic arc had the same relative orientation and geological effects as that which produced New Caledonia. A new subduction system formed probably at least several hundred kilometres east of the collision zone and produced the Macquarie arc, in which the oldest lavas were erupted ca 480 Ma. Continued slab rollback induced regional extension and the growth of narrow linear troughs in the Macquarie arc, which persisted until terminal deformation of this fold belt in the late-Middle to Late Devonian. A similar pattern of tectonic development generated the New England Fold Belt between the Late Devonian and Late Triassic. Parts of the New England Fold Belt have been broken from Australia and moved oceanward to locations in New Zealand, and on the Lord Howe Rise and Norfolk-New Caledonia Rise, during the post- 120 Ma breakup. Given that the Tasman Fold Belt System grew between 600 and 220 Ma by crustal accretion like the southwest Pacific since 120 Ma, facing the open Pacific Ocean, we question whether the eastern (Australia-Antarctica) part of the Neoproterozoic Rodinian supercontinent was joined to Laurentia

Magnetic anisotropy determination and magnetic hyperthermia properties of small Fe nanoparticles in the superparamagnetic regime

We report on the magnetic and hyperthermia properties of iron nanoparticles synthesized by organometallic chemistry. They are 5.5 nm in diameter and display a saturation magnetization close to the bulk one. Magnetic properties are dominated by the contribution of aggregates of nanoparticles with respect to individual isolated nanoparticles. Alternative susceptibility measurements are been performed on a low interacting system obtained after eliminating the aggregates by centrifugation. A quantitative analysis using the Gittleman s model allow a determination of the effective anisotropy Keff = 1.3 * 10^5 J.m^{-3}, more than two times the magnetocristalline value of bulk iron. Hyperthermia measurements are performed on agglomerates of nanoparticles at a magnetic field up to 66 mT and at frequencies in the range 5-300 kHz. Maximum measured SAR is 280 W/g at 300 kHz and 66 mT. Specific absorption rate (SAR) displays a square dependence with the magnetic field below 30 mT but deviates from this power law at higher value. SAR is linear with the applied frequency for mu_0H=19 mT. The deviations from the linear response theory are discussed. A refined estimation of the optimal size of iron nanoparticles for hyperthermia applications is provided using the determined effective anisotropy value

Naturaliste plateau: constraints on the timing and evolution of the Kerguelen Large Igneous Province and its role in Gondwana breakup

Volcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ∼70 000 km2 Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ∼90 000 km2 Naturaliste Plateau, located ∼170 to ∼500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. 40Ar/39Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high 87Sr/86Sr (up to 0.78), high 207Pb/204 Pb ratios (15.5–15.6), low 143Nd/144Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km2

Paleozoic tectonics of the southern Chinese Tianshan: Insights from structural, chronological and geochemical studies of the Heiyingshan ophiolitic mélange (NW China)

International audienceIn the southern Chinese Tianshan, the southernmost part of the Central Asian Orogenic Belt (CAOB), widespread ophiolitic mélanges form distinct tectonic units that are crucial for understanding the formation of the CAOB. However, the timing of tectonic events and subduction polarity are still in controversy. In order to better understand these geological problems, a comprehensive study was conducted on the Heiyingshan ophiolitic mélange in the SW Chinese Tianshan. Detailed structural analysis reveals that the ophiolitic mélange is tectonically underlain by sheared and weakly metamorphosed pre-Middle Devonian rocks, and unconformably overlain by non-metamorphic and undeformed lower Carboniferous (Serpukhovian) to Permian strata. The igneous assemblage of the mélange comprises OIB-like alkali basalt and andesite, N-MORB-like tholeiitic basalt, sheeted diabase dikes, cumulate gabbro and peridotite. Mafic rocks display supra-subduction signatures, and some bear evidence of contamination with the continental crust, suggesting a continental marginal (back-arc) basin setting. Zircons of a gabbro were dated at 392 ± 5 Ma by the U-Pb LA-ICP-MS method. Famennian-Visean radiolarian microfossils were found in the siliceous matrix of the ophiolitic mélange. Mylonitic phyllite which displays northward-directed kinematic evidence yielded muscovite 40Ar/39Ar plateau ages of 359 ± 2 Ma and 356 ± 2 Ma. These new data, combined with previously published results, suggest that the mafic protoliths originally formed in a back-arc basin in the Chinese southern Tianshan during the late Silurian to Middle Devonian and were subsequently incorporated into the ophiolitic mélange and thrust northward during the Late Devonian to early Carboniferous. Opening of the back-arc basin was probably induced by south-dipping subduction of the Paleo-Tianshan Ocean in the early Paleozoic, and the Central Tianshan block was rifted away from the Tarim block. Closure of the back-arc basin in the early Carboniferous formed the South Tianshan Suture Zone and re-amalgamated the two blocks

Geodynamic Significance of the Mesoproterozoic Magmatism of the Udzha Paleo-Rift (Northern Siberian Craton) Based on U-Pb Geochronology and Paleomagnetic Data

The emplacement age of the Great Udzha Dyke (northern Siberian Craton) was determined by the U-Pb dating of apatite using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). This produced an age of 1386 +/- 30 Ma. This dyke along with two other adjacent intrusions, which cross-cut the sedimentary units of the Udzha paleo-rift, were subjected to paleomagnetic investigation. The paleomagnetic poles for the Udzha paleo-rift intrusions are consistent with previous results published for the Chieress dyke in the Anabar shield of the Siberian Craton (1384 +/- 2 Ma). Our results suggest that there was a period of intense volcanism in the northern Siberian Craton, as well as allow us to reconstruct the apparent migration of the Siberian Craton during the Mesoproterozoic.Peer reviewe