Structural evolution of the 40 km wide Araguainha impact structure,central Brazil

Abstract— The 40 km wide Araguainha structure in central Brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation. The dominant structural features preserved at Araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity. Slumping of the transient cavity walls resulted in inward‐verging inclined folds and a km‐scale anticline in the outer ring of the structure. The folding stage was followed by radial and concentric faulting, with downward displacement of kilometer‐scale blocks around the crater rim. The central uplift records evidence for km‐scale upward movement of crystalline basement rocks from the transient cavity floor, and lateral moment of sedimentary target rocks detached from the cavity walls. Much of the structural grain in the central uplift relates to structural stacking of km‐scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks. Outward‐plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets. Each individual sheet records an early stage of folding and thickening due to non‐coaxial strains, shortly before sheet imbrication. We attribute this folding and thickening phase to the kilometer‐scale inward movement of the target strata from the transient cavity walls to the central uplift. The outer parts of the central uplift record additional outward movement of the target rocks, possibly related to the collapse of the central uplift. An inner ring structure at 10–12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks.     

Re-Os and Sm-Nd Isotope and Trace Element Constraints on the Origin of the Chromite Deposit of the Ipueira-Medrado Sill, Bahia, Brazil

The chromite deposit of the Paleoproterozoic Ipueira–Medradosill is hosted in a single, thick (5–8 m), massive layer,which sets severe constraints for the origin of chromitites.It is divided from bottom to top into: (1) a Marginal Zone (5–20m); (2) an Ultramafic Zone (<250 m) consisting of duniteand harzburgite that host the chromitite layer, in which intercumulusamphibole is important and more abundant toward the top; (3)a Mafic Zone (<40 m). The parental magma was large ion lithophileelement and light rare earth element enriched and high fieldstrength element depleted. Sm–Nd isotopic compositionsare consistent with a 2 Ga age, but suggest a variable initialNd isotopic composition that correlates with the abundance ofamphibole. The more negative     

NUMERICAL SOLUTION OF THE ACOUSTIC AND ELASTIC WAVE EQUATIONS BY A NEW RAPID EXPANSION METHOD1

We present a new rapid expansion method (REM) for the time integration of the acoustic wave equation and the equations of dynamic elasticity in two spatial dimensions. The method is applicable to spatial grid methods such as finite differences, finite elements or the Fourier method. It is based on a Chebyshev expansion of the formal solution to the appropriate wave equation written in operator form. The method yields machine accuracy yet it is faster than methods based on temporal differencing. Its disadvantages are that it does not apply to all types of material rheology, and it can also require much storage when many snapshots and time sections are desired. Comparisons between numerical and analytical solutions for simple acoustic and elastic problems demonstrate the high accuracy of the REM.