Rob Govers

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.

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.

Two-dimensional simulations of surface deformation caused by slab detachment

Detachment of the deeper part of subducted lithosphere causes changes in a subduction zone system which may be observed on the Earth’s surface. Constraints on the expected magnitudes of these surface effects can aid in the interpretation of geological observations near convergent plate margins where detachment is expected. In this study, we quantify surface deformation caused by detachment of subducted lithosphere. We determine the range of displacement magnitudes which can be associated with slab detachment using numerical models. The lithospheric plates in our models have an effective elastic thickness, which provides an upper bound for rapid processes, like slab detachment, to the surface deformation of lithosphere with a more realistic rheology. The surface topography which develops during subduction is compared with the topography shortly after detachment is imposed. Subduction with a non-migrating trench system followed by detachment leads to a maximum surface uplift of 2–6 km, while this may be higher for the case of roll-back preceding detachment. In the latter situation, the back-arc basin may experience a phase of compression after detachment. Within the context of our elastic model, the surface uplift resulting from slab detachment is sensitive to the depth of detachment, a change in friction on the subduction fault during detachment and viscous stresses generated by sinking of the detached part of the slab. Overall, surface uplift of these magnitudes is not diagnostic of slab detachment since variations during ongoing subduction may result in similar vertical surface displacements. D 2002 Elsevier Science B.V. All rights reserved.

Initiation of asymmetric extension in continental lithosphere

Govers R. and Wortel, M&R., 1993. Initiation of asymmetric extension in continental lithosphere. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth’s Surface. Tectonophysics, 223: X-96. The physical conditions are investigated under which the lithospheric-scale style of extension is pure shear or simple shear. We focus on the initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions. Continental collision, magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension. We use a finite element method to model the thermo-mechanical evolution of continental lithosphere. Experimental flow laws are used to model the elastic, hutle. power law creep or diffusion creep rheology of lithospheric rocks. Our results indicate that if in-plane forces change from compressive to tensile immediately after a rapid mountain building phase, initiation of a lithosphere-scale detachment fault is possible. We find a strong dependence of the extensional style on the distribution with depth of residual stresses from the collision phase. This result is consistent with observations of gravitational collapse in regions, like the Aegean and the Basin and Range Province, where detachment faults have exhumed lower crustal rocks. The predicted dip direction of the fault also agrees with observations in these areas. Intrusion of magma into continental lithosphere, which is subject IO in-plane tensile forces, will cause localization of pure shear deformation. The style of deformation resulting from mantle plumes impinging to the base of the lithosphere is symmetric. Delamination of lithospheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase, when in-plane forces change sign from compressive to tenstle. This result also strongly depends on the assumed residual stress distribution. If delamination occurs during the mountam huilding phase, the style of deformation will be pure shear. Another interesting outcome from our modeling is that dramatic strain weakening as a result of a deformation mechanism change from dislocation creep to diffusion creep, reduces the tendency to strain localization. A large volume of the published work on continental extension has been concentrated on quantifying the stretching factor p of the pure shear model of McKenzie (1978a). In some basins it was found that the kinematics of deformation could be more accurately described by depth-dependent stretching factors (Royden and Keen, 19801, which introduced asymmetry near basin edges. In the simple shear model of Wemicke

Ephemeral crustal thickening at a triple junction: The Mendocino crustal conveyor

As the North American crust interacts with the migrating Mendocino triple junction, the crust is first significantly thickened and then equivalently thinned over a distance of a few hundred kilometers (within a time frame of 5 m.y. or less). This process of ephemeral crustal thickening is proposed to result from viscous coupling between the northward-migrating Gorda slab and the base of North America south of the triple junction. A time-dependent, thermal-mechanical finite-element model is developed to test this hypothesis of plate-boundary tectonics. Results of the numerical simulations show patterns of crustal deformation consistent with the mapped sequence of folding and faulting in the area, the observed crustal structure and triple junction regional seismicity, and localized regions of crustal extension coincident with the position of a hypothesized lower-crustal melt zone.

Extension of stable continental lithosphere and the initiation of lithospheric scale faults

We address the physical conditions which control the style of continental extension. Geological evidence suggests that once lithospheric scale zones of localized deformation have been formed, they strongly affect continental deformation. It is the purpose of this paper to investigate mechanisms which may cause lithospheric scale faults to initiate in stable continental lithosphere which is laterally fairly homogeneous. Faults and shear zones cutting strong layers in the lithosphere will have a very significant influence on the evolution during extension. Based upon experimental flow laws, a strength maximum can be expected in the mantle directly beneath the Moho. Strain localization in the shallow upper mantle is therefore expected to have a very pronounced effect on the evolution of the extending lithosphere. Low viscosities in the lower crust decouple the crust mechanically from the upper mantle. Therefore causes for strain localization in the sub-Moho mantle must be found in the mantle itself. Two potential causes satisfying this requirement are boudinageing and strain weakening. We use thermal-mechanical finite element models which incorporate the elastic, visco-plastic, and viscous response of lithospheric rocks. The results of our model experiments suggest that boudins do not evolve during extension of continental lithosphere in most situations; only when the extension rate is fast relative to thermal reequilibration may homogeneous boudinage result. By its very nature, however, this type of boudinage cannot produce lithospheric scale faults. Our model results suggest that shear zones may evolve after strain weakening. However, the style of extension on the scale of the lithosphere is pure shear like because the shear deformation in localized zones is balanced; in most cases the shear zones occur in conjugate pairs. Initiation of lithospheric scale faults is concluded to be unlikely in stable and homogeneous lithosphere in interior parts of continental plates.

Choking the Mediterranean to dehydration: The Messinian salinity crisis

Strait uplift due to isostasy played a pivotal role in the development and maintenance of a desiccated Mediterranean basin during the Late Miocene Messinian salinity crisis. New three-dimensional flexure models for the western Mediterranean suggest that most of the giant evaporite body was deposited before sea-level lowering. Any mechanism that subsequently caused a Mediterranean sea-level lowering by a few hundred meters during several thousand years sufficed to cause isostatic uplift and closure of the gateways, thus choking the Mediterranean to desiccation. Due to this isostatic feedback, multiple desiccation and reflooding phases driven by Milankovitch cycles are unlikely. When the Atlantic connection had been lost and sea level had dropped, a sizeable regional uplift of the Strait of Gibraltar developed that presented a formidable obstacle to reflooding. Rollback and steepening of the Gibraltar slab may have played a key role at the end of the Miocene by dynamically lowering the sill region, thus reinstating the oceanic connection.

Dynamics of the lithosphere and the intraplate stress field

We outline the methodology of our numerical studies aimed at increasing the understanding of the relation between dynamics and stress field of the lithosphere with particular reference to oceanic lithosphere. The ridge-push force is modelled as a pressure gradient integrated over all contributing parts of the lithosphere. The slab-pull force is modelled as being dependent on the age of the subducting lithosphere. We parametrize the resistive forces and determine the unknown parameters by requiring the total torque of all forces acting on the plate to vanish. We illustrate the approach by the presentation and discussion of new modelling results for the Pacific plate.

The effect of plate stresses and shallow mantle temperatures on tectonics of northwestern Europe

Abstract Northwestern Europe is tectonically more active, in terms of seismicity, vertical motions and volcanism, than would be expected from its location far from any plate boundaries. In the context of the Netherlands Earth System Dynamics Initiative, we investigated the implications of two recent modeling efforts, of Eurasian plate forces and European mantle structure, for our understanding of the dynamics of these intraplate tectonics. We find that: (1) a simple balance between ridge push and collision forces along the southern European boundary does not seem sufficient to explain the observed direction of maximum horizontal compression in northwestern Europe. Our stress model, which imposes dynamical equilibrium on the full Eurasian plate, predicts that collision forces along the African–European boundary are relatively weak and have only a minor effect on the stress field in northwestern Europe; (2) seismic velocity anomalies in the shallow mantle imply 100–300°C variations in temperature under northwestern Europe. This regional mantle structure probably plays a significant role in the high level of intraplate tectonic activity and the regional variations in stress and tectonic style. For most tectonically active areas in Europe, observed surface heat flow anomalies coincide with anomalies in mantle velocity. Low velocity anomalies under northwestern Europe coincide with areas of recent volcanism and uplift, but are offset from the regions of maximum surface heat flow. This suggests that the thermal regime of the central European lithosphere is not in a steady state, probably due to changing mantle conditions. The effect of strong variations in lithospheric strength, predicted from the modeled thermal gradients in the shallow mantle, and of dynamic stresses induced by proposed active mantle upwellings may account for (some of) the differences between the observed and modeled stress field and will be investigated in future stress models.

Continental Collision and the STEP-wise Evolution of Convergent Plate Boundaries: From Structure to Dynamics

Particularly interesting stages in the evolution of subduction zones are the two main transient stages: initiation and termination. In this contribution the focus is on the second of these: terminal stage subduction, often triggered by continental collision or arc-continent collision. The landlocked basin setting of the Mediterranean region, in particular the western-central Mediterranean, provides unique opportunities to study terminal stage subduction and its consequences