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

Abstract

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.

Liquefaction of sedimentary rocks during impact crater development

“…The nature of this process is unknown to any tectonic environment operative on Earth and, consequently, the mechanism that best explains the sudden reduction in rock strength remains obscure. One explanation is that and the Araguainha structure in central Brazil (Lana et al, 2003a(Lana et al, , 2004(Lana et al, , 2006(Lana et al, , 2008Abels, 2005;Wieland et al, 2005;Salisbury et al, 2008;Tohver et al, 2013). These folds are the best expression of extreme compressional strain recorded in central uplifts of a number of complex craters in sedimentary and mixed (sedimentary+crystalline) target rocks (e.g., Lana et al, 2003aLana et al, , 2006Wieland et al, 2005).…”

“…The Araguainha structure is located in the northern margin of the Paraná Basin (Lana et al, 2006(Lana et al, , 2007(Lana et al, , 2008. It is morphologically marked by a 10-12 km diameter central uplift surrounded by a 15-20 km wide annular trough, with two concentric rings and the crater rim (Lana et al, 2007).…”

“…1A-C). These ridges comprise a 400 m to 800 m-thick pile of clastic quartzose layers segmented into fault-bounded blocks that are rotated independently, forming a megabreccia zone around the granite core (Lana et al, 2006(Lana et al, , 2008.…”

“…The Araguainha central uplift comprises a 5 km-wide granitic core surrounded by a collar of upturned to overturned strata of the Furnas and Passa Dois Formations (e.g., Lana et al, 2007Lana et al, , 2008. The highest intensity of impact-related folding is in the older stratigraphic units (inner collar strata) that have been thickened by a factor of 3 to 5 (Lana et al, 2006(Lana et al, , 2008. Thickening is particularly evident for the Devonian sandstone unit of the central uplift (Furnas Formation), which is exceptionally thick to the north, west, and southwest sectors of the core-collar contact (Figs.…”

Geophysical Studies of Impact Craters

“…Grids of two-dimensional (2D) seismic reflection profiles spaced a few hundred metres apart can provide good images of the subsurface, but may be contact between the core and sediment is occasionally covered by polymict and monomict impact breccias. Some of the sediments are overturned, and duplications within the sedimentary layer are responsible for the thickening of these strata (Lana et al, 2008). A suite of resistivity and gravity data were acquired across the central crater with the aim of obtaining a geological model of the structure below the surface (Tong et al, 2010).…”