The effect of impact-generated heterogeneities and discontinuities on
the subsequent weathering of impactites: insights from laboratory
experiments
Abstract
Impact cratering is an important geological process that affects all
planetary bodies in our solar system. As rock breakdown plays a vital
role in the evolution of landforms and sediments on a planetary scale,
it is crucial to assess the role of inheritance in the subsequent
breakdown of impactites (impact rocks). The shock pressure of several
gigapascals generated during the impact can exceed the effective
strength of target lithology by three to four orders of magnitude and is
responsible for melting, vaporisation, shock metamorphism, pore
collapse, vesiculation fracturing and fragmentation of rocks.
Environmental conditions and heterogeneities in rock properties exert an
important control in rock breakdown. Similar to other subaerial rocks,
impactites are affected by a range of rock breakdown processes. In order
to better understand the role of low-shock inheritance on rock
breakdown, a rock breakdown experiment was conducted in a simulated
environmental cabinet under conditions similar to terrestrial semi-arid
conditions. We cycled temperature (-2 to 35°C) and relative humidity
(13-45%) through 39 accelerated diurnal cycles (each of 8 hours
duration). We used 41 impactite samples in the experiment that included
low shocked sedimentary and crystalline rocks, impact melt rocks and
impact breccias. Mechanical (Equotip and weighing), photographic
(photographic monitoring), microscopic and solid-state methods
(petrographic microscopy, powder X-ray diffraction, scanning electron
microscopy, X-ray computed tomography) were used to characterise the
rock samples relative to unshocked rocks, and to assess the shock
related changes before and after the experiments. The low shocked
sedimentary rocks showed a decrease in porosity by 38% (Coconino
Sandstone) and 88% (Moenkopi Sandstone) compared to unshocked
counterparts. Macrofractures of 0.1-0.2 mm and microfractures 0.1-5 µm
in aperture were observed in all types of impactites. The results showed
that impactites exhibited an accelerated decline in strength compared to
non-impacted control samples. However, rock type and impact deformation
history were key parameters controlling the rate of deterioration.