Tectonic Flow Research Articles

Structural and kinematic assemblies in sedimentary rocks of the Phanerozoic cover of the Mid-Russian Dislocation Zone

The Mid-Russian Dislocation Zone is a large within-plate structural element of the East European Platform, which extends for more than 1100 km from the Timan Foredeep to the Orsha Basin. This deep, long-lived zone was formed against a background of changeable geodynamic settings, including (1) Late Paleoproterozoic collision events, (2) Late Riphean-Early Vendian epicontinental rifting, (3) Late Vendian-Early Triassic intraplatform tectogenesis with formation of horst-like uplifts within the zone against the background of general subsidence, and (4) Mesozoic-Cenozoic within-plate reactivation. At the final Kimmerian-Alpine stage of its evolution, the Mid-Russian Zone developed as a left-lateral transpressional structure with penetrative dissipative shear deformation resulting in the general horizontal transfer of Phanerozoic sedimentary rocks. The dislocations were manifested as two dynamically conjugate structural forms: a zone of scattered shearing and a bedding-plane tectonic flow. The dynamic manifestation of the Mid-Russian and the conjugate Belomorian-Dvina zones, which make up a common arcuate structure (in plan view), allowed us to outline the Dvina-Sukhona plate-flow with horizontal mass transfer in the southeastern direction. The tectonics of the Mid-Russian Dislocation Zone is considered in this paper with particular emphasis on the structural and kinematic assemblies in sedimentary rocks of the Phanerozoic cover.

10.1007/s11479-008-1001-4

The middle mantle as a separate geosphere within a depth interval of 840 to 1700 km was recognized in 1995 by Yu.M. Pushcharovsky. The structure, energetics, and tectonics of the middle mantle, as well as phase transformations inherent to this geosphere, are characterized in this paper. The distribution of seismic heterogeneities established by seismic tomography is a definitive attribute of the given geosphere. The middle mantle differs from other geospheres by greater dimensions of seismic heterogeneities, especially as concerns the low- and medium-velocity domains. The high-velocity heterogeneities are round and oval in shape and in some cases reach a few thousand kilometers in size. The distribution of such heterogeneities is nonuniform and varies from one depth level to another. A high lateral contrast of anomalous domains distinguished by elastic wave velocities is expressed in gradient zones hundredths of kilometers wide. Specific general patterns of middle-mantle anomalies in the Pacific and Indian-Atlantic sectors of the Earth reflect their difference in geological history. The consideration of heterogeneities in terms of tectonics leads to the conclusion that role of the tectonic flow of mantle masses in the form of shearing and thrusting is important. The middle mantle is characterized by a special mineral composition with the prevalence of MgSiO3 crystallized as an orthorhombically distorted perovskite-type structure. The transformations of stishovite into a poststishovite modification at a depth of ∼1500 km and of aragonite into the postaragonite phase with an unusual structure at a depth of ∼1050 km are inherent to this geosphere. A change of the electronic structure of alkali cations is assumed in the middle mantle. Thus, the recognition of the middle mantle as a special geopshere is emphasized by its crystal chemistry.

Numerical simulation of tectonic deformation-driven fluid flow: Implications for ore genesis in the Dachang district, Guangxi, China

Numerical simulation of tectonic deformation-driven fluid flow: Implications for ore genesis in the Dachang district, Guangxi, China

Lateral protrusions in the structure of the Earth’s lithosphere

The factual material and modeling results concerning the geology of specific structural elements defined as lateral protrusions, or flowing layers, are considered. The formation of such structural elements is a fundamental phenomenon that controls many features of the structural evolution and geodynamics of platform basement and foldbelts. A lateral protrusion, or flowing layer, is a spatially constrained, nearly horizontal geological body with attributes of 3D tectonic flow (rheid deformation) and lateral transport of rock masses. Flowing layers are large lateral protrusions that play important role in the structure of the continental and oceanic lithosphere. They embody the internal mobility of huge rock bodies and confirm the possibility of their lateral redistribution at different depths of the continental lithosphere. The lateral displacement of rocks within such assemblies may occur in the regime of cold deformation, heating, metamorphism, and ductile flow of rocks under subsolidus conditions or in the process of their partial melting.

The middle mantle of the earth

The middle mantle as a separate geosphere within a depth interval of 840 to 1700 km was recognized in 1995 by Yu.M. Pushcharovsky. The structure, energetics, and tectonics of the middle mantle, as well as phase transformations inherent to this geosphere, are characterized in this paper. The distribution of seismic heterogeneities established by seismic tomography is a definitive attribute of the given geosphere. The middle mantle differs from other geospheres by greater dimensions of seismic heterogeneities, especially as concerns the low- and medium-velocity domains. The high-velocity heterogeneities are round and oval in shape and in some cases reach a few thousand kilometers in size. The distribution of such heterogeneities is nonuniform and varies from one depth level to another. A high lateral contrast of anomalous domains distinguished by elastic wave velocities is expressed in gradient zones hundredths of kilometers wide. Specific general patterns of middle-mantle anomalies in the Pacific and Indian-Atlantic sectors of the Earth reflect their difference in geological history. The consideration of heterogeneities in terms of tectonics leads to the conclusion that role of the tectonic flow of mantle masses in the form of shearing and thrusting is important. The middle mantle is characterized by a special mineral composition with the prevalence of MgSiO3 crystallized as an orthorhombically distorted perovskite-type structure. The transformations of stishovite into a poststishovite modification at a depth of ∼1500 km and of aragonite into the postaragonite phase with an unusual structure at a depth of ∼1050 km are inherent to this geosphere. A change of the electronic structure of alkali cations is assumed in the middle mantle. Thus, the recognition of the middle mantle as a special geopshere is emphasized by its crystal chemistry.

Structural-kinematic evolution of the central Belomorian-Lapland Belt in the Paleoproterozoic

A model of the evolution of the central Belomorian-Lapland Granulite-Gneiss Belt is proposed on the basis of analysis of the Paleoproterozoic structural-kinematic assemblages. It is shown that this tectonic zone is a long-lived mobile structural unit that evolved through several stages of tectonic transformation and metamorphism of rocks, including (1) the Reboly stage, which comprises subduction (2.88-2.82 Ga) and col- lision (2.74-2.53 Ga) substages; (2) the Selet stage of rifting and extension of the continental crust according to the model of simple shear (2.45-2.35 Ga); and (3) the Svecofennian stage, characterized by collision and general transpression (1.94-1.75 Ga). The results of structural-kinematic study indicate that tectonic flow in the Svecofennian time was nonuniform and related to the formation of the Kolvitsa-Umba near-horizontal pro- trusion. The propagation of this protrusion was caused by transpressional extrusion of plastic lower-crustal masses to the surface as a gently ascending tectonic flow directed to the northwest (in present-day coordinates). Thereby, a thrust-normal-fault kinematic effect was expressed in pushing-out of deep-seated complexes con- temporaneously with tectonic erosion of the upper portions of the sequence owing to the development of low- angle normal faults.

Space geodesy validation of the global lithospheric flow

SUMMARY Space geodesy data are used to verify whether plates move chaotically or rather follow a sort of tectonic mainstream. While independent lines of geological evidence support the existence of a global ordered flow of plate motions that is westerly polarized, the Terrestrial Reference Frame (TRF) presents limitations in describing absolute plate motions relative to the mantle. For these reasons we jointly estimated a new plate motions model and three different solutions of net lithospheric rotation. Considering the six major plate boundaries and variable source depths of the main Pacific hotspots, we adapted the TRF plate kinematics by global space geodesy to absolute plate motions models with respect to the mantle. All three reconstructions confirm (i) the tectonic mainstream and (ii) the net rotation of the lithosphere. We still do not know the precise trend of this tectonic flow and the velocity of the differential rotation. However, our results show that assuming faster Pacific motions, as the asthenospheric source of the hotspots would allow, the best lithospheric net rotation estimate is 13.4 ± 0.7 cm yr −1 . This superfast solution seems in contradiction with present knowledge on the lithosphere decoupling, but it matches remarkably better with the geological constraints than those retrieved with slower Pacific motion and net rotation estimates. Assuming faster Pacific motion, it is shown that all plates move orderly ‘westward’ along the tectonic mainstream at different velocities and the equator of the lithospheric net rotation lies inside the corresponding tectonic mainstream latitude band ( ≈± 7 ◦ ), defined by the 1σ confidence intervals.

A new generation of the tectonic-geodynamic model of the earth: Review of the problem

Presented are the review and analysis of a series of works of Russian scientists devoted to the motion of continents and to global tectonics and geodynamics. The increasing number of publications on this subject is accounted for by controversy and imperfection of some theoretical aspects of plate tectonics. This is especially true of processes in deep mantle geospheres. The analysis performed shows that a transition to creating tectonogeodynamic models of a new generation has already started. Important components of the models are the development of the problem of a detached continent drift, primarily, mechanisms of this motion. The publications under consideration provide insight into several such mechanisms: the effect of the tidal forces of the Moon and Sun, which induce discrete wave motion in all mantle geospheres down to the core; the difference in density of oceanic and continental masses; the continental drift over the mantle as a result of cohesion; and the tectonic flow, which implies a structure-forming motion of both minor (at the microlevel) and major rock masses in the Earth’s crust and mantle. The globalization of tectonic and geodynamic models has already allowed the advance of such a timely problem as the history of the geologic evolution of deep geospheres.

Quantitative correlation between geodynamic systems and geodynamic cycles of various ranks

A concept of geodynamics of hierarchically subordinate geospheres developed by the author is briefly characterized. Geospheres of different ranks—(i) the Earth as a whole, (ii) the mantle and the crust, (iii) the upper mantle and the crust, (iv) the asthenosphere + the lithosphere, and (v) sedimentary cover—are successively enclosed in one another. Convective geodynamic systems of the respective ranks function in each of these geospheres. Such systems are composed of cells controlled by balanced arrangement of the tectonic flow and consisting, in turn, of domains of horizontal compression and horizontal extension alternating in chessboard order. The hierarchical organization of geodynamic systems implies that the domain of a lower rank (a larger scale) is simultaneously a system of a higher rank (a smaller scale). The interference of geodynamic systems of various ranks generates the entire spectrum of tectonic units of different scales, from the substantially continental Northern Hemisphere and the Indo-Atlantic segment of the Earth to small folds in orogenic belts. The geodynamic systems of various ranks reveal quantitative correlation with the hierarchical series of geodynamic cycles established by V.E. Khain [29]. The ratios of the thicknesses of the geospheres, where geodynamic systems of various ranks work, to the duration of the geodynamic cycle of the respective rank fall within a very narrow range of 0.45 ± 0.10 cm/yr. Therefore, this ratio may be regarded as an out-of-rank geodynamic constant that has dimensionality of velocity. Its formal physical meaning is a time-averaged rate of uplift of a minor elementary body of a geosphere from bottom to top during the geodynamic cycle. This rate may be deemed a characteristic velocity of the ascending convective flow in any of the aforementioned geospheres. The physical and geologic reality of this velocity is confirmed by realistic (within an order of magnitude) estimates of rates of continental drift and oceanic spreading deduced from this value. In contrast to the average velocity of tectonic flow, the average rate of deformation is variable and inversely proportional to the thickness of a geosphere of a respective rank. Not only does the thickness of a geosphere decrease with increasing rank but also the respective geodynamic system functions within more local areas. This is a specific realization of the abstract computer model of cascade convection in geospheres [24]. The increase in the deformation rate within increasingly local areas is a manifestation of the weak link effect. The essence of this effect is clearly demonstrated by a combination of extensive but slow ductile deformation and more local but faster brittle failure resulted from small-scale deformation in the Earth’s crust.

Average strain rate in the Italian crust inferred from a permanent GPS network - II. Strain rate versus seismicity and structural geology

SUMMARY High-quality, weekly solutions for the coordinates of 45 permanent GPS stations spanning 1–6 years and covering the Alpine Mediterranean area are used to estimate a horizontal velocity field aligned with the ITRF2000 velocity datum. After subtracting model velocities for the Eurasian Plate, we show that the residual velocity field north of the Apennines, in the Adriatic/Dinaric region, describes a NNE-trending tectonic flow with a negative velocity gradient, from 6 mm yr−1 NE of the Apennines to virtually zero NE of the Dinarids. In the Calabria region we observe a divergent pattern, with NNW orientation, which turns to west as one moves north, towards Tuscany. Very small residual velocities are observed for stations in the Sardinia Corsica block, Spain, Southern France and north of the Alps. This regional pattern of velocities defines areas of intraplate strain. On the basis of good station coverage and of a strain rate value above a threshold, we define five provinces: Calabria, Central Apennines, both in the extensional regime, and North Adriatic, Eastern Alps and South Adriatic, under compression. We show that the orientations of the principal strain rate axes agree with the stress pattern that is inferred from local seismicity. A comparison of the geodetically determined strain rate with the strain rate associated with the seismic moment of earthquakes in the past 30 yr shows that each province may differ considerably from the others in the amount of seismic moment that is accommodated by surface strain. For example, the Umbria seismic sequence of 1997 seems to have released a seismic moment that was not exceptionally large, but concentrated over a short time interval, so that the equivalent coseismic strain rate was larger than the value observed geodetically. On average, depending on the assumptions on the seismogenic volume and time span, 70–100 per cent of the surface strain seems to be accommodated by the release of seismic moment in the upper Italian crust.

Computer simulations of Alaskan neotectonics

Ongoing deformation and fault slip (neotectonics) in the Alaskan/Bering Sea region were modeled with thin‐plate finite‐element methods by computing anelastic strain rate, stress, and velocity fields which represent time averages over several seismic cycles. Models included variable crust and lithosphere thickness, heat flow, and elevation and incorporated all faults known or thought to be active. Three sets of experiments, totaling 46 models, were computed with systematic variations of shear traction on the megathrust, fault friction within North America, internal friction of lithospheric blocks, and mantle creep strength. Model quality was scored by comparing predictions to geologic data on fault slip rates, the most compressive horizontal principal stress direction , and geodimeter and very long baseline interferometry (VLBI) baseline rates. Best results were obtained with a driving traction of only 10∼15 MPa on the Aleutian megathrust and fault friction of only 0.17 within North America; this is clearly a “low‐stress” model. The fanning of away from the syntaxis was reproduced but does not imply high deviatoric stress magnitudes within the North American plate. The Aleutian forearc west of Kodiak Island moves westward along strike at rates which increase to 65 mm yr−1 in the Komandorsky region. However, no crust is presently escaping westward from the Alaskan syntaxis. Instead, it is shortened and stacked by thrusting, principally in the vicinity of the Chugach and Wrangell Mountains. Because stresses in this model are low and are delicately balanced against topography, it is likely that the present regime represents a transient response to Plio‐Pleistocene glacial mass redistribution and differs from typical tectonic flow in the Tertiary.

History of rift propagation and magnetization intensity for the Cocos‐Nazca sspreading Center

Analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the Cocos‐Nazca spreading center. Forward models of the magnetic anomalies accounting for spreading rate variations, ridge jumps, asymmetric spreading, magnetization intensity variations, and bathymetry show excellent agreement with observed anomalies. Spreading rates can be constrained to a common finite rotation history through anomaly 4A with three changes in rates. Rate changes at about 1.5 Ma and 4.1 Ma correspond to changes in rate gradients and occur during the well‐calibrated part of the reversal timescale, so they can unquestionably be identified as true changes in plate motion. A ∼15% rate decrease at about 5.2 Ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale. The change at 4.1 Ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0–6 Ma. Rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis. Propagation has been predominantly away from the hotspot, with jumps predominantly south‐ward. Propagation rates have ranged from 30 to 120 mm/yr, commonly near 70 mm/yr. Origin of most propagation sequences is difficult to interpret, but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot. Magnetic anomaly amplitude appears to be a reliable tracer of Fe content of lavas. Several generalizations can be drawn about along‐axis variations in magnetization intensities since 8 Ma: high magnetizations are only observed at the far ends (relative to the Galapagos hotspot) of segments at least 150 km long; offset at the end of a high‐magnetization segment is at least 15 km; and there are no offsets larger than 30–45 km between high‐magnetization segments and the reconstructed position of the hotspot. We interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along‐axis flow of evolved magma. The gradients in supply result from subaxial flow of hotspot‐derived asthenosphere in a narrow conduit. This flow is only partly obstructed by an offset of 20–30 km but entirely blocked by an offset of 50 km.

Self-indentation of a heterogeneous continental lithosphere

The Neoproterozoic Ribeira-Aracuai belt stretches along the southeastern edge of the Sao Francisco craton of southeastern Brazil and extends south of the cratonic domain for >1000 km. The termination of the craton is spatially correlated with a significant modification of the deformation pattern in the belt: (1) the structural trend bends from due north to northeast, (2) the dominant tectonic flow shifts from orogen-transverse to orogen-normal, and (3) the metamorphic conditions of deformation decrease southwestward from high to medium grade. Finite-element modeling suggests that the presence of a craton within a continent favors strain localization, initiation of continental-scale shear zones, and differential vertical deformations. The southward termination of the Sao Francisco craton may have triggered the development of the complex deformation pattern that characterizes the Ribeira-Aracuai belt.

Deformation of the oceanic crust between the North American and South American Plates

Fracture zone trends and magnetic anomalies in the Atlantic Ocean indicate that the North American plate must have moved with respect to the South American plate during the opening of the Atlantic. A comparison of plate tectonic flow lines with fracture zones identified from Geosat and Seasat altimeter data suggests that the North American‐South American plate boundary migrated northward from die Guinea‐Demarara shear margin to the Vema Fracture Zone before chron 34 (84 Ma), to north of the Doldrums Fracture Zone before chron 22 (51.9 Ma), and to north of the Mercurius Fracture Zone between chron 32 (72.5 Ma) and chron 13 (35.5 Ma). The paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area, indicate that the triple junction may have been located between the Mercurius and the Fifteen‐Twenty fracture zones after 67 Ma (chron 30). Plate reconstructions indicate a Late Cretaceous phase of transtension, followed by transpression in the Tertiary for the Tiburon/Barracuda Ridge area south of the Fifteen‐Twenty Fracture Zone. The ocean floor in this area is characterized by a series of ridges and troughs with large Bouguer gravity anomalies (up to ∼135 mGal). We use smoothing spline estimation to invert Bouguer anomalies for crustal layer structure. Our model results suggest that the Moho is uplifted 2–4 km over short wavelengths (∼70 km) at the Barracuda and Tiburon ridges and imply large anelastic strains. The severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted. We propose that the North‐South American plate boundary migrated to the latitude of the Tiburon Ridge, bounded by the Vema and Marathon fracture zones, before chron 34 (84 Ma). Post‐chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the Vema, Marathon, Mercurius, and Fifteen‐Twenty fracture zone troughs, but reaching the Fifteen‐Twenty Fracture Zone and future Barracuda Ridge area only after chron 32 (72.5 Ma). This interpretation concurs with our crustal structural model, which shows stronger crustal thinning underneath the Tiburon Ridge than at the Barracuda Ridge. Subsequent transpression may have continued along the existing zones of weakness in the Tertiary, creating the presently observed crustal deformation and uplift of the Moho, accompanied by anelastic failure of the crust. Middle‐Eocene‐Upper Oligocene turbidites on the slope of the Tiburon Ridge, now located 800 m above the abyssal plain, suggest that most of its uplift occurred at post‐Oligocene times. The unusually shallow Moho underneath the Tiburon and Barracuda ridges represents an unstable density distribution, which may indicate that compressive stresses are still present to maintain these anomalies, and that the North American‐South American plate boundary may still be located in this area.

Fracture zones in the North Atlantic from combined Geosat and Seasat data

We present a new tectonic fabric map of the northern and central North Atlantic Ocean floor from 10°S to 66°N based on combined Geosat and Seasat altimetry data. Short wavelength vertical gravity along‐track was derived by Hilbert transforming and high‐pass filtering Geosat and Seasat deflection of the vertical profiles. The along‐track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and V‐shaped structures. We traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible. We also mapped a number of unnamed fracture zones and fracture zone segments, many of which were previously uncharted. We have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the North, central North and South Atlantic. Most fracture zones in this area correlate quite well with synthetic flow lines, supporting the proposition that Geosat/Seasat altimetry data serve as a useful tool to map fracture zones, where insufficient bathymetric data are available. Many fracture zones, including some small‐offset fracture zones in the central North Atlantic, have been stable and follow tectonic flow lines for tens of millions of years. However, many other fracture zones do not reflect a ″railroad track″ pattern of plate motions. The geometry of many small‐ and medium‐offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge, causing off‐axis traces that deviate from tectonic flow lines. Velocity‐space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low‐angle V‐shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame. Other low‐angle V‐shaped structures reflect slow migrations/oscillations of small‐offset ridge axis discontinuities that are distinct from both tectonic flow lines and V‐shapes reflecting “absolute” plate motions. High‐angle V‐shaped structures are the result of fast migrating ridge axis discontinuities (propagating rifts).

Finite element modeling of the three-dimensional tectonic flow and stress field beneath the Kyushu island, Japan.

Three-dimensional patterns of tectonic deformations and stresses are calculated to simulate the tectonics of Kyushu and its surrounding regions in southwestern Japan using a finite element method. The configuration of the subducted Philippine Sea plate is modeled by means of three-dimensional finite elements. Viscosities in the crust and upper mantle assigned are taken from the data of postglacial or postseismic rebounds. Several possible loads are taken into consideration: a slab pull force arising from the density contrast between the subducted slab and the surrounding asthenosphere; a ridge push force due to evolution of oceanic plates; a traction due to flows in the asthenosphere; a crustal buoyancy acting on a low-density crust; a buoyant force in the back-arc region which may simulate the spreading of a back-arc basin; a positive buoyancy acting on the aseismic ridge; compressive forces in the E-W direction exerting on the crust and subducted slab.The results suggest that the tensile stress field observed in the crust of Kyushu may be generated by the interaction between the slab pull force, crustal buoyancy, and flows in the asthenosphere. It may also be concluded that the stress field of down-dip extension in the Wadati-Benioff zone can be explained mainly by the slab pull force. The asthenospheric flows might have a significant effect on the genesis of near-horizontal extension which yields normal faulting in the uppermost mantle, and such stresses would be more likely to appear on the oceanic side than on the continental side. In order to explain the dip angle of the subducted slab, if it is a manifestation of the flow pattern at a subduction zone, a traction due to asthenospheric flow of about 200 bar is required in the asthenosphere. The predicted values of velocities of tectonic flows and of maximum shear stresses are a few cm/yr and 500 bar at most in the crust and the subducted slab beneath Kyushu, though these are dependent on viscosity.

Plane-stress finite-element models of tectonic flow in southern California

The continuing, distributed crustal deformation in southern California was modelled as creeping flow in a nonlinear continuum. The crust was assumed to be in a membrane state of stress, except for the weight of topography, and subjected to plate-tectonic velocity boundary conditions. The flow law of this membrane contained a rigid-plastic term to represent the frictional faulting in the upper crust and a power-law term to represent dislocation creep in the lower crust. In addition to regional strength variations based on heat flow, some models had a weak belt representing the San Andreas fault. When calculated strain-rates were tested by cross-correlation with actual seismic moment rates, directions of shortening, and rates of vertical movement, the best model was found to be one in which the San Andreas fault is eight times weaker than the surrounding crust, perhaps because of clay gouges or fine-grained mylonites within it. This model is consistent with the absence of a detectable heat-flow anomaly at the San Andreas, and also with the difference between geologic and paleomagnetic plate velocities. The rest of the crust must be at least half as strong as is indicated by laboratory deformation, or the crust would slump southwest toward the continental shelf. However, the failure of all the models to match many details of local tectonics suggests that other faults share the anomalous weakness of the San Andreas. This is expected to limit the success of any future continuum models of the region.

Numerical investigation of tectonic flow in island-arc areas

Numerical investigation of material flow induced by a descending oceanic plate has been carried out. Asymmetrical flow is modeled in two wedge-shaped regions located on different sides of the downgoing slab. The calculation of induced flow is two-dimensional, time dependent, with viscosity dependent on temperature. The calculation is very sensitive to rheological properties of material. Modeling of island-arc volcanism could be realized only by taking into account non-linear rheological behaviour of material like yielding, plasticity, etc. Induced flow then, may be a probable explanation for island-arc volcanism and high heat flow in marginal basins.

Finite element modeling of lithosphere deformation: The Zagros collision orogeny

In the Zagros Mountains a formerly stable continental margin is being suddenly deformed. Finite element models of this tectonic flow are used to relate known surface deformations, earthquake locations, and fault plane solutions to unknown rock flow parameters and driving forces, in order to determine the latter. These two-dimensional plane-strain flow models incorporate the effects of nonlinear dislocation creep, frictional faulting, geological inhomogeneity, density anomalies, and varying temperature. All models driven by a subducting slab are unsuccessful because they require subduction of continental crust, which does not match present seismicity. Therefore the former oceanic slab must be detached, and the orogeny must be driven by horizontal compression in the lithosphere. Models also show that the subcrustal lithosphere is not shortened but acts as a stabilizing foundation. These results imply a simple geometry of crustal deformation which can be analytically modeled. The creep strength of the lower crust (75–100 bars) determines the topographic slope of the Zagros. The fact that subduction is not occurring on the old plate boundary places a limit on the shear stress deforming the cold upper crust. This limit is 300 bars if there is limestone at depth in the Crush Zone; otherwise 800 bars. These results are confirmed by a final finite element model. The total driving force of the orogeny associated with these limits is 2.8–5.5 · 10 15 dyne/cm, and the smaller amount could be provided by the gravitational spreading of the Red Sea rift. Shear-strain heating caused by the orogeny to date is less than 20°C. These results imply that even unheated continental crust is considerably weaker than laboratory friction measurements imply, and that it is mechanically decoupled at the Moho from the stronger mantle lithosphere.

Lead isotope relations in oceanic Ridge basalts from the Juan de Fuca-Gorda Ridge area N.E. Pacific Ocean

Lead isotopic analyses of a suite of basaltic rocks from the Juan de Fuca-Gorda Ridge and nearby seamounts confirm an isotopically heterogeneous mantle known since 1966. The process of mixing during partial melting of a heterogeneous mantle necessarily produces linear data arrays that can be interpreted as secondary isochrons. Moreover, the position of the entire lead isotope array, with respect to the geochron, requires that U/Pb and Th/Pb values are progressively increased over the age of the earth. Partial melting theory also dictates analogous behavior for the other incompatible trace elements. This process explains not only the LIL element character of MOR basalts, but also duplicates the spread of radiogenic lead data collected from alkali-rich oceanic basalts. This dynamic, open-system model of lead isotopic and chemical evolution of the mantle is believed to be the direct result of tectonic flow and convective overturn within the mantle and is compatible with geophysical models of a dynamic earth.