Constraints on the origins of lunar magnetism from electron reflection measurements of surface magnetic fields

Apollo 15 and 16 subsatellite measurements of lunar surface magnetic fields by the electron reflection method are summarized. Patches of strong surface fields ranging from less than $\text{1}\text{4}$° to tens of degrees in size are found distributed over the lunar surface, but in general no obvious correlation is observed between field anomalies and surface geology. In lunar mare regions a positive statistical correlation is found between the surface field strength and the geologic age of the surface as determined from crater erosion studies. However, there is a lack of correlation of surface field with impact craters in the mare, implying that mare do not have a strong large-scale uniform magnetization as might be expected from an ancient lunar dynamo. This lack of correlation also indicates that mare impact processes do not generate strong magnetization coherent over ～ 10 km scale size. In the lunar highlands fields of >100 nT are found in a region of order 10 km wide and >300 km long centered on and paralleling the long linear rille, Rima Sirsalis. These fields imply that the rille has a strong magnetization (>5 × 10^?6 gauss cm³ gm^?1 associated with it, either in the form of intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock. However, a survey of seven lunar farside magnetic anomalies observed by the Apollo 16 subsatellite suggests a correlation with inner ejecta material from large impact basins. The implications of these results for the origin of lunar magnetism are discussed.     

Lunar magnetic anomalies detected by the Apollo substatellite magnetometers

Properties of lunar crustal magnetization thus far deduced from Apollo subsatellite magnetometer data are reviewed using two of the most accurate presently available magnetic anomaly maps — one covering a portion of the lunar near side and the other a part of the far side.

The largest single anomaly found within the region of coverage on the near-side map correlates exactly with a conspicuous, light-colored marking in western Oceanus Procellarum called Reiner Gamma. This feature is interpreted as an unusual deposit of ejecta from secondary craters of the large nearby primary impact crater Cavalerius. An age for Cavalerius (and, by implication, for Reiner Gamma) of 3.2 ± 0.2 × 10⁹ y is estimated. The main (30 × 60 km) Reiner Gamma deposit is nearly uniformly magnetized in a single direction, with a minimum mean magnetization intensity of 7 × 10⁻² G cm³/g (assuming a density of 3 g/cm³), or about 700 times the stable magnetization component of the most magnetic returned samples. Additional medium-amplitude anomalies exist over the Fra Mauro Formation (Imbrium basin ejecta emplaced 3.9 × 10⁹ y ago) where it has not been flooded by mare basalt flows, but are nearly absent over the maria and over the craters Copernicus, Kepler, and Reiner and their encircling ejecta mantles.

The mean altitude of the far-side anomaly gap is much higher than that of the near-side map and the surface geology is more complex, so individual anomaly sources have not yet been identified. However, it is clear that a concentration of especially strong sources exists in the vicinity of the craters Van de Graaff and Aitken. Numerical modeling of the associated fields reveals that the source locations do not correspond with the larger primary impact craters of the region and, by analogy with Reiner Gamma, may be less conspicuous secondary crater ejecta deposits. The reason for a special concentration of strong sources in the Van de Graaff-Aitken region is unknown, but may be indirectly related to the existence of strongly modified crustal terrain which also occurs in the same region. The inferred directions of magnetization for the several sources of the largest anomalies are highly inclined with respect to one another, but are generally depleted in the north-south direction. The north-south depletion of magnetization intensity appears to continue across the far-side within the region of coverage.

The mechanism of magnetization and the origin of the magnetizing field remain unresolved, but the uniformity with which the Reiner Gamma deposit is apparently magnetized, and the north-south depletion of magnetization intensity across a substantial portion of the far side, seem to require the existence of an ambient field, perhaps of global or larger extent. The very different inferred directions of magnetization possessed by nearly adjacent sources of the Van de Graaff-Aitken anomalies, and the depletion in their north-south component of magnetization, do not favor an internally generated dipolar field oriented parallel to the present spin axis. A variably oriented interplanetary magnetizing field that was intrinsically strong or locally amplified by unknown surface processes is least inconsistent with the data.     

On deducing the magnetization of the lunar surface from orbital surveys

A combination of orbital photographic, selenochemical and magnetic surveys may elucidate the mechanism by which the lunar surface became magnetized and possibly yield an estimate of the intensity of the ancient magnetizing field and its time variation. The determination of the size and shape of the magnetized regions requires the measurement of the altitude dependence of field, especially at low altitudes (< 100 km) and with a high enough sampling rate to resolve the profile at the edges of magnetized bodies. The planned Lunar Polar Orbiter may well provide the necessary data.     

Ejecta from large craters on the Moon: Comments on the geometric model of McGetchin et al.

Amendments to a quantitative scheme developed by T.R. McGetchin et al. (1973) for predicting the distribution of ejecta from lunar basins yield substantially thicker estimates of ejecta, deposited at the basin rim-crest and at varying ranges byond, than does the original model. Estimates of the total volume of material ejected from a basin, illustrated by Imbrium, also are much greater. Because many uncertainties affect any geometric model developed primarily from terrestrial analogs of lunar craters, predictions of ejecta thickness and volume on the Moon may range within at least an order of magnitude. These problems are exemplified by the variability ofT, thickness of ejecta at the rim-crest of terrestrial experimental craters. The proportion ofT to crater rim-height depends critically upon scaled depth-of-burst and whether the explosive is nuclear or chemical.     

Linear magnetization feature associated with Rima Sirsalis

Measurements of the magnetic fields by the electron reflection method in the neighborhood of the long structural rille Rima Sirsalis show that a magnetic field of strength ? 100 nT (100 γ) is present over a region on the order of 10 km in width and at least 300 km long. The center of the magnetized region closely parallels and is centered on the rille. The linear magnetization feature extends at least to latitude 8°S, 60 km beyond the place where the rille disappears at the edge of Oceanus Procellarum. This extension is coincident with the extrapolation of the rille based on photographs. However, the magnetization is much weaker or entirely absent at 5°S and has vanished at 0° latitude. These results suggest that the rille is indeed a structural feature and has associated with it magnetization, either in the form of intrusive, magnetized rock, or else in the form of a gap in an otherwise more or less uniformly magnetized layer of rock of large extent in two dimensions. Furthermore, the rille structure evidently is present for some distance beneath the lava flows of the Oceanus Procellarum basin.     

Upper Eocene ejecta of the New Jersey continental margin reveal dynamics of Chesapeake Bay impact

Evidence for the Chesapeake Bay Crater as the source for New Jersey continental margin ejecta is provided by fine-grained tektites and coarse-grained unmelted ejecta. The Upper Eocene ejecta deposit, now demonstrated to be part of the North American strewn field, occurs on the New Jersey continental margin at Ocean Drilling Program (ODP) Sites 904 and 903. The mineralogy, major oxide composition of the ejecta materials, and biostratigraphic age of the enclosing sediments link the origin of these ejecta to the recently recognized Chesapeake Bay impact crater, located only 330 km away. Sediments associated with the ejecta provide information about the dynamics of impact events. The 35-cm-thick ejecta-bearing layer can be subdivided into three subunits that indicate a sequence of events. Bottom subunit III documents sediment failure and deposition of gravel-sized fragments, middle subunit II records deposition of abundant sand-sized ejecta by gravity settling, and upper subunit I contains a 12-cm-thick sedimentary deposit containing rare silt-sized tektites and evidence of waning currents. These events are interpreted by linking sediment deposition to seismic ground motion and subsequent tsunami waves triggered by both the Chesapeake Bay impact and slope failures.     

Magnetic field and shock effects and remanent magnetization in a hypervelocity impact experiment

The impact of aluminum projectiles onto high-alumina terrestrial basalt blocks at 13–15 km s^?1 in the presence of a variable magnetic field has been studied. Low-frequency search coil data show that plasma is produced. causing local compression of the ambient field. Although field production is expected, it was not detectable with the existing apparatus. Measurements of the remanence of the shocked basalt show that magnetization was acquired in the material near the craters. The acquired remanence is predominantly soft, but also contains a component not demagnetized by 500 Oe AC field treatment. Material shocked in a 10-Oe vertical field exhibits inverse dependence of magnetization upon distance from the crater center. Examination of the shocked basalt in thin section reveals a general lack of shock metamorphism in the material surrounding the crater, except for the presence of a high-pressure melt glass which was splashed onto the crater walls. Micro-probe analyses show that the glass is a whole-rock melt of fairly uniform composition, and is contaminated with aluminum from the projectile. The mineralogical data support the view that the acquired magnetization is shock remanence, since negligible shock heating occurred in the magnetized material. These results bear on the problem of lunar magnetism, suggesting that shock effects or possibly thermoremanence in ejecta fragments may be responsible for part of the magnetization of the lunar surface.     

Subophitic lithologies in KREEP-rich poikilitic impact melt rocks from Cayley Plains,Apollo 16 — remnants of a volcanic highland crust?

KREEP-rich poikilitic impact melt rocks 65777,11, 65015,88, and 62235,66 are the only mafic impact melt rocks from Cayley Plains stations, Apollo 16, from which areas of subophitic texture can be reported.The bulk chemistry of these unique subophitic areas and the surrounding poikilitic matrices, as well as mineral compositions (olivine, plagioclase, pyroxene, Fe-Ni metal) were determined by electron microprobe analysis. All subophitic areas could be undoubtedly identified as impact melt rockclasts. Inclusion 65777,11 II is of uniquely KREEP-rich composition, 62235,66 II can be classified as anorthositic. Therefore our attempt to identify pristine volcanic basement rocks of the Cayley regions among these inclusions of basaltic texture failed.However, the absence of pristine volcanic target rock fragments and the existence of KREEP-rich and anorthositic impact melt clasts in KREEP-rich impact melt rocks from Cayley Plains favors the theory that the Cayley Plains formation is part of the ejecta blanket from a large basin-type impact crater (Imbrium?), which is underlain by anorthositic material (Nectaris ejecta?), and has been reworked by local impacts in post-Imbrian times.     

Crater ejecta: Markers of impact catastrophes

The role of impact crater ejecta in the evolution of the Earth and Solar System planets is considered. Using the methods of numerical modeling, the accretion and erosion of planets, the redistribution of planetary material due to impacts of large cosmic bodies and related geospheric interactions, the asymmetrical deposition of ejecta around the crater rim, and the possibility of nonballistic transportation of ejecta are analyzed. The influence of impact events on the Earth’s envelopes is compared with the effects of volcanic eruptions.     

Far-field tectonics associated with a large impact basin: applications to Caloris on Mercury and Imbrium on the Moon

Lithospheric readjustments after the formation of a large impact crater have been computed. The models predict tectonic perturbations over a major portion of the surface of the planet. The weight of the ejecta and the mechanical perturbations in the crater area give rise to membrane stresses. If the planetary lithosphere is in compression, the direction of maximum compression after the impact is perpendicular to a meridian (in a referential where the basin would be at the North pole). On Mercury the preferential orientation of compressive scarps radial to Caloris is explained in this manner. For a planet in extension grabens perpendicular and parallel to a meridian can appear successively as the mechanical state of the crater area is modified. On the Moon this result is consistent with the large-scale tectonics associated with Imbrium.     

Three-dimensional structure of the Laguna Salada Basin and its thermal regime

A comprehensive reinterpretation of the available gravity, magnetic, geothermal, geological and borehole information has been made of the Laguna Salada Basin to establish a 3D model of the basement and sedimentary infill. According to statistical spectral analysis, the residual gravity anomaly is due to sources with a mean regional depth of 2.8 km. The topography of the basement was obtained from a three‐dimensional inversion carried out in the wavenumber domain using an iterative scheme. The maximum density contrast of ?300 kg/m³ estimated from previous studies and the mean depth of 2.5 km finally constrained this inversion. The resulting model indicated that the sedimentary infill is up to 4.2 km thick at its deepest point. According to the gravity‐derived basement topography, the basin presents an asymmetry (i.e. it is of the half‐graben type). It is deeper to the east, where it is delimited from the Sierra Cucapah by a step fault. By contrast, the limit with the Sierra de Juarez is a gently sloping fault (i.e. a listric fault). The basement is not even, but it comprises a series of structural highs and lows. N–S to NW–SE and E–W to NE–SW faults delimit these structural units. The magnetic modelling was constrained by (i) the gravity‐derived basement topography; (ii) a Curie isotherm assumed to be between 7 km and 10 km; (iii) assuming induced magnetization only; (iv) the available geological and borehole information. The magnetic anomalies were interpreted successfully using the gravity‐derived basement/sedimentary interface as the top of the magnetic bodies (i.e. the magnetic modelling supports the gravity basement topography). An elongated N–S to NW–SE trending highly magnetized body running from south to north along the basin is observed to the west of the basin. This magnetic anomaly has no gravity signature. Such a feature can be interpreted as an intrusive body emplaced along a fault running through the Laguna Salada Basin. Treatment of the gravity and magnetic information (and of their horizontal gradients) with satellite image processing techniques highlighted lineaments on the basement gravity topography correlating with mapped faults. Based on all this information, we derived detailed geological models along four selected profiles to simulate numerically the heat and fluid flow in the basin. We used a finite‐difference scheme to solve the coupled Darcy and Fourier differential equations. According to our results, we have fluid flow in the sedimentary layers and a redistribution of heat flow from the basin axis toward its rims (Sierra de Juárez and Sierra Cucapah). Our model temperatures agree within an error of 4% with the observed temperature profiles measured at boreholes. Our heat‐flow determinations agree within an error of ±15% with extrapolated observations. The numerical and chemical analyses support the hypothesis of fluid circulation between the clay–lutite layer and the fractured granitic basement. Thermal modelling shows low heat‐flow values along the Laguna Salada Basin. Deep fluid circulation patterns were observed that redistribute such flow at depth. Two patterns were distinguished. One displays the heat flow increasing from the basin axis towards its borders (temperature increase of 20°C). The second pattern shows an increasing heat flow from south to north of the basin. Such behaviour is confirmed by the temperature measurements in the thermometric boreholes.     

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.     

The behaviour of ulvöspinel samples containing magnetized inclusions

Composite samples containing either magnetized iron or a ferrite magnet embedded in ulvöspinel have been cooled through the ulvöspinel Curie point. The observed changes in magnetic moment which occur are shown to be a result of TRM acquisition in the ulvöspinel rather than an effect of high permeability. Nevertheless by defining an effective permeability of about (1 + TRM/H), magnetostatic theory relevant to magnetized inclusions in a permeable medium can be applied to explain the main features observed in this system.     

磁赤道处化极方法

化向地磁极(化极)是最基本的磁测资料处理方法之一,化极能消除或减少斜磁化影响,提高对磁测资料的认识程度和解释水平,对研究地壳产生的磁异常具有重要意义.但低纬度地区特别是磁赤道处,化极处理很不稳定甚至奇异,一直是位场研究的难点.针对地磁纬度较低特别是磁赤道地区磁异常化极的困难,利用从磁北极处垂直磁化向低纬度地区水平磁化方向转换稳定的特点,提出"狭义化赤"概念,并将其与低纬度磁异常"倒相"解释方法结合,提出专门用于磁赤道处化极的方法.该方法扩展了现有的化极理论,实现了磁赤道处的稳定化极.区别于目前任何方法,专门用于(近)水平磁化条件下的化极计算,具有原理简单,实现方便,收敛速度快等特点.对理论模型和实际资料计算表明这种针对磁赤道地区磁异常的化极处理方法是稳定、可靠的.     

Flood frequency under the influence of trends in the Pearl River basin,China: changing patterns,causes and implications

Using a nonstationary flood frequency model, this study investigates the impact of trends on the estimation of flood frequencies and flood magnification factors. Analysis of annual peak streamflow data from 28 hydrological stations across the Pearl River basin, China, shows that: (1) northeast parts of the West and the North River basins are dominated by increasing annual peak streamflow, whereas decreasing trends of annual peak streamflow are prevailing in other regions of the Pearl River basin; (2) trends significantly impact the estimation of flood frequencies. The changing frequency of the same flood magnitude is related to the changing magnitude or significance/insignificance of trends, larger increasing frequency can be detected for stations with significant increasing trends of annual peak streamflow and vice versa, and smaller increasing magnitude for stations with not significant increasing annual peak streamflow, pointing to the critical impact of trends on estimation of flood frequencies; (3) larger‐than‐1 flood magnification factors are observed mainly in the northeast parts of the West River basin and in the North River basin, implying magnifying flood processes in these regions and a higher flood risk in comparison with design flood‐control standards; and (4) changes in hydrological extremes result from the integrated influence of human activities and climate change. Generally, magnifying flood regimes in the northeast Pearl River basin and in the North River basin are mainly the result of intensifying precipitation regime; smaller‐than‐1 flood magnification factors along the mainstream of the West River basin and also in the East River basin are the result of hydrological regulations of water reservoirs. Copyright © 2014 John Wiley & Sons, Ltd.     

The origin of high-amplitude magnetic anomalies at the intersection of the Juan de Fuca ridge and Blanco fracture zone

The intersection of the Juan de Fuca ridge and Blanco fracture zone is characterized by unusually high amplitude magnetic anomalies (over 1500 nT) which appear to be associated with a body roughly 50 km in length and 20 km in width aligned along the fracture zone. Simple three-dimensional magnetic models indicate that this anomaly is probably caused by a highly magnetized block of material situated in the western end of the Blanco fracture zone near its intersection with the Juan de Fuca ridge. Rock magnetization studies of tholeiitic basalts dredged from this area confirm the presence of highly magnetized basalts near the ridge crest/transform fault intersection. These tholeiitic basalts are enriched in iron and titanium relative to “normal” oceanic tholeiites, apparently the result of extensive shallow fractionation involving olivine, plagioclase, and clinopyroxene. Magnetic model studies indicate that an average thickness of no more than 500 m of these iron-rich basalts is necessary to produce the observed anomaly pattern. Comparison of these basalts with samples previously dredged from the Juan de Fuca ridge crest suggests that these Fe-rich, highly magnetized basalts probably “leaked” out of the southernmost portion of the Juan de Fuca ridge.     

SIMPLIFIED EXPRESSIONS FOR THE MAGNETIC ANOMALIES DUE TO VERTICAL RECTANGULAR PRISMS*

The magnetic anomaly due to a uniformly magnetized vertical rectangular prism and that due to an arbitrary structure which can be divided into a number of such prisms are expressed in forms suitable for rapid computation. Both two-dimensional and three-dimensional cases are considered. The simplified expressions will find use in interpretation techniques where repeated computations have to be made of the anomaly due to prisms as in automated fitting of prism anomalies to observed magnetic anomalies using non-linear optimization techniques or related methods.     

Chicxulub impact ejecta from Albion Island, Belize

Impact ejecta from the Albion Formation are exposed in northern Belize. The ejecta come from the outer portion of the continuous ejecta blanket of the Chicxulub crater, which is located 360 km to the northwest. The basal unit of the Albion Formation is a 1-m-thick clay and dolomite spheroid bed composed of up to four discrete flows. The clay spheroids are altered impact glass, and the dolomite spheroids are accretionary lapilli. The upper unit is a 15-m-thick coarse diamictite bed containing altered glass, large accretionary blocks, striated, polished, and impacted cobbles, and rare shocked quartz. The abundance of accretionary clasts, evidence for atmospheric drag sorting, and the presence of multiple flows in the Albion Formation indicate that atmospheres play an important role in the formation of the outer portions of continuous ejecta blankets of large craters.     

Identification des anomalies magnétiques sur les dorsales à faible taux d'expansion: Méthode des taux fictifs

The fit between calculated and observed magnetic anomalies from slow-spreading centers is improved when allowing for a transition zone between two inversely magnetized blocks. In this paper it is shown that these models are very easily computed by choosing a fictitious spreading rate which is slower than the real spreading rate and by changing the distance scale appropriately. With these slow fictitious spreading rates, the models are very sensitive to the relative position of the successive inversions and could be used to adjust these positions in the magnetic time scales.     

Delayed impact of new volcanic ejecta on ground water quality

We observed long-term changes in the concentrations of dissolved ions in ground water caused by leachate from new volcanic ejecta deposited on the ground surface of the volcanic Miyakejima Island, Japan. Water samples were collected from nine wells and two rain collectors over a period of more than 10 years, and samples of runoff water were collected periodically. The samples were analyzed for temperature, pH, alkalinity, Cl⁻, and SO₄²⁻; some of the samples were also analyzed for δ¹³C. Because the leachate from the volcanic ejecta contained sulfate, we recorded an increase in SO₄²⁻ concentrations in the (unconfined) well water. The increase in SO₄²⁻ was initially detected between less than 1.4 and 5.2 years after the eruption, showing peak concentrations from 2.4 to 6.4 years after the eruption. This delayed response reflects the transit time of downward-moving SO₄²⁻ in the vadose zone, corresponding to an apparent movement rate of 0.4 to 7.2 cm/d. The rate relates to the mean recharge, represented as a fraction of local mean rainfall, and is calculated using the Cl⁻ balance method. The magnitude of the recorded increases reflects the volume of volcanic mudflow on the ground surface within the basin. For the management of ground water after an eruption, it is therefore important to know the chemical properties of the volcanic ejecta and the spatial distribution of mudflow to estimate the magnitude of the effect of ejecta on ground water quality.