The thermal structure of the crust strongly influences deformation, metamorphism and plutonism. Models for the geothermal gradient in stable crust predict a steady increase of temperature with depth. This thermal structure, however, is incompatible with observations from high-temperature metamorphic terranes exhumed in orogens. Global compilations of peak conditions in high-temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure, for both isothermal decompression and isobaric cooling paths. Here we develop simple one-dimensional thermal models that include the effects of melt migration. These models show that long-lived plutonism results in a quasi-steady-state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust. The models also predict that the upward advection of heat by melt generates granulite facies metamorphism, and widespread andalusite-sillimanite metamorphism in the upper crust. Once the quasi-steady-state thermal profile is reached, the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions, thus setting the stage for gravitational collapse, exhumation and isothermal decompression after the onset of plutonism. Near-isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that, in part, control the shape of the geotherm.

The Vaca Muerta-Quintuco system (uppermost lower Tithonianlower Valanginian) is a thick shallowing-upward sedimentary cycle consisting of dark bituminous shales, marlstones, limestones, and sandstones, cropping out in the Neuquén Basin, west-central Argentina. This paper analyzes three outcrop sections in Chos Malal area, northem Neuquén province. Detailed facies analysis allows us to differentiate six facies associations, representing basinal to proximal outer ramp facies of a homoclinal carbonate ramp system (Vaca Muerta Formation) and basinal to shoreface facies of a mixed carbonate-siliciclastic shelf system (Quintuco Formation), prograding westward from the eastern margin of the basin. Two sequence hierarchies were recognized: 5 composite depositional sequences (third order) and 15 high-frequency sequences (fourth order). Fluctuations in organic matter content within the Vaca Muerta Formation suggest a relationship with depositional sequences, finding the highest values associated with transgressive systems tract, whereas the transition to the Quintuco Formation shows a strong decrease in total organic carbon. The xray diffraction studies show an increase of clay minerals and quartz in the transgressive systems tract of the Vaca Muerta Formation and an increase in the content of calcite in highstand systems tracts. This pattern is reversed in the Quintuco Formation. Our sequence stratigraphic approach contributes to the understanding of the A U T H O R S

No abstract