Cenozoic plate driving forces

Past studies of plate driving forces have concluded that the forces due to subducted slabs in the upper mantle and those due to the thickening of the oceanic lithosphere are the principal driving forces. We reexamine the balance of driving forces for the present-day and extend our analysis through the Cenozoic, using an analytical torque balance method which accounts for interactions between plates via viscous coupling to the induced mantle flow, We use an evolving mantle density heterogeneity field based on the last 200 Myr. of subduction to drive plate motions, an approach which has proven successful in predicting the present-day mantle heterogeneity field. We find that for plausible upper mantle viscosities the forces due to subducted slabs in the Cenozoic and Mesozoic account for in excess of 90% of plate driving forces and those due to lithospheric thickening for less than 10%

Triggered seismicity associated with the 1990 Nicoya, Costa Rica, M-w=7.0 earthquake

The 25 March 1990 (M-w = 7.0) subduction megathrust earthquake that occurred offshore the Nicoya Peninsula, Costa Rica, produced a large number of aftershocks on the subduction plate interface as expected and preceded an unusual sequence of earthquakes 75 km inland that had two periods of significant increase, one at 60-90 days and one near 270 days, following the main shock. This inland sequence of events would not typically fall within the classification of aftershocks given their spatial and temporal distance, and we show here that this sequence was likely triggered by the 25 March main shock. We compute stress changes on representative faults within this inland region using both a simple half-space model as well as with a 2-D finite element model that incorporates variable rheologic properties. The half-space model predicts a minor increase in Coulomb stress changes and a large amount of unclamping in this region, likely enough to cause triggering on the inland right-lateral strike-slip faults. Models that include a viscoelastic response also indicate stress increases that may link to triggering, particularly related to the time delay. Earthquakes on the subduction zone thrust along Costa Rica should be considered in hazard assessments for the inland populated region as several sets of strike-slip faults have been mapped in the fore-arc region

Dynamics and excess temperature of a plume throughout its life cycle

Measurements of the velocity field associated with plumes rising through a viscous fluid are performed using stereoscopic Particle-Image Velocimetry in the Rayleigh number range 4.4 × 105 − 6.4 × 105. The experimental model is analogous to a mantle plume rising from the core-mantle boundary to the base of the lithosphere. The behaviour of the plume is studied throughout its life cycle, which is broken up into four stages; the Formation Stage, when the plume forms; the Rising Stage, when the plume rises through the fluid; the Spreading Stage, when the plume reaches the surface and spreads; and finally the Declining Stage, when the heat source has been removed and the plume weakens. The latter three stages are examined in terms of the Finite-Time Lyapunov Exponent fields and the advection of passive tracers throughout the flow. The temperature at the heater and near the fluid surface are measured using thermocouples to infer how the presence of a mantle plume would produce excess temperature near the lithosphere throughout the various stages of its life cycle. In all experiments a time lag is observed between the removal of the heat source and the decline in the excess temperature near the surface, which is proportional to the rise time. A simple analytical model is presented, which suggests that under mantle conditions (i.e. negligible thermal diffusion), the relationship between the time lag and the rise time is robust and independent of the Rayleigh number; however, the constant of proportionality is closer to unity in the absence of diffusion. Once the heat source is removed, the excess temperature near the surface declines exponentially at a rate that is inversely proportional to the rise time. The implications of this result are discussed in terms of the decline in volcanism in the Louisville hotspot chain over the past 20 Ma. The rise velocity of material in the plume is examined; the rise velocity is found to vary significantly with the plume height in a manner that is inconsistent with many of the common semi-analytical models of thermal plumes in the literature. It is also argued that this height-dependency will cause estimates of the rise velocity based on the decay series of Uranium isotopes to significantly underestimate the true value

Why is Africa rifting?

Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model

Orphaning Regimes: The Missing Link Between Flattened and Penetrating Slab Morphologies

Slab orphaning is a newly discovered phenomenological behavior, where the slab tip breaks off at the top of the lower mantle (~660 km depth) and is abandoned by its parent slab. Upon orphaning, subduction continues uninterrupted through the lateral motion of the parent slab above 660 km depth. In this work, we present a regime diagram for the range of conditions under which slabs can orphan at the top of the lower mantle. Our models show that a viscosity jump at 1,000 km depth not coincident with the endothermic phase change responsible for the 660 km seismic discontinuity, is necessary for orphaning as is the presence of a low viscosity channel between 660 and 1,000 km depth. We show that orphan slabs, similar to other deep slab morphologies, can be the end result for a wide range of physical parameters governing slab dynamics: slab orphaning persists across wide variations in slab dip, slab yield stress/strength, Clapeyron slope values, and overriding plate nature. The diversity in orphan slab sizes and orphaning periods is tied to the orphaning regime space, which describes a hitherto unexplored region between deflected and penetrating deep-subduction modes. Orphaning provides a simple dynamic link between the well-known deflection and penetration, and provides one possible way for slabs to switch from direct penetration to deflection, littering the mantle with abandoned fragments. Orphan slabs are therefore the intermediary between these two extensively studied slab morphologies

Surface exposure constraints on the mantle water budget

Mantle water content estimates range from 0.5 to 15 oceans of water. Its evolution is even more unclear. Rapid degassing during mantle solidification likely released much of the water to the surface, initially flooding Earth. However, evidence for subaerial land from at least 3.5 Ga means that much of this water must have been rapidly cycled back into the mantle. Here, we used a parameterized convection model and hypsometric curve to assess how much water could have been taken into the mantle and still satisfy evidence for subaerial land. Even if only the highest peaks were exposed, the initial ocean must have been less than 1.5 current oceans to explain subaerial exposure throughout most of Earth history. Today, this implies any water in the mantle?>0.5 oceans must be primordial and has been isolated from the convecting mantle for most of Earth’s history

The dynamics of Cenozoic and Mesozoic plate motions

Our understanding of the dynamics of plate motions is based almost entirely upon modeling of present-day plate motions. A fuller understanding, however, can be derived from consideration of the history of plate motions. Here we investigate the kinematics of the last 120 Myr of plate motions and the dynamics of Cenozoic motions, paying special attention to changes in the character of plate motions and plate-driving forces. We analyze the partitioning of the observed surface velocity field into toroidal (transform/spin) and poloidal (spreading/subduction) motions. The present-day field is not equipartitioned in poloidal and toroidal components; toroidal motions account for only one third of the total. The toroidal/poloidal ratio has changed substantially in the last 120 Myr with poloidal motion decreasing significantly after 43 Ma while toroidal motion remains essentially constant; this result is not explained by changes in plate geometry alone. We develop a self-consistent model of plate motions by (1) constructing a straightforward model of mantle density heterogeneity based largely upon subduction history and then (2) calculating the induced plate motions for each stage of the Cenozoic. The "slab" heterogeneity model compares rather well with seismic heterogeneity models, especially away from the thermochemical boundary layers near the surface and core-mantle boundary. The slab model predicts the observed geoid extremely well, although comparison between predicted and observed dynamic topography is ambiguous. The midmantle heterogeneities that explain much of the observed seismic heterogeneity and geoid are derived largely from late Mesozoic and early Cenozoic subduction, when subduction rates were much higher than they are at present. The plate motion model itself successfully predicts Cenozoic plate motions (global correlations of 0.7-0.9) for mantle viscosity structures that are consistent with a variety of geophysical studies.We conclude that the main plate-driving forces come from subducted slabs (>90%), with forces due to lithospheric effects (e.g., oceanic plate thickening) providing a very minor component (<10%). For whole mantle convection, most of the slab buoyancy forces are derived from lower mantle slabs. Unfortunately, we cannot reproduce the toroidal/poloidal partitioning ratios observed for the Cenozoic, nor do our models explain apparently sudden plate motion changes that define stage boundaries. The most conspicuous failure is our inability to reproduce the westward jerk of the Pacific plate at 43 Ma implied by the great bend in the Hawaiian-Emperor seamount chain. Our model permits an interesting test of the hypothesis that the collision of India with Asia may have caused the Hawaiian-Emperor bend. However, we find that this collision has no effect on the motion of the Pacific plate, implying that important plate boundary effects are missing in our models. Future progress in understanding global plate motions requires (1) more complete plate reconstruction information, including, especially, uncertainty estimates for past plate boundaries, (2) better treatment of plate boundary fault mechanics in plate motion models, (3) application of numerical convection models, constrained by global plate motion histories, to replace ad hoc mantle heterogeneity models, (4) better calibration of these heterogeneity models with seismic heterogeneity constraints, and (5) more comprehensive comparison of global plate/mantle dynamics models with geologic data, especially indicators of intraplate stress and strain, and constraints on dynamic topography derived from the stratigraphic record of sea level change

Abrupt upper-plate tilting during slab-transition-zone collision

The sinking remnant of a surface plate crosses and interacts with multiple boundaries in Earth's interior. Here, we specifically investigate the prominent dynamic interaction of the sinking plate portion with the upper-mantle transition zone and its corresponding surface elevation signal. We unravel, for the first time, that the collision of the sinking slab with the transition zone induces a sudden, dramatic downward tilt of the upper plate towards the subduction trench. Unraveling this crucial interaction was only possible thanks to state-of-the-art numerical modelling and post-processing. The new model that is introduced here to study the dynamically self-consistent temporal evolution of subduction features accurate subduction-zone topography, robust single-sided plate sinking, stronger plates close to laboratory values, an upper-mantle phase transition, and simple continents at a free surface. To distinguish the impact of the new physical model features, three different setups are used: the simplest model setup includes a basic high-viscosity lower mantle, the second adds a 660-km phase transition, and the third includes, additionally, a continental upper plate. Common to all models is the clear topographic signal upon slab-transition-zone interaction: the upper plate tilts abruptly towards the subduction trench by about 0.05° and over around 10. Ma. This dramatic increase in upper-plate tilt can be related to the slab-induced excitation of the high-viscosity lower mantle, which introduces a wider flow pattern. A large change in horizontal extent of inundation of up to 900. km is observed as a direct consequence of the upper-plate tilting. Such an abrupt variation in surface topography and inundation extent should be clearly visible in temporal records of large-scale surface elevation and might explain continental tilting as observed in Australia since the Eocene and North America during the Phanerozoic

Towards Inverting Seismic Waveform Data for Temperature and Composition in the Earth's Upper Mantle

B: Unraveling the physical state of the upper mantle, including the transition zone, is one of the key factors for understanding the Earth's mantle dynamics. Knowledge of mantle temperature and composition is mainly based on the interpretation of seismological observations based on insights from mineral physics. Despite the progress made to image the 3-D seismic structure of the upper mantle, its interpretation in terms of physical parameters is still challenging and it requires a truly interdisciplinary approach. Due to the better knowledge of the elastic and anelastic properties of mantle minerals at high temperatures and pressures, such an approach is now becoming feasible. We propose a new waveform inversion procedure, based on a formalism previously developed at Berkeley for global elastic and anelastic tomography, and using our existing collection of long-period fundamental and higher mode surface waveforms. Here, we incorporate mineral physics data at an early stage of the process to directly map lateral variations in temperature and composition, using recent estimates of the temperature and composition derivatives of seismic velocities (∂lnV/∂lnT,C). Anelasticity introduces a non-linear dependence of the seismic velocities with temperature throughout the upper mantle, and phase-transitions confer a non-linear character to the compositional derivatives as well, therefore the kernels should be re-computed after each iteration of the inversion. We discuss ways to address the non-linearities, as well as uncertainties in the partial derivatives. In addition to constraining the lateral variations in temperature or composition, the models can have implications on the average structure of the upper mantle. The most-common accepted physical 1-D structure had problems to satisfactorily fit seismic travel time data, requiring a slower TZ to improve the fit. However, these data do not have sufficient coverage (and resolution) in the TZ. A complementary outcome of our models will be to shed light on whether the seismic data require a modification of the physical structure in the transition zone and if the three-dimensional heterogeneity introduces a significant shift of the average physical structure away from adiabatic pyrolite

Modification of the lithospheric stress field by lateral variations in plate-mantle coupling

[1] The presence of deeply penetrating continental roots may locally increase the magnitude of basal shear tractions by up to a factor of 4 compared to a layered viscosity structure. Here we examine how these increases in mantlelithosphere coupling influence stress patterns in the overlying elastic lithosphere. By coupling a mantle flow model to a model for the elastic lithosphere, we show that the amplification of mantle tractions beneath cratons increases elastic stress magnitudes by at most a factor of only 1.5 in a pattern not correlated to local basal traction changes. This disconnect is explained by the transmission of elastic stresses across large distances, which makes them sensitive to regionally-averaged changes in basal tractions, but not local variations. Our results highlight the importance of regional variations in lithospheric strength, which could allow stress patterns to more closely match regional changes in basal shear. Citation: Naliboff, J. B., C. P. Conrad, and C. Lithgow-Bertelloni (2009), Modification of the lithospheric stress field by lateral variations in plate-mantle coupling, Geophys. Res. Lett., 36, L22307