Boulder, Colo., USA: The Geological Society of America regularly publishes articles online before print. GSA Bulletin topics covered this month include the nature and dynamics of China and Tibet; Atlantic Canada; the arc of New Caledonia; the Farallon dish; and the Hikurangi subduction margin, New Zealand. You can find these articles at .
Unconformity-bounded rift sequences in Terrestrial-Miaolingian strata of the Caledonian Highlands, Atlantic Canada
J. Javier Alvaro; Susan C. Johnson; Sandra M. Barr; Soren Jensen; Teodor Palaus…
Abstract: Cambrian syn-rift strata preserved in western Avalonia provide a distinctive example of how unconformity-bounded sequences are diachronous in proximal to marginal rift branches. Terrestrial–Miaolingian Tertiary sequences of the Caledonian Highlands, New Brunswick, Canada, reflect a complex interplay between syn-rift tectonic events, denudation pulses, and sea-level fluctuations. Unconformably overlying the rift-related volcano-sedimentary Coldbrook Group (ca. 560-550 Ma), the Ratcliffe Brook, Glen Falls, Hanford Brook, and Forest Hills Formations can be subdivided into two transgressive systems (TST)-tracts of high systems (HST) (each ∼10 my) and an incomplete TST sequence that are separated by stratigraphic gaps. They reflect uplift and dip events affecting the basement, transgressive and drowning surfaces, and condensed sections. Arid to semiarid climatic episodes are supported by excellent preservation of mafic to felsic volcanic clasts in breccias and non-marine conglomerates, which are derived from Ediacaran basement, and local precipitation of marine chalk by evaporation of pore fluids . Early Miaolingian episodes of microbial carbonate/cocoon production preserved precipitates of coeval evaporite (gypsum pseudomorphs after calcite drus mosaics) and ikaite (star-shaped glendonitic aggregates and crusts). Both minerals, traditionally considered indicators of contrasting climatic conditions, can coexist in temperate water substrates that record high rates of microbial activity. Early rift phases preserved in the western Avalonian rift transect include the gradual uplift and unfolding of rift shoulders, which are related to diachrons, angular unconformities and paraconformities bounded by slope syntectonic deposits. Facies homogenization was achieved during Miaolingian times as a result of widespread flooding, sealing of paleotopographies, and covering with offshore dominant monotonous shales.
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Fault zone architecture and lithology-dependent deformation mechanisms of the Himalayan frontal fold belt: insights from the Nahan Thrust, India
Dyuti Prakash Sarkar; Jun-ichi Ando; Gautam Ghosh; Kaushik Das; Prabir Dasgupta…
Abstract: Brittle shallow crustal faults typically develop a complex architecture of fault zones with distinct structural domains that display diverse microstructures, mineralogy, and deformation mechanisms. The development of these domains is typically controlled by protolith strength and composition, physical conditions of deformation, fluid input, and diachronic fault growth in response to stress accumulation and co-seismic slip. . Here, we studied the microstructure-mineralogy-kinematics of fault rocks on the Nahan thrust, in the vicinity of the main thrust front that represents a tectonically active zone in the Himalayan orogen. The Nahan Thrust is characterized by alternating layers of red and gray gouge and a single layer of black gouge. Our electron microscopy and X-ray diffractometry results indicate that the protolith of the red gouge layers is argillaceous sandstone, while that of the gray and black gouge layers is sandstone. The microstructures suggest initially distributed deformation (aseismic creep), followed by a prolonged brittle deformation event and a later stage of aseismic creep. The brittle stage is marked by progressive localization of stress, development of fractures, cataclasis, frictional slip and seismic slides. The black gouge layer acted as the main slip zone and showed ultrafine bands of micrometer-scale slip zones with steam escape structures and clay clast aggregates, indicating seismic faulting and frictional heating during the seismic slides. Preferential seismic rupture nucleation in the black gouge layer indicates a strong lithological dependence of seismic slip on the Nahan thrust. We also conclude that the heterogeneity within the Nahan Thrust resulted from primary lithological variations of the protoliths.
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A-type magma temperature constraint from the contact metamorphic aureole, Biesituobie batholith, Western Junggar NW China, Central Asian Orogenic Belt
Yichao Chen; Jiahui Liu; Renjie Zhou; Wenjiao Xiao; Ji’en Zhang…
Abstract: The Biesituobie A-type batholith at West Junggar in northwest China of the Central Asian Orogenic Belt contains metapelite xenoliths derived from the contact metamorphic aureole. These xenoliths could be divided into two types: biotite-muscovite-Andalusian gneiss and cordierite-alkaline feldspar-sillimanite-corundum, indicating prograde metamorphism. Phase equilibrium modeling on the cordierite–alkali feldspar–sillimanite–corundum gneiss sample gave a maximum condition of P = 3.0−4.0 kbar, T = 760−800 °C. Similarly, the Na-in-cordierite geothermometer gave a maximum condition of T = 771-780 °C. The temperature of maximum metamorphism could be considered as a lower limit of the temperature of the A-type batholith of Biesituobie. Conversely, the Ti-in-zircon geothermometer applied to Biesituobie A-type batholic zircons yields an average temperature of 672 ± 22 °C. He suggested that the application of the Ti-in-zircon geothermometer to A-type magma may involve an underestimation by >100 °C. U-Pb zircon dating indicates that the age of contact metamorphism is between 263 and 286 Ma, consistent with the age of Pluto at 274 Ma. The result of 760−800 °C calculated from the contact metamorphic aureole of the Biesituobie batholith set a lower limit for considering the temperature condition of the A-type granite series from a metamorphic constraint.
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A case of Ampferer-type subduction beneath the New Caledonia arc: evidence for inefficient subduction of hydrous lithologies in the upper mantle
Kirsten N. Nicholson; Mohammed Ali Abu El-Rus
Abstract: New Caledonia is one of the best exposed subduction/obduction complexes in the world and is central to understanding the geodynamic evolution of the SW Pacific region. We present new geochemical and Ar/Ar age dates for in situ eruptive sequences of the La Conception basaltic-andesitic lavas (ca. 29.12 Ma) and correlate the generation of these lavas with the generation of the Saint Louis Massifs and Koum/Borindi ( 24 Ma) to provide information on the magmatic processes operating within the mantle wedge over time and to indicate the direction of the subducted slab during the Oligocene. La Conception basaltic andesite lavas were emplaced in a trench-arc direction in southwestern New Caledonia due to partial melting of metasomatized garnet peridotites with amphiboles at the base of the mantle wedge (∼112− 118 km). However, both the Saint Louis and Koum/Borindi granodiorite massifs were derived from melting within the lower crust of an island arc over a mantle wedge. These temporal and spatial variations in magmatism in New Caledonia are consistent with a northeastern subduction zone during the Oligocene. The absence of Oligocene subduction-related voluminous arc magmatism is a consequence of the low Tp temperatures of the mantle wedge, the age and high rate of subsidence of the downgoing slab, and absence of stress within the main plate. The Oligocene subduction beneath New Caledonia is therefore an ideal example of Ampferer-type subduction, which is an area of magmatic closure due to inefficient subduction of hydrous lithologies in the convective upper mantle. Once Paleocene–Miocene contraction in the SW Pacific region ceased, subduction west of New Caledonia closed, the system relaxed, and no further melting occurred during the Oligocene.
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Intensified and rapid bauxitization on the paleokarst surface linked to volcanism
Xuefei Liu; Qingfei Wang; Yongbo Peng; Runsheng Yin; Yao Ma…
Summary: Recent studies indicate that volcanism may be a potentially important factor in the formation of karst bauxite, but more evidence is needed. Six billion tons of bauxite formed in the paleokarst terrain in a short time in the Late Carboniferous in the North China Basin (NCB) and the Late Permian in the Youjiang Basin (YB) of China. The factors that trigger their rapid formation are still unclear. Here, we proposed that extensive volcanic eruptions have explained this large-scale bauxite formation based on mineralogy, Hg isotopes, and Hg concentration enrichment proxies of bauxite sequence samples in the NCB and YB. NCB bauxite generally has three layers, namely lower Fe-bearing claystone, intermediate bauxite ore and upper claystone, while Fe-bearing lower claystone is usually absent in bauxite YB, which directly overlies carbonate rocks. The…