Ana M. Negredo

Neotectonic modelling of the western part of the Africa^Eurasia plate boundary: from the Mid-Atlantic ridge to Algeria

Abstract In this work we use the thin-shell approximation to model the neotectonics of the western part of the Africa–Eurasia plate boundary, extending from the Mid-Atlantic ridge to Tell Atlas (northern Algeria). Models assume a nonlinear rheology and include laterally variable heat flow, elevation, and crust and lithospheric mantle thickness. Including the Mid-Atlantic ridge permits us to evaluate the effects of ridge push and to analyse the influence of the North America motion on the area of the Africa–Eurasia plate boundary. Ridge push forces were included in a self-consistent manner and have been shown to exert negligible effects in the neotectonics of the Iberian Peninsula and northwestern Africa. Different models were computed with systematic variation of the fault friction coefficient. Model quality was scored by comparing predictions of anelastic strain rates, vertically integrated stresses and velocity fields to data on seismic strain rate computed from earthquake magnitude, most compressive horizontal principal stress direction, and seafloor spreading rates on the Mid-Atlantic ridge. The best model scores were obtained with fault friction coefficients as low as 0.06–0.1. The velocity boundary condition representing spreading on the Mid-Atlantic ridge is shown to produce concentrated deformation along the ridge and to have negligible effect in the interior of the plates. However, this condition is shown to be necessary to properly reproduce the observed directions of maximum horizontal compression on the Mid-Atlantic ridge. The maximum fault slip rates predicted by the model are obtained along the Mid-Atlantic ridge, Terceira ridge and Tell Atlas front. Relatively high slip rates are also obtained in the area between the Gloria fault and the Gulf of Cadiz. We infer from our modelling a significant long-term seismic hazard for the Gloria fault, and interpret the absence of seismicity on this fault as possibly due to transient elastic strain accumulation. The present study has also permitted better understanding of the geometry of the Africa–Eurasia plate boundary from the Azores triple junction to the Algerian Basin. The different deformational styles seem to be related to the different types of lithosphere, oceanic or continental, in contact at the plate boundary.

TEMSPOL: A MATLAB thermal model for deep subduction zones including major phase transformations

Abstract TEMSPOL is an open MATLAB code suitable for calculating temperature and lateral anomaly of density distributions in deep subduction zones, taking into account the olivine to spinel phase transformation in a self-consistent manner. The code solves, by means of a finite difference scheme, the heat transfer equation including adiabatic heating, radioactive heat generation, latent heat associated with phase changes and frictional heating. We show, with a few simulations, that TEMSPOL can be a useful tool for researchers studying seismic velocity, stress and seismicity distribution in deep subduction zones. Deep earthquakes in subducting slabs are thought to be caused by shear instabilities associated with the olivine to spinel phase transition in metastable olivine wedges. We investigate the kinematic and thermal conditions of the subducting plate that lead to the formation of metastable olivine wedges. Moreover, TEMSPOL calculates lateral anomalies of density within subducting slabs, which can be used to evaluate buoyancy forces that determine the dynamics of subduction and the stress distribution within the slab. We use TEMSPOL to evaluate the effects of heat sources such as shear heating and latent heat release, which are neglected in commonly used thermal models of subduction. We show that neglecting these heat sources can lead to significant overestimation of the depth reached by the metastable olivine wedge.

Numerical modeling of simultaneous extension and compression: The Valencia trough (western Mediterranean)

A two-dimesional kinematic model, which incorporates lateral accommodation of extension and differential stretching, is used to investigate the geodynamic evolution of the Valencia trough. This Neogene basin, located between the Iberian Peninsula and the Balearic Promontory, underwent two rifting episodes, the first one being coeval with compression. Model predictions are compared with observations of tectonic subsidence, Bouguer gravity anomaly, surface heat flow, and the present-day crustal structure. The best fitting model assumes three stages in the evolution of the basin. During the first stage (late Oligocene-early Miocene), extension is mainly restricted to the central and northwestern parts of the basin, whereas its southeastern margin is locked by the Alpine deformation front. The material expelled from the axial area is partly accommodated under the Balearic Promontory, producing compression and crustal thickening. During the second rifting stage (middle Miocene), extension affects a much broader area, including the Balearic Promontory and the Algero-Provencal basin. The third stage (late Miocene to Recent) corresponds to a postrift thermal relaxation phase. Our results put further constraints on the geodynamic evolution of the area.

Evidence for eastward mantle flow beneath the Caribbean plate from neotectonic modeling

AN was supported by the Spanish Ministerio de Ciencia y Tecnologia research projects BTE2002-02462 and ‘Ramon y Cajal’

Influence of cratonic lithosphere on the formation and evolution of flat slabs: Insights from 3‐D time‐dependent modeling

Several mechanisms have been suggested for the formation of flat slabs including buoyant features on the subducting plate, trenchward motion and thermal or cratonic structure of the overriding plate. Analysis of episodes of flat subduction indicate that not all flat slabs can be attributed to only one of these mechanisms and it is likely that multiple mechanisms work together to create the necessary conditions for flat slab subduction. In this study we examine the role of localized regions of cratonic lithosphere in the overriding plate in the formation and evolution of flat slabs. We explicitly build on previous models, by using time-dependent simulations with three-dimensional variation in overriding plate structure. We find that there are two modes of flat subduction: permanent underplating occurs when the slab is more buoyant (shorter or younger), while transient flattening occurs when there is more negative buoyancy (longer or older slabs). Our models show how regions of the slab adjacent to the subcratonic flat portion continue to pull the slab into the mantle leading to highly contorted slab shapes with apparent slab gaps beneath the craton. These results show how the interpretation of seismic images of subduction zones can be complicated by the occurrence of either permanent or transient flattening of the slab, and how the signature of a recent flat slab episode may persist as the slab resumes normal subduction. Our models suggest that permanent underplating of slabs may preferentially occur below thick and cold lithosphere providing a built-in mechanism for regeneration of cratons.

Thermo-mechanical constraints on kinematic models of lithospheric extension

Abstract Two-dimensional kinematic modelling of extension based on the numerical solution of the heat transport equation is used to investigate the lateral evolution of lithospheric yield strength during rifting and during postrift relaxation. Two yield strength minima are shown to exist, one beneath the rift centre and the other beneath the undeformed area adjacent to the rift. These are separated by a relative maximum in the transition zone between the rift and the outer area. Initially cold (thick) lithosphere and fast extension give rise to an absolute strength minimum beneath the rift, leading to a narrow rift. On the other hand, initially hot (thin) lithosphere and slow extension lead to an absolute strength minimum beneath the rift sides, which could cause outward migration of the area of principal strain and formation of a wide rift. In contrast to some existing one-dimensional analyses, no lithospheric hardening is necessary for the formation of a wide rift and the conditions for its formation are consequently less restricted. The maximum of strength in the transition zone suggests a third mode of extension characterized by the activation of extension parallel to the rift, with undeformed areas in between. During postrift evolution, the strength of the thinned area progressively increases until the zone adjacent to the rift becomes the weakest area, inducing a widening of the rift in subsequent rifting episodes. The minimum time required for migration of deformation in successive rifting stages is termed the critical relaxation time (CRT) and is shorter for an initially hot lithosphere and low strain rate. CRT values range from few million years to more than 130 Ma for a stretching factor of 1.65. This study highlights major differences with respect to previous one-dimensional analyses when considering lateral heat transport and a progressive crustal and lithospheric thinning from the rift flanks.

EXTENSION WITH LATERAL MATERIAL ACCOMMODATION : 'ACTIVE' VS. 'PASSIVE' RIFTING

Abstract In many rifted areas of the world, differential thinning is observed between upper and lower crust or between crust and lithospheric mantle. A kinematic model based on a finite element algorithm is presented which allows thermal modelling of differential stretching. In order to avoid space problems, the velocity field is defined under the assumption that the total amount of extension is the same for every lithospheric layer. Several combinations of far-field velocity and local extensional velocities in the rift area have been used to simulate basin-formation under different conditions of active (far field velocity smaller than extensional velocities) and passive rifting. Consequences on elevation, surface heat flow and lithospheric yield strength are investigated. Far-field velocities lower than local extensional velocities produce narrow basins bordered by high shoulders, whereas a wide basin without shoulders is obtained when the far-field velocity is similar to the maximum extensional velocity. A detailed study is presented for the first velocity distribution. When the detachment is located in the lower crust, a thickening of the crust occurs below the rift shoulders, which continue to rise during the post-rift phase. Lithospheric strength below the shoulders increases strongly during rifting due to cooling of the lithospheric caused by the down-welling of material in this area. In contrast, post-rift thermal relaxation leads to a rapid decrease of strength resulting eventually in a minimum located at the rift shoulders.