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Shock Metamorphism of Bosumtwi Impact Crater Rocks, Shock Attenuation, and Uplift Formation

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Science  12 Dec 2008:
Vol. 322, Issue 5908, pp. 1678-1681
DOI: 10.1126/science.1166283

Abstract

Shock wave attenuation rate and formation of central uplifts are not precisely constrained for moderately sized complex impact structures. The distribution of shock metamorphism in drilled basement rocks from the 10.5-kilometer-diameter Bosumtwi crater, and results of numerical modeling of inelastic rock deformation and modification processes during uplift, constrained with petrographic data, allowed reconstruction of the pre-impact position of the drilled rocks and revealed a shock attenuation by ∼5 gigapascals in the uppermost 200 meters of the central uplift. The proportion of shocked quartz grains and the average number of planar deformation feature sets per grain provide a sensitive indication of minor changes in shock pressure. The results further imply that for moderately sized craters the rise of the central uplift is dominated by brittle failure.

During the contact and compression phase of hypervelocity impact, a spherical shock wave is generated, propagates through the target rocks (1), and is attenuated rapidly with increasing distance. Consequently, a variety of shock effects are produced in rock-forming minerals, including formation of planar deformation features (PDFs) and high-pressure phases. The relative spatial distribution of these shock transformations and deformations formed at different pressures and temperatures [e.g., (2, 3)] in autochthonous rocks, at the scale of the impact structure, can be used to estimate maximum shock pressures and, consequently, the rate of shock attenuation. However, many parameters—such as rock type as well as lithological contrasts, texture, fabric, grain size, preshock orientation of grains, porosity, and volatile content—influence the shock levels attained locally. Furthermore, and typical in the case of complex impact structures (i.e., craters with diameters ≥2 to 4 km on Earth) (4), the original position and distribution of the shocked rocks is modified when, because of gravitational instability of the transient cavity rim, rebound of the crater floor leads to formation of a central uplift. Redistribution of rock is also associated with the collapse of the initially oversteepened central uplift (5).

There have been several efforts to estimate shock wave decay, mainly from nuclear and explosion crater studies or by numerical modeling [e.g., (68)]. Few studies have tried to quantify shock pressure distribution in simple (9) and complex impact structures (916) [see supporting online material (SOM text)] by using the frequency of shock-induced deformations in quartz grains from samples taken along profiles across the erosion surface of an impact structure. Only three investigations have explored the relative vertical decay of recorded shock pressure in complex impact structures, two for the ∼40-km-diameter Puchezh-Katunki impact structure (13, 15) and another for the ∼65-km-diameter Kara crater (11).

We characterized the shock wave attenuation in the uppermost part of the central uplift of the Bosumtwi impact structure, a moderately sized (10.5-km diameter) and well-preserved complex impact structure [e.g., (17)], and attempted to reconstruct the original position of the sampled section in the target before crater modification. This is possible by combining petrographic investigations (at the microscale) with modeling of inelastic rock deformation (at the mesoscale) and modification processes during uplift (at the megascale). This approach allows us to constrain shock wave attenuation and rock deformation during central uplift formation.

We studied drill core samples from the central part of the structure retrieved in the 2004 International Continental Scientific Drilling Program (ICDP) Bosumtwi drilling project (1820) (SOM text). The central uplift extends ∼130 m above the crater moat, is ∼1.9 km wide (21), and is composed of metasedimentary rocks [mostly metagraywacke (MGW) and shale] (19). Two cores were retrieved by the ICDP project: core LB-08A from the outer flank of the central uplift and core LB-07A from the deep crater moat (Fig. 1 and SOM text).

To derive the distribution of shock metamorphic effects with depth, we studied 18 MGW samples from 271.4- to 451.2-m depth in core LB-08A (Fig. 1), recording modal analysis and shock deformation properties of 8991 quartz grains (22) (tables S1 and S2). We determined 1056 crystallographic orientations of sets of PDFs in 602 quartz grains (table S3) because predominance of specific orientations of PDFs in quartz has long been considered to indicate different shock pressures [e.g., (23)].

The abundance of shocked quartz grains (i.e., quartz grains with planar fractures and/or PDFs) decreases with increasing depth (for PDFs, compare with Fig. 2). We interpret this decrease to reflect the variation of shock pressure in the uppermost part of the central uplift. Similarly, the average number of PDF sets per quartz grain (N) decreases steadily with depth, from ∼2.1 at 271-m depth to ∼1.6 at 442 m (Fig. 3). We interpret the decrease of N as evidence of the shock pressure attenuation with distance from shock wave origin.

No clear change was evident over this interval in the relative frequencies of the main crystallographic orientations of PDF sets in quartz grains with increasing depth (Fig. 2 and table S3). Most of the poles to the PDF planes correspond to the $Math$ orientation, and only a few, typically less than 5 relative-percent (rel%) of the orientation data obtained per section, correspond to basal [i.e., parallel to (0001)] planes. Only two samples, KR8-85 and KR8-89, from 390- to 400-m depth, show a higher proportion of basal PDF, at about 27 and 19 rel%, respectively (table S3). These two MGW samples originated just below a 10-m-thick layer of comparatively fine-grained metasedimentary rocks (phyllite and slate; Fig. 2); thus, it is possible that shock wave interference at the transition from metapelite to MGW resulted in the local formation of a larger proportion of basal PDFs [basal PDFs are formed at lower pressures (from 8 to 10 GPa) than $Math$ orientations (>10 GPa); e.g., (2)]. However, about 50% of the basal PDFs measured in these two samples occur in association with $Math$ PDF (i.e., dual sets of PDFs in the same quartz grain), which indicates relatively high shock pressures. Thus, minor variations of the relative proportions of crystallographic orientations of PDF sets in quartz grains with depth cannot be used to quantify the slight attenuation of the shock pressure along the ∼200-m-long bedrock section investigated here. Because the relative proportion of shocked quartz grains, as well as the number of PDF sets per quartz grain, steadily decrease with depth (Figs. 2 and 3), we infer that the shock wave was attenuated but not by much.

On the basis of the relative abundances of quartz grains with PDFs, the presence of multiple sets of PDFs in quartz grains (up to four sets), and the occurrence of various PDF set orientations, we estimate that the studied section experienced peak shock pressures of up to ∼25 to 30 GPa [e.g., (2, 2425)]. The upper limit of peak shock pressure (i.e., 30 GPa) is further constrained by the lack of isotropization of the quartz grains, which typically begins at this shock pressure [e.g., (2)]. The estimated pressure range is much too low for either partial or total shock melting of the whole rock, which is in agreement with macroscopic and microscopic observations on these metasedimentary rocks that have preserved their preimpact texture (19) (SOM text). Thus, the rise of the central uplift was not facilitated by shock-induced plasticity, and brittle failure was dominant. This finding is also supported by the presence of abundant faults in the central uplift, as indicated by reflection seismic data (21).

To further evaluate the apparent shock attenuation, we calculated the expected shock pressure attenuation and reconstructed the preimpact position of the investigated section of the central uplift by numerical modeling, by using a variant of the SALEB numerical code (26) (SOM text). To model the crater and central uplift properly, we did a direct parametric fit of our model to the observed crater geometry without usage of scaling relations for the crater size (SOM text and fig. S3). After modeling, tracers in the vicinity of the exact location of the two drill holes were identified (allowing the tracking of their initial positions), and lastly the position of the top of the “model core” was defined by the maximum recorded shock pressure of 30 GPa (based on our petrographic investigations). Then, the initial position of these tracers was plotted (Fig. 4). Our calculations show that, despite quite similar present-day depths, the initial depth of rocks in the crater moat (core LB-07A) was above that of the rocks in the central uplift corehole (Fig. 4). This specific displacement of the target rocks during crater modification is interesting because it indicates that rocks occurring in the outer flank of the central uplift were originally more deeply buried than the basement rocks drilled below the deep crater moat and, thus, were subjected to somewhat lower shock pressures. Thus, before estimating shock attenuation in complex impact structures, it is important to consider the differential movements of the target rocks, which cause changes in the spherical shock-attenuation scheme. The numerical model (Fig. 4 and SOM text) indicates that the samples investigated in core LB-08A have been uplifted by about 1.2 to 1.5 km. By using the relationship SU = 0.086D1.03 (27) (where D is the crater diameter), we estimated the amount of structural uplift (SU) for the Bosumtwi central uplift at about 1 km, in good agreement with the modeling result.

Taking into account the uplift of target rocks, the calculated apparent shock attenuation along the about-200 m of investigated core is ∼5 to 10 GPa (fig. S4). On the basis of the general absence of changes in the statistics of PDF orientations, a maximum shock attenuation of 5 GPa seems to be the more realistic value.

The percentage of shocked quartz grains and the number of PDF sets per grain are more sensitive indicators of minor changes in shock pressure than pure PDF orientation statistics. The combination of detailed petrographic investigation and numerical modeling indicates that both of these approaches are essential to reconstruct the preimpact position of rocks and to characterize properly the shock pressure distribution at the scale of an impact structure. Our observations suggest that, in the case of the 10.5-km-diameter Bosumtwi impact structure, the uppermost rocks of the central uplift experienced shock pressures below 30 GPa, whereas pressures up to 40 to 45 GPa were recorded for the about-four-times-larger Puchezh-Katunki impact structure (15). Shock attenuation in the uppermost part of a central uplift has been, for the first time, constrained by detailed shock degree profiling at the microscale. Numerical modeling of this section of the central uplift has then established where this section of the central uplift was located before uplift formation, which was only possible once the shock regime had been established by micropetrography. The results imply that, for moderately sized impact craters, the rise of the central uplift is dominated by brittle failure, whereas in the case of larger impact structures, and also depending on rock proprieties, the uplifted, relatively stronger shocked rocks may behave in a more ductile manner.

Supporting Online Material

Materials and Methods

SOM Text

Figs. S1 to S4

Tables S1 to S4

References

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