Nonlinear Seismic Wavespeeds Under Confining Pressure up to 200 MPa for Shocked Granitoids of the Chicxulub Impact Basin Peak Ring and Anisotropy Investigations.
Abstract
We describe mechanical measurements on shock-damaged granitoids collected from the 1334 m IODP-ICDP Expedition 364 Chicxulub Impact Structure drilling program. Modelling suggests impact-induced uplift of these crustal rocks in excess of 10 km. Earlier work, conducted both in situ and under ambient laboratory conditions on core samples, demonstrates that this material exhibits anomalous porosity (8-13%) and P-wave speeds ( 4.0 - 4.2 km/s). Microanalysis indicates that this microporosity exists in the form of planar fractures and localized cataclastic zones that developed during shock deformation. Ultrasonic ( 1 MHz) P- and S-wave speeds were measured subject to confining pressures of 200 MPa. The observed speeds increase nonlinearly with confining pressure, as might be expected from such cracked materials. This can be approximated by two linear trends meeting at a deviation point which may suggest essentially complete closure of one family of microcracks. Despite the high confining pressures, the maximum wave speeds observed are still only 5.1 km/s and 2.7 km/s for P- and S-waves, respectfully. These remain significantly lower than the 6 km/s and 4 km/s expected for pore-free intact granite and indicate the persistence of open porosity at 200 MPa, which may suggest more spheroidal porosity. Hysteresis is seen in wave speeds upon decompression that reverses at lower pressures; unconfined uniaxial quasi-static mechanical testing on the shocked granitoid samples also reveals a hysteresis effect for stress-strain behavior in the elastic regime. Uniaxial compressive strength values for the granites typically lie between 20-30 MPa, but can reach values up to 45 MPa. These are still less than values >150 MPa typically observed for intact granites. Current work includes He-pyncnometry, Hg-porosimetry, gas adsorption, and thin section analysis with electron microscopy to further characterize the pore structures, allowing for interpretation of their development.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP51D1168N
- Keywords:
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- 0410 Biodiversity;
- BIOGEOSCIENCESDE: 4901 Abrupt/rapid climate change;
- PALEOCEANOGRAPHYDE: 6240 Meteorites and tektites;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTSDE: 5420 Impact phenomena;
- cratering;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS