Burial and exhumation of the Hoh Xil Basin,northern Tibetan Plateau: Constraints from detrital (U-Th)/He ages

The uplift and associated exhumation of the Tibetan Plateau has been widely considered a key control of Cenozoic global cooling. The south-central parts of this plateau experienced rapid exhumation during the Cretaceous–Palaeocene periods. When and how the northern part was exhumed, however, remains controversial. The Hoh Xil Basin (HXB) is the largest late Cretaceous–Cenozoic sedimentary basin in the northern part, and it preserves the archives of the exhumation history. We present detrital apatite and zircon (U-Th)/He data from late Cretaceous–Cenozoic sedimentary rocks of the western and eastern HXB. These data, combined with regional geological constraints and interpreted with inverse and forward model of sediment deposition and burial reheating, suggest that the occurrence of ca. 4–2.7 km and ca. 4–2.3 km of vertical exhumation initiated at ca. 30–25 Ma and 40–35 Ma in the eastern and western HXB respectively. The initial differential exhumation of the eastern HXB and the western HXB might be controlled by the oblique subduction of the Qaidam block beneath the HXB. The initial exhumation timing in the northern Tibetan Plateau is younger than that in the south-central parts. This reveals an episodic exhumation of the Tibetan Plateau compared to models of synchronous Miocene exhumation of the entire plateau and the early Eocene exhumation of the northern Tibetan Plateau shortly after the India–Asia collision. One possible mechanism to account for outward growth is crustal shortening. A simple model of uplift and exhumation would predict a maximum of 0.8 km of surface uplift after upper crustal shortening during 30–27 Ma, which is insufficient to explain the high elevations currently observed. One way to increase elevation without changing exhumation rates and to decouple uplift from upper crustal shortening is through the combined effects of continental subduction, mantle lithosphere removal and magmatic inflation.     

Erosive water level regime and climatic variability forcing of beach–dune systems on south‐western Vancouver Island,British Columbia,Canada

Increases in the frequency and magnitude of extreme water levels and storm surges are correlated with known indices of climatic variability (CV), including the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), along some areas of the British Columbia coast. Since a shift to a positive PDO regime in 1977, the effects of ENSO events have been more frequent, persistent, and intense. Teleconnected impacts include more frequent storms, higher surges, and enhanced coastal erosion. The response of oceanographic forcing mechanisms (i.e. tide, surge, wave height, wave period) to CV events and their role in coastal erosion remain unclear, particularly in western Canada. As a first step in exploring the interactions between ocean–atmosphere forcing and beach–dune responses, this paper assembles the historic erosive total water level (TWL) regime and explores relations with observed high magnitude storms that have occurred in the Tofino‐Ucluelet region (Wickaninnish Bay) on the west coast of Vancouver Island, British Columbia, Canada. Extreme events where TWL exceeded an erosional threshold (i.e. elevation of the beach–foredune junction) of 5·5 m aCD are examined to identify dominant forcing mechanisms and to classify a regime that describes erosive events driven principally by wave conditions (61·5%), followed by surge (21·8%), and tidal (16·7%) effects. Furthermore, teleconnections between regional CV phenomena, extreme storm events and, by association, coastal erosion, are explored. Despite regional sea level rise (eustatic and steric), rapid crustal uplift rates have resulted in a falling relative sea level and, in some sedimentary systems, shoreline progradation at rates approaching +1·5 m a^–1 over recent decades. Foredune erosion occurs locally with a recurrence interval of approximately 1·53 years followed by rapid rebuilding due to high onshore sand supply and often in the presence of large woody debris and rapidly colonizing vegetation in the backshore. Copyright © 2012 John Wiley & Sons, Ltd.     

THE KRAMER CREEK,COLORADO METEORITE: A NEW L4 CHONDRITE

The Kramer Creek, Colorado, chondrite was found in 1966 and identified as a meteorite in 1972. Bulk chemical analysis, particularly the total iron content (20.36%) and the ratio of Fe_total/SiO₂ (0.52), as well as the compositions of olivine (Fa_21.7) and orthopyroxene (Fs_18.3) place the meteorite into the L-group of chondrites. The well-defined chondritic texture of the meteorite, the presence of igneous glass in the chondrules and of low-Ca clinopyroxene, as well as the slight variations in FeO contents of olivine (2.4% MD) and orthopyroxene (5.6% MD) indicate that the chondrite belongs to the type 4 petrologic class.     

EXPERIMENTAL VAPORIZATION OF THE HOLBROOK CHONDRITE

Knudsen cell-quadrupole mass spectrometry has been used to quantitatively determine the composition of the vapor phase produced by heating samples of the Holbrook chondrite to 1300 °C. Maximum observed vapor pressures (atm) of metals are 10^{?5.3 ± 0.3} Na, 10^{?5.8 ± 0.3} K, 10^{?5.3 ± 0.3} Fe, and 10^{?6.6 ± 0.3} Ni at 1200 °C. S₂ (with minor SO₂), H₂O, and CO₂ were also observed in the high-temperature gas phase. Release of intrinsically derived volatiles produced abundant vesicles in the heated sample residues. Some possible implications for chondrite evolution are briefly discussed     

The nature and genesis of marginal Cu–PGE–Au sulphide mineralisation in Paleogene Macrodykes of the Kangerlussuaq region, East Greenland

The Kangerlussuaq region of East Greenland hosts a variety of early Tertiary extrusive and intrusive igneous rocks related to continental break up and the passage of the ancestral Iceland plume. These intrusive bodies include a number of gabbroic macrodykes, two of which—the Miki Fjord Macrodyke, and the newly discovered Togeda Macrodyke—contain Cu–PGE–Au sulphide mineralisation along their margins. Sulphides occur as disseminated interstitial blebs and rounded globules of chalcopyrite and pyrrhotite with some Fe–Ti oxides and platinum-group minerals, comprising largely Pd bismuthides and tellurides. The globules are interpreted to have formed from fractionation of trapped droplets of an immiscible Cu- and Pd-rich sulphide melt and show geopetal indicators. Sulphur isotopes imply a local crustal source of S in these from pyritic sediments of the Kangerlussuaq Basin. Thus, generation of these sulphide occurrences was controlled by local country rock type. Low Ni/Cu and Pt/Pd ratios, also present in the Platinova reefs in the Skaergaard Intrusion, indicate that early fractionation of olivine may have depleted the magma of Ni and suggest the likely presence of a large magma chamber at depth. Xenoliths of Ni-rich olivine cumulates in the Miki Fjord Macrodyke may have been sourced from such a body. The location of thus far unidentified conduit or feeder zones to the macrodykes beneath the present day surface may represent potential targets for more massive sulphide orebodies.     

Application of Geochemistry and VNIR Spectroscopy in Mapping Heavy Metal Pollution of Stream Sediments in the Takab Mining Area, NW of Iran

This study considered the possibility of using visible and near infrared (VNIR) spectral absorption feature parameters (SAFPs) in predicting the concentration and mapping the distribution of heavy metals in sediments of the Takab area. In total, 60 sediment samples were collected along main streams draining from the mining districts and tailing sites, in order to measure the concentration of As, Co, V, Cu, Cr, Ni, Hg, Ti, Pb and Zn and the reflectance spectra (350–2500 nm). The quantitative relationship between SAFPs (Depth500nm, R610/500nm, R1344/778nm, Area500nm, Depth2200nm, Area2200nm, Asym2200nm) and geochemical data were assessed using stepwise multiple linear regression (SMLR) and enter multiple linear regression (EMLR) methods. The results showed a strong negative correlation between Ni and Cr with Area2200nm, a significant positive correlation between As and Asym2200nm, Ni and Co with Depth2200nm, as well as Co, V and total values with Depth500nm. The EMLR method eventuated in a significant prediction result for Ni, Cr, Co and As concentrations based on spectral parameters, whereas the prediction for Zn, V and total value was relatively weak. The spatial distribution pattern of geochemical data showed that mining activities, along with the natural weathering of base metal occurrences and rock units, has caused high concentrations of heavy metals in sediments of the Sarough River tributaries.     

On the perturbations of a close-Earth satellite due to lunar inequalities

The lunar disturbing function for a close-Earth satellite is expressed as a sum of products of harmonics of the satellite's position and harmonics of the Moon's position, and the latter are expanded about a rotating and precessing elliptic orbit inclined to the ecliptic. The deviations of the Moon from this approximate orbit are computed from Brown's lunar theory andthe perturbations in satellite orbital elements due to these inequalities are derived. Numerical calculations indicate that several perturbations in the position of the satellite's node and perigee have magnitudes on the order of one meter.The author is supported in part by a National Science Foundation Graduate Fellowship.     

Biodiversity in Hudson River shore zones: influence of shoreline type and physical structure

The shore zones of the Hudson River, like those of many developed waterways, are highly varied, containing a mix of seminatural and highly engineered shores. Our goal was to document the biodiversity supported by different kinds of shore zones in the Hudson. We chose six common types of shore zones, three of them ??natural?? (sand, unconsolidated rock, and bedrock), and three of them engineered (riprap, cribbing, and bulkheads). We measured selected physical characteristics (shore zone width, exposure, substrate roughness and grain size, shoreline complexity) of three examples of each of these shore types, and also sampled communities of terrestrial plants, fishes, and aquatic and terrestrial invertebrates. Community composition of most taxa differed across shore types, and frequently differed between wide, sheltered shores and narrow, exposed shores. Alien plant species were especially well represented along engineered shores. Nevertheless, a great deal of variation in biological communities was not explained by our six-class categorization of shore zones or the physical variables that we measured. No single shore type supported the highest values of all kinds of biodiversity, but engineered shore zones (especially cribbing and bulkheads) tended to have less desirable biodiversity characteristics than ??natural?? shore zones.     

P-waveform analysis for local subduction geometry south of Puget Sound,Washington

The local subduction geometry at a site south of Puget Sound in western Washington is investigated using teleseismicP-waveforms recorded on a three-component event triggered seismograph. The data are processed using source equalization deconvolution in order to isolate locally convertedP-to-S arrivals and stacked to improve the signal-to-noise ratio. Stable arrivals in the radial component indicate an oceanic Moho within the subducted slab at a depth of about 53 km beneath the station. Observed amplitude variations with azimuth in the radial data, as well as qualitative aspects of the tangential data, are used to establish a slab dip of 16° to the southeast. Our results are compatible with previous results from a site 60 km to the west, and further confirm a substantial warp in the regional geometry of the subducted Juan de Fuca plate.     

On yoderite: Using calculated phase equilibria to investigate its rarity in the geological record of whiteschists

A new activity–composition model is presented for green (Mn³⁺‐absent) yoderite for use in the latest internally consistent thermodynamic data set used by THERMOCALC, for calculations primarily in MgO–Al₂O₃–SiO₂–H₂O–O system, where O is a proxy for Fe₂O₃. P–T grids calculated with our model in the MASH and MASHO system feature invariant points and univariant reaction bundles that are consistent with existing experimental results. Using this new model, we have explored the stability of yoderite in whiteschists, a rare type of high‐pressure rock that conforms closely to the MASHO system. Using a series of calculated models in which composition varies, it is shown that yoderite stability is a function of bulk‐rock SiO₂, MgO and Al₂O₃, where the most important component for stabilizing yoderite is a function of pressure and temperature. The rarity of yoderite in naturally occurring whiteschists is largely related to these compositional factors, with most whiteschists having rock compositions that are too SiO₂‐rich and Al₂O₃‐poor to allow yoderite formation. However, in addition to compositional factors, the calculated P–T stability field of yoderite occurs over thermal gradients that are generally too high to occur in modern‐style subduction zones. As nearly all known whiteschist occurrences are Phanerozoic in age, the near‐complete absence of yoderite in late Neoproterozoic–Phanerozoic whiteschists may be at least partially due to modern subduction systems failing to produce the hotter thermal gradients needed to stabilize yoderite. The provision of this new a–x model for green yoderite allows for more rigorous P–T–X investigations of all whiteschists.