W. Uwe Reimold

Experimental constraints on shock-induced microstructures in naturally deformed silicates

Abstract Planar deformation features (PDFs) in various minerals have long been accepted as evidence of impact-induced deformation. The uniqueness of this association was challenged in the context of the K/T Boundary extinction debate, after mosaicism and microstructures similar to PDFs were reported from the products of explosive volcanism. As a result of this debate, a significant volume of new experimental and observational data on the development of shock-induced microstructures has become available over the last ten years. The results reveal that factors such as pre-shock temperature, pulse duration, and crystallographic orientation of target minerals to the shock wave have a primary influence on how these microstructures develop. Data from diamond anvil cell and high-pressure friction experiments reveal that the same solid-state amorphization process that produces shock-induced PDFs at low temperatures also occurs at much lower strain rates in static experiments. The experimental data indicate that the amorphization process is thermally activated and that the character of the resulting PDFs is a function of the applied strain rate. Shock-induced amorphization occurs along those crystallographic planes that are most readily transformed to the high-pressure phase during very short pulse durations and produces PDFs that are visible at the optical scale. Lower strain rate deformation produces TEM scale amorphization with orientations that are more homogeneously distributed throughout the target mineral and produces no optically visible PDFs. The data confirm the uniqueness of multiple intersecting sets of optically visible PDFs as a diagnostic indicator of hypervelocity impact. The data also support the hypothesis that the amorphization process can occur at a wide range of strain rates, and that the limiting pressure for the process is controlled by the phase stability of the target mineral under the applied loading conditions, not by the HEL. The data also suggest that the onset pressure, the maximum pressure, and the pressure range for producing optically visible PDFs decrease with increasing temperature and decreasing strain rate. As the pressure range for optically visible PDF formation decreases, it is replaced by homogeneous amorphization as observed in the anvil cell experiments. Thus, the uniqueness of multiple intersecting sets of optically visible PDFs to hypervelocity impact is not due to a unique process, but rather to a specific set of loading conditions that produce an optically visible microstructure. Likewise, single sets of microdeformations observed in volcanic rocks are produced by the same process, but at different loading conditions that preclude the development of multiple intersecting sets of PDFs. The data also indicate that shock mosaicism, which occurs above the HEL, represents a plastic response of the target mineral to loading rates that are too large to be accommodated by crystal plastic mechanisms. Observational data for some naturally deformed samples from the K/T Boundary, the Vredefort Dome, and volcanic rocks, along with the experimental observations, are used to constrain the range of conditions under which the natural microstructures form, and to understand the differences between microstructures produced by impact, volcanic, and other natural processes. Finally, some possible mechanisms for producing the microstructures observed in volcanic rocks are proposed.

Thermal-metamorphic signature of an impact event in the Vredefort dome, South Africa

Pseudotachylitic breccias and shock deformation features related to the 2.02 Ga formation of the Vredefort dome by meteorite impact were overprinted by a static metamorphic event, the intensity of which decreased from granulite-facies ( T ≥700 °C) in the center of the dome to greenschist-facies ( T ≤400 °C) around its margins. Geobarometric estimates of 0.2–0.3 GPa for the metamorphic parageneses indicate some 8–11 km of erosion since the impact event. The strong lateral thermal gradient implied by these P-T results is attributed to the combined effects of differential uplift of mid-crustal rocks heated along a pre-impact geotherm and increased shock heating of the target crust toward the center of the impact structure. We suggest that the exceptionally high grade of metamorphism in the center of the dome may, in part, reflect an elevated regional geothermal gradient of ∼25 °C/km in the target crust due to lingering thermal effects related to the 2.05–2.06 Ga Bushveld magmatic event.

Archean crustal structure of the Kaapvaal craton, South Africa – evidence from the Vredefort dome

Abstract Crystalline Archean basement rocks in the core of the Vredefort dome present a profile through a substantial part of the middle and lower crust of the Kaapvaal craton. Previously, this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal-scale Late Archean brittle–ductile thrust zone. Lithological and structural mapping across the dome indicates, however, that the basement lithologies share a common polyphase tectonic history culminating in high-grade metamorphism and melting at ∼3.1 Ga. No evidence was found of the postulated tectonic terrane boundary, but the alleged boundary does coincide with a 1–2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses. The implications of these results for Archean regional tectonic models for the Kaapvaal craton are discussed.

The microstructural response of quartz and feldspar under shock loading at variable temperatures

Shock recovery experiments were carried out on Westerly granite and Hospital Hill quartzite targets in the peak pressure range 8 to 25 GPa, preshock temperatures of 25°, 450°, and 750°C and pulse durations of 2 to 7 μs using internally heated momentum traps and explosive plane wave generators. Optical and transmission electron microscopy analyses of quartz and feldspar shocked at 25°C revealed the previously documented progression, with increasing pressure: (1) fracturing; (2) planar fractures and shock mosaicism; (3) shock mosaicism and planar deformation features (PDFs); and (4) isotropization. This same sequence is observed for experiments at elevated preshock temperature but with specific microstructures occurring at lower pressures than those in previous experiments at room temperature. At 750°C, strong shock mosaicism, partially thermally recovered, is characteristic of feldspar shocked to 8 GPa, whereas 15 GPa is required for its development in quartz and for the generation of PDFs in both minerals. The results suggest that threshold pressures for formation of the various microstructures and phases are expected to vary systematically as a function of the preshock temperature of the target material. We suggest that PDFs are generated in the shock transition by progressive, heterogeneous, phase transformation of the crystal structure to form dense glass or high pressure polymorphs. The onset pressures for PDFs in specific crystallographic orientations is not influenced strongly by temperature, but the character of the PDFs does change as preshock temperature is increased at the same peak shock stress. The change from multiple sets of thin PDFs at low temperature to thicker single sets of PDFs at moderate temperature, and finally to complete isotropization at high temperatures, reflects a change in the phase transformation mechanism as a function of temperature. In contrast, development of shock mosaicism in quartz and feldspar occurs throughout the duration of shock loading and is better developed at elevated temperatures where the kinetics are enhanced by the additional thermal energy in the target.

Metamorphism on the Moon: A terrestrial analogue in the Vredefort dome, South Africa?

A new model is proposed to explain the origin of enigmatic fine-grained granulite facies rocks sampled from the Moon, based on observations from the Vredefort dome, South Africa. The dome is the deeply eroded central uplift of the ;300-km-diameter Vre- defort impact structure. In the dome, fine-grained granulites dis- playing poikilitic or granoblastic microstructures were formed by relatively slow cooling of shock 6 friction melts derived at T. 1350 8C. Slow cooling was achieved owing to the .7 km depth of burial of the rocks following the impact. At least some of the lunar granulitic impactites may also have formed by shock heating and slow cooling at deep levels within the central uplifts of large impact structures, without the need for additional heating by younger in- trusive or impact melt bodies.

Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures: Comment and Reply

Hart et al. (1995) postulated that large meteorite impacts may generate characteristic magnetic signatures due to thermal resetting of remanent magnetism, as a consequence of the impact process. Their postulation is based on the results of a study of the ca. 2 Ga Vredefort structure which shows that the remanent magnetism of rocks from the core of the structure was reset penecontemporaneously with the formation of the structure. While we agree with Hart et al. (1995) that the Vredefort structure is the product of a large meteorite impact at ca. 2 Ga and that the reset remanent magnetism in the rocks is consistent with the 2 Ga paleopole orientation for the region, we believe that Hart et al. failed to satisfactorily discount an alternative possibility: that the postimpact thermal event manifested by the reset magnetism reflects high, preimpact, ambient rock temperatures that are related to an earlier regional metamorphic event. Hart et al. (1995, p. 279) mentioned this possibility briefly but dis

Landscape and Landforms of the Vredefort Dome: Exposing an Old Wound

A striking 100 km-long crescent of ridges and valleys straddling the Vaal River along the border between North West and Free State provinces near the towns of Parys and Vredefort is the most obvious remnant of one of the most remarkable geological events in Earth‘s history. The Vredefort impact event 2,020 Ma ago into the ancient rocks of the Kaapvaal craton is estimated to have left a crater that was originally at least 250 km wide and over 1 km deep. The crater and its infill of broken and melted rocks have long since been stripped away by erosion, rendering the crater margins largely invisible today. However, a central region of rock that was domed upward during the impact event and that bears numerous scars of the catastrophe is still visible. The crescentic Vredefort Mountainland forms a portion of this geological feature, which is referred to as the Vredefort Dome . The landscape of the Dome owes much of its current dramatic topographic relief to the 300 Ma Dwyka glaciation , evidence of which is now being exhumed by the modern Vaal River. Large potholes , sand-blasted rock pavements and the remnants of ancient dune fields testify to more recent shifts in climate in the Mesozoic and Cenozoic. Part of the Vredefort Dome was inscribed as a UNESCO World Heritage Site in 2005.