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 – The Bossòst dome is an E–W-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 E–W-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 staurolite–garnet 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 Bossòst 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

How to move ionized gas: an introduction to the dynamics of HII regions

This review covers the dynamic processes that are important in the evolution and structure of galactic HII regions, concentrating on an elementary presentation of the physical concepts and recent numerical simulations of HII region evolution in a non-uniform medium. The contents are as follows: (1) The equations (Euler equations; Radiative transfer; Rate equations; How to avoid the dynamics; How to avoid the atomic physics). (2) Physical concepts (Static photoionization equilibrium; Ionization front propagation; Structure of a D-type front; Photoablation flows; Other ingredients - Stellar winds, Radiation pressure, Magnetic fields, Instabilities). (3) HII region evolution (Early phases: hypercompact and ultracompact regions; Later phases: compact and extended regions; Clumps and turbulence).Comment: To be published as a chapter in 'Diffuse Matter from Star Forming Regions to Active Galaxies' - A volume Honouring John Dyson. Eds. T. W. Harquist, J. M. Pittard and S. A. E. G. Falle. 25 pages, 7 figures. Some figures degraded to meet size restriction. Full-resolution version available at http://www.ifront.org/wiki/Dyson_Festschrift_Chapte

Two stage tectonic history of the SW Amazon craton in the late Mesoproterozoic: identifying a cryptic suture zone

The history of the SW Amazon craton during late Mesoproterozoic times is marked by two separate tectonic events, the first related to collision with southern Laurentia and the second caused by suturing of the Paragua craton. The polycyclic basement rocks of the SW Amazon craton exposed in the Brazilian state of Rondônia were deformed at lower amphibolite conditions during early Grenville times (ca. 1.2–1.15 Ga). This deformation episode is the last of several tectonometamorphic events that affected the granitoid rocks of the Amazon basement throughout the Mesoproterozoic. The southern margin of the Amazon craton during late Mesoproterozoic times is defined by the E–W trending Nova Brasilândia metasedimentary belt, where upper amphibolite to granulite facies rocks from a younger (ca. 1.09 Ga) collisional event are preserved. Temperature–time (T–t) paths for each domain (craton and metasedimentary belt) are constructed using U–Pb, 40Ar/39Ar, and Rb–Sr data for minerals with different blocking temperatures. The T–t paths demonstrate no overlap in the timing or spatial distribution of tectonic and metamorphic activity. The separate cooling histories indicate the presence of a major tectonic boundary between the polycyclic basement rocks and the metasedimentary belt. This structure marks the suturing of the Paragua craton in the late Mesoproterozoic and is evidence that the accretionary history of the present outline of the Amazon craton was completed during the final stages of the amalgamation of Rodinia