Low-viscosity Channel Research Articles

Emergence of a low-viscosity channel in subduction zones through the coupling of mantle flow and thermodynamics

We use a petrological model (pHMELTS), coupled with a 2D thermal and variable viscosity flow model (ConMan), to describe and compare fundamental processes occurring within subduction zones. We study the thermal state and phase equilibria of the subducting oceanic slab and adjacent mantle wedge and constrain fluid flux. Using a Lagrangian particle distribution to perform thousands of thermodynamically equilibrated calculations, the chemical state of the domain is continuously updated. Compositionally and thermally dependent buoyancy and viscosity terms provide a consistent linkage between the effect of water addition to and flow within the mantle wedge. We present seven model cases that span normal ranges in subducting slab age, convergence velocity, and slab dip angle. In all models, the coupling between chemistry and dynamics results in behavior previously unresolved, including the development of a continuous, slab-adjacent low-viscosity channel (LVC) defined by hydrous mineral stability and higher concentrations of water in nominally anhydrous minerals (NAM). As the LVC evolves to fluid saturation, slab-derived components are able to migrate vertically upwards to the water-saturated solidus, forming a melting region that bounds the top of the LVC. The LVC develops due to fluid ingress into the mantle wedge from the dehydrating slab, and can be responsible for slab decoupling, large-scale changes in the wedge flow field, and a mechanism by which hydrated slab-adjacent mantle material can be transported to the deep mantle. Varying model parameters indicates that slab age and slab dip angle exert primary control over LVC shape and thickness, due to changing fluid release patterns within the slab. Younger slabs tend to have thinner, more uniform LVCs, while older slabs tend to have a thinner LVC at shallow depths with a large increase in LVC thickness at ∼ 100 km depth. Slab convergence velocity appears to have a secondary role in controlling LVC shape.

Partial melt in the upper-middle crust of the northwest Himalaya revealed by Rayleigh wave dispersion

Seismic shear-wave velocities are sensitive to the partial melts that should be present in the Himalayan orogen if low-viscosity channel flow is active at the present day. We analyzed regional earthquakes in the western Himalaya and Tibet recorded on 16 broadband seismometers deployed across the NW Indian Himalaya, from the Indian platform to the Karakoram Range. We used a multiple filter technique to calculate the group velocity dispersion of fundamental-mode Rayleigh waves, and then inverted the dispersion records to obtain separate one-dimensional shear-wave velocity models for five geologic provinces: the Tibetan plateau, Ladakh arc complex, Indus Tsangpo suture zone, Tethyan Himalaya, and Himalayan thrust belt. Our velocity models show a low-velocity layer (LVL) with 7–17% velocity reduction centered at ~ 30 km depth and apparently continuous from the Tethyan Himalaya to the Tibetan plateau. This LVL shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile. Of the possible explanations for low velocity and low resistivity in the mid-crust, only the presence of melts or aqueous fluids (or both) satisfactorily explains both sets of observations. Elevated heat flow observed in the NW Himalaya implies that if aqueous fluids are present in the mid-crust, then the mid-crust is well above its solidus. Comparison of our results with laboratory measurements and theoretical models suggests 3–7% melt is present in a channel in the upper-middle crust of the NW Himalaya at the present day, and the physical conditions to enable active channel flow may be present.

Viscosity stratification and the horizontal scale of end-driven cellular flow

We analyze isothermal viscosity-stratified creeping flow in a horizontal semi-infinite slot. The slot top and bottom are traction-free, and a cellular motion is driven by a given uniform shear stress applied at the vertical end. The viscosity is a prescribed function of depth: A thin low-viscosity channel of uniform viscosity mμ and thickness αD overlies a thick sublayer of viscosity μ and thickness D. By coupling a lubrication analysis of the channel to an exact solution for the sublayer, we show that in the limit of α→0, with ε=m∕α3 fixed, the flow has horizontal scale given for ε<5 by ℓ=2D∕3ε. The analysis is motivated by the long-standing suggestion that the presence of a low-viscosity layer could explain the existence of long end-driven cells beneath the Pacific and the Indo-Australian plates. Our formula supports that suggestion; applied to published viscosity profiles, it predicts that viscosity stratification could allow a cell length of up to seven times that for isoviscous flow.

Continental Lower-crustal Flow: Channel Flow and Laminar Flow

Numerous geological, geophysical and geochemical investigations and finite element modeling indicate that crustal flow layers exist in the continental crust. Both channel flow model and laminar flow model have been created to explain the flow laws and flow mechanisms. As revealed by the channel flow model, a low-viscosity channel in middle to lower crust in orogen or plateau with thick crust and high elevation would flow outward from mountain root in response to lateral pressure gradient resulted from topographic loading or to denudation. However, according to the laminar flow model proposed based on investigation of the Qinghai-Tibet plateau, circulative movement of crustal lithologies with different rheological properties between basin and orogen would occur, under the driving forces resulted from dehydration and melting of subduction plate on active continental margin and from thermal energy related to upwelling and diapiring of intercontinental mantle plume or its gravitational interactions. Similarly, when driven by gravity, the softened or melted substances of the lower crust in a basin would flow laterally toward adjacent mountain root, which would result in a thinned basin crust and a thickened orogenic crust. Partially melted magma within the thickened orogenic lower crust would cause vertical movement of metamorphic rocks of lower to middle crust due to density inversion, and the vertical main stress induced by thermal underplating of lower crust would in turn lead to formation of metamorphic core complexes and low-angle detachment fault systems. Lateral spreading of uplifting mountain due to gravitation potential would result in thrust fault systems on the border between mountain and basin. Meanwhile, detritus produced synchronously by intense erosion of uplifting mountain would be transported and deposited along the marginal deep depression in the foreland basin dragged by lower crust flow. Channel flow is similar to laminar flow in a variety of aspects, covering continental intraplate deformation, ductile extrusion of middle to lower crust, synchronous transition from orogenic extrusion to extension, ductile deformation and exhumation of deep metamorphic rocks, partial melting, magmatism, etc. However, radical differences exist between the two models, such as in tectonic setting, flow domain, flow surface, flow scale, flow pattern, flow regime, flow direction, flow substance, flow behavior or flow effects, flow time, and flow mechanism. Channel flow can be regarded as part of a spatial-temporal structure of laminar flow, but lower crustal laminar flow is actually driven by thermal energy and gravity and not by surface processes such as denudation or topographic loading. Therefore, from a global viewpoint, laminar flow is only a small part of multi-grade or multi-scale circulative flow system of the earth.

The Seismic Structure and Dynamics of the Mantle Wedge

Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, slab dehydration, arc volcanism, and backarc spreading. The mantle wedge is characterized by a low attenuation forearc, an inclined zone of low velocity and high attenuation underlying the volcanic front, and a broad region of low velocity and high attenuation beneath the backarc spreading center when present. Seismic velocities, bathymetry, and basalt chemistry suggest mantle temperature variations of ∼100°C between different backarc regions. Rock physics experiments and geodynamic modeling are essential for interpreting seismic observations. Seismic anisotropy indicates a complex pattern of mantle flow that can be modeled with along-strike flow in a low viscosity channel beneath the arc and backarc. Comparison of geodynamic models with seismic tomographic results using experimentally derived relations between velocity, attenuation, and temperature suggests the existence of small melt fractions in the mantle at depths of 30–150 km.

Rapid mantle flow beneath the Tonga volcanic arc

Flow in the Earth's asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates, suggesting flow rates comparable to plate velocities. However, arc-parallel shear-wave splitting and geochemical observations imply along-strike flow in the Lau basin, presenting a challenge to geodynamic models of plate-driven subduction systems as plate-driven flow models predict arc-perpendicular fast direction with simple (A-type) mineral texturing. Although B-type mineral texturing would result in the fast directions developing 90° relative to A-type and likely occurs in the forearc mantle, such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail. By tracking mineral texturing development within geodynamic flow models patterned after the Tongan system in the Lau basin that include along-strike flow, we find that the observations are best explained by models including rapid along-arc flow within the subarc mantle. The effect of melt on the anisotropy does not appear to be a primary control, but needs to be investigated more fully. The best fit to the observations is a model with along-strike flow within a low-viscosity channel beneath the arc at a rate of nearly 50 cm/yr, several times the plate motion velocities.

Subduction zone evolution and low viscosity wedges and channels

Dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased. Such a decrease in viscosity could form a low viscosity wedge (LVW) or a low viscosity channel (LVC) on top of the subducting slab. Using numerical models, we investigate the influence of low viscosity wedges and channels on subduction zone structure. Slab dip changes substantially with the viscosity reduction within the LVWs and LVCs. For models with or without trench rollback, overthickening of slabs is greatly reduced by LVWs or LVCs. Two divergent evolutionary pathways have been found depending on the maximum depth extent of the LVW and wedge viscosity. Assuming a viscosity contrast of 0.1 with background asthenosphere, models with a LVW that extends down to 400 km depth show a steeply dipping slab, while models with an LVW that extends to much shallower depth, such as 200 km, can produce slabs that are flat lying beneath the overriding plate. There is a narrow range of mantle viscosities that produces flat slabs (5 to10 × 10 19 Pa s) and the slab flattening process is enhanced by trench rollback. Slab can be decoupled from the overriding plate with a LVC if the thickness is at least a few 10 s of km, the viscosity reduction is at least a factor of two and the depth extent of the LVC is several hundred km. These models have important implications for the geochemical and spatial evolution of volcanic arcs and the state of stress within the overriding plate. The models explain the poor correlation between traditional geodynamic controls, subducting plate age and convergence rates, on slab dip. We predict that when volcanic arcs change their distance from the trench, they could be preceded by changes in arc chemistry. We predict that there could be a larger volatile input into the wedge when arcs migrate toward the trench and visa-versa. The transition of a subduction zone into the flat-lying regime could be preceded by changes in the volatile budget such that the dehydration front moves to shallower depths. Our flat-slab models shed some light on puzzling flat subduction systems, like in Central Mexico, where there is no deformation within the overriding plate above the flat segment. The lack of in-plane compression in Central Mexico suggests the presence of a low viscosity shear zone above the flat slab.

Ophiolite mélange zone records exhumation in a fossil subduction channel

Research Article| June 01, 2007 Ophiolite mélange zone records exhumation in a fossil subduction channel Laura Federico; Laura Federico 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Search for other works by this author on: GSW Google Scholar Laura Crispini; Laura Crispini 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Search for other works by this author on: GSW Google Scholar Marco Scambelluri; Marco Scambelluri 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Search for other works by this author on: GSW Google Scholar Giovanni Capponi Giovanni Capponi 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Laura Federico 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Laura Crispini 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Marco Scambelluri 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Giovanni Capponi 1Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, 16132 Genova, Italy Publisher: Geological Society of America Received: 13 Jul 2006 Revision Received: 09 Jan 2007 Accepted: 15 Jan 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2007) 35 (6): 499–502. https://doi.org/10.1130/G23190A.1 Article history Received: 13 Jul 2006 Revision Received: 09 Jan 2007 Accepted: 15 Jan 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Laura Federico, Laura Crispini, Marco Scambelluri, Giovanni Capponi; Ophiolite mélange zone records exhumation in a fossil subduction channel. Geology 2007;; 35 (6): 499–502. doi: https://doi.org/10.1130/G23190A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Recent models propose that the exhumation of high-pressure rocks occurs by means of return flow inside a low-viscosity channel of serpentinite situated between the plates. To test this hypothesis, we investigated a serpentinite mélange in the Western Alps, which contains exotic mafic and metasedimentary tectonic blocks, recording heterogeneous metamorphic evolutions and variable high-pressure ages. The peak metamorphic conditions range from eclogite- to garnet-blueschist-facies. The structural evidence and the pressure-temperature paths of the different blocks suggest coupling between blocks and matrix, at least in the blue-schist facies. 39Ar-40Ar dating indicates eclogite-facies peak at ca. 43 Ma and blueschist-facies peak at ca. 43 and 40 Ma in different blocks, respectively. These data point to diachronous metamorphic paths resulting from independent tectonic evolutions of the different slices. We therefore propose that this mélange formed during exhumation of subducted rocks equilibrated at different depths inside a subduction channel. This mechanism can be extended to other serpentinite mélanges in the Alps and other orogens (e.g., the Cyclades, the Coast Ranges of California) for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure-temperature paths can be expected with further investigations. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High-resolution estimates of lithospheric thickness from Missouri to Massachusetts, USA

This paper presents a new three-dimensional (3-D) model, NA00, of the S-velocity of the upper mantle beneath North America. The model differs from its predecessor NA95 in that it exploits seismograms recorded by a recent dense, broadband array, MOMA, and from independent measurements of North American crustal thickness. Model NA00 is derived by fitting the waveforms of broadband seismic S and surface waves recorded by the MOMA array and inverting them together with the database of waveform fits used for NA95 and the crustal thickness estimates. It is demonstrated that including data from the dense, broadband MOMA array yields a resolving power beneath the array that is of unprecedented quality and relatively constant over a large depth range. This improved resolution provides a unique opportunity for quantifying the structure of the upper mantle in and below the lower, thick Precambrian lithosphere. The high-resolution seismic structure of the imaged high-velocity lithosphere is compared with the thermal structure (estimated from heat flow), compositional structure (estimated from xenoliths and electrical conductivity) and the elastic structure (estimated from gravity and topography). There is a remarkable agreement between the seismic, thermal, and compositional estimates. The seismic lithosphere is 180 km thick below Missouri and Illinois, 200 km thick below Indiana, Ohio and Pennsylvania, practically undefined below New York, and 80 km below Massachusetts and the Atlantic continental shelf. The thick lithosphere is underlain by a layer with lower S-velocities that could represent a relatively low-viscosity channel. However, the S-velocities in this layer are much higher than those of typical oceanic asthenosphere.

Reply to comment by W. Fjeldskaar ‘What about the asthenosphere viscosity? Sea-level change, glacial rebound and mantle viscosity for northern Europe’

viscosity profiles that best match the observational database. include a low-viscosity channel in the upper mantle (Fjeldskaar Neither point to a well-developed low-viscosity channel in the 2000). In our paper we clearly noted that we examined only a upper mantle, and the second model compares well with the part of the possible earth-model space and that we restricted results from the sea-level analysis only. ourselves to a three-mantle-layer model comprising a lithoThe second reason for not pursuing the more detailed sphere, an upper mantle extending from the base of the solutions for the Scandinavian region is that a major limitation lithosphere down to the 670 km seismic discontinuity, and a in the modelling is the inadequate knowledge of the temporal lower mantle. A broad range of parameters defining these three and spatial distribution of the ice cover over the region, and, layers was explored and no a priori values were attached to based on tests with different ice models, we concluded that this any one layer. This is important in that considerable trade-off was more important than differences in predictions between between parameters can result. We were careful to note that the threeand five-layer mantle models: unless some modification analysis yielded only first-order results and that the resulting of the ice sheet was permitted in the inversion, this would earth-model parameters are effective values that, while they result in a very considerable dependence of earth-model results appear to give an adequate description of the rebound evidence on the adopted ice model (for example, compare Figs 16(a) (the relative sea-level change and shoreline migration), may and (b) and Fig. 33(b) in LSJ). We concluded that further not be a true reflection of the actual physical layering within the improvements in the knowledge of the spatial and temporal Earth. We also noted that further work is required to examine distribution of the ice are required before it becomes worthwhile whether a higher degree of mantle layering is appropriate to explore the earth structure in greater detail. Subsequent (e.g. p. 135). inversions in LSJ therefore placed greater emphasis on the ice The reason we did not conduct a more exhaustive search of model than on the detailed earth structure, although care was the model space was two-fold. The first was that our earlier taken at all times to ensure that an adequate separation of the experience with the British rebound modelling (Lambeck et al. earthand ice-model parameters occurred. 1996) led to the conclusion that five-layer mantle models, Three inversions of independent sea-level data for ice-sheet including the low-viscosity asthenosphere option, yielded only parameters and (three-layered) mantle parameters led to a marginal improvement in the overall comparison of preconsistent conclusions about likely ice thicknesses during Latedictions of sea-level change with observations. Fig. 1 illustrates glacial times. These data sets are the Lateand postglacial the two sets of effective earth-model parameters that were geological evidence for sea-level change across the region found to be optimal for the threeand five-layer models. The (LSJ), the tide-gauge data for the region (Lambeck et al. five-layer model is physically more intuitive but, in the British 1998b), and the timing and elevation data of various Baltic lake Isles case, it does not lead to a significant improvement in the stages (Lambeck 1999). In all cases, thick, quasi-parabolic ice ability to predict Lateand postglacial sea levels and shoreline models, in which ice thickness increases rapidly with distance locations compared to the three-layer model. In this particular from the ice margin, yield predictions that are inconsistent case the observational database has only limited resolving with the sea-level and shoreline observations for the southern power for isolating the higher-resolution structure in the mantle, and southeastern Scandinavia and Baltic regions (see also and for ‘all predictive purposes of sea-level change and shoreTushingham & Peltier 1991 and Lambeck et al. 1990). With line evolution, three-layer or five-layer models are essentially the exception of some of the Norwegian coastal zones, the

Inferences of viscosity from the oceanic geoid: Indication of a low viscosity zone below the 660-km discontinuity

We have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the geoid and tomographic structure beneath the oceans. In order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents, we have considered only the intermediate degrees of the oceanic geoid (ℓ = 12–25). A genetic algorithm has been used to determine the family of viscosity models which give the best correlation with the observed geoid. Our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms the viscosity increase in the lower mantle predicted by previous inferences, but suggests that the main viscosity jump occurs at a depth of about 1000 km and not at the usually stated 660-km boundary. Somewhere in the depth range of 400–1000 km, a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere. Existence of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern with a strong horizontal component in the top part of the lower mantle. Unfortunately, the resolution of the inversion as well as the quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accuracy for the viscosity of this low viscosity channel.

The non-linear initiation of diapirs and plume heads

A simple theory is devised to describe the non-linear feedback mechanisms involved in the initial growth of a single diapir or plume head from a low viscosity channel overlain by a much more viscous layer. Such feedbacks arise primarily from the relation between the growth of a proto-diapir (i.e. an undulation on the upper boundary of the low viscosity channel) and the draining of the low viscosity channel. In the period of time between its initial exponential growth (characterized by linear stability analysis) and its separation from the low viscosity channel as a fully formed diapir, the proto-diapir can undergo a significant cessation in its development due to deflation of the low viscosity channel; i.e. the proto-diapir's growth can essentially stall for a long period of time before it separates and begins its ascent through the overlying medium. The theory is used to determine a criterion for separation of the diapir from the low viscosity channel that is in terms of the geometrical and mechanical properties of the channel, instead of the ad hoc volume flux widely used in many models of mantle plumes and plume heads (e.g. Whitehead and Luther, 1975; Richards et al., 1989; Olson, 1990; Sleep, 1990; Bercovici and Mahoney, 1994). From this separation criterion, self-consistent scaling laws can be formulated to relate the size of the fully developed diapir and its trailing conduit to the properties of the initial channel, instead of to the ad hoc volume flux. Basic laboratory experiments involving highly viscous fluids are presented and demonstrate that the so-called ‘stalling’ period between initial growth and separation does indeed occur. These results suggest that nascent mantle plume heads may stall for extended periods at the base of the mantle and thereby contribute to variations in thickness of the D″ layer.

The 1995 Kobe, Japan, earthquake: A long-delayed aftershock of the offshore 1944 Tonankai and 1946 Nankaido earthquakes

AbstractThe 1995 Kobe earthquake produced predominantly right-lateral slip along the northeast-southwest-trending Nojima fault approximately 200 km landward of the Nankai trough, southwest Japan. The last major earthquakes along the Nankai trough, the M8.0 1944 Tonanakai and M8.2 1946 Nankaido earthquakes, involved about 4 m of interplate slip, each releasing about 100 years of accumulated stress from the northwest-southeast convergence of the Philippine Sea (PHS) plate. By calculating the change in Coulomb failure stress on the Nojima fault due to 1944/46 to 1995 postseismic relaxation of a thin low-viscosity channel beneath southwest Japan, we show that stress diffusion from the 1940s interplate earthquakes increased the likelihood of future failure on the Nojima fault by slowly but progressively unloading the normal stress on the fault over a 50-yr period.

Cenozoic subsidence and uplift of continents from time-varying dynamic topography

Time-varying global dynamic topography has been computed for the entire Cenozoic. The spherical viscous-flow model is driven by a distribution of density heterogeneity based on Mesozoic and Cenozoic subduction. By using the calculated topography, the uplift and subsidence histories of five regions fixed to the North American, Indian, Australian, Indonesian, and South American continents are computed. Predicted vertical motions are in qualitative agreement with observed values except for India and South America. For Australia and North America, the predicted periods of uplift and subsidence are out of phase with respect to observations, but display the correct overall trend. The latest phase of uplift for North America is directly related to the cessation of Kula plate subduction after 48 Ma. Dynamic topography depends on the viscosity structure of the mantle; a lower mantle that is 50 times more viscous than the upper mantle yields the best agreement between modeled and predicted vertical motions. The presence of a low-viscosity channel beneath the lithosphere has only a small influence on our results. Compared to dynamic topography, the time-varying geoid is insignificant in controlling relative sea level.

Global post-seismic deformation

SUMMARY We quantify the effects of post-seismic deformation on the radial and horizontal components of the displacement, in the near- and far-field of strike- and dip-slip point dislocations; these sources are embedded in the elastic top layer of a spherical, self-gravitating, stratified viscoelastic earth. Within the scheme of the normal mode technique, we derive the explicit analytical expression of the fundamental matrix for the toroidal component of the field equations; this component is propagated, together with its spheroidal counterpart, from the core-mantle boundary to the earth's surface. Viscosity stratification at 670km depth influences the radial and horizontal deformation accompanying viscoelastic relaxation in the mantle over time-scales of 103-104 yr, both in the near-field, ranging from 100 to 500 km and in the far-field, from 103 to 5 X 103 km. If the upper mantle is differentiated into a low-viscosity zone beneath the lithosphere and a normal upper mantle, faster relaxation is obtained. For an asthenospheric viscosity of 1020 Pa s we obtain, for a strike-slip dislocation and a seismic moment of 1022 N m characteristic of an average large earthquake, horizontal rates of 1-4 mm yr-1 in the near-field and 0.05-0.4 mm yr-1 in the far-field; these values are maintained over time-scales of 10-103 yr. Larger rates, with shorter duration, are obtained if the viscosity is reduced in the low-viscosity channel. As expected, strike-slip dislocations are the most effective in driving horizontal deformation in the far-field in comparison with dip-slip ones. It is noteworthy that horizontal velocities are maintained longer in the far-field in comparison with radial ones, which is not surprising since momentum is propagated in far regions essentially in the horizontal direction; radial deformation is generally lower in the far-field. VLBI techniques, with a precision of a few parts per billion over distances of 103 km, can detect global post-seismic deformation induced by large earthquakes. Our results affect the interpretation of the transfer of stress and seismic activity among different plate boundaries.

Flattening of the sea-floor depth-age curve as a response to asthenospheric flow

THE flattening of sea-floor depths from the √age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1. Previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than ∼80 Myr (ref. 2), thermal rejuvenation of lithosphere that passes over stationary hotspots3–5, or a whole-mantle flow forced by two converging plates6. We suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere. Applying the model to the Pacific plate, we show that a solution for flow in an asthenosphere low-viscosity channel which is 'consumed' by plate accretion and the subduction of lithosphere at trenches, and replenished by near-ridge upwelling and near-ridge hotspots, generates the observed ∼ 1 km of sea-floor flattening for asthenospheric viscosities of 2 × 1018 Pa s and an absolute Pacific plate motion of 100 mm yr−1. Our model can also explain the asymmetric subsidence of the South American and African plates away from the Mid-Atlantic Ridge.

The effect of a shallow low-viscosity zone on the mantle flow, the geoid anomalies and the geoid and depth-age relationships at fracture zones

A two-dimensional FEM is used to investigate the flow driven by the horizontal temperature gradient at a fracture zone and to calculate the resulting geoid and topography anomalies. Using a three-layered viscosity structure for the upper mantle, results are presented for the effects of varying: (1) the viscosity contrast between the fluid layers: (2) the Rayleigh number based on the viscosity of the bottom layer; and (3) the thickness of the low-viscosity channel. Good agreement is obtained with the results of geoid anomalies over the Udintsev fracture zone when the viscosity of the top layer is greater than one order of magnitude less than post-glacial rebound values.

Deep Seismic Reflections and the Deformational Mechanics of the Continental Lithosphere

Deep seismic reflection profiling of the continents shows the existence of significant dipping reflectors within the upper lithosphere which are intepreted as major low angle faults. These low angle faults do not cut or offset the Moho; to date no unequivocal example of a low angle fault passing continuously from the surface through into the mantle has been observed. Instead these major low angle fault reflections either merge into or are obscured by the sub-horizontal reflections of the lower crust. Thermo-rheological models predict the existence of low-strength, low-viscosity layers within and at the base of the lower continental crust related to the vertical changes in composition and rheology between the quartzo-feldspathic crust and the peridotitic mantle, and the quartz dominated upper crust and the plagioclase dominated lower crust. The major low angle faults imaged by deep reflection seismology are interpreted to sole out into horizontal detachments within these low viscosity regions m the lower crust. Lithosphere deformation within these low viscosity regions is most probably achieved by anastomosing networks of sub-horizontal shear zones. Deep seismic reflection and refraction studies show that mountain root Moho topography created during continental collisional tectonics is absent for old mountain belts (e.g., Caledonides and Variscides). Thermo-rheological models suggest that the decay and annihilation of mountain root Moho topography can be achieved within 100 Ma by an outward ductile flow in the low viscosity channel of the lower crust and an inward ductile half space flow in the upper mantle. The ductile flow process within the lower crust and mantle, and surface erosion are isostatically coupled. Moho topography is also generated by crustal thinning during continental extensional tectonics and the formation of extensional sedimentary basins. Thermo-rheological models suggest that whether or not such basin anti-root Moho topography persists or decays by ductile flow depends critically on the lower crustal and upper mantle viscosity structure prior to extension which in turn depends on crustal thickness and geothermal gradient. If the crustal thickness was of average thickness or less (≤ 35 km) prior to extension, Moho topography is predicted to persist. In contrast, if the crust was substantially thickened prior to extension (≥ 45 km) then basin anti-root Moho topography will rapidly be annihilated by ductile flow. These model predictions are consistent with observation of extensional Moho topography from deep seismic reflection profiling in the North Sea basin and the Basin and Range, USA. The decay of Moho topography by ductile flow is predicted to be extremely wavelength dependent. Short and long wavelength components are predicted to persist through geological time with only intermediate wavelength components being annihilated by ductile flow.

A mechanism for tectonic deformation on Venus

In the absence of identifiable physiographic features directly associated with plate tectonics, alternate mechanisms are sought for the intense tectonic deformation observed in radar images of Venus. One possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior. Spectral Green's function solutions have been obtained for stresses in an elastic lithosphere overlying a Newtonian interior with an exponential depth dependence of viscosity, and a specified surface‐density distribution driving the flow. At long wavelengths and for a rigid elastic/fluid boundary condition, horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid. The depth and strength of density anomalies in the venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 MPa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses. The great apparent depth of compensation of topographic features argues against this, however, thus supporting the importance of the coupling mechanism. If there is no elastic lid, stresses will also be very high near the surface, providing also that the viscosity gradient is negative.

The effects of post-seismic motions on the moment of inertia of a stratified viscoelastic earth with an asthenosphere

Summary. We give the analytical formulation for calculating the transient displacement of fields produced by earthquakes in a stratified, selfgravitating, incompressible, viscoelastic earth. We have evaluated the potential of viscous creep in the asthenosphere in exciting the Chandler wobble by a four-layer model consisting of an elastic lithosphere, a two-layer Maxwell viscoelastic mantle, and an inviscid core. The seismic source is modelled as an inhomogeneous boundary condition, which involves a jump condition of the displacement fields across the fault in the lithosphere. The response fields are derived from the solution of a two-point boundary value problem, using analytical propagator matrices in the Laplace-transformed domain. Transient flows produced by post-seismic rebound are found to be confined within the asthenosphere for local viscosity values less than 1OZoP. The viscosity of the mantle below the low-viscosity channel is kept at 1OZ2P. For low-viscosity zones with widths greater than about 100 km and asthenospheric viscosities less than lOP, we find that viscoelasticity can amplify the perturbations in the moment of inertia by a factor of 4-5 above the elastic contribution within the time span of the wobble period. We have carried out a comparative study on the changes of the inertia tensor from forcings due to surface loading and to faulting. In general the global responses from faulting are found to be much more sensitive to the viscosity structure of the asthenosphere than those produced from surface loading.