Shallow Lithosphere Research Articles

Predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics

The way in which basal tractions, associated with mantle convection, couples with the lithosphere is a fundamental problem in geodynamics. A successful lithosphere‐mantle coupling model for the Earth will satisfy observations of plate motions, intraplate stresses, and the plate boundary zone deformation. We solve the depth integrated three‐dimensional force balance equations in a global finite element model that takes into account effects of both topography and shallow lithosphere structure as well as tractions originating from deeper mantle convection. The contribution from topography and lithosphere structure is estimated by calculating gravitational potential energy differences. The basal tractions are derived from a fully dynamic flow model with both radial and lateral viscosity variations. We simultaneously fit stresses and plate motions in order to delineate a best‐fit lithosphere‐mantle coupling model. We use both the World Stress Map and the Global Strain Rate Model to constrain the models. We find that a strongly coupled model with a stiff lithosphere and 3–4 orders of lateral viscosity variations in the lithosphere are best able to match the observational constraints. Our predicted deviatoric stresses, which are dominated by contribution from mantle tractions, range between 20–70 MPa. The best‐fitting coupled models predict strain rates that are consistent with observations. That is, the intraplate areas are nearly rigid whereas plate boundaries and some other continental deformation zones display high strain rates. Comparison of mantle tractions and surface velocities indicate that in most areas tractions are driving, although in a few regions, including western North America, tractions are resistive.

A heat flow based cooling model for tectonic plates

Plate tectonic theory predicts subsidence and decreasing heat flow as young, hot buoyant lithosphere ages and cools. Oceanic cooling models are re-calibrated to a new ‘hydrothermal-free’ heat flow dataset to provide a better fit to heat flow on old (>80Ma) seafloor while simultaneously allowing for a more reasonable mantle potential temperature (1350°C). The plate model (constant basal temperature) and chablis (constant basal temperature and constant basal heat flow) are tested as viable solutions to the observed data. Both models fit to reasonable values of misfit, but only the plate cooling model is acceptable after considering additional statistical constraints. The best-fitting plate model results in a thinner lithospheric thickness (90km) than previous estimates. These results improve estimates of global heat loss rate through oceanic crust (29.4TW, ∼44TW globally) and improves the estimated temporal evolution of the thermal structure of oceanic lithosphere suggesting a shallower lithosphere–asthenosphere boundary. This model can serve as a reference for estimating the global redistribution of heat by ventilated hydrothermal circulation through young seafloor.

Bending stress and dissipation in subducted lithosphere

Oceanic lithosphere undergoes permanent deformation during subduction once the stresses exceed the elastic limit. Departures from elastic behavior occur by brittle failure in the shallow lithosphere and by a combination of low‐ and high‐temperature creep at greater depths. We combine laboratory‐based rheological models with estimates of slab shape from earthquake hypocenters to quantify the bending stress and dissipation in subduction zones. The peak stress occurs at the depth of the brittle‐ductile transition, which is controlled mainly by lithospheric age. Integrals of the stress over the thickness of the plate are used to evaluate the resistive bending force and the bending moment. A representative value for the resistive force on old oceanic lithosphere is 3 × 1012 N m−1, which is comparable in magnitude to ridge push but opposite in direction. Both the bending force and moment are remarkably insensitive to the rate of subduction. In fact, the bending moment can be approximated using a simple power law rheology and a stress exponent of n ≈ 14. Such a large exponent implies that the lithosphere behaves like a perfectly plastic solid. For most subduction zones the bending moment saturates along the entire plate. As a consequence, the bending stress does not influence the development of curvature during subduction. This behavior may explain why the curvature of subducted lithosphere is nearly independent of age.

Gas hydrate pingoes: Deep seafloor evidence of focused fluid flow on continental margins

Gas hydrates in the shallow subsurface form one of the largest reservoirs of methane in the global organic carbon cycle. Seafloor seeps and associated features represent the venting points of methane released from the shallow lithosphere to the hydrosphere and atmosphere. Here we document the discovery of seep-related seafloor mounds in the Kwanza Basin, offshore Angola, and employ high-resolution three-dimensional seismic analysis to unravel the subsurface plumbing system and the origin of mounds. Mounds with distinct morphologies and geophysical signatures illustrate different development stages associated with the formation and dissociation of shallow gas hydrate, linked to thermogenic fluid migration along salt diapir flanks draining deeply buried salt minibasins. The mounds are more than an order of magnitude larger than previously described submarine hydrate pingoes, and comparable to hydraulic pingoes commonly found in terrestrial periglacial environments, suggesting hydrate volumes of individual mounds up to 1.1 × 106 m3 (equivalent to 2.0 × 108 m3 of methane gas). The interpretation of seismically well-defined seep-related seafloor mounds brings new insight to the occurrence and development of concentrated near-surface gas hydrate accumulations and their relationship with thermogenic fluid migration and host sediment properties along continental margins.

On the origin of the asthenosphere

Abstract Various models of the origin of the asthenosphere are reviewed based on the latest observations on mineral physics of mechanical properties, melting relationships and the thermal models of the upper mantle. Partial melting likely occurs throughout the upper mantle except for the lithosphere. However, in order to explain the inferred shallow lithosphere–asthenosphere boundary (LAB) with a large and sharp velocity reduction in the old oceanic mantle by partial melting, a high end of the geotherm and some mechanisms of melt accumulation are required. Purely thermal sub-solidus model of the asthenosphere is inconsistent with the sharp LAB. A sub-solidus model invoking a role of hydrogen content stratification explains the shallow and sharp LAB but it fails to explain a large velocity drop if the absorption band model of anelasticity is assumed. A review of recent literatures shows that grain-size sensitive anelastic relaxation inevitably has two successive processes, high-frequency elastically accommodated grain-boundary sliding followed by low-frequency diffusion-accommodated anelasticity. Anelastic relaxation caused by elastically accommodated grain-boundary sliding leads to a large velocity reduction (~ 5% or more). Assuming a plausible temperature and water content dependence of the peak frequency of this relaxation, it is shown that a substantial velocity reduction occurs at a nearly constant depth (~ 70 km) in the old oceanic region but at the age-dependent depth corresponding to the ~ 1300 K isotherm in the young oceans. This model also provides an explanation for the seismological observations on the cratonic upper mantle including the observed large velocity drop at the mid-lithosphere discontinuity and a small velocity change at the LAB. Implications for seismic anisotropy and for the geochemical characteristics of the asthenosphere are also discussed.

Comparative Study of Magmatism in East Pacific Rise Versus Nearby Seamounts: Constraints on Magma Supply and Thermal Structure Beneath Mid‐ocean Ridge

Abstract:Major elements of 2202 basalts from the East Pacific Rise (EPR) and 888 basalts from near‐EPR seamounts are used to investigate their differences in magma crystallization pressures and mantle melting conditions. Crystallization pressure calculation from basalts with 5.0wt%<MgO<8.0wt % shows that magma crystallization pressures beneath near‐EPR seamounts are positively and negatively correlated with Na8 and Fe8, respectively. However, these correlations are indistinct in axial lavas, which can be explained by chemical homogenization induced by extensive mixing processes. In each segment divided by major transforms and over‐lapping spreading centers (OSCs), near‐EPR seamount lavas have higher magma crystallization pressures, higher Fe8 and lower Na8 than the EPR lavas, which indicate cooler lithosphere, lower degrees and shallower melting depths beneath near‐EPR seamounts than the EPR. The correlations between magma crystallization pressures and melting conditions beneath near‐EPR seamounts imply that the source thermal state controls the melting degree and melt flux, and then melting process controls the shallow lithosphere temperature and magma crystallization depth (pressure). The cooler mantle sources beneath near‐EPR seamounts produce a lower degree of melting and a less robust magma supply, which results in a deep thermal equilibrium level and high magma crystallization pressure. The magma crystallization pressure decreases significantly as spreading rate of the EPR increases from ∼80 mm/year in the north (16°N) to ∼160 mm/year in the south (19°S), while this trend is unobvious in near‐EPR seamounts. This suggests that the magma supply controlled by spreading rate dominates the ridge crust temperature and magma crystallization depth, while the near‐EPR seamount magma supply is not dominated by the axial spreading rate. Because most seamounts form and gain most of their volume within a narrow zone of 5–15 km from ridge axis, they provide good constraint on magma supply and thermal structure beneath the EPR. High magma crystallization pressures in seamounts indicate dramatic temperature decrease from the EPR. The crystallization pressures of seamount lavas are well correlated with mantle melting parameters but in a blurry relationship with axial spreading rate. Despite the adjacency of the EPR and nearby seamounts, the thermal structure beneath the near‐EPR seamounts are controlled by their own magma supply and conductive cooling, chemically and thermally unaffected by magmatism beneath the ridge axis.

Information theory–based measures of similarity for imaging shallow-mantle discontinuities

Mode conversions, such as the P-to-s (Ps) converted waves, are now employed in a routine manner to image the velocity boundaries in Earth9s interior. However, there exists an ambiguity in establishing shallow-mantle velocity discontinuities in the depth range 100–300 km through the Ps receiver function approach. This primarily stems from overlap in the time windows of arrivals of direct converted phases from the target depth boundaries (∼100–300 km depth) and reverberations originating from shallow structures (e.g., from crust and/or shallow lithosphere layers). Attempts have been made to address this problem. Classically, limited success has been achieved through methods such as the move-out, which are essentially performed in the measurement space. Recognizing this, we explore generic space-related information theory measures of similarity to extract diagnostics of discrimination between the primary converted waves and the multiply reflected arrivals in Ps receiver functions. Various measures of similarity, such as the mutual information (MI), and associated normalized distance measures, like the normalized variation of information (NVI) and normalized information distance (NID), are successfully applied to receiver function data sampling regions of different tectonic regimes of wide antiquity. Our seismological stations are located in the Archean-Proterozoic craton–mobile belt regions of SE India, Canada, and Phanerozoic United States. Significantly, at several locations in SE India, we interpret the unambiguous presence of midlithospheric discontinuities for the first time. We also either reconfirm or negate the presence of midlithospheric discontinuities beneath stations FFC (Flin Flon) and HRV (Harvard) located in North America. This study reinforces the presence of significant velocity contrast features related to the Lehmann discontinuity depth beneath the Precambrian cratonic stations of India (Hyderabad and Cuddapah) and the Phanerozoic Pasadena station in the United States. Further, multiple deep discontinuities of opposing velocity contrasts are delineated at depths of ∼275 km and ∼320 km beneath station PAS (Pasadena). Our results therefore show tremendous potential to unambiguously detect and distinguish between direct converted seismological phases and multiple reverberations. These information theory approaches discriminate the seismic phases unequivocally. This new approach thus complements the S-to-p (Sp) receiver function technique, which is suitable for detection of shallow-mantle discontinuities (midlithospheric discontinuities, lithosphere-asthenosphere boundary, Lehmann, etc.).

High-resolution imaging of the deep anisotropic structure of the San Andreas Fault system beneath southern California

SUMMARY Geological and geodetic records show that motion is distributed over a broad and complex system of faults in southern California, with many large earthquakes occurring off the main San Andreas fault. This unusually complex plate boundary geometry is imposed by the big bend of the San Andreas Fault and the dynamics of the underlying lithosphere, but lithosphere rheology and distribution of applied forces are still poorly constrained. Geodynamic modelling is thus very uncertain, and distribution of lithospheric deformations highly controversial. Here, exploiting the property that deformation orients olivine crystals, we show that deep lithospheric deformations can be mapped directly with a new imaging approach of seismic anisotropy, with a resolution fine enough for detailed geodynamic interpretation. This method relies entirely on finite-frequency effects in the splitting of SKS waves. We build an extensive data set of more than 3400 SKS splitting measurements from the analysis of SKS waves recorded by all the broad-band stations in southern California. Resolution tests demonstrate that, using this data set, we are able to resolve anisotropic structures smaller than the size of the first Fresnel zone of SKS waves. The good vertical resolution in our tomographic images results from the short inter-station spacing in southern California. In our 3-D anisotropy model, we find no evidence for localized lithospheric shear deformation beneath the San Andreas Fault. Instead, the lithospheric plate boundary is localized along a broad shear zone beneath the East California Shear Zone. Therefore, surface and deep deformation patterns are poorly correlated and most likely decoupled. Active N–S convergence in the Transverse Ranges results in strongly coherent E–W alignment of olivine fast axes in the shallow lithosphere. At the top of the asthenosphere, the observed low level of anisotropy suggests weak lithospheric basal tractions and thus a strong decoupling between the lithosphere and the asthenosphere.

Flash weakening of serpentinite at near-seismic slip rates

[1] To investigate the processes responsible for dynamic frictional weakening in antigorite serpentinite, we conducted single-velocity and velocity-stepping friction experiments at slip rates (V) from 0.01 to 0.4 m s−1, sliding displacements up to 40 mm, and a normal stress of 5 MPa. Single-velocity experiments demonstrate an approximately 1/V dependence of friction on velocity above a characteristic weakening velocity Vw ≈ 0.1 m s−1, consistent with theoretical predictions for flash heating and subsequent weakening of asperities. Velocity-stepping experiments impose stepwise increases in slip rate show stepwise weakening at slip rates above Vw. Scanning electron microscopy of experimental fault surfaces reveals nanoparticle gouge textures at raised sites (∼10 μm in diameter) in tests that exhibit dramatic weakening. Furthermore, X-ray diffraction analyses of fault gouge from the sliding surface document the formation of significant talc in these tests, indicating that weakening temperatures reached 500°C–700°C. In contrast, no talc is observed in samples for which V remained significantly below Vw. The observed value for Vw is consistent with published microphysical models for flash weakening when independent constraints on the thermal stability and contact strength of antigorite are considered. Finally, while serpentinite displays velocity-strengthening behavior at plate tectonic slip rates, our results indicate that seismic ruptures propagating into serpentinized regions in the shallow lithosphere may trigger seismicity or slow earthquakes, after limited amounts of displacement.

Rapid carbon sequestration at the termination of the Palaeocene–Eocene Thermal Maximum

The Palaeocene–Eocene Thermal Maximum warm event about 56 million years ago was caused by the release of large amounts of carbon to the ocean and atmosphere. Estimates of the rate of recovery from the event suggest that about 2,000 Pg of the carbon released was sequestered as organic carbon. The Palaeocene–Eocene Thermal Maximum (PETM), an approximately 170,000-year-long period of global warming about 56 million years ago, has been attributed to the release of thousands of petagrams of reduced carbon into the ocean, atmosphere and biosphere1,2. However, the fate of this excess carbon at the end of the event is poorly constrained: drawdown of atmospheric carbon dioxide has been attributed to an increase in the weathering of silicates or to increased rates of organic carbon burial1,3,4,5. Here we develop constraints on the rate of carbon drawdown based on rates of carbon isotope change in well-dated marine and terrestrial sediments spanning the event. We find that the rate of recovery is an order of magnitude more rapid than that expected for carbon drawdown by silicate weathering alone. Unless existing estimates of carbon stocks and cycling during this time are widely inaccurate, our results imply that more than 2,000 Pg of carbon were sequestered as organic carbon over 30,000–40,000 years at the end of the PETM. We suggest that the accelerated sequestration of organic carbon could reflect the regrowth of carbon stocks in the biosphere or shallow lithosphere that were released at the onset of the event.

Thermal‐mechanical behavior of oceanic transform faults: Implications for the spatial distribution of seismicity

To investigate the spatial distribution of earthquakes along oceanic transform faults, we utilize a 3‐D finite element model to calculate the mantle flow field and temperature structure associated with a ridge‐transform‐ridge system. The model incorporates a viscoplastic rheology to simulate brittle failure in the lithosphere and a non‐Newtonian temperature‐dependent viscous flow law in the underlying mantle. We consider the effects of three key thermal and rheological feedbacks: (1) frictional weakening due to mantle alteration, (2) shear heating, and (3) hydrothermal circulation in the shallow lithosphere. Of these effects, the thermal structure is most strongly influenced by hydrothermal cooling. We quantify the thermally controlled seismogenic area for a range of fault parameters, including slip rate and fault length, and find that the area between the 350°C and 600°C isotherms (analogous to the zone of seismic slip) is nearly identical to that predicted from a half‐space cooling model. However, in contrast to the half‐space cooling model, we find that the depth to the 600°C isotherm and the width of the seismogenic zone are nearly constant along the fault, consistent with seismic observations. The calculated temperature structure and zone of permeable fluid flow are also used to approximate the stability field of hydrous phases in the upper mantle. We find that for slow slipping faults, the potential zone of hydrous alteration extends greater than 10 km in depth, suggesting that transform faults serve as a significant pathway for water to enter the oceanic upper mantle.

Stratigraphy of the Archean western Superior Province from P- and S-wave receiver functions: Further evidence for tectonic accretion?

The Archean western Superior Province in Canada represents the nucleus of the North American continent whose origin has been speculated to be the result of widespread crustal accretion some 2.7 Ga ago. In this paper, crustal and upper-mantle seismic discontinuities beneath the western Superior Province of the Canadian shield are imaged with teleseismic P-to-S and S-to-P converted phases using the receiver function method. Three crustal discontinuities are observed: the Moho, ranging in depth between 38 and 47 km and dipping to the south; and two intra-crustal discontinuities having depths of approximately 15 and 30 km. The crustal discontinuities undulate laterally and often lose continuity, possibly indicating an imbricated structure and/or regions of velocity gradients. In the shallow lithosphere, a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide-angle reflection results. Additionally, two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low P- and S-wave velocities as well as a zone of high electrical conductivity. The images for the crust and shallow upper-mantle, when integrated with previous geophysical studies, are consistent with ideas of continental root formation due to imbrication of Archean subducted material and accretion of island arcs observed in surface geology.

Seismic evidence for large‐scale compositional heterogeneity of oceanic core complexes

Long‐lived detachment faults at mid‐ocean ridges exhume deep‐seated rocks to form oceanic core complexes (OCCs). Using large‐offset (6 km) multichannel seismic data, we have derived two‐dimensional seismic tomography models for three of the best developed OCCs on the Mid‐Atlantic Ridge. Our results show that large lateral variations in P wave velocity occur within the upper ∼0.5–1.7 km of the lithosphere. We observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology, and we use these correlations to show that gabbros are heterogeneously distributed as large (tens to >100 km2) bodies within serpentinized peridotites. Neither the gabbros nor the serpentinites show any systematic distribution with respect to along‐isochron position within the enclosing spreading segment, indicating that melt extraction from the mantle is not necessarily focused at segment centers, as has been commonly inferred. In the spreading direction, gabbros are consistently present toward the terminations of the detachment faults. This suggests enhanced magmatism during the late stage of OCC formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation. Heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults, and it therefore may explain eventual abandonment of detachments that form OCCs. Detailed seismic studies of the kind described here, when constrained by seafloor morphology and geological samples, can distinguish between major lithological units such as volcanics, gabbros, and serpentinized peridotites at lateral scales of a few kilometers. Thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced.

Seismic anisotropy in northern and central Gulf of California region, Mexico, from teleseismic receiver functions and new evidence of possible plate capture

We present anisotropic models for the shallow lithosphere in the northern and central Gulf of California obtained from receiver function (RF) analysis. This region has undergone intense plate reorganization over the past 25 Ma that led to the transfer of the plate boundary east of the modern Baja California Peninsula posterior to the cease of the subduction of the Farallon plate beneath the North American plate. Our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary. We calculate RFs from teleseismic P waves recorded by the Network of Autonomously Recording Seismographs(NARS)‐Baja array, and we stack them in 20° wide back azimuthal bins to enhance coherent information. These RFs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping Moho. Therefore we interpret the conspicuous RFs energy of the transverse component as convincing evidence of anisotropy. We present forward models obtained by fitting the radial and transverse RFs with synthetic seismograms calculated with a reflectivity code. Our analysis suggests substantial variations in the structure of the shallow lithosphere along the Baja California Peninsula and the presence of stalled fragments of the extinct Farallon plate beneath its central part. The results at station NE81 in Sonora are interpreted as frozen crustal anisotropy inherited from Miocene Basin and Range extension, consistent with previous observations of the upper mantle anisotropy beneath this station.

Nonvolcanic seafloor spreading and corner‐flow rotation accommodated by extensional faulting at 15°N on the Mid‐Atlantic Ridge: A structural synthesis of ODP Leg 209

Drilling during ODP Leg 209, dredging, and submersible dives have delineated an anomalous stretch of the Mid‐Atlantic Ridge north and south of the 15°20′N Fracture Zone. The seafloor here consists dominantly of mantle peridotite with gabbroic intrusions that in places is covered by a thin, discontinuous extrusive volcanic layer. Thick lithosphere (10–20 km) in this region inhibits magma from reaching shallow levels beneath the ridge axis, thereby causing plate accretion to be accommodated by extensional faulting rather than magmatism. The bathymetry and complex fault relations in the drill‐core suggest that mantle denudation and spreading are accommodated by a combination of high‐displacement, rolling‐hinge normal faults and secondary lower‐displacement normal faults. These extensional faults must also accommodate corner flow rotation (up to 90°) of the upwelling mantle within the shallow lithosphere, consistent with remnant magnetic inclinations in denuded peridotite and gabbro from Leg 209 core that indicate up to 90° of sub‐Curie‐temperature rotation.

Surface wave tomography of the Barents Sea and surrounding regions

The goal of this study is to refine knowledge of the structure and tectonic history of the European Arctic using the combination of all available seismological surface wave data, including historical data that were not used before for this purpose. We demonstrate how the improved data coverage leads to better depth and spatial resolution of the seismological model and discovery of intriguing features of upper-mantle structure. To improve the surface wave data set in the European Arctic, we extensively searched for broad-band data from stations in the area from the beginning of the 1970s until 2005. We were able to retrieve surface wave observations from regional data archives in Norway, Finland, Denmark and Russia in addition to data from the data centres of IRIS and GEOFON. Rayleigh and Love wave group velocity measurements between 10 and 150 s period were combined with existing data provided by the University of Colorado at Boulder. This new data set was inverted for maps showing the 2-D group-velocity distribution of Love and Rayleigh waves for specific periods. Using Monte Carlo inversion, we constructed a new 3-D shear velocity model of the crust and upper mantle beneath the European Arctic which provides higher resolution and accuracy than previous models. A new crustal model of the Barents Sea and surrounding areas, published recently by a collaboration between the University of Oslo, NORSAR and the USGS, constrains the 3-D inversion of the surface wave data in the shallow lithosphere. The new 3-D model, BARMOD, reveals substantial variations in shear wave speeds in the upper mantle across the region with a nominal resolution of 1° × 1°. Of particular note are clarified images of the mantle expression of the continent-ocean transition in the Norwegian Sea and a deep, high wave speed lithospheric root beneath the Eastern Barents Sea, which presumably is the remnant of several Palaeozoic collisions.

Anisotropy in Crust and Upper Mantle Beneath China Continent and Its Adjacent Seas

AbstractUsing the difference of shear wave velocity structures derived respectively from Love and Rayleigh waves, the polarization anisotropy in crust and upper mantle beneath China continent and adjacent seas is studied. The result indicates that the distribution of anisotropy is spatially uneven. (1) VSH > VSV anisotropy is dominant in the upper 150 km, which indicates that the horizontal stress in shallower lithosphere and the horizontal mass flow in upper asthenosphere are the leading factors in the formation of polarization anisotropy. In the land area the anisotropy varies markedly with depth. It is generally very weak in upper crust and upper mantle lid, while it becomes intense in the rheologically weak lower crust and asthenosphere, which indicates the lower crust may take a role of decoupling in the lithosphere deformation. (2) VSH < VSV anisotropy is the main characteristic of the lower asthenosphere beneath 200 km, which reflects the vertical movement of the mantle mass. (3) The overall pattern of anisotropy in eastern China continent is that in stable blocks VSH > VSV anisotropy is more prominent in lithosphere and VSH < VSV anisotropy is weaker in asthenosphere, in tectonically active areas it is contrary. (4) In west China the anisotropy is apparently related with the subduction of India plate beneath Tibetan Plateau. The conspicuous VSH > VSV anisotropy is the result of the collision and compression between India plate and Eurasia continent which causes orientated recrystallization of mantle material.

Spatial and temporal constraints on sources of seismic anisotropy: Evidence from the Scottish highlands

Routine SKS splitting analyses of seismic anisotropy usually suffer from limitations in station density that hinder attempts to place spatial constraints on anisotropic fabrics. Data from a ∼20 km station spacing network in Scotland show that splitting parameters vary considerably (δt = 0.45–1.5 s; ϕ = 49–128°) over short (10–20 km) length scales. Improved spatial constraints then lead to tighter temporal constraints on the anisotropic sources. Asthenospheric fabrics due to Tertiary rifting and present day plate motions do not strongly influence our results that instead correlate with lithospheric scale trends inferred from surface geology. Splitting observations track Scotland's tectonic history from the Precambrian emplacement of crustal basement and through the activation of large‐scale faulting and thrusting during the Caledonian Orogeny. The shallow lithosphere beneath Scotland has preserved a fossil anisotropic signature up to hundreds of millions of years after it was formed.

Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites

Deformation and hydration processes are intimately linked in the oceanic lithosphere, but the feedbacks between them are still poorly understood, especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and/or mass transfer. Serpentinization is accompanied by abundant veining marked by different generations of vein‐filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation. We use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow‐spreading ridges. We have selected a representative set of veins from ocean floor serpentinites of the Mid‐Atlantic Ridge near Kane transform fault (23°N) and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy (SEM, TEM) with electron microprobe analyses. Four main veining episodes (V1 to V4) accompany the serpentinization. The first episode, identified as vein generation V1, is interpreted as the tectonically controlled penetration of early seawater‐dominated fluid within peridotites, enhancing thermal cracking and mesh texture initiation at 3–4 km up to 8 km depth and at T <300–350°C. The two following vein stages (V2 and V3) formed in a closed, diffusive system and accommodate volume expansion required to reach almost 50% serpentinization of the protolith. The cracks exploited by these veins were caused by the progressive unroofing at depths of ∼4 to ∼2 km along a detachment fault. Degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization, and this capacity is in turn linked to the exhumation rate and local tectonics. During this period, water consumed by hydration may prevent the establishment of convective hydrothermal cells. The onset of an open hydrothermal system in the shallow lithosphere (<2 km), where brittle fracturing and advective transfer dominate and enable the completion of serpentinization, is marked by the last vein generation (V4). These results show a complete history of alteration, with the crystallization of different types of serpentine recording different tectonic events, chemical conditions, and modes of hydrothermal alteration of the lithosphere.

Improving depth resolution of teleseismic tomography by simultaneous inversion of teleseismic and globalP-wave traveltime data-application to the Vrancea region in Southeastern Europe

Over the years, teleseismic tomography has developed to be a sophisticated method to study the Earth's upper mantle on a regional scale. Using data from tomographic experiments with temporary station networks, one faces some inherent problems, which include limited resolution at depth and artefacts due to a plane-wave approximation at the bottom of the model volume. Simultaneous inversion of dense regionally recorded teleseismic and global P-wave traveltime data provides an opportunity to overcome these specific problems. The calculation of the entire ray path using a 3-D ray tracing algorithm and a non-linear iterative inversion scheme allow to localize heterogeneities in the Earth's mantle and to improve resolution at depth. Application of a variable parametrization scheme provides not only a regional high-resolution model but additionally allows to include a priori constrained structures such as a crustal model derived from independent studies. We investigated the effect of different inversion strategies for a priori constrained model parameters and found that, for upper-mantle studies, one must allow further perturbation of the known velocity structure during inversion to avoid artefacts down to the mantle transition zone. We apply this approach to the Romanian Vrancea region in Southeastern Europe. The results show a near-vertical, narrow high-velocity body underneath that region extending down to 280 km depth, approximately outlining the narrowly spaced seismogenic volume and a deeper, differently oriented positive anomaly coupled to the shallower segment at the latter's southwestern edge. At north, northwest and west of the Vrancea region, we find an extended region of decreased seismic P-wave velocity down to 200 km depth, being interpreted as a shallow lithosphere–asthenosphere boundary and asthenospheric mantle flow due to lateral migration of the high-velocity body. From synthetic reconstruction tests, we found that inversion of the combined data set of regionally recorded teleseismic and global traveltime data enhances resolution up to depths greater than could be resolved by the teleseismic data alone.