Lower Crustal Bodies Research Articles

Understanding the crustal architecture beneath the Bangui magnetic anomaly and its interactions with central African tectonic megastructures based gravity and magnetic analysis

The Bangui magnetic anomaly (BMA) in Central Africa is one of the largest continental magnetic anomalies on Earth in terms of amplitude and lateral size. Determining the sources of the BMA can lead to an increased understanding of the crustal dynamic in the Central African sub-region and the African continent as a whole. Magnetic and gravity analysis-based derivative, two-dimensional forward modelling and a Curie isothermal depth, showed that (a) the bottoms of the magnetic sources were between 15 and 35 km; (b) the BMA is a coalescence of several anomalies that trend E-W and roughly NE-SW. These directions coincide with regional Pan African-aged shear zones along the Central African orogenic belt and to thrust sheets at the northern edge of the Congo Craton. The depth of magnetization does not exceed 35 km with the amplitude of magnetization becoming smaller in the Central African Republic. The potential magnetic susceptibility sources have an average density of 2850 kg/m3 and magnetic susceptibilities between 0.06 and 0.25 SI. The BMA is interpreted to be a combination of middle and lower crustal bodies that are not continuous and consist of magnetic mineral rich granulites and banded iron formations. The gravity and magnetic modelling indicate that the entire crust was involved in the Pan African collisional event similar to what is seen in the Mozambique belt in East Africa. Combined with geological and geochemical studies, the models add evidence that one or two subduction zones were involved in accreting terranes on the northern edge of the Congo Craton. The tectonic accretions caused a crustal remobilization along major shear zones that has locally contributed to a probable circulation of fluids enriched in ferromagnesian minerals during late Neoproterozoic magmatism that created the BMA sources.

Transition from amagmatic to volcanic margin: Mantle exhumation in the Vøring Basin before the Icelandic plume influence

The outer Vøring Basin, Norwegian Sea, is an abnormally deep and distal part of the Norwegian passive margin, known as “magma-rich”. Yet, the lastest borehole drilled in its northernmost part led to a drastic change in the interpretation of seismic reflection data. This update motivates a new model for the entire Mesozoic-Eocene rifting period. Here I evaluate the link between tectonics, serpentinization and magmatism in the Norwegian margin from the Ryazanian-Valanginian to the Palaeocene-Eocene magmatic breakup. Maps of the regional Nise sandstones and main regional unconformities specifically highlight the fundamental role of the Nyk-Vema Structure to understand the basin and to question the deformation timing related to deep structures. The top of partially serpentinized mantle is supported by high-amplitude reflectors clearly visible on certain seismic lines, known as “T-Reflector”, covered with gravity-driven pre-exhumation sediments and further deposits. A new structural scheme based on the interpretation of 2D and 3D seismic data shows the complex organization of fault networks and constrains a major deformation period between the Late Campanian and the Late Palaeocene. Ocean-scale complementary arguments are in good agreement with Lower-Cretaceous mantle exhumation. Before becoming a volcanic margin in the Palaeocene, this segment of the Norwegian margin appears as amagmatic until the Late Campanian, at least. I propose an integrated model since the onset of hyperextension in the end of the Jurassic until the cessation of activity of the Aegir Ridge in the Miocene. This model has implications regarding the context-dependent interpretation of lower-crustal bodies, the ubiquity of serpentinization and mantle exhumation worldwide, and on the hydrocarbon system.

The magmatic plumbing system of the Askja central volcano, Iceland, as imaged by seismic tomography

AbstractThe magmatic plumbing system beneath Askja, a volcano in the central Icelandic highlands, is imaged using local earthquake tomography. We use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the P wave velocity (Vp) and the Vp/Vs ratio. Extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is ~4 km in the upper crust (depth < 8 km below sea level (bsl)), increasing to ~10 km in the lower crust at a depth of 20 km bsl. The plumbing system of Askja is revealed as a series of high‐Vp/Vs ratio bodies situated at discrete depths throughout the crust to depths of over 20 km. We interpret these to be regions of the crust which currently store melt with melt fractions of ~10%. The lower crustal bodies are all seismically active, suggesting that melt is being actively transported in these regions. The main melt storage regions lie beneath Askja volcano, concentrated at depths of 5 km bsl with a smaller region at 9 km bsl. Their total volume is ~100 km3. Using the recorded waveforms, we show that there is also likely to be a small, highly attenuating magmatic body at a shallower depth of about 2 km bsl.

Wilson cycle passive margins: Control of orogenic inheritance on continental breakup

Rifts and passive margins often develop along old suture zones where colliding continents merged during earlier phases of the Wilson cycle. For example, the North Atlantic formed after continental break-up along sutures formed during the Caledonian and Variscan orogenies. Even though such tectonic inheritance is generally appreciated, causative physical mechanisms that affect the localization and evolution of rifts and passive margins are not well understood.We use thermo-mechanical modeling to assess the role of orogenic structures during rifting and continental breakup. Such inherited structures include: 1) Thickened crust, 2) eclogitized oceanic crust emplaced in the mantle lithosphere, and 3) mantle wedge of hydrated peridotite (serpentinite).Our models indicate that the presence of inherited structures not only defines the location of rifting upon extension, but also imposes a control on their structural and magmatic evolution. For example, rifts developing in thin initial crust can preserve large amounts of orogenic serpentinite. This facilitates rapid continental breakup, exhumation of hydrated mantle prior to the onset of magmatism. On the contrary, rifts in thicker crust develop more focused thinning in the mantle lithosphere rather than in the crust, and continental breakup is therefore preceded by magmatism. This implies that whether passive margins become magma-poor or magma-rich, respectively, is a function of pre-rift orogenic properties.The models show that structures of orogenic eclogite and hydrated mantle are partially preserved during rifting and are emplaced either at the base of the thinned crust or within the lithospheric mantle as dipping structures. The former provides an alternative interpretation of numerous observations of ‘lower crustal bodies’ which are often regarded as igneous bodies. The latter is consistent with dipping sub-Moho reflectors often observed in passive margins.

Asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern South Atlantic

The southern South Atlantic has often been considered a classic example of continental break-up in the presence of a starting mantle plume. Evidence for a mantle plume includes the Paranà-Etendeka continental flood basalts, which are associated with the Rio Grande Rise and Walvis Ridge, and the wide-spread presence of seaward dipping reflectors and high-velocity lower-crustal bodies along the conjugate margins. Observations from seaward dipping reflector distributions suggested that lithospheric segmentation played a major role in the pattern of volcanism during break-up in this region, and consequent numerical modelling was used to test this. We tested this hypothesis ourselves by measuring the thickness of the earliest oceanic crust generated. This was done through the use of 37 measurements of initial oceanic crustal thickness from wide-angle and multichannel seismic profiles collected along the conjugate margins. These measurements show that at 450kmsouth of the Paranà-Etendeka flood basalts the oceanic crust is thicker than the global average at 11.7km. Farther south the oceanic crust thins, reaching 6.1km at a distance of 2300km along-strike. Overall, the along-strike trend of oceanic crustal thickness is linear with a regression coefficient of 0.7 and little indication of segmentation. From numerical models representing extension of the lithosphere, we find that observed melt volumes are matched with the presence of a hot layer. If we assume this region of hot mantle has a thickness of 100km, its excess temperature relative to the asthenosphere has to decrease from 200 to 50°C, north to south. This decrease in temperature, also seen in published thermobarometry results, suggests that temperature was the primary control of volcanism during the opening of the southern South Atlantic.

Deep structure of the Argentine margin inferred from 3D gravity and temperature modelling, Colorado Basin

Following previous work on the Colorado Basin using a 3D crustal structural model, we now investigate the presence of lower crustal bodies at the base of the crust using 3D lithospheric gravity modelling and calculations of the conductive thermal field.Our first study highlighted two fault directions and depocentres associated with thinned crust (NW–SE in the West and NE–SW at the distal margin). Fault relative chronology argues for two periods of extension: (1) NW–SE faulting and thinning in the western Colorado Basin and (2) NE–SW faulting and thinning related to the continental breakup and formation of the NE–SW-striking volcanic margins of the Atlantic Ocean.In this study, the geometry of modelled high-density Lower Crustal Bodies (LCBs) enables the reproduction of the gravimetric field as well as of the temperature measured in wells down to 4500m. The modelled LCBs correlate with geological observations: (1) NW–SE LCBs below the deepest depocentres in the West, (2) NE–SW LCBs below the distal margin faults and the seaward dipping reflectors. Thus the proposed poly-phased evolution of the margin could as well correspond to two emplacement phases of the LCBs. The calculated conductive thermal field fits the measured temperatures best if the thermal properties (thermal conductivity and radiogenic heat production) assigned to the LCBs correspond to either high-grade metamorphic rocks or to mafic magmatic intrusions. To explain the possible lithology of the LCBs, we propose that the two successive phases of extension are accompanied by magma supply, emplaced (1) in the thinnest crust below the older NW–SE depocentres, then (2) along the NE–SW continentward boundary of the distal margin and below the volcanic seaward dipping reflectors. The South African conjugate margin records only the second NE–SW event and we discuss hypotheses which could explain these differences between the conjugate margins.

Lower crustal bodies in the Møre volcanic rifted margin: Geophysical determination and geological implications

Understanding nature, structure and age of Lower Crustal Bodies (LCBs) and their relation to the crustal structure of the Møre margin (Mid-Norwegian margin), and in a more general way, of magma-rich rifted margins, is a key issue to decipher the tectono-magmatic processes found in volcanic rifted margins. In light of 2D potential field modelling combined with reflection and refraction seismic data, we reinvestigated the crustal nature of the Møre margin and adjacent Jan Mayen corridor. In the proximal domain, our study shows that the LCBs most likely represent inherited crustal bodies and not necessarily rift-related serpentinised mantle as previously proposed. To fit all geophysical observations, both lower and middle crustal layers need to be preserved over a large part of the Møre Basin. For the distal margin, the interpretation of the LCBs is more difficult. Our preferred interpretation is that they are mainly made of boudins of hyper-extended, pre-rift lower continental crustal rocks more or less intruded by Early Tertiary magmatic material. Our seismic, magnetic and gravity data does not easily support large scale exhumation of serpentinised mantle in the inner and is unlikely in the outer parts of the Møre Basin. The deep structures of the Mid-Norwegian magma rich rifted margin result from the poly-phase stretching and thinning of complex inherited crustal structures, locally intruded by Early Tertiary magmatic material.

Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation—including examples from the Norwegian Sea

The conditions permitting mantle serpentinization during continental rifting are explored within 2‐D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature‐dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2‐D transects have been reconstructed through the 3‐D data set by Scheck‐Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle.

Properties and distribution of lower crustal bodies on the mid-Norwegian margin

Abstract Anomalously high velocity and high density bodies have been detected in the lower crust on the mid-Norwegian margin. The lower crustal bodies (LCB) are pronounced on the Møre and Vøring margins segments and have mainly been interpreted as either magmatic or high-grade metamorphic in origin. Evolutionary models of the whole margin are heavily affected by the interpretation of the LCB and so are estimates of vertical movements and thermal structure in the area. A 3D gravity and magnetic model of the mid-Norwegian margin was constructed to map the main geological features of the margin and acquire the distribution of the LCB. The model utilizes the most recent potential field compilations on the margin and is constrained by extensive reflection seismic data and published refraction profiles. Further constraints on the model were attained from studying the isostatic state of the lithosphere. We present a map showing the distribution of the different LCB and discuss the implications for the structural and thermal evolution of the margin. The properties of the LCB vary across the margin and at least three different processes may be responsible for their existence. The LCB is commonly interpreted as igneous rock either intruded into the lower crust or underplated beneath it. The distribution of the LCB along the Vøring margin has an apparent correlation with the offshore prolongations of major onshore detachments stemming from Late Caledonian orogenic collapse. This may point towards some relation between the LCB and these old zones of weakness and that the LCB represents high-grade metamorphic rocks. Detailed modelling on the Møre margin shows a spatial link between parts of the LCB and extremely thin crustal thickness, suggesting a serpentinized exhumed mantle origin.

The structure of the lower crust at the Argentine continental margin, South Atlantic at 44°S

It is well established that the Argentine passive margin is of the rifted volcanic margin type. This classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope, manifested by seismic data as a seaward dipping reflector sequence (SDRS). Here, we investigate the deep structure of the Argentine volcanic margin at 44°S over 200 km from the shelf to the deep oceanic Argentine Basin. We use wide-angle reflection/refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times. The resulting P-wave velocity-depth model confirms the typical volcanic margin structure. An underplated body is resolved as distinctive high seismic velocity ( v p up to 7.5 km/s) feature in the lower crust in the prolongation of a seaward dipping reflector sequence. A remarkable result is that a second, isolated body of high seismic velocity ( v p up to 7.3 km/s) exists landward of the first high-velocity feature. The centres of both bodies are 60 km apart. The high-velocity lower-crustal bodies likely were emplaced during transient magmatic–volcanic events accompanying the late rifting and initial drifting stages. The lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the South Atlantic rift evolved by instantaneous breakup of longer continental margin segments. These segments are confined by transfer zones that acted as rift propagation barriers. A lower-crustal reflector was detected at 3 to 5 km above the modern Moho and probably represents the lower boundary of stretched continental crust. With this finding we suggest that the continent–ocean boundary is situated 70 km more seaward than in previous interpretations.

A moderate melting model for the Vøring margin (Norway) based on structural observations and a thermo-kinematical modelling: Implication for the meaning of the lower crustal bodies

High P-wave velocities (7.1–7.8 km/s) lower crustal bodies (LCBs) imaged along volcanic margins are commonly interpreted as plume and breakup-related thick mafic underplating. This interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer Vøring Basin (Norway). An exceptional strong amplitude reflection, the T Reflection, is particularly well defined below the North Gjallar Ridge (NGR) between 7and 8 s TWT. The T Reflection is located near the volcanic lava flows emplaced during the NE Atlantic breakup (∼ 55–54 Ma ago) and coincides with the top of the LCB, forming a mid-crustal dome. Based on structural and temporal relationships, we show that the dome clearly influences the structural development of the NGR and predates the continental breakup at least by 10–15 Ma. Using a thermo-kinematical model, we tried also to investigate and quantify the relationships between the extension, LCB and the magmatic production. Modelling suggests that significant Paleocene–Early Eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the breakup initiation. The conclusion of 2D thermo-kinematical parametric analysis is that the magmatic model predicts, either little extension ( β < 2) with no melting or high extension ( β > 5) with significant melting along the outer Vøring Basin. We suggest that the continental part of the LCB could not necessarily be breakup-related and so magmatic, as has often been stated previously. It is concluded here that the continental part of the LCB observed beneath the outer Vøring Basin may be partly (or fully) attributed to inherited, high-pressure granulite/eclogite lower crustal rocks. The real amount of mafic material emplaced along the outer Vøring Basin could be 20–40% less than thought.

North Atlantic volcanic margins: Dimensions and production rates

Early Tertiary lithospheric breakup between Eurasia and Greenland was accompanied by a transient (∼3 m.y.) igneous event emplacing both the onshore flood basalts of the North Atlantic Volcanic Province (NAVP) and huge extrusive complexes along the continent‐ocean transition on the rifted continental margins. Seismic data show that volcanic margins extend &gt;2600 km along the early Eocene plate boundary, in places underlain by high‐velocity (7.2–7.7 km/s) lower crustal bodies. Quantitative calculations of NAVP dimensions, considered minimum estimates, reveal an areal extent of 1.3×106 km2 and a volume of flood basalts of 1.8×106 km3, yielding a mean eruption rate of 0.6 km3/yr or 2.4 km3/yr if two‐thirds of the basalts were emplaced within 0.5 m.y. The total crustal volume is 6.6×106 km3, resulting in a mean crustal accretion rate of 2.2 km3/yr. Thus NAVP ranks among the world's larger igneous provinces if the volcanic margins are considered. The velocity structure of the expanded crust seaward of the continent‐ocean boundary differs from standard oceanic and continental crustal models. Based on seismic velocities this “volcanic margin” crust can be divided into three units of which the upper unit corresponds to basaltic extrusives. The regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust, including the high‐velocity lower crust which extends some distance landward of the continent‐ocean boundary, was emplaced during and subsequent to breakup. The volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling, probably reflecting the interaction of a mantle plume and a lithosphere already extending.