International Symposium on Triassic Chronostratigraphy and Biotic Recovery     

Tong Jinnan Mike Orchard 《《幕》》2005,28(4):298-299

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Geochemistry and Provenance of Springs in a Baja California Sur Mountain Catchment     

Jory C. Lerback  Brenda B. Bowen  C. E. Humphrey  Diego. P. Fernandez  Jeremiah A. Bernau  Shane J. Macfarlan  Eric Schniter  J. J. Garcia 《Ground water》2022,60(2):295-308

Fractured rock aquifers cover much of Earth's surface and are important mountain sites for groundwater recharge but are poorly understood. To investigate groundwater systematics of a fractured-dominated aquifer in Baja California Sur, Mexico, we examined the spatial patterns of aquifer recharge and connectivity using the geochemistry of springs. We evaluate a range of geochemical data within the context of two endmember hypotheses describing spatial recharge patterns and fracture connectivity. Hypothesis 1 is that the aquifer system is segmented, and springs are fed by local recharge. Hypothesis 2 is that the aquifer system is well connected, with dominant recharge occurring in the higher elevations. The study site is a small <15 km² catchment. Thirty-four distinct springs and two wells were identified in the study area, and 24 of these sites were sampled for geochemical analyses along an elevation gradient and canyon transect. These analyses included major ion composition, trace element and strontium isotopes, δ¹⁸O and δ²H isotopes, radiocarbon, and tritium. δ¹⁸O and δ²H isotopes suggest that the precipitation feeding the groundwater system has at least two distinct sources. Carbon isotopes showed a change along the canyon transect, suggesting that shorter flowpaths feed springs in the top of the transect, and longer flowpaths discharge near the bottom. Geochemical interpretations support a combination of the two proposed hypotheses. Understanding of the connectivity and provenance of these springs is significant as they are the primary source of water for the communities that inhabit this region and may be impacted by changes in recharge and use.  相似文献   

National lacustrine core repository (LacCore)     

Douglas W. Schnurrenberger  Kerry R. Kelts  Thomas C. Johnson  Linda C.K. Shane  Emi Ito 《Journal of Paleolimnology》2001,25(1):123-127

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A multiple-parameter approach to andesitic tephra correlation, Ruapehu volcano, New Zealand     

Shane J. Cronin  Vincent E. Neall  Robert B. Stewart  Alan S. Palmer 《Journal of Volcanology and Geothermal Research》1996,72(3-4)

A multi-parameter approach was used to correlate andesitic tephras in a complex tephra sequence ranging in age from ca. 23 to ca. 75 ka on the eastern ring plain of Ruapehu volcano, North Island. Field properties, combined with ferromagnesian mineral assemblages and mineral compositions, were required to map and correlate this sequence. Three tephra units could be identified based on their unique physical appearance, but other tephras could not be correlated on this basis alone. Hornblende and olivine proved to be valuable marker minerals enabling further distinction of two of the marker units recognised by field properties, as well as defining two further marker tephras. Unweathered titanomagnetite crystals, present in all of the tephras, were subjected to major-element analysis by electron microprobe. Canonical discriminant function analysis (DFA) of these analyses enabled the grouping and discrimination of tephra units, further aiding the identification of defined marker units, as well as defining new marker units. The titanomagnetite chemistry showed a strong relationship to the ferromagnesian mineralogy, showing that the ferromagnesian phenocrysts formed from the same melt or under the same melt conditions prior to eruption of each tephra. Canonical DFA was also applied to hornblende and olivine mineral analyses to identify further marker beds and to confirm identifications of previously defined units. This statistical analysis was found to be invaluable in reducing the large amount of compositional data from this study into a useable form for andesitic tephra correlation and mapping.  相似文献   

The Es 511 Y Job Application Form: An Unconventional Data Source For The Study Of Disadvantaged Urban Migrants     

Shane Davies  Gary L. Flower 《The Professional geographer》1972,24(3):253-256

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Implementation of Particle-scale Rotation in the 3-D Lattice Solid Model     

Yucang Wang  Steffen Abe  Shane Latham  Peter Mora 《Pure and Applied Geophysics》2006,163(9):1769-1785

In this study, 3-D Lattice Solid Model (LSMearth or LSM) was extended by introducing particle-scale rotation. In the new model, for each 3-D particle, we introduce six degrees of freedom: Three for translational motion, and three for orientation. Six kinds of relative motions are permitted between two neighboring particles, and six interactions are transferred, i.e., radial, two shearing forces, twisting and two bending torques. By using quaternion algebra, relative rotation between two particles is decomposed into two sequence-independent rotations such that all interactions due to the relative motions between interactive rigid bodies can be uniquely decided. After incorporating this mechanism and introducing bond breaking under torsion and bending into the LSM, several tests on 2-D and 3-D rock failure under uni-axial compression are carried out. Compared with the simulations without the single particle rotational mechanism, the new simulation results match more closely experimental results of rock fracture and hence, are encouraging. Since more parameters are introduced, an approach for choosing the new parameters is presented.  相似文献   

Oligocene to Recent tectonic history of the Central Solomon intra-arc basin as determined from marine seismic reflection data and compilation of onland geology     

Shane Cowley  Paul Mann  M. F. Coffin  Thomas H. Shipley 《Tectonophysics》2004,389(3-4):267

Systematic analysis of a grid of 3450 km of multichannel seismic reflection lines from the Solomon Islands constrains the late Tertiary sedimentary and tectonic history of the Solomon Island arc and its convergent interaction with the Cretaceous Ontong Java oceanic plateau (OJP). The OJP, the largest oceanic plateau on Earth, subducted beneath the northern edge of the Solomon arc in the late Neogene, but the timing and consequences of this obliquely convergent event and its role in the subduction polarity reversal process remain poorly constrained. The Central Solomon intra-arc basin (CSB), which developed in Oligocene to Recent time above the Solomon arc, provides a valuable record of the tectonic environment prior to and accompanying the OJP convergent event and the subsequent arc polarity reversal. Recognition of regionally extensive stratigraphic sequences—whose ages can be inferred from marine sedimentary sections exposed onland in the Solomon Islands—indicate four distinct tectonic phases affecting the Solomon Island arc. Phase 1: Late Oligocene–Late Miocene rifting of the northeast-facing Solomon Island arc produced basal, normal-fault-controlled, asymmetrical sequences of the CSB; the proto-North Solomon trench was probably much closer to the CSB and is inferred to coincide with the trace of the present-day Kia-Kaipito-Korigole (KKK) fault zone; this protracted period of intra-arc extension shows no evidence for interruption by an early Miocene period of convergent “soft docking” of the Ontong Java Plateau as proposed by previous workers. Phase 2: Late Miocene–Pliocene oblique convergence of the Ontong Java Plateau at the proto-North Solomon trench (KKK fault zone) and folding of the CSB and formation of the Malaita accretionary prism (MAP); the highly oblique and diachronous convergence between the Ontong Java plateau and the Solomon arc terminates intra-arc extension first in the southeast (Russell subbasin of the CSB) during the Late Miocene and later during the Pliocene in the northwest (Shortland subbasin of the CSB); folds in the CSB form by inversion of normal faults formed during Phase 1; Phinney et al. [Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc-Ontong Java Plateau convergent zone)] show a coeval pattern of southeast to northwest younging in folding and faulting of the MAP. Phase 3: Late Pliocene–early Pleistocene arc polarity reversal and subduction initiation at the San Cristobal trench. Effects of this event in the CSB include the formation of a chain of volcanoes above the subducting Australia plate at the San Cristobal trench, the formation of the broad synclinal structure of the CSB with evidence for truncation at the uplifted flanks, and widespread occurrence of slides and “seismites” (deposits formed by seismic shaking). Phase 4: Pleistocene to Recent continued shortening and synclinal subsidence of the CSB. Continued Australia-Pacific oblique plate convergence has led to deepening of the submarine, elongate basin axis of the synclinal CSB and uplift of the dual chain of the islands on its flanks.  相似文献   

Biotite composition as a tool for the identification of Quaternary tephra beds     

Phil Shane  Victoria SmithIan Nairn 《Quaternary Research》2003,59(2):262-270

Stratigraphically important Quaternary rhyolitic tephra deposits that erupted from the Okataina and Taupo volcanic centers in New Zealand can be geochemically identified using the FeO and MgO contents of their biotite phenocrysts. The FeO/MgO ratio in biotite does not correlate with FeO/MgO in the coexisting glass phase so that tephra beds with similar glass compositions can be discriminated by their different biotite compositions. Some individual tephra deposits display sequential changes in biotite composition that allow separate phases of the eruption to be identified, greatly increasing the potential precision for correlation. In addition, devitrified lavas that are unsuitable for glass analysis can be correlated to coeval tephra deposits by their biotite compositions. Biotite is common in high-K₂O (>4 wt%) tephra beds and is widely dispersed in ash plumes because of its platy form, thus making it important in correlation studies.  相似文献   

Multiple Sources for Sea-Rafted Loisels Pumice, New Zealand     

Phil Shane  Paul Froggatt  Ian Smith  Murray Gregory 《Quaternary Research》1998,49(3):271-279

Sea-rafted Loisels Pumice is one of the few stratigraphic markers used to correlate late Holocene coastal deposits in New Zealand. Along with underlying sea-rafted products of the local Taupo eruption of ca. 1800 yr B.P., these events have been used to bracket the first arrival of humans at New Zealand. Loisels Pumice is dacitic to rhyolitic (SiO₂63–78 wt%) in composition, but individual clasts are homogeneous (SiO₂range ± 1 wt%). Characteristics include very low K₂O (0.5–1.75 wt%) and Rb (<25 ppm) and a mineralogy dominated by calcic and mafic xenocrysts. Similar features are shared by pumices of the Tonga–Kermadec arc, suggesting a common tholeiitic oceanic source. Interclast diversity of Loisels Pumice suggests that it is the product of several eruptive events from different volcanoes. The differences in glass and mineral compositions found at various sites can be explained if the deposits are from different events. A multisource origin can also partially explain the discrepancy in reported¹⁴C ages (ca. 1500–600 yr B.P.) from different localities. Therefore, the value of Loisels Pumice as a stratigraphic marker is questionable, and it does not constrain the arrival of humans. The predominant westward drift of historic Tonga–Kermadec arc pumices and prevailing ocean currents suggest a long anticlockwise semicircular transport route into the Tasman Sea before sea-rafted pumice arrival in New Zealand. The diversity of the pumices indicates that silicic eruptions frequently occur from the predominantly basic oceanic volcanoes.  相似文献   

The time-space distribution of Eocene to Miocene magmatism in the central Peruvian polymetallic province and its metallogenetic implications     

Thomas Bissig  Thomas D. Ullrich  Richard M. Tosdal  Richard Friedman  Shane Ebert 《Journal of South American Earth Sciences》2008,26(1):16-35

Eocene to late Miocene magmatism in the central Peruvian high-plain (approx. between Cerro de Pasco and Huancayo; Lats. 10.2–12°S) and east of the Cordillera Occidental is represented by scattered shallow-level intrusions as well as subaerial domes and volcanic deposits. These igneous rocks are calc-alkalic and range from basalt to rhyolite in composition, and many of them are spatially, temporally and, by inference, genetically associated with varied styles of major polymetallic mineralization. Forty-four new ⁴⁰Ar–³⁹Ar and three U/Pb zircon dates are presented, many for previously undated intrusions. Our new time constraints together with data from the literature now cover most of the Cenozoic igneous rocks of this Andean segment and provide foundation for geodynamic and metallogenetic research.The oldest Cenozoic bodies are of Eocene age and include dacitic domes to the west of Cerro de Pasco with ages ranging from 38.5 to 33.5 Ma. South of the Domo de Yauli structural dome, Eocene igneous rocks occur some 15 km east of the Cordillera Occidental and include a 39.34 ± 0.28 Ma granodioritic intrusion and a 40.14 ± 0.61 Ma rhyolite sill, whereas several diorite stocks were emplaced between 36 and 33 Ma. Eocene mineralization is restricted to the Quicay high-sulfidation epithermal deposit some 10 km to the west of Cerro de Pasco.Igneous activity in the earliest Oligocene was concentrated up to 70 km east of the Cordillera Occidental and is represented by a number of granodioritic intrusions in the Milpo–Atacocha area. Relatively voluminous early Oligocene dacitic to andesitic volcanism gave rise to the Astabamba Formation to the southeast of Domo de Yauli. Some stocks at Milpo and Atacocha generated important Zn–Pb (–Ag) skarn mineralization. After about 29.3 Ma, magmatism ceased throughout the study region. Late Oligocene igneous activity was restricted to andesitic and dacitic volcanic deposits and intrusions around Uchucchacua (approx. 25 Ma) and felsic rocks west of Tarma (21–20 Ma). A relationship between the Oligocene intrusions and polymetallic mineralization at Uchucchacua is possible, but evidence remains inconclusive.Widespread magmatism resumed in the middle Miocene and includes large igneous complexes in the Cordillera Occidental to the south of Domo de Yauli, and smaller scattered intrusive centers to the north thereof. Ore deposits of modest size are widely associated with middle Miocene intrusions along the Cordillera Occidental, north of Domo de Yauli. However, small volcanic centers were also active up to 50 km east of the continental divide and include dacitic dikes and domes, spatially associated with major base and precious metal mineralization at Cerro de Pasco and Colquijirca. Basaltic volcanism (14.54 ± 0.49 Ma) is locally observed in the back-arc domain south of Domo de Yauli approximately 30 km east of the Cordillera Occidental.After about 10 Ma intrusive activity decreased throughout Central Perú and ceased between 6 and 5 Ma. Late Miocene magmatism was locally related to important mineralization including San Cristobal (Domo de Yauli), Huarón and Yauricocha.Overall, there is no evidence for a systematic eastward migration of the magmatic arc through time. The arc broadened in the late Eocene to early Oligocene, and thereafter ceased over wide areas until the early Miocene, when magmatism resumed in a narrow arc. A renewed widening and subsequent cessation of the arc occurred in the late middle and late Miocene. The pattern of magmatism probably reflects two cycles of flattening of the subduction in the Oligocene and late Miocene. Contrasting crustal architecture between areas south and north of Domo de Yauli probably account for the differences in the temporal and aerial distribution of magmatism in these areas.Ore deposits are most abundant between Domo de Yauli and Cerro de Pasco and were generally emplaced in the middle and late Miocene during the transition to flat subduction and prior to cessation of the arc. Eocene to early Oligocene mineralization also occurred, but was restricted to a broad east–west corridor from Uchucchacua to Milpo–Atacocha, indicating a major upper-plate metallogenetic control.  相似文献