CAN PT-TIME PATHS FROM EXHUMED METAMORPHIC ROCKS REVEAL MANTLE PROCESSES BENEATH OROGENS?

Geodynamic processes involving the lithospheric mantle beneath orogens are commonly invoked to explain a variety of tectonic and metamorphic effects. These processes include delamination, convective downwelling, ablative subduction, slab breakoff, slab retreat, and the formation of slab windows. Most of these involve the removal of part or all of the lithospheric mantle beneath the orogen, and its replacement by asthenosphere. This should result in a significant increase in the heat flux at the Moho, and a transient thermal pulse in the overlying crust. Whether this effect can be detected and identified depends on several factors. (1) The level at which lithospheric mantle is removed. Delamination or slab breakoff should cause drastic heating near the Moho and extensive crustal melting, whereas convective downwelling may only remove part of the lithospheric mantle, so that the amount of heating will be less, and the effect will be delayed. (2) The rate and amount of exhumation following removal of lithosphere. Removal of negatively buoyant mantle is likely to cause (a) an increase in surface elevation, enhancing erosion, and (b) an increase of gravitational potential energy, which may result in vertical shortening. Thinning reduces the thermal time constant for conductive heat transfer, enhancing the effect of a transient heat source at the base of the lithosphere, as well as heat loss towards the surface. (3) High radiogenic heat production in orogenic crust composed mainly of upper crustal rocks may mask the effects of subcrustal heat sources. (4) Advective heat transfer by magmatism or lower crustal flow may also mask the contribution of heat flow from the upper mantle.

The Betic-Alboran system in the western Mediterranean is an example of a system where a clear thermal signal produced by the removal of lithospheric mantle can be identified. Rapid extensional exhumation allowed the conduction of a transient thermal pulse to shallow levels. Continued temperature increase during rapid decompression to depths of <15km is difficult to explain by any other mechanism in the absence of magmatism. The lack of such a clearly identifiable thermal signal in other regions where lithospheric mantle may have been removed, such as the Basin & Range Province, may reflect lower total amounts or rates of crustal thinning.