Saskia Goes

Shallow mantle temperatures under Europe from P and S wave tomography

Temperature is one of the key parameters controlling lithospheric and mantle dynamics and rheology. Using recent experimental data on elastic parameters and anelasticity, we obtain models of temperature at 50 to 200 km depth beneath Europe from the global P wave velocity model of Bijwaard et al. [1998] and the regional S wave velocity model of Marquering and Snieder [1996]. Forward modeling of seismic velocity allows us to assess the sensitivity of velocity to various parameters. In the depth range of interest, variations in temperature (when below the solidus) yield the largest effects. For a 100°C increase in temperature, a decrease of 0.5–2% in VP and 0.7–4.5% in VS is predicted, where the strongest decrease is due to the large effect of anelasticity at high temperature. The effect of composition is expected to give velocity anomalies 80 km the relative amplitudes of the European VP and VS anomalies are consistent with a thermal origin. At shallower depths, variations in crustal thickness and possibly the presence of partial melt appear to have an additional effect, mainly on S wave velocity. In regions where both P and S anomalies are well-resolved, VP- and VS-derived thermal models agree well with each other and with temperatures determined from surface heat flow observations. Furthermore, the thermal models are consistent with known tectonics. The inferred temperatures vary significantly, from around 400°C below an average mantle adiabat at 100 km depth under the Russian Platform and a 300°C increase from east to west across the Tornquist-Teisseyre zone to temperatures around the mantle adiabat in the depth range 50–200 km under areas with present surface volcanism. In spite of the uncertainties in the calculation of temperatures due to uncertainties in the experimental elastic parameters and anelasticity and uncertainties associated with tomographic imaging, we find that the tomographic models of the shallow mantle under Europe can yield useful estimates of the thermal structure.

Small-scale convection at the edge of the Colorado Plateau: Implications for topography, magmatism, and evolution of Proterozoic lithosphere

The Colorado Plateau of the southwestern United States is characterized by a bowl-shaped high elevation, late Neogene–Quaternary magmatism at its edge, large gradients in seismic wave velocity across its margins, and relatively low lithospheric seismic wave velocities. We explain these observations by edge-driven convection following rehydration of Colorado Plateau lithosphere. A rapidly emplaced Cenozoic step in lithosphere thickness between the Colorado Plateau and adjacent extended Rio Grande rift and Basin and Range province causes small-scale convection in the asthenosphere. A lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized, resulting in magmatism. Edgedriven convection also drives margin uplift, giving the plateau its characteristic bowl shape. The edge-driven convection model shows good consistency with features resolved by seismic tomography.

Evidence of lower-mantle slab penetration phases in plate motions.

It is well accepted that subduction of the cold lithosphere is a crucial component of the Earth’s plate tectonic style of mantle convection. But whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate, which has substantial implications for the chemical and thermal evolution of the mantle. Here we identify lower-mantle slab penetration events by comparing Cenozoic plate motions at the Earth’s main subduction zones with motions predicted by fully dynamic models of the upper-mantle phase of subduction, driven solely by downgoing plate density. Whereas subduction of older, intrinsically denser, lithosphere occurs at rates consistent with the model, younger lithosphere (of ages less than about 60 Myr) often subducts up to two times faster, while trench motions are very low. We conclude that the most likely explanation is that older lithosphere, subducting under significant trench retreat, tends to lie down flat above the transition to the high-viscosity lower mantle, whereas younger lithosphere, which is less able to drive trench retreat and deforms more readily, buckles and thickens. Slab thickening enhances buoyancy (volume times density) and thereby Stokes sinking velocity, thus facilitating fast lower-mantle penetration. Such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle. Thus we identify a direct expression of time-dependent flow between the upper and lower mantle.

Active deformation in eastern Indonesia and the Philippines from GPS and seismicity data

In this study we combine Global Positioning System (GPS) velocities with information on the style of regional seismicity to obtain a self-consistent model velocity and strain rate field for the entire eastern Indonesia and Philippines region. In the process of interpolating 93 previously published GPS velocities, the style and direction of the seismic strain rate field, inferred from earthquakes with M0 < 1 × 1020 N m (from the Harvard centroid moment tensor catalog), are used as constraints on the style and direction of model strain rates within the plate boundary zones. The style and direction of the seismic strain rate field are found to be self-similar for earthquakes up to M0 = 1 × 1020 N m (equivalent to Mw < 7.3). Our inversion result shows the following: The Java Trench, which lacks any significant (historic) seismicity, delineates the Australian plate (AU) - Sunda block (Sunda) plate boundary west of the island of Sumba. East of Sumba, convergence is distributed over the back arc and Banda Sea, and there is no subduction at the Timor Trough, suggesting that the northern boundary of the AU plate runs north of this part of the Banda arc through the Banda Sea. In New Guinea most motion is taken up as strike-slip deformation in the northern part of the island, delineating the Pacific plate (PA) - AU boundary. However, some trench-normal convergence is occurring at the New Guinea Trench, evidence that the strain is partitioned in order to accommodate oblique PA-AU motion. PA-AU motion is consistent with NUVEL-1A direction, but ∼ 8 mm yr−1 slower than the NUVEL-1A estimate for PA-AU motion. The Sulawesi Trench and Molucca Sea delineate zones of high strain rates, consistent with high levels of active seismicity. The Sulawesi Trench may take up some of the AU-Sunda motion. Philippine Sea plate motion is in a direction slightly northward of the NUVEL-IA estimate and is partitioned in some strike-slip strain rates along the Philippine Fault and relatively larger trench-normal convergence along the Philippine Trench and on the Philippine mainland in the southern Philippines and along the Manila Trench in the northern Philippine islands. The high level of strain rate along the Manila Trench is not released by any significant (historic) seismic activity. For the entire eastern Indonesia-Philippines region, seismicity since 1963 has taken up ∼40% of the total moment rate inferred from our model.

Thermal structure of continental upper mantle inferred from S-wave velocity and surface heat flow

Results from seismic tomography provide information on the thermal structure of the continental upper mantle. This is borne out by the good agreement between tectonic age, surface heat flow and a tomographic S-wave velocity model for depths less than 180 km. The velocity anomalies of tomographic layers deeper than 230 km have relatively small amplitudes and show little correlation with surface heat flow or shallow velocities. We associate the drop in correlation and amplitude of the velocity perturbations between 180 and 230 km depth with the maximum thickness of the thermal boundary layer (TBL), in which larger variations in temperature and possibly composition than in the underlying convecting mantle can be sustained. Velocity profiles for different tectonic provinces are converted to temperature using mineralogical data. Both anharmonic and anelastic effects on the wave speeds are taken into account. The resulting geotherms differ most at depths of 60^120 km with variations of up to 900‡C. Below 230 km, differences do not exceed 300‡C. These geotherms agree well with one-dimensional conductive geotherms for the observed range of continental heat flow values using the empirical relationship that 40% of the surface heat flux stems from upper crustal radiogenic heat production. The S-wave velocity in the continental upper mantle appears to be adequately explained (within the uncertainties of the tomography and the conversion to temperature) by a thermal signature. A compositional component can, however, not be ruled out as it may have only a minor effect on the velocity and the heat flow. The surface heat flow is controlled by the shallow heat production and the thickness of the TBL. Seismology helps to determine the relative importance of the two factors and our results confirm the similar importance of both factors. Variations of TBL thickness could be controlled by compositional differences and/or by the effect of temperature on the rheology. fl 2000 Elsevier Science B.V. All rights reserved.

Interaction of subducted slabs with the mantle transition‐zone: A regime diagram from 2‐D thermo‐mechanical models with a mobile trench and an overriding plate

Transition zone slab deformation influences Earths thermal, chemical and tectonic evolution. However, the mechanisms responsible for the wide-range of imaged slab morphologies remain debated. Here, we use 2-D thermo-mechanical models with a mobile trench, an overriding plate, a temperature- and stress-dependent rheology, and a 10, 30 or 100-fold increase in lower mantle viscosity, to investigate the effect of initial subducting- and overriding-plate ages on slab transition-zone interaction. Four subduction styles emerge: (i) a “vertical folding” mode, with a quasi-stationary trench, near-vertical subduction and buckling/folding at depth (VF); (ii) slabs that induce mild trench retreat, which are flattened/“horizontally deflected” and stagnate at the upper-lower mantle interface (HD); (iii) inclined slabs, which result from rapid sinking and strong trench retreat (ISR); (iv) a two-stage mode, displaying backward-bent and subsequently inclined slabs, with late trench retreat (BIR). Transitions from regime (i) to (iii) occur with increasing subducting-plate age (i.e. buoyancy and strength). Regime (iv) develops for old (strong) subducting and overriding plates. We find that the interplay between trench motion and slab deformation at depth dictate the subduction style, both being controlled by slab strength, which is consistent with predictions from previous compositional subduction models. However, due to feedbacks between deformation, sinking rate, temperature and slab strength, the subducting-plate buoyancy, overriding-plate strength and upper-lower mantle viscosity jump are also important controls in thermo-mechanical subduction. For intermediate upper-lower mantle viscosity jumps (×30), our regimes reproduce the diverse range of seismically imaged slab morphologies.

Synthetic seismic signature of thermal mantle plumes

Abstract The first seismic images of mantle plumes have been a source of significant debate. To interpret these images, it is useful to have an idea of a plume’s expected seismic signature. We determined a set of dynamic thermal whole-mantle plumes, with parameters appropriate for the Earth’s mantle and shallow-mantle temperature contrasts compatible with surface observations. We explore the sensitivity of amplitude and width of thermal plume anomalies to model parameters. The conversion of thermal to seismic structure accounts for effects of temperature, pressure, an average mantle composition including phase transitions, and anelasticity. With depth-dependent expansivity and temperature- and depth-dependent viscosity, these relatively weak plumes have lower-mantle diameters of 300–600 km at one half of the maximum temperature anomaly. To attain the narrow upper-mantle plumes inferred from surface observations and tomography, viscosity reduction by a factor 30–100 is necessary, either as a jump or as a strong gradient. All model plumes had buoyancy fluxes ≥4 Mg/s and it seems difficult to generate whole-mantle thermal plumes with fluxes much lower. Due to changing seismic sensitivity to temperature with depth and mineralogy, variations in the plumes’ seismic amplitude and width do not coincide with those in their thermal structure. Velocity anomalies of 2–4% are predicted in the uppermost mantle. Reduced sensitivity in the transition zone as well as complex velocity anomalies due to phase boundary topography may hamper imaging continuous whole-mantle plumes. In the lower mantle, our plumes have seismic amplitudes of only 0.5–1%. Unlike seismic velocities, anelasticity reflects thermal structure closely, and yields plume anomalies of 50–100% in dln(1/QS).

Small-scale convection during continental rifting: Evidence from the Rio Grande rift

Recent seismic imaging across the Rio Grande rift, western United States, revealed unexpected structures in the underlying mantle. Low seismic wave velocity anomalies below the Rio Grande rift have been interpreted as being partially of melt origin, and high-velocity structures below the western Great Plains have been proposed to be the result of small-scale convection, i.e., cold downwelling lithospheric material with probably a compositional contribution. We perform a dynamic test of these interpretations using a passive rift model for isochemical convection. The models self-consistently produce a rift localized at approximately the right distance from the border to the nearby thicker Great Plains lithosphere. With realistic upper mantle rheologies, small-scale convection forms, aided by the lithospheric step. The resulting thermal anomalies produce seismic low-velocity anomalies below the rift of amplitudes similar to those imaged seismically, requiring the presence of only small amounts of melt. The lateral extent of the observed low velocities below the Rio Grande rift is as in the models, where it is controlled by the spacing between downwelling limbs of the small-scale convection. The fast velocity structure below the western Great Plains can be produced by cold downwelling lithosphere. The thermal rifting models can predict the amplitudes and size of the main seismic anomalies; compositional heterogeneity may contribute to some of the smaller features observed.

Fate of the Cenozoic Farallon slab from a comparison of kinematic thermal modeling with tomographic images

Abstract After more than 100 million years of subduction, only small parts of the Farallon plate are still subducting below western North America today. Due to the relatively young age of the most recently subducted parts of the Farallon plate and their high rates of subduction, the subducted lithosphere might be expected to have mostly thermally equilibrated with the surrounding North American mantle. However, images from seismic tomography show positive seismic velocity anomalies, which have been attributed to this subduction, in both the upper and lower mantle beneath North America. We use a three-dimensional kinematic thermal model based on the Cenozoic plate tectonic history to quantify the thermal structure of the subducted Farallon plate in the upper mantle and determine which part of the plate is imaged by seismic tomography. We find that the subducted Farallon lithosphere is not yet thermally equilibrated and that its thermal signature for each time of subduction is found to be presently detectable as positive seismic velocity anomalies by tomography. However, the spatially integrated positive seismic velocity anomalies in tomography exceed the values obtained from the thermal model for a rigid, continuous slab by a factor of 1.5 to 2.0. We conclude that Farallon fragments that subducted since 50 to 60 Ma are still residing in the upper mantle and must be heavily deformed. The deformation of the slab in the transition zone is probably caused by the same mechanisms that were responsible for flat subduction around 60 Ma.

The April 22, 1991, Valle de la Estrella, Costa Rica (Mw = 7.7) earthquake and its tectonic implications: A broadband seismic study

The rapture process of a large back arc thrusting earthquake, the April 22, 1991, Valle de la Estrella, Costa Rica (Mw = 7.7), earthquake, is investigated using broadband body waves and long-period surface waves. We find the source process to be relatively simple, with the source models separately obtained from body and surface waves being very consistent. The event occurred on a shallow, southwest dipping rapture plane on which most energy is released updip of the hypocentral location (10–20 km deep). High-frequency radiation appears to have been released over a relatively small source area. Our preferred model has a focal mechanism with strike 102 ± 10°, dip 17 ± 14° and rake 63 ± 17°, a seismic moment of 3.8 ± 1.5 × 1020 N m, and a total rupture duration of 40 ± 6 s. The earthquake appears to be associated with the North Panama Deformed Belt (NPDB), a thrust and fold complex that has accommodated the oroclinal deformation of Panama. This event, along with previous large events north of Panama in 1882 and 1916, indicates that there is substantial convergence along the NPDB, marking the NPDB as a probable emerging plate boundary. It remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity.