In-Situ Seismic and Ex-Situ Laboratory Analysis  from  Expedition 364: Chicxulub Impact Basin Peak Ring Hole M0077a
Creators: Nixon, Christopher
Publisher: name: University of Alberta Library
Year: 2022

<p>Although craters with central peak ring morphology are common throughout the solar system, intact examples are rare terrestrially. The central rebound in complex craters is attributed to compressional rebound, but the mechanism that causes these uplifts to form a peak ring above a certain threshold has only been hypothesized. IODP/ICDP Expedition 364 sought to elucidate the mechanisms behind peak ring formation by conducting offshore drilling directly into the Chicxulub Impact Basin’s peak ring. By directly sampling 700 m of peak ring impactite core, analysis of Expedition 364 data largely supports the dynamic collapse model of peak ring formation.  The work here builds further understanding of the structure in terms of its seismic responses via an in situ vertical seismic profile (VSP) and ex situ by laboratory measurements ultrasonic velocity measurements on representative core samples. <br>
Analysis of the downgoing  VSP seismic wavefield indicates that the peak ring impactites display both elevated attenuation and anomalously low seismic wavespeeds relative to that expected for the original granitic rock.  These observations are attributed  to damage from the hypervelocity impact that is quantified via  Grady-Kipp damage parameters that depend on the deficits in the elastic moduli relative to the intact, pore free material.  These suggest that the shear elastic moduli have been affected more significantly than the bulk moduli, and this may have implications for the original location of target materials at the time of impact. <br>
The VSP data are further processed to isolate the origin of seismic reflectivity within the structure to assist in calibration of the depths to reflecting events in the surface seismic profiles.  The  high amplitude seismic reflector,  originally interpreted to be the  K-Pg boundary, is more complicated as it  results from seismic  tuning between between an upper dense and high velocity carbonate cap,  a middle  low velocity layer of highly altered impact breccia, and the lowermost  melt and highly damaged peak ring granitoids. As the marine environment is inseparable from the sedimentary cap and hydrologically altered impactites below, interpretation is discussed regarding hydrological considerations, which may prove extremely relevant in Martian exploration. <br>
A variety of petrophysical characterizations of candidate breccia, melt, and damaged granitoid as well as P- and S-wave velocity measurements were made.   All these materials are generally porous with porosities more than 0.1.  However, Hg-injection porosimetry and scanning electron microscopy reveal that the breccia and melt pore space is equant with well-defined diameters of ~ 1 m while the pore space in the granitoids consists of microcracks with a continuum of apertures from ~1 m to 10 nm. Interpretation of the petrographic results suggests that a niche for microbial life may exist within crystalline impactites.<br>
Further, the ultrasonic velocity determinations, carried out under confining pressure, indicate non-linear and heterogeneous relations between the wave speeds and pressure. Poisson’s ratio at both seismic and sonic frequencies are analyzed within the impactites. These values, too, are anomalously high when compared to similar terrestrial lithology. Taken together, and incorporating thin section analysis, these properties show an extensive and interconnected pore network and attempts to invert velocity data for pore structure have been unsuccessful with existing effective medium theories.  This likely occurs because most rock physics theoretical models that all rely on dilute and isolated pores, which is not the case in the highly damaged granitoids.  </p>