Experimental Study on Wave Velocity of Rocks from Tarim Basin at High PT Conditions and Its Geological Implications
3
Citation
0
Reference
20
Related Paper
Citation Trend
Abstract:
The results of experimental measurements on elastic wave velocity (Vp) for rocks of different types from Tarim Basin at high temperatures and high pressures are presented here. The Vp of samples at ordinary temperature ranges from 6. 007 km·s-1 to 6. 803 km·s-1, given the maximum pressure at 2. 0 GPa, and the andesitic volcanoclastic rock has the largest Vp. At the temperature of 600℃, the Vp varies between 5. 871 km·s-1 and 6. 658 km·s-1, and the largest Vp value is obtained also for andesite. Combined with the velocity structure derived from seismic converted wave in study area, here we propose that the crustal structure in the most areas of Tarim Basin is of three layers: the upper crust consists mainly of granitic metamorphic rocks, the middle crust is mainly made of granitic diorite, and the lower crust is mainly composed of andesitic basalts.Keywords:
Diorite
Tarim basin
Molasse
Cite
Stishovite
Ringwoodite
Cite
Citations (5)
Adakite
Cite
Citations (193)
Seismic velocity
Radioactive decay
Exponential decay
Cite
Citations (32)
In order to constrain the crustal wave velocity structure in the southern Tibetan crust and provide insight into the contribution of crustal composition, geothermal gradient and partial melting to the velocity structure, which is characterized by low average crustal velocities and widespread presence of low-velocity zone(s), the authors model the crustal velocity and density as functions of depth corresponding to various heat flow values in light of velocity measurements at high temperature and high pressure. The modeled velocity and density are regarded as comparison standards. The comparison of the standards with seismic observations in southern Tibet implies that the predominantly felsic composition at high heat flow cannot explain the observed velocity structure there. Hence, the authors are in favor of attributing low average crustal velocities and low-velocity zone(s) observed in southern Tibet mainly to partial melting. Modeling based on the experimental results suggests that a melting percentage of 7-12 could account for the low-velocity zone(s).
Seismic velocity
Felsic
Earth crust
Low-velocity zone
Heat flow
Cite
Citations (3)
Two recent onshore-offshore seismic transects across the Namibian passive margin reveal a thick (to 20 km) prism of material at the base of the crust with high seismic velocity (Vp = 7.1-7.6 km/s). To better understand the nature of this material and the processes that formed it, we estimate the bulk chemical composition of the high-velocity crust by relating its seismic velocity to a petrophysical model that links basalt composition and conditions of partial melting of peridotite. Observed average seismic velocities in the igneous crust are consistent with basaltic material with ∼14-18 wt% MgO. This conclusion is not affected by the presence of cumulate minerals because it integrates over the full thickness of the body; however, the highest Vp values of 7.6 km/s are consistent with velocities expected for cumulate minerals produced by fractional crystallization of a 14%-18% MgO parental melt. The subsolidus growth of garnet is unlikely to be a significant factor for the crustal velocity above the Moho depth of 30 km. Garnet growth in a magnesium-rich basaltic crust can be expected to limit the crustal thickness to ∼30 km because bulk densities at deeper levels may exceed those of the peridotite mantle. The relationship between MgO content of partial melts and the potential temperature of a fertile peridotite source suggests that the estimated 14-18 wt% MgO basalts were generated from mantle at ∼1440-1560 °C potential temperature, which may be a good estimate for the potential temperature of the ancestral Tristan mantle plume at the Namibian margin. The igneous crust has the greatest volume, highest MgO contents, and highest inferred mantle potential temperatures at the location of the northern transect, which is closest to the Walvis Ridge hotspot trace. The mantle potential temperature estimated for the southern transect is 50-100 °C lower, suggesting cooling of the plume material during its flow southward.
Margin (machine learning)
Petrophysics
Seismic velocity
Cite
Citations (25)
Felsic
Ultramafic rock
Underplating
Cite
Citations (36)
Abstract To investigate crustal structures related to melt storage, Ps converted waves were used to image crustal velocity discontinuities across the Tohoku region of northeast Japan. Ps receiver functions from 127 permanent seismic stations were migrated to depth, accounting for variations in crustal velocity structure. In the 5-15 km depth range, negative Ps phases indicative of velocity decreases with depth are prevalent in central and western Tohoku, where Holocene volcanoes occur and surface wave tomography indicates low crustal velocities. Large negative Ps phases are largely absent in the crust of the old mountain terranes in eastern Tohoku where crustal velocities are higher. In central and western Tohoku, velocity gradients inferred from Ps phases and shear velocity anomalies are consistent with models for crustal melt storage that involve trans-crustal zones of crystal-melt mush where the highest concentrations of melt are localized at the top of the mush column. Negative Ps phases are concentrated at depths of 5-10 km, and many correlate with the upper margins of localized low velocity zones beneath volcanoes. These correlated features are consistent with the roofs of high melt fraction layers. At depths of 10-30 km, positive Ps phases are consistent with gradual velocity positive gradients that correspond to decreasing melt fraction with depth in the mush column, although some groups of deeper negative Ps phases at depths of 10-20 km may reflect local maxima in melt fraction. The amplitudes of the negative Ps phases at 5-10 km depth vary significantly. In a higher amplitude example, waveform modeling of Ps receiver functions from a station near Hijori volcano indicates a 16% ± 5% Vs drop at 10 km depth, implying melt fractions of 10% ± 3% if Gassmann's equations are assumed, above a velocity increase from 10 to 20 km that indicates decreasing melt fractions with depth. However, modeling of lower amplitude Ps phases in the 5-10 km depth range indicates a 7% ± 3% velocity drop and a maximum melt fraction of 5% ± 2%. The geographic distribution of apparent melt fraction at 5-10 km depth suggests that in some cases the same upper crustal source can supply multiple volcanoes, and in others high melt fraction zones are significantly laterally offset from the nearest volcano.
Classification of discontinuities
Receiver function
Cite
Citations (0)
Felsic
Eclogitization
Lithology
Cite
Citations (130)
The crustal low velocity layers (CLVL) occuring in the middle-lower crust have been found in many regions around the world and have been explained in different ways. In this paper, the compressional wave velocities (VP) of 138 rock samples selected from Qinling and North China were measured under high temperatures (up to 1500 °C) and high pressures (up to 3 GPa) simultaneously. The general feature of the experimental data were discussed and the Vp decrease phenomenon (54 samples involved) were found. Detailed studies, mineral constitutions and textures of experimental products examined by microscope and EPMA, indicated that the dehydration, phase transition and partial melt of water-bearing minerals (hornblende and biotite), are responsible for the Vp decrease. The results suggest that dehydration melting of hydrous minerals might be an important mechanism for CLVL observed in the continental crust of the study area and probably other parts of the world.
Wave velocity
Cite
Citations (0)
Abstract We derive a well‐resolved 1D model of shear‐wave velocity ( V S ) through both linear, and nonlinear joint inversions of Rayleigh‐ and Love‐wave group velocity dispersion data. Our best model from the deterministic approach suggests a two‐layered crust over a half‐space: the upper crust is 13.8 km thick with a shear velocity of 3.22 km/s; for the lower crust they are 24.9 km and 3.62 km/s, respectively. Finally, we use a global optimization method (very fast simulated annealing) to estimate shear‐wave velocity. Multiple models give acceptable fits to the observed data. A common feature of these models is a low‐velocity zone (LVZ) with its upper boundary at 60–70 km depth. At this depth, the temperature–depth profile of Kachchh cuts the wet peridotite geotherm, resulting in increased water content due to partial melting, and thus the decreased shear velocity. This LVZ extends down to a depth of 120–130 km, where the temperature–depth profile of Kachchh cuts the dry peridotite geotherm that leads to a sharp decrease in water content, thereby, a sharp increase in shear velocity. The maximum V S perturbation of −3% in the upper mantle can be explained by the presence of 100–150 K excess temperature beneath northwestern India at 70–120 km depth relative to the adjacent mantle along with the presence of 1% penny‐shaped melt inclusions (probably CO 2 ‐rich carbonatite melts) in the upper mantle. This is likely related to the imprints of the initial Deccan/Reunion plume head that might have moved with India since the Deccan volcanism at 65 Ma.
Group velocity
Wave velocity
Shear velocity
Cite
Citations (10)