Eogenetic Karst in Interbedded Carbonates and Evaporites and Its Impact on Hydrocarbon Reservoir: A New Case from Middle Triassic Leikoupo Formation in Sichuan Basin, Southwest China Abstract Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.

Coronas around Olivine in the Miaowan Olivine Norite, Yangtze Craton, South China Abstract Coronitic microstructures have been used to interpret the late-stage solidification history of igneous rocks and to constrain the corresponding chemical and/or physical changes. Coronas with three shells were also recognized in the Miaowan olivine norite, Yangtze Craton, South China. In our study, orthopyroxene intergrowth with vermicular magnetite in the inner shell is in optical continuity with magnetite-free orthopyroxene in the middle shell. In the outer shell of brown amphibole remaining magnetite-free orthopyroxene inclusions sporadically occur. Meanwhile Mg# values of orthopyroxene (76–80) in the inner and middle shells are basically consistent with olivine (78–81). In this paper, we propose a multi-stage genetic model for the formation of coronas in the Miaowan olivine norite. In the first stage, the magnetite-free orthopyroxene shell formed through reaction between primocrystal olivine with the residual Si-rich melt at 990–1 053 °C and 6.2–6.5 kbar. In the second stage, the orthopyroxene-magnetite symplectite shell formed when primocrystal olivine reacted with the late-stage residual Fe-rich melt promoted by high oxygen fugacity condition at 927–1 035 °C and 6.0–6.5 kbar. In the third stage, the brown amphibole shell formed as the presence of residual hydrous melt and replaced the middle shell at 821–900 °C and 5.5–6.0 kbar.

Magnitude of the 23 January 2018 M 7.9 Alaska Earthquake Estimated from Local Dense Seismic Records in Alaska Abstract We apply a novel method to estimate the magnitude of the 23 January 2018 M7.9 Alaska earth-quake using seismic stations recorded at local to regional distances in Alaska, US. We determine the source duration from back-projection results derived from the Alaska stations in a relatively compact azimuth range. Then we calculate the maximum P-wave displacements recorded on a wide azimuth range at distances of 8 to 15 degrees. Combining the source duration and the maximum P-wave displacements, we obtain magnitudes of 7.86–8.03 for the 23 January 2018 earthquake in 3–5 min, very close to the Mw 7.9 determined by the USGS and GCMT. This example validates the new approach for determining magnitude of large earthquakes using local to regional stations, and its time efficiency that magnitudes of large earthquakes can be accurately estimated within in 3–5 min after origin time. Therefore, further application of this new method would help accurate estimation of size of earthquakes that occur off shore and might cause tsunami hazards.

Nd Isotopic and Model Age Study of the Shandong Province, North China Craton: Implications for Correlation with South Korea Abstract The geological units in Shandong Province, North China are important parts of the North China Craton and offer important insights into their crustal evolutionary history. This work compiled 611 sets of Nd isotopic data of Archean-Mesozoic rocks from Shandong including the Luxi, Jiaobei, and Sulu terranes, which provided important constraints for crustal growth and reactivation. Nd-depleted mantle model ages (TDM) of Archean rocks with positive εNd(t) values showed that ca. 2.9 and 2.8-2.7 Ga were the most important periods of crustal growth in the Jiaobei and Luxi terranes, respectively, while the period of ca. 2.6-2.5 Ga in the Jiaobei terrane likely indicated a coherent event of crustal growth and reworking. During the Proterozoic, multi-stage rifting and collisional orogenic events possibly led to the reworking of Archean crust in the source region. The Nd isotopic data of the Paleoproterozoic and Neoproterozoic rocks from Sulu indicated significant reworking of older crust with juvenile magmatic input. Crustal reactivation occurred during the Mesozoic. The younger TDM ages of the Mesozoic rocks with low negative εNd(t) values indicated that a juvenile crustal/mantle component was added to the ancient basement. The reactivation reflected significant crust-mantle interaction via the mechanism of crustal subduction and mantle-derived magma un-derplating, or possibly asthenospheric upwelling. Our work also attempted to establish the crustal correlation between Shandong and Korea (including the Gyeonggi massif, Ogcheon belt, and Yeongnam massif). The TDM age distribution provided evidence favoring the affinity relationship between the Gyeonggi massif and Ogcheon belt of South Korea and the Jiaobei and Sulu terranes of Shandong, while the Yeongnam massif was more correlated with the South China Block.

Early-Middle Triassic Intrusions in Western Inner Mongolia, China: Implications for the Final Orogenic Evolution in Southwestern Xing-Meng Orogenic Belt Abstract The end-Permian to Early-Middle Triassic magmatic rocks in Inner Mongolia can provide valuable insights into the relationships between the collisional processes and the magmatic responses during the final orogenic evolution of Xing-Meng orogenic belt (XMOB). This paper presents zircon U-Pb ages and Hf isotopes, whole rock geochemical and Sr-Nd-Pb isotopic data for the Early-Middle Triassic diabases and monzogranites from the Langshan area, southwestern XMOB. Our results suggest that the studied diabases and monzogranites were respectively formed during Early Triassic and Middle Triassic. The Early Triassic diabases are characterized by “arc-like” geochemical signatures, including enrichment in Rb, U and K, and depletion in Nb, Ta, P and Ti. They have negative to weak positive εNd (t) values (−3.1 to +1.5) and relatively high initial ratios of 208Pb/204Pb (35.968–37.346), 207Pb/204Pb (15.448–15.508) and 206Pb/204Pb (16.280–17.492), indicating a subduction-metasomatized enriched lithospheric mantle source. Their low Ba/Rb (2.72–6.56), Ce/Y (0.97–1.39) and (Tb/Yb)N ratios (1.31–1.45) suggest that the parental magma was likely originated from low degree partial melting of the phlogopite-bearing lherzolite in a spinel-stability field. The Middle Triassic monzogranites show high Sr/Y ratios, low MgO, Cr and Ni contents, high Zr/Sm ratios (40–64), negative zircon εHf(t) values (−25.8 to −8.8), as well as relatively flat heavy rare earth element patterns. They were likely derived from low degree partial melting of a moderately thickened ancient lower crust. The diabases and the slightly postdated high Sr/Y granites in this study represent the magmatic responses to the final orogenic evolution in the southwestern XMOB. Together with regional works, we propose that the slab break-off of the Paleo-Asian oceanic lithosphere following the terminal collision between the North China Craton and the South Mongolia terranes triggered asthenospheric upwelling, and the ongoing convergence further initiated moderately crustal thickening and uplift in the XMOB.

Latest Wuchiapingian to Earliest Triassic Conodont Zones and δ 13 C carb Isotope Excursions from Deep-Water Sections in Western Hubei Province, South China Abstract Deep-water facies sections have advantages of recording complete information across the Permian-Triassic boundary (PTB). Here we present a detailed study on the conodont biostratigraphy and carbon isotope profile ranges from the Wuchiapingian-Changhsingian boundary (WCB) to the PTB of two deep-water facies sections at Zhuqiao and Shiligou in the Middle Yangtze region, western Hubei, South China. Fifteen species and three genera are identified. Eight conodont zones are recognized, in ascending order, they are the Clarkina orientalis, C. wangi, C. subcarinata, C. changxingensis, C. yini, C. meishanensis, Hindeodus parvus and Isarcicella isarcica zones. The onset of deposition of the deep-water siliceous strata of the Dalong Formation in western Hubei began in the Late Wuchiapingian and persisted to the Late Changhsingian. Carbon isotope negative excursions occur near both the WCB and PTB in both sections. The WCB δ13Ccarb negative excursion is in the C. orientalis and C. wangi zones. The PTB δ13Ccarb negative excursion began in the C. yini Zone and extended to the I. isarcica Zone. The absence of several Changhsingian zones may indicate the difficulty of extracting conodonts from siliceous strata or the presence of an intra-Changhsingian hiatus.

Chemical Compositions and Distribution Characteristics of Cements in Longmaxi Formation Shales, Southwest China Abstract Shale gas resources have been regarded as a viable energy source, and it is of great significance to characterize the shale composition of different cements, such as quartz and dolomite. In this research, chemical analysis and the multifractal method have been used to study the mineral compositions and petrophysical structures of cements in shale samples from the Longmaxi Formation, China. X-ray diffraction, electron microprobe, field emission scanning electron microscopy, cathodoluminescence microscopy and C-O isotope analyses confirmed that cements in the Longmaxi Formation shales are mainly composed of Fe-bearing dolomite and quartz. Fe-bearing dolomite cements concentrate around dolomite as annuli, filling micron-sized inorganic primary pores. Quartz cements in the form of nanoparicles fill primary inter-crystalline pores among clay minerals. Theoretical calculation shows that the Fe-bearing dolomite cements formed slightly earlier than the quartz cements, but both were related to diagenetic illitization of smectite. Moreover, multifractal analysis reveals that the quartz cements are more irregularly distributed in pores than the Fe-bearing dolomite cements. These results suggest that the plugging effect of the quartz cements on the primary inoraganic pore structures is the dominant factor resulting in low interconnected porosity of shales, which are unfavorable for the enrichment of shale gas.

New Fractal Evidence of Pacific Plate Subduction in the Late Mesozoic, Great Xing’an Range, Northeast China Abstract Late Mesozoic granitoids are widespread in the Great Xing’an Range (GXR), which is part of a large igneous province in eastern China. The geodynamic setting of the Late Mesozoic granitoids is still debated, and there have been two dominant models proposed, subduction and thermal erosion. This study discusses the geodynamic mechanisms from a new perspective on ages of the granitoids and fractal dimensions of their shape. Our results show that granitoids become gradually older from South GXR to North GXR to Erguna Block (EB) in the Jurassic, and opposite in the Cretaceous. The fractal dimensions of the Perimeter-area model (DAP) exhibit the same features. The values of DAP are smaller from South GXR (0.673 1) to North GXR (0.628 0) to EB (0.607 9) in the Jurassic, and larger from South GXR (0.609 6) to North GXR (0.630 2) to EB (0.639 9) in the Cretaceous. This implies that the geometrical irregularities of the granitoids were shaped by subduction, rather than thermal erosion. These spatial variations could be best explained by the subduction of the Pacific Plate and consequent granitoid magmatism in the late Mesozoic, thus providing a new fractal evidence for Pacific Plate subduction mechanism and opening a new possibilities method for study plate movement.

In-situ High-Temperature XRD and FTIR for Calcite, Dolomite and Magnesite: Anharmonic Contribution to the Thermodynamic Properties Abstract In-situ powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra were measured on the natural crystals of calcite (Ca0.996Mg0.004CO3), dolomite (Ca0.497Mg0.454Fe0.046Mn0.003CO3) and magnesite (Mg0.988Ca0.010Fe0.002CO3), with a temperature up to 796 K. The thermal expansion coefficients were evaluated for these carbonate minerals, resulting in the values of 2.7×10-5, 3.3×10-5 and 3.5×10-5 K-1 for calcite, dolomite and magnesite, respectively. The magnitude of these coefficients is in the same order as those for the isothermal and elastic moduli of these carbonates (e.g., calcite<dolomite<magnesite). The IR-active internal modes of the CO3 group systematically shift to lower frequencies at elevated temperature, and the isobaric (γiP) and isothermal (γiT) Gruneisen parameters for the internal modes are generally smaller than 0.5. The corresponding anharmonic parameters (ai) are typically within the range of -1.5.+1×10-5 K-1, which are significantly smaller in magnitude than those for the external modes. We also calculate the thermodynamic properties (internal energy, heat capacities and entropy) at high temperatures for these carbonates, and the anharmonic contribution to thermodynamics shows an order of calcite>dolomite>magnesite. The Debye model (harmonic approximation) would be valid for magnesite to simulating the thermodynamic properties and isotope fractionation β-factor at high P-T condition.

Applicability of Two Newmark Models in the Assessment of Coseismic Landslide Hazard and Estimation of Slope-Failure Probability: An Example of the 2008 Wenchuan M w 7.9 Earthquake Affected Area Abstract This paper presents the landslide hazard assessment and slope-failure function using two Newmark displacement models regressed by regional and global station records. Taking the 2008 Wenchuan Mw 7.9 earthquake area as an example, based on the topographic and geological data of the study area, we prepared a factor-of-safety (Fs) map and a critical acceleration (ac) map, respectively. Then using these two simplified Newmark models, two displacement maps were compiled by combining the ac map and peak ground acceleration (PGA) map. By virtue of the actual landslide inventory of the W enchuan earthquake, we constructed the slope-failure probability curves of the two Newmark models. The results show that the abilities to predict landslide occurrence of the two simplified Newmark models are largely identical, by which the assessment results can well delineate the macroscopic distribution of coseismic landslides, and most predicted landslide cells are distributed on the two sides of the Beichuan-Yingxiu fault, especially Pengguan complex rock mass in the hanging wall of this fault. The probability equations of two Newmark models are roughly the same, though the parameters vary slightly. The probability equation proposed in this paper can be applied to the Wenchuan region and other areas with similar tectonic environments.