GRAVITATIONAL INSTABILITY OF CONTINENTAL LITHOSPHERE AND RELATED

MAGMATISM

Kiselev A.I.*, Gordienko I.V.**

*Institute of the Earth’s Сrust SB RAS, Irkutsk, Russia, akiselev@crust.irk.ru

**Institute of Geology SB RAS, Ulan-Ude, Russia, gord@pres.bscnet.ru

Mechanical exfoliation and foundering of the mantle lithosphere are used to call delamination. Firstly the term “delamination” has been proposed by Bird (1979) according to whom the lithospheric mantle is peeled off from the overlying crust due to uplift and emplacement of asthenospheric material between them. Houseman et al. (1981) have proposed another mechanism that results from gravitational (convective) instability of the tectonically thickened lithosphere accompanied by complete or partial separation of its mantle root and its foundering into asthenosphere.

Continental lithosphere is characterized by varying thickness (150-300 km) and more subdivided stratification as compared with oceanic one. It is composed by high buoyant quartz-rich 2-3 layered crust (30-70 km) and negatively or neutrally buoyant olivine-rich mantle part of lithosphere (60-250 km). The upper crust is brittle-elastic, the lower one is plastic and its viscosity may be 10²⁰-10²¹ poises at T=250-400^oC. The dactile creep became active at more high temperature (750-800^oC). This suggests the presence of thin layer (lower and intermediate crust) between a solid upper crust and lithospheric mantle that results in their deformation independently from one another up to mechanical separation.

Delamination as a result of tectonic thickening in collisional orogens. When combining thermal and compositional parameters of the mantle lithosphere, it is less dense then underlying asthenosphere. However during compression, lithosphere becomes more thick and instable due to its quasithermal “pressing” into asthenosphere. Under these conditions, the mantle lithosphere may be delaminated together with lower crust in the case of its eclogitization after critical amounts of shortening. Density inversion may be realized in collisional orogens, where compressive lithosphere thickening give way to extension collapse. A complete orogenic cycle includes three stages of development: 1) collision, thickening, uplift of topographic surface, formation of crustal and lithospheric root; 2) metamorphism of crustal root and/or delamination of crustal root or lithospheric mantle; 3) collapse of orogen extension and reequilibration of Moho.

Delamination of lower crust. Basalt-eclogite phase transition. Phase transition of basalt in more thick modification, i.e. eclogite significantly contributes to the density inversion between lithosphere and asthenosphere with low-pressure Px+Pl+Ol mineral paragenesis turning into high-pressure eclogite composed by pyrope garnet and omphacite. Density of eclogite is by 6% more than that of initial substrate. It is similar to or may be higher than the underlying mantle density. Within continental areas with thin crust (< 45 km,) the lower crust with any compositional variations must have the density less than that of the mantle. And on the contrary, in areas where the compression leads to crustal thickening more than 50 km ( in Tibet up to 70 km), the rocks of basaltic composition in lower crust experience a great density increase at transition into eclogite and tend to foundering. In areas with thick crust, the delamination of lithospheric mantle causes the delamination of the crust lower part.

Fluids play the key role in eclogitization and delamination of lower crust in collisional orogens. Lower crust may be metastable for a long period with extremely low fluids quantity. Some researchers believe that fluids and deformation are of greater importance for eclogitic metamorphism than temperature and pressure (M. Leech, 2001). Eclogitization is followed by solidity decrease and in this regard, eclogite is less solid than its protolith. Eclogites are schistose and plastically deformed in highly deformed zones. The presence of water intensifies these processes. Metamorphic reactions increase the plasticity at the expense of the granularity decrease and the presence of metamorphic fluid. High eclogite density and the presence of the above weakened layer destabilize the crust basement. The lower crust delaminates itself and thus it increases a negative buoyancy of underlying lithospheric mantle.

Metamorphism of the dry lower crust is stimulated by the fluid infiltration: under P-T conditions satisfying the eclogite facies, the reactions will proceed rapidly with the volume decrease by 10-15%. This decrease causes the further fluid infiltration, completion of the lower crust eclogitization and its delamination. If the fluid infiltration does not occur, the thickened lower crust of orogens may be metastable for indefinitely long time (hundreds of m.y). Thus, the orogens evolution in final stage of its development is determined by the fluid regime of the lower crust. It is reasonable that processes of intracrustal magma formation in collisional orogens subjected to delamination may be essentially different from those in orogens, where the delamination did not occur.

Weighting of lithospheric mantle due to magmatic underplating and refertilization. If lithosphere is located within the eclogite stability field, the intruding melts or mafic cumulates may be transformed within its limits in the form of eclogite. Every 10% of eclogite will increase the mantle lithosphere density by about 1% and can result in its instability.

Delamination at the plume-lithosphere interaction. During the plume rise to the lithosphere basement, the destabilization of cold, more dense lithosphere (Relay-Taylor instability) occurs, it is expressed as follows: 1) regional dome uplift; 2) mechanical erosion of lithosphere above the head plume center and its differential thinning at lateral spreading; 3) delamination of lithospheric mantle, in addition, the reverse flow of the plume substance “pushes” the collapsed lithospheric parts into depth up to 400-500 km in the plume head marginal parts (Burov et al., 2007). Upwelling of hot plume matter can reach the Moho boundary with its adiabatic melting, heating of crust and formation of intracrustal melts.

Possibility of delamination depended on compositional differences between convective and lithospheric mantle. By numerical modeling, we have determined that only fluid-bearing asthenosphere corresponding to DMM is the most suitable medium for delamination of thickened lithosphere relatively to other more fertile model compositions of asthenosphere like the primitive mantle or KH lherzolite (Kiselev et al., 2004).

This work was supported by RFBR (grants 08-05-225, 08-05-00290).