Continental-Scale Rheological Heterogeneities and Complex Intraplate Tectono-Metamorphic Patterns: Insights from a Case-Study and Numerical Models
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ELSEVIER Tectonophysics 279 (1997) 327±350 Continental-scale rheological heterogeneities and complex intraplate tectono-metamorphic patterns: insights from a case-study and numerical models AndreÂa Tommasi *, Alain Vauchez Laboratoire de Tectonophysique, ISTEEM, CNRS/Universite de Montpellier II, F-34095 Montpellier cedex 5, France Accepted 2 May 1997 Abstract Continental plates are built over long periods of time through successive extensional and compressional cycles. They are therefore rheologically heterogeneous. This heterogeneity should signi®cantly in¯uence the mechanical response of the continental lithosphere during collision processes. The study of the Neoproterozoic Borborema shear zone system of northeast Brazil highlights a systematic link between marked changes in its tectono-metamorphic pattern and the pre-existing structure of the plate, that is characterized by juxtaposition of continental domains either comprising an old basement (Palaeo- to Eoproterozoic) or accreted during an extensional event between 1.0 and 0.7 Ga. In Neoproterozoic time, when the shear zone system was developed, these domains displayed different geotherms and lithospheric thicknesses, and therefore contrasted rheological behaviours. We use numerical models simulating the mechanical evolution of a continental plate comprising multiple thermally-induced rheological heterogeneities submitted to compression to investigate how these heterogeneities may affect strain localization and the distribution of deformation regimes and vertical strain within the plate. From the very beginning of the deformation, weak and stiff heterogeneities induce strain localization, due to a lower initial effective viscosity or to stress concentrations at their tips, respectively. Shear zones propagate from the heterogeneities and ®nally coalesce, forming a network of high-strain zones bounding almost undeformed blocks. Within this network, shear zones transfer strain between the different heterogeneities and model boundaries. The evolution of the system depends essentially on the geometrical distribution of heterogeneities and on their strength contrast relative to the surrounding lithosphere. The resulting ®nite-strain ®eld is heterogeneous and displays rapid lateral variations in vertical and/or rotational deformation. Such a heterogeneous strain distribution may induce contrasted magmatic, metamorphic and uplift evolutions within an orogenic belt, as observed in the Borborema shear zone system and other collisional belts. Keywords: continental deformation; rheological heterogeneity; shear zones; numerical modeling; inversion tectonics; NE Brazil Ł Corresponding author. Fax: 33-67143603; e-mail: [email protected] 0040-1951/97/$17.00 1997 Elsevier Science B.V. All rights reserved. PII S0040-1951(97)00117-0 328 A. Tommasi, A. Vauchez / Tectonophysics 279 (1997) 327±350 1. Introduction Continental collision zones often extend far in- land and display signi®cant spatial variation of strain intensity, deformation regime, and vertical strain. These complex tectono-metamorphic patterns may result from particular plate boundary con®gura- tions, like oblique convergence and indentation (e.g., Sumatra and New Zealand margins and the India± Asia collision). They may also be induced by a vari- ation of the bulk stress ®eld through time, due to changes in convergence direction or velocity, to an increase of buoyancy forces resulting from a large lithospheric thickening (e.g., Molnar and Tappon- Fig. 1. An example of heterogeneous continental lithosphere: lithospheric thicknesses inferred from P-wave residuals (Babuska nier, 1988), or to activation of processes like mantle and Plomerova, 1992) and surface heat ¯ows (Hurtig et al., 1991) delamination (e.g., England and Houseman, 1989). in Europe. Finally, in old orogens, where information on con- vergence directions and original plate geometry is often lost and knowledge of the timing of the de- logical heterogeneities, we investigate the effect of formation and metamorphism is limited to sparse these heterogeneities on strain localization and shear geochronological data, complex kinematic patterns zone development, and on the spatial organization and metamorphic histories were frequently taken as of deformation regimes and vertical strains. Then, evidence of polycyclic evolution. we discuss the implications of this process for the Boundary conditions-related processes are essen- tectono-thermal evolution of collisional belts. tial for producing heterogeneous strain ®elds within the assumption, often implicit in conceptual geody- 2. Intraplate rheological heterogeneities namic models, that plates react to external solicita- tions as homogeneous media. However, this assump- Intraplate rheological heterogeneities may be in- tion is highly questionable for continental plates duced by lateral variations in structural fabric, crustal whose evolution involves successive accretions and thickness, and geothermal gradient. Ancient high- dispersions resulting in domains with different ages deformation zones should induce a local anisotropy and tectonic histories. This mosaic-like structure im- in the mechanical properties of the lithosphere plies that continental plates display lateral variations (Vauchez et al., 1997a). However, limited experimen- of lithospheric thickness and composition, geother- tal data prevent a quanti®cation of this anisotropy. mal gradients and structural fabric. This heterogene- Ranalli (1986) and Dunbar and Sawyer (1989) in- ity is clearly illustrated by maps of surface heat ¯ow vestigated the effect of a lateral variation in crustal and lithospheric thickness of present-day continental thickness on the strength of the lithosphere. They plates, as shown in Fig. 1 for western Europe, or by show that for similar geothermal gradients, domains tomographic images of the uppermost mantle layer with a thickened crust will display lower lithospheric beneath continental plates (e.g., Grand, 1994; Polet strengths than those characterized by a normal crust and Anderson, 1995; VanDecar et al., 1995). due to replacement of stiff upper mantle material What is the effect of these heterogeneities on by weaker crustal rocks. Although this may result the mechanical response of a continental plate sub- in a signi®cant perturbation of the ®nite-strain ®eld jected to a continental collision? From the analysis (e.g., Dunbar and Sawyer, 1989), we will focus on of a case-study, the Borborema shear zone system the mechanical effect of lithospheric-scale thermal of northeast Brazil, and numerical models simu- heterogeneities. lating the mechanical evolution of a continental Intraplate thermal heterogeneities are commonly plate comprising multiple thermally-induced rheo- associated with the existence, within a continental A. Tommasi, A. Vauchez / Tectonophysics 279 (1997) 327±350 329 plate, of old blocks displaying a thicker lithosphere and a lower geothermal gradient than the surround- ing lithosphere, and of younger and thinner domains, showing higher geothermal gradients. These lateral variations in geothermal gradients within a continen- tal plate should induce changes in rheology, because under lithospheric P±T conditions and geological strain rates rock-forming minerals deform domi- nantly by dislocation creep, which is exponentially dependent on temperature. Thus lateral variations in geothermal gradients should imply either an increase or a decrease of the integrated yield strength of the lithosphere (Fig. 2), which corresponds to the force supported by a lithospheric column of unit width (England, 1983). 2.1. Stiff intraplate heterogeneities Ð cratons Continental plates develop by successive accre- tion around cratonic nuclei (e.g., African, South American and North American plates). Seismic to- mography data (e.g., Grand, 1994; Polet and Ander- son, 1995) suggest that these cratonic nuclei have a deep, cold lithospheric root. Moreover, in spite of the long time elapsed since the episode responsible for the main assembly of these continents (¾600 Ma for the African and South American plates), these Fig. 2. Geothermal gradient (dotted line), one-dimensional strength pro®le (solid line) and lithospheric strength for dif- ferent geodynamic settings characterized by typical surface heat ¯ows (top right): (a) a cratonic block, (b) a normal lithosphere, (c), and (d) a thinned lithosphere. Geotherms are calculated us- 1 1 ing thermal conductivities, kc D 2:5Wm K and km D 3:35 Wm1 K1 for crustal and mantle rocks, respectively, and a 6 3 surface heat production, Hs D 2ð10 Wm , which decreases exponentially with a length scale of 10 km (a, b, and c) or 7.5 km (d). Strength pro®les are calculated using: the Sibson (1974) frictional law for the upper crust, a quartzite ¯ow law (Paterson and Luan, 1990) for the quartz-rich upper to middle crust, a felsic granulite ¯ow law (Wilks and Carter, 1990) for the lower crust, and the Aheim dunite ¯ow law (Chopra and Paterson, 1981) for the upper mantle. For the cratonic block (a), the crust is considered as entirely formed by felsic granulite. zM indicates Moho depth. (b), (a), (c), and (d) correspond to the thermal and compositional pro®les used for calculation of the integrated rheological parameters (Table 1) for the normal lithosphere, the stiff domain, the weak domain (in low viscosity contrast mod- els), and the weak domain (in high viscosity contrast models), respectively. 330 A. Tommasi, A. Vauchez / Tectonophysics 279 (1997) 327±350 cratonic nuclei (e.g., the Archaean Kapvaal