M. ter Voorde

Thermomechanical evolution of the South-Alpine rifted margin (North Italy): constraints on the strength of passive continental margins

Abstract We have reconstructed the kinematic, thermal and rheological evolution of the Triassic-Cretaceous South Alpine passive continental margin (Northern Italy) and estimated lateral strength variations associated with rifting. Rifting initially caused the formation of a roughly symmetrical, ca. 100 km wide graben located 150–200 km away from the future break-up site. 30 Myr after the onset of rifting, the rift axis shifted laterally and eventually led to oceanic crust formation. The thermal evolution of the margin was first controlled by the waning of a thermal anomaly, which was emplaced shortly before rifting and was cooling during the first extensional stages. Numerical modelling of rifting and drifting, constrained by field data, is used to trace thermal and rheological changes affecting the rifted margin. The effects of the thermal anomaly are restricted to the first 10–20 Myr of rifting. Subsequently, isotherms are slightly affected by lithospheric thinning and show very little distortion after break-up. Rheological modelling demonstrates that the South Alpine lithosphere preserved substantial strength even shortly before break-up and became stronger thereafter. The modelling also shows that, at break-up and 100 Myr later, thinned domains of the margin are stronger than the less thinned ones.

Thermal effects of normal faulting during rifted basin formation, 1. A finite difference model

Abstract A finite difference model is presented, describing the thermal structure of a region undergoing fault-controlled extension. The model is two-dimensional and time-dependent. We combine temperature changes resulting from conduction, advection due to fault movements, sediment blanketing and heat production. Cooling curves can be derived at chosen points in the area. This study mainly investigates the effect of the extension rate on the thermal field, which turns out to be very important. 20 km of extension with extension rates of 0.2, 2.0 and 20 mm/yr yield maximum deviations in temperatures from the steady-slate situation of 2, 26 and 84%. The sediment blanketing effect can even be reversed by variations in the sedimentation rates. Fast sedimentation leads to crustal cooling, slow sedimentation to crustal heating. The turning-point value is lithology-dependent and corresponds to 2.0 mm/yr for shale, 2.2 mm/yr for clay, and 1.4 mm/yr for sand; a sandstone will cause an increased cooling at both high and low velocities. Our modelling shows that footwall cooling during or immediately after a rifting event cannot be explained by the downwards movement of the adjacent, relatively cold hanging wall. Consequently, in areas where such a cooling is observed, other cooling mechanisms must play an important role. These can be phenomena like footwall uplift or the fading out of an older thermal anomaly.

Thermal effects of normal faulting during rifted basin formation, 2. The Lugano-Val Grande normal fault and the role of pre-existing thermal anomalies

Abstract We investigate the thermal consequences of rift-related normal faulting and compare the results with a well-studied natural example, the Lugano-Val Grande normal fault (Southern Alps). Only limited heating of the crust is caused by lithospheric thinning. In the simple but realistic situation where heat conduction is substantially faster than heat advection, no major thermal disturbance is associated with the downward movement of the hanging wall. Radiometric ages and fault rocks associated with the Lugano-Val Grande normal fault demonstrate that cooling rather than heating affected the crust during normal faulting. This pattern is not compatible with such a simple numerical model and is explained by a waning thermal anomaly induced by a magmatic intrusion immediately preceding or overlapping with the first stages of normal faulting. The magmatic body must have been emplaced at depths greater than 15–18 km, and probably started to cool in the Carnian i.e. few million years before the onset of normal faulting along the Lugano-Val Grande fault.

An integrated modelling study of the central and northern Baikal rift: evidence for non-uniform lithospheric thinning?

Abstract We employ a kinematic model of continental extension to assess the lithospheric structure across the Baikal rift zone. The model includes simple shear extension of the upper crust along detachment faults and independent pure-shear thinning of the lower crust and mantle. Model predictions of topography, gravity anomalies and surface heat flow are compared with observations along two sections, one crossing the central Baikal rift and the other across northern Baikal. For the central Baikal profile, topography and gravity anomaly patterns are fit by a uniform extension model: upper crustal extension of approximately 19 km is equalled by a predicted mantle thinning factor δ 10 mW m−2) increase in regional surface heat flow. The short-wavelength, large-amplitude variability of the observed surface heat flow requires crustal redistribution processes involving large-scale ground water flow. The modelled topography and gravity anomaly patterns suggest that a possible asthenospheric upwarp is more likely to be situated below the northern than below the central Baikal basin, in contrast with earlier inferences from teleseismic and gravity studies.

Mechanical (de-)coupling of the lithosphere in the Valencia Through (NW Mediterranean): What does it mean?

We study the mechanics of lithospheric decoupling in continental extensional basins in relation to the distribution of (non-)competent mechanical layers within the lithosphere and the position of the isostatic compensation level. We specifically address the different modes of deformation taking place in crustal levels according to a self-consistent formulation of the concept of mechanical decoupling. Subsequently, we investigate the style of lithospheric decoupling in the Valencia Trough (NW Mediterranean), a prime example of a young continental rift basin. During its evolution, the lower crust (or at least part of it) acted as a weak, non-competent layer that eventually flowed laterally to accommodate deformation in the subcrustal lithosphere and overlying crust, which became mechanically decoupled. We use a numerical model to discern whether these two layers deformed fully independently (vertical decoupling), or maintaining a mechanical link (horizontal and partial decoupling). Results of our study, constrained by a high-quality database, exclude fully decoupled mode and favor isostatic compensation level in the asthenosphere. Interpretation of our results in light of geological and geophysical data suggests that the present Valencia Trough is best described by partial lithospheric decoupling. 6 2003 Elsevier Science B.V. All rights reserved.

Towards the development of a new generation of basin models

The contribution of the Tectonics/Structural geology group to the IBS project is focusing on the development of modelling concepts and tools incorporating the role of stresses and non-linear rheology in basin formation models [2,3,4,5] Basinwide modelling of the kinematics of extension demonstrates the key role played by the finite strength of the lithosphere in the development of rift shoulder topography [11,13].

Tectonic and eustatic control on basin fill - Constraints from basin modelling

Advances in quantitative understanding of basin formation mechanisms and integration of these concepts in forward stratigraphic modelling tools provide increasingly accurate constraints on predictions for basin fill in extensional and compressional basins (Cloetingh et al., 1993, 1994). An important step in this direction is provided by the incorporation of effects of changes in stress regime and compressional reactivation in the modelling of basins formed in an extensional regime.

Quantitative modelling of tectonic controls on the sedimentary records

Advances in quantitative understanding of basin formation mechanisms coupled with the collection of high quality data on the basin fill, increase prospects of discriminating tectonic and eustasy controls on the sedimentary record.