A Seismic Hazard Map Based on Geology and Shear-wave Velocity in Rawalpindi–Islamabad,Pakistan

The ‘twin cities' district of Rawalpindi–Islamabad is among the most endangered seismic regions in Pakistan,with the seismic hazard assessed(0.32 g) to intensity IX MMI for a 475-year return period. A seismic hazard map for Rawalpindi–Islamabad is presented herein, based on 85 shear-wave velocity(VS) profiles obtained through geophysical H/V measurements and from the geological map of the region. Relationships between the average top thirty-meter shear-wave velocities(VS30) and surficial geological units have been determined. The peak ground acceleration(PGA) maps for 150,475 and 2475-year return periods were converted into a seismic intensity map. Intensity increments for different soils were used to compute PGA values for 150, 475 and 2475-year return periods. Sites located on softer ground experienced a higher degree of damage from moderate earthquakes. Due to the presence of soft clay or lique?able soil and lateral spreading, a few locations may be classified as hybrid sites class C and D. This map is a critical step in facilitating code-based site classification and seismic design throughout Rawalpindi–Islamabad. Although the seismic hazard map based on seismic intensities is no longer used in engineering geology, it is still important in seismological analysis and for civil protection purposes.     

Timing of collision of the Kohistan–Ladakh Arc with India and Asia: Debate

The Kohistan–Ladakh Arc in the Himalaya–Karakoram region represents a complete section of an oceanic arc where the rocks from mantle to upper crustal levels are exposed. Generally this arc was regarded as of Jurassic–Cretaceous age and was welded to Asia and India by Northern and Southern Sutures respectively. Formation of this arc, timings of its collisions with Asia and India, and position of collision boundaries have always been controversial. Most authors consider that the arc collided with Asia first during 102–75 Ma and then with India during 55–50 Ma, whereas others suggest that the arc collided with India first at or before 61 Ma, and then the India–arc block collided with Asia ca 50 Ma. Recently published models of the later group leave several geological difficulties such as an extremely rapid drifting rate of the Indian Plate (30 ± 5 cm/year) northwards between 61–50 Ma, absence of a large ophiolite sequence and accretionary wedge along the Northern Suture, obduction of ophiolites and blueschists along the Southern Suture, and the occurrence of a marine depositional environment older than 52 Ma in the Indian Plate rocks south of the Southern Suture. We present a review based on geochemical, stratigraphic, structural, and paleomagnetic data to show that collision of the arc with Asia happened first and with India later.     

Discrete lithofacies discrimination of Jurassic strata using Advanced Spaceborne Thermal Emission and Reflection Radiometer Data, Bighorn Basin, Wyoming, USA

An investigation of the carbonate sedimentology of the Middle Jurassic strata, Bighorn Basin, North-central Wyoming, proved to be an ideal test of new spaceborne remote sensing platforms with high spatial and spectral resolution. To evaluate these new tools, it was necessary to: (i) map the occurrence of different lithofacies in at least a part of the area to establish ground truth; (ii) establish the reflectance characteristics of the different lithofacies using portable, on the ground, systems; (iii) produce maps based on spaceborne remote sensing data that discriminate between the lithological types found in the field; and, lastly, (iv) evaluate the discrimination by visiting previously unvisited areas in which these rock types are predicted to occur based upon their remote signatures. Field checking of previously unvisited outcrops demonstrated whether these remote sensing techniques allowed distinction of different lithological outcrops. This investigation determined that the new spaceborne tools could distinguish between outcrops of sparse silty oomicrites from sorted biosparites as small as 10 m by 10 m. In a study area as large as the Bighorn Basin, the ability to identify very small outcrops with a predicted lithology a priori proved to be a major advantage in maximizing field effort.     

Petrogenesis of the Late Cretaceous Tholeiitic Volcanism and Oceanic Island Arc Affinity of the Chagai Arc, Western Pakistan

The Late Cretaceous Chagai arc outcrops in western Pakistan, southern Afghanistan and eastern Iran. It is in the Tethyan convergence zone, formed by northward subduction of the Arabian oceanic plate beneath the Afghan block. The oldest unit of the Chagai arc is the Late Cretaceous Sinjrani Volcanic Group. This is composed of porphyritic lava flows and volcaniclastic rocks, and subordinate shale, sandstone, limestone and chert. The flows are fractionated low-K tholeiitic basalts, basaltic-andesites, and andesites. Relative enrichment in their LILE and depletion in HFSE, and negative Nb and Ta and positive K, Ba and Sr anomalies point to a subduction-related origin. Compared to MORB, the least fractionated Chagai basalts have low Na2O, Fe2O3T, CaO, Ti, Zr, Y and 87Sr/86Sr. Rather than an Andean setting, these results suggest derivation from a highly depleted mantle in an intraoceanic arc formed by Late Cretaceous convergence in the Ceno-Tethys. The segmented subduction zone formed between Gondwana and a collage of small continental blocks (Iran, Afghan, Karakoram, Lhasa and Burma) was accompanied by a chain of oceanic island arcs and suprasubduction ophiolites including Semail, Zagros, Chagai-Raskoh, Kandahar, Muslim Bagh, Waziristan and Kohistan-Ladakh, Nidar, Nagaland and Manipur. These complexes accreted to the southern margin of Eurasia in the Late Cretaceous.     

巴基斯坦穆斯林巴赫铬铁矿床成矿环境、 地质特征及控矿因素

巴基斯坦境内蛇绿岩广泛分布,且大多发育铬铁矿化。穆斯林巴赫铬铁矿床位于巴基斯坦中部,是新特提斯缝合带中大型豆荚状铬铁矿床之一,目前已经发现数百个矿体,储量超过4000 kt,是巴基斯坦境内最重要的铬铁矿产地。张洪瑞等（2013）总结了中部瓦济里斯坦-贝拉蛇绿岩带的地质特征和穆斯林巴赫铬铁矿床的产出特征,认为穆斯林巴赫铬铁矿床成因仍存在争议,其形成构造环境、控矿因素有待于进一步研究。本文论述穆斯林巴赫矿床的地质特征,通过矿床地球化学和铬尖晶石矿物化学等研究,结合我国典型的罗布莎豆荚状铬铁矿研究成果,分析了主要控矿因素,为巴基斯坦境内相同类型矿床的找矿勘查提供依据。     

Biodiverse rotifer assemblage(Rotifera: Eurotatoria) of floodplain lakes of the Brahmaputra basin of lower Assam,northeast India: composition and ecosystem diversity

The study aims to ascertain the hypothesis on the rich rotifer biodiversity of the floodplain lakes(beels) of the Brahmaputra river basin and to use these metazoans to assess trophic status or to characterize habitat variations of wetlands. The plankton samples collected from four beels of lower Assam revealed 160 Rotifera species belonging to 35 genera and 19 families. The richness is of biodiversity value as ~38.0% and ~57.0% of the rotifer species known till date from India and northeast India(NEI), respectively. One species each is new to the Oriental region and NEI, and three species are new to Assam; 23 species merit global biogeography interest and several exhibit distribution values in the Indian sub-region. The diverse Lecanidae Brachionidae Lepadellidae Trichocercidae and speciose littoral-periphytic L ecane Lepadella Trichocerca, and richness of Brachionus spp. following removal of aquatic macrophytes are noteworthy. Overall rotifer composition showed homogeneity amongst beels while lower monthly richness and community similarities affirmed heterogeneity within individual beels. We propose L/B quotient based on L ecane : Brachionus species ratios to characterize habitat variations of the sampled wetlands. Sláde?ek's B/T quotient based on B rachionus : T richocerca species ratios affirmed general 'meso-trophic' status of diff erent beels. Our results provided little insight on the influence of individual abiotic factors but the canonical correspondence analyses asserted higher cumulative influence of ten abiotic parameters on Rotifera richness in each beel.     

Depositional Environment and Lithofacies Analyses of Eocene Lacustrine Shale in the Bohai Bay Basin: Insights from Mineralogy and Elemental Geochemistry

The effect of various depositional parameters including paleoclimate, paleosalinity and provenance, on the depositional mechanism of lacustrine shale is very important in reconstructing the depositional environment. The classification of shale lithofacies and the interpretation of shale depositional environment are key features used in shale oil and gas exploration and development activity. The lower 3^rd member of the Eocene Shahejie Formation(Es₃^x shale) was sel...     

Sedimentary Processes and Depositional Characteristics of Coarse-grained Subaqueous Fans along Steep Slopes in a Lacustrine Rift Basin: A Case Study from the Dongying Depression, Bohai Bay Basin, China

Coarse-grained subaqueous fans are vital oil and gas exploration targets in the Bohai Bay basin, China. The insufficient understanding of their sedimentary processes, depositional patterns, and controlling factors restricts efficient exploration and development. Coarse-grained subaqueous fans in the Yong′an area, Dongying Depression, are investigated in this study. These fans include nearshore subaqueous fans, and sublacustrine fans, and their sedimentary processes, depositional patterns and distribution characteristics are mainly controlled by tectonic activity and paleogeomorphology. Nearshore subaqueous fans developed near the boundary fault during the early–middle deposition stage due to strong tectonic activity and large topographic subsidence. Early sublacustrine fans developed at the front of the nearshore subaqueous fans in the area where the topography changed from gentle to steep along the source direction. While the topography was gentle, sublacustrine fans did not develop. During the late weak tectonic activity stage, late sublacustrine fans developed with multiple stages superimposed. Frequent fault activity and related earthquakes steepened the basin margin, and the boundary fault slopes were 25.9°–34°. During the early–middle deposition stage, hyperpycnal flows triggered by outburst floods developed. During the late deposition stage, with weak tectonic activity, seasonal floods triggered hyperpycnal flows, and hybrid event beds developed distally.