LATERALLY FORCED OVERTURNS AND HORIZONTAL CHANNEL FLOW OF OROGENIC LOWER CRUST IN OROGENS

A large database of structural, geochronological and petrological data is combined with the gravity and seismic geophysical surveys of the subsurface basement to develop a two stage exhumation model of deep seated rocks in the Paleozoic orogen in Europe. An early sub-vertical fabric originated in orogenic lower and middle crust during intractrustal folding process followed by vertical extrusion of weak lower crustal rocks. This event is responsible for exhumation of the orogenic lower crust from 18−20 kbar to 8−10 kbar and contemporaneous burial of upper crustal rocks to 8–9 kbar. The folding and vertical extrusion event is followed by the vertical shortening leading to development of sub-horizontal fabrics at medium to low pressures. Corresponding fabrics originated during building of a crustal orogenic root as a response to the SE directed oceanic and later continental subduction. The second exhumation mechanism is related to indentation of a continental plunger at the opposite side of the orogen into rheologically weaker rocks of the orogenic root that started to flow over the basement within a northward directed sub-horizontal channel. This flow is responsible for heterogeneous mixing of lumps of lower and middle crustal rocks and for their progressive thermal re-equilibration. The depth level as well as the degree of reworking decreases from the south to the north, pointing to an outflow of lower crustal material to the surface and its sedimentation in the foreland basin. We discuss and model time scales and length scales of different heat sources which influence bulk rheological properties of thickened roots and trigger both vertical extrusion and channel flow processes. It is shown that emplacement (re-lamination) of felsic crust with high radioactive heat production at Moho depth above lithospheric mantle generates softening of orogenic lower crust which becomes weak and mobile thanks to vigorous heating and partial melting. Combined action of lateral forces and crustal scale gravity instabilities generates large scale overturns called “Laterally Forced Overturns” that are responsible for emplacements of felsic and granulitic lower crust in mid-crustal levels and development of shallow channel flow underneath rigid lid.