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Institución detectada Período Navegá Descargá Solicitá
No detectada desde ene. 1961 / hasta dic. 2023 GeoScienceWorld


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Late Cretaceous to Miocene volcanism, sedimentation, and upper-crustal faulting and folding in the Principal Cordillera, central Chile: Field and geochronological evidence for protracted arc volcanism and transpressive deformation

Jesse G. Mosolf; Phillip B. Gans; Andre R. Wyss; John M. Cottle; John J. Flynn

Palabras clave: Geology.

Pp. No disponible

Fault zone architecture and lithology-dependent deformation mechanisms of the Himalayan frontal fold-thrust belt: Insights from the Nahan Thrust, India

Dyuti Prakash Sarkar; Jun-ichi Ando; Gautam Ghosh; Kaushik Das; Prabir Dasgupta; Naotaka Tomioka

<jats:p>Brittle shallow crustal faults typically develop a complex fault zone architecture with distinct structural domains that display diverse microstructures, mineralogy, and deformation mechanisms. The development of such domains is typically controlled by the strength and composition of the protoliths, physical conditions of deformation, fluid ingress, and diachronous fault growth in response to stress accumulation and co-seismic slip. Herein, we studied the microstructure-mineralogy-kinematics of fault rocks in the Nahan Thrust, in the vicinity of the Main Frontal Thrust that represents a tectonically active zone in the Himalayan orogen. The Nahan Thrust is characterized by alternating red and gray gouge layers, and a single black gouge layer. Our results from electron microscopy and X-ray diffractometry indicate that the protolith of the red gouge layers is argillaceous sandstone, whereas that of the gray and black gouge layers is sandstone. Microstructures suggest an initially distributed deformation (aseismic creep), followed by a protracted brittle deformation event, and a later aseismic creep stage. The brittle stage is marked by progressive localization of stress, fracture development, cataclasis, frictional sliding, and seismic slips. The black gouge layer acted as the principal slip zone and exhibited ultrafine bands of micrometer-scale slip zones with vapor escape structures and clay clast aggregates, indicating seismic faulting and frictional heating during seismic slips. The preferential seismic rupture nucleation in the black gouge layer indicates a strong lithological dependence on seismic slip in the Nahan Thrust. We also conclude that heterogeneity within the Nahan Thrust resulted from primary lithological variations of the protoliths.</jats:p>

Palabras clave: Geology.

Pp. No disponible

Constraint on the temperature of A-type magma from contact metamorphic aureole, Biesituobie batholith, West Junggar in NW China, Central Asian Orogenic Belt

Yichao Chen; Jiahui Liu; Renjie Zhou; Wenjiao Xiao; Ji’en Zhang; Zhiyong Zhang; Qian W.L. Zhang; Zhen M.G. Li; Chunming Wu

<jats:p>The Biesituobie A-type batholith in West Junggar in NW 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-andalusite hornfels and cordierite-alkaline feldspar-sillimanite-corundum gneiss, indicating prograde metamorphism. The phase equilibrium modeling on the cordierite-alkaline feldspar-sillimanite-corundum gneiss sample yielded a peak condition of P = 3.0−4.0 kbar, T = 760−800 °C. Similarly, the Na-in-cordierite geothermometer yielded a peak condition of T = 771−780 °C. The temperature of the peak metamorphism could be considered as a lower limit of the temperature of the Biesituobie A-type batholith. On the contrary, the Ti-in-zircon geothermometer applied to the Biesituobie A-type batholith zircons yields a mean temperature of 672 ± 22 °C. It suggested that the application of Ti-in-zircon geothermometer on A-type magma may involve a &amp;gt;100 °C underestimation. Zircon U-Pb dating indicates that the age of the contact metamorphism is between 263 and 286 Ma, consistent with the age of the pluton at 274 Ma. The result of 760−800 °C calculated from the contact metamorphic aureole of the Biesituobie batholith put a lower limit on considering the temperature condition of the A-type granite series from a metamorphic constraint.</jats:p>

Palabras clave: Geology.

Pp. No disponible

Unconformity-bounded rift sequences in Terreneuvian‒Miaolingian strata of the Caledonian Highlands, Atlantic Canada

J. Javier Álvaro; Susan C. Johnson; Sandra M. Barr; Sören Jensen; Teodoro Palacios; Deanne van Rooyen; Chris E. White

<jats:p>The Cambrian syn-rift strata preserved in western Avalonia provide a distinctive example of how unconformity-bounded sequences are diachronous throughout proximal to marginal rift branches. Terreneuvian‒Miaolingian third-order sequences of the Caledonian Highlands, New Brunswick, Canada, reflect a complex interplay among syn-rift tectonic events, denudation pulses, and sea-level fluctuations. Unconformably overlying the early, rift-related volcanosedimentary Coldbrook Group (ca. 560‒550 Ma), the Ratcliffe Brook, Glen Falls, Hanford Brook, and Forest Hills Formations can be subdivided into two transgressive systems tract (TST)‒highstand systems tract (HST) sequences (each ∼10 m.y.) and an incomplete TST sequence that are separated by stratigraphic gaps. They reflect uplift and tilting events affecting the basement, transgressive and drowning surfaces, and condensed sections. Arid to semi-arid climatic episodes are supported by the excellent preservation of mafic to felsic volcanic clasts in non-marine breccias and conglomerates, which are derived from the Ediacaran basement, and the local precipitation of marine gypsum through the evaporation of pore fluids. Early Miaolingian episodes of microbial/shelly carbonate production preserved precipitates of coeval evaporite (gypsum pseudomorphs after drusy mosaics of calcite) and ikaite (glendonitic, star-shaped aggregates and crusts). Both minerals, traditionally considered to be indicators of contrasting climate conditions, potentially co-occur in temperate-water substrates recording high rates of microbial activity. The early rift phases preserved in the western Avalonian rift transect comprise stepwise uplift and unroofing of rift shoulders, which are related to diachronous, angular discordances and paraconformities bounded by syntectonic slope-apron deposits. Facies homogenization was attained during Miaolingian times as a result of generalized flooding, sealing of paleotopographies, and blanketing with monotonous offshore-dominant shales.</jats:p>

Palabras clave: Geology.

Pp. No disponible

Structure, geochronology, and petrogenesis of Permian peraluminous granite dykes in the southern Chinese Altai as indicators of Altai−East Junggar convergence

Tan Shu; Yingde Jiang; Karel Schulmann; Yang Yu; Chao Yuan; Sheng Wang; Zhiyong Li; Lingzhu Kong

<jats:p>How and at which thermal conditions the convergence between the Chinese Altai and East Junggar operated remain poorly understood. This issue is addressed in the current study by focusing on the timing and petrogenesis of syntectonic granite dykes from the representative areas of Fuyun (convergent front) and Kalasu-Aletai (Chinese Altai interior). It is shown that Fuyun and Kalasu-Aletai dykes are fractionated I- and S-type granites, with zircon and monazite U-Pb ages of 300−291 Ma and 281−265 Ma, respectively. Geochemically, the Fuyun dykes have lower contents of aluminous (ASI: 0.97−1.13) and light rare earth element-enriched features, while the Kalasu-Aletai dykes have ASI = 1.01−2.17 and show overall flat rare earth element patterns with tetrad effects. The Fuyun dykes exhibit less evolved Sr-Nd isotopic characteristics (87Sr/86Srinitial: 0.7039−0.7048, εNd(t): + 5.7 to + 6.1) with respect to those of the Kalasu-Aletai dykes (87Sr/86Srinitial: 0.6978−0.7183, εNd(t): −7.6 to +3.0). The Fuyun and Kalasu-Aletai dykes are geochemically compatible with isotopically less evolved East Junggar arc components and heterogeneous Ordovician wedge sediment of the Chinese Altai, respectively, implying genetic links. We propose that the late Paleozoic Chinese Altai−Junggar convergence created a local perturbation of weak mantle beneath the southern Chinese Altai, causing partial melting of the underthrusting East Junggar and the overriding Altai components successively. The resulting magmas were emplaced along northward propagating syn-tectonic tensional fractures perpendicular to the Chinese Altai−East Junggar deformation front that serves as an excellent indicator of the convergent-shortening process.</jats:p>

Palabras clave: Geology.

Pp. No disponible

Deoxygenation and organic carbon sequestration in the Tethyan realm associated with the middle Eocene climatic optimum

Margot J. Cramwinckel; Robin van der Ploeg; Niels A.G.M. van Helmond; Niels Waarlo; Claudia Agnini; Peter K. Bijl; Annique van der Boon; Henk Brinkhuis; Joost Frieling; Wout Krijgsman; Tamsin A. Mather; Jack J. Middelburg; Francien Peterse; Caroline P. Slomp; Appy Sluijs

<jats:p>The middle Eocene climatic optimum (ca. 40 Ma) stands out as a transient global warming phase of ∼400 k.y. duration that interrupted long-term Eocene cooling; it has been associated with a rise in atmospheric CO2 concentrations that has been linked to a flare-up in Arabia-Eurasia continental arc volcanism. Increased organic carbon burial in the Tethys Ocean has been proposed as a carbon sequestration mechanism to bring the middle Eocene climatic optimum to an end. To further test these hypotheses, we assessed the sedimentary and geochemical expression of the middle Eocene climatic optimum in the northern Peri-Tethys, specifically, the organic-rich Kuma Formation of the Belaya River section, located on the edge of the Scythian Platform in the North Caucasus, Russia. We constructed an age-depth model using nannofossil chronobiostratigraphy. Throughout the studied middle Eocene interval (41.2−39.9 Ma), we documented sea-surface temperatures of 32−36 °C based on the tetraether index of tetraethers consisting of 86 carbons (TEX86), depending on proxy calibration, and during the early middle Eocene climatic optimum, we observed sea-surface warming of 2−3 °C. Despite the proximity of the section to the Arabia-Eurasia volcanic arc, the hypothesized source of volcanic CO2, we found no evidence for enhanced regional volcanism in sedimentary mercury concentrations. Sedimentary trace-element concentrations and iron speciation indicate reducing bottom waters throughout the middle Eocene, but the most reducing, even euxinic, conditions were reached during late middle Eocene climatic optimum cooling. This apparent regional decoupling between ocean warming and deoxygenation hints at a role for regional tectonics in causing basin restriction and anoxia. Associated excess organic carbon burial, extrapolated to the entire regional Kuma Formation, may have been ∼8.1 Tg C yr−1, comprising ∼450 Pg C over this ∼55 k.y. interval. Combined with evidence for enhanced organic carbon drawdown in the western Peri-Tethys, this supports a quantitatively significant role for the basin in the termination of the middle Eocene climatic optimum by acting as a large organic carbon sink, and these results collectively illustrate that the closing Tethys Ocean might have affected global Paleogene climate. Moreover, this study highlights the importance of the interplay between global climate and regional oceanic gateway evolution in determining local climate and oceanographic change.</jats:p>

Palabras clave: Geology.

Pp. No disponible

New insights into deep carbon recycling and formation of nepheline-bearing alkaline rocks from Sr-Nd-Mg isotope compositions

Weiliang Kong; Zhaochong Zhang; Dongyang Zhang; Changhong Wang; M. Santosh; Bingxiang Liu; Bowen Wei

<jats:p>Magnesium isotopes are widely used to trace recycled carbonates in the mantle source. We recently recognized extremely light Mg isotope values (δ26Mg = −0.50 to −0.62‰) in nepheline syenites in the Tarim large igneous province (TLIP), NW China. To evaluate the significance of the light Mg isotopes, we conducted petrological, mineral chemical, zircon U-Pb ages, and geochemical and isotopes (Sr-Nd-Mg) analyses on the nepheline syenite to understand its petrogenesis. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating yields an age of 272.5 ± 1.4 Ma for the nepheline syenite. Petrographic and geochemical studies show that the nepheline syenite and nephelinite in the TLIP display similar mineral assemblages, clinopyroxene Sr isotope compositions and bulk-rock Sr and Nd isotope compositions (87Sr/86Sr(i) = 0.70364−0.70399, εNd(t) = +3.51 to +4.49 versus 87Sr/86Sr(i) = 0.70348−0.70371, εNd(t) = +3.28 to +3.88 for nepheline syenite and nephelinite, respectively), indicating they are possibly co-magmatic. Rhyolite-MELTS modeling shows that the nepheline syenite formed from nephelinite by fractional crystallization of spinel, olivine, clinopyroxene, apatite, and biotite. In combination with information from previous studies, we correlated the extremely light magnesium isotopes of nepheline syenite to “genetic genes” of nephelinite (δ26Mg = −0.35 to −0.55‰) which were produced by the reaction between peridotite and carbonated silicate melt derived from the carbonated eclogite. We invoke a three-stage model for the genesis of the nepheline syenite in the TLIP. Initially, the subduction of oceanic crust delivered the sedimentary carbonate rocks into the deep mantle and formed carbonated eclogite. The carbonated silicate melt derived by the melting of the carbonated eclogite reacted with ambient peridotite to form primary nephelinitic magma. Finally, fractional crystallization of nephelinitic melt during ascent produced the nepheline syenite. Our study provides insights into the implication of light magnesium isotopes for deep carbon recycling in the origin of alkaline rocks.</jats:p>

Palabras clave: Geology.

Pp. No disponible

East Antarctic Ice Sheet variability during the middle Miocene Climate Transition captured in drill cores from the Friis Hills, Transantarctic Mountains

Hannah Chorley; Richard Levy; Tim Naish; Adam Lewis; Stephen Cox; Sidney Hemming; Christian Ohneiser; Andrew Gorman; Margaret Harper; Aline Homes; Jenni Hopkins; Joe Prebble; Marjolaine Verret; Warren Dickinson; Fabio Florindo; Nicholas Golledge; Anna Ruth Halberstadt; Douglas Kowalewski; Robert McKay; Stephen Meyers; Jacob Anderson; Bob Dagg; Pontus Lurcock

<jats:p>This study describes 16 well-dated, terrestrial glacial sedimentary cycles deposited during astronomically paced climate cycles from the termination of the Miocene Climatic Optimum (MCO) through the middle Miocene Climate Transition (MMCT) (15.1−13.8 Ma) in the Friis Hills, Transantarctic Mountains, Antarctica. Three locations were continuously cored (79% recovery) to a maximum depth of 50.48 m through a succession of interbedded till sheets and fossil-bearing, fluvio-lacustrine sediments. A composite chronostratigraphic framework is presented for the cores based on the previous mapping, a seismic refraction survey that defines basin geometry, and a new, integrated age model based on paleomagnetic stratigraphy that is constrained by radioisotopic 40Ar/39Ar numeric ages on two newly identified silicic tephra.</jats:p> <jats:p>The paleoecologic and sedimentologic characteristics of organic-rich lithologies are relatively consistent up-section, which implies that successively younger interglacial deposits during the MMCT represented broadly similar environmental and climatic conditions. During these interglacials, the Friis Hills hinterland was likely ice-free. Major disconformities in the section suggest a transition to colder climates, and after ca. 14.6 Ma, thicker, more extensive and erosive ice cover occurred across the Friis Hills during glacial episodes. Diamictites in the upper three cycles suggest that climate cooled and became drier after ca. 14.2 Ma. However, cyclical retreat of the ice and a return to warm climate conditions during interglacials continued through ca. 13.9 Ma. These direct records reflect a highly variable East Antarctic Ice Sheet margin but show that the ice margin became progressively more extensive during successive glacial intervals, which is consistent with a cooling trend toward more glacial values in the far-field benthic foraminifera δ18O proxy ice volume and temperature record.</jats:p> <jats:p>Age constraints show that glacial-interglacial variability at the terrestrial margin of the East Antarctic Ice Sheet was primarily paced by astronomical precession (∼23 k.y.) through the onset of the MMCT (15−14.7 Ma). Precession-driven cycles are modulated by short-period (∼100 k.y.) eccentricity cycles. Intervals of maximum eccentricity (high seasonality) coincide with sedimentary cycles comprising thin diamictites and relatively thick interglacial sandstone and mudstone units. Intervals of minimum eccentricity (low seasonality) coincide with sedimentary cycles comprising thick diamictites and relatively thin interglacial sedimentary deposits. Major disconformities in the Friis Hills succession that span more than ∼100 k.y. reflect episodes of expansion of erosive ice across, and well beyond, the Transantarctic Mountains and coincide with nodes in eccentricity (∼400 k.y.). These relationships suggest that during relatively warm intervals in the middle Miocene, the East Antarctic Ice Sheet expanded and contracted over 100 k.y. cycles, while its margins continued to fluctuate at higher (∼23 k.y.) frequency. After 14.5 Ma, obliquity is the dominant frequency in δ18O records, marking a period during which large regions of the Antarctic Ice Sheet grounded in marine environments.</jats:p>

Palabras clave: Geology.

Pp. No disponible

Geological architecture and history of the Antigua volcano and carbonate platform: Was there an Oligo−Miocene lull in Lesser Antilles arc magmatism?

Leny Montheil; Mélody Philippon; Jean-Jacques Cornée; Marcelle BouDagher-Fadel; Douwe J.J. van Hinsbergen; Pierre Camps; Marco Maffione; Franck Audemard; Brechtje Brons; Koen J.R. Van der Looij; Philippe Münch

<jats:p>Since the acceptance of plate tectonics, the presence of calc-alkaline magmatic rocks has been recognized as evidence of subduction. But under specific geodynamic circumstances, subduction may occur without generating magmas. Here, we investigate the Cenozoic northern Lesser Antilles arc where, from sparsely exposed magmatic records, Eocene−Oligocene and Pliocene magmatic flare-ups and a Miocene lull were postulated. Nevertheless, most of the arc is submarine, so it is challenging to discern lulls and flare-ups from sampling bias. We review the magmatic evidence exposed onshore in the Lesser Antilles and investigate in detail the island of Antigua, which exposes an Eocene to Miocene volcanic sequence and platform carbonate series that coincide with the postulated lull. By combining lithostratigraphic analysis, structural mapping, 40Ar/39Ar geochronology, and biostratigraphy, we refine the magmatic history of the island and date the arrest of extensive arc magmatism at 35 Ma, with minor activity until 27 Ma. No magmatic products are interleaved with the platform sequence until the latest Oligocene, which confirms a lull in northern Lesser Antilles arc magmatism that may have lasted ca. 20 Ma. Flare-up of magmatic activity contributed to crustal thickening and land emersion, whereas magmatic lulls and related thermal cooling induced subsidence/submersion. Thus, we propose that the paleo-(bio)-geographical evolution of the eastern Caribbean region has been partly controlled by magmatic activity.</jats:p> <jats:p>Fault kinematic analysis, along with anisotropy of magnetic susceptibility, suggest that, at the island scale, magmatic arrest is not associated with a change in stress field during the Oligocene. We speculate that slab flattening triggered by progressive curvature played a role in the temporal shutdown of the northern Lesser Antilles arc.</jats:p>

Palabras clave: Geology.

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Late Cenozoic deepening of Yosemite Valley, USA

Kurt M. Cuffey; Alka Tripathy-Lang; Matthew Fox; Greg M. Stock; David L. Shuster

<jats:p>Although Yosemite Valley, USA, catalyzed the modern environmental movement and fueled foundational debates in geomorphology, a century of investigation has failed to definitively determine when it formed. The non-depositional nature of the landscape and homogeneous bedrock have prevented direct geological assessments. Indirect assumptions about the age of downcutting have ranged from pre-Eocene to Pleistocene. Clarity on this issue would not only satisfy public interest but also provide a new constraint for contentious debates about the Cenozoic tectonic and geomorphologic history of the Sierra Nevada in California. Here we use thermochronometric analysis of radiogenic helium in apatite crystals, coupled with numerical models of crustal temperatures beneath evolving topography, to demonstrate significant late Cenozoic deepening of Tenaya Canyon, Yosemite’s northeastern branch. Approximately 40%−90% of the current relief has developed since 10 Ma and most likely since 5 Ma. This coincides with renewed regional tectonism, which is a long-hypothesized but much debated driver of Sierran canyon development. Pleistocene glaciation caused spatially variable incision and valley widening in Yosemite Valley, whereas little contemporaneous erosion occurred in the adjacent upper Tuolumne watershed. Such variations probably arise from glacial erosion’s dependence on opographic focusing of ice discharge into zones of rapid flow, and on the abundance of pre-existing fractures in the substrate. All available data, including those from our study, are consistent with a moderately high and slowly eroding mid-Cenozoic Sierra Nevada followed by significant late Cenozoic incision of some, but not all, west-side canyons. A likely driver of this event was range-crest uplift accompanied by fault-induced beheading of some major drainages, although other mechanisms such as drainage reorganization following volcanic deposition are plausible.</jats:p>

Palabras clave: Geology.

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