Giorgio Ranalli

Rheological stratification of the lithosphere

Abstract The rheology of the lithosphere is estimated on the basis of pore-pressure dependent frictional failure in the brittle regime, and power-law steady-state creep in the ductile regime. Different petrological models of the continental and oceanic lithosphere, combined with different geotherms and thicknesses, are used to generate models covering a variety of geodynamic situations. Results show that the depth-dependence of lithospheric rheology varies with the tectonic province, and that in many instances the continental lithosphere has one (at the bottom of the crust) or two (the previous one plus another at mid-crustal levels) soft ductile layers sandwiched between brittle layers. The depth distribution of geophysical parameters (seismicity, seismic wave velocity, and electrical conductivity) matches the theological predictions well. The lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern Canadian Cordillera. The large-scale characteristics of the system show a satisfactory agreement with the inferred rheological structure. Sub-horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified.

Rheology of the lithosphere in space and time

Abstract The rheology of the lithosphere is a factor of primary importance in the kinematics and dynamics of mountain belts. This paper attempts to clarify the role of rheology in orogenesis, by applying simple physical principles to the analysis of tectonic processes. The emphasis is on broad generalizations leading to order-of-magnitude estimates. Since the rheology of lithospheric materials is strongly dependent on temperature, the discussion opens with a review of continental and oceanic geotherms and an assessment of their reliability. Then the brittle (frictional) and ductile (high-temperature creep) properties of the lithosphere are considered. In the brittle field, particular attention is paid to the problem of fault reactivation, which is shown to be more likely in extensional than in compressional regimes. In the ductile field, a summary of creep parameters for the most common lithospheric materials is presented. The central concept of rheological profiles (strength envelopes), essential to the estimation of the depth variations of lithospheric rheology, is discussed with reference not only to its applicability but also to its limitations (a two-dimensional example from the Canadian Cordillera is given). Processes related to the rheological properties and layering of the lithosphere — gravitational collapse in thickened and softened crust, tectonic inversion following the detachment of a lithospheric root, lower crustal ductile flow with consequent relaxation of Moho topography — are analysed semi-quantitatively, mainly to show that they are indeed likely to be important players in geodynamics. Finally, in a brief Archaean detour, some possible important differences between present-day and Archaean oceanic lithosphere are examined, and the conclusion is reached that in all likelihood plate tectonics, although at different rates, has been the main agent of orogenesis during most of the history of the planet.

The role of rheology in extensional basin formation modelling

The rheology of the lithosphere determines its deformation under given initial and boundary conditions. This paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling. Since strength envelopes are often used in models, we review the uncertainties (in temperature and rheological parameters) and assumptions (in type of rheology and mode of deformation) involved in their construction. Models of extensional basins are classified into three groups: kinematic, kinematic with rheological constraints, and dynamic. Rheology enters kinematic models only implicitly, in the assumption of an isostatic compensation mechanism. We show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere, which requires a flexural response. Kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension. Dynamic models are the only ones to explicitly introduce rheological constitutive equations (usually in plane strain or plane stress). They usually, however, require the presence of an initial perturbation (thickness variations, pre-existing faults, thermal inhomogeneities, rheological inhomogeneities). The mechanical boundary conditions (kinematic and dynamic) and the thermal boundary conditions (constant temperature or constant heat flux at the lower boundary of the lithosphere) may result in negative/positive feedbacks leading to cessation/acceleration of extension. We conclude that, while kinematic models (with rheological constraints if possible) are very successful in accounting for the observed characteristics of sedimentary basins, dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution.

Extensional shear zones as imaged by reflection seismic lines: the Larderello geothermal field (central Italy)

The Larderello geothermal field is located in the Inner Northern Apennines, in an area which has been subject to extension since the Early Miocene. The latest extensional episode (Pliocene–Present) has resulted in the formation of NW-trending, NEdipping listric normal faults, whose geometry is controlled down to f3 km by borehole data. In this paper, we integrate a new interpretation of seismic reflection lines with existing seismic, field, and borehole data to analyse the relations among listric normal faults, the top of the brittle–ductile transition, and the migration of geothermal fluids. In accordance with previous interpretations, we consider the strong reflector (K-horizon) marking the top of the reflective mid-lower crust, and located at a depth of 3–5 km in the geothermal area, to represent the top of the brittle–ductile transition. Its reflectivity most probably derives from the presence of overpressured fluids. We identify three main NW-trending, NEdipping extensional brittle shear zones, showing listric geometry and soling out in the vicinity of the K-horizon. The latter appears to be dislocated in correspondence of the soling out of the shear zones. These shear zones, because of the associated intense fracturing, represent the most natural channels of upward migration of geothermal fluids from the magmatic sources located below the K-horizon. We suggest that these two conclusions—that listric normal faults root at or near the brittle–ductile transition, and that they act as preferential upward migration paths for magmatic fluids—may be of general validity for geothermal fields located in extensional settings. D 2002 Elsevier Science B.V. All rights reserved.

Critical stress difference, fault orientation and slip direction in anisotropic rocks under non-Andersonian stress systems

Abstract The Coulomb-Navier failure criterion is applied to geological faulting in the general three-dimensional case of rocks containing arbitrarily oriented strength anisotropies and subject to non-Andersonian stress systems (i.e. with none of the principal stresses acting in a vertical direction). General expressions for the critical stress difference necessary to cause failure as a function of depth are given in terms of material parameters, pore fluid pressure, orientation of the stress field and orientation of the strength anisotropy. The range of angles between a plane of anisotropy and the maximum principal stress direction, for which slip occurs along the pre-existing anisotropy rather than along a new fault, is calculated as a function of depth for different stress regimes. When the stress field is non-Andersonian and/or strength anisotropies not containing the intermediate stress direction occur in the rock, faulting generally is oblique-slip. A kinematic classification of faulting is given on the basis of the angle between the strike direction and the slip direction on the fault plane. Triangular diagrams, analogous to those used in petrology, are introduced to describe (i) faulting in isotropic rock subject to arbitrarily oriented stress fields, and (ii) faulting in anisotropic rock when one principal stress direction is vertical. The type of faulting as a function of stress field and anisotropy orientation can be read off directly from these diagrams.

Rheology and seismotectonic regime in the northern central Mediterranean

Abstract The connection between thermal field and mechanical properties is analysed in the northern central Mediterranean region, extending from the Ligurian-Provencal basin to the Adriatic foredeep. As the thermal regime is still far from equilibrium in most of the tectonic units, transient thermal models are used. The temperature-depth distribution is estimated in four areas affected by the volcanic activity, which from the Neogene to the Present shifted from Corsica to the Apenninic arc. In the Adriatic foredeep, the thermal effects of the recent thrust-faulting phase in the buried sectors of the northern Apennines are taken into account. The general context consists of convergence involving westward subduction of the Adriatic plate. This process caused anti-clockwise rotation of Corsica and Sardinia, which led to formation of the Ligurian marginal basin, and also resulted in crustal doubling and overthrusting in the northern Apennines and rifting in the northern Tyrrhenian. Seismic activity is focused in the internal and external zones of the Apenninic arc, where low surface heat flux is observed, and in the western margin of the Ligurian-Provencal basin. This is a consequence not only of lateral variations in the thermal field but also of the different tectonic settings. Regional extensional patterns in the shallow crust, with minimum principal stress axes trending N60°E and E-W, are observed in the northern and in the southern sectors of the Apenninic arc, respectively. A compressional regime at depths greater than 30 km is observed below the northern sector of the arc, while to the south a change in the structure of the lithosphere is marked by a decrease in deeper seismic activity. Thrust faults and strike-slip faults with a thrust component support a compressional regime along the western margin of the Ligurian basin with maximum principal stress axis oriented N120°E. Two lithospheric cross-sections across the study region are constructed, based on structural, thermal, gravity, rheological and seismic data. There is clear evidence of the presence of the subducting slab of the Adriatic plate, corresponding to a thickening of the uppermost brittle layer. The crustal seismicity cut-off corresponds to temperatures of 320–390°C. A brittle layer of considerable thickness is present in the uppermost mantle beneath Variscan Corsica and the Adriatic foredeep, with estimated seismic cut-off temperature of about 550 ± 50°C.

Heat flow, deep temperatures and extensional structures in the Larderello Geothermal Field (Italy): constraints on geothermal fluid flow

The Larderello geothermal field is located in the inner Northern Apennines (southern Tuscany), an area which has been affected by extensional tectonics since the Early–Middle Miocene. The structure of the Larderello field is characterised by NW-trending, NE-dipping Pliocene to Present normal faults. Their geometry down to depths of 4–5 km is constrained by field, borehole, and reflection seismic data. An association between extensional structures and heat flow maxima (up to 1000 mW/m2) is recognisable from detailed surface heat flow mapping. In order to investigate the relationships among extensional structures and heat flow, subsurface isotherms were traced, subject to borehole control, along variously oriented geological cross-sections. The isotherms show vertical displacements associated with the recent normal faults and related deformation zones, which reach the brittle/ductile transition. Estimates of the relative importance of convective and conductive components of heat flow suggest that fluid circulation is particularly important in correspondence with the normal faults, accounting for the correlation between isotherm perturbations and extensional structures. In this view, extensional shear zones are interpreted as the main structural pathways for the flow of hot geothermal fluids.

Time dependence of negative buoyancy and the subduction of continental lithosphere

Abstract The time evolution of negative buoyancy of a subducting slab is modelled from the beginning of subduction under various kinematic conditions (dip angle and subduction velocity). The calculations take into account the thermal and density effects of the variations of the thermophysical parameters with temperature and pressure, and of phase transitions. The magnitude of the negative buoyancy increases during subduction of oceanic lithosphere, up to values in the (2–4) × 10 13 N m −1 range when the tip of the slab reaches a depth of 600–700 km. If continental material arrives at the trench and is subducted, the downward buoyancy decreases by an amount proportional to the volume of the subducted continental crust. Assuming that subduction stops when the buoyancy becomes zero, and that delamination of the continental crust or slab breakoff do not occur, the maximum downdip length of the subductable continental crust is estimated as a function of the dip angle, subduction velocity and geometry of the margin. In most cases, subduction of continental material down to depths of 100–250 km is possible, and continental subduction can continue for times up to 10–15 Ma if the velocity is low. These estimates are not significantly affected by the hypothetical occurrence of a metastable olivine wedge within the slab, and could be lower bounds if the lower continental crust is mafic and transforms to eclogite.

Density structure and buoyancy of the oceanic lithosphere revisited

[1] The density structure of the lithospheric and sublithospheric oceanic mantle is assessed with an integrating methodology that incorporates mineral physics, geochemical, petrological, and geophysical data. Compressibility, partial melting, and compositional layering are considered in addition to the standard thermal modelling. The results indicate that due to differences in the degree of melt depletion and crust segregation, the depth-averaged density of old oceanic plates with thermal thicknesses of ∼ 105 ± 5 km is always lower than the density of the underlying sublithospheric mantle. Moreover, representative depth-averaged density contrasts between the plate and the adiabatic mantle, Ap, do not exceed values of ∼40 kg m -3 , in contrast to what is assumed (∇p > 70 kg m -3 ) in many geodynamic models. Thus, the role of ∇p in triggering/assisting processes such as subduction initiation may be less critical than previously thought.

Thermal and rheological constraints on the earthquake depth distribution in the Charlevoix, Canada, intraplate seismic zone

The Charlevoix Zone is the most active seismic area of eastern Canada. For the period 1978–1993, 99% of earthquakes occurred at less than 25 km depth and 80% at less than 15 km. This depth distribution is compared with the estimated brittle/ductile and velocity weakening-velocity strengthening transitions. Using realistic ranges of thermal parameters and a 41 ± 10 mW/m2 surface heat flow, one-dimensional thermal models show that 90% of 22,000 computed geotherms fall between 215 and 355°C at 25 km. For the central value of heat flow, this range is reduced to 280 and 340°C. These temperatures and the inferred mafie mid- and lower-crustal composition imply a brittle-ductile transition deeper than 25 km. With a higher than average geotherm, the maximum depth of seismicity could correspond to the velocity weakening-velocity strengthening boundary. The basic lower crust of the area precludes a correlation of this depth with the onset of ductility of quartz at around 300°C. However, it may correspond to the onset of ductility for hydrated feldspar at about 350°C if the geotherm is relatively high. Since the maximum possible crustal stress difference is unlikely to be larger than 200 MPa, high pore-fluid pressures and/or low static coefficient of friction are required for the occurrence of lower-crustal earthquakes.