ALBERT

Description: Itacolumites are very special rocks due to their high flexibility. The investigated Brazilian itacolumites and associated non-flexible quartzites are of comparable composition but differ in their rock fabrics. The shape and size of quartz is mainly controlled by the mica fabric. Quartz textures and grain boundary migration features are indications for deformation at temperatures of about 500{degrees}C. The flexibility is mainly related to a penetrative network of open grain boundaries which enable a limited body rotation of individual quartz grains. Continuous layers of white mica display deformation features indicative of shear along the layer-parallel cleavage planes. As demonstrated by simple bending experiments, the flexibility is a highly anisotropic phenomenon which can be related to a directional dependence of grain shape fabrics and corresponding grain boundary pore spacing. According to quantitative estimates, the amount and anisotropy of bending can be explained by the rotation of separated quartz grains between layers of mica which act as flexural slip planes and are also responsible for the observed elastic rebound. Solution along grain boundaries, volumetric strain by thermal contraction of quartz and bulk extension are processes discussed for the origin of the extreme values of secondary grain boundary porosity.

Description: Eastward Andean orogenic growth since the late Oligocene led to variable crustal loading, flexural subsidence and foreland basin sedimentation in the Chaco basin. To understand the interaction between Andean tectonics and contemporaneous foreland development, we analyse stratigraphic, sedimentologic and seismic data from the Subandean Belt and the Chaco Basin. The structural features provide a mechanism for transferring zones of deposition, subsidence and uplift. These can be reconstructed based on regional distribution of clastic sequences. Isopach maps, combined with sedimentary architecture analysis, establish systematic thickness variations, facies changes and depositional styles. The foreland basin consists of five stratigraphic successions controlled by Andean orogenic episodes and climate: (1) the foreland basin sequence commences between ∼27 and 14 Ma with the regionally unconformable, thin, easterly sourced fluvial Petaca strata. It represents a significant time interval of low sediment accumulation in a forebulge-backbulge depocentre. (2) The overlying ∼14-7 Ma-old Yecua Formation, deposited in marine, fluvial and lacustrine settings, represents increased subsidence rates from thrust-belt loading outpacing sedimentation rates. It marks the onset of active deformation and the underfilled stage of the foreland basin in a distal foredeep. (3) The overlying ∼7-6 Ma-old, westerly sourced Tariquia Formation indicates a relatively high accommodation and sediment supply concomitant with the onset of deposition of Andean-derived sediment in the medial-foredeep depocentre on a distal fluvial megafan. Progradation of syntectonic, wedge-shaped, westerly sourced, thickening- and coarsening-upward clastics of the (4) ∼6-2.1 Ma-old Guandacay and (5) ∼2.1 Ma-to-Recent Emborozú Formations represent the propagation of the deformation front in the present Subandean Zone, thereby indicating selective trapping of coarse sediments in the proximal foredeep and wedge-top depocentres, respectively. Overall, the late Cenozoic stratigraphic intervals record the easterly propagation of the deformation front and foreland depocentre in response to loading and flexure by the growing Intra- and Subandean fold-and-thrust belt. © 2006 Blackwell Publishing Ltd.