ISSN:0148-0227    

DOI:10.1029/JB076i023p05381

年代:1971

数据来源: WILEY

摘要:

The morphology and geology of the Lappajärvi structure, Finland, suggest that it is a meteorite impact structure (astrobleme). The high central portion has been protected from glacial erosion by a resistant rock that is believed to be an impact melt. Preferential erosion of the softer basement rock has produced marginal troughs. The impact melt zone is approximately circular, with a diameter of 5–6

ISSN:0148-0227    

DOI:10.1029/JB076i023p05382

年代:1971

数据来源: WILEY

摘要:

Two circular structures in the Northern Territory, Australia, are identified as being of probable impact origin. At Liverpool Crater, 1.6 km in diameter, a ring of sandstone breccia with tangentially striking sandstone at the base is exposed. Quartz grains in the breccia exhibit planar features, mosaic structure, and cleavage, indicating moderate shock deformation. At Strangways crater, tangentially striking sandstone and siltstone units define a circular pattern about 16 km in diameter. In the interior are large outcrops of melt breccia with highly shocked inclusions and an outcrop of basement rock, which may mark a central uplift.

ISSN:0148-0227    

DOI:10.1029/JB076i023p05387

年代:1971

数据来源: WILEY

摘要:

In southern Germany a prominent north‐northwest‐facing scarp is interrupted by a roughly circular depression known as the Ries basin. Surface topography and geophysical investigations reveal a complicated crater geometry consisting of an inner zone roughly 8 km in diameter with only modest surface expression, followed outward by a concentric zone of flat to hummocky relief, which itself is bordered by the above‐mentioned scarp to the south, and by a less prominent rim to the north. The diameter of the entire depression is 22–24 km. The present crater floor is a flat plain as a result of crater filling by fallback breccias as well as fresh‐water lake and alluvial deposits. The distribution, structure, texture, composition, and mechanism of deposition of the various ejecta are important clues for reconstructing the cratering mechanism. The undisturbed crystalline basement of the Ries originally was overlain by a sequence of Mesozoic sedimentary rocks, roughly 600 meters thick. The ejecta consist of a variety of sedimentary breccias (Bunte Trümmermassen) and crystalline breccias (monomict breccias, polymict breccias, suevite) derived from these preexisting rocks. Volumetrically the sedimentary breccias dominate, forming a discontinuous ejecta blanket especially in the south. Most crystalline breccias are confined to the crater itself or to the proximity of the rim, though sporadic patches occur as far away as one crater radius. Ejecta block size decreases with increasing distance from the crater. In general, where discernible the pre‐Ries stratigraphic sequence tends to be reversed within the ejecta blanket. Suevite is the single most important breccia derived from basement rocks. It contains a variety of crystalline rock types in all stages of shock metamorphism. Its most characteristic components are aero dynamically shaped glass bombs. Detailed petrographic investigations indicate peak shock pressures in excess of 600 kb and 2000°C. Suevite is responsible for strong local negative magnetic anomalies. Oriented samples show reversed remanent magnetism. Gravity measurements have revealed a strong negative anomaly corresponding to about 7×106g mass deficiency. The Ries event has been dated by K‐Ar and fission track techniques at 14.8±0.7 m.y. and 14.0±0.6 m.y., respectively, and these dates are compatible with stratigraphic evidence

ISSN:0148-0227    

DOI:10.1029/JB076i023p05394

年代:1971

数据来源: WILEY

摘要:

A deeply eroded impact structure of probable Late Paleozoic or Early Mesozoic age is located in granites and gneisses of Hercynian age in south‐central France near the town of Rochechouart. No circular feature is now observable, but geological study of the shock‐metamorphosed rocks suggests that the original structure was about 15 km in diameter, possibly with a central uplift about 4 km in diameter. Postcrater deformation and erosion have exposed a wide variety of shock‐metamorphosed rocks in both the original crater fill and the underlying autochthonous basement. Several distinct units, exposed at different localities, form a nearly complete stratigraphic sequence similar to that observed at other impact structures. From the base upward, five units are distinguished: (1) shocked autochthonous basement rocks (granites and gneisses, containing breccias and shatter cones); (2) finely crystalline igneous impact melt; (3) red welded breccia with numerous shocked inclusions; (4) lithic breccia composed of shocked basement rock fragments without glassy inclusions; (5) glassy breccia (= suevite) containing both shocked basement rock fragments and heterogeneous glassy bodies (Fladen). Shock‐metamorphic features are ubiquitous in the rocks and include shatter cones, planar features (shock lamellae) in quartz, partly to completely isotropized feldspar, and heterogeneous glasses. The maximum age of the Rochechouart structure is established by the Hercynian orogeny (275–300 m.y.). Preliminary K‐Ar ages of 150–170 m.y., determined on the Babaudus impact melt, are considered as mini

ISSN:0148-0227    

DOI:10.1029/JB076i023p05407

年代:1971

数据来源: WILEY

摘要:

The Charlevoix structure contains exposures of rocks that solidified at the surface and that have the superficial aspect of volcanic rocks. However, they were formed in a milieu of diverse debris by agglomeration of small melted masses. K/Ar age determinations link the time of the meteoritic impact with that of the formation of this rock. Chemical analyses and Rb/Sr age determinations prove that what occurred was fusion of the basement rocks. The rock is therefore an impactite, that is, a rock formed from rocks melted or vaporized by meteoritic impact. The rock was preserved from erosion in a small depression formed by readjustment of the structure.

ISSN:0148-0227    

DOI:10.1029/JB076i023p05414

年代:1971

数据来源: WILEY

摘要:

Potassium‐argon ages obtained for the apparently igneous monzonite body of the Manicouagan‐Mushalagan Lakes structure yield an unambiguous age of 210±4 m.y. Ages obtained for a series of shock‐metamorphosed anorthosites are best interpreted as yielding a shock outgassing age of 280 to 300 m.y., and the 70‐ to 90‐m.y. age difference is thought to tentatively support the theory that this monzonite was produced by a later, impact‐induced magmatic event. One highly shocked anorthosite yielded an age essentially the same as the monzonite age, casting some doubt on this conclusion. A thermal release study of a maskelynitized anorthosite indicates that maskelynite is a fairly argon‐retentive material. The shock age of 320 m.y. determined for this material is not significantly in error because of diffusion loss subsequent to the shock event, but the effect of incomplete outgassing during the shock ev

ISSN:0148-0227    

DOI:10.1029/JB076i023p05424

年代:1971

数据来源: WILEY

摘要:

Potassium‐argon ages have been established for 34 samples related to the Brent crater in Ontario, Canada, to determine the effect of shock metamorphism on the apparent potassiumargon age of rocks. Complete radiogenic argon loss occurred in rocks shocked to peak pressures of 200 kilobars and above. Most of this argon loss, from samples shocked below the melting point, is attributed to diffusion driven by sustained high temperatures after the shock event. Rocks shocked to lower peak pressures lost variable amounts of argon, but commonly produced a potassium‐argon age nearer the time of rock crystallization than the time of the shock event. Samples from a rapidly crystallized shock melt produced low ages indicating incomplete postevent argon retention by a minimum of 15 to 30%. The most coarsely crystallized sample from the impact‐produced melt zone yielded a minimum age for the Brent impact event (414±20 m.y.). Potassium enrichment has occurred in rocks near the boundaries of the melt zone and in highly shocked (heated) rocks in the overlying breccia lens. Potassium‐argon dating results suggest that alnoite dikes occurring in the area are related to neither the potassium enrichment nor the Brent crater‐for

ISSN:0148-0227    

DOI:10.1029/JB076i023p05437

年代:1971

数据来源: WILEY

摘要:

A study of the shocked Coconino sandstone from Meteor Crater, Arizona, was undertaken to examine the role of porosity in the compression of rocks and in the formation of highpressure phases. A suite of shocked Coconino specimens collected at the crater is divided into five classes, arranged in order of decreasing quartz content. The amounts of coesite, stishovite (measured by quantitative X‐ray diffraction), and glass vary systematically with decreasing quartz content. Coesite may comprise 1/3 by weight of some rocks, whereas the stishovite content does not exceed 1%. The five classes of rocks have distinct petrographic properties, correlated with the presence of regions containing coesite, stishovite, or fused silica. Very few occurrences of diaplectic glass are observed. In the lowest stages of shock metamorphism (class 1), the quartz grains are fractured, and the voids in the rock are filled with myriads of small chips derived from neighboring grains. The fracture patterns in the individual quartz grains are controlled by the details of the initial morphology of the colliding grains. In one weakly shocked rock, it was possible to map the general direction of shock passage by recording the apparent direction of collision of individual grains. The principal mechanism of energy deposition by a shock wave in a porous material is the reverberation of shock and rarefaction waves through grains due to collisions with other grains. A one‐dimensional model of the impact process can predict the average pressure, volume, and temperature of the rock if no phase changes occur but cannot predict the observed nonuniformity of energy deposition. In all rocks shocked to higher pressure than was necessary to close the voids, high‐pressure and/or high‐temperature phases are present. Locally high pressures enduring for microseconds and high temperatures enduring for milliseconds controlled the phases of SiO2that formed in the rock. Collapsing pore walls became local hot spots into which initial deposition of energy was focused. Microcrystalline coesite in class 2 rocks occurs in symplektic regions on quartz grain boundaries that were regions of initial stress and energy concentration, or in sheared zones within the grains. The occurrence and morphology of the coesite‐rich regions can be explained only if the transformation from quartz to coesite proceeds slowly in the shock wave. In class 3 rocks, microcrystalline coesite occurs in opaque regions that surround nearly isotropic cores of cryptocrystalline coesite. The cores are interpreted to be the products of the inversion of stishovite (or a glass with Si in sixfold coordination) that initially formed in the shock front in regions of grains shocked to pressures near 300 kb. Stishovite is preserved only in the opaque regions, which are believed to have been cooler than the cores. In class 4 rocks vesicular glass occurs in core regions surrounded by opaque regions containing coesite. The relation of the glass to the coesite and quartz suggests that the glass was formed by inversion of stishovite formed above 350 kb on release to lower pressure. Class 5 rocks are composed almost entirely of glass, with vesicles uniformly distributed in the glass. These vesicles were probably formed by exsolution of water that had been dissolved in melted SiO2during passage of

ISSN:0148-0227    

DOI:10.1029/JB076i023p05449

年代:1971

数据来源: WILEY

摘要:

Quantitative determination of the coesite and stishovite content of nonporous crystalline rocks of variable composition and degree of shock metamorphism taken mainly from the Ries crater was made by chemical concentration and X‐ray powder diffractometry. The stishovite abundances range between 4 and 0.005% of the primordial quartz. The coesite abundances are between 7 and 40% of the primordial quartz. Coesite occurs as very fine grained aggregates embedded in diaplectic quartz or quartz glass with mean refractive indices below 1.48. Stishovite is included within the planar deformation structures of diaplectic quartz having mean refractive indices between 1.546 and 1.472. Traces of stishovite are also present within diaplectic glass. On the basis of these data, of microscopical observations, and of experimental data on the shock compression of quartz, as well as the thermal stability of coesite and stishovite, it is concluded that stishovite and coesite are formed and metastably preserved in the peak pressure ranges from about 120 kb to 450 kb and from about 300 kb to 550 kb, respectively. Stishovite is considered to crystallize during shock compression from a high‐pressure silica phase with silicon in sixfold coordination. Coesite obviously crystallizes behind the shock front, that is, during pressure release, from a stishovite‐like high‐pressure phase and/or after pressure release from a silica phase of short‐range order with fourfold coordination of silicon. It is only formed by shocks transforming the primary quartz completely or almost completely to the high‐pressure phase within the period of shock compression. The influence of ‘secondary’ thermal metamorphism of shocked rocks on the metastable persistence of coesite and stishovite in various kinds of impact breccias shows that both high‐pressure polymorphs may be used as very sensitive temperature indicators for any an

ISSN:0148-0227    

DOI:10.1029/JB076i023p05474

年代:1971

数据来源: WILEY