ELSEVIER Tectonophysics 296 (1998) 31–46 Mantle unrooting in collisional settings A.M. Marotta a,Ł , M. Ferna `ndez a , R. Sabadini b a Institute of Earth Sciences ‘J. Almera’ – CSIC – Lluis Sole i Sabaris s=n, 08028 Barcelona, Spain b Universita ´ degli Studi di Milano, Department of Earth Sciences, Via L. Cicognara 7, 20129 Milano, Italy Accepted 5 February 1998 Abstract We present a two-dimensional numerical model to study the thermo-mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break-up and consequent unrooting. A Newtonian rheology with a temperature-dependent viscosity is considered. The system is not closed and horizontal flow through lateral boundaries is permitted. A horizontal velocity is imposed at the top of the model to simulate compression, whereas velocity vanishes at the bottom of the model. The initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust. The analysis of variations of maximum shear stress, strain rate, and total kinetic energy allowed us to define four major stages during the mantle unrooting process: orogenic growth, initiation of gravitational instability until lithospheric failure, sinking of the detached lithosphere, and relaxation of the system. Numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity, the wideness of the deformation zone, and the imposed rheology.  1998 Elsevier Science B.V. All rights reserved. Keywords: orogenic collapse; gravitational instability; shear stress; strain rate; kinetic energy 1. Introduction The removal of lower lithospheric material is often reported in the literature as delamination, with- out specifying the particular mechanism involved. The term delamination sensu stricto refers to the mechanism envisaged by Bird (1979). In this case, lithospheric mantle is peeled away from the overly- ing crust owing to the intrusion of asthenospheric material between them (Fig. 1a). This mechanism is different from that envisaged by McKenzie (1978), Houseman et al. (1981) and Fleitout and Froidevax (1982), involving the break-up of the thickened lower Ł Corresponding author. Tel.: C34 (3) 4900552; Fax: C34 (3) 4110012; E-mail: mfernandez@ija.csic.es lithospheric mantle and its growth as a whole into the asthenosphere because of its gravitational instability (Fig. 1b). In this paper we used the term lithospheric unrooting to refer to this latter mechanism. In regions of plate convergence, lithospheric shortening produces crustal and lithospheric thick- ening with a consequent downward deflection of the isotherms at deep crustal and lithospheric levels. Un- der these conditions the lithospheric root, colder and thus heavier than the surrounding mantle, becomes gravitationally unstable. It has been postulated that the lithospheric root can detach and sink into the sublithospheric mantle (Platt and England, 1994). The replacement of lithospheric mantle by hotter as- thenosphere would induce a rapid isostatic rebound and could change the surficial stress regime from 0040-1951/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII:S0040-1951(98)00134-6