Shock experiments were carried out with dry Seeberger sandstone at 5, 7.5, 10, and 12.5 GPa. The ... more Shock experiments were carried out with dry Seeberger sandstone at 5, 7.5, 10, and 12.5 GPa. The shocked samples show subplanar fractures, melting, and local shock features along shear zones, e.g., diaplectic quartz glass, SiO_2 melt, microbreccia, and PDF.
We present shock petrography for porous sandstone shocked tup to 17.5 GPa and compare results wit... more We present shock petrography for porous sandstone shocked tup to 17.5 GPa and compare results with numerical models of impact on porous rock.
Porosity plays an important role in impact crater formation and shock wave propagation. Where pre... more Porosity plays an important role in impact crater formation and shock wave propagation. Where present, it causes fast attenuation of shock pressure. In the framework of the "MEMIN" (Multidisciplinary Experimental and Modeling Impact crater research Network) project, the effect of porosity in dry and water-saturated sandstone on shock wave loading is investigated [1]. We are focusing on shock recovery experiments that have been carried out within one sub-project of MEMIN. The experiments are subject to investigate shock effects in experimentally shocked quartz at low shock pressure (5 - 12.5 GPa) where diagnostic shock features and calibration data are lacking at the moment. The influence of porosity on progressive shock metamorphism is investigated. The laboratory impact experiments were accompanied by meso-scale numerical modeling in order to quantify processes beyond the optical and electron optical observational capabilities. The model enables a detailed description and...
The influence of porosity on shock metamorphism was investigated with impact experiments (2.5-17.... more The influence of porosity on shock metamorphism was investigated with impact experiments (2.5-17.5 GPa) with sandstones and a quartzite. The results are compared to mesoscale numerical models quantifying the processes upon single-pore collapse.
A total of 184 confirmed impact structures are known on Earth to date, as registered by the Earth... more A total of 184 confirmed impact structures are known on Earth to date, as registered by the Earth Impact Database. The discovery of new impact structures has progressed in recent years at a rather low rate of about two structures per year. Here, we introduce the discovery of the approximately 10 km diameter Santa Marta impact structure in Piau ı State in northeastern Brazil. Santa Marta is a moderately sized complex crater structure, with a raised rim and an off-center, approximately 3.2 km wide central elevated area interpreted to coincide with the central uplift of the impact structure. The Santa Marta structure was first recognized in remote sensing imagery and, later, by distinct gravity and magnetic anomalies. Here, we provide results obtained during the first detailed ground survey. The Bouguer anomaly map shows a transition from a positive to a negative anomaly within the structure along a NE-SW trend, which may be associated with the basement signature and in parts with the signature developed after the crater was formed. Macroscopic evidence for impact in the form of shatter cones has been found in situ at the base around the central elevated plateau, and also in the interior of fractured conglomerate boulders occurring on the floor of the surrounding annular basin. Planar deformation features (PDFs) are abundant in sandstones of the central elevated plateau and at scattered locations in the inner part of the ring syncline. Together, shatter cones and PDFs provide definitive shock evidence that confirms the impact origin of Santa Marta. Crystallographic orientations of PDFs occurring in multiple sets in quartz grains are indicative of peak shock pressures of 20-25 GPa in the rocks exposed at present in the interior of the crater. In contrast to recent studies that have used additional, and sometimes highly controversial, alleged shock recognition features, Santa Marta was identified based on well-understood, traditional shock evidence.
As part of the MEMIN research program this project is focused on shock deformation experimentally... more As part of the MEMIN research program this project is focused on shock deformation experimentally generated in dry, porous Seeberger sandstone in the low shock pressure range from 5 to 12.5 GPa. Special attention is paid to the influence of porosity on progressive shock metamorphism. Shock recovery experiments were carried out with a highexplosive set-up that generates a planar shock wave, and using the shock impedance method. Cylinders of sandstone of average grain size of 0.17 mm and porosity of about 19 vol%, and containing some 96 wt% SiO 2 , were shock deformed. Shock effects induced with increasing shock pressure include: (1) Already at 5 GPa the entire pore space is closed; quartz grains show undulatory extinction. On average, 134 fractures per mm are observed. Dark vesicular melt (glass) of the composition of the montmorillonitic phyllosilicate component of this sandstone occurs at an average amount of 1.6 vol%.
ABSTRACT Impact experiments (2.5-17.5 GPa) with porous sandstone were compared to mesoscale numer... more ABSTRACT Impact experiments (2.5-17.5 GPa) with porous sandstone were compared to mesoscale numerical models quantifying the processes during single pore collapse.
ABSTRACT Particle size distribution (PSD) is an often used parameter to describe and quantify fra... more ABSTRACT Particle size distribution (PSD) is an often used parameter to describe and quantify fragmentation of deformed rock. Our analyses of shock deformed sandstone show that dynamic fragmentation influences the PSD, expressed as fractal dimension (D-value). Image analysis was used to derive fractal dimensions from a hypervelocity impact cratering experiment (2.5 mm steel sphere, 4.8 km/s) and a planar shock recovery experiment (2.5 GPa). The D-values in the cratering experiment decrease from 1.74 at the crater floor to 0.84 at a distance of 7.2 mm to the crater floor. The D-values found in this experiment are closely related to the microstructural features found at distinct distances from the crater floor. The obtained values are in good agreement with the D-values reported for fault zones, impact sites and deformation experiments. The D-value measured in the shock recovery experiment is 2.42. Such high D-values were usually attributed to abrasive processes related to high strain. Since the strain in our experiment is only ∼23% we suggest that at highly dynamic deformation very high d-values can be reached at small strain. To quantify this, numerical impact modelling has been used to estimate strain rates for the impact experiment. This is related to the activation of more inherent flaws and fracture bifurcation at very high strain rates ∼>102 s−1.
Shock experiments were carried out with dry Seeberger sandstone at 5, 7.5, 10, and 12.5 GPa. The ... more Shock experiments were carried out with dry Seeberger sandstone at 5, 7.5, 10, and 12.5 GPa. The shocked samples show subplanar fractures, melting, and local shock features along shear zones, e.g., diaplectic quartz glass, SiO_2 melt, microbreccia, and PDF.
We present shock petrography for porous sandstone shocked tup to 17.5 GPa and compare results wit... more We present shock petrography for porous sandstone shocked tup to 17.5 GPa and compare results with numerical models of impact on porous rock.
Porosity plays an important role in impact crater formation and shock wave propagation. Where pre... more Porosity plays an important role in impact crater formation and shock wave propagation. Where present, it causes fast attenuation of shock pressure. In the framework of the "MEMIN" (Multidisciplinary Experimental and Modeling Impact crater research Network) project, the effect of porosity in dry and water-saturated sandstone on shock wave loading is investigated [1]. We are focusing on shock recovery experiments that have been carried out within one sub-project of MEMIN. The experiments are subject to investigate shock effects in experimentally shocked quartz at low shock pressure (5 - 12.5 GPa) where diagnostic shock features and calibration data are lacking at the moment. The influence of porosity on progressive shock metamorphism is investigated. The laboratory impact experiments were accompanied by meso-scale numerical modeling in order to quantify processes beyond the optical and electron optical observational capabilities. The model enables a detailed description and...
The influence of porosity on shock metamorphism was investigated with impact experiments (2.5-17.... more The influence of porosity on shock metamorphism was investigated with impact experiments (2.5-17.5 GPa) with sandstones and a quartzite. The results are compared to mesoscale numerical models quantifying the processes upon single-pore collapse.
A total of 184 confirmed impact structures are known on Earth to date, as registered by the Earth... more A total of 184 confirmed impact structures are known on Earth to date, as registered by the Earth Impact Database. The discovery of new impact structures has progressed in recent years at a rather low rate of about two structures per year. Here, we introduce the discovery of the approximately 10 km diameter Santa Marta impact structure in Piau ı State in northeastern Brazil. Santa Marta is a moderately sized complex crater structure, with a raised rim and an off-center, approximately 3.2 km wide central elevated area interpreted to coincide with the central uplift of the impact structure. The Santa Marta structure was first recognized in remote sensing imagery and, later, by distinct gravity and magnetic anomalies. Here, we provide results obtained during the first detailed ground survey. The Bouguer anomaly map shows a transition from a positive to a negative anomaly within the structure along a NE-SW trend, which may be associated with the basement signature and in parts with the signature developed after the crater was formed. Macroscopic evidence for impact in the form of shatter cones has been found in situ at the base around the central elevated plateau, and also in the interior of fractured conglomerate boulders occurring on the floor of the surrounding annular basin. Planar deformation features (PDFs) are abundant in sandstones of the central elevated plateau and at scattered locations in the inner part of the ring syncline. Together, shatter cones and PDFs provide definitive shock evidence that confirms the impact origin of Santa Marta. Crystallographic orientations of PDFs occurring in multiple sets in quartz grains are indicative of peak shock pressures of 20-25 GPa in the rocks exposed at present in the interior of the crater. In contrast to recent studies that have used additional, and sometimes highly controversial, alleged shock recognition features, Santa Marta was identified based on well-understood, traditional shock evidence.
As part of the MEMIN research program this project is focused on shock deformation experimentally... more As part of the MEMIN research program this project is focused on shock deformation experimentally generated in dry, porous Seeberger sandstone in the low shock pressure range from 5 to 12.5 GPa. Special attention is paid to the influence of porosity on progressive shock metamorphism. Shock recovery experiments were carried out with a highexplosive set-up that generates a planar shock wave, and using the shock impedance method. Cylinders of sandstone of average grain size of 0.17 mm and porosity of about 19 vol%, and containing some 96 wt% SiO 2 , were shock deformed. Shock effects induced with increasing shock pressure include: (1) Already at 5 GPa the entire pore space is closed; quartz grains show undulatory extinction. On average, 134 fractures per mm are observed. Dark vesicular melt (glass) of the composition of the montmorillonitic phyllosilicate component of this sandstone occurs at an average amount of 1.6 vol%.
ABSTRACT Impact experiments (2.5-17.5 GPa) with porous sandstone were compared to mesoscale numer... more ABSTRACT Impact experiments (2.5-17.5 GPa) with porous sandstone were compared to mesoscale numerical models quantifying the processes during single pore collapse.
ABSTRACT Particle size distribution (PSD) is an often used parameter to describe and quantify fra... more ABSTRACT Particle size distribution (PSD) is an often used parameter to describe and quantify fragmentation of deformed rock. Our analyses of shock deformed sandstone show that dynamic fragmentation influences the PSD, expressed as fractal dimension (D-value). Image analysis was used to derive fractal dimensions from a hypervelocity impact cratering experiment (2.5 mm steel sphere, 4.8 km/s) and a planar shock recovery experiment (2.5 GPa). The D-values in the cratering experiment decrease from 1.74 at the crater floor to 0.84 at a distance of 7.2 mm to the crater floor. The D-values found in this experiment are closely related to the microstructural features found at distinct distances from the crater floor. The obtained values are in good agreement with the D-values reported for fault zones, impact sites and deformation experiments. The D-value measured in the shock recovery experiment is 2.42. Such high D-values were usually attributed to abrasive processes related to high strain. Since the strain in our experiment is only ∼23% we suggest that at highly dynamic deformation very high d-values can be reached at small strain. To quantify this, numerical impact modelling has been used to estimate strain rates for the impact experiment. This is related to the activation of more inherent flaws and fracture bifurcation at very high strain rates ∼>102 s−1.
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Papers by A. Kowitz