Determining kinematic order and relative age of faulting via flexural‐kinematic restoration: A case study in far western Nepal

Forward modeled, balanced cross sections that account for the flexural response to thrust loading and erosional unloading can verify and refine the kinematic sequence of deformation in fold‐thrust belts as well as help assess the validity of a balanced cross section. Results from flexural‐kinematic reconstructions that indicate either the cross section, the kinematic order or both are invalid include: (a) a predicted final topography that is dramatically different from the actual topography; (b) large normal fault or thrust fault bounded synorogenic basins that are not present in the mapped geology; and/or (c) an exhumation history that is not consistent with provenance records in the basin or measured thermochronometers. Where detailed measured foreland basin sections exist, flexural‐kinematic modeling of fold‐thrust belt deformation, including out‐of‐sequence (OOS) faults can predict a foreland basin evolution that can be compared to measured data. The modeling process creates a “pseudostratigraphy” in the modeled foreland. The pseudostratigraphy and predicted provenance of each modeled stratigraphic increment can be directly compared to measured stratigraphic sections. We present a case study using two cross sections through the Himalaya of far western Nepal (Api and Simikot) to assess the validity of the section geometries and the resulting kinematic histories, displacement rates, flexural wave response and predicted provenance for both sections. Insights from combining the flexural‐kinematic models with existing stratigraphic data include: (a) Changing the order of proposed OOS and normal faults to earlier in the evolution of the fold‐thrust belt was necessary to reproduce the foreland provenance data. We argue that OOS thrust and normal faults in the Api section occurred between 11 and 4 Ma. (b) Published shortening estimates for the Simikot cross section are too high (>50 km), resulting in unrealistic shortening rates up to 80 mm/yr between 25 and 20 Ma. (c) Flexural forward models with and without an additional sediment loading modeling step indicate that while sediment loading does not have a measurable effect on the magnitude and location of erosion within the fold‐thrust belt, it does have a small effect on accumulation rates and thus the predicted age of stratigraphic boundaries when compared to measured stratigraphic thicknesses and age. Thickness difference range from 0.2 to 0.5 km and can result in predicted age differences of ca. 1 Ma. Accounting for both flexural isostacy and erosion can eliminate unviable kinematic sequences and when combined with provenance data from measured stratigraphic sections, can provide insight into the order, age and rate of deformation.     

Geochemical Characterizations of Source and Depositional Environment of Hydrocarbons in the Lake Albert Rift Basin

Despite the upsurge in hydrocarbon exploration in the Lake Albert Rift Basin(LARB)over the past three decades,systematic characterization of hydrocarbon compositions remains lacking,leading to uncertainties in source rock and oil generation determination.We characterized crude oil compositions and oil sand samples in the northern and southern subbasins of LARB.The relative abundance of normal and branched linear alkanes,hopanes,steranes,and aromatic hydrocarbon suggest that northern and southern hydrocarbons were deposited in anoxic to suboxic lacustrine environments and share similar biological source compositions(i.e.,a mixture of plants and aquatic algae and bacteria).Relative to southern samples,northern samples show more negative δ¹³C values for oils,saturates and aromatics,indicating longer migration paths,and exhibit higher MPI-1,DNR-1 and 4-/1-MDBT ratios,indicating higher maturity.Between the two possible sets of source rocks(upper Miocene and Jurassic strata),the positive δ¹³C values of saturated hydrocarbons(average=-20.5‰)suggest that the upper Miocene lacustrine shale is the most likely candidate.Oleanane index(<5% in our samples)does not exclude either source rock possibility,and C₂₈/C₂₉ regular sterane(average=0.63)may be biased by high terrestrial inputs in a lacustrine setting.Together,our data show that northern and southern oils originate from the same source rocks but different oil kitchens.Given the similar geochemical characteristics of southern and northern oils,previous exploration successes in the northern subbasin likely suggest similar potential in the southern sector,while other elements influencing exploration success must be also evaluated.     

A comparison of the hydrology of moorland under natural conditions,agricultural use and forestry

Abstract

Long term research has been conducted into the hydrological effects of different land usage of a wetland mire in southern Germany. Drainage for agriculture lowered the water table and reduced evaporation from about 110% of open water losses to just under the Penman short grass potential rate. The runoff regime was altered and peak flows increased. Afforestation of agricultural land increased evaporation losses to much higher levels than open water evaporation, and annual runoff was nearly halved. Forest growth reduced soil water and baseflows. Peak flows became smaller; the rate of reduction was particularly rapid in the early years of tree growth.     

Compositional variations in mafic phyllosilicates from regional low-grade metabasites and application of the chlorite geothermometer

Abstract Mafic phyllosilicates in metabasites affected by low-grade regional metamorphism from Wales and eastern North Greenland show variations in their structure and chemistry. These variations are related to four mineral zones in these metabasites, which are recognized on the presence/absence of various key calc-silicate minerals and also actinolite. Zones 1 and 2 equate with the zeolite facies, zone 3 with the prehnite–pumpellyite facies (or prehnite–actinolite facies in rocks with appropriate bulk rock composition) and zone 4 with the greenschist facies. Whilst variations in Fe/(Fe + Mg) in chlorite correlate closely with Fe/(Fe + Mg) ratios in the whole-rock, other chemical variations are clearly unrelated to whole-rock compositions. Contents of Al^iv are seen to increase systematically in samples from zone 1 through to zone 4, which relate to an increase in temperature. Calibration of alteration temperatures, calculated using the chlorite geothermometer (based on Al^iv contents) developed for meta-andesites in the Los Azufres geothermal system (Mexico), against x values (an estimate of the proportion of chlorite to swelling component in the mafic phyllosilicates) shows a decrease in the swelling component in passing from zone 1 to zone 4, i.e. with an increase in temperature. Calculated temperatures compare favourably with published stability estimates for the various key calc-silicates and actinolite. These data indicate that the chlorite geothermometer, although developed for meta-andesites from a hydrothermal system, does show a correlation with temperatures estimated from calc-silicate assemblages in metabasites affected by low-grade metamorphism developed on a regional scale.     

Porosity and density of ordinary chondrites: Clues to the formation of friable and porous ordinary chondrites

Abstract— Densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors. We report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study. Based on our measurements, we find that no significant correlation exists between porosity and petrologic grade, chemical group, sample mass, bulk and grain density, or shock level. To investigate variations in porosity and density between pieces of a meteorite, we examined stones from two showers, Holbrook and Pultusk. Examination of nine samples of Holbrook suggests relative homogeneity in porosity and density between pieces of this shower. Measurements of three samples of Pultusk show homogeneity in bulk density, in contrast to Wilkison and Robinson (2000), a study that reported significant variations in bulk density between 11 samples of Pultusk. Finally, examination of two friable ordinary chondrites, Bjurböle and Allegan, reveal variability in friability and porosity among pieces of the same fall. We suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid.     

Diamond and Recycled Mantle:A New Perspective——Introduction of IGCP 649 Project

<正>The IGCP 649 project entitled "Diamonds and Recycled Mantle"was approved by UNESCO and IUGS in March 2015.This project is led by an international team of researchers,including Prof.Yang Jingsui of Institute of Geology of CAGS(China),Prof.Yildirim Dilek of Miami     

Petrology of Cr-rich Podiform Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

<正>The ultramafic massif of Bulqiza,which belongs to the eastern ophiolitic belt of Albania,is a major source of metallurgical chromitite ore.The massif consists of a thick(4 km)sequence,composed from the base upward of tectonized harzburgite with minor dunite,a transitional zone of dunite,and a magmatic sequence of wehrlite,     

EFFECTS OF CLIMATE CHANGE ON FRESHWATER ECOSYSTEMS OF THE SOUTH-EASTERN UNITED STATES AND THE GULF COAST OF MEXICO

The south-eastern United States and Gulf Coast of Mexico is physiographically diverse, although dominated by a broad coastal plain. Much of the region has a humid, warm temperate climate with little seasonality in precipitation but strong seasonality in runoff owing to high rates of summer evapotranspiration. The climate of southern Florida and eastern Mexico is subtropical with a distinct summer wet season and winter dry season. Regional climate models suggest that climate change resulting from a doubling of the pre-industrial levels of atmospheric CO₂ may increase annual air temperatures by 3–4°C. Changes in precipitation are highly uncertain, but the most probable scenario shows higher levels over all but the northern, interior portions of the region, with increases primarily occurring in summer and occurring as more intense or clustered storms. Despite the increases in precipitation, runoff is likely to decline over much of the region owing to increases in evapotranspiration exceeding increases in precipitation. Only in Florida and the Gulf Coast areas of the US and Mexico are precipitation increases likely to exceed evapotranspiration increases, producing an increase in runoff. However, increases in storm intensity and clustering are likely to result in more extreme hydrographs, with larger peaks in flow but lower baseflows and longer periods of drought. The ecological effects of climate change on freshwaters of the region include: (1) a general increase in rates of primary production, organic matter decomposition and nutrient cycling as a result of higher temperatures and longer growing seasons: (2) reduction in habitat for cool water species, particularly fish and macroinvertebrates in Appalachian streams; (3) reduction in water quality and in suitable habitat in summer owing to lower baseflows and intensification of the temperature–dissolved oxygen squeeze in many rivers and reservoirs; (4) reduction in organic matter storage and loss of organisms during more intense flushing events in some streams and wetlands; (5) shorter periods of inundation of riparian wetlands and greater drying of wetland soils, particularly in northern and inland areas; (6) expansion of subtropical species northwards, including several non-native nuisance species currently confined to southern Florida; (7) expansion of wetlands in Florida and coastal Mexico, but increase in eutrophication of Florida lakes as a result of greater runoff from urban and agricultural areas; and (8) changes in the flushing rate of estuaries that would alter their salinity regimes, stratification and water quality as well as influence productivity in the Gulf of Mexico. Many of the expected climate change effects will exacerbate current anthropogenic stresses on the region's freshwater systems, including increasing demands for water, increasing waste heat loadings and land use changes that alter the quantity and quality of runoff to streams and reservoirs. Research is needed especially in several critical areas: long-term monitoring of key hydrological, chemical and biological properties (particularly water balances in small, forested catchments and temperature-sensitive species); experimental studies of the effects of warming on organisms and ecosystem processes under realistic conditions (e.g. in situ heating experiments); studies of the effects of natural hydrological variation on biological communities; and assessment of the effects of water management activities on organisms and ecosystem processes, including development and testing of management and restoration strategies designed to counteract changes in climate. © 1997 John Wiley & Sons, Ltd.