Small-scale mantle flows and induced lithospheric stress near island arcs

Summary. This paper explores the middle ground between complex thermally-coupled viscous flow models and simple corner flow models of island arc environments. The calculation retains the density-driven nature of convection and relaxes the geometrical constraints of corner flow, yet still provides semianalytical solutions for velocity and stress. A novel aspect of the procedure is its allowance for a coupled elastic lithosphere on top of a Newtonian viscous mantle. Initially, simple box-like density drivers illustrate how vertical and horizontal forces are transmitted through the mantle and how the lithosphere responds by trench formation. The flexural strength of the lithosphere spatially broadens the surface topography and gravity anomalies relative to the functional form of the vertical flow stresses applied to the plate base. I find that drivers in the form of inclined subducting slabs cannot induce self-driven parallel flow; however, the necessary flow can be provided by supplying a basal drag of 1–5 MPa to the mantle from the oceanic lithosphere. These basal drag forces create regional lithospheric stress and they should be quantifiable through seismic observations of the neutral surface. The existence of a shallow elevated phase transition is suggested in two slab models of 300 km length where a maximum excess density of 0.2 g cm⁻³ was needed to generate an acceptable mantle flow. A North New Hebrides subduction model which satisfies flow requirements and reproduces general features of topography and gravity contains a high shear stress zone (75 MPa) around the upper slab surface to a depth of 150 km and a deviatoric tensional stress in the back arc to a depth of 70 km. The lithospheric stress state of this model suggests that slab detachment is possible through whole plate fracture.     

The dynamical origin of subduction zone topography

Summary. Subduction zones are expressed topographically by long linear oceanic trenches flanked by a low outer rise on the seaward side and an island arc on the landward side. This topographic structure is reflected in free air gravity anomalies, suggesting that much of the topography originates from dynamical forces applied at the base of the crust. We have successfully reproduced the general topographic features of subduction zones by supposing that the stresses generated by the bending of the viscous lower lithosphere as it subducts are transmitted through the thin elastic upper portion of the lithosphere. The trench is due to a zone of extensional flow (associated with low pressure) in the upper part of the viscous lithosphere.
The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90°) were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 × 10²² P. This low value is probably due to the stress dependence of the effective viscosity. However, these models also show that the topography of the subduction zone depends primarily upon the geometry of the subducting slab (dip, radius of curvature of the bend) rather than upon its rheology. Shear stresses beneath the trench reach maxima of approximately 50 MPa. An interesting feature of some solutions is a dynamically supported bench or platform between the trench and island arc.     

Gravity anomalies and the origin of the Puerto Rico Trench

The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and more dense than it, pulls the lithosphere down at the trench. Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different) and that the Atlantic Ocean floor seaward of the outer rise, Puerto Rico, and the Caribbean Sea floor are each in isostatic equlibrium. The residual gravity anomalies are then consistent with the existence of a subcrustal dense mass, that could be the hanging slab of lithosphere. The value of this excess mass depends upon more arbitrary assumptions for the crustal mass in the Puerto Rico Trench and its landward wall, but if the other assumptions above are realistic, the dense mass is required and is adequate to bend the surface down at the trench.     

Towards understanding the lithospheric structure of the southern Chilean subduction zone (36°S–42°S) and its role in the gravity field

The Southern Andes differ significantly from the Central Andes with respect to topography and crustal structures and are, from a geophysical point of view, less well known. In order to provide insight into the along-strike segmentation of the Andean mountain belt, an integrated 3-D density model was developed for the area between latitudes 36°S and 42°S. The model is based on geophysical and geological data acquired in the region over the past years and was constructed using forward density modelling. In general, the gravity field of the South American margin is characterized by a relatively continuous positive anomaly along the coastline and the forearc region, and by negative anomalies along the trench and the volcanic arc. However, in the forearc region of the central part of the study area, located just to the south of the epicentre of the largest ever recorded earthquake (Valdivia, 1960), the trench-parallel positive anomaly is disrupted. The forearc gravity anomaly differences thus allow the study area to be divided into three segments, the northern Arauco-Lonquimay, the middle Valdivia-Liquiñe, and the southern Bahía-Mansa-Osorno segment, which are also evident in geology. In the proposed model, the observed negative gravity anomaly in the middle segment is reproduced by an approximately 5 km greater depth to the top of the slab beneath the forearc region. The depth to the slab is, however, dependent upon the density of the upper plate structures. Therefore, both the upper and lower plates and their interaction have a significant impact on the subduction-zone gravity field.     

The gravity field of a dipping plate in Greece

Summary. The surface gravity anomalies for a three-dimensional density model of a dipping lithospheric plate under the Aegean island arc and the Aegean Sea in Greece in the eastern Mediterranean Sea have been calculated. Such a dipping plate has been suggested in geophysical investigations. The calculated gravity maximum over the dipping plate is located in the area of the largest observed Bouguer anomalies inside the island arc. It is suggested that this should be taken into account in studies of crustal structure and gravity in the area.     

Gravity constraints on the dynamics of the crust–mantle system during Calabrian subduction

The thermomechanic evolution of the lithosphere–upper mantle system during Calabrian subduction is analysed using a 2-D finite element approach, in which the lithosphere is compositionally stratified into crust and mantle. Gravity and topography predictions are cross-checked with observed gravity and topography patterns of the Calabrian region. Modelling results indicate that the gravity pattern in the arc-trench region is shaped by the sinking of light material, belonging to both the overriding and subduction plates. The sinking of light crustal material, up to depths of the order of 100–150 km is the ultimate responsible for the peculiar gravity signature of subduction, characterized by a minimum of gravity anomaly located at the trench, bounded by two highs located on the overriding and subducting plates, with a variation in magnitude of the order of 200 mGal along a wavelength of 200 km, in agreement with the isostatically compensated component of gravity anomaly observed along a transect crossing the Calabrian Arc, from the Tyrrhenian to the Ionian Seas. The striking agreement between the geodetic retrieved profiles and the modelled ones in the trench region confirms the crucial role of compositional stratification of the lithosphere in the subduction process and the correctness of the kinematic hypotheses considered in our modelling, that the present-day configuration of crust–mantle system below the Calabrian arc results from trench's retreat at a rate of about 3 cm yr⁻¹, followed by gravitational sinking of the subducted slab in the last 5 Myr.     

Free-surface formulation of mantle convection—II. Implication for subduction-zone observables

Viscous and viscoelastic models for a subduction zone with a faulted lithosphere and internal buoyancy can self-consistently and simultaneously predict long-wavelength geoid highs over slabs, short-wavelength gravity lows over trenches, trench-forebulge morphology, and explain the high apparent strength of oceanic lithosphere in trench environments. The models use two different free-surface formulations of buoyancy-driven flows (see, for example, Part I): Lagrangian viscoelastic and pseudo-free-surface viscous formulations. The lower mantle must be stronger than the upper in order to obtain geoid highs at long wavelengths. Trenches are a simple consequence of the negative buoyancy of slabs and a large thrust fault, decoupling the overriding from underthrusting plates. The lower oceanic lithosphere must have a viscosity of less than to²⁴ Pa s in order to be consistent with the flexural wavelength of forebulges. Forebulges are dynamically maintained by viscous flow in the lower lithosphere and mantle, and give rise to apparently stiffer oceanic lithosphere at trenches. With purely viscous models using a pseudo-free-surface formulation, we find that viscous relaxation of oceanic lithosphere, in the presence of rapid trench rollback, leads to wider and shallower back-arc basins when compared to cases without viscous relaxation. Moreover, in agreement with earlier studies, the stresses necessary to generate forebulges are small (∼ 100 bars) compared to the unrealistically high stresses needed in classic thin elastic plate models.     

A critical assessment of viscous models of trench topography and corner flow

Summary. We obtain stresses for Newtonian viscous flow in simple geometries (e.g. corner flow, bending flow) in order to study the effect of imposed velocity boundary conditions. Stress for a delta function velocity boundary condition decays as 1/ r ²; for a step function velocity, stress goes as 1/ r ; for a discontinuity in curvature, the stress singularity is logarithmic. For corner flow, which has a discontinuity of velocity at a certain point, the corresponding stress has a 1/ r singularity. However, for a more realistic circular-slab model, the stress singularity becomes logarithmic. Thus the stress distribution is very sensitive to the boundary conditions, and in evaluating the applicability of viscous models of trench topography it is essential to use realistic geometries.
Topography and seismicity data from northern Honshu, Japan, were used to construct a finite element model, with flow assumed constant speed and tangent to the top of the grid, for both Newtonian and non-Newtonian flow (power law 3 rheology). Normal stresses at the top of the grid are compared to the observed trench topography. There is poor agreement. Purely viscous models of subducting slabs with simple, geometrically consistent velocity boundary conditions do not predict normal stress patterns compatible with observed topography. Elasticity and plasticity appear to be important in determining trench topography.     

Extensional tectonic regimes in the Aegean basins during the Cenozoic

Abstract Kinematics of faults in the Northern Aegean show three extensional tectonic regimes the tensional directions of which trend (1) WNW-ESE, (2) NE-SW and (3) N-S. These were active during the Upper Miocene, Pliocene-Lower Pleistocene and Mid Pleistocene-Present day, respectively. The main characteristics of the stress patterns (1) and (2) on the overall Aegean is tentatively explained by variations of the horizontal lithospheric stress value σ_zz due to the slab push and of the vertical lithospheric stress value σ_zz due to mass heterogeneities. During the Mid Pleistocene-Present, due to the slab push, tectonics were compressional along the arc boundary: σ_zz was σ1. In the Aegean basins, tectonics were extensional, c_2Z was σ1 as a consequence of the thickness of the continental crust and, possibly of an updoming asthenosphere; thus σ_zz became σ2, allowing tension σ3 to be orthogonal to the compression along the arc, i.e. to be roughly parallel to the arc trend. During the Pliocene-Lower Pleistocene, the extensional regime was distinctly different. The tensional directions were roughly radial to the arc. It is suggested that σ_zz was weakly compressional, or eventually tensional, due a seaward migration of the slab so that σ_zz became σ3. In the Northern Aegean, the stress pattern has been also controlled by the westward push of the Anatolian landmass. During the Mid Pleistocene-Present day, this was typically extensional (al was vertical) and the right lateral strike-slip motion on the North Anatolian Fault transformed into a N-S-stretching, E-W-shortening of the Northern Aegean. Dextral strike-slip motions along the North Aegean Trough fault zone were possible on NE-SW-striking faults. During the Pliocene-Lower Pleistocene, normal fault components were higher; however, because the angle between the NE-SW trend of the tensional axis and the strike of the fault zone was acute, dextral strike-slip components were possible on all the faults striking NE-SW to E-W. A clockwise 15^o rotation of Limnos with respect to Samothraki, Thraki and Thassos, suggested by structural data, was probably associated with these dextral motions. The WNW-ESE trending tension during the Upper Miocene indicates that the dextral North Anatolian Fault had not yet merged into the North Aegean Trough fault zone at that time. We propose that the formation of Aegean basins during the Cenozoic was related to the activity of two major Hellenic arcs. The ‘Pelagonian-Pindic Arc’ resulted in the formation of the subsident Aegean basins of Middle Eocene-Lower Miocene age and of the older Northern Aegean orogenic volcanism. The ‘Aegean Arc’ resulted in the formation of the subsident Aegean basins of Middle Miocene to Present day age and of the Southern Aegean orogenic volcanism. Were these arcs associated with a unique subduction zone or with two such zones ? In the first case, the slab is no more than 16 Myr old, in the second it may be as old as 45–50 Myr. The answer depends on the accuracy of the seismic tomography profiles.     

Regional vertical tectonics in the Eastern Mediterranean

Summary. New gravity observations from a systematic survey of the Eastern Mediterranean Sea and from a reconnaissance land survey in Central and Western Turkey have been compiled with existing data. Lack of sufficient geological and geophysical information precludes an analysis of the local anomalies or crustal structure; however, implications of the topography and gravity field at long wavelengths have been examined. Negative free-air anomalies characterize almost the entire Eastern Mediterranean basin and positive anomalies predominate in Turkey and the Aegean Sea. The change in sign coincides with the northern boundary of the African plate, and the wavelength and amplitude of the gravity variation are of the order of 1000 km and 100 mgal respectively. The lithosphere is probably unable to support such anomalies because the implied shear stresses are too large. The source of the anomalies is concluded to be in the asthenosphere where the low finite strength of material suggests that some sort of flow must exist to maintain the stresses. A good correlation is observed between the gravity and topography at wavelengths greater than 300 km; and the relationship is the same as that observed in the North Atlantic and the Central Pacific, as well as that computed for simple models of mantle convection. The gravity and topography of the Eastern Mediterranean can be explained in terms of flow in the upper mantle. This is the first region of subsidence for which this interpretation has been made.     

Late Cenozoic tectono‐stratigraphic sequences of the Crotone Basin: insights on the geodynamic history of the Calabrian arc and Tyrrhenian Sea

The Crotone Basin was generated in the late Cenozoic as a forearc basin of the Ionian arc‐trench system. New data are gained through detailed field mapping, high‐resolution stratigraphic analysis of a key area and examination of offshore well data and seismic reflection profiles. Major unconformities divide the basin fill into major sequences, which reveal a three‐stage internal organization thought to reflect geodynamic events of the Calabrian arc and backarc area closely. The first stage is characterized by extensional block faulting and uplift followed by rapid drowning during high subsidence and transtension in the basin along a major NNW‐ to NW‐striking fault system. This stage is interpreted to reflect resumption of rollback after an episode of slab tearing triggered by transitory docking of continental lithosphere in the trench. The initial uplift is inferred to reflect decoupling and rebound after the transitory coupling phase. The second stage is characterized by increased subsidence and continued extension/transtension. This trend presumably reflects a decreasing rate of rollback resulting from a tendency towards viscous coupling after acceleration of slab downwelling. The third stage is characterized by short‐lived transpression along major shear zones and local inversion of former basins. This is inferred to reflect entrance into the trench of buoyant continental lithosphere, resulting in significant deceleration of slab rollback and consequently a break in, or slowing of, backarc extension, and predominance of the effects of compression related to Africa–Europe convergence. Overall, the above evolution resulted in the formation of a progressively narrower and rapidly retreating slab, inducing extreme rates of backarc extension, and may have played a critical role in determining the intermittent nature of the backarc rifting.     

On the elastic-perfectly plastic bending of the lithosphere under generalized loading with application to the Kurd Trench

Summary. The bending of the lithosphere as it approaches an oceanic trench is modelled using an elastic-perfectly plastic rheology. The rock within the elastic lithosphere is assumed to behave elastically until a yield stress is reached; for larger strains the stress is assumed to remain at the yield value (no strain hardening). The deflection of the lithosphere is obtained numerically for a range of applied moments, vertical forces and horizontal forces at the trench axis. The results of the analysis are compared with a series of topographic and gravity profiles across the Kuril Trench. In order to model the large observed curvatures of the lithosphere on the outer trench slope plasticity is required. The location of the region of high curvatures gives a value for the compressive axial load. One conclusion is that the lithosphere is under compression seaward of the Kuril Trench.     

Roll-back controlled vertical movements of outer-arc basins of the Hellenic subduction zone (Crete, Greece)

Crustal thickening north of the Hellenic subduction zone continued in the most external zones (e.g. Crete) probably until the late middle Miocene. The following period of predominant extension has been related by various workers to a number of causes such as: (1) trench retreat (roll back) driven by the pull of the African slab and (2) gravitational body forces associated with the thickened crust, both in combination with NNE motion of the African plate combined with westward extrusion of the Anatolian block along the North Anatolian Fault. To verify these hypotheses an inventory of fault orientations and fault-block kinematics was carried out for central and eastern Crete and adjoining offshore areas by combining satellite imagery, digital terrain models, and structural, seismic, sedimentary and stratigraphical field data, all set up in a GIS. The GIS data set enabled easy visualization and combination of data, which resulted in a relatively objective analysis. The geological results are discussed in the light of a numerical model that investigated the intraplate stresses resulting from the above mentioned forces. Our tectonostratigraphic results for the late Neogene of central and eastern Crete show three episodes of basin extension following a period of approximately N–S compression. In the earliest Tortonian, N130E- to N100E-trending normal faults developed, resulting in a roughly planar, arc-parallel fault system aligning strongly asymmetric half-grabens. The early Tortonian to early Messinian period was characterized by an orthogonal fault system of N100E and N020E faults resulting in rectangular grabens and half-grabens. From the late Tortonian to early Pleistocene, deformation occurred along a pattern of closely spaced, left-lateral oblique N075E faults, orientated parallel to the south Cretan trenches. Deformation phases younger than early Pleistocene are dominated by normal to oblique faulting along WSW–ENE (N050E) faults and dextral, oblique motions along NNW–SSE (N160E) faults. Many faults that were generated during previous deformational episodes appear to be reactivated in later periods. Our tectonostratigraphy points to a three step anticlockwise rotation of active fault systems since the late middle Miocene compressional phase. We suggest here that the rotation is associated with a reorganization of the stress field going from SSW–NNE tension in the early late Miocene to NE–SW left-lateral shear in the Quaternary. The rotation is likely to be a response to arc-normal pull forces combined with a progressive increase of the curvature of the arc. During the Pliocene to Recent period, the SSW-ward retreat of the arc and trench system relative to the African plate was accomplished by transform motions in the eastern (Levantine) segment of the Hellenic Arc, resulting in, respectively, NNW–SSE and NE–SW left-lateral shear on Crete.     

Initiation and growth of salt-based thrust belts on passive margins: results from physical models

Scaled sandbox models simulated primary controls on the kinematics of the early structural evolution of salt‐detached, gravity‐driven thrust belts on passive margins. Models had a neutral‐density, brittle overburden overlying a viscous décollement layer. Deformation created linked extension–translation–shortening systems. The location of initial brittle failure of the overburden was sensitive to perturbations at the base of the salt. Salt pinch‐out determined the seaward limit of the thrust belt. The thrust belts were dominated by pop‐up structures or detachment folds cut by break thrusts. Pop‐ups were separated by flat‐bottomed synclines that were partially overthrust. Above a uniformly dipping basement, thrusts initiated at the salt pinch‐out then consistently broke landward. In contrast, thrust belts above a seaward‐flattening hinged basement nucleated above the hinge and then spread both seaward and landward. The seaward‐dipping taper of these thrust belts was much lower than typical, frictional, Coulomb‐wedge models. Towards the salt pinch‐out, frictional resistance increased, thrusts verged strongly seawards and the dip of the taper reversed as the leading thrust overrode this pinch‐out. We attribute the geometry of these thrust belts to several causes. (1) Low friction of the basal décollement favours near‐symmetric pop‐ups. (2) Mobile salt migrates away from local loads created by overthrusting, which reduces the seaward taper of the thrust belt. (3) In this gravity‐driven system, shortening quickly spreads to form wide thrust belts, in which most of the strain overlapped in time.     

Phase boundaries in finite amplitude mantle convection

Summary. Phase boundaries are included in dynamical finite element models of mantle convection. They are represented by point chains which act as additional sources of buoyancy forces when distorted, and as additional source or sink of heat. The influence of the exothermic olivine-spinel transition is studied in both shallow and deep convection models. The flow is only slightly enhanced by the transition. The increase of temperature due to latent heat release is step-like in the deep model, in the case of shallow convection it is more diffuse. Other quantities like ocean-floor topography, gravity anomalies, and stress distribution are no more than moderately affected. In a further investigation the effect of spinel post-spinel transition, whether endothermic or exothermic, on deep convection is examined. The effect on the flow is negligibly small in both cases.     

Seismic Delays in the Eastern Caribbean

i
The intervals between the arrivals of the same body wave phases from distant earthquakes at a close network of stations are compared with those expected from the known surface speeds of the phases. The arrival at the station on the island of Barbados is shown to be
2.94 = 0.34 s
later than expected relative to the other stations.
This is believed to be due to differences in crustal structure associated with the belt of negative gravity anomalies east of the West Indian arc.
A crustal section consistent with the seismic delays, gravity anomalies and seismic refraction profiles is presented.     

A technique for computing geoid height anomalies over two-dimensional earth models

Summary A technique is presented for calculating geoid height anomalies over two-dimensional models of Earth structure. The method consists of convolving gravity anomalies over the structure with filters which take into account the finite size of the structure in the third dimension and the curvature of the Earth. Similar filters are also developed for a flat earth case. The method is applied to a sea-surface gravity profile crossing the Tonga-Kermadec trench and is found to give good agreement with a Geos-3 radar altimetry profile in the same region. The example demonstrates that introducing arbitrary offsets in computing gravity anomalies can result in spurious long-wavelength effects in the computed geoid. Comparison of the results obtained using flat earth and spherical earth filters suggests that the effects of the curvature of the Earth only become significant for wavelengths in the gravity field greater than about 1000 km.     

Large-scale gravity profiles across subducted plates

Summary. The mean gravity profiles, across Central and South America and Eurasia, in the direction normal to the subduction zone are deduced from the Gem 10B gravity model. They have a typical shape: a maximum close to the trench, a negative slope towards the interior of the plate over a 3000 km wide distance, usually followed by a local maximum. It is found that large convective cells driven by the heat sink of the sinking slab have an associated gravity signal having such a typical shape. A detailed comparison between observed and theoretical data supports this point of view and thus constrains the possible structure of the convective flow under these plates.     

The palaeomagnetic field as inferred from magnetic studies on magmatic arc zones

Summary. Processes involved in the generation of arc maginatism, which are associated with the evolution of subduction zones, may in certain cases be able to record the variations of the Earth's magnetic field with time. A mechanism is suggested which may generate lineated magnetic anomaly patterns, similar to those observed over oceanic areas, with bands of alternating normal and reverse polarity parallel to the subduction zone. It is suggested that magmatic arc rocks acquire remanent magnetizations creating a zone of normal or reverse polarity in the magmatic arc zone, and if the geometric arrangement of plate subduction changes, i.e. the region of primary magma generation is displaced normal to the trench by changes of either the Wadati—Benioff zone dip angle or the trench position, then active arc magmatism is displaced accordingly, creating another magnetic anomaly zone. An important factor is the rate of displacement of arc magnatism with time; estimates for the south-western North America magmatic arc gives a value of about 1.2 cm yr^−l for the interval 100–55 Myr ago, which is comparable to sea-floor spreading rates. The elongated patterns of positive and negative magnetic anomalies, trenchward of the Japan trench and the Kurile—Kamchatka trench may have been produced by this process. Elongated intense magnetic anomalies are also observed in other magmatic arc assemblages, such as in western North America. Processes which may obscure these lineated anomaly patterns include, e.g. tectomc rotations within the magmatic arc zone, changes in thermal conditions and igneous activity, remagnetization events and decay of remanent magnetism. The anomalies may be related to, and better preserved by, intrusive rocks or buried rocks.     

Polarization anomaly of Love waves caused by lateral heterogeneity

We calculate surface waves propagating in a laterally heterogeneous structure beneath the Kuril trench, where significant Love-wave polarization anomalies, called quasi-Love waves, are generated. Since 3-D wave propagation in the two-dimensionally heterogeneous structure can be assumed, we apply the 2.5-D finite difference method to the surface-wave calculations. The calculations show that a velocity contrast of 7 per cent at depths of less than 210 km beneath the Kuril trench cannot generate quasi-Love waves, and that an unlikely contrast of 20 per cent is required to generate clear quasi-Love waves. The possible cause of the quasi-Love waves inferred from previous studies on coupled free oscillations is a lateral variation in azimuthal anisotropy. The lateral variation in azimuthal anisotropy beneath the Kuril trench suggests a change in the mantle flow induced by the subducting slab.