796 research outputs found
Shrimp-RG-U-Pb zircon geochronology of mesoproterozoic metamorphism and plutonism in the southwesternmost United States
Mesoproterozoic intrusive and granulite-grade metamorphic rocks in southern California have been inferred to be exotic to North America on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks. Ion microprobe geochronology indicates that zircons in granulite-grade gneisses, dated at 1.4 Ga using conventional methods, are composed of 1.68-1.80-Ga cores and 1.19-Ga rims. These Early Proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1.19-Ga San Gabriel anorthosite complex. The lack of a 1.4-Ga metamorphic event suggests that Proterozoic rocks in this region, rather than being exotic to North America, may in fact be a midcrustal window into Mesoproterozoic crustal evolutionary processes in southwestern North America
Geochronology of the proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province
The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 Âą 12 and 1675 Âą 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at âź1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia
Assembling and disassembling california: A zircon and monazite geochronologic framework for proterozoic crustal evolution in southern California
The Mojave province in southern California preserves a comparatively complete record of assembly, postorogenic sedimentation, and rifting along the southwestern North American continental margin. The oldest exposed rocks are metasedimentary gneisses and amphibolite, enclosing intrusive suites that range from tonalite and quartz mon-zodiorite to granite with minor trondhjemite. Discrete magmatic episodes occurred at approximately 1790-1730 and 1690-1640 Ma. Evidence from detrital and premagmatic zircons indicates that recycling of 1900-1790 Ma Paleopro-terozoic crust formed the unique isotopic character of the Mojave province. Peak metamorphic conditions in the Mojave province reached middle amphibolite to granulite facies; metamorphism occurred locally from 1795 to 1640 Ma, with widespread evidence for metamorphism at 1711-1689 and 1670-1650 Ma. Structures record early, tight to isoclinal folding and penetrative west-vergent shear during the final metamorphic event in the west Mojave province. Proterozoic basement rocks are overlain by siliciclastic-carbonate sequences of Mesoproterozoic, Neoproterozoic, and Cambrian age, recording environmental change over the course of the transition from stable Mojave crust to the rifted Cordilleran margin. Neoproterozoic quartzites have diverse zircon populations inconsistent with a southwest North American source, which we infer were derived from the western conjugate rift pair within Rodinia, before establishment of the miogeocline. Neoproterozoic-Cambrian miogeoclinal clastic rocks record an end to rifting and establishment of the Cordilleran miogeocline in southern California by latest Neoproterozoic to Early Cambrian time
Invariant Distances
In this chapter we shall define the (invariant) distance we are going to use, and collect some of its main properties we shall need later on
Light-Front Approach for Heavy Pentaquark Transitions
Assuming the two diquark structure for the pentaquark state as advocated in
the Jaffe-Wilczek model, there exist exotic parity-even anti-sextet and
parity-odd triplet heavy pentaquark baryons. The theoretical estimate of
charmed and bottom pentaquark masses is quite controversial and it is not clear
whether the ground-state heavy pentaquark lies above or below the strong-decay
threshold. We study the weak transitions of heavy pentaquark states using the
light-front quark model. In the heavy quark limit, heavy-to-heavy pentaquark
transition form factors can be expressed in terms of three Isgur-Wise
functions: two of them are found to be normalized to unity at zero recoil,
while the third one is equal to 1/2 at the maximum momentum transfer, in
accordance with the prediction of the large-Nc approach or the quark model.
Therefore, the light-front model calculations are consistent with the
requirement of heavy quark symmetry. Numerical results for form factors and
Isgur-Wise functions are presented. Decay rates of the weak decays Theta_b+ to
Theta_c0 pi+ (rho+), Theta_c0 to Theta+ pi- (rho-), Sigma'_{5b}+ to
Sigma'_{5c}0 pi+ (rho+) and Sigma'_{5c}0 to N_8+ pi- (rho-) with Theta_Q,
Sigma'_{5Q} and N_8 being the heavy anti-sextet, heavy triplet and light
octet pentaquarks, respectively, are obtained. For weakly decaying Theta_b+ and
Theta_c0, the branching ratios of Theta_b+ to Theta_c0 pi+, Theta_c0 to Theta+
pi- are estimated to be at the level of 10^{-3} and a few percents,
respectively.Comment: 33 pages, 3 figures, version to be published in Phys. Rev.
Be our guest/worker: reciprocal dependency and expressions of hospitality in Ni-Vanuatu overseas labour migration
Whilst there has been renewed interest in the development potential of temporary migration programmes, such schemes have long been criticized for creating conditions for exploitation and fostering dependence. In this article, which is based on a case study of Ni-Vanuatu seasonal workers employed in New Zealandâs horticultural industry, I show how workers and employers alike actively cultivate and maintain relations of reciprocal dependence and often describe their relation in familial terms of kinship and hospitality. Nevertheless, workers often feel estranged both in the Marxian sense of being subordinated to a regime of time-discipline, and in the intersubjective sense of feeling disrespected or treated unkindly. I show how attention to the ânon-contractual elementâ in the work contract, including expressions of hospitality, can contribute to anthropological debates surrounding work, migration, and dependence, and to interdisciplinary understandings of the justice of labour migration.ESRC scholarship (project reference ES/H034943/1
Depositional setting, provenance and tectonic-volcanic setting of Eocene-Recent deep-sea sediments of the oceanic Izu-Bonin forearc, NW Pacific (IODP Expedition 352)
New biostratigraphical, geochemical, and magnetic evidence is synthesized with IODP Expedition 352 shipboard results to understand the sedimentary and tectono-magmatic development of the IzuâBonin outer forearc region. The oceanic basement of the IzuâBonin forearc was created by supra-subduction zone seafloor spreading during early Eocene (c. 50â51 Ma). Seafloor spreading created an irregular seafloor topography on which talus locally accumulated. Oxide-rich sediments accumulated above the igneous basement by mixing of hydrothermal and pelagic sediment. Basaltic volcanism was followed by a hiatus of up to 15 million years as a result of topographic isolation or sediment bypassing. Variably tuffaceous deep-sea sediments were deposited during Oligocene to early Miocene and from mid-Miocene to Pleistocene. The sediments ponded into extensional fault-controlled basins, whereas condensed sediments accumulated on a local basement high. Oligocene nannofossil ooze accumulated together with felsic tuff that was mainly derived from the nearby IzuâBonin arc. Accumulation of radiolarian-bearing mud, silty clay, and hydrogenous metal oxides beneath the carbonate compensation depth (CCD) characterized the early Miocene, followed by middle MioceneâPleistocene increased carbonate preservation, deepened CCD and tephra input from both the oceanic IzuâBonin arc and the continental margin Honshu arc. The IzuâBonin forearc basement formed in a near-equatorial setting, with late Mesozoic arc remnants to the west. Subduction-initiation magmatism is likely to have taken place near a pre-existing continentâoceanic crust boundary. The IzuâBonin arc migrated northward and clockwise to collide with Honshu by early Miocene, strongly influencing regional sedimentation
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
Measurement of the Lambda(b) cross section and the anti-Lambda(b) to Lambda(b) ratio with Lambda(b) to J/Psi Lambda decays in pp collisions at sqrt(s) = 7 TeV
The Lambda(b) differential production cross section and the cross section
ratio anti-Lambda(b)/Lambda(b) are measured as functions of transverse momentum
pt(Lambda(b)) and rapidity abs(y(Lambda(b))) in pp collisions at sqrt(s) = 7
TeV using data collected by the CMS experiment at the LHC. The measurements are
based on Lambda(b) decays reconstructed in the exclusive final state J/Psi
Lambda, with the subsequent decays J/Psi to an opposite-sign muon pair and
Lambda to proton pion, using a data sample corresponding to an integrated
luminosity of 1.9 inverse femtobarns. The product of the cross section times
the branching ratio for Lambda(b) to J/Psi Lambda versus pt(Lambda(b)) falls
faster than that of b mesons. The measured value of the cross section times the
branching ratio for pt(Lambda(b)) > 10 GeV and abs(y(Lambda(b))) < 2.0 is 1.06
+/- 0.06 +/- 0.12 nb, and the integrated cross section ratio for
anti-Lambda(b)/Lambda(b) is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are
statistical and systematic, respectively.Comment: Submitted to Physics Letters
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