Trace elements in ocean ridge basalts

A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.     

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.     

Geophysics and geology of an explosion crater in the Ethiopian rift valley

Geophysical and geological studies of an Ethiopian maar, Haro Maja, demonstrate that its eruptive history is more complex than surface geology alone suggests. The crater is 750 m by 1000 m in diameter and varies in depth from 70 m to 110 m. A strong magnetic anomaly is caused by a central basaltic mound, but a broader crater-wide anomaly is best modelled by a 50 m thick frozen lava lake, 30 m below the crater floor. The central mound was not erupted directly onto the lava lake, but was extruded onto top of the sedimentary infill after a quiescent depositional interval. Electrical resistivity measurements further indicate that other basaltic intrusions failed to reach the surface during that eruptive period.     

Delineation of Canadian sedimentary basins from MAGSAT data

A crustal scalar magnetic anomaly map of Canada and the northern United States is derived using data collected from the MAGSAT satellite. The anomalies are correlated to geological features. Basins associated with failed arms of old rifts have high magnetic anomalies. The Rocky Mountains, the Appalachian Mountains, the suture zone of the Grenville province, modern hotspots and ocean ridges have low magnetic anomalies.     

Variation of magnetic directions within pillow basalts

Relative directions of magnetization have been measured within individual pillow basalts collected from the Atlantic Ocean and Caribbean Sea. The angle between the magnetic directions was determined and is referred to as the directional difference. Although one pillow contained a directional difference of 44°, the remaining ten pillows had differences less than 14°. The maximum orientation and measurement error was 7°. Dispersion on the scale found in these fine-grained pillow basalts would not appreciably affect the magnetic anomaly pattern on the sea floor. We detected no reversals of magnetization despite the sometimes large and variable low-temperature oxidation. Comparison of directions within homogeneous segments of the pillow, viscous remanent magnetization (VRM) acquisition experiments, and alternating field (AF) demagnetization indicate a large portion of the dispersion was due to the acquisition of a viscous component in the larger grained, less oxidized portion of the pillows. Evidence from one variably weathered pillow suggests that extreme low-temperature oxidation may lead to the acquisition of a secondary component with high coercivities (20–80 mT). We could not determine whether this was a chemical remanent magnetization (CRM) or a VRM acquired by single domain grains near the superparamagnetic threshold. Hysteresis properties confirmed by microscopic examination indicated that the magnetic grain size in all the pillows was at least as small as pseudo-single domain.     

Impact of micro-organisms activity on the natural remanent magnetization of the young oceanic crust

Microbes have been widely reported in the deep subseafloor environment. Still it is difficult to detect a global chemical signature of bacterial activity in the oceanic crust. We carried out experiments up to 355 days exposing very young oceanic basalts to anaerobe sulfate reducing organisms in an in-vitro marine environment. The Natural Remanent Magnetization of samples was monitored during the whole duration of experiments and within this time frame the most magnetized sub-samples lost up to 30% of their original signal. Scanning electron microscope observations show cycling of iron from the titanomagnetites to iron sulfide phases. Our results suggest that microbes can have a major and fast impact on the magnetization of young oceanic basalts and could contribute to a global signal as the central anomaly magnetic high seen along ridges axis.     

A proterozoic subduction zone in southern Sweden

Optical microscopy and chemical analyses for major and trace elements have been performed on 27 Proterozoic metavolcanics from southern Sweden. The metavolcanics and associated metasediments are part of a large arced structure around a vast region with granitic batholiths.The compositional data show that the rocks are altered basalts and andesites and the Si, Ti, Zr and Cr relations demonstrate that almost half of the metabasalts are in accordance with present day arc volcanics and that another large fraction represents ocean floor tholeiites. These relations suggest that major plate tectonic processes occurred already during the Proterozoic in southern Sweden and that during these processes arc volcanics and ocean floor basalts were mixed at a subduction zone.     

Interpretation of the Gravity and Magnetic Anomalies of the Cappadocia Region,Central Turkey

The Cappadocia region, located in Central Turkey, is characterized by widespread lava flows and volcanoclastic deposits dating from Miocene to Quaternary. Gravity and aeromagnetic anomalies of the region appear to present similar high and low amplitude regions, although the aeromagnetic anomalies exhibit a rather complex pattern which is thought to be caused by remanent magnetization. The low-pass filtered aeromagnetic map shows a deep-seated magnetic anomaly which may be linked to the widespread volcanic activity at the surface. The pseudogravity transformation of the upward continued anomaly has been constructed. The pseudogravity anomaly demonstrates some form of clockwise rotation. This anomaly was modelled by means of a three-dimensional method. The top and bottom of the body are at 6.3km and 11km (including the flight height) from the ground surface, respectively. This deep body is ellipsoidal and extends along an E-W direction, which is in line with the regional stress direction deduced from GPS measurements. A new mobilistic dynamo-tectonic system appears to explain the body’s E-W elongation. The modelled body may be the source for the inferred geothermal energy of the region. Magnetic measurements were carried out on oriented rock samples collected from outcrops of ignimbrites and basalts, providing directions and intensities of remanent magnetization, susceptibilities and Koeningsberger (Q) ratios. Standard deviations of remanent directions of the Natural Remanent Magnetization (NRM) display a wide scatter implying unreliability of the surface data. Reduction to pole (RTP) transformation of magnetic anomalies was successful with the induced magnetization angle despite the complex pattern of magnetic anomalies.     

On the abnormally high natural remanent magnetization of certain lavas

Summary The rock specimens found to have natural remanent magnetization of abnormally high intensity, have been generally from hill sides or tops, or from ridges high up from the local surroundings. A field of several hundred oersteds has been found sufficient to produce in some of the artificially demagnetized specimens an isothermal magnetization of the same order as the abnormal natural ones, and this magnetization has shown a similar degree of stability as the natural one. Variations in the direction and magnitude of the natural magnetic vector have been found over a distance of a few centimetres. Laboratory tests indicate a normal chemical composition for the specimens. The lightning discharge seems to be the plausible cause of abnormally high magnetization of rocks, which is generally isothermal and might have originated in the recent past, but the magnetization is sometimes complicated probably by the thermal effect of the discharge.     

Origin of a magnetic lineation on Kyushu Island, Japan

Abstract Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of ∼1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.     

Magnetic properties of basalts and geodynamics of the rift zone in the southern Red Sea

Magnetic properties of young rift basalts in the southern Red Sea are studied in detail. It is found that basalt samples dredged in the rift zone are characterized by a large spread in magnetic parameters. The magnetic properties of the basalts are shown to indicate a complex evolution of the Red Sea rift zone. Titanomagnetite grains of the basalts are mostly affected by single-phase oxidation processes and have preserved paleomagnetic information. However, basalts discovered near the rift axis yield evidence of multiphase oxidation of titanomagnetite grains, which is untypical of young oceanic basalts. These basalts have high Curie points and large values of the natural remanent magnetization and Koenigsberger parameter. The corresponding samples were taken in nontransform zones where rocks have experienced the action of significant tectonic forces and, moreover, anomalous geomagnetic field patterns correlate with the position of these zones. Using the magnetic properties of the basalts, the northern segment of the rift axis is relocated.     

南海北部磁异常特征及对前新生代构造的指示

为了研究南海北部前新生代构造,利用新近的船载磁力测量数据,对磁异常进行变纬度化极,并反演计算视磁化强度和磁源重力异常,以及对三条OBS剖面进行重磁拟合.结果认为东沙隆起高磁异常带是浙闽沿海火山岩带向西的延续,其间被NW向古老的转换边界断裂F10错断;NE向的F2断裂是高磁异常带的南界,并限制了底侵活动的北界;F3断裂在...     

Seafloor spreading history of the Agulhas Basin

Data gathered by recent “Islas Orcadas” cruises reveal the seafloor spreading pattern for a region south of the Agulhas/Falkland fracture zone system. The presence of a magnetic anomaly bight about the Agulhas Plateau indicates that the Agulhas Plateau may have developed at the site of a tectonic plate triple junction during the Late Cretaceous. A westward jump in the seafloor spreading center during the Late Maestrichtian (anomaly 34?31) reduced the offset across the Falkland/Agulhas fracture zone system and resulted in the formation of two conjugate aseismic ridges here described as the Meteor and Islas Orcadas Rises. The magnetic lineation pattern in the Agulhas Basin suggests that a tectonic plate (Malvinas Plate) existed during Campanian to Maestrichtian times. Relative rates of motion are calculated for Antarctica, South America, and Africa for the Late Cretaceous.     

Oceanic versus island arc origin of ophiolites

The silica content of basaltic rocks is an unreliable variable with which to distinguish ultramafic-mafic complexes developed at ocean ridges from those potentially formed beneath volcanic island arcs. Data from Appalachian ophiolites supports the view that silica metasomatism is responsible for the high silica content of supposed calc-alkaline basaltic rocks found in ophiolites such as Troodos, and that the high-silica (70 wt.%) leucocratic rocks associated with ophiolites are of tholeiitic rather than calc-alkaline parentage. The use of titanium as a discriminant of tectonic environment is also suspect because the titanium content of basalts associated with Appalachian ophiolites as well as those recently recovered from the Atlantic ocean floor ranges from values even lower than those typical of island arc tholeiites to values typical of abyssal tholeiites. However, the internal stratigraphy of ophiolites in both the Appalachian and Tethyan systems can only be explained on the basis of the postulate that ophiolites originate at oceanic spreading centres rather than beneath island arcs.     

Mantle-driven geodynamo features – Effects of compositional and narrow D″ anomalies

Lower mantle heterogeneity could cause deviations from axial symmetry in geodynamo properties. Global tomography models are commonly used to infer the pattern of core–mantle boundary heat flux via a linear relation that corresponds to a purely thermal interpretation of lower mantle seismic anomalies, ignoring both non-thermal origins and non-resolved small scales. Here we study the possible impact on the geodynamo of narrow thermal anomalies in the base of the mantle, originating from either compositional heterogeneity or sharp margins of large-scale features. A heat flux boundary condition composed of a large-scale pattern and narrow ridges separating the large-scale positive and negative features is imposed on numerical dynamos. We find that hot ridges located to the west of a positive large-scale core–mantle boundary heat flux anomaly produce a time-average narrow elongated upwelling, a flow barrier at the top of the core and intensified low-latitudes magnetic flux patches. When the ridge is located to the east of a positive core–mantle boundary heat flux anomaly, the associated upwelling is weaker and the homogeneous dynamo westward drift leaks, precluding persistent intense low-latitudes magnetic flux patches. These signatures of the core–mantle boundary heat flux ridge are evident in the north–south component of the thermal wind balance. Based on the pattern of lower mantle seismic tomography (Masters et al., 2000), we hypothesize that hot narrow thermal ridges below central Asia and the Indian Ocean and below the American Pacific coast produce time-average fluid upwelling and a barrier for azimuthal flow at the top of the core. East of these ridges, below east Asia and Oceania and below the Americas, time-average intense geomagnetic flux patches are expected.     

喜马拉雅东构造结周边地区地壳磁异常分布特征研究

根据波茨坦地磁场模型(POMME6.2),研究喜马拉雅东构造结周围地区地壳磁异常的空间分布、磁异常随高度的衰减特征.利用二维小波变换方法对地表磁异常进行分解,分析小波细节组合和逼近信号的异常特点.讨论磁异常与地质构造的联系.结果表明,研究区内地壳磁异常分布相当不均匀.喜马拉雅—东构造结—龙门山—大巴山地区分布着较强的负磁异常;四川盆地为正磁异常,其他地区磁异常较弱.东构造结对周围地区磁异常有重要影响,它及其周围地区的地壳磁异常都是在负磁或弱磁异常背景上,叠加着中短波长的正负磁异常.这些中小尺度磁异常由中、上层地壳磁性物质产生,走向与地质构造基本一致.沿金沙江、红河断裂带分布着清晰的弱磁异常带.龙门山断裂带、丽江—小金河断裂带和红河断裂带是磁异常强弱过渡带.青蒇高原中部东西向的磁异常,在东构造结弧顶地区呈弧形分布.青藏高原中部和滇中地块带状、团状磁异常具有相同的衰减规律.     

Satellite and surface geophysical expression of anomalous crustal structure in Kentucky and Tennessee

An equivalent layer magnetization model obtained from inversion of long-wavelength satellite magnetic anomaly data indicates a very magnetic source region centered in south central Kentucky. The magnetization maximum nearly coincides with a gravity high elongated north-south and extending into Tennessee. Previous refraction profiles suggest that the source of the gravity anomaly is a large mass of rock occupying much of the crustal thickness. The outline of the source delineated by gravity contours is also discernible in aeromagnetic anomaly patterns. Taken together, the geophysical data suggest a large, localized mass of intracrustal rock which is both dense and very magnetic. A simple magnetization/density model is given which accounts for the gravity and long-wavelength aeromagnetic anomalies due to the body. We interpret it as a mafic plutonic complex, and several lines of evidence are consistent with a rift association. The body is, however, clearly related to the inferred position of the Grenville Front. It is bounded on the north by the fault zones of the 38th Parallel Lineament. The inferred mean magnetization (4 A/m) of the body is large, but not inconsistent with values reported by others for deep crustal bodies associated with long-wavelength magnetic anomalies. Such magnetization levels can be achieved with magnetic mineralogies produced by normal oxidation and metamorphic processes and enhanced by viscous build-up, especially in mafic rocks of alkaline character.     

Ophiolitic breccias associated with allochthonous oceanic crustal rocks in the East Ligurian Apennines,Italy — a comparison with observations from rifted oceanic ridges

In the East Ligurian segment of the North Apennines, eugeosynclinal sequences which contain ophiolitic rocks have been tectonically emplaced onto approximately coeval miogeosynclinal sediments. These allochthonous sequences represent the floor of a Mesozoic ocean which closed during the early Tertiary. The ophiolitic rocks consist of serpentinite, gabbro, pillowed and massive basalts, and breccias derived from these lithologies. They are overlain with depositional contacts by Upper Jurassic-Cretaceous pelagic cherts, limestones, and a shale/limestone sequence.The ophiolitic breccias attain thicknesses up to 100 m and strike lengths up to a few kilometres, and consist largely of unorganized accumulations of sand- to block-sized clasts. Compositions at specific horizons may range from oligomict breccias containing gabbro, basalt, or serpentinite fragments, to polymict breccias consisting of any mixture of these lithologies. Most of the breccias probably represent slow talus accumulations at the base of major submarine fault scarps which have exposed gabbro and serpentinite to submarine erosion. Direct exposure of gabbro and serpentinite on the ocean floor is also indicated by the occurrence of stratigraphically intact contacts between these lithologies and overlying pelagic sediments (generally cherts). The distribution and thickness of the breccias and volcanics, and the distribution of the gabbro and serpentinite, can vary greatly within distances of a few kilometres, thus producing complex heterogeneous sequences consisting of laterally impersistent lithological units.Recent observations and deep drilling of the Mid-Atlantic Ridge and other rifted ridges have revealed occurrences of significant thicknesses of basaltic, serpentinitic, and gabbroic breccias upon and within the volcanic layer of the oceanic crust, as well as the direct submarine exposure of plutonic rocks. It is therefore likely that the East Ligurian sequences represent parts of rifted ridge-generated crust. If so, then the complexity of the East Ligurian sequences suggests that the upper part of rifted ridge-generated crust may in places possess large variations in its stratigraphy over small (<10 km²) areas.Smooth, non-rifted (fast-spreading) ridges, which have very reduced topography and lack major fault scarps, should form ophiolitic complexes deficient in breccias containing fragments of plutonic igneous rocks. Most large ophiolitic complexes do not contain plutonic rock-bearing breccias, and were therefore probably formed at smooth ridges. The apparently preferential preservation of this type of ophiolitic complex, as opposed to the rifted ridge-type crust in East Liguria, may be related to the less pervasive and less intense fracturing of smooth ridges. This resulted in greater “cohesion” and lateral continuity of smooth ridge-generated crust during later tectonic emplacement into allochthonous positions in orogenic belts.     

An integrated geodynamic model of the Nankai subduction zone and neighboring regions from geophysical inversion and modeling

I investigate large-scale deep crustal structures of the Nankai subduction zone and neighboring region using regional magnetic and gravity anomalies, heat flow measurements, and earthquake hypocenters. It is found that ages, dip angles, and geothermal states of the subducting slab have direct influences on mantle wedge serpentinization. The weakest serpentinization observed in the Nankai forearc region is associated with the youngest downgoing plate of the Shikoku Basin. Conspicuous gravity anomalies identified in the forearc region are coincidental spatially with magnetic anomalies after the reduction to the pole, a mathematical procedure that helps relocate magnetic sources and boundaries, and allows us to more easily interpret magnetic data. It is argued that these patches of magnetic and gravity anomalies are caused by the same sources of anomalous density and magnetization, and are linked directly to preexisting structures such as magnetic anomalies and their boundaries in the subducting oceanic crust. Since the gravity and magnetic anomaly patches are found to be closely related to interplate seismogenic behaviors in the Nankai subduction zone, I suggest that major magnetic boundaries in the Shikoku Basin are likely weak places for slab tears that trigger seismic segmentations along the subduction zone.     

鲜水河断裂带南东段的深部孕震环境与2014年康定M_S6.3地震

2014年11月22日16时55分在四川省甘孜藏族自治州康定县发生的6.3级地震,结束了鲜水河断裂带近30多年以来没有较大地震发生的历史,其潜在的地震危险性再次引起国内外地学工作者的关注.为了研究鲜水河断裂带南东段深部孕震环境和探求康定MS6.3地震的成因,本文先利用四川区域数字地震台网和康定地区及周边所布设的流动地震台阵在2009年1月1日至2014年12月5日期间所记录到7397次区域地震事件的99287条P波到时资料,反演得到了鲜水河断裂带南东段上地壳范围内不同深度的三维P波速度结构特征;再对康定震区及周边的重力、航磁数据进行视密度、视磁化强度反演,得到了壳内不同深度密度的横向变化信息和视磁化强度的分布特征;在此基础上综合研究鲜水河断裂带南东段的深部孕震环境.研究结果表明,雅江—九龙一带的低速区与泸定—宝兴高速区的速度结构特征表明了鲜水河断裂带南东段两侧壳内物质存在显著的横向介质差异,康定MS6.3地震发生在该高低速异常区的分界线上;结合康定MS6.3地震的1028个余震序列的精确定位结果可以看出,重新定位后的余震沿着鲜水河断裂带南东段呈条带状分布,且震源深度优势分布层位深度为8~15km,该余震序列的空间分布特征与鲜水河断裂南东段的深部介质条件密切相关.鲜水河断裂带南东段特有的视密度和视磁化强度异常分布特征反映了康定地区东西两侧块体的基底性质存在明显差异,康定—石棉及其以东地区所表现出的磁异常高和重力高的位场特征,反映该区域由强磁性、高密度物质组成,而康定MS6.3地震就发生在康定—石棉重力梯度变化带上、雅安—泸定磁性穹窿区的西边界线上.随着川青块体向南东方向滑移,受到四川盆地西缘边界刚性基底对川青块体的强烈阻挡,加剧了康定—石棉及其以东地区基底岩层的褶皱变形并产生了强烈的应力积累,所积累的应力突然释放导致了康定MS6.3地震的发生,这正是此次鲜水河断裂带南东段康定地区强震孕育和发生的深部构造环境和介质特征.根据本文对鲜水河断裂带南东段深部孕震环境的综合研究成果可知,石棉段处于重磁异常梯级带上且其北东侧表现出的高密度、强磁性和高波速等物性特征有利于区域应力的相对集中,因此,鲜水河断裂带南东段石棉地区的地震活动趋势和地震危险性背景值得进一步关注和研究.