Km In Thickness Research Articles

Structure of the North Kara Shelf from results of seismostratigraphic analysis

The sedimentary cover section of the North Kara Shelf had been subdivided based on extensive seismic data, and seismic correlation was carried out. The predominant role of Upper Riphean-Middle Paleozoic rocks has been corroborated. A series of relatively deepwater basins filled with primarily terrigenous fly-schoid rocks up to 7–9 km in thickness existed in the Late Riphean-Vendian at the place of the shelf. In the Cambrian, isolated basins merged into a wide and shallow-water basin as a result of the Baikalian reactivation in southeast Severnaya Zemlya and probably in Taimyr. After the pre-Ordovician hiatus, a vast sedimentation basin with a regressive section was formed on the shelf from Ordovician to Late Devonian. Shallow-water marine and near-shore carbonate and carbonate-terrigenous sequences accumulated in this basin and gave way to continental and less frequent near-shore, marine, variegated, and red beds in the Devonian. The thickness of the Ordovician-Devonian sequence reaches 6 km. Since the Mid-Carboniferous, block emergence and deep erosion of Ordovician-Devonian complexes have occurred in the north Kara shelf as a result of Hercynian events in northern Taimyr, Severnaya Zemlya, and in the southern Kara Sea. No Permian-Triassic rifts existed on the North Kara Shelf. At that time, the shelf was an area of erosion. The thickness of the Middle Carboniferous-Cretaceous sequence was insignificant and gradually increased toward Barents Sea troughs. The newly formed Svyataya Anna and Voronin troughs arose due to opening of the Eurasia Basin of the Arctic Ocean. The terrane concept has been subjected to criticism. The available data show that a large epi-Grenvillian continental block existed, and the North Kara region was part of it. Collision of the northern continent with the Paleosiberian Platform in the Late Paleozoic resulted in the formation of the Hercynian fold arc to the south of the North Kara Shelf, and an inverted orogenic arch arose at the place of shelf basin. The individual geological features that distinguish the North Kara Shelf from the Barents Sea troughs and the South Kara Syneclise are emphasized. The ancient pre-Riphean basement, a system of Late Riphean-Vendian relatively deepwater troughs and basins, Hercynian tectonic inversion, deep erosion of the most uplifted part of the arch, and significant block motions are the distinguishing features of the North Kara Shelf.

High frequency seismic waves and slab structures beneath Italy

Tomographic images indicate a complicated subducted slab structure beneath the central Mediterranean where gaps in fast velocity anomalies in the upper mantle are interpreted as slab tears. The detailed shape and location of these tears are important for kinematic reconstructions and understanding the evolution of the subduction system. However, tomographic images, which are produced by smoothed, damped inversions, will underestimate the sharpness of the structures. Here, we use the records from the Italian National Seismic Network (IV) to study the detailed slab structure. The waveform records for stations in Calabria show large amplitude, high frequency (f>5 Hz) late arrivals with long coda after a relatively low-frequency onset for both P and S waves. In contrast, the stations in the southern and central Apennines lack such high frequency arrivals, which correlate spatially with the central Apennines slab window inferred from tomography and receiver function studies. Thus, studying the high frequency arrivals provides an effective way to investigate the structure of slab and detect possible slab tears. The observed high frequency arrivals in the southern Italy are the strongest for events from 300 km depth and greater whose hypocenters are located within the slab inferred from fast P-wave velocity perturbations. This characteristic behavior agrees with previous studies from other tectonic regions, suggesting the high frequency energy is generated by small scale heterogeneities within the slab which act as scatterers. Furthermore, using a 2-D finite difference (FD) code, we calculate synthetic seismograms to search for the scale, shape and velocity perturbations of the heterogeneities that may explain features observed in the data. Our preferred model of the slab heterogeneities beneath the Tyrrhenian Sea has laminar structure parallel to the slab dip and can be described by a von Kármán function with a down-dip correlation length of 10 km and 0.5 km in thickness with ∼2.5% Vp fluctuations within the slab. This suggests that the heterogeneities are inherited from the melt shear bands formed during the original formation of the oceanic lithosphere near the mid-ocean ridge.

Low‐pressure–high‐temperature metamorphism during extension in a Jurassic magmatic arc, Central Pontides, Turkey

AbstractMagmatic arcs are zones of high heat flow; however, examples of metamorphic belts formed under magmatic arcs are rare. In the Pontides in northern Turkey, along the southern active margin of Eurasia, high temperature–low pressure metamorphic rocks and associated magmatic rocks are interpreted to have formed under a Jurassic continental magmatic arc, which extends for 2800 km through the Crimea and Caucasus to Iran. The metamorphism and magmatism occurred in an extensional tectonic environment as shown by the absence of a regional Jurassic contractional deformation, and the presence of Jurassic extensional volcaniclastic marine basin in the Pontides, over 2 km in thickness, where deposition was coeval with the high‐T metamorphism at depth. The heat flow was focused during the metamorphism, and unmetamorphosed Triassic sequences crop out within a few kilometres of the Jurassic metamorphic rocks. The heat for the high‐T metamorphism was brought up to crustal levels by mantle melts, relicts of which are found as ultramafic, gabbroic and dioritic enclaves in the Jurassic granitoids. The metamorphic rocks are predominantly gneiss and migmatite with the characteristic mineral assemblage quartz + K‐feldspar + plagioclase + biotite + cordierite ± sillimanite ± garnet. Mineral equilibria give peak metamorphic conditions of 4 ± 1 kbar and 720 ± 40 °C. Zircon U–Pb and biotite Ar–Ar ages show that the peak metamorphism took place during the Middle Jurassic at c. 172 Ma, and the rocks cooled to 300 °C at c. 162 Ma, when they were intruded by shallow‐level dacitic and andesitic porphyries and granitoids. The geochemistry of the Jurassic porphyries and volcanic rocks has a distinct arc signature with a crustal melt component. A crustal melt component is also suggested by cordierite and garnet in the magmatic assemblage and the abundance of inherited zircons in the porphyries.

SHRIMP U–Pb Zircon Triassic Intrusion Age of the Finero Mafic Complex (Ivrea–Verbano Zone, Western Alps) and its Geodynamic Implications

Interstitial, subhedral, slightly pink zircons have been separated from a diorite of the External Gabbro unit of the Finero Mafic Complex, which crops out in the northernmost tip of the Ivrea^Verbano Zone (IVZ, Southern Alps).They are characterized by oscillatory zoning under cathodoluminescence (CL), a highTh/U ratio (0·9^0·4), and associated high U,Th, Pb, rare earth element (REE),Ta, Nb, Li and P contents, consistent with a magmatic origin.The mean of the single concordant ages is 232 3 Ma, which is here proposed to constrain the timing of intrusion of the Finero Mafic Complex. Most of these magmatic zircons exhibit an outer rim, up to 50 mm thick, with a bright emission, with transgressive and/or sub-concordant contacts to the internal, oscillatory-zoned domains, in which the Th/U ratio is decreased, as are the U,Th, Pb, REE,Ta and Nb contents.These rims give younger, slightly discordant Pb/ U ages, between 219 3 Ma and 205 3 Ma, which are interpreted to highlight a fluid-assisted recrystallization event. The gabbros also have an interstitial zircon population consisting of rounded, small, colourless, bright to milky crystals with blurred zoning patterns in CL. These zircons have low U, Th, Pb, Nb and Ta contents, and lowTh/U ratio (down to 0·08).The overall metamorphic character, together with field evidence of the presence of blocks of paragneiss belonging to the crystalline basement at the roof of the External Gabbro unit, indicates that these zircons are inherited from local crustal rocks. They have U and Pb isotopic compositions that are strongly altered and discordant, showing an upper limit of the timing of recrystallization consistent with the MiddleTriassic emplacement age obtained from the magmatic zircons. It is concluded that the Finero Mafic Complex can no longer be considered as a part of the huge Permian Mafic Complex of the central IVZ. It follows that the intrusive record preserved in the IVZ allows us to look at the interplay between the continental crust and mantle melts through time (i.e. Permian toTriassic). In this scenario, the modest crustal assimilation shown by the Finero Mafic Complex can be interpreted in the first instance as the consequence of the limited volume of intruded melt (less than 2 km in thickness). However, the possible role played by the decreased fertility of the metamorphic country rocks owing to a inferred Permian magmatic event must be better constrained.The intrusion of the Finero Mafic Complex may represent the deep-crustal counterparts of the Ladinian volcanism that is widespread throughout the Southern Alps. The geodynamic

Thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the Northern Gulf of California: Analysis of seismic reflection profiles

AbstractThe transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern Gulf of California across the obliquely conjugate Tiburón‐Upper Delfin basin segment. Structural mapping on a 5–20 km grid of seismic reflection lines of Petroleos Mexicanos demonstrates that ~1000% extension is accommodated on a series of NNE striking listric‐normal faults that merge at depth into a detachment fault. The detachment juxtaposes a late‐Neogene marine sequence over thinned continental crust and contains an intrabasinal divide due to footwall uplift. Two northwest striking, dextral‐oblique faults bound both ends of the detachment and shear the continental crust parallel to the tectonic transport. A regional unconformity in the upper 0.5 s (two‐way travel time) and crest erosion of rollover anticlines above the detachment indicates inversion and footwall uplift during the lithospheric rupture in the Upper Delfin and Lower Delfin basins. The maximum length of new crust in both Delfin basins is less than 40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath a wedge‐shaped upper sequence that reaches >5 km in thickness. A fundamental difference exists between the Tiburón‐Delfin segment and the Guaymas segment to the south in terms of presence of low‐angle normal faults and amount of new oceanic lithosphere, which we attribute to thermal insulation, diffuse upper‐plate extension, and slip on low‐angle normal faults engendered by a thick sedimentary lid.

An assessment of cirrus heights from MISR oblique stereo using ground‐based radar and lidar at the Tropical Western Pacific ARM sites

We compare cirrus presence and heights (CTHs) using oblique stereo by the Multiangle Imaging SpectroRadiometer (MISR) with measurements from ground‐based cloud radar and lidar sensors at the Tropical Western Pacific (TWP) sites operated by the U.S. Department of Energy Atmospheric Radiation Measurement Program. Precise point‐wise comparisons, limited to only 195 coincident cases, showed that the total number of cirrus retrieved using oblique‐stereo analysis improved to 70% from 39% using the standard‐stereo technique. The stereo technique detects cloud with the highest contrast, which is often at lower altitude. The oblique‐stereo technique's efficiency depends on the thickness and number of underlying cloud layers. A histogram approach allowed similar regions to be compared statistically with many more samples and showed three distinct peaks at ≈13 km, 15 km, and 19 km related to deep convective clouds, tropical tropopause layer (TTL) cirrus, and overshooting convective clouds, respectively. Most differences between the satellite and ground‐based measurements resulted from a number of cases of invalid cloud comparisons (14%), blunders from edges and broken clouds (7%), low contrast stereo mismatches (4%), and under‐estimation of CTHs (3%). Overall, the oblique‐stereo analysis detected a cirrus‐top layer in 65% of all the valid coincident cases, mostly <1 km in thickness. The oblique‐stereo derived cirrus CTHs differed from the heights of cirrus‐top layers from ground‐based cloud radar and lidar by −0.5 ± 1.0 km, validating the MISR retrievals. This suggests global thin cirrus retrievals are possible with the oblique‐stereo technique after the screening of occasional blunders.

Geodynamic nature of the Baikal Rift Zone and its sedimentary filling in the Cretaceous–Cenozoic: the effect of the far-range impact of the Mongolo-Okhotsk and Indo-Eurasian collisions

The problem of the Late Meso-Cenozoic tectonics and geodynamics of the southern framing of the Siberian Platform is discussed. This area abounds in Late Meso-Cenozoic structures composed of sedimentary deposits. Analysis of voluminous geologo-stratigraphic, geophysical, and geochronological (track dating) data showed that these structures, including the Baikal Rift Zone, resulted from the far-range impact of two collisions related to the prolonged convergence of the North China and Indian continents with Eurasia, which took place in the Late Jurassic–Paleocene and Cenozoic, respectively. The synchronous occurrence of two great tectonic events led to the formation of a complex structure and a sedimentary basin, which are of different geodynamic nature and cannot be united into the same sequence of Late Cretaceous–Cenozoic formation of the Baikal Rift Zone. The Baikal system of shears and conjugate rifts formed in the Pliocene–Quaternary as a result of the far-range impact of the Indo-Eurasian collision. The latter led to the accumulation of the upper layered undeformed seismostratigraphic complex in all three Baikal basins. The sediments in Central Baikal are up to 3 km thick, and in the Selenga River valley they reach 5–6 km in thickness. The active intracontinental rift structures are characterized by zonal sedimentation. The middle layered deformed seismostratigraphic complex 1–1.5 km thick is recognized in all three Baikal basins and is similar to the Upper Oligocene–Lower Pliocene sediments of lacustrine-littoral, deltaic, and lacustrine facies widespread throughout the Baikal and Altai–Sayan regions and in Mongolia, where they formed in the environments of large lake systems. As a result of the deformations caused by the Indo-Eurasian collision, the Upper Oligocene–Lower Pliocene sediments were involved in the Pliocene–Quaternary ramp and unilateral-ramp structures in intermontane basins of the Altai–Sayan region and in Mongolia. The lower seismically transparent seismostratigraphic complex occurs only in South Baikal and Central Baikal. It is 1 km thick in the east and up to 45 km thick in the west. The complex is a fragment of the Late Cretaceous–Paleogene lacustrine-river sediments of the large fore-Baikal piedmont basin, which formed at the final stage of evolution of the vast Mongolo-Okhotsk orogen.

Fabric evidence for granodiorite emplacement with extensional shear zones in the Variscan Gredos massif (Spanish Central System)

Three granitoid bodies in the central part of the Gredos massif (Spanish Central System batholith) are tabular, around 1 km in thickness, and intruded into a migmatitic middle crust during the D3 deformation phase of the Variscan Orogeny. Petrologically, they are composed of Bt-granodiorite and Crd-monzogranite, and they show varying abundance of large (cm-scale) feldspar megacrysts. A detailed study of the shape preferred orientation (SPO) magmatic fabric defined by these megacrysts, together with a kinematic analysis of the structures due to interactions between them, and the measurement of quartz c-axis fabrics in migmatites and granitoids, suggests that granitic magma and country rocks were mechanically coupled during deformation. The emplacement took place along large-scale, extensional shear zones active during the first stages of the D3 phase. The shape of the SPO ellipsoids varies from constrictional at the centre of the granitic bodies, to flattening or even oblate at their external contacts with the migmatitic host rocks. The favoured interpretation of this spatial fabric variation is the overprinting of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments, followed by shear zone development commonly at the top of the granitic bodies. The entire structure was later folded during the last stages of the D3 phase.

Sources and formation conditions of sulfide-silicate magmas in the Noril’sk district

Geology, tectonomagmatic reactivation of the Noril’sk district, as well as stratigraphy and geochemistry of the volcanic sequence are considered. Sources and formation mechanism of ore-bearing magma and the scope of ore formation are discussed. The Permian-Triassic flood-basalt magmatism of the Noril’sk district developed in part of the Siberian Platform with Archean-Paleoproterozoic basement broken into blocks and overlapped by a sedimentary cover up to 13 km thick and a volcanic sequence reaching 3.7 km in thickness. The geophysical data show that remnants of the subducted ancient oceanic crust exist in the mantle and fragments of transitional magma chambers and conduits are retained at different levels of the Earth’s crust. The cyclic tectonomagmatic evolution of the territory was characterized by alternation of extension with intense volcanic activity and compression accompanied by waning of volcanic eruptions. The early rifting, transitional stage, and late dispersed spreading are distinguished. The associations of volcanic (lavas and tuffs) and intrusive rocks were formed during each stage. The volcanic sequence is subdivided into 11 formations. The intrusions of the Talnakh and Noril’sk ore fields are distinguished by two-level structure with the Upper Noril’sk ore-bearing intrusions above and the Lower Noril’sk barren intrusions below. Two types of primary magmas differ in geochemistry of lavas and intrusions: (1) OIB-type high-Ti magma (iv, sv, gd formations of the first stage from bottom to top) and (2) low-Ti magma (hk, tk, nd formations of the second stage and mr-mk formations of the third stage). The nd formation depleted in ore elements and the ore-bearing cumulus composed of silicate and sulfide melts in combination with early silicate minerals and chromite are products of the fractionation of the primary low-Ti magma. As follows from geochemical parameters, intrusions of the Lower Noril’sk type are comagmatic to the evolved lavas of the nd3 subformation, whereas intrusions of the Upper Noril’sk type are comagmatic to the lavas of the mr-mk formations. Geochemical similarity with volcanic rocks provides evidence for the composition of the initial magma and the time of intrusion emplacement. The ore-bearing intrusions of the Upper Noril’sk type were formed at the onset of the third stage, when the primitive low-Ti magma similar to the lavas of mr-mk formations in composition was emplaced. When intruding, this melt captured and transported ore-bearing cumulus (drops of sulfide melt, early olivine and chromite grains) into the magma chamber. Separate portions of sulfide liquid were involved into movement as a self-dependent intrusive subphase during formation of the Talnakh and Kharaelakh intrusions. An extremal effect of pressure on sulfur concentration in fluid-bearing and sulfide-saturated mafic magmas has been established in experiments to be P = 1−2 GPa. In this interval of pressure, the S concentration in sulfide-saturated magmas increases in the following sequence: dry magma ≤(H2O + CO2)-bearing magma 2.5 GPa) pressures, the S contents (0.1–0.2 wt %) are commensurable. The extremal baric relationship of S concentration in fluid-bearing and sulfide-saturated mafic magmas may be important for the formation of ore-bearing magmas. The calculation results show that the amount of sulfides in the known deposits does not exceed 2% of geological resources of the sulfides separated from the flood basalts. Therefore, the chance of discovery of new deposits remains rather high. Proceeding from the conditions of ore-bearing magma formation and geological setting of the known deposits, criteria for recognition of potentially ore-bearing areas are proposed and such areas are outlined.

The crustal structure of southern Baffin Bay: implications from a seismic refraction experiment

Baffin Bay represents the northern extension of the extinct rift system in the Labrador Sea. While the extent of oceanic crust and magnetic spreading anomalies are well constrained in the Labrador Sea, no magnetic spreading anomalies have yet been identified in Baffin Bay. Thus, the nature and evolution of the Baffin Bay crust remain uncertain. To clearly characterize the crust in southern Baffin Bay, 42 ocean bottom seismographs were deployed along a 710-km-long seismic refraction line, from Baffin Island to Greenland. Multichannel seismic reflection, gravity and magnetic anomaly data were recorded along the same transect. Using forward modelling and inversion of observed traveltimes from dense airgun shots, a P-wave velocity model was obtained. The detailed morphology of the basement was constrained using the seismic reflection data. A 2-D density model supports and complements the P-wave modelling. Sediments of up to 6 km in thickness with P-wave velocities of 1.8–4.0 km s-1 are imaged in the centre of Baffin Bay. Oceanic crust underlies at least 305 km of the profile. The oceanic crust is 7.5 km thick on average and is modelled as three layers. Oceanic layer 2 ranges in P-wave velocity from 4.8 to 6.4 km s-1 and is divided into basalts and dykes. Oceanic layer 3 displays P-wave velocities of 6.4–7.2 km s-1. The Greenland continental crust is up to 25 km thick along the line and divided into an upper, middle and lower crust with P-wave velocities from 5.3 to 7.0 km s-1. The upper and middle continental crust thin over a 120-km-wide continent-ocean transition zone. We classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data. The profile did not reach continental crust on the Baffin Island margin, which implies a transition zone of 150 km length at most. The new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern Baffin Bay.

Regional-scale structural role of Permian salt within the Central European Basin System

Abstract The Central European Basin System (CEBS) includes the former Northern and Southern Permian Basins together with superimposed Meso-Cenozoic sub-basins and contains a thick layer of Upper Permian (Zechstein) salt. This salt was mobilized in response to several post-Permian tectonic events. In order to analyse the regional relationship between the structural pattern of the Meso-Cenozoic sedimentary cover and the distribution of the Upper Permian salt, a 3D structural model of the CEBS has been constructed. In this model, the Permian salt is resolved as an extra layer for the entire basin system. According to the 3D structural model, the salt layer is strongly deformed as a result of halokinetic activity. The thickest salt is localized within salt walls and diapirs, reaching up to 9 km of thickness. A regional structural 3D analysis of the overburden in relation to underlying ductile salt demonstrates that the geometry of the sedimentary cover is strongly complicated by a variety of salt structures. The withdrawal of the Permian salt appears to have played a key role in both deposition and deformation of Meso-Cenozoic deposits in addition to tectonically forced regional subsidence.

Anatomy of a 10 km scale sheath fold, Mount Hay ridge, Arunta Region, central Australia: The structural record of deep crustal flow

Mount Hay ridge, Arunta Region, central Australia, exposes a 10 km scale, antiformal sheath fold formed in the deep crust (P ≈ 0.6–0.9 GPa) in Proterozoic time. Tilting and exhumation during the Devonian Alice Springs orogeny resulted in an oblique cross section, such that the once subhorizontal sheath fold now has a moderately dipping, gently folded axial plane and overturned southern limb. Fabric types (e.g., S = L, L > S) were mapped in the field, and finite strain data were collected from a widely distributed gabbroic granulite unit. Fabric and finite strain data sets exhibit remarkable correspondence and confirm the common assumption that fabric type is directly related to finite strain type for deep crustal rocks. Both data sets reveal a distinct pattern of oblate strain on fold limbs grading into prolate strain in the fold hinge. There is no field evidence for significant strain localization, and measured strain magnitudes are consistent across the fold. Thus, we infer that different structural domains record different though coeval strain paths and that strain compatibility was maintained across domain boundaries. The uniformity of foliation, lineation, and mesoscale fold orientations across Mount Hay and the consistent magnitude of maximum stretch across structural domains indicate that the sheath fold and subsidiary structures formed during a single, progressive deformational event. Based on its geometry, the inferred scale of the enclosing shear zone (≥8 km in thickness), and regional tectonic correlations, we infer that the Mount Hay sheath fold formed in a subhorizontal, crustal‐scale flow zone during the Paleoproterozoic Strangways orogeny (1740–1690 Ma). The record of deformation is consistent with fold formation either on one side of a channel or through detachment flow.

Deep seismic structure of the Tonga subduction zone: Implications for mantle hydration, tectonic erosion, and arc magmatism

We present the first detailed 2D seismic tomographic image of the trench-outer rise, fore- and back-arc of the Tonga subduction zone. The study area is located approximately 100 km north of the collision between the Louisville hot spot track and the overriding Indo-Australian plate where ~80 Ma old oceanic Pacific plate subducts at the Tonga Trench. In the outer rise region, the upper oceanic plate is pervasively fractured and most likely hydrated as demonstrated by extensional bending-related faults, anomalously large horst and graben structures, and a reduction of both crustal and mantle velocities. The 2D velocity model presented shows uppermost mantle velocities of ~7.3 km/s, ~10% lower than typical for mantle peridotite (~30% mantle serpentinization). In the model, Tonga arc crust ranges between 7 and 20 km in thickness, and velocities are typical of arc-type igneous basement with uppermost and lowermost crustal velocities of ~3.5 and ~7.1 km/s, respectively. Beneath the inner trench slope, however, the presence of a low velocity zone (4.0–5.5 km/s) suggests that the outer fore-arc is probably fluid-saturated, metamorphosed and disaggregated by fracturing as a consequence of frontal and basal erosion. Tectonic erosion has, most likely, been accelerated by the subduction of the Louisville Ridge, causing crustal thinning and subsidence of the outer fore-arc. Extension in the outer fore-arc is evidenced by (1) trenchward-dipping normal faults and (2) the presence of a giant scarp (~2 km offset and several hundred kilometers long) indicating gravitational collapse of the outermost fore-arc block. In addition, the contact between the subducting slab and the overriding arc crust is only 20 km wide, and the mantle wedge is characterized by low velocities of ~7.5 km/s, suggesting upper mantle serpentinization or the presence of melts frozen in the mantle.

Thickness of proximal ejecta from the Orientale Basin from Lunar Orbiter Laser Altimeter (LOLA) data: Implications for multi-ring basin formation

Quantifying the ejecta distribution around large lunar basins is important to understanding the origin of basin rings, the volume of the transient cavity, the depth of sampling, and the nature of the basin formation processes. We have used newly obtained altimetry data of the Moon from the Lunar Orbiter Laser Altimeter (LOLA) instrument to estimate the thickness of ejecta in the region surrounding the Orientale impact basin, the youngest and best preserved large basin on the Moon. Our measurements yield ejecta thicknesses of approx.2900 m near the Cordillera Mountains, the topographic rim of Orientale, decaying to approx.1 km in thickness at a range of 215 km. These measurements imply a volume of ejecta in the region from the Cordillera ring to a radial range of one basin diameter of approx.2.9 x 10(exp 6)cu km and permit the derivation of an ejecta-thickness decay model, which can be compared with estimates for the volume of excavation and the size of the transient cavity. These data are consistent with the Outer Rook Mountains as the approximate location of the transient cavity s rim crest and suggest a volume of approx.4.8 x 10(exp 6)cu km for the total amount of basin ejecta exterior to this location.

Spin crossover and iron-rich silicate melt in the Earth’s deep mantle

A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation. The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core-mantle boundary region. Recent theoretical calculations combined with estimates of iron partitioning between (Mg,Fe)SiO(3) perovskite and melt at shallower mantle conditions suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments. Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than ∼76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg(0.95)Fe(0.05))SiO(3) glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO(3) liquid becomes more dense than coexisting solid at ∼1,800 km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to ∼1,000 km in thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO(3) perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core-mantle boundary region.

The lunar magma ocean: Reconciling the solidification process with lunar petrology and geochronology

The Moon is thought to have originated with a magma ocean that produced a plagioclase flotation crust as solidification proceeded. Ages of anorthositic crust range over at least 200 million years. The model for solidification presented here integrates chemical and physical constraints of lunar magma ocean solidification to determine (1) the final thickness of flotation crust generated by a fractionally solidifying magma ocean, (2) the timescale of crystallization before plagioclase is a stable phase, (3) the timescale of solidification after the formation of the plagioclase flotation crust, and (4) the post-overturn lunar mantle composition and structure. We find that magma oceans of as much as 1000 km depth are consistent with creating an anorthositic crust 40 to 50 km in thickness. Solidification of the magma ocean prior to formation of the flotation crust may occur on the order of 1000 years, and complete solidification would require additional ten to tens of millions of years. Reconciling these short model timescales with radiometric dates of crustal samples requires either a very late-forming Moon combined with finding older crustal ages to be incorrect, or calling on tidal heating of the crust by the early Earth to prolong solidification. Gravitationally driven overturn of cumulates during tidal heating provides a mechanism for creating the compositions and ages of the lunar Mg suite of crustal rocks. Further, we find that upon crystallization, the Moon likely began with an azimuthally heterogeneous, gravitationally stable mantle, after magma ocean cumulate overturn. This result may help explain the experimentally determined origin of picritic glasses at similar depths but from different source materials.

Pre-Cenozoic sedimentary section and structure as reflected in the JavaSPAN TM crustal-scale PSDM seismic survey, and its implications regarding the basement terranes in the East Java Sea

Abstract A new long-offset, long-record crustal-scale seismic survey of 9600 km called JavaSPAN was acquired in the Java Sea and Makassar Strait. The East Java Sea is underlain by continental basement with a prolonged multiphase history of deposition punctuated by extensional and compressional events. This East Java Terrane is a major component of SE Sundaland lying between the Meratus suture, the contemporary Java arc, and the west Sulawesi orogenic belt, but is poorly constrained on the north under the North Makassar Basin and in Kalimantan. A Precambrian to Permo-Triassic sedimentary section up to some 8.5 km in thickness overlies crystalline basement in a number of fault blocks and synformal structures below a strong angular unconformity. A thin overlap assemblage of Cretaceous to early Cenozoic sediments overlies that unconformity. Middle Eocene to Neogene clastic and carbonate rocks overlie another angular unconformity that marks the initiation of a well known history of Palaeogene extension, sag, and Neogene inversion. The East Java Terrane rifted from the Bonaparte-Arafura sector of northern Australia in the Jurassic and accreted onto a magmatic arc on the SW flank of what is now Kalimantan in the Cretaceous.

Reconstruction of a multi-vent kimberlite eruption from deposit and host rock characteristics: Jericho kimberlite, Nunavut, Canada

The Jericho kimberlite (173.1 ± 1.3 Ma) is a small (~ 130 × 70 m), multi-vent system that preserves products from deep (> 1 km?) portions of kimberlite vents. Pit mapping, drill core examination, petrographic study, image analysis of olivine crystals (grain size distributions and shape studies), and compositional and mineralogical studies, are used to reconstruct processes from near-surface magma ascent to kimberlite emplacement and alteration. The Jericho kimberlite formed by multiple eruptions through an Archean granodiorite batholith that was overlain by mid-Devonian limestones ~ 1 km in thickness. Kimberlite magma ascended through granodiorite basement by dyke propagation but ascended through limestone, at least in part, by locally brecciating the host rocks. After the first explosive breakthrough to surface, vent deepening and widening occurred by the erosive forces of the waxing phase of the eruption, by gravitationally induced failures as portions of the vent margins slid into the vent and, in the deeper portions of the vent (> 1 km), by scaling, as thin slabs burst from the walls into the vent. At currently exposed levels, coherent kimberlite (CK) dykes (< 40 cm thick) are found to the north and south of the vent complex and represent the earliest preserved in-situ products of Jericho magmatism. Timing of CK emplacement on the eastern side of the vent complex is unclear; some thick CK (15–20 m) may have been emplaced after the central vent was formed. Explosive eruptive products are preserved in four partially overlapping vents that are roughly aligned along strike with the coherent kimberlite dyke. The volcaniclastic kimberlite (VK) facies are massive and poorly sorted, with matrix- to clast-supported textures. The VK facies fragmented by dry, volatile-driven processes and were emplaced by eruption column collapse back into the volcanic vents. The first explosive products, poorly preserved because of partial destruction by later eruptions, are found in the central-east vent and were formed by eruption column collapse after the vent was largely cleared of country rock debris. The next active vent was either the north or south vent. Collapse of the eruption column, linked to a vent widening episode, resulted in coeval avalanching of pipe margin walls into the north vent, forming interstratified lenses of country rock-rich boulder breccias in finer-grained volcaniclastic kimberlite. South vent kimberlite has similar characteristics to kimberlite of the north vent and likely formed by similar processes. The final eruptive phase formed olivine-rich and moderately sorted deposits of the central vent. Better sorting is attributed to recycling of kimberlite debris by multiple eruptions through the unconsolidated volcaniclastic pile and associated collapse events. Post-emplacement alteration varies in intensity, but in all cases, has overprinted the primary groundmass and matrix, in CK and VK, respectively. Erosion has since removed all limestone cover.

GEOPHYSICAL MODELLING OF THE MOLOPO FARMS COMPLEX IN SOUTHERN BOTSWANA; IMPLICATIONS FOR ITS EMPLACEMENT WITHIN THE 2 GA LARGE IGNEOUS PROVINCES OF SOUTHERN AND CENTRAL AFRICA

The Molopo Farms Complex is an extremely poorly exposed, major, ultramafic-mafic layered intrusion straddling the southern border of Botswana with South Africa. It lies within the south-western part of the ~2.0 Ga large igneous province of southern and central Africa that includes the better known Bushveld Complex. Integrated interpretation of regional gravity data and new high-resolution airborne magnetic data have constrained the geometry of the Molopo Farms Complex in southern Botswana as a strongly faulted, polyphase intrusion compartmentalised by regional ductile shear zones. Previous models showing that the Complex was emplaced in at least two discrete stages are supported. Ultramafic rocks were initially emplaced as a semi-coherent lopolithic sheet up to about 4 km in thickness cutting across Transvaal Supergroup strata that had already been folded into open eastwest trending dome and basin structures with wavelengths of about 4 km. Steeply dipping, dyke-like ultramafic bodies adjacent to, and within major shears are inferred to be solidified feeders to the main lopolithic part of the MFC. It is likely that the initial ultramafic sheet was emplaced at a high crustal level (<3 km depth) into an attenuated Transvaal Supergroup sequence. This lack of a thick hanging wall sequence is thought to be significant for the emplacement of the succeeding mafic sheets. The ultramafic sheet thermally altered its wall rock and also created a complex fracture system in its hanging wall rocks. Differentiation within the ultramafic sheet produced basal harzburgites overlain by bronzites and possibly mafic sheets. Later mafic/basic sheets and dykes, again fed along shear-controlled, steeply dipping zones, spread into the fracture network created by the initial emplacement of the ultramafic lopolith to form a distinctive spider’s-web pattern on high-resolution airborne magnetic maps. It is proposed that either post-emplacement regional folding or gravitational collapse of the basal ultramafic lopolith produced a major basin with a ~40 km eastwest diameter, north of the Jwaneng-Makopong Shear Zone and smaller basin to the southeast. The newly postulated, steeply dipping ultramafic/mafic feeders, as well as the ultramafic lopolith and areas with anomalous nickel values in soils are considered to be prospective for PGE-bearing magmatic nickel-copper sulphide mineralisation. Magmatic rocks dated at about 2.0 Ga are a common feature, not only of the Kaapvaal Craton, but of all the African cratonic blocks south of the Equator. Reactivated intracratonic faults and shears appear to control emplacement of individual magmatic complexes although a sub-continental thermal anomaly unconfined by lithospheric plate boundaries is a likely driving force for the widespread magmatism.

Late Paleoproterozoic to early Mesoproterozoic Dongchuan Group in Yunnan, SW China: Implications for tectonic evolution of the Yangtze Block

The Dongchuan Group including the Yinmin, Luoxue, Etouchang, and Luzhijiang Formations from the base upward has extensive outcrops in the western Yangtze Block. It was previously thought to be the lower part of the late Mesoproterozoic to early Neoproterozoic Kunyang Group, but there was a lack of precise age determination. The whole sequence is more than 4 km in thickness and consists of conglomerate, sandstone, slate, dolostone, carbonaceous slate, and minor tuffaceous volcanic rocks. The basal unit, the Yinmin Formation, is a continental red-bed deposit thought to have been formed in an intra-continental rift basin. Zircons from the sandstone and volcanic rocks of the Yinmin Formation are analyzed for U–Pb and Lu–Hf isotopes in this study. Detrital zircons from the sandstone have U–Pb age populations at 1.8–1.9 Ga, 2.25–2.35 Ga, ∼2.5 Ga, and 2.7–2.9 Ga with the oldest age of ∼3.7 Ga. Most zircons have high Th/U ratios (>0.1) and were probably derived from igneous rocks, whereas a few 2.05–1.95 Ga grains with low Th/U ratios (<0.1) may have a metamorphic origin. The age groups are roughly consistent with those of known tectonothermal events in the northern Yangtze Block. The youngest detrital zircons of the Yinmin Formation have 207Pb/ 206Pb ages of ∼1780 Ma and dolerite dykes intruding the formation have a zircon 207Pb/ 206Pb age of 1690 ± 32 Ma (2 σ). A zircon 207Pb/ 206Pb age of 1742 ± 13 Ma (2 σ) was obtained from a tuff sample of the Yinmin Formation and is considered to represent the deposition age. In combination with other age data of the Dongchuan Group, the newly obtained U–Pb age data thus constrain the deposition of the Dongchuan Group to the period between ∼1.7 Ga and 1.5 Ga, significantly older than previously thought. Presence of late Paleoproterozoic continental rift in the western Yangtze Block is contemporaneous with the break-up of the Columbia supercontinent, and provide further evidence to support the idea that the Yangtze Block was likely part of the supercontinent. Detrital zircons of the Yinmin Formation mostly have Hf continental modal ages ( T DM C) between 3.5 Ga and 2.6 Ga with the oldest T DM C of 3.9 Ga, indicating that the western Yangtze Block experienced significant continental crustal growth during the Paleo- to Neo-archean and contains Eoarchean crustal rocks. In combination with data from others, our results suggest that the Yangtze Block have a widespread Archean basement. The Archean zircons have both positive and negative ɛ Hf( t) values (−10.1 to +6.5) suggesting both juvenile crustal growth and reworking of older materials. In contrast, the Paleoproterozoic zircons have highly variable but mostly negative ɛ Hf( t) values implying episodic crustal reworking of Archean crustal rocks. Unlike the Paleoproterozoic detrital zircons, igneous zircons from the tuff and dolerite dyke samples mostly have positive ɛ Hf( t) values (−0.8 to +10.1), indicating contributions of juvenile materials from a depleted mantle source due to continental rifting.