Lauri J. Pesonen

The terrestrial impact cratering record

Approximately 130 terrestrial hypervelocity impact craters are currently known. Due to variations in preservation and in geologic knowledge, this sample is biased towards young ( x 20 km) craters on the cratons of Australia, Europe (including the former U.S.S.R.) and North America. The rate of discovery of new craters is 3–5 craters per year. Although modified by erosion, terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies, such as the Moon. Terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters, which can be used to constrain interpretations of lunar samples and models of crater formation. Due to erosion and its effects on form, terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects. These include: shatter cones, microscopic planar deformation features, solid-state and fusion glasses, high pressure polymorphs and whole rock melting and vaporization. Shock recovery experiments indicate that these features occur over shock pressures of ⩾5 GPa to >x 100 GPa. Terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact-induced fracturing. They include gravity and magnetic lows and reductions in seismic velocity. The gravity anomalies are seldom greater than ~ 30 mGal, due to the limiting effects of lithostatic pressure on fracturing. At large complex craters, the gravity signature may include a central relative gravity high, due to uplift, and short wavelength central magnetic anomalies, due to a variety of processes. Much current work is focused on the effects of impact on earth evolution. Previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from Cretaceous-Tertiary boundary sites as resulting from a major impact. Suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record. By analogy with the lunar record and modelling of the effects of very large impacts, it has been proposed that biological and atmospheric evolution of the Earth could not stabilize before the end of the late heavy bombardment ~ 3.8 Ga ago. The present terrestrial cratering rate is 5.4 ± 2.7 × 10−15 km−2a−1 for a diameter ⩾ 20 km. This represents a local threat on historic time scales. On a global scale, a major impact sufficient to cripple human civilization severely will occur on time scales of ~ 106 a.

Crustal evolution of Fennoscandia—palaeomagnetic constraints

Abstract Palaeomagnetic poles from Fennoscandia, ranging in age from Archaean to Tertiary, are compiled and graded using a modified Briden-Duff classification scale. An new “filtering” technique is applied to select only the most reliable poles for analysis. The filtering takes into account the following information: 1. (1) source block of rock unit, 2. (2) age of rock, 3. (3) age of magnetization component, 4. (4) scatter of palaeomagnetic directions, 5. (5) information from multicomponent analysis of natural remanent magnetization (NRM), 6. (6) whether the pole considered belongs to a cluster or subcluster of poles, 7. (7) magnetic polarity and 8. (8) the authors original assignment of results. Data are still insufficient for the drawing of separate Apparent Polar Wander Paths (APWP) for different blocks or cratons of Fennoscandia. Treating Fennoscandia as a single plate, a new APWP from Archaean to Permian is constructed. From the five previously drawn APWP loops (or “hairpins”), only one, the Jatulian loop (2200-2000 Ma), disappears in filtering. The loops during 1925-1700 Ma and during 1100-800 Ma ago are linked to Svecofennian and Sveconorwegian orogenies, respectively. Palaeomagnetic data support the concept that these orogenies took place episodically; three distinct orogenic pulses (early, middle and late) can be distinguished in the cluster plots of palaeopoles. The drift history of Fennoscandia from Archaean to Permian is presented. During most of geological history, Fennoscandia has occupied low to moderate latitudes and undergone considerable latitudinal shifts and rotations. The Svecofennian and Sveconorwegian orogenies have different kinematic characteristics. During the Svecofennian orogeny, Fennoscandia drifted slowly while rotating a large amount in an anticlockwise sense. During the Sveconorwegian orogeny, it drifted rapidly and rotated first clockwise and then anticlockwise. The most striking feature in the drift velocity curves is, however, the pronounced maxima in the latitudinal drift and rotation rates (~ 9 cm/yr and ~ 0.8°/Ma, respectively) during the late Subjotnian-Jotnian anorogenic magmatism and rifting phase (~1450-1250 Ma ago), possibly reflecting the passage of Fennoscandia across a thermal upwelling (hotspot) at equatorial latitudes. The use of palaeomagnetism in delineating and dating movements between blocks is demonstrated with three examples from the POLAR Profile area, the northernmost section of the European Geotraverse.

The drift of the Fennoscandian and Ukrainian Shields during the Precambrian: a Palaeomagnetic analysis

Abstract A revised Precambrian (2.85−0.6 Ga) Apparent Polar Wander Path (APWP) for the Fennoscandian Shield, based on a new compilation and analysis of data, is presented. In fitting the APW path to successive Grand Mean Palaeomagnetic poles (GMPs), we applied the spherical spline technique originally developed by Jupp and Kent in 1987. The position and orientation of the Fennoscandian Shield during 2.85−0.6 Ga was determined from the GMPs. Major palaeoclimatological findings are used to constrain the palaeomagnetic interpretation of palaeolatitudes. The general drift of Fennoscandia, from relatively high latitudes in the late Archaean-Early Proterozoic to nearly equatorial latitudes in the Middle Proterozoic, correlates with palaeoclimatological indications that a period of cold climate was followed by one of warm climate during this time interval. From the continuous APWP the APW velocities and latitudinal drift velocities of the shield were calculated. An accumulated APW curve was also calculated. The palaeomagnetic data are irregularly distributed and some periods are rather poorly represented. This means that the calculated velocities can sometimes be artifacts of sampling. Late Archaean and Early Proterozoic (2.85−1.90 Ga) data are too sparse to make these calculations meaningful and velocity calculations are therefore restricted to data of 1.90 Ga and younger ages. The accumulated APW curve shows a number of linear segments with varying slopes, indicating sudden changes in drift rate. During the Middle Proterozoic (1.90−1.35 Ga) there was a period when the rate of APW was constant and low and that of latitudinal drift also was low. This pattern changed at ca. 1.35 Ga, and the following Middle-Late Proterozoic period can be described by rapid APW and strongly fluctuating drift velocities. Jotnian rifting and the intrusion of numerous dyke swarms (at ca. 1.25 Ga) correlate with this shift in rate. These changes are attributed to changes in plate configuration. A new database for the Ukrainian Shield is also presented, and GMPs in the 2.32−1.20 Ga range are defined. The database is still inadequate and the comparison of the Ukrainian and Fennoscandian drift histories is therefore tentative. Similarities in position, latitudinal drift and rotation during the Early-Middle Proterozoic are, nevertheless, evident. A close relationship between the shields in this period is consistent with the low APW rate of Fennoscandia, indicating that Fennoscandia may have been part of a larger continent, including the Ukraine, at that time. At ca. 1.2 Ga, the latitudinal position of Ukraine differed significantly from that of Fennoscandia, suggesting that the large shield split up between ca. 1.35 and 1.2 Ga. This would explain the change in APW rate at 1.35 Ga. The subsequent increase in rate was due to a reduction in the size of the shield. The discrepancy in palaeopositions of Fennoscandia and Ukraine at 1.2 Ga led Mikhailova and Kravchenko to suggest a late Precambrian time (1.07−0.57 Ga) for the accreation of Fennoscandia to the East European Platform (EEP). This may be correct as the rate of APW for Fennoscandia decreased in the late Precambrian, reflecting such a consolidation.

Impact craters and craterform structures in Fennoscandia

Abstract Sixty-two, nearly circular, topographic, morphological or geophysical structures in Fennoscandia and Estonia were studied to discover their origin. The structures were divided into five classes. Fifteen were caused by meteorite impacts (class A and a). Four of them (Lappajarvi, Janisjarvi, Mien and Dellen) contain large volumes of impact melt. The age of the recognized impact craters varies from pre-historic (3500 B.C.) to 700 Ma and their diameters from 0.04 km to 55 km. The majority of the other structures are probable (class B) or possible (class C) craters for which there is not yet sufficient proof of impact origin. There is increasing evidence that some of the large arcuate morphological or geophysical structures (class E) represent the deeply eroded scars of very old craters but, owing to the lack of identified shock metamorphic features or impact-generated rocks (e.g., allogenic breccias), they cannot so far be classified as impact craters. So far, no craterform structure or ejecta layer of Archaean or Early Proterozoic age has been found in Fennoscandia, although, statistically, remnants of ancient cratering events should be found in the Fennoscandian shield. New ways of searching for these structures are proposed. The impact cratering rate in Fennoscandia is 2.4 × 10−14 km−2a−1 (N = 12, the very small craters are omitted), which corresponds to about two events per 100 Ma for the last 700 Ma. This is a minimum estimate and is higher than the global cratering rate.

Catalogue of palaeomagnetic directions and poles from Fennoscandia: Archaean to tertiary

Abstract Palaeomagnetic data from Fennoscandia ranging from the Archaean to the Tertiary have been compiled into a catalogue. The data are presented in table format, listing Precambrian data according to tectonomagmatic blocks and Late Precambrian-Phanerozoic data according to geological periods. Each pole is graded with the modified Briden-Duff classification scheme. The catalogue (complete to the end of 1988) contains 350 entries from 31 tectonomagmatic blocks and/or geological periods. Normal and reversed polarity data are listed separately to allow polarity asymmetries to be studied. Each entry also has an indexed abstract summarizing relevant information, such as the age of the rock, the age of the natural remanent magnetization and the basis for the assigned reliability grade. All the data are stored in the palaeomagnetic data bank, which will be updated annually with new data. The catalogue is the basic source of data for the microcomputer-based palaeomagnetic database for Fennoscandia now being compiled.

Impacts in Precambrian shields

The Impact Cratering Record of Fennoscandia - A New Look at the Database.- Re-examining Structural Data from Impact Craters on the Canadian Shield in the Light of Theoretical Models.- Popigai Crater: General Geology.- Geophysics and Petrophysics of the Popigai Impact Structure, Siberia.- U-Pb Analyses of Zircons from the Popigai Impact Structure, Russia: First Results.- Impact-Generated Hydrothermal Systems: Data from Popigai, Kara, and Puchezh-Katunki Impact Structures.- Geology and Petrography of the Zapadnaya Impact Crater in the Ukrainian Shield.- Bosumtwi Impact Crater, Ghana: A Remote Sensing Investigation.- Geochemistry of Soils from the Bosumtwi Impact Structure, Ghana, and Relationship to Radiometric Airborne Geophysical Data.- Tektite Origin in Oblique Impacts: Numerical Modeling of the Initial Stage.- Geology and Magnetic Signatures of the Neugrund Impact Structure, Estonia.- Natural Resources of the Kardla Impact Structure, Hiiumaa Island, Estonia.- Seismic Correlation of the Mjolnir Marine Impact Crater to Shallow Boreholes.- Numerical Modeling of Impacts into Shallow Sea.

The petrophysical classification of meteorites

SummaryPetrophysical properties (susceptibility, intensity of the Natural Remanent Magnetisation (NRM) and bulk density) of 489 meteorite samples from 368 meteorites are discussed. The samples, obtained from Finnish meteorite collections, represent all chemical-petrological meteorite classes and their groups. This meteorite petrophysical database has many potential applications in the geophysical studies of extraterrestrial bodies (planets and their moons, asteroids, meteorite parent bodies, etc.). Here we use the database to classify meteorites rapidly and non-destructively by applying the petrophysical classification scheme developed by Kukkonen and Pesonen [10]. For example, the main classes and many groups form distinct clusters in petrophysical relation diagrams such as susceptibility vs. density or NRM vs. susceptibility. The petrophysical classification method was tested on 24 meteorites from Czechoslovak, 3 from Swedish and one from Australian collections. The chemical-mineralogical classifications of these meteorites were previously known. The subjective classification method was also compared with a mathematical cluster analysis. The subjective classification technique was successful in 64% to 93% of the cases whereas the mathematical analysis was successful in 57% to 82% of the cases. The failures can be attributed to (i) non-uniqueness problems (cluster plots overlap) and (ii) effects of porosity, self-demagnetisation, electrical conductivity and frequency on measured values, or to biasing caused by small sample size.

Palaeomagnetism of the Lappajärvi impact structure, western Finland

Abstract Palaeomagnetic and rock magnetic results are presented for rocks from the Lappajarvi meteorite impact structure, western Finland. Oriented samples were collected from the melt rocks in the central island of the crater and of the Palaeoproterozoic (Svecofennian) target rocks at the crater rim. The natural remanent magnitization (NRM) of the melt rocks is generally weak and metastable during demagnetization. The characteristic NRM component, carried by pyrrhotite, yields a palaeomagnetic pole (latitude 55.4°N, longitude 152.6°E, dp = 6.2°, dm = 8.7/dg, 6 sites) suggesting an age of ~ 195 Ma for the impact. The results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations. The palaeomagnetic age differs from the 40Ar-3939Ar age of 77 Ma for the impact. Assuming that the 40Ar-39Ar age is correct, the discrepancy between palaeomagnetic and radiometric ages is best explained by post-impact (and post-cooling) tilting of the melt layer. The tilting (15°) may be due to isostatic rebounds along joints in the melt. Impact remanence has also been isolated in a few target rocks at the rim. This poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1.9 Ga old Svecofennian direction. This interpretation is supported by the observation that the NRM intensity is enhanced in specimens having the impact overprint. The palaeomagnetic results are consistent with earlier interpretations based on geophysical, morphological, mineralogical and geochemical data that the Lappajarvi structure is a complex meteorite impact site less than 200 Ma old.

The impact cratering record of Fennoscandia

The current database of craterform structures in Fennoscandia contains 22 structures of impact origin and about fifty other structures which lack sufficient evidence for impact. The discovery rate of new structures has been one or two per year during the past ten years. The proven impact structures are located in southern Fennoscandia and the majority have been found in Proterozoic target rocks. The age of the structures varies from prehistoric to ≤ 1000 Ma and their diameters (D) from 0.04 km to 55 km. Nine of the structures contain impact melt. A characteristic feature of the Fennoscandian impact record is a relatively large number of small (≤ 5 km) but old (> 200 Ma) structures: this is a result of success of geophysical methods to discover small but old impact structures in an eroded shield covered with relatively thin overburden. Some of the large circular structures in satellite images and/or in geophysical maps may represent deeply eroded scars of very old impacts, but due to the lack of shock metamorphic features, impact-generated rocks or identified ejecta layers, they cannot yet be classified as impact sites. Two huge structures are proposed here as possible impact sites on the basis of circular satellite images and distinct geophysical anomalies: the Lycksele structure in northern Sweden (D ~ 120 km, see also Witschard, 1984) and the Valga structure in Latvia/Estonia (D ~ 180 km). However, endogeneous explanations, like buried granites, basement domings, or fault-bounded blocks are also possible for these structures. Hints, such as distal ejecta layers or impact produced breccia dykes, of an Archaean or Early Proterozoic impact structure have not been found in Fennoscandia so far. New ways of searching for these structures are proposed with particular emphasis on high-resolution integrated geophysical methods. The impact cratering rate in Fennoscandia is ~ 2.0 · 10−14 km−2 a−1 (for craters with D > 3 km) corresponding to about two events per every 100 Ma for the last 700 Ma. Due to erosion, this is a minimal estimate but is higher than the global rate probably due to strong research activity for finding impact structures in Fennoscandia.

Preliminary results of a palaeomagnetic and rock magnetic study of the Proterozoic Tsuomasvarri intrusions, northern Fennoscandia

Abstract Preliminary palaeomagnetic results are presented for the Proterozoic Tsuomasvarri central ultramafic intrusion and surrounding, older gabbro-diorite intrusion in the northeastern Fennoscandian shield. In the ultramafic intrusion, five remanence components, reflecting different geological processes, were isolated in the alternating field (AF) and thermal demagnetization of 55 samples from ten sites. A reversed polarity remanence component C (pole: Plat=26°N, Plon=245°E, dp=5°, dm=9°, n =32 samples) dominates in the ultramafic intrusion. The normal polarity pole A (Plat=44°N, Plon=232°E, dp=4°, dm=6°, n =9 samples) corresponds to an age of ∼ 1.88 Ga on the Apparent Polar Wander Path (APWP), and is probably a thermochemical remanence acquired during the Svecokarelian orogeny or in the late stages of magma cooling. Pole B (Plat=35°N, Plon=158°E, dp=9°, dm=15°, n =5 samples, normal polarity) corresponds to the ∼ 1.75 Ga old post-orogenic activity in the area, and is also seen in results of KAr (whole rock) isotopic age determinations from the vicinity. The normal polarity pole D (pole: Plat=25°S, Plon=285°E, dp=12°, dm=19°, n =11 samples) has a palaeomagnetic age of ∼ 2.44 Ga which is in contrast with the isotopic age determination of 1.93 Ga (UPb age on zircon) of the gabbro-diorite intrusion. Pole F (Plat=3°N, Plon=212°E, dp=10°, dm=18°, n =5 samples) indicates an age of 0.5-0.4 Ga on the Palaeozoic APWP segment of Fennoscandia and probably reflects a weak Caledonian overprint in northeastern Finland. In the gabbro-diorite intrusion, two remanence components were isolated in 46 samples from eight sites. The palaeomagnetic age, ∼ 1.93 Ga, of the dominant normal polarity component, A′ (pole: Plat=40°N, Plon=247°E, dp=5°, dm=8°, n =28 samples) is consistent with the isotopic age of 1.93 Ga (UPb determination on zircon). The normal polarity component D (pole: Plat=20°S, Plon=285°E, dp=14°, dm=23°, n =5 samples), which was also isolated in the ultramafic intrusion, contradicts the UPb isotopic age determination.