Shock deformation confirms the impact origin for the Cerro do Jarau,Rio Grande do Sul,Brazil, structure

Cerro do Jarau is a conspicuous, circular morpho‐structural feature in Rio Grande do Sul State (Brazil), with a central elevated core in the otherwise flat “Pampas” terrain typical for the border regions between Brazil and Uruguay. The structure has a diameter of approximately 13.5 km. It is centered at 30^o12′S and 56^o32′W and was formed on basaltic flows of the Cretaceous Serra Geral Formation, which is part of the Paraná‐Etendeka Large Igneous Province (LIP), and in sandstones of the Botucatu and Guará formations. The structure was first spotted on aerial photographs in the 1960s. Ever since, its origin has been debated, sometimes in terms of an endogenous (igneous) origin, sometimes as the result of an exogenous (meteorite impact) event. In recent years, a number of studies have been conducted in order to investigate its nature and origin. Although the results have indicated a possible impact origin, no conclusive evidence could be produced. The interpretation of an impact origin was mostly based on the morphological characteristics of the structure; geophysical data; as well as the occurrence of different breccia types; extensive deformation/silicification of the rocks within the structure, in particular the sandstones; and also on the widespread occurrence of low‐pressure deformation features, including some planar fractures (PFs). A detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with PFs and feather features (FFs), as well as a number of quartz grains exhibiting planar deformation features (PDFs). While most of these latter grains only carry a single set of PDFs, we have observed several with two sets, and one grain with three sets of PDFs. Consequently, we here propose Cerro do Jarau as the seventh confirmed impact structure in Brazil. Cerro do Jarau, together with Vargeão Dome (Santa Catalina state) and Vista Alegre (Paraná State), is one of very few impact structures on Earth formed in basaltic rocks.     

Gravity and Magnetic Investigations in the Haughton Impact Structure,Devon Island,Canada

Abstract— The results of a new gravity survey show that the Haughton impact structure is associated with a 24 km diameter negative Bouguer gravity anomaly with a maximum amplitude of ?12 mgal. A local minimum with a half-width of 2 km and an amplitude of ?4 mgal is located at the center of the structure. A positive magnetic total field anomaly with a half-width of 0.6 km and an amplitude of 700 nT coincides with the local central gravity anomaly. The overall negative gravity anomaly is explained by lowered rock densities due to impact-related fracturing in the crater area. The central gravity and magnetic anomalies are believed to be due to highly shocked and heated sedimentary and crystalline basement rocks forming the unexposed peak of the central uplift in the Haughton impact structure.     

Geophysical signature of the Tunnunik impact structure,Northwest Territories,Canada

In 2011, the discovery of shatter cones confirmed the 28 km diameter Tunnunik complex impact structure, Northwest Territories, Canada. This study presents the first results of ground‐based electromagnetic, gravimetric, and magnetic surveys over this impact structure. Its central area is characterized by a ~10 km wide negative gravity anomaly of about 3 mGal amplitude, roughly corresponding to the area of shatter cones, and associated with a positive magnetic field anomaly of ~120 nT amplitude and 3 km wavelength. The latter correlates well with the location of the deepest uplifted strata, an impact‐tilted Proterozoic dolomite layer of the Shaler Supergroup exposed near the center of the structure and intruded by dolerite dykes. Locally, electromagnetic field data unveil a conductive superficial formation which corresponds to an 80–100 m thick sand layer covering the impact structure. Based on the measurements of magnetic properties of rock samples, we model the source of the magnetic anomaly as the magnetic sediments of the Shaler Supergroup combined with a core of uplifted crystalline basement with enhanced magnetization. More classically, the low gravity signature is attributed to a reduction in density measured on the brecciated target rocks and to the isolated sand formations. However, the present‐day fractured zone does not extend deeper than ~1 km in our model, indicating a possible 1.5 km of erosion since the time of impact, about 430 Ma ago.     

Geophysical research on the Kärdla impact structure,Hiiumaa Island,Estonia

Abstract— The 4 km wide and 500 m deep circular Kärdla impact structure in Hiiumaa Island, Estonia, of middle Ordovician age (～455 Ma), is buried under Upper Ordovician and Quaternary sediments. To constrain the geophysical models of the structure, petrophysical properties such as magnetic susceptibility, natural remanent magnetization (NRM), density, electrical conductivity, porosity and P-wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area. The results were used to interpret the central gravity anomaly of ?3 mGal and the magnetic anomaly of ?100 nT and also the surrounding weak positive anomalies revealed by high precision survey data. The unshocked granitic rocks outside the structure have a mean density of ～2630 kgm^?3. Their shocked counterparts have densities of ～2400 kgm^?3 at a depth of ～500 m, increasing up to 2550 kgm^?3 at a depth of 850 m. Porosity and electrical conductivity decrease, but P-wave velocity increases as density increases away from the impact point. Thus, the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for Kärdla. As in many other impact structures, the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks. A new geophysical and geological model of the Kärdla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks, together with structural data from numerous boreholes. An important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores.     

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Cover: Residual gravity anomaly map (displayed in relief) of the Cerro do Jarau structure. Bruno B. Giacomini et al. discuss the structure in their article on pp. 565–583 . Image courtesy of Bruno B. Giacomini and Emilson P. Leite.     

Hydrothermally enhanced magnetization at the center of the Haughton impact structure?

Haughton is a ~24 Myr old midsize (apparent diameter 23 km) complex impact structure located on Devon Island in Nunavut, Canada. The center of the structure shows a negative gravity anomaly of ?12 mGal coupled to a localized positive magnetic field anomaly of ~900 nT. A field expedition in 2013 led to the acquisition of new ground magnetic field mapping and electrical resistivity data sets, as well as the first subsurface drill cores down to 13 m depth at the top of the magnetic field anomaly. Petrography, rock magnetic, and petrophysical measurements were performed on the cores and revealed two different types of clast‐rich polymict impactites: (1) a white hydrothermally altered impact melt rock, not previously observed at Haughton, and (2) a gray impact melt rock with no macroscopic sign of alteration. In the altered core, gypsum is present in macroscopic veins and in the form of intergranular selenite associated with colored and zoned carbonate clasts. This altered core has a natural remanent magnetization (NRM) four to five times higher than materials from the other core but the same magnetic susceptibility. Their magnetization is still higher than the surrounding crater‐fill impact melt rocks. X‐ray fluorescence data indicate a similar proportion of iron‐rich phases in both cores and an enrichment in silicates within the altered core. In addition, alternating‐field demagnetization results show that one main process remagnetized the rocks. These results support the hypothesis that intense and possibly localized post‐impact hydrothermal alteration enhanced the magnetization of the clast‐rich impact melt rocks by crystallization of magnetite within the center of the Haughton impact structure. Subsequent erosion was followed by in situ concentration in the subsurface leading to large magnetic gradient on surface.     

Shatter cones and planar deformation features confirm Santa Marta in Piauí State,Brazil, as an impact structure

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 Piauí 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.     

Geophysical studies of the Montagnais impact crater,Canada

Abstract— The 45-km diameter Montagnais impact structure, Nova Scotia, Canada, is characterized by a positive, circular 8 mGal gravity anomaly associated with its central uplift. The negative gravity anomaly, which is expected for a complex crater of this size, is not observed within the structure, and magnetic data lack any well-defined, crater-related signature. The absence of a negative gravity anomaly implies that no low-density zone generally related to fracturing and brecciation exists. Since Montagnais appears well preserved, this zone has not been removed by erosion. Its formation may have been impeded due to the lack of competency in the target rocks. The crater was formed in a shallow marine environment where the lack of strength in the unconsolidated sediments may have prevented the preservation of voids and fractures that cause a negative gravity anomaly as observed over other impact craters. Additionally, the efficient absorption of impact energy by unconsolidated target material may have inhibited fracture/void development. Although the gravity signature of impact craters formed on land is well known, structures occurring in unconsolidated target material, such as continental shelf environments, constitute another signature that should also be recognized.     

Reconstruction Of The Subsurface Structure Of The Marquez Impact Crater In Leon County,Texas, Usa,Based On Well‐Log And Gravity Data

Abstract— In Leon County, Texas, USA, the Marquez Dome, an approximately circular 1.2 km diameter zone of disturbed Cretaceous rocks surrounded by shallow dipping Tertiary sediments, has been interpreted by Gibson and Sharpton (1989) and Sharpton and Gibson (1990) as the surface expression of a buried complex impact crater. New gravity and magnetic anomaly data collected over the Marquez Dome have been combined with well‐log and seismic reflection information to develop a better estimate of the overall geometry of the structure. A three‐dimensional model constructed to a depth of 2000 m from all available information indicates a complex crater 13 km in diameter with an uplift in the center of at least 1120 m. The zone of deformation associated with the cratering event is limited to a depth of <1720 m. No impact breccias were recovered in drilling at two locations, 1.1 and 2 km from the center of the structure, and the central uplift may be the only prominent remnant of this impact into unconsolidated, water‐rich sediments. The magnetic anomaly field shows no correlation with the location and extent of the structure.     

Geophysical investigation of the Colônia structure,Brazil

The origin of the nearly circular Colônia structure, located at the southwestern edge of the city of São Paulo, Brazil, has been the subject of a long‐standing debate, ever since the 1960s when the structure was first investigated by geophysical methods. The structure still raises interest for geological research, as its sedimentary infill holds important paleoclimatic information about the evolution of the tropical rainforest, as well as the interplay between the South American summer monsoon, the Intertropical Convergence Zone, and the southern Westerly wind belt—for possibly as long as several million years. In addition, the search for evidence to conclusively establish the origin of this structure continues, and the answer most likely lies in the lower portions of the basin's sedimentary infill, which also holds a significant potential for underground water resources. Here, we present the results from recent seismic (reflection and HVSR), gravimetric, and geoelectrical surveys. They have provided a reliable image of the sedimentary infill, and the maximum depth to basement within the structure has been constrained consistently by the different methods to approximately ?400 m. The geophysical data have also allowed to map the lateral contact between the crystalline basement rocks and the sedimentary infill, which indicates a diameter of approximately 2.8 km for the sedimentary basin, with 3.6 km being the diameter of the outermost limit of the structure. A total of six seismic stratigraphic boundaries were identified within the sedimentary infill, providing a framework for the planning of a deep drilling campaign and subsequent sampling program aimed at geological and paleoclimatology studies.     

A lunar crustal gravity model across the Triesnecker-Hyginus area,Mare Tranquillitatis,and Mare Fecunditatis

LOS Bouguer gravity anomalies have been calculated from a low altitude LOS free air Doppler gravity profile across northern Mare Fecunditatis, southern Mare Tranquillitatis and the Aridaeus Rille. The Hyginus-Triesnecker area has been included in model calculations, though here only free air anomalies are present. A crustal density model has been fitted to the Bouguer anomalies and to the free air anomalies in the case of the Hyginus-Triesnecker area.On a regional scale northern Fecunditatis has Bouguer anomalies up to 80 mgal and lithostatic stresses of 29 bar and thus is nearly in isostatic equilibrium. Tranquillitatis can be divided into three regions of different crustal structure: (1) northern Tranquillitatis with only minor free air gravity anomalies is more or less in isostatic balance, (2) the southeastern region with Bouguer anomalies to –100 mgal and lithostatic stresses of –73 bar has a considerable mass deficit, (3) the southwestern basin is dominated by the local structure Lamont with a Bouguer maximum of 200 mgal and extremely high lithostatic stresses of 285 bar.The Bouguer minimum of –180 mgal of the Aridaeus area has been modelled by two alternative models: (i) a crustal thickening of 33 km and associated lithostatic stresses of –164 bar, and (ii) a crustal thickening of 20 km plus a low density intrusion. The free air maximum of the Hyginus-Triesnecker area has been fitted by a mantle plug connected with stresses of 116 bar.As the old irregular maria could not sustain large mascon stresses, it has been concluded that the local high stresses of Lamont, Aridaeus, and Hyginus-Triesnecker have been evolved after the impacts of the circular maria. Intrusional activities in these areas could have proceeded to fault zones generated by the large impacts.Contribution No. 211, Institut für Geophysik der Universität Kiel, F.R.G.     

Geology and impact features of Vargeão Dome,southern Brazil

Abstract– Vargeão Dome (southern Brazil) is a circular feature formed in lava flows of the Lower Cretaceous Serra Geral Formation and in sandstones of the Paraná Basin. Even though its impact origin was already proposed in the 1980s, little information about its geological and impact features is available in the literature. The structure has a rim‐rim diameter of approximately 12 km and comprises several ring‐like concentric features with multiple concentric lineaments. The presence of a central uplift is suggested by the occurrence of deformed sandstone strata of the Botucatu and Pirambóia formations. We present the morphological/structural characteristics of Vargeão Dome, characterize the different rock types that occur in its interior, mainly brecciated volcanic rocks (BVR) of the Serra Geral Formation, and discuss the deformation and shock features in the volcanic rocks and in sandstones. These features comprise shatter cones in sandstone and basalt, as well as planar microstructures in quartz. A geochemical comparison of the target rock equivalents from outside the structure with the shocked rocks from its interior shows that both the BVRs and the brecciated sandstone have a composition largely similar to that of the corresponding unshocked lithologies. No traces of meteoritic material have been found so far. The results confirm the impact origin of Vargeão Dome, making it one of the largest among the rare impact craters in basaltic targets known on Earth.     

An integrated geophysical and geological study of the Monturaqui impact crater,Chile

Abstract— –The Monturaqui impact crater (350–370 m in diameter and 0.1 Ma old), located in a remote area in northern Chile, was surveyed in December 2003 with detailed geophysics (gravity and magnetics), topography, petrophysics, and geology. The geology of the Monturaqui area is characterized by a basement of Paleozoic granites overlain by Pliocene ignimbrite units. No impact breccia was found in the area. The granites are the main lithology affected by the impact. Although the granite samples analyzed did not show evidence of shock metamorphism, quartz, and to a lesser extent feldspar and biotite grains from impactite samples exhibit different degrees of shock, ranging from planar microdeformation and cleavage to the development of intense planar deformation features (PDFs) and diaplectic glasses in some grains. The differential GPS survey allowed the creation of a detailed digital elevation model of the crater. Its dimensions are 370 m along the east‐west direction, 350 m along the north‐south direction, and ～～34 m deep. The crater exhibits a circular morphology with a preferred northwest‐southeast elongation that coincides with the steepest slopes (～～35°) on the southeast edge. The newly acquired gravity data shows a negative anomaly of ～～1 mGal at the center and allowed the creation of a 3‐D model with a RMS error of <0.1 mGal, which supports the predictions of a fracturing‐induced low‐density granitic layer on top of the unfractured basement.     

An appraisal of the Serra da Cangalha impact structure using the Euler deconvolution method

Abstract— The applicability of the Euler deconvolution method in imaging impact crater structure vis‐à‐vis delineation of source depth of the circular magnetic anomaly and/or basement depth beneath the crater is addressed in this paper. The efficacy of the method has been evaluated using the aeromagnetic data obtained over the Serra da Cangalha impact crater, northeastern Brazil. The analyses of the data have provided characteristic Euler deconvolution signatures and structural indices associated with impact craters. Also, through the interpretation of the computed Euler solutions, our understanding of the structural features present around the impact structure has been enhanced. The Euler solutions obtained indicate shallow magnetic sources that are interpreted as possibly post‐impact faults and a circular structure. The depth of these magnetic sources varies between 0.8 and 2.5 km, while the Precambrian basement depth was found at ?1.5 km. This is in good agreement with the estimates of the Precambrian basement depth of about 1.1 km, calculated using aeromagnetic data. The reliability of the depth solutions obtained through the implementation of the Euler method was confirmed through the use of the existing information available in the area and the result of previous studies. We find that the Euler depth solutions obtained in this study are consistent with the results obtained using other methods.     

The Corossol structure: A possible impact crater on the seafloor of the northwestern Gulf of St. Lawrence,Eastern Canada

We report on a 4.1 (±0.2) km diameter and 185 m deep circular submarine structure exposed on the seabed in >40 m water depths in the northwestern Gulf of St. Lawrence (Eastern Canada) from the analysis of high‐resolution multibeam bathymetric and seismic data. The presence of a circular form characterized by a central uplift and concentric rings resembles the morphology and geometry of complex meteorite impact structures. Also, other origins, such as kimberlites, intrusions, karsts, or diapirs, can be eliminated on geological criteria. A single 4 cm long breccia fragment recovered from the central uplift has numerous glassy droplets of fluorapatite composition, assumed to be impact melts, and a single quartz grain with planar intersection features thought to be shock‐induced planar deformation features (PDFs). The absolute age of this possible impact structure is unknown, but its geological setting indicates that it was formed long after the Mid‐Ordovician and before regional pre‐Quaternary sea‐level lowstands. Present results outline the need for further examination to confirm an impact origin and to precisely date the formation of the structure.     

Cloud Creek structure,central Wyoming,USA: Impact origin confirmed

Abstract— The circular Cloud Creek structure in central Wyoming, USA is buried beneath ?1200 m of Mesozoic sedimentary rocks and has a current diameter of ?7 km. The morphology/morphometry of the structure, as defined by borehole, seismic, and gravity data, is similar to that of other buried terrestrial complex impact structures in sedimentary target rocks, e.g., Red Wing Creek in North Dakota, USA. The structure has a fault‐bordered central peak with minimum diameter of ?1.4 km, composed predominantly of Paleozoic carbonates thickened by thrust faulting and brecciation, and is elevated some 520 m above equivalent strata beyond the outer rim of the structure. There is a ?1.6 km wide annular trough sloping away from the central peak (maximum structural relief, 300 m) and terminated by a detached, fault‐bounded, rim anticline. The youngest rocks within the structure are Late Triassic (Norian?) clastics and these are overlain unconformably by post‐impact Middle Jurassic (Bathonian?) sandstones and shales. Thus, the formation of the Cloud Creek structure is dated chronostratigraphicly as ?190 ± 20 Ma. Reported here for the first time are measurements of planar deformation features (PDFs) in shocked quartz grains in thin sections made from drill cuttings recovered in a borehole drilled at the southern perimeter of the central peak. Other, less definitive microstructures consistent with impact occur in samples collected from boreholes drilled into the central peak and rim anticline. The shock‐metamorphic evidence confirms an impact origin for the Cloud Creek structure.     

Geophysical survey of the proposed Tsenkher impact structure,Gobi Altai,Mongolia

Abstract– We have performed forward magnetic and gravity modeling of data obtained during the 2007 expedition to the 3.7 km in diameter, circular, Tsenkher structure, Mongolia, in order to evaluate the cause of its formation. Extensive occurrences of brecciated rocks, mainly in the form of an ejecta blanket outside the elevated rim of the structure, support an explosive origin (e.g., cosmic impact, explosive volcanism). The host rocks in the area are mainly weakly magnetic, silica‐rich sandstones, and siltstones. A near absence of surface exposures of volcanic rocks makes any major volcanic structures (e.g., caldera) unlikely. Likewise, the magnetic models exclude any large, subsurface, intrusive body. This is supported by an 8 mGal gravity low over the structure indicating a subsurface low density body. Instead, the best fit is achieved for a bowl‐shaped structure with a slight central rise as expected for an impact crater of this size in mainly sedimentary target. The structure can be either root‐less (i.e., impact crater) or rooted with a narrow feeder dyke with relatively higher magnetic susceptibility and density (i.e., volcanic maar crater). The geophysical signature, the solitary appearance, the predominantly sedimentary setting, and the comparably large size of the Tsenkher structure favor the impact crater alternative. However, until mineralogical/geochemical evidence for an impact is presented, the maar alternative remains plausible although exceptional as it would make the Tsenkher structure one of the largest in the world in an unusual setting for maar craters.     

Geology and impact features of Riachão structure,northern Brazil

Riachão, located at S7°42′/W46°38′ in Maranhão State, northeastern Brazil, is a complex impact structure of about 4.1 km diameter, formed in Pennsylvanian to Permian sedimentary rocks of the Parnaíba Basin sequence. Although its impact origin was already proposed in the 1970s, information on its geology and shock features is still scarce in the literature. We present here the main geomorphological and geological characteristics of the Riachão impact structure obtained by integrated geophysical and remote sensing analysis, as well as geological field work and petrographic analysis. The identified lithostratigraphic units consist of different levels of the Pedra de Fogo Formation and, possibly, the Piauí Formation. Our petrographic analysis confirms the presence of shock‐diagnostic planar microdeformation structures in quartz grains of sandstone from the central uplift as evidence for an impact origin of the Riachão structure. The absence of crater‐filling impact breccias and melt rocks, shatter cones, as well as the restricted occurrence of microscopic shock effects, suggests that intense and relatively deep erosion has occurred since crater formation.     

Morphologic and gravimetric investigations of Bell and Eisila Regiones on Venus

Bell Regio is a highland fragment south of Ishtar Terra, extending 1300 km in N-S direction and 900 km in E-W direction. South of this region Eisila Regio is located with an E-W extension of 8000 km and a width of 2000 km. Bell Regio consists of two large massifs: a northern massif with maximum altitudes of 2.5 to 3.0 km above the 6051 km datum and with a semi-corona (other coronae on Venus are associated with volcanic-tectonic processes) and a southern massif with a maximum of 4 to 4.5 km above the datum. The possible shield volcano Tepev Mons of 250 km in diameter is superimposed on the southern massif. It shows a radar dark crater of 40 km diameter on its eastern flank, a crater-like feature of 15 km diameter on the top and a radar bright area extending from the dark crater across the summit. South of Tepev Mons are several volcanic structures with summit depressions. The crest of Bell Regio exhibits a N-S extending fossa system. The whole fresh appearing plain-like area has been classified as rather young compared to other units. Gravity data show a maximum of 33 mGal at Bell Regio and 35 mGal at eastern Eisila Regio. The basins north and south of the highland fragments are associated with gravity lows.Density models have been calculated along the gravity profile Rev. 163 of Pioneer Venus Orbiter across Bell and Eisila Regiones assuming Airy isostatic compensation of the topography and considering several boundary conditions (e.g. mean crustal thickness T<- 100 km). There are two groups of density models in the case of Airy compensation. In the first group global total compensation is assumed along the profile and regional partial compensation for Bell and Eisila Regiones. This solution gives a range of possible models with 10 km <- T <- 100 km and a partial compensation for Bell and Eisila Regiones between 12% and 55%. Thus these two highland fragments show subsurface surplus masses.The second group of models considers for the whole profile total compensation with a global T <- 100 km and a regional very large depth of compensation for Bell and Eisila Regiones, i.e. T > 100 km.The highland of Beta Regio has, like Bell Regio, a N-S rifting system, volcanic structures, a fresh appearing plain-like surface and either deep-seating compensating masses or near surface surplus masses. Bell can be considered as little sister of Beta. The geological and geophysical results imply a volcanic-tectonic uplift over a hot spot. The conditions of Atla Regio in eastern Aphrodite Terra are similar. Thus the existence of volcanic-tectonic uplifts support the important role of hot spot volcanism on Venus.Contribution Nr. 343, Institut für Geophysik der Universität Kiel, F.R.G.     

The Bosumtwi meteorite impact structure,Ghana: A magnetic model

Abstract— A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (～1.07 Ma) structure with a diameter of ～10.5 km is exposed within early Proterozoic Birimian—Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high‐resolution, low altitude (～70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, ～12 km in diameter. The central‐north part of the lake reveals a central negative magnetic anomaly with smaller positive side‐anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact‐melt rock or melt‐rich suevite bodies. Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.