Geophysics Appearance of The South China Sea

<p class="AbstractText">South China Sea (SCS) is underlain by sediments of an average density 2.10 g/cm³ of 2 km thickness at its central part up to 10 km in the margins. The basement rock is the upper and lower crust of densities 2.67 and 2.85 g/cm³ respectively of varying thicknesses. The thinnest crustal rock is at the centre of SCS that is called the South China Sea Basin (SCSB). The Mohorovicic discontinuity is about 15 km depth below the SCSB. Heatflow values in this basin vary from 2 to 3.5 HFU.</p><p class="AbstractText">Lineations of total magnetic anomaly are generally in a west-east direction covering the whole study area. However, an elongated northeast-southwest lineation of dipole anomaly separates the west-east anomaly patterns in the north from those in the south. This feature is also observed in the gravity map. These elongated patterns of the total magnetic features are in coincident with the occurrences of seamounts inferred being remnant of extinct seafloor spreading. Because of this spreading a crustal extension had taken place that separated Kalimantan from the mainland of China to restore its present position. A paleomagnetic study result confirms this hypothesis.</p><p class="AbstractText">The Palawan trench is marked by north-east trending magnetic and gravity anomaly that is inferred being traces of a remnant subduction zone. This anomaly forms a boundary between the Zengmu also called the Sarawak basin and the SCSB. Here, heat flow value is 1 to 2 HFU. This value in coincident with gravity gradient of 2.5 mGal/km also represents an active subduction of the Manila trench north of the Palawan Island. The Manila trench is supposed to be the energizing source of volcanism and earthquake in the Phillippines. Free-air and Bouguer anomaly of the order of 50 to 60 mGal and magnetic of about 100 nT represent the Zengmu basin in the Sunda Shelf. This basin is underlain by sediments of 2.10 g/cm³ of 8 km thickness and also crustal rock which is much thicker than the one underneath the SCSB. Strong topographic relief at the surfaces of sedimentary layer and the crustal rock is very my much associated with normal faulting that may cause fluctuation of the free-air values.</p><p class="AbstractText">The continental margins of Sarawak and the Sunda Shelf are areas of hydrocarbon deposits now still in production, whereas the offshores Vietnam and Hainan are promising target for hydrocarbon exploration.</p>

Elastic anisotropies of deformed upper crustal rocks in the Alps

The crust within collisional orogens is very heterogeneous both in composition and grade of deformation, leading to highly variable physical properties at small scales. This causes difficulties for seismic investigations of tectonic structures at depth since the diverse and partially strong upper crustal anisotropy might overprint the signal of deeper anisotropic structures in the mantle. In this study, we characterize the range of elastic anisotropies of deformed crustal rocks in the Alps. Furthermore, we model average elastic anisotropies of these rocks and their changes with increasing depth due to the closure of microcracks. For that, pre-Alpine upper crustal rocks of the Adula Nappe in the central Alps, which were intensely deformed during the Alpine orogeny, were sampled. The two major rock types found are orthogneisses and paragneisses; however, small lenses of metabasites and marbles also occur. Crystallographic preferred orientations (CPOs) and volume fractions of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. The sample set shows a wide range of different seismic velocity patterns even within the same lithology, due to the microstructural heterogeneity of the deformed crustal rocks. To approximate an average for these crustal units, we picked common CPO types of rock forming minerals within gneiss samples representing the most common lithology. These data were used to determine an average elastic anisotropy of a typical crustal rock within the Alps. Average mineral volume percentages within the gneiss samples were used for the calculation. In addition, ultrasonic anisotropy measurements of the samples at increasing confining pressures were performed. These measurements as well as the microcrack patterns determined in thin sections were used to model the closure of microcracks in the average sample at increasing depth. Microcracks are closed at approximately 740 MPa yielding average elastic anisotropies of 4 % for the average gneiss. This value is an approximation, which can be used for seismic models at a lithospheric scale. At a crustal or smaller scale, however, local variations in lithology and deformation as displayed by the range of elastic anisotropies within the sample set need to be considered. In addition, larger-scale structural anisotropies such as layering, intrusions and brittle faults have to be included in any crustal-scale seismic model.

Deformation-induced Japan twinning in quartz during incipient mylonitization

Many crystalline rocks of the continental crust contain coarse-grained quartz as a main mineral (e.g., granitoids). Incipient deformation of coarse quartz, which likely controls the accumulation of bulk strain in heterogeneously deformed crustal rock volumes, commonly develops microshear zones (MSZs) of localized recrystallization. At mid-crustal conditions, where quartz deformation is mostly accomplished by subgrain rotation recrystallization, grains of MSZs can show an abrupt change in crystallographic orientation (large misorientation angle) with respect to the host quartz that is still not fully understood. We analyzed MSZs (20–200 μm thick) from deformed coarse-grained (millimeter grain size) quartz veins in the Austroalpine Schobergruppe (Eastern Alps). Electron backscatter diffraction analysis reveals that the MSZs are characterized by a nearly 90° misorientation angle between the c-axes of the host and new grains, which also share one {m} and one {1122} pole, compatible with Japan twinning. This abrupt switch of the c-axis orientation can promote geometrical softening and shear localization. So far, Japan twinning has been interpreted as a growth feature. We show that deformation-induced twinning in quartz, including Japan and Dauphiné twinning, can play an important role in initiation of crystal-plastic deformation within the crust.

Composition of the Primordial Ocean Just after Its Formation: Constraints from the Reactions between the Primitive Crust and a Strongly Acidic, CO2-Rich Fluid at Elevated Temperatures and Pressures

The Hadean was an enigmatic period in the Earth’s history when ocean formation and the emergence of life may have occurred. However, minimal geological evidence is left from this period. To understand the primordial ocean’s composition, we focused on the ocean’s formation processes from CO2- and HCl-bearing water vapor in the high-temperature atmosphere. When the temperature of the lower atmosphere fell below the critical point, high-temperature rain reached the ground surface. Then, hydrothermal reactions between the subcritical fluid and primordial crust started. Eventually, a liquid ocean emerged on the completely altered crust as the temperature decreased to approximately 25 °C. Here, we conducted two experiments and modeling to simulate the reactions of hypothetical primordial crustal rock (basalt or komatiite). The results indicate that the primordial ocean was mildly acidic and rich in CO2, Mg, and Ca relative to Na, irrespective of the rock type, which is different from the modern equivalents. Therefore, unlike the present seawater, the primordial seawater could have been carbonic, bitter, and harsh rather than salty.

Elastic anisotropies of deformed upper crustal rocks in the Alps

The upper crust within collisional orogens is very heterogeneous both in composition and grade of deformation, leading to very variable physical properties at small scales. This yields difficulties for seismic investigations of tectonic structures at depth since local changes in elastic anisotropy cannot be detected. In this study, we show elastic anisotropies of the range of typical lithologies within deformed upper crustal rocks in the Alps. Furthermore, we aim to model average elastic anisotropies for these rocks and their changes with increasing depth due to the closure of microcracks. We therefore sampled rocks in the Adula Nappe of the central Alps, which is typical for upper crust in collisional orogens. The two major rock types found are orthogneisses and paragneisses, however, small lenses of metabasites and marbles also occur. Crystallographic preferred orientations (CPOs) and volume fractions of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. The sample set shows a wide range of different seismic velocity patterns even within the same lithology, due to the heterogeneity of deformed upper crust. To approximate an average for these upper crustal units, we picked common CPO types of rock forming minerals within the gneiss samples, which represent the most common lithology. These data were used to determine an average elastic anisotropy of a typical upper crustal rock within the Alps. Average mineral volume percentages within the gneiss samples were used for the calculation. In addition, ultrasonic measurements of elastic anisotropies of the samples at increasing pressures were performed. These measurements, as well as the microcrack pattern determined in thin sections of the samples were used to model the closure of microcracks in the average sample at increasing depth. At ≈740 MPa microcracks are assumed to be closed yielding average elastic anisotropies of 4 % for the average gneiss. This value is an approximation, which can be helpful for seismic models at a lithospheric scale. At a crustal or smaller scale, however, it is an oversimplification and local lithological as well as deformational changes shown by the range of elastic anisotropies within the sample set have to be considered.

Synconvergent and coherent (ultra)high-pressure crustal rock exhumation

&lt;p&gt;Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in 1984 and have since then been observed at many convergent plate boundaries worldwide. Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustal rocks is key to understand the geodynamic evolution of orogens such as the Alps.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (&gt;80 km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent (U)HP units, involving significant tectonic mixing forming so-called m&amp;#233;langes. Furthermore, these predictions often rely on excessive erosion or periods of divergent plate motion as important exhumation mechanism. Inconsistent with field observations and natural data, application of these models to the Western Alps was recently criticised.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Here, we present models with continuous plate convergence, which exhibit local tectonic-driven upper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HP units along the subduction interface, involving feasible erosion.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The two-dimensional petrological-thermo-mechanical numerical models presented here predict both subduction initiation and serpentinite channel formation without any a priori prescription of these two features. The (U)HP units are exhumed coherently, without significant internal deformation. Modelled pressure and temperature trajectories and exhumation velocities of selected crustal units agree with estimates for the Western Alps. The presented models support previous hypotheses of synconvergent exhumation, but do not rely on excessive erosion or divergent plate motion. Thus, our predictions provide new insights into processes leading to the exhumation of coherent (U)HP crustal units consistent with observations and natural data from the Western Alps.&lt;/p&gt;

Thermomechanical Controls on the Timing of Magma Reservoir Failure in a Viscoelastic Crust

&lt;p&gt;As volcanic systems undergo unrest, understanding the conditions required for reservoir failure, the associated timescales, and the links to geophysical observations are critical when evaluating the potential for eruption. The characteristics and dynamics of a pressurised magmatic system can be inferred from episodes of surface deformation, but this process is heavily reliant on the assumed crustal rheology. In volcanic regions, shallow or long-lived magmatic systems can significantly perturb the regional geothermal gradient, altering the rheology of the surrounding crustal rock. Viscoelasticity incorporates a time-dependent viscous deformation response, accounting for the increased ductility and thermomechanical heterogeneity induced by the modelled reservoir.&lt;/p&gt;&lt;p&gt;Here, we investigate the influence of an imposed thermal regime on the critical reservoir overpressure (OPc) required to facilitate failure in elastic and viscoelastic models, alongside the predicted critical surface uplift (Uc). By evaluating tensile and Mohr-Coulomb failure criteria on the reservoir walls, we can determine the mechanical stability of the magma reservoir and identify the conditions that are susceptible to failure. We explore a range of reservoir temperatures (representing felsic, intermediate, and mafic magma compositions) and background geothermal gradients, to simulate varied volcanic regions, and use the Standard Linear Solid viscoelastic rheology together with a temperature-dependent viscosity structure, calculated from the thermal constraints. The models incorporate mechanical heterogeneity in the form of a temperature-dependent Young&amp;#8217;s modulus, accounting for the thermal weakening of the surrounding crustal rock. We use an overpressure rate of 10 MPa yr&lt;sup&gt;-1&lt;/sup&gt;, in excess of lithostatic pressure, that produces an average elastic volumetric strain rate of ~3-7x10&lt;sup&gt;-12&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;, depending on the imposed thermal regime.&lt;/p&gt;&lt;p&gt;We show that reservoir failure is systematically inhibited by incorporating viscoelasticity, with OPc for Mohr-Coulomb failure increasing by up to 65% with respect to the corresponding elastic model. The greatest increases in OPc, and Uc, are observed when pairing cool reservoir temperatures (i.e., felsic composition) with low background geothermal gradients. In contrast, stress partitioning due to the viscoelastic crustal rheology promotes failure at the ground surface, decreasing the required OPc for tensile failure by up to 32%. The greatest reductions in OPc are produced in models that couple a hot reservoir temperature (i.e., mafic composition) with low background geothermal gradients. By resisting mechanical failure on the reservoir wall, temperature-dependent viscoelasticity impacts the conditions required for dyke nucleation and propagation. Further to this, a viscoelastic crustal rheology dramatically reduces the timescales for throughgoing failure; complete brittle failure connecting the reservoir to the ground surface. This occurs much earlier than suggested by elastic models, which could have implications for interpreting the conditions, and onset, of a potential eruption.&lt;/p&gt;

Evolution of chemical weathering processes and CO2 sequestration in the glaciated basins of Western Himalayas

&lt;p&gt;Understanding of chemical weathering process involved in ionic elution helps in distinguishing the CO&lt;sub&gt;2&lt;/sub&gt; sequestration rate at the different micro-climatic setup of Himalayan catchments. In the present study, we have selected three glaciated basins from two different climatic zones of Western Himalayas (Lato and Phutse from the cold-arid zone of Ladakh and Chhota Shigri from the monsoon-arid zone of Himachal Pradesh, India) for determining various solute sources, CO&lt;sub&gt;2&lt;/sub&gt; sequestration rate and its control over melt-water quality. Solute sourcing models used in this work shows major cations like Ca&lt;sup&gt;2+&lt;/sup&gt;&amp;#160; and Mg&lt;sup&gt;2+ &lt;/sup&gt;are from crustal rock-weathering while Na&lt;sup&gt;+&lt;/sup&gt; and K&lt;sup&gt;+&lt;/sup&gt; sourced out from the sea-salt origin. However, major anions like SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt; (&gt; 85%) were derived from the crustal origin and HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; mostly derived from atmospheric sources (39% to 45 %) in all catchments except HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; contribution from carbonation dissolution and silicate weathering is ~29% and ~16% for Ladakh catchments compared to ~9 % and ~29% in Chhota Shigri respectively. The solute model also reveals that the contribution of sulphate oxidative mediated carbonate dissolution (SOCD) in HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; flux is relatively higher in Chhota Shigri (~16%) than others (~9%). It is also observed that catchment like Chhota Shigri having a combined network of channelized and distributed drainage patterns with lower specific discharge, more glacierized area, low pH, high pCO&lt;sub&gt;2&lt;/sub&gt;, Low molar ratio [Ca&lt;sup&gt;2+&lt;/sup&gt; + Mg&lt;sup&gt;2+&lt;/sup&gt;]/[ Na&lt;sup&gt;+&lt;/sup&gt; + K&lt;sup&gt;+&lt;/sup&gt;], high SMF (~ 0.4), low CO&lt;sub&gt;2 carbonate&lt;/sub&gt;/CO&lt;sub&gt;2 silicate&lt;/sub&gt; ratio (~1.3) show relatively more sulphide oxidative and silicate weathered products than other catchments. Conversely, presence of excess non-glaciated areas in Stok and Phutse having well-channelized subsurface discharge with high CO&lt;sub&gt;2 carbonate&lt;/sub&gt;/CO&lt;sub&gt;2 silicate &lt;/sub&gt;ratio (~10 to ~5) show enhanced carbonation via atmospheric CO&lt;sub&gt;2&lt;/sub&gt; (CAC) and carbonate dissolution with high annual CO&lt;sub&gt;2&lt;/sub&gt; sequestration. Thus, varying subglacial drainage system, specific discharge pattern and reactive rock-types with distinct hydro-micro-climatic set up alters the chemical weathering mechanism in these catchments and control meltwater quality.&lt;/p&gt;

Reconstruction of the entire rift architecture of Theistareykir Fissure Swarm (northeast Iceland): integration between extensive UAV and field surveys

&lt;p&gt;Reconstructing the origin and kinematics of structures along active rifts is essential to gain a deeper knowledge on rifting processes, with important implications for the assessment of volcanic and seismic hazard. Here we reconstruct the architecture of an entire rift, the 70-km-long Theistareykir Fissure Swarm (ThFS) within the Northern Volcanic Zone of Iceland, through the collection of an extensive amount of 7500 quantitative measurements along extension fractures and normal faults, thanks to the integration between Unmanned Aerial Vehicles (UAV) mapping with centimetric resolution through Structure from Motion (SfM) techniques and extensive field surveys with classical methods. Quantitative measurements, collected across a wide area during several campaigns, comprise strike, opening direction and amount of opening at extension fractures, and strike and offset values at faults, along 6124 post-Late Glacial Maximum (LGM) and 685 pre-LGM structures.&lt;/p&gt;&lt;p&gt;The extent of the area covered by our data allowed us to pinpoint differences in the structural architecture of the rift. From south to north: i) extension fractures and faults strike ranges from mainly N10&amp;#176;-20&amp;#176;, to N00-10&amp;#176;, to N30-40&amp;#176;; ii) the opening direction starts from N110&amp;#176;, reaches N90-100&amp;#176; in the center and amounts to N125&amp;#176; in the northernmost sector; and iii) the dilation amount is in the range 0.1&amp;#8211;10 m, then 0.1&amp;#8211;9 m and it finally reaches 0.1&amp;#8211;8 m. We explore such differences as due to the interaction with the WNW-ESE-striking Husavik-Flatey transform fault and the Gr&amp;#237;msey Oblique Rift (Gr&amp;#237;msey lineament), and to the structural inheritance of older NNE- to NE-striking normal faults. The reconstruction of the stress field resulting from such data allows the interpolation of the &amp;#963;&lt;sub&gt;hmin&lt;/sub&gt;&amp;#160;values, through the unpublished&amp;#160;software ATMO-STRESS, prepared in the framework of the EU NEANIAS project, in order to plot and examine the strain field.&lt;/p&gt;&lt;p&gt;Furthermore, mechanisms of rift propagation and the relation between magma systems are here investigated through the analysis of 281 slip profiles of the main Pleistocene-Holocene faults. Our data show a mechanism of along-axis propagation of the rift outward from the volcano: in fact, north of the volcano, 75% of the asymmetric faults propagated northward, whereas south of the volcano 47% of the asymmetric faults propagated southward. This can be due to the combination between the development of faults following lateral dyke propagation outward from the magma chamber, and faults nucleation near the volcano as a consequence of the different crustal rock rheology produced by a higher heat flux.&lt;/p&gt;