79 research outputs found
Polymetamorphism and ductile deformation of staurolite-cordierite schist of the Bossost Dome: indication for Variscan extension in the Axial Zone of the central Pyrenees
Abstract ā The BossoĢst dome is an EāW-trending elongated structural and metamorphic dome developed in Cambro-Ordovician metasedimentary rocks in the Variscan Axial Zone of the central Pyrenees. A steep fault separates a northern half-dome, cored by massif granite, from an EāW-trending doubly plunging antiform with granitic sills and dykes in the core to the south. The main foliation is a flat-lying S1/2 schistosity that grades into a steeper-dipping slaty cleavage at the dome margins. Three major deformational and two metamorphic phases can be differentiated. S1/2 schistosity is an axial planar cleavage to W-vergent recumbent folding that probably occurred in mid-Westphalian time. Peak regional metamorphism M1 is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust. Strong non-coaxial deformation recording uniform top-to-the-SE extension during D2a is preserved in stauroliteāgarnet schists in a 1.5 km thick, shallowly SE-dipping zone in the southeastern dome. A 500 m thick contact aureole (M2) was imprinted on the regionally metamorphosed rocks following the intrusion the BossoĢst granite during D2b. More coaxial deformation prevailed during synkinematic growth of M2 phases in the inner part of the contact aureole around the northern part of the dome, where it obliterated D2a fabrics. Progressive non-coaxial deformation continued in the southeastern antiform and is recorded by late-synkinemati
The Rwenzori Mountains, a Paleoproterzoic crustal shear belt crossing the Albertine rift system
This contribution discusses the development of the Paleoproterozoic Buganda-Toro belt in the Rwenzori mountains and its influence on the western part of the East African Rift System in Uganda. The Buganda-Toro belt is composed of several thick-skinned nappes consisting of Archaean Gneisses and Palaeoproterozoic cover units that are thrusted northwards. The high Rwenzori mountains are located in the frontal unit of this belt with retrograde greenschist facies gneisses towards the north, which are unconformably overlain by metasediments and amphibolites. Towards the south the metasediments are overthrust by the next migmatitic gneiss unit that belongs to a crustal scale nappe. The southwards dipping metasedimentary and volcanic sequence in the high Rwenzori mountains shows an inverse metamorphic grade with greenschist facies conditions in the north and amphibolite facies conditions in the south. Early D1 deformation structures are overgrown by cordierite, which in turn grows into D2 deformation, representing the major northwards directed thrusting event. We argue that the inverse metamorphic gradient develops because higher grade rocks are exhumed in the footwall of a crustal scale nappe whereas the exhumation decreases towards the north away from the nappe leading to a decrease in metamorphic grade. The D2 deformation event is followed by a D3 E-W compression, a D4 with the development of steep shear zones with a NNE-SSW and SSE-NNW trend including the large Nyamwamba shear followed by a local D5 retrograde event and D6 brittle inverse faulting. The Paleoproterozoic Buganda-Toro belt is relatively stiff and crosses the NNE-SSW running rift system exactly at the node where the highest peaks of the Rwenzori mountains are situated and where the lake George rift terminates towards the north. Orientation of brittle and ductile fabrics show some similarities indicating that the cross-cutting Buganda-Toro belt influenced rift propagation and brittle fault development within the Rwenzori mountain and that this stiff belt may form part of the reason why the Rwenzori mountains are relatively high within the rift.
Keywords: East African Rift, Basement, Buganda Toro, Inverse Metamorphic Gradient, Microtectonics, Rwenzori mountain
First electron beam polarization measurements with a Compton polarimeter at Jefferson Laboratory
A Compton polarimeter has been installed in Hall A at Jefferson Laboratory.
This letter reports on the first electron beam polarization measurements
performed during the HAPPEX experiment at an electron energy of 3.3 GeV and an
average current of 40 A. The heart of this device is a Fabry-Perot cavity
which increased the luminosity for Compton scattering in the interaction region
so much that a 1.4% statistical accuracy could be obtained within one hour,
with a 3.3% total error
Comparison between children and adolescents with and without chronic benign pain: consultation rate and pain characteristics
The aim of the study was to determine whether children with chronic benign
pain are in contact with their general practitioner (GP) more frequently
than those without chronic benign pain. A random sample of children and
adolescents aged between 0 and 18 years of age was drawn from the records
of ten general practices. According to their responses to a pain
questionnaire, subjects were assigned to the chronic benign pain group (n
= 95) if they had pain of more than three months' duration, or to the
control group (n = 105) if they had pain of less than three months'
duration or no pain at all. All the subjects had an average GP
consultation rate of 2.6 contacts per year. No significant age and sex
differences were found. Chronic benign pain in childhood and adolescence
is not related to increased use of healthcare services, suggesting that
somatisation does not play a major role in children with chronic benign
pain
The Charge Form Factor of the Neutron from the Reaction \pol{2H}(\pol{e},e'n)p
We report on the first measurement of spin-correlation parameters in
quasifree electron scattering from vector-polarized deuterium. Polarized
electrons were injected into an electron storage ring at a beam energy of
720~MeV. A Siberian snake was employed to preserve longitudinal polarization at
the interaction point. Vector-polarized deuterium was produced by an atomic
beam source and injected into an open-ended cylindrical cell, internal to the
electron storage ring. The spin correlation parameter A^V_{ed} was measured for
the reaction \pol{2H}(\pol{e},e'n)p at a four-momentum transfer squared of 0.21
(GeV/c)^2 from which a value for the charge form factor of the neutron was
extracted.Comment: 4 pages, 5 file
Polyphase deformation and metamorphism of the CuiabƔ group in the PoconƩ region (MT), Paraguay Fold and Thrust Belt: kinematic and tectonic implications
Petrologia do Granito Chasqueiro, regiĆ£o de Arroio Grande, sudeste do Escudo Sul-Rio-Grandense
Efficient use of the velocity gradients tensor in flow modelling
For models of fabric development in rocks, with vorticity as a variable parameter, the choice of an unsuitable
reference frame for instantaneous flow can hamper clear presentation of results. The orientation of most fabric
elements which develop in deforming rocks is attached to some principal direction of instantaneous flow: syntaxial
fibres, crystallographic preferred orientation patterns, steady state foliations and immobilised rigid objects seem to
orient themselves in a fixed position relative to instantaneous stretching axes (dĀ”), while shape fabric elements and
mica preferred orientation approach parallelism with the extensional apophysis (lā) of the flow. Models for development
of such fabric elements benefit from a choice of reference frame attached to either dĀ”, or lā
Identification of an underfilled foreland basin system in the upper Devonian of the Central Pyrenees: implications for the Hercynian orogeny" by Souquet et al
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