Quantitative Characterization of Chicxulub Impact Basin Peak Ring Materials

The exceptionally low seismic wave speeds determined by tomographic image models of the upper portions of the peak ring of the Chicxulub Impact Structure were largely confirmed by sonic and seismic borehole measurements from the IODP/ICDP Expedition 364 scientific drilling campaign. These low wave speeds result from the extensive damage to the peak ring rocks that, based on the results of numerical modelling, were likely displaced 20 km outwards and 10 km upwards after being subject to initial shock pressures as high as 60 GPa. Here, we describe a series of detailed petrophysical and wave speed measurements on select `granite' and `impact melt' cores from the borehole in order to better understand the nature of this damage To date, two granite and two impact melt samples have been studied. X-ray CT scans (30 mm voxel) reveal extensive micro-fracturing between and within the mineral grains in the granites (Fig. 1a) while highlighting the existence of relatively large, 1000 mm3, occluded pores (Fig. 1b). The porosities, as determined using a Hg-injection porosimeter, were also remarkably high with values of <10% and <25% observed in the granites and the melts, respectively. Grain densities, measured using a helium pycnometer, were similar between both samples ranging between 2.581-2.654 g/cc. Both VP and VS were determined under hydrostatic confining pressures between 3 MPa and 200 MPa by ultrasonic transit time measurements (Fig. 1c). The velocities increase linearly with pressure in the impact melt rock suggesting this material lacks microcrack porosity. In contrast, the velocities in the granites depend strongly on pressure further indicating the existence of pervasive microcracking. Even at the highest confining pressure of 200 MPa available to us, the observed VP is 4.5 km/s, a value much below the 6.0 km/s expected for the same nonporous rock. These observations suggest that the low wave speeds and densities within the peak ring differ based on whether the material is the original displaced granite or the intruded impact melt. Samples and data provided by IODP. Samples can be requested at http://web.iodp.tamu.edu/sdrm after 19 October 2017. Expedition 364 was funded by ECORD, IODP and ICDP with contributions and logistical support from the Yucatan State Government and Universidad Nacional Autónoma de México (UNAM).